Better bonds with bone

Researchers in the UK hope to increase the lifetime of prosthetic joints by allowing them to form bonds with bone

Surgeons often only offer replacement joints such as hips to older patients as the growth of fibrous tissue around them can lead to failure within 15 years of fitting and revision surgery is complex. Introducing a degree of randomness to a regular pattern on the titanium surfaces of prosthetic joints could increase their life span by letting bone tissue grow into them instead.

Dr Matt Dalby, lecturer in cell engineering at Glasgow University, is leading research combining stem-cell biology and material science with the aim of encouraging bone to grow onto the implant.

‘Currently, the problems are twofold,’ he said. ‘You get the wrong tissue type forming on the bone, and the modulus [stiffness] of the material is so much higher than that of bone you don’t get bending of the implant with the bone.’

Dalby previously carried out research with a fabricator at Glasgow, which showed that bone marrow stem cells could be encouraged to develop into bone by creating nanoscale topography — surface roughness — on polymers, and wanted to apply the same idea to titanium.

There are currently two different ways of fitting prosthetic joints. A cemented prostheses uses similar chemicals to epoxy resin, but sets at a high temperature that causes tissue damage and could leave toxic monomers if it does not completely react. The alternative is cement-less stems, which have textured surfaces to encourage tissue growth.

‘With a random surface topography you can’t work out what’s having a good effect, what’s having a bad effect and you can get conflicting results in surface roughness studies,’ Dalby explained. ‘The problem is that two surfaces with similar average roughness might look very different. We are trying to use defined surfaces that we know will definitely give us an appropriate response.’ Surface topography is one of three factors that affect stem-cell growth alongside the chemistry of the environment and the stiffness of the implant. Surface chemistry can get washed off and bioactive fillers can affect the strength of the implant. Load-bearing implants need to be very hard so the choice of material is restricted. Changing the topography is a practical approach as it uses the same materials and does not introduce foreign chemicals into the body.

One of the collaborators on the project, senior research fellow Dr Bo Su at Bristol University, will use electron-beam lithography to pattern titanium surfaces. This will introduce a concept developed in previous work at Glasgow University combining precisely defined patterns with the total randomness of anodising or sand blasting to create the ideal reproducible surface for bone growth.

‘Cells do not like a precise pattern as it is not natural,’ said Dalby. ‘We added a slight amount of disorder to the electron beam pattern by telling the writing programme to stick in plus or minus 50 nanometres disorder from placing our regular 300 nanometre square patterned pits. Cells loved it, and formed bone with a very similar efficiency to chemical treatment.’

This process can produce well-defined features in terms of height, diameter and packing with a balance of order and randomness. It will build tiny 15-nanometre islands and micro machining will be used to put grooves onto the surface.

‘Our research shows that if we get the groove size right, they should also be osteoinductive so help with the production of bone,’ added Dalby. ‘The other thing I hope they’ll do is allow bone to grow into the implants a bit — a process known as interdigitation. We want not just to encourage bone growth, but to encourage bone growth right into the implant.’

Other project partners are Southampton University and the AO Research Institute. Southampton’s School of Medicine will supply stem cells and expertise in pre-clinical and clinical trials. The AO Research Institute is an orthopaedic trauma research centre where an electron microscope will be used to examine focal adhesions of cells — how the cells are adhering to the materials. A consultant orthopaedic surgeon at Glasgow will steer the technology into a format for clinical and pre-clinical trials.

The project runs until 2012, at the end of which the researchers hope to produce a prototype.

‘The ultimate aim is to be able to make stems of implants that will allow surgeons to work on younger people who are suffering from osteoarthritis due to sports injuries or early onset rheumatoid arthritis,’ said Dalby. ‘This is clearly important as we’re all living longer and more of us will need replacement joints. We want to create an implant that will bond directly to the bone and last for the lifetime of the patient.’

Berenice Baker