Metallic endplates and vertebral implantation
The same observations can be made on the preparation of the bony implantation site as flexural stiffness of these monobloc implants differentiates them from the first-generation articulated implants.
The size of the implant must be selected carefully to avoid subsidence. Over-distraction of disc space increases the load on facet joints and can cause postoperative neck pain or limited motion within the artificial disc. In case with too loose disc space or too low-height implant, suboptimal device function or migration may occur. Adaptation of the implantation site may be challenging in some cases as the vertebral endplates can be flat or concave. Aggressive bone preparation can be unfavorable with a risk of heterotopic ossifications (HO). The motion characteristics of the different designs, the extent and quality of end-plate coverage, an inadequate position of the center of rotation or a kyphotic adaptation may be significant factors.
These are also questions raised for the evaluation of HO occurrence. To date HO risk is difficult to precise as the length of follow-up is a key point. The use of plain radiographs in the literature may underestimate the rate and grade of HO. In addition, some surgeons use post operatively non-steroidal anti-inﬂammatory medications to prevent HO formation despite the lack of evidence for this strategy. The published rate ranges from 0 at 29 months follow-up (25 patients), or at 34 months (43 patients) to 15.1% McAfee-Suchomel grade I and 10.7% grade II in (112 patients, 36 months follow-up), or 29% no HO 13% grade I and 58% grade II or more. As for lumbar levels, the implantation must avoid any anteroposterior shift of one metal endplate relative to the other which can damage the viscoelastic component. Due to its miniaturization the elastomeric component is more sensitive to parasitic translation of the endplates linked to the operating technique.
The “viscoelastic” components
As for lumbar implants, 2 types of implants must be individualized according to the mode of connection with or without local mobility at the interface.
The design of M6-C Cervical Disc Prosthesis is similar to the one of M6-L with a mobile central core and peripheral ultrahigh-molecular weight polyethylene fibers for stabilization. The same observations and comments can be done especially for the potential over solicitation of the implant in case of increased sagittal inclination of vertebral endplates and high rotational mobility. As at the lumbar level the weak point may be linked to the friction of the polyethylene fibers on the metal endplates with granulomatous reaction or the unexpected locking of the mobile nucleus[33,34].
The other 3 implants claim a direct and stable fixation of the viscoelastic component on the metal endplates. The FCD and the Rhine cervical disc (RCD) have an homogeneous elastic core. The influence of the height of the core on mobility must be carefully monitored on the long term as the available range of motion could be affected. This point should be followed carefully on multi-level implantations. In addition, the original endplates design of FCD implies an 8° lordosis in the metal endplates, the impact of which is unknown in the event of cervical kyphosis posture. Surprisingly, the impact of cervical posture is not analyzed for cervical implants despite normal cervical spinal alignment may vary from lordosis to neutral or to kyphosis[35,36]. Due to variations in sagittal posture, the "neutral position" of the disc prostheses when the gaze is horizontal may involve more kyphosis or lordosis inside the mechanically active cushion. This phenomenon may have different consequences according to the endplate’s designs.
The CP ESP design is based on the experience with the LP ESP implant. The viscoelastic core is one piece but not homogeneous: a mobility limiter is positioned in the center of the cushion to control translation and compression. In addition, the shape of the cushion is asymmetrical to control rotation and translation. These two particularities are intended to avoid the implant kyphosis described for the Bryan type viscoelastic prostheses and potential asymmetry of sagittal ROM in case of kyphotic spine. Like for the LP ESP implant, the combination of central limiter and the asymmetric shape of the cushion claims better control of the coupling of the 6 functional 6 DOFs. Due to the small size of the cushion and its axial limiter, the introduction of this implant must be very rigorous to avoid the relative sliding of the plates and the lesion of the central stabilizer. Like RCD implant CP ESP does not provide intrinsic lordosis in the endplates.
The long-term adaptability of the specific viscoelastic cushion according to the level and the anatomical and postural particularities must be confirmed.
The restitution of the amplitude of movement (ROM) for the flexion- extension is similar to that of the first-generation implants. But it is not the only relevant parameter to characterize the functional quality of these implants. The location of the axis of rotation is another clinically relevant measure of quality of motion for flexion–extension, lateral bending, and axial rotation. These implants claim adaptability of the sagittal axis of rotation according to the disc levels. This could be a relevant argument to advocate for the protection of adjacent levels. One experimental study on the RCD prosthesis showed a fair restitution for centers of rotation on 2 specimens for C5-C6 and C6-C7 levels.
Another experimental study on M6-C reported the location of C5-C6 COR before and after implantation on 12 specimens for flexion–extension mode. The location of the COR for the implanted segments was posterior to the midpoint of the C6 superior endplate (similar to vivo data reported in the literature) but more cranial than intact controls. Currently, there is insufficient evidence to assess the long-term effects of a more cranial COR location. In addition, no information was available for the other segments
A recent study on the evolution of centers of rotation in vivo for CP ESP implants at 2 years of follow-up reports an adaptability of the mean center of rotation at the prosthesis level and adjacent disks as a function of time and posture of the cervical spine.
The restoration of lateral bending motion and motion coupling is also a relevant parameter for the functional quality of cervical disc prostheses. This can also be a determining factor in the longevity of viscoelastic implants depending on the shape and characteristics of the mechanically active cushion. As for the first-generation disc protheses, longer follow-up is needed to understand and interpret the effects of coupled motions on the PCU component and its interface with metal endplates.
Aseptic loosening related to debris deposit in the area surrounding the prosthesis could also be a problem affecting the implant survival curve. These prostheses are supposed to avoid any friction with the exception of M6 implants (mobility of the nucleus and contact of the peripheral fibers of the annulus with the metal plates). A longer follow-up is necessary to accurately assess the rate of aseptic loosening and the possibility of osteolysis due to wear or deterioration of the mechanically active cushion.
Anterior bone loss (ABL) in cervical disc arthoplasties (CDA) has been reported for first-generation implants[39,40]. According to the literature, osteolysis evolution is not consistent with wear debris or low-grade infection. For Kieser et al ABL is common (57.1% of CDA), occurs within 3 months and remains stable after the first year. Osteolysis only affects the not loaded bone. It could be linked to the resection of the anterior longitudinal ligament and likely caused by avascular necrosis of the anterior subchondral bone. ABL could be expected in viscoelastic CDA as this bone remodeling does not seem related to the implant.