Failure strength of lumbar spinous processes loaded in a tension band model: Laboratory investigation

Object. There has been increasing interest in spinous process tension band devices, as distinct from spinous process spacers and plates. The purpose of this study was to load spinous processes caudally at L-4 and cranially at L-5 parallel to the long axis of the spine in a biomechanical model of tension band loading. The goal was to provide normative data for the design of a spinous process tension band device after varying degrees of surgical decompression and across varying bone mineral densities (BMDs). Methods. Fresh-frozen L4-5 lumbar vertebrae pairs were divided into 3 surgical groups: intact, midline-sparing decompression (laminotomy and medial facetectomy), and midline decompression with foraminotomy (one-half of spinous process resected, laminotomy, and medial facetectomy). After decompression, specimens were disarticulated into isolated L-4 and L-5 vertebrae. Each vertebra was loaded to failure in a caudal (L-4) or cranial (L-5) direction parallel to the long axis of the spine via a 6-mm-wide strap looped around the spinous process. Failure strength and mode were recorded. Results. Seventeen L-4 and L-5 lumbar vertebrae were tested from 17 cadavers. There were 10 male (59%) and 7 female (41%) cadavers, with a mean age of 66.6 ± 16.5 years (range 41-100 years) and a mean BMD of 1 ± 0.23 g/ cm ² (range 0.66-1.34 g/cm ²); the mean is expressed ± SD throughout. For data analysis, specimens were grouped into those with no or midline-sparing decompression (Group 1: 11 of 17) and those with midline decompression (Group 2: 6 of 17). At L-4, the mean failure strength for Group 1 was 453 ± 162 N, and for Group 2 it was 264 ± 99 N (p = 0.02; Cohen's d = 1.4). At L-5, the mean failure strength for Group 1 was 517 ± 190 N, and for Group 2 it was 269 ± 184 N (p = 0.02; Cohen's d = 1.3). There was no significant difference in failure strength between the intact and midline-sparing decompression groups at L-4 (p = 0.91) or L-5 (p = 0.41). Conclusions. Across specimens with a wide range of BMDs, midline-sparing decompression was not found to decrease the mean failure strength of the L-4 and L-5 spinous processes (453 and 517 N, respectively), whereas midline surgical decompression decreased the failure strength of these processes (264 and 269 N, respectively) in a biomechanical model of tension band loading relevant to the design of a tension band device.