Abstract: BACKGROUND: Most unilateral knee compartment lesions are accompanied by varus deformities of varying degrees. If the lower limb force line is over-corrected, it is easy to increase the stress of the lateral compartment and cause osteoarthritis of the lateral compartment; if the correction is insufficient, the prosthesis will loosen due to uneven force.  
OBJECTIVE: To observe the effect of lower extremity line correction after mobile-bearing unicompartmental knee arthroplasty on the clinical efficacy of patients in early and middle stages.
METHODS:  Totally 137 cases undergoing unicompartmental knee arthroplasty at Wangjing Hospital of China Academy of Chinese Medical Sciences from January 2013 to December 2016 were enrolled in this study, aged from 45 to 83 years. Lower limb force line in the coronal plane was evaluated using X-ray films at 3 days after arthroplasty. The patients were divided into an obvious varus (5°-10°) group (n=11), a mild varus (1°-5°) group (n=59), a standard neutral position (0°-1°) group (n=65), and mild valgus (eversion more than 0°) group (n=2). Oxford knee score and American knee society score were analyzed before operation, 3, 6, 12, 36, and 60 months after operation.  
RESULTS AND CONCLUSION: (1) All 137 patients were followed up for 60 months. Among them, few patients with mild valgus were not included in the result analysis. There were one case of aseptic prosthesis loosening, two cases of unexplained pain, and one case of compartment osteoarthritis in the obvious varus group. There were one case of infection, one case of joint instability, one case of aseptic prosthesis loosening, one case of unexplained pain, and one case of lateral compartment osteoarthritis in the mild varus group. There were one case of infection, one case of polyethylene liner dislocation, one case of joint instability, three cases of unexplained pain, and two cases of lateral compartment osteoarthritis in the standard neutral position group. There were significant differences in the incidence of lateral compartment osteoarthritis among the three groups (P < 0.05). (2) Compared with the score before operation, Oxford knee score was lower (P < 0.05) and American knee society score was higher (P < 0.05) at various time points after operation in the three groups. At 12 months after operation, Oxford knee score was higher in the obvious varus group than that in the mild varus and standard neutral position groups (P < 0.05); American knee society score was lower in the obvious varus group than that in the mild varus and standard neutral position groups (P < 0.05). At 3, 6, 36, and 60 months after operation, no significant difference was found in Oxford knee score and American knee society score among the three groups (P > 0.05). (3) At 60 months after operation, survival rates of prosthesis were 81.8%, 96.9%, and 94.9% in the obvious varus, mild varus, and standard neutral position groups, respectively. No significant difference in survival rate of the prosthesis was found among the three groups (P > 0.05). (4) The results confirm that early and mid-term clinical effects are better when the lower limb force line is maintained at 0°-5° after unicompartmental knee arthroplasty.

Key words: knee, osteoarthritis, unicompartmental knee arthroplasty, lower limb force line, varus, medial compartment of the knee, curative effect analysis