Is the rotation of the femoral head a potential initiation for cutting out? A theoretical and experimental approach

Abstract

Background: Since cut-out still remains one of the major clinical challenges in the field of osteoporotic proximal femur fractures, remarkable developments have been made in improving treatment concepts. However, the mechanics of these complications have not been fully understood.We hypothesize using the experimental data and a theoretical model that a previous rotation of the femoral head due to de-central implant positioning can initiate a cut-out.

Methods: In this investigation we analysed our experimental data using two common screws (DHS/Gamma 3) and helical blades (PFN A/TFN) for the fixation of femur fractures in a simple theoretical model applying typical gait pattern on de-central positioned implants. In previous tests during a forced implant rotation by a biomechanical testing machine in a human femoral head the two screws showed failure symptoms (2-6Nm) at the same magnitude as torques acting in the hip during daily activities with de-central implant positioning, while the helical blades showed a better stability (10-20Nm).To calculate the torque of the head around the implant only the force and the leverarm is needed (N [Nm] = F [N] * × [m]). The force F is a product of the mass M [kg] multiplied by the acceleration g [m/s2]. The leverarm is the distance between the center of the head of femur and the implant center on a horizontal line.

Results: Using 50% of 75 kg body weight a torque of 0.37Nm for the 1 mm decentralized position and 1.1Nm for the 3 mm decentralized position of the implant was calculated. At 250% BW, appropriate to a normal step, torques of 1.8Nm (1 mm) and 5.5Nm (3 mm) have been calculated.Comparing of the experimental and theoretical results shows that both screws fail in the same magnitude as torques occur in a more than 3 mm de-central positioned implant.

Conclusion: We conclude the center-center position in the head of femur of any kind of lag screw or blade is to be achieved to minimize rotation of the femoral head and to prevent further mechanical complications.

Figure 1

Schematic 2D representation of the femoral head showing an a. central, b. inferior, c. posterior positioning of the load carrier and d. the example the occurrence of torque Nx with the Force F and the lever x.

Figure 2

A) Clockwise rotation (-) and B) Anticlockwise rotation (+) of the human femoral heads with fixed blades (PFNa/TFN) or screw (DHS/Gamma 3) [5,6]

Figure 3

Summary of all rotation experiments for various implants, Dynamic hip screw (DHS), Gamma 3 (G3), Trochanteric Femur Nail (TFN) and Proximal Femur Nail A (PFN A) in human femoral heads as previously published [5,6]compared to theoretical torques acting in the hip during daily activities as standing and walking according to [1o] .

Figure 4

Anatomical bone slice of the proximal femur (85J old male) and x-ray in a.p. projection of the proximal femur