We’ve all seen television crime shows where the good guys locate the bad guys at night using heat-sensing devices. The crook’s bodies appear as glowing figures on the good guys’ detector screens. This same technology also makes a terrific diagnostic tool called thermography, which shows variations in the temperature of body tissues and allows veterinarians to detect inflammation at its earliest stages.
This, of course, allows therapy to begin earlier, often even before swelling and pain is noted, helping our horses back into work sooner. It can also help prevent secondary injuries that occur when a horse tries to protect an undetected painful or sore area.
It’s no surprise that sophisticated thermography equipment, which generates full-color, hard-copy pictures of the injured area, costs tens of thousands of dollars — definitely a big-ticket item appropriate only for large veterinary hospitals. But wait. We found a reasonably priced, hand-held — and accurate — thermography gun you can use right in your own barn.
Raytek Infrared Thermometer
We used the Raytek Infrared Thermometer to scan the legs of five horses in race training over an eight-week period. The gun is lightweight, fits well in your hand and is operated by a system about as simple as you can get — point and shoot.
The temperature of the area being scanned is detected on a digital screen. A chart on the “barrel” of the gun shows the size of an area you are scanning compared to your distance from the leg. This unit is sensitive to within 0.1 degree centigrade and sells for $295 from Equine Racing Systems (www.equineracing.com or 360/837-3700).
When the trigger of the Raytek gun is depressed, the center of the area where temperature readings are taken is marked by a red laser-beam dot on the leg. We found a routine for covering the legs and distances from the leg: Examine legs from the front surface, beginning at the feet and working up to just above the hocks and knees, then examine from the back and finally from both sides, so we scan all four surfaces of the leg.
The closer you are to the leg, the smaller the area measured. A distance of six inches away at the top of the legs, tapering down to four inches for the lower leg, worked well for general scanning to detect hot areas. If areas of significant temperature difference were noted, we moved in to within two inches away from the leg to try to pinpoint the center of the “hot” reading more precisely.
Normal vs. Abnormal Findings
In a normal horse’s leg, the coronary-band temperature is highest, unless you are right over a superficial blood vessel. As you scan the foot from the coronary band down, the temperature progressively drops.
On the soles, the clefts scan warm, while the frog and soles should scan cooler. Muscular areas normally read warmer than bone and tendon/ligament. The unit will detect areas of muscle damage to the superficial layers of muscles the same way it does for the lower legs — by a higher temperature over the injured area. The legs from the coronary bands, up to and including the hocks and knees, will scan a relatively consistent temperature in a normal horse. One variation of this is what is called a “bleed down” from above, where the higher temperatures found in the muscle groups located above the joints may extend down to give higher readings over the top part of the knee or hock. This is still normal.
Some places on the legs are consistently warmer than other parts. The most dramatic areas are directly over blood vessels or areas with a high normal circulation. When learning how to use and interpret these infrared thermometers, have a good anatomy reference book close by.
Although the Raytek unit literature suggests a temperature increase as slight as 1 to 2 degrees centigrade marks a potential problem, it wasn’t unusual to pick up readings this much higher over joints. When scanning joints, it is therefore important to first define the outlines of the entire joint — extent of the joint capsule. Do this on both sides and compare the reading obtained when focusing on the center of the joint to one taken at the same location on the opposite leg. If these are different, scan the entire joint on the warmer side to look for a localized problem area.
You must then come in closer to scan the entire joint surface from side to side and top to bottom to look for spots of higher temperature that could signal a localized problem underneath such as a bone chip. If no clearly identifiable hotter spots can be found within the limits of the warmer joint, a more diffuse problem, such as a synovitis, might be the cause. If you find well-localized hot spots in a joint, a cartilage or bone problem (chip) may be the cause.
Problems like splints and bucked shins are easily detected with thermography. By moving in close and slowly scanning within the detected hot area, you can pinpoint the location of the problem as the spot that has the highest temperature. With a tendon or ligament problem, the entire length of the structure may “light up” with a high temperature, but you can pinpoint spots that have the highest temperatures within the lesion by moving in closer.
As useful as thermography is in diagnosis, it’s even more important in preventing injury. To feel abnormal heat by hand, you need a difference of 5 to 7 degrees centigrade between the problem area and the surrounding tissue or opposite normal leg.
The gun will detect differences of 2 degrees, which warrant keeping an eye on. Temperature differences from 3 to 5 degrees almost always indicate a significant problem of some sort underneath. By routine leg scanning in high-risk horses, e.g. young stock, horses in heavy work, after a strenuous training session or competition, you can detect problems before they progress to an obvious lameness, back off work and consider more detailed diagnostic work.
Thermography measures differences in temperature at the skin level. It therefore will accurately reflect what is going on directly beneath the surface but not much deeper. For example, it won’t accurately pick up problems with the internal surfaces of the stifle or shoulder. However, we’d still scan these areas as a diffusely hot joint might show up along the joint lines. We were able to detect problems with more superficial structures, such as the patellar ligaments.
Also, weather will affect the readings. On extremely hot days, we found thermography virtually useless, as there is no gradient between the air temperature and the horse.
Remember, if you have just unwrapped a leg, to wait several minutes for the trapped heat to dissipate or the leg will read hot. The same is true for a recently iced leg, although it takes longer to return to “normal.”
Paints and liniments may make the leg diffusely warmer than the rest, but the underlying patterns of greater heat over inflamed areas are preserved. Wet legs can be scanned with no problems and the amount of hair on the leg makes no difference. All areas must be free of straw, mud and manure, however.
These are all minor limitations. What we find to be the biggest concern with thermography is that it only tells you where the problem is, not what it is. It doesn’t replace your veterinarian. If a suspensory ligament shows hot, this isn’t much of a diagnostic challenge, but to determine what’s actually going on inside joints or the feet, you will need further work up.
A University of Minnesota study looked at Thoroughbreds in training for 10 weeks to determine if thermography could predict developing injuries before they became evident clinically and how well the findings correlated with areas of concern to trainers and with diagnoses by vets. A total of 225 thermal studies on 45 horses from seven trainers were evaluated during the trial.
Of the scans, 60% identified possible problems. Trainers had similar concerns in about half the cases, with the suspected area matching thermography findings in 88%.
When vets examined the horses, thermography findings agreed with their diagnoses 95% of the time. Nine horses were lost to the study due to injuries. In each case, thermography had predicted the injury at least two weeks prior to the diagnosis.
Three horses were taken out of training because they were “sore all over.” Again, thermography had detected multiple areas of inflammatory response, two to four weeks before the horses were withdrawn.
Icing and the use of liniments did not interfere with the ability to use thermography in this study. Corticosteroid therapy did block inflammatory readings for a week or so. The authors also found you have to wait two hours after exercising for the legs to properly cool before attempting thermography.
While thermography may give you some false alarms, it is still an excellent early warning system. The Ray-Tek Infrared Thermometer is a reasonably priced and accurate scanning device, suitable for barn use by anyone. It allows you to not only help localize lameness problems but also pick them up in their early stages, before serious problems occur.