Infographic: Impact of implant materials and malalignment in unicompartmental knee arthroplasty

Unicompartmental knee arthroplasty (UKA) survival remains variable across implants and institutions. A strong relationship has been proven between UKA survival and surgeon volume,¹ implying that achieving well-aligned UKAs is important for maximizing survival. In nearly half of UKA revisions, the indication for revision is unexplained pain,² and elevated proximal tibial strain and microdamage are thought to contribute to this.^3,4

A finite element model of a composite tibia (experimentally validated using digital image correlation and acoustic emission⁵) was used to investigate the effect of tibial component alignment in cemented metal-backed (MB) and cemented all-polyethylene (AP) fixed-bearing medial UKAs.⁶ A linearly elastic analysis was performed with loads up to 2500 N medially (4170 N total load). Standard alignment (medial proximal tibial angle 90°, 6° posterior slope), coronal malalignment (3°, 5°, 10° varus; 3°, 5° valgus), and sagittal malalignment (0°, 3°, 6°, 9°, 12°) were analyzed. The primary outcome measure was the volume of compressively overstrained cancellous bone (VOCB) < -3000 µε. The secondary outcome measure was maximum cortical bone stress (MCBS) over a medial region of interest.

Malalignment had less effect on the VOCB than implant selection. Well-aligned AP implants displayed greater volumes of overstrained cancellous bone and greater anteromedial MCBS than poorly aligned MB implants at both low and high loads. Consistent with previous studies of MB implants,⁷ varus malalignment increased MCBS but decreased VOCB in both implants. Varus malalignment of 10° reduced the VOCB by 10% and 3% in AP and MB implants, respectively, but increased the MCBS by 14% and 13%. Valgus malalignment of 5° increased the VOCB by 8% and 4% in AP and MB implants, respectively, with reductions in MCBS of 7% and 10%. Sagittal malalignment displayed negligible effects.

Supportive of previous work showing AP implants to be more sensitive to polyethylene thickness than MB implants,⁸ this finite element study has shown that UKA tibial component material has a greater effect on proximal tibial bone strain than malalignment. Cancellous bone strain and cortical bone stress had a reciprocal relationship: varus malalignment reduced cancellous bone strain but increased anteromedial cortical bone stress; valgus malalignment did the reverse. Well-aligned AP implants display greater bone strains than malaligned MB implants.