Background: The quality of flexion-extension motion after cervical disc arthroplasty has been a subject of interest due to the number of available devices with different designs and kinematics. Previous studies of motion quality have been limited to measuring the range of motion using two radiographs taken at the extremes of flexion and extension. This is the first study evaluating the live quality of index segment motion using fluoroscopic images collected over the arc of flexion-extension after M6-C cervical disc arthroplasty surgery.

Methods: Eligible participants had previously undergone a single-level cervical disc arthroplasty surgery for degenerative cervical spine disease performed by the senior author. Study participants underwent dynamic lateral fluoroscopic imaging to capture the C2-C7 motion between maximal flexion and extension. The amount of motion contribution by individual segments to the C2-C7 motion (termed segmental motion fraction) and its variation throughout the arc of flexion-extension were compared between the index and adjacent segments. The shift of centre of rotation during the arc of motion was also assessed.

Results: Ten subjects with a mean age of 43.8 years old were recruited, with an average follow-up of 16.2 months at the time of fluoroscopy. The C2-C7 cervical spine had an average flexion-extension range of 66.7 degrees. The contribution of the index segment averaged over the flexion-extension arc of motion was 18.9% (peak contribution 24.4%) for the C5-C6 group; and 15.5% (peak contribution 25.5%) for the C6-C7 group. The mean cranial-caudal location of the centre of rotation progressively shifted in the cranial direction from C2-C3 to C6-C7 motion segment.

Conclusions: Our results demonstrated physiologic quality of motion at the index segment and harmony among its neighbouring segments following cervical disc arthroplasty, without gearshift-like intermittent locking of the prosthesis during the arc of flexion-extension motion. This pilot study provides a basis for the design of future long-term studies with larger sample size to compare the quality of motion between different cervical disc prostheses using the concept of segmental motion fraction as a motion-quality metric.