Pre- and post-operative knee alignment phenotypes in restricted kinematic alignment, mechanical alignment total knee arthroplasty, and unicompartmental knee arthroplasty

Purpose

The coronal plane alignment of the knee (CPAK) classification has been introduced as a simple system to classify knee phenotypes based on leg alignment [hip–knee–ankle angle (HKA) and joint line obliquity (JLO)]. Differences in post-operative phenotype between mechanical alignment (MA) and kinematic alignment (KA) total knee arthroplasty (TKA) have been reported, but not the phenotypes after unicompartmental knee arthroplasty (UKA). Moreover, most studies have focused upon Western populations. This study compared pre- and post-operative knee phenotypes between MA-TKA, KA-TKA, and UKA in a Japanese cohort.

Methods

In this study, 230 knees in 165 patients who underwent UKA, 124 knees in 80 patients who underwent MA-TKA, and 80 knees in 51 patients who underwent restricted KA-TKA in Takatsuki General Hospital between March 2019 and January 2021 were retrospectively investigated. Pre- and post-operative lateral distal femoral angle (LDFA) and medial proximal tibial angle (MPTA) were measured, and JLO (MPTA ​+ ​LDFA) and arithmetic HKA (aHKA) (MPTA – LDFA) were calculated and compared between the surgeries.

Results

Pre-operative JLO and aHKA did not significantly differ between the three groups (pre. JLO, p ​= ​0.164; pre. aHKA, p ​= ​0.13). Pre-operatively, 62.0% of knees were categorized as type I (varus leg alignment and medially sloped joint line). After UKA, 59.1% of type I inherited the pre-operative phenotype, whereas most cases were altered in MA-TKA and KA-TKA (4.8% and 30.0%, respectively, inherited the pre-operative phenotype).

Conclusions

The pre-operative phenotype was mostly inherited after UKA and to a lesser extent after KA-TKA, whereas it tended to be altered after MA-TKA.