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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Stability and failure of internal fractrure fixation systems

Author: SHELTON, Julia

Year: 1989

Supervisor(s): Bill Bonfield

The mechanical properties of internal fracture fixation systems were investigated using three complementary methods. First, a technique applying double exposure holographic interferometry was used to investigate the overall structural stiffness of fractures repaired by internal fracture fixation plates. Secondly, the screw-holding performance of orthopaedic screws in bone was assessed by measuring the direct pull-out force from sheets and cylinders of bone. The effect in vitro of a cyclic loading regime on screw pull-out was also tested. Thirdly, the bone-screw interface was characterised by sectioning and optical microscopy.
The technique of double exposure holographic interferometry allowed a qualitative assessment of fracture fixation systems. When cyclic loads of 1 kN were applied to the system - comprising a fractured or osteotomised human tibia and a plate - no significant change in the overall structural stiffness of the system was noted until after 150,000 cycles. An angulated osteotomy or natural fracture was found to eliminate changes in bone-end separation when a tibia was loaded in torsion. However, only by applying compression and hence introducing friction across the fractured bone ends, or by using a plate of greater torsional stiffness, was the relative movement between the fracture ends reduced.

It was established that the threads on the screws had an important role in keeping the plate close to the bone surface and hence ensuring effective load transfer across a fracture. The pull-out force of screws from bone was found to correlate primarily with bone thickness, although the outside diameter and screw design were also statistically significant. Three failure mechanisms for pull-out were identified namely - cylindrical pull-out when a core of bone the diameter of the outside thread of the screw was pulled out with the screw, radial cracking when cracks propagated from the screw thread causing the destruction of the whole bone, or cone pull-out when a graduated, laminated cone of bone was pulled out with the screw. A theoretical analysis of these three modes of failure was carried out. The bone-screw system was found to fatigue; however, loading at 30 per cent of the ultimate pull-out force produced no failure at 500,000 cycles.

The bone-screw interface was found to give little contact between the bone and the screw. Cyclically loading whole tibias up to 150,000 cycles did not produce cracking or degradation in the bone-screw interface, however loading at 80 per cent of the ultimate pull-out load led to cracks being formed in the bone.