It is crucial to conduct long-term follow-up studies to see whether, over time, artificial discs hold up and meet the primary goals of good motion and fewer problems with adjacent segment degeneration . Even 2 year results need to be interpreted with caution: Historically, hip arthroplasty results seemed very encouraging early on. However, 10 year follow-up of patients demonstrated disappointing outcomes, particularly for adults under 55 years of age, who had a surgical revision rate of 28% .
Most importantly, no randomized clinical controlled trials have evaluated the long-term results of intervertebral disc replacement . Its success needs to be evaluated and compared with the clinical outcomes for spinal arthrodesis, especially perioperative complications, implant survivability, and measures of patient improvement in pain, disability, and activity level. In addition it is important to understand the acceptability and safety of subsequent salvage procedures by examining the rates of complications for patients who require further surgery and/or device removal.
As with the hip, a primary end point of interest will be implant survivability, i.e. the absence of failures requiring reoperation, revision, or surgical removal of the disc. There may be disc extrusion or explantation, and wear particles or host inflammatory response or reaction may occur. Currently there are insufficient data to adequately assess the long-term performance of total disc arthroplasty. Review of the literature revealed a case series study of 50 patients who received the Charite III disc device . It found that 24 (48%) of the patients required reoperation, with the majority having complications at the site of the prosthesis or adjacent vertebral sites.
Another study that followed a case series of 27 patients for a mean time period of 53 months found problems with polyethylene wear, subsidence of the prosthesis, and adjacent vertebral deterioration . Metal sensitivity and immune reaction to materials of the artificial discs have been understudied [64,65]. The carcinogenic potential of orthopedic implants has been an area of concern, partly as a result of animal studies . While there appears to be no concern for long-term risk of excess cancer for patients with hip arthroplasty, the occurrence of cancer among persons receiving spinal implant devices has not been studied [67,68].
Another important endpoint is the clinical outcome for the patient. Full assessment of pain, neurological impairment, disability, activity level, and patient satisfaction should be performed. Standard measurement tools for patient evaluation of persons with spinal surgeries include the Oswestry disability index (ODI), short-form health-related quality of life questionnaire (SF-36), and pain assessment using the Visual Analogue Score
(VAS). Additional information before and after surgery is collected regarding employment and activity level, clinical neurological examination findings, and measurements of vertebral range of motion with flexion/extension/rotation. Data about hospitalizations and use of medications (especially pain and antiinflammatory medications) also are important in demonstrating the effect of the surgery on the clinical status of the patient.
In addition to the need for better understanding of the overall long-term success of disc implant surgery, we need a better understanding of what population subgroups will likely benefit or fail. Evaluation of patient factors should include age, underlying conditions, prior back surgery, duration of back pain, occupation, activity level, smoking status, body mass index, and severity of disease. These all may have an effect on the survivability of the device and clinical patient outcome. Degenerative disease factors, such as trauma, osteoporosis, osteoarthritis, and cancer, will alter the success rates and rates of complications. It seems likely that the numbers of complications would increase with longer follow-up time, as was seen in total hip arthroplasty . In addition, the type of surgical approach, e.g. percutaneous or open procedure, and number of levels of operated discs have also been shown to be important predictive factors in various spinal treatments. Several studies suggest that multilevel disc procedures have worse clinical outcomes, especially with respect to disability and activity level. One study revealed that college athletes who underwent two-level disc surgery had higher rates of drop-out for persistent back symptoms than those with one-level procedures . Another study of elderly patients found worse results for those who had more than one-level surgery . Similar results are emerging for total disc replacement, as found in some short-term studies . Long-term prospective studies will be needed to adequately assess whether artificial disc replacement will lead to many of the same complications and adjacent vertebral problems that have been noted with spinal arthrodesis.
Radiographic assessment provides data regarding device loosening, subsidence, displacement or migration, presence of radiolucencies (representing separation), disc extrusion, disc height, nearby soft tissue reaction and evaluation of adjacent level degeneration. Diagnostic criteria to define degenerative disc disease is not standard, and while X-rays and CT scans may be used in evaluations, MRIs appear to offer more accurate assessments of disc disorders and have become the gold standard for spine imaging. However, there is no consensus about possible prognostic radiological indicators [72,73]. Discography can be a very painful procedure, with variable results. It has problems with accuracy and precision and many centers report high false positive rates of disease . Many experts feel it is seldom helpful prior to surgery.
It is also important to assess specific surgical risks. Overall rates of perioperative complications of total disc replacement may vary from 2% to 10%, depending on the study [62,75,76]. The anterior placement of an artificial disc is highly technical surgery that may pose several safety concerns. Vascular complications are rare (< 0.17%), but potentially fatal [77,78]. These may include bleeding, phlebitis, emboli, and acute leg ischemia. They may occur because the surgical approach is anterior to the spinal column in close proximity to the great vessels. To clear the degenerative disc area for insertion, retracting surgical instruments are placed under the bifurcation of the great vessels (aorta and iliac arteries and veins). This calls for specialized training.
Perioperative complications vary according to the surgeon's experience. One study that looked at complication rates according to the surgeon's experience found that the most experienced surgeon's complication rate (2.2%) was statistically different from that of less experienced surgeons (10.7%) . Possible gastrointestinal injuries include peritoneal or intestinal tears leading to bowel perforation, peritoneal scarring, and adhesions. Genitourinary complications such as retrograde ejaculation, incontinence, or ureter damage might occur. Peripheral nerve damage, spinal cord injuries, dural tear, epidural hematoma, or herniated nucleus pulposus may occur . Postoperative complications include deep vein thrombosis, pulmonary embolus and infection. Late complications may include pain and leg edema.
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