Effect of varus alignment on the bone‐implant interaction of a cementless tibial baseplate during gait

Abstract

Component alignment in total knee arthroplasty is a determining factor for implant longevity. Mechanical alignment, which provides balanced load transfer, is the most common alignment strategy. However, a retrospective review found that varus alignment, which could lead to unbalanced loading, can happen in up to 18% of tibial baseplates. This may be particularly burdensome for cementless tibial baseplates, which require low bone‐implant micromotion and avoidance of bone overload to obtain bone ingrowth. Our aim was to assess the effect of varus alignment on the bone‐implant interaction of cementless baseplates. We virtually implanted 11 patients with knee OA with a modern cementless tibial baseplate in mechanical alignment and in 2° of tibial varus alignment. We performed finite element simulations throughout gait, with loading conditions derived from literature. Throughout the stance phase, varus alignment had greater micromotion and percentage of bone volume at risk of failure than mechanical alignment. At mid‐stance, when the most critical conditions occurred, the average increase in peak micromotion and amount of bone at risk of failure due to varus alignment were 79% and 59%, respectively. Varus alignment also resulted in the decrease of the surface area with micromotion compatible with bone ingrowth. However, for both alignments, this surface area was larger than the average area of ingrowth reported for well‐fixed implants retrieved post‐mortem. Our findings suggest that small varus deviations from mechanical alignment can adversely impact the biomechanics of the bone‐implant interaction for cementless tibial baseplates during gait; however, the clinical implications of such changes remain unclear.

Impact of patient, surgical, and implant design factors on predicted tray–bone interface micromotions in cementless total knee arthroplasty

“…More varus alignments shifted the resultant contact force medially to the joint (Figure 3), increased the moment arm to the location of peak micromotion and resulted in more tray tilting at the anterolateral aspect. These findings were in line with previous studies 18,30 . However, a previous study 18 reported 79% increase in peak micromotion during the GT activity due to a 2° varus alignment, whereas the increment was approximately 13% with a similar perturbation in this study.…”

“…These findings were in line with previous studies 18,30 . However, a previous study 18 reported 79% increase in peak micromotion during the GT activity due to a 2° varus alignment, whereas the increment was approximately 13% with a similar perturbation in this study. The difference was thought to be because this study considered deformable polyethylene, and femoral‐insert and insert‐tray contact locations off the midline of the tray.…”

“…The loading conditions applied in these TKA studies were typically simple vertical loads which do not account for the more complex physiological loading at the knee joint during daily activities. Glenday et al studied the effect of varus alignment on the micromotion with consideration of the loading variations 18 . However, the loading was applied to the center of tibial tray without consideration of physiological femoral‐insert contact.…”

“…Glenday et al studied the effect of varus alignment on the micromotion with consideration of the loading variations. 18 However, the loading was applied to the center of tibial tray without consideration of physiological femoral-insert contact.…”

The influence of body weight index on initial stability of uncemented femoral knee protheses: A finite element study

“…A negative/positive value meant the bone was under compressive/tensile stress. Bone was considered to be at risk of failure if the compressive or tensile strain exceeded −7300 με or 6500 με [ 41 ]. Von-Mises stress was used to assess the risk of femoral component failure.…”