5 Possible Prevention of Adjacent Segment Degeneration and Disease Avoidance of Iliac Crest Bone Graft Donor Site Morbidity Potential Avoidance of Postoperative Dysphagia Cervical Total Disk Replacement in the Treatment of Adjacent Segment Disease Prestige Cervical Disk Replacement (Medtronic Sofamor Danek, Memphis, TN) Bryan Cervical Disc (Medtronic Sofamor Danek, Memphis, TN) Porous Coated Motion (Cervitech, Rockaway, NJ) ProDisc-C (Synthes, Inc., West Chester, PA) CerviCore (Stryker Spine, Allendale, NJ) Anterior cervical diskectomy and fusion (ACDF) is one of the most reliable procedures in spine surgery for relief of symptoms due to a herniated disk and produces satisfactory results in 90 to 95% of patients with cervical radiculopathy or myelopathy.1 Despite the excellent results of this existing treatment, there has been much recent interest in cervical total disk replacement (C-TDR) as an alternative to ACDF. A portion of the drive to accept C-TDR can be attributed to the interests of the spine implant industry. Unlike fusion in the peripheral joints, short or single-level fusions and subsequent loss of mobility in the subaxial cervical spine result in no functional limitations. However, there exist multiple legitimate problems associated with ACDF that C-TDRs have the potential to address: adjacent segment degeneration and disease, iliac crest bone graft donor site morbidity, pseudarthrosis, and postoperative dysphagia. Retrospective radiographic studies have demonstrated that many patients develop degenerative changes at levels adjacent to segments treated by ACDF.2–5 In a series of 180 patients treated with ACDF with greater than 5-year follow-up, 92% of the patients were found to have new degeneration or progression of degeneration at levels adjacent to the fusion.4 In a recent study of 44 patients who had anterior cervical corpectomy and fusion, magnetic resonance imaging was performed after mean 18 months of follow-up. New degenerative changes adjacent to the fused segment were seen in 75% of the patients. The majority of these patients with radiographic changes (referred to as adjacent segment degeneration), however, remained asymptomatic.5 In contrast, adjacent segment disease refers to the subset of patients who develop new radiculopathy or myelopathy due to degeneration of a motion segment adjacent to the site of a previous anterior cervical fusion. Hilibrand et al reviewed 374 patients with 409 noninstrumented anterior cervical fusions who were followed for up to 21 years. They found that adjacent segment disease, symptomatic enough to warrant further surgical treatment, occurred at a relatively constant rate of 2.9% per year. C5–C6 and C6–C7 were the levels most likely to develop adjacent segment disease. Survivorship analysis predicted that adjacent segment disease may affect more than one fourth of all patients within 10 years after an anterior cervical fusion.6 Whether these degenerative changes are the result of increased mechanical forces on the levels adjacent to a fusion or whether these changes merely represent the natural progression of the degenerative disease process is yet to be determined. Likely, both factors have a role in the etiology of adjacent segment disease. A long-term 5- to 9-year follow-up after ACDF with plating for trauma detected a 60% incidence of asymptomatic adjacent segment degeneration. The high incidence of adjacent degeneration in a patient population without preexisting spondylosis points to the significance of the effect of the fusion on the adjacent levels.7 However, it must be noted that adjacent level degeneration may also be related to the use of instrumentation in these patients. Park and Riew have described the entity of adjacent level ossification secondary to encroachment of the adjacent disk space by cervical plates.8 Multiple studies suggest that the immobility of the fused segment has detrimental biomechanical effects on the adjacent levels. These detrimental effects have been quantified as compensatory increases in adjacent segmental motion and elevation in adjacent intradiskal pressures during cervical motion. In two cadaveric studies, intradiskal pressures at adjacent levels were measured before and after ACDF at C5–C6. Both studies found that intradiskal pressures were increased during flexion and extension at both the caudal and cephalad adjacent levels. Dmitriev et al found that C4–C5 experienced a 48% increase in intradiskal pressure and the C6–C7 experienced a 125% increase in intradiskal pressure. Eck et al found that the pressure increased by 73% at C4–C5 and by 45% at C6–C7.9,10 Other cadaveric studies have also shown that segmental motion is increased at levels adjacent to a fusion.10–14 Eck et al found that during flexion, motion increased at both adjacent levels, with greater increases at the cephalad level. However, during extension, greater increases in motion occurred at the caudal level.10 Summers et al found that increases in adjacent motion were greater after a two-level fusion compared with a single-level fusion.12 Using slightly different testing methods, Fuller et al similarly demonstrated increased motion in nonfused levels and that increased bending moments were required to achieve the same amount of global sagittal motion. However, they found that motion was not increased disproportionately at the motion segments immediately adjacent to the fusion.11 Additional cadaveric studies have suggested that the adverse biomechanical effects of fusion on adjacent levels can be prevented by C-TDR. Dmitriev et al found that the intradiskal pressures adjacent to a C-TDR when using a Porous Coated Motion (PCM) prosthesis (Cervitech, Rockaway, NJ) were significantly less than the pressures adjacent to a fusion. No differences in intradiskal pressure were recorded between the native spine and the spines implanted with a C-TDR under any loading conditions.9 Two additional cadaveric studies using two different C-TDR implants (Prestige I; Medtronic Sofamor Danek, Memphis, TN, and ProDisc-C; Synthes, Inc., West Chester, PA) found that C-TDRs maintained the normal kinematics of the cervical spine. Simulation of fusion significantly reduced motion at the fusion level, which was compensated for by increased motion at the adjacent segments. In contrast, the use of a C-TDR did not alter the motion patterns at either the C-TDR level or adjacent segments.13,14 The findings of these biomechanical cadaveric studies have been verified by the results of clinical studies. Wigfield et al performed a prospective randomized study and compared the motion at levels adjacent to a single-level C-TDR (12 patients, Prestige I) to that of a fusion (13 patients).15 In the fusion group, adjacent-level motion increased from preoperative levels by 5% at 6 months to 15% at 12 months. The increase in motion mainly occurred in disks that were preoperatively normal. At 1 year after surgery, the levels next to a fusion had significantly greater motion than the levels next to a C-TDR. However, a slight reduction in adjacent-level motion was observed in the C-TDR group when compared with preoperative range of motion (ROM). The significance of the decrease in adjacent motion with C-TDR is unknown. In contrast, Duggal et al found that patients with single-level C6–C7 C-TDRs (Bryan Cervical Disc; Medtronic Sofamor Danek, Memphis, TN) had restoration of normal physiological cervical spine kinematics. Motion at both the C-TDR level and adjacent levels was unchanged from the preoperative measurements.16,17 Current clinical evidence to support the contention that C-TDR can prevent adjacent segment degeneration/disease consists of a recent study comparing the rate of adjacent segment degeneration in two prospective single-level cervical diskectomy cohorts. One cohort of 187 patients were treated by fusion with a cage, whereas the other cohort of 80 patients were treated with a C-TDR (Bryan). The two cohorts were followed using similar methodologies. At 2-year follow-up, 27% of the fusion patients had developed new radiographic changes of disk degeneration, compared with 14% in C-TDR patients. Adverse events of neck, shoulder, or arm pain developed in 32% of the fusion patients compared with 1% of the C-TDR patients. The rates of reoperation for adjacent segment disease were similar (3.2% in fusion vs 2.5% in C-TDR).18 Unfortunately, because of the nonrandomized nature of the study, only limited conclusions could be made. The authors also did not examine whether the quantity of preserved motion in the C-TDR group correlated with the incidence of adjacent segment degeneration.18 Only long-term follow-up of ongoing prospective randomized studies comparing C-TDR to ACDF will determine whether C-TDR can prevent or decrease the incidence of adjacent segment disease. The rate of pseudarthrosis following ACDF depends on multiple patient and nonpatient factors such as smoking status, use of anti-inflammatory medications, and host immunocompetency, and surgical factors such as number of levels fused, use of autograft versus allograft bone, and use of instrumentation. In a series of studies by Wang et al, for a one-level ACDF using autograft, the nonunion rate was 4% if plated and 8% if not plated.19 For a two-level ACDF, the nonunion rate was 0% if plated and 25% if not plated.20 For a three-level ACDF the pseudarthrosis rate was 18% if plated and 37% if not plated.21 Fortunately, ~50% of the patients with nonunion will remain asymptomatic at 2 years and 33% will remain asymptomatic at 5 years.22 However, ~50% of the patients will require revision surgery.23 With C-TDR, the risk of pseudarthrosis is virtually eliminated. However, for the C-TDR to be stable long term, bony ingrowth into the prosthesis end plates is required. The porous coating of the Bryan C-TDR has demonstrated bony ingrowth ranging from 10 to 50% in chimpanzee models and 20 to 50% in human explant retrieval studies.24 The porous surface of the PCM C-TDR has been shown to have 48% surface ingrowth in a baboon model.25 This amount of ingrowth compares favorably to that seen in stable total hip arthroplasties and is likely sufficient for long-term C-TDR implant stability. A current advantage of C-TDR over ACDF is that C-TDR does not require autologous iliac crest bone grafting and thus bone graft harvesting morbidities can be avoided.26 In a retrospective, questionnaire-based investigation of 134 patients who underwent single-level ACDF, acute morbidities related to the iliac crest bone graft were reported at the following rates: ambulation difficulty, 51%; extended antibiotic usage, 8%; persistent drainage, 4%; wound dehiscence, 2%; and need for incision and drainage, 1.5%. At an average of 4 years after surgery, pain at the donor site was reported by 26% of patients with a mean Visual Analogue Scale (VAS) score of 3.8. Chronic use of pain medication for iliac crest pain was required by 11.2% of patients. Abnormal sensation at the donor site was reported by 16%, but only 5.2% reported discomfort with clothing. A functional assessment revealed iliac crest-related impairments at the following rates: ambulation, 13%; recreational activities, 12%; work activities, 10%; activities of daily living, 8%; sexual activity, 8%; and household chores, 7%. The current gold-standard graft source for anterior cervical fusion is the use of autologous iliac crest bone graft. However, the recent introduction of fusion-enhancing biologics may decrease the need for autograft once this technology is adapted for use in the cervical spine.27,28 In addition, increasing evidence shows that the use of allograft and plate fixation provides a safe and effective alternative to autograft.29–32 One month after anterior cervical surgery, ~50% of patients still experience dysphagia.33 This subjective symptom has been validated by abnormal postoperative videofluoroscopic swallow evaluations.34 Although the prevalence of dysphagia gradually decreases to ~18% at 6 months, 12% of patients are still symptomatic at 1 year. Age, female gender, and multiple operated levels have been identified as risk factors for persistent postoperative dysphagia. A possible cause of postoperative dysphagia is the retraction pressure placed on the esophagus during the anterior cervical procedure. A recent study revealed that implantation of C-TDRs may place less pressure against the esophagus than ACDF plating. ACDFs or C-TDR (PCM, with no fixation screws) implantations were performed in cadavers through a 4 cm transverse incision while esophageal pressures were monitored. Cervical plating resulted in significantly greater intraesophageal pressures than C-TDR. It was postulated that cervical plating required more traction to insert the convergent contralateral drill, tap, and screws. Because disk arthroplasty does not require more contralateral retraction, less pressure is theoretically placed against the esophagus.35 One possible additional risk factor for postoperative dysphagia relates to the bulkiness of anterior surface internal fixation. In a recent prospective study, the prevalence of dysphagia was compared in patients who underwent ACDF with use of a thin, narrow plate versus a wide, thicker plate.36 The two plates had very similar rates of dysphagia at 1 month postop (~50%). However, at 2 years, the prevalence of dysphagia was significantly higher for the bulkier plate (14% vs 0%). Because bulky anterior hardware may have a causative role in postoperative persistent dysphagia, the use of C-TDRs with no anterior profile (Prestige STLP, Bryan, ProDisc-C, and PCM) may lower the incidence of this complication. Several potential disadvantages of cervical disk arthroplasty exist. It is important to realize that symptomatic radiculopathy and myelopathy are caused by combined static and dynamic neural compression.37–40 Because motion at the diseased level is retained with C-TDRs, there may be greater potential for failure to relieve symptoms due to dynamic factors or for recurrent symptoms at the same level (“same level disease”). With ACDF, the static component can be decompressed and the dynamic component can be eliminated by fusion. With C-TDR, dynamic neural compression will remain unless a more aggressive decompression is performed. More aggressive decompression may mean greater blood loss and higher risks of neural or vascular injury. In addition, there exist no objective criteria for what represents an adequate decompression. Even if individual surgeons have developed their own set of criteria, these criteria are based on the amount of decompression necessary to achieve good clinical results with fusion. Current criteria for adequate decompression may become obsolete in the setting of a C-TDR. Contrary to these contentions, short-term results of randomized trials suggest equivalent clinical outcomes of ACDF and C-TDR. However, these trials are being conducted by surgeons with vast experience in performing an anterior cervical decompression. With wider release of these implants to the general population, good results may become less predictable. With ACDF, the room for error in performing a decompression is high. In fact, it has been shown that equally good outcomes can be achieved regardless of whether direct uncovertebral joint decompression is performed.41 With C-TDR, performing a decompression may be more critical in achieving successful short-term outcomes. Reports of C-TDR revisions for inadequate decompression have already begun to surface.42–44 In the long term, successful ACDF also eliminates motion and thus halts progression of spondylotic spurs. In fact, fusion often leads to spur resorption. With motion preserved in C-TDR, however, spondylotic spurs may recur, leading to late symptom recurrence at the same level.45 Further follow-up may reveal that we have traded a relatively low incidence of adjacent segment disease for a higher incidence of same-level disease. The current indications for cervical disk replacement are identical to those for ACDF (i.e., radiculopathy or myelopathy caused by one or more levels of anterior compression). These are patients who present with a neural compressive lesion causing upper extremity weakness, paresthesias, and pain, with or without lower extremity hyperreflexia. The varied diagnosis may include soft tissue disk herniation with radiculopathy, spondylotic radiculopathy, disk herniation with myelopathy, and spondylotic myelopathy. As more experience with these devices accrues, however, the indications may expand or contract. For example, C-TDRs may encounter problems with same-level disease and symptom recurrence in older patients with spondylotic spurs and degenerated facet joints. Indications may then become narrowed to younger patients with soft disk herniations. These patients are typically younger with normal facet joints and may have more predictable results with motion preservation. On the other hand, indications could also expand to include patients with diskogenic axial neck pain. Interestingly, the indication for lumbar total disk replacement is primarily diskogenic axial low back pain. In contrast to the indications for C-TDR, neural compressive lesions such as spinal canal stenosis or herniated nucleus pulposus are considered to be contraindications to lumbar TDR.46 There are limited data to indicate that patients with refractory axial neck pain and degenerative disk disease limited to one or two levels can benefit from ACDF. These same patients may benefit from C-TDR.47,48
Cervical Disk Arthroplasty: Rationale, Indications, and Clinical Experience
Rationale for Cervical Total Disk Replacement
Potential Disadvantages of Cervical Total Disk Replacements
Indications for Cervical Total Disk Replacement
Current Designs of Cervical Total Disk Replacements
Pros and Cons of the Various C-TDR Designs
Rationale for Cervical Total Disk Replacement
Possible Prevention of Adjacent Segment Degeneration and Disease
Avoidance of Pseudarthrosis
Avoidance of Iliac Crest Bone Graft Donor Site Morbidity
Potential Avoidance of Postoperative Dysphagia
Potential Disadvantages of Cervical Total Disk Replacements
Indications for Cervical Total Disk Replacement
ConclusionRelated posts:
Prosthetic Disk Nucleus Partial Disk Replacement: Pathobiological and Biomechanical Rationale for Design and Function
Bryan Cervical Disc Device
Spinal Kinetics Cervical Disc
Biomechanical Testing Protocol for Evaluating Disk Arthroplasty
DIAM (Device for Intervertebral Assisted Motion) Spinal Stabilization System
Nonfusion Stabilization of the Degenerated Lumbar Spine with Cosmic
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