Robert Charles Mulholland

Fulcrum assisted soft stabilization system: a new concept in the surgical treatment of degenerative low back pain.

Study Design. An experimental study on cadaver spine and spine model for biomechanical evaluation of a novel dynamic stabilization device. Objectives. First, to test the hypothesis that in dynamic stabilization of a lumbar spine using pedicle screws and ligament, addition of a fulcrum in front of the ligament can unload the disc. Second, to determine the relationship between the length and stiffness of the fulcrum and the ligament on disc unloading, lordosis and motion preservation. Summary of Background Data. Activity related low back pain may be attributable to abnormal disc loading or abnormal movement. Spinal fusion addresses both the mechanisms, but it has limitations. Soft stabilization with Graf ligament restricts abnormal movement but increases the disc pressure. The Dynesys system uses a plastic cylinder around the ligament to prevent overloading the disc, but it restricts extension and loses lordosis. Methods. A novel dynamic stabilization system (fulcrum assisted soft stabilization or FASS) was developed in which a flexible fulcrum was placed in front of a ligament between the pedicle screws. It was hypothesized that the fulcrum should transform the compressive force of a ligament behind into a distraction force in front and unload the disc. Three spine models were developed using wooden blocks for vertebral bodies and neoprene rubber of different hardness for disc. Their load-deformation character was tested and compared with that of the cadaver spine in a spine tester. The spine model with the closest load-deformation property to cadaver spine was then tested for the effect of a FASS system, consisting of high density polythene rod as fulcrums and rubber “O” rings as ligaments. The disc pressure in the spine models were recorded with strain gauge in the center. Results. Application of ligaments alone across the pedicle screws increased the disc pressure, produced a lordosis, and reduced the range of motion. Application of fulcrums reduced the disc pressure and maintained thelordosis. Increasing the fulcrum length resulted in progressive unloading of the disc but increased stiffness of the motion segment. As the fulcrum length approximatedthe height of the motion segment, the lordosis was lost, and the disc was completely unloaded. Decreasing the lateral bending stiffness of the fulcrum had minimal effect on disc unloading and motion-segment stiffness. Conclusion. The novel FASS system can unload the disc, control the range of motion, and maintain lordosis. These parameters may be controlled with a suitable combination of ligament and fulcrum system. The study provides an indication toward the desirable biomechanical properties of the fulcrum and ligament for future development of a clinically applicable prototype.

Functional anatomy of the deer spine: An appropriate biomechanical model for the human spine?

The object of this study was to create a database for the biomechanical and certain functional anatomical parameters of the deer spine, for comparison with the human spine. This was done with a view toward using the deer spine as an alternative model for various biomechanical experiments, as it is difficult to procure nonembalmed, fresh human spine specimens. Bovine spongiform encephalopathy (BSE) and its human variant, Creutzfeld Jakob disease (CJD), prevent us from using bovine and sheep spine. There is a risk of transmission of disease through direct inoculation to the researcher working with infected bovine or sheep spine, and a theoretical possibility of transmission through the food chain if proper precautions for specimen disposal are not taken. We chose deer spine as an alternative for testing nonembalmed fresh human spine because, to date, there have been no reported cases of deer being carriers of prion diseases. Fifteen deer spine specimens were sectioned appropriately to obtain six functional spinal units for each level in the thoracic and lumbar spine. Each unit was tested in a Dartec materials testing machine (Dartec Ltd., Stourbridge, UK) under pure moments in three main anatomical planes. The range of motion (ROM), neutral zone (NZ), and stiffness parameters of the functional unit were determined in flexion‐extension, right/left lateral bending, and axial rotation. The data obtained were compared with the corresponding human spine data in the literature. Deer spine specimens were also studied for bone mineral density (BMD) using a DEXA scan. The results revealed the overall ROM was greater for deer spine compared to the human spine in the upper thoracic region, but less compared to human spine in the lower lumbar spine region. The only comparable region for ROM was in the lower thoracic/upper lumbar region. The stiffness coefficients were also comparable in this region. The BMD was also comparable in the two species. We conclude that the lower thoracic/upper lumbar region in the deer spine can be used as a model for some human biomechanical experiments because of its biomechanical and material similarities to the human spine of the corresponding region. Anat Rec 266:108–117, 2002.

Biomechanical evaluation of immediate stability with rectangular versus cylindrical interbody cages in stabilization of the lumbar spine

BackgroundRecent cadaver studies show stability against axial rotation with a cylindrical cage is marginally superior to a rectangular cage. The purpose of this biomechanical study in cadaver spine was to evaluate the stability of a new rectangular titanium cage design, which has teeth similar to the threads of cylindrical cages to engage the endplates.MethodsTen motion segments (five L2-3, five L4-5) were tested. From each cadaver spine, one motion segment was fixed with a pair of cylindrical cages (BAK, Sulzer Medica) and the other with paired rectangular cages (Rotafix, Corin Spinal). Each specimen was tested in an unconstrained state, after cage introduction and after additional posterior translaminar screw fixation. The range of motion (ROM) in flexion-extension, lateral bending, and rotation was tested in a materials testing machine, with +/- 5 Nm cyclical load over 10 sec per cycle; data from the third cycle was captured for analysis.ResultsROM in all directions was significantly reduced (p < 0.05) with both types of cages. There was no significant difference in reduction of ROM in flexion-extension (p = 0.6) and rotation (p = 0.92) between the two cage groups, but stability in lateral bending was marginally superior with the rectangular cages (p = 0.11). Additional posterior fixation further reduced the ROM significantly (p < 0.05) in most directions in both cage groups, but did not show any difference between the cage groups.ConclusionsThere was no significant difference in immediate stability in any direction between the threaded cylindrical cage and the new design of the rectangular cage with endplate teeth.