Many studies have reported posterolateral lumbar fusion with rhBMP-2 in animal models [7
]. Sandhu et al. [17
] reported that they transplanted rhBMP-2 with a collagen sponge as a carrier in their study of posterolateral lumbar fusion in canines and that 100% bone union was obtained at 12 weeks. In addition, Sandhu et al. [18
] also reported that rhBMP-2 precluded the need for decortication. Fischgrund et al. [7
] reported that a larger fusion mass could be obtained by adding rhBMP-2 to an autogenous bone graft. Martin et al. [15
] found that a larger amount of rhBMP-2 compared with what had been effective in other animal models would be needed to demonstrate effectiveness in primates.
Based on these findings, a few studies have been performed with human subjects [5
]. In 2002, rhBMP-2/ACS INFUSE® Bone Graft received FDA approval as an autograft replacement for an interbody spinal fusion procedure after a number of preclinical and clinical investigations [16
This approval was just based on the clinical studies for anterior lumbar interbody fusion (ALIF). After this FDA approval, this rhBMP-2/ACS INFUSE® Bone Graft was applied for posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) [16
This is the first study in which the monosegmental PLF at the same level was performed and patients were followed up for a mean period greater than 60 months. In addition, the bone union rates within individual patients were compared between rhBMP-2 and an autogenous bone graft, which eliminated the influence of interindividual differences and also provided a more accurate comparison.
Boden et al. [5
] reported a prospective, randomised clinical pilot trial that evaluated the use of rhBMP-2 with a biphasic calcium phosphate carrier in single-level posterolateral lumbar fusion. The mean follow-up period of their study was 17 months. Three cohorts were used in this study: five patients were treated with iliac crest bone graft with posterior instrumentation (control group), 11 patients were treated with rhBMP-2/biphasic calcium phosphate with instrumentation and nine patients were treated with rhBMP-2/biphasic calcium phosphate without instrumentation. At 17 months only two of five patients with iliac crest bone graft exhibited fusion compared with fusion in all of the patients treated with rhBMP-2.
Glassman et al. [8
] performed posterolateral lumbar fusion with pedicle screw implantation in 38 patients. In their study, 20 mg rhBMP-2 was used bilaterally with bovine collagen/hydroxyapatite and tricalcium phosphate as a carrier. Bone union was originally scored from 1 (no fusion) to 5 (solid bilateral fusion) for evaluation. One year after surgery, the score was 4.62, which was better than the 3.77 of PLF with autogenous iliac bone graft and pedicle screw that was simultaneously performed. They reported that rhBMP-2 provided a more rapid, firmer bone union than autogenous iliac bone after posterolateral fusion.
In our study, bone fusion progressed for two years after surgery, with a bone fusion rate of 9, 73 and 82% at 6, 12 and 24 months, respectively, for the right rhBMP-2 side, while the rate was 91% on the left autogenous bone side. Statistically significant differences in bone fusion were observed only six months after surgery. The use of rhBMP-2 would eliminate the harvesting of autologous bone graft from a patient and the resulting potential complications. Therefore, rhBMP-2 could reduce complications. rhBMP-2 can be used as the sole source of osteogenesis with success equivalent to an autologous graft of the PLF.
Our bone fusion rate with rhBMP-2 was less than the rate reported by Boden et al. [5
], which was virtually the same as the results reported by Glassman et al. [8
]. In our study, the autologous bone graft side was able to obtain better fusion rate (over 90%) compared with another study. We applied 8 g autologous bone from the iliac crest, and we consider that this amount of bone might be appropriate for good posterolateral fusion.
Patients were followed up for a mean of 61 months. Clinical improvement was observed within the first six months after surgery and was likely to then show slight deterioration over time; however, the JOA score six years after surgery was as favourable as 26.6
4.3. In our study, bony formation with rhBMP-2 progressed up to two years after surgery. This suggests that posterolateral bone fusion should be determined by two years after surgery. During the follow-up period, no complications, such as surgical wound inflammation or allergic reactions to rhBMP-2, were observed.