Abstract
In clinical Orthopaedic research we often need better tools for follow up investigations and evaluation of new methods. One alternative is Radiostereometric analyses (RSA) which can be used for high precision measurements of migration, micro movements and wear. Since developed 25 years ago it has now been used in a few thousand patients and made into a comprehensible computerized, PC based system. Recent development has made it much faster, more accurate and user friendly enough for more common use.
RSA can basically measure 3D movements between rigid bodies as bone or implants and is used for many sorts of applications as bone growth, fracture healing, joint kinematics, bone elasticity, spinal fusion etc where a high accuracy is needed. It has, however, mostly been used for research in hip and knee arthroplasty since early migration has been found a good predictor for later implant failure. As also wear in artificial joints can be accurately measured the technique is definitely a useful tool for implant research.
Method: A minimum of 3 tantalum markers is inserted in bone and preferably also the implant. Two X-rays are taken with a 40° angle between them and with a RSA calibration cage beneath the X-ray table. The films are then digitized and measured with dedicated software, UMRSA ®. The same procedure is repeated at another occasion and the change in positioning calculated.
With modern digital x-ray technique we obtain an in vivo precision of about 50 microns longitudinally, 80 horizontally and 200 sagittaly, for rotations 0.1°–0.3°depending on direction, (95% confidence limit).
Studies: We have done around 30 different hip and knee studies since 1986 in Umea and this actual speech will give an overview of what is achievable in terms of outcome and some clinical implications found. In short and generally spoken we have also in Umea found early RSA measurements to very accurately predict coming clinical loosening or high wear rates in implants.
Some general findings are: In cemented stems the loosening starts at the stem-cement interface and the cement mantles are very well fixed to bone, loosening being a secondary phenomenon.
A low temperature curing, non-vacuum mixed cement had equal fixation to bone and stem as a standard vacuum mixed.
Some stem designs move a lot inside the cement, possibly with a big risk for cement fractures and abrasion. Especially subsidence and retro version seems ominous.
Repeatedly HA coating has shown excellent implant stability, in the same range as cemented components and better than porous coated ones.
We have found good and reproducible stability with impaction grafting in both acetabulum and femur using both cemented and uncemented non tapered implants. Structural grafts seem to imply increased migration.
Wear has been increased with non irradiated plastic components, in younger patients, if cement contains ZrO as opacifier and together with unstable cemented stems. No correlation has been found between wear and HA coating, head or stem material or weight but decreased wear found for high cross-linked plastics.
RSA has been a big asset for Implant research over the years. With the more stable implants and modern bearings of today a high accuracy method is even more needed for to measure fixation and wear, or the actual results will be lost in a lot of noise. The new focus and interest in synergistic effects of implant micro movements, interface stress, hydrostatic pressure and particles for the development of osteolyses is a new area were RSA should be a useful tool to study inducible implant movements and fixation quality in vivo.
The abstracts were prepared by Professor Jegan Krishnan. Correspondence should be addressed to him at the Flinders Medical Centre, Bedford Park 5047, Australia.
