Enhanced suture repair of the ACL using a collagen-platelet composite has been previously shown to stimulate the healing response of the torn ligament in an animal model.10, 12
In this study, we assessed whether providing initial protection to the healing ligament using the bone-to-bone suture technique would further improve healing. We found that the bone-to-bone method improved the structural properties of the ligament following enhanced ACL repair when compared to traditional suturing to the tibial stump of the ACL (Marshall procedure) after 15 weeks of healing. These data suggest that the concept of providing some protection to the healing ligament using a temporary stint during the initial healing stage may be beneficial. Although significant improvements in graft structural properties were measured, there was no detectable change in the AP laxity of the joint. Thus, long-term translational studies are needed to establish if these initial improvements will affect outcome.
Using the porcine model, we have previously shown that enhanced suture repair with a collagen-platelet composite increases the structural properties of the ligament when compared to traditional suture repair without the collagen-platelet composite after three months of healing.12
When using a collagen-platelet composite to enhance the repair, there was a 59% and 221% improvement in the yield load (126 vs 79 N) and linear stiffness (40.2 vs 12.5 N/mm), respectively.12
The LIGAMENT suture technique, as described in the present study, was used for both the enhanced and traditional ACL repair groups in the previous study.12
When using the TIBIA suture technique, further improvements in the yield load and stiffness were obtained (267 N and 66 N/mm, respectively).
We have also used the porcine model to assess healing following ACL reconstruction with a bone-patellar tendon-bone allograft in age, species and gender matched animals.16
The yield load and stiffness of the reconstruction after 3 months of healing were 279 N and 48.2 N/mm, respectively.16
For the enhanced ACL repairs using the TIBIA suturing technique, these values are comparable to those following ACL reconstruction. Thus, it would appear that if early temporary bony stabilization of the knee is accomplished with a suture technique, improved functional healing may progress to levels that approximate that of ACL reconstruction with bone-patellar tendon-bone allograft.
The failure load of the healing ACL with the collagen-platelet composite when sutures were placed in the tibial stump (LIGAMENT group) was improved from those reported for traditional suture repair in animal models.2–4
With complete transection and no suture repair, it has been shown there was no connection between the two ligament stumps, and no strength returned to the ACL in the rabbit and canine models.1, 4
Although an ACL transected group was not included in the present study, it is likely that the same response would occur in the porcine model given the limited improvement seen in unenhanced repair.12
In long-term canine studies, the strength of a repaired ACL was noted to be less than 25% of the intact control ACL from 1 to 5 years.2, 3
In contrast, the maximum loads of the repairs enhanced with collagen-platelet composite reported here were over 30% of the intact ACL at 3 months from surgery, a significant improvement over the “unenhanced repairs” previously reported.
The optimal surgical technique for enhanced repair is not yet known. Our recent ex vivo work of traditional suture repair to the tibial stump (i.e. LIGAMENT group) (methods previously used in both clinical and animal studies) resulted in knees with over 5 mm greater AP laxity when compared to the ACL-intact knee at the time of surgery whereas those with direct suturing between the femur and tibia restored normal AP laxity.13
However, it remained unknown if these ex vivo results would lead to improved biomechanical healing in-vivo. It seems reasonable to assume that an ACL suture repair technique that restores laxity at the time of surgery would be optimal for healing.
Although the structural properties were different between the two groups, AP laxity values were not. For example, the mean difference in AP laxity between the repaired and intact knees was 10.8 and 10.3 mm for the TIBIA and LIGAMENT groups, respectively, with the porcine knee at 60° of flexion (which is equivalent to 30° of flexion in the human). These differences are also similar to the 10.6 mm difference between the reconstructed and intact knee 15 weeks after ACL reconstruction in the same model.16
It should be noted that the AP laxity values of the ACL deficient porcine knee are greater than 25 mm.16
Thus the repaired ACL does restrain some of the anterior shear applied to the tibia. The large magnitudes of AP laxity following ACL repair are similar to that of other animal models of ACL reconstruction, including the goat.15
The primary mode of suture fixation failure in the three animals assigned to the TIBIA group was pull out of the suture anchors within the first four weeks post-operatively. Unfortunately, two of them were euthanized because we thought the repairs had failed. Upon dissection, it appeared that these ligaments were healing well, thus the third animal was not euthanized. We also inspected the integrity of the sutures used to repair the ACL in both groups of animals at time of harvest. None of the absorbable sutures were present in the LIGAMENT group, and only two of the animals in the TIBIA had partially intact non-absorbable sutures after fifteen weeks of healing. It should be noted that these were released prior to biomechanical testing. It is unknown exactly when or how the sutures failed, but these data suggest that the concept of temporary bony fixation is feasible and effective.
Absorbable sutures were selected for the repairs performed on the animals in the LIGAMENT while non-absorbable sutures were utilized for those in the TIBIA group. Typically absorbable sutures would be used for a traditional repair since they are secured in the soft tissue of the ligament. Non-absorbable sutures for the TIBIA group were selected in hopes that they would remain intact for the 15 week duration of the study. Although this proved not to be the case, they remained intact long enough to improve the healing response of the ACL for the animals in the TIBIA group. Further work will be necessary to determine the optimum suture material, design and the time joint support will be required to prevent failure.
The porcine model was chosen because of its size, because it is anatomically equivalent to other large animal models of ACL reconstruction:20
it is similar knee anatomy to humans, it is functionally dependent on the ACL,21
and because it has baseline coagulation values and platelet characteristics similar to those in humans.22
The latter feature is particularly important when looking at treatments involving platelets and wound healing. Therefore, we feel that the porcine model is superior to other animal models for this study. We have successfully used the porcine model to show that the addition of a collagen-platelet composite to a suture repair of the ACL improves its healing capacity,10, 12
while treatment with either the platelets or collagen sponge alone does not.17, 23
We have also used it to evaluate graft healing following ACL reconstruction.16
Thus, the porcine model represents a translational model of ACL surgery.
One of the weaknesses of the porcine model is the rapid growth of the animals during the healing period. During the three months of this study, the pigs doubled in size, and the maximum failure load of the ACL almost tripled. The effects of rapid growth on the repair strength and knee laxity are unknown. It is also possible that the rapid growth rate could distract the joint or transected ACL during healing damaging the sutures of both treatment groups over a relatively short time frame. Nonetheless, a difference was detected between treatments. For this study the contralateral knee was used as a control to normalize the structural properties and joint laxity and account for changes due to growth. Future studies in adolescent or adult animals using a minipig strain may help shed light on the relative contributions of growth and time on suture repair strength.
The porcine model has other weaknesses that are common to all large animal studies in quadrupeds – predominantly that these animals weight bear on four limbs, there is minimal ability to control the rehabilitation after surgery, and that there are certainly likely to be subtle differences in the wound healing cascade that are not yet appreciated. These factors may explain why all animal models of ACL reconstruction show large increases in post-operative laxity. These are weaknesses that must be considered when interpreting the results of this study.
In this study, both experimental groups experienced loss of flexion. To determine the etiology of this change, the 15 week MR images were examined for evidence of physeal arrest, including narrowing or localized closure. There was no gross evidence of physeal closure or sagittal dimension deformity in the distal femur; however, it is possible that subtle deformities were developing that we were unable to appreciate with this imaging modality. We also did not notice any cyclops lesion formation at the 15 week time point, rather, the ACL scar was remodeled and no extension block due to scar formation was noted either clinically or on gross examination of the retrieved joints. Interestingly, thigh girth measured at 5 cm above the superior pole of the patella increased 25% (from approximately 20 inches to 25 inches) during the period of the study, likely due to the rapid growth of the animals. It is possible that this increased soft tissue in the thigh contributed to the decrease in maximum flexion noted at the 15 week time point.
When dissecting the knee for tensile testing, it was not possible to separate the scar mass from the ACL because the two are integrated. It may be possible that the scar tissue could influence the mechanical testing results but it is part of the repaired ACL that will support the loads applied to the joint. Long-term studies are needed to see how the structural properties changes with the amount of scar tissue that is present.
A limitation of the study was the relatively small sample size. We intended to have six animals in each group but due to failure of some of the sutures within four weeks of surgery, two of the animals in the TIBIA group were euthanized. We found that the AP laxity were comparable after 15 weeks of healing between the two treatment groups. A power analysis suggests that laxity results would not have reached statistical significance by increasing the original sample size three-fold. Given the relatively small difference in mean values, running additional animals would be hard to justify. On the other hand, differences in structural properties (i.e. yield load, linear stiffness) were significantly improved in the TIBIA group compared to the LIGAMENT group even with the small sample size.