Background
Cervical disc replacement (CDR) is a relatively new technology in spinal surgery, which allows the preservation of the mobility at the implanted segment, and could reduce the stress sustained by adjacent levels and slow down the progression of degeneration of adjacent segments, compared with fusion [
1‐
3]. However, it is still unknown that whether CDR reduces rates of adjacent segment degeneration, compared with the natural history of the disease [
4,
5]. Moreover, artificial disc prostheses should be constantly improved and designed more scientifically, in order to reduce the prosthesis-related complications including subsidence, loosening, migration, dislocation, device wear and heterotopic ossification which have been widely reported [
6,
7].
To our knowledge, most of the current available cervical disc prostheses with various design concepts present a flat surface instead of an arcuate surface. However, as the morphology of inferior endplates of the cervical spine is mainly concave [
8,
9], the mismatch between the prosthesis surface and the endplate geometry gives rise to an inadequate load distribution across the prosthesis-endplate interface, which may be responsible for prosthesis subsidence [
10]. Besides, size mismatch in the current available cervical disc prostheses is another noteworthy issue, since an undersized prosthesis is unable to cover the peripheral marginal zones of the endplate whose biomechanical strength is much larger than that of the central areas so as to increase the probability of prosthesis subsidence [
11‐
13]. Therefore, according to the anatomy of cervical vertebra and the people’s physical size of cervical disc in China [
14,
15], we designed a novel artificial disc prosthesis (Pretic-I, Trauson) based on the physiological curvature of cervical endplate, with an advantage of increasing the contact area between the prosthesis and endplate to disperse the axial load.
In this study, a novel cervical disc prosthesis is tested in vitro, whose biomechanical behaviour is compared with intact cervical spines, and implanted spines with the Prestige LP (Medtronic, Memphis, TN, USA) prosthesis whose surface is flat. Our aim is to test whether or not CDR with the novel prosthesis preserves, or is most similar to, the normal range of motion (ROM) of the cervical spine, and sustains the intradiscal pressure (IDP) on adjacent segments. A secondary objective is to analyse the biomechanical differences between the two prostheses.
Discussion
CDR is a successful, promising, non-fusion technique aimed at restoring the disc space height and spine kinematics. Previous studies [
21‐
23] have demonstrated that CDR confers more benefits than anterior cervical discectomy and fusion, the gold standard technique. Prosthetic devices of the correct sizes and supported by scientific design processes are crucial to the success and long-term survival of CDR. Therefore, this novel artificial disc prosthesis warrants further investigation, as it is designed based on the physiological curvature of the cervical endplate and the people’s anatomy of the cervical vertebra in China. Human spinal specimens are often used for in vitro testing. After being implanted into the segments, we also tested the ROM of all three segments and IDP on adjacent segments to assess the function of this novel prosthesis, and in comparison with the Prestige LP prosthesis.
For the biomechanical testing, although the application of a cervical follower load would decrease the ROM under lateral bending of functional spinal units [
24], we still adopted a 75 N follower load to increase clinical practicability. Furthermore, there has always been a dispute about using a load-controlled, or displacement-controlled, protocol during biomechanical testing [
25]. We deemed it to be the case that the load-controlled testing mode may better reflect the axial load from the weight of the head and muscle forces in the neck, making the testing of spinal specimens a better representation of prevailing physiological functional conditions. Consequently, we adopted the load-controlled testing mode for these biomechanical tests.
In the present study, we tested the ROM at the target segment and adjacent segments under three conditions (intact spine and CDR with two types of artificial disc prostheses). In addition, the IDP on adjacent segments was also analysed. There were minor changes in ROM after CDR with the novel prosthesis and the Prestige LP prosthesis (
P > 0.05). Similar to previous studies, the differences in ROM at target segment between the intact spine and CDR with the novel prosthesis were not statistically significant; the ROM at adjacent segments after CDR with the novel prosthesis also approached the values of the intact spine without significant differences between them [
26,
27]. Therefore, the novel artificial disc prosthesis was similar to the Prestige LP disc prosthesis in mimicking the motion function of a normal cervical disc. With regard to IDP, the superior IDP after CDR with two types of artificial disc prostheses exhibited no significant difference; however, the inferior IDP after CDR with the novel prosthesis was significantly lower than with the Prestige LP disc prosthesis in almost all situations, except axial rotation. During axial rotation, the inferior mean IDP after CDR with the novel prosthesis was slightly lower than with the Prestige LP disc prosthesis, which may be statistically significant given a larger sample size. The result of the pressure analysis demonstrated that the novel artificial disc prosthesis could reduce the IDP on the inferior segment to some extent, compared with the Prestige LP prosthesis.
The above results may be mainly attributed to the differences between the two disc prostheses. First, compared with the Prestige LP prosthesis with a flat surface, the novel prosthesis is designed based on the physiological curvature of the cervical endplate. The arcuate surface of the novel prosthesis can provide a greater effective contact area between the prosthesis and cervical endplate, in order to disperse the axial load more evenly. Second, the novel prosthesis is designed according to the anatomy of cervical vertebra and the people’s physical size of the cervical disc in China [
14,
15], providing a better size match between the prosthesis and cervical vertebra. Size match between the prosthesis and cervical vertebra can not only provide a greater contact area between the prosthesis and cervical endplate but also can cover the peripheral marginal zones of cervical endplate which provide a much stronger support than the central areas [
11‐
13]. However, one previous study [
28] investigated the most common available artificial disc prostheses and found that 53.5% of the largest device footprints were smaller in their anterior-posterior diameter, and 51.1% in the mediolateral diameter were smaller than the cervical endplate diameters. Again, compared with the Prestige LP prosthesis with a metal-on-metal design, the novel prosthesis with a metal-on-polymer design possesses much better wear resistance and stress cushioning effect [
16,
29]. Thus, given the same loading from the superior vertebral body, the novel prosthesis can disperse the axial load more evenly, resulting in a smaller pressure on the inferior segment, compared with the Prestige LP prosthesis.
With regard to the IDP measurement in the fresh-frozen cadaveric specimens, there are several aspects that should be expounded. First, there is no doubt that the fresh cadaveric specimens are the optimal choice for in vitro biomechanical testing. Second, freezing may affect the IDP measurement, especially multiple freeze/thaw cycles, as IDP is based on the hydrostatic behaviour of the nucleus pulposus [
30]. Again, repeated measurements (up to ten) on a single specimen at different testing conditions do not significantly affect the IDP [
30]. However, factors affecting IDP measurement during in vitro biomechanical testing are complex and diverse; further investigations are still needed.
As for the deficiency of any in vitro biomechanical test using a cadaveric cervical spine, our study was mainly devoted to the investigation of the extent of motion without considering the quality, therefore, and could not reflect the long-term effect of CDR. Besides, as the human cadaver specimens were difficult to obtain, the sample size was small. Therefore, further studies with a larger sample size are still needed to evaluate more comprehensively the function of this novel prosthesis.