Robert Elfring

Assessment of optical localizer accuracy for computer aided surgery systems

The technology for localization of surgical tools with respect to the patients reference coordinate system in three to six degrees of freedom is one of the key components in computer aided surgery. Several tracking methods are available, of which optical tracking is the most widespread in clinical use. Optical tracking technology has proven to be a reliable method for intra-operative position and orientation acquisition in many clinical applications; however, the accuracy of such localizers is still a topic of discussion. In this paper, the accuracy of three optical localizer systems, the NDI Polaris P4, the NDI Polaris Spectra (in active and passive mode) and the Stryker Navigation System II camera, is assessed and compared critically. Static tests revealed that only the Polaris P4 shows significant warm-up behavior, with a significant shift of accuracy being observed within 42 minutes of being switched on. Furthermore, the intrinsic localizer accuracy was determined for single markers as well as for tools using a volumetric measurement protocol on a coordinate measurement machine. To determine the relative distance error within the measurement volume, the Length Measurement Error (LME) was determined at 35 test lengths. As accuracy depends strongly on the marker configuration employed, the error to be expected in typical clinical setups was estimated in a simulation for different tool configurations. The two active localizer systems, the Stryker Navigation System II camera and the Polaris Spectra (active mode), showed the best results, with trueness values (mean ± standard deviation) of 0.058 ± 0.033 mm and 0.089 ± 0.061 mm, respectively. The Polaris Spectra (passive mode) showed a trueness of 0.170 ± 0.090 mm, and the Polaris P4 showed the lowest trueness at 0.272 ± 0.394 mm with a higher number of outliers than for the other cameras. The simulation of the different tool configurations in a typical clinical setup revealed that the tracking error can be estimated to be 1.02 mm for the Polaris P4, 0.64 mm for the Polaris Spectra in passive mode, 0.33 mm for the Polaris Spectra in active mode, and 0.22 mm for the Stryker Navigation System II camera.

A new approach to implant alignment and ligament balancing in total knee arthroplasty focussing on joint loads.

Abstract Preservation and recovery of the mechanical leg axis as well as good rotational alignment of the prosthesis components and well-balanced ligaments are essential for the longevity of total knee arthroplasty (TKA). In the framework of the OrthoMIT project, the genALIGN system, a new navigated implantation approach based on intra-operative force-torque measurements, has been developed. With this system, optical or magnetic position tracking as well as any fixation of invasive rigid bodies are no longer necessary. For the alignment of the femoral component along the mechanical axis, a sensor-integrated instrument measures the torques resulting from the deviation between the instrument’s axis and the mechanical axis under manually applied axial compression load. When both axes are coaxial, the resulting torques equal zero, and the tool axis can be fixed with respect to the bone. For ligament balancing and rotational alignment of the femoral component, the genALIGN system comprises a sensor-integrated tibial trial inlay measuring the amplitude and application points of the forces transferred between femur and tibia. Hereby, the impact of ligament tensions on knee joint loads can be determined over the whole range of motion. First studies with the genALIGN system, including a comparison with an imageless navigation system, show the feasibility of the concept.

Vascular electromagnetic tracking: experiences in phantom and animal cadaveric models

This study assesses the feasibility of navigated vascular interventions in a rapid prototyped anthropomorphic phantom and swine cadaveric model using electromagnetic tracking (EMT) in combination with a CT image data set. In the phantom model overall feasibility and handling of the EMT navigation system were evaluated and first experiments performing visceral vessel catheterization were carried out. In the cadaveric model a predefined structure was targeted using an electromagnetically tracked guidewire. In the phantom model the catheterization of visceral vessels was reproducibly possible. In the cadaveric model a catheter placement accuracy of 0.97 ± 0.88 mm was achieved. The feasibility of EMT based vascular interventions could be shown and a landmark concerning the accuracy of such systems in a model setting could be determined. Further ex- and in-vivo experiments are needed to preevaluate applicability of such systems in clinical settings.

Determination of the Mechanical Leg Axis Using a Force-Torque Sensor

The correct determination of the mechanical leg axis (so called Mikulicz-Line) is cruicial step for the correct alignment of prosthesis components in Total Knee Arthroplasty (TKA). The newly developed “genALIGN”-system uses a force-torque sensor to directly measure the torques induced by a deviation from the mechanical axis. During surgery, the device is fixed to the knee center and a force is applied in the approximate direction of the hip center. An unstable system is created which can only reach equilibrium, when both the device and the mechanical leg axis are coaxial, i.e. the device points exactly in direction of the hip center. The result can be easily fixed and the bone cuts can be performed using an attachable cutting jig. To account for the leg’s weight, the sensor is adjusted to zero before starting the actual measurement.

Accuracy of Optical Localizers for Computer Aided Surgery

The localization of surgical tools and the patient’s reference coordinate system in three to six degrees of freedom is one of the key technologies in computer aided surgery. Several tracking methods are available, among which optical tracking is the most widespread in clinical use. Optical tracking technology has well proven to be a reliable method for intra-operative position and orientation acquisition for many clinical applications. However, the accuracy of these localizers is still a topic of discussion. In this paper, the accuracy of three optical localizer systems, the NDI Polaris P4, NDI Polaris Spectra (active and passive mode) and the Stryker Navigation System II Camera, is assessed and critically compared. Static tests revealed that only the Polaris P4 shows a significant warm up behavior with a significant shift of accuracy within 42 minutes after switching it on. Furthermore, the intrinsic localizer accuracy was determined for single markers as well as for typical tools using a volumetric measurement protocol on a coordinate measurement machine. To determine the relative distance error within the measurement volume, the Length Measurement Error (LME) was determined at 35 test lengths. The two active localizer systems, the Stryker Navigation System II camera and the Polaris Spectra (active mode), showed the best results with a trueness of 0.058±0.033mm and 0.089±0.061 mm, respectively (mean±std. dev.). The Polaris Spectra (passive mode) showed a trueness of 0.170±0.090mm and the Polaris P4 showed the lowest trueness with 0.272±0.394mm and a higher number of outliers than all the other cameras.

A new Intra-Articular Load Measuring Device for Ligament Balancing and Prothesis Alignment in Total Knee Arthroplasty

Stable ligamentous guidance of the knee joint movement, the preservation or recovery of the mechanical knee joint axis and the rotational alignment of the femoral component significantly influence the longevity of knee joint prostheses. A new intra-articular load measuring device was developed to support the surgeon during ligament balancing procedure. This device can as well be used during the rotational femoral component alignment and - integrated into the genALIGN [1] system - during axial alignment. The first feasibility study showed promising results.