Cervical degenerative disc disease (CDDD) results from degeneration of cervical intervertebral disc(s) and/or the adjoining vertebral bodies. This causes clinical symptoms of cervical myelopathy, radiculopathy or myeloradiculopathy. Surgical treatment can be an option if non-surgical treatment options provide insufficient relief. The standard surgical technique for treating single or multilevel CDDD is anterior cervical discectomy, either without (ACD) or with fusion (ACDF). Both ACD and ACDF have good short-term clinical results in 90-100% of patients. Both techniques also have a high rate of fusion, respectively 70-80% and 95-100% 1. After 7-20 years, patient satisfaction slowly drops to 68-96% 2. The reason for this decline is thought to be due to the development of adjacent segment disease (ASDis). This is defined as the development of new complaints of radiculopathy or myelopathy due to degeneration one level above or below the previously operated segment. This occurs in approximately 25% of patients during 10 years follow-up and more than 2/3 of these patients need additional surgery 3. The underlying mechanism is thought to be compensation of loss of motion in the fused segment, resulting in overstraining of the adjacent segments 4. Anterior cervical discectomy with arthroplasty (ACDA) is developed in an effort to reduce the incidence of ASDis by preserving physiological motion in the operated segment.

Although the term 'physiological motion' is commonly used, a proper definition has been lacking for a long time. Segmental range of motion (sROM, e.g. the amount of sagittal rotation in a segment between maximal flexion and maximal extension position of the entire cervical spine) is most commonly used to study motion. SROM, however, suffers from large intra- and interindividual variability 5,6. Therefore, in the lower cervical spine, Boselie et al have recently described a consistent sequence of segmental contribution in sagittal rotation during flexion and extension in 80-90% of 20 healthy participants7,8. The normal sequence of segmental contributions in the lower cervical spine during the second half of extension was C4-C5 followed by C5-C6, and then C6-C7.

Moreover, Boselie et al performed a randomized controlled trial (RCT) to compare the presence of this physiological motion pattern and clinical outcomes for ACDA (n=12) and ACD (n=12) patients. Before the randomization 3 patients were operated in a pilot group (all with ACDA), so in total 27 patients with CDDD and radiculopathy were operated. In both groups 10 patients were available for follow-up at one year and fusion rate was 0% in the ACDA group and 70% in the ACD group. The majority of patients in the ACDA group (80%) showed a normal sequence of segmental contribution, which was not the case in the ACD group. This is the first method described which can reliably differentiate between normal or abnormal movement of the cervical spine in an individual subject. There were no differences in patient reported outcome measures, however the study population was small. These data have been submitted but not been published yet since follow-up duration is considered too short for most journals. The primary goal when performing the study was to analyze the sequence of segmental contribution of motion for which a follow-up duration of one year seemed appropriate. However, the expected advantage of the ACDA lies in the long-term, since it should lead to less ASDis by preserving physiological motion in the operated segment. Therefore, longer follow-up is needed to be able to determine if this physiological motion pattern remains present in the ACDA group at the long term. In the RCT performed by Boselie et al, the first patient was operated in December 2007 and the last patient in September 2014. At this moment the follow-up duration is therefore 6 to almost 13 years. By using the same method for analyzing the sequence of segmental contribution of motion in the cervical spine 8, the investigators want to analyze these 27 patients again in the long-term (at an average follow up of 9 years).

The investigators hypothesize that normal motion of the cervical spine will also be preserved by ACDA at the long-term, whereas ACD shows a decline in motion due to the induced fusion at the operated segment. As a consequence, the investigators expect ACDA to induce a lower risk on developing ASDis and thus re-operations.