Making waves

28 October 2017



Mechanical engineer Dr Geoff Rodgers from New Zealand’s University of Canterbury has worked with the healthcare industry to create an acoustic emission-monitoring system that can detect early signs of wear and tear in joint implants. Andrew Putwain looks into how this technique could improve the lives of patients and improve other areas of diagnostic medicine.


The early detection of wear and tear in joint implants can lead to proactive intervention that reduces the number of invasive surgeries needed and improves patient suffering.

The monitoring technique designed by Dr Geoff Rodgers measures vibrations that are created by the patient’s implant and travel through tissue to the skin’s surface. By listening to these ultrasonic vibrations, it is possible to relate them to the condition of the implant. The procedure is entirely non-invasive and can detect issues when a patient is moving and the implant is loaded.

Andrew Putwain: What is your background, and what was your role in this research?

Dr Geoff Rodgers: I originally studied mechanical engineering, but I wanted a change of scene and went on to apply the skill set I had developed on mechanics, signal processing and dynamic simulation to a new area of research. That new area was a postdoctoral fellowship where I worked in the department of orthopaedic surgery and musculoskeletal medicine. It was during this postdoctoral fellowship that I began working on the concept of acoustic hip joint monitoring. In 2012, I took up an academic position at the University of Canterbury, where I am the senior supervisor of a PhD student working on developing this system further.

I am leading this research project in collaboration with several others. More recently, we have been working with Dr Justin Fernandez at the University of Auckland Bioengineering Institute to develop the biomechanical modelling and gait analysis aspect of the project.

What are the specific issues that occur with these implants and what are the common themes regarding patient complaints?

Total joint replacement surgery is typically the last resort for people with a degenerative joint disease, such as osteoarthritis. Hip replacement surgery typically has a high success rate in returning patients to pain-free activities. The ageing population has seen the incidence of primary total hip replacement (THR) increase dramatically, and the predictions for the next 20 years suggest a significant increase in primary surgeries. As a consequence, the number of patients requiring revision surgery is also expected to rise proportionately due to eventual failure of the THR implants.

The most common cause of long-term failure of hip replacement implants is a condition called aseptic loosening. It involves the failure of the interface between a person’s bone and the hip replacement implant components in the absence of infection. It is typically related to the body’s response to wear debris that is created by years of movement at the primary bearing/ articulating surfaces of the replacement. As the bearing surfaces wear, the inorganic wear debris creates a biological reaction that causes bone destruction (known as osteolysis) and prosthesis loosening.

This process ultimately leads to prosthesis failure and the requirement of a revision procedure. A typical revision procedure for a loosened implant costs on average about four times that of the initial procedure and consequently places a large burden on health expenditure. Therefore, the ability to predict failure early and allow proactive surgical intervention has the potential to significantly help patients and the healthcare system.

Early versions of hip replacement implants suffered the highest occurrence of wear of the bearing surfaces and so had the highest rates of wear debris. To improve the long-term outcomes of hip replacements, different bearing surfaces have been developed using ceramicon- ceramic or metal-on-metal bearing surfaces. These have a much lower coefficient of friction-and-wear rate than metal-on-polyethylene, in an attempt to reduce polyethylene wear debris.

However, these bearing combinations have introduced the relatively new complication of audible ‘squeaking’ from the patient’s implant. These abnormal and irritating acoustic emissions caused by these bearing combinations, although troublesome to the patient, have not been associated with significant wear or failure of the prosthesis to date. Yet, there remains controversy as to their underlying cause and the potential impact on the longevity of the implant.

What is the goal of the study and how does the acoustic emission-monitoring system work?

The goal of this study is to develop a diagnostic method that can augment existing methods such as X-ray and MRI to provide additional insight into what is happening within a patient’s hip implant. Current methods provide a static image of a patient’s hip, and detection of deteriorating implants is largely limited to reviewing sequential changes on radiographs, which is often non-specific and prolonged.

By the time the deterioration is advanced enough to be clearly identified on the static images, the ability to retain the implant components that interface with the bone can be compromised. Early detection of implant wear and bone resorption (destruction) – which can occur around the implant from the body’s reaction to the wear debris – can potentially allow proactive surgical intervention before severe loosening occurs. In such proactive surgery, it may be possible to simply change the bearing surfaces without changing the components that interface with the patient’s bone. Such surgery is less invasive and typically leads to improved patient outcomes.

The monitoring system currently being developed uses a set of four ultrasonic sensors that are placed against a patient’s skin, near the implant. The patient then undergoes a range of standard orthopaedic motions, such as walking, rising from a chair and climbing steps. The sounds that are emitted from the implant while it is under load during the patient’s movements are recorded and later related to the clinical outcomes for the patient. The goal of this process is to establish relationships between the recorded vibration characteristics and the clinical outcome, so that emitted implant vibrations in future patients can be used to diagnose failures.

Historically, have acoustics often been used in medicine?

The broad idea of using acoustic emissions to detect conditions within the human body has been around for a long time. Medical practitioners routinely use stethoscopes to diagnose lung conditions and there has been a lot of research into using this concept more broadly in other areas of the human body. There have been other studies into the possibility of using acoustic emissions from hip joints to diagnose implant condition, including some work done by a local orthopaedic surgeon in the early 1990s. At that time, technology was limited and recent advances in sensors and signalprocessing methods have enabled us to pursue avenues of investigation that were not previously possible.

Part of your research focuses on using in-vivo procedures to analyse gait and joint motion. What new things do you hope to learn in this area?

The research we have done over the past few years has been focused primarily on relating the characteristics of the recorded vibrations to broad categories of patient motion, implant type and implant-failure mode. We could broadly relate the signal characteristics to the type of patient motion, such as walking or climbing steps, but could not identify at what point within that motion the vibrations occurred. As such, there is information we are still missing, which restricts our ability to get a full understanding of what is happening at the implant.

If we are able to undertake gait analysis and determine joint angle, we will be better able to understand the mechanisms that lead to implant vibrations, such as audible squeaking. For example, we would like to clearly determine whether implant behaviour – such as audible squeaking – typically happens during the stance phase of walking, at low-flexion angles and with large implant loading, or during the swing phase of walking when implant loads are very low. To better understand when these unusual and undesirable events occur, we will have a much better understanding of the mechanics that create them. This outcome would lead to improved understanding of the biomechanics of a hip implant within a patient.

The gait analysis will be driven by a set of small accelerometers that are strapped to a patient’s limb while they move. By recording the accelerations and rotations of the small sensors on the patient’s leg, we can develop a model that relates these measurements to the limb angles, and approximate the joint loads. The challenge we have here is to infer as much information as possible without undertaking any invasive measurements; to ensure we do not cause any pain or discomfort to the patient by the monitoring system.

How do you envision this technology developing in the future?

In the next few years, I hope to see the gait analysis developed further so that we can reliably determine joint angle and implant loads, and better understand the biomechanics within a patient’s implant, based only upon simple, non-invasive measurements. If we can couple that with an increased understanding of vibration characteristics, we should be able to provide new insight into what is happening within a hip joint, using only non-invasive measurements.

I am encouraged by the array of different medical sensing methods being developed worldwide. Many current medical treatments are restricted by our inability to fully understand what is going on within a patient, so a full suite of new diagnostic methods has to be a positive outcome for medical practitioners and patients.

Dr Geoff Rodgers: In 2012, Dr Geoff Rodgers moved to a lecturing position within mechanical engineering at the University of Canterbury, where he is currently an associate professor. He was awarded a Fulbright Graduate Award in 2008 to study at Texas A&M University, and is a Royal Society of New Zealand Rutherford Discovery fellow.
The monitoring systems uses untrasonic sensors to record emissions from a hip implant while the patient completes a variety of physical movements.
The acoustic emission-monitoring system can lessen patient discomfort and save healthcare systems money by reducing the need for invasive surgeries.


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