TP4.4 A Three-Dimensional Morphological Analysis of Cam Lesions in Femoroacetabular Impingement Syndrome Yields Three Distinct Phenotypes: Moving Beyond the Alpha Angle

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ID: 331402
2026
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Abstract
Abstract Introduction The alpha angle remains the standard metric for quantifying cam deformity in femoroacetabular impingement syndrome (FAIS), although this metric reduces a three-dimensional deformity to a two-dimensional angular measurement. A three-dimensional volumetric approach offers superior reliability and clinical correlation during femoroplasty. The purpose of this study is to three-dimensionally phenotype cam deformity to best target surgical resection and characterize the breadth of FAIS morphology. Methods After processing 2 GB of data across 156 radial CT scans of patients indicated for hip arthroscopy for FAIS at a tertiary referral center, we trained an nnU-Net segmentation model on 56 manually annotated hip CTs and applied it to a separate cohort of 100 FAIS patients indicated for arthroscopy. Two fellowship-trained hip specialists virtually performed femoroplasty with an agreed upon planned resection. For each patient, we automatically isolated the femoral head, fit a best-fit sphere through iterative refinement, and extracted morphological descriptors spanning four domains: head size (fitted sphere diameter), cam deformity (4 deviation metrics), resection geometry (volume, depth, surface area, solid angle), and spatial localization (resection centroid, extent, and span along the proximal-distal axis of the femoral neck). We performed Pearson correlation analyses, principal component analysis (PCA), and k-means clustering to characterize the morphology-resection relationship. We acquired pre- and postoperative patient-reported outcome scores (mHHS). Results K-means clustering yielded three cam phenotypes (Figure 1A, Figure 2): Type A/Hillock (33%), characterized by focal, proximal, gentle elevations necessitating relatively shallow resections closer to the femoral head; Type B/Hill (24%), characterized by broader surface coverage reflective of femoral head size with intermediate elevation; and Type C/Mountain (43%), characterized by distal, extreme elevations with intermediate surface area. Resection breadth depended upon head size (diameter vs. surface area r = 0.67, p < 0.001; Figure 1B), while resection depth was tightly coupled with distal extension (depth vs. centroid distal position r = 0.87, p < 0.001; Figure 1C). Breadth and depth were only moderately correlated (surface area vs. max depth r = 0.50; Figure 1D), confirming these as semi-independent dimensions. Patient-reported outcome scores did not differ significantly across phenotypes at any postoperative timepoint (all p > 0.05). Discussion Surgical resection in cam FAIS is governed by two orthogonal dimensions: head size determines cam surface area, while cam elevation is tightly coupled with distal cam extent. Three distinct three-dimensional cam phenotypes exist (Hillock, Hill, and Mountain) and establish a foundation for three-dimensional diagnostic evaluation and arthroscopic management, enabling heightened diagnostics beyond the alpha angle and individualized preoperative planning to best execute the femoroplasty.
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openalex_W7220459441 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors Mr Andrew Yang, Mr Joshua Woo, Danyal H. Nawabi, Dr Bryan Kellly, Prem N. Ramkumar
Journal Journal of hip preservation surgery
Year 2026
DOI
10.1093/jhps/hnag043.157
URL
https://doi.org/10.1093/jhps/hnag043.157
Keywords Keywords not found

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