Abstract
INTRODUCTION
In order to address high failure rates following rotator cuff repairs, a greater understanding is required of the underlying structural changes so that treatments can be appropriately targeted and biomarkers of failure can be identified. As collagen is the primary constituent of tendon and determines force transmission, collagen structural changes may affect responses to loading. For example changes in collagen 1 and 5 are associated with the hyperelastic Ehlers-Danlos syndrome, which is diagnosed by looking for pathopneumonic altered collagen fibres or ‘collagen flowers’ in skin using transmission electron microscopy (TEM). To date no study has been performed on the microstructure of torn human rotator cuff tendons using TEM.
It was hypothesized that normal, small and massive human rotator cuff tendons tears will have altered microscopic structures. The unique study aimed to use TEM to compare the ultrastructure of small and massive rotator cuff tears, to normal rotator cuff tendons.
METHODS
Samples from 7 human rotator cuff tendons repairs were obtained, including 4 massive (>5 cm) and 3 small (< 1 cm) tears, and 3 matched normal controls with no history of connective tissue disorders. Specimens were fixed in 4% glutaraldehyde in 0.1M phosphate buffer, processed and examined blind using routine TEM examination.
To assess whether changes in the relative expression of collagen 1 and 5 (COL1A1, COL5A1 and COL5A2) occurred in all tears, qPCR was performed on another 6 phenotypically matched patients.
RESULTS
The basic structure of the normal tendon consisted of tightly packed clumps of dense packed parallel running collagen fibers with few fibroblasts and small amounts of fine filamentous material between clumps. In contrast, torn samples were more variable with areas of less dense packing of collagen fibers and larger areas of filamentous material plus variable numbers of lipid droplets both within the fibroblast and between the collagen bundles. There was also evidence of twisting and random orientation of individual collagen fibers.
All torn tendons showed evidence of a proportion of the fibers within the collagen bundles being enlarged with a serrated outline, similar in appearance to ‘collagen flowers’. Clear differences between the small and massive tears were not identified.
qRT-PCR of torn rotator cuff tendon specimens demonstrated no altered collagen expression compared to normal tendons.
DISCUSSION
This novel study has identified the previously unreported presence of atypical collagen fibers with focal swelling resulting in the appearance of ‘collagen flowers’ in torn rotator cuff tendons only. This appearance is considered pathognomonic of Ehlers-Danlos syndrome, classical type 1 and 2. Torn tendons also showed an increase in filamentous material, and infiltration with fat droplets.
These novel findings may offer insight into the mechanisms of structural damage that contribute to rotator cuff failure. Further examination is required, to evaluate the significance of these observations.
