- Email: [email protected]
Bilateral Catastrophic Metacarpal Fractures in a Quarter Horse Gelding
Journal of Equine Veterinary Science 44 (2016) 17–20
Contents lists available at ScienceDirect
Journal of Equine Veterinary Science journal homepage: www.j-evs.com
Case Report
Bilateral Catastrophic Metacarpal Fractures in a Quarter Horse Gelding Robert Cole a, *, Robyn Wilborn a, Alex Gillen a, Joe Newton b, Heather Walz c a
Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL Department of Pathobiology, Auburn University, Auburn, AL c Alabama State Diagnostic Laboratory, Auburn, AL b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2016 Received in revised form 24 February 2016 Accepted 24 February 2016 Available online 27 May 2016
A 5 year-old, 450 kg, American Quarter Horse gelding was presented for postmortem examination. The horse had sustained multiple fractures of the metacarpal bones of both fore limbs while being ridden at a canter. Based on the radiographic presence of parasagittal fractures in the condyles of the distal third metacarpus, we theorized that the fractures in this horse likely originated in the metacarpophalangeal joint and propagated into the diaphysis resulting in catastrophic failure. The unique finding in this case is that these injuries occurred in a horse that was not being trained for racing. Owners and trainers should be aware that horses trained for disciplines other than racing can develop condylar fractures similar to racing Thoroughbreds and Quarter Horses. Ó 2016 Elsevier Inc. All rights reserved.
Keywords: Catastrophic Metacarpal Fractures Quarter Horse
1. Introduction The carcass of a 5-year-old, 450 kg, American Quarter Horse gelding was presented to the Alabama State Diagnostic Laboratory for postmortem examination. The horse had sustained multiple fractures of the metacarpal bones of both fore limbs while being ridden at a canter on a hardpacked, dirt road. The rider was thrown from the horse as it fell. The colt was found sternally recumbent and unable to rise because both third metacarpal bones were fractured. The horse was immediately euthanized by gunshot. The horse had been in training for pleasure riding for 20 days but had been ridden for 1 hour approximately twice per week by the owner since being broken for riding 2 years previously. During its 20 days with the trainer, the horse was ridden in sets of walk, trot, and canter across the same ground for approximately an hour daily, 5 days a week, and then taken on a 2.5- to 3-hour trail ride during the
* Corresponding author at: Robert Cole, Department of Clinical Sciences, Auburn University, Auburn, AL 36849. E-mail address: [email protected] (R. Cole). 0737-0806/$ – see front matter Ó 2016 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jevs.2016.02.235
weekends. The horse was shod for the first time 10 days before being injured. The rider, who weighed 68 kg, reported that the horse did not seem to trip or stumble, but instead “fell out” from beneath him at a canter. Neither the owner nor the rider had observed lameness before the incident. During examination of orthogonal radiographs of the metacarpi (Fig. 1), severely comminuted distal diaphyseal fractures of both second, third, and fourth metacarpal bones were observed. In the left forelimb, a fracture extended from the medial aspect of the midsagittal ridge through the medial condyle proximally toward the comminuted diaphyseal fracture. The medial and lateral condyles of the right forelimb were fractured in multiple planes that radiated proximally into the comminuted segment. The fracture of this limb spiraled proximally into the carpometacarpal joint. The trabecular pattern appeared to be normal in the intact portions of the third metacarpal bones and in the proximal phalanges. No periosteal new bone or evidence of change in the bone density was identified radiographically. The proximal phalanges were radiographically normal. The suspensory ligaments were
18
R. Cole et al. / Journal of Equine Veterinary Science 44 (2016) 17–20
Fig. 1. Distal diaphyseal fractures in the third metacarpal bones were severely comminuted. A spiral fracture extended proximally into the carpometacarpal joint of the right forelimb. Fissure lines were seen extending into both metacarpophalangeal joints. The overall trabecular pattern appeared to be normal in the intact portions of the third metacarpal bones and in the proximal phalanges. No periosteal changes were identified, and no changes in bone density were visible.
intact and visually normal. A cortical fragment taken from a fracture site in each of the third metacarpal bones was demineralized, and sections processed for standard paraffin embedding. Slides were stained with H and E according to standard methods (Kiernan 2008). The sections of bone were histologically normal. 2. Discussion Few apparently spontaneous fractures occurring in athletic horses are actually spontaneous. Instead, nearly all are the end result of progressive deterioration of focal areas of the skeleton from repetitive injury [1]. Predilection sites for repetitive injuries (i.e., stress or fatigue injuries) of the third metacarpal bone include the dorsomedial or dorsolateral cortex (types 1 and 2 dorsal metacarpal disease, respectively) [2], the palmaroproximal cortex [3], the palmarolateral aspect of the distal end of the diaphysis [4], the third metacarpal condyles [5–7], and the distal third of the metacarpus [8]. Most often these stress injuries are reported to occur in the racing Thoroughbreds, and in nearly all cases, lameness, rather than catastrophic injury, is the result. This case is unusual in that the horse was an American Quarter Horse being trained for pleasure riding. Exercise programs affect the shape and substance of bone and influence the likelihood of a horse developing a
stress fracture of the metacarpus [9]. Most metacarpal stress fractures are reported to occur in horses trained at high speeds [1]. Two types of dorsal metacarpal disease are recognized: type 1 or bucked shins, which occurs in 2-yearold horses and is often associated with lameness and radiographic evidence of a subperiosteal callus at the dorsal cortex of the third metacarpal bone; and type 2, which occurs in older (3 to 5 years old) horses and is characterized by incomplete, oblique fracture of the dorsolateral cortex of the third metacarpal bone [10]. A number of experiments in racehorses lead Nunamaker [9] to conclude that, whereas high-speed exercise decreases the incidence of type 1 dorsal metacarpal disease, long galloping exercise increases the risk for this disease. Nunamaker [9] also concluded that horses that do not develop bucked shins (i.e., type 1 dorsal metacarpal disease) do not develop stress fractures of the dorsal cortex (i.e., type 2 dorsal metacarpal disease) and, in addition, that horses that do not develop these cortical stress fractures do not develop midshaft fractures of the third metacarpal bone. Because the horse had no history of lameness and had no radiographic evidence of periostitis seen with type 1 dorsal metacarpal disease, dorsal cortical fractures seem to be an unlikely cause of the injuries seen in this horse. The transverse metacarpal stress fracture reported by Ramzan (2009) and the distal third palmar metacarpal cortical fracture reported by O’Sullivan and
R. Cole et al. / Journal of Equine Veterinary Science 44 (2016) 17–20
Lumsden [4] occurred proximal to the physeal scar. The fractures seen in this case appear to have occurred near the physeal scar and lacked evidence of a periosteal callus often seen with transverse fractures. Parasagittal fractures of the distal metacarpal condyles is a common injury in racing Thoroughbred horses and is most commonly seen in the lateral condyles of the third metacarpal bones. Medial condylar fractures of this bone are at in increased risk for catastrophic failure. Condylar fractures can be seen in the radiographs of this horse (Fig. 2). We believe the catastrophic fractures experienced by this horse were most likely caused by a progression of subclinical parasagittal fractures of the distal third metacarpal condyles [5–7]. The etiopathogenesis of parasagittal fractures of the distal condyles of the third metacarpal/ metatarsal bone was proposed by Riggs et al [7] who theorized that these fractures are caused by underlying cyclic fatigue damage in subchondral bone. Sclerosis of the subchondral bone in the condyles occurs as a response to increased intensity of training (cumulative stress–induced bone injury) and makes them more rigid than surrounding bone, leading to creation of shear strains and fatigue damage (especially at the parasagittal groove). More recently, it was shown that intense training in equine athletes suppresses the normal remodeling response of the third metacarpal subchondral bone, thus limiting damage repair which leads to bone fragility [11]. Microfractures of
19
the articular surface develop during high cyclic loading, especially if applied rapidly. The propagation of microfractures in the subchondral bone plate may lead to an articular condylar stress fracture [12]. Although lameness would be anticipated with parasagittal stress fractures and subchondral fatigue, some horses will show minimal lameness [13,14]. Lameness caused by preexisting injury, if present, may not have been noticed by the rider because the horse was not used for performance and because lameness would have been bilateral. Lack of recognition of lameness allowed for continued work leading to catastrophic break down. Based on the presence of parasagittal fractures in the condyles of the distal third metacarpus, we theorize that the fractures in this horse likely originated in the metacarpophalangeal joint and propagated into the diaphysis resulting in catastrophic failure. The horse in this report was a Quarter Horse being trained for pleasure riding and was not subject to the routine repetitive stress typical of a racehorse. It is possible that processes other than repetitive stress could have caused similar bone fatigue. Morphologic abnormalities in the subchondral bone of the parasagittal groove in a small population of yearling Thoroughbreds were detected before entering race training [15]. This suggests that there may be a developmental component in some horses that predispose to microfractures and thus development of condylar fractures. Ideally in this case, both
Fig. 2. Close up DP (Dorso-palmar) view, the condylar components of the fractures are visible and located close to the midsagittal ridge noted at the arrows.
20
R. Cole et al. / Journal of Equine Veterinary Science 44 (2016) 17–20
a complete radiographic examination and computerized tomography of the metacarpophalangeal joints would have been performed. This would have allowed for better assessment of the subchondral bone to establish if underlying densification or signs of abnormal stress were present, typical of fatigue fractures. The incidence of fatal skeletal injuries in the racing Quarter Horse appears to be similar to that of the Thoroughbred race horse with the most common site of injury in either breed being the fetlock region [16]. The unique finding in this case was not the breed difference but rather that these injuries occurred in a horse that was not being trained for racing. Owners and trainers should be aware that horses trained for disciplines other than racing can develop condylar fractures similar to racing Thoroughbreds and Quarter Horses. References [1] Riggs CM, Pilsworth R. Repetitive strain injuries of the skeleton in high performance equine athletes. In: Hinchcliff KW, Kaneps AJ, Geor RJ, editors. Equine sports medicine & surgery. 2nd ed. Philadelphia: Saunders Elsevier; 2014. p. 457–71. [2] Bassage II LH. Metacarpus/metatarsus. In: Hinchcliff KW, Kaneps AJ, Geor RJ, editors. Equine sports medicine & surgery. 2nd ed. Philadelphia: Saunders Elsevier; 2014. p. 297–315. [3] Morgan R, Dyson S. Incomplete longitudinal fractures and fatigue injury of the proximopalmar medial aspect of the third metacarpal bone in 55 horses. Equine Vet J 2012;44:64–70. [4] O’Sullivan CB, Lumsden JM. Distal third metacarpal bone palmar cortical stress fractures in four Thoroughbred racehorses. Equine Vet Educ 2002;14:70–6. [5] Ellis DR. Some observations on condylar fractures of the third metacarpus and third metatarsus in young Thoroughbreds. Equine Vet J 1994;26:178–83.
[6] Zekas LJ, Bramlage LR, Embertson RM, Hance SR. Characterisation of the type and location of fractures of the third metacarpaI/metatarsaI condyles in 135 horses in central Kentucky (1 986-1 994). Equine Vet J 1999;31:304–8. [7] Riggs CM, Whitehouse GH, Boyde A. Pathology of the distal condyles of the third metacarpal and third metatarsal bones of the horse. Equine Vet J 1999;31:140–8. [8] Ramzan PHL. Transverse stress fracture of the distal diaphysis of the third metacarpus in six Thoroughbred racehorses. Equine Vet J 2009;41:602–5. [9] Nunamaker DM. On bucked shins. Proc Am Ass Equine Practnrs 2002;48:76–89. [10] Labens R, Schramme MC. Orthopaedics 2. Diseases of the foot and distal limbs. In: Mair TS, Love S, Schumacher J, Smith RKW, Frazer G, editors. Equine medicine, surgery and reproduction. 2nd ed. Philadelphia: Saunders Elsevier; 2013. p. 329–68. [11] Whitton CR, Trope GD, Chasem-Zadeh A, Anderson GA, Parkin TD, Mackie EJ, Seeman E. Third metacarpal condylar fatigue fractures in equine athletes occur within previously modelled subchonrdal bone. Bone 2010;47:826–31. [12] Muir P, McCarthy J, Radtke CL, Markel EM, Santschi EM, Scollay MC, Kalsheur VL. Role of endochondral ossification of articular cartilage and functional adaptation of the subchondral plate in the development of fatigue microcracking of joints. Bone 2006;38:342–9. [13] Tull TM, Bramlage LR. Racing prognosis after cumulative stressinduced injury of the distal portion of the third metacarpal and third metatarsal bones in Thoroughbred racehorses: 55 cases (20002009). J Am Vet Med Assoc 2011;238:1316–22. [14] Ramzan PH, Palmer L, Powell SE. Unicortical condylar fracture of the Thoroughbred fetlock: 45 cases (2006–2013). Equine Vet J 2015;47: 680–3. [15] Firth EC, Duble M, Boyde A. Changes in mineralized tissue at the site of origin of condylar fractures are present before athletic training in Thoroughbred horses. N Z Vet J 2009;57:278–83. [16] Sarrafian TL, Case JT, Kinde H, Daft BM, Read DH, Moore JD, Uzal FA, Stover SM. Fatal musculoskeletal injuries of Quarter Horse racehorses: 314 cases (1990–2007). J Am Vet Med Assoc 2012;241: 935–42.
Contents lists available at ScienceDirect
Journal of Equine Veterinary Science journal homepage: www.j-evs.com
Case Report
Bilateral Catastrophic Metacarpal Fractures in a Quarter Horse Gelding Robert Cole a, *, Robyn Wilborn a, Alex Gillen a, Joe Newton b, Heather Walz c a
Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL Department of Pathobiology, Auburn University, Auburn, AL c Alabama State Diagnostic Laboratory, Auburn, AL b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2016 Received in revised form 24 February 2016 Accepted 24 February 2016 Available online 27 May 2016
A 5 year-old, 450 kg, American Quarter Horse gelding was presented for postmortem examination. The horse had sustained multiple fractures of the metacarpal bones of both fore limbs while being ridden at a canter. Based on the radiographic presence of parasagittal fractures in the condyles of the distal third metacarpus, we theorized that the fractures in this horse likely originated in the metacarpophalangeal joint and propagated into the diaphysis resulting in catastrophic failure. The unique finding in this case is that these injuries occurred in a horse that was not being trained for racing. Owners and trainers should be aware that horses trained for disciplines other than racing can develop condylar fractures similar to racing Thoroughbreds and Quarter Horses. Ó 2016 Elsevier Inc. All rights reserved.
Keywords: Catastrophic Metacarpal Fractures Quarter Horse
1. Introduction The carcass of a 5-year-old, 450 kg, American Quarter Horse gelding was presented to the Alabama State Diagnostic Laboratory for postmortem examination. The horse had sustained multiple fractures of the metacarpal bones of both fore limbs while being ridden at a canter on a hardpacked, dirt road. The rider was thrown from the horse as it fell. The colt was found sternally recumbent and unable to rise because both third metacarpal bones were fractured. The horse was immediately euthanized by gunshot. The horse had been in training for pleasure riding for 20 days but had been ridden for 1 hour approximately twice per week by the owner since being broken for riding 2 years previously. During its 20 days with the trainer, the horse was ridden in sets of walk, trot, and canter across the same ground for approximately an hour daily, 5 days a week, and then taken on a 2.5- to 3-hour trail ride during the
* Corresponding author at: Robert Cole, Department of Clinical Sciences, Auburn University, Auburn, AL 36849. E-mail address: [email protected] (R. Cole). 0737-0806/$ – see front matter Ó 2016 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jevs.2016.02.235
weekends. The horse was shod for the first time 10 days before being injured. The rider, who weighed 68 kg, reported that the horse did not seem to trip or stumble, but instead “fell out” from beneath him at a canter. Neither the owner nor the rider had observed lameness before the incident. During examination of orthogonal radiographs of the metacarpi (Fig. 1), severely comminuted distal diaphyseal fractures of both second, third, and fourth metacarpal bones were observed. In the left forelimb, a fracture extended from the medial aspect of the midsagittal ridge through the medial condyle proximally toward the comminuted diaphyseal fracture. The medial and lateral condyles of the right forelimb were fractured in multiple planes that radiated proximally into the comminuted segment. The fracture of this limb spiraled proximally into the carpometacarpal joint. The trabecular pattern appeared to be normal in the intact portions of the third metacarpal bones and in the proximal phalanges. No periosteal new bone or evidence of change in the bone density was identified radiographically. The proximal phalanges were radiographically normal. The suspensory ligaments were
18
R. Cole et al. / Journal of Equine Veterinary Science 44 (2016) 17–20
Fig. 1. Distal diaphyseal fractures in the third metacarpal bones were severely comminuted. A spiral fracture extended proximally into the carpometacarpal joint of the right forelimb. Fissure lines were seen extending into both metacarpophalangeal joints. The overall trabecular pattern appeared to be normal in the intact portions of the third metacarpal bones and in the proximal phalanges. No periosteal changes were identified, and no changes in bone density were visible.
intact and visually normal. A cortical fragment taken from a fracture site in each of the third metacarpal bones was demineralized, and sections processed for standard paraffin embedding. Slides were stained with H and E according to standard methods (Kiernan 2008). The sections of bone were histologically normal. 2. Discussion Few apparently spontaneous fractures occurring in athletic horses are actually spontaneous. Instead, nearly all are the end result of progressive deterioration of focal areas of the skeleton from repetitive injury [1]. Predilection sites for repetitive injuries (i.e., stress or fatigue injuries) of the third metacarpal bone include the dorsomedial or dorsolateral cortex (types 1 and 2 dorsal metacarpal disease, respectively) [2], the palmaroproximal cortex [3], the palmarolateral aspect of the distal end of the diaphysis [4], the third metacarpal condyles [5–7], and the distal third of the metacarpus [8]. Most often these stress injuries are reported to occur in the racing Thoroughbreds, and in nearly all cases, lameness, rather than catastrophic injury, is the result. This case is unusual in that the horse was an American Quarter Horse being trained for pleasure riding. Exercise programs affect the shape and substance of bone and influence the likelihood of a horse developing a
stress fracture of the metacarpus [9]. Most metacarpal stress fractures are reported to occur in horses trained at high speeds [1]. Two types of dorsal metacarpal disease are recognized: type 1 or bucked shins, which occurs in 2-yearold horses and is often associated with lameness and radiographic evidence of a subperiosteal callus at the dorsal cortex of the third metacarpal bone; and type 2, which occurs in older (3 to 5 years old) horses and is characterized by incomplete, oblique fracture of the dorsolateral cortex of the third metacarpal bone [10]. A number of experiments in racehorses lead Nunamaker [9] to conclude that, whereas high-speed exercise decreases the incidence of type 1 dorsal metacarpal disease, long galloping exercise increases the risk for this disease. Nunamaker [9] also concluded that horses that do not develop bucked shins (i.e., type 1 dorsal metacarpal disease) do not develop stress fractures of the dorsal cortex (i.e., type 2 dorsal metacarpal disease) and, in addition, that horses that do not develop these cortical stress fractures do not develop midshaft fractures of the third metacarpal bone. Because the horse had no history of lameness and had no radiographic evidence of periostitis seen with type 1 dorsal metacarpal disease, dorsal cortical fractures seem to be an unlikely cause of the injuries seen in this horse. The transverse metacarpal stress fracture reported by Ramzan (2009) and the distal third palmar metacarpal cortical fracture reported by O’Sullivan and
R. Cole et al. / Journal of Equine Veterinary Science 44 (2016) 17–20
Lumsden [4] occurred proximal to the physeal scar. The fractures seen in this case appear to have occurred near the physeal scar and lacked evidence of a periosteal callus often seen with transverse fractures. Parasagittal fractures of the distal metacarpal condyles is a common injury in racing Thoroughbred horses and is most commonly seen in the lateral condyles of the third metacarpal bones. Medial condylar fractures of this bone are at in increased risk for catastrophic failure. Condylar fractures can be seen in the radiographs of this horse (Fig. 2). We believe the catastrophic fractures experienced by this horse were most likely caused by a progression of subclinical parasagittal fractures of the distal third metacarpal condyles [5–7]. The etiopathogenesis of parasagittal fractures of the distal condyles of the third metacarpal/ metatarsal bone was proposed by Riggs et al [7] who theorized that these fractures are caused by underlying cyclic fatigue damage in subchondral bone. Sclerosis of the subchondral bone in the condyles occurs as a response to increased intensity of training (cumulative stress–induced bone injury) and makes them more rigid than surrounding bone, leading to creation of shear strains and fatigue damage (especially at the parasagittal groove). More recently, it was shown that intense training in equine athletes suppresses the normal remodeling response of the third metacarpal subchondral bone, thus limiting damage repair which leads to bone fragility [11]. Microfractures of
19
the articular surface develop during high cyclic loading, especially if applied rapidly. The propagation of microfractures in the subchondral bone plate may lead to an articular condylar stress fracture [12]. Although lameness would be anticipated with parasagittal stress fractures and subchondral fatigue, some horses will show minimal lameness [13,14]. Lameness caused by preexisting injury, if present, may not have been noticed by the rider because the horse was not used for performance and because lameness would have been bilateral. Lack of recognition of lameness allowed for continued work leading to catastrophic break down. Based on the presence of parasagittal fractures in the condyles of the distal third metacarpus, we theorize that the fractures in this horse likely originated in the metacarpophalangeal joint and propagated into the diaphysis resulting in catastrophic failure. The horse in this report was a Quarter Horse being trained for pleasure riding and was not subject to the routine repetitive stress typical of a racehorse. It is possible that processes other than repetitive stress could have caused similar bone fatigue. Morphologic abnormalities in the subchondral bone of the parasagittal groove in a small population of yearling Thoroughbreds were detected before entering race training [15]. This suggests that there may be a developmental component in some horses that predispose to microfractures and thus development of condylar fractures. Ideally in this case, both
Fig. 2. Close up DP (Dorso-palmar) view, the condylar components of the fractures are visible and located close to the midsagittal ridge noted at the arrows.
20
R. Cole et al. / Journal of Equine Veterinary Science 44 (2016) 17–20
a complete radiographic examination and computerized tomography of the metacarpophalangeal joints would have been performed. This would have allowed for better assessment of the subchondral bone to establish if underlying densification or signs of abnormal stress were present, typical of fatigue fractures. The incidence of fatal skeletal injuries in the racing Quarter Horse appears to be similar to that of the Thoroughbred race horse with the most common site of injury in either breed being the fetlock region [16]. The unique finding in this case was not the breed difference but rather that these injuries occurred in a horse that was not being trained for racing. Owners and trainers should be aware that horses trained for disciplines other than racing can develop condylar fractures similar to racing Thoroughbreds and Quarter Horses. References [1] Riggs CM, Pilsworth R. Repetitive strain injuries of the skeleton in high performance equine athletes. In: Hinchcliff KW, Kaneps AJ, Geor RJ, editors. Equine sports medicine & surgery. 2nd ed. Philadelphia: Saunders Elsevier; 2014. p. 457–71. [2] Bassage II LH. Metacarpus/metatarsus. In: Hinchcliff KW, Kaneps AJ, Geor RJ, editors. Equine sports medicine & surgery. 2nd ed. Philadelphia: Saunders Elsevier; 2014. p. 297–315. [3] Morgan R, Dyson S. Incomplete longitudinal fractures and fatigue injury of the proximopalmar medial aspect of the third metacarpal bone in 55 horses. Equine Vet J 2012;44:64–70. [4] O’Sullivan CB, Lumsden JM. Distal third metacarpal bone palmar cortical stress fractures in four Thoroughbred racehorses. Equine Vet Educ 2002;14:70–6. [5] Ellis DR. Some observations on condylar fractures of the third metacarpus and third metatarsus in young Thoroughbreds. Equine Vet J 1994;26:178–83.
[6] Zekas LJ, Bramlage LR, Embertson RM, Hance SR. Characterisation of the type and location of fractures of the third metacarpaI/metatarsaI condyles in 135 horses in central Kentucky (1 986-1 994). Equine Vet J 1999;31:304–8. [7] Riggs CM, Whitehouse GH, Boyde A. Pathology of the distal condyles of the third metacarpal and third metatarsal bones of the horse. Equine Vet J 1999;31:140–8. [8] Ramzan PHL. Transverse stress fracture of the distal diaphysis of the third metacarpus in six Thoroughbred racehorses. Equine Vet J 2009;41:602–5. [9] Nunamaker DM. On bucked shins. Proc Am Ass Equine Practnrs 2002;48:76–89. [10] Labens R, Schramme MC. Orthopaedics 2. Diseases of the foot and distal limbs. In: Mair TS, Love S, Schumacher J, Smith RKW, Frazer G, editors. Equine medicine, surgery and reproduction. 2nd ed. Philadelphia: Saunders Elsevier; 2013. p. 329–68. [11] Whitton CR, Trope GD, Chasem-Zadeh A, Anderson GA, Parkin TD, Mackie EJ, Seeman E. Third metacarpal condylar fatigue fractures in equine athletes occur within previously modelled subchonrdal bone. Bone 2010;47:826–31. [12] Muir P, McCarthy J, Radtke CL, Markel EM, Santschi EM, Scollay MC, Kalsheur VL. Role of endochondral ossification of articular cartilage and functional adaptation of the subchondral plate in the development of fatigue microcracking of joints. Bone 2006;38:342–9. [13] Tull TM, Bramlage LR. Racing prognosis after cumulative stressinduced injury of the distal portion of the third metacarpal and third metatarsal bones in Thoroughbred racehorses: 55 cases (20002009). J Am Vet Med Assoc 2011;238:1316–22. [14] Ramzan PH, Palmer L, Powell SE. Unicortical condylar fracture of the Thoroughbred fetlock: 45 cases (2006–2013). Equine Vet J 2015;47: 680–3. [15] Firth EC, Duble M, Boyde A. Changes in mineralized tissue at the site of origin of condylar fractures are present before athletic training in Thoroughbred horses. N Z Vet J 2009;57:278–83. [16] Sarrafian TL, Case JT, Kinde H, Daft BM, Read DH, Moore JD, Uzal FA, Stover SM. Fatal musculoskeletal injuries of Quarter Horse racehorses: 314 cases (1990–2007). J Am Vet Med Assoc 2012;241: 935–42.