Is collagen fiber damage the cause of early softening in articular cartilage?

Objective: Because collagen damage and cartilage softening have not yet been determined simultaneously in one study for the very early onset of osteoarthritis (OA), it remains questionable whether they are associated. The aim of the present study is therefore to evaluate whether indeed, initial collagen damage can be found when tissue softening occurs as a result of excessive mechanical loading. Methods: To investigate this aim, a series of specific indentation loading protocols were designed to induce and monitor cartilage softening in osteochondral explants of bovine carpometacarpal joints. The experiment contained one control group (n = 6) in which no damage was induced and four experimental groups in which samples received either a constant load of 3 (n = 5), 6 (n = 5) or 15 N (n = 6), or an increasing load (n = 7) from 2 to 13 N in 11 steps. Moreover, to determine mechanically induced collagen damage, Col2-3/4M (cumulative collagen damage) and Col2-3/4C(short) (only enzymatic damage) staining were compared. Results: The normalized peak and equilibrium reaction forces decreased in the groups that received increasing and 15 N peak loading. However, Col2-3/4M staining was negative in all samples, while enzymatic damage (Col2-3/4C(short)) appeared similar in experiments and in unloaded control groups. Conclusion: It was shown that a loading magnitude threshold exists above which softening occurs in cartilage. However, in samples that did show softening, we were unable to detect collagen damage. Thus, our results demonstrate that cartilage softening most likely precedes collagen damage. (C) 2012 Osteoarthritis Research Society International. Published by Elsevier Ltd. All rights reserved.