K. Radermacher

Concept and development of an orthotropic FE model of the proximal femur

PURPOSEnIn contrast to many isotropic finite-element (FE) models of the femur in literature, it was the object of our study to develop an orthotropic FE model femur to realistically simulate three-dimensional bone remodelling.nnnMETHODSnThe three-dimensional geometry of the proximal femur was reconstructed by CT scans of a pair of cadaveric femurs at equal distances of 2mm. These three-dimensional CT models were implemented into an FE simulation tool. Well-known density-determined bony material properties (Youngs modulus; Poissons ratio; ultimate strength in pressure, tension and torsion; shear modulus) were assigned to each FE of the same CT-density-characterized volumetric group. In order to fix the principal directions of stiffness in FE areas with the same density characterization, the cadaveric femurs were cut in 2mm slices in frontal (left femur) and sagittal plane (right femur). Each femoral slice was scanned into a computer-based image processing system. On these images, the principal directions of stiffness of cancellous and cortical bone were determined manually using the orientation of the trabecular structures and the Haversian system. Finally, these geometric data were matched with the CT-density characterized three-dimensional femur model. In addition, the time and density-dependent adaptive behaviour of bone remodelling was taken into account by implementation of Carters criterion.nnnRESULTSnIn the constructed model femur, each FE is characterized by the principal directions of the stiffness and the CT-density-determined material properties of cortical and cancellous bone. Thus, on the basis of anatomic data a three-dimensional FE simulation reference model of the proximal femur was realized considering orthotropic conditions of bone behaviour.nnnCONCLUSIONSnWith the orthotropic model femur, the fundamental basis has been formed to realize realistic simulations of the dynamical processes of bone remodelling under different loading conditions or operative procedures (osteotomies, total hip replacements, etc).

CT-Based Planning and Individual Template Navigation in TKA

Among the various modalities of alloarthroplasty of the knee joint the total endoprosthesis is the most often exe-cuted intervention in the world. Severe arthrosis and axial deformities up to 30o varus or valgus can thereby be equalized. Today’high quality standard of surgical treatment requires anatomically correct orientation and fitting of the implant aiming at optimal biomechanical functionality. Thus, the single components must be taken into account as well as functional changings of tension in the surrounding soft tissue.

CRANIO—computer-assisted planning for navigated and robot-assisted surgery on the skull

Abstract Computer-assisted planning, navigation, and robotic craniotomy, with optional skull reconstruction using customized implants are of increasing clinical interest in craniofacial and neurosurgery. The aim of the CRANIO project presented in the first part of this paper is to develop a system that integrates the entire surgical process, from planning, customized implant manufacturing, optional robot programming, up to intraoperative navigation and robot-assisted execution. In the second part of the paper, we present new algorithms developed in the framework of CRANIO project. This work is especially related to the manipulation of 3-D geometry data of the skull for planning and definition of craniotomy. The results of our work are presented, together with results and objectives for further development.

Pedicle Screw Implantation Using the DISOS Template System

In computer-aided pedicle-screw implantation, exactly as in conventional procedures, correct positioning of the screw is only possible based on a precise knowledge of the anatomical structure in question. The choice of screw length and caliber depends to a great extent on detailed knowledge about this structure [6]. Generally, the greater the screw length and caliber, the better the anchoring [4]. In addition, particular attention must be paid to ventral boundaries and the constant danger of perforation in the area of the spinal cord. The proximity of the spinal column means the greatest care is required in introducing the screw in order to avoid damage to surrounding neural and vascular structures. Placement of a pedicle screw is therefore difficult since there is no direct view into the spine. In particular, problems can arise with medial or caudal placement of the screw, possibly resulting in serious neurological dysfunction. In the literature, faulty placements (perforations) using conventional procedures are reported to range from between 8.5% [2], 15.9% [9], 39.9% [3], and 42% [5].

Pelvic Osteotomy with Template Navigation

Various surgical techniques are used for the treatment of hip dysplasia in young people and adults. In contrast to acetabuloplasty in children, the position of the complete acetabulum is corrected. The repositioning serves to enlarge the weight bearing zone of the dysplastic acetabulum covering the femoral head, in order to reduce pressure on this area to physiological levels. The major goals of this therapy are relief from pain as well as preventing premature osteoarthritis with the necessity for artificial hip replacement. The basics of the biomechanics as well as diagnosis and indications are described and discussed in detail elsewhere [9,11]. Apart from the triple osteotomy according to Tonnis, practiced by our clinic since more than 15 years, particular mention should be made of the periacetabular osteotomy technique described by Ganz [3]. In contrast, the spherical osteotomy procedure of Wagner [12] has not become established due to the danger of acetabular necrosis.

An interface for the data exchange between CAS and CAD/CAM systems

Abstract In Computer Assisted Surgery (CAS), there is an increasing need for the data exchange between the planning system and external Computer Aided Design (CAD)/Computer Aided Manufacture (CAM) process to simulate the intraoperative execution or to verify and optimize the implant design. An interface for such data exchange was therefore implemented and integrated in the CAS system developed in our group. We also propose a simple, robust and fast algorithm for the voxelization of surface models with triangle meshes. The tools have been used successfully for the integration of standard geometry implant data for total knee arthroplasty. A surgical simulation and planning module for the robot assisted craniectomy will also use this interface to simulate intraoperative robotic milling and support the iterative design of the individual implant.

DEVELOPMENT OF THE PLANNING SYSTEM FOR COMPUTER ASSISTED CRANIOTOMY

S l I M M A R Y · In the framework l ihc CRANIOριομνι ( l ) K i SIMM 124 RA54S/2-I) problcms of compulcr and rohot assistcd craniotomy / cranieciom arc addressed. The aim of thc project is the mievj iu t ion of thc following modules into onc sysicm: Computer assistcd planning, Computer assisted unplant planning and manufacturing and robot assistcd trepanation. In this paper we present advanccs and futurc work in development of the planning system.

BENUTZERSCHNITTSTELLE FÜR DIE FREIHAND-NAVIGIERTE RÖNTGENBILDBASIERTE ZEMENTENTFERNUNG

Die Revision einer gelockerten Hüft-Endoprothese, hesoiulers die Entfernung des festsitzenden Knochen/emenies. ist /eitaulwändig und führt in einigen Fällen /u IVrlorutioncn und Frakturen des FernurSchaftes. Besonders der distale Zementköcher ist schwer zu erreichen und aufgrund der schlechten Sichtverhältnisse isi im die intraoperative Orientierung eine wiederholte i tera t ive Röntgen-Durchleuchtung notwendig. Im Rahmen des DFG-Projektes MINARO entwickeln wir einen neuartigen Ansatz zur Identifikation, SDRekonstruktion und Entfernung des femoralen Knochen/ementes auf der Basis intraoperativer ROnlgenprojektionen [1,2,3,4]. Nach der Akquisition einer geringen Anzahl kalibrierter Röntgenbilder und einer semiautomatischen Segmentierung wird von dem System ein dreidimensionales Model der ZementKnochen-Grenzschicht rekonstruiert. Neben der optionalen Anwendung eines robotergeführten Fräsinstrumentes |5| untersuchen wir die Möglichkeiten einer Freihand-Navigierten Zemententfernung. Im Gegen sät/ /u herkömmlichen Navigationsaufgaben, bei denen zwei multiplanare Bilder ausreichend sind, benötigen komplexe dreidimensionale Fräsaufgaben zusätzliche Informationen. Für eine effektive und zeitelfizientc Führung des Benutzers ist daher die Entwicklung einer auf die speziellen Bedürfnisse angepassten Mensch-Maschine-Schnittstelle erforderlich.