[Finite element biomechanical analysis of coronal alignment correction in unicompartmental knee arthroplasty for severe varus deformity].

OBJECTIVE: To investigate the effects of different correction angles of the coronal lower limb alignment (represented by the hip-knee-ankle, HKA ) on the stress of prostheses and ligaments in patients with severe genu varum undergoing unicompartmental knee arthroplasty (UKA), and to clarify the optimal reconstruction strategy for postoperative coronal alignment.

METHODS: Based on the computed tomography (CT) and full-ength lower limb X-ray data of 12 patients with severe genu varum (HKA angle:15°to 20°), individualized three-dimensional finite element models were established. By adjusting the HKA angle, four types of UKA correction models were constructed with HKA angles of 0° (neutral), 3°, 6°, and 9°of varus. Seven physiological knee postures (erect, varus, valgus, internal rotation, external rotation, flexion, and hyperextension) were simulated, with a vertical load of 750 N and a bending moment of 7.5 N·m applied. The maximum stress values of the femoral prosthesis, tibial prosthesis, polyethylene insert, medial/lateral collateral ligaments(MCL/LCL), and anterior/posterior cruciate ligaments(ACL/PCL) were calculated. The Mann-Whitney U test was used to analyze the biomechanical differences among different correction angles and postures.

RESULTS: A total of 48 groups of models were established (4 HKA angles×12 patients× 1 prosthesis type). In terms of prosthetic stress, the prosthesis bore the maximum stress when the HKA angle was corrected to 9° of varus, which was significantly higher than those at other angles (P<0.01). For soft tissue stress, the 6° model exhibited the largest number of combinations with maximum soft tissue stress (10 combinations), which was significantly higher than that of the 3° model (1 combination);the 0° and 9° models showed 8 and 9 combinations, respectively. The differences in maximum stress values of the same model components among different varus models and across different postures were statistically significant (P<0.01). The 9° varus model presented peak prosthetic stress in 3 out of the 7 postures (1 posture each for the 0° and 6° models, and 2 postures for the 3° model), with the most significant effect observed in the valgus posture. The lateral compartment stress was the highest in the 0° model and the lowest in the 3° model.

CONCLUSION: Finite element analysis demonstrated that for patients with severe genu varum undergoing UKA, correcting the postoperative HKA angle to approximately 3° of varus results in an optimal stress distribution across the prosthesis-ligament system, and the soft tissue load is closer to the physiological state. The pursuit of a completely neutral alignment (0°) or residual large-angle varus (≥6°) should be avoided. The former increases lateral compartment stress and accelerates joint degeneration, while the latter significantly elevates the stress load on the prosthesis and some ligaments (e.g., ACL), thereby increasing the risks of prosthetic wear, loosening, and ligament injury. It is recommended that the coronal lower limb alignment be controlled within the range of approximately 3° of mild varus.