Abstract
Introduction
Balancing of joint gap is a prerequisite in total knee arthroplasty (TKA). Recently, the tensor has been developed which can measure the joint gap with the patellofemoral joint reduced for more physiological assessment, and the results for osteoarthritis (OA) patients indicated that the flexion gap is larger than the extension gap during posterior-stabilized (PS) TKA. However with respect to the rheumatoid arthritis (RA) patients, the soft tissue balance in TKA is still unknown. Therefore, the purpose of this study was toinvestigate thecharacteristics of thejoint gap during TKAsurgeryforpatients with RA.
Methods
We implanted 90 consecutive knees with a PS TKA using a NexGen LPS-flex (Zimmer, Warsaw, IN). OA was the underlying disease in 60 knees and RA was the disease in30 knees.
Surgical procedure
We performed all operations with a measured resection technique. The rotational position of the femoral component was determined based on the epicondylar axis of the femur with anterior reference for anteroposterior sizing.
Joint gap measurements
After bone cuts and soft tissue balancing, we measured the joint gap with the femoral component in position using seesaw-type tenser device with the patella reduced position after repair of the medial arthrotomy with a few stitches. The center width and asymmetry (tilting) of joint gaps under 40-lb distracting force were measured at 0 degree extension and 90 degrees of knee flexion.
Results
The changes in the joint gap from 0 to 90 degrees were 3.2 ± 0.3 mm in OA group and 4.3 ± 0.4 mm in RA group. The increase of joint gap from 0 to 90 degrees in RA was significantly larger than that in OA group (Figure 1). The tilting angle of the joint gap (varus gap expressed as positive values) at 90 degrees of knee flexion in RA group (5.3 ± 0.5 degrees) was significantly larger than that in OA group (2.6 ± 0.4 degrees) (Figure 2). In RA group, there was a positive correlation (r= 0.34, p <0.05) between the increase of joint gap from 0 to 90 degrees and the tilting angle of the joint gap at 90 degrees of knee flexion (Figure 3).
Discussion
In this study, the increase of joint gap from 0 to 90 degrees in RA group was significantly larger than that in OA group. In addition, the lateral gap in knee flexion, calculated from the tilting angle of the joint gap, was significantly larger in RA group and was correlated with the increase of joint gap from 0 to 90 degrees of knee flexion. These differences could be attributed to reduced stiffness of the lateral structure, such as lateral collateral ligament and popliteofibular ligament, as well as the extensor mechanism in patients with RA. Therefore, it is necessary to considerthe individual stiffness of soft tissues, together with the applied tension, to decide the rotation of femoral component by reference to the flexion gap during TKA for RA patients.
