The effect of vertebral body stapling on spine biomechanics and structure using a bovine model.

BACKGROUND: Adolescent idiopathic scoliosis is a common condition affecting 2.5% of the general population. Vertebral body stapling was introduced as a method of fusionless growth modulation for the correction of moderate idiopathic scoliosis (Cobb angles of 20-40°), and was claimed to be more effective than bracing and less invasive than fusion. The aim of this study was to assess the effect of vertebral body stapling on the stiffness of a thoracic motion segment unit under moment controlled load, and to assess the vertebral structural damage caused by the staples. METHODS: Thoracic spine motion segments from 6 to 8 week old calves (n=14) were tested in flexion/extension, lateral bending, and axial rotation. The segments were tested un-instrumented, then a left anterolateral intervertebral Shape Memory Alloy (SMA) staple was inserted and the test was repeated. Data were collected from the tenth load cycle of each sequence and stiffness was calculated. The staples were carefully removed and the segments were studied with micro-computed tomography to assess physical damage to the bony structure. Visual assessment of the vertebral bone structure on micro-CT was performed. FINDINGS: There was no change in motion segment stiffness in flexion/extension nor in axial rotation. There was a reduction in stiffness in lateral bending with 30% reduction bending away from the staple and 12% reduction bending towards the staple. Micro-CT showed physeal damage in all the specimens. INTERPRETATION: Intervertebral stapling using SMA staples cause a reduction in spine stiffness in lateral bending. They also cause damage to the endplate epiphyses.

The effect of repeated loading and freeze-thaw cycling on immature bovine thoracic motion segment stiffness.

There is growing interest in the biomechanics of "fusionless" implant constructs used for deformity correction in the thoracic spine; however, there are questions over the comparability of in vitro biomechanical studies from different research groups due to the various methods used for specimen preparation, testing and data collection. The aim of this study was to identify the effect of two key factors on the stiffness of immature bovine thoracic spine motion segments: (1) repeated cyclic loading and (2) multiple freeze-thaw cycles, to aid in the planning and interpretation of in vitro studies. Two groups of thoracic spine motion segments from 6- to 8-week-old calves were tested in flexion/extension, right/left lateral bending and right/left axial rotation under moment control. Group A was tested with continuous repeated cyclic loading for 500 cycles with data recorded at cycles 3, 5, 10, 25, 50, 100, 200, 300, 400 and 500. Group (B) was tested after each of five freeze-thaw sequences, with data collected from the 10th load cycle in each sequence. Results of testing showed that for Group A: flexion/extension stiffness reduced significantly over the 500 load cycles (-22%; p = 0.001), but there was no significant change between the 5th and 200th load cycles. Lateral bending stiffness decreased significantly (-18%; p = 0.009) over the 500 load cycles, but there was no significant change in axial rotation stiffness (p = 0.137). Group B: there was no significant difference between mean stiffness over the five freeze-thaw sequences in flexion/extension (p = 0.813) and a near-significant reduction in mean stiffness in axial rotation (-6%; p = 0.07). However, there was a statistically significant increase in stiffness in lateral bending (+30%; p = 0.007). Study findings indicate that comparison of in vitro testing results for immature thoracic bovine spine segments between studies can be performed with up to 200 load cycles without significant changes in stiffness. However, when testing protocols require greater than 200 cycles, or when repeated freeze-thaw cycles are involved, it is important to account for the effect of cumulative load and freeze-thaw cycles on spine segment stiffness.