Roles of sagittal anatomical parameters in femoral component alignment of primary posterior stabilized total knee arthroplasty for osteoarthritis

Abstract

Purpose: Total knee arthroplasty (TKA) is one of the important methods to treat severe osteoarthritis (OA) of knee joint. The alignments of the knee joint are pivotal in ensuring patient satisfaction and functional ability after TKA. Compared with the balance of coronal plane, more attention is paid to the balance of sagittal plane. Moreover, the probability for sagittal malposition of the femoral component is higher than that of tibial component. Normal sagittal spine-pelvis-lower extremity alignment is crucial in humans for maintaining a balance of motion, including the motion of spine, hip, knee and ankle joints. There has been little study on balance of pelvis-knee joint alignment with regard to the sagittal alignment of the femoral component. The objective of our study is to determine the role of the impact of pelvis-knee joint sagittal alignment parameters on the femoral component in primary posterior stabilized (PS) TKA for OA. Methods: This retrospective study was approved by the First Affiliated Hospital of Sun Yat-sen University Clinical Research Ethics Committee (Guangzhou, China). The Knee Society Score (KSS), the Western Ontario and McMaster Universities (WOMAC) Osteoarthritis Index and patient satisfaction were evaluated before surgery and 1 years after surgery, respectively. Surgimap® software (Nemaris, Inc) was applied to investigated the following parameters: pre-operation:pelvic incidence (PI), sacral slope (SS), pelvic tilt (PT), pelvic femoral angle (PFA), sacrum femoral angle (SFA), femoral inclination(FI), femoral anterior bowing angle (FABA), distal femoral flexion angle(DFFA), distal femoral anterior cortex angle (DFACA, Figure 1); post-operation: femoral prosthesis flexion angle (FPFA),αangle ( the angle between the distal femur anterior cortex line and flange of femoral component, βangle (the angle between a line parallel to the distal cement interface of the femoral component and the femoral machanical axis),γangle (the proximal angle between a line drawn perpendicular to the distal cement interface of the femoral component and the femoral anatomical axis, , Figure 2) , rang of knee flexion and extension. Data were analyzed using SPSS version 20.0 (IBM Co., Armonk, NY, USA). Multivariate regression analysis, Pearson correlation analysis, χ2 tests and t-tests were used to compare differences between groups and means. Cohorts were checked for statistical homogeneity at baseline. Results: The study comprised a total of 54 Chinese patients (73 knees) who received TKA surgery (Smith &Nephew, PS) during January to July 2018, including 13 men and 41 women with a mean age of 69.1 years (range, 57-87 years). The mean BMI of all patients was 25.8 (range, 17.6-34.5 years). The results of function scales are as follow:pre-operation:KSS:61.3±14.7 (37⁓92), functional scores:38.6±12.5(15⁓95), WOMAC:48.9±7.2(37⁓65); post-operation:KSS:86.3±10.0 (40⁓100), functional scores:62.2±13.9(35⁓90), WOMAC:29.3±5.5(21⁓40), patient satisfaction: 7.6±1.1(5⁓10). Sagittal anatomical parameters was evaluated before surgery as follows: PI:56.1±14.1°(30.2⁓94.6°), PT:17.7±10.2°(0.3⁓45.5°), SS:38.8±10.5°(10.4⁓61.2°), PFA:11.1±9.2°(0.4⁓35.2°), SFA:58.6±14.2°(17.9⁓102.0°), FI:15.7±9.1°(1.2⁓62.2°), FABA:10.9±4.0°(3.2⁓20.7°), DFFA: 3.3±1.9°(0.3⁓8.5°), DFACA:2.8±2.1°(0.1⁓10.2°). The parameters was assessed one year after surgery as follows: FPFA:6.0±4.9°(0.4⁓20.7°),α angle:2.5±5.4°(-13.7⁓20.0°), β angle:86.4±10.3°(63.8⁓113.3°),γangle:6.3±5.2°(0.0⁓19.0°), rang of knee flexion: 109.2±12.0 (85.0 ⁓125.0), rang of knee extension: 3.2±3.5(-5.0⁓10.0).(Table 1) It was found that PI=PT+SS,t tests revealed that the scale scores of KSS and WOMAC after surgery improved significantly than that of pre-surgery(p<0.001), which indicated most patients gained a good clinical outcome after surgery. Pearson correlation analysis showed that PI was positively related with the absolute value of α angle (r=-0.5696, p=0.0001, Figure 3A), which suggested that increased PI would probably lead to a greater α angle.Moreover, the absolute value of α angle was found negatively related with the KSS and patient satisfaction, respectively( r= -0.4906, -0.5204; p=0.0001,p=0.0002, respectively, Figure 3B, C). However, multivariate regression analysis and Pearson correlation analysis didn`t show significance relationship among functional scales , WOMAC and other sagittal anatomical parameters(p>0.05). Furthermore, the patients was divided into four cohorts according the α angle: A:α angle extension >0°;B:0°≤α angle flexion≤3°; C:3°≤α angle flexion≤7°; D: α angle flexion>7°(Figure 5). Bonferroni paired comparison showed KSS, functional scales and rang of knee flexion in B cohort were highest (p=0.025) while these above scales both in A and D cohorts were lower (p=0.033, p=0.005) among these four cohorts. Conclusions: Increased PI would probably lead to a greater α angle, which suggested that older patients with greater PI might have greater probability for femoral malposition. The distal femur anterior cortex and α angle might be useful index for sagittal alignment of femoral component in TKA. The patients with neutral-to-mild flexion of femoral component( 0°≤α angle flexion≤3°) would possibly gain a good clinical outcome.