Laxity phenotyping in total knee arthroplasty: A current concept article

PURPOSE: Phenotypic variation is determined by both bony anatomy and ligamentous laxity, which are closely interrelated. Therefore, assessment of patient-specific laxity phenotypes should begin with characterisation of the underlying bony phenotype. The purpose of this manuscript is to develop a clinically applicable classification of knee laxity phenotypes and a corresponding treatment algorithm for total knee arthroplasty (TKA) balancing. METHODS: Patient-specific laxity phenotypes were evaluated in the context of the underlying bony phenotype. Based on coronal gap characteristics, three laxity phenotypes in extension and three in flexion were identified, resulting in a 3 × 3 classification system comprising nine distinct phenotypes. A treatment algorithm was subsequently developed to guide correction strategies according to the magnitude of gap imbalance. RESULTS: In flexion, the most common phenotype demonstrated lateral laxity (>60% of knees), followed by a balanced flexion gap (approximately 30%). A medially lax flexion phenotype was also identified and may be clinically relevant because of its association with potential TKA failure mechanisms. Only two of the nine laxity phenotypes (Types 4 and 5) demonstrated currently accepted gap balance criteria in both extension and flexion. The remaining seven phenotypes required correction towards either Types 4 or 5. CONCLUSION: A treatment algorithm is presented outlining different strategies to address imbalance based on the magnitude of the gap difference. Minimal differences (2-3 mm) may be managed through targeted bone cuts, moderate differences (3-5 mm) benefit from adding soft-tissue releases and specific insert types. Lastly, larger differences (>5 mm) may require the addition of further implant constraint. This work demonstrates that one standardised balancing goal will not be able to reconstruct patient-specific laxity phenotypes without imposing variable compromises on either bone reconstruction or soft-tissue balancing. LEVEL OF EVIDENCE: Level V.