Nuclear scintigraphy or 'bone scanning' attempts to take lameness diagnosis one stage further by predicting rather than just diagnosing fractures. It is an imaging technique that searches for an increase in bone production and thus can often pinpoint the cuase of minor lameness problems before they become catastrophic injuries.
Nuclear scintigraphy works via the highly ingenious
idea of linking a mildly radioactive substance to bone turnover so that
it can be measured. A radioactive substance called technetium is joined
together with a phosphorous compound and then injected intravenously
into the horse. The bones of the horse use this phosphorous compound to
make more bone cells at different rates depending on what is going on in
each individual bone. A ‘normal' bone in an adult horse will therefore
only take up a small amount of the phosphorous compound and thus only
emit a small amount of radiation. However, a bone with a stress
fracture in it will take up lots of the phosphorous compound and
therefore emit a large amount of radiation as it attempts to make lots
of new bone cells to try and ‘fix' itself. As a result, the amount of
bony remodeling taking place in the equine skeleton can be measured by
the amount of radiation emitted from a particular site using a
sophisticated radioactivity measurement device called a gamma camera.
After the intravenous injection, providing that the horse is not too lame, it is exercised gently to distribute the compound evenly before its radiation is measured. Thirty minutes later, the injection leaves the blood and soft tissue and heads for bone, therefore radioactivity readings are taken two to five hours after administration. Horses must stand still long enough to obtain good readings and so they receive a standing sedation but no general anesthetic is required. Although simple handheld ‘point' radioactivity scanners can be used to measure the radioactivity, large expensive gamma cameras are much better as they are situated on a steady crane, move all around the horse smoothly and take more accurate readings in a shorter period of time, thus reducing the risk of movement errors. Bone scans can easily be carried out and analyzed in a day, although horses must remain in controlled areas overnight as they remain slightly radioactive until the next day. This does lead to slight practical disadvantages of nuclear scintigraphy – the safety precautions required when working with radiation and the necessity to stop the horse's work whilst it resides at a nuclear facility.
Nuclear scintigraphy allows for the evaluation of the entire equine skeleton, although specific regions can be imaged as required. A computer then processes the information from the gamma camera and generates an image of the horse's bones. Areas of increased radioactivity, which reflect increased bony remodeling, are represented as ‘hot spots.' Although these can simply reflect a normal area of increased bone turnover such as a growth plate in a young horse, a large uptake in a certain place may signify a ‘stress fracture.' A stress fracture is simply a very early fracture that is not displaced in any way. The bone turnover is high because the bone is trying to ‘fix' itself. Once a ‘hot spot' has identified where the problem is, x-rays and ultrasound scans can be brought in to further investigate the specific area.
Due to the expense of the equipment and the practical safety issues associated with bone scanning, it is not the sort of equipment that is found at every training center or racetrack. Even though the radioactivity of the substance is very short-lived, many safety precautions have to be taken. The syringe containing the radioactive injection is protected from the veterinarian administering it by way of a lead shield. All those coming into contact with the horse from that point onwards wear protective clothing, and the horse's dirty bedding is stored and then disposed of in accordance with strict radiation regulations.
Although many veterinary centers may own a handheld point scanner, the superior gamma cameras are generally found at universities and large veterinary hospitals. The result is that bone scans are not carried out as routinely as some other less expensive, more readily available imaging techniques. However, when ‘conventional' imaging techniques such as x-rays and ultrasound scans either fail to find abnormalities or more serious fractures are feared but not seen, then horses should undergo a bone scan. They are most useful in young horses with severe, acute lameness and they have a number of important uses. Firstly, their most common use is in the case of a lame horse whose specific problem has not been found by conventional veterinary medicine. For example, ‘nerve blocks,' which ‘freeze' the leg in specific locations, may have found the area of pain but x-rays and ultrasound scans have not revealed a specific problem. If the bone scan reveals a hot spot in, for example, the lower cannon bone, then the horse is likely to have a stress fracture here. Stress fractures can be so small that in the initial stages they are not visible on x-ray and it is only when the bone has remodeled around the small fracture line that some changes can be seen. In fact, sometimes a fracture line is never seen at all on x-ray and so nuclear scintigraphy really is the only method by which it can be diagnosed.
Nuclear scintigraphy is also useful in horses with suspected spinal or pelvic pain where x-rays and ultrasound images are inconclusive. A bone scan can reveal a hot spot that proves the activity of the bone at the suspected location, thus confirming it as the source of pain. The horse's back is a very difficult area to assess both clinically and using x-rays and ultrasound scans, hence the gap in the market for an influx of miraculous ‘back manipulators' and chiropractors, many of whom have very little scientific basis behind their technique. A bone scan can prove whether there really is any bony problem behind the horse's pain, for example, a ‘kissing spine' where a horse's back vertebrae ‘rub' together or a pelvic stress fracture.
Trainers also send horses that are moving and performing poorly for full body bone scans as these animals can have multiple sites of pain. Rather than freezing joints one by one with nerve blocks to try and ascertain which joints hurt most, the bone scan can highlight several mild hot spots, which might be troubling that particular horse. Following assessment of these areas either clinically or using x-rays and ultrasound scans, some trainers may then choose to have several joints ‘medicated' with anti-inflammatories, lubricants and substances to increase joint health in an attempt to make the horse move more fluently and win more races.
The only drawback with using scintigraphy in this way is that bone turnover does not necessarily correlate perfectly with painful joints. Nuclear scintigraphy has a tendency to over-diagnose problems and label ‘normal' bony remodeling as injuries. Some joints have lots of bony changes in them but actually cause very little pain or reduction in performance, whilst some very painful joints are actually caused by inflammation of the joint capsule, joint fluid and joint ligaments and thus bony turnover may not actually be increased. Therefore, proving the site of pain by nerve blocking may have in fact been more effective. When a horse is diagnosed with two sore knees and a sore hind fetlock, we will probably believe it. However, when horses are diagnosed with three sore joints and four stress fractures, I personally find it hard to believe.
The final important asset of nuclear scintigraphy is the speed with which it can diagnose a fracture. Sometimes it is urgent to find out immediately whether or not a horse has a fracture. The veterinarian dealing with the horse suspects a fracture but cannot see one on x-ray. Whilst it would be possible to wait and re-x-ray the horse in a few days or weeks, the bone scan gives an instant answer and thus connections know what the problem is with their horse and how it should be treated – as a mild lameness or a fracture that must be rested in order to prevent a catastrophic injury.
At this point, readers are probably wondering why there are not more bone scanning facilities and why they are not used more regularly. This again brings us back to the fact that nuclear scintigraphy measures bone turnover and unfortunately this does not always correlate with fractures. Whilst a bone scan is highly unlikely to miss a fracture, it may diagnose one when there is not one there. Examples of this include areas of ‘normally' high bone turnover such as growth plates in young horses (bone remodeling associated with growing), ‘bucked' shins that are remodeling but should not be treated as fractures, and some changes associated with large bones such as the radius and the tibia. The tibia is the equivalent of the human shinbone and in the same way as we can get sore shins, horses can get sore tibias. When horses begin training, tibias may be remodeling at quite a high rate (and thus will be picked up by a bone scan) but they should not always be treated as fractures. If I were to start road running tomorrow, my shins might become slightly sore after a few days and start to remodel to the increased work. However, rather than stopping, it would actually be better for me to carry on with my running until my shins adapt to their new work. Similarly, bone scans can make us stop training some horses fearing a fracture when they are actually at no higher a risk of fracturing than the horse in the next stall, and in fact, we are just making their bones ‘softer' for when we recommence their training.
In summary, nuclear scintigraphy may be hard for many of us to pronounce but by measuring bone turnover in the equine skeleton, it has become a very useful tool in equine lameness diagnosis. As legendary Breeders' Cup winning trainer Michael Dickinson (my uncle) says: "the phrase that sums up bone scanning is ‘peace of mind.'" There are numerous examples of horses that have had potentially fatal fractures prevented by undergoing a bone scan, which revealed that a minor lameness was actually being caused by a potentially catastrophic fracture.
Thanks to the late Dolly Green, the Southern California Equine Foundation was able to build a nuclear scintigraphy facility at Santa Anita racetrack. It was this facility that enabled 2007 Kentucky Derby hopeful Ravel to be diagnosed with a stress fracture that could not be found on x-ray. As trainer Todd Pletcher said, "…it would have turned into a condylar fracture if we had breezed him." Similarly, Halfbridled, the Champion two-year-old filly of 2003, was diagnosed with a stress fracture in a cannon bone and is now safely undertaking her new role as a broodmare. Nevertheless, bone scans are not perfect. They can over-diagnose stress fractures, they do come with certain practical safety disadvantages and they are perhaps not one hundred percent accurate at diagnosing joint pain. However, despite these limitations, they have been a great addition to veterinary medicine. They may prevent one or two horses from being trained when they are actually fit to work, but they also prevent great horses like Johar, Ouija Board, Ravel and Halfbridled from fracturing on the racetrack and for this we should be grateful.