Proximal Sesamoid Bones
The athletic horse is a resilient, yet vulnerable animal. When racing, jumping, or competing at other sports that require speed, equine athletes are at increased risk of injury. While great strides have been made in preventing these injuries by learning more about exercise physiology, training, and bone formation, there still are instances when an athlete becomes injured and has to be treated. Because of this, researchers and veterinarians have studied why and how equine athletes get injured, and what the best courses of treatment are for various injuries.
Following is information on one area of the competitive horse that has shown a propensity for injury in athletes competing at speed, especially Thoroughbred and Standardbred racehorses--the sesamoids. This information will help you as a breeder, owner, or trainer understand better the area of the sesamoids, how they can be injured, and how they can be treated.
The stay apparatus consists of ligaments, tendons, and muscles, and enables the horse to sleep in an standing position by stabilizing the joints of the forelimb and hindlimb. The suspensory apparatus, a portion of the stay apparatus in the lower limb, is a ligament that extends from the top of the cannon bone to the first (long) and second (short) phalanx (pastern) of the lower limb. The suspensory apparatus consists of the suspensory ligament, the sesamoid bones, and the distal sesamoidean ligaments, and is located in front of the flexor tendons and behind the cannon bone. The sesamoid bones of the lower limbs consist of the paired proximal sesamoid bones that are embedded within the suspensory ligament, and the distal sesamoid (navicular) bone that is contained within the hoof capsule. The proximal sesamoid bones are roughly triangular in shape, with the proximal most portion acting as a point of insertion for the suspensory ligament, and the base acting as a point of origin for the distal sesamoidean ligaments. The intersesamoidean ligament is a dense ligament that firmly secures the proximal sesamoid bones together along their axial aspect. The sesamoid bones provide stability to the suspensory apparatus as it courses around the back of the fetlock joint, and they function to prevent extreme overextension of the fetlock joint when the horse is performing.
Cause of the Problems
The sesamoid bones become injured during the weight-bearing phase of the limb. When the horse is racing, galloping, or landing after a jump, these weight-bearing forces transfer a tensile or pulling force to the suspensory ligament, proximal sesamoid bones, and the distal sesamoidean ligament. When these tensile forces exceed the inherent strength of the suspensory apparatus, a suspensory ligament tear (suspensory desmitis), or a sesamoid bone fracture can occur. Previous training might be an important determinant as to which type of injury will occur to the suspensory apparatus. One researcher compared horses being trained or raced with untrained horses from the pasture. When fetlocks were experimentally loaded to failure in vitro, he noted that 83% of trained or raced animals sustained a sesamoid fracture; 92% of untrained pasture horses sustained a suspensory ligament tear.
Sesamoiditis, or inflammation of the proximal abaxial aspect of the sesamoid bones, usually is divided into articular and non-articular components. Articular sesamoiditis is associated with marginal osteophyte (spur) formation on the apical and basilar portions of the sesamoid bone. These usually are secondary to arthritis within the fetlock joint. Non-articular sesamoiditis is primarily associated with diseases of the suspensory apparatus, is characterized by increased intraosseous channels on the abaxial surface of the sesamoid bone, and is considered to be true sesamoiditis. The pain from this disease is believed to originate from the point of insertion of the suspensory ligament onto the sesamoid bone. The dimensions of the sesamoid bone are variable among individuals, and the bones themselves vary by size, shape, and degree of "lysis" or decomposition seen on radiographs along its abaxial border. This lysis, also called "sesamoiditis" or "vascular channels," was evaluated in a recent study.
During a one-year period, the researcher radiographed all four fetlocks of 71 2-year-old Standardbreds in training every three months. Radiographic findings were classified into three types, and correlations were made with clinical findings. Twenty-two percent of horses had no radiographic evidence of lysis during the year-long study. Fifty-five percent of horses had Type 1 lesions (one or two linear lytic defects 1mm wide) at the start of training; clinical signs of disease were not manifested, and lesions did not become clinically relevant.
Type 2 lesions (three or more linear defects 1mm wide) were noted in 13% of horses not manifesting clinical signs of disease. These lesions were more frequently observed after three months of training and were more likely to be observed in horses with injuries to the suspensory apparatus than the sesamoid bone.
Type 3 lesions (wide, abnormally shaped linear defects) were detected in 10% of the horses at the beginning of the study. These horses had chronic lameness, and the lesions were considered to be indicative of significant proximal sesamoid bone disease. The researcher concluded that greater than 75% of 2-year-old Standardbred horses might have radiographic changes in the proximal sesamoid bone at the start of training. Type 1 lesions might be considered normal. Training could increase the number of linear defects observed (Type 2), but the defects should remain well defined and 1mm wide. The researcher also noted that horses which fractured their sesamoid bone while training did not have lytic defects, and also concluded that the radiographic presence of lytic defects did not seem to predispose the sesamoid bone to fracture.
Another study described the results of 18 cases of Thoroughbred foals with fractures to the proximal sesamoid bones. Fifteen of the foals were less than two months of age. Seventeen of the 18 cases involved the front legs, and most histories described a foal which was galloping to near exhaustion trying to keep up with its dam in the paddock. Thirteen cases were treated by confinement to a box stall and bandage support. Surgical removal was the treatment of choice when the fracture fragment was displaced. Foals in which the fractures healed by fibrous (i.e., not bone) union were not lame, although there was considerable thickening of the fetlock joint. The researchers suggested that a small nursery paddock to restrict exercise in mares which run excessively might be beneficial in decreasing the incidence of these injuries.
Fractures of the proximal sesamoid bones are common among racing Thoroughbreds, Standardbreds, and Quarter Horses. Forelimb sesamoid fractures are noted more frequently among Thoroughbreds and Quarter Horses, while hindlimb fractures are most frequently encountered in Standardbreds. Six basic types of sesamoid fractures have been described, and include apical, axial, mid-body, basilar, sagittal, and comminuted.
Apical fractures are defined as proximal fractures that occur in the upper one-third of the sesamoid bone. They clearly represent the majority of fractures observed, and usually extend into the joint. Several theories exist as possible explanations for the high incidence of fractures seen in the proximal sesamoid bone. A slower remodeling response of the proximal sesamoid bone relative to the remodeling response of the suspensory ligament is seen when these tissues are stressed by training and racing. The presence of structural weak points within the proximal sesamoid bone also has been suggested.
One study evaluated the effect of training on the proximal sesamoid bone using eight untrained Thoroughbred racehorses. The researcher noted that training caused the sesamoid bone to become most dense in the region where apical sesamoid fractures occur, and speculated that the increased density noted in this region was in response to the increased stresses that training and racing placed on this location. He suggested that apical sesamoid fractures might have a higher incidence than other types of sesamoid bone fractures because forces could be concentrated in this region.
Clinical signs of fractures include lameness that is noticed within 24 hours of the race, and joint distension. Pain can be elicited in the acute case when digital pressure is applied to the apex of the sesamoid bone, and standard radiographic projections of the fetlock joint usually provide a diagnosis. When left untreated, the continued distraction of the fractures due to loading of the limb causes these fractures to heal slowly and usually with a fibrous union. Because this fibrous union is weaker than the union created between the remaining sesamoid bone and the suspensory ligament, surgical removal is recommended for the fastest return to performance and the best long-term prognosis. Removal can be accomplished by either arthroscopy or arthrotomy in cases where fracture fragments are large. In a study of 109 apical sesamoid fractures that were diagnosed in Standardbreds, 67% were seen in 2- and 3-year-olds. Horses which had raced before surgery, which had surgery within 30 days of the injury, and which did not have concurrent damage to the suspensory ligament had the best prognosis for return to athletic performance.
Mid-body fractures are defined as fractures that occur in the middle of the sesamoid bone. Surgery is the treatment of choice because conservative treatments result in a fibrous union that often leads to chronic pain due to motion at the fracture site. Surgical repair includes placing a compression lag screw through the sesamoid bone, or placing a Cerclage wire around the circumference of the fractured sesamoid bone. The aim of both techniques is to eliminate fracture motion at the fracture site so that a bony union will result. A study evaluated the results of 25 horses with mid-body fractures that were treated with compression lag screw fixation and a bone graft. Nine of 12 Standardbred racehorses, and five of eight Thoroughbred racehorses, raced successfully after surgery, with a mean recovery time of 11.3 months.
Abaxial sesamoid fractures involve the outermost (abaxial) aspect of the sesamoid bone, and surgery is not recommended unless the fracture extends into the fetlock joint. Horses with joint involvement often have joint distension and are more painful when the fetlock joint is manipulated than are horses without joint involvement. Diagnostic ultrasound and radiography can help to make the distinction between articular and non-articular abaxial sesamoid fractures.
Basilar fractures are defined as fractures that occur in the bottom third of the proximal sesamoid bone and usually extend into the fetlock joint. The clinical signs of these fractures are similar to other types of proximal sesamoid bone fractures. Clinical signs include lameness within 24 hours of the race, joint distension, and pain on manipulation of the sesamoid bone and fetlock joint. Radiographs provide a definitive diagnosis. Previous studies have suggested that these fractures have a poor prognosis for return to athletic performance.
One researcher evaluated 57 horses with basilar fractures of the sesamoid bone. In this study, 49 horses were Thoroughbreds; seven were Standardbreds. Fifty of the horses had sustained an injury to the forelimb. Twenty-three horses had the fracture fragment removed surgically, and 29 horses were treated with prolonged stall rest. Fifty-nine percent of horses returned to racing at least one time, and 41% finished first, second, or third.
Horses with fragments that were only mildly displaced (less than 3 mm displacement) had a significantly better outcome than horses with severe displacement. Of horses which were treated surgically,
73% returned to racing, while only 48% of horses which were treated conservatively returned to racing. After careful comparison of the results and the results of other studies on proximal sesamoid fractures, the researchers noted that the prognosis for a horse with a basal sesamoidean fracture
is not dramatically different than the prognosis for horses with other types of ses-amoid fractures. The researchers also noted that any injury to the fetlock joint of the forelimb of a Thoroughbred racehorse had a less favorable prognosis than the same injury in a Standardbred racehorse.
Sagittal fractures are defined as fractures that occur on the axial margin of the sesamoid bone. Some of these fractures might involve the intersesamoidean ligament. These injuries are reported to be rare in occurrence, and are thought to be associated with other injuries, such as lateral condylar fractures.
Comminuted (biaxial) sesamoid fractures are severe fractures that often are associated with a traumatic disruption of the suspensory apparatus. These are classically referred to as a "breakdown" or catastrophic injury. Traumatic disruption of the suspensory apparatus might be due to rupture of the suspensory ligament, transverse fracture of both sesamoid bones, or rupture of the distal sesamoidean ligaments. The highest incidence is in Thoroughbred racehorse, and it is due to extreme over-extension of the fetlock joint. The trauma associated with these fractures can cause the fetlock joint to become open to the exterior and become contaminated by the environment. This complication severely compromises the prognosis for recovery of any joint. When evaluating these injuries, it is crucial to ensure that the nerve and blood supply to the lower limb is functioning normally before surgery is attempted. Stability is returned to the fetlock joint by creating a permanent bony fusion of this joint (fetlock arthrodesis). This is a salvage procedure that is reserved for valuable broodmares, stallions with breeding potential, and horses of significant sentimental value.
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About the Author
John G. Peloso, DVM, MS, Dipl. ACVS, is owner and surgeon of Equine Medical Center of Ocala in Fla.
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