Introduction Cam-type femoroacetabular impingement syndrome (FAIS) is a pre-arthritic hip condition, defined as a bony growth on the proximal femur that causes abnormal joint contact. The tissue presentation of the cam deformity and capsule in FAIS remains understudied. The purpose of this study was to 1) evaluate histopathological features in cam deformity and capsule from FAIS patients, 2) assess the extent of local inflammation within the capsule, and 3) determine relationships between cam deformity tissue composition versus α angle and patient factors.

Methods Cam deformity and capsular tissues were collected from FAIS patients undergoing arthroscopic surgery. Samples were histologically processed, imaged using light and polarized light microscopy, and assessed with point counting. Correlation-based statistics were performed to identify features correlated with α angle and patient factors.

Results Across 21 cam deformity samples assessed, a total of 16,259 points were counted. The tissue within the cam deformity was observed to be heterogeneous between specimens, comprised of 16 distinct structures spanning different states of viability. In samples with articular cartilage, the tissue was highly disrupted and/or calcified. The presence of fibrocartilage, necrotic cartilage, and vasculature had significant low-moderate correlations with α angle. During assessment of capsular tissue quality, synovitis was observed in most samples.

Conclusion The cam deformity is complex and heterogeneous, both within individual cam deformities and between individuals with FAIS. Several cam deformity tissue features were correlated with α angle, age, sex, and BMI. The heterogeneity observed in these samples indicates that tissue properties within the cam deformity varies between patients with FAIS, which may contribute to outcomes of hip arthroscopic surgery and a patient’s level of risk for the subsequent development of osteoarthritis. Our findings suggest distinct tissue phenotypes of FAIS exist, which may be an important consideration for FAIS treatment strategies.

The authors have declared no competing interest.

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by a NIAMS-funded postdoctoral fellowship awarded to the first author (Yuh, NIH T32AR073157).

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This study received ethical approval through a Notification of Exemption from IRB Review, from the Rush University IRB, on 7/29/2021.

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Declaration of Conflicting Interests: The authors declare that there is no conflict of interest.

Funding Statement: The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by a NIAMS-funded postdoctoral fellowship awarded to the first author (Yuh, NIH T32AR073157).

Ethics Approval Statements: This study received ethical approval through a Notification of Exemption from IRB Review, from the Rush University IRB, on 7/29/2021.

All data produced in the present study are available upon reasonable request to the authors.