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EDITORIAL Table of Contents   
Year : 2003  |  Volume : 37  |  Issue : 3  |  Page : 1
Bone substitutes

Department of Orthopaedics, Institute of Medical Sciences, Banaras Hindu University, Varanasi, India

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How to cite this article:
Goel S C. Bone substitutes. Indian J Orthop 2003;37:1

How to cite this URL:
Goel S C. Bone substitutes. Indian J Orthop [serial online] 2003 [cited 2019 Dec 11];37:1. Available from:
Autograft continues to be gold standard for bone replacement but bone substitutes are gaining ground as sufficient bone graft may not always be available and there is associated morbidity involved. Allografts carry the risk of infection and compromised biological and mechanical properties.

Bone substitutes may be osteoconductive, osteoinductive or both. An osteoconductive material promotes bone apposition to its surface, functioning as a receptive scaffold to facilitate enhanced bone formation. An osteoinductive material provides a biological stimulus that induces local or transplanted cells to differentiate into osteoblasts. Common osteoconductive biomaterials being used currently are hydroxyapatite, calcium sulphate, tricalcium phosphate etc.

Hydroxyapatite is an osteoconductive calcium phosphate that can be prepared as granules, blocks, or coatings on implants. For example, Pro Osteon® is synthesized by removing the protein and replacing the calcium carbonate of sea coral with hydroxyapatite.1 Pro Osteon have interconnecting pores that allow ingrowth of blood vessels, fibrous tissue, and bone and has a compressive strength similar to that of cancellous bone. Although relatively brittle at the time of implantation, the material gains strength as bone apposition and ingrowth progress. If used for metaphyseal fractures defects, it is to be used in conjunction with rigid internal fixation. A different macroporous hydroxyapatite product derived from cancellous bovine bone as well as granules or blocks of hydroxyapatite also are available as a bone graft substitute or as a carrier for autologous bone marrow cells. Hydroxyapatite also can be applied as a coating to the surface of implants by plasma spray or precipitation techniques. Hydroxyapatite coatings are thought to enhance early bone apposition and fixation of total joint prostheses.2 Blocks, granules and coatings of high hydroxyapatite content and high crystallinity can be dissolved in the acidic pH created by osteoclasts, but are relatively stable at neutral pH and usually resorbed very slowly in vivo.

Although hydroxyapatite is osteoconductive, its resorption rate is very slow and is interpreted as a limitation in certain clinical applications. Calcium sulphate readily dissolves at neutral pH in vivo and serves as a source of calcium ions that become incorporated in bone. It dissolves too rapidly to act as an osteoconductive matrix, but calcium sulfate preparations are reported to be effective in filling unloaded skeletal defects. Calcium phosphates other than hydroxyapatite also have been advocated for use in bone voids. For example, granules composed of a mixture of hydroxyapatite and tricalcium phosphate, are osteoconductive and undergo dissolution or resorption more rapidly than sintered hydroxyapatites. When mixed with autogenous bone marrow, it is indicated for use in acute long bone fractures that are fixed internally or externally, and traumatic osseous defects smaller than 30 mm. A macroporous formulation of tricalcium phosphate is osteoconductive and completely resorbable.

Calcium phosphates also can be precipitated under conditions that favour the development of a carbonated mineral with very low crystalline order similar to bone mineral. This type of mineral can be adapted for use as injectable cement, or precipitated onto cross linked bovine collagen to form a composite for use as a bone graft substitute or carrier for bone marrow cells. Bioactive cements that are injectable and cure to develop variable compressive strength are being developed for potential use as bone graft substitutes, as grouting materials to enhance fixation of orthopaedic hardware in osteoporotic bone, for fracture fixation, and vertebroplasty. [3] The bioactive components of most cements are based on either osteoconductive calcium phosphates or silica. The recognition that the osteoinductive properties of demineralized bone matrix are caused by a group of proteins led to the isolation, purification, and eventual synthesis of BMPs. The BMPs are proteins grouped into the TGF b family. They produce bone by a complex series of events. BMP-7 / BMP-2 and BMP-2/BMP-6 have 5 to 10 fold more potency for bone cartilage formation than BMP-2 homodimens. [4] Several purified bovine BMPs mixed with Type I collagen are being studied, especially for periodontal and spine fusion applications. Recombinant DNA technology has led to the synthesis of several human BMPs. BMP-7 has been combined with a bovine collagen (demineralised bone) carrier and is available for selected applications, whereas BMP-2 with a collagen sponge carrier is being investigated for potential use in the spine and other clinical applications. Other osteoinductive agents eg. Fibroblast growth factor­2, Insulin like growth factor I and II and growth and differentiation factor -5 are promising molecules on which lot of research is going on.

   References Top

1.Bucholz RW. Nonallograft osteoconductive bone graft substitutes. Clin Orthop 2002; 395: 44-52.  Back to cited text no. 1  [PUBMED]  [FULLTEXT]
2.Bauer TW, Geesink RC, Zimmerman R et al. Hydroxya-patite coated femoral stems. J Bone Joint Surg [Am] 1991; 73-A: 1439-1452.  Back to cited text no. 2    
3.Bauer TW, Smith ST. Bioactive materials in Orthopaedic surgery. Clin Orthop 2002; 395: 11-22.   Back to cited text no. 3  [PUBMED]  [FULLTEXT]
4.Yoon ST, Boden SD. Osteoinductive molecules in Ortho-paedics. Clin Orthop 2002; 395: 33-43.  Back to cited text no. 4  [PUBMED]  [FULLTEXT]

Correspondence Address:
S C Goel
Department of Orthopaedics, Institute of Medical Sciences, Banaras Hindu University, Varanasi
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Source of Support: None, Conflict of Interest: None

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