Thomas Webster and colleagues at Brown University, Providence, US, explain why today's bone implants are so much more than your grandparent's hip replacement thanks to nanotechnology

Coating traditional implant materials, such as titanium, with selenium nanoclusters produces implants that can inhibit cancer

Bone fracture is very common among the elderly as bones become more brittle as we age. Active young people also have a high risk of bone fracture through every day life and sporting activities. If a fracture is small, it can be filled with bone cement, such as polymethylmethacrylate. However, if the fracture is large, more durable metal implants, such as titanium and titanium-based alloys, are used. The goal is to not only fill the fracture space with a strong material that can support the body's weight, but also to promote new bone growth to fully restore the bone's functions.

In the past, bone implants were made of inert materials, chosen because they didn't severely influence bodily functions or generate scar tissue, which is a thick, insensitive tissue layer that can form around an implant. But this simple design principle causes implants to loosen from the surrounding bone after around 10 to 15 years. Loosening becomes worse with time and can cause significant pain. As a result, patients often undergo additional surgery (called revision surgery) to remove the loose implant and insert a new one. Revision surgery is clearly undesirable as it is costly, painful and requires therapy all over again for the patient.

It is unsurprising that there has been an on-going effort to create implants that can integrate into the surrounding natural bone for the patient's lifetime. Using their understanding of bone composition and the bone-forming process, scientists have developed various methods to transform these once inert implants into implants that can promote bone growth.

One of the first approaches to make more proactive bone implants uses surface chemistry to encourage the implant to interact with osteoblasts (bone-forming cells). This method has resulted in a number of implant materials, such as bioactive glass, that show good bone formation. However, scientists often need to resort to trial and error processes to find an implant material that not only increases bone growth but also has good mechanical properties for use in cementless implants, such as the hip implant. Such combinations are not always easy to find in one material or even a composite of materials.

Nanotechnology has taken a bold new step towards improving orthopedic implant devices. Orthopedic nanotechnology is based on understanding cell-implant interactions. Cells do not interact directly with an implant but instead interact through a layer of proteins that absorb almost instantaneously to the implant after insertion. Scientists have improved numerous implant materials, including titanium and titanium alloys, porous polymers, bone cements and hydroxyapatite, by placing nanoscale features on their surfaces. The bulk materials' properties remain unchanged, maintaining their desirable mechanical properties, but the surface changes enhance the interactions with proteins. This causes bone-forming cells to adhere to the implant and activates them to grow more bone.

Scientists are also creating 'smarter' implants that can sense what type of tissue is growing on them, communicate the information to a hand-held device and release drugs on demand to promote tissue growth. These implants are designed to help avoid complications frequently observed after bone implantation, such as infection, inflammation (or scar tissue growth), implant loosening and, in the case of bone cancer, cancer reoccurrence. Scientists have been investigating implants that have inherent mechanisms to protect the body from infection (such as silver and zinc) or inhibit cancer growth (such as selenium).

Significant promise can be drawn from recent advances in biomaterials research, especially where nanotechnology is involved. But discovering the perfect biomaterial that can last the lifetime of a patient is still a challenge.

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