Imaging for Nonarthritic Hip Pathology

Take-Home Points

• Be sure to have a well centered AP pelvis without rotation.
• Get at least 3 plain radiographs—AP pelvis, false profile, and lateral hip view.
• Ensure that there is sufficient acetabular coverage, LCEA >20° on AP pelvis and ACEA >20° on false profile view.
• CT scans are helpful for precise hip pathomor­phology but must be weighed against risk of radiation exposure.
• MRI or MRA can be helpful to diagnose intra-articular as well as extra-articular hip and pelvis abnormalities.

In the work-up for nonarthritic hip pain, the value of diagnostic imaging is in objective findings, which can support or weaken the leading diagnoses based on subjective complaints, recalled history, and, in some cases, elusive physical examination findings. Morphologic changes alone, however, do not always indicate pathology.^1,2 At presentation and at each step in the work-up, it is imperative to evaluate the entire clinical picture. The prudent clinician uses both clinical and radiographic findings to make the diagnosis and direct treatment.

Radiography

The first step in diagnostic imaging is radiography. Although use of plain radiographs is routine, their value cannot be understated. Standard hip radiographs—an anteroposterior (AP) radiograph of the pelvis and AP and frog-leg (cross-table lateral) radiographs of the hip—provide a wealth of information.^3-6

Evaluated first is the radiograph itself. For example, the ideal AP radiograph of the pelvis (Figure 1) is centered on the lower sacrum, and the patient is not rotated.

Signs of rotation on the supine AP radiograph of the pelvis include but are not limited to the asymmetric appearance of the obturator foramina, the disproportionate spacing of the ischial spines from the midsagittal plane of the pelvis, the pubic symphysis off the midsagittal plane, and the clear imbalance of iliac wings or greater trochanters from the edges of the radiograph. Pelvic rotation can affect image interpretation and be detrimental to patient care.^7-9 Further, 15° internal rotation of the hips should be confirmed to ensure that the femoral necks are to length and that the measured femoral neck–shaft angle is accurate.

AP radiographs allow for evaluation of fractures, intraosseous sclerosis, acetabular depth, inclination and version, acetabular overcoverage, joint-space narrowing, femoroacetabular congruency, femoral head sphericity, and femoral head–neck offset.^7,8,10 Inspection for labral calcification is important, as it can indicate repetitive damage at the extremes of range of motion.

On AP pelvis radiographs, it is important to distinguish coxa profunda from acetabular protrusion. These entities are on the same pathomorphologic spectrum and are similar but distinctively different. Coxa profunda refers to the depth of the acetabulum relative to the ilioischial line, and acetabular protrusion refers to the depth (or medial position) of the femoral head relative to the ilioischial line. Each condition suggests—but is not diagnostic for—pincer-type femoroacetabular impingement (FAI).¹¹Acetabular rotation is another important entity that can be evaluated on well-centered, nontilted AP pelvic radiographs. Acetabular rotation refers to the opening direction of the acetabulum. It may be anterior (anteverted), neutral, or posterior (retroverted). Anteversion is present when the anterior acetabular rim does not traverse the posterior rim shadow⁴; in other words, the ring formed by the acetabulum is not twisted. When the walls overlap but do not intersect, the cup has neutral version. Retroversion is qualitatively determined by the crossover (figure-of-8) and posterior wall signs¹² and is associated with pincer-type FAI and the development of hip osteoarthritis.¹²Dunn lateral radiographs (Figure 2A), taken with 90° hip flexion, were originally used to measure femoral neck anteversion.¹³

Modified Dunn lateral radiographs (Figure 2B), taken with 45° hip flexion, have largely replaced their 90° counterparts. In addition to being used to measure femoral version (Figure 3), the modified radiographs can be used to detect head–neck offset and bony prominence at the head–neck junction.

Head–neck offset is qualitatively determined by comparing the symmetry of the anterior and posterior femoral head–neck concavities.

Dunn and modified Dunn lateral radiographs can be used to assess femoral head asphericity, which can be overlooked on standard AP or cross-table radiographs.¹⁴

Both femoral head–neck offset (Figure 4) and α angle (Figure 5) can be measured on Dunn and modified Dunn radiographs.

False-profile radiographs (Figure 6), valuable in evaluating anterior acetabular coverage and femoral head–neck junction morphology,^14,15 allow characterization of both cam-type and pincer-type FAI.

These weight-bearing radiographs are standing oblique radiographs of the pelvis and lateral radiographs of the proximal femur. Pincer-type FAI is indicated by increased anterior center-edge angle (ACEA), and dysplasia is indicated by decreased ACEA (<20°). To appreciate cam-type FAI, arthroscopists look for a convex bony prominence of the femoral head–neck junction.

Quantitative measures warrant specific consideration (Table). Femoroacetabular morphology is quantitatively measured by α angle, Tönnis angle (acetabular inclination angle), and lateral center-edge angle (LCEA).^7,8,10 The α angle (Figure 4) detects the loss of normal anterosuperior femoral head–neck junction concavity caused by a convex osseous prominence. An α angle >50° represents a cam deformity.¹⁶ In a cohort study of 338 patients, Nepple and colleagues¹⁷ qualitatively associated increased α angle with severe intra-articular hip disease. Murphy and colleagues¹⁸ found a Tönnis angle >15° to be a poor prognostic factor in untreated hip dysplasia. LCEA quantifies superolateral femoral head coverage,¹⁹ and its normal range is 20° to 40°.²⁰ LCEA <20° indicates dysplasia of the femoroacetabular joint, and LCEA >40° indicates overcoverage and pincer-type FAI. As with any quantitative radiographic measurement, results should be interpreted within the presenting clinical context.

Radiographic findings, even findings based on these special radiographs, may underestimate the pathologic process.

Repeat radiographs are recommended to address symptoms that persist after treatment. If technique is consistent, repeat radiographs reveal subtle changes. The other option is to proceed with cross-sectional imaging.