The effect of an interspinous process implant on facet loading during extension.

STUDY DESIGN Facet loading parameters of lumbar cadaver spines were measured during extension before and after placement of an interspinous process implant. OBJECTIVE The study was undertaken to quantify the influence of an interspinous implant on facet loading at the implanted and adjacent levels during extension. SUMMARY OF BACKGROUND DATA Facet loading is increased during extension and decreased during flexion. Previous studies have demonstrated that interspinous process decompression relieves disc pressure at the implanted level and does not alter disc pressure at the adjacent levels. Facet joints are believed to play a key role in back pain, especially in patients with collapsed discs and increased motion segment mobility resulting in increased facet loading. METHODS Seven cadaver spines (L2-L5) were loaded to 15 Nm of extension and 700 N compression with and without an interspinous process implant (X STOP) placed between the L3-L4 spinous processes. Pressure-sensitive film was placed in the facet joints of the implanted and adjacent levels. After loading, the film was digitally analyzed for peak pressure, average pressure, contact area, and force. These values were compared between the intact and implanted specimens at the adjacent and implanted levels using a paired t test (P < 0.05). RESULTS The implant significantly reduced the mean peak pressure, average pressure, contact area, and force at the implanted level. The mean peak pressure, average pressure, contact area, and force at the adjacent levels were not significantly different between the intact and implanted specimens with the exception of contact area at the L2-L3 level. CONCLUSIONS Interspinous process decompression will unlikely cause adjacent level facet pain or accelerated facet joint degeneration. Furthermore, pain induced from pressure originating in the facets and/or posterior anulus of the lumbar spine may be relieved by interspinous pro-cess decompression. Clinical results from patients with a component of lower back pain suggest that this is a valid conclusion.