Introduction: Recently, the correct interpretation of anteroposterior (AP) pelvic radiographs has regained increased attention, particularly in the field of joint preserving hip surgery. The diagnosis of acetabular retroversion associated with femoroacetabular impingement or hip dysplasia is made regardless the individual pelvic orientation due to the lack of a method of correction. Furthermore, it is known that a substantial number of the most common radiographical hip parameters can vary with the individual pelvic orientation. The goal of the study was to evaluate which parameter can be measured accurately on an AP radiograph.

Methods: Digital AP pelvic radiographs of 100 consecutive hips were used for evaluation. The blinded and randomized x-rays were examined by two independent observers with special software that has been validated previously. The software is able to correct the projected acetabular rim and the associated parameters for pelvic malpositioning. The following parameters were investigated: femoral head coverage in craniocaudal and anteroposterior direction (in total and for each single quadrant of the femoral head), the lateral center edge angle, the acetabular index, the ACM-angle, the extrusion index, the cross-over sign, the retroversion index, and the posterior wall sign. All parameters were first measured regardless to the individual tilt and rotation. These non-standardized values were then compared to the standardized values for a neutral pelvic orientation. This was defined with a pelvic inclination of 60 degrees which was detected with one single strong lateral pelvic radiograph.

Results: There were no differences in evaluation of the radiographs between the two observers concerning the significance of standardized and non-standardized values for the measured features. All but three parameters were significantly different when measured to the anatomically reference neutral orientation. The only parameters that did not change after standardization were the total femoral coverage, the acetabular index and the ACM.

Discussion: Except from the ACM and the acetabular index, basically all parameters change when standardized to a neutral orientation. Although from a statistical point of view, the total craniocaudal femoral coverage did not change, it is likely that this is due to an inverse effect of the anterior and posterior part of the acetabulum. We conclude that the most common hip parameters can not be reliably measured without standardization. It remains to be proven that the standardization of the parameters correlates with the clinical symptoms.

Introduction: It could be shown that an ample number of classical hip parameters for radiographic quantification of hip morphology on anteroposterior (AP) pelvic radiographs vary significantly with individual pelvic tilt and rotation. This could be proven not only for classical hip parameters (e.g. the lateral centre edge angle) but also for more recently described radiographic features such as acetabular retroversion. The resulting misdiagnosis and misinterpretation can potentially impair a correct therapy for the patient.

We developed fast and easy-to-use computer software to perform three-dimensional (3D) analysis of the individual hip joint morphology using two-dimensional (2D) AP pelvic radiographs. Landmarks extracted from the radiograph were combined with a cone beam x-ray projection model and a strong lateral pelvic radiograph to reconstruct 3D hip joints. Twenty-five parameters including quantification of femoral head coverage can be calculated for a neutral orientation. The aim of the study was to evaluate the validity of this method for tilt and rotation correction of the acetabular rim and associated radiographic parameters.

Methods: The validation comprised three steps:

External validation;

A series of x-rays of 30 cadaver pelves mounted on a flexible holding device were available for step 1 and 2. External validation comprised the comparison of radiographical parameters of the cadaver hips when determined with our software in comparison with CT-based measurements or actual radiographs in a neutral pelvic orientation as gold standard. Internal validation evaluated the consistency of the parameters when each single pelvis was calculated back from different random orientations to the same neutral pelvic position. The intra-/interobserver analysis investigated the reliability and reproducibility of all parameters with the help of 100 randomized, blinded AP pelvic radiographs of a consecutive patient series.

Results:

All but one parameter (acetabular index) showed a substantial to almost perfect correlation with the CT-measurements.

Conclusion: The software could be shown to be an accurate, reliable and reproducible method for correction of AP pelvic radiographs. This computer-assisted method allows standardized evaluation of all relevant radiographic parameters for detection of anatomic morphologic differences. It will be used to study the influence of pelvic malorientation on the radiographic appearance of each individual parameter. In addition, it allows evaluating the clinical significance of standardizing pelvic parameters.

Background: Successful total knee arthroplasty requires component alignment according to the mechanical axes and restoration of ideal knee kinematics. This requires adequate ligament balancing, stable tibia-femoral and patello-femoral joints, and a non-restricted range of motion.

We developed a computer assisted total knee arthroplasty system to help the surgeon achieving more intra-operative accuracy.

Material and methods: An OPTOTRAK camera is used to track relative motions between femur, tibia, and instruments. In contrast to other systems we avoid fixation of reference bases onto acetabulum and foot. The surgeon generates a representation of the patient’s anatomy using the technique of “surgeon defined anatomy”. Based on recorded landmarks the system calculates the femoral and tibial mechanical axes, the position of the knee joint line, the level of the defects on femoral and tibial side, the anatomically best fitting femoral component size, the femoral ventral level, and the natural tibial rotation. These values enable an initial planning situation, which features alignment of the tibial and femoral distal resection planes according to the mechanical axes as well as the definition of the anterior and posterior femoral resection planes with respect to the ventral cortex and the prosthesis design. To consider soft-tissue behaviour the surgeon loads both collateral ligaments in extension and flexion, a

Results: During a clinical study we performed thirteen total knee arthroplasties. Postoperatively passive extension was 0.8-4.2° (mean 1.9°) in the coronal plane and 0.2-3.9° (mean 1.8°) in the sagittal plane. Varus-valgus instability was 7.2°. The results of the subsequent patients of this ongoing study will be available during the conference.

The most common reason for possible complications after total hip replacement (THR) surgery is improper positioning of the implant components within the hip joint. Systems for computer assisted planning and navigation during THR have been developed. However, these established modules focus on the acetabular implant component only; disrespecting the fact that proper implant functioning relies upon correct placement of both components relative to each other. Therefore, we developed an extension to the existing CT-based SurgiGATE-Prosthetics system (Medivision, Oberdorf, Switzerland) for planning and placing of the acetabular component to give the surgeon a tool, which can help him/her to also plan and insert the femoral implant.

Preoperatively, the appropriate size and position as well as the orientation of both implants components were planned. Following navigated cup placement a dynamic reference base (DRB) was fixed to the thighbone and the registration procedure was executed. For the preparation of the femoral cavity a modular PPF rasp system (Biomet-Merck, Darmstadt, Germany) was developed. All surgical action was visualised graphically within the patient’s image data. In addition, the surgeon was provided with real-time information about the depth of tool insertion, antetorsion angle, varus/valgus deviation, and the postoperative change in leg length and lateralisation of the hip joint.

After extensive validation and accuracy analyses performed on plastic models the presented system was used during one operation. An extended clinical study is currently being started.

We expect that the developed application will help the surgeon to better plan the appropriate size and position of the both parts of a hip endoprosthesis and will supply intraoperative feedback of the position of the surgical instruments relative to the patients’ anatomy and to the preoperative plan. Safer and more accurate placement of the implants components during free-hand THR surgery may be expected from this technology.

Correct placement of the total elbow endoprosthesis is a critical factor for the long-term success of an artificial joint. Correct restoration of the centre of rotation is essential for optimal outcome. To evaluate whether surgical navigation has the potential to improve accuracy during Total Elbow Arthroplasty (TEA), an existing CAS system was applied on one plastic model and three patients.

The spine module of the SurgiGATE™ navigation system (Medivision, Oberdorf, Switzerland) was used. To apply it during TEA, a standard 3.5-mm drill guide was instrumented with infrared LEDs and calibrated. A dynamic reference base (DRB) was developed. Its base consisted of an X-shaped, scissors-like construct that could be clamped rigidly onto the distal humerus after exposure. On a plastic model, the DRB design was evaluated, and three landmarks suitable for intraoperative matching were identified. Subsequently, the Spine system was applied during three TEA surgeries. For the first surgery no pre-operative CT scan was acquired, but the design of the DRB, its camera visibility, and the accessibility of the landmarks were verified. For the other cases, the elbows were CT-scanned preoperatively. Planning consisted of 3-D segmentation as well as the definition of matching landmarks and a trajectory representing the position of a Steinmann pin, with which the humeral implant position is defined. Intraoperatively, the DRB was fixated, and matching was performed. Using the navigation system, the drill guide could then be aligned with the planned trajectory.

For the second patient, no accurate matching was achieved, hence surgery was completed conventionally. The last patient could be registered precisely, and the Steinmann pin was placed as planned.

Preliminary results show that CT-based navigation can be applied during TEA. Given a positive output of an ongoing clinical study, the development of a special TEA navigation system is planned.