Biomaterials and bone

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tant to the place of the prime implant. Composite materials such as carbon fiber are also widely used and they have excellent mechanical charac- teristics.
Aging Clinical and Experimental Research

Biomaterials and bone Daniele Pili and Paolo Tranquilli Leali AOU, Clinica Ortopedica, Università degli Studi di Sassari, Sassari, Italy ABSTRACT. The healing process of bone is influenced by several biochemical, biomechanics, cellular, hormonal and pathological mechanisms. The ideal biomaterials should therefore guarantee the same mechanisms and be able to "heal". At present we do not have such materials at our disposal. We can anyway select among several biomaterials with different characteristics to best suit our need. In this paper we will overview some of the biomaterials used today in bone surgery with regard to their main biological properties as well as their compatibility. (Aging Clin Exp Res 2011; 23 (Suppl. to No. 2): ##-##) ©2011,

Editrice Kurtis

The ideal biomaterial in bone surgery should be able to be repaired by biological mechanisms. To do so, they should provide the adequate environment for the three key factors: osteogenesis, osteoconduction and osteoinduction. Osteogenesis is defined as the process of laying down new bone material by specialised cells and is mainly determined by the presence of osteoblasts and bone precursors such as proteins and minerals. Osteoconduction is the property of a surface to make bone grow on its surface or down into pores, channels or pipes (1). It involves the presence of a scaffolding which allows the migration of cells and development of blood vessels. Osteoinduction is the process by which osteogenesis is induced, leading to bone tissue growth into the structure of an implant or graft. It is regularly seen in any type of bone forming and bone healing process, and in the case of bone implants. Implant materials of low biocompatibility such as copper, silver and bone cement show little or no osteoconduction (1). Osteoinduction implies the recruitment of immature cells and the stimulation of these cells to develop, when stimulated by growth factors, into osteoblasts which will deposit new bone, also outside bone compartment. Several biomaterials with high biocompatibility are available today, but none of them has all the characteristics to be considered ideal. Among biomaterials we have

biodegradable materials such as poly-L-Lactic acid (PLLA). PLLAs have different kind of biodegradability and a good compatibility, but are not well integrated as they are always separated from bone tissue by a fibrous membrane (2). Polyethylene is not biodegradable and is always separated by a fibrous layer from the surrounding bone. In addition, polyethylene is notorious for his attitude to release micro and macro particles causing a local reaction in the area of deposit. The area of deposit can also be distant to the place of the prime implant. Composite materials such as carbon fiber are also widely used and they have excellent mechanical characteristics. Their drawbacks are friction wearing and their attitude to release their fibers, especially with regard to carbon fibers, which can elicit massive inflammatory reactions (3). Among ceramics, hydroxyapatite is the most widely used as a coating and provides an excellent interface between material and bone. Nonetheless, hydroxyapatite can cause a local lytic reaction when the coating is too thick making the coating itself delaminate. Bioactive glasses are less used as coating than hydroxyapatite. They form a silicate gel around their surface, which is slowly reabsorbed conducting the bone tissue towards the surface (4). Titanium alloys have already proved their outstanding biocompatibility and osteoconductive properties. Intervertebral cages made of titanium alloys have been used in spinal surgery for interspinal fusion. They are designed to distract the intervertebral spaces to achieve stability by stretching the intervertebral ligaments and annulus and to allow new bone formation through custom cavities in the cage itself. The fusion process between the two vertebrae is achieved through new bone growing into and around the cages and forming bone bridges between the two vertebrae. The fusion process usually occurs within 24 months and this is also possible due to the osteoconductive properties of this biomaterial. The new forming bone is not in contact with the device itself but through a fibrous membrane which is in intimate contact

Key words: ????????????????????? Correspondence: Paolo Tranquilli Leali, AOU, Clinica Ortopedica, Università degli Studi di Sassari, Viale San Pietro, 07100 Sassari, Italy. E-mail: [email protected]

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D. Pili and P. Tranquilli Leali

with the device allowing good adherence to the bone tissue. Another widely used biomaterial in bone surgery is PMMA (poly methyl methacrylate). It has several applications such as prosthesis implants and, as bone filler, it has been used in vertebroplasty and kyphoplasty. In this last application it has been tested in comparison with calcium phosphate, another bone filler, with regard to inflammatory reaction and osteointegration. In this studies, calcium phosphate shows a less intensive inflammatory reaction with a better osteointegration (5). At present there are not long-term clinical studies on calcium phosphate, but it has shown to have better biological properties (6). Calcium phosphate shows a better integration and a gradual replacement of the filler with new bone tissue with preserved mechanical properties, but it shows to be inferior, from a mechanical point of view, when compared to PMMA. Therefore, calcium phosphate may have a better indication in young patients with vertebral fractures, in which is more suitable a biomaterial with good biocompatibility. In conclusion, we would also like to mention the use of growth factors to facilitate the use of biomaterials. In literature there are just preliminary and controversial data on the employment of growth factors with synthetic materials or bone. Some studies show that PDGF is not able to improve bone formation nor quality of bone when used alone or, when used as a growth factor, on allotransplants or xenotransplants (7). Some other studies show that PDGF has a relevant effect on the early phases of bone regeneration when used with autologous bone with a possible osteoinductive effect (8).

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To suggest the use of growth factors is also a study based on the use of PDGF on tibial osteotomies, which has shown significant improvement in early healing processes and that the graft is more rigid and resistant after the use of PRP than without the use of any growth factor. Disclosure statement ??????????????????????????

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