Abstract: BACKGROUND: Appropriate soft tissue laxity is required for a successful total knee arthroplasty; however, different patients present with different soft tissue changes after surgery.  
OBJECTIVE: To investigate the effect of knee ligamentous laxity on patient satisfaction during medium and long-term follow-up after total knee arthroplasty, and to preliminarily test the effectiveness of measuring knee ligamentous laxity based on length measurement.
METHODS:  A total of 107 patients who underwent total knee arthroplasty in the First Affiliated Hospital of Zhengzhou University and had been followed up for 5-10 years after surgery were selected. Stress X-rays were taken in each patient, and the coronal laxity of the bilateral knee joints was measured by different methods. Method A: The knee joint laxity was expressed by the sum of the changes in the distance between the medial and lateral openings of the knee joint in the varus and valgus stress test: A1 group (≥ 0 mm and ≤ 5 mm, n=23), A2 group (> 5 mm and ≤ 7 mm, n=38), A3 group (> 7mm and ≤ 9 mm, n=24), and A4 group (> 9 mm, n=22). Method B: The knee joint laxity was expressed by the sum of the knee joint varus and valgus angles in the varus and valgus stress test: B1 group (≥ 0° and ≤ 4°, n=20), B2 group (> 4° and ≤ 6°, n=31), B3 group (> 6° and ≤ 9°, n=23), and B4 group (> 9°, n=33). Method C: The difference in laxity between the knee joints described in Method A was calculated to indicate the difference in laxity between the bilateral knee joints: C1 group (≤ -5 mm, n=22), C2 group (> -5 mm and ≤ -1 mm, n=33), C3 group (> -1 mm and ≤ 1 mm, n=32), and C4 group (> 1 mm, n=20). Method D: The difference in the laxity between the knee joints in Method B was calculated to indicate the difference in laxity between the knee joints: D1 group (≤ -3°, n=27), D2 group (> -3° and ≤ 0°, n=33), D3 group (> 0° and ≤ 3.5°, n=27), and D4 group (> 3.5°, n=20). The Oxford knee score and the modified Lysholm score were used for knee function assessment, to find out the soft tissue laxity range with the highest patient satisfaction. By comparing the sensitivity and specificity of Methods A and B, we could get a better measurement method in this follow-up. By analyzing the results of Methods C and D, the influence of difference in laxity between the bilateral knee joints on patient satisfaction could be explained.  
RESULTS AND CONCLUSION: In Method A, there were significant differences in Oxford knee scores and modified Lysholm scores among groups (P < 0.05), and the range of the best function was 5-9 mm. In Method B, there were significant differences in Oxford knee scores and modified Lysholm scores among groups (P < 0.05), and the range of the best function was 4°-9°. In Method C, there was no significant difference in Oxford knee scores and modified Lysholm scores among groups (P > 0.05). In Method D, there were significant differences in the Oxford knee scores and the modified Lysholm scores (P < 0.05), and the range of the best function was -3°-3.5°. The sensitivity in Method A was slightly lower than that in the Method B, but the specificity was higher than that in Method B. These findings indicate that the range of knee joint laxity with the highest patient satisfaction in 5-10 years after total knee arthroplasty is 4°-9° when measured by angle, and 5-9 mm when measured by length. Length and angle measurements have similar effectiveness in measuring the coronal laxity of the knee joint. When the knee joint laxity is expressed in angle, the smaller the angle difference, the higher the patient’s satisfaction. When the difference is significant, the satisfaction of patients whose knee joint laxity on the surgical side is higher than that on the contralateral side is worse. When the knee joint laxity is expressed in length, the difference in laxity between the bilateral knee joints is not well denoted.

Key words: osteoarthritis, total knee arthroplasty, knee ligamentous laxity, soft tissue balance, knee joint function score, satisfaction, implantation of prosthesis, orthopedic implant