PRELIMINARY DESIGN AND EXPERIMENTAL STUDIES OF A NOVEL SOFT IMPLANT FOR CORRECTING SAGITTAL PLANE INSTABILITY IN THE LUMBAR SPINE

Study Design. A novel soft implant design for resisting the instabilit y of the lumbar spine in the sagittal plane was mechanically tested. O bjectives. To ascertain whether a soft preformed implant made of diffe ring grades of silicone would contribute to stabilizing the lumbar spi ne in the sagittal plane. Summary of Background Data. Methods of stabi lizing the lumbar spine in patients who present with chronic low back pain have usually concentrated on rigidly fixing the associated segmen t. This has many inherent problems with both the surgical methods and the long-term rigidity at and away from the stabilized site. To the au thors' knowledge, no ''soft'' interspinous spacer that would allow a c ertain amount of flexion but still stabilize the movements associated with instability at the level of the lesion has been investigated mech anically as an alternative to rigid fixation or prosthetic replacement . Methods. The apparatus was designed to allow a cadaveric lumbar moti on segment to be tested in compression at four angles of flexion with loads up to 700 N. The intradiscal pressure and sagittal plane stiffne ss were recorded during loading, with and without Various sizes of the soft silicone implants placed between the spinous processes. Results. Insertion of the silicone implants between the spinous processes redu ced the intradiscal pressure under load at the angles of flexion teste d. The size of the interspinous space determines the optimal diameter of the implant that afforded sagittal stability, the load-bearing cont ribution of the implant, and the prevention of disc space narrowing at the level investigated. Conclusions. A circular silicone spacer place d between the spinous processes appears to contribute to the stability of the cadaveric lumbar spine. There are many attractions to using a simple, soft implant that can be placed with minimal surgery between t he spinous processes.