Abstract
The assessment of knee laxity by application of varus and valgus stress is a subjective clinical manoeuvre often used for soft tissue balancing in arthroplasty or for diagnosis of collateral ligament injuries. Quantitative adjuncts such as stress radiographs have enabled a more objective measurement of angular deviation but may be limited by variations in examination technique. The aim of this study was to quantify clinical knee laxity assessment by measurement of applied forces and resultant angulations.
A novel system for measuring the manually-applied forces and moments was developed. Both hardware and software components underwent laboratory validation prior to volunteer testing. Two clinicians performed multiple blinded examinations on two volunteers and the corresponding angular deviations were measured using a validated non-invasive system with a repeatability of ±1° for coronal alignment. The distance between the kinematically-determined knee and ankle centres was used as the moment arm.
Comparison of single measurements of laxity showed a wide intra- and inter-observer variation (up to 3°). However, when the median value of repeated measurements was used there was good repeatability for both a single surgeon on different days and between the two clinicians with angular measurements agreeing within 1°. In spite of this agreement, the magnitudes of the tangential forces and moments applied varied between clinicians and did not correlate with the corresponding angular deviations.
It was not possible to standardise clinical examination using the current system. Orientation of the applied force with respect to the leg was not quantified and during force measurement it became apparent that the assumed tangential direction of application was not true. This may explain the lack of correlation between the force and angulation data. However, for quantitative measurement of coronal knee laxity using non-invasive laxity measurements, the use of a repeated measures protocol may be accurate enough for clinical application.
