Which Imaging Modalities, Parameters, and Diagnostic Thresholds Best Define Hip Instability? Consensus Statements From a Scoping Review and an International Multidisciplinary Delphi Study.

BACKGROUND: The diagnosis of hip instability remains challenging because of its multifactorial etiology and the variability of imaging practices across institutions. Hip instability refers to a clinical condition of insufficient osseous and/or soft tissue constraint with abnormal femoral head motion and symptoms, which may occur along a morphologic spectrum that overlaps with-but is not synonymous with-developmental dysplasia of the hip. Although numerous imaging modalities and imaging parameters have been proposed, there is limited consensus on which are most relevant, how they should be measured, and what diagnostic thresholds best correlate with instability. To address this gap, a Delphi consensus study with an international panel of experts to establish standardized clinical statements for the diagnostic criteria of hip instability was conducted.

QUESTIONS/PURPOSES: (1) What are the imaging modalities and protocols for investigating hip instability? (2) Which imaging parameters are most strongly associated with clinical hip instability? (3) What are the quantitative imaging thresholds for diagnosing hip instability?

METHODS: A consensus study using a modified Delphi technique was conducted in accordance with the Accurate Consensus Reporting Document (ACCORD) guideline. Following a scoping review, 27 statements addressing imaging modalities and protocols (question 1), imaging parameters associated with hip instability (question 2), and quantitative imaging thresholds (question 3) were developed. These statements were evaluated by a multidisciplinary international expert panel during a structured consensus meeting, with two rounds of anonymous voting and consensus defined a priori as ≥ 75% agreement. Consensus was achieved for 81% (22 of 27) of statements.

RESULTS: There was strong agreement that radiography (AP pelvis, false-profile, Dunn view) and MRI and CT constitute the foundational imaging modalities for the assessment of hip instability, whereas cartilage mapping techniques and dynamic ultrasound were considered adjunctive modalities with a more limited and evolving role. Radiographic measurements such as lateral center-edge angle (LCEA), anterior wall index (AWI), posterior wall index, and Tönnis angle reached consensus as key morphologic indicators. Several quantitative thresholds were endorsed. An LCEA of < 20° was considered diagnostic for hip dysplasia, a condition in which clinical hip instability is highly prevalent, whereas values between 20° and 25° were considered concerning for possible clinical hip instability. Additional thresholds included an AWI of < 0.30, PWI of < 0.8, and acetabular version < 5° or > 30°. In contrast, no consensus was reached for less-established imaging metrics such as the FEAR index, crescent sign, anterior capsule thickness, and dynamic femoral head translation.

CONCLUSION: This international Delphi consensus defined core imaging approaches and threshold values for the evaluation of hip instability. Although agreement was reached on key morphometric parameters and diagnostic thresholds, a lack of consensus exists around newer soft tissue and dynamic measures, highlighting important areas for future research.

CLINICAL RELEVANCE: In patients with suspected clinical hip instability, standard radiography should be used as the first-line imaging modality, followed by targeted use of CT and MRI to evaluate three-dimensional bony morphology and soft tissue stabilizers. Imaging findings should be interpreted within a stepwise, algorithmic framework that integrates clinical assessment with multiple complementary imaging parameters rather than relying on isolated measurements. Ultimately, accurate diagnosis of hip instability depends on the integration of findings from multiple modalities. Additionally, it must be recognized that hip instability may exist even in the absence of characteristic radiologic signs.