In Far East, including Japan and the Middle East, daily activities are frequently carried out on the floor. Deep flexion of the knee joint is therefore very important in these societies. Some patients who underwent total knee arthroplasty (TKA) in these countries often perform deep flexion activity, such as squatting, cross-leg sitting and kneeling. However it is still unknown that deep flexion activity affects long term durability after TKA. The purpose of this study was to examine the correlation between deep flexion and long term durability.

Between December 1989 and May 1997, 507 total knee arthroplasties were carried out in 371 patients using the Bi-Surface Knee System (Japan Medical Material, Osaka, Japan) at two institutions and routine rehabilitation program continued for one to two months after TKA. One patient who underwent simultaneous bilateral TKA was excluded because of pulmonary embolism within one month. The other 505 knees (370 patients) were divided into two groups according to the range of flexion after our routine rehabilitation program; one group (Group A: 207 knees) consisted of more than 135 degrees flexion knees and the other group (Group B: 298 knees) consists of less than 135 degrees flexion knees. Patients whose follow-up period was less than 10 years were excluded from this clinical evaluation. Range of flexion was measured preoperatively, at the time after routine rehabilitation program, and at the latest follow-up. Knee function was evaluated on the basis of Knee Society knee score and functional score preoperatively and at the latest follow-up. Kaplan-Meier survivorship analysis was performed with revision for any operation as the end point.

In Group A, the mean preoperative range of flexion was 133.0±16.3 degrees, and at the time after routine rehabilitation program, this improved to 139.7±5.1 degrees. This angle maintained to 136.2±14.3 at the latest follow-up. In Group B, the mean preoperative range of flexion was 111.6±20.4 degrees, and at the time after routine rehabilitation program, this improved to 114.5±13.6 degrees. This angle maintained to 118.2±17.8 at the latest follow-up. The Knee Society knee score and functional score was improved from 43.0±16.9 points and 39.0±20.2 points preoperatively to 95.1±5.8 points and 51.8±21.2 points at the latest follow-up, respectively in Group A. The Knee Society knee score and functional score was improved from 37.1±16.7 points and 31.9±18.4 points preoperatively to 92.5±8.7 points and 53.1±26.1 points at the latest follow-up, respectively in Group B. Kaplan-Meier survivorship at 10-year was 95.5% in Group A and 96.2% in Group B with any operation as the end point. The survivorship between Group A and Group B was not statistically significant.

Good range of flexion was maintained and Knee society score was excellent after a long time follow-up for the patients who achieved deep flexion after TKA. Deep flexion was proved not to affect long term durability in this Bi-Surface Knee System.