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Equine Veterinary Journal ISSN 0425-1644 DOI: 10.1111/evj.12227
Magnetic resonance imaging and foot lameness. Problem solved? Or do we know we know less now that we know more? The first description of ‘navicular disease’ is reputed to date from 1701, by an unknown author, in the Grand Marechal, Expert et Francais. In the mid 19th century, the term gained common usage, and the use of ‘neurotomy’ as a treatment, originally attributed to the same unknown French author, was popularised by Professor Sewell of London [1]. In 1904, Brown noted that the action of the horse had all the appearance of lameness being seated in the shoulder, but that dissection in numerous cases had confirmed that the ‘navicular joint’ was the seat of this lameness, ‘which had deceived and puzzled so many persons, both learned and unlearned’ [2]. Brown also suggested that the disease was of so delicate a nature, that its cure should not be attempted by an unskilful person. Yet over 100 years later, the cure appears as remote as ever, no matter how skilful the person, and we are still debating the seat of this lameness, which has deceived so many persons. There have been many theories as to the cause of navicular disease. The description of the thrombosis theory as a unifying explanation for lameness and ‘navicular changes’ as seen on radiographs was one such example [3]. This was confirmed by other authors who showed consistent arterial occlusion in horses affected by navicular disease, caused by organised thrombi, while there were isolated occlusions resulting in vessel narrowing only in control horses [4]. However, a very careful and elegant experiment occluding the ramus navicularis with a polyvinyl alcohol foam resulted only in temporary lameness, between 10 and 29 days [5]. It was in the late 1990s that attention shifted to the deep digital flexor (DDF) tendon. Core lesions of the DDF tendon were first described in a post mortem study of horses with ‘navicular syndrome’ [6]. These authors noted that fibrillation of the dorsal border of the DDF tendon and erosion of the palmar fibrocartilage of the navicular bone were the commonest lesions of significance. They also concluded that investigation of alternative in vivo imaging modalities of the DDF tendon should be a research priority. Attempts to image these lesions were made ultrasonographically, and it proved technically possible to achieve images of the DDF tendon, both distal to the navicular bone, and proximal to it [7–9]. Images distal to the navicular bone can be achieved by paring the frog until it has a rubbery texture, and then soaking the foot. Images proximal to the navicular bursa can be achieved by scanning the palmar surface of the pastern, immediately proximal to the bulbs of the heels. This technique can provide useful diagnostic information, but has not gained great acceptance [10]. Magnetic resonance imaging (MRI) was described in 1973, with further rapid advances made to arrive at the 2D and 3D Fourier transform methods used in all modern scanners [11]. Such has been the impact of this imaging modality that Paul Lauterbur and Peter Mansfield won Nobel Prizes for their work in 2003. Magnetic resonance images of equine cadaver limbs were published in 1987 [12]. In 1997 images were first obtained from a live horse, using a ‘high field’ MRI scanner, with the horse under general anaesthesia [13]. In 2000, a 1.5 T MRI scanner was installed at the Animal Health Trust in Newmarket, and was used to scan horses as well as small animals [14]. These machines use superconductors to produce a high strength magnetic field, which is enclosed within a ‘tube’, and thus inevitably require general anaesthesia to accommodate a horse’s limb. With the inevitable cost and risk of general anaesthesia, it was evident that to have widespread and practical application for equine clinicians, MRI would need to be possible in the standing sedated horse. Open coil technology, using a C shaped magnet, was originally developed to prevent the claustrophobia some people experience in these tube magnets. Several such machines were installed for veterinary use, including for horses. The Italian company Esaote marketed the VET-MR machine and a Siemens Magnetom Open 0.2 T system was installed at CIRALE in France specifically for equine applications in the 1990s. Hallmarq was formed in the summer of 2000 by a small group of MRI scientists, who had an interest in horses. After just over a year of development the first MRI images were collected from a standing horse at
264
the Bell Equine Veterinary Clinic, Kent, UK in early 2002 [15]. The technology has been embraced by equine veterinary practice, and there are now 71 Hallmarq MRI scanners around the world and, by October 2013 40,000 horses had been scanned (Hallmarq.net). The large majority of MRI scans conducted on horses are undertaken in the Hallmarq Eq2 standing scanner. Has this interest and commercial activity been matched by scientific results? While the majority of publications relate to ‘high field’ general anaesthesia magnets, there have been a wide number of publications from standing magnets as well [16]. It has been established that the diagnoses produced and the frequency of them are comparable, no matter which scan system is employed [17]. Magnetic resonance imaging has unquestionably contributed to a significant advance in unravelling the enigma of foot lameness. The shining example of the use of MRI is the increased understanding of the DDF tendon and its role in foot lameness. It is now well established that DDF tendonitis is an important cause of foot lameness [13,18]. Deep digital flexor tendonitis is reported to account for lameness in 59% of horses undergoing MRI scans for foot lameness, and up to 73% of horses when combined with abnormalities of the navicular bone [19]. Lesions of the DDF tendon were described in 83% of limbs from 264 horses with foot lameness [14]. Further, it has been clearly established that DDF tendonitis has a poor prognosis for return to athletic function [20]. Of 47 horses with primary DDF tendonitis 13 (28%) returned to original activity and a further 9 (19%) became sound but did not return to work. Twenty-five horses (53%) had persistent or recurrent lameness [19]. In another study, only 25% of horses returned to their original function [21]. Refinements of these early studies are now being published. It has been clearly shown that the type of lesion is relevant to the prognosis. The only form of DDF tendon lesion that is observed to shrink over time is a lesion of the dorsal border of the tendon [22]. Building on this work, the authors established that the prognosis was also different for this form of tendon lesion [23]. Of 160 horses with DDF tendonitis, 51 (31%) had dorsal border lesions, as opposed to core lesions (31%) or parasagittal tendon splits (36%). The overall prognosis was similar to previous studies, with 25% of horses returning to full levels of ridden exercise, while 55% returned to some ridden exercise. However, there was a significant difference between lesion types, with 73% of horses with dorsal border lesions, 50% of horses with parasagittal splits and 41% of horses with core lesions returning to some level of athletic activity. Multivariate analysis confirmed that horses with complete splits or core lesions of the DDF tendon were significantly less likely to return to some level of athletic activity than horses with dorsal border lesions P20% of the tendon cross-sectional area invariably remained lame [24], while those longer than 30 mm or >10% of tendon cross-sectional area invariably did not return to previous levels of athletic activity. Magnetic resonance imaging of the foot has also resulted in the description of other conditions. Some of these have been suspected from radiography. Deep erosions of the palmar surface of the navicular bone were reported with equivocal radiographic findings in 7/16 (44%) of horses. The prognosis with this condition, as might be surmised, is poor, with 7/16 (44%) of horses subjected to euthanasia, and only one (6%) able to return to previous levels of ridden activity [25]. Other conditions were previously undocumented. Bone marrow lesions, colloquially termed bone oedema, of the navicular bone have been reported [14]. Other indications for MRI have also become accepted. In the foot in particular, the use of MRI scans for the diagnosis of sepsis as a result of penetrating trauma has become widely accepted [26]. In a series of 55 horses, it was determined that the penetrating tract could usually be identified if the scan was undertaken within 7 days of injury. However, the correlation of surgical findings and MRI was poor, and wood foreign bodies were identified surgically in 2 horses, which had not been suspected on MRI scans [27]. Equine Veterinary Journal 46 (2014) 264–266 © 2014 EVJ Ltd
B. Bladon
All imaging modalities, particularly in the early stages of their adoption, suffer from 2 principle problems. The first is the presence of numerous artefacts that are not fully appreciated, and the second is the detection of abnormalities that are of limited clinical significance. Magnetic resonance imaging has been no exception. Bone marrow lesions have been reported in the middle phalanx. The prognosis with this condition is good, with 5/7 (71%) of horses able to return to full athletic activity [28]. These authors considered that 2 of the diagnosed bone marrow lesions were ‘non pathological’. Flexor surface resorption of the pedal bone has been described, and careful evaluation has confirmed that this finding is visible on radiographs [29]. This finding has often been described as a bone cyst in the insertion of the impar ligament or DDF tendon. However, the authors favoured the term ‘osseous resorption’ due to the variation in shape and lack of fluid within the cavitary lesions identified with MRI. The authors illustrated that flexor surface resorption is a common finding on MRI scans of the horse’s foot, but the finding was usually associated with other abnormalities of the distal sesamoidean impar ligament, DDF tendon or the navicular bone. Therefore the significance of these resorptive lesions as a primary cause of lameness was uncertain [29]. Nowhere has this problem, the detection of lesions of uncertain significance, been more marked than distal border fragmentation of the navicular bone [30]. This finding is clearly visible on MRI scans, and it has been shown that horses with distal border fragments are more likely to be lame than horses without [31]. The presence of distal border fragments can be predicted by radiographic lucency of the lateral or medial angles of the distal border of the bone [32]. However, it also clearly documented that sound horses can have distal border fragmentation [33]. The significance of this finding, in the absence of abnormalities of the navicular bone or impar ligament, remains difficult to interpret. The other key issue with new imaging modalities, that of hitherto unsuspected image artefacts, has also been true with MRI. At 54.7° to the magnetic field (B0), the magnetic dipole forces (between 2 magnets, the attraction of the north pole for the south pole, balanced with the repulsion of the north pole for the north pole) fall to zero. This results in a prolongation of the T2 time – the time taken for the protons to spin out of phase with each other. This results in an artefactual hyperintensity of ligaments and tendons when they are between 30° and 75° to B0, with increasing hyperintensity closer to 55°. This artefact was first identified in the horse in the insertion of the DDF tendon on scans obtained in ‘high field’ magnets under general anaesthesia [34]. The presence of the artefact has been clearly documented in the lateral collateral ligament of the distal interphalangeal joint, and the medial oblique distal sesamoidean ligament [35]. Desmitis of the collateral ligaments of the distal interphalangeal joint has been described as a cause of lameness, diagnosed in ‘high field’ scanners [36]. However, the presence of this artefact can make the diagnosis challenging in standing MRI, and ideally the diagnosis is confirmed by the presence of osseous abnormalities of the insertion or origin of the ligament [37]. Similarly, desmitis of the sesamoidean ligaments has been described [38–40]. However, the presence of this artefact in these ligaments can make the diagnosis particularly challenging using a horizontal magnetic field in the standing horse. Has this increased knowledge of the lesions and their associated prognosis had an impact on the ‘cure’ for ‘navicular disease’? It is fair to say that therapy has not kept pace with diagnosis, although there have been developments in this field. Direct medication of the navicular bursa is a logical step when imaging suggests that this is the location of the pathology. Two studies have now documented the effect of bursal medication [41,42]. Combined, these papers have shown that 70–80% of horses will respond to medication of the navicular bursa, and the mean duration of soundness was 7.3 and 9.3 months. Both studies identified that abnormalities of the navicular bone, such as flexor surface erosions or adhesions, were associated with a worse outlook. One study concluded that horses with increased fibrous scar formation in the navicular bursa were associated with a worse outlook [42]. This finding is interesting, as this condition is potentially responsive to minimally invasive surgery, to examine the navicular bursa [43]. More accurate medication of the affected site has also been reported [44]. Finally, the use of MRI screening for accurate medication of the relevant tissues has also been described [45]. However, regenerative medicine is a discipline in its infancy. There are few simpler lesions to medicate accurately than the core lesion of the Equine Veterinary Journal 46 (2014) 264–266 © 2014 EVJ Ltd
Problem solved? Or do we know we know less now we know more?
superficial digital flexor tendon in the mid metacarpus, yet the prognosis with this lesion remains guarded [46]. Magnetic resonance imaging has contributed enormously to the knowledge surrounding foot lameness. The frequency and prognosis of previously recognised conditions has been established. New conditions have been reported, and a prognosis established for these. New or adapted therapeutic approaches have been described. However, many questions remain to be answered. More objective methods for classification of lesions are needed, particularly with the varying prognoses. Therapeutic interventions are still limited, and critically these may require greater understanding of the ageing process in orthopaedic tissues, and how to manipulate it. Finally, it is likely that the significance of lesions will continue to be refined for many years to come. After all, radiographic findings are still being reported, over 100 years after the development of the x-ray. B. Bladon Donnington Grove Veterinary Surgery, Newbury, Berkshire, UK
References 1. Fleming, G. (1869) Horse Shoes and Horse Shoeing, Their Origins, History, Uses and Abuses, Chapman and Hall, London. 2. Brown, T. (1904) The Complete Modern Farrier, John Grant, Edinburgh. 3. Colles, C.M. and Hickman, J. (1977) The arterial supply of the navicular bone and its variations in navicular disease. Equine Vet. J. 9, 150-154. 4. Fricker, C., Riek, W. and Hugelshofer, J. (1982) Occlusion of the digital arteries – a model for pathogenesis of navicular disease. Equine Vet. J. 14, 203-207. 5. Rijkenhuizen, A.B., Németh, F., Dik, K.J., Goedegebuure, S.A. and Van de Brom, W.E. (1989) The effect of artificial occlusion of the ramus navicularis and its branching arteries on the navicular bone in horses: an experimental study. Equine Vet. J. 21, 425-430. 6. Wright, I.M., Kidd, L. and Thorp, B.H. (1998) Gross, histological and histomorphometric features of the navicular bone and related structures in the horse. Equine Vet. J. 30, 220-234. 7. Hauser, M.L., Rantanen, N.W. and Modransky, P.D. (1982) Ultrasound examination of distal interphalangeal joint, navicular bursa, navicular bone and deep digital tendon. J. Equine Vet. Sci. 2, 95-97. 8. Sage, A. (2002) Ultrasonography of the soft tissue structures of the equine foot. Equine Vet. Educ. 14, 221-224. 9. Kristoffersen, M. and Thoefner, M. (2003) Ultrasonography of the navicular region in horses. Equine Vet. Educ. 15, 150-157. 10. Rabba, S., Bolen, G. and Verwilghen, D. (2011) Ultrasonographic findings in horses with foot pain but without radiographically detectable osseous abnormalities. Vet. Radiol. Ultrasound 52, 95-102. 11. Mansfield, P. (1977) Multi-planar image formation using NMR spin echoes. J. Phys. C. Solid State Physics 10, 55-58. 12. Park, R.D., Nelson, T.R. and Hoopes, P.J. (1987) Magnetic resonance imaging of the normal equine digit and metacarpophalangeal joint. Vet. Radiol. 28, 105-116. 13. Schneider, R.K., Gavin, P.R. and Tucker, R.L. (2003) What MRI is teaching us about navicular disease (21-Nov-2003). Ivisorg, Proceedings of 49th Annual Convention of the American Association of Equine Practitioners, 2003, New Orleans, Louisiana. 14. Dyson, S. and Murray, R. (2007) Magnetic resonance imaging evaluation of 264 horses with foot pain: the podotrochlear apparatus, deep digital flexor tendon and collateral ligaments of the distal interphalangeal joint. Equine Vet. J. 39, 340-343. 15. Mair, T.S. and Bolas, N.M. (2002) MRI of the distal limbs in the standing sedated horse, 41st Congress of the British Equine Veterinary Association. p 206. 16. Mair, T.S., Kinns, J., Jones, R.D. and Bolas, N.M. (2005) Magnetic resonance imaging of the distal limb of the standing horse. Equine Vet. Educ. 17, 74-78. 17. Murray, R., Mair, T., Sherlock, C. and Blunden, A. (2009) Comparison of high-field and low-field magnetic resonance images of cadaver limbs of horses. Vet. Rec. 165, 281-288. 18. Dyson, S., Murray, R., Schramme, M. and Branch, M. (2003) Lameness in 46 horses associated with deep digital flexor tendonitis in the digit: diagnosis confirmed with magnetic resonance imaging. Equine Vet. J. 35, 681-690.
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Problem solved? Or do we know we know less now we know more?
19. Dyson, S., Murray, R. and Schramme, M. (2005) Lameness associated with foot pain: results of magnetic resonance imaging in 199 horses (January 2001-December 2003) and response to treatment. Equine Vet. J. 37, 113-121. 20. Schramme, M.C. (2011) Deep digital flexor tendonopathy in the foot. Equine Vet. Educ. 23, 403-415. 21. Boswell, J. (2009) Does a DDFT injury in the foot mean the end of the horse’s athletic career? Proceedings of the 48th British Equine Veterinary Congress. p 20. 22. Milner, P.I., Sidwell, S., Talbot, A.M. and Clegg, P.D. (2012) Short-term temporal alterations in magnetic resonance signal occur in primary lesions identified in the deep digital flexor tendon of the equine digit. Equine Vet. J. 44, 157162. 23. Cillan-Garcia, E., Milner, P.I., Talbot, A., Tucker, R., Hendey, F., Boswell, J., Reardon, R.J. and Taylor, S.E. (2013) Deep digital flexor tendon injury within the hoof capsule; does lesion type or location predict prognosis? Vet. Rec. 173, 70-76. 24. Vanel, M., Olive, J., Gold, S., Mitchell, R.D. and Walker, L. (2012) Clinical significance and prognosis of deep digital flexor tendinopathy assessed over time using MRI. Vet. Radiol. Ultrasound 53, 1-7. 25. Sherlock, C., Mair, T. and Blunden, T. (2008) Deep erosions of the palmar aspect of the navicular bone diagnosed by standing magnetic resonance imaging. Equine Vet. J. 40, 684-692. 26. Kinns, J. and Mair, T.S. (2005) Use of magnetic resonance imaging to assess soft tissue damage in the foot following penetrating injury in 3 horses. Equine Vet. Educ. 17, 69-73. 27. Del Junco, C.I., Mair, T.S., Powell, S.E., Milner, P.I., Font, A.F., Schwarz, T. and Weaver, M.P. (2012) Magnetic resonance imaging findings of equine solar penetration wounds. Vet. Radiol. Ultrasound 53, 71-75. 28. Olive, J., Mair, T.S. and Charles, B. (2009) Use of standing low-field magnetic resonance imaging to diagnose middle phalanx bone marrow lesions in horses. Equine Vet. Educ. 21, 116-123. 29. Young, A.C., Dimock, A.N., Puchalski, S.M., Murphy, B. and Spriet, M. (2012) Magnetic resonance and radiographic diagnosis of osseous resorption of the flexor surface of the distal phalanx in the horse. Equine Vet. J. 44, Suppl. 43, 3-7. 30. Biggi, M., Blunden, T. and Dyson, S. (2013) Can distal border fragments of the navicular bone be a primary cause of lameness? Equine Vet. Educ. 25, 347-351. 31. Biggi, M. and Dyson, S. (2012) Distal border fragments and shape of the navicular bone: radiological evaluation in lame horses and horses free from lameness. Equine Vet. J. 44, 325-331. 32. Biggi, M. and Dyson, S. (2010) Comparison between radiological and magnetic resonance imaging lesions in the distal border of the navicular bone with particular reference to distal border fragments and osseous cyst-like lesions. Equine Vet. J. 42, 707-712. 33. Yorke, E.H., Judy, C.E., Saveraid, T.C., McGowan, C.P. and Caldwell, F.J. (2013) Distal border fragments of the equine navicular bone: association between
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magnetic resonance imaging characteristics and clinical lameness. Vet. Radiol. Ultrasound 55, 35-44. Busoni, V. and Snaps, F. (2000) Effect of deep digital flexor tendon orientation on the MRI signal intensity in equine isolated limbs. Vet. Radiol. Ultrasound 43, 428-430. Smith, M., Dyson, S. and Murray, R. (2008) Is a magic angle effect observed in the collateral ligaments of the distal interphalangeal joint or the oblique sesamoidean ligaments during standing magnetic resonance imaging? Vet. Radiol. Ultrasound 49, 509-515. Dyson, S., Murray, R., Schramme, M. and Branch, M. (2004) Collateral desmitis of the distal interphalangeal joint in 18 horses (2001-2002). Equine Vet. J. 36, 160-166. Dakin, S.G., Dyson, S.J., Murray, R.C. and Tranquille, C. (2010) Osseous abnormalities associated with collateral desmopathy of the distal interphalangeal joint: part 1. Equine Vet. J. 41, 786-793. Smith, S., Dyson, S. and Murray, R. (2008) Magnetic resonance imaging of distal sesamoidean ligament injury. Vet. Radiol. Ultrasound 49, 516-528. Sampson, S., Schneider, R., Tucker, R., Gavin, P., Zubrod, C. and Ho, C. (2007) Magnetic resonance imaging features of oblique and straight distal sesamoidean desmitis in 27 horses. Vet. Radiol. Ultrasound 48, 303-311. King, J.N., Zubrod, C.J., Schneider, R.K., Sampson, S.N. and Roberts, G. (2013) MRI findings in 232 horses with lameness localized to the metacarpo (tarso)phalangeal region and without a radiographic diagnosis. Vet. Radiol. Ultrasound 54, 36-47. Bell, C., Howard, R., Taylor, D., Voss, E. and Werpy, N. (2009) Outcomes of podotrochlear (navicular) bursa injections for signs of foot pain in horses evaluated via magnetic resonance imaging: 23 cases (2005-2007). J. Am. Vet. Med. Ass. 234, 920-925. Marsh, C.A., Schneider, R.K., Sampson, S.N. and Roberts, G.D. (2012) Response to injection of the navicular bursa with corticosteroid and hyaluronan following high-field magnetic resonance imaging in horses with signs of navicular syndrome: 101 cases (2000-2008). J. Am. Vet. Med. Ass. 241, 1353-1364. Smith, M.R.W. and Wright, I.M. (2012) Endoscopic evaluation of the navicular bursa: observations, treatment and outcome in 92 cases with identified pathology. Equine Vet. J. 44, 339-345. Anderson, J.D.C., Puchalski, S.M., Larson, R.F., Delco, M.L. and Snyder, J.R. (2008) Injection of the insertion of the deep digital flexor tendon in horses using radiographic guidance. Equine Vet. Educ. 20, 383-388. Lamb, M.M., Barrett, J.G., White, N.A. II and Were, S.R. (2013) Accuracy of low-field magnetic resonance imaging versus radiography for guiding injection of equine distal interphalangeal joint collateral ligaments. Vet. Radiol. Ultrasound Epub ahead of print; doi: 10.1111/vru.12109. O’Meara, B., Bladon, B., Parkin, T., Fraser, B. and Lischer, C. (2010) An investigation of the relationship between race performance and superficial digital flexor tendonitis in the Thoroughbred racehorse. Equine Vet. J. 42, 322-326.
Equine Veterinary Journal 46 (2014) 264–266 © 2014 EVJ Ltd
Equine Veterinary Journal ISSN 0425-1644 DOI: 10.1111/evj.12227
Magnetic resonance imaging and foot lameness. Problem solved? Or do we know we know less now that we know more? The first description of ‘navicular disease’ is reputed to date from 1701, by an unknown author, in the Grand Marechal, Expert et Francais. In the mid 19th century, the term gained common usage, and the use of ‘neurotomy’ as a treatment, originally attributed to the same unknown French author, was popularised by Professor Sewell of London [1]. In 1904, Brown noted that the action of the horse had all the appearance of lameness being seated in the shoulder, but that dissection in numerous cases had confirmed that the ‘navicular joint’ was the seat of this lameness, ‘which had deceived and puzzled so many persons, both learned and unlearned’ [2]. Brown also suggested that the disease was of so delicate a nature, that its cure should not be attempted by an unskilful person. Yet over 100 years later, the cure appears as remote as ever, no matter how skilful the person, and we are still debating the seat of this lameness, which has deceived so many persons. There have been many theories as to the cause of navicular disease. The description of the thrombosis theory as a unifying explanation for lameness and ‘navicular changes’ as seen on radiographs was one such example [3]. This was confirmed by other authors who showed consistent arterial occlusion in horses affected by navicular disease, caused by organised thrombi, while there were isolated occlusions resulting in vessel narrowing only in control horses [4]. However, a very careful and elegant experiment occluding the ramus navicularis with a polyvinyl alcohol foam resulted only in temporary lameness, between 10 and 29 days [5]. It was in the late 1990s that attention shifted to the deep digital flexor (DDF) tendon. Core lesions of the DDF tendon were first described in a post mortem study of horses with ‘navicular syndrome’ [6]. These authors noted that fibrillation of the dorsal border of the DDF tendon and erosion of the palmar fibrocartilage of the navicular bone were the commonest lesions of significance. They also concluded that investigation of alternative in vivo imaging modalities of the DDF tendon should be a research priority. Attempts to image these lesions were made ultrasonographically, and it proved technically possible to achieve images of the DDF tendon, both distal to the navicular bone, and proximal to it [7–9]. Images distal to the navicular bone can be achieved by paring the frog until it has a rubbery texture, and then soaking the foot. Images proximal to the navicular bursa can be achieved by scanning the palmar surface of the pastern, immediately proximal to the bulbs of the heels. This technique can provide useful diagnostic information, but has not gained great acceptance [10]. Magnetic resonance imaging (MRI) was described in 1973, with further rapid advances made to arrive at the 2D and 3D Fourier transform methods used in all modern scanners [11]. Such has been the impact of this imaging modality that Paul Lauterbur and Peter Mansfield won Nobel Prizes for their work in 2003. Magnetic resonance images of equine cadaver limbs were published in 1987 [12]. In 1997 images were first obtained from a live horse, using a ‘high field’ MRI scanner, with the horse under general anaesthesia [13]. In 2000, a 1.5 T MRI scanner was installed at the Animal Health Trust in Newmarket, and was used to scan horses as well as small animals [14]. These machines use superconductors to produce a high strength magnetic field, which is enclosed within a ‘tube’, and thus inevitably require general anaesthesia to accommodate a horse’s limb. With the inevitable cost and risk of general anaesthesia, it was evident that to have widespread and practical application for equine clinicians, MRI would need to be possible in the standing sedated horse. Open coil technology, using a C shaped magnet, was originally developed to prevent the claustrophobia some people experience in these tube magnets. Several such machines were installed for veterinary use, including for horses. The Italian company Esaote marketed the VET-MR machine and a Siemens Magnetom Open 0.2 T system was installed at CIRALE in France specifically for equine applications in the 1990s. Hallmarq was formed in the summer of 2000 by a small group of MRI scientists, who had an interest in horses. After just over a year of development the first MRI images were collected from a standing horse at
264
the Bell Equine Veterinary Clinic, Kent, UK in early 2002 [15]. The technology has been embraced by equine veterinary practice, and there are now 71 Hallmarq MRI scanners around the world and, by October 2013 40,000 horses had been scanned (Hallmarq.net). The large majority of MRI scans conducted on horses are undertaken in the Hallmarq Eq2 standing scanner. Has this interest and commercial activity been matched by scientific results? While the majority of publications relate to ‘high field’ general anaesthesia magnets, there have been a wide number of publications from standing magnets as well [16]. It has been established that the diagnoses produced and the frequency of them are comparable, no matter which scan system is employed [17]. Magnetic resonance imaging has unquestionably contributed to a significant advance in unravelling the enigma of foot lameness. The shining example of the use of MRI is the increased understanding of the DDF tendon and its role in foot lameness. It is now well established that DDF tendonitis is an important cause of foot lameness [13,18]. Deep digital flexor tendonitis is reported to account for lameness in 59% of horses undergoing MRI scans for foot lameness, and up to 73% of horses when combined with abnormalities of the navicular bone [19]. Lesions of the DDF tendon were described in 83% of limbs from 264 horses with foot lameness [14]. Further, it has been clearly established that DDF tendonitis has a poor prognosis for return to athletic function [20]. Of 47 horses with primary DDF tendonitis 13 (28%) returned to original activity and a further 9 (19%) became sound but did not return to work. Twenty-five horses (53%) had persistent or recurrent lameness [19]. In another study, only 25% of horses returned to their original function [21]. Refinements of these early studies are now being published. It has been clearly shown that the type of lesion is relevant to the prognosis. The only form of DDF tendon lesion that is observed to shrink over time is a lesion of the dorsal border of the tendon [22]. Building on this work, the authors established that the prognosis was also different for this form of tendon lesion [23]. Of 160 horses with DDF tendonitis, 51 (31%) had dorsal border lesions, as opposed to core lesions (31%) or parasagittal tendon splits (36%). The overall prognosis was similar to previous studies, with 25% of horses returning to full levels of ridden exercise, while 55% returned to some ridden exercise. However, there was a significant difference between lesion types, with 73% of horses with dorsal border lesions, 50% of horses with parasagittal splits and 41% of horses with core lesions returning to some level of athletic activity. Multivariate analysis confirmed that horses with complete splits or core lesions of the DDF tendon were significantly less likely to return to some level of athletic activity than horses with dorsal border lesions P20% of the tendon cross-sectional area invariably remained lame [24], while those longer than 30 mm or >10% of tendon cross-sectional area invariably did not return to previous levels of athletic activity. Magnetic resonance imaging of the foot has also resulted in the description of other conditions. Some of these have been suspected from radiography. Deep erosions of the palmar surface of the navicular bone were reported with equivocal radiographic findings in 7/16 (44%) of horses. The prognosis with this condition, as might be surmised, is poor, with 7/16 (44%) of horses subjected to euthanasia, and only one (6%) able to return to previous levels of ridden activity [25]. Other conditions were previously undocumented. Bone marrow lesions, colloquially termed bone oedema, of the navicular bone have been reported [14]. Other indications for MRI have also become accepted. In the foot in particular, the use of MRI scans for the diagnosis of sepsis as a result of penetrating trauma has become widely accepted [26]. In a series of 55 horses, it was determined that the penetrating tract could usually be identified if the scan was undertaken within 7 days of injury. However, the correlation of surgical findings and MRI was poor, and wood foreign bodies were identified surgically in 2 horses, which had not been suspected on MRI scans [27]. Equine Veterinary Journal 46 (2014) 264–266 © 2014 EVJ Ltd
B. Bladon
All imaging modalities, particularly in the early stages of their adoption, suffer from 2 principle problems. The first is the presence of numerous artefacts that are not fully appreciated, and the second is the detection of abnormalities that are of limited clinical significance. Magnetic resonance imaging has been no exception. Bone marrow lesions have been reported in the middle phalanx. The prognosis with this condition is good, with 5/7 (71%) of horses able to return to full athletic activity [28]. These authors considered that 2 of the diagnosed bone marrow lesions were ‘non pathological’. Flexor surface resorption of the pedal bone has been described, and careful evaluation has confirmed that this finding is visible on radiographs [29]. This finding has often been described as a bone cyst in the insertion of the impar ligament or DDF tendon. However, the authors favoured the term ‘osseous resorption’ due to the variation in shape and lack of fluid within the cavitary lesions identified with MRI. The authors illustrated that flexor surface resorption is a common finding on MRI scans of the horse’s foot, but the finding was usually associated with other abnormalities of the distal sesamoidean impar ligament, DDF tendon or the navicular bone. Therefore the significance of these resorptive lesions as a primary cause of lameness was uncertain [29]. Nowhere has this problem, the detection of lesions of uncertain significance, been more marked than distal border fragmentation of the navicular bone [30]. This finding is clearly visible on MRI scans, and it has been shown that horses with distal border fragments are more likely to be lame than horses without [31]. The presence of distal border fragments can be predicted by radiographic lucency of the lateral or medial angles of the distal border of the bone [32]. However, it also clearly documented that sound horses can have distal border fragmentation [33]. The significance of this finding, in the absence of abnormalities of the navicular bone or impar ligament, remains difficult to interpret. The other key issue with new imaging modalities, that of hitherto unsuspected image artefacts, has also been true with MRI. At 54.7° to the magnetic field (B0), the magnetic dipole forces (between 2 magnets, the attraction of the north pole for the south pole, balanced with the repulsion of the north pole for the north pole) fall to zero. This results in a prolongation of the T2 time – the time taken for the protons to spin out of phase with each other. This results in an artefactual hyperintensity of ligaments and tendons when they are between 30° and 75° to B0, with increasing hyperintensity closer to 55°. This artefact was first identified in the horse in the insertion of the DDF tendon on scans obtained in ‘high field’ magnets under general anaesthesia [34]. The presence of the artefact has been clearly documented in the lateral collateral ligament of the distal interphalangeal joint, and the medial oblique distal sesamoidean ligament [35]. Desmitis of the collateral ligaments of the distal interphalangeal joint has been described as a cause of lameness, diagnosed in ‘high field’ scanners [36]. However, the presence of this artefact can make the diagnosis challenging in standing MRI, and ideally the diagnosis is confirmed by the presence of osseous abnormalities of the insertion or origin of the ligament [37]. Similarly, desmitis of the sesamoidean ligaments has been described [38–40]. However, the presence of this artefact in these ligaments can make the diagnosis particularly challenging using a horizontal magnetic field in the standing horse. Has this increased knowledge of the lesions and their associated prognosis had an impact on the ‘cure’ for ‘navicular disease’? It is fair to say that therapy has not kept pace with diagnosis, although there have been developments in this field. Direct medication of the navicular bursa is a logical step when imaging suggests that this is the location of the pathology. Two studies have now documented the effect of bursal medication [41,42]. Combined, these papers have shown that 70–80% of horses will respond to medication of the navicular bursa, and the mean duration of soundness was 7.3 and 9.3 months. Both studies identified that abnormalities of the navicular bone, such as flexor surface erosions or adhesions, were associated with a worse outlook. One study concluded that horses with increased fibrous scar formation in the navicular bursa were associated with a worse outlook [42]. This finding is interesting, as this condition is potentially responsive to minimally invasive surgery, to examine the navicular bursa [43]. More accurate medication of the affected site has also been reported [44]. Finally, the use of MRI screening for accurate medication of the relevant tissues has also been described [45]. However, regenerative medicine is a discipline in its infancy. There are few simpler lesions to medicate accurately than the core lesion of the Equine Veterinary Journal 46 (2014) 264–266 © 2014 EVJ Ltd
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superficial digital flexor tendon in the mid metacarpus, yet the prognosis with this lesion remains guarded [46]. Magnetic resonance imaging has contributed enormously to the knowledge surrounding foot lameness. The frequency and prognosis of previously recognised conditions has been established. New conditions have been reported, and a prognosis established for these. New or adapted therapeutic approaches have been described. However, many questions remain to be answered. More objective methods for classification of lesions are needed, particularly with the varying prognoses. Therapeutic interventions are still limited, and critically these may require greater understanding of the ageing process in orthopaedic tissues, and how to manipulate it. Finally, it is likely that the significance of lesions will continue to be refined for many years to come. After all, radiographic findings are still being reported, over 100 years after the development of the x-ray. B. Bladon Donnington Grove Veterinary Surgery, Newbury, Berkshire, UK
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