Inducible displacements of cemented tibial components during weight-bearing and knee extension. Observations during dynamic radiostereometry related to joint positions and 2 years history of migration in 16 TKR

The inducible displacements of the tibial component caused by active extension were studied in 16 knees 1 yr after an AMK total knee arthroplasty with either flat, concave or posterior-stabilised (PS) designs of the joint area. Continuous change of the position of the tibial component occurred with proceeding extension. Rocking, subsidence and lift-off at different localisations were observed. In 3 of 4 knees with flat inserts the tibial component tilted anteriorly from 45° to 35° of flexion. A similar anterior tilt was seen in 2 of 6 with concave inserts and 5 of 6 with the PS design, but the tilting started later, when the knee had 5–20° more extension. From 45–15° of flexion most components tilted into valgus. Three knees (1 concave with, 1 concave without PCL and 1 PS) showed a sudden tilt into varus direction followed by a rocking motion in the opposite direction. The other types of displacements studied showed a more uniform pattern. The inducible maximum translation (MTPM) at 20° of extension tended to be associated with increased migration between 0 and 2 yr when measured with the same parameter (Spearman’s rho = 0.54, P = 0.03). Increased medial displacement of the center of the proximal tibia at 25° was associated with increased anterior tilt. This type of motion was most commonly seen with the concave design. Our observations demonstrate that the forces acting on the tibial component vary during active extension, which results in rocking movements. This will influence the migration and the patttern of wear, factors of importance for the clinical longevity of a total knee replacement (TKR).