Varus placement of the tibial component reduces the potential risk of fracture with adequate bony coverage in the Oxford unicompartmental knee arthroplasty

Abstract

A short keel-cortex distance (KCD), especially to the posterior cortex, is a potential risk factor for tibial plateau fracture after Oxford mobile-bearing unicompartmental knee arthroplasty (OUKA). This study aimed to evaluate the effect of tibial component alignment in the coronal plane and tibial proximal morphology on the KCD. Included in this study were 51 patients scheduled for primary Oxford medial unicompartmental knee arthroplasty (UKA). The anterior and posterior KCD were preoperatively assessed using 3D simulation software with the component set perpendicular to the tibial mechanical axis (neutral), 3° valgus, 3° varus, and 6° varus, relative to neutral alignment. We evaluated the existence of overhanging medial tibial condyle where the medial eminence line, the line including the medial tibial eminence parallel to the tibial axis, passes outside of the tibial shaft. In all component alignments, patients with a medial overhanging condyle had significantly shorter posterior KCD than those without. In patients with a medial overhanging condyle, the posterior KCD significantly increased when the tibial component was placed in 3° varus (4.6 ± 1.5 mm, P = 0.003 vs neutral, P < 0.001 vs 3° valgus) and 6° varus (5.0 ± 1.4 mm, P < 0.001 vs neutral, P < 0.001 vs 3° valgus) compared with in neutral (3.5 ± 1.9 mm) or 3° valgus (2.8 ± 1.8 mm). In OUKA, varus implantation increased the KCD. This could potentially decrease the risk of fracture, even in knees with the overhanging medial condyle. Conversely, valgus implantation of the tibial component shortened the KCD, and should therefore be avoided.

Figure 1

Images showing the setting of the tibial component coronal alignment. The tibial component neutral position was set perpendicular to the tibial mechanical axis (A). The tibial component varus (B)/valgus (C) alignment was adjusted with the tip of medial intercondylar eminence.

Figure 2

(A) The distance between the keel and tibial cortex was measured along the axial plane in various regions: anterior (a), anteromedial (b), posterior (c), and posteromedial (d). (B) Amount of over-coverage was measured by calculating area within the enclosed line.

Figure 3

(A) Intramedullary type in the medial eminence line (MEL) classification. (B) Extramedullary type in the MEL classification. (C) Measurement of proximal tibia vara angle.

Figure 4

Comparison of KCDs in four regions (A. anterior B. anteromedial C. posterior D. posteromedial) within-factors (valgus3, neutral, varus3, and varus6) in both extramedullary and intramedullary groups. *Statistically significant difference (P < 0.05).

Figure 5

Comparison of over-coverage within-factors (valgus3, neutral, varus3, and varus6) in both extramedullary and intramedullary groups. *Statistically significant difference (P < 0.05).