THE EFFECT OF GREATER EXTERNAL ROTATION OF THE FEMORAL COMPONENT AS A RESULT OF GAP LIGAMENT BALANCING ON PATELLAR TRACKING, FLEXION STABILITY AND FUNCTION OF TWO DIFFERENT TKA IMPLANT DESIGNS

INTRODUCTION Use of a novel ligament gap balancing instrumentation system in total knee arthroplasty (TKA) resulted in femoral component external rotation values which were higher on average, compared to measured bone resection systems. In one hundred twenty knees in 110 patients the external rotation averaged 6.9 degrees (± 2.8) and ranged from 0.6 to 12.8 degrees. The external rotation values in this study were 4° and 2° larger, respectively, than the typical 3° and 5° discrete values that are common to measured resection systems. The purpose of the present study was to determine the effect of these greater external rotation values for the femoral component on patellar tracking, flexion stability and function of two different TKA implant designs. METHODS In the first arm of the study, 120 knees in 110 patients were consecutively enrolled by a single surgeon using the same implant design (single radius femur with a medial constraint tibial liner) across subjects. All patients underwent arthroplasty with tibial resection first and that set external rotation of the femoral component based upon use of a ligament gap balancing system. Following ligament tensioning / balancing, the femur was prepared. The accuracy of the ligament balancing system was assessed by reapplying equal tension to the ligaments using a tensioning bolt and torque wrench in flexion and extension after the bone resections had been made. The resulting flexion and extension gaps were then measured to determine rectangular shape and equality of the gaps. Postoperative Merchant views were obtained on all of the patients and patellar tracking was assessed and compared to 120 consecutive total knee arthroplasties previously performed by the same surgeon with the same implant using a measured resection system. In the second arm of the study, 100 unilateral knees in 100 patients were consecutively enrolled. The same instrumentation and technique by the same surgeon was used, but with a different implant design (single radius femur without a medial constraint tibial liner). RESULTS Rectangular flexion and extension gaps were obtained within ± 0.5mm in all cases. Equality of the flexion and extension gaps was also obtained within ± 0.5mm in all cases. Merchant views of the total knee arthroplasties showed central patellar tracking with no tilt or subluxation in 90% of the ligament gap balanced knees and 74% of the measured resection knees. Arthrofibrosis resulting in a closed manipulation under anesthesia was required in 6% of the knees with single radius femurs and medial constaint tibial liners, but only in 1% of the single radius femur knees without medial constraint liners. DISCUSSION AND CONCLUSION External rotation values are higher on average, when ligament tensioning / balancing is employed with this novel system compared to measured resection systems. In this study this resulted in consistent matching of the flexion gap to the extension gap and better patellar tracking. These findings suggest that limiting the surgeon to discrete rotation values may be at odds with where the femur “desires” to be, given soft tissue considerations for each patient. Also, even with ideal soft tissue balancing, TKA implant design can have a significant affect on the outcome measure of development of arthrofibrosis.