Development of a force-determining tensor to measure "physiologic knee ligament gaps" without bone resection using a total knee arthroplasty approach

BACKGROUND: Ligament balancing is a challenging but essential part of successful total knee arthroplasty. There is general agreement that flexion and extension gaps should be equal and symmetrical. However, to date there are no available comparisons to physiologically normal knee joints that have not undergone bone resection. There are also no standards specifying which instruments (e.g., spacers, distractors, trial components, navigation systems) and particularly what degree of force should be used for gap distraction. METHODS: To measure the physiological extension and flexion gaps, a prototypical force-determining tensor (Aesculap, Tuttlingen, Germany) was constructed and adapted so that force could be applied directly through Schanz screws inserted medially and laterally on the tibia and femur, independent of each other and without the need for bony resection. Ten normal cadaveric knees were assessed using a standard medial parapatellar total knee arthroplasty approach with patellar subluxation. Gap measurements were carried out twice, alternating distraction forces of 100 and 200 N each time. RESULTS: The prototype was implemented successfully. Repeat measurements showed only slight deviation from the original, resulting in minimal standard error. Precision did not vary with the application of greater force (200 N), but gap size increased significantly (p > 0.001). CONCLUSIONS: The success of this assessment with cadaveric knees indicates that this prototype can be applied to measure flexion and extension gaps without the need for bony resection. Increasing the distraction force to 200 N does not improve results, so 100 N per compartment appears adequate. Most likely, the extension and flexion gaps are physiologically asymmetric and unequal, and the kinematics are markedly altered after resection of the PCL and/or ACL. This new force-determining tensor can be used for further analyses, e.g., to explore the effects of selective ligament resection.