Properties of the cartilage layer from the cam-type hip impingement deformity.

Femoro-acetabular impingement (FAI) is associated with significant acetabular cartilage damage and degenerative arthritis. To understand the contact stress and thus biomechanical mechanisms that may contribute to degeneration, the material behaviour of the cartilage layer is required. The objective of this study is to determine the fibril-reinforced poroelastic properties and composition of cartilage from cam deformities and to compare to those of normal cartilage. Patients undergoing surgical treatment of a symptomatic cam FAI deformity were recruited from the clinical practice of one of the authors. Osteochondral specimens were retrieved from the deformity during surgery using a trephine. Control specimens were retrieved from the anterior femoral head bearing surface during autopsy procedures. Indentation stress-relaxation tests were performed to determine the modulus (E), Poisson's ratio (ν) and permeability (k) of the poroelastic component, and the strain-independent (E) and -dependent (E) moduli of the fibril-reinforcement using finite element analysis and optimization. Safranin-O staining was used to quantify proteoglycan content. E and ν were 71% and 37% lower, respectively, in Cam specimens compared to controls, and k was approximately triple that of Control specimens (p<0.05). No significant differences were seen in the fibrillar components, E and E. Proteoglycan content was substantially depleted in Cam specimens, and was correlated with E, ν and k. This study showed that cartilage from the cam deformity exhibits severe degeneration in terms of the mechanical behaviour and composition changes, and is consistent with osteoarthritis. This further supports the hypothesis that FAI is a cause of hip osteoarthritis.