Summary

This study aims to determine the impact of altering patients' CPAK phenotype on early functional outcomes after robotic-assisted TKA.

Abstract

Background

The Coronal Plane Alignment of the Knee (CPAK) classification enables patients' preoperative alignment prior to total knee arthroplasty (TKA) to be classified into one of nine phenotypes. Little is known, however, about the interaction between pre- and postoperative CPAK change and patient outcomes. The primary objective of this study is to determine the impact of altering patients' CPAK phenotype on early functional outcomes after robotic-assisted TKA.

Methods

A retrospective multicentre cohort study of patients undergoing primary robotic-assisted TKAs was conducted. Standardised standing pre- and post-operative long-leg radiographs were obtained for each patient. Radiographic measures included the lateral distal femoral and medial proximal tibial angles (LDFA and MPTA). Pre- and post-operative CPAK classification was calculated by determining the arithmetic hip-knee-ankle (HKA) angle and joint line obliquity (JLO) by the sum or difference of the two coronal measurements. Oxford Knee Scores (OKS) were assessed at preoperative baseline and at 3 months, 6 months and 1 year postoperatively.

Results

A total of 297 patients were included across three sites, with an average age of 66.6 years and mean BMI of 33. Postoperatively, 31.3% of patients remained in the same preoperative CPAK classification, while 62.0% moved one CPAK classification and 6.7% moved two phenotypes. The mean change in MPTA was 0.25 ± 5.93 degrees, LDFA 0.86 ± 7.76 degrees, HKA 0.90 ± 3.51, and JLO 0.81 ± 15.16 degrees when pre- and postoperative values were compared. Of note, patients that moved across two CPAK phenotypes after robotic-assisted TKA demonstrated less improvement in their OKS at 3 months, compared to those that remained in the same CPAK phenotype (preoperative OKS 7.5 ± 8.9 vs. postoperative OKS 14.0 ± 10.6, p=0.02). Improved OKS at 3 months was seen among patients with lower changes in their LDFA postoperatively when compared to preoperative OKS (r^2=0.008, p=0.13).

Conclusions

Altering a patient's coronal alignment across two CPAK phenotypes after robotic-assisted TKA appears to negatively influence early functional outcomes at 3 months when compared to those that remained in the same preoperative CPAK type. Increased changes in femoral component coronal alignment also appear to negatively influence early functional outcomes postoperatively. Future research is needed to determine the impact of subtype variation and direction of change on outcomes after TKA.