The use of X-rays to assist fracture-repair surgery has been routine for many years, enabling operations to be performed through small skin incisions and negating the need for more invasive approaches that reveal the entire surface of the bone.
When repairing a simple pastern fracture, for example, X-rays are taken with markers (needles) placed in the horse’s skin to help identify where screws should be inserted. Further X-rays at the end of surgery then confirm that the screws are in the correct position and are of an appropriate length.
While this X-ray guidance works for many types of surgery, the technique has its limitations. An X-ray flattens the complex three-dimensional structure of bones into a single, two-dimensional image. The latest innovation uses computed tomography (CT) to obtain three-dimensional images for more sophisticated guidance of orthopaedic surgery — usually the placement of screws for treatment of fractures or bone cysts.
CT-guided surgery allows us to “see” inside a limb in far greater detail, enabling the surgeon to perform procedures that until now have not been possible. Operations previously carried out using X-rays can now be executed with more accuracy, while more complex surgeries can be performed in a minimally invasive fashion, reducing patient discomfort and potential complications.
Instant assessment
The technique requires a portable, lightweight CT scanner that can be moved around the patient in the operating theatre.
Most scanners are fixed units that are bolted to the floor, whereas a moveable in-theatre scanner, on wheels, allows us to position the horse for surgery on a standard operating table. Multiple scans can then be performed throughout the surgery without breaking the sterility of the operating environment.
The CT system produces cross-sectional images, or “slices”, through the limb. Each CT scan takes only minutes to perform. The images are transferred wirelessly to a computer, patient-side, so that the operating surgeon can assess them and make decisions without delay.
The surgical procedure is planned using these reconstructed images. Markers are placed on the limb, corresponding to where the surgeon plans to place one or more screws, and a further scan is performed to assess their location. The markers are then moved, as necessary, and scans are repeated until the screws can be placed in exactly the right location.
Before surgery begins, the markers are replaced with a visual drill guide. Scans are then performed to check the positioning and length of each screw as it is inserted.
Hitting the target
With the majority of fracture repair surgeries there is a degree of flexibility as to where screws are positioned — a few millimetres one way or another from the planned location is of no consequence. In some surgeries, however, a few millimetres can mean the difference between success and failure.
A good example is fracture of the navicular bone, a small bone within the hoof with a diameter of approximately 5mm when measured in cross-section. To repair fractures of the navicular bone, a screw measuring 3.5mm in diameter is used.
It is crucial that the screw does not emerge from the front or back of the bone — if it did, it would end up in the coffin joint or navicular bursa, which could damage the joint surfaces and cause arthritis. Not only is there a small target area (only 1.5mm larger than the screw), but the angle of the screw must also be exactly as planned.
The three-dimensional imaging capability of CT is also invaluable when operating on highly curved bones, such as the pedal bone. The fact that X-rays will only create a two-dimensional “silhouette” of the bone makes it impossible to judge the length of screw needed and whether it has been positioned correctly.
CT-guided surgery is particularly helpful with fractures of the cannon bone, which are often seen in athletes such as racehorses and eventers. The fracture, which can spiral up the bone from the fetlock joint, can be repaired effectively with screws — as long as these are placed following the spiral pattern of the fracture.
It is impossible to do this confidently using X-rays alone. As a consequence, more invasive surgery has traditionally been performed — either by opening up the leg to allow the surgeon to see the surface of the bone and the fracture, or by inserting a bone plate.
Because CT is able to map the fracture with accuracy as it spirals around the bone, it is possible to place screws through short incisions in the skin with the knowledge that they are in exactly the right place. This technique reduces patient discomfort and time spent in the hospital, compared to more invasive surgery, and lessens the risk of complications, such as wound infections.
Injuries within the hoof have benefited most from this technology. Unlike other parts of the limb, where larger incisions can be made to see the bone, it is not possible to open up the hoof to complete a surgery as the incision cannot be stitched up afterwards. CT allows us to assess the internal structures of the hoof, enabling more effective surgery to repair fractures.
The development of CT-guided surgery has revolutionised fracture repair, particularly in cases that require exceptionally high levels of precision.
Due to the ready availability and ease of use of our portable CT in-theatre scanner, it has become routine at Newmarket Equine Hospital to undertake CT scans of all horses undergoing fracture repair. The amount of information obtained from these scans has been eye-opening, resulting in real differences to patient outcome. The number of uses for CT-guided surgery is constantly growing: the accurate placement of screws across bone cysts in the foot is just one recent example.
CT-guided surgery
Currently, Newmarket Equine Hospital is the only equine hospital in the UK offering CT-guided surgery. Other clinics have CT scanners and are able to perform scans before surgery, but the horse must then be moved from the CT area into the theatre while anaesthetised (which is easier said than done), or operated on in the CT room — which compromises sterility and the ability to perform surgery. Neither situation enables repeated scanning during the operation, which is required for the CT guided surgery described here.
To be successful, the technique relies not only on the equipment used but also on a high level of surgical expertise and experience. Typically, the fee would be twice that of a routine surgery performed using X-rays — to take into account both the extra time required for surgery and the cost of the necessary equipment, which can run into hundreds of thousands of pounds.
Because of this expense, and the need for a sufficient number of patients requiring surgery to make investment worthwhile, CT guided surgery is unlikely to be found in many equine hospitals in the near future. Hopefully, however, availability will increase with time.
Ref Horse & Hound; 31 January 2019
