There is controversy on the ideal treatment modality for mCDD.11 The frequency of mCDD increases with age, more than 85% of the population older than 60 may suffer from severe degeneration at least at one cervical level. In the light of current literature, HC seems to be a safe and feasible treatment alternative for some patients with mCDD scheduled for operation using the anterior approach.12,13
Even though posterior cervical decompression, including laminectomy and laminoplasty and posterior instrumentation are effective for the achievement of backward mobility of the spinal cord, C5 nerve root palsy and axial neck pain constitute the major disadvantages. The advantages of anterior decompression are the direct removal of the lesion, including removal of osteophytes of the posterior longitudinal ligament, and correction of the cervical alignment.14
Clinical and experimental trials yielded that fusion of cervical segments can remarkably expand the ROM of adjacent segments and intradiscal pressure, in this way increasing the risk of adjacent segment disorders particularly at the levels around the fusion.15 In cadaver models, intradiscal pressures adjacent to a fused level was found to be as much as 73%.16
The spondylotic spine is mostly linked with multiple-level degeneration. C-TDR may constitute an adequate alternative to fusion owing to the theoretical advantages of C-TDR, such as reduction of the non-physiological biomechanics of adjacent segments, preservation of ROM, maintenance of the functional spinal unit, and prevention of ASD. However, since C-TDR indications are more restricted than ACDF due to various criteria, it cannot be applied to every pathological segment.17
ACDF is an accepted, safe and reliable strategy for single-level or multi-level cervical disc disease. Nonetheless, fusion can cause a reduction in ROM and increase the stress on adjacent levels.1,2,4,14,18−21 Moreover, numerous publications have demonstrated an increase in the number of segments involved in fusion was associated with amplification of the compensatory motion and biomechanical stress in adjacent segments. This process may eventually lead to a more prominent ASD. Biomechanical and clinical studies have shown the occurrence of symptomatic disc disease at adjacent segments.4 A meta-analysis conducted by Tian et al. yielded that hybrid construction provided excellent clinical and radiological outcomes. Postoperative cervical ROM was found to be similar with the physiological state and no reduction was detected in ROM of the adjacent segment in HC cases.15
There is no consensus on the segment for the performance of HC. Hybrid construction was indicated in patients with cervical spondylotic radiculopathy or myelopathy caused by continuous degeneration from C3 to C7, which were unresponsive to conservative treatment for at least 6 weeks.22 To select and perform the optimal treatment strategy of HC, parameters such as decreasing the motion and facet force compensation at adjacent segments should be taken into account.2
There are many reports on the clinical outcomes of ASD after ACDF in the current literature. A 10-year radiographic follow-up review indicated that there were hypermobility and degenerative changes in the non-fused segments of the spine, including disc space narrowing, end-plate sclerosis, and osteophyte formation in 50% of patients after ACDF. The rate of re-operation ranged from 5–20% due to symptomatic ASD.23 Hilibrand et al. reported that ASD occurred at an annual rate of 2.9% for 10 years after ACDF.24 C-TDR may diminish the stress on adjacent discs and thereby, potentially reducing the rate of ASD, which is estimated to be 3% per year.25
Compensatory hyperkinesis is less likely to occur in adjacent segments if the segment involved in surgery maintains mobility.16 A systematic review by Lu et al. had shown that the Rom at levels C2 - C7 was significantly higher than ACDF after HC, and adjacent upper ROM and lower ROM were significantly lower.20 Hybrid construction resulted in a better recovery of the NDI score at 2 years of follow-up and a similar improvement of the VAS score was noted compared to ACDF.21
Heterotopic ossification (HO) is the bone formation outside the skeletal system. The occurrence rate of HO ranged from 16.1–85.7%, and the overall prevalence was 46.4% (95% CI, 40.1–52.8%) by the random-effects model. It is supposed to be an inevitable postoperative complication after cervical ADR. It can decrease the ROM of the index segment, which is in contrast with the fundamental goal of the artificial disc. The prevalence of both HO and severe HO exhibited a trend of progression. The factors associated with HO occurrence are obscure. The influence of prosthesis on the occurrence of HO needs to be elucidated in future trials.26 In the present study, we determined a HO incidence of 5.12% and this relatively low rate may be attributed to our selection criteria or loss for follow-up.
Our data indicated that the fused segment may be overloaded on the lower or upper adjacent disc prosthesis, which may result in impairment or dislodgement. The wide range of ROM recovery measurements may be attributed to the types of prosthesis used, as well as the X-ray shooting technique and the position of the patient at that moment. The flexibility of the cervical region compared to the thoracic and lumbar regions may also remarkably influence the angle measurements.