The effects of an interspinous implant on intervertebral disc pressures

Study Design. Measurement of intradiscal pressure was performed after placement of an interspinous implant in a cadaver model. Objective. To understand the likelihood of accelerated adjacent-level disc degeneration as a result of the implant. Summary of Background Data. An interspinous implant has been developed to treat lumbar neurogenic claudication secondary to spinal stenosis that places the stenotic segment in slight flexion and prevents extension. Previous biomechanical studies demonstrated that fusing one level may significantly increase the intradiscal pressures at adjacent levels. Moreover, clinical studies have reported an increased incidence of adjacent-level degeneration after lumbar spinal fusion. Methods. Eight cadaver lumbar specimens (L2-L5) were loaded in flexion, neutral, and extension. A pressure transducer measured intradiscal pressure and annular stresses during each of the three positions at each of the three disc levels. An appropriately sized implant was placed at L3-L4 and the pressure measurements were repeated. Results. The pressures at the adjacent discs were not significantly affected by the interspinous implant insertion. There was a significant decrease in intradiscal pressure at the L3-L4 disc in the posterior annulus and nucleus in the neutral and extended positions. Conclusions. The implant does not significantly change the intradiscal pressures at the adjacent levels, yet it significantly unloads the intervertebral disc at the instrumented level in the neutral and extended positions. On the basis of the current findings, it does not appear that the implant causes accelerated disc degeneration at the adjacent levels.