The impact of changing Coronal Plane Alignment of the Knee classification on early functional outcomes after primary robotic-assisted total knee arthroplasty.

AIMS: We aimed to determine the impact of changing a patient's Coronal Plane Alignment of the Knee (CPAK) phenotype on early functional outcomes after robotic-assisted total knee arthroplasty (RA-TKA).

METHODS: We conducted a retrospective multicentre cohort study of 297 patients undergoing primary RA-TKA. Standardized pre- and postoperative long-leg radiographs were obtained. Radiological measures included the lateral distal femoral angle (LDFA) and medial proximal tibial angle (MPTA). Pre- and postoperative CPAK classification was derived by determining the arithmetic hip-knee-ankle (HKA) angle and joint line obliquity (JLO). Oxford Knee Scores (OKSs) were assessed at preoperative baseline and at three months, six months, and one year postoperatively. We also investigated the isolated impact of changes in LDFA and MPTA on OKS.

RESULTS: Preoperatively, the most common classification was CPAK type II (30.0%; n = 89), followed by type I (27.6%; n = 82). Postoperatively, the majority of patients were CPAK type V (30.0%; n = 89), followed by type IV and type II. Overall, 30.6% (n = 91) of patients remained in the same preoperative CPAK classification, while 62.3% (n = 185) shifted by one CPAK classification and 6.4% (n = 19) moved two phenotypes following RA-TKA. Patients who moved across two CPAK phenotypes after RA-TKA demonstrated significantly smaller improvements in OKS at three months (p = 0.008) and six months (p = 0.036) compared with those who remained in the same CPAK phenotype. Patients with LDFA changes in excess of 4° from the native value demonstrated the smallest improvement in mean three-month (7.14 (SD 9.9)) and six-month (10.95 (SD 7.0)) OKS compared with all other groups.

CONCLUSION: In this exploratory analysis, altering coronal alignment across two CPAK classifications after RA-TKA was associated with lower early functional outcomes as measured by OKS at three months and six months, though this finding was based on a small subgroup (n = 19) and warrants further investigation in larger cohorts. Furthermore, femoral coronal deviation in excess of 4° from native alignment appears to have a greater negative impact on functional outcomes than equivalent tibial changes.