Aims: Comparing periprosthetic bone remodeling in 5 stem designs and the effects of the bone remodeling on stem migration. Methods: Bone remodeling at the proximal femur in 89 cemented THA of 5 stem designs (Exeter,Sp2_aluminum,Sp2_CoCr,SHP,Spectron) were followed up at 1 week, 2 years, and 5 years postoperatively by measuring bone mineral density (BMD) in 7 Gruen zones using dual energy x-ray absorptiometer (DEXA). The migrations of the femoral stem measured by Radiostereometric Analysis (RSA) during the same time period were compared with the changes in BMD. Results: An 8% to 25% decrease in BMD at the proximal femur, especially at the medial cortex, was found 5 years after cemented THA. Exeter showed the most favourable bone remodeling, whereas the SHP and Sp2_CoCr stems showed the worst bone remodeling (with bone loss of 15–27%), and the bone loss in Sp2_aluminum and Spectron stems were between (8–13%). The majority of the bone loss occurred within 2 years without major progress thereafter. The rate of the bone loss was significantly related to the design of the stem as well as gender and body mass index (BMI). A significant relation between the rate of bone loss and the migration of the femoral stem could not be established by the current study. Conclusions: The stiffness of the stem as well as the concept of stem design significantly affected the bone remodeling at the proximal femur. However, the rate of bone loss seemingly did not affect the migration of the femoral stem in the current study.

Post operative stability is of paramount importance to obtain bone in growth and a tight interface in uncemented implants. Although hemispherical press fit cups are widely used different opinions exists according optimal fixation and a variety of principles are preferred. Lab studies show better stability if a cup is augmented by screws or pegs. However, cups with screws and holes increases penetration of joint fluid, pressure and particles to the interface with a risk for osteolyses. HA coating is in many studies favourable to obtain a quick in growth but is by many regarded unnecessary or even a risk for increased wear. This RSA studie was done to investigate stability and wear in cups with different fixation.

Material: 80 hips in 75 patients with a mean age of 58 years (36–70) were operated with a cemented Spectron stem and a porous coated Reflection cup of titanium (Smith a Nephew) All cups were oversized 1–2 mm and fixed with press fit by experienced surgeons. Great care was taken to achieve a good rim fit. They hips were by randomisation allocated to one of four groups with different fixation methods. One group was done with only press fit technique, one with additional screws, one additional pegs and another with HA coating on the porous surface. Migration and wear was investigated with RSA at 2, 12 and 24 months and standard radiography was done post op and at 2 years.

Mann-Whitneys U-test was used on signed values for evaluation of group differencies.

Results: At 2 years the mean cup migration for the whole group was 0.2 mm longitudinally, 0.3 mm horizontally and 0.3 mm anterior-posteriorly. (SEM 0.03–0.05) The inclination changed 0.4 dgr and anteversion 0.4 dgr as well. Most cups moved laterally, proximally and anteriorly with increased anteversion and decreased inclination. No major differences in mean values were found between the groups according translations or rotations and no significant differences. Nor did we find differences between the press fit only and the augmented cups.

Wear was 0.45 mm proximally and in total 0. 6 mm without any sign of differences between the HA and porous coated groups. HA coated cups had less radiolucent lines after 2 years. (p=0.01)

Discussion: The Reflection cups were stable fixed ad we found no indication of inferior stability for cups without augmentation as might have been anticipated. The reason is probably the firm press fit fixation obtained with a stable cup, good bone and forceful impaction. Many use screws for safety but such cups are not sealed for leakage of joint fluid and pressure gradients and the risk for backside osteolyses is higher… Pegs are easier and safer to insert and tighten the holes better. A tight, no hole cup with HA is preferred by us in normal younger patients since HA gave a better interface. Such a solution should minimise the risk for penetration of particles and pressure gradients with less risk for lyses.

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.