Biomechanics of bone-fracture fixation by stiffness-graded plates in comparison with stainless-steel plates

Abstract

Background: In the internal fixation of fractured bone by means of bone-plates fastened to the bone on its tensile surface, an on-going concern has been the excessive stress-shielding of the bone by the excessively-stiff stainless-steel plate. The compressive stress-shielding at the fracture-interface immediately after fracture-fixation delays callus formation and bone healing. Likewise, the tensile stress-shielding of the layer of the bone underneath the plate can cause osteoporosis and decrease in tensile strength of this layer.

Method: In order to address this problem, we propose to use stiffness-graded plates. Accordingly, we have computed (by finite-element analysis) the stress distribution in the fractured bone fixed by composite plates, whose stiffness is graded both longitudinally and transversely.

Results: It can be seen that the stiffness-graded composite-plates cause less stress-shielding (as an example: at 50% of the healing stage, stress at the fracture interface is compressive in nature i.e. 0.002 GPa for stainless steel plate whereas stiffness graded plates provides tensile stress of 0.002 GPa. This means that stiffness graded plate is allowing the 50% healed bone to participate in loadings). Stiffness-graded plates are more flexible, and hence permit more bending of the fractured bone. This results in higher compressive stresses induced at the fractured faces accelerate bone-healing. On the other hand, away from the fracture interface the reduced stiffness and elastic modulus of the plate causes the neutral axis of the composite structure to be lowered into the bone resulting in the higher tensile stress in the bone-layer underneath the plate, wherein is conducive to the bone preserving its tensile strength.

Conclusion: Stiffness graded plates (with in-built variable stiffness) are deemed to offer less stress-shielding to the bone, providing higher compressive stress at the fractured interface (to induce accelerated healing) as well as higher tensile stress in the intact portion of the bone (to prevent bone remodeling and osteoporosis).

Figure 1

Fracture fixation plates (length 60 mm and thickness 5 mm) with grading (a) Stiffness graded along thickness and is given by Young's modulus = 36X+20 (b)Stiffness graded along length Young's modulus = -6X+200.

Figure 2

Normal stress S_xx (MPa) at the fracture interface for different stages of healing with SS plate.

Figure 3

Normal stress S_xx (GPa) at the fracture interface during 1% healing (a) comparison of stresses for SGT and SS (b) comparison of stresses for SGL and SS.

Figure 4

Normal stress distribution at the fracture interface (during 50% healing) (a) comparison of stresses for SGT and SS (b) comparison of stresses for SGL and SS.

Figure 5

Normal stress distribution at the fracture interface during 75% healing stage (a) comparison of stresses for SGT and SS (b) comparison of stresses for SGL and SS.

Figure 6

Stresses along the bone-plate interface (top layers of bone) during the initial stages of healing (a) comparison of stresses for SGT and SS (b) comparison of stresses for SGL and SS.

Figure 7

Stresses at the bottom layers of bone during the initial stages of healing (a) comparison of stresses for SGT and SS (b) comparison of stresses for SGL and SS.

Figure 8

Stresses along the bone-plate interface during 50% healing (a) comparison of stresses for SGT and SS (b) comparison of stresses for SGL and SS.

Figure 9

Stresses along the bottom of the bone during 50% healing (a) comparison of stresses for SGT and SS (b) comparison of stresses for SGL and SS.

Figure 10

Stresses along the bone-plate interface during the final stages of healing (a) comparison of stresses for SGT and SS (b) comparison of stresses for SGL and SS.

Figure 11

Stresses along the bottom layer of the bone during the final stages of healing (a) comparison of stresses for SGT and SS (b) comparison of stresses for SGL and SS.