Restoration of CPAK type in total knee arthroplasty does not lead to superior clinical outcome: A study based on arthroplasty registry data

PURPOSE: To assess the pre- and post-operative distribution of patients undergoing total knee arthroplasty among the nine categories of the Coronal Plane Alignment of the Knee-CPAK classification (Aim 1). Another goal was to assess whether the clinical outcome (patient-reported outcome and implant survival) differed between those whose CPAK type changed vs. those who maintained their CPAK type (Aim 2). Most previous studies investigated only small cohorts and provided only pre-operative data. Moreover, no studies related the change in CPAK type to robust clinical information such as implant survival. METHODS: A retrospective, comparative design was applied and included patients who had undergone primary total knee arthroplasty. A digital analysis of whole-leg radiographs was performed to allocate the cases to the CPAK classes pre- and post-operatively. Likewise, the patient-reported outcomes were assessed pre- and 1 year post-operatively using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). RESULTS: 2056 patients (742 men, 1314 women; age 69.55 ± 8.61 years) were analysed. Pre-operatively, the most frequent CPAK types were V, II and I, constituting 18.6%, 18.3% and 17.6% of cases, respectively. The mean follow-up period for the whole study population regarding implant survival was 8.36 years. Following TKA, CPAK type V was dominant with 46.1% followed by types IV and VI with 15% and 14.2%, respectively. Of the study patients, 80.0% (n = 1645) underwent a change in CPAK type. One year post-operatively, WOMAC total was 12.29 (4.77-26.97) for CPAK changers and 11.76 (4.31-25.53) for CPAK maintainers (p = 0.399). Moreover, no differences were seen in the WOMAC subscores pain, stiffness or function (0.128 < p < 0.678). The 10-year implant survival was 91.2% among the CPAK changers and 92.9% among the maintainers (p = 0.506). CONCLUSIONS: The pre-operative CPAK knee alignment was preserved in 20% of patients. However, this did not seem to have a negative impact on clinical outcome, as CPAK type changers and CPAK type maintainers did not show significant differences in WOMAC scores or implant survival. LEVEL OF EVIDENCE: Level III.