By: 1 February 2008

An Introduction to Bioactive Materials in Orthopaedics

Metaphyseal defects are a common complication of periarticular trauma, where the cancellous bone is subject to compressive forces. In order to address this situation, a mechanical pillar of support (buttress) is required to uphold the periarticular cortical bone. Traditionally autogenous bone graft has been the material of choice when faced with the problem of metaphyseal bone defects however the associated problems with harvesting the graft and the extra surgical time required obtaining it, have opened the door for the use of artificial graft substitutes.

These substitutes can be divided into three main types.

1. Osteogenic: these materials contain living cells with the capacity to differentiate into bone e.g. autogenous bone graft and bone marrow stromal cells (both are also osteoconductive)
2. Osteoinductive: these materials provide biological stimulus for local or transplanted cells to differentiate into osteoblasts e.g. bone morphogenic proteins
3. Osteoconductive: these materials act as a scaffold to promote bone growth into the artificial graft e.g. hydroxyapatite and calcium bone cements

It should be pointed out that many graft substitutes display a combination of these properties. Demineralised bone matrix is both osteoconductive and osteoinductive and the other prime example is autogenous bone graft which is both osteogenic and osteoconductive as mentioned above.

In the application of primarily osteoconductive substances, one should bear in mind that they are not appropriate for either the treatment of non-unions or for large segmental defects by themselves. The more appropriate choice would be either to use them in conjunction with an osteogenic or osteoinductive material or to choose a substitute which displays multiple properties. In general osteoconductive substances are also contraindicated in the presence of osteomyelitis and in children with open growth plates1.

So Why Choose Calcium Phosphate Bone Cements?

Returning to basic fracture treatment principles, management of periarticular trauma with metaphyseal compromise requires stable fixation. Stable fixation in turn allows you to commence early movement and weight bearing which is beneficial in the return of joint function. The complicating factors common to achieving stable fixation in older patients with periarticular trauma are lack of a buttress to support the cortical bone and the presence of osteoporosis.

There are several options for filling the metaphyseal defect. Autogenous bone graft is the current gold standard. But in addition to the surgical morbidity involved in its extraction it is further compromised by the lack of immediate support it provides to the periarticular cortical bone. This can lead to failure of the hardware before the bone has healed. Acrylic bone cements such as commonly used in arthroplasty e.g. (polymethylmethacrylate) have been shown to provide fracture stability in osteoporotic patients. But there are several concerns with its use relating to the exothermic reaction during its curing process, the inability for the cement to be remodelled and the difficulty with its removal in revision surgery2.

Calcium bone cements show several advantages in these respects. Firstly, they provide immediate support to the fracture site, preventing early hardware failure. Secondly, they effect hardware support by increasing screw pull out strength either directly in cancellous bone or in combination with plate fixation3. Thirdly they undergo osteoclastic reabsorption and are eventually replaced by circumferential lamellae of new bone. Fourthly, the rate of reabsorption matches that of host bone formation which prevents premature fracture collapse.

Biomechanical & Biochemical Properties Of Calcium Bone Cements

In the early 1980s, at the National Institute of Standards and Technology in America, researchers were working on developing mineralising pastes for the treatment of dental caries based on calcium phosphate. When some of these pastes were inadvertently left for a few hours in test tubes with water, they were found to have hardened into a solid mass of hydroxyapatite. Many experiments followed in the subsequent years substituting various calcium compounds and other solvents which resulted in the first calcium phosphate cement being licensed for the repair of cranial defects in 1996. The properties of calcium cements which make them so attractive include their superb biocompatibility, self hardening and their gradual resorption by bone.

The calcium phosphate cements were prepared in a similar fashion to polymethylmethracylate (PMMA) cements involving a powder and a liquid. Tricalcium phosphate (TCP), monocalcium phosphate (MCP) and calcium carbonate in powder form were mixed with a liquid containing a solution of sodium phosphate.

Preparation of the cement involves five stages.

1. Dry Mix: grinding of the powders into a uniform consistency
2. Wet Mix: add liquid, mix and then place into the appropriate delivery system e.g. syringe
3. Working Time: it remains a injectable paste for about 5 min (at 20

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