Goal of the Study?
In this study 1 the authors explore the impact of cervical implant height on facet joint pressure and range of movement (ROM). They hypothesize that a higher artificial disc would result in greater facet pressure and lower ROM and should not be used in total cervical disc replacement (TDR).
Why are they doing this study?
Increasingly, total cervical disc replacement (TDR) is being used in clinical practice as it has the ability to maintain the biomechanical state of the cervical spine, ROM and has an accelerated rehabilitation process. However, there is no clear consensus on how to choose the most appropriate height of prosthesis. Some research suggests that a higher artificial disc is better for relieving neurological deficits. Other research suggests that lower (smaller) implants provide a closer to normal biologic function than higher implants. To date, there has been no research that reports on the biomechanical impacts of different artificial disc heights on facet joints during TDR.
What was done?
This is an in vitro biomechanical study using six fresh-frozen cadaveric cervical spines (C2-C7) with 5mm intervertebral disc height at C5/6. Specimens with flaws, including fractures, deformities, tumours and other injuries, were excluded. Biomechanical testing was done with intact specimens first, and then implants with different heights were inserted. These were broken down into 4 groups: 1. Intact specimen; 2. 5mm insert; 3. 6mm insert; 4. 7mm insert. Facet joint pressure and range of motion (ROM) for each group were recorded.
What did they find?
Overall, the researchers found that a 7mm prosthesis resulted in significantly lower ROM and increased facet joint loading compared with specimens of other implant heights. For example, facet joint pressure at the index level increased by 77% during flexion, 53% during extension and 40% during lateral bending. In comparison, specimens with 5mm implant height had a similar ROM and facet joint pressure as intact specimens. Additionally, they found that facet joint pressures increased with implant heights, with the most significant pressure during flexion for implants of 7mm.
While the authors acknowledge that a higher artificial disc could enlarge the volume of the intervertebral foramen and relieve neurological symptoms, it also has the potential to increase the risk of post-surgical complications such as arthritis and neck pain. Therefore, they suggest that a prosthesis with 2mm height than normal should not be used in TDR.
Why do these findings matter?
As TDR is becoming more common in clinical practice, it is critical that patients and their health care professionals chose an appropriately sized artificial disc. This research illustrates the importance of choosing an appropriate artificial disc height to achieve near-normal biomechanical outcomes. In particular, this study provides evidence to support the choice of a smaller implant ( ≥1mm) to achieve almost normal ROM and facet joint loads.