Abstract
Introduction
ACL reconstruction using hamstring tendons has gained general acceptance. However, it has been recommended to seek a tight fit of the tendon in the bone canal in order to provide circumferential contact and healing of the graft, and to prevent secondary tunnel widening. Recent findings show, that the graft dynamically adapts to pressure in the canal resulting in a potentially loose graft-bone contact. It was the goal of this study to understand the viscoelastic behaviour of hamstring grafts under pressure and to develop a new method for tendon pre-conditioning to reduce the graft volume before implantation, in order to reduce the necessary bone canal diameter to accommodate the same graft.
Material and Methods
Flexor digitorum tendons of calf and extensor digitorum tendons of adult sheep were identified to be suitable as ACL grafts substitutes for human hamstring tendons in vitro. The effect of different compression forces on dimensions and weight of the grafts were determined. Further, different strain rates (1mm/min vs 10mm/min), compression methods (steady compression vs. creep) and different compression durations(1, 5, 10min) were tested to identify the most effective combination to reduce graft size by preserving its macroscopic structure.
Results
The effect of compression on volume reduction (25% of initial volume) reached a plateau at 6000N. Both, steady compression and creeping were able to reduce dimensions of the graft, however, creeping was more effective. There was no difference in effect with different durations for compression (p>0.05) in both methods. With a strain rate of 1mm/min no macroscopic destruction was documented, however with 10mm/min some parts were ruptured. During all pressure tests, considerable amounts of liquid were pressed out from the tendons, and if the graft was submersed in saline solution overnight, the volume reduction was mostly reversible.
Conclusion
Compression reduces the dimensions of the ACL graft reversibly, to the greatest part by squeezing out of interstitial water. It is reasonable to assume that this effect also occurs if tendons are under constant pressure in the body, such as at the bend where entering a bone tunnel or under the pressure of interference screws. This in vitro experiment suggests that preconditioning of a 8mm hamstring graft is achieved best by creeping compression with 6kN at a strain rate of 1mm/min. By using this technique, indeed a canal of approximately 10–15% less diameter (i.e. 7 instead of 8mm) may be drilled for the same tendon, resulting in a tight fit of the graft in the bone.
