Dante A. Implicito, M.D., James Zucherman, M.D., Ken Hsu, M.D.
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As the use of threaded fusion cages (BAK) as a method of instrumented lumbosacral fusion nears FDA approval, we undertook a review of our initial experience, in order to determine whether there was any relationship between pre-operative disc height and disc volume on rate of failure of fusion consolidation. Forty-two levels in 29 patients (12 males, 17 females) were reviewed at an average of 20 months follow up (range: eight to 37 months). Fusion was defined as absolute absence of motion on lateral flexion/extension plain x-rays and/or sagittal midline flexion/extension tomograms. Fusion status was determined by an independent observer. Disc volumes were calculated from pre-operative plain x-rays on all operated levels. Clinical response was measured using the Oswestry Score.
Results
In disc spaces where the volume was less than or equal to 15,000 cubic millimeters, the fusion rate (at 8-24 months) was 87.5%, while in discs where the volumes were greater than or equal to 20,000 cubic millimeters, the fusion rate was approximately 25 %. There was an overall fusion rate of 59.5 % (fused levels per attempted fusion levels). At L4-5, the fusion rate was at 27 % (4/15 patients). At L5-S 1, the fusion rate was 81 % (21/26 patients). The average pre-operative Oswestry score was 67.3. The average improvement in Oswestry scores was 18.2, with only two patients experiencing a decline in function as a result of surgery.
Discussion
Two most intriguing results from this small, clinical series are that time-to-absolute solid fusion is at least delayed in patients with disc volumes greater than 20,000 cubic millimeters, and that unlike our experience with posteriorly internally-fixed fusions, absolute fusion solidity does not appear necessary to obtain good clinical outcomes. This may be due to the absence to the post-operative fibrosis, secondary to the extensive dissection required for posterior surgery. We hypothesize that post-operative fibrosis may increase the nociceptor to mechanical stimulation. |
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