Figure 1.
Flow diagram for limb inclusion and exclusion.
Of the 152 condyles from 76 limbs horses initially screened, 38 condyles with naturally occurring subchondral fatigue cracks were ultimately studied in detail.
Figure 2.
Subchondral fatigue crack dimensions were measured in transverse (A) and reconstructed frontal plane (B) computed tomography (CT) images. In addition, the area of the crack array was measured in the parasagittal plane (C).
Figure 3.
Photograph illustrating the custom-made jig that secures the distal end of the MC3 bone to the platen of a materials testing machine (MTS 858, Minneapolis, MN).
(A) Drill holes 1.25 mm in diameter were made on the lateral and medial side of the artificial slot or crack array in the parasagittal condylar groove. (B) Hypodermic needles (18 g) were then placed in the drill holes and attached to an extensometer to measure motion across the slot or crack. The actuator consisted of a metal rod that was contoured to conform to the curved surface of the condyle.
Figure 4.
Representative computed tomography images of the distal end of the third metacarpal bone illustrating variation in appearance of parasagittal subchondral crack arrays in the condylar grooves of racing Thoroughbreds.
(A) A large subchondral crack is evident within the lateral condylar groove. The crack is evident in all three planes. (B) In this horse, a small radiolucency is evident in the subchondral plate in the condylar groove (arrows) associated with the presence of a smaller array of fatigue cracks in the transverse and frontal plane images (arrows). (C) In this horse, a small well-defined crack is present in the subchondral bone of the medial condylar groove (arrow), which was associated with a large parasagittal crack area that exceeded 30 mm2. Fracture of the adjacent proximal sesamoid is also evident.
Figure 5.
Representative images of the distal end of the third metacarpal bone illustrating the appearance of a large parasagittal subchondral crack array in the lateral condylar groove of the joint surface (arrows) evident on transverse (A), frontal (B), and parasagittal reconstructed computed tomography images (C). A large array of subchondral cracks is present in the subchondral bone of lateral condylar groove (arrows) with relatively little change in the overlying cartilage (D, E). An array of fatigue cracks that extend into the proximal part of the subchondral plate is also evident (arrows) on a microradiograph of an oblique frontal bone section (F).
Figure 6.
Representative images of the distal end of the third metacarpal bone illustrating articular saucer stress fracture formation in a racing Thoroughbred horse with severe palmar osteochondral disease.
(A) In reconstructed computed tomography images, a circular articular saucer fracture is present in both condyles. A halo of reduced bone density adjacent to the saucer fracture is caused by the reparative remodeling response in the subchondral plate. Extensive adaptive subchondral sclerosis is also evident. (B) Hyaline cartilage overlying the stress fracture has been replaced by fibrocartilage. (C) Extensive fatigue damage to the underlying subchondral plate is also evident, including parasagittal cracks in the condylar grooves (arrows). (D) Propagation of the fracture line at the interface of the vascular remodeling response to the fatigue injury in the subchondral plate is evident (arrows) on a microradiograph of an oblique frontal bone section.
Table 1.
Distribution of fetlock joint abnormalities in Thoroughbred racehorses identified by radiographic imaging and direct observation.
Table 2.
Identification of parasagittal subchondral crack arrays in the distal end of the third metacarpal bone of Thoroughbred racehorses by computed tomography and direct observation.
Figure 7.
Representative images of the distal end of the third metacarpal bone before and after cartilage digestion illustrating fetlock abnormalities commonly associated with the presence of subchondral fatigue crack arrays.
Extensive wear lines in hyaline cartilage (A), subchondral bruising (A–C) and flattening of the palmar region of the condyles (C) were commonly found. In some horses parasagittal defects in the hyaline articular cartilage of the condylar grooves were of mild severity and associated with varying degrees of fatigue cracking of the underlying subchondral plate (arrows) (A–C). Palmar osteochondral disease was commonly associated with condylar groove fatigue cracks (* in C). Images obtained from the same horses as Figure 4.
Figure 8.
In our ex-vivo biomechanical model, extensometer micromotion associated with creation of shallow parasagittal slots in the palmar region of the condylar groove was used to model naturally occurring arrays of fatigue cracks.
In both the 3-corrected extensometer motion was significantly different from zero (no motion) (p<0.001). Increased slot depth was associated with increased micromotion. In our model, the condyle was loaded at −7,500 N in compression. In the scatterplot, the horizontal bar represents the median value for each group.
Figure 9.
In our ex-vivo biomechanical model, extensometer micromotion associated with naturally occurring parasagittal fatigue cracks in the palmar region of the condylar groove was also determined.
(A) There was a significant correlation between extensometer motion and parasagittal crack area measured from reconstructed computed tomographic images (SR = 0.32, p<0.05). (B) Extensometer motion corrected for baseline. Micromotion 15% above baseline was detected in 18 of 38 condyles (47%).
Table 3.
Relationship between computed tomography measurement of parasagittal fatigue crack dimensions and detection of crack micromotion during mechanical loading.
Table 4.
Relationship between athletic history and severity of pathologic change observed in the joint surface of the distal end of the third metacarpal of racing Thoroughbreds.