Bilayered Calcium Phosphate/Calcium Sulfate Bone Graft Sustitutes
Autologous bone is considered the 'gold standard' in bone grafting, but it is not without downfalls. In addition to limited amounts that can be obtained, harvesting autologous bone is associated with up to 30% risk for donor site morbidity.1 Alloplastic bone grafting materials are being developed as an alternative to autologous bone that can be utilized in nearly infinite amounts without the occurrence of a second surgery or donor site morbidity.1 With the potential for release of bioactive agents and customizable erosion, combined with the structural functionality to prevent infiltration of soft tissue, these synthetic materials may offer a suitable substitute to the current 'gold standard'.
The objective of this study was to characterize the physical, chemical, and mechanical properties of a novel bilayered synthetic bone graft. The layers were composed of dicalcium phosphate dihydrate (DCPD) and calcium sulfate hemihydrate (CS). The DCPD and CS were separately incorporated into a cylindrical mold comprising concentric core and shell layers and allowed to set in preparation for testing.
Physical, chemical, and mechanical properties were characterized by microcomputed tomography (microCT), mass loss measurement, and compression testing. Samples were scanned using a Scanco µCT-40. For destructive degradation tests, samples were weighed and incubated in PBS at 37 degrees C. Every 4 days, replicate samples were collected, dried, and re-weighed to determine mass loss. The difference in rate of mass loss was calculated using linear regression. Mechanical tests were conducted using a Bose ELF 3300, with calculation of the ultimate compressive strength (UCS) and compressive elastic modulus (M). Mechanical data were analyzed using one-way ANOVA with a Tukey post-test.
MicroCT imaging of DCPD-core/CS-shell samples demonstrated surface erosion of the CS-shell towards the core, with complete loss of the shell around day 16. Meanwhile, the DCPD core persisted throughout the 60 day study, with formation of internal voids. The pattern of erosion was similar for the DCPD-shell/CS-core samples, with the CS core eroding from the ends of the samples towards the center, forming a hollow cylinder of DCPD. The bilayered DCPD-shell/CS-core samples had a significantly (p<0.05) higher UCS than what has been reported for mandibular trabecular bone (3.9 MPa).2 All the sample types had similar M that was significantly (p<0.001) higher than what has been recorded for trabecular bone (96.2 MPa).2 The single layer DCPD and DCPD-shell/CS-core samples experienced the least mass loss (20% and 27%, respectively) during incubation, while single layer CS samples were entirely eroded by day 32. The DCPD-core/CS-shell samples demonstrated an initial mass loss rate similar to the CS samples until day 16, and then exhibited a rate comparable to that of the single layer DCPD samples, ending with a mass loss of 80% after 60 days.
The combination of DCPD with CS in a layered system provides adequate compressive strength comparable to single layer DCPD and CS. Incorporating CS and DCPD into separate layers also allows for customizable erosion. For example, structures can exhibit fast initial erosion of a CS shell while maintaining the DCPD core and vice versa. The erosion pattern combined with the layers creates potential for tunable drug release, such as to facilitate osteogenesis and decrease inflammation, which could make a layered arrangement useful for a bone graft substitute.
References:
1. Choi, B.-H., Lee, S.-HR., Huh, J.-Y., Han, S.-G. Use of the sandwich osteotomy plus an interpositional allograft for vertical augmentation of the alveolar ridge. Journal of Cranio-Maxillofacial Surgery. 2004;32: 51-54.
2. Carl E Misch, Zhimin Qu, Martha W Bidez. Mechanical properties of trabecular bone in the human mandible: Implications for dental implant treatment planning and surgical placement. Journal of oral and maxillofacial surgery. 1999;57: 700-706.