Review ArticlesDecision-making in the Borderline HipNepple, Jeffrey J. MD, MS*; Fowler, Lucas M. MPHS*; Larson, Christopher M. MD†Author Information *Department of Orthopaedic Surgery, Washington University School of Medicine, St. Louis, MO †Twin City Orthopaedics, Edina, MN Disclosure: J.J.N. is a consultant and received research support from Smith and Nephew; is a consultant and received royalties from Responsive Arthroscopy; received research support from Zimmer-Biomet, Stryker, and Department of Defense. C.M.L. is a paid consultant at Smith and Nephew. L.M.F. declare no conflicts of interest. Reprints: Jeffrey J. Nepple, MD, MS, Washington University Orthopaedics, 1 Barnes-Jewish Hospital Plaza, Campus Box 8233, St. Louis, MO 63110. Sports Medicine and Arthroscopy Review: March 2021 - Volume 29 - Issue 1 - p 15-21 doi: 10.1097/JSA.0000000000000298 Buy Metrics Abstract Borderline acetabular dysplasia represents a “transitional acetabular coverage” pattern between more classic acetabular dysplasia and normal acetabular coverage. Borderline dysplasia is typically defined as a lateral center-edge angle of 20 to 25 degrees. This definition of borderline dysplasia identifies a relatively narrow range of lateral acetabular coverage patterns, but anterior and posterior coverage patterns are highly variable and require careful assessment radiographically, in addition to other patient factors. Treatment decisions between isolated hip arthroscopy (addressing labral pathology, femoroacetabular impingement bony morphology, and capsular laxity) and periacetabular osteotomy (improving osseous joint stability; often combined with hip arthroscopy) remain challenging because the fundamental mechanical diagnosis (instability vs. femoroacetabular impingement) can be difficult to determine clinically. Treatment with either isolated hip arthroscopy or periacetabular osteotomy (with or without arthroscopy) appears to result in improvements in patient-reported outcomes in many patients, but with up to 40% with suboptimal outcomes. A patient-specific approach to decision-making that includes a comprehensive patient and imaging evaluation is likely required to achieve optimal outcomes. Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.