Neck Fusion: Which Way Do We Go?

When it comes to fusion of the cervical spine (neck), there are now many ways to surgically fuse one or more segments. Surgeons are interested in knowing which method provides stability without significant changes in biomechanics.

In this study from Italy, four different fixation (fusion) methods were compared. The research was done in a lab using four simulation models of the C4-C7 vertebral segments. Using models is the first step in this type of research before performing similar testing on humans.

The four models used included 1) stand-alone cage, 2) cage with anterior locking plate, 3) cage with an anterior dynamic plate, and 4) dynamic combined plate-cage device. The models were made to test the segments right after surgery (before bone fusion actually takes place).

The authors describe in detail the surgical methods used to implant each cage and/or plate device. Removal of the discs, preparation of the surrounding ligaments, type of screws used, and preparation of the vertebral body were all considered in the experiment.

All four models used the same type of intervertebral cage made of polyetheretherketone (PEEK; also known as polyketones). This material is an elasticized plastic to give support with some give. Plates used to supplement the cages were made of titanium. Locking plates do not allow motion between the various parts of the device. Dynamic locking plates use a nonbonding contact between the screws and plate and between the surface of the plate and the vertebral bones.

Tests were done by exposing each model to a compressive load. The load was applied in four different directions (flexion, extension, side bending, and rotation). Range-of-motion was measured for each trial at the level of the fusion and the adjacent levels (segments above and below). The researchers also measured how much load was placed on the cage and plate.

They found that the cage with the locking plate had the best ability to prevent motion in all directions at the fused levels. Dynamic plates reduce, but do not stop, motion at the fusion site. The locking plate was able to bear the most load (90 per cent compared to 40 per cent for the dynamic plate).

Amount of motion detected with each model varied. Motion was the greatest with the stand-alone cage. The dynamic plate-cage device (model number four) was the least stable of the combined options. The authors think this may be because this model only has two screws to hold it in place (compared with four screws in the cage with locking or dynamic plate).

All four models showed elevated pressure where the devices come in contact with the upper endplate of the vertebral segment. The endplate is a transition area of cartilage between the intervertebral disc and the next vertebral bone. The stand-alone cage put the most pressure in the endplate area. This is a concern because of subsidence (cages sinking down into the endplate). Cages used with plates seem to spread out the pressure more evenly across the entire endplate.

The authors conclude that even though they could measure differences in the biomechanics of the fused spine, differences in load sharing between the four models, and differences in motion and stability, these are only models. Actual clinical trials with patients randomly placed one of these four groups for comparison is the next step. There is no question that single-level fusions work well. The focus now is really on success and stability of multiple segmental fusions.

Fabio Galbusera, MSEng, et al. Anterior Cervical Fusion: A Biomechanical Comparison of 4 Techniques. In Journal of Neurosurgery: Spine. November 2008. Vol. 9. No. 5. Pp. 444-449.

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