Abstract: BACKGROUND: The long-term follow-up report shows that the progress of lateral compartment osteoarthritis is an important reason for the revision of unicompartmental knee arthroplasty, and the force line of lower limbs is considered to be an important factor leading to the progress of lateral compartment osteoarthritis after unicompartmental knee arthroplasty.
OBJECTIVE: To explore the influence of lower limb force line on the progression of lateral compartment arthritis in unicompartmental knee arthroplasty with mobile bearing.
METHODS: From March 2014 to March 2017, a retrospective analysis was conducted in 84 patients who underwent unicompartmental knee arthroplasty in the Department of Arthrology, Foshan Hospital of Traditional Chinese Medicine. Kellgren-Lawrence X-ray grading was used to evaluate the osteoarthritis of the lateral compartment during the follow-up. According to whether osteoarthritis of the lateral compartment was more advanced than that of the operation during the last follow-up, it was divided into the advanced group and the non-advanced group. The force lines of the lower limbs, such as hip-knee-ankle angle and Kennedy area distribution of the mechanical axis of the lower limbs, were compared between the two groups. Simultaneously, the knee joint function of the two groups was compared by the Hospital for Special Surgery knee score, visual analogue scale score of the knee joint, and motion range of the knee. The relationship between the changes of lower extremity force lines and the progress of lateral compartment arthritis was analyzed.
RESULTS AND CONCLUSION: (1) All patients were followed up for 36-72 months, and no complications such as infection, poor wound healing, periprosthetic fracture, polyethylene gasket dislocation occurred. (2) Among the 84 patients, 27 cases were in the advanced group and 57 cases were in the non-advanced group. Significant differences in Hospital for Special Surgery knee score and visual analogue scale score were detected at the last follow-up between the advanced group and the non-advanced group (P < 0.05). There was no significant difference in knee motion range between the two groups at the last follow-up (P > 0.05). (3) At the last follow-up, the average hip-knee-ankle angle in the advanced group was (-1.02±3.13)°, while that in the non-advanced group was (3.94±1.56)°. The difference between the two groups was statistically significant (P < 0.05). Meanwhile, there was a significant difference in hip-knee-ankle angle between the last follow-up and the preoperation between the two groups (P < 0.05). (4) The regional distribution of lower limb mechanical axis Kennedy was compared between the two groups at the last follow-up, and the difference was statistically significant (P < 0.05). The postoperative lower limb force lines were mostly located in zone 3 and C in the advanced group, and mostly located in zone 2 in the non-advanced group. (5) Good lower limb alignment is the key factor affecting the clinical efficacy after unicompartmental knee arthroplasty with mobile bearing. Average varus angle of mechanical axis was approximately 3.94° in patients with well-functioning unicompartmental knee arthroplasty at follow-up, whereas patients for progression of osteoarthritis were in more valgus (mean 1.02° of valgus).

Key words: joint, unicompartment, mobile bearing, lower limb line of force, osteoarthritis, lateral compartment, genu valgus