Scientific Name(s): Phylum coelenterata
Common Name(s): Coral
Corals are a broad group of marine invertebrate animals (phylum Coelenterata) that deposit a mineral skeleton as they grow, eventually producing coral reefs. Corals used for medical application are limited to a select number of genera. Goniopora and Porites appear to be the most commonly utilized. Others include Acropora, Lobophyllia, Polyphyllia, Pocillopora and Foraminifera. Areas of harvest include the Caribbean Sea, the New Calendonia island area of the Pacific Ocean, the Red Sea, the east coast of Africa, the Gulf of Thailand, the coast of Hainan Island, and the coastline of Australia.Chou 2013, Demers 2002
While coral has been used by the inhabitants of Pacific regions as cutting tools and as the basis of jewelry and amulets, it was not until the mid-1980s that its value in surgery was fully recognized. The natural material derived from the matrix of sea coral serves as an effective substrate for the growth of new bone in areas damaged by trauma or requiring reconstruction. Coral may be more durable than bone and appears to eliminate some of the complications inherent in traditional bone graft surgery.Smith 1989
Coral polyps absorb calcium ions and carbonic acid present in seawater to produce aragonite crystals of calcium carbonate, representing 97% to 99% of the coral exoskeleton. The remainder is made up from various elements, including magnesium (0.05% to 0.2%), sodium (0.4% to 0.5%), and traces of potassium (0.02% to 0.03%), strontium, fluorine, and phosphorus in the phosphate form. The 3 elements in coral are known to play a critical role in the bone mineralization process and in the activation of enzymatic reactions with osteoid cells. Strontium contributes to the mineralization process and protects calcification. Fluorine, present 1.25 to 2.5 times more in coral than bone, is thought to help bone formation through its effect on osteoblast proliferation.Demers 2002
The main differences between natural coral and bone are the organic content and mineral composition. One third of bone is made up of organic components, compared with 1% to 1.5% of coral. The mineral content of bone is mainly hydroxyapatite and amorphous calcium phosphate associated with calcium carbonate, while coral is essentially calcium carbonate. Most of the elements found in bone can be found in coral but in a different distribution.Demers 2002
Although the structural and mineral composition of coral is very similar to that of bone, coral is not implanted in its natural state. Following its harvest, coral is treated chemically together with heat and high pressure to convert the calcium carbonate matrix to hydroxyapatite (calcium phosphate hydroxide). Hydroxyapatite is the normal mineral portion of bone.
Natural coral has a porous structure that offers a substantial surface exchange area. The size and interconnectivity of the coral pores are critical factors in the rate of coral resorption and in the role of coral in bone regeneration. The pores of the processed coral exoskeletal matrix range from 150 to 600 microns in diameter, with interconnecting pore sizes averaging approximately 260 microns in diameter.Green 2013, Ripamonti 1992, Zeng 1991 These dimensions are in the range for normal bone, making coral an excellent base for the spread of new bone growth.
In coral, mechanical properties are mainly influenced by the direction and growth of the polyps and the porosity of the skeleton. Corals have better mechanical properties in the direction of their growth, but overall, those growing vertically as opposed to horizontally have a better resistance to mechanical strains. Mechanical integrity can be maintained if appropriate rate of coral resorption is matched to bone formation rate of each implant site.Demers 2002Goniopora and Porites have an open porosity of 80% and 50%, respectively, resembling that of spongy bone where the pores are interconnected longitudinally and transversally. This allows for a rapid vascularization as well as the invasion and apposition of newly formed bone.
Uses and Pharmacology
The 3-dimensional structure, porosity, pore interconnections, and composition of commonly used natural coral confers its osteoconductive capacity and make it suitable for hard tissue regeneration. Its osteoconductive capacity allows cell attachment and growth through the scaffold of the material, characteristic of a good support for cells. The initial invasion of coral by blood and bone marrow cells with subsequent vascularization is a determinant factor for bone regeneration. Research has clearly demonstrated that coral is only osteoconductive and is not an osteoinductive material.Demers 2002
Coral possesses all the principal properties of an adequate bone graft substitute, with the exception of its lack of osteoinductivity and osteogenesis, which can be provided with the addition of growth factors such as bone morphogenic proteins and bone marrow cells. The addition of growth factors or bone marrow cells to coral grafts generally improves bone formation when compared with implantation of coral alone. Coral scaffolds act as good carriers of growth factors and good supports for cell transplantation onto a bony site. Animal models have shown increased osteogenesis when using appropriate osteoinductive protein, such as bone morphogenic protein.Demers 2002
During surgery, the processed coral is shaped to fit the patient's bone defect. Sea coral has several advantages over human bone. Coral does not require the surgical removal of bone matrix from elsewhere in the patient's body (eg, hip) for grafting, it retains its shape well, and it provides a long-lasting matrix that closely resembles natural bone.Hippolyte 1991, Smith 1989
A comparative study of bioceramic, coral, and processed bone graft substitutes concluded that while these materials provide a scaffold for the ingrowth of bone from the adjacent host bone, they have no inherent bone-forming ability and provide only limited mechanical strength.Begley 1995
Natural coral has been shown to be a good scaffold for bone tissue engineering. Coral implants in the shape of a human mandibular ramus were seeded with bone marrow-derived osteoblasts and implanted into rabbits. After 2 months, a bone graft had formed that kept the initial shape of the mandibular ramus, both macroscopically and microscopically. Histological examination showed that the scaffolds were covered with new bone or osteoid tissue.Chen 1994
Similar results were shown in another study in which a coral implant in the shape of a human mandibular condyle was seeded with marrow-derived osteoblasts and implanted into mice for 2 months. The results showed new bone grafts developed and maintained the initial shape of the coral, with new bone observed histologically on the surface and in the pores of the natural coral in all specimens.Chen 2002
The success and efficacy of guided tissue regeneration (GTR) therapy using barrier devices has been shown to be dependent on the space-providing capacity of the device. Failure to provide space has resulted in impaired or hindered regeneration.
A study in dogs evaluated the efficacy of calcium carbonate coral implant (CI) in the presence and absence of GTR for bone regeneration. Greater bone formation was seen in sites receiving CI/GTR compared with CI alone. CI particles remained embedded in newly formed bone and fibrovascular marrow and connective tissue apparently unrelated to new bone formation.Koo 2005 A study in rats was designed to evaluate a composite of natural coral exoskeleton (CC), autologous bone marrow cells (BMC), and bone morphogenetic protein (BMP) as a bone substitute for the reconstruction of massive craniofacial tissue loss; the intent was to determine their relative contribution on osteogenesis. A 9 mm craniotomy was performed using a dental surgical drilling unit in rats. The defects were either left empty (control defects) or filled with 1 of the composites (CC, CC-BMC, CC-BMP, or CC-BMC-BMP). After 2 months, the results demonstrated an increase in bone formation by filling the cavities with CC-BMP or CC-BMP-BMC; osteogenesis being sufficient to obtain union. No increase in osteogenesis was observed by the addition of BMC alone. Further investigations are needed to determine the kinetics of BMP release and the most appropriate quantity of growth factor.Arnaud 1999
In baboon studies, surgically made bone defects that were grafted with coral demonstrated substantial bone growth (P < 0.01) compared with bone grafts as early as 3 months after surgery, culminating with complete penetration of bone into the tridimensional porous spaces of the coral.Ripamonti 1992 Similar results were observed when the material was implanted in the mandibles of rabbits.Zeng 1991 In dogs, bone regrowth in experimentally created proximal tibia defects demonstrated that the stereological distribution of regenerated bone in the porous hydroxyapatite was the same as in normal tibial bone; after 12 months, 66% of the surface of the coral was covered with new bone ingrowth.Holmes 1986
Coral has been used as a bone graft substitute to treat a wide range of bone-related problems in humans. The applications tested include spinal fusion, fracture repair caused by trauma, replacement of harvested iliac bone and treated bone tumors, and filling bone defects mainly in periodontal and cranial-maxillo-facial areas. Overall, reported results appear satisfactory with infection rates ranging from 0% to 11%, which is comparable to those obtained when autologous bone is used for treatment.Demers 2002
Coral-based material is used to aesthetically enhance the facial skeleton in cosmetic surgery and is used as a surgical aid in maxillofacial reconstructive surgery.Servera 1987
Although experience is somewhat limited, published results suggest that in man, the use of coral in maxillofacial surgery results in good bone conduction into the surgical site.Zeng 1991
As for any bone graft substitute, some specific rules have to be followed when using coral implants:
There must be protection of the coral architecture during handling and shaping;
the coral graft must be in intimate contact on all sides with bone tissue or minimally have a high surface area in contact with bone tissue;
stability of the graft in the implant site must be evident;
the positioning of the coral must be in a viable and aseptic area distant from infection sites treated with solutions capable of destroying the coral architecture or preventing cell invasion, away from synovial liquid in cases of dura-mater breach, away from zones other than extra-articular, and away from devascularized sites where osseous necrosis can occur.
Deviation from these rules appears to cause complications and results in higher rates of infection.Demers 2002
The use of coral to treat fractures of the calcaneus requires great expertise as infection rates of 38% and 75% have been reported. The calcaneous sustains high levels of mechanical load and because of this, unless a type of coral exoskeleton other than Porites is used, coral application in the site is not recommended. Retrospective studies of cervical interbody fusion in humans have shown that the use of coral grafts with lower porosity such as Acropora is preferable. Porites particles of mainly 300 to 450 microns (instead of blocs) have been used successfully to treat bone defects of the maxilla and mandible areas. Coral has been applied to treat cranial-maxillo-facial defects. Most studies report the use of blocks of coral to treat these defects except for some facial reconstruction in which the high facial contours make shaping of coral blocs difficult. In the case of cranial applications, especially in the area of the cranial base bone, it is necessary to find a coral implant with a lower resorption rate (lower porosity) because high porosity Porites resorbs faster than the very slow rate of bone formation in that region.Demers 2002
A review of 158 patients who had received coralline hydroxyapatite orbital implants showed that over a follow-up period of 6 to 130 months (average, 39 months), complications reported after the placement of the implant included discharge, implant exposure, conjunctival thinning, pyogenic granuloma formation, implant infection, and persistent pain or discomfort. The problems seen were similar to those with synthetic FCI3 HA and aluminum oxide (bioceramic) implants.Jordan 2004
Twenty patients with mechanical low back pain consistent with discogenic pain symptomology, each with loss of disc height and disc dehydration consistent with degenerative disc disease on magnetic resonance imaging, had anterior lumbar interbody fusions (ALIF) using a coralline hydroxyapatite bone graft. The mean preoperative disability score of 64% was reduced to 35% at latest follow-up. Clinical success was demonstrated in 16 patients who reported pain relief of 50% or more.
The authors concluded that coralline hydroxyapatite performed similarly to autograft and allograft as the anterior component in an instrumented circumferential lumbar fusion model. The major drawback to its use is that it has no osteoinductive properties. The implant is osteoconductive, so it requires a large interface directly with the bone or fusion will not occur.Thalgott 2002
Another study has shown that natural coral blocks placed in the iliac crest defect resorbed centripetally and are smaller than 50% of their original size, on average, at the end of approximately 2 years. None of the coral blocks resorbed completely. Coral served as a scaffold for soft tissue and to some extent for bone ingrowth, but the original form of the iliac crest was not achieved.Vuola 2000
Bone defects in humans have been shown to heal rapidly following reconstruction with coral microgranules. Biopsies at 8 and 18 months showed good bone formation around the coral particles.Issahakian 1989
Coral is used for its mechanical properties and is not administered as a drug; therefore, there is no specific dose.
Pregnancy / Lactation
Information regarding safety and efficacy in pregnancy and lactation is lacking.
None well documented.
Adverse reactions from coral have not been reported.
A follow-up of patients for 6 to 24 months found no deleterious host responses and good tolerability to coral implants.Zeng 1991 Data is not sufficient to confirm the benefit of coral products in assisting bone growth in severely damaged weight-bearing bones.
No immunologic or giant cell reaction is seen surrounding the bulk implants. The implants initially are mechanically weaker than the host bone, but with host tissue invasion, the strength of the implant increases proportionally with the amount of tissue ingrowth.
From the mid-1980s to the present, bulk implants of coraline hydroxyapatite have been used successfully to reconstruct cancellous metaphysical defects, but they are not recommended for comminuted diaphyseal defects because of incomplete incorporation and lack of remodeling. This compromises the ultimate mechanical strength of the diaphysis. The poor initial strength and nonconforming handling properties of coraline implants also constitute a disadvantage with respect to their use as cortical diaphyseal implants.Cornell 1998
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