Navigation

6.1.1 Equipment, installation and modalities

A complete navigation module includes the following units (O Fig. 6.1):

4 Computer workstation with monitor 4 Camera 4 Reference bases 4 Navigated instruments

The reference bases (RB) are marked with LED dots or reflecting materials which are recognised by the camera. The RBs are affixed to the bone being operated in alignment with the camera. Signals are transmitted between camera, patient and navigated systems by means of infrared signals.

Before starting the operation and actual registration process, it is vital to stipulate exactly how the system is to be arranged, i.e. the exact position of all equipment in the navigation system in relation to each other. This also includes the C-arm or Iso-C-arm. The equipment should be arranged before starting or parallel to the positioning of the patient.

The attachment of the RBs must be rotationally stable during the operation to avoid relative movements; if the RBs work loose, this causes inaccuracies (O Fig. 6.2). If the RBs work loose during the operation after registration of the system, this must be repeated. The alignment and side-dependency of the RBs and instruments should be kept the same to guarantee optimum communication to the camera during the navigation process. After registration of the RBs and the C-arm, the patient can be moved freely. The instruments are moved relative to the RBs on the patient.

Fig. 6.1. View of the equipment

D Fig. 6.2. The attachment of the RBs must be rotationally stable during the operation

Fig. 6.1. View of the equipment

D Fig. 6.2. The attachment of the RBs must be rotationally stable during the operation

At present there are various different imaging modalities in use for navigation; these are as follows: 4 CT

4 Fluoroscopy 4 Iso-C

4 Kinematic (non-imaging) navigation

In CT-based navigation, during the operation attention only has to be given to the positioning of the workstation and possibly also the camera. Pictures produced before the operation are used while the operation is taking place and as a rule, no further pictures are taken during the operation. Fluoroscopy and Iso-C navigation entails consideration of the C-arm and image intensifier monitor. The C-arm or camera must be positioned to allow for unimpaired communication for registration during the scan. In particular for Iso-C navigation, this must be guaranteed throughout the whole scanning process. Before the operation it is important to check whether troublefree scanning without artefacts will be possible in the necessary anteroposterior and lateral projections. It is sensible to put the monitor in an ergonomic position directly next to the workstation. Kinematic navigation does not require additional imaging. Various anatomic regions are depicted on the basis of non-picture data obtained during the operation. In this case, the camera and workstation are positioned together or separately depending on the system (D Fig. 6.3).

Various different navigation systems are currently available; in many cases the camera is integrated directly at the workstation. The corresponding angles and settings of the camera can be changed at short notice using a handle (D Fig. 6.4).

Other models have an independent mobile camera unit with correspondingly different arrangements in the operating theatre. Details can be found in the special section.

Fig. 6.3. Fluoroscopy-based navigation

Fig. 6.5. C-arm and monitors on the side opposite the surgeon

Fig. 6.4. Workstation with camera

Fig. 6.6. Navigated instruments

It must be possible for the surgeon to look at the monitor easily without special effort during the whole operation. In most cases it is preferable to position it on the side opposite the surgeon. Some indications deviating from this arrangement are described in the special section. In the case of fluoroscopy or Iso-C navigated operations, the image intensifier monitor can be positioned next to the navigation module. Generally, the C-arm should also be placed on the side opposite the surgeon. In the case of necessary control scans, the position of the C-arm is defined and the control scans can be performed without complicated repositioning (O Fig. 6.5).

Before the operation it is important to stipulate whether the surgeon will control the workstation himself, e.g. using a sterile touch screen or special handling instruments, or whether an assistant performs this directly in sterile/non-sterile conditions at the system (O Fig. 6.3).

Basically for all fluoroscopy or Iso-C navigation, care is required to ensure that there are no X-ray aprons in the region being scanned. Consideration should also be given to partly adjoining joints, e.g. hip or knee joint when defining the navigated leg axis.

6.1.2 Iso-C3D general

A solid carbon (CRP) table should always be used. The region being scanned should be positioned centrally in the middle of the table where possible (O Figs. 6.7, 6.8).

If this is not available, the region being scanned must be arranged in the middle of the table, away from all metal braces/brackets. In the case of peripheral extremities such as the hand or foot, the extremity can be hung over the end of the table.

When positioning the patient, it is important to ensure that side supports, leg holders and other supports do not interfere with the direct X-ray path or in the area of the orbital movement of the device. When the patient is positioned on the side, the side supports in particular must be moved towards the thorax. For abdominal positioning, padded cushions should be given preference over metal bolsters. In the case of deep solid carbon (CRP) tables, lateral positioning is only conditionally possible because of the restricted clearance to the C-arm. Similarly, abdominal positioning with high bolsters/cushions is difficult with obese patients.

Only exact preoperative adjustment of the Iso centre allows for complete orbital movement. Additional intraoperative covers, cloths and equipment restrict the clearance even further.

Before the operation it is important to check whether the operating site is exactly in the Iso centre in both anteroposterior and lateral projections. The possibility of performing the full orbital movement through 190° should be checked by swivelling through this angle once. Bumping against the table or the operating site causes the automatic scan to abort.

Before being brought to the operating table, the system should be protected with specific sterile covers for the Iso-C system. It is also advisable to cover the site additionally with sterile cloths for the actual scan itself. For example, here the extremities can be wrapped in stockinette.

To guarantee sterility while the system is rotating, the table can also be wrapped in a sterile cloth from below. All cloth covers used in this way can be removed again easily after the scan (O Figs. 6.9, 6.10).

All instruments and cables in the X-ray path should be removed before the scan to avoid any artefacts.

For surgical procedures to the extremities, the contralateral side interferes a little in the X-ray path; the calculation and display of the multiplanar reconstructions is based on the 12x12x12 cm cube in the Iso centre (O Figs. 6.11, 6.12).

6.1.3 Iso-C3D navigation

In the case of ISO-C navigation, the RBs affixed to the bones for registration must not be covered by the sheets during the scan. The monitor should be positioned next to the navigation workstation. During the scan, all operating staff should leave the immediate area of the operation to guarantee that the camera has a permanent view of the C-arm.

Fig. 6.10. Swivelling movement

□ Fig. 6.9. Scanning procedure with the lower extremities in sterile covering

Fig. 6.10. Swivelling movement

D Fig. 6.11. Supine position with knee in a middle position on the carbon (CRP) patient board

Fig. 6.12. Swivelling movement

Fig. 6.13. Iso-C3D during the scanning process

Basically the Iso-C can then still be used as a normal scanning unit; if necessary, another scan can be performed as a direct control on success after the end of navigation (O Fig. 6.13).

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