BIOMECHANICAL EVALUATION OF NORMAL ARTICULAR CARTILAGE IN THE HUMAN KNEE

Abstract

Articular cartilage has compressive stiffness determined primarily by the matrix which is quite characteristic and distinct from that of degenerative articular cartilage or regenerative fibrocartilage. Alterations evident when articular cartilage begins to degenerate include a decrease in proteoglycan content and water content and resultant reduction in stiffness. Regenerative fibro-cartilage has greatly reduced stiffness with functional implications. Identification of cartilaginous stiffness for various sites of normal articular cartilage in the knee is important to enable comparison measures of suspected degenerative cartilage and regenerative articular cartilage either hyaline, fibrocartilage or mixed. The aim of this study was to map the in vivo biomechanical properties of normal human articular knee cartilage using the Artscan 1000 arthroscopic cartilage stiffness tester (Artscan Oy, Finland). It has been shown that the Artscan 1000 is reliable when measuring the stiffness of thin articular cartilage (Lyra et al., 1999). Over a period of 12 months, 94 patients (age 15–69 yr) undergoing a knee arthroscopy consented to having their normal articular surfaces biomechanically evaluated for stiffness. Cartilage stiffness (N) was defined by the mean indenter force at each site where the applied force on the measurement rod equalled 10 ±1.5 N. Medial femoral condyle stiffness (M ±SD; 3.71 ±1.28 N) was greater than all other sites and was significantly greater than mean values obtained for proximal, distal and lateral trochlea (1.87 ±0.91, 2.44 ±1.02 and 2.69 ±1.52 N, respectively); medial (1.71 ±0.70 N) and lateral patella (2.18 ±1.03 N); and medial and lateral tibial plateau for all subjects (2.33 ±.1.26 and 2.27 ±1.19 N, respectively; p < 0.05). There were no significant differences between sexes for each site. There was no trend for cartilage stiffness to be lower in patients over forty compared to younger patients for both sexes for all sites. There was, however, statistically significant less stiffness of the distal trochlea for females under 40 years when compared to that of females older than 40 years. The clinical significance of this is under review. Further research involving the characterisation of cartilage stiffness in pathological situations and evaluation of stiffness following articular cartilage repair is now possible.