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Predicting fetlock fractures in the racehorse
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Introduction
The fetlock joint is the most common site of musculoskeletal disease that results in reduced performance, premature retirement, catastrophic failure, and euthanasia of Thoroughbred racehorses in the United States, Canada, United Kingdom, Australia, and Hong Kong (1,2). A 15-year summary of Thoroughbred racehorses submitted to a US necropsy program revealed that 57% of all submissions involved the fetlock joint: 34% (651/1,915) involved the proximal sesamoid bones (PSB), 19% (364/1,915) involved the cannon bone and 4% (77/1,915) involved the first phalanx (3).
Recently it was identified that 55% of horse spills injure the jockey and that falls were most common with fetlock injuries (4). When the fatal injury rate is 1.9 horses per 1000 starts, a credible worldwide average, 1000 starts are attained when 10 horses per race compete in 10 races per day. In this hypothetical 2 horses would be destroyed and 1 human injured every 10 days. These facts underscores the urgent need to develop additional surveillance strategies of the fetlock for the health and welfare of the racehorse and there jockeys, owners and the thoroughbred industry,
In engineering it is know that the failure of more than 80 % of all structural materials (metallic or ceramic) can be traced to cyclic fatigue and bone is no exception (5). No athlete inflicts higher force and cyclic fatigue on bone that the thoroughbred racehorse. At weights of 500 kilograms and speeds of 60 kilometers per hour, the repetitive cycle of the gallop has each limb supporting this entire load unassisted at some point during the 4 beats of this gait. Metacarpal / metatarsal (MC3/MT3) condylar (condyle) and first phalangeal (P1) fracture (herein MC3), palmar osteochondral disease (POD) and biaxial proximal sesamoid bone (PSB) fracture occur due to cyclic fatigue and failure of the subchondral bone (SCB) in the fetlock (6).
Identifying the signs of fatigue in the SCB of the fetlock pre-in the fracture could significantly reducing injuries to the thoroughbred racehorse and their jockeys. We hypothesized that the standing MRI (a) would identify significantly more severe (i.e., higher grades) of bone change in Thoroughbred racehorses with catastrophic fracture of the fetlock when compared to thoroughbred racehorses without fetlock fracture (controls) (11,18).
MC3 Condylar Fracture
A catastrophic condylar fracture is the final event of a bone stress injury (BSI) that began in the SCB of MC3 where the PSB oppose the stress created by the downward movement of MC3 during the stance phase of the gallop (11-13). Understanding BSI (14,15) and its progression is essential if strategies to prevent MC3 condylar fractures are to be developed and applied (11-15).
Bone can fail monotonically by a maximal load that exceeds the failure stress of bone, or gradually by a repetitive submaximal load that creates fatigue. Overuse injury to bone occurs in the absence of a radiographically apparent fracture because lower strain (submaximal) creates microscopic damage and microcracks (5,14,15). Fatigue is dangerous because the single application of the submaximal load does not produce any obvious ill effects and leads to the erroneous assumption that safety exists when it does not (5).
Bone is a solid material that changes its internal architecture when stressed (Wolff’s law). By adapting to increased loads through remodelling, an increase in bone density enables a greater tolerance to the higher demands of racing. The microdamage of BSI is a normal and physiological event that initiates remodelling (14-16). With continued stress and without appropriate rest, repair mechanisms are overwhelmed and BSI can become pathologic when the pace of fatigue damage is faster than the pace of remodelling (14-16).
The pathologic continuum of a BSI has an exact sequence of events identified below and is well recognized in human athletes (14-16). A stress reaction is a pre-failure event that is distinguished by the appearance of bone marrow edema (BME) and provides the earliest evidence of BSI on a cellular level. MRI is the only advanced imaging modality that can identify BME (16). A stress fracture is a pre-failure event that is distinguished by the presence of an incomplete fracture line and provides evidence of BSI on a macroscopic level. A stress fracture can be identified by MRI, gamma scintigraphy (bone scan) and less frequently with computed tomography (14-16). A complete fracture is a failure event that occurs when macrocracks grow into a complete fracture: evidence of a BSI on a gross level. A complete fracture is can be identified with digital radiography. A stress fracture has opportunity to become a catastrophic fracture if the stress fracture is unidentified and propagates during training or racing. MRI is the recommended advanced imaging modality in human medicine for BSI because it has the best combined specificity and sensitivity of available imaging modalities and is the only modality that can identify BME (14 -16).
The standing MRI (sMRI) was used to compare 26 horses with condylar fracture (26 fractured and 26 non-fractured limbs) with 88 horses (controls) without condylar fracture (11). Bone marrow edema (figure 1) was significantly more common in the fractured limbs verses the non-fractured limb, and in the fractured limb verses horses without condylar fracture (controls). Density in either the medial or lateral condyles (figure 2) was not significant different when comparing fractured limbs with non-fractured limbs in horses with condylar fracture (11). Density was significantly greater when fractured limbs were compared to horses without condylar fracture (controls). This study also identified that control horses were significantly more likely to have a POD lesion verses horses with a condylar fracture (11).
Palmar Osteochondral Disease and Biaxial Proximal Sesamoid Bone Fracture
Palmar osteochondral disease (POD) (6-10) and biaxial proximal sesamoid bone (PSB) fracture (17,18) are the final event of bone fatigue that began in the SCB of the PSB and MC3 condyle where the PSB oppose the stress created by the downward movement of MC3 during the stance phase of the gallop. Understanding the pathologic progression of these conditions is critical if injuries are to be identified pre-fracture and alternate training and racing recommendations instituted. Like a BSI, POD and biaxial PSB fracture are associated with material fatigue of the SCB following repetitive overload trauma in horses undergoing cyclic high intensity exercise (6-10). In the PSB’s, histomorphometric analysis identified that the bony material is more compacted in horses with biaxial PSB fracture when compared to horses without biaxial PSB fracture. The authors concluded that the early identification of these structural changes could provide an opportunity for prevention of PSB fractures (17).
In horses with POD that were evaluated using micro-CT, mild lesions were identified as bones with sclerosis and mild focal or coalescing radiolucent areas of MC3 without involvement of the articular surface (19). Severe POD lesions were defined as bones with more severe multifocal radiolucent areas traversing the subchondral bone plate and articular surface (19). Muir identified mild POD lesions containing normal articular surface in horses with grossly discolored superficial SCB containing diffuse damage with microcracks and microfractures surrounded by sclerotic trabecular bone (20). In severe POD lesions, Bani Hassan evaluated racing Thoroughbred horses using high-resolution CT and identified articular surface collapse and suggested it was sequel to fatigue injury of subchondral bone, and that focal subchondral bone resorption appears to contribute to the collapse of the calcified articular cartilage layer. Articular surface collapse with intact overlying cartilage is a feature of advanced POD (21).
Monitoring the progressive densification of the PSB’s and POD, the progressive damage to the SCB of the MC3 condyle with subsequent failure of the SCB plate, was evaluated in 21 horses with biaxial PSB fracture and 53 horses without biaxial PSB fracture using the standing MRI (18). Horses with marked densification of the PSB (figure 3) in the ipsilateral (fractured) limb were 10.4 times more likely to have a biaxial PSB fracture verses horses without marked PSB densification (18). Horses with severe POD (figure 4) of the contralateral (not fractured) limb were 20.8 times more to have a biaxial PSB fracture in the ipsilateral limb verses horses without severe POD in the contralateral limb (18).
A. METACARPAL III CONDYLAR FRACTURE
1) Bone Marrow Edema Grading System
Figure 1: The STIR MRI transverse image of a normal horse (left) and the transverse (middle) and frontal (right) image of a horse with an MC3 condylar fracture is identified above. The white arrow identifies the MC3 condylar fracture and the red arrow identifies the generalized accumulation of bone marrow edema (BME). Bone marrow edema was graded as absent (left image) or present (middle and right image). In the normal horse to the left, the medullary cavity is distinctly black due to the absence of BME and was classified as absent. In the horse with an MC3 condylar fracture (middle and right), the red arrows identify the BME and were classified as present.
2) Metacarpal Condylar (MC3) Densification Grading System
Figure 2: Surface area measurement of dense bone in the MC3 condyle is identified in the T1W sagittal image above. Using T1W sagittal images, surface area measurement of dense bone (green area) was calculated by tracing the cross section area of the condyle that had a decrease in signal intensity. This measurement was standardized by a second surface area measurement (blue area) that measured the epiphysis of MC3. The sum of the 4 medial condylar numerator measurements was divided by the sum of the 4 medial condylar denominator measurements to determine the dense bone volume percentage (DBVP) in the MC3 medial condyle. The same formula was used to calculate the DBVP of the lateral MC3 condyle.
B. BIAXIAL PROXIMAL SESAMOID BONE FRACTURE
1) Proximal Sesamoid Bones Densification Grading System
Figure 3: Representative T1W standing MRI images of 3 equine forelimb fetlock joints in the transverse plane (top row) and sagittal plane (bottom row) to describe the PSB grading system developed for a study of bony changes in the forelimbs of Thoroughbred racehorses with and without catastrophic biaxial PSB fracture (17).
1) PSB grade of 0 (panel A and B)………no densification of the PSB (i.e., normal). The signal intensity of the medullary bone is derived from a combination of porous bone filled with bone filled with fatty marrow (white) and similar in intensity to the medullary bone of the first phalanx in panel B;
2) PSB grade of 1 (not provided)…………< 50% densification of sesamoid bones;
3) PSB grade of 2 (panel C and D….…….≥ 50% to < 75% densification of sesamoid bones.
Notice most of the PSB has a hypointense (black) signal (white arrows), compared with the reference area of the first phalanx. In the transverse image (image C), the remaining fatty marrow in the PSB created a hyperintense “Y” shape (white arrows) as densification (black) front’s form on the dorsal, palmar axial and palmar abaxial aspects of the PSB.
4) PSB grade of 3 (panel E and F)…………≥ 75% densification or continuous dorsal to palmar densification identified in sagittal slices.
Notice that the PSB contains only a small amount of clinically normal bone (red arrow) because the densification and trabecular reinforcement in the marrow space (blue arrows) and the hyperintense “Y” from panel C is no longer apparent. In panel E, only 1 PSB is visible because the other PSB was fractured and retracted from view (black arrow)
C. PALMAR OSTEOCHONDRAL DISEASE (POD) GRADING SYSTEM
Figure 4—Representative T1W standing MRI images obtained in the sagittal plane from 6 different equine forelimb fetlocks is provided to describe the palmar osteochondral disease (POD) MRI grading system developed for a study of bony changes in the forelimbs of Thoroughbred racehorses with and without catastrophic biaxial proximal sesamoid bones (PSB) fracture (17). This grading system is focused on the integrity of the subchondral bone (SCB) plate in the contact area of the PSB with the palmar aspect of the MC3 condyles of the cannon (red arrows in panel A). The SCB plate (blue arrows in panel A) curves from the dorsal to the palmar aspect of the cannon bone and is a continuation of the dense cortical bone of the distal portion of the cannon bone. It creates a symmetric linear hypointense (black) signal that is sandwiched superficially between the symmetric linear hyperintense (white) signal of the articular cartilage and the deeper symmetric linear medullary trabecular bone and fatty marrow. The specifics of each panel are described below.
A: POD grade 0: The SCB plate is normal and there are no signal abnormalities in the SCB deep to the SCB plate. This is an example of an untrained (normal) horse.
B: POD grade 1. The SCB plate has normal signal intensity and remains symmetric and intact. There is an area of superficial hyperintensity immediately beneath the SCB plate (blue arrow) and an increase in densification of the medullary bone marrow (red arrows) towards the periphery of the lesion (vs., panel A). Joint fluid does not extend into the SCB plate, which implies that the overlying articular cartilage of the cannon bone is intact.
C: POD grade 2. The SCB plate has an abnormal hyperintensity and a contour deformity (red arrow) that is suggestive of focal depression or fracturing of the SCB plate or resorption or fracturing of the deeper trabecular bone. There is a progression from panel B with a larger area of superficial hyperintensity immediately beneath the SCB plate and a larger area of increase in densification of the medullary bone marrow towards the periphery of the lesion (vs., panel A and B). Joint fluid does not extend into the SCB plate, which implies that the overlying articular cartilage is intact.
D: POD grade 3. There is full-thickness defect in the SCB plate (red arrow) with a signal intensity similar to that of joint fluid, which implies that the overlying articular cartilage is not intact, and an increase in the densification of the subjacent SCB marrow. There is a progression from panel C with an increase in hyperintense superficial signal near the SCB plate and an increase in densification towards the periphery of the lesion.
E: POD grade 4. There is a partial or complete detachment of an osteochondral fragment that is not displaced (red arrows), an increase in superficial hyperintense signal and an increase in the densification of the deeper SCB marrow. In this image, the signal intensity around the fragment is not as hyperintense as that of joint fluid, which implies that the overlying articular cartilage is still intact.
F: POD grade 5. Notice that an osteochondral fragment from panel E is displaced from the original defect in this panel F, which has left a large area of exposed SCB of the MC3 cannon bone (red arrows), and there is an increase in the densification of the subjacent SCB marrow.
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[1] Peloso JG, et al. (1994) Prevalence of, and factors associated with musculoskeletal racing injuries of Thoroughbreds. J Am Vet Med Assoc, 204:620–626.
[2] Parkin TD, et al. (2004) Risk of fatal distal limb fractures among Thoroughbreds involved in the five types of racing in the United Kingdom. Vet Rec 154:493–497.
[3] Stover SM, et al. (2008) The California postmortem program: leading the way. Vet Clin North Am Equine Pract 24:21–36.
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