gms | German Medical Science

Objectives: Shear loading of the distal femur plays a crucial role for distal femur fractures and implant failures, especially after total knee replacement revision and distal femur reconstruction in tumour patients [1], [2], [3]. While tibio-femoral (TF) loading acting in vivo relative the proximal tibia has been previously reported [4], knowledge of the individual load components acting relative to the distal femur is still limited. We hypothesised that the anterior-posterior (AP) shear forces at the femur are substantially greater than at the tibia during high flexion activities.

Methods: Six TKA patients (aged 65-80y), performed 5-10 cycles of level walk (LW), stair descent (SD), sit-stand-sit (SSS) and squat (SQ). An instrumented tibial component measured in vivo TF loading, while a mobile video-fluoroscope captured the TF kinematics [5]. A femoral bone fixed coordinate system (FBS) was defined: origin at midpoint of the femoral epicondyles, Z-axis towards the hip centre, and the Y-axis anterior, normal to the plane containing all 3 landmarks. The relative 3D positions and rotations of the implant components from fluoroscopy were used to transform the measured 6 load components from the tibial-implant-system (TIS) to FBS. The peak force and moment magnitudes in TIS and FBS were compared using the Wilcoxon signed-rank test (* p<0.05) and reported as median of absolute and bodyweight (BW) normalized values.

Results and conclusion: Peak axial loads on the femur didn't exceed those on the tibia during any activity, and were greatest during SD (TIS: 2816N, 343%BW; FBS: 2317N, 292%BW) (Figure 1A [Fig. 1]). Peak femoral AP forces were substantial greater than peak tibial AP forces during SD, SQ, and showed the greatest difference during SSS (TIS: 151N, 18%BW; FBS: 2384N, 271%BW) (Figure 1B [Fig. 1]). Peak torsional moments on the femur exceed those on the tibia during SD, SSS and showed the greatest difference during SQ (TIS: 7Nm; FBS: 21Nm) (Figure 1C [Fig. 1]).

For the first time, this study quantifies the role that femoral segment rotation relative to the tibia plays for translating joint contact forces into shear loading. Using a worldwide unique data set [5], we confirmed the hypothesis that the AP loading of the distal femur can be ~10-fold of the tibial AP loading, and can exceed 3000N during daily activities. Additionally, we found that the femoral torsional moments can exceed 30Nm (~3 times the tibial torsional moment).