2.7. Coral-derived granules
An ideal bone graft substitute should be biologically inert, readily available, and easily adaptable to the recipient site in terms of size and shape. It should be biodegradable and replaceable by host bone (Bajpai 1983). Coral-derived granules (CDG) exhibit a number of these properties.
CDG are xenograft coral exoskeletons harvested from the French part of the Great Barrier Reef in New Caledonia (Guillemin et al. 1987). The organisms used, are all part of the Madreporian group belonging to the genera Acropora. Corals of this group are formed by a colony of individual coral organisms, each consisting of a soft part, the polyp and its hard exoskeleton. The coral exoskeleton consists of 99% calcium carbonate (CaCO3) in the form of aragonite crystals, the high pressure form of calcite. The crystal is 100µm long and is prismatic in shape (Guillemin et al. 1981, 1995). The remaining 1% is composed of simple amino acids (Issahakian et al. 1987a, Ouhayoun et al. 1992) and has osteoconductive properties (Sautier et al. 1990).
The porosity of coral has been shown to be an important physical property for its behaviour as an implant. Coral skeletons present with different size porosities. The volume of porosity affects the rates of alloplast resorption and bone formation. Two genera of coral Acropora and Porites differ in the architecture of their exoskeletons. They both exhibit an open porosity, as all the pores communicate with each other. The Porites porosity volume is 49 2% and their mean pore diameter is 250 µm (range 150–400 µm) while Acropora porosity volume is 12 4% with a mean pore diameter of 500 µm (range 200–800 µm). The smaller the porosity of the coral exoskeleton is, the greater the density per volume unit and the greater the compressive strength and modulus of elasticity become. The rate of coral resorption and bone deposition is faster with larger porosity volumes and smaller pore diameters both in pig and in sheep models (Guillemin et al. 1989). Coral skeletons of higher porosity volume allow larger cellular infiltrate and ion exchange promoting a faster resorption and bone apposition (Guillemin et al. 1989, Jammet et al. 1994).
CDG are thought to be resorbed through the enzymatic action of carbonic anhydrase (CA) (Chétail & Fournie 1969, 1970). CA is known to catalyze the reversible reaction:
CO2 + H2O <=> HCO3– + H+
CA has been shown to be present in the actively resorptive part of the osteoclast in its ruffled border (Simanski & Yagi 1960, Gay & Muller 1974). If the specific inhibitor of CA, acetazolamide is given to dogs implanted with coral exoskeleton grafts, the resorption of the coral is delayed and fractures treated with coral-derived grafts fail to heal (Guillemin et al. 1981, 1987, 1995).
Figure 1. Photomicrographs showing the gradual incorporation of coral exoskeleton into mammalian bone. 1 Fibrovascular ingrowth. 2 Bony apposition onto coral exoskeleton. 3. Osteoclast resorbing coral skeleton 4. Woven bone replacing coral exoskeleton.
Implanted coral is well tolerated in a variety of animal models (Issahakian et al. 1987b, Ouhayoun et al. 1991, Shababana et al. 1991), and also in humans (Souyris et al. 1985, Issahakian & Ouhayoun 1988, Ouhayoun et al. 1991). In orthopaedic indications coral-derived xenografts have been used to treat spinal fusions in children and for cervical fusions in adults, for femoral lengthening and for acetabular reconstruction (Patel et al. 1980, Pouliquen et al. 1989, Kehr et al. 1991).
CDG have been used as a bone graft substitute in the cranio-maxillofacial skeleton for situations ranging from small periodontal lesions to large cranio-maxillofacial defects and forehead recontouring (Issahakian et al. 1987b, Issahakian & Ouhayoun 1988, Roux et al. 1988a,b, Yukna & Yukna 1998). They are completely resorbable and replaceable by host bone (Chiroff et al. 1975., Guillemin et al. 1987, Roux et al. 1988a, Ouhayoun et al. 1991). Previous experience augmenting defects of the human craniofacial skeleton with CDG has been satisfactory (Brasnu et al. 1988, Levet & Jost 1983, Robier et al. 1987, Besins & Philipe 1988, Levet et al. 1988, Roux et al. 1988a,b).
The use of a bone graft substitute like CDG avoids the need to create a second surgical site for harvesting a bone graft, along with the morbidity associated with this additional procedure. These granules could be useful both in the osseous reconstruction of the cranio-maxillofacial skeleton and could also serve potentially, as a safe and effective means to preserve the dimensions of the alveolar process, until such time as an implant can be placed. The time for total replacement of this implant by bone in the human craniofacial skeleton is approximately 18 months (Roux et al. 1988a). This could make CDG particularly useful in the paediatric population as an alveolar sparing material, where one would have to delay implant reconstruction until growth is complete.