Migration is one of the factors that can lead to the failure of the surgical implant of a medical device. RSA (roentgen stereophotogrammetric analysis) is a technique that is used to predict long-term implant survival for devices that are attached to bone. It is commonly used in early stage clinical trials for implants for knee and hip replacements for example. Because it relies on X-rays, the use of RSA to sample smaller numbers of patients with a higher degree of predictive accuracy is desired. Thus, the development of the new ISO 16087 standard, “Implants for surgery – Roentgen stereophotogrammetric analysis for the assessment of migration of orthopaedic implants.”
RSA was developed in 1974 by Goran Selvic to measure migration and micromotion of joint replacement prosthesis from the bone to which it is attached. This new ISO publication was generated to provide a standardized format for the results of these clinical trials. The goal? To make the results of these tests as useful and as widely applicable as possible.
The new standard supports a process that involves taking two X-rays of the implant from different directions. Software that’s a part of the equipment involved then provides the tester with a 3-D image of the implant. Tiny markers inserted during the original surgical procedure are used to measure the movement of the device itself in comparison with the attachment location.
The ISO 16087 begins with the usual scope and definition sections. It then moves on to the measurement requirements, based on the inclusion of markers in the implant surgery itself. The markers are implant grade metal with a high radio-opacity. “Virtual Markers” may be defined as well, different positions on the implant that can map the device should one or more of the metal markers themselves be obscured or loosened.
Since RSA involves X-rays, there are sections regarding the radiographic arrangement, the reference plates, and radiographs. Software, coordinate systems,the actual migration data and the total point motion are also covered. The standard concludes with validation requirements, practical issues, and of course, the standardized presentation of the output. You’ll also find all the relevant formulas you’ll need in the informative Annex A and a bibliography with further documentation to review.
RSA testing is an important tool in evaluating the lifespan of any given orthopedic implant. As these implants are being used on younger populations, the need for longer lasting joint replacements is highlighted. Standardizing the process by which these outcomes are measured and reported supports the more effective development and usage of these products.
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