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Synopsis: Sequential MRI scans at various time periods following hamstring tendon harvest revealed progressive regrowth of the tendons toward their tibial attachment.
Source: Rispoli DM, et al. Magnetic resonance imaging at different time periods following hamstring harvest for anterior cruciate ligament reconstruction. Arthroscopy. 2001;17(1):2-8.
Much has been written regarding how the patellar tendon defect regenerates and can potentially be used as another graft source for ACL reconstruction. However, few people are willing to actually use this potentially weakened tissue. Only a few scattered case reports have suggested hamstring tendons may also regenerate after they are harvested for ACL reconstruction. In this study, 45 patients had the semitendinosus and gracilis tendons harvested in standard fashion for quadruple hamstring ACL reconstruction by Rispoli and colleagues over a 20-month period. Of these, 21 volunteered to have MRI scans at time points ranging from 2 weeks to 32 months following tendon harvest. MRIs were performed with a 1.5 tesla magnet and prospectively evaluated by 2 musculoskeletal radiologists who were blinded to the time interval between graft harvest and MRI.
The results showed a progressive regrowth of the tendons toward the tibial attachment. Only fluid was evident at the early time points, but by 6 weeks there were disconcernable tendons at the level of the superior pole of the patella. Tendon regrowth progressed from the level of the joint line at 3 months, to within 1-3 cm of the tibial attachment by 12 months, to within 1 cm of the tibial attachment at 32 months. Although the regenerate tissue was a bit ill-defined and variable in the first 6 weeks, by 3 months the tendons appeared to have normal signal characteristics as compared to native tendon. Semitendinosus appeared to regenerate more predictably than the gracilis.
Hamstring ACL reconstruction is becoming increasingly popular as fixation methods improve. There is limited donor-site morbidity, and the tendon has been shown to be stronger than the patella tendon biomechanically. A major advantage also is the lack of detrimental effect on the extensor mechanism and quadricep strength. Often after patellar tendon harvest, quadricep strength takes 6-12 months to return to normal. Interestingly, patients who have their hamstring tendons harvested rapidly regain hamstring strength. This led Rispoli et al to postulate that the tendons regrow and actually function. Their study effectively demonstrates by MRI criteria that the tendons do regrow analogous to a lizard tail. The mechanism of this is unknown but intriguing. The study did not attempt to assess if the new tendons were functional, however. Cross-sectional area of the muscles showed no significant atrophy of the sartorius or semitendinosus muscle. The gracilis seemed to regenerate a little less predictably and did demonstrate some degree of atrophy.
The normal hamstring tendons lie invested in a fascial plane along the medial aspect of the knee. This tissue plane may help explain the regeneration of the harvested tendon. It appears to initiate proximally in a more vascular area and proceed distally along the fascial plane toward the tibial attachment. Although these somewhat random time points for MRI evaluation of different patients are not as valuable as prospective, sequential MRIs on the same patient, this certainly appears to demonstrate that the lizard tail phenomenon exists. A major unanswered question is whether this tendon achieves a functional tibial attachment and whether it can be harvested again as a graft source. Isokinetic testing suggests that the tendons are functional, as strength has been shown by many investigators to return to normal within a few months.
This is a major finding with implications for surgeons, therapists, trainers, and patients. It is likely to shape the way we perform ACL reconstructions in the future as more is learned. I understand that some of the investigators are now proceeding to an animal model to better understand the physiology of this lizard tail phenomenon.
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