1. Computer-assisted bone segment navigation

2. Markerless laser registration in image-guided surgery

 

 

1. Computer-assisted bone segment navigation

Repositioning osteotomy is a frequently used method correcting malpositions in orthopedic surgery and traumatology. Computer tomography, stereolithography models and tele-X-rays are used in planning. However, the precision achieved in the planning phase is usually not translated to patients.
The segment navigator SSN is a navigation system which allows for the computer-assisted correction of malpositions. It consists of an infrared-positioning device, two dynamic reference frames DRF, an infrared-pointer and an infrared-camera. All data are displayed numerically and graphically on the monitor of the SSN workstation.
A Laboratory Unit for Computer Assisted Surgery LUCAS is used for planning surgery in the laboratory. LUCAS requires only a scout view CT. A preparatory operation to implant bone markers visible in X-rays and a further planning CT scan showing the bone markers - which were necessary in previous systems - are not required for the LUCAS- and SSN-system. This reduces significantly the radiation exposure of the patient and the costs of surgical planning.
Even the measurements of anatomical landmarks in the surgical site which are time consuming and reduce the accuracy are not required for the SSN-system since the position of the infrared-transmitters is already known during surgical planning on the LUCAS-workstation. This makes the surgical approach faster and much more precisely. The surgical planning data are transferred to the surgical site using a data file and an individual surface pattern which fits to the surface of the navigated bone segment:
The data file is exported from the LUCAS-workstation to the SSN-workstation. The planned spatial displacement of the infrared-transmitters is saved in this file.
The individual surface pattern carries the infrared-transmitters. This pattern is the mechanical interface between infrared-transmitters and navigated bone segment.
The individual surface pattern can be polymerized directly on a small stereolithographic model of the navigated bone segment. The surface pattern can as well be generated as negative form from a CT data set using a CAD-CAM-system.
To summarize, LUCAS and SSN allow for the computer-assisted correction of malpositions, positioning of artificial joints and implants. In principle, the systems can be used in all fields of surgery.

 

 

Technical Concept

The concept of the Surgical Segment Navigator SSN was found with support by Carl Zeiss in 1997.
The SSN is based on an infrared positioning device such as the Surgical Tool Navigator (STN) and the Surgical Microscope Navigator (SMN) manufactured by Carl Zeiss.
The infrared positioning device is connected to a Microsoft Windows NT 4.0 Workstation on a Hewlett Packard NetServer LD Pro.
The software of the Surgical Segment Navigator (SSN) and the Laboratory Unit for Computer Assisted Surgery (LUCAS) is written by Rüdiger Marmulla and compiled with Microsoft Visual Studio 6.

 

 

Navigation with the SSN

Fig. 1

The SSN workstation with its monitor is shown in left left side of the figure, DRF(1) is connected to a halo frame on the patient's head, DRF (2) is connected to the dental splint between maxilla and mandible. The head of the patient is centered in the focus of the infrared camera.

SSN in the operating room
Fig. 2

Surgical plan on the LUCAS workstation. The orbital roof, lateral orbital wall, orbital floor and the facial wall of the maxillary sinus are lifted, the upper pole of the segment is rotated anticlockwise. The virtual osteotomy lines can be seen. The surgical plan corrects the volume of the left orbit to 21 cm3, so it is equal in size with the right orbit.

Surgical planning on the LUCAS workstation
Fig. 3

Bone segment navigation of the left orbit via a bicoronary approach: This figure corresponds to the view of Figure 4. A bone segment with a posttraumatic malposition is to be fixed in a new position after osteotomy. A part of the frontal bone, the orbital roof, the lateral orbital wall, the orbital floor and the facial wall of the maxillary sinus belong to this bone segment.
DRF1 on the right is located on the patient's head in an arbitrary - but stable - position. DRF2 on the left is connected to the surface pattern which allows for a reproducible transfer from the laboratory model to the surgical site. The positioning of the bone segment is checked by DRF2. The lower levels of the bone segment (orbital floor, facial wall of the maxillary sinus) have not been visible via the bicoronary approach - but their position has been displayed clearly on the SSN monitor. Neighboring bone segments had to be osteotomied additionally, in order to dissect scar adherences of the dura mater. The SSN showed the target for the navigated bone segment clearly, although the neighboring structures had lost their former position. The neighboring bone segments had been refixed after positioning the navigated bone segment.

Surigical site of a computer assisted repositioning osteotomy
Fig. 4

Elements of the Surgical Segment Navigator SSN: DRF1 for defining a three-dimensional orthogonal coordinate system, ‚ DRF2 for navigating the osteotomied bone segment, surface pattern which connects DRF2 with the bone segment to be navigated, infrared-camera, SSN workstation. In comparison to Fig. 3, this scheme shows even the invisible levels which are hidden under the bicoronary flap.

Scheme to Fig. 3
Fig. 5

Monitor-view of the SSN workstation: This figure corresponds to figure 3 and 4. The yellow polygon represents the bone segment in the preoperative position, the green polygon shows the current position and the blue polygon the target position. Delta 1, 2 and 3 in the left part of the window indicate the distance from the target position for selected points of the bone segment. The other parts of the window provide information about the distance of the infrared transmitters from their target position numerically and graphically. The unit for all numerical values is mm. All distance values must be approximated to "0mm" in navigation. The green polygon which respresents the current (=intraoperative) position of the bone segment must be matched with the blue polygon of the target position.

Intraoperative monitor view on the SSN workstation

 

The method of computer assisted bone segment navigation is described with interactive software, multimedia files, movies and source code on a CD which is available via

Rüdiger Marmulla:
Computer Assisted Bone Segment Navigation
Chicago, Berlin: Quintessence Publ, 2000, ISBN 3-87652-869-0

You can download a short demoversion of the interactive software.

 

 

 

2. Markerless laser registration in image-guided surgery

In computer-assisted surgery a correlation between a volume data-set and the surgical site is required in order to localize the patient's head in the operating room. 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 a data-set acquisition and have to be kept in their position until the patient enters the operating room.

Several studies deal with a new markerless registration method in cranio-maxillofacial surgery that is based on a high-resolution laser-scan of the patient's skin surface.

For this purpose, the SSN++ was developed, which is based on a Surgical Segment Navigator (SSN) that was enlarged by an additional 3d laser scanner (VI 900 from Minolta, Japan).

The clinically applied accuracy of the laser-scan-based registration was measured through additionally placed evaluation markers. The clinically evaluated mean accuracy of the SSN++ was 1 mm.

Thus, the facial skin surface serves as a sufficiently stable and invariant structure in order to register patients for computer-assisted cranio-maxillofacial surgery.

Other studies deal with the congruence between the facial soft tissue in CT scans and laser scans. Beside of the facial soft tissue, other surfaces - maxilla, mandible and auricle - are used for markerless laser registration.

 

Fig. 6

Concept of SSN++: Operating room (1), infrared camera (2), 3D-laser scanner from Minolta (3), Dynamic Reference Frame (4) for a fast tracking of head movements, SSN++ workstation (5), LUCAS workstation for surgical planning and simulation (6).

Scheme of the SSN++ system

 

Fig. 7

SSN++ in the operating room: laser scanner, infrared camera and SSN++ workstation.

SSN++ with laser scanner, infrared camera and workstation

 

Fig. 8

Laser scan on the SSN++ workstation: the system specifies the third orthogonal axis with different colors. Red indicates distant points (eg lateral cheek), blue and purple indicate close points (eg tip of the nose). An extraoral bow with five evaluation markers for determination of the accuracy of the markerless patient registration is visible.

Laser scan on the SSN++ workstation

 

Fig. 9

Matched data sets of the surfaces generated from CT scan and laser scan on SSN++. Yellow: Raw voxels of the CT data set. Rainbow colors represent the laser scan in those areas, in which it is on top of the CT-skin surface.
The spatial volume enclosed is represented in blue and is visible in the plane of the sagittal cut.

Data set alignment after surface matching of CT scan and laser scan

 

You can download a short demo of the navigation system SSN++ (the media files are available in german language only).