4 Hip

The clinical examination consists of measuring leg length (see p. 144), observing gait, measuring the circumference of the muscles at standardized measuring points, and assessing the active and’ passive ranges of motion. Functional tests provide further information in certain lines of inquiry.

In newborns and infants, provocative tests (Orto-lani and Barlow) can be also performed in the presence of suspected hip instability. The dynamic ultrasound examination complements these tests.

Patient History

The patient’s history can provide valuable information about the presence and causes of a hip disorder. The patient’s age is the first clue that the history provides. In newborns and infants, the disorders encountered are most often due to abnormal development such as developmental hip dysplasia. Risk factors for developmental hip dysplasia in newborns may include:

– Positive family history

– First pregnancy

– Breech presentation

– Female sex

– Other anomalies such as club feet or torticollis

– Subsidence of spontaneous kicking toward the end of pregnancy

Children and adolescents with a disturbed gait may have Legg-Calvé-Perthes disease or a slipped capital femoral epiphysis. Symptoms of degenerative diseases such as idiopathic chondrolysis or degenerative joint disease are more prevalent in adults and older patients.

Questions to ask the patient include:

• Location of pain
Pain is usually felt in the groin or in a retrotrochan-teric location, occasionally radiating to the inner thigh and extending as far as the knee. This means, especially in children, that a hip disorder can masquerade as knee pain. The differential diagnosis should consider pathology in the adductor insertions and involvement of the lumbar spine, particularly the sacroiliacjoints.

• Onset of pain
Acute pain is indicative of inflammation, infection, or involvement of the femoral head as in an acute slipped capital femoral epiphysis, a subchondral collapse in Legg-Calvé-Perthes disease, or pathologic fractures. Pain that develops slowly suggests a degenerative disorder or tumor.

• Time and duration of pain
Transient morning pain that subsides after the patient has taken a few steps, but which reappears with exercise such as walking longer distances or climbing stairs (exercise pain), is typical of degenerative hip disorders.

The differential diagnosis should consider neuro-genic claudication (in the presence of stenosis of the spinal canal) and vascular claudication (in the presence of peripheral vascular disease). With neurogenic claudication, the patient feels no pain when beginning exercise, and bending forward in a sitting position lessens the pain. Generally, the pain radiates symmetrically into both the front and back of the thigh. With vascular claudication, the patient reports more rapid onset with exercise, and distal rather than proximal pain. This will be accompanied by alterations in perfusion, and occasionally by murmurs detectable by auscultation over the femoral arteries.

Fig. 4.1 Adduction contracture

• Pain provocation
If a hip disorder is present, pain can usually be provoked by exercise (standing up from a sitting position, climbing stairs) or detected during the examination (pain felt upon forced internal rotation). In contrast to disorders of the lumbar spine, advanced hip disease will not allow extension of the affected leg; active flexion may only be possible, at the hip, causing pain.

– Pelvic obliquity is the result of functional leg shortening due to contracture of the adductors or abductors in the hip (Figs. 4.1 and 4.2).

In an adduction contracture of the hip, the affected leg will appear shorter because the patient must raise the pelvis on the affected side when trying to place the foot of the affected side beside that of the nonaf-fected side. Conversely, an abduction contracture requires the patient to raise the contralateral side to place both feet together parallel to each other.

Trendelenburg gait due to weakness. In the Trendelenburg gait (Fig. 4.3), weakness of the hip abductors, primarily the gluteal musculature, causes the pelvis to dip toward the unaffected side in the stance phase. Again, the patient attempts to relieve the weakened gluteal musculature by shifting the upper body over the affected side. However, the stance phase is not as sharply truncated as in an antalgic gait, and motion is more uniform. A bilateral Trendelenburg gait produces typical waddling. Testing for the Trendelenburg sign will determine whether such a gait is a true Trendelenburg gait.

Compensatory limp with leg shortening. The upper body is shifted slightly over the leg in the stance phase. Otherwise, the gait is relatively smooth.

Compensation for hip fusion. Arthrodesis of the hip does not produce a true limp in the sense that the pelvis dips in the stance phase. Rather the increased tilt of the pelvis in the sagittal plane, as it moves from hyperlordosis into lumbar kyphosis, produces femoral anteversion in the swing phase. Since most hip fusions result in some degree of femoral shortening, the post hip fusion gait may demonstrate upper body shift to the affected side in the stance phase as well as femoral anteversion in the swing phase.

Fig. 4.3 Trendelenburg limp

A number of computerized methods have been developed to supplement clinical examination of gait. These methods, some of which are used in combination with electromyographic studies, may allow the examiner to define the anatomical and functional cause of gait irregularities with far greater precision.

Palpation

To palpate the hip, first locate the origins and insertions of the hip muscles to evaluate tendon disorders involving the muscular insertions (Table 4.1). Muscle pulls in the iliopsoas often occur in athletes (Fig. 4.4).

It is almost impossible to palpate the joint capsule in adults. Ultrasound studies should be performed if an effusion is suspected. Further palpation should attempt to exclude disorders such as inguinal and femoral hernias, varicoceles, and lymph node disorders. Palpate the femoral arteries and listen to them through a stethoscope to exclude arterial peripheral vascular disease.

Assessing the Range of Motion in Adults

In the initial position, the patient is supine with the hip fully extended. The medial aspects of both knees are barely touching, and both feet are parallel. The legs are correctly positioned when the axes of support (the straight lines connecting the midpoints of the hip, knee, and ankle) are parallel. The pelvis is horizontal when the straight line connecting the anterior superior and anterior inferior iliac spine is at an angle of approximately 12° from horizontal. In the neutral position, the pelvis is tilted slightly anteriorly by the lumbar lordosis.

Perform the examination with the patient supine on a stable surface. Ensure that the pelvis is in a neutral position by placing your hand under the sacrum. Now flex the contralateral leg at the hip until the lumbar lordosis is neutralized and the sacrum touches your hand. In this position, the patient holds his or her own leg (Thomas grip; Fig. 4.5). Now flex the leg to be examined until the pelvis begins to move. Immobilize the pelvis with your hand on the sacrum. The angle between the thigh and the examining table corresponds to the angle of flexion. Evaluate extension with the patient supine by flexing the contralateral leg past the position in the Thomas grip until the leg being examined is lifted off the table (Fig. 4.6).

Fig. 4.4 Palpating the lesser trochanter at the insertion of the iliopsoas while rotating the leg.

The difference in the angle between both femurs corresponds to extension.

Hip flexion and extension can also be evaluated in the lateral position. Place the patient on the contralateral side while immobilizing the pelvis in the neutral position of 12° anterior tilt with one hand. To evaluate flexion, flex the leg until the pelvis begins to move. To evaluate extension, place the patient supine, immobilize the contralateral leg with the Thomas grip, and extend the hip until the pelvis begins to move.

Make sure that the knee is flexed when evaluating hip flexion so that the hamstrings are relaxed. The knee should be extended to relax the rectus when extension is evaluated. Contractures in the knee can simulate limited hip motion.

Normal values in adults:
Extension: 10°–15°
Flexion: 130°–140°

Fig. 4.5 Evaluating flexion using the Thomas grip.

Fig. 4.6 Evaluating hip extension.

Fig. 4.7 Evaluating abduction in extension (one hand holds the anterior superior iliac spine).

Fig. 4.8 Evaluating adduction in flexion (with slight flexion so that the leg may be lifted over the contralateral leg).

Fig. 4.9 Evaluating rotation in 90° flexion with the patient supine.

Abduction and Adduction

Abduction and adduction are also evaluated with the patient supine. In children, this examination is performed with the hip extended; in adults, the hip is usually flexed 90°. Two lines are used for orientation in the neutral position. The first is the line connecting the two anterior superior iliac spines, which should be perpendicular to the axis of the body. The second is the line connecting the midpoints of the hip, knee, and ankle joints. This line should be parallel to the axis of the body.

Examination in extension. Immobilize the pelvis with one hand so that your thumb and small finger palpate the anterior superior and inferior iliac spines. With this action, compensatory movement of the pelvis is readily detectable. The extended leg is abducted until passive motion of the pelvis is detected (Fig. 4.7). To test adduction, place the leg being examined in slight flexion and lift it over the contralateral leg. Another option is to immobilize the contralateral leg with the Thomas grip. This makes it possible to adduct the leg even in full extension (Fig. 4.8).

Normal values in adults:
Abduction: 30°–45°
Adduction: 20°–30°
Examination in 90° flexion

This examination can be used in adults in addition to the examination in extension. In newborns, it is the only way to measure abduction and adduction because of physiologic flexion contracture. As in the examination in extension, immobilize the two anterior superior iliac spines with one hand while abducting the flexed leg from the vertical position until the pelvis begins to tilt in the transverse plane. Proceed similarly to evaluate adduction.

Normal values in adults:
Abduction: 60°–70°
Adduction: 20°–30°

Internal Rotation and External Rotation

Hip rotation is usually evaluated in children with the patient prone and the hip extended (Fig. 4.9). In adults, the examination may also be performed with the patient supine and the hip flexed 90°. When performing the examination with the patient supine, make sure that the axis through the two iliac spines is perpendicular to the axis of the body. Move the hip while monitoring the movement of the iliac spines until passive motion of the pelvis is detected. When performing the examination with the patient prone and the knee flexed, place your hand on the sacrum to immobilize the pelvis.

In a slipped capital femoral epiphysis, the thigh will move into compensatory external rotation as hip flexion increases (Fig. 4.10).

Ludloff-Hohmann Test

The knee can be completely extended when the hip is in flexion and adduction because the relative shortening of the thigh relaxes the hamstrings. Positive results are a sign of hip dysplasia.

Fulcrun or Stinchfield Test

In the presence of proximal femur, femor neck, or subcapital pathology such as stress fractures, holding the heel with the hip extended and pressing down on the midfemur will elicit groin pain.

Clinical Examination of Newborns and Infants

When examining the hip in children and newborns, obtaining a precise history is essential (see also Patient History). Inquire about a history of hip dysplasia in the family, whether the child is the first born, about breech presentation, details of delivery, the size of the child, the presence of other position anomalies, and differences in kicking motions. The type of pain can also be a sign because small children especially will present with referred pain into the knee. Even temporary pain with exercise can be an early sign of hip dysplasia. Sometimes “start-up” pain will be present, which children may barely notice. In some cases, the only sign will be gait irregularity. Other causes of such symptoms aside from hip dysplasia include an undiagnosed neurologic disorder, spasticity, or flaccid paralysis. Limps are categorized in various forms as discussed above. The Trendelenburg gait, Duchenne antalgic gait, leg shortening, and a fused joint should be distinguished. Examination of leg length can provide crucial information, particularly in the case of unilateral hip dislocation, where the affected leg appears shortened. Leg length is best evaluated with the child supine and the hips and knees flexed 90°. If this type of apparent leg shortening is present, one could estimate the difference between the legs by the level of the knees. The appearance of the soft tissue would also alter in a fully dislocated hip. Additional skin folds would be present.

Assessing Range of Motion

The differences in technique in the examination of newborns are due primarily to the physiologic flexion contracture in newborns, which precludes evaluation of hip abduction and rotation in extension. The examination is performed with the baby supine with hips and knees in 90° flexion. Make sure that the axis through the two superior iliac spines is horizontal to the examining table and perpendicular to the axis of the baby’s body. Estimate the flexion contracture before performing the examination. It can vary, but will generally be about 20°–35°.

Normal values for abduction and rotation in newborns fluctuate and are difficult to specify. Haas’ studies cite the following ranges of motion:

Normal values in newborns:
Internal rotation: 50°–70°
External Rotation: 75°–105°
Abduction: 65°–90°

According to studies by Harris, the range of abduction decreases in the first 9 months of life to 60°–70°. A range of abduction of less than 50° at birth can be regarded as abnormal. It is important to compare findings with the contralateral side in each case. Tonnis cites a difference between sides of 10°–30° for hip dysplasia and 30°–40° for dislocation. Differences between the sides in the range of abduction can also be the result of a deformity with pelvic obliquity. Symmetric bilateral limited abduction occurs in bilateral hip dysplasia or dislocation.

Asymmetric skin folds in the adductor and buttocks region are the result of superior protrusion of the femoral head on the affected side; the soft tissue of the thigh is too long for this pathologic anatomical configuration and forms folds (Fig. 4.11). However, this is not a specific sign (Table 4.2) as it can occur in up to 30% of children with normal hips. Asymmetrical folds may also be observed in scoliosis.

• Palpation
There are three situations in developmental dysplasia of the hip. The first is a dislocated hip, the second a dislocatable hip, and the third a subluxatable hip.

• Ortolani Test
A snapping sound in the first few days and weeks of life suggests an unstable hip (Fig. 4.12). Sit in front of the supine newborn. With one hand, grasp the leg to be examined by the flexed knee so that your thumb is touching the medial thigh and your second finger and ring finger are touching the greater trochanter. Bring the contralateral hip into maximum flexion with your other hand by grasping the knee so that the pelvis is immobilized. Now flex and slightly adduct the leg to be examined. In this position, apply slight anteroposterior (AP) pressure to the knee you are holding with your hand. This will cause posterior subluxation of the femoral head. Abduct the hip under slight pressure. Apply slight pressure to the greater trochanter. In a positive test, you will hear a characteristic snapping sound during the abduction motion as the femoral head reduces into the acetabulum.

• Barlow test
In this examination, the hips are brought into intermediate abduction, and pressure is applied to the greater trochanter to evaluate reduction (Fig. 4.13). Then try to dislocate the hip by applying posterior and lateral pressure to the femoral head.

Neurologic Examination
Examining the Muscles

Not every nerve root has a specific corresponding muscle.

To maximize clinical information, examining the muscles according to their function is recommended. They are then evaluated according to the muscle grading chart.

Flexors
Primary flexor: iliopsoas (femoral nerve, Ll, 2, and 3).

The iliopsoas is the primary flexor. To test it, have the patient sit and let his or her legs dangle over the side of the examining table. Stabilize the pelvis by placing your hand on the iliac crest and having the patient lift the thigh off the table. Press on the distal portion of the thigh with your other hand to determine maximum resistance. Use both hands with strong patients (Fig. 4.14). Repeat the test on the contralateral side for comparison.

Fig. 4.14 Evaluating the iliopsoas.

Extensors
Primary extensor: gluteus maximus (inferior gluteal nerve, S1).

To test the gluteus maximus in isolation, have the patient lie prone with the knee flexed. Place your arm over the patient’s iliac crest to stabilize the pelvis before instructing the patient to lift the leg. Provide resistance to this motion by pressing against the distal thigh with your other hand. Palpate the muscle tone during this examination. Repeat the test for the contralateral side.

Abductors
Primary abductor: gluteus medius (superior gluteal nerve, L5).

To evaluate abduction, place the patient in a lateral position and stabilize the pelvis at the iliac crest. Have the patient abduct the leg against your resistance. Another option is to examine the patient supine with the leg abducted about 20°. Again, instruct the patient to abduct the leg against your resistance.

Adductors
Primary adductor: adductor longus (obturator nerve, L2, 3, and 4).

To evaluate adduction, have the patient adduct the abducted leg against your resistance (Fig. 4.15). Another option is to examine the patient supine. Again, instruct the patient to adduct the abducted leg against your resistance.

Examining Sensation

Findings on sensation testing are easier to evaluate than muscular findings because the sensory supply is divided into demarcated dermatomes (Fig. 4.16) that are relatively similar among all patients.

Dermatome L1 covers a transverse band along the anterior aspect of the proximal thigh directly distal to the inguinal ligament.

Dermatome L3 extends in an oblique band proximal to the knee.

Dermatome L2 lies precisely between these two. Dermatome L4 includes the area of skin on the medial thigh; dermatome L5 supplies the lateral calf including the dorsum and medial edge of the foot.

Dermatome S2 supplies a long strip along the posterior aspect of the thigh from the fold of the buttocks to the popliteal fossa.

Dermatome S3, often clinically significant because of the lateral femoral cutaneous nerve, is a wide oval area located on the lateral aspect of the thigh.

Dermatomes S3 through S5 are arranged in three concentric rings around the anus, from medial to lateral.

Fig. 4.15 Evaluating the hip adductors.

4.3 Radiology

Standard Views

Standard views of the hip include the AP radiograph of the hip and lateral radiograph of the hip.

The AP radiograph is used to determine the neck-shaft angle and assess possible varus or valgus deformity. It can be used for detecting fractures of the ipsi-lateral anterior pelvic ring, acetabulum, and femoral head and neck; for confirming dislocations of the hip; and for preoperative evaluation of the vertical origin of the lumbar spine where it exits the sacrum. It may be used in preoperative planning for leg-length operations.

The lateral radiograph demonstrates the anteversion of the femoral neck, the anterior and posterior cortex of the femoral neck, the ischial tuberosity, and rotation and displacement of the femoral head in fractures.

The AP radiograph is prepared with the patient supine with the legs in slight internal rotation to compensate for anteversion of the femoral necks. If the entire pelvis is to be imaged, the ray is centered on the middle of the pelvis; if a hip is to be imaged, it is centered on the respective femoral head.

The lateral radiograph is taken with the patient supine. The affected leg is extended, and the opposite leg is lifted and abducted (Fig. 4.17). The cassette is placed along the side of the affected hip with the ray centered on the groin and inclined 20° cranially from horizontal. This permits calculating the angle of ante-version as described by Dunn, Muller, and Rippstein (see section Hip Dyplasia, p. 153).

Fig. 4.16 Sensory dermatomes of the lower extremity.

Fig. 4.17 Positioning for the lateral radiograph of the hip.

Fig. 4.18 Technique of the anterior oblique radiograph (obturator view).

Fig. 4.19 Obtaining the posterior oblique radiograph (iliac wing view).

Special Views

Special views include inlet and outlet views of the pelvis, variations of the lateral radiographs, various oblique views, Lauenstein views, AP radiographs with altered settings, views for preoperative planning, and special views for precise evaluation of the acetabulum and the femoral neck.

In a trauma setting, inlet and outlet views of the pelvis can be helpful in defining a fracture detected in the AP radiograph. The patient is supine with the legs extended for both views. For the inlet view (Fig. 9.37), the central ray is angled 40° caudad and enters the midline at the level of the anterior superior iliac spine. Forthe outlet view (Fig. 9.39), the central ray is angled 40° cephalad and enters at the inferior aspect of the pubic symphysis.

The lateral radiographs mentioned in the previous section can be varied by altering the projection. Epiphyseal dislocation can be documented precisely in a straight lateral projection. A vertical projection, i.e., inclining the projection 20° cranially from horizontal (see above), is especially suitable for documenting anteversion and a slipped epiphysis. A horizontal projection (Sven Johnson) is useful as a second imaging plane if fractures of the femoral neck are to be internally fixed.

The oblique radiographs (Judet views see also p. 434) include anterior (internally rotated) and posterior (externally rotated) views. These views are most suitable for diagnostic purposes and preoperative planning for treatment of acetabular fractures. The anterior oblique view shows fractures of the anterior column or posterior acetabular margin, and the linea terminalis. The posterior view shows fractures of the posterior column or anterior acetabular margin, and the quadrilateral plate.

The anterior oblique radiograph (obturator view) is prepared with the patient positioned supine and rolled 45° onto the healthy contralateral hip (Fig. 4.18). The ray is centered vertically on the affected hip.

The posterior oblique radiograph (iliac wing view) is also prepared with the patient supine, but rolled 45° onto the affected side (Fig. 4.19). Here too, the ray is centered vertically on the affected hip.

The Lauenstein view is particularly suited for visualizing fractures of the femoral head or neck and of the greater or lesser trochanter. For a Lauenstein radiograph, the patient is positioned supine with the knees flexed, pressing the soles of the feet together at maximum abduction. If both hips are to be imaged, the ray must be centered vertically or inclined about 10° cranially and centered on a point slightly superior to the pubic symphysis. If one of the hips is to be imaged, the ray is centered directly on that hip.

The Ferguson view and Letournel view represent modifications of AP radiographs in which the X-ray tube is inclined 30°−40° cranially or caudally (Fig. 4.20) The cranially inclined view visualizes fractures of the pubic rami, sacral fractures, and ruptures of the pubic symphysis and sacroiliac joint. The caudally inclined view permits the surgeon to assess the pelvic ring and the sacroiliac joints.

Preoperative planning studies are prepared if a varus or valgus correction in abduction or adduction is planned. The surgeon may often elect views in flexion or hyperextension for preoperative planning in the presence of defects in the femoral head.

The Faux profile view is recommended for precise evaluation of the anterior acetabulum. The patient stands with the pelvis against a diagonal wall fixed at an angle of 65° to the X-ray plate (Fig. 4.21). The leg bears the patient’s weight during the exposure and must remain strictly parallel to the X-ray plate. The center of the film plate is positioned behind the patient’s hip. When using this technique, it should be noted that increased anteversion may increase the anterior inclination of the roof of the acetabulum.

Abnormal Findings
Hip Dysplasia

Developmental dysplasia of the hip can only be imaged in AP views, which can be altered by placing the hips in abduction or internal rotation. However, a variety of reference lines can be used to quantify this disorder.

Fig. 4.21 Technique of the Faux profile view for evaluation of the roof of the acetabulum.

The initial purpose of the radiograph is to demonstrate the relationship between the femoral head and the acetabulum. Determining reference angles, lines, etc. can provide more precise information about the severity of the dysplasia. Evaluation of the ossification center in the femoral capital epiphysis will provide clear signs of delayed development even during the first years of life.

When evaluating radiographs of newborns and children, always bear in mind that about a quarter of all girls and about half of all boys do not yet have an ossified core in the femoral head. Differences in shape and size often represent physiologic rather than pathologic findings. At age three or four, the ossified core of the femoral head will usually be recognizable as a hemispheric shape. The epiphysis between the femoral neck and head courses obliquely from superior and lateral to inferior and medial. Its physiologic appearance is slightly wavy. The younger the child, the wider the joint will be, opening at the margins. By the age of 18, the head fuses completely with the femoral neck.

As there is a wide variety of reference lines, we will limit our discussion to a selected few in the interests of clarity. Proper positioning of the child’s pelvis is crucial to all of these methods as unreliable findings might otherwise result.

Hilgenreiner’s line is also known as the triradiate cartilage line since it connects these two structures (Fig. 4.22). This line is tangent to the most distal points of the iliac wings, at the distal margins of the acetabula.

Perkins’ line intersects Hilgenreiner’s line since it is drawn down to Hilgenreiner’s line from the most lateral point of the acetabulum. It forms four quandr-ants used to determine the degree of hip dislocation.

Shenton’s line follows the superior curve of the obturator foramen and continues in a curve along the inferior margin of the femoral neck (Fig. 4.23). This line is interrupted in early dislocations and in Legg-Calvé-Perthes disease.

Calve’s line (Fig. 4.23) extends the lateral margin of the iliac wing from the acetabular convexity to the femoral neck. Normally it should touch the lateral margin of the femoral neck.

Hilgenreiner’s acetabular index can be influenced by patient positioning (Fig. 4.24). A proven technique is to place the patient with the legs extended in a strict neutral position. The angle is obtained by marking a point in the triradiate cartilage at the lowest lateral corner of the ilium on both sides and connecting these points with a horizontal line. A second line is drawn from this point on the ilium to the lateral acetabular convexity. This point should lie within the sclerotic area.

If the angle of pelvic inclination is great, the anterior and posterior margins of the acetabulum may project separately. In this case, the tangent extending to the acetabular convexity should not be drawn at the inferior projection line but should be drawn directly to the intersection of the two projection lines at the acetabular convexity. The desired angle is located between the triradiate cartilage and the line to the acetabular convexity.

Defining normal values in the first few months of life is difficult. Hilgenreiner defined a value of 35°, although fluctuation occurs within a wide range. In practice, a system of classification according to one or two standard deviations was introduced, which determines how observation of the hip should continue. It became apparent that hips ranging between both values (20°-25° and 35°-40°) can remain normal or become abnormal, whereas hips with values exceeding the standard deviation should generally be regarded as pathologic.

Wiberg’s center/corner angle can be used to evaluate the position of the femoral head relative to the acetabulum (Fig. 4.25).

This angle is measured between a line drawn parallel to the longitudinal axis of the body and a line connecting the midpoint of the femoral head and the lateral apex of the superior acetabular margin. This angle increases as the depth of the acetabulum increases, and decreases in a shallow acetabulum.

Normal values are 25° between the ages of five and eight, 30° between the ages of nine and 12, and 35° above that age.

The neck-shaft angle described by M. E. Müller as the collum-center-diaphysis or CCD angle is an important angle in the femoral neck (Fig. 4.26). This angle is formed by an arc intersecting the most lateral point of the epiphysis and the diaphyseal “spine” as the medial reference point on the section of the femoral neck that forms the head. Another lateral point, which lies on the arc around the midpoint of the femoral head in the narrowest part of the femoral neck, is marked and included. The points of intersection are then connected with the cortex of the femoral neck. A line drawn form the center of the head perpendicular to this line represents the axis of the femoral neck. The axis of the femur is the midline between the margins of the shaft. The neck-shaft angle is measured medially between the femoral neck and the axis of the femur. Since the physiologic anteversion of the femur will result in an excessively high value, the measured value should be converted using Muller’s tables to determine the actual value.

Fig. 4.25 Wiberg’s center/corner angle.

Fig. 4.26 Neck-shaft angle described by M. E. Muller.

Fig. 4.27 Radiographic image of Legg-Calvé-Perthes disease in the condensation and fragmentation stage.

Legg-Calvé-Perthes Disease

The radiologic diagnosis of Legg-Calvé-Perthes disease is usually made with an AP pelvic radiograph and with a Lauenstein view of both hips.

If indicated, these views can be supplemented with anteversion or lateral radiographs.

For documenting early stages of the disease, we recommend a radiograph with the hip flexed 30° and in neutral rotation, with the tube centered if necessary.

Throughout the course of the disease, the radio-logic diagnosis is made on the basis of a few features that may be very unspecific, particularly in the initial stages of the disease. Upon initial presentation it may be difficult, except in retrospect, to identify at which stage of the disease process a patient is.

A soft-tissue shadow can be an early sign of the disorder.

Widening of the joint space or lateralization of the femoral epiphysis may be a further sign.

Changes in the epiphysis become increasingly apparent as the disorder progresses; the epiphysis shrinks and loses its sphericity. These changes affect the anterolateral quadrant in particular and can be seen in a Lauenstein view.

Kite and French described the flattening of the lateral and medial epiphyseal dome as a roof sign.

Köhler referred to the enlargement of the acetabular teardrop with an increase in the width of the anterior acetabular margin.

In the early stages of the disorder, the Lauenstein view can reveal a transient subchondral radiolucent line, which has been described as a fracture line, resorption zone, or crescent sign.

The condensation and fragmentation stage (Fig. 4.27) is usually characterized by compression of the core of the head and loss of roundness, later by a chaotic nodular structure and collapse of the epiphysis.

Lateralization (sometimes referred to as decentralization) also occurs as the femoral head enlarges and subluxes. At this time, the epiphysis is insufficiently covered. Decentralization results in disturbed anterolateral growth and creates a rounded recess inferior to the roof of the acetabulum. At times the epiphysis will appear to have sunk laterally in the shape of a saddle. Later, this will be accompanied by calcification-like image fogging and bony islands in the lateral aspect.

The reossification stage is characterized by the formation of new trabeculae, reossification of the cartilaginous mantle, growth of anterolateral bone islands, and fusion of the epiphysis (Fig. 4.28).

The final stage is usually characterized by a flattened, broadened, enlarged, and lateralized femoral head. Particularly in young children, restoration of joint integrity is sometimes possible. The femoral neck is shortened and widened, and sometimes the trochanter protrudes superiorly. This form is collectively referred to as coxa plana.

The so-called "sagging rope sign" has been described by Apley and Weintroub. This sign gets its name from the dense, inferiorly concave line at the upper femoral metaphysis, which is reminiscent of a sagging rope.

Catterall has suggested classifying radiologic findings into the four groups according to their prognosis:

Group 1: The anterior aspect of the epiphysis is only slightly affected; no metaphyseal changes are present, and the prognosis is good (Fig. 4.29).

Group 2: The anterior aspect of the epiphysis is significantly affected, with profound involvement of the medial and lateral aspects (Fig. 4.30); small cystic changes are present in the metaphysis. A subchondral fracture line is present, but does not extend beyond the tip of the epiphysis and lies in the anterior half. The prognosis is still good, especially in a younger child.

Group 3: The entire epiphysis is involved, and the condition is most acute in the anterior lateral aspect (Fig. 4.31). A long, prominent fracture line is present that covers the tip of the epiphysis on the AP radiograph. The AP radiograph in particular reveals the head-within-a-head phenomenon; the femoral neck is widened. The prognosis is considerably worse.

Group 4: The entire epiphysis is necrotic, and the head is mushroom shaped. A triangular shape is discernible medially and laterally, and there is generalized metaphyseal involvement. The prognosis is very poor (Fig. 4.32).

Fig. 4.31 Catterall, group III.

Fig. 4.32 Catterall, group IV.

Fig. 4.33 Schematic diagram of the Gage sign.

Salter and Thompson have described another similar classification system that emphasizes the significance of the subchondral fracture line.

Catterall has supplemented his classification system with risk factors, the four “head-at-risk signs” visible on the AP radiograph. Their presence denotes a substantially worse prognosis, and they are more significant than the extent of epiphyseal necrosis.

One of these signs is the Gage sign (Fig. 4.33), that represents an area of lysis in the lateral epiphyseal margin and the adjoining metaphysis. According to Catterall, this sign is indicative of a lateral deformation of the femoral head.

Lateral calcification is another sign. Lateral subluxation is regarded as an important criterion and is almost always indicative of an unfavorable outcome.

Metaphyseal changes should also be regarded as indicative of an extremely unfavorable development.

Additional risk factors can only be determined by measurement. These include containment of the femoral head, increase in radius or hypertrophy of the femoral head in the early fragmentation stage, coxa magna and coxa brevis, early epiphyseal closure, and lateral closure of the epiphysis of the femoral head.

Obtaining objective results from radiographic studies requires the use of indices that are partly specific to Legg-Calvé-Perthes disease and partly unspecific. We will discuss only the most common indices.

The epiphyseal index described by Eyre-Brook specifies a measure for epiphyseal flattening in terms of the ratio of the height of the epiphysis to the width of the epiphyseal line. This index is calculated from the quotients of the height and width of the epiphysis. In children below the age of seven, values between 45% and 55% are normal; in older children, values between 35% and 45% are normal.

The epiphyseal quotient described by Sjovall compares the healthy side with the affected side. The quotient is calculated by division, using the indices of the affected side and the healthy side. Normal values range from 90% to 100%.

The head-neck quotient represents a measure for the compaction of the femoral neck. It is the percentage quotient of the head-neck index of the affected and healthy sides. The head-neck index itself consists of the quotient of the length and shortest width of the femoral neck multiplied by 100. Normal values lie between 190 and 150.

The acetabulum-head index is a measure for the disproportion between head and acetabulum, particularly for the completeness of coverage. The index is calculated from the quotient of the horizontal diameter of the covered section of the head (i.e., from the medial epiphysis to the perpendicular of the ace-tabular convexity) and the horizontal diameter of the entire head. Normal values lie between 90% and 70%.

The comprehensive index according to Heyman and Herndon is an additional index for Legg-CalvéPerthes disease that covers the shape of the head, neck, and acetabulum. Results from 100% to 90% are regarded as having an excellent prognosis, from 90% to 80% good, from 80% to 70% satisfactory, from 70% to 60% poor, and below 60% very poor.

Capener’s triangle sign in the AP radiograph is early evidence of a slipped capital femoral epiphysis. The medial side of the femoral neck will be seen to overlap the posterior wall of the acetabulum, forming a triangle. In addition to this, the cranial S-shaped line marking the boundary of the femoral neck and head is flattened. Occasionally, proliferations of bone will be seen at the inferior posterior angle between the neck and epiphysis. The height of the epiphysis may be reduced as it begins to slip posteriorly (Fig. 4.34). The lines of the epiphysis may close prematurely.

The lateral projection can show increased bone resorption in the posterior metaphysis of the femoral neck. The contour of the head and neck may also be flattened. A common radiographic finding is loss of the smooth curve from the obturator foramen along the inferior femoral neck- a break in Shenton’s line.

The late stages of the disorder are characterized by convex deformation of the head, widening and shortening of the femoral neck, and structural changes in the metaphysis of the femoral neck. Bone remodeling and formation of osteophytes can occur in the femoral neck.

Osteoarthritis of the Hip

Osteoarthritis of the hip is diagnosed by narrowing of the joint space, increased sclerosis, formation of osteophytes at non-weight-bearing sites (i.e., usually at the edge of the joint), and formation of cysts or pseudocysts as a sign of micro fractures and penetration of synovial fluid into the cancellous bone (Fig. 4.35). Acetabular cysts of this sort are referred to as Egger cysts.

Special craniolateral oblique views are essential for evaluating osteophytes on the anterior circumference of the acetabulum and femoral head. Posterior or medial cysts can only be imaged in Faux profile views.

Appositions of osteophytes on the medial contour femoral neck (which Lequesne refers to as a “hammock”) show a double contour with a convex outer layer of ossification.

The Faux profile view will show a circular area of cartilage necrosis with a tendency to dislocate anteriorly, or primarily inferior and posterior cartilage necrosis with ossification of the interacetabular ligament.

In addition to these signs of arthritis, migration of the femoral head will be observed: the direction is generally superolateral.

The type of deformation and the displacement of the head relative to the acetabulum provide further information. Views in adduction and abduction, Faux profile views (see section Special Views, p. 152), and views in internal and external rotation may be helpful.

Hip dysplasia with osteoarthritis is a special case. In its early stages, it is characterized by an excessively small roof of the acetabulum, a center/corner angle of less than 25°, a pathologic acetabular index, an excessively large neck-shaft angle, and pathologic anteversion (see section Hip Dysplasia, p. 153). The femoral head may show flattening around the fovea, an abnormal position in the acetabulum, or the beginning of cartilage necrosis with congruity of the femoral head and acetabulum.

The late stage is characterized by arthritis and subluxation. Shenton’s line (see section Hip Dysplasia, p. 153) is disrupted, and there is increased sclerosis of the acetabular convexity, joint-space narrowing, and deformation of the femoral head.

Avascular Necrosis of the Femoral Head

Early signs of this disease that have been described include thickening and loss of roundness or subsidence of the anterosuperior contour of the femoral head.

Occasionally these discrete changes will only be visible in Schneider tangential views as these views visualized the largest part of the circumference.

Four radiographs should be obtained: a 30° oblique view demonstrating the posterosuperior sector, an AP view for the superior sector, and views in 30° and 60° flexion to demonstrate the anterosuperior portions.

Table 4.3 Ficat’s stages of osteonecrosis

Arlet and Ficat have developed a standard system for classifying the entire course of the disorder (Table 4.3).

— Stage 1 describes a normal radiograph while MRI reveals early changes.

— Stage 2 describes nodular changes in the cancellous bone with porous or sclerotic areas. At this time the joint space and the contour of the femoral head are still normal.

— Stage 3: the contour of the femoral head is interrupted. Sequestration as a result of collapse is observed while the joint space remains unchanged.

— Stage 4 represents the complete clinical syndrome. The joint space appears narrow and shallow. Massive joint destruction is present.

Rheumatoid Disease

Radiologie characteristics of rheumatoid hip disease include erosion, osteoporosis, and soft-tissue swelling. In contrast to degenerative hip disease, sclerosis or osteophytes can rarely be demonstrated.

Osteoporosis is a significant characteristic of the disorder. Initially it is localized; later, generalized osteoporosis can be demonstrated.

Narrowing of the joint space usually causes the femoral head to migrate axially, occasionally medially. In some cases, protrusion of the acetabulum will occur (Fig. 4.36).

This is a synovial-based process. Areas of joint erosion occur on both sides of the joint at the synovial insertions. These areas destroy the joint without any repair processes such as formation of osteophytes or sclerosis.

Synovial cysts and pseudocysts can be demonstrated in the immediate vicinity of the joint. Occasionally there will be radiographic evidence of an effusion.

Fig. 4.36 Radiographic image of protrusion of the acetabulum.

Fig. 4.37 Radiographic image of pigmented villonodular synovitis.

Pigmented Villonodular Synovitis

This rheumatic disorder is characterized by a lack of joint changes. Only in a few cases can small diffuse cysts be demonstrated (Fig. 4.37).

Femoral Neck Fractures

Fractures of the femoral neck are generally visible in the plain pelvic radiograph, although some can only be demonstrated in Lauenstein or lateral axial views. Extracapsular, intracapsular, and articular fractures are differentiated. Fractures can be described using the AO/ASIF classification system (Fig. 4.38).

The first step in diagnosing a fracture is to differentiate between intracapsular and extracapsular fractures. Intracapsular fractures show a fracture line in the femoral neck, occasionally involving the femoral head or the base of the femoral neck.

Intracapsular fractures can be classified according to the system defined by Garden.

Garden’s classification system is based on the position of the main medial weight-bearing trabecula. This system differentiates four degrees of severity (Fig. 4.39a-d).

— Type 1: describes an incomplete subcapital non-dislocated fracture. The distal fragment is externally rotated, and the proximal fragment is in a valgus position. The trabeculae of the medial fernoral head and those of the medial femoral neck form an angle of 180°. The prognosis is good.

— Type 2: defined as a complete subcapital fracture without deformity. The distal fragment is in a normal position with respect to the proximal fragment. The trabeculae of the medial femoral head form an angle of 160° with those of the medial femoral neck. The prognosis for these fractures is also good.

— Type 3: involves a complete subcapital fracture with a certain degree of deformity. The proximal fragment is twisted, abducted, and tilted into a varus position. This form of fracture is regarded as unstable, and the prognosis is unfavorable.

— Type 4: involves a complete subcapital fracture with pronounced deformity. In these fractures, the distal fragment is externally rotated, superiorly displaced, and lies anterior to the proximal fragment. However, the medial fragment is in a correct position in the acetabulum. The prognosis is least favorable for these fractures.

Extracapsular fractures are either intertrochanteric or subtrochanteric. Usually the fracture line can be followed from the lesser trochanter to the greater trochanter.

Intertrochanteric fractures can be described according to the AO/ASIF classification as simple fractures with respect to the number of fragments, according to the Boyd-Griffin classification or the classification of Fielding and Zickel.

The Boyd-Griffin classification shows a linear intertrochanteric fracture line in type 1, a comminuted fracture in the trochanter region with type 2, a comminuted fracture including a subtrochanteric component in type 3, and an oblique fracture extending into the subtrochanteric region in type 4 (Figs.4.40a-d).

Subtrochanteric fractures are divided into five types according to Seinsheimer’s classification as shown in Table 4.4 and Figure 4.41.

Fig. 4.38 AO/ASIF classification of proximal femoral fractures.

Positioning for ultrasound examination of the hip depends on the patient’s age. Newborns are examined in the lateral position in special examination tubs. The examiner stands to the right of the baby so that the acetabular region is imaged on the right and the trochanter region is imaged on the left (Fig. 4.42). High-contrast image settings should be used to better demonstrate cartilage and bone structures.

Older children or adults are positioned supine, rarely laterally. At this age, ultrasound examination can only provide information about periarticular structures so that softer image settings should also be preferred in these patients.

A distinction is made between the anterior and lateral imaging planes.

The examiner attempts to image hyaline cartilage structures. These appear as hypoechoic or anechoic in the sonogram. If the core of the femoral head has formed completely, this examination will no longer be possible because the bone will completely eliminate sound reflection and produce what is known as an acoustic shadow. This will occur within the first year of life (Fig. 4.43).

In healthy adults the hip appears in the sonogram as a hyperechoic triangular acetabular labrum, a sharp-edged hyperechoic acetabular convexity, and a hyperechoic band representing the joint capsule. The femoral head also appears as a hyperechoic structure. Occasionally, pseudolesions will be visible on the femoral neck.

Fig. 4.43 Sonogram of a 3-month-old infant.
1 Gluteus maximus
2 Gluteus médius
3 Gluteus minimus
4 Acetabular labrum
5 Joint capsule
6 Femoral head
7 Epiphyseal center of the femoral head
8 Chondro-osseous boundary in the femoral neck
9 Inferior margin of the ilium
10 Bony acetabular convexity
11 Proximal perichondrium (rectus tendon)
12 Silhoutte of the ilium

This clinical syndrome is diagnosed with the aid of ultrasound studies in infants, i.e., from birth until the end of the first year of life (Figs. 4.44–4.50). Developmental hip dislocation is a congenital disorder of the hip involving disturbed maturity, delayed development, or underdevelopment of the elements forming the hip. Subluxation or dislocation of the hip will usually occur in the first 6 months of life as a result. Rarely, a genuine congenital dislocation will be present; frequently this disorder will occur in combination with other deformities. Genetic and geographic factors influence etiology, as do exogenous factors such as breech presentation.

Legg-Calvé-Perthes Disease

Ultrasound examination of Legg-Calvé-Perthes disease is based on observation of the effusion (Fig. 4.51). This distends the joint capsule in the superior middle section of the femoral neck. The capsule distension is less than for infections, and can be detected for a significantly longer period of time.

Fig. 4.45 The alpha value is obtained from images in the longitudinal plane in a normal infant. The control range lies within the standard deviation; the therapeutic range lies within twice the standard deviation.

Fig. 4.47a Schematic diagram of sonographic type II hip. Total joint coverage is insufficient; relationship between the bony and cartilaginous parts of the acetabular roof shifted in favor of the cartilage.
1 Baseline 2 Line of the cartilage roof
3 Line of the roof of the acetabulum
α: Bone angle
β: Cartilage angle

Fig. 4.49a Schematic diagram of a type IIIb hip. Pressure on the cartilage roof alters its histologic structure, causing increased echogenicity.
1 Baseline 4 Transition point
2 Line of the cartilage roof 5 Superiorly elongated labrum
3 Bony roof line 6 Fulcrum
α: Bone angle
β: Cartilage angle

Fig. 4.51 Ultrasound image of Legg-Calvé-Perthes disease.

This disorder is demonstrated in the sonogram by a reduction in the height of the epiphysis, and especially by slippage (Fig. 4.52). An effusion may be present in acute cases.

Effusion

A hip effusion is characterized by hypoechoic widening of the intra-articular space with separation of the joint capsule from the femoral neck (Fig. 4.53). In children, the cartilaginous portion of the joint and the iliopsoas may also appear hypoechoic. This can be misinterpreted as an effusion. Comparison of both hips is recommended.

Trochanteric Bursitis

The lateral imaging plane demonstrates the trochanter. In this disorder, a widened hypoechoic bursa will be seen to extend far distally.

Problems and Sources of Error

In adults, but particularly in infants, imaging planes must be painstakingly followed to permit precise evaluation of the images and accurate measurements. Topographic structures must be clearly identified, and the images should be evaluated immediately so that the examination can be repeated if necessary. High image quality is particularly important in adults. This can require the use of a low-frequency transducer.

4.5 Arthrography

Indications and Contraindications

Arthrography of the hip is used primarily in children and rarely in adults.

The main indication of arthrography in children is to identify impediments to reduction in a dysplastic hip.

Arthrography can also provide information about the position of the femoral head and acetabulum in Legg-Calvé-Perthes disease, and can provide information about the articular surfaces. It is used to confirm needle placement in aspiration arthrograms for suspected septic arthritis in children.

Contrast-medium allergy is a contraindication. One must also consider the relatively high degree of radiation exposure and, especially in children, the risks of anesthesia. However, tolerance of a single application of anesthesia represents a minor risk in light of the definitive information arthrography provides for further therapy. The use of arthrography for other indications has decreased with the advent of CT and MRI. However, arthrography continues to offer the advantages of a low-cost method that is easy to use and permits dynamic examination.

Technique, Instrumentation, and Examination Procedure

Arthrography involves injection of a negative contrast medium, such as air or some other gas, or a positive contrast medium. Occasionally, one may elect to use a double-contrast method using air and a positive contrast medium.

The contrast medium is usually injected from an inferior approach (Fig. 4.54). Other possible approaches are from a superior, anterior, or medial approach, and via the greater trochanter.

The inferior approach is obtained by positioning the anesthetized child with the hip flexed 110° and abducted approximately 40°. The needle is inserted lateral and anterior to the ischial tuberosity, and is aimed at the acetabulum. Leakage of the injected Ringer solution shows that the proper position has been achieved before contrast medium is injected. Fluoroscopic control images are then obtained in the initial position and after reduction (Fig. 4.55).

Fig. 4.54 Arthrography

Fig. 4.55 Intraoperative arthrogram

Fig. 4.56 Arthrogram of Legg-Calvé-Perthes disease.

Normal Findings

Arthrography in infants should permit evaluation of articular cartilage. The acetabular labrum appears as a pointed gap in the contrast medium in the joint. Adjacent to the labrum lies a small recess, and inferior to the gap in the contrast medium lies a recess caused by the transverse acetabular ligament. This ligament can be found on the inferior margin of the acetabulum as the continuation of the acetabular labrum. This gap bridges the acetabular fossa, which is visible above the transverse ligament as a widened area of contrast medium. Contrast medium on the floor of the acetabulum should only be visible as a fine line.

Abnormal Findings
Legg-Calvé-Perthes Disease

With the aid of arthrography, the surgeon can assess the position of the femoral head, surface texture, and width of the joint cavity (Fig. 4.56). Images that can demonstrate containment are recommended for pre-operative planning.

Osteoarthritis of the Hip

The surface of the femoral head can also be demonstrated in various imaging planes. This is useful for preoperative diagnosis.

Hip Dysplasia (Fig. 4.57)

The primary indication for arthrography is to identify an impediment to reduction. The femoral head is increasingly displaced superiorly and laterally. There may be a fold in the acetabular labrum that prevents the femoral head from entering the acetabulum.

Implant Loosening

In cases of implant loosening, contrast medium can penetrate between the implant and the surrounding bone. This can provide a clear visual image of implant loosening.

Problems and Sources of Error

The primary sources of error in arthrography lie in inaccurate interpretation of the images. This is because structures are often difficult to identify. Another source of error is the evaluation of images obtained in inaccurately positioned imaging planes. Periarticular injection of contrast medium should be avoidable using the technique described.

See chapter 1 (Shoulder, p. 54) for a general discussion on the utility of nuclear medicine studies.

4.7 Computed Tomography

Indications

Indications for CT of the hip include diagnosis of fractures or dislocations, tumors, inflammatory or rheumatoid arthritis, and destructive changes. CT can demonstrate the relative positions of hip structures and image joint effusions. Changes to bony structures and soft tissue can be imaged.

Technique and Examination Procedure

The patient is supine in the CT gantry. In contrast to MRI, the examiner can only choose between horizontal and three-dimensional CT, although different slice thicknesses may be selected.

Normal Findings

The image demonstrates the acetabulum, the femoral head, the neck of the femur, and the obturatorius internus as an axial slice. The bony structures appear light, and the soft-tissue structures appear in various shades of gray.

Abnormal Findings
Abnormal Version of the Femoral Neck

To measure the angle of anteversion of the femoral neck and head with respect to the coronal plane, the angle of relative anteversion and the angle of internal or external rotation are determined to obtain the true angle of anteversion.

The patient is supine with the legs placed firmly together.

First define a straight line through the femoral neck in a slice through the femoral neck using the femoral head and the trochanter as landmarks. The angle between this line and the level of the table or horizontal is referred to as the angle of relative anteversion.

The angle of internal or external rotation is measured between the horizontal and a tangent along the posterior margins of the condyles.

In internal rotation, the true angle of anteversion is measured as the sum of the angle of internal rotation and the angle of relative anteversion. In external rotation, this angle is subtracted from the angle of relative anteversion.

Osteoarthritis of the Hip

In osteoarthritis with hip dysplasia, the major advantage of CT is its ability to faithfully reproduce three-dimensional relationships in a manner suitable for preoperative planning. One may also obtain measurements for designing total hip implants.

CT can image osteophytes and the extent of cartilage destruction and accompanying synovitis in degenerative or traumatic osteoarthritis of the hip.

Fractures

Fractures about the hip are divided into fractures of the pelvis, femoral head, and femoral neck.

CT studies can determine whether pelvic fractures are stable (i.e., when the pelvic ring is not compromised) or unstable. The acetabulum in particular can be readily visualized. Thinner slice thickness may be used where required to visualize fractures in this region that are otherwise difficult to image. It is important to identify intra-articular fragments or structures that block a concentric reduction of an ace-tabular fracture or hip dislocation.

A change in the width of the joint space at the anterior or posterior margin of the acetabulum can suggest a dislocation.

Intramuscular changes such as hematomas may be indirect signs of a fracture.

Three-Dimensional Reconstruction for Preoperative Planning

Three-dimensional images of bony surfaces (Fig. 4.58) are obtained by processing adjacent CT slices using image-processing equipment and software.

The image can be observed from any perspective. For clinical purposes, horizontal perspectives in 45° increments are standard.

However, any direction may be chosen if warranted by the specific line of inquiry. It is also possible to break the image of the joint down into its individual components and view them separately.

Three-dimensional reconstruction in the hip and pelvis is suitable for visualization of changes in osteo-arthritis of the hip, acetabular dysplasia, and axial deformities. Acetabular imaging is helpful in preoperative planning for treatment of acetabular fractures, corrective osteotomies, or total hip arthroplasty. It also permits custom design of hip implants.

4.8 Magnetic Resonance Imaging

Indications

The most frequent indication to perform an MRI is the avascular necrosis of the femoral head in adults, and Legg-Calvé-Perthes disease in children. MRI discovers pathologic changes earlier than other imaging modalities. Septic arthritis can be distinguished from aseptic arthritis. Soft-tissue injuries associated with fractures can be visualized. Furthermore, MRI can detect so-called occult osseous injuries. Osteoarthritis is an indication whenever the evaluation of the cartilage is needed to determine the therapeutic approach. Tumors of the hip region are primarily examined to assess size and location, less often to obtain a tissue diagnosis.

Examination Protocol

The examination is generally performed with the body coil, though the head coil is preferable for children. As for other joints, proton density weighted sequences are used for delineating the morphology, and T1-weighted and T2-weighted sequences, including fat suppression sequences, for displaying the pathology.

Fig. 4.58 Three-dimensional reconstruction of the hip and pelvis.

Pathologic Findings
Arthritis (Fig. 4.59)

Transient synovitis (irritable hip) is a painful condition occurring in the pediatrie age-group (Fig. 4.59). It resolves spontaneously with rest. MRI reveals an effusion and synovial thickening, best seen on the STIR image and on the T1-weighted phase-contrast image after intravenous administration of gadolinium-based contrast medium. This condition must reliably be separated from septic arthritis, which, in addition to a joint effusion, shows involvement of the osseous structures and periarticular soft tissue. The STIR image reveals linear areas of high signal intensity, which are close to the cortex and indistinctly demarcated from the bone marrow. They are mostly in the transition zone between femoral head and neck, but can also be found in the acetabulum. The periarticular soft tissues also show a high signal intensity that frequently spreads along the muscle fibers. The synovial membrane as well as the affected soft tissues and osseous areas show intense contrast enhancement, usually less extensive than the area of increased signal intensity seen on the STIR image.

Fig. 4.59 Coronal image (GRE 280/6.8 out of phase after application of intravenous gadolinium contrast medium) showing transient synovitis with a hypertrophic synovial membrane.

Fig. 4.60a Coronal image (GRE 280/6.8 out of phase) showing transient osteoporosis with medullary edema in the right femoral head with low signal intensity.

Fig. 4.60b Coronal image (GRE 280/6.8 out of phase after administration of intravenous gadolinium contrast medium) showing transient osteoporosis with contrast-medium enhancement in the femoral head and neck accompanied by synovitis.

Transient osteoporosis of the hip is a condition found in adults (Figs. 4.60a, b). MRI shows striking edema of the femoral head and neck with increased signal intensity on the STIR and T2-weighted images and decreased signal intensity on the T1-weighted image, accompanied by a small effusion.

Perthes Disease (Fig. 4.61)

This is an infarction of the femoral head in childhood and MRI shows a circumscribed zone of decreased signal intensity on the T1-weighted image. The infarcted area is in the center of the epiphysis. In the early stage, the femoral head keeps its spherical shape and the articular cartilage is unremarkable. The T2-weighted and STIR images show the infarcted area as a central increase in signal intensity. Enhancement is very strong. A small effusion is usually encountered. With progression of the disease, edema spreads through the femoral head and can extend into the neck. An enhancing demarcating zone can be recognized. The epiphysis collapses, with resultant epiphy-seal extrusion and flattening. A cartilaginous lesion is generally not found. A subchondral zone of decreased signal intensity is seen in all sequences as a sign of increased sclerosis. The peak incidence of this condition is between the ages 5 and 8. Its diagnosis is difficult before the fatty conversion of the epiphysis, since the epiphysis still has a low signal intensity on the T1-weighted and a high signal intensity on the T2-weighted image.

Avascular Necrosis of the Femoral Head
(Figs. 4.62a, b)

This is an infarction of the femoral head in adulthood. In the early stages, the infarcted area is best seen as decreased signal intensity on the T1-weighted image.

Fig. 4.61 Coronal image (SE 650/14) showing necrosis of the epiphysis with low signal intensity.

Fig. 4.62a Coronal image (SE 650/14) showing avascular necrosis in the left femoral head with subchondral infarction. Beginning avascular necrosis may be seen in the right femoral head with diffuse subchondral edema.

Osteoarthritis (Fig. 4.63)

Osteoarthritis is caused by cartilage degeneration and induces subchondral sclerosis and osteophyte formation along the acetabulum and also around the circumference of the femoral head, associated with more or less severe reactive synovitis. Initially, the cartilage is slightly thickened, followed by thinning and the formation of defects. These changes are best observed on high resolution T2*-weighted sequences. Osteophytes are easily seen on the T1-weighted image. The accompanying inflammatory changes are best seen on the STIR image or after intravenous administration of gadolinium-based contrast medium on the T1-weighted phase contrast image, with the synovial changes exceptionally well discernible.

Fractures (Fig. 4.64)

MRI is not the primary modality for diagnosing fractures. It contributes to the detection and evaluation of pseudoarthroses since the non-ossified tissue is well demarcated from the osseous structures on the phase contrast image. Occult fractures, however, are a domain of MRI. They are characterized on the STIR image as indistinct zones of high signal intensity in the trabecular bone.

Fig. 4.64 Coronal image (GRE 500/10 out of phase after application of intravenous gadolinium contrast medium). The hyperemia causes the contrast medium to enhance the fracture line in the femoral neck.

Muscular Injuries (Fig. 4.65)

They are primarily sport injuries ranging from sprains, usually involving the adductors, to tears. In sprains, the STIR or heavy T2-weighted sequences show a high signal intensity along the fascicular orientation of the muscle. The same sequences show the muscular fascicles to be interrupted with more or less severe hemorrhage in the created gap. After about 4 days, high signals relative to the low signal musculature are seen on the T1-weighted image. Intramuscular abscesses, usually appearing after intramuscular injections, have a homogeneously high signal intensity on the STIR and T2-weighted images. After intravenous administration of a gadolinium-based contrast medium, strong peripheral enhancement is seen on the T1-weighted image.

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