Robot-assisted mechanically aligned total knee arthroplasty: comparing early outcomes between varus and non-varus knees, and between CPAK Type I and Type IV subtypes

OBJECTIVE: This study aims to compare early pain relief and functional recovery between varus and non-varus phenotypes following robot-assisted mechanically aligned total knee arthroplasty (MA-TKA), with a specific focus on the two most common varus subtypes: Type I and Type IV. METHODS: A retrospective analysis was performed on 124 patients who underwent primary unilateral robot-assisted MA-TKA. Patients were classified according to the Coronal Plane Alignment of the Knee (CPAK) classification based on preoperative arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO). Based on aHKA, patients were categorized into a varus group (CPAK Types I, IV, and VII, n = 93) and a non-varus group (neutral and valgus phenotypes, n = 31). Within the varus cohort, the two most prevalent subtypes - Type I (n = 64) and Type IV (n = 27) - were compared; Type VII (n = 2) was excluded from subtype-specific analyses due to small sample size. Patient-reported outcomes (VAS pain, KSS clinical/function, WOMAC) were evaluated preoperatively and at 1, 3, and 6 months postoperatively. Radiographic parameters (aHKA, MPTA, LDFA, JLO) were measured preoperatively and at 1 month. RESULTS: Preoperatively, the varus group had a significantly lower aHKA than the non-varus group (- 6.9° ± 2.4° vs. -0.1° ± 1.1°, p < 0.001). Postoperatively, both groups achieved neutral mechanical alignment with no significant difference in aHKA (p = 0.231). However, residual joint line obliquity differed significantly between Type I and Type IV knees (176.2° ± 0.2° vs. 179.3° ± 0.3°, p < 0.001). Non-varus patients had significantly better functional recovery at 1 month than varus patients (KSS‑Function: 60.5 ± 2.8 vs. 53.8 ± 3.1, p = 0.042; WOMAC: 36.2 ± 1.9 vs. 43.0 ± 2.3, p < 0.001), and these differences attenuated by 6 months. Within the varus subgroup, Type IV patients had lower 1‑month VAS scores and higher KSS‑Function scores than Type I patients (VAS: 3.5 ± 0.4 vs. 4.2 ± 0.4, p = 0.018; KSS‑Function: 55.0 ± 2.5 vs. 53.3 ± 2.9, p = 0.039). CONCLUSIONS: Preoperative CPAK phenotype is associated with early recovery after robot-assisted MA‑TKA. Non-varus patients had higher 1‑month KSS‑Function scores than varus patients. Among varus knees, Type IV patients had lower 1‑month VAS scores and higher 1‑month KSS‑Function scores than Type I patients, whereas Type I knees retained greater residual postoperative joint line obliquity. Distinguishing varus subtypes may help refine prognostic expectations and guide individualized alignment strategies.