Poster P16 - Selective Bundle ACL Reconstruction Does Not Initially Restore the Normal ACL Force Distribution in an Adolescent Porcine Model: Implications for Treatment of Partial ACL Injuries

Introduction: There has been an increase in the occurrence of anterior cruciate ligament (ACL) injuries among children and adolescents, with approximately 25% of these being partial tears [1]. Partial injury typically happens to either the anteromedial (AM) or posterolateral (PL) bundles of the ACL, which　share　in　providing　overall ACL function. When only one bundle is disrupted, reconstruction surgery may be performed on the injured bundle while preserving the healthy bundle [2]. Clinical studies have shown that selective bundle ACL reconstruction preserves the intact ACL fibers and enhances vascularization of the graft, as well as significantly reduces the differential joint laxity relative to the healthy side at 1-3 years follow-up [3,4,9], with similar values after AM or PL bundle reconstruction [5,6]. Yet, there is a lack of research comparing the relative forces within the graft and remaining intact bundle, which may impact re-injury risk and long-term degenerative outcomes. In the porcine model, the ACL bundles undergo changes in size and function throughout skeletal growth [7], thus age-specific bundle function should be controlled for in experimental studies, which is difficult in clinical studies. Therefore, the objective of this study was to evaluate the initial joint stability and bundle function following selective AM and PL bundle ACL reconstruction within an adolescent porcine model.

Materials and

Methods: A total of 8 pairs of deep digital flexor tendons and 8 pairs of normal hindlimbs were collected from adolescent (6 months old) pigs, which were divided into two groups for AM bundle reconstruction (AMR) (n=8) and PL bundle reconstruction (PLR) (n=8). A 6-degree-of-freedom robotic testing system with a universal force-moment sensor was used to test joint function (Kuka, simVITRO) (Figure 1A). The intact joint was separately subjected to a 100 N anterior-posterior (AP) load, a 150 N compression, and a 5 N·m varus-valgus (VV) torque at 60° of flexion. Kinematics were recorded, then repeated while recording forces for the intact joint. The same loading conditions were then applied after AM bundle transection (AMT) or PL bundle transection (PLT) while recording the kinematics, which was then repeated with forces/moments recorded. Selective bundle reconstruction was then performed for AM and PL groups by using flexor tendon grafts: 1) AMR while preserving the remaining intact PL bundle; 2) PLR while preserving the remaining intact AM bundle. The same AP, compression, and VV loads were then applied. The recorded kinematics from intact, AMT or PLT, and AMR or PLR states were repeated after removing the AM or PL bundle graft, followed by removal of the remaining intact PL or AM bundle. Joint laxity and tissue force contributions were compared between different states using one-way repeated measures ANOVA with Tukey’s HSD. Paired t-test were utilized to compare differential joint laxity relative to intact state from different groups. Significance was set at 0.05.

Results, Conclusions, and Discussions: Both PL bundle injured and reconstructed joint showed AP joint laxity within 1 mm of intact state, which was more stable than AM bundle injured and reconstructed joint (within 3 mm) (Figure 1).
AMT and AMR increased anterior instability under compression, while both PLT and PLR showed similar ATT as the intact joint (Figure 2A). AMT and PLT resulted in joint destabilization under VV torque (Figure 2B). Varus-valgus stability was restored to intact state after both AM and PL bundle reconstruction (Figure 2B).
After AM bundle injury, the PL bundle bore higher forces under anterior tibial loading and varus-valgus torques (Figure 3). AMR was able to restore the normal force distribution under varus-valgus loading (Figure 3B) but not anterior loading (Figure 3A). After PL bundle injury, the AM bundle also bore higher forces. PLR partially returned these forces under anterior loading (Figure 3A). Under varus and valgus loading, PLR showed a dramatic shift such that the graft carried more force than the intact AM bundle (Figure 3B).
In this study, selective bundle reconstruction showed the ability to partially restore the minor changes in joint laxity after partial ACL injury, but the force distribution within the graft and remaining ACL bundle did not fully restore the normal distribution within the intact ACL bundles. The APTT kinematic data in the present work aligns with results reported in previous human cadaveric data and clinical data, indicating better initial and long-term joint stability following PL injury [7,8]. Additionally, the restoration of varus-valgus laxity through selective bundle reconstruction may be relevant in preventing secondary meniscus or collateral ligament injuries. In terms of tissue-level forces, neither AM nor PL bundle reconstruction restored the normal ACL force distribution under all loading conditions tested. It is unclear how these initial imbalances in force distribution may change once the in-vivo graft remodeling process occurs [8]. Our data advances the understanding of selective bundle reconstruction in pediatric cases by providing biomechanical insights into the roles of AM and PL bundles. Ongoing and future in-vivo work will be needed to compare joint laxity and function after graft and intact bundle remodeling.

Acknowledgements (Optional): We would like to thank the NC State College of Veterinary Medicine and Laboratory Animal Resources for their contribution to this work. Funding provided by NIH (R01 AR071985).
References
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