When a person has a bone fracture, a surgeon can attempt to remedy the fracture by performing a bone graft, surgically transplanting bone to the fractured area so it can heal. The graft, the transplanted tissue, can be taken from the patient’s pelvis or from a tissue bank. So reports journalist E.B. in an article in the periodical Medical Update.
As E.B. describes, other types of grafts have included ceramics paste and cloned proteins. However, Nature provides an interesting alternative in the form of a xenograft, a graft transplanted from a member of one species to a member of another species. And it happens that coral is a fitting source. It is biocompatible with human tissue (Demers et al), meaning it can be juxtaposed without causing toxicity, injury, or immunological rejection.
In a previous article, I described the basic biology of corals. One thing I mentioned was the difference between soft corals and hard corals. Soft corals lack any hard skeleton. Corals that possess that solid skeleton are called hard corals or stony corals. Their calcareous exoskeleton consists of calcium carbonate, which becomes the predominant constituent in the sedimentary rock limestone. A portion of this skeleton is the “coral” utilized as a bone graft substitute.
E.B. remarks that unprocessed coral is unable to support a human limb while it holds weight, although it has been used as graft material for non-ambulatory patients. However, a coralline graft can be processed before grafting it, by changing the chemical composition of the coral skeleton.
Such a graft can be obtained from marine stony coral skeleton with poriferous structures similar to those in cancellate bone, bone with a spongy or porous structure. Then the coral is converted to a solid chemical called coralline hydroxyapatite (Jahangir et al; Demers et al). Hydroxyapatite is a mineral that is the principal structural component of vertebrate bone. (Humans are vertebrates.)
In a 2002 article, Caroline Demers et al depict how scientists began considering coral as a bone graft substitute for animals in the early 1970s and for humans in 1979. In an article on the Web site of the American Academy of Orthopaedic Surgeons, A. Alex Jahangir et al report that coralline hydroxyapatite was approved by the U.S. Food and Drug Administration in 1992.
Jahangir et al also explicate that coralline hydroxyapatite is osteoconductive, meaning that it facilitates the formation of bone structure, albeit the chemical is not osteoinductive, meaning it does not stimulate bone formation. Nor is it osteogenic: it does not form bone (Demers et al). Nonetheless, Demers et al note that osteogenesis around a coralline graft can be achieved by the addition of growth factors (a type of protein) or by the addition of bone marrow cells.
Another issue is that the coralline graft should be capable of resorption, of being broken down and assimilated by the body. Demers et al relate that the procedure used to produce coralline hydroxyapatite has been adapted to make the product resorbable, when using specific coral species. Despite this, they say, “natural coral” continues to be applied as a bone graft substitute, as it apparently has different advantages than coralline hydroxyapatite.
Many other complexities of coralline grafts are covered in their extensive review.
Author: E.B. Article: New bones from the sea. Periodical: Medical Update. Volume: 20. Issue: 7. Date: January 1997. Pages: 2.
Author: Caroline Demers; C. Reggie Hamdy; Karin Corsi; Fatiha Chellat; Maryam Tabrizian; L’Hocine Yahia. Article: Natural coral exoskeleton as a bone graft substitute: A review. Periodical: Bio-Medical Materials and Engineering. Volume: 12. Issue: 1. Date: 2002. Pages: 15-35.
Author: A. Alex Jahangir; Ryan M. Nunley; Samir Mehta; Alok Sharan; the Washington Health Policy Fellows. Web page: Bone-graft substitutes in orthopaedic surgery. Web site: American Academy of Orthopaedic Surgeons – AAOS. Date: January 2008. Institution: American Academy of Orthopaedic Surgeons. Date of access: November 22, 2011. Web address: http://www.aaos.org/news/aaosnow/jan08/reimbursement2.asp.
Further Scholarly Reading
Author: Biswanath Kundu; Mithlesh K Sinha; Santanu Mitra; Debabrata Basu. Article: Synthetic Hydroxyapatite-based Integrated Orbital Implants: A Human Pilot Trial. Periodical: Indian Journal of Ophthalmology. Volume: 53. Issue: 4. Date: December 2005. Pages: 235-241.
Author: S.K. Nandi; S. Roy; P Mukherjee; B. Kundu; D.K. De; D. Basu. Article: Orthopaedic applications of bone graft & graft substitutes: a review. Periodical: Indian Journal of Medical Research. Volume: 132. Issue: 1. Date: July 2010. Pages: 15-30.
Author: LI Zhaolin; LI Wen; BO Zunzhao; YIN Qingshui; ZHANG Yu. Article: Study on the physical properties and chemical compositions of artificially synthesized coral hydroxyl apatite (CHA) bone. Periodical: Chinese Journal of Geochemistry. Volume: 28. Issue: 4. Date: 2009. Pages: 421-426.