Bilateral implantation of low-profile interbody fusion cages: subsidence, lordosis, and fusion analysis.

BACKGROUND CONTEXT: The use of interbody fusion cages as a treatment for degenerative disc disease has become widespread. Low-profile cages have been developed to allow a closer fit when implanting bilateral cages in patients with smaller vertebral bodies. Some surgeons feel the open design also allows better bone contact and visualization. This is particularly true when two low-profile cages are used adjacent to one another. Because of the open design of low-profile interbody fusion cages, there has been concern regarding such issues as subsidence, lordosis and fusion rates.
PURPOSE: This retrospective review of paired bilateral reduced profile interbody fusion cages was completed to assess changes in subsidence, lordosis and fusion. As a secondary goal, patient outcomes were measured to determine overall health since surgery and the patient's satisfaction with the spine surgery, in an attempt to assess the effect of the outcome variables cited supra. STUDY
DESIGN: This was a retrospective evaluation of patients who underwent anterior lumbar interbody fusion with low-profile interbody fusion cages. PATIENT SAMPLE: Seventy-one consecutive patients who underwent bilateral implantation of low-profile interbody fusion cages were evaluated. OUTCOME MEASURES: A patient self-evaluation, which included a Short Form (SF)-36 and questions regarding patient satisfaction were administered to patients who were at least 1 year postoperative. Subsidence and lordosis measurements were completed. Fusion was assessed by the operating surgeon.
METHODS: Low-profile interbody fusion cages (BAK/Proximity, Centerpulse Spine-Tech, Inc., Minneapolis, MN) were implanted bilaterally in at least one level from L3-L4 to L5-S1. Most patients had degenerative disc disease with leg and back pain that was not responsive to conservative treatment and demonstrated segmental instability or collapse. A small percentage of patients had either a degenerative spondylolisthesis (7.0%) or an isthmic spondylolisthesis (4.2%). Autograft harvested from the iliac crest was used in all cases. Demographic, surgical and follow-up data were retrospectively collected from patient charts. A clinical outcome questionnaire that included an SF-36 as well as questions regarding patient satisfaction was either mailed to each patient who was at least 1 year postsurgery or given to patients to complete at their 1-year visit. Patients were routinely followed radiographically before surgery, immediately after surgery and at 3, 6, 12 and 24 months after surgery. Fusion was assessed by the operating surgeon using lateral radiographs often in conjunction with a thin-slice computed tomography (CT) scan. Criteria for a successful fusion were lack of motion, anterior bridging bone and lack of lucencies on flexion/extension X-rays and/or contiguous bone through the cage using a thin-cut sagittal CT scan. Lateral X-rays on each patient were also measured for subsidence and lordosis changes.
RESULTS: A total of 71 patients (45 men, 26 women) with a mean age of 43.4 years (range, 25 to 74) were evaluated. Thirty-six percent of the patients were smokers, and 96% were worker's compensation patients. Thirty-two percent of the patients had previous lumbar surgery. A total of 100 operative levels were evaluated. There were 45 one-level, 23 two-level and three three-level cases. Forty-nine percent were level L5-S1, 43% were L4-L5 and 8% were L3-L4. The mean duration of symptoms was 31.5 months. Mean surgical time, mean blood loss and mean hospital stay were 139 minutes, 186 cc and 3.34 days, respectively. There were no intraoperative or postoperative complications attributable to the construct and no cases of cage migration or collapse. Patients who were at least 1-year postsurgery and had follow-up X-rays or had undergone a CT scan at this time point were evaluated for fusion status. Sixty-three patients were assessed for fusion. Fifty-four (86%) of these patients were determined to have a solid fusion. Mean time to fusion was 10 months. Fusion was assessed as solid only if all operative levels were fully fused. Mean subsidence of the anterior region was 1.97 mm, whereas the mean subsidence of the posterior region was 0.82 mm. Lordosis was unchanged at all surgical levels with mean lordosis in L3-L4 decreasing only slightly from 13 degrees before surgery to 12 degrees after surgery. L4-L5 and L5-S1 showed only slight increases in lordosis changing from 17 to 18 degrees at L4-L5 and from 17 to 19 degrees at L5-S1. These changes were not statistically significant. The clinical outcome questionnaires had a return rate of 68%. Of the 48 patients who completed the questionnaire, 75% responded that they were happy with the surgical results and would definitely recommend the surgery to a friend. Sixty-seven percent agreed that surgery met their expectations or that surgery improved their condition enough that they would go through it again for the same outcome. The results of the SF-36 portion of the survey revealed that the physical and mental composite scores were within normal range of the US population that has experienced back pain or sciatica.
CONCLUSION: Bilateral implantation of low-profile cages in this patient population led to satisfactory outcomes. Subsidence and changes in lordosis were minimal. Fusion rates were good, especially for one-level cases. Patient satisfaction was relatively high, considering the population consisted of 96% worker's compensation cases. With proper surgical technique, bilateral low-profile cages can be used effectively to treat patients with degenerative disc disease.