Text

Objectives and questions: Varus knee osteoarthritis (OA) progresses from anteromedial to posteromedial wear and is frequently accompanied by apparent ligament imbalances and flexion contractures. Distinguishing true ligament pathologies from bone- & osteophyte-driven pseudolaxities remains challenging. Using a real-time robotic protocol for articular laxities evaluation (REAL-Classification) during total knee arthroplasty (TKA) after systematic osteophyte removal, aim was primarily given to define a sequential pseudolaxity-model (SEQ) for varus OA and to analyse possible behaviours of gap-laxities, contractures, and substance losses. Secondarily, a SEQ-based management is unfolded pointing different approach-tendencies.

Material and methods: In 254 varus robotic-assisted functionally aligned TKAs, the REAL laxity-phenotypes with gap-opening side were defined. Bone resections in mm and joint line orientations in extension and flexion were retrieved from the robotic database. Mid-flexion laxity ranges was assessed in degrees of axis-deviation. Preoperative ROM-data were also defined. Knees were classified into five sequential phenotypes: Seq0 [lateral-opening, extension-deficit(ED)≤5°], Seq1 (medial-opening, ED≤5°), Seq2 (medial-opening, ED>5°), Seq3 (lateral opening, ED>5° with low lateral laxity (LL) bellow the top tertile <7.07mm), and Seq4 (lateral opening, >5° with high LL>7.07mm). Preoperative ROM, midflexion laxity range at 45°, gap imbalances, REAL distributions, and robotic management parameters were analysed.

Results: Seq0 represented an early anteromedial OA stage with no contracture and minimal cartilage loss (extension deficit 0.5 ± 3.0°), preserved ROM (126 ± 9°), small lateral opening (−1.7 ± 1.1 mm in extension), and predominantly low REAL-grades (REAL 1A-1B: 75%). Seq1 represented a posteromedial OA with more cartilage but no contractures. Seq2 represented a progressed varus OA with progressed cartilage-/ bone-losses and fixed-flexion deformities. Despite increasing contractures from Seq1 to Seq2, REAL severity did not decrease, indicating osteophyte-related pseudo-contractures rather than true ligament tightening. However, in the more progressed Seq3 phenotype a fixed-flexion varus OA with possible MCL contracture illustrated an increasing lateral opening but with suppressed REAL-imbalances. In Seq4 through an progressing lateral elongation, a sequent increase in lateral REA-imbalances could be illustrated.

Discussion and conclusions: Varus knee OA demonstrates a reproducible sequence of pseudolaxity phenotypes. Early apparent contracture is predominantly osteophyte-driven, whereas true ligament imbalance emerges later. REAL classification provides a real-time, alignment-independent framework to distinguish pseudolaxity from true ligament pathology and to guide personalized robotic TKA strategies.