A Pressure Sensitive Film Study on the Effect of Screw Omission on Bone-plate Interface Mechanics in Cadaveric Bone

Summary

Minimalising surgical exposure and the amount of hardware implanted has become the aim in promoting the biological healing of fractures. Increased spacing of fewer bone screws can increase the mechanical strength of fracture fixation without jeopardizing the stability of the construction. The omission of selective screws has also been shown to increase bone strain and has been suggested as a means of treating bone plate induced osteoporosis.

Using Fuji prescale pressure sensitive film we evaluated the effect of selective screw omission on the platebone interface mechanics. Testing was performed using 10-hole, 4.5 mm DCP applied to equine cadaveric third metacarpal bones. Film was interposed between the bone and the bone plate which were applied to a constant level of screw torque. The resultant digitized bone plate patterns enabled computation of the interface contact area (%) and the average force (Newtons-N) at the interface and a qualitative assessment of the pattern of pressure distribution.

The symmetrical omission of 40% of the total screw number resulted in significant differences between the control (10 screws) and each treatment for both interface contact area (p <0.05) and average force (p <0.02). In one pattern of screw omission there was a marginally non-significant difference between the control and treatment interface contact area (p = 0.0518).

The results suggest that a dramatic reduction in the number of screws applied to a given plate (40%) evokes significant changes in the interface mechanics. Of interest is the effect the pattern of screw omission had on the interface mechanical features; certain screw omission patterns induced a reduction in contact area and force of the same magnitude as the number of screws omitted (40%), whilst in other patterns of screw omission the effect on interface mechanical features was much less than expected (20%).

A number of implications can be postulated from these findings;

1. Subsequent to the omission of screws the level interface friction may remain at a level sufficient as to not alter the ability of the fixation, to maintain fracture reduction and stability.

2. The interface bone will benefit from a reduction in interface contact presumably resulting in less impairment to blood outflow through the cortex.

3. The use of fewer screws in a given construction may augment the inherent bone strain.

The effect on plate-bone interface mechanics of the symmetrical omission of screws was evaluated. This was accomplished using Fuji pressure sensitive film and provided an indication of interface contact area and average force. It was found that the omission of 40% of screws from a given plate significantly altered the interface contact area and the average force, however, the extent of these changes was influenced by the pattern of screw omission. This suggests that the selective omission of screws from a plate-bone construction may provide a means of limiting plate-induced osteopaenia during fracture healing, through a reduction in contact area and improved inherent bone strain.

Keywords

Internal fixation - osteopaenia - staged removal

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