Abstract
Backgrounds
It is well accepted that gap balancing is one of the important step for total knee arthroplasty (TKA). In order to evaluate gap balancing during operation, many tension devises have been used and developed. However, during operation, proper load to be applied, ideal gap amount, appropriate angle formed between femoral component and tibial cut surface are not clearly defined. Understanding the relationship between applied load and gap pattern will provide important information. The purpose of this study is to precisely analyze gap amount and inclination in extension and flexion using digital analyzer during TKA and characterize gap pattern.
Methods
We analyzed 39 knees in 39 cases that underwent TKA with Scorpio NRG PS knee prosthesis operated by modified gap balancing technique. A customized digital knee balancer was manufactured applying load cell, angle sensor, and gap sensor in the selected part within offset seesaw type balancer (Fig 1). It can measure three values (gap, angle and force) at the same time and automatically record the values. After bone cut for femur, tibia, and patella, femoral component trial was inserted to the femur. Then gap length and inclination angle between femoral condyle surface and tibial cut surface was analyzed in extension and at 90 degrees knee flexion with gradually increasing opening torque. Inclination was expressed by positive degrees when lateral side opened. Serial data was recorded automatically and analyzed.
Results
In extension, average gap between femoral implant and tibial cut surface increased gradually from 7.3mm to 13.6mm with increasing load from 10 lbs. to 75 lbs. During this load increase, average inclination changed from 0.2 degrees to 0.7degrees. In flexion, average gap between femoral implant and tibial cut surface increased gradually from 7.4mm to 15.9mm with increasing load from 10 lbs. to 75 lbs. During this increase, average inclination change was from 0 to 3.5 degrees (Fig 2). Lateral opening was observed over 25lbs and this opening angle increased gradually thereafter. When the identical load is applied, average gap difference between extension and flexion was 0.1 to 2.38mm. Linear relationship between extension gap and flexion gap was obtained when each applied load was identical (Fig 3).
Discussions
In this study, we have reported gap patterns using digital knee analyzer in vivo for the first time. This digital analyzer provides gap length, angle and applied force between tibia and femur with accuracy. We conclude that in extension lateral laxity is not affected even with increasing load up to 75lbs. On the other hand, in flexion, lateral laxity became remarkable with load increase and can cause more gap increase in flexion compared in extension. In determining the rotational alignment using modified gap technique, this tendency has to be kept in mind.
Conclusions
Digital knee balancer provided precise gap pattern in TKA with femoral component in place. Gap length in extension and flexion has linear relationship Lateral laxity in flexion need to be analyzed carefully in TKA.
