Abstract
This technical note demonstrates the benefits of preoperative planning, involving the use of rapid prototype models and rehearsal of the surgical procedure, using image-guided navigational surgery. Optimum reconstruction of large defects can be achieved with this technique.
The advent of computer technology has permeated all aspects of medical care and is revolutionizing aspects of modern surgery. The advances made in modern imaging have all but eliminated the unexpected event, and consequently the opportunity has arisen to move towards more minimally invasive procedures.
Primary intraosseous meningiomas are rare tumours that arise in the skull. They are often benign, and the fronto-parietal and orbital regions are the most common locations. In adults, they usually present with a painless firm swelling. There are often no associated neurological symptoms or signs. The optimum treatment is a wide excision and this is usually curative. However a detailed and complex reconstruction is often needed for intraosseous tumours that involve the orbit, to ensure an excellent functional and cosmetic outcome. The tumours are typically slow-growing and as such there is time to carefully plan the radical excision of the tumour and reconstruction of the cranial defect. The borders of an intraosseous meningioma are often easier to identify from the preoperative magnetic resonance (MR) and high-resolution head computed tomography (CT) imaging than at the time of surgery. This underlines the importance of the preoperative planning to ensure a radical excision of the tumour with careful reconstruction of the wide cranial defect and also an optimum functional and cosmetic outcome.
These advances have been facilitated by the introduction of navigational surgical systems, where detailed CT/MRI/angiographic images can be superimposed on the computer image of the patient in real time.
The ability to take the digital data from CT scans and form an exact replica of the skull and facial bone in plastic using rapid prototyping technology introduces a new dimension to all disciplines in modern surgery. There is the opportunity in complex cases to use the navigation system to rehearse the proposed operation in the laboratory. This allows the operative procedure to be refined and reveals the full extent of the subsequent defect. Consequently the reconstructive phase of the procedure can be planned in advance. Specially prepared templates can be constructed that allow accurate reconstruction of complex shapes rather than trust to experience and fortune on the day. The present case illustrates the successful amalgam of these two technologies to facilitate the surgery and repair of a significant craniofacial defect.
Case
In July 2009 a 56-year-old female was diagnosed with an intraosseous meningioma originating in the region of the left pterion. Symptoms were minimal; the patient reported occasional sensation of pulsation in the left ear over a period of about 3 years. There was no relevant medical history.
The problems that faced the surgical team were first to establish the full extent of the lesion within the bone and the challenge of repairing the resulting defect. The tumour involved the temporal bone together with both the greater and lesser wings of the sphenoid bone. Consequently the function of the left eye was threatened, as the roof, posterior and lateral walls of the orbit had to be sacrificed in the operation. The extent of the tumour was mapped by overlaying CT/MRI and angiography images within the Neurosign tracking system (Brainlab, UK). In addition, the diseased skull was reproduced ( Fig. 1 ) in both plaster (easy to cut) and nylon (autoclavable but not easy to cut) by rapid prototyping technology (Cavendish Imaging, London, UK).
The CT images were obtained with the patient wearing a dental plastic bite splint containing four localizing titanium screws. The rapid prototyping models could then be co-localized with the CT images in the Neurosign system, which in turn permitted the operation to be rehearsed in the laboratory. The advantage of this option was that the full extent of the excision could not be determined by virtual images alone and that the tumour margin was more easily determined when the operation was rehearsed. The model was produced in plaster of Paris that had architectural features that accurately represented the diseased bone. Therefore excision margins could be determined more accurately ( Fig. 1 b).
The advantage was that the reconstruction could be planned in detail. The orbital rim, roof and lateral wall was to be reconstructed in bone and the larger temporal defect restored by a custom-made titanium plate ( Fig. 2 ).
In the present case, any postoperative visual dysfunction was a threat to the patient’s professional career (dentist). A virtual image of the diseased bone in the orbit was produced. A rapid prototyping model of this bone was made. This was then divided into three segments that acted as a template to harvest bone. Using computer software, a donor site was selected on the contralateral cranium where the curvature of the inner table matched that of the orbital walls, and a rapid prototyping model reproduced the harvested bone from the donor site. A second craniotomy was designed with bone harvested from the inner table. An exact replica of the harvested bone was produced again by rapid prototyping technology ( Fig. 3 ).
