Abstract
Summary
The cartilage layer from cam-type femoroacetabular impingement deformities had lower stiffness and increased permeability compared to normal cartilage. This is consistent with osteoarthritis and supports the hypothesis of abnormal contact stresses.
Introduction
Femoroacetabular impingement (FAI) has recently been associated with osteoarthritic (OA) degeneration of the hip and may be responsible for up to 90% of adult idiopathic OA cases. FAI results from deformities in the hip joint which may lead to abnormal contact stresses and degeneration. The more common cam-type deformity consists of a convex anterior femoral head-neck junction which impinges the anterosuperior acetabular rim during flexion and internal rotation of the hip. Increased subchondral bone density has been reported in this region which may be a bone remodelling response to increased contact stress. The abnormal contact is expected to cause degeneration of the cartilage layer. The goal of this study was to assess the mechanical properties of cartilage retrieved from the cam deformity and to compare this with normal articular cartilage from the femoral head. It is hypothesised that the cartilage will have a lower elastic modulus and higher permeability than normal cartilage.
Patients & Methods
Osteochondral biopsies were retrieved from nine patients undergoing surgical correction of a symptomatic cam deformity as well as 10 fresh cadaveric specimens (10 hips, 6 donors). An indentation stress relaxation test was performed on each specimen to 10% of the estimated cartilage thickness. A needle penetration test was performed to accurately measure the thickness. The equilibrium modulus was calculated per Hayes et al. A specimen-specific axisymmetric finite element model was used in a non-linear optimization to obtain the fibril-reinforced poroelastic properties of the cartilage that best fit the experimental data. The material properties were non-fibrillar modulus (Es), Poisson's ratio (νs) and permeability (k) and strain-independent and –dependent moduli (E0, Eε)[4].
Results
The equilibrium modulus was 0.14 MPa and 0.63 from surgical and cadaver specimens, respectively (p=0.002). Compared to cadaver specimens, Es in surgical specimens was 73% lower (p=0.01), νs was 43% lower (p=0.01) and k was an order of magnitude higher (p=0.02). Fibril moduli were not significantly different (p>0.35).
Discussion/Conclusions
This study showed decreased elastic modulus and increased permeability in cartilage from cam deformities compared to cadaver controls. These differences are consistent with changes expected in osteoarthritic cartilage degeneration. Fibril moduli were 14% to 57% lower in surgical specimens consistent with fibrillation, however results were not significant due to high variability. Altered cellular activity and proteoglycan depletion has been reported in cartilage of cam deformities, which are similar to changes expected in osteoarthritis. The altered mechanical and biochemical properties of this cartilage therefore support the hypothesis that osteoarthritis is secondary to cam FAI deformities and is a result of abnormal contact stresses between the deformity and acetabular rim.
