Variation of the spatial position computed by Roentgen Stereophotogrammetric Analysis (RSA) under non-standard conditions

Changes in knee motion over the first 3 years with a mobile-bearing prosthesis

The present work aims to evaluate the mobility of the polyethylene bearing in 25 patients implanted with a mobile-bearing PCL-retaining TKR (Interax ISA, Striker Howmedica) under in vivo, weight-bearing conditions over 3 years' follow-up. We compared the bearing motion in response to specific tests in two consecutive follow-ups with Roentgen Stereophotogrammetric Analysis (RSA). Results showed that polyethylene continued to displace on the tibial baseplate over time: longitudinal rotations and medio-lateral translations tended to increase at the second follow-up (mean increases 0.7+/-2.5 degrees and 0.34+/-1.08 mm, respectively), while sagittal translations did not show any relevant change. Longitudinal rotations and AP translations were preserved when present at both follow-ups. Nevertheless, no statistical evidence of repeatable pattern of motion was found, since bearings frequently reversed their pattern of motion between the two follow-ups. The unconstrained polyethylene mobility resulted in an unrepeatable, erratic pattern of bearing motion intra-patient and over time. The changes in knee motion were found not to affect the clinical results over the first 3 years of follow-up. As in a fixed-bearing design, mobile-bearing design was not able to reproduce normal knee kinematics.

Investigation into the detection of marker movement by biplanar RSA

Roentgen Stereophotogrammetric Analysis (RSA) is the most accurate technique to measure micromotion between rigid bodies and it has been widely used in the orthopedic field to investigate lower limb prostheses. The aim of this paper was to investigate the factors affecting the accuracy of measurements performed on the knee joint by the biplanar RSA system. Errors were assessed by means of a test object. A particular goal was to verify the reliability of the system when the patient cannot stand in the ideal position during the examination, i.e. in the centre of the calibration space. Results showed that detection of micromotion is not correlated to the position of markers inside the calibration space. The influence of different materials able to simulate human tissues, both hard and soft, on the data acquisition phase was also investigated. This additional exam was performed to assess whether the results obtained were similar also under as close as possible conditions to the clinical trials. Different experimental conditions were shown to not affect the acquisition phase of RSA procedure; markers were acquired with equal accuracy independently of tissues examined. Since the acquisition procedure is often performed by different operators, the difference between acquisitions made by different operators was compared. This study confirms the reliability of RSA and its accuracy, also in non-standard clinical setup and acquisition procedures.

Fixation of total knee arthroplasty improved by mobile-bearing design

We examined the in vivo displacement of mobile-bearing polyethylene and its effect on prosthesis-bone fixation. Thirty-five patients with the Interax ISA mobile platform total knee arthroplasties were examined by roentgen stereophotogrammetric analysis. Our protocol included weightbearing to measure the range of motion of the mobile platform in relation to the tibial component. We also studied the micromotions of the tibial component in relation to the tibia during a 2-year followup. The analysis of micromotion (maximum total motion mean, 0.5 mm; subsidence mean, 0.15 mm) showed this mobile platform design is stable and improves fixation at the prosthesis-bone interface. Longitudinal rotations of the mobile platform occurred in all patients and followed a homogeneous pattern of motion, depending on the examination. However, anteroposterior translations did not show any patterns with specific examination. This suggests the meniscal movement adapts itself to different weightbearing conditions. We measured the in vivo mobile-bearing displacement under static loading conditions. We also observed improved fixation at the bone-prosthesis interface.

Marker Configuration Model-Based Roentgen Fluoroscopic Analysis

It remains unknown if and how the polyethylene bearing in mobile bearing knees moves during dynamic activities with respect to the tibial base plate. Marker Configuration Model-Based Roentgen Fluoroscopic Analysis (MCM-based RFA) uses a marker configuration model of inserted tantalum markers in order to accurately estimate the pose of an implant or bone using single plane Roentgen images or fluoroscopic images. The goal of this study is to assess the accuracy of (MCM-Based RFA) in a standard fluoroscopic set-up using phantom experiments and to determine the error propagation with computer simulations. The experimental set-up of the phantom study was calibrated using a calibration box equipped with 600 tantalum markers, which corrected for image distortion and determined the focus position. In the computer simulation study the influence of image distortion, MC-model accuracy, focus position, the relative distance between MC-models and MC-model configuration on the accuracy of MCM-Based RFA were assessed. The phantom study established that the in-plane accuracy of MCM-Based RFA is 0.1 mm and the out-of-plane accuracy is 0.9 mm. The rotational accuracy is 0.1 degrees. A ninth-order polynomial model was used to correct for image distortion. Marker-Based RFA was estimated to have, in a worst case scenario, an in vivo translational accuracy of 0.14 mm (x-axis), 0.17 mm (y-axis), 1.9 mm (z-axis), respectively, and a rotational accuracy of 0.3 degrees. When using fluoroscopy to study kinematics, image distortion and the accuracy of models are important factors, which influence the accuracy of the measurements. MCM-Based RFA has the potential to be an accurate, clinically useful tool for studying kinematics after total joint replacement using standard equipment.

How Cyclic Loading Affects the Migration of Radio-Opaque Markers Attached to Tendon Grafts Using a New Method: A Study Using Roentgen Stereophotogrammetric Analysis (RSA)

An increase in anterior laxity following reconstruction of the anterior cruciate ligament (ACL) can result from lengthening of the graft construct in either the regions of fixation and/or the region of the graft substance between the fixations. RSA could be a useful technique to determine lengthening in these regions if a method can be devised for attaching radio-opaque markers to soft tissue grafts so that marker migration from repeated loading of the graft is limited. Therefore, the objectives of this study were 1) to develop a method for attaching radio-opaque markers to an ACL graft that limits marker migration within the graft, 2) to characterize the error of an RSA system used to study migration, and 3) to determine the maximum amount of migration and the time when it occurs during cyclic loading of ACL grafts. Tendon markers were constructed from a 0.8-mm tantalum ball and a stainless steel suture. Ten double-looped tendon grafts were passed through tibial tunnels drilled in bovine tibias and fixed with a tibial fixation device. Two tendon markers were sewn to one tendon bundle of each graft and the grafts were cyclically loaded for 225,000 cycles from 20 N to 170 N. At specified intervals, simultaneous radiographs were obtained of the tendon markers and a radiographic standard of known length. The bias and imprecision in measuring the length of the radiographic standard were 0.0 and 0.046 mm respectively. Marker migration was computed as the change in distance between the two tendon markers along the axis of the tibial tunnel. Marker migration was greatest after 225,000 cycles with a root mean square (RMS) value of less than 0.2 mm. Because the RMS value indicates the error introduced into measurements of lengthening and because this error is small, the method described for attaching markers to an ACL graft has the potential to be useful for determining lengthening of ACL graft constructs in in vivo studies in humans.