Functional and Quantitative Evaluation of the Vasculature Supplying the MCL of ACL Deficient Rabbit Knees: Localized Development of Endothelial and Vascular Dysfunction

Introduction: Rupture of the anterior cruciate ligament (ACL) results in significant joint instability, inflammation and degeneration. Chronic inflammation associated with ACL deficiency may alter normal vascular responses rendering the vasculature unable to respond to the local tissue compromising tissue viability. Vascular responses to vasodilatory and vasoconstrictive mediators were evaluated in the medial collateral ligament (MCL) of the ACL deficient knees at 6 and 14 weeks post ACL transection. Altered vasoconstrictive and vasodilatory responsiveness may be due to excessive neural activity, vascular immaturity, or inflammation. These mechanisms were investigated in the ACL deficient knee.;Results: ACL deficiency was shown to impair normal vascular responses. Altered responses were found to not be the result of neural activity or vascular immaturity, but rather were partially attributable to iNOS activity. These changes in vascular responses persisted through 14 weeks after ACL transection. Further deficient vascular responses were found to correlate with degenerative changes in the MCL. Vascular dysfunction was also found to vary by region within the MCL consistent with changes in mechanical load in the MCL of ACL deficient knees.;Conclusion: Altered responsiveness of the MCL vasculature to these vasoactive mediators indicates that the vasculature surrounding the MCL of ACL deficient knees and particularly the endothelium is dysfunctional. Deficient vascular responses are the result of iNOS activity associated with inflammation in the ACL deficient knee and are not the result of neural stimulation or vascular immaturity. Deficient vascular responses and increased vascularity were found to be correlated with pathological degeneration of the MCL and may have a role in MCL pathology. Vascularity and vascular function were found to be related to regional variations within the MCL which experience difference levels of mechanical loading. Vascular dysfunction is therefore a complex pathology with a multitude of processes involved.;Methods: Chronic joint instability and progressive OA were induced in rabbit knees by surgical transection of the ACL. Under halothane anesthesia, laser speckle perfusion imaging (LSPI) was used to measure MCL blood flow in unoperated control, 6-week ACL-transected knees, 14 week ACL-transected knees, MCL-transected knees, and femoral nerve denervated ACL deficient knees. Vascular responses to various vasoactive mediators were applied topically and blood flow was then measured using LSPI. Measurement of vascular maturity was evaluated through immunohistochemical volume analysis of the endothelium and smooth muscle in MCL's using CD31 and alpha-smooth muscle actin antibodies. Ultrasound imaging of MCL's was conducted using MHF-1 high frequency ultrasound unit and was graded by two blinded independent observers.