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Keywords:

  • horse;
  • third metacarpus;
  • lateral condyle;
  • fracture;
  • MRI

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Authors' declaration of interests
  9. Source of funding
  10. Acknowledgements
  11. Manufacturers' addresses
  12. References

Reasons for performing the study: Lateral condylar (LC) fractures of the third metacarpus (McIII) are a common reason for euthanasia in racehorses, and may be the result of repetitive overloading or cumulative pathological change. Magnetic resonance imaging (MRI) allows monitoring of bone and cartilage to detect pathological and adaptive changes that may be precursors of fracture.

Objectives: To describe bone and cartilage MRI features in the distal condyles of McIII of Thoroughbred racehorses, with and without condylar fracture.

Hypotheses: 1) A greater degree of bone and cartilage adaptation or pathology will be seen in fractured McIIIs compared with their respective contralateral McIIIs. 2) Contralateral McIIIs will have a greater degree of bone and cartilage adaptation or pathology than McIIIs from control horses that did not sustain a LC fracture.

Methods: The McIIIs from 96 horses subjected to euthanasia at racecourses were divided into 3 groups: Group 1: nonfractured bones from horses without LC fracture; Group 2: nonfractured bones from horses with unilateral LC fracture; and Group 3: fractured bones from horses with unilateral LC fracture. The MR images were examined and graded for bone and cartilage changes.

Results: Nine percent of Group 1 (n = 9) and 11% of Group 2 bones (n = 5) had incomplete LC fractures. Focal palmar necrosis was most frequently detected in bones from Group 1 (12%) compared with Groups 2 (9%) and 3 (4%). The prevalence of bone and/or cartilage abnormalities tended to increase from Group 1 to Group 2 to Group 3.

Conclusions: Magnetic resonance imaging is able to detect cartilage and bone changes that may be associated with LC fracture. There was no significant difference in bone/cartilage changes between bones from Groups 1 and 2, despite increased pathology in Group 2 bones.

Potential relevance: Periodic monitoring of bone and/or cartilage changes in distal McIII of Thoroughbred racehorses may help to prevent catastrophic LC fractures.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Authors' declaration of interests
  9. Source of funding
  10. Acknowledgements
  11. Manufacturers' addresses
  12. References

Lateral condylar (LC) fractures of the third metacarpus (McIII) are the most common reason for euthanasia on UK racecourses and are associated with 44% (150/340) of all fatal distal limb fractures that occur during racing in the UK [1,2]. In the USA, the lateral condyle of McIII is the second most common site of catastrophic fracture [3]. Catastrophic LC fractures typically occur during high-speed exercise [4–6] but there is evidence suggesting that they are the result of pre-existing pathological defects [7–10]. Underlying bone pathology has been associated with damage at the level of the articular surface in fetlock joints of racehorses [11].

Magnetic resonance imaging (MRI) is increasingly being used in the examination of the equine limb for diagnosis of lameness [12]. With improvements in technology allowing acquisition of scans in the standing horse, there is potential opportunity to use MRI as a screening tool to monitor bone and cartilage changes over time to detect prefracture pathology and prevent catastrophic LC fracture in racehorses. However, to do this, it is important to identify which MR changes are associated with, and potentially predictive for, catastrophic LC fracture. In man, MRI is routinely used in the diagnosis of osteochondral lesions and fracture [13–16].

Contralateral limbs from horses with a unilateral fracture experience the same loading environment, genetic makeup and other environmental factors as the fractured limb, so it could be expected that early pathological changes would be similar to but potentially less severe than changes seen in the fractured limb. There is limited evidence of pathological symmetry in unsound horses. However, a number of studies have demonstrated symmetry in sound horses [17–20].

The aim of this study was to identify MRI-detectable factors associated with catastrophic LC fracture by comparing MRI findings in limbs with LC fracture, the contralateral nonfractured limbs from these horses and control limbs from horses with a similar exercise history but subjected to euthanasia on the racecourse for other reasons.

It was hypothesised that: 1) a greater degree of bone and cartilage adaptation or pathology will be seen in fractured McIIIs compared with their respective contralateral McIIIs; and 2) contralateral McIIIs will have a greater degree of bone and cartilage adaptation or pathology than McIIIs from control horses that did not sustain a LC fracture.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Authors' declaration of interests
  9. Source of funding
  10. Acknowledgements
  11. Manufacturers' addresses
  12. References

Sample collection

One-hundred-and-ninety-two McIIIs were collected from 96 Thoroughbred horses (70 males and 26 females) aged 2–11 years. Bones were collected from 3 different groups of horse: Group 1 included 98 nonfractured McIIIs from 49 horses that were subjected to euthanasia on the racecourse for reasons other than LC fracture (cardiovascular catastrophe [n = 22] or neck/back fracture [n = 27]); Group 2 included apparently nonfractured contralateral McIIIs from 47 horses that had been subject to euthanasia following unilateral catastrophic LC fracture while racing; and Group 3 included all fractured McIIIs from the same 47 horses in Group 2. All limbs were collected from horses that were subjected to euthanasia at UK racecourses between 1999 and 2005, as part of a Horserace Betting Levy Board funded study, conducted at the University of Liverpool [1,2]. One bone from Group 2 was identified as having a screw in place in distal McIII, which prevented MRI. Therefore 98 bones from Group 1, 46 bones from Group 2 and 47 bones from Group 3 were imaged and analysed during this study. Limbs were examined within 36 h of death, and McIIIs were dissected away from the rest of the limb, wrapped in moist paper and cling film and stored frozen at -20°C.

High-field MRI

The bones were defrosted at room temperature for 24 h prior to scanning. The distal condyles of the McIIIs were examined using a human extremity radiofrequency coil, with the bones positioned in the isocentre of a 1.5 Tesla GE Signa Echospeed MRI systema and scanned with a standardised protocol. Sagittal, dorsal and transverse MR images were obtained using 3D T1-weighted spoiled gradient echo (SPGR), 3D T2*-weighted gradient echo (GRE) and short tau inversion recovery (STIR) scans. The scan parameters used were the same as those used in a clinical examination (Table 1). To obtain sufficient signal and fat/water peak separation from the isolated bones, it was necessary to wrap the bones in modelling clay and surround them with vegetable oil filled bags for the fat saturated images, to simulate soft tissue coverage.

Table 1. Magnetic resonance imaging (MRI) parameters used in the high- and low-field systems
ScanTETR
  1. TE = echo delay time; TR = repetition time; SPGR = spoiled gradient echo; GRE = gradient echo; STIR = short tau inversion recovery; FSE = fast spin echo.

High-field MRI  
 T1 SPGR3.78.9
 T2* GRE3.710.1
 STIR13.610,500.0
Low-field MRI  
 T1W GRE850
 T2*W GRE1370
 STIR FSE221,700
 T2W FSE881,400

Image analysis

A grading system was developed to assess bone and cartilage changes (Table 2). Bone shape abnormality was determined using sagittal, dorsal and transverse images. This was to ensure that the shape was representative in all 3 planes. Focal palmar necrosis lesions were defined as focal high signal intensity in all image sequences, in the palmarodistal aspect of McIII at approximately 35° to the longitudinal axis, extending into the subchondral and trabecular bone. This area of high signal intensity was surrounded by an area of low signal intensity. The operator was blinded to the group status of all nonfractured bones. It was obviously not possible to blind the status of the fractured bones.

Table 2. The grading system used for magnetic resonance image assessment
FactorScale
0123456
  1. STIR = short tau inversion recovery.

Bone grading on T1 and T2* images       
 Signal intensityHighGrey/mediumLow    
 Signal homogeneity HomogeneousHeterogeneous    
 FractureNoneIncompleteComplete nondisplacedComplete displacedComminuted  
 Focal/generalised abnormalityNoneFocalGeneralised    
 Shape of abnormalityNoneLinearRoundSaucer/ovalTriangularWhole condyleOther
 Margin of abnormalityNoneSmoothIrregular    
Bone grading on STIR images       
 Signal intensityLowGrey/mediumHigh    
 Signal homogeneity HomogeneousHeterogeneous    
 FractureNoneIncompleteComplete nondisplacedComplete displacedComminuted  
 Focal/generalised abnormalityNoneFocalGeneralised    
 Shape of abnormalityNoneLinearRoundSaucer/ovalTriangularWhole condyleOther
 Margin of abnormalityNoneSmoothIrregular    
Cartilage grading on T1 scans       
 Signal intensityHighGrey/mediumLow    
 Signal homogeneity HomogeneousHeterogeneous    
 Surface appearanceSmoothIrregularDefect    
 Defect typeNoneFocalMultiple focalGeneralised   
 Defect shapeNoneOvalTriangularSquare   
 Width of defectNone<1 mm1–2 mm>2 mm   
 Depth of defectNoneSuperficialDeep onlyFull thickness   

Low-field MRI

Bones with suspected incomplete LC fractures also underwent low-field MRI in a 0.27 Tesla Hallmarq MR imaging systemb. As with the high-field MRI, the bones were wrapped in modelling clay and bags of vegetable cooking oil to create sufficient signal and fat/water peak separation in the absence of soft tissue coverage. Sagittal, dorsal and transverse MR images were obtained using T1-weighted GRE, T2*-weighted GRE, STIR fast spin echo (FSE) and T2-weighted FSE scans. The protocol that was selected replicated a clinical examination (Table 1).

High-resolution radiography

Bones with suspected incomplete LC fractures also underwent high-resolution radiography to confirm the presence of an incomplete fracture. Radiographic examination was performed using a Siemens Polydoros 150 kV 1250 mA tubec at an exposure of 68 kVp and 16 mAs to create high-resolution images. Four views were obtained: lateromedial, dorsolateral-palmaromedial oblique, dorsomedial-palmarolateral oblique and dorsopalmar to replicate the views obtained in a routine clinical examination.

Data analysis

Descriptive statistics were used to investigate the distribution of MRI-detected adaptation or pathology using statistical softwared. The proportion of bones in each group that demonstrated different adaptive or pathological changes were compared using Chi-squared or Fisher's exact tests, where appropriate, with the significance level set at P<0.05.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Authors' declaration of interests
  9. Source of funding
  10. Acknowledgements
  11. Manufacturers' addresses
  12. References

The difference in mean ages of the 47 case horses (6.1 years) and of the control horses in Group 1 (6.7 years) was not statistically significant (P = 0.1). Of the case horses, 82% (39) were geldings and 18% (8) were female. Of the horses in Group 1, 75% (37) were geldings and 25% (12) were female. There was no statistically significant difference in the proportions of each gender between case and control horses (P = 0.54). At the time of fracture 17% (8) of case horses were racing in a flat race, 34% (16) in a hurdle race, 43% (20) in a steeplechase race and 6% (3) in a National Hunt flat race. At the time of death or euthanasia 10% (5) Group 1 horses were racing in a flat race, 43% (21) in a hurdle race, 41% (20) in a steeplechase race and 6% (3) in a National Hunt flat race. There was no statistically significant difference in the proportions of horses that were racing in flat or National Hunt races (P = 0.33).

High-field MR image analysis

The findings on high-field MR image analysis are summarised in Table 3. One bone (1/47:2%) from Group 3 had a complete nondisplaced LC fracture and 98% (46/47) of bones had complete displaced LC fractures; 19 of these complete fractures were from left McIIIs and 28 were from right McIIIs. Nine percent of bones in Group 1 (9/98; 5 left McIIIs and 4 right McIIIs) and 11% of bones in Group 2 (5/46; 2 left McIIIs and 3 right McIIIs) had incomplete LC fractures. None of the bones in Groups 1 or 2 had evidence of medial condylar fracture, but 11% (5/47) of bones in Group 3 had incomplete medial condylar fractures, as well as complete LC fractures (Fisher's exact P<0.001) (Fig 1).

Table 3. Results of the high-field magnetic resonance image assessment showing the number of bones in each group detected as having specific features
Factor Group
1 (n = 98) 2 * (n = 46) 3 (n = 47)
MedLatMedLatMedLat
T1 and T2* scansn (%)n (%)n (%)n (%)n (%)n (%)
  • *

    One bone from Group 2 had previously had all cartilage removed. Med = medial; Lat = lateral.

Bone appearance and signal intensity      
 1. Heterogeneous, dark/low39 (40)27 (28)22 (48)26 (57)28 (60)45 (96)
 2. Heterogeneous, grey/medium49 (50)63 (64)24 (52)20 (43)16 (34)2 (4)
 3. Heterogeneous, bright/high6 (6)5 (5)001 (2)0
 4. Homogeneous, dark/low2 (2)00000
 5. Homogeneous, grey/medium2 (2)3 (3)0000
 6. Homogeneous, bright/high00002 (4)0
Bone abnormality shape      
 1. None13 (13)00
 2. Linear7 (7)3 (7)1 (2)
 3. Round12 (12)4 (9)2 (4)
 4. Saucer/oval14 (14)3 (7)4 (9)
 5. Triangular20 (20)13 (28)0
 6. Whole condyle13 (13)12 (26)31 (66)
 7. Other19 (19)11 (24)9 (19)
Cartilage appearance and signal intensity   
 1. Heterogeneous, dark/low008 (17)
 2. Heterogeneous, grey/medium30 (31)18 (39)32 (68)
 3. Heterogeneous, bright/high41 (42)14 (30)7 (15)
 4. Homogeneous, dark/low000
 5. Homogeneous, grey/medium2 (2)3 (7)0
 6. Homogeneous, bright/high25 (26)10 (22)0
Cartilage appearance and defects   
 1. Smooth4 (4)1 (2)0
 2. Irregular63 (64)23 (50)0
 3. Focal defects   
  Superficial5 (5)8 (17)0
  Deep01 (2)0
  Full thickness8 (8)3 (7)5 (11)
 4. Multi-focal   
  Superficial4 (4)2 (4)0
  Deep000
  Full thickness13 (13)7 (15)33 (70)
 5. Generalised   
  Superficial000
  Deep000
  Full thickness1 (1)09 (19)
STIRMedLatMedLatMedLat
Signal intensity changes      
 1. Increase5 (5)6 (6)1 (2)4 (9)2 (4)22 (47)
 2. Decrease41 (42)46 (47)12 (26)12 (26)7 (15)1 (2)
 3. No change52 (53)46 (47)33 (72)30 (65)38 (81)24 (51)
GeneralMedLatMedLatMedLat
Fracture type      
 1. No fracture98 (100)89 (91)46 (100)41 (89)42 (89)0
 2. Incomplete condylar fracture09 (9)05 (11)5 (11)0
 3. Complete condylar fracture0000047 (100)
  a. Displaced     46 (98)
  b. Nondisplaced     1 (2)
Focal palmar necrosis      
 1. Yes5 (5)7 (7)3 (7)1 (2)2 (4)0 (0)
 2. No93 (95)91 (93)43 (93)45 (98)45 (96)47 (100)
image

Figure 1. Images of a Group 2 bone with an incomplete lateral condylar fracture (arrow). There is disruption at the articular surface. a) High-field dorsal T1 spoiled gradient echo magnetic resonance (MR) image. b) Low-field dorsal T1W gradient echo MR image. (c) High-resolution dorsopalmar radiograph.

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Focal palmar necrosis was defined as focal high signal intensity in all image sequences, in the palmarodistal aspect of McIII at approximately 35° to the longitudinal axis, extending into the subchondral and trabecular bone. This area of high signal intensity was surrounded in all cases by a rounded or triangular shaped area of low signal intensity. There was frequently an indentation into the articular surface at this location in bones that exhibited these changes in Groups 1 (75%; 9/12) and 2 (50%; 2/4). Focal palmar necrosis was found in either condyle, but in the lateral condyle there was a trend for it to be more frequently detected in the lateral condyle of bones from Group 1 (7%; 7/98) and Group 2 (2%; 1/46) than Group 3 (0%; 0/47). Focal palmar necrosis in either condyle was more frequently detected in bones from Group 1 (12%; 12/98) and Group 2 (9%; 4/46) than bones from Group 3 (4%; 2/47) (Fig 2). However, there were statistically significant differences in these proportions.

image

Figure 2. High-field sagittal T1 spoiled gradient echo magnetic resonance images of 2 different Group 1 bones with focal palmar necrosis (arrow) in the medial condyle. a) There is a triangular low signal intensity area surrounding a smooth rounded focus of high signal intensity adjacent to the cartilage. There is no indentation of the articular surface. This lesion was graded as mild. b) There is a triangular low signal intensity region surrounding an irregular heterogeneous high signal intensity area adjacent to the cartilage invagination. This lesion was graded as severe.

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T1 and T2 scans

Trabecular bone appearance and signal intensity: At least 90% of bones (in both lateral and medial condyles) in all of the 3 groups had either ‘heterogenous, dark/low’ or ‘heterogenous, grey/medium’ appearance. (Fig 3). In the lateral condyles, a ‘heterogeneous, dark/low’ appearance was significantly more common than a ‘heterogeneous grey/medium’ appearance in bones from Group 3 (Fig 4) compared with bones from Group 1 (P<0.001), or bones from Group 2 (P<0.001), or bones from Groups 1 and 2 combined (P<0.001). In addition, bones from Group 2 were more likely to have a ‘heterogeneous dark/low’ appearance than bones from Group 1 (P = 0.005). In the medial condyles, none of the equivalent comparisons were statistically significant (Group 3: compared with Group 1 P = 0.06; compared to Group 2 P = 0.19; compared with Groups 1 and 2 combined P = 0.06).

image

Figure 3. High-field dorsal T1 spoiled gradient echo magnetic resonance (MR) image of a Group 1 bone with no MR detected changes.

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image

Figure 4. High-field dorsal T1 spoiled gradient echo magnetic resonance image of a Group 3 bone with a complete lateral condylar fracture. The lateral condyle has a ‘heterogeneous, dark/low’ appearance whereas the medial condyle has a ‘heterogeneous grey/medium’ appearance.

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Trabecular bone abnormality shape: All of the bones from Groups 2 or 3 had a bone abnormality detected whereas 13% (13/98) of bones from Group 1 had no bone abnormality detected (Fisher's exact P<0.001). In bones from Group 1 there was no bone abnormality shape that was predominant. In bones from Group 2‘triangular’ (28%; 13/46) (Fig 5) or ‘whole condyle’ (26%; 12/46) (Fig 6) bone abnormality shapes were more common and in bones from Group 3 the majority of bones demonstrated a bone abnormality shape that included the ‘whole condyle’ (66%; 31/47). Bones from Group 3 were significantly more likely to demonstrate a bone abnormality appearance that included the whole condyle, compared with bones from Group 1 (P<0.001), from Group 2 (P<0.001), or bones from Groups 1 and 2 combined (P<0.001). There was no statistically significant difference in the likelihood of a whole condyle bone abnormality in bones from Group 1 compared with bones from Group 2 (P = 0.09).

image

Figure 5. High-field dorsal T1 spoiled gradient echo magnetic resonance image of a Group 2 bone with a low signal intensity triangular shaped bone reaction in both condyles. The reaction in the lateral condyle (on the left of the image) is more pronounced than that of the medial condyle.

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image

Figure 6. High-field dorsal T1 spoiled gradient echo magnetic resonance image of a Group 2 bone with low signal intensity bone reaction spanning both condyles. Decreased signal intensity extends to the articular surface in the lateral condyle but is less extensive in the medial condyle.

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Cartilage appearance and signal intensity: The majority of bones had a heterogeneous cartilage appearance (72% of Group 1, 71% of Group 2 and 100% of Group 3). A heterogeneous appearance was significantly more common than a homogenous appearance in bones from Group 3 compared with bones from Groups 1 or 2 (P<0.001). More specifically, a heterogeneous, grey/medium appearance was more common than other heterogeneous appearances in bones from Group 3 compared with bones from Groups 1 or 2 (P = 0.02). A homogeneous appearance (the majority of which were ‘bright/high’) was only seen in Groups 1 or 2. A heterogeneous bright/high appearance was significantly more common than other appearances in bones from Group 1 or 2 compared with bones from Group 3 (P<0.001).

Cartilage appearance and defects: Interpretation of cartilage appearance and defects for bones from group 3 was difficult due to the large amount of post fracture trauma. In Group 1, 66% (65/98) of the bones had an irregular cartilage surface and 34% (33/98) had either superficial or full thickness cartilage defects. In bones from Group 2, 50% (23/46) had an irregular cartilage surface appearance and 50% (23/46) had superficial or full thickness cartilage defects. All bones (47/47) in Group 3 had full thickness cartilage defects.

STIR images

Signal intensity changes: In the lateral condyle, an increase in signal intensity was observed in 5% (5/98) of bones in Group 1, 9% (4/46) of bones in Group 2 and in 47% (22/47) of bones in Group 3. An increase in signal intensity in the lateral condyle was significantly more common than a decrease or no change in signal intensity in bones from Group 3 compared with bones from Group 1 (P<0.001) or Group 2 (P<0.001) or Groups 1 and 2 combined (P<0.001). In the medial condyle, an increase in signal intensity was seen in 4% (4/98) of bones in Group 1, 2% (1/46) of bones in Group 2 and in 4% (2/47) of bones in Group 3. In the medial condyle the equivalent analytical comparisons were not statistically significant (Fisher's exact P = 1.0).

Low-field MR image analysis

All of the 14 bones that were detected with an incomplete LC fracture line using the high-field MRI scanner were also identified as having incomplete LC fracture lines when scanned using the low-field MRI scanner. In addition, signal intensity patterns in the trabecular bone and cartilage, were identical to those seen on the high-field images.

Radiological analysis

Incomplete LC fracture lines were detected in 100% (14/14) of the bones that underwent radiographic examination. Features seen included disruption of the articular surface, regions of increased mineralisation (sclerosis) within the trabecular bone, linear lucency, cortical thickening and endosteal irregularity at the abaxial margin of the affected bone.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Authors' declaration of interests
  9. Source of funding
  10. Acknowledgements
  11. Manufacturers' addresses
  12. References

The results of this study support the hypotheses tested. In general, the degree of pathological or adaptive change increased from bones in Group 1, through bones in Group 2 to bones in Group 3. Such changes were significantly more frequently detected when comparing Group 3 with Groups 1 and 2 (and Groups 1 and 2 were not significantly different) suggesting that these are bone-level changes. Other changes were more common in Group 3 compared with Groups 1 and 2, but also more common in Group 2 compared with Group 1, suggesting that some changes may be regarded as being both bone- and horse-level adaptations or pathology.

Incomplete medial condylar fractures were more common in LC fractured bones (Group 3) than nonfractured bones from either Group 1 or 2. This suggests that some bones are more susceptible to fracture than others either in the same horse or a different horse. The fact that incomplete LC fractures were also detected in 9% and 11% of bones from Groups 1 and 2, respectively, and that none of these fractures were identified in the same horses (i.e. none from Group 1 were bilateral), also suggests that individual bones are susceptible to fracture rather than individual horses. This is despite the fact that bones from the same horse are exposed to the same training and management regimens. The distribution between left and right limb catastrophic and incomplete LC fracture was not significantly different, supporting the suggestion of bone rather than horse susceptibility to fracture. Horses can use their limbs differently either due to ‘handedness’ or conformation, which could also be a contributory factor in bone-level changes [21–23].

Three other pathological/adaptive changes were more common in Group 3 than Groups 1 and 2, but not more common in Group 2 than Group 1. These were the shape of detected bone abnormalities, a heterogeneous cartilage appearance and increased signal intensity of the lateral condyle on STIR images. The correlation between full thickness cartilage changes in the different MR systems was not assessed. However, they are thought to be true findings as a number of them underwent histological assessment and features seen histologically correlate with what was described on MR. It is not clear if these changes are adaptive or pathological responses to loading, although there is evidence that a certain amount of it is adaptive [24,25]. It has previously been shown that cartilage and subchondral bone thickness are related to loading patterns [26–34] and it could be assumed that trabecular bone thickness patterns would change in the same way. The altered signal intensity seen on STIR and T2* GRE sequences has previously been described as a reflection of bone trauma in horses [35–39] and in man [40–44] that could represent cumulative microdamage.

The percentage of bones with heterogeneous dark/low appearance on T1- and T2*-weighted images increased from Group 1, to Group 2 to Group 3. This appearance was significantly more common in Group 3 compared with Groups 1 and 2 and also more common in Group 2 compared with Group 1. This suggests that this appearance may be both a bone- and horse-level pathological or adaptive change, associated with complete LC fracture. A reduced signal intensity in both T1- and T2*-weighted images has been reported in clinical cases with lameness associated with bone trauma in the fetlock joint [35,39,45,46].

It is interesting to note that the statistically significant differences on T1-, T2*-weighted and STIR images were apparent in the lateral condyles, but not the medial condyles of distal McIII. This perhaps provides greater evidence that such changes are truly associated with fracture and not merely coincidental findings that could develop independently of fracture risk but as a consequence of the same training regimen.

Lesions classified as focal palmar necrosis had MRI findings that were indicative of a focal area of higher proton mobility than undamaged bone, consistent with abnormality that could include bone necrosis, proteinaceous fluid accumulation, cartilage thickening and infolding into the subchondral bone. The surrounding low signal intensity on all image sequences would be consistent with increased bone density and loss of medullary fat. Focal palmar necrosis has been reported as a post mortem finding and on MR evaluation of racehorses [39,47–52]. Previously, osteonecrosis has been shown to be caused by a variety of factors in man. These include: alcoholism, excessive steroid use, disease, trauma, increased bone pressure, vascular compression or damage [53,54]. Magnetic resonance imaging is considered the most sensitive imaging tool for diagnosing osteonecrosis at an early stage [55,56] but further histological assessment is required to establish the bone alterations at these locations.

Although no statistically significant differences in the prevalence of focal palmar necrosis were identified, the frequency decreased from Group 1, to Group 2 to Group 3, being more common in nonfractured bones. This raises the interesting possibility that focal palmar necrosis may be protective against LC fracture. Perhaps it is a pathological process that is not severe enough to prevent the horse from training and racing but is sufficient to prevent the horse from exercising to maximal capacity. If exercise at maximal capacity is associated with an increased risk of LC fracture, low grade damage or pain that prevents maximal exercise may prevent the development of LC prefracture pathology. In support of this hypothesis is the fact that this pathology has often been identified as an incidental finding on post mortem examination of racehorses subjected to euthanasia for unrelated reasons [47,49]. These horses were apparently racing and training without problem even in the presence of focal palmar necrosis and other condylar pathology.

The results from this study suggest that there are certain MR features associated with LC fracture. An increase in signal intensity on STIR sequences and a decrease on T1- and T2*-weighted sequences spanning over both condyles or in a triangular shape were associated with LC fracture, so could potentially be warning signs for fracture. If a horse underwent MRI and multiple of these changes were detected then its training regimen could be altered, based on previous epidemiological studies [1,2,47], potentially reducing prevalence of fracture.

Although this is the largest study of its kind to date, there are some limitations. A small number of bones had been thawed and frozen more than twice prior to MRI, which may have led to MR artifact due to crystal formation in the bone spaces [57,58]. The study was not longitudinal in nature, so all the measurements and descriptions only represent a snapshot in time. However, clinical experience with repeated MRI in racehorses with condylar pathology clearly demonstrates that there is reduction in STIR signal intensity and reduction in the area of low signal intensity on T1 and T2 GRE sequences following rest and in association with resolution of an incomplete fracture line [45,59]. A future longitudinal study to assess pathological or adaptive changes over time is required to determine the progression or improvement in association with altered training regimens.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Authors' declaration of interests
  9. Source of funding
  10. Acknowledgements
  11. Manufacturers' addresses
  12. References

Magnetic resonance imaging is able to detect cartilage changes and subchondral and trabecular bone changes associated with LC fracture of the distal condyles of McIII. The results from this study may in the future allow veterinarians to carry out screening examinations of at risk horses prior to the development of catastrophic LC fracture.

Source of funding

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Authors' declaration of interests
  9. Source of funding
  10. Acknowledgements
  11. Manufacturers' addresses
  12. References

The project was funded by The Horserace Betting Levy Board and partial funding of the collection of control horse limbs by the University of Liverpool.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Authors' declaration of interests
  9. Source of funding
  10. Acknowledgements
  11. Manufacturers' addresses
  12. References

The authors would like to thank the Jockey Club, the British Horseracing Authority and the racecourse veterinary surgeons who assisted in providing the samples, Dr Meredith Smith for assisting with the low-field MRI and Miss Nia Turley for carrying out the radiography.

Manufacturers' addresses

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Authors' declaration of interests
  9. Source of funding
  10. Acknowledgements
  11. Manufacturers' addresses
  12. References

a General Electric, Milwaukee, Wisconsin, USA.

b Hallmarq Veterinary Imaging Ltd, Guildford, Surrey, UK.

c Siemens Medical Solutions, Bracknell, Berkshire, UK.

d Analyse-It Software Ltd, Leeds, West Yorkshire, UK.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Authors' declaration of interests
  9. Source of funding
  10. Acknowledgements
  11. Manufacturers' addresses
  12. References
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