Because we found no difference in the gliding resistance after the tendons had been soaked in saline solution for two hours but had not been treated with hyaluronidase (group 3), we believe that our findings are the result of changes in a hyaluronidase-sensitive substance on the surface of the tendon and not to autolysis. As washing of the tendon in saline solution did not remove this substance, we believe that the substance is probably bound to the surface.
There may be hyaluronate and other alcian blue-positive, hyaluronidase-sensitive materials on the surface of the flexor digitorum profundus tendon either in a complex with fibronectin or as a part of the proteoglycan of the synovial membrane or the surface of the tendon2,19,21. Proteoglycan may also be present between collagen-fiber bundles on the volar part of the tendon, as a result of adaptive differentiation of the fibrocytes into cartilaginous cells in response to repetitive compression11,13,24. These substances may play a role in the surface lubrication of the tendon-pulley interaction. At lower loads, there was no significant increase in the gliding resistance of tendons that had been treated with hyaluronidase; this may have been because a fluid film of saline solution acted as a lubricant between the tendon and the pulley. As the load increased, an increase in the gliding resistance became evident. The higher loads that we used were similar to those that have been noted in tendons moving actively without resistance17. Thus, we believe that these loads may be more representative of the in vivo conditions than the lower loads were. We hypothesize that the fluid film may have been squeezed out of the tendon-pulley interface under high loads, after which direct contact became the dominant interaction. Under these conditions, any potential lubricant bound to the surface of the tendon, such as hyaluronate or proteoglycan, may act to reduce the frictional resistance as a boundary lubricant. We believe that our findings support the hypothesis that treatment with hyaluronidase removed these surface lubricants and therefore increased the gliding resistance.
The gliding resistance at the tendon-pulley interface may depend on the surface conditions18, macroscopic shape2, or compression properties of the tendon and the pulley. This is evidenced by the shapes of the curves for the F2 loads (Fig. 1-A, 1-B and 1-C); a peak of resistance was associated with a change in the cross-sectional profile of the flexor digitorum profundus tendon as it moved beneath the A2 pulley23. Because the force pattern as a function of excursion of the tendon did not change even after treatment with hyaluronidase, we believe that a change in the surface conditions was responsible for the increase in gliding resistance that we observed. If the compression properties or the shape of the tendon had changed substantially, we would have expected a change not only in the magnitude of force but also in the pattern of force.
Several limitations of this study should be mentioned. The tendons were washed with saline solution to remove synovial fluid from their surface so that the hyaluronidase could react only to any hyaluronate that was chemically combined or included on the surface. However, it was unclear how much synovial fluid was present before and after the tendons were washed. We believe that most of the synovial fluid was washed away by this procedure20. Although there was a tendency for the gliding resistance to increase after the tendons had been washed, no significant difference was detected. This suggests that the synovial fluid may have a coating effect, which may have partially blocked the action of hyaluronidase on the tendons that were not washed before they were treated with this enzyme. Although washing with saline solution did not increase the gliding resistance significantly in our study, given the wide standard deviations many more specimens would need to be examined to test the hypothesis that washing causes a significant difference in the measured resistance. Our sample size of ten in each group was sufficient to provide roughly an 80 per cent chance of detecting a difference of one standard deviation. With standard deviations of 100 per cent of the mean and more, our sample size does not have the statistical power to test for the presence of a difference in the range of 50 per cent or less. Approximately forty specimens would be needed to produce an 80 per cent chance of detecting a difference of this size (that is, one-half of the standard deviation).
We do not know exactly what the hyaluronidase dissolved in the tendon, or the quantity that it dissolved, as the implications of hyaluronidase digestion are unclear and the specificity of the hyaluronidase preparation can be questioned. This may explain the relatively large standard deviations in the gliding resistance of the tendons that had been treated with hyaluronidase. Of course, we cannot rule out completely the possibility that the collagen-fiber structure on the surface of the tendon was damaged by treatment with hyaluronidase and that this contributed to the increase in the gliding resistance that we observed. However, we believe that such an effect would have been negligible because no obvious damage was seen with scanning electron microscopy, and treatment with hyaluronidase is not thought to affect the biomechanical properties of the collagen network substantially16.
Finally, there are other potential lubricants in the synovial fluid or the synovial membrane as well as lubrication mechanisms other than boundary lubrication. This study did not rule out the possibility that other lubrication mechanisms were involved in the reduction of the resistance between the tendon and the pulley.
The superior gliding ability of intrasynovial tendons, such as the flexor digitorum profundus, compared with that of extrasynovial tendons, has been the subject of much recent research. Intrasynovial tendon grafts have been shown to have less early necrosis, better intrinsic healing, fewer adhesions, and better gliding ability1,5,6,18 than extrasynovial grafts. Tenocyte metabolism appears to differ between intrasynovial and extrasynovial tendons1. In this study, we explored another possible difference, surface lubrication. Our observations suggest that, in addition to the potential factors previously elucidated, there also is a difference in surface lubrication. We, as well as others1,5,6,18, believe that intrasynovial tendon grafts have better gliding abilities than extrasynovial tendon grafts; we believe that one contributing factor is a difference in surface lubrication.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were the Mayo Foundation and National Institutes of Health Grant AR17172.
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