Restoration of quadriceps muscle mass and strength following ACL (anterior cruciate ligament) injury and reconstruction is a major focus of rehabilitation in this patient population. Numerous groups have recommended restoration of quadriceps strength to within 10% of that of the uninjured contralateral limb prior to clearance for return to sport1. Such recommendations recognize associations between impaired quadriceps strength and altered functional movement patterns2 that may place patients at increased risk for reinjury3,4 and development of osteoarthritis5.
In spite of the desire by surgeons, therapists, and patients to restore quadriceps strength following ACL reconstruction and the targeting of modern rehabilitation protocols to achieve this goal, strength deficits at 6 and even 12 months postoperatively are quite common1. The reasons for these persistent deficits are not clear and are likely multifactorial.
The recent work by Noehren et al. provides a detailed look at quadriceps muscle changes following ACL injury and reconstruction through MRI (magnetic resonance imaging) and muscle biopsy. Post-injury but pre-reconstruction, they noted a reduction in type-IIA fast twitch fibers in the vastus lateralis compared with the uninjured side, a reduction that actually decreased further by 6 months post-reconstruction. The authors also noted increased fibrosis and decreased satellite cells, muscle volume, and physiological cross-sectional area post-injury. These values did not exhibit significant changes from pre-reconstruction to 6 months postoperatively. While more work is needed to understand how reversible (if at all) these changes may be, the implications of these structural changes on muscle function may help to explain the persistent quadriceps weakness that some patients note following ACL reconstruction—in spite of completion of a focused rehabilitation program.
It is important not to overstate the results of this interesting, but preliminary, work. The relatively small number of patients in the study (10) precludes the evaluation of factors that may influence changes to muscle following ACL injury and surgery. Such factors include graft type (both hamstring and patellar tendon grafts were included in the study), time from injury to biopsy and from injury to reconstruction, the use of perioperative nerve blocks, and details of the rehabilitation protocol, among others. It should be noted that the patients in this study had not yet achieved the quadriceps strength goals mentioned above when the final biopsy sample was taken at 6 months postoperatively (the average isometric strength deficit of the surgically treated limb at this point was 39%). Also, the small number of patients precluded any meaningful comparison between morphological alterations of muscle and resultant strength. It is unknown whether similar findings would be noted in patients who achieved full restoration of quadriceps strength.
In spite of these limitations, this study represents an important step forward in the understanding of quadriceps muscle deficits following ACL injury and reconstruction. Further work will likely build on these findings to evaluate the reversibility of the noted changes and evaluate interventions to potentially minimize the morphological changes noted in this study as well as maximize muscle function following the development of these changes.
↵* The author indicated that no external funding was received for any aspect of this work. The Disclosure of Potential Conflicts of Interest form is provided with the online version of the article.
- Copyright © 2016 by The Journal of Bone and Joint Surgery, Incorporated