Borderline acetabular dysplasia remains a controversial topic in hip preservation, with poor current comparative literature to guide accurate diagnosis and treatment decision making. Borderline dysplasia represents a "transitional acetabular coverage" pattern between more classic acetabular dysplasia and normal coverage. Traditionally, borderline dysplasia has been defined by a lateral center-edge angle between 20° and 25°, whereas more recently, some authors have used 18° to 25°. Treatment decisions between isolated hip arthroscopy (addressing labral tears, femoroacetabular impingement morphology, and capsular laxity) and periacetabular osteotomy (improving joint stability, often combined with arthroscopy) remain challenging because the fundamental mechanical diagnosis (instability vs femoroacetabular impingement) can be difficult to determine clinically. Obtaining an accurate diagnosis to direct surgical treatment relies on comprehensive assessment of additional bony anatomy features (including femoral version) and patient characteristics (including sex, soft-tissue laxity, and range of motion). Future research efforts in borderline dysplasia should better characterize the role of disease- and patient-specific factors that will inform accurate diagnoses, leading to the development of optimal treatment strategies in distinct patient subgroups through comparison of treatment outcomes.
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