It was the primary objective of this study to compare the functional outcome of bio-enhanced ACL repair, a method under development, with ACL reconstruction, the current standard of care for patients with ACL injuries. The secondary objective was to ensure that both treatments were effective by comparing them with a control group of animals with an untreated ACL transection. In this study we found that the structural properties of the ligament after bio-enhanced ACL repair were not significantly different from the grafts after ACL reconstruction. In addition, both treatments offered improvement in AP knee laxity, ACL linear stiffness, and displacements to yield and failure over ACL transection.
The rationale behind bio-enhanced ACL repair is to augment a normal suture repair with a bioactive collagen-platelet composite. Earlier studies have identified the lack of clot formation in the knee as a key reason for the high rate of failure of primary suture repair.2,3
The collagen-platelet composite serves both as a scaffold for cell-based tissue remodeling and as a source of anabolic growth factors, thus stimulating healing. Using collagen or platelets alone led to significantly worse biomechanical outcomes in prior in vivo experiments.14,22
It has also been shown that the addition of a collagen-platelet composite can also significantly improve biomechanical outcomes after ACL reconstruction in a porcine model,15
but again, the addition of platelets alone has no beneficial effect on ACL reconstruction in human trials or in animal models.23
These results for bio-enhanced ACL reconstruction support our proposed technique. However, we used conventional ACL reconstruction in this study because we wanted to use a clinically accepted positive control with which to compare bio-enhanced ACL repair, rather than another experimental technique.
No significant differences were observed between ACL reconstruction and bio-enhanced ACL repair for any of the measured biomechanical outcomes. From prior studies, it is also known that the type of suture repair that we use in this model is not effective in improving AP laxity unless both collagen and platelet-rich plasma are combined.13,14,21
For this study, AP laxity of bio-enhanced ACL repair was not significantly different from ACL reconstruction at any knee flexion angle. However, whereas surgical treatment produced significantly less AP laxity than ACL transection at 30° and 60°, there was no difference at 90°. When one is interpreting these results, it should be noted that AP laxity testing was performed with the joint capsule and the menisci in situ. These secondary stabilizers may play more of a role in AP stability with the porcine knee in deeper flexion.24–28
None of the animals in this study had postoperative flexion or extension contractures develop. In addition, all thigh circumferences increased over time, consistent with what would be expected for a juvenile, growing animal; however, the ACL reconstruction group had a significantly lower rate of increase of almost 50% of circumference. This could possibly be because of increased pain in this group because of the larger tunnels required for graft implantation; the persistence of a subclinical effusion in reaction to the allograft material, which could potentially limit leg usage in the postoperative period; or loss of the pro-prioceptive function of the ACL when the ligament is removed before reconstruction. Further work to determine the cause of this observation is required but may point to other advantages of repair over reconstruction.
It should be noted that the animals used in this study were skeletally immature. In earlier studies it was shown that young age is associated with increased cellular migration and proliferation, translating into better biomechanical outcomes in a porcine model at 3 months.19,29–31
Thus the results for bio-enhanced ACL repair might be less striking in adults. However, cellular invasion of the graft is thought to be essential for successful long-term function of the graft; thus both methods may change in efficacy with age. An important point that steered us toward the use of skeletally immature animals is the current, particularly high need for improved ACL treatment options in pediatric patients. ACL reconstruction is still avoided in many skeletally immature patients because of fear of growth disturbances, leading to secondary cartilage and meniscus injury in these conservatively treated patients.10
Extra-articular stabilization is a valuable proposition, but there are only limited long-term data and it is not an anatomic reconstruction of the knee kinematics.10,32
The technique presented herein could be an effective option for skeletally immature patients leading to an anatomic repair without affecting the growth plate.33
Bio-enhanced ACL repair has several potential advantages over ACL reconstruction if equivalent efficacy can be shown. Bio-enhanced ACL repair obviates the need for graft harvest and can thus be a less invasive procedure. In addition, bio-enhanced ACL repair offers the opportunity to retain the insertion sites of the ligament, as well as the proprioceptive function of the ligament.2,3,34
Finally, whereas bio-enhanced ACL repair shows promise in this in vivo study, there are multiple ways in which this basic technique can be improved. The deliberate use, and controlled release, of selected growth factors is 1 potential way to improve tissue healing, as is the improvement of the collagen scaffold and suture technique. Further work to refine and improve this technique may now be warranted with this proof-of-principle study.
Our study has potential shortcomings. First, although results at 15 weeks have been proven to be likely predictors of long-term outcome,20
the long-term outcomes of these procedures remain unknown. Second, an animal model cannot fully reproduce the human situation. For example, we could not control postoperative rehabilitation in the animals. Likewise, the ACL injury was simulated with sharp transection in the midsubstance. It is possible that a frayed disruption would heal differently. Third, allografts were used in this study because harvesting the patellar tendon can compromise the extensor mechanism of the porcine knee. In addition, the allograft used was a complete patellar tendon rather than the middle third of the tendon, which may result in a relatively stronger time 0 graft than would be used clinically. Finally, the animals were adolescents, and whether these results will translate to older individuals remains to be seen.29,30