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.

The purpose of this research was to determine the feasibility of radiostereometric analysis (RSA) as a diagnostic tool for assessing non-union following spinal arthrodesis procedures. Further, to estimate clinical thresholds for precision and accuracy of the proposed method in the cervical and lumbar spine.

A three-level lumbo-sacral and a four-level cervical posterior arthrodesis procedures were performed on an artificial spine model (Sawbones, WA). Using a spring loaded inserter (RSA Biomedical, Sweden), eight to ten RSA markers were placed within each of the L4 and L5 segments in the spinous process (L4 only), lamina, transverse processes, posterior and anterior (down the pedicle) wall of the vertebral body. Eight to ten markers were placed within the proximal sacrum (S1) at the medial and lateral crests, tuberosity, and within the sacral canal wall. Four to eight RSA markers were placed into the C3-C6 lateral masses. Titanium screws and rods were applied to the spinal segments. Identical procedures were then performed on a cadaveric spine using similar bead placement and hardware.

RSA imaging consisted of 12 double exams (24 exams) of the cervical and lumbar regions for both the Sawbones and cadaveric spine to assess precision of measurement under zero-displacement conditions. The most distal vertebrae were considered the datum against which the movement of all other vertebrae was compared.

The artificial spine was then dismantled for accuracy assessment in which the middle vertebrae (L5 and C4-C5) were moved relative to the superior (L4 and C3) and inferior (S1 and C6) vertebrae by known, incremental displacements on an imaging phantom device. Displacements occurred along the superior-inferior, anterior-posterior, and flexion-extension (rotational) axes of motion. RSA images were obtained at each displacement.

Image analysis was performed using model-based software (RSACore v3.41, Leiden, Netherlands) to visualize implanted RSA beads in 3-D space. Precision was defined as the 95% confidence interval of error in measuring zero-displacement. Accuracy was defined as the mean difference (with 95% confidence interval) between the known and measured displacement.

The rate of RSA bead detection was high with 5–8 implanted beads being visible in both the lumbar and cervical regions of the artificial and cadaveric spines.

Translational RSA precision for both spines was better than 0.25 mm and 0.82 mm for the lumbar and cervical regions, respectively. Rotational precision was better than 0.40° and 1.9° for the lumbar and cervical regions, respectively. RSA accuracy for the artificial spine overall demonstrated less than 0.11 mm translational bias (margin < ±0.02 mm) and less than 0.22° rotational bias (margin < ±0.15°).

This study demonstrates that RSA achieves sufficient precision and accuracy to detect intervertebral micromotion for the purpose of assessing arthrodesis. Well dispersed RSA bead placement is critical to achieving sufficient accuracy as well as avoiding occlusion by metal hardware. Cervical bead implantation is particularly sensitive to bead clustering due to small vertebrae size and proximity to critical structures. The results of this work will aid in the development of a clinical study to assess arthrodesis in patients.

Introduction

This research determines the feasibility of radiostereometric analysis (RSA) as a diagnostic tool for assessing fusion following spinal arthrodesis. Further, to estimate clinical thresholds for precision and accuracy of the proposed method in the cervical and lumbar spine.

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.

Articulation of the polyethylene (PE) insert between the metal femoral and tibial components in total knee replacements (TKR) results in wear of the insert which can necessitate revision surgery. Continuous PE advancements have improved wear resistance and durability increasing implant longevity. Keeping up with these material advancements, this study utilises model-based radiostereometric analysis (mbRSA) as a tool to measure in vivo short-term linear PE wear to thus predict long-term wear of the insert.

Radiographic data was collected from the QEII Health Sciences Centre in Halifax, NS. Data consisted of follow-up RSA examinations at post-operative, six-, 12-, and 24-month time periods for 72 patients who received a TKR. Implanted in all patients were Stryker Triathlon TKRs with a fixed, conventional PE bearing of either a cruciate retaining or posterior stabilised design. Computer-aided design (CAD) implant models were either provided by the manufacturer or obtained from 3D scanned retrieved implants. Tibial and femoral CAD models were used in mbRSA to capture pose data in the form of Cartesian coordinates at all follow-ups for each patient. Coordinate data was manually entered into a 3D modeling software (Geomagic Studio) to position the implant components in virtual space as presented in the RSA examinations. PE wear was measured over successive follow-ups as the linear change in joint space, defined as the shortest distance between the tibial baseplate and femoral component, independently for medial and lateral sides. A linear best-fit was applied to each patient's wear data; the slope of this line determined the annual wear rate per individual patient. Wear rates were averaged to provide a mean rate of in vivo wear for the Triathlon PE bearing.

Mean linear wear per annum across all 72 patients was 0.088mm/yr (SD: 0.271 mm/yr) for the medial condyle and 0.032 mm/yr (SD: 0.230 mm/yr) for the lateral condyle. Cumulative linear wear at the 2-year follow-up interval was 0.207mm (SD: 0.565mm) and 0.068mm (SD: 0.484mm) for the medial and lateral condyles, respectively.

Linear PE wear measurements using mbRSA and Geomagic Studio resulted in 0.056mm/yr additional wear on the medial condyle than the lateral condyle. Large standard deviations for yearly wear rates and cumulative measurements demonstrate this method does not yet exhibit the accuracy needed to provide short-term in vivo wear measurement. Inter-patient variability from RSA examinations is likely a source of error when dealing with such small units of measure. Further analysis on patient age and body mass index may eliminate some variability in the data to improve accuracy. Despite high standard deviations, the results from this research are in proximity to previously reported linear wear measurements 0.052mm/yr and 0.054mm/yr. Linear wear analysis will continue upon completion of >100 patients, in addition to volumetric PE wear over the entire articulating surface.