Overview

Anterior cervical diskectomy and fusion is a well-established, commonly performed surgical procedure for cervical spondylosis. Since its introduction in the 1950s by Robinson and Smith and also Cloward, excellent clinical results have been reported in the treatment of degenerative disorders of the cervical spine. The primary disadvantage of the procedure is that interbody fusion converts a functionally mobile, mechanically stable spinal unit into a fixed, nonfunctional unit. Analysis of the strain distribution of intervertebral disks after anterior cervical disk fusion has shown an increase in longitudinal strain, most frequently at the levels immediately adjacent to the fused segment. The resulting increase in stress on adjacent disks is thought to lead to accelerated disk degeneration and mechanical instability. Radiographic changes consistent with spondylosis and instability at levels above and below cervical fusions have been described by several authors, but these changes have not always manifested as clinical symptoms.

Recently, cervical arthroplasty performed with artificial cervical disks has gained attention as an alternative to traditional arthrodesis, and this can be used to restore and maintain mobility and function of the involved cervical spinal segments. The theoretical advantages of disk arthroplasty include maintenance of range of motion (ROM), avoidance of adjacent-segment degeneration, reconstitution of disk height and spinal alignment, and greater maintenance of maneuverability. Furthermore, the procedure shows decreased surgical morbidity and avoidance of complications from instrumentation or postoperative immobilization, and it allows an earlier return to the previous level of function.

Indications and Contraindications

The indications for cervical artificial disk replacement (C-ADR) are single-level or multilevel disk herniations between C3–C4 and C6–C7 with radiculopathy, myelopathy, or both with minimal spondylosis and no substantial adjacent-level degeneration. The indications for cervical disk replacement are similar to those for anterior cervical diskectomy and fusion (ACDF). These are patients who present with a neural compressive lesion causing upper extremity weakness, paresthesias, and pain, with or without lower extremity hyperreflexia, who are refractory to conservative treatment. Diagnoses may include spondylotic radiculopathy, spondylotic myelopathy, disk herniation with myelopathy, and soft-tissue disk herniation with radiculopathy. Patients with predominantly anterior compression of the cervical spinal cord or nerve roots are good candidates. The radiologically documented presence of motion at the level for which the procedure is proposed is a prerequisite for arthroplasty.

As more experience with cervical arthroplasty accrues, the inclusion criteria may expand. At the beginning of the arthroplasty era, cervical spondylotic myelopathy and spondylotic radiculopathy were not included in the surgical indications. Only young patients under age 50 without spondylotic changes were accepted for this procedure. Since that time, inclusion criteria have evolved to include cervical spondylotic radiculopathy without progressive myelopathy. For example, reconstitution of disk height by C-ADR could be beneficial to some patients with narrowed disk height caused by spondylotic change resulting in foraminal stenosis and nerve root entrapment. The indications could be further expanded to include patients with diskogenic axial neck pain. Interestingly, the indication for lumbar total disk replacement (TDR) is primarily diskogenic axial low back pain. In contrast to the indications for C-ADR, neural compressive lesions such as spinal canal stenosis or herniated nucleus pulposus are considered contraindications to lumbar TDR. Limited data indicate that patients with refractory axial neck pain and degenerative disk disease limited to one or two levels can benefit from ACDF, and these same patients may benefit from C-ADR. Contraindications for C-ADR include axial neck pain related to facet arthropathy, cervical myelopathy caused primarily by posterior compression, deformity (cervical scoliosis, postlaminectomy kyphosis), potential for C-ADR instability as posterior element insufficiency, potential for inadequate end plate integrity (osteoporosis, metabolic bone disease), infection or inflammatory disease (prior infection, ossification of the posterior longitudinal ligament, ankylosing spondylitis, rheumatoid arthritis), insufficient cervical motion at the indexed level or bridging osteophytes, and intervertebral disk space collapse greater than 50% of the normal height.

One of benefits of C-ADR expected in the near future includes its usefulness in the treatment of adjacent-segment disease after ACDF. Revision fusion for adjacent disease is challenging because of high rates of pseudarthrosis and postoperative dysphagia. The difficulty in achieving fusion adjacent to a prior fusion may be due to an unfavorable biologic milieu and a substantial difference in stiffness between the fusion and the adjacent open disk space. With C-ADR, the need for fusion is eliminated, although bony ingrowth is still necessary. C-ADR may also decrease the potential for disease progression to the next adjacent level. With revision fusion for adjacent disease, the index plate is removed to provide space for a new plate to be extended to the additional level. The multilevel dissection required to remove the index plate likely contributes to a higher incidence of postoperative dysphagia and respiratory compromise. If C-ADR is used, there is no need to remove the previously operated plate, and multilevel dissection can be avoided.

Potential Disadvantages of Cervical Artificial Disk Replacement

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. Because motion at the diseased level is retained with C-ADR, there may be greater potential for failure to relieve symptoms at the same level. With ACDF, the static and dynamic component can be eliminated by fusion. With C-ADR, dynamic neural compression will remain unless a more aggressive decompression is performed. More aggressive decompression may mean greater blood loss and a higher risk of neural or vascular injury. Contrary to these contentions, short-term results of randomized trials suggest the clinical outcomes of ACDF and C-ADR are equivalent. However, these trials are being conducted by surgeons with vast experience in performing anterior cervical decompression. Wider use of these implants in the general population may decrease the predictability of good results. 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. With C-ADR, performing a decompression may be more critical in achieving successful short-term outcomes. Reports of C-ADR revisions for inadequate decompression have already begun to surface.

In the long term, successful ACDF also eliminates motion and thus halts the progression of spondylotic spurs. Fusion often leads to spur resorption. However, with motion preserved in C-ADR, spondylotic spurs may recur and lead to late symptom recurrence at the same level. Heterotopic ossification or osteophyte growth at the operated level during long-term follow-up after arthroplasty indicates the role of motion preservation in the progression of spondylotic change. 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.

Other potential disadvantages of C-ADRs include increased cost, neurologic injury as a result of posterior implant dislodgment, implant failure, and need for revision. Fortunately, the approach and potential need for corpectomy in revision C-ADR are known to most spine surgeons.

Preoperative Radiologic Evaluation

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  C-spine plain radiographs should include anteroposterior (AP), lateral (neutral, flexion, extension), and bilateral oblique views.

  
  
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  Visibility must be checked on a C-arm lateral view preoperatively: short neck, high shoulder, C6–C7 level.

  
  
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  Assess preexisting spondylosis (anterior or posterior osteophytes, ossification of ligaments).

  
  
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  Proper disk height for arthroplasty must be ensured.

  
  
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  A prerequisite for arthroplasty is identification of motion in operation segments.

  
  
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  Biomechanical properties must be checked.

  
  
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  Sagittal balance, whole cervical (C2–C7) ROM, and segmental motion (functional spinal unit, FSU) must be evaluated.

  
  
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  Cervical spine computed tomography (CT) and magnetic resonance imaging (MRI) are essential and used to identify both pathology and surrounding structures.

Equipment

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  Radiolucent operating table

  
  
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  C-arm fluoroscopy (essential) or intraoperative radiograph

  
  
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  Operating microscope

  
  
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  Bipolar electrocautery

  
  
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  Cervical retractor system

  
  
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  High-speed drill system

  
  
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  Caspar distraction pins, 12 to 14 mm

  
  
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  Straight and angled curettes

  
  
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  1- to 3-mm Kerrison punches

Operating Room Setup

Proper setup of the operating room is essential and is the most important step before operation. The difference compared with conventional cervical diskectomy surgery is the use of C-arm fluoroscopy in real time during surgery. Special considerations for arrangement are required to avoid improper C-arm imaging, unexpected contamination, and anesthesia difficulty during surgery. Additional considerations include sufficient space for the surgeon, assistants, scrub nurse, and other medical personnel to participate in the surgery ( Fig. 19-1 ).

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  The C-arm fluoroscope is initially located on the cranial side, separated from the patient with a drape, and it is only used during the arthroplasty procedure after microscopic diskectomy. The C-arm fluoroscope base is located on the ipsilateral side of the operator.

  
  
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  The breathing circuit and electrical lines for patient monitoring connected to the anesthesia machine are arranged such that they do not interfere with the caudal location of the C-arm fluoroscope with respect to the patient’s cervical spine. An extension tube for the breathing circuit is commonly required.

  
  
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  The anesthesia machine is located on the lateral side of the operating table opposite the surgeon.

  
  
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  The assistant stands on the opposite side of the operating table as the surgeon during the diskectomy procedure and stands aside to make way for the C-arm fluoroscope. The scrub nurse is positioned below the iliac crest.

Operating room setup for artificial disk replacement. Special considerations for arrangement are required to avoid improper C-arm imaging, unexpected contamination, and anesthesia difficulty during surgery. Additional considerations include sufficient space for the surgeon, assistants, scrub nurse, and other medical personnel to participate in the surgery.

Patient Positioning

The patient is placed in a supine position, and general endotracheal anesthesia is used. The patient is asked to extend his or her neck to the point of pain or onset of radicular or myelopathic symptoms before the induction of anesthesia. A neck roll to facilitate neck support is used in most cases.

The head is placed in a foam cradle headrest, and the arms are tucked to the sides. The patient’s neck is positioned neutrally, not in hyperlordosis, which is routinely used for anterior fusion techniques. Positioning of the patient’s neck in hyperlordosis can result in inappropriate positioning of the prosthesis, because intraoperatively, the alignment of the prosthesis and the spinal segment may appear correct. However, as soon as the spine returns to a neutral position, the segment and prosthesis can fall into a kyphotic position.

Before the patient is draped, C-arm fluoroscopic examination is required to gain a true AP lateral view and to visualize the upper and lower end plate at the operation segment. Because the inserted artificial disk should be properly centered and placed at the appropriate biomechanical depth and height, the position of the patient, operating table, or C-arm fluoroscope should be modified to achieve perpendicular alignment for arthroplasty before draping. If the target segment is not visible because of the lower cervical level (C6–C7), a short neck, high shoulders, and shoulder traction in the caudal direction using tape is required. The patient is covered with a warm-air blanket to maintain body temperature ( Fig. 19-2 ).

Patient positioning. A, The patient’s neck is positioned neutrally. B, Before the patient is draped, C-arm fluoroscopic examination is required to gain a true anteroposterior lateral view and to visualize the upper and lower end plate at the operation segment.

Operative Technique

Incision, Soft Tissue Dissection, and Exposure of the Vertebra

A local anesthetic is injected subcutaneously at the incision site, and a transverse skin incision is made from the midline to the lateral edge of the sternocleidomastoid (SCM) muscle ( Fig. 19-3, A ).

Operative technique. A, Transverse skin incision is made from the midline to the lateral edge of the sternocleidomastoid (SCM) muscle after local anesthesia subcutaneous injection. B through D, The subcutaneous layer, platysma muscle, subplatysmal areolar layer, medial border of the SCM muscle, anterior cervical fascia on the medial border of the SCM muscle, and the areolar plane between the SCM muscle and the omohyoid and sternothyroid muscles. E, The carotid artery pulse is palpated, and the artery is freed from the surrounding connective tissue medially by blunt dissection, and it is retracted laterally. F and G, Exposure of the vertebra and needle insertion to localize the desired level. H, Self-retaining retractor placement.