[Influence of the tibial slope on tibial translation and mobility of non-constrained total knee prosthesis].

PURPOSE OF THE STUDY: We determined retrospectively the influence of posterior tibial slope and anterior cruciate ligament (ACL) sparing on anterior tibial translation in 68 Cloutier total knee prosthesis. We also precised the influence of posterior tibial slope on knee functional score and appearance of tibial prosthetic interfaces.

MATERIAL: 38 Cloutier total knee prosthesis (62 patients mean aged 62 +/- 10 years (36-76) at surgery) reviewed at systematic follow-up control, after a mean period of 5.5 +/- 3 years (2-15), were included in the study. The ACL was preserved in 38 knees and sacrified in 30 knees, the posterior cruciate ligament was preserved in all cases. The prosthetic design was the same whatever the number of cruciate ligament preserved. Osteoarthritis was the reason for surgery in 54 knees, and rheumatoid arthritis in 14 knees. Mean HSS knee score was 54 +/- 10 (29-80) before surgery and 89 +/- 10 (35-100) at follow-up. The mean range of motion was 103 +/- 24 degrees (30-130) before surgery and 110 +/- 14 degrees at follow-up (40-130).

METHODS: Anterior tibial translation was determined on two profil x-rays (non weight bearing and weight bearing) at 20 degrees of flexion by comparing the position of tibial tray with regard to posterior edge of femoral prosthesis. Tibial slope was measured on lateral view with regard to peroneus axis. Appearance of tibial prosthetic interface was studied in 48 knees on AP and lateral x-rays orientated with an image intensifier in order to obtain the x-ray would be parallel to the tibial interface.

RESULTS: Posterior tibial slope (mean value 6.2 degrees +/- 4.2 degrees) was the main factor influencing the anterior tibial translation (mean value 3.9 +/- 4.6 mm) (p = 0.0007). A 10 degree increase of posterior tibial slope makes the anterior tibial translation rise by 5.6 mm in weight bearing situation. When ACL was preserved, the anterior tibial translation was lower but the decrease was not significant. Likewise, preservation of ACL or the degree of posterior tibial slope had no influence on: 1) HSS knee functional score, 2) range of motion. Radiolucent lines were observed in 18 out of 48 knees, but their occurrence was not influenced by the degree of posterior tibial slope or preservation of ACL.

DISCUSSION: Posterior tibial slope has a higher influence than ACL preservation on anterior tibial translation. The increase of posterior tibial slope in order to improve range of motion and to protect the bone-prosthetic tibial interface appeared unjustified with this non-constrained prosthesis. Moreover, implantation of tibial tray (whatever the preservation of ACL) with an important posterior inclination exposes to high anterior tibial translation in weight bearing situation. This last condition could reduce the survivorship of tibial polyethylene.