Longer Lasting Artificial Joint Designs with Higher Patient Satisfaction

Urs P. Wyss, Ph.D., P.Eng
.Professor, Mechanical Engineering
University of Manitoba, Orthopaedic Innovation Centre
Winnipeg, MB

Eric R. Bohm, BEng, M.D., MSc, FRCSC
Director for Arthroplasty Research, Concordia Joint Replacement Group
Associate Professor, Faculty of Medicine University of Manitoba
Winnipeg, MB

Jan M. Brandt, Ph.D.
Professor, Mechanical Engineering
University of Manitoba, Orthopaedic Innovation Centre
Winnipeg, MB

The quality and performance of artificial joints have improved sufficiently enough over the last years that they are more often used today in younger patients with joint degeneration. Younger patients, however, are generally more active and also have higher expectations of artificial joints, such as being able to do sports. After many apparent improvements, these newer joints have a survival rate of over 80% after 15 years, and this has not improved much over time. Therefore, younger patients still require one or more revisions in their lifetime. It is also a well-known fact that patients with artificial knee joints have a lower satisfaction score than patients with artificial hip joints. This can be partially explained by unmatched patient expectations, but also by the fact that knee joints are not as simple as hip joints, which are a ball and socket type of joint. There are also new demands on artificial joints from emerging markets in Asia, the Middle East and Africa that require different ranges of motion and sizes, due to different priorities in activities of daily living, and differences in overall body type, size and weight. Squatting, kneeling and cross-legged sitting is much more common in some of those countries than in so-called Western countries, requiring designs that are more tailored for such activities. The performance of ankle, elbow, shoulder, finger and toe joints also leaves room for improvement, as the survival rate of these joints is not as good as those of hip and knee joints.

 

Data from joint registries in Sweden, Norway, Australia and Canada are excellent indicators of the performance of implants. It is interesting to note that joint registries are showing that many so called improvements result in a similar or even inferior performance as compared with devices that have been used for a long time. Implant retrieval analysis on single devices, or of groups of the same device, further indicate aspects of artificial joints that are not performing optimally. It is clear that there is still room for improvements to make artificial joints that function better, last longer and achieve higher patient satisfaction.

One area that needs continued attention is wear reduction of  the metallic and non-metallic components, and implant bone or cement interfaces.   Less wear will minimize the long-term adverse tissue reactions. Less metallic wear and ion release, in particular, appears to be important in the light of recent recalls with some metal-on-metal joints, corrosion issues at head-neck and neck-stem junctions of hip joints, and other modular connections of artificial joints. Another area for improvement are modified and extensively tested new implant materials with better matching mechanical properties to the surrounding bone, and higher fatigue resistance to make the devices thinner and more bone sparing. Knee designs with more physiological kinematics constitute another area of possible improvement. Most improvements would also require different instruments that would allow a surgeon to attach and align the artificial joints more accurately onto the bone. Furthermore, we will probably see more patient-specific artificial joints and instruments to take full advantage of new designs. Patient-specific joints are more feasible today at competitive costs thanks to novel manufacturing techniques, such as plastic and metallic rapid prototyping.

Rigorous computer modelling and in vitro testing program based on existing ASTM, ISO and other standards, and what is learned from the analysis of retrieved implants, should be studied before regulatory approval for sale is requested. Tests should include kinematics and kinetics from various activities of daily living, such as walking, stair-climbing, walking up and down inclines, getting up and down from a chair, lifting objects onto shelves, carrying shopping bags and so on. It is also important that the feedback from joint registries and retrieval analysis be on-going, as new findings are fed back into further refinements and improvements of artificial joints.

There is still ample room for improvements to make artificial joints last longer, lead to better performance and higher patient satisfaction. Artificial joints with all of these improvements should only be introduced into clinics with well-controlled multicentre clinical trials, possibly in addition to Roentgen Stereophotogrammetric Analysis (RSA). Lastly, it is crucial that regulatory bodies, such as Health Canada, very carefully assess new or modified designs, and release them in a well-controlled manner. It is also advisable that regulatory bodies increasingly work with independent laboratories to verify claims made by manufacturers, rather than rely solely on data that companies are providing.

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