Tendon pain remains an enigma. Many clinical features are consistent with tissue disruption-the pain is localised, persistent and specifically associated with tendon loading, whereas others are not-investigations do not always match symptoms and painless tendons can be catastrophically degenerated. As such, the question 'what causes a tendon to be painful?' remains unanswered. Without a proper understanding of the mechanism behind tendon pain, it is no surprise that treatments are often ineffective. Tendon pain certainly serves to protect the area-this is a defining characteristic of pain-and there is often a plausible nociceptive contributor. However, the problem of tendon pain is that the relation between pain and evidence of tissue disruption is variable. The investigation into mechanisms for tendon pain should extend beyond local tissue changes and include peripheral and central mechanisms of nociception modulation. This review integrates recent discoveries in diverse fields such as histology, physiology and neuroscience with clinical insight to present a current state of the art in tendon pain. New hypotheses for this condition are proposed, which focus on the potential role of tenocytes, mechanosensitive and chemosensitive receptors, the role of ion channels in nociception and pain and central mechanisms associated with load and threat monitoring.

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S]sulfate lost to the medium and retained in the matrix was determined for each day in culture. It was shown that the rate of catabolism of radiolabelled small proteoglycans (decorin and biglycan) was significantly slower (T ½ > 20 days) compared with the radiolabelled large proteoglycans (aggrecan and versican) that were rapidly lost from the tissue (T ½ 2 days). Both the small and large newly synthesized proteoglycans were lost from the matrix with either intact or proteolytically modified core proteins. When explant cultures of tendon were maintained either at 4°C or in the presence of the lysosomotrophic 2 agent ammonium chloride, inhibition of the cellular catabolic pathway for small proteoglycans was demonstrated indicating the involvement of cellular activity and lysosomes in the catabolism of small proteoglycans. It was estimated from these studies that approximately 60% of the radiolabelled small proteoglycans that were lost from the tissue were degraded by the intracellular pathway present in tendon cells. This work shows that the pathways of catabolism for large aggregating and small leucine-rich proteoglycans are different in tendon and this may reflect the roles that these two populations of proteoglycans play in the maintenance of the extracellular matrix of tendon.

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Objective. To determine differences in the metabolism of proteoglycans and the gene expression of proteinases and their inhibitors between patellar tendons exhibiting chronic overuse tendinopathy and normal patellar tendons in humans.Methods. Rates of loss and synthesis of proteoglycans were determined. Radiolabeled and total proteoglycans retained in and lost from the tissue were analyzed by fluorography and Western blotting. Levels of messenger RNA for matrix metalloproteinase 1 (MMP-1), MMP-2, MMP-3, MMP-9, MMP-13, ADAMTS-1, ADAMTS-4, ADAMTS-5, tissue inhibitor of metalloproteinases 1 (TIMP-1), TIMP-2, TIMP-3, and TIMP-4 were determined in fresh tissue.Results. The rate of loss of 35 S-labeled proteoglycans was greater in abnormal tendons, as was the rate of synthesis of proteoglycans. Fluorography and Western blotting revealed the presence of greater amounts of large proteoglycans (aggrecan and versican) in abnormal tendons, and these proteoglycans were rapidly lost from the matrix of abnormal tendons. There was no significant difference in the expression of ADAMTS-1, ADAMTS-4, ADAMTS-5, MMP-1, MMP-2, MMP-3, MMP-13, TIMP-2, TIMP-3, or TIMP-4. There was a significant increase in the expression of MMP-9 and TIMP-1 in abnormal tendons.Conclusion. Our findings suggest that a change in the proteoglycan content of the extracellular matrix in abnormal tendons results from the altered metabolism of the cells, reflected in the enhanced synthesis of the large proteoglycans aggrecan and versican, and does not appear to result from changes at the level of gene expression.