Although the survivorship of total-ankle replacement (TAR) has improved, long-term results of TAR are not as reliable as are those of total hip and knee replacements. Its overall failure rate was reported approximately 10% at 5 years, which required revision arthroplasty or conversion to a tibiotalar or tibiotalocalcaneal fusion (19). The choice of the revision technique will mostly depend on the cause of failure, the bone defect, quality of the bone itself, and the experience and expertise of the surgeon. Based on the reviews of literature, ankle fusion is carried out more frequently than is revision replacement for failed TAR (19, 20). It is often preferred because it is currently more reliable at providing relief from pain and gives a stable platform to bear weight even if ankle fusion limits movement and makes the limb short. The major challenge is the severe bone defect due not only to resection for exposure of the prosthesis but also to wear with secondary periprosthetic osteolysis (21). Therefore, the development of treatment for the bone defect is a clinical challenge to foot and ankle surgeons. In spite of advances in reconstruction techniques, the complication rate has remained high. The subsequent amputation was required in 19% of patients with revision surgery (2).
Bone grafting is one of the best options to augment bone-defect regeneration in orthopedic procedures. Recently, among all clinically available grafts, autogenous bone is still being approved as the gold standard, since all important properties required in bone healing in terms of osteoconduction, osteoinduction, and osteogenesis are combined, and an autograft is completely histocompatibility with no risk of disease transmission as well (22, 23). However, autogenous bone has largely been associated with donor site morbidity, a longer hospital stay, limited quantity, and concerns for quality in a high risk patient (24).
In fact, the effect of rhBMP-2 on bone regeneration has been established in both preclinical studies and clinical trials with various models (25). However, for the foot and ankle, very few studies have reported the use of rhBMP-2 and its effectiveness, especially in ankle fusion with a bone-defect model. Therefore, within this study, we combined rhBMP-2 with HA (HA plays a role as a synthetic bone substitute as well as a delivery system for rhBMP-2) and then evaluated whether rhBMP-2/HA can be an acceptable alternative to autogenous bone graft in ankle fusion with a bone defect in a rabbit model.
According to new bone formation’s promising results from radiographs, micro-CT, and histological analysis, once again, the rhBMP-2/HA delivery system shows remarkable efficiency in forming new bone on defect site when compared with other groups. These results are similar to those of many published studies that have demonstrated the effectiveness of rhBMP-2 in combination with various carrier materials in producing bone formation and healing of segmental bone defects in different types of animals and defect models. The results of BMP were equivalent to or better than those of autogenous bone-grafting (13, 14, 26). Similar results were observed by Gerhard and co-workers following grafting of rhBMP-2 in sheep femoral defects. They showed that new bone formation in the defect sites treated with rhBMP-2 first appeared one month after implantation and complete bone healing at four months postoperative (13). In another study, Juan Hou et al. showed that the rhBMP-2-loaded composite scaffold bridged the defects rapidly at 4 weeks, healed the defects, and presented recanalization of the bone-marrow cavity at 12 weeks (27), Elizabeth Wang et. al. demonstrated the efficacy of rhBMP-2 in healing large defects in dog mandible and sheep femoral (26). In clinical applications, in a retrospective review study by Schwartz Niles et al., the authors revealed that rhBMP-2 can heal critically large bone defects in a variety of patients, with a success rate of 84% in their study (28). Moreover in the ankle foot surgery field, based on the results observed in a retrospective cohort study of Schuberth John et al., bone morphogenetic proteins were used in a wide variety of high-risk clinical situations in the foot and ankle and the incidence of successful bone healing was 84.21% (29). Christophera Bibbo et al. did a total of 32 ankle fusions, which were treated with rhBMP-2 and achieved a 100% union rate in a mean time of 10 weeks (30).
Interestingly, the good results of new bone formation on a bone defect site likely relate to ankle-joint fusion results in this study. In fact, according to fusion ratio analysis by micro-CT, the percentage of bone fusion was higher in the rhBMP-2/HA group than in the auto-bone group at 12 weeks with a significant difference (p < 0.001), as well as at 8 weeks postoperative, although this difference was not statistically significant (Figure 4B). Moreover, based on the classification of fusion rate on micro-CT images, Caroll Jones et al. established ankle fusion status, with nonunion as 0% to 33% fusion, partial union as 34% to 66% fusion, and complete union as 67% to 100% fusion (31). In this study, at 8 weeks postoperative, partial union was observed for both the rhBMP-2/HA and auto-bone groups, but the control group showed nonunion. However, at 12 weeks, the rhBMP-2/HA group showed complete union, with above 84.66% fusion; in contrast, partial union was observed for the auto-bone and control groups, with 60.22 % and 41.71% fusion, respectively. This result was encouraged by radiographic as well as micro-CT and histological images. In the radiographic results, the ankle-joint space became narrower with time, from 4 to 12 weeks. The consolidation status in the ankle joint was greater when comparing the control and, auto-bone groups to the rhBMP-2/HA group (Figure 2). At 12 weeks postoperative, micro-CT images showed a complete union in the rhBMP-2/HA group, whereas the auto-bone group showed partial union, and nonunion was seen for the control group (Figure 3). Finally, according to histological images at 12 weeks postoperative, the rhBMP-2/HA group showed complete union with stronger integration between talus and distal tibia including new bone formation when compared with the other groups (Figure 5B). Compared with other studies, in the results on joint’s fusion ratio as well as union status with rhBMP-2 treatment, similar results were shown., whereas ankles receiving an auto-bone graft achieved union at a mean time of 13.3 weeks (30). In the study of KB Lee et al., a comparison of fusion rates and time to fusion on spinal fusion, the authors demonstrated that time to fusion in the rhBMP-2 group was significantly faster than in the autograft group, and fusion rates in the rhBMP-2 group were also higher than in the autograft group, although there was no significant difference (32).
According to promising results of new bone formation as well as fusion rate (%) from radiographs, micro-CT, and histological analysis in this study, the results of the rhBMP-2/HA group were equivalent to or better than those of the auto-bone grafting group on fusion rate, quality and time to fusion as well. This difference between the two groups is most likely due to the osteoinductive properties of the graft materials as well as the osteoinduction process of the two graft materials after implantation. When autograft was implanted on a side defect, the osteoinduction process is affected by the characteristics of the autograft itself and the environment of the fusion site, such as the inflammatory response, angiogenesis, and creeping substitution. Commonly, the implanted autograft has two periods of osteoinduction. In the initial 4 weeks after implantation, osteoinduction occurs from osteocytes or osteoprogenitor cells of the implanted autograft itself, and then from the host bone after 4 weeks. Capillary invasion takes place in the new bone formed by the osteoinduction process, and then bony incorporation is completed by bone remodeling and creeping substitution. In contrast when the rhBMP-2/HA was implanted, some studies have shown a sequence of cellular events leading to the formation of new bone with all of its cellular elements. Immediately, the rhBMP-2 stimulates the stem cells to proliferate and differentiate into chondrocytes. This process takes 5 to 7 days, after which capillary invasion takes place. The chondrocytes subsequently undergo hypertrophy and become calcified, and the osteoblasts appear at the implant site. The new bone is formed at 9 to 12 days. The subsequent remodeling, formation of ossicles, and bone formation take place in the next 14 to 21 days (32-34).
The rhBMP-2 has proved to be one possible strategy for inducing bone formation, but controversy exists. The optimal therapeutic dosage, delivery system, and local circumstances for bone repairs are still under examination. Local inflammation, seroma formation, bone overgrowth, retrograde ejaculation, and increased risk of neoplastic changes are the most common complications associated with rhBMP-2 dosage(25) . The rhBMP-2 dose must be sufficient for adequate bone formation, yet not cause the known complications. Abnormal bone formation has been observed in rats after 2 weeks in critical-sized femoral bone defects treated with high doses of BMP-2 (> 150 µg/ml) (35). Therefore the rhBMP-2 was used at an average of 40 µg/mL based on previous research (16, 17) and the baseline dose used in human spinal fusion. No side effects such as skin infection, necrosis, or local inflammation were observed for up to 10 days; that is, the acute complications possible for wound sites did not occur. Besides, during the sample harvesting operation, we did not observe any evidence of local infections inflammations, or ectopic bone.
Much more work needs to be done if more BMPs-based tissue-engineering constructs are to become available for routine clinical use. It would require elucidation of optimal therapeutic dosage, development of more efficient carriers, and better understanding of the local bone-repair environment. In addition, there have been limited clinical trials in comparison to a large pool of preclinical studies for evaluating rhBMP-2 for routine clinical application in humans, but many promising results have demonstrated that BMPs are effective, and there is evidence that in some situations, their efficacy is comparable to or even better than autografts. (36) Our results help reinforce the above theoretical considerations, especially since this is the first investigation performed on ankle fusion with a bone defect in an animal model.
There are several limitations to this study. First, we placed a priority on imaging and histology to observe and quantify the newly formed bone on the bone defect site as well as the joint consolidation status. A mechanical strength test should be required, and this study was performed with short-term follow up. The second limitation is related to the singular rhBMP-2 dose, carrier, and fixation method used in this study, which did not compare the effect of different rhBMP-2 doses on bone formation, such as fracture healing, bone defect, or distraction osteogenesis, as well as different carriers and fixation methods.