Improved Design of Artificial Disc TestedArtificial disc replacement (ADR) has become a way to replace a degenerated disc without losing motion. Young patients need an implant that will hold up under constant loads for many years. In this study, two ADR designs are compared. The fixed-core and the mobile-core lumbar ADR.
Forces through the vertebrae and the discs between the vertebrae affect the load placed on the face joints of the spine. The facet joints can generate pain in the lumbar spine so increased loads there are important. Surgeons want to find the best placement of the ADR to avoid shear force and stress on the facet joints.
The fixed-core implant is a ball and socket design. The vertebral body moves around a fixed center of rotation (the ball). A mobile-core implant has a moving center of rotation. This is created by two plates that move around a central core.
It appears that the type of ADR and where it's placed inside the disc space may effect motion and loading at that segment. A 3-D computer model was created to test this theory and find out more about the best placement for ADRs. The model was of the L45 lumbar spinal segment.
The geometric center of the disc nucleus (center part) was used for all implant placements. Viewed from the side, this is located two millimeters (2 mm) behind the center of the vertebral body. The load placed on the facet during motion using this placement with both types of ADRs was calculated. All tests were repeated with the implant misplaced (four mm behind the center instead of two mm).
The authors report that the mobile-core ADR can be put anywhere in the disc space and still decrease the load on the facet by at least 50 per cent. The same was not true for the fixed-core implant. When placed in the center, the fixed-core ADR increased the load by 10 per cent. A misplaced fixed-core implant increased the load by 40 per cent.
The results of this study suggest that the mobile-core implant may be more forgiving than the fixed-core when misplaced. In other words, errors in placement may not be such a problem with the mobile-core ADR compared to the fixed-core model. Long-term studies must be done to confirm what happens over time. When it comes to reducing load on the facet joints, this study shows an advantage of the mobile-core ADR.
Missoum Moumene, PhD, and Fred H. Geisler, MD, PhD. Comparison of Biomechanical Function at Ideal and Varied Surgical Placement for Two Lumbar Artificial Disc Implant Designs. In Spine. August 1, 2007. Vol. 32. No. 17. Pp. 1840-1851.
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