Functional consequences of cartilage degeneration in the equine metacarpophalangeal joint: quantitative assessment of cartilage stiffness

Reasons for performing study: No quantitative data currently exist on the relationship of the occurrence of cartilage degeneration and changes in site-specific biomechanical properties in the metacarpophalangeal (MCP) joint in the horse. Objectives: To gain insight into the biomechanical consequences of cartilage deterioration at 2 differently loaded sites on the proximal articular surface of the proximal phalanx (P1). Hypothesis: Static and dynamic stiffness of articular cartilage decreases significantly in degenerated cartilage. Methods: Cartilage degeneration index (CDI) values were measured at the lateral dorsal margin (Site 1), lateral central fovea (Site 2) and entire joint surface of P1 (CDI(P1)) in 30 horses. Group I contained joints without (CDI(P1) values < 25%, n = 22) and Group 2 joints with (CDI(P1) values > 25%, n = 8) signs of cartilage degeneration. Cartilage thickness at Sites I and 2 was measured using ultrasonic and needle-probe techniques. Osteochondral plugs were drilled out from Sites I and 2 and subsequently tested biomechanically in indentation geometry. Young's modulus at equilibrium and dynamic modulus were determined. Results: Cartilage thickness values were not significantly different between the 2 groups and sites. Young's modulus at Site I was significantly higher in Group I than in Group 2; at Site 2, the difference was not significant. Dynamic modulus values were significantly higher in Group I than in Group 2 at both sites. Conclusions: Degenerative cartilage changes are clearly related to loss of stiffness of the tissue. Absolute changes in cartilage integrity in terms of CDI are greatest at the joint margin, but concomitant changes are also present at the centre, with a comparable decrease of the biomechanical moduli at the 2 sites. Therefore, significant cartilage degradation at the joint margin not only reflects local deterioration of biomechanical properties, but is also indicative of the functional quality in the centre. Potential relevance: These findings may be important for improving prognostication and developing preventative measures.