Abstract
Introduction: The lordosis of the lumbar spine, flexion angle and body weight result in significant shear forces through the lumbar and lumbosacral disc spaces. These shear forces result in translational motion across the disc space, which is resisted but not completely abolished by pedicle screw stabilisation. Failure of lumbar interbody fusions through non-union may be related to translational micromotion at the vertebral endplate / bone graft interface. A porcine in vitro model was established to test whether variations in the design of inter-body implants and in particular, the presence of surface serrations would assist in resisting shear forces – especially those causing anterior translation.
Methods: Measurements of anterior vertebral translation were recorded on porcine cervical spine segments, subjected to 25 N antero-posterior shear load while under a 300 N compressive pre-load. Baseline testing was firstly performed on the intact specimens and following removal of the facet joints. The annulus, disc nucleus and cartilaginous endplates were then removed and the specimens were divided into two groups for testing using interbody implants. Four stainless steel blocks measuring 15 mm (length) × 5 mm (height) × 4 mm (width) were manufactured to act as intervertebral disc spacers. Two were made with smooth surfaces and two were made with 1 mm deep serrations on the upper and lower surfaces. One group was tested with two smooth and one with two serrated implants.
Results: Under 25 N shear load, the specimens tested with the serrated implants showed anterior vertebral translation of 0.046 ± 0.013 mm while those tested with the smooth surfaced implants measured 0.152 ± 0.075 mm (p < 0.01). A significant difference was also found between the stiffness of the specimens implanted with smooth surfaced (432.8 N/mm) and serrated (1088.4 N/mm) implants (p < 0.01). The value for peak load at failure for the specimens with smooth surfaced implants (150.43N) was less than those implanted with serrated implants (175.48 N), but not significantly different.
Discussion: The presence of surface serrations on the interbody implants significantly increased the resistance to shear forces in this model. In the clinical setting, we postulate that the degree of micromotion generated by anterior shear forces at interbody fusion sites should be substantially less when serrated implants are used and reduce the incidence of non-union. This may explain the improved fusion rates reported by contemporary authors when using some interbody implants. Further research is needed to clarify the combined effects of pedicle screw stabilisation and interbody implants upon shear displacement and variations in implant design.
The abstracts were prepared by Dr Robert J. Moore. Correspondence should be addressed to him at The Spine Society of Australia, Institute of Medical and Veterinary Science, The Adelaide Centre for Spinal Research, Frome Road, Adelaide, South Australia 5000
