Summary. Comparison of accuracy and precision in measuring wear using 4 commonly used uncemented cup designs shows small differences in mean and data scatter for marker and model-based RSA. Introduction. The disadvantage with conventional RSA is that implant has to be supplied with tantalum markers, which may be difficult to visualise. This problem can be resolved with model-based RSA, but it is uncertain if this method has the same precision as marker-based RSA to measure wear. We compared these methods and studied different prosthesis geometries represented by four different uncemented cup designs (Trilogy, TMT-Trabecular Metal, Zimmer, Warsaw, USA, Ringloc, Biomet, Inc., Warsaw, Indiana, and ABG, Howmedica International, Staines, UK). Patients and Methods. Stereoradiographs of 75 patients (19 Trilogy, 17 TMT, 20 Ringloc, 19 ABG) were exposed postoperatively (2 examinations) and after 2 years. The patients were selected from prospective clinical studies. During operation tantalum markers had been inserted into the liner in all cases. The measurements and analysis of all radiographs were performed with UmRSA Digital Measure and UmRSA Analysis 6.0. We used the differences between the postoperative double-examinations to compute the precision for the two methods and for the different implant designs. The proximal and the total (vectorial sum of medial/lateral, proximal/distal and anterior posterior) femoral head penetration up to 2 years were compared. Results. The mean differences and the standard deviation of mean obtained from calculations between the double examinations in the total material did not differ between the 2 methods for any of the designs studied. The mean values and SD for marker and model-based RSA were −0, 00 mm ± 0.09 mm and 0.02 mm ± 0.08 mm, respectively (p>0.05). The comparison between Classical marker-based RSA and Model-based RSA in measuring wear up to 2 years did not showed any statistically significant differences for the Trilogy, TMT and ABG cups (p>0.05). However the mean difference of the postoperative double examinations were slightly higher for the Ringloc design (p=0, 02) and the data scatter (SD) at 2 years was higher (p=0,004) with use of model-based RSA. Conclusions. We found small differences between marker and model-based RSA for measurements of proximal and total wear (penetration). In 3 of the 4 cup designs studied the data scatter was about equal for the 2 methods. In the 4. th. design (Ringloc) the data scatter was higher when model-based RSA was used

Objectives. To validate the precision of digitally reconstructed radiograph (DRR) radiostereometric analysis (RSA) and the model-based method (MBM) RSA with respect to benchmark marker-based (MM) RSA for evaluation of kinematics in the native hip joint. Methods. Seven human cadaveric hemipelves were CT scanned and bone models were segmented. Tantalum beads were placed in the pelvis and proximal femoral bone. RSA recordings of the hips were performed during flexion, adduction and internal rotation. Stereoradiographic recordings were all analyzed with DRR, MBM and MM. Migration results for the MBM and DRR with respect to MM were compared. Precision was assessed as systematic bias (mean difference) and random variation (Pitman’s test for equal variance). Results. A total of 288 dynamic RSA images were analyzed. Systematic bias for DRR and MBM with respect to MM in translations (p < 0.018 mm) and rotations (p < 0.009°) were approximately 0. Pitman’s test showed lower random variation in all degrees of freedom for DRR compared with MBM (p < 0.001). Conclusion. Systematic error was approximately 0 for both DRR or MBM. However, precision of DRR was statistically significantly better than MBM. Since DRR does not require marker insertion it can be used for investigation of preoperative hip kinematics in comparison with the postoperative results after joint preserving hip surgery. . Cite this article: L. Hansen, S. De Raedt, P. B. Jørgensen, B. Mygind-Klavsen, B. Kaptein, M. Stilling. Marker free model-based radiostereometric analysis for evaluation of hip joint kinematics: A validation study. Bone Joint Res 2018;7:379–387. DOI: 10.1302/2046-3758.76.BJR-2017-0268.R1

To measure micromotion of an orthopaedic implant with respect to its surrounding bone, Roentgen Stereo-photogrammetric Analysis (RSA) was developed. A disadvantage of conventional RSA is that it requires the implant to be marked with tantalum beads. This disadvantage can potentially be resolved with model-based RSA, whereby a 3D model of the implant is used for matching with the actual images and the assessment of position and rotation of the implant. In this study, an improved model-based RSA algorithm is presented and validated in phantom experiments. This algorithm is capable to process projection contours that contain drop-outs. To investigate the influence of the accuracy of the implant models that were used for model-based RSA, we studied both Computer Aided Design (CAD) models as well as models obtained by means of Reversed Engineering (RE) of the actual implant. The results demonstrate that the RE-models provide more accurate results than the CAD models. If these RE models are derived from the very same implant, it is possible to achieve a maximum standard deviation of the error in the migration calculation of 0.06 mm for translations in x- and y-direction and 0.14 mm for the out of plane z-direction, respectively. For rotations about the y-axis, the standard deviation was about 0.1 degree and for rotations about the x- and z-axis 0.05 degree. For the femur component, it was also possible to reach these accurate results for non-scanned components. The results show that the new algorithm is an improvement with respect to a study we presented earlier [. 1. ]. Studies with clinical RSA-radiographs must prove that these results can also be reached in a clinical setting, making model-based RSA a possible alternative for marker-based RSA

Introduction. Model-based radiostereometric analysis (MBRSA) allows the in vivo measurement of implant loosening (i.e. migration) from a host bone by acquiring a pair of biplanar radiographs of the patient's implant over time. Focusing on total knee replacement patients, the accuracy of MBRSA in calculating tibial baseplate migration depends on the accuracy in registering a 3D model onto the biplanar radiographs; thus, the shape of the baseplate and its orientation relative to the imaging planes is pertinent. Conventionally, the baseplate coordinate system is aligned with the laboratory coordinate system, however, this reference orientation is unnecessary and may hide unique baseplate features resulting in less accurate registration (Figure 1). Therefore, the primary objective of this study was to determine the optimal baseplate orientation for improving accuracy during MBRSA, and an acceptable range of orientations for clinical use. A second objective was to demonstrate that a custom knee positioning guide repeatably oriented the baseplate within the acceptable range of orientations. Materials and Methods. A tibia phantom consisting of a baseplate rigidly fixed to a sawbone was placed in 24 orientations (combination of six rotations about X (i.e. knee flexion) and four rotations about Z (i.e. hip abduction)) with three pairs of radiographs acquired at each orientation. The radiographs were processed in MBRSA software, and the mean maximum total point motion (MTPM), an indicator of bias error during model registration, was plotted as a function of the two rotations to determine the optimal orientation and a range of acceptable orientations (Figure 2). A custom knee positioning guide was manufactured with the goal of orienting the baseplate close to the optimal orientation and within the acceptable range of orientations (Figure 3). Ten independent pairs of biplanar radiographs were acquired by repeatedly placing a knee model in the knee positioning guide, and the images were processed in MBRSA software to determine the baseplate orientation. Results and Discussion. Results showed an 85% decrease in bias error between the reference orientation (i.e. no rotation) and the optimal orientation (10° rotation about X and 5° rotation about Z). An acceptable range of orientations from 5° − 20° rotation about an axis perpendicular to the sagittal imaging plane and from 5° − 15° rotation about an axis perpendicular to the coronal imaging plane was defined as these orientations decreased the bias error by more than 50%. Additionally, the custom knee positioning guide controlled the mean orientation ± one standard deviation within the acceptable range of orientations. Conclusions. The accuracy of MBRSA is significantly improved if the tibial baseplate is placed in the range of acceptable orientations as opposed to the conventional reference orientation. A custom knee positioning guide can be used during a clinical study to repeatably position the patient's knee within the range of acceptable orientations. For any figures or tables, please contact the authors directly

Aseptic loosening of the total TMC joint prosthesis occurs frequently and may depend on the design of the prosthesis. Numerous TMC prosthesis designs are available, and new designs are being developed and tested. One of the problems in the clinical studies of TMC prostheses is identifying and predicting prosthetic loosening at an early stage. Roentgen Stereophotogrammetric Analysis (RSA). allows assessment of three-dimensional micromotion of orthopaedic implants with high accuracy. Early micromotion (in the first two postoperative years) of most prostheses is strongly correlated with the development of aseptic loosening. We studied if RSA assessment was possible after total TMC joint arthroplasty. In five cadaveric hands the TMC joint was replaced by the SR-TMC prosthesis. Tantalum beads of 0.8 mm were implanted in the trapezium and first metacarpal bone without extending the standard surgical exposure. The metacarpal prosthesis component was provided with 0.5 mm beads. A three-dimensional surface model of the trapezium component of the SR-TMC prosthesis was prepared to facilitate model-based RSA. After the surgical procedure, RSA radiographs were made of all hands in two commonly used positions for imaging of the TMC joint. The number of visually detected markers for each bone/implant was recorded. Of one cadaver hand, RSA radiographs were made in ten different positions to calculate the measurement error of the performed technique. For the metacarpal bone, all beads were visible in all positions and both (L+R) RSA radiographs. For beads in the polyethylene metacarpal prosthesis component three beads seem sufficient, however in exceptional cases the most proximal placed bead might be invisible due to overprojection by the metal trapezium prosthesis component. Therefore the X-rays should be carefully checked at the radiology department before the patient leaves the ward. Alternatively, an extra bead can be placed in the prosthesis, although this is a lesser option due to possible weakening of the component caused by the placement of the beads. The use of different sizes of beads (0.5/0.8 mm) in the metacarpal bone and metacarpal prosthesis made the interpretation for the analyser easier. The accuracy analysis is currently carried out. First results of these measurements are promising and placement of tantalum beads for RSA analysis during TMC-joint replacement seems feasible

Aims. Both the femoral and tibial component are usually cemented at revision total knee arthroplasty (rTKA), while stems can be added with either cemented or press-fit (hybrid) fixation. The aim of this study was to compare the long-term stability of rTKA with cemented and press-fitted stems, using radiostereometric analysis (RSA). Methods. This is a follow-up of a randomized controlled trial, initially involving 32 patients, of whom 19 (nine cemented, ten hybrid) were available for follow-up ten years postoperatively, when further RSA measurements were made. Micromotion of the femoral and tibial components was assessed using model-based RSA software (RSAcore). The clinical outcome was evaluated using the Knee Society Score (KSS), the Knee injury and Osteoarthritis Outcome Score (KOOS), and visual analogue scale (pain and satisfaction). Results. The median total femoral translation and rotation at ten years were 0.39 mm (interquartile range (IQR) 0.20 to 0.54) and 0.59° (IQR 0.46° to 0.73°) for the cemented group and 0.70 mm (IQR 0.15 to 0.77) and 0.78° (IQR 0.47° to 1.43°) for the hybrid group. For the tibial components this was 0.38 mm (IQR 0.33 to 0.85) and 0.98° (IQR 0.38° to 1.34°) for the cemented group and 0.42 mm (IQR 0.30 to 0.52) and 0.72° (IQR 0.62° to 0.82°) for the hybrid group. None of these values were significantly different between the two groups and there were no significant differences between the clinical scores in the two groups at this time. There was only one re-revision, in the hybrid group, for infection and not for aseptic loosening. Conclusion. These results show good long-term fixation with no difference in micromotion and clinical outcome between fully cemented and hybrid fixation in rTKA, which builds on earlier short- to mid-term results. The patients all had type I or II osseous defects, which may in part explain the good results. Cite this article: Bone Joint J 2022;104-B(7):875–883

Aims. Highly polished stems with force-closed design have shown satisfactory clinical results despite being related to relatively high early migration. It has been suggested that the minimal thickness of cement mantles surrounding the femoral stem should be 2 mm to 4 mm to avoid aseptic loosening. The line-to-line cementing technique of the femoral stem, designed to achieve stem press-fit, challenges this opinion. We compared the migration of a highly polished stem with force-closed design by standard and line-to-line cementing to investigate whether differences in early migration of the stems occur in a clinical study. Methods. In this single-blind, randomized controlled, clinical radiostereometric analysis (RSA) study, the migration pattern of the cemented Corail hip stem was compared between line-to-line and standard cementing in 48 arthroplasties. The primary outcome measure was femoral stem migration in terms of rotation and translation around and along with the X-, Y-, and Z- axes measured using model-based RSA at three, 12, and 24 months. A linear mixed-effects model was used for statistical analysis. Results. Results from mixed model analyses revealed a lower mean retroversion for line-to-line (0.72° (95% confidence interval (CI) 0.38° to 1.07°; p < 0.001), but no significant differences in subsidence between the techniques (-0.15 mm (95% CI -0.53 to 0.227; p = 0.429) at 24 months. Radiolucent lines measuring < 2 mm wide were found in three and five arthroplasties cemented by the standard and line-to-line method, respectively. Conclusion. The cemented Corail stem with a force-closed design seems to settle earlier and better with the line-to-line cementing method, although for subsidence the difference was not significant. However, the lower rate of migration into retroversion may reduce the wear and cement deformation, contributing to good long-term fixation and implant survival. Cite this article: Bone Joint J 2022;104-B(1):19–26

Summary

In this study we validate that weight-bearing images are needed for accurate polyethylene liner wear measurement in total knee prostheses by measuring the difference in minimum joint space width between weight-bearing and non-weight-bearing RSA views.

The purpose of this study was to examine the influence of weight-bearing on the measurement of in vivo wear of total knee replacements using model-based RSA at 1 and 2 years following surgery. Model-based RSA radiographs were collected for 106 patients who underwent primary TKR at a single institution. Supine RSA radiographs were obtained post-operatively and at 6-, 12-, and 24-months. Standing (weight-bearing) RSA radiographs were obtained at 12-months (n=45) and 24-months (n=48). All patients received the same knee design with a fixed, conventional PE insert of either a cruciate retaining or posterior stabilized design. Ethics approval for this study was obtained. In order to assess in vivo wear, a highly accurate 3-dimensional virtual model of each in vivo TKA was developed. Coordinate data from RSA radiographs (mbRSA v3.41, RSACore) were applied to digital implant models to reconstruct each patient's replaced knee joint in a virtual environment (Geomagic Studio, 3D Systems). Wear was assessed volumetrically (digital model overlap) on medial and lateral condyles separately, across each follow-up. Annual rate of wear was calculated for each patient as the slope of the linear best fit between wear and time-point. The influence of weight-bearing was assessed as the difference in annual wear rate between standing and supine exams. Age, BMI, and Oxford-12 knee improvement were measured against wear rates to determine correlations. Weight bearing wear measurement was most consistent and prevalent in the medial condyle with 35% negative wear rates for the lateral condyle. For the medial condyle, standing exams revealed higher mean wear rates at 1 and 2 years, supine, 16.3 mm3/yr (SD: 27.8) and 11.2 mm3/yr (SD: 18.5) versus standing, 51.3 mm3/yr (SD: 55.9) and 32.7 mm3/yr (SD: 31.7). The addition of weight-bearing increased the measured volume of wear for 78% of patients at 1 year (Avg: 32.4 mm3/yr) and 71% of patients at 2 years (Avg: 48.9 mm3/yr). There were no significant (95% CI) correlations between patient demographics and wear rates. Volumetric, weight-bearing wear measurement of TKR using model-based RSA determined an average of 33 mm3/yr at 2 years post-surgery for a modern, non-cross-linked polyethylene bearing. This value is comparable to wear rates obtained from retrieved TKRs. Weight-bearing exams produced better wear data with fewer negative wear rates and reduced variance. Limitations of this study include: supine patient imaging performed at post-op, no knee flexion performed, unknown patient activity level, and inability to distinguish wear from plastic creep or deformation under load. Strengths of this study include: large sample size of a single TKR system, linear regression of wear measurements and no requirement for implanted RSA beads with this method. Based on these results, in vivo volumetric wear of total knee replacement polyethylene can be reliably measured using model-based RSA and weight-bearing examinations in the short- to mid–term. Further work is needed to validate the accuracy of the measurements in vivo

Purpose. The purpose of this study was to examine the influence of weight-bearing on the measurement of in vivo wear of total knee replacements using model-based RSA at 1 and 2 years following surgery. Methods. Model-based RSA radiographs were collected for 106 patients who underwent primary TKR at a single institution. Supine RSA radiographs were obtained post-operatively and at 6-, 12-, and 24-months. Standing (weight-bearing) RSA radiographs were obtained at 12-months (n=45) and 24-months (n=48). All patients received the same knee design with a fixed, conventional PE insert of either a cruciate retaining or posterior stabilized design. Ethics approval for this study was obtained. In order to assess in vivo wear, a highly accurate 3-dimensional virtual model of each in vivoTKA was developed. Coordinate data from RSA radiographs (mbRSA v3.41, RSACore) were applied to digital implant models to reconstruct each patient's replaced knee joint in a virtual environment (Geomagic Studio, 3D Systems). Wear was assessed volumetrically (digital model overlap) on medial and lateral condyles separately, across each follow-up. Annual rate of wear was calculated for each patient as the slope of the linear best fit between wear and time-point. The influence of weight-bearing was assessed as the difference in annual wear rate between standing and supine exams. Age, BMI, and Oxford-12 knee improvement were measured against wear rates to determine correlations. Results. Weight bearing wear measurement was most consistent and prevalent in the medial condyle with 0–4% of calculated wear rates being negative compared to 29–39% negative wear rates for the lateral condyle. For the medial condyle, standing exams revealed higher mean wear rates at 1 and 2 years; supine, 16.3 mm. 3. /yr (SD: 27.8) and 11.2 mm. 3. /yr (SD: 18.5) versus standing, 51.3 mm. 3. /yr (SD: 55.9) and 32.7 mm. 3. /yr (SD: 31.7). The addition of weight-bearing increased the measured volume of wear for 78% of patients at 1 year (Avg: 32.4 mm. 3. /yr) and 71% of patients at 2 years (Avg: 48.9 mm. 3. /yr). There were no significant (95% CI) correlations between patient demographics and wear rates. Discussion and Conclusion. This study demonstrated TKA wear to occur at a rate of approximately 10 mm. 3. /year and 39 mm. 3. /year in patients imaged supine versus standing, respectively, averaged over 2 years of clinical follow-up. In an effort to eliminate the effect of PE creep and deformation, wear was also calculated between 12 and 24 months as 9.3 mm. 3. (standing examinations), This value is comparable to wear rates obtained from retrieved TKRs. Weight-bearing exams produced better wear data with fewer negative wear rates and reduced variance. Limitations of this study include: supine patient imaging performed at post-op, no knee flexion performed, and unknown patient activity level. Strengths of this study include: large sample size of a single TKR system, linear regression of wear measurements and no requirement for implanted RSA beads with this method. Based on these results, in vivo volumetric wear of total knee replacement polyethylene can be reliably measured using model-based RSA and weight-bearing examinations in the short- to mid–term. For any figures or tables, please contact authors directly

Vitamin E-doped cross-linked polyethylene (VEPE) has encouraged the use of larger heads in thinner liners in total hip arthroplasty (THA). However, there are concerns about wear and mechanical failure of the thin liner, especially when metal heads are used. The aim of this randomized controlled trial was to investigate if the use of a large metal head in a thin VEPE liner would increase polyethylene wear compared with a standard 32-mm metal head and to compare periacetabular radiolucencies and patient-reported outcomes in THA. 96 candidates for uncemented THA were randomly allocated to either the largest possible metal head (36–44 mm) that could be fitted in the thinnest available VEPE liner (intervention group) or a standard 32-mm metal head (control group). The primary outcome was proximal head penetration (PHP) measured with model-based radiostereometric analysis (RSA). Secondary outcomes were periacetabular radiolucencies and patient-reported outcomes. The mid-term results of the trial at 5 years are presented. Median total PHP (interquartile range) was -0.04 mm (−0.12 to 0.02) in the intervention group and -0.03 mm (=0.14 to 0.05) in the control group (p=0.691). The rates of periacetabular radiolucencies were 1/44 and 4/42 (p=0.197), respectively. Patient-reported hip function and health-related quality of life did not differ between the groups, but participants in the intervention group reported a higher level of activity (median UCLA rank 7 vs 6, p=0.020). There were 5 revisions caused by dislocations (2), periprosthetic fracture (1), stem subsidence (1), or iliopsoas impingement (1). Large metal heads in thin VEPE liners did not increase liner wear and were not associated with liner failure 5 years after THA

This study used model-based radiostereometric analysis (MBRSA) to compare migration of a recently introduced cementless hip stem to an established hip stem of similar design. Novel design features of the newer hip stem included a greater thickness of hydroxyapatite coating and a blended compaction extraction femoral broach. Fifty-seven patients requiring primary total hip arthroplasty (THA) were enrolled at a single centre. Patients were randomized to receive either an Avenir collarless stem and Trilogy IT cup (ZimmerBiomet) or a Corail collarless stem and Pinnacle cup (DePuy Synthes) via a posterior or lateral approach. Both stems are broach-only femoral bone preparation. RSA beads (Halifax Biomedical) were inserted into the proximal femur during surgery. Patients underwent supine RSA imaging a 6 weeks (baseline), 6, 12, and 24 months following surgery. The primary study outcome was total subsidence of the hip stem from baseline to 24 months as well as progression of subsidence between 12 and 24 months. These values were compared against published migration thresholds for well-performing hip stems (0.5mm). The detection limit, or precision, of MBRSA was calculated based on duplicate examinations taken at baseline. Patient reported outcome measures were collected throughout the study and included the Oxford-12 Hip Score (OHS), EuroQoL EQ-5D-5L, Hip Osteoarthritis Score (HOOS) as well as visual analogue scales (VAS) for thigh pain and satisfaction. Analysis comprised of paired and unpaired t-tests with significance set at p≤0.05. Forty-eight patients (30 males) were included for analysis; 7 patients received a non-study hip stem intra-operatively, 1 patient suffered a traumatic dislocation within three weeks of surgery, and 1 patient died within 12 months post-surgery. RSA data was obtained for 45 patients as three patients did not receive RSA beads intra-operatively. Our patient cohort had a mean age of 65.9 years (±;7.2) at the time of surgery and body mass index of 30.5 kg/m2 (±;5.2). No statistical difference in total stem migration was found between the Avenir and Corail stems at 12 months (p=0.045, 95%CI: −0.046 to 0.088) and 24 months (p=0.936, 95% CI: −0.098 to 0.090). Progression of subsidence from 12-24 months was 0.011mm and 0.034mm for the Avenir and Corail groups which were not statistically different (p=0.163, 95%CI: −0.100 to 0.008) between groups and significantly less than the 0.5mm threshold (pNo statistically significant differences existed between study groups for any pre-operative function scores (p>0.05). All patients showed significant functional improvement from pre- to post-surgery and no outcome measures were different between study groups with exception of EQ-5D-5L health visual analogue scale at 12 months which showed marginally superior (p=0.036) scores in the Avenir group. This study was not powered to detect differences in clinical outcomes. This study has demonstrated no statistical difference in subsidence or patient-reported outcomes between the Corail hip stem and the more recently introduced Avenir hip stem. This result is predictable as both stems are of a triple-tapered design, are coated with hydroxyapatite, and utilize a broach-only bone preparation technique. Both stem designs demonstrate migration below 0.5mm suggesting both are low-risk for aseptic loosening in the long-term

Objectives. The aim of this study was to determine the polyethylene wear rate of Phase 3 Oxford Unicompartmental Knee Replacement bearings and to investigate the effects of resin type and manufacturing process. Methods. A total of 63 patients with at least ten years’ follow-up with three bearing types (1900 resin machined, 1050 resin machined, and 1050 resin moulded) were recruited. Patients underwent full weight-bearing model-based radiostereometric analysis to determine the bearing thickness. The linear wear rate was estimated from the change in thickness divided by the duration of implantation. Results. The wear rate for 1900 resin machined (n = 19), 1050 machined (n = 21), and 1050 moulded bearings (n = 23) were 60 µm/year (. sd. 42), 76 µm/year (. sd. 32), and 57 µm/year (. sd. 30), respectively. There was no significant difference between 1900 machined and 1050 machined (p = 0.20), but 1050 moulded had significantly less wear than the 1050 machined (p = 0.05). Increasing femoral (p < 0.001) and tibial (p < 0.001) component size were associated with increasing wear. Conclusion. Wear rate is similar with 1050 and 1900 resin, but lower with moulded bearings than machined bearings. The currently used Phase 3 bearings wear rate is low (1050 moulded, 57 µm/year), but higher than the previously reported Phase 2 bearings (1900 moulded, 20 µm/year). This is unlikely to be due to the change in polyethylene but may relate to the minimally invasive approach used with the Phase 3. This approach, as well as improving function and thus increasing activity levels, may increase the risk of surgical errors, such as impingement or bearing overhang, which can increase wear. Surgeons should aim to use 4 mm thick bearings rather than 3 mm thick bearings in young patients, unless they are small and need conservative bone resections. Cite this article: Bone Joint Res 2019;8:535–543

Model-based Roentgen Stereophotogrammetric Analysis (RSA) measures micromotion of an orthopaedic implant with respect to its surrounding bone, without the use of markers on the implant. In previous studies with a total knee prosthesis, Model-based RSA showed to be very accurate. In this study, Model-based RSA is validated in a phantom experiment of a total hip prosthesis. A metal backed, elliptical shaped EP-FIT PLUS ®cup was used in combination with a SL-PLUS ® hip-stem from PLUS Endoprothetik AG. In vivo conditions were simulated by using sawbones and perspex plates to mimic the bones and soft tissue. Virtual projections of the CAD models of the implant were fitted on the automatically detected contours in nine RSA radiographs and the error inmigration calculation was determined. The standard deviations of the error in translation for the cup were: 0.03, 0.05, and 0.21 mm. (x, y, z-direction) The standard deviations of the error in orientation were respectively 0.56, 0.48, and 0.18 degrees (n = 10). For the stem, the standard deviations of the error in translation are: 0.09, 0.11, and 0.29 mm and for the orientation: 0.63, 2.03, and 0.24 degrees (n = 0). The results for the cup are satisfactory, and make Model-based RSA a good alternative for conventional RSA. Especially for this type of metal backed, non hemispherical cup for which no markerless alternative is available. The error in orientation around the y-axis of the stem is of concern. Experiments with models from Reversed Engineering had similar low accuracy. We expect that the cause of these inaccuracies is the rectangular cross sectional shape of this specific hip stem, and we expect better results from experiments with differently shaped stems. The results of this study make very clear that Model-based RSA is avaluable and accurate technique, but phantom studies are always necessary to validate the accuracy for a specific implant shape

Early micromotion of joint prostheses with respect to the bone can be assessed very accurately by a method called Roentgen Stereophotogrammetric Analysis (RSA); a method that uses two simultaneous X-ray exposures of the joint and has an accuracy of 0.1 mm for translations and 0.3 degree for rotations [. 1. ]. In order to reach this accuracy, metallic markers are inserted into the bone and attached to the surface of the prosthesis. These markers can then be identified automatically in the two radiographs [. 2. ]. Since the adjustments to the prosthesis are difficult, time-consuming and expensive, RSA has only been applied in a limited number of clinical trials. In a previous study we have developed a Model-based RSA algorithm, which does not require the attachment of markers to the prosthesis [. 3. ]. This algorithm is based on minimisation of the non-overlapping area (NOA) between the automatically detected contour of the prosthesis from the roentgen image, with the virtually projected contour of a three-dimensional model of the prosthesis. Because the accuracy of this NOA algorithm was not as high as the accuracy of the currently used Marker-based RSA, we have studied alternative algorithms for Model-based RSA. From a simulation study in which we used models of the Interax Total Knee Prosthesis (Stryker-Howmedica) and the G2 Hip Prosthesis (Johnson & John-son), we found that the results of the NOA algorithm can be improved substantially. The newly developed Model-based RSA algorithm is based on minimisation of the mean distance between the points of the actual contour and the virtually projected contour. The simulation study shows that the new algorithm is superior to the NOA-algorithm in situations where part of the contour is occluded, as well as in situations where the contour is distorted by noise. With the new algorithm, the residual position error can be reduced to 0.1 mm. and also the residual orientation error can be reduced to 0.3 degree, making Model-based RSA a future alternative to Marker-based RSA

Introduction. Surgeons performing a total knee replacement (TKR) have two techniques to assist them achieve proper bone resections and ligament tension – gap balancing (GB) and measured resection (MR). GB relies on balancing ligaments prior to bony resections, whereas bony resections are made based on anatomical landmarks in MR. Many studies have been done to compare the implant migration and kinematics between the two techniques, but the results have been varied. However, these studies have not been done on modern anatomically designed prostheses using radiostereometric analysis (RSA). Anatomical designs attempt to mimic the normal knee joint structure to return more natural kinematics to the joint, with emphasis on eliminating both paradoxical anterior motion and reduced posterior femoral rollback. Given the major design differences between anatomical and non-anatomical prostheses, it is important to investigate whether one surgical technique may have advantages another. We hypothesize that there would be no difference between GB and MR techniques in implant migration, but that GB might provide better knee kinematics. Methods. Patients were recruited to receive an anatomically designed prosthesis and randomized to groups where the GB or MR technique is used. For all patients in the study, RSA images were acquired at a 2 week baseline, as well as at 6 weeks, 3 months, and 6 months post-operatively. These images were used to collect the maximum total point motion (MTPM) of the tibial and femoral implant components relative to the bone using a model-based RSA software. A series of RSA images were also acquired at 3-months post-operatively at different knee flexion angles, ranging in 20° increments from 0° to 100°. Model-based RSA software was used to obtain the 3D positions and orientations of the femoral and tibial components, which were used to obtain the anterior-posterior (AP) contact locations for each condyle. Results. Results from 47 patients (27 GB, 20 MR) were analyzed. No significant differences were present between the two surgical techniques for tibial component MTPM at 6 weeks (mean difference=0.02 mm, p=0.61), 3 months (mean difference=0.01 mm, p=0.92), and 6 months (mean difference=0.01 mm, p=0.93) post-operatively. No significant differences were present between the two surgical techniques for femoral component MTPM at 6 weeks (mean difference=0.12 mm, p=0.08), 3 months (mean difference=0.05 mm, p=0.54), and 6 months (mean difference=0.13 mm, p=0.05) post-operatively. On the medial condyle, no significant differences in AP contact location were found at all angles between 0° and 80° of flexion (p-values from 0.28 to 0.95). There was a significant difference medially between the AP contact location of the two surgical techniques at 100° of flexion (p=0.01), indicating more posterior rollback on the medial condyle in the GB technique. On the lateral condyle, no significant differences in AP contact location were found at all angles of flexion (p-values from 0.13 to 0.62). On the medial condyle of the GB group, the AP contact location moved posteriorly 5.83 mm from 0° to 20°, anteriorly 2.60 mm from 20° to 60°, and posteriorly 7.40 mm from 60° to 100°. On the medial condyle of the MR group, the AP contact location moved posteriorly 5.36 mm from 0° to 20°, anteriorly 2.87 mm from 20° to 60°, and posteriorly 3.65 mm from 60° to 100°. On the lateral condyle of the GB group, the AP contact location moved posteriorly 6.87 mm from 0° to 20°, 0.30 mm from 20° to 60°, and 3.61 mm from 60° to 100°. On the lateral condyle of the MR group, the AP contact location moved posteriorly 6.86 mm from 0° to 20°, anteriorly 0.02 mm from 20° to 60°, and posteriorly 3.56 mm from 60° to 100°. Conclusions. The GB and MR techniques are very similar in terms of implant migration and overall kinematics when an anatomical prosthesis design is used for TKR. This study suggests that surgeon preference should be used when deciding which technique to use for implanting this anatomically designed knee replacement. For figures, tables, or references, please contact authors directly

Introduction. The most common method for accurate kinematic analysis of the knee arthroplasty uses bi-planar fluoroscopy and model-based RSA. The main challenge is to have access to reverse-engineered CAD models of the implant components, if not provided by the company, making this method impractical for a clinical study involving many types or sizes of implants. An alternative could be to reconstruct the 3D primitive features of the implant, such as cylindrical pegs, flat surfaces and circular boundaries, based on their 2D projections. This method was applied by Kaptein et al. (2006) for hip implants. However, despite its broad potential, it has not yet been applied for studying TKA kinematics. This study develops a methodology for feature-based RSA of TKA and investigates the range of accuracies in comparison to model-based RSA. Methods. Joint-3D software was developed in the MATLAB programming language to segment and fit elementary 2D features such as circles, lines, and ellipses to the edges of the parts on the radiographs (Figure 1). The software has the capability to reconstruct the 3D location and orientation of the components based on their 2D projections. To test the accuracy of the system a standard primary knee replacement system (Zimmer NexGen) was implanted on bone replica models, and positioned at 0° to 120° flexion at 30° intervals, simulating a lunge activity. For each pose, a multi-planar radiography system developed in our lab (Amiri et al., 2011) was used to take a sagittal and a 15° distally rotated radiograph (Figure 2a). Figure 1 shows the features C, L, and E segmented on the tibia and femur. The 3D reconstruction is performed based on a number of functions: Functions ‘f’ and ‘g’ reconstruct a 3D point or line based on their 2D projections. Function ‘h’ finds the plane containing the 3D circular edge based on its two projection ellipses. Function ‘i’ finds the 3D location of a line based on one projection line, and a known 3D vector normal to the solution 3D line. Based on these, the coordinate systems of the components were reconstructed (Figure 2b):. Femur_Origin=f(C1A,C1B);. Femur_Anteroposterior=g(L1A, L1B);. Femur_Proximodistal=g(L2A,L2B);. Femur_Mediolateral=i(L,C1A–C1B),{L=L1: if flexion<45°; L=L2: if flexion>45°};. E_3D=h(E1A,E1B);. Tibia_Origin=f(E1A_Centre,E1B_Centre);. Tibia_Anteroposterior=g(L3A,L3B);. Tibia_Mediolateral=cross(E_3D, Tibia_Anteroposterior);. Tibia_Proximodistal=cross(Tibia_Anteroposterior, Tibia_Mediolateral). To determine the errors, model-based RSA measures were used as the reference using the reverse-engineered models of the components in JointTrack software (University of Florida). Results. The overall accuracies in terms of bias (the mean error) and precision (standard deviation of the errors) are shown in Figure 3. The bias was within 0.5–1 mm and 0.9–1.2°, and the calculated precision was in the range of 0.4–0.6 mm and 0.7–1.0°. The overall accuracy was 0.8±0.6 mm and 1±0.7°. Discussion. The very good accuracies obtained show the practicality of the methodology. The methodology can be easily worked out for any type of implant based on the primitive geometric features at the bone-implant interface. This method can be extremely useful in a large clinical study by eliminating the need for having the 3D models of many types and sizes of the implant available

Purpose. The purpose of this study was to assess the biomechanical stability of the a total ankle arthroplasty system using longitudinal migration (LM) and inducible displacement (ID) measures. This study is the first study of its kind to assess total ankle arthroplasty (TAA) implant micromotion using model-based radiostereometric analysis (MBRSA). Method. Twenty patients underwent TAA that implanted the Mobility(TM) (DePuy, Warsaw IN). The mean (SD) age was 60.4 (12.5) and BMI was 29.1 (2.8) kg/m. 2. One surgeon performed all surgeries. All patients included in this study had given informed consent. Capital Health Research Ethics Board had approved this study. Uniplanar medial-lateral RSA X-ray exams were taken postop (double exam), at six wk, three mth, six mth, one yr and two yr followup times using a supine, unloaded position. Standing medial-lateral exams were taken at three mth, six mth, one yr and two yr followup intervals. LM and ID micromotions were assessed using Model-based RSA 3.2 software (Medis specials, Leiden, The Netherlands). Implant micromotions (x, y, z, Rx, Ry, Rz, MTPM) were determined and assessed for each subject using model-based pose estimation, and the implant-based coordinate system. The Elementary Geometric Shapes module from the Model-based RSA 3.2 software was used to assess the micromotion of the tibial component spherical tip due to implant symmetry. Results. The median (range) maximum total point motion (MTPM) for the implants at 2 year followup were 1.23 mm (0.39–1.95 mm) for the talar implant and 0.96 mm (0.17–2.28 mm) for the spherical tip of the tibia implant. Generally for each subject and implant component, the slopes of the migration curves decreased over time. The talar and tibial implants mean LM showed initial subsidence in the y-direction (migration into the bone) followed by stabilization patterns at one year followup. The median (range) of two year MTPM ID for the talar component was 0.39 (0.27–1.06) mm. At the one year and two year followup times the ID were almost all below the detection limit of 0.85 mm. The highest measured displacement for any one talar component at either of these times was 1.06 mm. Hence, the implant was displaced at least 0.21 mm under loading. The median (range) of one year and two year MTPM ID for the tibial component spherical tip was 0.08 (0.03–0.19) mm. The tibial component spherical tip demonstrates no ID in terms of MTPM greater than the 0.22 mm detection limit. Conclusion. The implant subsides directly into the bone in the line of primary loading during standing or walking. For most of the patients the two year LM for the Mobility(TM) demonstrates a typical subsidence-stabilization behaviour seen in many RSA studies of orthopaedic implants. Based on the results of this study the Mobility(TM) components show no measurable ID. This is the first study of its kind internationally for total ankle arthroplasty and offers novel insight into the need for prosthetic design change

Objectives. The accuracy and precision of two new methods of model-based radiostereometric analysis (RSA) were hypothesised to be superior to a plain radiograph method in the assessment of polyethylene (PE) wear. Methods. A phantom device was constructed to simulate three-dimensional (3D) PE wear. Images were obtained consecutively for each simulated wear position for each modality. Three commercially available packages were evaluated: model-based RSA using laser-scanned cup models (MB-RSA), model-based RSA using computer-generated elementary geometrical shape models (EGS-RSA), and PolyWare. Precision (95% repeatability limits) and accuracy (Root Mean Square Errors) for two-dimensional (2D) and 3D wear measurements were assessed. Results. The precision for 2D wear measures was 0.078 mm, 0.102 mm, and 0.076 mm for EGS-RSA, MB-RSA, and PolyWare, respectively. For the 3D wear measures the precision was 0.185 mm, 0.189 mm, and 0.244 mm for EGS-RSA, MB-RSA, and PolyWare respectively. Repeatability was similar for all methods within the same dimension, when compared between 2D and 3D (all p > 0.28). For the 2D RSA methods, accuracy was below 0.055 mm and at least 0.335 mm for PolyWare. For 3D measurements, accuracy was 0.1 mm, 0.2 mm, and 0.3 mm for EGS-RSA, MB-RSA and PolyWare respectively. PolyWare was less accurate compared with RSA methods (p = 0.036). No difference was observed between the RSA methods (p = 0.10). Conclusions. For all methods, precision and accuracy were better in 2D, with RSA methods being superior in accuracy. Although less accurate and precise, 3D RSA defines the clinically relevant wear pattern (multidirectional). PolyWare is a good and low-cost alternative to RSA, despite being less accurate and requiring a larger sample size

Background. Migration analysis after total joint arthroplasty are performed using EBRA analysis (Krismer et al., 1997) or - more accurate but also much more cost-intensive and time-consuming – via radiostereometric analysis (RSA). For the latter, additional radiographs from two inclined perspectives are needed in regular intervals in order to define the position of the implant relative to tantalum bone markers which have been implanted during surgery of the artificial joint (Fig. 1). Modern analysis software promises a migration precision along the stem axis of a hip implant of less than 100 μm (Witvoet-Brahm et al., 2007). However, as the analysis is performed semi-automatically, the results are still dependent on the subjective evaluation of the X-rays by the observer. Thus, the present phantom study aims at evaluating the inter- and intra-observer reliability, the repeatability as well as the precision and gives insight into the potential and limits of the RSA method. Materials and Methods. Considering published models, an RSA phantom model has been developed which allows a continuous and exact positioning of the prostheses in all six degrees of freedom (Fig. 2). The position sensitivities of the translative and rotative positioning components are 1 μm and 5 to 24, respectively. The roentgen setup and Model-Based RSA software (3.3, Medis specials bv, Leiden, Netherlands) was evaluated using the SL-PLUS® standard hip stem (size 7, Smith & Nephew, Baar, Switzerland). The inter-observer (10 repetitions) and intra-observer (3 observers) reliability have been considered. Additionally, the influences of the model repositioning and inclination as well as the precision after migration and rotation along the stem axis are investigated. Results and Discussion. Precision along the stem axis was determined to 161 μm (± 230 μm), in the lateral plane 100 μm (± 85 μm) and maximal rotations to 0.524° (± 1.268°). High reproducibility (intra-observer reliability) is reported with relevant influences of the inclination of the implant on the radiograph, in particular for the first clinical scene which serves as a reference. Deviations after translations along the stem axis are 0.37 ± 1.92% and −3.28 ± 6.62% after rotations. In conclusion, the precision given by the software producer of less than 100 μm could not be verified. Beside the limitations from the software, potential sources of errors are the subjective analysis by the observer, a small number of bone markers and the positioning of the implant (patient) during X-ray examination. Though, Model-Based RSA largely outmatches the EBRA approach in terms of measuring implant migration. However, standardization of the X-rays and RSA analysis is recommended