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Description
Adolescent idiopathic scoliosis is a complex three-dimensional deformity of the spine where there is increased loading on the concave side. Fusionless spinal growth modulation strategies such as intervertebral stapling attempt to reverse the imbalance of forces while limiting curve progression and preserving spinal motion. The objective of this study was to characterize the thermo-mechanical behavior of nitinol staples, a shape-memory alloy currently utilized in medical staples, to evaluate the force generated by the staples on the vertebrae, and to measure the forces on implanted staples in motions in an ex vivo and in vitro set up on immature porcine model. A total of three separate experiments were conducted to characterize the behavior of the staple in the study. First, the effect of temperature (heating) on staple force was investigated using a strain gauge attached to the surface of the staple to record changes in strain as the staple was heated. Nitinol is a unique material in that it exhibits both shape-memory and superelasticity characteristics which allows factors such as temperature and stress to affect its mechanical properties. The second experiment investigated the mechanical characteristics of nitinol through temperature-controlled, ex vivo mechanical testing of the staple. Strain-gauge instrumented staples were mounted in a servo-hydraulic testing apparatus and subjected to a constant load as strain was recorded. Lastly, the force generated by the staples due to the shape-memory effect was measured. Two staples were first stretched to different initial displacement lengths and then heated while the subsequent compression force generated was measured. In addition to these ex vivo experiments, staples were instrumented into post-mortem porcine spine specimens and subjected to in situ testing with spinal motions in an in vitro setting. The forces on the staple were measured during these tests and the effect of stapling on spinal biomechanics was assessed. A notable change in strain on the staple was found at 30°C, which is below the critical threshold of 37°C (body-temperature), indicating that the staple will exhibit the proper shape-memory effect characteristics vital to its use in the body. In the study where the force generated by the staple was measured, the average peak force was found to be 24.7 N, correlating to 5-15% of body weight for a growing adolescent. For testing in porcine spines, a significantly lower force obtained from the strain gauge was seen in lateral bending towards the staple (0.46 N/deg) compared to away from the staples (3.43 N/deg). In flexion (2.44 N/deg) and extension (2.92 N/deg), similar forces were observed, with little difference in force noted in axial rotation towards (0.50 N/deg) and away from the implanted staples (0.48 N/deg). These results suggest that intervertebral stapling might be able to significantly alter the biomechanics of the spine as evidenced by the level of force generated by the staple when measured ex vivo as well as the smaller staple force observed in motions directed towards the implanted staple.
