Mechanical and chemical properties of rotator cuff tendons
Keywords: shoulder
supraspinatus
gene expression
biology
rheology
tendon
rotator cuff
microarray

Shoulder disease is the third most common musculoskeletal problem, and rotator cuff tendon tears account for the greatest proportion of shoulder complaints. Rotator cuff tears are estimated to affect between 5-30% of adults, with higher incidences of tearing and failure to heal in elderly patients, placing a huge socioeconomic burden on an ageing British population. Serious concern arises as a large proportion of technically correct surgical repairs re-rupture. The intra-articular environment of the tendon often precludes normal healing and surgical repair is often necessary to improve pain and restore some function. It is feasible that there may be an inherent physiological or biomechanical defect in the tissue that prevents complete healing without some further augmentation to the surgical repair. Improved understanding of the biochemical and biomechanical changes in torn rotator cuff tendons may help to reduce the high rerupture rates. This study aimed to characterise normal, and different sized rotator cuff tendon tears from small samples obtained intraoperatively to try to use tests that may potentially be clinically useful in the future.
Tendon samples were mechanically tested using dynamic shear analysis, a form of rheology, to overcome gripping and slippage problems of very small specimens. It was found that torn tendons had a significantly reduced storage modulus compared to normal tendons, particularly for massive tears. Chemical analysis of tendons using Fourier transform infrared spectroscopy revealed that partial and different sized rotator cuff tendon tears are chemically distinguishable. The onset of rotator cuff tear pathology is mainly due to an alteration of the collagen structural arrangements, with associated changes in lipids and carbohydrates. Collagen structural changes in small and massive tendons were quantified using differential scanning calorimetry, which allows measurement of collagen thermal properties as a reflection of their structural integrity. Small and massive tendon tears had reduced thermal properties and hence reduced collagen integrity when compared to normal tendons, although there was no difference between the two tear groups. Gene expression differences between the small, massive tears and normal tendons were studied using microarray analysis, and revealed that the different groups were biologically distinguishable. Microarray gene profiles of human rotator cuff tendon tears suggested a key pathogenesis role for various extracellular matrix (ECM) genes, such as aggrecan, matrix metalloproteinases (MMPs) and a disintegrin and metallopeptidases (ADAMs).
Rotator cuff tendon tears involve complex gene and biochemical changes, which particularly affect collagen and ECM components. These may interact and result in reduced mechanical properties of torn tendons. A growing interest has been shown in augmenting rotator cuff tendon repairs, for example with repair patches. Mechanical assessment of four commercially available repair patches has demonstrated wide variations between all patches and at least some reduced mechanical properties when compared to human rotator cuff tendons. Surgeons should be aware of the need to address and improve the biology and the mechanical properties of torn rotator cuff tendons when treating and possibly repairing these defects. The differences in different sized rotator cuff tendon tears should also be appreciated, as all tears are not a uniform homogenous group. It is possible that the future may increasingly involve tailoring treatments to specific tear characteristics, although much greater work is required in this area.