Effects of a Cervical Disc Prosthesis on Segmental and Cervical Spine Alignment

Gwynedd E. Pickett, M.D., F.R.C.S.(C); Demytra K. Mitsis; Lali H. Sekhon, M.B.B.S., Ph.D., F.R.A.C.S.; William R. Sears, M.B.B.S., F.R.A.C.S.; Neil Duggal, M.D., M.Sc., F.R.C.S.(C)

Disclosures

Neurosurg Focus. 2004;17(3) 

In This Article

Discussion

It is controversial whether cervical kyphosis influences clinical outcome after anterior cervical discectomy with or without fusion.[1,11] In several studies researchers have examined the impact of surgery on sagittal alignment of the cervical spine.[1,11,17,19,23] Cervical arthroplasty, a recent advance in spinal instrumentation, promises to expand the surgical armamentarium for the management of degenerative disc disease. Early clinical results from European trials of single- and two-level arthroplasty in which the Bryan disc was used have included outcomes of excellent, good, or fair in more than 90% of patients at 1 year.[7] The ROM was reportedly preserved in 88% of patients with single-level and 86% of those with two-level prostheses at 1 year. Nevertheless, although the ROM is an important feature of an artificial disc, it is only a single measure of spinal biomechanics. The effect of the artificial disc on angulation at the treated level and on the overall spinal alignment may be important to long-term clinical outcomes and rates of adjacent-segment disease.

Our results demonstrated that the endplate or shell angle of the Bryan Cervical Disc assumed a kyphotic angulation postsurgery (mean change –8.2°), and that this resulted in kyphosis of the FSU. Despite the kyphosis seen at the level of the endplates of the disc, the overall alignment between C-2 and C-7 did not change significantly, indicating that the untreated segments of the cervical spine were compensating for a focal kyphosis. Kyphotic angulation in the neutral position did not prevent the disc from providing sagittal plane motion (also referred to as ROM) equivalent to the preoperative ROM.

With most of the available disc replacement systems, including the Bryan disc, the angle of disc insertion can significantly alter the orientation of the prosthesis. This is related to bone removal and endplate preparation for the prosthesis. The angle of disc insertion is arbitrary, with no precise measure available to predict accurately the impact of the prosthesis on the sagittal alignment. A number of different methods have been used to calculate the angle of insertion for the Bryan disc. In its description of the surgical technique for the Bryan Cervical Disc system, the product monograph suggests that the disc space angle can be calculated by measuring the angle between the plumb line and a line connecting the posterior inferior corner of the caudal VB and the posterior superior corner of the cephalad VB. The results obtained using this method can be quite variable, depending on the level of the target disc space and the method of intraoperative positioning. Furthermore, with this method of angle calculation, the angle of insertion is determined when the patient is supine with muscles relaxed from anesthesia. Again, this may not represent the true cervical alignment.

In another method used to calculate the angle of insertion, an attempt is made to place the prosthesis in parallel with the angle of the disc space. Because of the passive nature of the prostheses, this strategy may minimize the risk of introducing kyphosis into the cervical spine. Changing the angle of disc insertion, however, is only effective in avoiding kyphosis in the prosthesis endplates. Unfortunately, one cannot correct an underlying straight or kyphotic cervical spine by changing only the angle of prosthesis insertion. Because in the majority of our patients the angle of disc insertion was that suggested by the product monograph, our results for kyphotic endplate and FSU angles may be technique- and/or surgeon-related. Further studies will be needed to address the issue of how to calculate the angle of disc insertion reliably.

The Cobb angle from C2–7 did not change significantly in the early or late postoperative period when compared with preoperative imaging. Neuroimaging studies performed on the 1st postoperative day indicated a tendency for the cervical spine alignment to become slightly more kyphotic immediately postoperatively, but it returns to the preoperative state by 6 weeks. This finding may reflect neck pain as an early response to surgery. We hypothesize that the preoperative global spinal alignment, as assessed by determining the Cobb angle from C2–7, may influence the observed shell angles. It should be noted that Cobb angles compare only the ends of the cervical curve, but do not describe what happens to the curve internally, as in segmental kyphosis.[8] Sekhon[16] attained some degree of correction of alignment in two of seven patients who suffered "loss of lordosis" preoperatively. This would not be predicted by the biomechanical parameters of the disc, and we speculate that it may relate to relief of pain and relaxation of spinal musculature, as was observed between the early and late postoperative visits in our patients.

Eight of our patients had cervical lordosis of 10° or more preoperatively; two had C2–7 Cobb angles between 0 and 10°; two had kyphotic spines (Cobb angle < 0°); and the Cobb angle could not be determined in the other two patients. In this small group we could not define a statistically clear relationship between the preoperative Cobb angle and postoperative alignment. Nevertheless, based on our observations pertaining to insertion technique and the disc kinematics, we propose three categories. In patients with normal preoperative lordosis, alignment is typically preserved, even if the prosthesis shells develop a kyphotic angulation. Patients with preoperative straightening of the cervical spine may have a preserved alignment after disc insertion, but are at risk for development of segmental kyphosis if the angle of disc insertion is not correct. In our limited experience, patients with a preoperative kyphosis may remain unchanged or they may experience worsening of the kyphosis. The prosthesis is passive, and hence one would expect that it would be unable to restore lordosis to a spine that has focal or global kyphosis. An artificial disc inserted in to a kyphotic segment is likely to take on the angle and local biomechanics determined by the preoperative diseased FSU. The Bryan Cervical Disc is not designed to correct kyphosis.

Although the artificial cervical disc is new, the problem of segmental kyphosis and its effects on the adjacent levels and sagittal spinal alignment is not. The numerous papers in the literature pertaining to the question of whether to fuse after anterior cervical discectomy often return to the question of alignment.[11,15,17,20,23] Proponents of fusion note that insertion of a bone plug, whether or not supplemented by anterior plate fixation, is the sole method available to restore or preserve cervical lordosis. Segmental kyphosis is significantly more common after anterior cervical discectomy without arthrodesis.[1]

In a retrospective study of 42 patients followed for a mean of 10 years after undergoing anterior cervical discectomy and fusion for symptomatic cervical spondylosis, Katsuura, et al.,[11] found that degeneration of adjacent levels was significantly associated with loss of physiological cervical lordosis. They speculated that fusion in kyphosis loads posterior slipping forces onto adjacent vertebral levels and may cause higher loads on the posterior column than a fusion in lordosis. The implication is that physiological lordotic fixation is required for prevention of neurological deterioration. Because artificial discs by definition are not intended to produce fixation, the question be comes whether the spine will tolerate kyphosis as long as motion is preserved.

Troyanovich, et al.,[19] performed a retrospective analysis of intersegmental and global sagittal alignment in 57 patients who had undergone single-level anterior cervical discectomy and fusion with or without anterior plating. They reported that the segmental angle became on average 2.5° more kyphotic in the group that did not receive plates, whereas the group in which plates were implanted gained 5.67° of lordosis. Interestingly, however, the overall cervical lordosis did not change significantly, indicating that compensation occurs at other levels. Our results similarly showed that Cobb angles from C2–7 did not change significantly in the postoperative period, regardless of changes in the endplate or segmental angles.

In patients with preoperative focal kyphosis, there is a risk that the kyphosis may worsen after insertion of the disc. This may represent progression of their disease at this level, rather than a causal relationship to the prosthesis. The disc preserves the existing motion of the spine, which has been altered by the underlying degenerative process: it thus maintains an abnormal condition rather than correcting it. Careful patient selection and attention to the angle of insertion should reduce the risk of postoperative kyphotic deformity and its attendant problems.

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