Stefan Hassfeld

Laser-scan-based navigation in cranio-maxillofacial surgery

BACKGROUND In computer-assisted surgery, a correlation between a volume data set and the surgical site is required in order to localize the patients head on the operating table. Registration markers are commonly used for this procedure. However, the marker registration is associated with high logistics, since the markers have to be placed prior to data set acquisition and have to be kept in their position until the patient enters the operating room. This study deals with a new markerless registration method in cranio-maxillofacial surgery that is based on a high-resolution laser-scan of the patients (relaxed) skin surface. PATIENTS 20 patients with tumours, bone malformations or foreign bodies, scheduled for computer-assisted surgery, were involved in the study. STUDY DESIGN The clinically applied accuracy of the laser-scan-based registration was measured through additionally placed registration markers. The inherent precision of the laser-scan registration system was controlled in phantom studies. RESULTS The clinically applied accuracy of the new laser-scan-based registration technique ranged between 0.2 and 1.8 mm with a mean deviation of 1.1mm and a standard deviation of 0.3 mm. CONCLUSION The facial skin surface can serve as a sufficiently stable and invariable reference base in order to register patients for computer-assisted cranio-maxillofacial surgery.

Artefacts in magnetic resonance imaging caused by dental material

Abstract.A common problem in computer tomography (CT) based imaging of the oral cavity is artefacts caused by dental restorations. The aim of this study was to investigate whether magnetic resonance imaging (MRI) of the oral cavity would be less affected than CT by artefacts caused by typical dental restorative alloys. In order to assess the extent of artefact generation, corresponding MRI scans of the same anatomic region with and without dental metal restorations were matched using a stereotactic frame. MRI imaging of the oral and maxillofacial region could be performed without reduction of the image quality by metallic dental restorations made from titanium, gold or amalgam. Dental restorations made from titanium, gold or amalgam did not reduce the image quality of the MRI sequence used in imaging of the oral and maxillofacial region for dental implant planning. In this respect MRI is superior to CT in implant planning.

Maxillofacial fractures resulting from falls

OBJECTIVE This study was performed to clarify the characteristics of facial fractures caused by falls with a particular focus on aetiology. PATIENTS Of 505 patients with facial fractures treated in the Department of Oral and Maxillofacial/Craniofacial Surgery, Heidelberg University Hospital from January 1997 to May 2001, 129 patients injured by falls were analysed. STUDY DESIGN Aetiological analysis was performed according to falling patterns. RESULTS The distribution of age showed two peaks in the 4th and 8th decades, and the male to female ratio was 1.1:1, unlike the overall facial fracture ratio of 3:1. Seventy-six patients had fallen from standing height or less and 44 patients had fallen from greater heights. The former cases were more often seen in older females. More severe injuries tend to be seen in the patients who have fallen from greater heights, and in the cases associated with acute medical disorders. CONCLUSION This retrospective study documents the higher risk of fractures in older females and the higher risk of severe injury in those patients who were unconscious. This is in agreement with recent orthopaedic studies.

Virtual surgery in a (tele-)radiology framework

Presents telemedicine as an extension of a teleradiology framework through tools for virtual surgery. To classify the described methods and applications, the research field of virtual reality (VR) is broadly reviewed. Differences with respect to technical equipment, methodological requirements and areas of application are pointed out. VR, desktop VR and augmented reality are differentiated and discussed in some typical contexts of diagnostic support, surgical planning, therapeutic procedures, simulation and training. Visualization techniques are compared as a prerequisite for VR and assigned to distinct levels of immersion. The advantage of a hybrid visualization kernel is emphasized with respect to the desktop VR applications that are subsequently shown. Moreover, software design aspects are considered by outlining functional openness in the architecture of the host system. A teleradiology workstation was extended by dedicated tools for surgical planning through a plug-in mechanism. Examples of recent areas of application are introduced, such as liver tumor resection planning, diagnostic support in heart surgery, and craniofacial surgery planning. In the future, surgical planning systems will become more important. They will benefit from improvements in image acquisition and communication, new image processing approaches and techniques for data presentation. This will facilitate pre-operative planning and intra-operative applications.

Tracing of Thin Tubular Structures in Computer Tomographic Data

For many applications in diagnostics and in the planning of surgical interventions, specific structures have to be identified in a patients volume data set. In this article we give an outline of how the detection of thin tubular structures (e.g., nerves and vessels) can be automated, requiring very little initialization from a human expert. We focused on the nervus alveolaris inferior in the lower jaw and were looking at three details: data acquisition, detection, and validation of accuracy. Our method can be easily adapted to many similar cases such as other nerves, arteries, and veins or bundles thereof.

Neuronavigation Methoden und Ausblick

ZusammenfassungMit den Fortschritten in der Computertechnologie und der neuroradiologischen Bildgebung wurde die Entwicklung der Neuronavigation möglich. Diese wird seither immer häufiger angewendet und hat die rahmenbasierte Stereotaxie zur intraoperativen Führung bei mikrochirurgischen, intrakraniellen Operationen weitgehend abgelöst. Mit Hilfe von dreidimensionalen Digitalisierinstrumenten werden dabei die präoperativ angefertigten CT- oder MRT- Bilder eines Patienten währen der Operation auf den Operationssitus übertragen. So wird eine genaue Lokalisation von Läsionen wie Tumoren oder anatomischen Strukturen ermöglicht. Die verwendeten Techniken wie positionsfühlende Gelenkarme, Infrarot- oder Robotersysteme werden ebenso wie die Anwendung in der klinischen Routine dargestellt. In 200 Eingriffen, die mit Neuronavigation durchgeführt wurden, hat sich die Methode als sehr hilfreich zur minimal invasiven Eingriffsplanung und -durchführung erwiesen. Mit Abweichungen zwischen 2 und 4 mm ist die Genauigkeit nur geringfügig schlechter als die Werte, wie sie für die rahmenbasierte Stereotaxie angegeben werden. Als besonders vorteilhaft wurde die Navigation von den Operateuren bei Eingriffen an der Schädelbasis sowie kleinen, tiefliegenden oder in eloquenten Arealen gelegenen Läsionen eingestuft. Hauptnachteil der Methode ist die Verwendung von präoperativem Bildmaterial, was zu Ungenauigkeiten führt, wenn es im Verlauf einer Operation durch zunehmende Tumorresektion zu Veränderungen der Anatomie kommt. Hier könnten Verfahren der intraoperativen Bildgebung wie der MRT eine Lösung darstellen. So ist es uns bereits gelungen, durch Aktualisierung der Navigationsdaten mit intraoperativer MRT Verschiebungen auszugleichen. Diese Entwicklungen werden zu einer zunehmenden Bedeutung der Neuronavigation in der operativen Routine führen, was durch eine Senkung der Operationsmorbidität auch erhebliche Auswirkungen auf die Indikationsstellung zu neurochirurgischen Eingriffen haben wird.SummaryWith the recent developments in computer technology and the improvements in modern neuroimaging, frame-based stereotactic guidance for open microsurgical procedures has been increasingly replaced by neuronavigation, also called frameless stereotaxy. It allows transfer of individual patientís images onto the operative field to assist the neurosurgeon intraoperatively in defining the tumor margins or identifying functionally important brain areas. The different localization techniques employed are articulated position-sensing arms, infrared or ultrasound systems working with the principle of satellite navigation and robotic systems integrated with the operating microscope. In 200 operations performed with different systems (arm-based, robotic and infrared) the method proved to be helpful, enabling fewer invasive procedures to be performed. With a mean deviation of 2.87±1.9 mm for intraoperative localization, the accuracy was only slightly worse than in frame-based stereotaxy with deviations below 2 mm. Neuronavigation was most helpful for operations on deeply seated lesions, skull-base tumors and lesions in brain areas with high functionality. The major disadvantage is the use of preoperative data for navigation, leading to inaccuracies when anatomical structures are altered during the operation by resection of tumors or shift of intracranial soft tissue. Intraoperative magnetic resonance imaging (MRI) might be a solution for this problem. With the method of intraoperative MRI developed in our department it has already been possible to update neuronavigation with images reflecting intraoperative changes in anatomy. Therefore, neuronavigation is definitely a method with growing importance in operative routine, and it will also spread into other surgical specialties.

Soft tissue scanning for patient registration in image-guided surgery.

Prior to an image-guided surgical intervention, a correlation between the patients data set and the surgical site is required. This study introduces a markerless registration method for craniomaxillofacial surgery that is based on a high-resolution laser scan of the patients skin surface. The Surgical Segment Navigator SSN++ rejects contaminated surface measurements in a way similar to the bluescreen technique. Acquisition of the spatial position and the corresponding surface color of each laser-scanned point facilitates this bluescreen method, removing points with a defined surface color, e.g., blue or green points. The accuracy of the laser-scan-based registration was measured via additional intraoral titanium-markers. These markers served only to check the accuracy of the markerless registration process. In twelve patients, the stability and accuracy of the data set alignment was evaluated for high-(300,000 surface points), medium-, and low-resolution (down to 3,750 surface points) laser scanning. The accuracy of the registration technique was best for high-resolution laser scanning (mean deviation 1.1 mm; maximum deviation 1.8 mm). Low-resolution laser scans revealed inaccuracies up to 6 mm.

Development and First Patient Trial of a Surgical Robot for Complex Trajectory Milling

Objective: Todays surgical robots normally perform “simple” trajectories, e.g., assisting as tool-holding devices in neurosurgery, or milling linear paths for cavities in total hip replacement. From a clinical point of view, it is still a complex undertaking to implement robots in the operating room. Until now, robot systems have not been used in patient trials to mill “complex” trajectories, which involve many positional and orientation changes and are often necessary in cranio-maxillofacial (CMF) surgery. This paper presents the RobaCKa surgical robot system, which allows more precise execution of surgical interventions and milling of “complex” trajectories. Materials and Methods: The main components of the RobaCKa system are a (former) CASPAR robot system, a POLARIS system, and a force-torque sensor. Results: In the first patient trial (April 2003) the planned trajectory was executed with an error of 0.66 ± 0.2 mm. Conclusions: The use of former industrial robots for surgical applications is possible but complex. The advantages are improved precision and quality and the possibility of documentation. The use of such systems is normally limited to research institutions or large clinics, because it is hardly possible to implement the necessary technical and logistic efforts in routine surgical work.

Factors Influencing Changes in Maxillary Arch Dimensions in Unilateral Cleft Lip and Palate Patients until Six Months of Age

OBJECTIVE The objectives of the present study were (1) to investigate whether growth increments until 6 months of age are influenced by particular factors, (2) to analyze whether anterior cleft reduction is dependent on the extent of the cleft width at birth, and (3) to examine the correlation between maxillary measurements at birth and the anterior cleft width at 6 months of age. DESIGN The study design was prospective and longitudinal. SETTING Heidelberg University Hospital Interdisciplinary Cleft/Craniofacial Center. PATIENTS AND METHOD The longitudinal records of 34 patients (24 male and 10 female) with complete unilateral cleft lip, alveolar ridge, and hard and soft palate were included in this study. All patients were treated with the same protocol. All participants were assessed at 0 and 6 months of age. Maxillary plaster casts of the patients were analyzed using a computer-controlled three-dimensional digitizing system. MAIN OUTCOME MEASURE Maxillary models were measured and compared to putative factors influencing growth. RESULTS No statistically significant differences were found between maxillary growth changes and increases in weight and length. Similarly, there was no significant interaction between the extent of the alveolar cleft width at birth and its reduction prior to lip closure. In contrast, significant differences of maxillary growth increments could be found between male and female patients. Stepwise regression analysis demonstrated a correlation between maxillary measurements at birth and growth increments. CONCLUSION The results of the study indicate that gender plays a certain role in growth changes within the first 6 months of age.

Intraoperative Computer-Assisted Neuronavigation in Functional Neurosurgery

The clinical experience with a frameless computer-assisted neuronavigational system (ISG. Canada) used in functional neurosurgery is described. The advantage of image-guided surgery is stressed for functional procedures of the cortex with delineation of the gyral pattern, e.g. motor cortex stimulation and procedures at the base of the skull with triplanar and three-dimensional reconstruction of the bony landmarks. A general use of the device for aiming at subcortical targets cannot be recommended. Limitations are the accuracy (< or = 2.2 mm) and software deficiencies and the lack of a reliable fixation of the position sensing arm (wand).