Influence of patella position on soft tissue balance and clinical outcomes in patients undergoing total knee arthroplasty via a midvastus approach

Abstract

Objective To compare the effects of patellar position on the balance of soft tissue during the operation and the postoperative outcomes in minimally invasive total knee arthroplasty via a midvastus approach. Methods From December 2018 to February 2019, a total of 55 patients were enrolled for primary total knee arthroplasty via a midvastus approach. All patients were randomly divided into 2 groups by random number table. During the operation, the gap balance technique was used with patella reduced or subluxated to complete the osteotomy and balance of soft tissue. The changes of gap and varus-valgus angle were compared between the patellar reduction group and the patellar subluxation group in both extension and flexion position. These data were also compared before and after reducing patellar in the patellar subluxation group. Furthermore, the differences of femoral prosthesis rotation, mechanical femoral axis to tibial axis angle, Knee Society score (KSS), visual analogue scale (VAS) and range of motion (ROM) were compared between the two groups. Results All patients finished 6 months follow-up, including 27 patients in the patellar reduction group and 28 patients in the patellar subluxation group. After osteotomy and soft tissue balance during the operation, there was no significant difference in gap and varus-valgus angle between two groups in either extension or flexion position. While in the patellar subluxation group, the flexion gap was 10.5±0.3 mm with patella subluxated, less than 11.0 ± 0.3 mm after reducing the patella. The varus-valgus angle was 1.5±0.3 with patella subluxated, less than 2.3±0.4 degree after reducing the patella. The difference was statistically significant (t=4.180, P 0.05). The rotation angle of the femoral component in the patellar subluxation group was -0.49°±1.2°, and the external rotation angle was smaller than that in the patellar reduction group (0.24°±1.3°). The difference was statistically significant (t=2.116, P=0.039). At one month after operation, ROM of the patellar reduction group was 109.6°±8.5° which was higher than that of the patellar subluxation group (104.9°±8.6°, t=2.048, P=0.046). There was no significant difference in ROM between the two groups at 3 and 6 months (P>0.05). Moreover, there was no significant difference in KSS and VAS at 1, 3 and 6 months after operation (P>0.05). Conclusion In the minimally invasive total knee arthroplasty, it is suggested to balance the soft tissue as much as possible with the patellar reduced. Otherwise, the consequence of increased flexion space, increased varus and increased internal rotation of femoral prosthesis should be considered. The patients undergoing soft tissue balance with patella reduced have better ROM in the early stage postoperatively. Key words: Arthroplasty, replacement, knee; Patella; Treatment outcome