Knee kinematics during staircase descent : native, after bi-cruciate retaining and bi-cruciate stabilized total knee arthroplasty

Aims The goal was to evaluate tibiofemoral knee joint kinematics during stair descent, by simulat-ing the full stair descent motion in vitro. The knee joint kinematics were evaluated for two types of knee implants: bi-cruciate retaining and bi-cruciate stabilized. It was hypothesized that the bi-cruciate retaining implant better approximates native kinematics.Methods The in vitro study included 20 specimens which were tested during a full stair descent with physiological muscle forces in a dynamic knee rig. Laxity envelopes were measured by apply-ing external loading conditions in varus/valgus and internal/external direction.Results The laxity results show that both implants are capable of mimicking the native internal/ external-laxity during the controlled lowering phase. The kinematic results show that the bi-cruciate retaining implant tends to approximate the native condition better compared to bi-cruciate stabilized implant. This is valid for the internal/external rotation and the anteropos-terior translation during all phases of the stair descent, and for the compression-distraction of the knee joint during swing and controlled lowering phase.Conclusion The results show a better approximation of the native kinematics by the bi-cruciate retain-ing knee implant compared to the bi-cruciate stabilized knee implant for internal/external rotation and anteroposterior translation. Whether this will result in better patient outcomes remains to be investigated.