Early Clinical & Radiographic Results of NFix II Posterior Dynamic Stabilization System

Background Complications associated with rigid posterior instrumented fusions of the lumbar spine include pseudarthrosis, accelerated adjacent-segment disease, hardware failure, and iatrogenic fixed sagittal imbalance. Posterior pedicle screw/rod-based dynamic stabilization systems, in which semirigid rods or cords are used to restrict or control, rather than completely eliminate spinal segmental motion, aim to reduce or eliminate these fusion-associated drawbacks. In this study, we analyzed the early radiographic and clinical outcomes of patients treated with the NFix II System (N Spine, Inc., San Diego, California), a novel pedicle screw/ rod-based system used as a nonfusion posterior dynamic stabilization system, and compared our results to those of similar systems currently in use.

Methods Seven sites participated in a retrospective assessment of 40 consecutive patients who underwent dynamic stabilization of the lumbar spine with the NFix II System at a single level. (One patient underwent 2 single-level dynamic constructs at noncontiguous levels (L3-4 and L5-S1).) Patients were included based on the presence of spinal stenosis, degenerative spondylolisthesis, adjacent segment degeneration, recurrent disc herniation, symptomatic degenerative disc disease, and degenerative scoliosis requiring dynamic stabilization at 1 level with or without instrumented rigid fusion at a contiguous level. Participants were evaluated preoperatively, with planned postoperative assessments at 3 and 6 weeks (1 center assessed patients at 4 weeks), 3 months, 6 months, and 12 months. The primary clinical outcome measures at each assessment were visual analogue scale (VAS) scores to measure back pain, and Oswestry Disability Index (ODI)¹ scores to measure function. Radiographic outcome measurements included evidence of instrumentation failure and range of motion (ROM) based on postoperative flexion-extension radiographs at 3, 6, and 12 months.

Results Forty patients (15males, 25 females) with a mean age of 55 years (range 21–81) were included. Average follow-up was 8.1 months (range 6–12). The mean VAS score improved from 7.6 preoperatively to 3.3 postoperatively (P < .001), and the ODI score from 47.3 to 22.8 (P < .001). Eighty percent of patients were severely disabled or worse (ODI ≥ 41) preoperatively, which was reduced to 13% postoperatively. Of the 10 patients with more than 6 months’ follow-up, only 4 demonstrated adequate flexion/extension effort. ROM measurements in those 4 patients showed that on average 53% of preoperative segmental motion was retained at the dynamically stabilized level 6 months postoperatively. There were no instrumentation-related complications.

Conclusions Results of this limited study indicate that the NFix II System when used as a nonfusion device for dynamic stabilization produces significant improvements in pain and function at short-term follow-up with outcomes comparable to other dynamic stabilization systems. The use of this system was not associated with an increased risk of instrumentation failure. The small number of patients with postoperative severe disability or worse compares favorably to long-term published data on posterolateral fusion. Lastly, in this small sample, ROM was preserved at 6-month follow-up.

Clinical Relevance Posterior pedicle screw/rod dynamic stabilization using the NFix II System seems very effective in improving pain and function scores, at least in the short term (mean postoperative ODI of 22.8). Preservation of ROM is also possible. Longerterm follow-up is necessary to assess sustained clinical improvement, hardware complications, and maintenance in segmental ROM. The NFix II System may be considered an effective alternative to existing dynamic stabilization systems. This device is cleared by the US Food and Drug Administration for use as an adjunct to fusion and has the European CE Marking for use in both fusion and nonfusion applications.