Radiology of the Hip

The pelvis is tilted anteriorly by about 45 degrees and normally has no rotation or obliquity. An AP pelvis X-ray is therefore an intermediate view — the outlet view is a true AP of the sacrum and the inlet view shows the entire pelvic ring in the axial plane. A 45-degree external or iliac oblique view shows the full profile of the ilium, anterior wall of the acetabulum, and the posterior column. The internal oblique or obturator view shows an orthogonal view of the ilium, posterior wall, and the anterior column.

Normal morphology is widely variable. It is better to describe each parameter in terms of ideal morphology so that deviations can be recognised. Morphological variation is often not pathological because several factors are interdependent. For example, coxa valga can cause increased joint reaction forces and contribute to instability, but is well tolerated if there is no acetabular deficiency or increased femoral anteversion.

Certain abnormalities — cysts in bone, fracture lines, joint space narrowing, slipped epiphysis, and fragmentation in early Perthes — are always pathological. Bone islands are incidental findings. Variations such as coxa vara, coxa valga, over-coverage, cam morphology, and dysplasia are deviations from ideal morphology that are abnormal only when they cause pathology and correlate with symptoms or influence the outcome of treatment.

Plain Radiographs

A radiograph provides a global two-dimensional view of the bony anatomy. With experience, interpretation of the three-dimensional structure becomes adequate. A full pelvis radiograph allows interpretation of the orientation of each acetabulum and determination of its version. Standing AP is better for showing subluxation in dysplastic hips; supine X-ray is easier to standardise for a perfect view.

The Perfect AP View

Beam centred on the pelvis, showing both walls of the acetabulum with no rotation or obliquity and normal pelvic tilt.

• Obliquity — none; both hips should be at the same level
• Rotation — the spinous process line passes through the pubic symphysis; obturator foramina are symmetrical
• Pelvic tilt — 2–3 cm between the sacrococcygeal junction and the top of the pubic symphysis (up to 6 cm accepted in females)

The ideal inlet view (for trauma) is when the anterior borders of S1 and S2 overlap. The ideal outlet view is when the top of the symphysis is just below the S1 body.

An inlet view falsely exaggerates anterior wall coverage and falsely decreases posterior wall coverage. The outlet view has the opposite effect. Right rotation of the pelvis exaggerates anterior coverage and falsely decreases posterior coverage of the right femoral head, and does the opposite to the left. Inlet and right rotation have an additive effect on the right and negate each other on the left.

AP View Findings

Both hemipelves are usually symmetrical. Radiographic asymmetry is not uncommon and has little importance in the absence of symptoms or high-risk signs such as subluxation, joint space narrowing, or abnormal bone density.

Pubic Symphysis

Widening and irregularities of the pubic symphysis are commonly associated with hip impingement.

Prominent AIIS

A healthy AIIS stays above the sourcil. A prominent AIIS extends inferiorly overlapping the acetabulum, causing a false crossover sign.

Ilio-inguinal and Ilio-ischial Lines

The ilio-inguinal line represents the anterior column and is used to confirm proper rotation of an acetabular osteotomy involving a pubic osteotomy. The ilio-ischial line (Kohler's line) represents the posterior column and defines the position of the hip joint. The acetabular fossa lies lateral to Kohler's line.

Teardrop

The teardrop is a U-shaped radiographic appearance formed by Kohler's line medially and the acetabular fossa line laterally. A wide teardrop is commonly associated with dysplasia. A narrow or reversed teardrop (acetabular fossa medial to the ilio-ischial line) is a sign of coxa profunda. Neither finding is highly specific — coverage is better assessed directly from the sourcil and the two walls.

Ischial Spine Sign

The ischial spines are ideally not visible on the AP view. Visibility on a well-positioned AP is an indirect sign of external rotation of the hemipelvis or acetabular retroversion. It is a normal finding if the X-ray is rotated to the same side or is an inlet view.

Acetabulum

Acetabular morphology is defined by three structures: the sourcil, the anterior wall, and the posterior wall — all visible on the AP pelvis.

Sourcil

The sourcil is the radiographic density of subchondral bone representing the weight-bearing dome. It is not significantly affected by mild pelvic tilt or rotation.

• Size — should cover about 80% of the femoral head. Less than 70% is under-coverage; more than 90% is over-coverage. Measured by lateral center-edge angle (LCE), ideally 25–35°. Medial center-edge angle (MCE) should be at least 20°. Lateral over-coverage with medial deficiency is a contraindication to rim trimming.
• Shape — should be concave to contain the convex head. Lateral upslope of the sourcil indicates early acetabular remodelling to accommodate an aspherical head — a sign of cam morphology, often seen in slipped epiphysis.
• Tönnis angle (sourcil angle / acetabular index) — angle between a transverse line connecting the bottoms of the teardrops and a line drawn from one end of the sourcil to the other. Normal: 0–10° upsloping, perpendicular to the primary compression trabeculae of the femur.

A good sourcil should extend 25–35° lateral and at least 20° medial to the centre of the head, be concave in shape, and be flat to 10° up. The sourcil angle is more important than size or shape — it imparts stability to the hip. A small sourcil placed flat on the head is better than a large tilted one. Stability is more important than surface area.

Anterior Wall

The anterior wall is more horizontal, has an S-shape, and leads to the inferior pubic ramus. Ideally it stays completely medial to the posterior wall and contacts it at the lateral edge of the sourcil, covering approximately 20% of the femoral head. Anterior wall crossing the posterior wall is always abnormal, though it does not always cause symptoms.

Posterior Wall

The posterior wall is more vertical, usually straight, and leads to the lateral ischium. It normally covers half of the head and stays lateral to the anterior wall — so LCE measures posterior coverage unless there is a crossover sign. With a crossover sign, posterior wall LCE should be measured separately to quantify posterior coverage at the dome level.

The posterior wall sign — posterior wall passing medial to the centre of the head — indicates posterior wall deficiency, though it is not quantitative. Posterosuperior deficiency (posterior wall deficient at sourcil level despite crossing the centre of the head) is better quantified by posterior wall LCE.

An ossified labrum can produce two lines representing the walls — the double wall sign.

Acetabular Coverage — Classification

Assess each of the three acetabular components individually. Ideally, the anterior wall covers 20% of the head, the posterior wall 50%, and the sourcil 80%. With each wall potentially being normal, deficient, or over-covered, nine types of acetabulae are possible — only one is ideal, and five can have a crossover sign.

Crossover Sign

The crossover sign means the anterior wall is more lateral than the posterior wall. It is not diagnostic of retroversion alone — it can be seen with:

1. True retroversion — anterior over-coverage with posterior under-coverage
2. Coxa profunda / global over-coverage / protrusio — anterior over-coverage exceeds posterior over-coverage
3. Global dysplasia — in one third of dysplastic hips, posterior coverage is more deficient than anterior
4. Posterior dysplasia — anterior coverage normal, posterior deficient
5. Anterior over-coverage with normal posterior coverage

Crossover sign, ischial spine sign, and teardrop shape are all indirect signs and not individually diagnostic. Together — crossover sign, prominent ischial spine, and posterior wall sign — they suggest acetabular retroversion.

Treatment should be determined by the degree of posterior deficiency, risk of arthritis, and the age and preferences of the patient. Patients over 25 may not be ideal candidates for anteverting PAO as there may already be anterior acetabular damage.

Radiology of the Proximal Femur

The normal femoral head is mostly spherical with thicker and wider cartilage superiorly. The epiphysis extends anterolaterally onto the neck, often producing cam morphology (alpha angle >55°). Anterolateral cam is seen on a 45° Dunn or shoot-through lateral with the hip in internal rotation. Lateral cam is seen on AP. Anterior and posterior cam are seen on frog lateral views. The best evaluation of head-neck contour is on radial MRI and 3D CT.

• The epiphyseal extension sign — physeal scar extending laterally — is a sign of cam morphology
• The sagging rope sign shows the lateral extent of the anterior head; a second sagging rope shows the lateral extent of the posterior head
• Fovea alta — fovea higher than 10° below the medial edge of the sourcil — is a risk factor for foveal impingement where the ligamentum teres rubs against the acetabular dome

The epiphysis can tilt posteriorly on the neck. The Southwick angle considers up to 12° of posterior tilt to be normal. In idiopathic cam morphology the head has an anterior tilt. In SCFE, posterior tilt is seen in 99.9%, varus tilt in 98%, and valgus tilt in 2%. Siebenrock defines a varus slip as when the neck axis passes above the fovea on the AP view.

Femoral Neck

The primary functions of the neck are to increase the abductor lever arm and allow greater range of motion. The neck has a neck-shaft angle of approximately 130° and 15–20° of anteversion.

Neck-shaft angle measurement is error prone and varies with hip rotation. The height of the tip of the greater trochanter relative to the centre of the head is a more reliable estimate — above the centre is coxa vara, below is coxa valga.

• Coxa vara — shortens the abductor lever arm causing weakness, limp, and fatigue pain; decreases abduction and rotations in flexion; Trendelenburg gait decreases joint reaction forces but increases shear forces on hip cartilage and valgus stress on the knee
• Coxa valga — increases joint reaction forces, makes the hip more unstable, brings the neck and lesser trochanter closer to the pelvis, predisposing to intra- and extra-articular impingement in extension
• Anteversion — increases flexion and internal rotation, decreasing the risk of anterior impingement but increasing the risk of posterior impingement
• Retroversion — increases the risk of anterior impingement
• Coxa breva — neck length less than one head diameter; similar effects to coxa vara, usually associated with vara

Head-Neck Junction — Alpha Angle

A narrow neck with a spherical head gives greater range without impingement. The alpha angle measures how medial the head-neck junction is; head-neck offset measures the depth of the neck relative to the head. None of the risk factors — over-coverage, cam morphology, or coxa vara — are independently diagnostic of hip impingement.

Hip dysplasia is always a radiographic diagnosis. FAI is mostly a clinical diagnosis treated by correcting radiographic abnormalities.

Radiographic Signs of Dysplasia

In addition to measurable decreased coverage, hip dysplasia is significant when the hip shows signs of instability: widened medial joint space, break in Shenton's line, acetabular rim fracture, and cyst formation from rim loading.

Radiographic Signs of Impingement

Early signs: sclerosis, ridge or groove formation at the head-neck junction, cyst formation under the cam bump. Late signs: narrowing of lateral joint space, medial osteoarthritis, subtle lateral subluxation of the head, double wall sign, labral calcification, and head-neck osteophytes.

Femoral Head and Acetabulum — Mutual Influence

It is common to see lateral joint space widening in idiopathic cam, SCFE, and Perthes from acetabular remodelling induced by an aspherical head in a growing child. Conversely, cam morphology is commonly seen with acetabular dysplasia, probably from inadequate rounding of the head because the socket was not deep enough to influence head shape. Protrusio hips have a very spherical head with a narrow neck (low alpha angle). The combination of protrusio and cam is less common but very likely to become symptomatic early.

Interaction of Morphological Features

Though radiographic variations are common, symptoms and damage are less common because most abnormalities can be compensated by another morphological feature:

1. Lateral dysplasia compensated by coxa vara or flattening of the sourcil angle
2. Coxa profunda compensated by coxa valga and increased head-neck offset
3. Cam morphology and mild SCFE compensated by increased anteversion or shallow acetabulum
4. Anterior dysplasia compensated by femoral retroversion
5. Posterior dysplasia compensated by femoral anteversion

Each morphological variation has a specific effect — effects can be additive to cause early disease, or can compensate for others and allow good function. As mechanical hip disease is activity-related, more active people become symptomatic earlier. Biology also dictates who becomes more symptomatic from the same activity level, making individual prognosis uncertain. Surgery is offered only to symptomatic and active patients who understand this. The exception is dysplasia in a growing child, which worsens without causing pain — surgery is recommended more strongly when the prognosis without treatment is clearly poor.

Pelvis Imaging for Trauma

AP pelvis is less useful in trauma because of anterior pelvic tilt.

• Inlet view — axial view of S1; anterior borders of S1 and S2 should overlap perfectly. Shows superior displacement, rotational deformity, and sacral alar buckling (as in LC1 injury).
• Outlet view — AP of S1; top of the pubic symphysis placed at the bottom of S1. Shows the entire sacrum and pubic rami in profile; vertical displacement and flexion injuries are clearly seen.

The standard angles for inlet and outlet views may not suit all patients given variable pelvic tilt. A useful tip: measure the required angles pre-operatively on supine pelvic CT to obtain true AP and axial views of S1, then start fluoroscopy at those angles and fine-tune as needed.

Almost all pelvic reduction and fixation can be done with these two views. A perfect lateral of the sacrum, though difficult to obtain, helps with starting point adjustment and placing transsacral screws in S1 and S2. The obturator outlet view is occasionally useful to visualise the teardrop above the acetabulum — the supra-acetabular column running from AIIS to PSIS, a corridor for large front-to-back screws or external fixator pins.

MRI

MRI rules out pathologies like AVN and tumour, shows labral tears and advanced cartilage damage. It is used when the clinical diagnosis is unclear and to differentiate between borderline dysplasia versus cam damage by assessing labral size. Circumferential alpha angle measurements are best performed on radial sequences.