Context:

Ortho-DX needed to improve the measurement tolerance obtainable with their sensor under development for monitoring micromotion in joint implants.

Solution:

TTP carried out an independent assessment of the sensor prototype and developed a multiphysics model that was used to refine the design.

Result:

TTP suggested design improvements indicating that a five-fold improvement in measurement tolerance is technically feasible, greatly improving the prospects of attracting funding from investors.

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Detecting micromotion of joint prostheses

Over ten percent of annual joint replacement procedures are due to revisions- primarily caused by component loosening. Ortho-DX is developing a system to precisely monitor the dynamic stability of a surgically placed orthopaedic implant in an effort to significantly reduce the annual number of revision surgeries by measuring its movements with respect to the host bone under varying loads representative of ordinary daily activity.

This is accomplished by use of wirelessly communicating induction sensors placed in proximity to, but not contiguous with, the surgically installed orthopaedic hardware. Subsequently, short, non-intrusive and radiation-free regular monitoring sessions collect data indicative of the healing process. Periodic monitoring provides early warning of implant loosening -generally defined as persistent motion greater than 150 microns. Should micro-motion exceed this threshold, therapeutic intervention may begin. Having already developed a prototype as well as two US patents and a peer reviewed paper, a second iteration prototype, designed to meet FDA requirements, is being pursued with the collaboration of TTP, a renowned research and development centre located in Cambridge, UK.

Through the combined efforts of Ortho-DX and TTP it is anticipated that a much-needed diagnostic tool will be developed, providing quantitative motion measurements of orthopaedic hardware at the bone/implant interface in real time, without use of x-rays, without batteries, with precision within twenty to fifty microns.

Post-operative micro-motion of hip and knee implants – of the order of 150 microns – can lead to pain for the patient and failure of the bone to grow into the prosthesis. But by the time that any loosening of the implant is apparent using standard X-ray tools (which have a detection limit of only 1–2 mm), it’s usually too late to offer anything to the patient except a ‘revision’ surgery. This is not only very expensive but has a reduced chance of success.

A new wireless induction sensing technology, developed and patented by US-based company Ortho-DX, has the potential to solve this problem, by enabling in situ measurement of micromotion. By 2024, Ortho-DX had developed a bench prototype that demonstrated ‘proof of principle’ for their design. But they needed to improve the accuracy of the measurement, and also have their design externally validated, in order to attract funding and proceed to the next stage of development. That’s where they turned to TTP.

Technical challenges in optimising sensor performance

Dr Sophie Meredith, a specialist in implantable medical devices and TTP’s team leader for the project, explains the scientific principle behind Ortho-DX’s innovation. “Their sensor is implanted in the bone about 5 mm from the metallic prosthesis, and is driven to resonate electrically by an external antenna. Resonance coupling to the prosthesis causes it to lose energy, and the rate of this loss varies as a function of the distance between the sensor and the implant. So, if the implant is loose, you would see a change in that distance when the joint bears weight, and that could be detected by the sensor”.

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Although the prototype sensor could detect displacements of as little as 40 microns – approximately the preferred level recommended for a new implant – it was very sensitive to external movement. To address this, Dr Meredith says that the first stage was to examine the existing bench prototype, and to consult closely with Ortho-DX’s staff to learn from their experience.

With that done, the TTP team were able to create a first-principles multiphysics model of the whole sensor system that allowed them to simulate the effect of all the system variables, such as the sensor design, tissue conductivity and temperature. They were then able to factor in the influence of signal processing, and so work towards their aim of making the measurement more robust in the face of external influences.

The result was that the team were able to propose a modified sensor and signal-processing system that together enhanced the response (per millimetre of movement) by a factor of five, boosting the signal-to-noise ratio of the detected signal for better detection of smaller displacements.

Rapid progress with a multi-disciplinary team

Working on this project required a cross-disciplinary suite of skills, says Dr Meredith: “Our project team included four TTP consultants specialising in different areas: implantable devices, wirelessly coupled sensors, mathematical modelling, and digital signal processing”.

And as project leader, Dr Meredith helped to bring this expertise together and ensure that the needs of the patient interface were kept in view – an aspect that she says is all too easily overlooked when the focus is on overcoming individual technical challenges. She also kept the project on track by arranging updates with Ortho-DX, and identifying and dealing with emerging questions.

The whole project, she says, was also enhanced by the team’s experience of using similar sensors in adjacent industries. This enabled them to take ‘short-cuts’ to performance-enhancing design tweaks that would have otherwise taken much longer to identify. “Having that input was really helpful to adhere to the tight timeline for the project, and deliver a rigorous assessment within a couple of months”, she says.

Independent assessment of sensor improvements

And that assessment report offered some very useful insights, explains John Lucey, M.D., Founder and Chairman of Ortho-DX. Whereas a 120 micron displacement on the existing prototype system could be measured with a tolerance of 60 microns, the design updates proposed by the team indicate that it is feasible to reduce that figure to less than 10 microns. “We needed to make our sensor much less sensitive to external influences, and TTP have shown us that this can indeed be achieved”, he says.

He’s also been impressed with the modelling work the team has done: “Thanks to the help from TTP, we’ve progressed from a proof-of-principle to a fully digitally simulated part… and we’re now in a much better position to attract funding and take our product to the next level”, he adds. That process is well underway, he explains, with the US FDA already involved on the regulatory front, and TTP ready to step in again when Ortho-DX give the go-ahead for construction of a life-realistic demonstration model.

Lucey says that it was especially useful that the expertise at TTP was all under one roof. This helped avoid the need to outsource operations to multiple suppliers, while increasing the pace of collaborating across multiple specialities. It was also beneficial that the team were able to keep the ultimate clinical application of the sensor in mind throughout. “TTP had an impeccable track record, and I’ve been very pleased with their work”, Lucey says.

A valuable tool for orthopedic surgeons

So what impacts could the Ortho-DX system have when it reaches the clinic? Lucey explains that there are two main situations in which it could be useful, with the first being for initial prosthesis placement. “Currently, a component is physically driven down until it ‘seats’, and that’s judged very subjectively, on the way it feels, the way it looks, the way it sounds. So if you can have our sensor in place providing a quantitative indication of when it’s no longer moving, I think that would be a great help to orthopedic surgeons”.

“Thanks to the help from TTP … we’re now in a much better position to attract funding and take our product to the next level”

The second aspect, he says, relates to post-operative care, when limiting micromotion is vital for promoting osteointegration. “At the moment, if a patient presents with pain in their joint, you can’t be sure that the prosthesis is the cause. But if you have a record of the micromotion, then it helps you to pinpoint the problem much sooner, and perhaps salvage the joint by tailoring and monitoring subsequent interventions”.

Lucey concludes with an assessment of what the future holds for Ortho-DX and their sensor: “Because of this wonderful collaboration, it looks like we could have a very rewarding product that would really help orthopedic surgeons. And, after having personally worked on this concept for over 10 years, it finally feels like it’s going to happen!”.

About TTP’s Biosensing Team

Specialising in the development of wearable and implantable biosensors, TTP’s Biosensing consulting team deploys multidisciplinary teams, combining expert capabilities from electrochemistry and optics to human factors, mechanical design, software, electronics, wireless power and communications to deliver tailored solutions that meet the most demanding project requirements.

Partnering with us means working with experts who understand the biosensing industry inside out. Whether navigating uncharted territories, supporting your next innovation or accelerating time to market, we solve complex challenges with engineering and scientific rigour, finding and retiring risks early to deliver high-impact, human-centred results. When your internal bandwidth is constrained, TTP is ready to deliver. With deep expertise and a 25-year track record, we provide the agility and expertise needed to ensure your commercial success.