Settable Polymeric Autograft Extenders in a Rabbit Radius Model of Bone Formation.

Autograft (AG) is the gold standard for bone grafts, but limited quantities and patient morbidity are associated with its use. AG extenders have been proposed to minimize the volume of AG while maintaining the osteoinductive properties of the implant. In this study, poly(ester urethane) (PEUR) and poly(thioketal urethane) (PTKUR) AG extenders were implanted in a 20-mm rabbit radius defect model to evaluate new bone formation and graft remodeling. Outcomes including µCT and histomorphometry were measured at 12 weeks and compared to an AG (no polymer) control. AG control examples exhibited new bone formation, but inconsistent healing was observed. The implanted AG control was resorbed by 12 weeks, while AG extenders maintained implanted AG throughout the study. Bone growth from the defect interfaces was observed in both AG extenders, but residual polymer inhibited cellular infiltration and subsequent bone formation within the center of the implant. PEUR-AG extenders degraded more rapidly than PTKUR-AG extenders. These observations demonstrated that AG extenders supported new bone formation and that polymer composition did not have an effect on overall bone formation. Furthermore, the results indicated that early cellular infiltration is necessary for harnessing the osteoinductive capabilities of AG.

Effects of nanocrystalline hydroxyapatite concentration and skeletal site on bone and cartilage formation in rats.

Most fractures heal by a combination of endochondral and intramembranous ossification dependent upon strain and vascularity at the fracture site. Many biomaterials-based bone regeneration strategies rely on the use of calcium phosphates such as nano-crystalline hydroxyapatite (nHA) to create bone-like scaffolds. In this study, nHA was dispersed in reactive polymers to form composite scaffolds that were evaluated both in vitro and in vivo. Matrix assays, immunofluorescent staining, and Western blots demonstrated that nHA influenced mineralization and subsequent osteogenesis in a dose-dependent manner in vitro. Furthermore, nHA dispersed in polymeric composites promoted osteogenesis by a similar mechanism as particulated nHA. Scaffolds were implanted into a 2-mm defect in the femoral diaphysis or metaphysis of Sprague-Dawley rats to evaluate new bone formation at 4 and 8 weeks. Two formulations were tested: a poly(thioketal urethane) scaffold without nHA (PTKUR) and a PTKUR scaffold augmented with 22 wt% nHA (22nHA). The scaffolds supported new bone formation in both anatomic sites. In the metaphysis, augmentation of scaffolds with nHA promoted an intramembranous healing response. Within the diaphysis, nHA inhibited endochondral ossification. Immunohistochemistry was performed on cryo-sections of the bone/scaffold interface in which CD146, CD31, Endomucin, CD68, and Myeloperoxidase were evaluated. No significant differences in the infiltrating cell populations were observed. These findings suggest that nHA dispersed in polymeric composites induces osteogenic differentiation of adherent endogenous cells, which has skeletal site-specific effects on fracture healing. STATEMENT OF SIGNIFICANCE: Understanding the mechanism by which synthetic scaffolds promote new bone formation in preclinical models is crucial for bone regeneration applications in the clinic where complex fracture cases are seen. In this study, we found that dispersion of nHA in polymeric scaffolds promoted in vitro osteogenesis in a dose-dependent manner through activation of the PiT1 receptor and subsequent downstream Erk1/2 signaling. While augmentation of polymeric scaffolds with nHA enhanced intramembranous ossification in metaphyseal defects, it inhibited endochondral ossification in diaphyseal defects. Thus, our findings provide new insights into designing synthetic bone grafts that complement the skeletal site-specific fracture healing response.

Nanocrystalline hydroxyapatite-poly(thioketal urethane) nanocomposites stimulate a combined intramembranous and endochondral ossification response in rabbits.

Resorbable bone cements are replaced by bone osteoclastic resorption and osteoblastic new bone formation near the periphery. However, the ideal bone cement would be replaced by new bone through processes similar to fracture repair, which occurs through a variable combination of endochondral and intramembranous ossification. In this study, nanocrystalline hydroxyapatite (nHA)-poly(thioketal urethane) (PTKUR) cements were implanted in femoral defects in New Zealand White rabbits to evaluate ossification at 4, 12, and 18 months. Four formulations were tested: an injectable, flowable cement and three moldable putties with varying ratios of calcium phosphate to sucrose granules. New bone formation and resorption of the cement by osteoclasts occurred near the periphery. Stevenel's Blue and Safranin O staining revealed infiltration of chondrocytes into the cements and ossification of the cartilaginous intermediate. These findings suggest that nHA-PTKUR cements support combined intramembranous and endochondral ossification, resulting in enhanced osseointegration of the cement that could potentially improve patient outcomes.

Compression-Resistant Polymer/Ceramic Composite Scaffolds Augmented with rhBMP-2 Promote New Bone Formation in a Nonhuman Primate Mandibular Ridge Augmentation Model.

PURPOSE: This study was designed to test the hypothesis that compression-resistant (CR) scaffolds augmented with recombinant human bone morphogenetic protein-2 (rhBMP-2) at clinically relevant doses in a nonhuman primate lateral ridge augmentation model enhances bone formation in a dose-responsive manner without additional protective membranes. MATERIALS AND METHODS: Defects (15 mm long × 8 mm wide × 5 mm deep) were created bilaterally in the mandibles of nine hamadryas baboons. The defect sites were implanted with poly(ester urethane) (PEUR)/ceramic CR scaffolds augmented with 0 mg/mL rhBMP-2 (CR control), 0.75 mg/mL rhBMP-2 (CR-L), or 1.5 mg/mL rhBMP-2 (CR-H). The primary outcome of ridge width and secondary outcomes of new bone formation, cellular infiltration, and integration with host bone were evaluated using histology, histomorphometry, and microcomputed tomography (micro-CT) at 16 weeks following implantation. RESULTS: New bone formation in the mandible was observed in a dose-responsive manner. CR-H promoted significantly greater new bone formation compared with the CR control group. In all groups, ridge width was maintained without an additional protective membrane. CONCLUSION: CR scaffolds augmented with a clinically relevant dose of rhBMP-2 (1.5 mg/mL) promoted significant new bone formation. These results suggest that a CR PEUR/ceramic composite scaffold without a protective membrane may be a potential new rhBMP-2 carrier for clinical use.

Poly(Thioketal Urethane) Autograft Extenders in an Intertransverse Process Model of Bone Formation.

IMPACT STATEMENT: The development of autograft extenders is a significant clinical need in bone tissue engineering. We report new settable poly(thioketal urethane)-based autograft extenders that have bone-like mechanical properties and handling properties comparable to calcium phosphate bone cements. These settable autograft extenders remodeled to form new bone in a biologically stringent intertransverse process model of bone formation that does not heal when treated with calcium phosphate bone void fillers or cements alone. This is the first study to report settable autograft extenders with bone-like strength and handling properties comparable to ceramic bone cements, which have the potential to improve treatment of bone fractures and other orthopedic conditions.

Injectable, compression-resistant polymer/ceramic composite bone grafts promote lateral ridge augmentation without protective mesh in a canine model.

OBJECTIVE: The objective of this study was to test the hypothesis that a compression-resistant bone graft augmented with recombinant human morphogenetic protein-2 (rhBMP-2) will promote lateral ridge augmentation without the use of protective mesh in a canine model. MATERIALS & METHODS: Compression-resistant (CR) bone grafts were evaluated in a canine model of lateral ridge augmentation. Bilateral, right trapezoidal prism-shaped defects (13-14 mm long × 8-9 mm wide × 3-4 mm deep at the base) in 13 hounds (two defects per hound) were treated with one of four groups: (i) absorbable collagen sponge + 400 µg rhBMP-2/ml (ACS, clinical control) protected by titanium mesh, (ii) CR without rhBMP-2 (CR, negative control), (iii) CR + 200 µg rhBMP-2 (CR-L), or (iv) CR + 400 µg rhBMP-2 (CR-H). All animals were euthanized after 16 weeks. Ridge height and width and new bone formation were assessed by µCT, histology, and histomorphometry. The release kinetics of rhBMP-2 from CR bone grafts in vitro and in vivo in a femoral condyle defect model in rabbits was also evaluated. RESULTS: All four bone grafts promoted new bone formation (11-31.6 volume%) in the lateral ridge defects. For CR grafts, ridge height and width increased in a dose-responsive manner with increasing rhBMP-2 concentration. Ridge height and width measured for CR-H without the use of protective mesh was comparable to that measured for ACS with a protective mesh. CONCLUSIONS: At the same dose of rhBMP-2, an injectable, compression-resistant bone graft resulted in a comparable volume of new bone formation with the clinical control (ACS). These findings highlight the potential of compression-resistant bone grafts without the use of protective mesh for lateral ridge augmentation.