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Boeing Test Model Used to Measure Results of Spinal Fusion

Many people with damaged and aging discs have pain from lumbar instability. Fusion is one way to treat this problem. Doctors know that the stress on the disc above or below the fusion often leads to disc damage at those levels. In this study, doctors in Japan used a special method to measure stresses in the disc above the fusion.

The finite element method (FEM) was used. The FEM was first used in 1956 by the Boeing Company to test the strength of new airplane wings. In this study, the FEM was used to measure the load on adjacent discs after lumbar spinal fusion. Three loads were applied to the level above the fusion: 1) compression load; 2) flexion load; and 3) extension load.

Spines with two different types of fusion were tested. One group had a posterolateral fusion (PLF). The PLF takes pressure off the spinal nerve by removing the disc and using bone graft across the side and back of two or more vertebrae. The second type of fusion was the posterior lumbar interbody fusion (PLIF). In this type of fusion, the problem disc is removed from the back of the spine and replaced with bone graft.

Stress was measured in two places: the vertebral endplate and the annulus. The endplate is the thick cartilage between the disc and the next vertebra. The annulus is the ligament ring around the disc.

The authors report a large increase in stress on both areas for both types of fusion methods. They suggest that fusing a spinal segment when the next segment is already degenerating may not be in the best interest of the patient. It appears that in cases of adjacent disc degeneration, the risk of increasing instability of the spine is greater with a fusion than without.

References: Etsuo Chosa, et al. Analysis of the Effect of Lumbar Spine Fusion on the Superior Adjacent Intervertebral Disk in the Presence of Disk Degeneration, Using the Three-Dimensional Finite Element Method. In Journal of Spinal Disorders. April 2004. Vol. 17. No. 2. Pp. 134-139.

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