The Surgeon’s Corner … Bucked Shins
by Dr. Antonio M. Cruz DVM, MVM, MSc, DrMedVe

Bucked shins seem to be associated with two-year-olds exercised at speed and as a disease/syndrome of Thoroughbred racehorses it has been described for generations.
The condition may be seen in Thoroughbred (most commonly), Standardbred, and racing Quarterhorses. It has been reported that 70% of young Thoroughbred racehorses in training develop a repetitive loading injury in their cannon bone known as “bucked shins”. This is a fatigue injury of bone and usually occurs in Two year-old horses during the first six monthsof their training and may be seen bilaterally. If the problem does occur bilaterally, the left limb is usually involved before the right because horses that train and race in a counterclockwise direction are on the left lead of their gait when in the turns. It costs millions to the industry.
Clinically, the condition is diagnosed by palpation of the cannon bone to reveal heat, pain, and tenderness, with or without swelling over the front or the inside surface of the cannon bone. The affected animals tend to be short strided, uncomfortable at exercise, or lame.
Radiographic diagnosis may be delayed from the clinical onset of signs but is evidenced by new bone formation over the front or the inside aspect of this bone. The proliferation of new bone may be extreme in some cases.
With few exceptions, once a horse “bucks” and resolves, the animals do not experience this problem again. This “vaccination” or adaptation phenomenon may be used in distinguishing animals at risk from those that are not. This observation has also been used in the past to intentionally buck horses’ shins to get past the problem.
One downside to this method, besides the lost training and racing days, is the risk that the horse will go on to develop a stress or saucer fracture of MCIII that could lead to a catastrophic fracture.
However, horses that buck their shins and stop training may re-buck when reintroduced to training. These animals are typically 2-yr-old horses and are equivalent to human adolescents. They are still growing and will have open growth plates. If by chance older adult horses are entered into training as 3- or 4-yr-olds, they may develop bucked shins as well. The classical description of its cause was associated with other fractures of bone. Basically, the story told of bone hemorrhage and micro-fractures on the surface of the cannon bone as a result of high-speed exercise, concussion, and external trauma but little observation or experimentation was performed to study the real cause.
The classical hypothesis of bone surface hemorrhage and micro-fractures seems to be the wrong interpretation of the bone’s morphology. The forces associated with race training on the cannon bone induce bone formation on the front of the cannon.
Classical training will induce this bone on the front and inside surfaces of the bone.‘
New’ bone formation over the cannon bone also called “Lamellar bone” can accrue on surfaces up to 1 to 2 microns/day. Faster bone formation may occur using fiber bone that forms the new bone that is vascular and porous. This normal new bone formation is interpreted by the clinician as bucked shin, and therefore, disease.
Some times the new bone formation is not disease per se but an appropriate response of bone to the load on the cannon bone. However when there is associated heat and pain and/or lameness we are dealing with an “abnormal” new bone production.
It is this injury that needs to be addressed as these horses are at an increased risk of fracturing their bones. The results of elegant studies performed by Dr. Nunamaker over a 20 year span supported the concept that exercise (training) could be designed to optimize the shape of the cannon bone. This, in turn, should influence (decrease) the incidence of bucked shins and therefore, the problem within the industry.
Dr. Nunamker developed and tested an exercise (training) regimen that modestly increased the small numbers of high load cycles (high speed) using peak load magnitudes and directions that are seen during racing, rather than galloping. Increasing the number of short distance works (breezes) from once every 7–10 days, as it occurs with classical training programs, to three times a week, produced large and positive changes in the cannon bone.
Classical training produced little progressive change in the mechanical properties of the bone seemingly and surprisingly no better than no training at all, whereas the new modified training program showed bone development that equaled or surpassed that observed in established older Thoroughbreds, those horses apparently no longer susceptible to bucked shins.
The above said program assumes that the young horse is broken to ride in the fall and is able to gallop 1 mile at 18 to 20 seconds/furlongs) by the end of December. The training program starts in January and can be broken into three stages. The principle involved is that the horse’s bones need to “see” the load environment of racing as soon as possible so that bone modeling and remodeling can begin in a timely manner.
The training program is six days a week with Sundays off. The horses walk to and from the racetrack.
On the track the horse walks one-half mile and jogs one-half mile to warm up. The horse then gallops 1 mile.
Stage 1
Using the above strategy, the horse will finish the gallops two times a week with a one-eighth mile open gallop in 15 seconds/per furlong.
This speed work is done on Tuesdays and Saturdays. This speed and distanceof the open gallop is repeated ten times over a five week period.
Stage 2
After five weeks of stage 1 gallops, the horse moves up in distance so that two times a week (Tuesday an Saturday) the horse will finish its gallop with a one-quarter mile in 30 seconds (15-s furlongs). This speed work is repeated 10 times, which takes up the next five weeks. All open gallops in stage 1 and stage 2 are at the end of the gallop and are included in the one mile gallop.
Stage 3
After the completion of stage 2, the animal continues its training using speed work once a week (Saturday), breezing one-quarter mile in approximately 26 seconds (13 seconds/furlongs). This is repeated four times over a four week period.
In stage 3, the daily gallops are extended to one and one-quarter miles twice a week.
After the fourth week the one-quarter mile breeze is continued with a strong “gallop out” for an additional furlong. This makes the three furlong total about 40 seconds. This is done for an additional three weeks, giving stage 3 a total time of seven weeks.
After stage 3, these horses have effectively established their MCIII’s shape and architecture for the longer high-speed workouts necessary for racing.
They can now go on to four- to six-furlong works as needed to develop their other body systems to complete fitness for racing.
The total time of this initial training program is 119–147 days, depending on the availability of a race for the individual. This does not include any downtime for sickness or injury. Gate work is started early and often in this program. The animals are introduced to the gate in January as the program starts. All young horses are turned out in a small paddock if available for one to one and a half hours of exercise before daily training. This training program has shown no increase in the injury rate of young horses and the incidence of buck shins has decreased to almost zero.