Imaging the Pelvis and Hip

Pelvis and hip imaging runs from acute trauma (neck of femur fracture, acetabular fracture, pelvic ring disruption) to chronic conditions (osteoarthritis, avascular necrosis, femoroacetabular impingement, developmental dysplasia). The AP pelvis radiograph is the essential first-line investigation for all of them, with advanced imaging selected according to the clinical question.

The scenario that matters most. An elderly patient with hip pain after a fall and normal radiographs needs urgent MRI to exclude an occult neck of femur fracture. Delayed diagnosis is one of the most common causes of medicolegal claims in orthopaedic practice.

Where MRI changes management.

• Occult neck of femur fracture: radiographs normal, clinical suspicion persists
• AVN staging: pre-radiographic disease
• Labral tears: MR arthrography in the FAI work-up
• Transient osteoporosis: diffuse bone marrow oedema without AVN changes
• Bone marrow pathology: metastases, myeloma infiltration, stress fractures
• Perthes disease staging: extent of epiphyseal involvement

Adequacy first. Both hips should be visible, the obturator foramina symmetric (no rotation) and the coccyx aligned with the pubic symphysis. Then trace the same lines in the same order every time; that is how subtle pelvic and acetabular injuries are not missed.

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Shenton's line and the sacroiliac joints. Shenton's line runs as a smooth arc along the inferior border of the superior pubic ramus and continues along the medial femoral neck. Trace it on both sides: a break or step signals a fracture or dislocation, and the arc is also disrupted in DDH and SCFE. The sacroiliac joints are checked for symmetry, widening and sclerosis (sacroiliitis).

The hip joint. Compare the two sides for joint space width (normal more than 4mm medially), congruence, femoral head sphericity, osteophytes and subchondral change.

The ring. Trace every cortex of the iliac wings, ischium and pubic rami, and look for disruption at the pubic rami, the sacroiliac joints and the acetabulum. The pelvic ring must break in two places, so one fracture found is a second to look for.

The proximal femur. Check cortical integrity, the trabecular pattern (Singh index) and fracture lines along the femoral neck, and the trochanters: in an older patient an apparently isolated greater trochanter fracture is an incomplete intertrochanteric fracture until MRI says otherwise.

The Judet Views: Which Oblique Shows What

An acetabular fracture is defined by which column and which wall are broken, and a single AP view cannot separate them because the two columns are superimposed. The Judet views are 45-degree obliques that rotate the pelvis to unwrap the acetabulum, and knowing which view shows what is the difference between describing a fracture and classifying it.

The pairing is deliberately crossed, each oblique showing one column and the opposite wall, and that is exactly what makes it easy to get backwards under pressure. Two ways to hold it:

• The view is named for what it opens up. The obturator oblique opens the obturator foramen; the iliac oblique opens the iliac wing. Each view then shows the column behind the structure it opened: opening the obturator foramen looks down the anterior column, opening the iliac wing looks down the posterior column.
• Then take the opposite wall. Anterior column goes with posterior wall, posterior column with anterior wall.

The columns on the AP film. The iliopectineal line is the anterior column and the ilioischial line is the posterior column, so a broken iliopectineal line with an intact ilioischial line already tells you the injury is anterior before any oblique is taken. CT with 3D reconstruction has largely replaced the Judet views for definitive Letournel-Judet classification and planning, but the obliques remain the fastest way to answer the question in an emergency department, and examiners still ask for them.

Having traced the ring on the AP pelvis (plus inlet/outlet views and CT), classify the injury: the classification predicts instability, haemorrhage and neurological risk.

Pelvic ring: Young-Burgess (mechanistic) and Tile (stability)

Young-Burgess groups injuries by force vector:

• APC (anteroposterior compression, "open book") I-III: symphyseal diastasis with or without progressive anterior then posterior sacroiliac disruption
• LC (lateral compression) I-III: sacral impaction with or without internal rotation of the hemipelvis (the "windswept" pelvis at LC-III)
• VS (vertical shear): vertical displacement of the hemipelvis (fall from height); vertically unstable
• CM (combined mechanism): features of more than one pattern

Why the pattern matters. APC is the pattern that bleeds. In complete ring disruptions, APC injuries required 10 or more units of blood in the first 24 hours in 15 of 18 patients, developed multi-organ failure in 11 of 18, and carried a 39% mortality against 0% for vertical shear. The mechanism you read off the film, not merely the fracture location, predicts the resuscitation.

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Tile classifies by stability: A stable (ring intact or minimally displaced), B rotationally unstable but vertically stable (open-book or lateral compression), C rotationally and vertically unstable (complete posterior disruption).

Sacral fractures: Denis zones

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Spinopelvic dissociation. A transverse sacral fracture with bilateral vertical components (a U- or H-shaped fracture) separates the upper sacrum and spine from the pelvis, and carries a high rate of neurological deficit.

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Radiographic detection. The AP pelvis radiograph detects approximately 95-98% of neck of femur fractures. The signs:

• Cortical discontinuity at the femoral neck
• Disruption of Shenton's line
• Trabecular pattern disruption (medial calcar compression)
• Valgus angulation (Garden I, impacted)
• Shortening and external rotation of the limb in a displaced fracture: a lesser trochanter seen in profile on a straight AP suggests that rotation

Garden classification. The grade describes displacement on the AP view, and displacement is what disrupts the retinacular vessels, so the grade carries the AVN risk.

Non-displaced fractures (I-II) are fixed; displaced fractures (III-IV) in the elderly are replaced.

The occult fracture and its two numbers. Two very different figures circulate here and they answer different questions. About 2-5% of all hip fractures are invisible on the first radiograph: that is the miss rate across everyone who turns out to have a fracture. The number that matters at the bedside is the rate in the group you are actually deciding about, elderly patients with hip pain, negative or equivocal films, and enough clinical suspicion to warrant advanced imaging. In pooled data from 35 studies (2,992 patients, mean age 77), 39% of that group had an occult fracture on MRI. Quoting 2-5% to justify the scan understates the risk by an order of magnitude and, said out loud in a viva, sounds like an argument for not scanning.

The isolated greater trochanter fracture. A second trap sits in the same population. In the same pooled data, 92% of apparently isolated greater trochanter fractures had occult intertrochanteric or femoral neck extension on MRI: the visible fragment is the tip of an incomplete intertrochanteric fracture, which is why these patients cannot weight-bear and why the fracture can complete under load.

Presentation and the MRI. These patients present with hip pain, inability to weight-bear and pain on internal rotation. Clinical suspicion must lead to MRI within 24 hours; MRI has 99-100% sensitivity for occult neck of femur fracture. The fracture appears as a low-signal line on T1 surrounded by marrow oedema (high signal on T2/STIR), and its configuration (subcapital, transcervical, basicervical) and displacement determine management.

CT when MRI is unavailable. CT has approximately 80% sensitivity for undisplaced neck of femur fractures. It is inferior to MRI for this indication but a reasonable alternative in emergency settings.

Why MRI. MRI is the investigation of choice for AVN detection. Pooled across 43 studies its sensitivity is 93% and specificity 91% for early osteonecrosis: excellent, but not the near-perfect figures sometimes quoted, and the imperfect specificity is exactly the transient-osteoporosis problem in the differential diagnosis below. It detects AVN at Stage I, typically 6-12 months before radiographic changes become visible. The necrotic zone shows low signal on T1, replacing the normal fatty marrow, and variable signal on T2.

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Staging. AVN is staged with Ficat-Arlet or Steinberg; the modified Ficat-Arlet stages are:

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Cam morphology. The alpha angle is measured on a cross-table lateral or Dunn lateral radiograph, or on radial MRI sequences centred on the femoral neck axis. An alpha angle more than 55-60 degrees is abnormal (an aspherical head-neck junction). For scale, the study that introduced the measurement found a mean of 74 degrees in symptomatic impingement hips against 42 degrees in asymptomatic controls, so 55 sits much closer to normal than to disease.

Pincer morphology. A lateral centre-edge angle (LCEA) more than 40 degrees indicates overcoverage, a crossover sign on the AP pelvis indicates focal retroversion, and coxa profunda and protrusio belong here too.

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MR arthrography for the labrum, and the half of the number that usually goes unquoted. Pooled across 19 studies, MR arthrography has a sensitivity of 87% against 66% for non-contrast MRI, which is why it is the test of choice. But its specificity is 64%, lower than the 79% of plain MRI. It trades false negatives for false positives: it finds more tears, and it also calls tears that are not there. Since asymptomatic labral tears are common, a positive MR arthrogram in a patient whose symptoms and examination do not fit is weak evidence, and the injection itself carries a procedural risk. Order it when a positive result would change what you do.

Dysplasia. A single angle should not substitute for a complete morphological assessment of coverage.

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When the AP pelvis is normal or near-normal but the patient has significant hip pain, distinguishing the possibilities is almost entirely an MRI exercise, because every discriminating feature lies in the bone marrow or the soft tissue rather than in the bone contour. Several of these conditions produce marrow oedema and nothing else, so the question is never "is there oedema" but "what shape is it, and is there a line inside it".

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Pelvis and hip imaging pathways are broadly concordant worldwide, but the named guidelines, resource availability and registry context differ. The AP pelvis radiograph is the universal first-line study; the principal global divergence is in how quickly and how often advanced imaging (MRI/CT) is added.

Global epidemiology

Hip fracture is the dominant high-volume indication. There are an estimated 1.5 million-plus hip fractures per year worldwide, projected to rise sharply with population ageing, with the largest absolute increase expected in Asia. Around 5-10% of fractures that ultimately require surgery are not seen on the initial radiograph, and meta-analysis data show that among elderly patients imaged for clinical suspicion with negative/equivocal films the occult-fracture rate is far higher (about 39%). Non-traumatic AVN of the femoral head presents most often in the 30-50 age group and is bilateral in roughly half to four-fifths of non-traumatic cases. Developmental dysplasia of the hip (DDH) affects roughly 1-3 per 1000 live births for frank dislocation, with higher rates of sonographic dysplasia.

Side-by-side guideline comparison

Registry and outcome context

National hip fracture registries and audits (e.g. the UK National Hip Fracture Database, and equivalents in Australia/New Zealand, the Nordic countries and elsewhere) tie prompt, accurate imaging diagnosis to time-to-surgery targets, because delayed diagnosis of an occult fracture delays surgery and worsens outcome. Arthroplasty registries (NJR, AJRR, AOANJRR, SHAR/Swedish, Norwegian, NZJR) capture downstream implant survival for the hemiarthroplasty and total hip replacements that displaced NOF fractures and end-stage AVN ultimately require.

High- versus limited-resource practice variation

In well-resourced systems 24-hour MRI for occult fracture, MR arthrography for labral pathology, and CT with 3D reconstruction for acetabular/ring planning are routine. Where MRI access is constrained, CT (sensitivity around 79% for occult fracture) is the accepted substitute, repeat/delayed radiographs and bone scanning are used, and ultrasonography has an expanding bedside role — for DDH screening, for hip effusion, and as a high-accuracy adjunct for occult femoral neck fracture where MRI is unavailable. DDH screening strategy itself varies globally between universal sonographic screening (common in parts of continental Europe) and selective screening of at-risk infants (breech, family history, clinical instability), with radiography reserved for after the femoral head ossifies (around 4-6 months).

The 24-hour target. The "MRI within 24 hours" standard is driven by avoiding surgical delay rather than by any loss of MRI sensitivity over time, and it is a service-delivery target that not all systems can meet.

CT versus MRI when access is limited. MRI is the reference standard, but modern CT detects most surgically relevant occult fractures (a high negative predictive value for femoral neck and intertrochanteric fractures) while missing a minority, including some undisplaced fractures. Ultrasonography of anterior joint capsule swelling shows promising accuracy for occult femoral neck fracture in dedicated series. The unresolved question is how to triage limited MRI capacity without missing the fractures CT under-calls.

Asymptomatic cam morphology. An alpha angle above 55-60 degrees defines cam morphology, but cam morphology is common in asymptomatic athletes and not all cam hips become symptomatic or develop osteoarthritis. Imaging numbers must be interpreted alongside symptoms and examination (the Warwick Agreement frames FAI as a clinical-imaging-symptom triad), and there is no single universally agreed threshold.

DDH screening strategy. Universal sonographic screening detects more dysplasia but increases treatment of hips that would have resolved spontaneously, whereas selective (risk-factor) screening misses some cases. The optimal balance between overtreatment and missed late-presenting DDH remains debated and is reflected in differing national programmes.