PATIENT SELECTION
Careful patient selection is critical to surgical outcomes. Only patients with discogenic back pain secondary to DDD are candidates for ADR. The traditional work-up for DDD includes a thorough history and physical examination and multiple imaging techniques.18 Provocative diskography can help distinguish asymptomatic disks from pain-generating disks. If significant facet joint disease is suspected, diagnostic injection techniques can eliminate facet arthrosis pain. Osteoporosis must be considered in the differential diagnosis and appropriate studies performed.
Furthermore, a patient is considered to be a candidate for ADR only after a minimum of 6 months of conservative treatment has failed to resolve the problem. Contraindications to ADR include prior lumbar fusion, facet joint arthrosis, osteoporosis, metabolic bone disease, segmental instability, spondylolisthesis, spondylolysis, lumbar scoliosis, and arachnoiditis. Other contraindications are morbid obesity, active infection, neoplasm, chronic steroid use, pregnancy, metal or polyethylene allergy, autoimmune disorders, more than three prior intra-abdominal procedures, and significant psychosocial disorder.5,7,19
The surgical approach varies depending on the segment involved. Typically, a Pfannensteil incision is used for surgery on the L5-S1 level in female patients, whereas a longitudinal (midline or paramedian) incision is utilized in male patients and is often necessary for procedures on the upper lumbar segments. The retroperitoneal approach is preferred.5,7 Dissection is performed with attention on mobilizing the great vessels and exposing the disk space. Once the damaged segment is verified by fluoroscopy, a complete diskectomy is performed to remove all disk material except the lateral annulus.
A critical member of the surgical team is the access surgeon, usually a general or vascular surgeon. The access surgeon must be aware that the anterior exposure required to safely and accurately position the prosthesis is far more extensive than that required for a typical transabdominal spine exposure. A complete exposure of the posterior aspect of the disk space must be provided. The spine surgeon needs to remove the dorsal and ventral osteophytes to properly position the prosthesis posterior to the middle of the disk space in the anteroposterior diameter. Proper size and alignment of the prosthesis is of critical importance.9 If the prosthesis is placed too anteriorly, it will not flex, negating its advantage as a motion-sparing device.5
Risks related to transperitoneal or retroperitoneal surgical exposure include vascular or bowel injury, postoperative ileus, deep venous thrombosis, embolic events, and retrograde ejaculation in males.7,20 Implant-related complications include vertebral body fracture, malpositioning of the device, infection, postoperative radiculopathy, expulsion of the implant or core, implant subsidence, mechanical failure, progressive facet rthrosis, and adjacent level disease.20
DISCUSSION
Can two surgical interventions with opposite technical goals (motion preservation versus motion prevention) effectively treat the same problem (discogenic pain)? Indeed, the availability of ADR for the treatment of DDD has further complicated the process of matching patients with this condition to the appropriate surgical intervention.11 Given the inclusion and exclusion criteria for arthroplasty, clinicians may find that many of their patients are, in fact, not good candidates for ADR. Futhermore, many failed procedures can be attributed to inappropriate surgical indications.6 Other comorbidities that have yet to be clinically evaluated include preoperative narcotic use and addiction, smoking history, and duration of pain before the surgical procedure. There is a well-acknowledged functional overlay that accompanies the workers' compensation patient population.21 Psychological factors associated with overcoming pain and returning to work, as well as the secondary gain factor related to a sense of entitlement and impairment ratings, have not been analyzed in the setting of a controlled prospective outcomes study comparing ADR with traditional spinal fusion surgery.
Spine surgeons have found that even after a technically perfect operation, a poor surgical candidate is not likely to have the desired outcome. Although cutting edge technology is exciting, long-term clinical outcomes for patients who undergo ADR are not yet known. Asymptomatic DDD is ubiquitous in the general population and well-documented on MRI. One study of asymptomatic patients found that 25% of them in their fifth decade had degenerative findings. This number increased to 75% by the seventh decade.22 Historically, this patient population does not have a high success rate with any spinal surgical procedure. Furthermore, a controversial test—diskography—is used to determine whether these patients are good candidates for ADR. In the end, ADR is offered to patients with no spinal column instability and no significant spinal cord or nerve root compression. Patients whose pain does not resolve have limited additional surgical options. Posterior instrumented fusion remains the primary revision strategy and is performed as a potential salvage procedure.23 An anterior revision carries with it even more significant risk because of the markedly increased likelihood of vascular injury and difficulty revising the device.
SUMMARY
The many unanswered questions surrounding ADR beget the question of whether patients would ultimately be better managed with lumbar fusion surgery or even no surgery at all. Lumbar fusion technology has made advances in recent years with the advent of transforaminal lumbar interbody fusion (TLIF) and the use of bone morphogenetic proteins. With a TLIF procedure, spine surgeons are able to obtain a 360-degree fusion through a single posterior incision with more predictable long-term results, especially when patients have confounding issues. Although early clinical results show ADR to be a promising alternative to current surgical options, long-term results are needed to adequately assess the procedure's efficacy.
There will always be a demand for new technology. Cost, quality, and access are performance benchmarks for hospital survival in the current health care industry. Today's health care environment forces surgeons to be fiscally responsible while still providing the best quality care possible. Therefore, the cost of ADR compared to a traditional pedicle screw instrumentation (about $12,000 per segment vs $3,500 per segment) must be taken into consideration. Is the additional cost of ADR justified when studies so far fail to show a statistically significant difference in long-term outcomes? How do we justify the added cost when conventional lumbar fusion, the gold standard for intractable cases, has more predictable outcomes? Ultimately, ADR may be a more promising alternative to spinal fusion for patients with multilevel DDD (ie, a three-segment lumbar disk replacement) because it can spare disk decompensation of the adjacent segments. JAAPA
George Midla practiced at Brook Army Medical Center, Fort Sam Houston, Texas, when this article was written. He is currently working toward an MA in education, adult learning, and teaching at the University of Texas at San Antonio. He has indicated no relationships to disclose relating to the content of this article.
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