Beyond the nine boxes: Introducing the X-type coronal plane alignment of the knee phenotype in knee arthroplasty

Background: The coronal plane alignment of the knee (CPAK) classification provides a nine-box framework to categorize knee phenotypes based on the hip–knee–ankle angle (HKA) and joint line obliquity. While widely adopted, it does not account for knees with intersecting joint lines. We describe a variant of the extreme phenotypes and report its incidence and characteristics in a consecutive series of primary total knee arthroplasties (TKAs). Materials and Methods: We retrospectively reviewed 100 consecutive knees undergoing primary TKA performed by a senior surgeon using the CORI robotic platform. Standard anteroposterior, lateral radiographs, and long-leg scannograms were evaluated for CPAK classification. Lateral distal femoral angle, medial proximal tibial angle, and joint line obliquity were measured. Cases, in which femoral and tibial joint lines crossed, were defined as X-type. Intraoperative data included surgical time, bone defect location, use of augments or grafts, and polyethylene insert thickness. Results: Six knees (6%) demonstrated the X-type phenotype. Five cases had tibial-sided bone defects, all associated with varus deformities, and one case had a femoral condylar defect. One tibial case required a wedge augment, three required screw fixation with cement, and one required no augmentation. The mean tourniquet time was 12 min longer than baseline (50 vs. 38 min). Polyethylene insert thicknesses were 9 mm in two cases, 11 mm in three cases, and 13 mm in one case. All final alignments were within 3° of the planned target (175°–182° HKA). Conclusion: The X-type CPAK phenotype is a rare but clinically relevant alignment pattern occurring in up to 6% of knees in our series of 100 consecutive knees, most commonly arising due to tibial bone defects in varus deformities. Recognition of this variant is important for preoperative planning and intraoperative management. Robotic assistance enabled reliable correction, suggesting this phenotype may represent a reproducible extension to the CPAK classification.