Does nitric oxide help explain the differential healing capacity of the anterior cruciate, posterior cruciate, and medial collateral ligaments?

This study compared the ability of rabbit medial collateral ligament, poste rior cruciate ligament, and anterior cruciate ligament tissue to synthesize nitric oxide, and determined its effects on matrix synthesis, an important component of ligament repair. It is not known whether ligament cells can p roduce nitric oxide and, if so, whether it influences healing of ligament i njuries. The anterior cruciate and posterior cruciate ligament tissue produ ced large amounts of nitric oxide in response to the inflammatory cytokine interleukin-1. Medial collateral ligament, in contrast, produced only modes t amounts of nitric oxide. Furthermore, anterior cruciate ligament and, to some degree, posterior cruciate ligament synthesized nitric oxide spontaneo usly in culture, whereas medial collateral ligament never did so. When nitr ic oxide was supplied to these tissues, it strongly inhibited collagen synt hesis by the two cruciate ligaments, but had little effect on collagen synt hesis by the medial collateral ligament. Endogenously synthesized nitric ox ide was also able to inhibit collagen synthesis as well as proteoglycan syn thesis by the hive cruciate ligaments, but had little effect on matrix synt hesis by the medial collateral ligament. We propose a novel hypothesis, bas ed on nitric oxide production and matrix synthesis, that may help explain w hy the two cruciate ligaments have such limited healing capacity compared w ith the medial collateral ligament.