The Role of In Vitro Testing and Retrieval Analysis in Improving Clinical Outcomes

Jan-M. Brandt, Dipl.-Ing., PhD
Director of Tribology, Concordia Joint Replacement Group
Assistant Professor, Faculty of Medicine, University of Manitoba
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

Urs P. Wyss, PhD, PEng
Professor, Department of Mechanical & Manufacturing Engineering
University of Manitoba
Winnipeg, MB

Over the past decade, the number of patients requiring total joint arthroplasty has been rapidly increasing, especially in the group of younger, more active patients. This change in patient demographics increasingly exposes total joint replacements to increased mechanical wear and tear. Feedback obtained from retrieved components has been shown to provide valuable information to the surgeon and engineer on how the total joint replacements performed clinically. Retrieval analyses have been shown to help to improve current in vitro test conditions with the goal of increasing the durability of total joint replacements.

 

In vitro testing of total joint replacements has become widely used to evaluate new material and designs for clinical applications. Despite considerable success in the development of new, more durable total joint replacements, current standards for in vitro testing only contain limited clinical test conditions. With the number of total joint arthroplasties increasing, it is crucial that in vitro test conditions should include a larger variety of clinical test scenarios. This will help to illustrate material and design limitations of new products prior to large scale clinical application and thus, hopefully, reduce the incidence of disastrous implant failures and expensive product recalls.

To make in vitro testing more clinically relevant, the current test environments for total joint replacements need to be modified. Currently, the standard conditions applied during in vitro wear testing only consider level walking. Adverse testing conditions such as loading and motions that occur during activities of daily living have been considered by various research laboratories and have shown to accelerate wear, causing damage that can be observed clinically. For example, simulating micro separation in wear testing of total hip replacements, a condition known to occur between the femoral head and acetabular liner during the swing phase, has been shown to create an additional, more adverse clinically relevant test scenario.

Besides modifying the standardized load and motion parameters, the lubricant composition in which the wear tests are being performed also need to be revised. The biochemical properties of the lubricants currently used differ from the properties of human synovial fluid surrounding total joint replacements. Not only does the lubricant composition affect the wear behaviour of total joint replacements in vitro, it also affects the corrosive behaviour of biomaterials. In particular, the corrosion at the taper junction between the stem and femoral head has led to increased metal ions in patients and has contributed to increased revisions in recent years.

The feedback on the clinical performance of total joint replacements through retrieval analyses can be used to monitor total joint replacements to serve as an early warning system for implant failures. During retrieval analysis, it is crucial that all clinical data is collected using an appropriate data management system, (e.g. www.ortechsystems.com) to relate the data to damage features and overall implant performance. If performed at centralized sites, the dissemination of the data on implant retrieval analysis could be published and provide feedback to the Canadian Joint Replacement Registry. Historically, funding for retrieval analyses have been mainly provided by grant agencies, although industry funding is known to occur as well. Ideally, the data collected through retrieval analyses should be communicated to Health Canada to close the loop between the patient and the service provider with the goal to reduce the financial burden for the health care system.

 

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