Author: James B. O'Donnell

Publisher:

Published: 2010

Total Pages: 134

ISBN-13:

Fusion is one of the most commonly performed surgeries in treatment for chronic, severe, low back pain. The current standard is to implant intervertebral cages with bone graft and supplement with posterior fixation using pedicle screws and rods to provide stability until the fusion takes place. This study is divided into two parts: an in vitro study testing six human cadaver L1-S1 lumbar spines and a follow-up study investigating the same conditions on an L1-S1 finite element model. The hypothesis is that for the devices tested, the Aspen spinous process fixation system provides at least the same level of stability to the spine as a pedicle screw and rod construct whenimplanting three consecutive levels of the lumbar spine. The first part of the study was an in vitro study of six human cadaver L1-S1 lumbar spines in which the Aspen spinous process fixation system was compared to a pedicle screw and rod fixation system for fusion. Each spine was tested intact, implanted with intervertebral cages at L2-L3, L3-L4, and L4-L5, cages supplemented with pedicle screws and rods from L2-L5, and finally cages supplemented with the Aspen spinous process system from L2-L5. For the intact case, moments of 10 Nm were applied in extension, flexion, lateral bending, and axial rotation. For the three instrumented cases, moments up to 15 Nm were applied according to the hybrid protocol. This states that loads shall increase until the range of motion of the instrumented spine reaches that of the intact spine. The second part of the study was a finite element analysis of the same cases. A previously validated L1-S1 spine model was used. Loads were applied in the same directions and steps as in the in vitro study. The implants were modeled and inserted into the spine model, modifying the intact spine accordingly to replicate the surgical procedure. Results were then compared between cases and between the two methods of study. The results show that the Aspen interspinous fixation system performs equally well to a pedicle screw and rod system in flexion and extension. The Aspen device allowed more motion in lateral bending and axial rotation than the pedicle screws and rods, but less than the intact spine or with intervertebral cages alone. The FE results are in agreement with the in vitro results. The pedicle screws and rods provide a more rigid construct, but the Aspen allows for more loading of the anterior column which may accelerate the fusion process.