Research in Veterinary Science 98 (2015) 92–97
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Research in Veterinary Science j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / r v s c
Do low field magnetic resonance imaging abnormalities correlate with macroscopical and histological changes within the equine deep digital flexor tendon? C.E. Sherlock a,*, T.S. Mair a, J. Ireland b, T. Blunden b a b
Bell Equine Veterinary Clinic, Mereworth, Kent ME18 5GS, UK Animal Health Trust, Lanwades Park, Kentford, Newmarket, Suffolk CB8 7UU, UK
A R T I C L E
I N F O
Article history: Received 20 November 2013 Accepted 4 December 2014 Keywords: Horse Lameness Foot Lesion Injury Penetration
A B S T R A C T
Correlating magnetic resonance (MR) imaging and histopathological findings is essential to validate low field MR imaging in lame horses. This study aimed to compare signal changes in the deep digital flexor tendon (DDFT) of the distal limb on low field MR imaging with macroscopical and histological findings. Cadaver limbs from lame horses with DDFT lesions were selected. The DDFT MR imaging findings and histopathological results were graded, and macroscopical abnormalities were recorded. There was a strong correlation between MR imaging and histopathology grades (rs = 0.76, p < 0.001) in the foot. There was moderate agreement (Kappa statistic 0.52) between the MR and histopathology grades; agreement was superior further proximal in the foot. The presence and severity of pathology in the DDFT are well represented by the presence and severity of MR imaging signal changes. The study supports the use of low field MR imaging for diagnosis of equine distal limb DDFT lesions. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Damage to the deep digital flexor tendon (DDFT) in the distal limb is recognised as a common cause of lameness in horses (Dyson and Murray, 2007; Dyson et al., 2003a, 2003b, 2005; Mair and Kinns, 2005; Mair et al., 2005; Murray et al., 2006b; Sherlock et al., 2007; Whitton et al., 1998) either alone or in combination with pathological changes in other structures. Magnetic resonance (MR) imaging has considerably contributed to the understanding and diagnosis of deep digital flexor (DDF) tendon lesions in the foot; high field MR signal intensity changes have been shown to correlate well with histological changes within the DDFT (Blunden et al., 2009; Murray et al., 2006a). MR imaging is also being used to monitor disease course and progression of DDF tendon lesions (Milner et al., 2012). For optimal clinical application, it is not only important to be able to identify abnormalities on MR imaging, but also to reliably interpret these abnormalities in terms of underlying pathology (Seewan et al., 2009). Compared to high field MR systems, low field MR systems are less expensive and can be used to image standing sedated horses which make them more practical in private practice and reduces the risks of general anaesthesia (Murray et al., 2009). However, low field images are acquired with different imaging
* Corresponding author. Bell Equine Veterinary Clinic, Mereworth, Kent ME18 5GS, UK. Tel.: +44 01622813700; fax: +11 01622812233. E-mail address:
[email protected] (C.E. Sherlock). http://dx.doi.org/10.1016/j.rvsc.2014.12.008 0034-5288/© 2014 Elsevier Ltd. All rights reserved.
parameters and there is a lower signal to noise ratio which limits image resolution (Murray et al., 2009). A study comparing high field and low field MR imaging of equine cadaver limbs showed that severe lesions of the DDFT could be identified by both imaging systems, but that small focal lesions identified by high field MR imaging were often not visible on low field MR images (Murray et al., 2009). Furthermore, the predictive value of low field MRI for identification of dorsal border lesions of the DDFT recognised during navicular bursoscopy has been questioned recently as MRI has revealed both false negative and false positive results compared with bursoscopic evaluation (Smith and Wright, 2012). Studies correlating results of low field MR imaging and histopathological changes in the DDFT in the foot are scarce (Murray et al., 2009), although it has been demonstrated that low field MR imaging is a more accurate way of demonstrating damage and histological changes than ultrasonography in the soft tissue structures of the palmar and plantar metacarpus/metatarsus (Kasashima et al., 2002; Rand et al., 1998). Postmortem MR imaging and histopathological correlation studies enable a direct translation of basic pathology to the clinical setting, and simultaneously serve as a biological validation of MR imaging techniques (Seewan et al., 2009). The aims of this study were to compare signal changes in the DDFT of the distal region of cadaver limbs on low field MR imaging with macroscopical and histological findings within the DDFT. It was hypothesised that changes in signal intensity on low field MR imaging would correlate with the presence and severity of histological changes in the DDFT. Furthermore, it was hypothesised that
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strong correlation and good agreement of MR and histological grades would exist at all anatomical locations of the DDFT within the foot. 2. Materials and methods The records of all MR examinations of the distal limb undertaken at Bell Equine Veterinary Clinic between 2005 and 2009 were reviewed. Horses were included in this study if: i) The site of pain causing lameness had been localised to the foot or digital flexor tendon sheath by observing a greater than 80% improvement in the degree of lameness following perineural analgesia of the palmar nerves (palmar digital or abaxial sesamoid nerve blocks), or intra-articular analgesia of the distal interphalangeal joint, or intrathecal analgesia of the digital flexor tendon sheath, or the horses had a known history of a penetrating injury involving the DDFT. ii) The horse was euthanised (for reasons unrelated to inclusion in the study) and the affected limbs were available for postmortem examination. iii) Informed owner consent for inclusion in the study was obtained. iv) Cadaver MR images of the area of interest and the results of macroscopical and histological examination of the DDFT were available for evaluation. The signalment, history and diagnostic work up of each horse were recorded. Limbs were either imaged fresh (within 12 hours of death) or after being frozen (−20 °C for a maximum period of 3 months) and defrosted. All cadaver limbs were positioned to simulate a standing stance and imaged using a 0.27 Tesla open magnet and dedicated
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extremity radio frequency coil (Hallmarq Veterinary Imaging Ltd, Guildford, UK) (Mair and Bolas, 2002; Mair et al., 2005). Images were obtained in sagittal, transverse (including perpendicular to the DDFT at all examined levels) and frontal planes using gradient echo (GRE) T1-weighted, GRE T2*-weighted, fast spin echo (FSE) T2-weighted and FSE short tau inversion recovery (STIR) sequences (Supplementary Table S1). Images were interpreted by an experienced analyst (TSM) who was blinded to the histology results. All osseous and soft tissue structures within the 13 cm field of view were examined. In the foot, the DDFT was divided into 4 levels as described previously (Busoni et al., 2005). Level 1 was proximal to the collateral sesamoidean ligaments (CSLs), level 2 was adjacent to the CSLs and proximal recess of the navicular bursa, level 3 was adjacent to the navicular bone and level 4 was distal to the navicular bone. Table 1 defines the grading system for the MR imaging findings at each anatomical level of the DDFT. The type (dorsal, core or parasagittal split [tear]) (Blunden et al., 2009) and the location at each level (proximal, distal, axial, abaxial, medial or lateral) of DDFT lesions were recorded. The limbs were dissected and macroscopical lesions were recorded and photographed. The DDFT was sectioned into the same anatomical divisions as used to grade the MR abnormalities, and each section was placed into 10% neutral-buffered formalin. The DDFT sections were softened in 4% phenol in 70% alcohol for 1–5 days before being embedded in paraffin wax. Five micron sections were stained with Harris’s haematoxylin and eosin (H&E) for examination. Histological abnormalities were described and graded (Table 1). The descriptive and quantified abnormalities detected within the DDFT on MR imaging and histology were then compared. Statistical analyses were performed using PASW version 18 (SPSS Inc, Chicago, IL, USA) and Stata version 9.2 (StataCorp LP, College Station, TX, USA). Spearman’s rank correlation coefficients were used to
Table 1 Table to show the grading system used to determine MR imaging and histology grades in the DDFT in the foot. Grade
MR findings
0
• • •
Uniform low signal intensity throughout DDFT + Excellent definition with adjacent structures + /− Symmetry between lobes
1
•
Small areas (<1 mm2) of mildly increased signal intensity (seen clearest on T1 GRE images) + /− Very mild irregularity of DDFT surface + /− Mild asymmetry between lobes
• •
2
• • • •
3
• • • •
Moderate areas (<1/3 of transverse DDFT tendon area) of moderately increased signal intensity on T1 GRE, T2* GRE +/− T2 FSE +/− STIR FSE images + /− Moderate irregularity of the tendon margins + /− Moderate asymmetry between lobes + /− Partial thickness parasagittal split
Large areas (>1/3 of transverse DDFT area) of marked increased signal intensity on T1 GRE, T2* GRE +/− T2 FSE +/− STIR FSE images + /− Marked disruption of tendon margins + /− Marked asymmetry between lobes + /− Full thickness parasagittal split
Histopathology findings
• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
Pallid central septum between lobes (where DDFT is lobulated) + /− Mild blood vessel ghosting + /− Very small areas of marginal fibrocartilage metaplasia + /− Very mild superficial fibrillation in the centre of the DDFT + /− Normal collagen fascicular structure Mild pallor of interfascicular septa Mild blood vessel ghosting + /− Occasional vascular occlusion Mild fibrovascular proliferation + /− Mild fibrocartilagenous metaplasia + /− Mild superficial dorsal fibrillation/crevicing/splitting + /− Mild disruption of collagen fascicular structure Moderate pallor of interfascicular septa Moderate blood vessel ghosting + /− Vascular occlusion + /− Prominent groups of blood vessels Moderate fibrovascular proliferation + /−Moderate focal fibrocartilagenous metaplasia + /− Moderate focal fibroplasia + /− Moderate superficial dorsal fibrillation, crevicing or splitting + /− Moderate disruption of collagen fascicular structure Marked pallor of interfascicular septa Marked blood vessel ghosting + /− Vascular occlusion + /− Prominent vascularisation + /− Marked fibrovascular proliferation + /− Marked focal fibrocartilagenous metaplasia + /− Marked focal fibroplasia + /− Marked dorsal fibrillation, crevicing or splitting that may extend into the deep dorsal layer of the DDFT + /− Marked disruption of collagen fascicular structure + /− Basophilic amorphous foci (mineralisation)
DDFT, Deep digital flexor tendon; GRE, Gradient Echo; STIR, Short tau inversion recovery; FSE, Fast Spin Echo.
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assess the degree of association between MR imaging and histology grades. Mann–Whitney U tests were used to test the statistical significance of differences in median values between MR imaging and histology grades and Kappa statistics were calculated to estimate the agreement between MR imaging and histology grades. 3. Results Thirty-four cadaver limbs from 26 horses were included in the study. Twenty-eight cadaver limbs from 20 horses had lameness isolated to the foot in the absence of penetrating injury (Cases 1–20). Three cadaver limbs from 3 horses had lameness isolated to the digital flexor tendon sheath (DFTS) in the absence of penetrating injury (Cases 21–23). Three cadaver limbs from 3 horses had lameness attributed to a penetrating injury that involved damage to the DDFT; the times from penetrating injury to euthanasia were 6 years (Case 24), 5 weeks (Case 25) and less than one day (Case 26). The 26 horses consisted of 22 geldings and 4 mares. The breeds included Thoroughbred or Thoroughbred crosses (9), Warmbloods (7), Irish draught crosses (7), Camargue (1) and ponies (2). These horses were used for general purpose riding (13), showjumping (3), dressage (1), hunting (4), 3 were retired and the use was not recorded in 2 horses. The median age of the horses was 11 years (Interquartile range (IQ) 8–13). All horses with lameness isolated to the foot had histories of chronic minimally responsive foot lameness that remained undiagnosed following routine evaluation, including standard radiographic views. Lameness grades in the most severely affected limb varied from 1/10 to 6/10 (mean 3/10) (where 1/10 is mild and 10/10 non weight-bearing) (Sherlock et al., 2007). 3.1. Descriptive data 3.1.1. Macroscopic findings The location of lesions detected on MR imaging were consistent with the position of the lesions detected macroscopically and histologically. The types and locations of lesions identified on gross examination are summarised in Supplementary Table S2. Of the 28 limbs from 20 horses with foot lameness not associated with penetrating injury, 13 limbs had only dorsal lesions of one or both lobes in at least one level, two limbs had only core lesions, 9 limbs had both dorsal lesions and core lesions, one limb had both dorsal lesions and a parasagittal split, and three limbs had combinations of dorsal lesions, core lesions and parasagittal splits. Of the three limbs with DDFT damage confined to the digital flexor tendon sheath, one had a core lesion and two had longitudinal splits. 3.1.2. Histological findings Limbs (n = 14) in horses (n = 11) with lesions categorised as core lesions on gross examination and MR imaging (well circumscribed increases in signal intensity (GRE T1+/− GRE T2* + /− FSE T2+/− FSE STIR sequences) within the DDFT and not extending to any surface) had degeneration and fragmentation of the tendon fascicles in the affected area (Figs. 1 and 2). There was core necrosis, hyalinised collagen, fibroplasia or fibrocartilagenous metaplasia in the place of the normal fascicular structure. There was a loss of interfascicular septa within the lesions and the surrounding tendon had pale and thickened septa, and occasionally demonstrated fibrocartilagenous metaplasia, blood vessel ghosting (partially degenerate-looking blood vessels that are still recognisable in outline form, but lack cellular detail and normal staining affinity; they are associated with increased matrix deposition of proteoglycans in the surrounding tissue) and vascular occlusion (Figs. 1 and 2). In some lesions the surrounding fascicles were also hypercellular, and neovascularisation extended towards the necrotic lesion.
Fig. 1. Case 21, DDFT damage (core lesion) within the digital flexor tendon sheath. Photomicrograph (H&E, ×200 magnification) of affected area of the DDFT corresponding with the abnormal signal intensity within the DDFT (white arrow) on the MR image insert (T1 weighted 3D transverse high resolution image). There is fibrocartilagenous metaplasia (surrounded by black arrow heads) that abuts onto area of core necrosis (black arrow).
Limbs (n = 26) in horses (n = 18) with lesions categorised as dorsal lesions on gross examination and MR imaging (focal increases in signal intensity (GRE T1+/− GRE T2* + /− FSE T2+/− FSE STIR sequences) and/or irregular signal intensity (GRE T1+/− GRE T2* + /− FSE T2+/− FSE STIR sequences) at the dorsal aspect of the tendon) had dorsal irregularity, crevices, craters, splits or fibrillation of the DDFT. Two horses had evidence of adhesion formation between the dorsal DDFT lesion and the navicular bursa (n = 1) or navicular bone (n = 1) on gross examination that was not definitively identified on MR imaging. In limbs with dorsal DDFT lesions, the normal fascicular structure of the dorsal aspect of the DDFT was replaced with fibroplasia and occasionally fibrocartilagenous metaplasia or fibrils of hyalinised collagen. Occasionally, the hyalinised fibrils and associated chondrones were detached. Some sections had increased
Fig. 2. Case 21, DDFT damage (core lesion) within the digital flexor tendon sheath. Photomicrograph (H&E, ×200 magnification) of affected area of the DDFT corresponding with the abnormal DDFT (white arrow) on the macroscopic image (Transverse cross section of DDFT). There is core necrosis (black arrows) that is surrounded by fibrocartilagenous metaplasia (small black arrow heads). Within the centre of the image, there is a chondrone (large black arrow head).
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vascularisation and some had blood vessel occlusion at the dorsal aspect of the tendon. The interfascicular septa of the dorsal tendon also demonstrated blood vessel ghosting, evidence of vascular occlusion, fibrocartilagenous metaplasia or early chondroid metaplasia. Limbs (n = 4) in horses (n = 4) with lesions categorised as parasagittal splits on gross examination and MR imaging (linear parasagittal increases in signal intensity (GRE T1+/− GRE T2* + /− FSE T2+/− FSE STIR sequences)) had a combination of deep crevices with disruption of normal fascicular structure and replacement with necrosis or occasionally replacement with fibroplasia or fibrocartilagenous metaplasia. There was sometimes peripheral blood vessel ghosting or occlusion and clustering of chondrones around fibrillated tissue. Horses that had a recent penetrating lesion as a cause for DDFT damage (Cases 25 and 26) demonstrated disruption of the epitendinum and tendon fascicular structure, and focal haemorrhage, inflammatory cell infiltration and a fibrovascular response (Fig. 3). One horse with a longer duration since the penetration (Case 24) also demonstrated a marked fibrovascular response. The MR imaging signal intensity changes were characterised by increases in signal intensity (GRE T1+/− GRE T2* + /− FSE T2+/− FSE STIR sequences) (Fig. 4). These signal intensity changes were similar to those seen in horses with acute DDFT lesions not associated with penetrations. Horses with lesions in the DDFT within the DFTS (n = 3) had similar MR imaging and histological abnormalities as those noted in the DDFT within the foot (Figs. 1 and 2). Of the 6 horses with fore foot lameness of ≤6 months duration in the absence of penetrating injury, there were 5 horses that had increased STIR signal intensity within the DDFT lesion, 4 of which had core necrosis on histopathological examination of the DDFT. There was one horse with fore foot lameness of less than 6 months duration that had no STIR signal intensity within the lesion and no evidence of core necrosis on histopathological examination. Of the 3 horses with fore foot lameness of 6–12 months duration, there was one horse with increased STIR signal intensity within the lesion but no core necrosis on histopathological examination, and two horses with no increased STIR signal intensity within the lesion and no core necrosis on histopathological examination. Of the 5 horses with lameness of >12 months duration, all had increased STIR signal within the lesion and 4 had core necrosis on histopathological
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Fig. 4. Case 25, Penetrating DDFT injury. MR image (T1 weighted 3D frontal) of affected area and corresponding macroscopic image of the DDFT. The black arrows demonstrate the abnormal area of DDFT characterised by high signal intensity and loss of definition of the lateral lobe on MR imaging and replacement of normal tendon tissue with friable gelatinous necrotic tissue on gross evaluation. The white arrows demonstrate the normal medial lobe of the DDFT. The medial and lateral branches of the superficial digital flexor tendon are abaxial to the DDFT (white arrow heads).
examination. The precise duration of lameness was not known in 6 horses. 3.2. Quantitative data (cases 1–20) 3.2.1. Overall MR imaging and histological grade The overall MR imaging grade (median 2; (IQ) 1–3) was not significantly different from the overall histological grade (median 2.5; IQ 2–3) (Mann–Whitney U test p = 0.1). Additionally, there was no statistically significant difference between horses classified as normal on MR imaging (Grade 0) and the histological grade assigned (Mann– Whitney U test p = 0.06). The results of a Spearman’s rank correlation coefficient (rs = 0.76, p < 0.001) demonstrated a strong correlation between MR imaging and histological grades. Using Stata weighting of 50% to 1 grade difference and 0% to >1 grade difference, there was moderate agreement (Kappa statistic 0.52) (Viera and Garrett, 2005) between the MR and histological grades. 3.2.2. Level specific MR imaging and histological grade While there remained a strong and significant correlation at all levels, using Stata weighting of 50% to 1 grade difference and 0% to >1 grade difference, there was a decreasing level of agreement between MR imaging and histological grades with increasingly distal levels of the DDFT (Table 2).
Table 2 Table to demonstrate the median MR imaging and histology grades and their correlation and strength of agreement at each anatomic level of the DDFT within the foot.
Fig. 3. Case 25, Penetrating DDFT injury. Photomicrographs (H&E) of affected area of DDFT 6 weeks following a penetrating injury. The insert (×20 magnification) demonstrates a necrotic tract (black arrow) surrounded by fibrovascular tissue. The main figure (×200 magnification) demonstrates granulation tissue containing mononuclear cells replacing normal tendon.
Location
Median MR imaging grade (IQ)
Median histology grade (IQ)
Rs value
P valuea
Kappa statistic
Level 1 Level 2 Level 3 Level 4
2 (1–2.5) 2 (1–3) 2 (1–3) 2.5 (1.5–3)
2 (0–3) 2 (0–3) 3 (2–3) 3 (2–3)
0.86 0.79 0.86 0.55
<0.001 <0.001 <0.001 0.009
0.71 0.48 0.43 0.36
a
P value for Spearman’s rank correlation coefficient (Rs).
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4. Discussion As DDFT injuries within the foot are recognised as a common cause of lameness in horses (Dyson and Murray, 2007; Dyson et al., 2003a, 2005; Mair and Kinns, 2005), non-invasive methods of identifying these lesions are essential in equine veterinary practice. Investigations into the aetiology and consequent histological changes of DDFT lesions are integral in the clinician’s ability to prevent and treat these lesions. In the DDFT, the histopathological changes in non-penetrating tendon injury have been described as degenerative in nature, possibly as a sequel to vascular compromise; inflammatory changes have not been recognised even in horses with lameness <6 months duration (Beck et al., 2011; Blunden et al., 2006a, 2006b, 2009; Busoni et al., 2005). Matrix degeneration characterised as an increase in proteoglycan content of tendon and attempts at repair characterised by an alteration in tenocyte subtypes and increased vascularisation have been described in the DDFT of lame horses compared with sound horses (Beck et al., 2011). High field MR imaging studies have demonstrated good correlation between signal intensity alterations and the gross (Schramme et al., 2005) and histopathological changes within the DDFT (Blunden et al., 2009; Murray et al., 2006a). The results of this study demonstrated that abnormalities identified on low field MR images also correlate well with histological lesions identified in the DDFT. This is important information, since low field standing MR imaging is currently commonly used to evaluate horses with foot lameness. Histopathological examinations in the 2 horses with histories of recent penetrating injuries (Cases 25 and 26) demonstrated inflammatory lesions within the tendon in contrast to the noninflammatory degenerative lesions noted in horses with lameness attributed to spontaneous atraumatic damage to the DDFT in the foot or DFTS (Beck et al., 2011; Blunden et al., 2006b, 2009). Although the MR imaging signal abnormalities were similar between horses with and without a history of acute penetrating injury, there were marked differences in the peritendinous soft tissues, thereby aiding differentiation of the aetiology of these two types of lesions. This is also important information since not all horses with penetrating injuries to the foot will have a known history of trauma, and in a significant number of horses with such injuries, the penetrating tract will not be evident on MR images (del Junco et al., 2012). Recent studies have evaluated variations in the soft tissue lesion appearance on MR imaging with time. Resolution of STIR signal within soft tissues has been significantly associated with a return to soundness (Holowinski et al., 2010), and a decrease in the size and T2* GRE signal intensity have been noted in dorsal border lesions re-evaluated at a 6 month interval after conservative treatment (Milner et al., 2012). In contrast to STIR and T2* GRE sequences, DDFT lesions persist on T1 GRE weighted sequences even when horses are sound and return to athleticism (Vanel et al., 2012). Although it was beyond the scope of this study to evaluate temporal changes in signal intensity with different imaging sequences in tendon healing, 5/6 horses (83%) with non-penetrating foot lameness of ≤6 months duration demonstrated increases in STIR signal intensity within the DDFT lesion consistent with the proposed hypothesis that STIR signal is present in recent DDFT lesions that are contributing to lameness (Holowinski et al., 2010). However, all horses with lameness duration >12 months also demonstrated increased STIR signal intensity, and there were horses in this study that had no evidence of intralesional STIR signal yet remained lame with lameness attributed to the DDFT lesion. Horses with lameness ≤6 month duration have been reported to be more likely to have intralesional core necrosis whereas horses with lameness of ≥6 months duration are more likely to have core fibroplasia and fibrocartilagenous metaplasia (Blunden et al., 2006b, 2009). The majority of the horses
in this study with lameness <6 months demonstrated core necrosis within the lesion supporting the results of previous studies (Blunden et al., 2006b, 2009). However, the majority of horses in this study with lameness duration >12 months also demonstrated core necrosis. An alteration from core necrosis in lesions <6 months to fibroplasia or fibrocartilaginous metaplasia is considered to be associated with tendon lesion ageing and has previously been noted in chronic tendon lesions in humans (Gigante et al., 2004; Jarvinen et al., 1997; Mosier et al., 1998). It seems likely that the persistence of the intralesional core necrosis (and increased STIR signal intensity) in these cases reflects the severity and persistence of the DDFT damage in these horses most of whom were euthanised as a consequence of their prolonged lameness. Further studies using postmortem material are warranted to correlate differences in signal characteristics using different MR imaging sequences with the histopathological changes in the DDFT. Although there was no statistically significant difference between the MR imaging grades and histological grades, and there was significant correlation between MR and histological findings overall, there was some variation in the strength of agreement between MR imaging and histological grade at the different anatomical levels. Agreement was superior further proximally in the limb (substantial agreement at Level 1) but decreased further distally (fair agreement at Level 4). The MR grades were lower than the histological grades at levels 3 and 4 although they remained significantly correlated. This may be associated with the intrinsic difficulties in identifying lesions in this area; pathology in the adjacent distal sesamoidean impar ligament (DSIL) can be difficult to identify on low field images (Murray et al., 2009). Recognition of lesions in the DSIL using the low field system is supported by the presence of increased soft tissue between the DSIL and DDFT (Murray et al., 2009). Additionally, there is an association between DSIL body lesions and abnormalities at the osseous origin and insertion (Dyson et al., 2010). In clinical cases, the appearance of the adjacent tissues would be taken into consideration when assessing the presence and severity of a lesion in the DDFT. This may improve agreement between MR imaging and histological grading in the distal DDFT but further studies are necessary for validation. Limitations of this study include the generation of MR images on cadaver limbs and not live horses. However, the sequences evaluated were those used in clinical cases and there is excellent correlation between live and cadaver images (Murray et al., 2004; Widmer et al., 2000). Imaging thawed limbs after freezing does not alter overall image quality and does not alter signal intensity within the DDFT (although signal to noise ratios in other soft tissues within the hoof capsule may be altered) (Bolen et al., 2011). Another potential limitation was the case selection. Most horses included in the study were euthanised due to the severity of their lameness and there were no sound control horses; however, there were areas of tendon that were graded normal on MR imaging which were not graded significantly differently on histological examination. Although all horses had their lameness localised to the area of tendon pathology by diagnostic analgesia, there remains poor anatomic specificity for these routinely utilised techniques. Therefore, the authors are unable to be certain of the clinical significance of the tendon lesions to the lameness and this topic needs further investigation. An additional limitation is the relatively small section of tendon examined histologically compared with the large segment of tendon graded by the MR imaging. The MR grade was based on evaluation of the tendon on multiple sequences through the entire level whereas the histological grade was based on evaluation of a small section of tendon within the level. This means that lesions may have been under-represented or missed on histological examination or that the area examined may have been chosen due to the appearance of a gross lesion in this area thereby overestimating the severity of the lesion throughout the tendon.
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5. Conclusion In conclusion, this study demonstrated that DDFT lesions identified on low field MR imaging were consistent with those identified on macroscopical evaluation of the tendon. There was a strong correlation between the presence and severity of DDFT lesions identified and graded on MR imaging and validated and graded on histology; this strong correlation existed at all anatomic levels of the DDFT within the foot. Overall, there was moderate agreement between the low field MR imaging grades and histology grades of the equine DDFT. The study therefore supports the use of low field MR imaging for diagnosis of DDFT lesions within the feet of lame horses. Acknowledgements The authors gratefully acknowledge the support of the RCVS Trust and Hallmarq Veterinary Imaging Ltd for this study. We thank the owners and veterinary surgeons of the horses used in the study, and Ray Wright for his technical assistance. Appendix: Supplementary material Supplementary data to this article can be found online at doi:10.1016/j.rvsc.2014.12.008. References Beck, S., Blunden, T., Dyson, S., Murray, R., 2011. Are matrix and vascular changes involved in the pathogenesis of deep digital flexor tendon injury in the horse? Veterinary Journal 189, 289–295. Blunden, A., Murray, R., Dyson, S., 2009. Lesions of the deep digital flexor tendon in the digit: a correlative MRI and post mortem study in control and lame horses. Equine Veterinary Journal 41, 25–33. Blunden, T., Dyson, S., Murray, R., Schramme, M., 2006a. Histological findings in horses with chronic palmar foot pain and age-matched control horses. Part1: navicular bone and related structures. Equine Veterinary Journal 38, 15–22. Blunden, T., Dyson, S., Murray, R., Schramme, M., 2006b. Histological findings in horses with chronic palmar foot pain and age-matched control horses. Part 2: deep digital flexor tendon. Equine Veterinary Journal 38, 23–27. Bolen, G.G., Haye, D., Dondelinger, R.F., Massart, L., Busoni, V., 2011. Impact of successive freezing-thawing cycles on 3-T magnetic resonance images of the digits of isolated equine limbs. American Journal of Veterinary Research 72, 780–790. Busoni, V., Heimann, M., Trenteseaux, J., Snaps, F., Dondelinger, R., 2005. Magnetic resonance imaging findings in the equine deep digital flexor tendon and distal sesamoid bone in advanced navicular disease – an ex vivo study. Veterinary Radiology and Ultrasound 46, 279–286. del Junco, C.I., Mair, T.S., Powell, S.E., Milner, P.I., Font, A.F., Schwarz, T., et al., 2012. Magnetic resonance imaging findings of equine solar penetration wounds. Veterinary Radiology and Ultrasound 53, 71–75. Dyson, S., 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 Veterinary Journal 39, 340–343. Dyson, S., Murray, R., Schramme, M., Branch, M., 2003a. Magnetic resonance imaging of the equine foot: 15 horses. Equine Veterinary Journal 35, 18–26. Dyson, S., Murray, R.S.M., Branch, M., 2003b. Lameness in 46 horses associated with deep digital flexor tendonitis in the digit: diagnosis confirmed with magnetic resonance imaging. Equine Veterinary Journal 35, 681–690. Dyson, S., Murray, R., 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 Veterinary Journal 37, 113–121.
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