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Listen to the Hips When They Can No Longer Hop

hip x-ray

Hip implants and their replacement are a growing consequence of an ageing population.

By Geoffrey Rodgers

Hip replacement implants wear out and need to be replaced, but determining when this is necessary is a significant challenge for orthopaedic surgeons. Now an ultrasound device has been developed that can detect the vibrations made by microscopic abrasions within implants.

The development of artificial hip replacement implants has been a godsend for patients suffering from degenerative joint diseases like osteoarthritis. However, the implanted components do not last forever and will eventually require revision surgery to replace the cup and ball.

Patients often come to surgeons complaining of pain or discomfort from their replacement hips, and in some cases even experience audible squeaking from the hip joint. However, it can be difficult for orthopaedic surgeons to diagnose what is happening within the joint to give their patients such trouble.

With an ageing population it is very important to enable surgeons to determine the implant’s condition and decide upon a correct course of action, without needing to operate unnecessarily. By listening to the ultrasonic vibrations of the implant, we think we might have found a solution.

Total hip replacement implants consist of four main components: the femoral stem, the femoral head (or ball), the acetabular liner (or cup) and the acetabular shell (Fig. 1). The femoral stem is the metal component that is placed down the centre of the upper leg bone, the femur. The femoral head is the spherical bearing component that is mounted into the stem. The acetabular shell is mounted into the pelvis and contains the hollowed bearing component that interfaces with the femoral head or ball. These components together replicate all the components and functions of a healthy natural hip joint.

Listening to the Hips

We are developing an acoustic emission monitoring system that uses four passive ultrasound receivers to record vibrations made by implants as patients go about their daily activities (Fig. 2). Much like in the engine in your car, whenever two surfaces move against each other and transfer load, they create small vibrations. These vibrations are small when everything is in good working order, but the vibrations can become much larger and more noticeable as the components deteriorate. The same is true of a hip replacement implant. Soon after surgery, the implant will operate well and vibrations will be small, but vibrations can increase markedly after 5–10 years as the bearing surface wears or the metallic implants loosen within the bone.

The four ultrasonic receivers are placed directly against the skin’s surface near a hip replacement implant, from near the pelvis to the mid-thigh. As a patient moves through a normal range of motion, such as standing up from a sitting position or climbing stairs, the implant components move against each other and create vibrations. If we record these vibrations in test patients that have implant problems and then observe their clinical outcomes during subsequent revision surgery, we can establish a link between the vibrations we hear on the skin surface and the condition of the implant. We can then use these relationships between the vibrations and the clinical outcome to better understand the condition of implants in future patients.

Overall, the concept is to develop a non-invasive monitoring system that can be worn by patients during their daily life and doesn’t expose them to radiation. The system can provide insights into what happens during normal daily activities. Much like taking your car to the mechanic and finding that you cannot reproduce a problem that you’ve been having, patients often complain that the implants “behave” during consultations with their surgeons, even if they’ve been having ongoing problems. Therefore, the ability to monitor a patient over several days during their normal life is an extremely important aspect of this monitoring technique.

The Hips Don’t Lie

Several mechanisms can necessitate the revision of an implant. For instance, the main bearing surface between the ball and cup can degrade and become rough. Instead of a smooth sliding interface, the rough surface can create small vibrations that cause discomfort for patients. The implants can also loosen within the bone, leading to unwanted movement of the implants and observable vibrations with different characteristics to those produce by wearing of the bearing surfaces.

Patients that are scheduled for revision surgery have been tested with our monitoring device, enabling us to listen to the vibrations made by the implants across a range of different motions. When these patients have then gone onto revision surgery, the implant components removed and replaced by the surgeon are kept and carefully examined in the laboratory. The implants are then put through motions that are similar to the motion they would undergo within the patient, and recordings are made.

Certain characteristics can be identified by breaking signals down into their frequency components. Some signals will show a strong dominance of a certain frequency, such as a squeak, enabling us to identify that implant’s behaviour and later refer to it when the same sounds are recorded in a future patient test. This linking of laboratory benchtesting results from retrieved implants to sounds recorded within current patients enables us to get a lot of additional information about what might be going on within these patients.

This information can be used by orthopaedic surgeons to help diagnose problems and decide on a treatment method. Our monitoring system can also obtain information about the implant’s response to patient motion that cannot be obtained from a static image, such as an X-ray or traditional ultrasound.

We have also looked very closely at the surfaces of implants that have been retrieved from patients to observe the surface roughness at a microscopic level. Scanning electron microscopy of wear surfaces at 3000 times magnification has shown that wear mode is due to inter-granular failure of the ceramic surface after very small grains of ceramic material have been torn out of the surface of the implant (Fig. 3).

This microscale failure of the surface increases the surface’s roughness and results in an increase in friction between the bearing surfaces. This may contribute to a loss of the lubrication film between the components and contribute to squeaking by the implant.

Conclusion

Our ultrasound monitoring system listens to very small vibrational sounds made by an implant as a patient moves, enabling us to hear what an implant is doing within a patient. It provides insights into the implant’s mechanics during motion that we cannot obtain from an X-ray or ultrasound, and helps guide the decisions made by surgeons. Possible mechanisms of squeaking we have identified could help improve implant designs, but more work is needed to provide additional information to surgeons and implant designers to get the best possible patient outcomes and the lowest cost to the health system.

Geoffrey Rodgers is a lecturer in Mechanical Engineering at the University of Canterbury.