All 7×-dose spinal levels demonstrated moderate-to-marked peri-implant bone resorption on radiographs, with histological analysis showing substantial end-plate changes and bone resorption extending well into osseous trabeculae of the adjacent vertebral bodies. Histological analysis showed the greatest amount of bone resorption at 4 weeks post-implantation (Fig. 5-A). Unmineralized osteoid formed via intramembranous ossification was frequently observed at the periphery and within the resorption zones. Early stages of de novo bone formation resulting in early histologically evident fusion were found at 1 of the 3 levels in the 7×-dose group (Fig. 5-B). One of the 3 levels in the 1×-dose group demonstrated moderate resorption radiographically and histologically, whereas the other 2 levels in the 1×-dose group showed less peri-implant resorption (Fig. 5-C). Granulation tissues were found within the PEEK implant and within peri-implant resorption zones. In 4 of the 6 treated levels, the ACS implant was observed within the PEEK interbody construct.
At 8 weeks, CT scans showed minimal end-plate changes at the 1×-dose spinal levels and moderate-to-marked end-plate and vertebral body resorption at 1 level that had received the 7× dose. This was confirmed by histological analysis as osseous resorption with osteoclasts present on the surfaces of osteopenic trabeculae. Histological analysis of the 7×-dose spinal levels showed substantial end-plate changes and bone-remodeling areas. Although end-plate changes and bone resorption extended well into osseous trabeculae of the vertebral bodies, osseous healing with hypodense mineralized trabeculae could be observed within these resorption zones and in the thrugrowth region of the disc space on the microradiograph (Fig. 6-A). Numerous foci of intramembranous ossification are seen on the surfaces of the developing osseous fusion mass within the thrugrowth region of the PEEK device at a 7×-dose level (Fig. 6-B). At higher magnification, hypertrophied osteoblasts in intramembranous ossification on osseous trabeculae of the developing fusion mass were observed within the PEEK interbody fusion device (Fig. 6-C). All 3 of the 7×-dose levels showed evidence of exostoses, which in some cases bridged the disc spaces. Intramembranous and endochondral ossification within the thrugrowth region of the PEEK interbody fusion device was a routine histological finding in both treatment groups. Osteoclastic activity on rarified trabeculae was observed in bone of the adjacent vertebral body, some distance away from the PEEK implant, and within the thrugrowth region of the PEEK device (Fig. 6-D). ACS was not observed at any treated levels at 8 weeks on histological analysis. An incidental histological finding was intracellular PEEK particulate debris and an attendant focal mild chronic inflammatory host response consisting primarily of macrophages with some foreign-body giant cells at a majority of the treated levels (Fig. 6-E). One unusual histological finding was a fluid-filled lined cyst adjacent to the PEEK device in the 1×-dose group (Fig. 6-F).
At 12 weeks, CT scans showed limited evidence of end-plate remodeling and demonstrated bone formation through the thrugrowth region of the PEEK interbody devices. Except for 1 of the 1×-dose spinal levels, all treated levels demonstrated complete bridging bone on CT. The CT findings correlated well with the findings of the histological analysis, which showed 1 fusion, 1 partial fusion, and 1 non-fusion at the 1×-dose levels and 2 fusions and 1 partial fusion at the 7×-dose levels. In both groups, the bone was isodense with respect to native trabeculae on microradiography (Fig. 7). With the exception of some pitting of the end plates at 1 of the 1×-dose levels, end-plate changes were not observed. Microradiography showed that previous bone-remodeling areas that had extended well into the vertebral bodies were now fully healed, with isodense-to-slightly hypodense trabeculae. Bridging and non-bridging exostoses were a frequent finding in both groups. All 6 treated levels had intracellular PEEK particulate debris and an attendant focal mild chronic inflammatory host response consisting primarily of macrophages with some foreign-body giant cells. One unusual histological finding was a small subclinical infection with numerous segmented neutrophils limited to tissue adjacent to the PEEK implant at the 1×-dose spinal level at which histological analysis showed non-fusion.
At 20 weeks, all treated levels showed osseous fusion across the disc space on CT scans. Except for 1 of the 1×-dose levels, all treated levels had either a partial or complete fusion seen on histological analysis. One level in the 7×-dose group demonstrated solid fusion outside the PEEK device and fusion progression within the device. In both treatment groups, the incorporated bone of the fusion mass was isodense with respect to native trabeculae on microradiography (Fig. 8). PEEK particulate debris was not observed.
In this end-plate-sparing sheep model, osteoclastic activity and corresponding peri-implant bone resorption was dose-dependent, with the 7×-dose spinal levels demonstrating moderate-to-marked resorption and the 1×-dose levels demonstrated minimal-to-no resorption. By 20 weeks, transient bone-resorption effects were not observed and bridging bone was seen regardless of the rhBMP-2 dose. The time to healing was not altered by increasing the dose. Findings similar to those in the current study were previously reported by Toth et al.35, whose chronologic study demonstrated bone resorption with subsequent osseous healing in association with application of rhBMP-2 in a corticocancellous sheep distal femoral model. With immediate access to cancellous bone in that distal femoral model, a 7× dose of rhBMP-2 caused increased osteoclastic activity and peri-implant bone resorption with marked resorption by 1 week35. In all of their treatment groups, the osteoclastic response was transient with no osteoclastic activity and substantial new bone formation by 4 weeks35. Immediate cancellous access caused osteoclastic resorption to peak at 1 week in that study, whereas the delayed cancellous access associated with the cortical end-plate-sparing model in our study resulted in osteoclastic resorption peaking at 3 to 4 weeks.
As can happen in any animal model, inter-animal variability with respect to bone remodeling was observed in the current study. Transient bone-resorption areas varied in both size and location, with the greatest variability in size occurring in the 7×-dose group. The location of the transient bone-resorption areas varied from cranial to caudal in relation to the thrugrowth region of the PEEK implant and was also seen posterior to the implant, particularly at the 3-week time point in the 7×-dose group. While settling of the PEEK interbody fusion device into the cortical end plate was observed in both groups, dose-dependent subsidence was not observed. A lateral 2-screw and single-rod construct was used to provide some postoperative stability and may have limited the potential for substantial subsidence to occur.
The literature suggests that BMPs can affect osteoclastic activity, with a stimulatory effect on the formation of osteoclasts or their bone resorptive capacity, or both37-44.
Although a number of FDA-approved clinical investigations have been conducted with rhBMP-2/ACS15-22,24, to our knowledge transient bone resorption has been reported in only 1. Burkus and colleagues21,22 reported that 14 (18%) of 79 patients developed remodeling zones between 3 and 12 months after treatment with 8.4 to 12 mg of rhBMP-2/ACS combined with 2 threaded allograft dowels. By 24 months, all remodeling zones had resolved and a 99% fusion rate had been achieved.
Other clinical studies have demonstrated transient bone resorption following implantation of rhBMP-2/ACS. Meisel and colleagues28 reported bone resorption following posterior lumbar interbody fusion with application of 12 mg of rhBMP-2/ACS within PEEK interbody spacers. At 3 months postoperatively, 17 patients were noted to have reduced mineral density on CT scans. However, these resorption zones were transient, resolving by 6 months. Pradhan et al.34 reported resorption of the graft and end plates following the use of rhBMP-2/ACS with ring allografts. Finally, Hansen and Sasso25 reported on a single patient who had a transient resorptive response following the use of rhBMP-2/ACS in anterior lumbar interbody fusion with an allograft ring.
One limitation of the current study is the fact that the variables of increasing rhBMP-2/ACS fill of the interbody fusion device and hyperconcentration of the rhBMP-2 were not examined individually as potential drivers of heightened osteoclastic resorption. However, it is important to recall that the 1× and 7× doses that we selected had been examined previously in an ovine corticocancellous distal femoral model35. As discussed above, these doses demonstrated the largest difference in transient osteoclastic activity and peri-implant bone resorption. The 7× dose represents a worst-case scenario in which the user underdilutes the rhBMP-2 to increase the solution concentration while overfilling the thrugrowth region of the interbody fusion device. While this is unlikely to occur in clinical practice, it is possible and was therefore included as a worst-case dosing scenario. Importantly, in the previous ovine distal femoral corticocancellous model35, overfilling with (2×) and hyperconcentration of (3.5×) the rhBMP-2 were examined as standalone variables and both resulted in heightened osteoclastic activity and peri-implant bone resorption compared with what was seen in the 1× group. As 1× and 7× doses performed similarly regardless of whether the distal femoral or cortical end-plate-sparing interbody fusion model was used, it can be assumed that both bone-forming environments are susceptible to rhBMP-2 dose-dependent transient osteoclastic activity and peri-implant bone resorption.
In conclusion, osteoclastic activity and corresponding peri-implant bone resorption were rhBMP-2-dose-dependent in an end-plate-sparing ovine interbody fusion model in which a custom-designed PEEK interbody construct had been implanted via a transpsoas retroperitoneal approach; 7×-dose levels resulted in moderate-to-marked resorption while 1×-dose levels resulted in minimal-to-no resorption. By 20 weeks, transient bone-resorption effects were not observed and bridging bone was seen by the blinded assessors regardless of the dose. Increasing the dose had no effect on the time to healing. Using the FDA-approved rhBMP-2 concentration and matching the volume of rhBMP-2/ACS with the volume of the desired bone formation may limit the occurrence of transient bone resorption.
NOTE: The authors thank Amy Rizzo, MT(ASCP), and Sharath Chandra Venkata Chedella, BDS, MS, for their technical expertise in histological processing of the tissues.
Investigation performed at the Colorado State University, Fort Collins, Colorado, and The Medical College of Wisconsin, Milwaukee, Wisconsin
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