The first artificial disc replacements in the cervical spine were introduced in the 1950s with the use of the Fernstrom stainless steel ball, which was also tried clinically by McKenzie in 1969. Two other South African doctors, Reitz and Joubert, also performed this so-called Fernstrom’s prosthesis in 1964 in patients with intractable headache and neck and shoulder pain, but no long-term follow-up has been reported. A major advance in cervical artificial disc research was the development of a prosthesis by Dr. Cummins at Frenchay Hospital in Bristol, Westport, England, in 1989, made of stainless steel and screwed to the vertebral body with unrestricted range of motion in the corresponding segment. The Bryan cervical artificial disc is composed of a metal-to-polymer sealed structure. The Bryan prosthesis does not require additional special fixation. Clinical trials were initiated in the United States in 2002 under the direction of the FDA. Rationale and benefits of artificial disc replacement in the cervical spine i. Prevention of adjacent segment degeneration and lesions Retrospective imaging studies have found that many patients after ACDF have degenerative changes in adjacent segments of their fusion zone. In a study of 180 ACDF patients followed for more than 5 years, a total of 92% of patients were found to have new degeneration or aggravation of existing degeneration in the segment adjacent to the fusion zone. In a more recent group of 44 patients who underwent anterior subtotal cervical resection and fusion with bone grafting, 75% had new degenerative changes in the adjacent segments of the fusion zone on MRI after a mean follow-up of 18 months. Although the majority of these patients showed imaging changes (specifically adjacent segment degeneration), none of them showed significant clinical symptoms. The concept of adjacent segment lesions specifically refers to radiculopathy or myelopathy due to degeneration of adjacent motor segments in the fusion zone after anterior cervical fusion.Hilibrand et al. summarized 409 patients with anterior cervical fusion without internal fixation, 374 of whom were followed up for 21 years. They found that adjacent segmental lesions that were symptomatic enough to require reoperation occurred at a relatively constant rate of 2.9% per year, with C5-C6 and C6-C7 being the segments most likely to have adjacent segmental lesions. They concluded that within 10 years after anterior cervical fusion, approximately more than one quarter of patients will develop adjacent segment lesions. It is inconclusive whether the degenerative changes that occur in the adjacent segments are the result of increased mechanical stress in the adjacent segments of the fusion zone or whether these changes are simply a manifestation of the natural course of the degenerative disease itself. It is likely that both factors play a role in the pathogenesis of adjacent segmental lesions. After 5-9 years of follow-up in a group of ACDF patients with internal plate fixation for cervical spine trauma, it was found that 60% of the patients developed adjacent segment degeneration without clinical symptoms. In such a group of patients without preoperative degenerative spondylolisthesis, such a high incidence of adjacent segment degeneration suggests the significance of fusion to the adjacent segment. However, it is important to note that the adjacent segment degeneration seen in these patients may also be related to the inappropriate use of internal fixation, as described by Park and Riew, who described adjacent segment ossification due to anterior cervical plate coverage of the adjacent spinal space. Several studies have shown that the loss of motion of the fused segment has a negative biomechanical effect on the adjacent segment. The compensatory increase in adjacent segmental kinematics during cervical spine motion and the increase in adjacent intradiscal pressure can be quantified as manifestations of this effect. Two studies measuring intra-disc pressures adjacent to C5-C6 before and after cadaveric simulation of ACDF showed an increase in upper and lower adjacent disc pressures during cervical flexion and extension. increased by 45%. Other cadaveric studies have also found an increase in segmental motion adjacent to the fusion zone, and Eck et al. found an increase in motion in adjacent segments above and below the cervical spine in flexion, with a greater increase in motion in adjacent segments above. Summers et al. found a greater increase in adjacent segmental momentum in two-segment fusion compared to single-segment fusion, and Fuller et al. showed that the increase in non-fused segmental flexion momentum was consistent with the overall sagittal mobility of the cervical spine. They also found a more balanced increase in adjacent segmental momentum above and below the fusion zone. Other studies of cadavers have concluded that the application of C-TDR eliminates the adverse biomechanical effects of fusion on adjacent segments.Dmitriev et al. found that the adjacent intradiscal pressure in the PCM prosthesis was much less than the adjacent intradiscal pressure in the fusion zone. There was no difference in intradiscal pressure between the normal spine and the spine with an implanted cervical disc prosthesis compared to both under any loading condition. Two studies using two different cervical disc prostheses (Prestige I and ProDisc-C) have shown that the cervical disc prosthesis does maintain normal motion in the cervical spine. C-TDR does not alter cervical motion patterns, either in the replaced segment or in its adjacent segments. In their prospective randomized study, Wigfield et al. compared two groups of patients who underwent single-segment C-TDR (12 patients, Prestige I) and single-segment fusion (13 patients) for changes in adjacent segmental motion. The fusion group showed a 5% increase in adjacent segmental motility at 6 months and a 15% increase at 12 months postoperatively compared to the preoperative group. The increase in motility occurred mainly at the level of the adjacent discs, which were normal before surgery. At 1 year postoperatively, the increase in adjacent segment motility was significantly higher in the fusion group than in the C-TDR group. They also found a mild decrease in adjacent segmental motion in the C-TDR group compared to the preoperative range of motion. It is not clear what the significance of the reduction in adjacent segmental motility is after C-TDR. In addition, Duggal et al. found that in patients who underwent single-segment C-TDR (C6-C7, Bryan disc), normal physiologic motion of the cervical spine was restored postoperatively. Neither the C-TDR replacement segment nor its adjacent segmental motion was altered compared to the preoperative period. The results of a recent prospective study on the incidence of adjacent segment degeneration may be used as clinical evidence to support the notion that C-TDR is effective in preventing the development of adjacent segment degeneration or lesions. Two groups of patients who underwent single-segment cervical discectomy were studied. One group of 187 patients underwent fusion with an intervertebral fusion device (cage), and the other group of 80 patients underwent C-TDR (Bryan). Both groups were followed up in the same way. At 2 years of follow-up, 27% of patients in the fusion group had new imaging changes of disc degeneration compared to 14% in the C-TDR group. The rate of appearance of neck and shoulder arm pain was 32% in the fusion group and 1% in the C-TDR group. Reoperation for adjacent segment degeneration was similar, with 3.2% in the fusion group and 2.5% in the C-TDR group. Since the study did not use the principle of randomization, it was used only as a reference point and not as a definitive conclusion. In addition, the authors did not analyze whether there was a correlation between the amount of preserved segmental momentum and the occurrence of adjacent segmental degeneration in the C-TDR group. Only a prospective randomized comparative study of C-TDR and ACDF in long-term follow-up can clarify whether C-TDR is effective in preventing or reducing the occurrence of adjacent segmental lesions. (ii) Avoidance of pseudarthrosis The incidence of pseudarthrosis after ACDF depends on a variety of factors within or outside the patient, such as the amount of smoking, the application of anti-inflammatory drugs, and the immune activity of antibodies. Surgically relevant factors include the number of fused segments, the use of autogenous or allogeneic bone graft, and the use of internal fixation devices; Wang et al. showed that for single-segment autogenous bone graft ACDF, the implant non-union rate was 4% with plate fixation and 8% without plate. For ACDF with fusion of two segments, the rate was 0% with plate fixation and 25% without plate. The rate of ACDF with fusion of three segments was 18% with plate fixation and up to 37% without plate fixation. Of the patients with non-healing implants, about 50% had no clinically significant symptoms 2 years after surgery and about 33% 5 years after surgery. However, approximately 50% of patients still require revision surgery. In fact, the use of C-TDR eliminates the risk of prosthetic joint formation. However, C-TDR also requires endplate bone ingrowth for long-term stability. Bone ingrowth has been observed in the porous surface layer of the Bryan prosthesis. In the chimpanzee model, the bone ingrowth area is approximately 10-50%, and in humans, 20-50% bone ingrowth is observed after removal of the prosthesis. The amount of bone ingrowth in the endplate of the C-TDR prosthesis is greater than that of the total hip replacement prosthesis, which provides good assurance of long-term stability of the C-TDR prosthesis. (iii) Avoidance of pain problems in the iliac bone donor area C-TDR has a distinct advantage over ACDF in that C-TDR does not require an autologous iliac bone graft, so several problems associated with bone grafting can be avoided. A retrospective study of 134 patients with single-segment ACDF in the form of a questionnaire reported the incidence of recent comorbidities associated with iliac implants: difficulty walking, 51%; overuse of antibiotics, 8%; prolonged drainage, 4%; incisional dehiscence, 2%; and the need for incisional drainage, 1.5%. At 4 years after surgery, 26% of patients still had residual donor pain, with a mean VAS score of 3.8. 11.2% of patients required long-term pain medication to relieve pain in the iliac donor area. About 16% of patients had abnormal sensation in the donor area, but only 5.2% of patients complained of discomfort when dressing. The incidence of various impaired daily behaviors due to iliac bone extraction was evaluated as follows: walking, 13%; recreational activities, 12%; work, 10%; daily activities, 8%; sexual life, 8%; and household chores, 7%. The current gold standard for the source of ACDF bone graft is the use of autologous iliac bone graft. However, there have been some recent reports of biologics that can facilitate fusion. If these techniques are used clinically in the future, it may reduce the need for autologous bone grafting. In addition, there is several evidence that the combined application of allograft implants and plate internal fixation can be a safe and effective alternative to autologous implants. (iv) Avoiding postoperative dysphagia One month after anterior cervical approach, about 50% of patients still complain of dysphagia. This subjective symptom has been confirmed by the presence of abnormal swallowing activity on postoperative radiographs. Although dysphagia had gradually decreased to 18% by 6 months postoperatively, 12% of patients still had residual symptoms at 1 year postoperatively. Age, female gender, and multisegmental surgery are considered risk factors for persistent postoperative dysphagia. The cause of postoperative dysphagia may be the strain on the esophagus during anterior cervical surgery. Recent studies have found less intraoperative disturbance of the esophagus with respect to C-TDR prosthesis implantation compared to ACDF plate implantation. The trial was performed by instrumenting ACDF and C-TDR (PCM, without fixation screws) through a 4-cm transverse incision in the cadaver while monitoring intraesophageal pressure. It was found that greater intraesophageal pressure was generated with the implantation of the cervical plate. It was hypothesized that this was due to the fact that more esophageal distraction was required to implant the anterior cervical plate so that the contralateral drilling, tapping, and screwing could be accomplished. Since C-TDR does not require more distraction of the esophagus to the contralateral side, the pressure on the esophagus is theoretically less. Another risk factor for postoperative dysphagia may be related to the size of the anterior endoprosthesis. A recent prospective comparative study of postoperative dysphagia in ACDF patients used two different volumes of anterior cervical plates, thin and narrow and wide. The results found that dysphagia occurred similarly with both plates at 1 month postoperatively (approximately 50% each). However, at 2 years postoperatively, more symptoms remained in the group with the larger volume anterior plate (14%-0%). Since excessive anterior endograft volume is an important cause of persistent postoperative swallowing, the application of a C-TDR prosthesis with small anterior incisions (Prestige STIP, Bryan, ProDisc-C, PCT) may reduce the incidence of this complication.