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Dual Mobility Hip Testing

How does dual mobility hip testing contribute to ensuring the durability and safety of joint replacement implants? We examine the significance of this innovative testing method and its pivotal role in advancing new innovations in joint replacements.

Our team shares a strong focus on advancing the medical device industry through innovative research, design, and testing. We have designed and developed reliable ways to test and analyze wear on complex dual mobility hip replacements effectively to bring your novel products to market quickly.

Figure 1: Graphic image of a dual mobility hip replacement.

What is a dual mobility hip replacement?

Dual mobility (DM) hip replacements were developed to improve stability and increase mobility in the hip joint by means of its unique dual articulating bearing design. In a dual mobility hip replacement, a fixed metal shell (cup) is implanted into the acetabulum, a femoral head is fixed onto the femoral stem, and a second larger head (polyethylene insert) articulates in between (Figure 1).

 

Daily low range-of-motion (ROM) activities such as walking initially triggers movement at the inner articulation (between the femoral head and the polyethylene insert) where friction is lower.

When actions that require a higher ROM are performed, like squatting, the outer articulation (between the shell and the polyethylene insert) is activated from the inner articulation constraints and provides an extended and unconstrained ROM to the hip joint.

This dual mobility mechanism significantly reduces the risk of dislocation compared to conventional hip replacements because it requires more distraction, or ‘jump height’, to dislocate from the acetabular cup (Figure 2).

Figure 2: Graphic demonstration of impingement leverage causing a ‘jump height’ in a standard hip replacement versus a DM hip replacement.

Important considerations when planning for a dual mobility wear test

The mechanics of a DM hip replacement imparts challenges and new methods for wear testing compared to testing standard hip replacements. The most important step when planning for a DM wear test is to evaluate and define the worst-case conditions effectively. You may ask, what implant size combinations should be tested? How do we maximize wear rates? And what test method should be used?

First, deciding the best implant size combination to test may have more than one answer. To guide this decision, it is important to consider device combinations that will maximize wear rate, which commonly suggests testing the largest inner and outer articulating surface diameters. However, you must also consider factors which risk inducing subsurface fatigue, such as thinner polyethylene inserts that could result in delamination wear. Therefore, finding the right balance between the perceived risk of delamination and maximizing wear rate is critical when selecting your optimal implant size combination for testing.

Second, the method of testing must also impart worst-case conditions. This requires modifications to standard wear test methods that would otherwise only consider typical wear at the inner articulation. We have experience in developing several custom test methods specific for DM hip replacements that apply worst-case conditions and can help you determine the right path for your DM hip device.

How do we modify test methods for dual mobility wear testing?

We have successfully modified standard hip wear test methods for DM hip replacements by directing movement to the outer articulation, enforcing full-ROM, and simulating rigorous adverse wear testing conditions. These methods model worst-case physiological conditions appropriate for DM hip replacements that deliver accurate and reliable wear results.

To implement full-ROM we have created a locking mechanism in our set-up at the inner articulation that forces all motion towards the outer articulation. The clinical allegory to this is the potential for scar tissue to form around the neck of the femoral stem that would limit movement of the inner articulation, creating a worst-case wear scenario.

Incorporating adverse wear testing (often with third-body particulates) into your DM wear test is strongly advised because it involves extreme test conditions that push the boundaries of implant durability and performance. We have developed several adverse wear testing techniques and the ‘know how’ to effectively adapt and combine these procedures with recognized standards for particulate testing suitable for DM hips. Our methods have proven to generate repeatable damage to both inner and outer articulations and have shown reproducible wear results.

Figure 3: Photograph showing an example of our locking mechanism that diverts motion to the outer articulation of a DM hip replacement.
Figure 4: Inner (top) and outer (bottom) articulations of a DM polyethylene insert measured using CMM and geometric deviation modeling to characterize wear scars.

Our reliable and repeatable wear analysis procedures

Alongside our experience with adapting standard wear test methods for DM hip replacements, we have addressed the challenges involved in dual mobility testing and wear analysis procedures, including: component disassembly and re-installment at test intervals for gravimetric analysis, as well as post-test dimensional analysis to assess wear scars, surface morphology, and define minimum polyethylene thickness (Figure 4). Our engineers routinely perform dimensional measurements using a high precision coordinate measuring machine (CMM) and a repeatable set-up.

Our expert testing support and services

Our specialized team of joint replacement engineers is here to support you through the planning, test execution, and analysis of your DM hip replacement. We work directly with you to determine the appropriate worst-case conditions and modify the test methods accordingly to obtain the results you need.

Wear testing is performed on our AMTI Hip Simulator using customized fixtures tailored to your device. In addition to wear testing, DM hip replacements can also be tested for stability through pull-out, lever-out, and impingement testing using our custom test methods adapted from ASTM F1820.

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