Hip Dysplasia

Acetabular dysplasia represents a structural pathomorphology associated with hip pain, instability, and osteoarthritis. The wide spectrum of dysplasia anatomically refers to a 3-dimensional volumetric- and surface area-based insufficiency in coverage and is classified based on the magnitude and location of undercoverage. Borderline dysplasia has been variably defined and leads to management challenges. In symptomatic dysplasia, treatment addresses coverage with periacetabular osteotomy. Concomitant simultaneous or staged hip arthroscopy has significant advantages to address intra-articular pathology. In nonarthritic individuals, there is evidence PAO alters the natural history of dysplasia and decreases the risk of hip arthritis and total hip arthroplasty.

Key points

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Acetabular dysplasia is a 3-dimensional osseous structural insufficiency of acetabular coverage of the femoral head.
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Acetabular dysplasia is characterized with a variety of 2- and 3-dimensional imaging modalities, including plain radiographs, MRI, and computed tomography scans.
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Acetabular dysplasia management includes a variety of different nonsurgical and surgical treatment options.
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Surgical treatment of acetabular dysplasia includes periacetabular osteotomy, with or without concurrent hip arthroscopy.
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The long-term natural history of untreated acetabular dysplasia is early hip osteoarthritis.

Introduction

Acetabular dysplasia represents a multiplanar structural pathomorphology associated with hip pain, instability, and osteoarthritis. The wide spectrum of dysplasia anatomically refers to a shallow acetabulum and is further classified based on the magnitude and location of undercoverage. Analogous to the Warwick agreement for femoroacetabular impingement (FAI) syndrome, the diagnosis of hip dysplasia requires the assessment and interpretation of patient symptoms, physical examination signs, and imaging. The definition of dysplasia has been and continues to be controversial, especially regarding its mild version, frequently termed “borderline” dysplasia or “transitional acetabular coverage.” Although many trials, articles, books, presentations, and guidelines often define dysplasia dichotomously via a lateral center edge angle (LCEA) of less than 20°, this is a shallow interpretation of a highly complex 3-dimensional volumetric- and surface area-based insufficiency in coverage. The natural history of dysplasia leads to chondral injury secondary to the shallow cup and upsloping sourcil, with a resultant increased risk of early hip arthritis. Thus, in symptomatic individuals with dysplasia, the cornerstone of treatment addresses coverage with increased structural stability via periacetabular osteotomy (PAO). The goals of PAO are multifaceted, with improvements in pain, function, and the natural history via arthritis risk reduction. With an increased recognition of structural hip abnormalities, especially cam morphology, concomitant simultaneous or staged hip arthroscopy has significant advantages to address intra-articular pathology, including the labrum, articular cartilage, head–neck offset, subspine impingement, and iliofemoral ligament. In nonarthritic individuals, there is evidence that PAO alters the natural history of dysplasia and reduces the risk of hip arthritis and subsequent total hip arthroplasty.

Patient presentation

History

The presentation of patients with acetabular dysplasia typically includes atraumatic, insidious (97%) onset groin (72%) or lateral hip (66%) pain. The pain location is usually deep in the hip, with a “C” sign or “between the fingers” sign frequently exhibited. As with most intra-articular problems, like FAI syndrome or arthritis, position- and motion-dependent pain with prolonged seated or deep flexion and rotational maneuvers is common. Activity-related (88%), weight-bearing lateral hip pain with ambulating may manifest with increasing load, often termed “abductor fatigue.” Objective abnormalities in gait include a limp (48%), apprehension with terminal hip extension at the end of stance phase, and Trendelenburg gait. These symptoms are largely due to structural instability owing to deficient acetabular coverage with resultant femoral head edge loading the acetabular rim, and subsequent increased long-term secondary osteoarthritis. Twisting and pivoting exacerbate pain, mainly with extension and external rotation. Hip flexor symptoms, including painful internal snapping (audible, palpable), occur owing to the anterior head instability seen with anterior undercoverage. Painful external snapping (visible, palpable), owing to the iliotibial band, also frequently occurs. Anterior undercoverage typically manifests as anterior hip and groin pain, whereas posterior dysplasia manifests as anterior and/or posterior hip pain (often concomitant diagnoses include sacroiliac dysfunction, piriformis syndrome, deep gluteal syndrome, sciatica, low back pain); and lateral dysplasia manifests as deep lateral pain.

Skeletally mature individuals with symptomatic dysplasia are typically young and active. In a group of 982 consecutive hips that underwent PAO, patients were primarily female (83%), with a mean age of 25.3 years (range, 9–54 years), mean body mass index of 24.6 kg/m ² , 87% Caucasian, family history of hip disease (27%), and significant preoperative low scores on validated hip-specific (the Hip injury and Osteoarthritis Outcome Score, modified Harris Hip Score), activity (University of California at Los Angeles), and general health (Short Form-12) patient-reported outcome scores. Interestingly, 71% of patients in the latter study had symptoms for at least 1 year, 30% for at least 3 years, and 17% for at least 5 years. In addition, 35% had undergone previous hip surgery (15% ipsilateral, 20% contralateral), with 50% of ipsilateral cases being hip arthroscopy. In a similar investigation of 65 symptomatic skeletally mature hips diagnosed with dysplasia, the mean time from symptom onset to diagnosis was 61.5 months (range, 5 months to 29 years), the mean number of health care providers seen before diagnosis was 3.3 (range, 0–11), and most patients had tried multiple treatments before diagnosis (rest [75%], oral nonsteroidal medications [57%], physical therapy [43%], activity modification [42%], surgery [18%], and opioids [8%]). These numbers are similar to that of FAI syndrome: patients saw 4.0 health care providers, had 3.4 imaging tests, had 3.1 previous treatments, spent $2456.97, and waited 32 months before diagnosis.

Physical Examination

A proper systematic physical examination in a patient with dysplasia should include observation (including gait), inspection, palpation, motion (supine, lateral, prone, and seated), strength, and special testing. Gait assessment should evaluate for a Trendelenburg gait (common with abductor fatigue), antalgic gait (concern for effusion, stress fracture), in-toeing or out-toeing (significant version abnormalities), and the use of gait aides (eg, crutches, cane, wheelchair). Inspection is frequently normal, without cutaneous abnormalities, such as atrophy or deformities. Palpation evaluates all osseous and soft tissue structures around the hip, thigh, pelvis, and lumbosacral spine. An obvious tenet to this evaluation mandates absolute consideration for modesty and genitourinary and gastrointestinal systems. A pre-examination discussion with the patient and their family (eg, parents, if applicable) about the latter ensures professionalism and decreases the risk of misperceived examination techniques, especially palpation. The hip joint is a deep structure impossible to palpate. Specific areas to be palpated and documented for tenderness include the greater trochanteric facets, abductor tendons, iliac crest (from anterior superior to posterior superior iliac spines), inguinal canal (including inguinal hernia evaluation), pubic symphysis, pubic bone, rectus abdominis, sacroiliac joint, spinous processes (including asymmetry in coronal and sagittal plane alignment [scoliosis, kyphosis, and lordosis], and rib humps with forward bend), deep gluteal space, ischiofemoral space (lateral to ischium), sciatic nerve (plus Tinel evaluation), proximal hamstring, ischial tuberosity, adductor longus, quadriceps muscle, and iliotibial band (from the hip to tibia Gerdy’s tubercle).

Hip (flexion, extension, abduction, adduction, and internal and external rotation) and knee (flexion, extension) motion should be measured and compared with the contralateral side. Importantly, hip flexion should be measured with the contralateral hip at 0° flexion (anterior pelvic tilt) and maximal hip flexion (posterior pelvic tilt). Hip rotation should also be measured in both the supine and prone (obviates cam morphology engagement) positions. Hip internal rotation is affected more by femoral version, whereas hip flexion is affected more by cam morphology. Frequently in dysplasia, hip internal rotation is significantly increased and external rotation is decreased with increased version. Strength (Medical Research Council classification, x /5) of the paraspinal muscles and all lower extremity muscles is measured. Special testing should include log roll, axial load (while supine), axial distraction, external rotation recoil, dial test, impingement testing (anterior via flexion, adduction, internal rotation; subspine via straight sagittal plane maximal flexion; lateral via straight coronal plane maximal abduction; and posterior via external rotation [pain for impingement; apprehension for instability]; and flexion, abduction, external rotation distance to table asymmetry [>4 cm for FAI syndrome] vs sacroiliac joint pain), iliopsoas evaluation (Ludloff, Stinchfield, iliopsoas test, and iliopsoas snap [audible pop]), iliotibial band snap (visible pop), Ober test, long- and short-lever adductor squeeze, resisted sit-up/crunch, and Valsalva examination (hernia, sports hernia, or core muscle injury). Most, but not all, patients with dysplasia demonstrate either a positive anterior (76%) or posterior impingement (27%) sign. A positive impingement test implies an intra-articular location of the pain, but is not the sine qua non for FAI syndrome. Thus, the impingement test is often positive in dysplasia as well. In patients with dysplasia, an assessment of hypermobility should always be performed; the Beighton score is the most common tool ( x /9; >4/9 indicates hypermobility).

Imaging

Plain radiographs

Imaging evaluation for individuals with dysplasia should include a combination of high-quality 2- and 3-dimensional modalities. Initial evaluation includes plain radiographs of the hip and pelvis. A weight-bearing anteroposterior (AP) pelvis and a variety of lateral views are typically performed. The AP pelvis and false profile views are the 2 primary views to characterize acetabular morphology. The LCEA is the most common measurement used to characterize dysplasia on the AP view, but should not be used to diagnose in isolation. The normal range of LCEA is 25° to 40°. Using a computed tomography (CT) scan as the reference standard in 474 asymptomatic hips, mean lateral coverage represents a mean LCEA of 31° ± 1°. The LCEA described by Wiberg is measured to the lateral edge of the sourcil. However, the LCEA may also be measured to the lateral edge of acetabular bone, which may be significantly different from the sourcil lateral edge ( Fig. 1 ). Using a 3-dimensional CT scan, the lateral edge of the sourcil (anterosuperior) is more anterior than the lateral edge of acetabular bone (superolateral) (1:30 ± 0:42 vs 12:06 ± 0:30, respectively). The bone LCEA has been reported to be as much as greater than 13° more than the sourcil LCEA (mean, 4°–5°; range, −4° to 13°). ^, In a study by Hanson and colleagues, out of the 14 patients with a sourcil LCEA of less than 20°, only 4 (29%) also had a bone LCEA of less than 20°; similarly, out of the 54 patients with a sourcil LCEA of less than 25° (dysplasia or borderline dysplasia), 32 (59%) had a bone LCEA of greater than 25° (normal coverage); further, 6 patients had a sourcil LCEA of less than 20°, but a bone LCEA of greater than 25°. This significant variation in the LCEA measurements alone can dramatically alter diagnostic and treatment decisions. Lateral coverage may also be assessed by the femoral head extrusion index, with values of greater than 20% to 25% indicating dysplasia.

The Tonnis angle, also known as the sourcil angle or acetabular index, is a measure that represents the slope of the weight-bearing surface of the acetabular dome from the medial to lateral sourcil ( Fig. 2 A). The normal range of Tonnis angle is typically 0° to 10°, with borderline from 10° to 15°. However, studies have shown a significantly increased risk of arthritis progression in patients with a Tonnis angle of more than even 7° to 8°, and a significantly increased risk of reoperation after arthroscopy (mean, 6.7° [range, 5.3°–8.1°] in the reoperation group vs 4.8° [range, 4.4°–5.3°] in the nonreoperation group). For every increased degree in Tonnis angle, the odds ratio was 1.12 for reoperation. In patients with a Tonnis angle of greater than 10°, the risk of reoperation was 84%. The upsloping sourcil places excessive shearing force on the acetabular articular cartilage with weight-bearing, hence the increased arthritis risk. In addition to the Tonnis angle measurement itself, an upsloping lateral sourcil ( Fig. 2 B, C) is significantly associated with generalized joint hypermobility (59% prevalence). Thus, an upsloping lateral sourcil may represent more than just osseous structural instability. A relatively recent measure of stability is the femoroepiphyseal acetabular roof (FEAR) index ( Fig. 2 D). The FEAR index has value in the evaluation of hip instability (femoral head migration on conventional plain radiographs or head recentering on an AP abduction view) in patients with transitional acetabular coverage. In patients with the latter (also known as borderline dysplasia), a FEAR index of less than 5° is likely to be stable (sensitivity, 78%; specificity, 80%; the stable borderline group’s mean FEAR index was −2.1° ± 8.4° vs the unstable group, 13.3° ± 15.2°). A greater degree of instability may be associated with head lateralization and incongruity, which can radiographically be observed with a broken Shenton’s line.

Although the LCEA represents lateral (or global) acetabular coverage, it largely ignores anterior and posterior coverage. ^, A variety of measures can be visualized on plain radiographs to characterize the magnitude of anterior coverage. On the false profile view, the anterior center edge angle (ACEA) is analogous to the LCEA in that there is commonly a discrepancy between its measurement at the anterior sourcil versus anterior bone ( Fig. 3 ). However, the magnitude of discrepancy in the ACEA measurement is much greater than in the LCEA. The bone ACEA has been reported to be as much as 22° more than sourcil ACEA (mean, 10°; range, −2° to 22°). An ACEA discrepancy of greater than 5° was observed in 78% of patients (107/137). All the patients with a sourcil ACEA less than 20° (7/7) had a bone ACEA of greater than 20°. The false profile view has the advantage of the ability to concurrently assess the anterior and inferomedial joint space and the anterior inferior iliac spine morphology. A disadvantage of the false profile is the significant challenge in patient positioning during the radiograph. The latter leads to large variation and low reliability in ACEA measurement, even with small changes in pelvic rotation during image acquisition. Thus, a low-dose pelvis CT scan (effective dose as low as 0.97–1.46 mSv) may obviate the false-profile view (1.0 mSv) altogether, with the addition of all the advantages of 3-dimensional analysis of coverage. ^, The low-dose CT protocol can result in a 90% decrease in the total effective dose radiation exposure versus a traditional CT scan.

Additional measures of anterior coverage include the anterior wall index (AWI) and percent anterior coverage. The AWI radiographically quantifies anterior acetabular coverage ( Fig. 4 ). The mean normal AWI is 0.41 (range, 0.30–0.51). In dysplasia, the mean AWI is 0.28 (range, −0.06 to 0.52). Anterior coverage is the percentage of the femoral head covered by the acetabulum in the AP direction and can be measured via CT scan or a variety of computerized collision detection software programs. Normal anterior coverage is 18.6% (range, 6.7%–28.9%). In dysplasia, the mean anterior coverage is 9.8% (range, 0%–22.2%). Using surface area regional coverage of the femoral head, anterior coverage of 40% ± 2% for a large asymptomatic group (474 hips; 61% ± 3% for superior coverage, 48% ± 3% for posterior coverage).

Analogous to the AWI, the posterior wall index (PWI) radiographically quantifies posterior acetabular coverage (see Fig. 3 ). The mean normal PWI is 0.91 (range, 0.81–1.14). In dysplasia, the mean PWI is 0.81 (range, 0.35–1.04). Posterior coverage is the percentage of femoral head covered by the acetabulum in the posteroanterior direction. Normal posterior coverage 42.9% (range, 31.6%–59.1%). In dysplasia, the mean posterior coverage is 36.6% (range, 15.3%–53.0%). A further analysis of posterior coverage includes an evaluation of the posterior wall and ischial spine signs, which indicate global acetabular retroversion, with or without dysplasia ( Fig. 5 ). Although the crossover sign has been used to characterize focal acetabular retroversion, its accuracy is poor given the frequent (50%) false-positive rate owing to a prominent anterior inferior iliac spine. In a study of 474 normal hips evaluated with a CT scan, only 15% of hips with true cranial retroversion displayed a crossover sign.