Introduction. Primary stability is an important factor for long-term implant survival in total hip arthroplasty. In revision surgery, implant fixation becomes especially challenging due the acetabular bone defects, which are often present. Previous studies on primary stability of revision components often applied simplified geometrical defect shapes in a variety of sizes and locations. The objectives of this study were to (1) develop a realistic defect model in terms of defect volume and shape based on a clinically existing acetabular bone defect, (2) develop a surrogate acetabular test model, and (3) exemplarily apply the developed approach by testing the primary stability of a pressfit-cup with and without bone graft substitute (BGS). Materials & Methods. Based on clinical computed tomography data and a method previously published [1], volume and shape information of a representative defect, chosen in consultation with four senior hip revision surgeons, was derived. Volume and shape of the representative defect was approximated by nine reaming procedures with hemispherical acetabular reamers, resulting in a simplified defect with comparable volume (18.9 ml original vs. 18.8 ml simplified) and shape. From this simplified defect (Defect D), three additional defect models (Defect A, B, C) were derived by excluding certain reaming procedures, resulting in four defect models to step-wise test different acetabular revision components. A surrogate acetabular model made of 20 PCF polyurethane foam with the main support structures was developed [2]. For the exemplary test, three series for Defect A were defined: Native (acetabulum without defect), Empty (defect acetabulum without filling), Filled (defect acetabulum with BGS filling). All series were treated with a pressfit-cup and subjected to dynamic axial load in direction of maximum resultant force during level walking. Minimum load was 300 N and maximum load was increased step-wise from 600 N to 3000 N. Total relative motion between cup and foam, consisting of inducible displacement and migration, was assessed with the optical measurement system gom Aramis (gom GmbH, Braunschweig, DE). Results. Total relative motion increased with increasing load, with a maximum of 0.63 mm for Native, 0.86 mm for Filled, and 1.9 mm for Empty. At load stage 1800 N, total relative motion in Empty was 11.0-fold increased in comparison to Native, but could be reduced to a 3.3-fold increase in Filled. Discussion. The objective of this study was to develop a simplified, yet realistic and modular defect model which could be used to step-wise test different treatment strategies. Applicability of the developed test setup was shown by assessing primary stability of a pressfit-cup in a native, empty, and filled situation. The presented method could potentially be used as a modular test setup to compare different acetabular revision components in a standardized way. For any figures or tables, please contact authors directly

INTRODUCTION. Deformation of modular acetabular press-fit shells is a topic of much interest for surgeons and manufacturer. Such modular components utilise a titanium shell with a liner manufactured from metal, polyethylene or ceramic. Initial fixation is achieved through a press-fit between shell and acetabulum with the shell mechanically deforming upon insertion. Shell deformation may disrupt the assembly process of inserting the bearing liner into the acetabular shell for modular systems. This may adversely affect the integrity and durability of the components and the tribology of the bearing. OBJECTIVE. Most clinically relevant data to quantify and understand such shell deformation can be achieved by cadaver measurements. ATOS Triple Scan III was identified as a measurement system with the potential to perform those measurements. The study aim was to validate an ATOS Triple Scan III optical measurement system against a co-ordinate measuring machine (CMM) using in-vitro testing and to check capability/ repeatability under cadaver lab conditions. METHODS. Two sizes of custom-made acetabular shells were deformed using a uniaxial/ two-point loading frame and measured repeatedly at different loads. Roundness measurements were performed using both the ATOS Triple Scan III optical system and a co-ordinate measuring machine and then compared. The repeatability was also tested by measuring shells pre and post insertion in a cadaver lab multiple times. RESULTS. The in-vitro comparison with CMM demonstrated a maximum difference of 5 µm at the rim and 9 µm at the measurement point closest to the pole of the shell. Deviation between the two systems increased towards the pole for the in-vitro measurements. However as press fit shells are designed to be loaded at the rim, this is likely where the maximum deflection will occur as a result of the highest force. Therefore, the increased difference between the systems towards the pole is of less importance compared with accuracy at the rim. Maximum repeatability was below 1 µm for the CMM and 3 µm for the ATOS Triple Scan III optical system. Repeatability of the ATOS Triple Scan III optical system was comparable between pre insertion (below 2 µm) and post insertion (below 3 µm) measurements in the cadaver lab. In addition these values were comparable to the repeatability measured during the in-vitro validation study (below 3 µm). This proves high repeatability not only for in-vitro conditions, but also for the cadaver lab as well. CONCLUSIONS. This study supports the view that the ATOS Triple Scan III optical system fulfils the necessary requirements to accurately measure shell deformation in cadavers. As a result, the authors propose further studies using cadavers to identify the impact of other factors upon shell deformation. Other factors to be measured include bone strength, shell diameter, under reaming and wall thickness

To develop a useful surgical navigation system, accurate determination of bone coordinates and thorough understanding of the knee kinematics are important. In this study, we have verified our algorithm for determination of bone coordinates in a cadaver study using skeletal markers, and at the same time, we also attempted to obtain a better understanding of the knee kinematics. The research was performed at the Medical Simulation Center of Tzu Chi University. Optical measurement system (Polaris® Vicra®, Northern Digital Inc.) was used, and reflective skeletal markers were placed over the iliac crest, femur shaft, and tibia shaft of the same limb. Two methods were used to determine the hip center; one is by circumduction of the femur, assuming it pivoted at the hip center. The other method was to partially expose the head of femur through anterior hip arthrotomy, and to calculate the centre of head from the surface coordinates obtained with a probe. The coordinate system of femur was established by direct probing the bony landmarks of distal femur through arthrotomy of knee joint, including the medial and lateral epicondyle, and the Whiteside line. The tibial axis was determined by the centre of tibia plateau localised via direct probing, and the centre of ankle joint calculated by the midpoint between bilateral malleoli. Repeated passive flexion and extension of knee joint was performed, and the mechanical axis as well as the rotation axis were calculated during knee motion. A very small amount of motion was detected from the iliac crest, and all the data were adjusted at first. There was a discrepancy of about 16.7mm between the two methods in finding the hip centre, and the position found by the first method was located more proximally. When comparing the epicondylar axis to the rotation axis of the tibia around knee joint, there was a difference of 2.46 degrees. The total range of motion for the knee joint measured in this study was 0∼144 degrees. The mechanical axis was found changing in an exponential pattern from 0 degrees to undetermined at 90 degrees of flexion, and then returned to zero again. Taking the value of 5 degrees as an acceptable range of error, the calculated mechanical axis exceeded this value when knee flexion angle was between 60∼120 degrees. The discrepancy between the hip centres calculated from the two methods suggested that the pivoting point of the femur head during hip motion might not be at the center of femur head, and the former location seemed closer to the surface of head at the weight bearing site. Under such circumstances, the mechanical axis obtained through circumduction of the thigh might be 1∼2 degrees different from that obtained through the actual center of femur head. During knee flexion, the mechanical axis also changed gradually, and this could be due to laxity of knee joint, or due to intrinsic valgus/varus alignment. However, the value became unreliable when the knee was at a flexion angle of 60∼120 degrees, and this should be taken into account during navigation surgery

INTRODUCTION. Over the last twenty years, image-guided interventions have been greatly expanded by the advances in medical imaging and computing power. A key step for any image-guided intervention is to find the image-to-patient transformation matrix, which is the transformation matrix between the preoperative 3D model of patient anatomy and the real position of the patient in the operating room. In this work, we propose a robust registration algorithm to match ultrasound (US) images with preoperative Magnetic Resonance (MR) images of the Humerus. MATERIALS AND METHODS. The fusion of preoperative MR images with intra-operative US images is performed through an NDI Spectra® Polaris system and a L12-5L60N TELEMED® ultrasound transducer. The use of an ultrasound probe requires a calibration procedure in order to determine the transformation between an US image pixel and its position according to a global reference system. After the calibration step, the patient anatomy is scanned with US probe. US images are segmented in real time in order to extract the desired bone contour. The use of an optical measurement system together with trackers and the previously-computed calibration matrix makes it possible to assign a world coordinate position to any pixel of the 2D US image. As a result, the set of US pixels extracted from the images results in a cloud of 3D points which will be registered with the 3D Humerus model reconstructed from MR images. The proposed registration method is composed of two steps. The first step consists of US 3D points cloud alignment with the 3D bone model. Then, the second step performs the widely-known Iterative Closest Point (ICP) algorithm. In order to perform this, we define the coordinate system of both the 3D Humerus model and the US points cloud. The frame directions correspond to the directions of the principal axes of inertia calculated from the matrices of inertia of both the preoperative 3D model and the US data obtained intra-operatively. Then, we compute the rotation matrix to estimate the transformation between the two coordinate systems previously calculated. Finally the translation is determined by evaluating the distance between the mass centres of the two 3D surfaces. RESULTS. In order to evaluate the performance of this registration method in terms of precision and accuracy, we performed the US/MRI fusion on 8 patients. The evaluation criterion used for the validation step was the fiducial registration error (FRE) estimation based on 8 anatomic fiducials detected on the Humerus of the patient. The mean, standard deviation, minimum and maximum values of the 8 Fiducial Registration Errors were 4.34, 2.20, 2.81 and 9.48 mm, respectively. DISCUSSIONS. In this work, we propose a robust registration method of MR and US data. Thanks to the optical system, this fusion will allow us for example to guide and assist surgeons in the positioning of the radiofrequency probe for bone tumor ablation. In addition to the fact that it is completely automatic, the proposed image-to-patient registration method is minimally invasive

INTRODUCTION. Modular knee implants are used to manage large bone defects in revision total knee arthroplasty. These implants are confronted with varying fixation characteristics, changes in load transfer or stiffen the bone. In spite of their current clinical use, the influence of modularity on the biomechanical implant-bone behavior (e.g. implant fixation, flexibility, etc.) still is inadequately investigated. Aim of this study is to analyze, if the modularity of a tibial implant could change the biomechanical implant fixation behavior and the implant-bone flexibility. MATERIAL & METHODS. Nine different stem and sleeve combinations of the clinically used tibial revision system Sigma TC3 (DePuy) were compared, each implanted standardized with n=4 in a total of 36 synthetic tibial bones. Four additional un-implanted bones served as reference. Two different cyclic load situations were applied on the implant: 1. Axial torque of ±7Nm around the longitudinal stem axis to determine the rotational implant stability. 2. Varus-valgus-torque of ±3,5Nm to determine the bending behavior of the stem. A high precision optical 3D measurement system allowed simultaneous measuring of spatial micromotions of implant and bone. Based on these micromotions, relative motions at the implant-bone-interface and implant flexibility could be calculated. RESULTS. Lowest relative micromotions were measured along the tibial base component and the sleeve; however, these motions varied depending on the implant construct used. Maximum relative micromotions were detected at the distal end of the implant for all groups, indicating a more proximal fixation of all modular combinations. Regarding varus-valgus-torque measurement, all groups showed a deviant flexibility behavior compared to the reference group. When referred to the un-implanted bone, implants without stems revealed the highest flexibility, whereas implants with shorter stems had lowest flexibility. DISCUSSION & CONCLUSION. All groups showed a more proximal fixation behavior; moreover, both extent and location of fixation could be influenced by varying the modular combination. Larger stems seemed to support a more distal fixation behavior, whereas the implant fixation moved proximal while extending the sleeve. Here the influence of the sleeve on fixation behavior seemed to be dominant compared to the influence of the stem. Concerning varus-valgus-torque, a strong connection between the used stem and implant-bone flexibility seemed to exist. In addition, the influence of the sleeve on flexibility seemed to be rather low. This study showed, that modularity can influence the biomechanical behavior of tibial implants. If these results can be transferred to other tibial implants still remains to be seen