Abstract
PURPOSE
Soft tissue balancing can be achieved by using spacer blocks, by distractors which measure tensile forces, or by instrumented devices which measure the forces on the lateral and medial condyles. However there is no quantitative method for assessment of balancing at clinical follow-up; to address this, we developed a Smart Knee Fixture (SKF) which measured the varus and valgus angles for a moment of 10 Nm. Our purpose was to determine if varus and valgus angles measured at clinical follow-up, was equivalent to the balancing parameters of distraction forces or contact forces measured at surgery. METHODS: The SKF, which measured VV angles using stretch sensors on each side of the knee, was validated by cadaver studies, fluoroscopy, and emg. The balancing parameters were:
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The lateral and medial contact forces at surgery, expressed as FL/FM
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The distraction tensions in the collateral ligaments at surgery, expressed as TL/TM
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The moments to cause lift-off when a varus or valgus moment is applied, MVAR/MVAL
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The varus and valgus angles measured at post-op follow-up, VAR/VAL
A force analysis, and measurements on 101 surgical cases & clinical follow-up in an IRB study, were carried out to determine the relationship between these parameters.
RESULTS
The ratio TL/TM was approx. equal to FL/FM, especially near to a balanced state
The ratio MVAR/MVAL (lift-off moments) was equal to FL/FM
The ratio VAR/VAL was approx. equal to FL/FM only if the collateral stiffnesses were equal;
otherwise the ratio was approx. proportional to the collateral stiffnesses.
In the clinical follow-ups, there was no significant linear relation between VAR/VAL and FL/FM.
CONCLUSIONS
While follow-up measurements of varus and valgus angles are useful indicators, their lack of correspondence to the intraoperative parameters is a limitation. However, lift-off moments at follow-up would correspond, and we have now modified the SKF to measure these moments.
SIGNIFICANCE
A method for measuring both lift-off moments and VV angles at follow-up would provide a valuable comparator to surgical balancing and quantitatively indicate clinical stability.
