The relative motion between a prosthesis, the cement mantle and its’ host bone during weight bearing is not well understood. Using Radiostereophotogrammetric Analysis (RSA), we examined the dynamically inducible micromotion that exists at these interfaces when an increased load is placed through the prosthesis. Dynamically inducible micromotion was measured in the femoral components of 21 subjects undergoing total hip replacement with polished Exeter stems. Two standing RSA studies were performed, at 3 and 12 months postoperatively. Firstly in double-leg stance, and secondly fully weight bearing through the operated hip. Subjects had no signs of clinical or radiological signs of loosening at 1 year. Significant micromotion was detected at the prosthesis-cement interface at 3 months. Similar patterns of micromotion were observed at 12 months. The prosthesis appeared to bend during single-leg stance weight bearing, however this accounted for less than half of the total observed movement. Conventional RSA studies were conducted at 3 months, 6 months and 1 year to confirm that the implants showed normal migration patterns. This study demonstrates that movement exists between the prosthesis and bone during cyclical weight bearing. This dynamically inducible micromotion probably occurs at the prosthesis-cement interface. It could account for the wear that is observed on the surface of retrieved secure prostheses. This may be a mechanism by which failure eventually occurs.
The aim of the study was to measure in-vivo the 10-year linear and volumetric polyethylene wear of a fully congruent mobile bearing unicompartmental knee arthroplasty (OUKA). We studied six OUKA’s that had all been implanted 10 years previously. Each patient was examined in even double leg stance at a range of knee flexion angles, in a calibration cage. A stereo pair of X-ray films was acquired for each patient at 0°, 15° and 30° of flexion. The films were analysed using an RSA style calibration and a CAD model silhouette-fitting technique. The position and orientation of each femoral and tibial component was found relative to each other and the bearing position inferred. Penetration of the femoral component into the original volume of the bearing was our estimate of linear wear. The volumetric wear is calculated from the measured linear wear and the known surface area of the bearing. In addition eight control patients were examined less than 3 weeks post-operation where no wear would be expected.and 30We studied seven OUKA’s that had all been implanted at least 10 years previously. A stereo pair of X-ray films was acquired for each patient at 0 Results: The control group showed no measured wear. The seven OUKA’s had an average maximum depth of linear penetration of 0.40 mm at a mean follow-up of 10.9 years. The linear wear rate was 0.033 mm/year. The volumetric material loss was 79.8 mm3. If a steady gradual material loss is assumed, 8 mm3 of UHMWPE was lost per year. Polyethylene particulate wear debris continues to be implicated in the aetiology of component loosening and implant failure knee following arthroplasty. The OUKA employs a spherical femoral component and a fully congruous meniscal bearing to increase contact area and theoretically reduce the potential for polyethylene wear. The results from this in-vivo study confirm that the device has low 10-year linear and volumetric wear in clinical practice. This may protect the device from component loosening in the long term.
The Birmingham reSurfacing Arthroplasty (BSA) is a metal on metal prosthesis with no published independent clinical studies. Despite this, it is increasing in popularity, especially as an alternative to stemmed prostheses in younger patients. This study presents the 1year migration results of the BSA femoral component using Roentgen Stereophotogrammteric Analysis (RSA). Twenty six subjects underwent a BSA, through the postero-lateral approach using CMW3G cement, with RSA marker balls placed intra-operatively. The femoral component migration was measured at intervals of 3, 6 and 12 months using the Oxford RSA system. Geometric algorhythms were used to identify the femoral component. The data was examined for distribution prior to analysis. All statistical analysis was performed using the t-test. The data was normally distributed. The 1 year migration results of the BSA femoral component are displayed below. All cemented implants migrate in vivo. The majority of cemented stemmed implant migration occurs within the first post-operative year. High rates of migration within the first post-operative year correlate with premature component failure in some instances. The BSA is a fundamentally different design to most cemented prostheses, despite this we know that very large migrations, those in excess of 2mm/year in any direction are generally regarded as poor indicators of long term outcome. These results suggest that the BSA femoral component is an inherently stable device as it does not migrate significantly within the first post-operative year. Only long-term independent clinical studies and continued RSA follow-up will enable a comprehensive evaluation of the device.
This early study examines the influence of a wider shoulder on the 1 year migration of a cemented, polished, tapered stem, using RSA. Polished, tapered stems (PTS) have excellent 10 year survival rates. RSA studies have demonstrated that these devices subside about 1 mm / year. Small amounts of subsidence are beneficial in stabilising a stem. Stem rotation (measured as posterior head migration) within the cement mantle is probably a more important mechanism of failure than subsidence. Stems with a wider proximal portion are thought to better resist rotation. The CPS (Endoplus, UK) is such a device; here we compare its’ stability with that of the Exeter. 20 patients received the CPS-plus stem and underwent RSA examinations at 3, 6 and 12 months postoperatively. The Exeter 1 year migration data was used as a comparison. Both groups underwent a Hardinge approach with CMW3G cement. Both stems subsided about 1mm. The CPS showed less medio-lateral and A-P movement of the proximal stem than the Exeter over 1 year, as shown below: The CPS internally rotates less than the Exeter, as demonstrated by the smaller amount of posterior head migration. It has a lateral flare of the shoulder; making its cross-section wider than the Exeter’s, this probably accounts for its’ greater resistance to rotation. The CPS also undergoes less medio-lateral proximal stem migration. Its’ lateral shoulder flare is probably responsible for this axial subsidence, as it prevents the shoulder from moving laterally whilst subsiding over the calcar. A PTS with a broad proximal section is more stable, this may confer an increased survival advantage.
Results at one year (TMK first): AKSS(Knee) 91.6 / 84.1 (p=0.003), OKS 39.8 / 37.6 (p=0.006), ROM 104 / 104 (p=0.364), Pain (AKSS) 47.3 / 41.7 (p=0.01), Pain (OKS) 3.5 / 2.9 (p=0.006).