Correlation of healing capacity with vascular response in the anterior cruciate and medial collateral ligaments of the rabbit

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Abstract
In clinical terms, functional recovery after anterior cruciate ligament (ACL) injury is generally poorer than after medial collateral ligament (MCL) injury. In experimental studies of injury, the early phases of ligament healing require an augmented blood supply. We hypothesized that the differences in healing properties of the ACL and MCL would be reflected in the magnitude of their vascular responses to partial injury. This study is the first to quantify and define the time course of changes in blood flow and vascular volume following hemisection of the rabbit ACL and MCL. Adult female rabbits were assigned to control, sham operation, ACL hemisection or MCL hemisection groups. Standardized ACL or MCL injuries were surgically induced. About 2, 6 or 16 weeks later, blood flow and vascular volume of the ACL and MCL were measured. The MCL of the rabbit responded to hemisection with a large significant increase in blood flow and a substantial angiogenic response associated with inflammation and scar formation. During subsequent matrix remodelling, blood flow and vascular volume returned towards control values. In contrast, the ACL showed only a 2-fold increase in vascular volume, no increase in blood flow and atrophied after hemisection. The superior capacity of the MCL to increase its blood supply through angiogenesis and increased flow is essential for ligament healing to occur, and may be the major difference in healing potential between the ACL and MCL.