Ulrich Langlotz

Anatomic referencing of cup orientation in total hip arthroplasty.

Pelvic tilt and rotation can drastically affect the apparent cup orientation on conventional anteroposterior pelvic radiographs. It was hypothesized that nonstandardized radiographic cup version and abduction can differ significantly from the corresponding anatomic angles if not measured to the anterior pelvic plane, defined by the pubic tubercles and the anterior superior iliac spine. Differences in preoperative and postoperative pelvic orientation and their influence on radiographic measurements of the two angles were analyzed. Conventional radiographs and preoperative and postoperative computed tomography scans of 37 total hip arthroplasties were compared. Calculations were made with the preoperative planning station of a computer navigation system. Significantly smaller values of cup version were seen on nonstandardized radiographs, whereas abduction could be measured reliably when referenced to horizontal pelvic landmarks seen on radiographs. The underlying cause for this difference was the variation of pelvic tilt that ranged 27° (range, −7°-20°). Influence of pelvic tilt on the apparent cup orientation can be seen with simple nomograms. The orientation of the anterior pelvic plane before and after surgery did not differ. We think that version measurement on nonstandardized radiographs without anatomic referencing is highly inaccurate. Level of Evidence: Diagnostic study, Level II-1 (development of diagnostic criteria on basis of consecutive patients-with universally applied reference “gold” standard). See Guidelines for Authors for a complete description of levels of evidence.

A hybrid CT-free navigation system for total hip arthroplasty.

OBJECTIVE To design and evaluate a novel CT-free image-guided surgical navigation system for assisting placement of both acetabular and femoral components in total hip arthroplasty (THA). MATERIALS AND METHODS The methodology in this paper is conceptually based on our previous work on CT-free cup placement. For femoral component placement, two patient-specific reference coordinate systems are first defined: One for the pelvis, based on the so-called anterior pelvic plane (APP) concept, and one for the femur, using the center of the femoral head, the posterior condylar tangential line, and the medullary canal axis of the proximal femur. A hybrid method is used for the associated landmark acquisition, which involves percutaneous point-based digitization and bi-planar landmark reconstruction using multiple registered fluoroscopy images. The following clinical parameters are computed in real time: cup inclination and anteversion, antetorsion and varus/valgus of the stem, lateralization, and change in leg length for complete THA. In addition, instrument actions such as reaming, impaction, and rasping are visualized for the surgeon by superimposing virtual instrument representations onto the fluoroscopic images. RESULTS A laboratory study of computer-assisted measurement of antetorsion and varus/valgus, change in leg length, and lateralization for femoral stem placement demonstrated the high precision of the proposed navigation system. Compared with CT-based measurement, mean deviations of 1.0 degrees, 0.6 degrees, 0.7 mm, and 1.7 mm were found for antetorsion, varus/valgus, change in leg length, and lateralization, respectively, with standard deviations of 0.5 degrees, 0.5 degrees, 0.6 mm, and 0.7 mm, respectively. A pilot clinical evaluation showed that THA could benefit from this newly developed CT-free hybrid system. CONCLUSIONS The proposed CT-free hybrid system promises to increase the accuracy and reliability of THA surgery.

Accuracy and potential pitfalls of fluoroscopy-guided acetabular cup placement

Using a total of 30 cadaveric hips, the accuracy of a fluoroscopy-based computer navigation system for cup placement in total hip arthroplasty (THA) was investigated and an error analysis was carried out. The accuracy of placing the acetabular component within a predefined safe zone using computer guidance was compared to the precision that could be achieved with a freehand approach. Accurate control measurements of the implanted cup were obtained using fiducial-based matching to a pre-operative CT scan with respect to the anterior pelvic plane. A significantly higher number of cups were placed in the safe zone with the help of the navigation system. The variability of cup placement could be reduced for cup abduction but not substantially for cup version. An error analysis of inaccurate landmark reconstruction revealed that the registration of the mid-pubic point with fluoroscopy was a potential source of error. Keeping this pitfall in mind, fluoroscopy-based navigation in THA is a useful tool for registration of the pelvic coordinate system, particularly those points that cannot be reached by direct pointer digitization with the patient in the lateral decubitus position.

Computer-assisted Orthopaedic surgery (AOS): from pedicle screw insertion to further applications

SummaryComputer assisted orthopaedic surgery is a new but rapidly evolving field. Based on previous research and development in the area of stereotactic neuronavigation a few groups have adapted these technologies for the image interactive insertion of pedicle screws. The present paper summarizes past and current work in the field of computer assisted orthopaedic surgery and describes the state of the art of research and future innovations, particularly in in vivo applications.ZusammenfassungComputerassistenz ist für den Bereich der Orthopädischen Chirurgie ein relativ neues und dennoch sich rasch entwickelndes Feld. Durch Modifizierung teilweise bereits aus der Neurochirurgie bekannter stereotaktischer Systeme haben mehrere Arbeitsgruppen diese Techniken zunächst für die bildinteraktive Insertion von Pedikelschrauben nutzbar gemacht. Der vorliegende Artikel faßt bisher geleistete Arbeiten insbesondere in der In-vivo-Applikation zusammen, beschreibt den aktuellen Stand der Forschung und gibt einen Ausblick auf zukünftige Entwicklungen.