INTRA-OPERATIVE FLEXION BALANCE DOES NOT NECESSARILY CORRELATE WITH POST-OPERATIVE INSTABILITY IN TOTAL KNEE ARTHROPLASTY.

Abstract

This study was conducted to investigate the correlation between intra-operative Flexion Balance (IFB) and post-operative Flexion instability in Posterior-Stabilized Total Knee Arthroplasty (TKA).

Eighty-three knees (4 males and 79 females, average 74 y/o) with primary TKA (Zimmer NexGen LPS flex fixed-bearing) for varus osteoarthritis in our hospital between January 2006 and December 2007, were included in this study. After bone-cutting independently and balancing manually, Extension Balance (EB) and IFB were measured with seesaw type tensor. Post-operative Flexion Balance (PFB) was evaluated as post-operative instability with Kanekasu’s Epicondylar view at the least more than 6 months postoperatively. Varus inclination (lateral joint opening) was indicated as plus. In addition, pre-operative standing FTA (femorotibial angle), the change of FB (CFB=PFB-IFB) and True Correction Angle (TCA=FTA-174-EB), we had defined, were calculated. The TCA was hypothesized to mean the extent of medial soft tissue release. With these data, the correlation between IFB and PFB, CFB and TCA were analyzed. Of these, furthermore, in the well-balanced knees (IFB ≥ ±2°), same analyses were done. Statistical analysis was performed with StatView software.

Each data (n=83) in all subjects was as follows (Mean ± SD, degrees.); EB: 2.74 ±2.74, IFB: 1.61 ±3.67, PFB: 1.73 ±2.66, CFB: 0.01 ±4.25, FTA: 185.3 ±6.7, TCA: 8.65 ±6.52, respectively. Though there was no correlation between IFB and PFB (r=−0.09, p=0.57), CFB was correlated with TCA (r=0.40, p< 0.01).

Each data in the well-balanced knees (n=43) was as follows, EB: 3.09 ±2.71, IFB: 0.70 ±1.30, PFB: 1.22 ±2.52, CFB: 0.57 ±2.3, FTA: 185.5 ±6.5, TCA: 8.42 ±6.09, respectively. There was a correlation between IFB and PFB (r=0.41, p< 0.01), however, FBC was not correlated with TCA (r=−0.26, p=0.10).

Same rectangular balance has been thought to be one the most important factors to obtain the good postoperative stability in TKA. For correcting alignment of lower extremity, medial or posteromedial release are generally needed to perform mainly in extended knee. Even if well-balanced EB was achieved, IFB does not necessarily prove to be well, rather than sparse. This might be because intra-operative balance was not measured under physiological condition, especially after wide posteromedial release. Soft tissues released for balancing would be repaired and shortened over time, so it seems to be natural that intra-operative balance would change. We have reported that EB was correlated with post-operative instability in the previous congress (ISTA 2006). However, it remains unknown as for FB. Our study demonstrated that CFB increased in accordance with the extent of soft tissue release (TCA), and that IFB was correlated with PFB only in the well balanced knees. This means that the measurement of IFB was not useful for predicting PFB in the imbalanced knees. That’s why we should achieve adequate balance & gap during operation and should recognize that FB was influenced by various factors, not only soft tissues but also rotation and inclination of components. In the future, how to measure IFB, including tensor and measurement condition, should be considered and established to predict knee balancing for good clinical results.