Introduction
Orbital pathology is an important part of spectrum of ophthalmic diseases that can be broadly divided into three main categories; neoplastic (50%), inflammatory (25%), trauma and other causes (25%).1 2 These heterogenous pathologies, vary in prevalence according to different age groups.1 2 In infants and up to 2 years of age neoplastic lesions and congenital structural or cystic lesions are more common. In older children from 2 to 16 years of age structural lesions (orbital trauma, dermoid and epidermoid cysts), neoplastic lesions, inflammatory lesions (mostly infectious) and vascular lesions are more common. In individuals aged from 17 to 64 years thyroid associated orbitopathy is by far the most common orbital pathology. Above the age of 65 neoplastic lesions are the most common orbital pathology.2 In this group, there is a high incidence of malignancy and it is imperative for these patients to have focused therapeutic and diagnostic care which often requires complex multi-disciplinary treatment and surgery.
The unique pyramidal shaped orbit is densely packed posterior to the eyeball with sensory and motor nerves, vasculature and extraocular muscles. These delicate structures are enveloped and supported by the orbital septum, which is a fibrous layer arising from the periosteum at the anterior orbital rim as a means of natural protection. There is further cushioning of these structures by unique almost fibrous orbital fat which fills the posterior orbit and is kept separate from anterior orbit by the orbital septum. Therefore, orbital lesions may result in a space occupying mass effect that will cause displacement and/or compression of the orbital contents, neurovascular structures and the eyeball. This manifests as neuro-ophthalmic clinical signs and symptoms such as proptosis, hypoglobus or hyperglobus, impaired ocular motility, diplopia, pupillary abnormalities, reduction or loss of vision, colour vision, and peripheral vision. Thus, orbital pathology and management of these conditions may result in prolonged morbidity3–5 and in some cases the manifested diseases causing mortality.6
The wide spectrum of pathologies faced in this small, dense and complex anatomical landscape makes safe and effective surgical access challenging. Therefore, interventions (ie, biopsy, debulking, resection) may result in complications due to intraoperative damage to the neurovascular structures, the globe and/or the anterior/middle cranial fossa which lie in close proximity. Complications include damage to the globe, optic nerve damage, a permanently dilated pupil and diplopia. One of the most devastating complications following orbital surgery is loss of vision, a serious risk resulting in profound morbidity.6 There are variable reports of blindness following orbital surgery from 0.84% to 7%, with the highest reported with vascular tumours of 4%–7%.3–5 Loss of vision with the latter is often attributable to central retinal artery occlusion secondary to excessive manipulation and bleeding.6
Therefore, advancements in surgical techniques for orbital surgery are required for a more accurate and precise surgical approach to minimise tissue damage and intraoperative complications. There is an imperative to find ways to perform minimally invasive surgery and reduce collateral damage.1 7 Modified surgical techniques have included the use of endoscopic surgery and of navigation image-guided systems (IGS).8 In a similar vein, the fields of neurosurgery and otorhinolaryngology have also looked to advance their surgical techniques by incorporating both of the above techniques.9
With IGS the patients’ actual anatomy is correlated with their preoperative scans via a navigation platform which allows intraoperative real-time tracking of surgical instruments in relation to bony and soft tissue structures. IGS has had its origins in stereotaxy a method developed in neurosurgery that involves the use of external rigid anatomical reference markers for location of internal anatomical surgical landmarks.10 11 There are two main types of IGS navigation systems, optical and electromagnetic, working on two different principles. The optical guided systems uses a light source from infrared or LED cameras that emit beams reflecting off the navigation probes using optical sensors to determine their position within the surgical field. Optical systems determine the position of instruments relative to the patient’s anatomy via these infrared cameras, the static patient reference frame and the specialised tracking instrument. The electromagnetic navigation system on the other hand, tracks instruments and the patient simultaneously and dynamically by generating an electromagnetic field encompassing the anatomical field.10 In electromagnetic image guidance systems (EM-IGS), the operating system is based on a generator that creates an electromagnetic field around the patient’s head which results in a coordinated system within which the reference point as well as the navigating instrument equipped with the built-in electromagnetic sensor are positioned. Spatial movement of the sensor changes the characteristics of the field, and this allows the navigating system to determine the coordinates of the instrument with respect to soft tissue and bone within its field. In EM-IGS platforms, the emitter is fixed to the operating table generating an electromagnetic field around the surgical site, navigation is conducted by a probe or an instrument’s relative position within this field. It is important for orbital surgery that the EM-IGS used are frameless giving unobstructed access to the surgical orbital field and that they do not consist of clamps to immobilise the head or invasive markers attached to the head to assist in navigation which is usually the case with optical systems.
The accuracy of EM-IGS’s surface registration has been questioned in the past as the electromagnetic field is based on narrow field surface registration rendering the depth of orbit distant and that in turn can affect accuracy of navigation, although evidence suggests surface registration is superior to point or fiducial based registration in related anatomical areas.12 13 There is additional interference with ferromagnetic instruments and devices thereby theoretically making the navigation less accurate.
There have been reports on the use of EM-IGS in orbital surgery comparing it to optical IGS, but none have systematically assessed the technology in a preclinical environment to determine if the use of this technology is feasible and consistent in orbital surgery setting. Currently, no bespoke software for orbital IGS exists, and preclinical assessment of the technology’s accuracy within the orbital space has not yet been explored. Therefore, we sought to assess the feasibility and accuracy of EM-IGS using high-fidelity physical orbital anatomy simulators.