THE EFFECT OF BALANCING VALUES AT TOTAL KNEE SURGERY ON SIX-MONTH FUNCTIONAL OUTCOMES

INTRODUCTION Ligament balancing aims to equalize lateral and medial gaps or tensions for optimal functional outcomes. Balancing can now be measured as lateral and medial contact forces during flexion (Roche 2014). Several studies found improved functional outcomes with balancing (Unitt 2008; Gustke 2014a; Gustke 2014b) although another study found only weak correlations (Meneghini 2016). Questions remain on study design, optimal lateral-medial force ratio, and remodeling over time. Our goals were to determine the functional outcomes between pre-op and 6 months post-op, and determine if there was a range of balancing parameters which gave the highest scores. METHODS This IRB study involved a single surgeon and the same CR implant (Triathlon). Fifty patients were enrolled age 50–90 years. A navigation system was used for alignments. Balancing aimed for equal lateral and medial contact forces throughout flexion, using various soft tissue releases (Meneghini 2013; Mihalko 2015). The patients completed a Knee Society evaluation pre-op, 4 weeks, 3 months and 6 months. The total (medial+lateral) force, and the medial/(medial+lateral) force ratio was calculated for 4 flexion angles and averaged. These were plotted against Pain, Satisfaction, Delta Function (postop – preop), and Delta Flexion Angle. The data was divided into 2 groups. 1. By balancing parameters. T-Test for differences in outcomes between the 2 groups. 2. By outcome parameters. T-Test for differences in Balancing Parameters between the two groups. RESULTS Ranges were: Balancing Parameters; Total Force 19–70 lbs; the Force Ratio 0.34 to 0.75. Outcome Measures; Pain 11–25, Satisfaction 15–40, Delta Function −20–70, Delta Flexion −3–29. The only significance was that higher Delta Flexion was associated with a higher Force Ratio. An unpaired t-test was carried out for cases with a balancing ratio between 0.48–0.68 versus cases outside that band (Fig 1). The mean gains were 27.2+/−20 versus 18.8+/−18.5. However the difference at p=0.104 was not significant, due to the large standard deviation. An odds ratio calculation was carried out for the above range, and 35 points Delta Function (Figure 2). The range of 0.48–0.68 and a gain of 35 was determined by optimizing. For patients in the balancing range, 39% achieved that; for patients outside the range, only 8% (Figure 2). This gave an odds ratio of 4.9 that within the balancing range 0.45–0.68, there would be a functional gain of 35 points or more. DISCUSSION A striking characteristic of the data was the wide range of the functional scores and the narrow band of balancing parameters. This explained the lack of significance between the sets of 2 groups, which might have demonstrated an association of higher scores with high or low balancing values or ranges. However by reverting to an odds ratio analysis, in this case for gain in functional score, there was a suggestion that a certain balancing range provided the best functional results. This suggests that the best average balancing target for surgery is around 0.58 (higher medial force than lateral) rather than 0.5. However further studies and longer follow-up will be needed to verify this. For any figures or tables, please contact authors directly (see Info & Metrics tab above).