PREVALENCE AND POSSIBLE PATHOLOGICAL SIGNIFICANCE OF CALCIUM-PHOSPHATE SALT ACCUMULATION IN TENDON MATRIX DEGENERATION

Objectives-To investigate the prevalence of calcium phosphate mineral salt accumulation in degenerative supraspinatus 'tendinitis' compared with a normal sample of human tendons, and to determine whether there is an association of calcium salt deposition with pathological changes in the tendon extracellular matrix. Methods-Cadaver tendons (supraspi natus and common biceps tendons, n=96) and fragments of supraspinatus tendons obtained during shoulder surgery (n=31) were analysed for calc ium content by atomic absorption spectroscopy, phosphorous content usi ng a spectrophotometric assay, and matrix composition (collagen, glyco saminoglycans and DNA) using standard biochemical techniques. Results- We established baseline values of calcium concentration in macroscopic ally normal cadaver tendons (mean 1.1 (SD 0.35) mu g/mg dry wt, n=60) and found that 33% (nine of 27) of ruptured tendons from patients with 'degenerative tendinitis' contained an excess of calcium (more than 2 SD greater than the normal sample mean). Five of these specimens had i ncreased concentrations of phosphorous and calcium:phosphorous (molar) ratios consistent with a variety of possible calcium crystals, includ ing calcium pyrophosphate, hydroxyapatite, and tricalcium phosphate, i n addition to mixed or amorphous calcium phosphate deposits. Four of t hese specimens contained normal concentrations of phosphorous, consist ent with deposits of calcium oxalate or calcium carbonate, although th is was not confirmed biochemically. In contrast, surgical specimens (n =4) from patients with 'calcifying tendinitis' (radiographically detec ted calcium deposits) all contained salts with a mineral composition c onsistent with hydroxyapatite. The presence and identity of crystal de posits was subsequently confirmed in five specimens by radiographic mi croanalysis. Analysis of the tendon matrix demonstrated a number of si gnificant differences between normal and degenerate (ruptured) tendons , including a reduction in collagen content, an increase in sulphated glycosaminoglycans (predominantly dermatan sulphate) and an increase i n DNA (cellular) content. However, there were no significant differenc es between degenerate tendons that were 'calcified' and those degenera te specimens that contained normal concentrations of calcium. Conclusi ons-Although there was a relatively high prevalence of calcium salts i n degenerate tendons, which might contribute to the pathological proce ss (such as increased matrix collagen degradation), these data are con sistent with the hypothesis that 'dystrophic calcification' of degener ate tendon matrix is a pathological entity distinct from cell mediated 'calcifying tendinitis'. Calcification is probably one possible outco me (or end point) of chronic tendon injury, although the possibilty ex ists that in many cases, the presence of calcium salts may contribute to the tendon matrix degeneration.