Investigator: Scott A. Yerby, PhD Project Staff: Derek P. Lindsey, MS and Jen Kreshak, BS Project Categories: Spinal Cord Injury / Osteoporosis - 2000 Although several spinal fixation systems utilize the spinous processes as a fixation point, there are very few reports on the strength of the spinous processes in relation to the in situ loads applied by the implants. The objective of this study was to measure the in situ loads of a novel interspinous process spacer and relate these implant loads to the failure loads of the spinous processes using a cadaver model. Eight L2-L5 human cadaver specimens were DEXA scanned in the anteroposterior direction and mounted in a spinal loading frame capable of applying pure bending moments in flexion and extension. The spacer, a thin-walled titanium alloy cylinder, was then placed in the interspinous process space between the 3rd and 4th lumbar vertebra. Each spacer was instrumented with two axial strain gages adhered to the inner wall, and calibrated to known loads. Once in the loading frame, each specimen was loaded to 7.5 Nm in flexion and extension for five cycles at a rate of 5 deg/sec. Spacer loads, bending angles, and bending moments were recorded at 20 Hz for all five cycles, and data were analyzed for the fifth cycle. Following testing in flexion and extension, each specimen was disarticulated into individual vertebrae, and the vertebral bodies and lamina of L3 and L4 were secured in bone cement. Each individual vertebra was placed in an axial loading frame. The spinous process of each L3 specimen was loaded along the caudal edge in the caudad/cephalad direction, and the spinous process of each L4 specimen was loaded along the cephalad edge in the cephalad/caudad direction until failure. In situ spacer loads were calculated as a percentage of the spinous process failure load, and the failure loads were correlated to the individual vertebral BMD. Differences between the mean spinous process failure loads of L3 and L4 specimens were analyzed using a paired t-test with a level of significance of 0.05. The mean maximum load of all spacers occurred in extension (mean: 109.5 N, sd: 65.3 N, range: 43.4 to 214.0 N) the mean minimum load of all spacers resulted from flexion (mean: 45.0 N, sd: 39.0 N, range: 3.5 to 106.2 N). The mean failure strength of the L3 spinous processes (mean: 1033 N, sd: 505 N, range: 498 to 2095 N) was significantly greater than the mean failure strength of the L4 spinous processes (765 N, sd: 374 N, range: 379 to 1472 N). During extension, the L3 spinous processes were loaded to a mean of 11.7% of their failure load (sd: 7.3%, range: 5.0 to 25.7%), and the L4 spinous processes were loaded to a mean of 16.3% of their failure load (sd: 11.4%, range: 4.7 to 40.7%). The difference between the L3 and L4 mean percent failure loads was significantly different. Finally, a significant regression between vertebral BMD and spinous process failure strength was established using a second order polynomial relationship (Fail Load=1136*BMD2, R2=0.388, p<0.01). With the increased interest in using the spinous process as a point of fixation for spinal instrumentation, the data presented in this study provides researchers and clinicians with a set of failure data by which decisions of patient selection, and construct integrity and strength can be based. Funding Source: St. Francis Medical Technologies |
