AAEP Convention: Lameness
MILNE LECTURE PART 1
The Milne Lecture is also known as the State of the Art Lecture because each year's topic is selected for its groundbreaking qualities and potential to change the paradigms by which veterinarians and researchers understand that topic. This year's Milne Lecture (which is named after Frank J. Milne, an AAEP past president) combined biology research and biomechanics principles into an insightful, landmark report of more than 20 years of bone and fracture research by David M. Nunamaker, VMD, Dipl. ACVS, Jacques Jenny Orthopedic Research Chair at the University of Pennsylvania's New Bolton Center.
Nunamaker began a two-part presentation with a discourse on his work on bucked shins in Thoroughbred racehorses, noting that an incidence of 70% has been reported in young Thoroughbreds in race training (and about 10% in Standardbreds). The annual cost of this problem in the United States has been estimated at $10 million per year in lost training and racing days.
"This is a fatigue injury of bone and usually occurs in 2-year-old horses during the first six months of their training and may be seen bilaterally (left and right limbs)," he explained. "If it does occur bilaterally, the left limb is usually involved before the right," due to the counterclockwise direction of racing in the U.S. and thus the greater forces on the left lead limb in turns.
"Clinically, the condition is diagnosed by physical examination using palpation of MC3 (the cannon bone) to reveal heat, pain, and tenderness, with or without swelling over the dorsal (front) or dorsomedial (front inside) surface of MC3," he continued. "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 periosteal new bone formation over the dorsal or dorsomedial aspect of this bone.
"With few exceptions, once this condition occurs and resolves, the animals do not experience this problem again," he explained. "This observation has 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 the horse will develop a stress or saucer facture of MC3 that could lead to catastrophic fracture. These fractures seem to only occur in horses that have previously bucked their shins."
Nunamaker said that while this problem is usually seen in 2-year-olds entering race training, it might be seen in older horses when they first enter training. He noted that racehorses trained on turf tracks might buck their shins when they race on harder dirt tracks, and that approximately 12% of horses with bucked shins will develop a stress or saucer fracture on the dorsolateral (front outside) surface of MC3 up to one year after the original bucked shin injury.
"Catastrophic complete midshaft fractures of MC3 can occur when these horses are exercised at speed or raced," he warned. These fractures comprise about 10% of the fatal catastrophic musculoskeletal injuries on North American racetracks annually.
Understanding Bone Strength
To quantify the strength of racehorses' cannon bones, Nunamaker and colleagues evaluated cannon bone samples from Thoroughbred and Standardbred horses of various ages. Their experiment yielded data for Imin (measurement of dorsopalmar, or front-to-back, bending) and Imax (lateromedial, or side-to-side, bending).
Nunamaker found that in Thoroughbreds, all section properties were much lower (weaker) for yearlings than any other age group. "The most significant changes in the bone occurred at the midsection between one and two years of age, but continued change took place until age four," he related. "No observable changes took place after four years of age. The Thoroughbred changed this property (Imin) to a greater extent than did the Standardbred during the first two to four years of life, just when the animal is at risk for bucked shins."
In Vivo Strain Testing
Armed with a better understanding of in vitro (outside the body) bone characteristics, the researchers set out to measure these strains in live animals. Four 2-year-old racehorses were purchased and trained for six months, and a 12-year-old racing veteran was also used. The horses had rosette strain gauges mounted on their cannon bones under general anesthesia, then they were exercised on a dry dirt track.
They found that when horses work slowly, the front of the cannon bone is under tension--in other words, the front of the bone is stressed by stretching force rather than compressing force--and compressive forces occur at an angle to the bone surface. At higher speeds, the forces become more compressive. Also, the older horse had lower measures of strain than the younger horses at the same speeds, although the strains were in the same directions from the bone. Nunamaker noted that the peak strains in horses are higher than previously reported in any animal species.
Nunamaker concluded that the difference between strain in younger vs. older horses is due to not yet having remodeled the bone to meet the demands of racing (thickening at the dorsal/dorsomedial aspect and thus becoming more resistant to bending; more on that later).
Cannon Bone Shape and Strength
We might think bones are shaped at birth and only changing in size as horses grow. That's not necessarily the case, although shape changes aren't extreme.
Eight untrained 2-year-old Thoroughbreds were purchased for this five-month study. The researchers found that untrained horses exhibited a symmetrical cannon bone in cross section. Classically trained horses (breezing every seven to 10 days) showed a relatively smaller marrow cavity due to significant bone addition on the medial aspect (and to a lesser degree on the lateral aspect). A harder dirt track stimulated remodeling more quickly than a softer wood chip track. Pastured horses and those in modified training (with more frequent breezing) had remodeling in the dorsal and dorsolateral areas that wasn't seen in the other groups.
"Classical training methods...did not effectively change the inertial properties (Imin) that influence bending of MC3 in a dorsopalmar direction," Nunamaker reported. "Exercise regimens that stressed MC3 in compression on its dorsal surface (with more breezing) did change Imin in a significant manner, consistent with adult racehorses evaluated previously that were no longer at risk for bucked shins.
"The results of this study supported the concept that exercise could be designed to optimize the shape of MC3. This, in turn, should influence (decrease) the incidence of bucked shins in this Thoroughbred racehorse model, and therefore the problem within the industry."
Testing Training Regimens
A larger study was required to test the efficacy of different training regimens; two training stables (numbers two and five) used the modified training program developed in the previously discussed study (breezing three times per week). The other three used classical training methods (breezing once every seven to 10 days). Eleven years of training data from 226 2-year-old Thoroughbreds were compiled, including daily records of accurate distances of jogging, galloping, and breezing. Fifty-six horses bucked their shins.
Stable two, with the highest breezing rate, had the lowest incidence of bucked shins (9.3%), while stables one and four had the highest galloping rates and the highest incidence of bucked shins (41.3% for stable one).
"Classical training produced little progressive change in the inertial properties of MC3, seemingly no better than no training at all; whereas the new modified training program showed inertial property (Imin and Imax) development that equaled or surpassed that observed in established older Thoroughbreds, those horses apparently no longer susceptible to bucked shins," said Nunamaker.
The Modified Training Program
The guiding principle of the training program developed by Nunamaker's team (with John Fisher, VMD, a Fairhill, Md., trainer) is that the young horse's bones need to see the strain environment of racing as soon as possible so that bone modeling and remodeling can begin in a timely manner.
Nunamaker noted a bucked shin incidence of less than 5% with this program, which takes 119-147 days. (The complete modified training program can be found in article #4066 at www.TheHorse.com.)
The Bottom Line
"It seems that horses are not born with the right bone structure for racing," Nunamaker concluded. "They must develop it. Bone can only develop based on its own experience. Training adapts bone to training, and training that mimics racing adapts bone to racing. Bucked shins do not have to be a part of normal training for racing of young Thoroughbred racehorses."
MILNE LECTURE PART 2
Bone and Fracture Treatment
Nunamaker continued the Milne lecture with a focus on bone properties and fracture treatment in horses.
"Bone is a unique and fascinating material," he began. "People often think of bone as being relatively inert, but I'd like to dispel that concept. Modeling and remodeling can be occurring in the same bone at the same time--bone is always in all stages of remodeling. Bone does not heal, incorporating the scar tissue as seen in most all other tissues; it regenerates itself. It changes its shape and structure based on its use, and if broken can resume 100% of its former strength and function."
Nunamaker described in great detail and with many microscopic views the modeling/remodeling processes of bone and its microstructure, beginning with bone composition and bone cell activity. "Since bone can only form on surfaces, resorption (by osteoclast cells) creates the surface on which the bone forms to replace itself," Nunamaker explained.
"Activation of osteoclasts may be influenced by physical activity and by drugs," he continued. "For example, many of the non-steroidal anti-inflammatory drugs will decrease bone formation somewhat, and some osteoporosis drugs (used in humans and not yet available for the horse) slow down resorption. As we saw in the first presentation, working horses harder stimulates remodeling in response to particular stresses...Bone formation rates seem to be independent of physical activity, but can be modified by drugs."
The new bone laid down will further calcify over time. Bone can be remodeled from within, creating new haversian systems (also called osteons, which can house small blood vessels), or it can be remodeled on its surface. A primary example of surface remodeling for growth is the equine rib; as a foal grows, bone is resorbed from the inner surface of each rib and deposited on the outer surface, "to allow the foal's narrow chest to become the very large adult's chest cavity," Nunamaker explained (see image at left).
Various densities and porosities are characteristic of different types of bone; cortical or compact bone (the normal outer tubular layer of bone) has a density of about 1.85 g/cm3 and about 5% porosity, while trabecular or cancellous bone (found deeper within bones at ends and some spinal vertebral bodies) has a density of about 0.9 g/cm3 and a porosity of 20% or more. Trabecular bone can compress a bit in response to stress via microfracture within the bone, thus avoiding larger fractures. "It might lose some connectivity, but the basic structure remains intact," Nunamaker said. "It's Nature's way of maintaining structural integrity in high-load areas."
However, this trabecular bone compression can affect the overlying cartilage and thus hasten arthritis. "Though this phenomenon is not yet fully explained, its existence is well accepted," he said.
"Bone is strongest in compression and weakest in tension," he added. "Bending forces produce tension on the convex (outwardly curved or bent) surface of the bone, hence bones are weak in bending. Torsion (twisting) forces will resolve into tension forces as well, so bones are also weak in torsion. Fatigue failure of bone may occur when bone is repeatedly loaded below its breaking strength."
Following the healing process using plates and screws (in dogs), the new bone is strongest at about 20 weeks, Nunamaker said. However, it then tends to weaken a bit following plate removal from the initial resorption phase of remodeling before becoming even stronger. The same strengthening, weakening, then strengthening process is also seen when a fracture repair plate is removed; he explained that the loss of support of the plate increases stress on the bone, thus causing activation, remodeling, and decreased strength before remodeling. "The bone doesn't return to its original strength at plate removal until eight to 12 weeks after removal," he said. "Four to six weeks out is the weakest time."
"Our problems (with fracture repair) relate to a 1,000-pound animal that is uncooperative and must have immediate full weight bearing to prevent catastrophic sequelae such as laminitis," he explained.
"When a horse first stands up after surgery, his first step (on the repaired limb) opens the fracture a bit, then it's a race between cycles to failure (of the plate) and fracture healing," he said. Thus, veterinarians have worked hard to refine better fracture repair materials and methods. One of the most widely used has been plate luting.
Plate Luting for Strength
This technique augments rigid plate fixation of a fracture with polymethylmethacrylate (PMMA), an adhesive compound that hardens completely and is sometimes used in hoof repair. The compound is placed between the plate and bone, and between the screw head and the plate, to improve contact between bone and plate, stability, and fatigue resistance to cyclic loading.
"With plate luting, the screws were loaded as one and the bone or plate was the usual failure site (rather than individual screws, as is often the case without plate luting)," said Nunamaker. "This improvement showed that plate luting could increase the fatigue life of the implants by an order of magnitude (300-1,200%; see photo on page 32). That meant that the horse could walk on its internal fixation repair three to 12 times longer before implant failure with plate luting than without it. Presently, the use of plate luting is a standard technique in treating long bone fractures with plates."
Concerns about PMMA's effect on the healing process of the bone it covers appear to be groundless; Nunamaker cited research that showed no significant difference in porosity of bone under the compound between horses with and without plate luting. He recommended adding an antibiotic to the PMMA.
Nunamaker also discussed new fixation hardware that's being used in humans, such as threaded screw heads for tighter anchoring of the screw to the plate as well as the bone. This hardware has not yet been used in horses and is quite expensive, although, he said, "The concept of mechanically locked screws using a threaded interface between the screw and the plate should provide stronger and more certain stability than plate luting."
External Skeletal Fixation
External skeletal fixation is not indicated for just any equine injury--only those where the horse couldn't survive any other way.
"These fractures include badly comminuted (shattered) fractures, open fractures, or fractures with 'bad skin' (damaged skin due to vascular compromise from bone fragments beneath it)," Nunamaker said.
He described his work with an open fixator that's now in its third iteration. The latest version (Model III, left) allows removal or reduction of weight bearing (on fractures below the knee or hock), along with the near total immobilization provided by gluing the hoof to the walking plate. This allows the fractured area to heal with greatly minimized stress and without surgical fracture repair.
"You can take a horse that looked like he just wanted to die, and after you put this device on him, he gets right up, walks to his stall, and starts eating," Nunamaker said. These horses can be shipped home for convalescence if the owner desires (after a three- to five-day observation period).
"This device can hold about 19 times the horse's body weight prior to bone failure," he reported. A non-bony advantage to this fixator is that it does not cover skin, allowing easy treatment of any skin injury (from an open fracture, for example). A cast would not allow this.
The holes remaining after fixator removal pose a valid concern. "The holes may never completely fill in," Nunamaker said. "The bone may or may not be as strong as an intact bone without any holes in it. With this in mind, presently treatment of fractures with large-diameter external skeletal fixation pins should be reserved for animals that would not be expected to return to athletic pursuits."
The Model III external fixator is commercially available through Ron Nash Engineering, Magnolia, Ark., who was involved in the design and testing (for more information, e-mail email@example.com or call 870/554-2236).
"The success of fracture treatment has progressed to the point that decisions involving treatment are often related to financial decisions and not to our ability to save the individual's life," Nunamaker concluded. "Euthanasia should no longer be thought of as a satisfactory treatment." (See complete articles #4066 and #4067 online.)
NEW RACEHORSE SHOE PREVIEW
David M. Nunamaker, VMD, Dipl. ACVS, Jacques Jenny Orthopedic Research Chair at the University of Pennsylvania's New Bolton Center, tantalized his audience with a discussion of a study in which his group measured accelerations of the hoof with toe-grab shoes. A measurement package was glued to the hoof for evaluation of toe-grab shoes attached with nails and attached by glue.
They found that the highest accelerations were on take-off, reaching as high as 460 Gs (gravitational force equivalents) with nailed-on toe-grab shoes. Landing deceleration forces reached around 250 Gs. The glued-on toe-grab shoe yielded about 260 Gs of take-off force, and about 160 Gs at landing.
Nunamaker then previewed the results of similar testing of a new shoe he designed. The entire ground surface of this shoe is reminiscent of a serrated knife with low-profile teeth hooking rearward, yielding take-off forces of less than 200 Gs and landing forces of less than 100 Gs. Nunamaker theorized that this design allowed the hoof to slide forward a bit on landing, thus avoiding the snatch of a "grabbier" shoe, while providing plenty of take-off traction--all without changing the angle of the foot as a toe grab does.
The University of Pennsylvania is currently negotiating a license agreement with a manufacturing company for production and distribution of this shoe design.
ONE JOINT NUTRACEUTICAL'S EFFECT ON HOCK LAMENESS
Horse owners often feed their horses supplements in an attempt to improve some part of those horses' functions, but little scientific research has been done to prove or disprove their faith in these products. Hilary Clayton, BVMS, PhD, MRCVS, McPhail Dressage Chair in Equine Sports Medicine at Michigan State University's Mary Anne McPhail Equine Performance Center, shed some light on one product's efficacy in minimizing joint lameness with her presentation.
The study looked at the effects of a joint supplement (Corta-Flx) on gait asymmetry caused by degenerative joint disease (DJD) of the distal intertarsal and/or tarsometatarsal joints of one or both hocks. Clayton stated, "The objective of this study was to objectively assess changes in gait variables in horses with tarsal DJD after administration of a joint supplement in a double-blind, placebo-controlled trial.
"DJD is a common pathology in older working horses, and the majority of affected horses show changes in more than one joint," she noted. "The horses used in this study were lame in multiple sites, leading to complex patterns of gait abnormalities and compensations, which are typical of the older working horse."
Eight horses used in this study were treated with Corta-Flx and a placebo solution (with the same taste, smell, color, and consistency) in a randomized, cross-over design. The horses received either Corta-Flx or the placebo orally via syringe for two weeks, then there was a two-week period without any treatment, followed by the other treatment for two weeks.
Gait analysis using joint markers and computerized video evaluation as horses trotted in-hand on a rubberized runway was combined with force plate data to calculate several measures for comparing lameness between groups. In horses receiving Corta-Flx, the left and right hind limbs showed significantly more symmetrical values for the peak vertical (weight-bearing) ground reaction force, vertical impulse, tarsal joint range of motion, and tarsal joint energy generation during the stance phase. Thus, "This product produced a more symmetrical gait pattern," Clayton said. "A more symmetrical pattern is considered to be characteristic of a sounder horse.
"The results are specific to the oral supplement used in this study and would not necessarily apply to other similar products," Clayton cautioned. She added, "It is unrealistic to expect that an oral supplement of this type will restore complete soundness, but an effective product might be expected to improve the lameness so that the horse's gait pattern more closely approaches left-right symmetry." (See complete article #4050 online.)
JUVENILE BOWED TENDONS
"Juvenile bowed tendons, or 'baby bows,' are not uncommon in yearlings and weanlings," said Johanna Reimer, VMD, Dipl. ACVIM, Dipl. ACVC (cardiology), Rood and Riddle Equine Hospital in Lexington, Ky. In her presentation "Enlarged Superficial Digital Flexor Tendons in Immature Thoroughbred Horses: Prognosis for Racing," she discussed her seven-year field study of Thoroughbred yearlings with visually enlarged superficial digital flexor tendons. Her goal was to define the prognosis for racing for immature, untrained Thoroughbreds with this problem.
These young horses were presented for examination with enlargement of one or both forelimb superficial digital flexor tendons, generally with a homogenous (uniform) appearance on ultrasound examination. For a control group to compare with the study group, she took the race records of the selected horses' maternal siblings from The Jockey Club. The likelihood of racing, age at first start, and earnings were evaluated for both groups.
There was no significant difference in likelihood of racing at all or racing at two, three, or four years of age compared to the control group. There was no difference in average earnings per start between the two groups.
Reimer concluded that the prognosis for racing for immature horses with thickened superficial digital flexor tendons appears to be favorable based on this research. However, she cautioned that this field study had limitations in that she had no knowledge of whether the siblings used as controls were unaffected by enlarged superficial digital flexor tendons and the subjects could have been managed differently because of the enlarged superficial digital flexor tendon(s), making direct comparison between groups possibly erroneous.
The cause of these enlarged superficial digital flexor tendons is unknown. The onset of this problem appears to be insidious (developing slowly and thus avoiding detection until it is fairly well developed). Reimer suggested that increased use of walkers to exercise and "muscle up" these youngsters intended for sale might increase the incidence of this problem, citing an increased number of these cases in the past year in her practice.
To avoid and manage enlarged superficial digital flexor tendons in young horses:
- Avoid heavy preparation of affected horses for 2-year-old in training sales. This can aggravate the problem.
- Don't anticipate heavy 2-year-old racing for affected horses.
- Monitor affected horses' superficial digital flexor tendons carefully during training and modify the training regimen if the problem appears to be getting worse. (See complete article #4024 online.)
LAMINITIS SUNRISE SESSION
"Never use only clinical assessment to estimate the progress of a laminitic horse," began Ric Redden, DVM, moderator of the Laminitis Sunrise Session and founder of the International Equine Podiatry Center in Versailles, Ky. "X rays and venograms (images of blood flow in the foot) are essential." He went on to discuss cases where horses had little to no blood flow in their feet but weren't very lame, and contrasted this with horses which were very lame but imaging showed that their feet were recovering. The implication was that the horse's outward presentation can often contradict what's really going on. (See complete article #3990 online.)
EXTRACORPOREAL SHOCK WAVE THERAPY
Scott McClure, DVM, PhD, reported on the effects of extracorporeal shock wave therapy (ESWT) on suspensory ligament desmitis. He noted that ESWT is being used for the treatment of equine musculoskeletal diseases. "Recent studies have demonstrated that shock waves induce neovascularization (new blood vessel growth) at the tendon-bone junction, which in turn relieves pain and improves tissue regeneration and repair. ESWT was also found to have a positive effect on the concentration of a cellular messenger that stimulates cells.
The result of the study was that the horses treated with ESWT showed a statistically significant decrease in lesion area and improved fiber alignment and echogenicity of the ligament when compared with control ligaments.
COFFIN BONE FRACTURES
Fractures of the distal phalanx (coffin bone) are common, especially in racehorses, noted Tara S. Rabuffo, DVM, of the University of Pennsylvania's New Bolton Center. She and her colleagues reviewed case records and related diagnostic materials on 73 racehorses (26 Thoroughbred and 47 Standardbred) admitted to the university's George D. Widener Hospital for Large Animals from Jan. 1, 1990, through Dec. 31, 2001.
"Fractures of the equine distal phalanx may be a form of stress-related bone injury much like other fractures in the racehorses," Rabuffo said. "Early scintigraphic examination of racehorses with palmar foot pain may identify stress-related bone injury before fracture and allow corrective shoeing to be instituted. These fractures most commonly occur in the lateral (outside) aspect of the left forelimb and the medial (inside) aspect of the right forelimb. Despite the fact that this injury can lead to osteoarthrits of the distal interphalangeal joint, there is a favorable prognosis for return to racing."
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