Biomechanics & Orthopaedic Interventions

Head of Research: Dr Jan Herman Kuiper

Orthopaedic Interventions

Collaborators: Ben Chatterton, Sagarika Haranatha and Derfel Williams

Joint replacement

The ability to replace painful hips and knees by artificial joints forms one of the success stories in orthopaedics. However, over a period of 20 years around 8-10% of these implants need revising, mainly because they have loosened and caused resorption of the surrounding bone. For hip prostheses, these results depend strongly on the type of bearing in the prosthesis. For prosthesis with a metal head and polyethylene cup, the typical 20-year revision rate is around 5-6%, but if both components are made of metal this revision rate is around 20%!

The higher revision rates of metal-on-metal prostheses were mostly the consequence of tiny metal particles or “nanoparticles” (mainly cobalt and chromium) that wear from these prostheses. These nanoparticles spread throughout the bod, which means the patients’ blood has higher levels of cobalt and chromium than the blood of people without implants or patients with a conventional prosthesis. In the past, many people expected that the extra chromium and cobalt would have harmful long-term effects, but this has never been demonstrated. As it turned out, a much bigger problem came from the nanoparticles that did not enter the bloodstream but remained inside the hip. These caused severe allergic reactions and “pseudo-tumours”, and hip patients suffering from this or at severe risk needed their implant being revised. However, not only metal-on-metal implants, but also hip implants that consists of loose parts or “modules” that need “modular connections”, such as a separate metal head connected to the main prosthesis, wear metal nanoparticles that can be measured in the bloodstream.

To understand how cobalt wear particles spread into the hip and then throughout the body, we developed a Physiologically Based Pharmacokinetic (PBPK) model (Fig. 1). This is a type of computer model, normally used to predict how medicines spread through and stay in the body. Our model predicted that the cobalt concentration rose quickly in the hip synovial fluid and the synovial membrane, where it stayed high (Fig. 2A). In blood and urine it would rise steadily in the first 6 months after implantation, after which it would start to go down slowly (Fig. 2B). This pattern corresponds to that found in humans (Fig. 2D). The metal concentration in liver and kidney was predicted to rise more slowly and reach a peak at 2 years, after which they started to decrease very slowly (Fig. 2C). We hope this model will help predicting which patients are most likely to be at risk from adverse reactions to high metal levels.

Limb lengthening

Children who have one leg longer than the other, are at risk to develop problems later in life once they are fully grown. Unfortunately, while the child is still young it is hard to decide which difference in length will cause problems later in life. This makes it hard to decide the benefit of complicated leg lengthening procedures, such as surgically cutting the bone followed by using an external fixator to lengthen the leg. A relatively simple procedure is to cut only the membrane that surrounds the bone, known as the periosteum. Bones whose periosteum has been cut grow slightly quicker, and over time this could resolve the difference in leg length. Children undergoing the procedure need only relatively small surgical cuts in the shorter bone, which quickly heal (Fig. 3). However, not much is known about this procedure. We studied 18 children whose periosteum around the shortest bone had been cut. On average, the difference in leg length quickly reduced after the procedure before settling on an average reduction by around 1 cm (Fig. 4). In a few children, the shorter leg grew so much that it ended being the longer leg after the procedure (Fig. 4, blue curves)! We concluded that this procedure might be helpful in children whose legs differ around 2-3cm in length, which is the difference where simple orthotic solutions such as a heel rise or thicker soles become problematic. Even reducing the leg length difference by 1 cm will make applying such simple solutions much easier.