It is agreed that if you have a disc replacement, it’s better than a fusion (in terms of motion). Movement of the spine is maintained with the implant (called a disc arthroplasty. But does a disc arthroplasty provide the same kind of spinal movement (called kinematics) as the normal, natural spine? That’s what this study was intended to find out.
The authors conducted a systematic review (collecting all the published materials on this topic) and a meta-analysis (combined all the data together). They compared the results of a disc arthroplasty with anterior cervical disc removal and fusion (called an anterior discectomy and fusion or ACDF). The cervical spine refers to the neck.
The main measure of results or outcomes was change in movement at the spinal level next to (adjacent) the implant. Kinematic (movement) measurements were made by looking at overall (global) neck range of motion. Special flexion-extension X-rays were used to measure changes in cervical spine motion.
They looked at the motion at the spinal segments above and below the arthroplasty and the fusion site. There was interest in knowing if either of these adjacent areas would end up with increased stress, force, and motion.
They also looked at movement of the vertebra (spinal bone) forward over the vertebra below it. This motion is called anterior translation. And they studied changes in the center of rotation of the discs and vertebra above and below the sites of fusion or arthroplasty.
There really wasn’t any significant difference in motion at the adjacent levels between the two groups (arthroplasty versus fusion). The studies followed patients for up to two years after the surgery. Longer-term results might show a difference but this wasn’t evident in the short-term.
Likewise, there were no measurable differences in center-of-rotation or anterior translation motion between the two groups in the first two years. Observing center-of-rotation motion gives us an idea of the quality (not just quantity) of motion. And that’s important because early joint degeneration leading to arthritic changes can occur when the center-of-rotation is off. Uneven load on the spinal joints and increased pressure through the disc can occur when there is a shift in the vertebral center-of-rotation.
What they did notice was a change in alignment in the group with the disc replacement. The cervical spine became more lordotic (backwards spinal curve). But even with this change, the overall (global) neck motion stayed the same among all the patients no matter what kind of treatment they had. Small changes in global motion in both groups were attributed to decreased pain, which allowed for increased neck function.
In conclusion, studies have already shown that compensatory motion occurs at adjacent spinal levels after spinal fusion. This increase in motion is accompanied by other effects such as increased pressure on the discs, a shift in the center of rotation, and increased vibrational stress on the spine.
Total disc replacement helps avoid these effects. But as this study shows there is no major difference in kinematics at the adjacent levels after disc replacement compared with spinal fusion. There were some significant changes in alignment after disc replacement as described above (i.e., increased lordosis in disc replacement group).
The authors offer their suggestions for future studies including: 1) longer follow-up time (more than two years), 2) compare results based on different implant designs, 3) study effects of muscular contraction, pain, and pain relief on results, and 4) effect of ever changing technology in these procedures and their effect on spinal biomechanics.