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Changes in Discs and Facet Joints After Total Disc Replacement
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Posted on: 11/30/1999
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Total disc replacement (TDR) is possible now for some patients in place of spinal fusion. Two of the disadvantages of spinal fusion are loss of motion at the fused segment and increased risk of adjacent level degeneration.
We know that disc replacements restore spine motion. But do they prevent accelerated degeneration at the level of replacement and at the next level segment above or below? That is the question that researchers attempted to answer with this study. It is the first study to compare degenerative changes in the discs and facet joints before and after TDR.
Patients with disc pain from degenerative disc disease were included in the study. One surgeon inserted a lumbar total disc replacement (TDR) in 46 patients. The ProDisc II device was used. The ProDisc is one of two types of total disc replacements. The other is called the CHARITÉ. Both types of implants are metal-on-polymer. The ProDisc is constrained in front-to-back and side-to-side shear but moves freely in twisting or rotational patterns. Reducing the shear force decreases the load on the facet joints.
Each patient was followed for at least two years. Results were measured first by comparing dynamic X-rays, MRIs, and CT images before and 26 months (or more) after surgery. Any changes in the discs or joints were compared with patient symptoms, range of motion, and function.
The researchers also looked at patient age, gender, height, weight, and vertebral level operated on. They also studied the relative size and position of the implant. They found improved pain levels and motion (flexion and extension) at the operated segment. There was very little sign of degeneration of the adjacent (next level) discs.
Facet (spinal) joint degeneration referred to as progression of facet arthrosis (PFA) was also evaluated. PFA usually occurs along with loss of disc height. As the discs degenerate, there is a loss of height or cushion between the vertebral bodies. This puts more pressure on the facet joints and creates changes such as bone spurs or hypertrophy (overgrowth) of the joint capsule.
It appears that PFA was more commonly seen in patients when disc replacements were placed at more than one level. PFA also occurred more often when the implant was placed off center. Women were affected more often than men. The reason for the gender difference is unknown at this time.
The most striking finding in this study was that disc and joint changes were fairly minimal at the adjacent segments. But changes were present in about one-third of the patients at the level of the TDR (also known as the index level). At the same time, there was improved pain, function, and quality of life even with the joint changes at the index level.
The authors point out two design flaws in their study. First, there was no control group (patients with degenerative disc disease who did not receive treatment). It's possible that what they saw with the group in this study was nothing more than what would have happened anyway. That's referred to as the natural history of a condition. And secondly, the sample size was fairly small. This same study should be repeated with a larger group of patients, including a control group.
For best results, surgeons are advised to make sure the implant is placed at the middle of the vertebral body on the frontal plane. This is especially important when TDRs are placed at more than one level at the same time. Malpositioning on the frontal plane puts an uneven load on the implant and on the facet joints. This can create further degeneration of an already compromised joint.
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References:
Chun-Kun Park, MD, PhD, et al. Degenerative Changes of Discs and Facet Joints in Lumbar Total Disc Replacement Using ProDisc II. In Spine. July 15, 2008. Vol. 33. No. 16. Pp. 1755-1761.
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