34th Annual Scientific Meeting proceedings

Computer-assisted surgery (CAS) refers to the use of computer technology for surgical planning and real-time perioperative guidance. CAS helps to perform surgeries more accurately and facilitates minimally invasive approaches. In human medicine, surgical navigation has become an integral part of advanced spinal and orthopaedic surgery.¹ Following this trend, the use of CAS in small animal surgery has recently been proposed for orthopaedic and spinal surgery^2,3 as well as for brain biopsies.⁴

A navigation system using an iso-centric C-arm was first introduced for equine computer-assisted orthopaedic surgery (CAOS).^5-8 More recently, a navigation system (Stealth Station^TM) coupled with a cone beam computed tomography (CBCT) unit (O-arm^TM; both by Medtronic) has been developed specifically for navigated surgery and we described numerous clinical applications using this system for equine orthopaedic surgery.⁹ The O-arm allows for a fast image acquisition, has a large field of view, is mobile and extremely versatile, making it useful for diagnostic imaging and various intraoperative applications. We routinely use the O-arm for preoperative imaging in standing horses to examine the equine extremities from the hoof up to (and including) the carpus/tarsus^10,11, and up to the distal humerus and distal femur in anaesthetized horses.⁹ Moreover, we use it for pre- or intraoperative imaging of the head¹² and neck of horses, as well as the pelvis of small equids.¹⁰

In equine surgery, CAS systems are mainly used to facilitate navigated drilling procedures, which demand maximal precision and optimal perioperative orientation. This includes lag screw repair of fractured sesamoid bones, phalangeal or cuboidal bones, but also trans-lesional drilling of subchondral bone cysts, less invasive arthrodesis techniques, plating procedures or toggle-pinning of reduced coxofemoral luxations.^9,13,14 Furthermore, navigation systems are used for real-time perioperative guidance to allow for minimally invasive removal of small bone or dental fragments, to facilitate complicated tooth extractions and paranasal sinus surgery.^9,15,16

CAS starts with pre- or perioperative image acquisition. In a second step, the acquired images are assessed to ensure that the region of interest is included in the data set and of adequate quality. Based on the imaging findings, an operative plan is established using the navigation workstation. To provide the operating surgeon with correct spatial orientation and real-time feedback of the performed surgical actions, the information is virtually documented on a monitor screen. This requires a tracking method that constantly assesses the position and orientation of the target and instrumentation. For equine CAS, optical tracking using infrared light is predominantly used.^5,9,13-16 Optical tracking utilizes dynamic reference bases, or “trackers,” holding either light-emitting diodes (active) or light-reflecting spheres (passive). An infrared optical digitizer and camera array detect the spatial position of the trackers.

To start the procedure, the navigation system must first align the preoperative images with the current position of the targeted anatomy, a step called registration. Instruments that are to be detected by the navigation system must be equipped with an instrument tracker and calibrated. Verification and adjustment throughout the procedure should be performed periodically using validation methods or repeated perioperative imaging. If necessary, adjustments are made to ensure that the alignment between the preoperative images and the patient's anatomy remains accurate. This plays a critical role in enhancing the accuracy and safety of CAS procedures and should always be considered if the surgeon suspects any discrepancies between the displayed virtual representation and the tactile feedback during the procedure. Another important feature of optical tracking systems is the need to securely anchor the patient tracker to the target bone. For this purpose, Schanz pins or screws are drilled into the target bone to create a rigid and angle-stable fixation. Depending on the integrity, size, and location of the target bone, this may lead to spatial interference between patient and instrument tracker,¹⁷ or cause morbidity of the bone or adjacent soft tissue structures.¹⁸ To overcome such disadvantages in CAOS applications involving the equine distal extremity, we developed a purpose-built frame (PBF).¹⁷ Alternatively, other methods to provide external stabilization may be explored. These methods allow positioning of the patient tracker at strategically advantageous sites, often remote from the surgical field.^9,13,14

With the increasing availability of three-dimensional imaging units in equine referral centres, and the associated increasing use of CAS in equine surgery, the spectrum of indications will become clearer, and more innovative solutions tailored to the special requirements of equine surgery will refine the applications of CAS. Recent developments are paving the way for introducing CAS to spinal and neurosurgery in horses. By merging preoperatively acquired magnetic resonance imaging data with perioperatively acquired CT data sets, diagnostic and surgical interventions in immediate proximity to the equine brain and spinal cord may become possible or facilitated.¹⁶

References

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