Named for the sport in which it often occurs, hunter's (or jumper's) bump is a sometimes painful pelvic condition that affects performance and gait. It's tricky to diagnose, occurs in horses which do many different disciplines, and can be an acute or chronic condition. Hunter's bump is a prominence of the tuber sacrale, a part of the hip bone that lies at the highest point of both sides of the horse's croup.
(Hunter's bump anatomy) "The pelvic bones consist of three distinct anatomical regions--ilium, pubis, and ischium," explains Nathan Slovis, DVM, Dipl. ACVIM, an internist at the Hagyard-Davidson-McGee referral hospital in Lexington, Ky. "The ilium consists of a wing and body. The wing can have normal anatomical variations. The dorsal (top) aspect of the ilium forms what is called the tuber sacrale. Normally the tuber sacrale is covered by fat and muscle, which could make it difficult to see on obese and/or well-muscled horses. Loss of fat or muscle will make the tuber sacrale more pronounced. A prominent tuber sacrale could be caused by a luxation or subluxation (dislocation or partial dislocation) of the sacroiliac joint." (See illustration at right.)
The sacroiliac joints are designed to combine firmness of attachment between the sacrum (a specific anatomical section of the spinal column) and the wing (ilium) of the pelvis. "The sacroiliac joint aids in a shock-absorbent capacity by transferring hind limb propulsive forces to the vertebral column," Slovis states. This dissipates the load placed upon the vertebral column from shear (created by two objects sliding parallel to each other), tension, and compression forces.
However, the sacroiliac joint is not meant to be mobile; stresses that produce motion might produce subluxation or tearing of the muscles/ligaments that hold the joint in place, resulting in distinct hind limb lameness.
"Once the joint has been injured, there may be chronic pain resulting from the partial displacement and instability," says Slovis.
Adds Marjorie W. Miller, DVM, a Tampa, Fla., equine practitioner and horse owner who has bred her own hunter/jumpers and racehorses, "Tearing of the ligaments causes motion of the sacroiliac joint or joints, resulting in pain from instability and muscle spasm. Stability to the joint can be restored when the ligaments scar. Therefore, it is important to remember that a hunter's bump is evidence of a prior injury to the sacroiliac joint, and the injury might not be causing clinical signs at this time.
"Conversely, a sacroiliac strain or subluxation may not be present with any visible abnormalities," continues Miller. "Because there are two sacroiliac joints (one on each side), the condition can be unilateral (on one side) or bilateral (on both sides)."
Sacroiliac subluxation can be caused acutely by a fall, slip, or twisting over a jump, or the injury can be sustained chronically through repetition of a motion or action, says Miller. "An acute injury to the sacroiliac joint can happen to any horse. The chronic forms are seen more frequently in horses that perform at gaits other than the gallop, such as horses that jump, trotters and pacers, draft horses, cutting horses, reining horses, and roping horses."
The clinical signs of hunter's bump are variable and can include lameness, stiffness, an abnormal head and neck posture, hip hike (a raised-hip carriage), and a noticeably shortened stride, Miller says.
"While trotting, the horse may keep the affected limb on the ground longer, then suddenly jerk the leg forward," she notes. "The toe of the affected limb may drag, and due to the outward rotation of the stifle, the foot is observed to slide in a diagonal motion. Riders often complain that there is a lack of impulsion, and hunters and jumpers may present with a complaint of refusal to jump and/or a loss of form."
The horse could have a prominent tuber sacrale over one or both sacroiliac joints, chronic intermittent lameness causing a shortening of stride, and spasticity of surrounding muscles in their attempt to reestablish the stability of the sacroiliac joint, says Slovis.
"Pain may also be elicited with downward pressure on the tuber coxae (bony prominence of the hip behind the flank), which causes rotation of the pelvis and movement of the sacroiliac joint," he adds. "The common complaint with the hunter/ jumper horse is that the horse is stiff and has difficulty jumping fences. Dressage horses will have difficulties with transitional changes (such as walk to trot, trot to walk, etc.) because the capacity to transfer hind limb forces to the vertebral column will be painful."
Arriving at a diagnosis of hunter's bump can be tricky; it is often a diagnosis of exclusion. Says Miller, "A veterinarian may detect motion in the joint via rectal palpation with the horse walking slowly. This motion results in crepitus--a grating of bone upon bone. Pain over the affected area may be present, but one must rule out other conditions. The presence of a hunter's bump with a history of decreased performance would certainly lead one to suspect exacerbation of a sacroiliac strain."
A proximal (upper) limb flexion test on the unaffected hind limb can also be done. Lifting of the unaffected hind limb for more than 60 seconds forces weight bearing on the suspect side and often results in an increase in lameness on the affected side during the jog.
Local anesthesia can aid the diagnosis. "A needle is inserted into the region of the sacroiliac joint and lidocaine (local anesthetic) infused," explains Slovis. "This procedure may ameliorate (improve) signs of the current lameness."
The most accurate diagnosis, though, is achieved through visual diagnostics, specifically nuclear scintigraphy, ultrasonography, and thermography. Unfortunately, radiography, which is more readily available for the farm veterinarian, is difficult and often unrewarding because of the physical mass and density of the horse pelvis.
Treatment and Prognosis
The foundation for treatment is complete stall rest for four to six weeks to allow scarring of the ligaments and healing. Hand-walking can be implemented beginning at the third week of rest, says Slovis.
"Complementary therapy, along with the use of Bute (phenylbutazone) or Banamine (flunixin meglumine), acupuncture, chiropractic manipulation, and massage therapy for help in controlling pain while the lesion is healing could also be considered," he says. "Injection of corticosteroids and serapin into the sacroiliac joint may also help relieve the immediate pain associated with subluxation."
Muscle relaxants might be utilized to reduce spasm of the back musculature, says Miller.
"One must always weigh risk versus benefit with any therapeutic approach," Miller cautions. "Introduction of infection into this region could result in disastrous consequences. Any time we stick a needle into anything, there is a slight chance of introducing infection. Aseptic preparation of the area generally prevents infection, but the nature of equine practice is that practitioners perform their work in barns and pastures that are not sterile environments. The depth of tissues required to penetrate into the area of the sacroiliac joint (through the overlying fat and musculature) via injection makes drainage difficult in the event of abscessation. Infection could affect the spine, spinal cord, and nerves, resulting in weakness or paralysis of the hind limb or hind limbs, urination problems, etc. Nothing in medicine is without risk."
The degree of recovery from hunter's bump depends on how quickly treatment--i.e., rest--is implemented and how frequently repeated injury has occurred. "Prognosis can be good if immediate rest is offered," Slovis states.
Prognosis might be poor in some cases in which there has been repeated injury, extensive injury, and/or weakening of the ligamentous attachment of the sacroiliac joint. Horses which make a poor recovery might have chronic or intermittent lameness, and retirement to pet or pleasure horse status should be considered, depending on the severity of the lameness.
Miller warns that the prognosis for returning to athletic activity after sacroiliac strain can be guarded due to the repeated weakening and widening of the ligaments and their attachments as a result of constant shearing, compression, and tension forces applied to the sacroiliac joint. However, if owners are patient and afford the horse proper time for treatment and healing, then the prognosis might be improved.
"If the cartilage of the joint has been damaged, healing occurs through bony fusion," she explains. "Evidence of a hunter's bump in the absence of lameness would lead to a more favorable prognosis."
These horses could return to the same activity or competition level with the same degree of soundness.
"A hunter's bump is indicative of prior pathology to the sacroiliac joint, but not necessarily active disease," Miller continues. "The area has been weakened by damage to the ligaments and perhaps the cartilage. I use the analogy of a re-tread tire in explaining prognosis to my clients. A tire that has been re-tread can be used on a vehicle, but this tire is more subject to a blowout at the weakened site. Intended use also factors into the equation as a re-tread will likely hold up under normal driving conditions, but one would not want to use re-tread tires on the NASCAR circuit! A horse that has sustained injury to its sacroiliac joints is at greater risk for re-injury to the joints due to the inelasticity of scar tissue."
Although there are no particular prevention techniques specific to hunter's bump, Slovis points out that fit horses are less prone to injury. Miller agrees.
"Good horsemanship minimizes the risk for most injuries in general," Miller says. "Proper conditioning of the animal for his intended use and proper shoeing and hoof balance allow the horse to perform comfortably. Training the horse on good footing minimizes wear and tear of all joints. If a rider or trainer detects a consistent change in the horse's way of going, have the horse evaluated. Videotapes of past and present competitions can be helpful in evaluation of changes in performance."
A Bump Is Sometimes Just A Bump
Keep in mind that if you see a protrusion on the croup, your horse might not necessarily have hunter's bump (nor does the absence of a protrusion on a horse with pain or gait problems mean that diagnosis is ruled out). Says Slovis, "Dissimilarity or protrusion of the tuber sacrale may be present in a horse not currently affected. This can happen if the sacroiliac subluxation has healed properly and no motion is currently present in the joints. Dr. James Rooney (DVM), an expert in regard to sacroiliac lameness, stated that in his experience, the asymmetries of the tuber sacrale are largely normal anatomical variations and rarely associated with sacroiliac subluxations."
If you notice such a protrusion, have a veterinarian examine your horse so you can make appropriate and well-informed decisions about modifying or continuing as usual with your horse's exercises and performance activities.
About the Author
Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.
POLL: Enriching Your Horse's Environment