Effects of a new shape-memory alloy interspinous process device on pressure distribution of the intervertebral disc and zygapophyseal joints in vitro

Objective:  To quantify the pressure distribution of lumbar intervertebral discs and zygapophyseal joints with different degrees of distraction of the interspinous processes by using a new shape-memory interspinous process stabilization device, and to research the relationship between changing disc and zygapophyseal joint loads and the degree of distraction of interspinous processes, and thus optimize usage of the implant. Methods:  Six cadaver lumbar specimens (L2–L5) were loaded. The loads in disc and zygapophyseal joints were recorded at each L3-4 disc level. Implants with different spacer heights were then placed by turn and the pressure measurements repeated. Results:  An implant with 10 mm spacer height does not significantly share the load. A 12 mm implant reduces the posterior annulus load, and meanwhile decreases the zygapophyseal joints pressure, but only in extension. A 14 mm implant shares the loads of posterior annulus, nucleus, and zygapophyseal joints in extension and the neutral position, but slightly increases the anterior annulus' load. Though 16–20 mm implants do decrease the loads in the posterior annulus and zygapophyseal joints, the anterior annulus' load was apparently increased. Conclusion:  Different degrees of distraction of the interspinous processes lead to different load distribution on the intervertebral disc. The implant tested is not appropriate in cases of serious spinal stenosis because of the contradiction that, while over-distraction of the interspinous processes decreases the posterior annulus and the zygapophyseal joints load and distracts the intervertebral foramina, it leads to a marked increase in the load of the anterior annulus, which is recognized to accelerate disc degeneration.