Osseous Trauma in the Fetlock Region of Mature Sports Horses

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1 Osseous Trauma in the Fetlock Region of Mature Sports Horses Sue J. Dyson, VetMB, PhD; and Rachel Murray, VetMB, PhD Osseous trauma of the fetlock is a potentially important cause of lameness in mature sports horses, and although the diagnosis may be achieved by radiographic examination in some horses, nuclear scintigraphy or magnetic resonance imaging may be required for other horses. Authors address: Centre for Equine Studies, Animal Health Trust, Lanwades Park, Kentford, Newmarket, Suffolk CB8 7UU, UK; sue.dyson@aht.org.uk (Dyson) AAEP. 1. Introduction The concept of stress-related bone injury is well recognized in immature equine athletes; however, there is little published data relating to subchondral bone trauma in mature sports horses. Nuclear scintigraphy has proven efficacious for the early identification of increased osteoblastic activity after bone trauma, and it has been helpful in the characterization of stress-related bone injury in the metacarpophalangeal (MCP) and metatarsophalangeal (MTP) joints (fetlock) of both Thoroughbred 1 and Standardbred 2 racehorses. Osseous cyst-like lesions (OCLLs) in the fetlock region have predominantly been documented in immature horses and debate continues as to whether osteochondrosis or trauma is the most likely cause. 3 Short, incomplete dorsal cortical stress fractures of the proximal aspect of the proximal phalanx have been well described in racehorses 4 6 but have been poorly documented in mature sports horses. A bone bruise is a well-recognized cause of joint pain in people and is identified using magnetic resonance imaging (MRI). 7 More recently, with the more widespread use of MRI in equine medicine, subchondral bone trauma in the fetlock region of mature horses has been recognized. 8 The aims of this study were to describe how different imaging modalities can lead to a diagnosis of osseous trauma in the equine fetlock region of mature sports horses, to describe the clinical manifestations and the results of diagnostic imaging, and to determine the outcome of treatment. 2. Materials and Methods Clinical records of horses referred to the Centre for Equine Studies, Animal Health Trust between January 2001 and December 2005 were reviewed. Horses selected for inclusion in the study had to meet the following criteria: a non-racehorse, 4 yr of age, history of sudden onset lameness, pain localized to the fetlock region using perineural and/or intra-articular analgesia, and examination using radiography and nuclear scintigraphy. Horses with unequivocal radiographic evidence of a fracture of the third metacarpal (McIII) or third metatarsal (MtIII) bones, the proximal phalanx, or a proximal sesamoid bone (PSB) were excluded along with horses with osteoarthritis or clinical signs referable to the periarticular soft tissues or the digital flexor NOTES AAEP PROCEEDINGS Vol

2 Table 1. Signalment and Clinical Characteristics of 12 Horses With Osseous Trauma of the Fetlock Age (yr) Breed* Sex Discipline Lame Limb Duration of Lameness (mo) Effusion Flexion Response Degree of Lameness Digital (AS) Nerve Block** Four- Point Nerve Block IA Analgesia Comments 1 12 WBL G Dr RF S NP S 2 9 WBL G SJ RH 8 4 NC S I 3 4 WBL G Dr LF 3 4 NC S I 4 10 Arab G End RF S NP NP 5 10 WBL M SJ LF S NP NP 6 8 TB M E LF 2 6 S NP NP 7 11 WBL G SJ LH 3 4 NC S NC 8 9 WBL G GP RH 4 4 NC S I 9 8 Conn M GP LF 2 6 NC S NP Cob M GP LH 1 5 S S NC 11 9 WBL M SJ LF 1 6 NP NP I 12 8 WBL M SJ RF NP NP I *TB, Thoroughbred; WBL, Warmblood; Conn, Connemara. M, Mare; G, Gelding. Dr, Dressage; SJ, Showjumping; GP, general purpose and unaffiliated competition; E, Eventing; End, Endurance riding. RF, right front; LF, left front; RH, right hind; LH, left hind. On a scale of 0 8 (0 sound; 2 mild; 4 moderate; 6 severe; 8 non weight-bearing). **S, sound; I, improved 50%; NC, no change; NP, not performed. No response to intra-articular medication. tendon sheath. Selected horses with joint effusion underwent exploratory arthroscopic examination of the fetlock joint. Selected horses with no evidence of joint capsule distension underwent MRI. Radiographic examination included a minimum of a dorsal 15 proximal-palmarodistal oblique (D15 Pr-PaDiO) view, a flexed lateromedial view, a dorsolateral-palmarodistal oblique view, and a dorsomedialpalmarolateral oblique view (or the equivalent views in a hindlimb). In selected horses, additional dorsoproximolateral-palmarodistal medial oblique views were obtained to highlight the trabecular bone of the palmar (plantar) condyles of the MtIII. 9 The horse was sedated using detomidine a (10 mcg/kg, IV), and scintigraphic examination was performed 2.5 h after injection of 99m technetium methylene diphosphonate (MDP; 10 MBq/kg, IV). Images were obtained using a low-energy, general-purpose collimator and a 500-mm circular field-of-view GE gamma camera linked to a dedicated nuclear medicine computer system. b All images (lateral, dorsal, or plantar) were acquired dynamically as a series of 35 2-s frames using a matrix with the gamma camera positioned as close as possible to the area of interest. Care was taken to shield gamma radiation from other limbs using a lead rubber sheet. For all images, motion correction software was used to align each frame before producing a final static image. Using a human extremity radiofrequency coil, MR images of the fetlock were obtained under general anesthesia with the feet positioned in the isocenter of a short-bore, flared-end 1.5-Tesla GE Signa Echospeed magnet. c10 Sagittal, dorsal, and transverse MR images were obtained using three-dimensional (3-D) T1-weighted spoiled gradient echo (SPGR), 3-D T2* gradient echo (GRE), and short inversionrecovery (fast STIR) or fat-saturated 3-D T2* GRE sequences with a slice thickness of 1.5 mm (SPGR and GRE images) or 4 mm (fast STIR). The majority of horses underwent clinical reassessment and appropriate diagnostic imaging at the Animal Health Trust. More recent follow-up information was obtained by telephone communication with the owners. 3. Results Twelve horses (Table 1) were identified with evidence of traumatically induced bone injury of the McIII or MtIII (n 6), proximal phalanx (n 5), or a proximal sesamoid bone (n 1); eight had forelimb lameness, and four had hindlimb lameness. All horses had sudden onset lameness, and in three showjumpers, this occurred immediately after a fall (cases 3, 11, and 12). The duration of lameness at the time of examination at the Animal Health Trust ranged from 2 wk to 8 mo. Effusion of the fetlock joint was only noted in two horses (cases 11 and 12), both of which had full-thickness cartilage defects of the distal aspect of McIII and associated bone trauma. Lameness varied in degree between 1 and 6 on a scale of 0 8 (0 sound; 2 mild; 4 moderate; 6 severe; 8 non-weight bearing). In five horses (cases 1, 4, 5, 6, and 10), lameness was abolished by digital nerve blocks performed at the base of the PSBs (2 ml mepivacaine per site). In five horses, lameness was abolished by perineural analgesia of the palmar (plantar) nerves performed at the junction of the proximal three-quarters and distal one-quarter of the metacarpal (metatarsal) region and the palmar (plantar) metacarpal (meta Vol. 52 AAEP PROCEEDINGS

3 tarsal) nerves. Perineural analgesia was not performed in the two horses with fetlock effusion. Intra-articular analgesia of the fetlock (6 ml mepivacaine; lameness reassessed 10 min after injection) was performed in eight horses, and it abolished the lameness in one horse (case 1). It produced substantial improvement in five horses (cases 2, 3, 8, 11, and 12), but it had no effect in one horse (case 10) with an injury of the lateral PSB. Radiography Radiographic examination revealed no detectable abnormality in seven horses (cases 1, 2, 3, 7, 8, 10, and 11) at the time of initial examination (Table 2). However, case 10 had developed a radiolucent defect in the axial proximal aspect of the lateral PSB when reexamined 4 wk later (2 mo after the onset of lameness). One horse (case 4) had subtle sclerosis of the proximal aspect of the proximal phalanx axially seen only in the D15 Pr-PaDiO projection (Fig. 1). Case 5 had a subtle, very short lucent line in the axial proximal aspect of the proximal phalanx, identified in one D15 Pr-PaDiO view (Fig. 2), but this could not be identified in subsequent similar views. Case 9 had an OCLL in the proximal axial aspect of the proximal phalanx, also only seen in the D15 Pr- PaDiO view. Case 12 had a small, ill-defined radiolucent area in the abaxial aspect of the medial condyle of McIII (Fig. 3). This was better defined when the horse was reexamined 4 wk later, at which time the horse had substantially improved. Nuclear Scintigraphy and MRI All horses except cases 7 and 8 had moderate or focal increased radiopharmaceutical uptake (IRU) corresponding with the site of major pathological change identified radiographically using MRI or arthroscopically (Table 2; Figs. 1 7). Six horses underwent MRI (cases 2, 3, 5, 6, 7, and 8). In case 5, the suspected incomplete dorsal cortical fracture of the proximal phalanx was confirmed (Fig. 2). In case 6, there was marked thickening of the subchondral bone of the proximal phalanx from the midline that extended medially from dorsal to palmar and distally to the level of the physis (Fig. 5). This was characterized by reduced signal intensity in both T1- and T2-weighted images and was also associated with increased signal intensity in fat-suppressed images. There was also increased subchondral bone thickness in the opposing aspect of McIII with an irregular margin of the proximal aspect of the subchondral bone plate. Case 2 had evidence of bone trauma of the plantar aspect of the lateral condyle of MtIII with low-grade abnormalities of the opposing aspects of the lateral PSB and the proximal phalanx (Fig. 7). This was characterized by diffuse low-signal intensity in T1-weighted images and mild increased signal intensity in fat-suppressed images. There was a more focal area of intense high-signal intensity in T1-weighted and STIR images and intermediate signal intensity in T2-weighted images in the subchondral bone of the distal plantar aspect of MtIII. Case 3 had more extensive areas of bone trauma involving the lateral and medial condyles of McIII (lateral medial) with areas of reduced signal intensity on T1- and T2-weighted images and increased signal intensity in fat-suppressed images (Fig. 8). In addition, there was a lesion in the lateral aspect of the lateral condyle in the approximate region of origin of the lateral collateral ligament of the metacarpophalangeal joint that was characterized by intense increased signal intensity in fatsuppressed images and low signal intensity on T1- weighted images (Fig. 8). Case 7 had a focal, nonarticular lesion in the central proximal aspect of the proximal phalanx that was characterized by low signal intensity in T1- and T2-weighted images and high signal intensity on fat-suppressed images. Case 8 had a focal subchondral defect in the lateral condyle of MtIII with mineralization extending proximally into the subchondral bone that was characterized by low signal intensity on T1- and T2- weighted images and no abnormalities on fatsuppressed images (Fig. 6). Surgery Three horses (cases 10, 11, and 12) underwent exploratory arthroscopy, and in two of these horses, a large (2 3 cm) diameter full-thickness cartilage defect was identified on the distal dorsal aspect of McIII; the defect was more dorsal in case 12 (Fig. 3) and more on the weight-bearing surface of McIII in case 11. Both lesions were debrided. In case 12, there was a cavity in the subchondral bone 1 cm deep that contained fibrous tissue and soft, granular bone that was debrided. Inspection of the palmar pouch of the joint revealed no abnormality in case 12, but in case 11, there were extensive wear lines on the axial aspect of both PSBs. In case 10, the plantar pouch was examined, and no abnormality was seen. However, after an incision in the abaxial aspect of the intersesamoidean ligament adjacent to the lateral PSB, a defect was identified in the cortex of the bone with an underlying area of bone necrosis that was debrided. Case 9, with a non-articular OCLL in the proximal phalanx, was treated by an extra-articular approach. The lesion was debrided through a 5.5-mm drill hole and packed with tricalcium phosphate. Case 2, with evidence of bone trauma of the lateral plantar condyle of MtIII, underwent arthroscopic exploration after MRI to try to determine if a cartilage defect could be identified, but no abnormality was observed in the areas accessible to inspection. Case 8, with sclerosis of the lateral condyle of MtIII, underwent exploratory arthroscopy at the owner s request, and as expected, no lesion was identified. Follow-up Nine horses have returned to full athletic function (cases 1, 2, 3, 4, 5, 6, 9, 10, and 12); however, in cases 2, 9, 10, and 12, it took between 5 and 9 mo for AAEP PROCEEDINGS Vol

4 Table 2. Results of Diagnostic Imaging and Response to Treatment in 12 Horses With Osseous Trauma of the Fetlock Radiography* Nuclear Scintigraphy Magnetic Resonance Imaging Arthroscopy Treatment Outcome 1 NAD Moderate focal IRU distal lateral palmar aspect McIII 2 NAD Moderate focal IRU plantarolateral aspect MtIII 3 NAD Intense IRU distal McIII laterally medially 4 Subtle sclerosis proximal aspect of proximal phalanx axially 5 Subtle short radiolucent line axial proximal aspect of proximal phalanx 6 Mild thickening of subchondral bone plate of proximomedial aspect of the proximal phalanx Moderate focal IRU proximodorsal aspect of proximal phalanx axially Moderate focal IRU proximal dorsal aspect of proximal phalanx axially Moderate focal IRU proximal axial aspect of proximal phalanx extending from dorsal to palmar 7 NAD Moderate focal IRU distal dorsal lateral aspect MtIII 8 NAD Subtle IRU distal lateral aspect MtIII 9 OCLL axial proximal aspect of proximal phalanx 10 NAD at first examination; 4 wk later radiolucent defect in axial proximal aspect of lateral PSB Focal intense IRU proximal axial aspect of proximal phalanx Focal intense IRU lateral PSB LH 11 NAD Focal intense IRU distal medial aspect McIII, dorsal palmar 12 Small ill-defined radiolucent zone abaxial aspect of medial condyle McIII; 4 wk later enlarged and better defined, although lameness improved substantially Focal intense IRU distal medial dorsal aspect McIII NP NP Rest S Decreased signal intensity in plantar aspect of lateral condyle of MtIII in T1- and T2-weighted images with increased signal intensity in fat suppressed images NAD Rest S Decreased signal intensity in NP Rest S T1-weighted images and increased signal intensity in fat suppressed images in lateral and medial condyles McIII; areas of increased signal intensity in fat suppressed images close to origin of lateral collateral ligament NP NP Rest S Incomplete short dorsal cortical sagittal fracture proximal aspect of proximal phalanx NP Rest S Decreased signal intensity in T1- and T2-weighted images in proximal aspect of proximal phalanx medial lateral and also distal McIII; increased signal intensity in fat suppressed images NP Rest S Focal area of increased signal NP HD intensity in proximal aspect of proximal phalanx axially, mid-dorsal to plantar in fat suppressed images with decreased signal intensity in T1- and T2-weighted images Focal mineralization of lateral NAD HD condyle of MtIII, with normal signal in fat suppressed images NP NP Extraarticular S drilling and curettage NP NAD, until incision made S in intersesamoidean ligament, revealing defect in cortex of lateral PSB and bone necrosis NP Large circular fullthickness R (broodmare) defect in articular cartilage distal aspect of McIII NP Large oval-shaped fullthickness S defect in articular cartilage distal dorsal aspect of McIII, with fibrous tissue and soft granular bone in subchondral defect *NAD, no abnormality detected; OCLL, osseous cyst-like lesion; PSB, proximal sesamoid bone; McIII, third metacarpal bone. IRU, increased radiopharmaceutical uptake; MtIII, third metatarsal bone. NP, not performed. S, sound; R, retired; HD, humane destruction because of persistent lameness Vol. 52 AAEP PROCEEDINGS

5 Fig. 1. (A) Dorsal 15 proximal-palmarodistal oblique radiographic view of the right metacarpophalangeal joint of case 4. Medial is to the left. There is slight thickening of the axial aspect of the subchondral bone plate of the proximal phalanx. (B) Lateral scintigraphic image of case 1. There is focal moderate IRU in the dorsoproximal aspect of the proximal phalanx. It was concluded that the horse had an incomplete dorsoproximal sagittal fracture of the proximal phalanx. lameness to resolve. Clinical improvement was much more rapid in cases 1, 4, 5, and 6. Cases 1, 3, and 5 were reexamined radiographically and scintigraphically 6 wk (cases 1 and 5) and 3 mo (case 3) after initial examination. No radiographic abnormality was seen, and IRU was normal. Case 4 underwent radiographic follow-up examination, and the previously identified sclerosis was no longer detectable. Although it was less well-defined, the cyst-like lesion in case 12 was still apparent radiographically after 3 mo. Case 11 improved clinically and is used as a broodmare. Cases 7 and 8 were euthanized because of persistent lameness. Case 7, with a non-articular osseous cyst-like lesion seen on MR images in the proximal axial aspect of the proximal phalanx, underwent post-mortem examination, which verified the presence of a cyst. 4. Discussion Bone trauma of the fetlock region is not common, and only 12 horses fulfilled the inclusion criteria for this study over a 5-yr period. During that time, 2500 horses were examined by the author for lameness or poor performance. Nonetheless, recognition of primary osseous trauma of the fetlock as a cause of lameness in mature equine athletes is considered important. Mares were over-represented, comprising 50% of the horses in this study compared with 28% of the normal clinic population. Lameness Characteristics and Response to Local Analgesia The degree of lameness ranged from 1 to 6, but, in general, the horses had moderate to severe lameness that was exacerbated by distal limb flexion in most. Joint effusion was only present in those horses with major cartilage pathology. Five horses (cases 1, 4, 5, 6, and 10) had lameness that was abolished by perineural analgesia of the digital nerves at the level of the base of the PSBs within 10 min of injection. It is important to recognize that this nerve block has the potential to AAEP PROCEEDINGS Vol

6 Fig. 2. (A) Dorsal 15 proximal-palmarodistal oblique radiographic view of the left MCP joint of case 5. Medial is to the left. There is a short axial radiolucent line in the proximal aspect of the proximal phalanx that could not be reproduced in other similar radiographic projections. (B) Lateral scintigraphic image of case 5. There is focal moderate IRU in the dorsoproximal aspect of the proximal phalanx. (C) Dorsal SPGR image of the left metacarpophalangeal joint of case 5. There is a short high signal-intensity line going through the proximal cortex of the proximal phalanx with generalized hypointense signal throughout the proximal aspect of the proximal phalanx and the distal aspect of the McIII. This verified the presence of an incomplete dorsal cortical fracture of the proximal aspect of the proximal phalanx. (D) Transverse SPGR image of the proximal aspect of the proximal phalanx of case 5. There is generalized hypointense signal in the dorsal third of the bone Vol. 52 AAEP PROCEEDINGS

7 Fig. 3. (A) Dorsal 15 proximal-palmarodistal oblique radiographic view of the right MCP joint of case 12. Medial is to the left. There is a radiolucent zone in the distal aspect of the McIII in the abaxial aspect of the medial condyle. (B) Dorsal and lateral scintigraphic images of the left and right MCP joints of case 12. In the dorsal image, the right forelimb is to the left. There is moderate IRU in the medial condyle of the left McIII, located dorsally in the lateral image. (C) Arthroscopic view of the right MCP joint. The McIII is to the bottom. The probe is inserted into the cyst-like lesion in the McIII. abolish fetlock region pain, including lesions of the McIII (MtIII), proximal phalanx, and PSBs. One horse (case 1) was sound after intra-articular analgesia; another five horses (cases 2, 3, 8, 11, and 12) with lesions of the distal McIII or MtIII showed 50% improvement in lameness. Case 1, rendered sound, and two of the horses with improvement in lameness (cases 2 and 8) were presumed to have intact articular cartilage. The mechanism by which primary subchondral bone pain is improved by intra-articular analgesia remains unclear. One horse (case 10) with a lesion of the axial aspect of the lateral PSB and an intact intersesamoidean ligament showed no change in lameness after intraarticular analgesia, which has been previously reported in other horses with axial lesions of the PSBs. 11 Location of Lesions The axial aspect of the proximal phalanx seemed to be a predilection site for injury. The dorsoproximal aspect of the proximal phalanx is a well-recognized AAEP PROCEEDINGS Vol

8 Fig. 4. (A) Lateral pool-phase, (B) lateral bone-phase, and (C) dorsal bone-phase images of the MCP joints of case 1. In the dorsal image, the right forelimb is to the left. There is moderate IRU in the palmar lateral aspect of the right McIII in both the pool-phase and bone-phase images. Lameness had been sudden in onset after the horse leaped into the air in a wash box and landed heavily. It was concluded that the horse had stress-related bone injury of the lateral palmar condyle of McIII. Lameness resolved, and IRU was normal 8 wk later Vol. 52 AAEP PROCEEDINGS

9 Fig. 5. (A) Dorsal and (B) lateral scintigraphic images of the MCP joints of case 6. In the dorsal image, the left forelimb is to the right. There is moderate IRU in the proximomedial aspect of the left proximal phalanx, extending from dorsal to palmar in the lateral image (compare with Figs. 1 and 2). (C) Dorsal SPGR, (D) T2* GRE, (E) STIR, and (F) transverse SPGR images of the left MCP joint of case 6. There is diffuse decreased signal intensity in the proximal medial aspect of the proximal phalanx (black arrows) in both T1- and T2-weighted images (A, B, and D) involving only the cancellous bone with more focal decreased signal intensity in the opposing aspect of the McIII (white arrow). There was mild increased signal intensity in STIR images in the same region (C) consistent with both mineralization and fluid. The horse returned to full athletic function. AAEP PROCEEDINGS Vol

10 site of incomplete dorsal cortical fractures in racehorses. 5,6 One of two mature horses with subchondral bone injury of the proximal phalanx identified using MRI had a lesion distal to the sagittal groove. 8 In the forelimb, the distal medial aspect of McIII was more commonly affected than the lateral aspect; however, in immature athletes, stress-related bone injury of the lateral aspect is more common. 12 In hindlimbs, the lateral aspect of MtIII was most at risk of injury, which is consistent with the stressrelated bone injury in MtIII in both Thoroughbred 1 and Standardbred racehorses 2 and with the normal biomechanical loading of the hind fetlock. Two mature horses with subchondral bone injury of MtIII identified by MRI had lesions of the lateral condyle. 8 In the MCP joints of normal horses, maximum IRU is located on the proximal aspect of the proximal phalanx. 13 This suggests that this is a site with high bone turnover, reflecting the mechanical loading of the area. In contrast in the MTP joints of normal horses, maximum IRU is in the lateral condyle of MtIII. Lesions of the Subchondral Bone of McIII, MtIII, and the Proximal Phalanx Lesions of the subchondral bone of the McIII, MtIII, or the proximal phalanx occurred with or without lesions of the articular cartilage. Variable signal intensity was observed in association with subchondral bone pathology and loss of trabecular architecture on MR images. In two horses (cases 2 and 3), there was focal increased signal intensity in the subchondral bone plate in T2-weighted and fat-suppressed images surrounded by a much larger area of reduced signal intensity extending into the cancellous bone. The nature of the tissue associated with the focal high signal intensity remains open to speculation; it may represent proteinaceous fluid, granulation tissue, or necrotic bone. In case 8, there was a generalized area of low signal intensity in the lateral condyle of MtIII in both T1- and T2-weighted images consistent with mineralization. This may possibly be the end result of long-term repetitive overload. Fig. 6. (A) Plantar scintigraphic image of the MTP joints of case 8. The right hindlimb is to the right. There is mild IRU in the lateral distal aspect of the right MtIII. (B) Dorsal SPGR image of the right MTP joint of case 8. Lateral is to the left. There is decreased signal intensity of the cancellous bone of the lateral condyle of the MtIII. This was also seen in T2-weighted images, which is consistent with sclerosis. (C) Parasagittal SPGR image of the right MTP joint of case 8. There is decreased signal intensity of the cancellous bone of the plantar aspect of the lateral condyle. This was also seen in T2-weighted images, which is consistent with sclerosis. Signal intensity was normal in STIR images. Lesions of the Articular Cartilage Joint effusion was present in the two horses (cases 11 and 12) that had major cartilage defects confirmed arthroscopically. Focal cartilage defects were suspected based on the MR images of cases 2 and 3, and we would have expected associated joint effusion if the cartilage surface had been broached. Case 2 was subsequently evaluated arthroscopically and no lesion was seen; however, the postulated site of a lesion was inaccessible to view. MRI permits identification of changes of signal intensity, which reflect chemical changes within a tissue without there necessarily being gross structural change. 14 The curved articular surfaces of the fetlock joint make the articular cartilage particularly susceptible to partial-volume averaging artifacts, and, there Vol. 52 AAEP PROCEEDINGS

11 Fig. 7. (A) Lateral scintigraphic image of the right hindlimb of case 2. There is moderate IRU in the plantar aspect of the MtIII. (B) Lateral parasagittal SPGR, (C) T2*GRE, (D) dorsal SPGR, (E) transverse SPGR, and (F) transverse STIR MR images of the right MTP joint of case 2. There is diffuse decreased signal intensity in the lateral plantar condyle of MtIII in T1- and T2-weighted images (white arrows), which is consistent with mineralization and fluid. There is also focal increased signal intensity in the cortex extending into the cancellous bone (black arrow), which is consistent with proteinaceous fluid. In the STIR image, there is a diffuse mild increase in signal intensity throughout the plantar two-thirds of the condyle (arrow heads) consistent with loss of fat signal. Subsequent arthroscopic evaluation through the plantar pouch of the MTP detected no cartilage surface abnormality. The horse has returned to competition after a prolonged rest period. AAEP PROCEEDINGS Vol

12 Fig. 8. (A) Lateral parasagittal SPGR, (B) dorsal SPGR, (C) dorsal T2*GRE, (D) lateral parasagittal STIR, and (E) transverse STIR images of the left MCP joint of case 3. In (A), there is diffuse decreased signal intensity extending from the dorsal to plantar aspects of the lateral condyle of McIII (white arrows). In (B) and (C), there is decreased signal intensity in the lateral and medial condyles of McIII and the proximomedial aspect of the proximal phalanx (white arrows), which is consistent with mineralization. There is also focal increased signal intensity laterally in the subchondral bone (arrowhead), which is consistent with proteinaceous fluid and altered signal intensity in the overlying cartilage. In (D), there is mild increased signal intensity on the dorsal aspect of McIII (arrows), and in (E), there is intense increased signal intensity laterally adjacent to the origin of the lateral collateral ligament of the MCP joint. No abnormality of the ligament was detected Vol. 52 AAEP PROCEEDINGS

13 fore, the accurate identification of lesions is not easy. 15 The cartilage lesions identified in cases 11 and 12 were similar; however, in case 12, there was a larger defect in the subchondral bone that was identified radiographically. This lesion was located more dorsally than in case 11 (dorsal to the region of maximum weight bearing), and this may explain why this horse ultimately made a complete recovery and remains in full athletic function 3 yr post-operatively. Case 11 also had evidence of osteoarthritis with wear lines on the articular cartilage of the PSBs. We suggest that the presence of joint effusion is a good clinical indicator of the potential value of arthroscopic surgery of the fetlock, whereas, in the absence of effusion, MRI is more likely to yield diagnostic information. However, it was notable that the horse with the axial defect in the lateral PSB (case 10) had no effusion. Lesions of the PSBs Axial radiolucent lesions in the PSBs have previously been attributed to either sepsis 11,16 or trauma related to the insertion of the intersesamoidean ligament. 11,17 In previously documented cases of traumatically induced injury, gross discoloration or structural abnormalities of the intersesamoidean ligament have been seen, whereas in case 10, the ligament appeared intact. Surgical debridement had a successful outcome in 100% of five horses. 11 In contrast, two horses with traumatically induced lesions that were treated conservatively by the author had persistent lameness. d Post-mortem examination revealed bone necrosis at the lesion site. Therefore, it is suggested that surgical debridement is the treatment of choice. Comparison of Imaging Modalities Radiography was a relatively insensitive means for detection of bone injury, and it generally underestimated the extent of osseous pathology. However, in two horses (cases 10 and 12), there was relatively rapid progression of radiographic abnormalities with sequential radiographic examinations. The position of IRU generally correlated well with the site of the primary lesion, and the intensity of IRU usually reflected the type of injury, being intense in many horses. In the current study, intense IRU was usually associated with increased signal intensity in STIR images; mild IRU was normally related to normal signal intensity in STIR images. In one horse (case 8), there was only subtle IRU that was associated with subchondral bone sclerosis and no evidence of active bony reaction. In contrast, in the study of Zubrod et al., 8 subchondral bone injury was described in the fetlock region of four horses, all of which had increased signal in fat-suppressed MRIs. Two of these underwent scintigraphic examination, and in both, IRU was described as mild and diffuse. In this study in both horses (cases 4 and 5) with an incomplete dorsal cortical stress fracture of the proximal aspect of the proximal phalanx, IRU was localized to the dorsal aspect of the bone in lateral scintigraphic images. In Case 6, which had abnormalities on MR images extending from the dorsal to palmar aspects of the bone, IRU also was present from the dorsal to palmar aspects. MRI allowed the characterization of osseous lesions and had the capacity to identify subchondral bone lesions that would not be detectable arthroscopically because of the intact overlying cartilage; this has previously been documented in the carpus. 18 Prognosis Many of the horses had acute-onset, severe lameness that took many months to improve. Nonetheless, 75% of horses returned to their former level of activity, including international-level competition. Lameness associated with an incomplete dorsal cortical fracture of the proximal phalanx resolved most quickly. 5. Conclusions In conclusion, osseous trauma of the fetlock region can cause moderate to severe lameness, but with appropriate diagnostic imaging, an accurate diagnosis can usually be achieved. With suitable treatment, a favorable outcome is possible for many horses; however, in some cases, the convalescent period is long. We thank Michael Schramme, Richard Payne, and Timothy Greet for performing surgery. References and Footnotes 1. Shepherd M, Pilsworth R. Stress reactions in the plantarolateral condyles of the third metatarsal bone in UK Thoroughbreds: 26 cases, in Proceedings. 43rd Annu Conv Am Assoc Equine Pract 1997; Ross M. Scintigraphic and clinical findings in the Standardbred metatarsophalangeal joint: 114 cases. Equine Vet J 1998;30: McIlwraith CW. Subchondral bone cysts in the horse: aetiology, diagnosis and treatment. Equine Vet Edu 1998; 10: Markel M, Richardson D. Noncomminuted fractures of the proximal phalanx in 69 horses. J Am Vet Med Assoc 1985; 186: Ellis D, Simpson D, Greenwood R, et al. Observations and management of fractures of the proximal phalanx in young Thoroughbreds. Equine Vet J 1987;19: Tetens J, Ross M, Lloyd J. Comparison of racing performance before and after treatment of incomplete mid sagittal fractures of the proximal phalanx in Standardbreds: 49 cases. J Am Vet Med Assoc 1997;210: Kapelov S, Teresi L, Bradley W, et al. Bone contusions of the knee: increased lesion detection with fast spin-echo MR imaging with spectroscopic fat saturation. Radiology 1993; 189: Zubrod C, Schneider R, Tucker R, et al. Diagnosis of subchondral bone damage using magnetic resonance imaging in eleven horses. J Am Vet Med Assoc 2004;24: Butler J, Colles C, Dyson S, et al. The foot, pastern and fetlock. In: Clinical radiology of the horse, 2nd ed. Oxford, UK: Blackwell Scientific, 2000; AAEP PROCEEDINGS Vol

14 10. Dyson S, Murray R, Schramme M, et al. Magnetic resonance imaging of the equine foot: 15 horses. Equine Vet J 2003;35: Dabareiner R, Watkins J, Carter G, et al. Osteitis/osteomyelitis of the axial border of the proximal sesamoid bone in horses, in Proceedings. 45th Annu Conv Am Assoc Equine Pract 1999; Riggs C. Structural variation of the distal condyles of the third metacarpal and metatarsal bones in the horse. Equine Vet J 1999;31: Weekes J, Murray R, Dyson S. Scintigraphic evaluation of metacarpophalangeal and metatarsophalangeal joints in clinically sound horses. Vet Radiol Ultrasound 2004;45: McRobbie D, Moore E, Graves M, et al. MRI: from picture to proton. Cambridge, UK: Cambridge University Press, Murray R, Dyson S. Image interpretation and artefacts. Clin Tech Equine Pract 2006 (in press). 16. Wisner E, O Brien T, Pool R, et al. Osteomyelitis of the axial border of the proximal sesamoid bone in seven horses. Equine Vet J 1991;23: Winberg F, Petterson H. Diagnosis and treatment of lesions in the intersesamoidean ligament and its adjoining structures. Vet Surg 1994;23: Murray R, Dyson S, Schramme M. Validation of magnetic resonance imaging use in equine limbs. Clin Tech Equine Pract 2006 (in press). a Domosedan, Pfizer Ltd., Sandwich, Kent CT13 9NJ, UK. b Hermes, Nuclear Diagnostics, Unit E1, Springhead Enterprise Park, Northfleet, Gravesend, Kent DA11 8HH, UK. c General Electric, Milwaukee, WI d Dyson, S. Unpublished data. January Vol. 52 AAEP PROCEEDINGS