A Total Knee Arthroplasty Is Stiffer When the Intraoperative Tibial Force Is Greater than the Native Knee

We hypothesized that a total knee arthroplasty (TKA) with an intraoperative tibial force greater than the tibial force of the native knee has signs of stiffness as measured by loss of extension and flexion, and anterior translation of the tibia. Intraoperative forces in the medial and lateral tibial compartments were measured during passive motion in 71 patients treated with calipered kinematically aligned TKA. Maximum extension, flexion, and the anterior-posterior position of the tibia with respect to the distal femur at 90 degrees of flexion were measured. Measurements were repeated after exchanging to a 2 mm thicker insert. The sum of the average of the medial and lateral compartment forces at 0, 45, and 90 degrees of flexion represented the tibial force through a 90-degree motion arc. For the implanted insert, the tibial force averaged 28 ± 17 lb, which is comparable to the 20 ± 7 lb reported for the native knee. At 6 months, patients reported an average 40 point Oxford Knee and 15 point Western Ontario and McMaster Universities Osteoarthritis (WOMAC) score. For the 2 mm thicker insert, the tibial force averaged 50 ± 28 lb. A 30 lb tibial force greater than native generated a 3-degree loss of extension, a 3-degree loss of flexion, and 3-mm anterior translation of the tibia. Because a TKA with a tibial force greater than native has signs of stiffness, a strategy for lowering this risk is to match the tibial force of the native knee when balancing a TKA as this restored high function.