Patterns of Gene Expression in a Rabbit Partial Anterior Cruciate Ligament Transection Model The Potential Role of Mechanical Forces

Background: The inconsistency in healing after anterior cruciate ligament (ACL) repair has been attributed to ACL fibroblast cellular metabolism, lack of a sufficient vascular supply, and the inability to form a scar or scaffold after ligament rupture because of the uniqueness of the intra-articular environment. Hypotheses: (1) Stress deprivation in the surgically transected ACL will increase matrix metalloproteinase (MMP) and alpha smooth muscle actin (alpha-SMA) expression. (2) Stress deprivation will decrease collagen expression. (3) The transected anteromedial bundle of the ACL will demonstrate a pattern of gene expression similar to the completely transected ACL, while gene expression profiles in the intact posterolateral bundle will be similar to the sham-operated controls. Study Design: Controlled laboratory study. Methods: Thirty-six New Zealand White rabbits underwent a partial ACL surgical transection separating the anteromedial (AM) and posterolateral (PL) bundles and transecting the AM bundle. Contralateral ACLs were either sham operated or completely transected. Ligament tissue was harvested at 1, 2, or 6 weeks after surgery, and real-time PCR was performed using primers for collagen I, collagen III, alpha-SMA, MMP-1, and MMP-13. Results: At 1 week, a 28- and 29-fold increase in MMP-13 expression was seen in the complete transection and the transected AM bundle specimens when compared with sham-operated controls (P = .049, P = .018), respectively. There was no significant difference in MMP-13 between the sham controls and the intact PL bundle specimens. A 22- and 23-fold increase in alpha-SMA was seen (P = .03, P = .009) in the complete transection and transected AM bundle specimens, respectively, while no difference was seen between the intact PL bundle and controls. No significant differences were seen in collagen I (Col I) or collagen III (Col III) gene expression at 1 week. At 6 weeks, Col I expression increased 5-fold in complete transection samples (P = 3.9 x 10(-6)), 3-fold in transected AM samples (P = 3.3 x 10(-6)), and 2-fold in the intact PL bundle samples as compared with controls. alpha-SMA was increased 7.5-fold and 5-fold in complete transection and transected AM samples, respectively (P = .004, P = 2.2 x 10(-6)), while no significant change was seen in the intact PL bundle samples compared with controls. Complete transection specimens showed a 3-fold increase in MMP-1 expression. Col III increased 5.4-, 2.6-, and 2.4-fold in the complete transection, transected AM, and intact PL groups, respectively (P = .003, P = .004, P =.04). Conclusion: Partial or complete surgical transection of the rabbit ACL with resultant loss of mechanical stimuli results in an increase in MMP-13 and alpha-SMA expression at the early time point (1 week) and an increase in alpha-SMA, Col I, and Col III expression at the later time point (6 weeks). These data provide support for the hypothesis that there is a time-dependent alteration of anabolic and catabolic matrix gene expression after injury/loss of ligament integrity. Clinical Relevance: Identification of pathways that respond to mechanical stress in the intact ACL and after surgical transection may permit development of novel therapies to alter healing of the partial ACL injury or to assist in the development of biomechanical active "smart'' scaffolds for tissue-engineered ligament replacements.