Measuring knee joint laxity: a review of applicable models and the need for new approaches to minimize variability.

Knee joint laxity can result from soft tissue injury, such as a ligament tear, or from genetic factors such as joint hypermobility syndrome and various forms of Ehlers-Danlos Syndrome. The location of a subject's passive knee laxity along a continuous spectrum is dependent on the mechanical properties of the existing structures, and the increased motion that often follows joint injury. At a threshold along the spectrum, a patient will be at risk for joint instability and further injury to joint structures. Links between instability and laxity may be better understood if laxity can be reliably and accurately quantified. Current measures of laxity have not been compared to a 'gold standard' in all cases, and when they have, were found to overestimate the laxity values. This is attributed to soft tissue deformation. Consequently, a noninvasive measure of laxity with improved accuracy and repeatability would be useful clinically and in the research sector. In this review, current clinical measures of laxity are critiqued, criteria for a measure of laxity are identified, and three theoretical models of knee laxity are outlined. These include contact, lumped parameter, and finite element models, with emphasis on applicability, strengths, and limitations of each. The long term goal is to develop a model and method able to differentiate subjects along a spectrum of laxity, and understand the functional implications of altered joint integrity. This would allow careful scrutiny of clinical interventions aimed at improving joint health and provide a valuable research tool to study joint injury, healing, and degeneration.