The impact of postoperative inclination of the joint line on clinical outcomes in total knee arthroplasty using a prosthesis with anatomical geometry

Abstract

The goal of this study was to investigate the impact of postoperative inclination of the joint line on clinical results after total knee arthroplasty (TKA) using a prosthesis with anatomical geometry. This study included 145 primary cruciate-retaining type of knee prosthesis with anatomical geometry. Three years postoperatively, clinical outcomes including the patient-reported outcomes (PROs) were recorded. Limb alignment was evaluated by the hip-knee-ankle (HKA) axis and inclination of the joint line was assessed by the joint line orientation angle (JLOA). Knees were divided into two groups according to the HKA: in-range (- 3 to 3°) and outlier group (< - 3° or > 3°) or the JLOA: in-range (2-4°) and outlier group (< 2° or > 4°), and clinical outcomes were compared between the groups. Postoperative Knee Society Function Score (KS-FS) was significantly higher in the HKA in-range group than the outlier group (p = 0.01). The Knee Society Knee Score and all subscales of the Knee injury Osteoarthritis Outcome Score were comparable between the groups. A multivariate analysis revealed a significant association between age at operation and postoperative KS-FS > of 80 points. Neither HKA in-range nor JLOA in-range were associated with the higher knee function. In conclusion, TKA-postoperative inclination of the joint line was not relevant to the short-term PROs. Treatment strategies that attempt to make joint line inclination in order to improve postoperative PROs should be avoided, and alignment goals such as kinematic alignment should be considered carefully.

Figure 1

The FINE total knee. The femoral condyle has an asymmetric shape and femorotibial joint line with an inclination of 3° both in coronal (left) and axial (right) planes. The medial and lateral condyle have different length of radii (medial larger than lateral), resulting in medially inclined joint line both in extension and flexion. The medial surface of the polyethylene insert has a convex curve while the lateral surface has a flat surface. The figure is reprinted with minor modifications from Fig. 1 in reference 13.

Figure 2

Joint line orientation angle (JLOA). The JLOA is the angle between the joint line and a line parallel to the floor, and medial inclination (medial side down and lateral side up) was defined as positive.

Figure 3

Distribution of the in-range and the outlier groups in a total of 145 knees. When the knees were classified according to the HKA axis, 81 knees (55.9%) were assigned to the in-range group (− 3 to 3°) (left, red bars), and 64 knees (44.1%) were assigned to the outlier group (< − 3° or > 3°) (left, blue bars). When classified according to the JLOA, 80 knees (55.2%) were assigned to the in-range group (2–4°) (right, red bars), and 65 knees (44.8%) were assigned to the outlier group (< 2° or > 4°) (right, blue bars). HKA: hip-knee-ankle; JLOA: joint line orientation angle.