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Keywords:

  • forelimb lameness;
  • horse;
  • magnetic resonance imaging;
  • navicular disease;
  • navicular syndrome

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. References

Seventy-two horses with recent onset of navicular syndrome and normal radiographs were assessed. Horses underwent magnetic resonance (MR) imaging of both front feet. All abnormalities were characterized and the most severe abnormality identified, if possible. Abnormal signal intensity in the navicular bone was the most severe abnormality in 24 (33%) horses. Pathologic change in the deep digital flexor tendon was the most severe abnormality in 13 (18%) horses. Pathologic change in the collateral sesamoidean ligament was the most severe abnormality in 11 (15%) horses. Pathologic change in the distal sesamoidean impar ligament was the most severe abnormality in seven (10%) horses. Multiple abnormalities were observed in 13 (18%) horses in which an abnormality that was more severe than the others could not be determined. Abnormalities were not observed in the navicular bone or its supporting soft tissues in four (5%) horses. Fifty-six horses had abnormalities that were most severe in one limb; in 52 (93%) horses, the most severe abnormalities were in the foot of the most lame limb. In 7% (4/56) of horses, the most severe findings were in the opposite limb, and in 16 horses, the findings on both limbs were similar. MR imaging is a useful technique for evaluating horses with navicular syndrome and can differentiate between multiple abnormalities. This provides a more specific diagnosis which affects further treatment of the horse. Pathologic changes in different locations in the foot can cause similar clinical signs that, before MR imaging, were categorized as one syndrome.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. References

Navicular syndrome, a chronic, progressive bilateral forelimb condition affecting the navicular bone and it's supporting soft tissue structures, with or without radiographic abnormalities, is a common cause of forelimb lameness in athletic horses.1–3 Despite its common occurrence, the initiating cause and pathogenesis of navicular bone degeneration is not understood.4–9 A vascular theory proposes that the pain and navicular bone degeneration are the result of ischemic necrosis caused by digital artery thrombosis.4 Intimal proliferation and arteriosclerosis of vessels supplying the navicular bone has also been suggested as a cause of ischemic necrosis.5,6 A biomechanical theory has been proposed based on histopathologic observations. Abnormal biomechanical forces that cause chronic sustained pressure of the deep digital flexor tendon against the navicular bone stimulate an abnormal remodeling process.7 This bone remodeling leads to increased medullary bone trabeculae, edema, and pain, frequently progressing to degeneration of the bone. Navicular syndrome has also been related to osteoarthritis, based on histologic and histomorphometric changes in the navicular bone and palmar fibrocartilage that are similar to abnormalities observed in articular hyaline cartilage and subchondral bone of affected joints.8,9

Most theories about the pathophysiology of navicular syndrome are based on clinical observations, radiographs, and post-mortem studies of chronically affected horses.7–9 Post-mortem evaluation of acutely affected horses has not been performed because many of these horses return to performance.3 While post-mortem studies have provided useful information about navicular syndrome, it has been difficult to identify the inciting causes of the pathologic changes.

The purpose of this report is to describe the abnormalities observed with magnetic resonance (MR) imaging of the front feet in horses that have recently developed clinical signs of navicular syndrome. There are reports of MR imaging findings in horses with pain in one or both front feet,10–29 but none focus on radiographically normal horses with bilateral forelimb lameness that was abolished by palmar digital nerve blocks. We evaluated horses with recent onset of bilateral clinical signs of navicular syndrome to determine the anatomic location of pathologic change in affected horses. Our hypothesis was that the initial site of pathologic change would be within the navicular bursa or navicular bone.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. References

Seventy-two horses with a recent onset of clinical signs of navicular syndrome that had MR imaging performed on both front feet between May 1998 and September 2007 were evaluated retrospectively. There were 51 geldings, 19 mares, and two stallions, with an average age of 8.2 years (median 8 years, range 3–17 years). Breed distribution was as follows: 42 Quarter horse, 17 Warmblood, seven Thoroughbred, four Paint, one Appaloosa and one Arabian. Thirty-four were western performance event horses, 15 were event horses or jumpers, 10 were pleasure or show horses, nine were dressage horses, and use was unknown in four horses. Inclusion criteria included bilateral forelimb lameness that was abolished by bilateral palmar digital nerve blocks, with or without a positive response to hoof testers applied to the frog. The duration of lameness was <6 months. A lameness evaluation was performed in all horses. Injection of 1.5–2 ml local anesthetic over the medial and lateral palmar digital nerves of the lame leg proximal to the cartilages of the foot caused each horse to switch to lameness in the opposite forelimb. Injection of local anesthetic over the medial and lateral palmar digital nerve of the opposite forelimb eliminated the lameness. All horses were examined by the same individual (R.K.S.) and lameness was graded at a trot in a straight line and in small circles in each direction before and after palmar digital nerve blocks. Lameness grades were 0—no lameness observed, 1—mild lameness without an observed head nod, 2—subtle head nod observed, 3—obvious head nod observed, 4—severe head nod with <50% of normal weight supported on the limb, and 5—nonweight bearing on the limb. Radiographs, including a dorsal 60° proximal–palmarodistal projection, a palmaroproximal to palmarodistal projection of the navicular bone, and a lateromedial projection of the digit, were evaluated and found to be normal for the age and use of the horse for all horses.30

MR imaging was performed on both front feet of every horse following general anesthesia. All horses were anesthetized in right lateral recumbency with the forelimbs in a 1.0 Tesla magnet.* A human knee quadrature receiver coil was positioned around the foot being imaged and proton density (PD), T2-weighted (T2W), short tau inversion recovery (STIR), and gradient echo (3DGE) images were acquired. Transverse, sagittal, and dorsal images were obtained using standard protocols (Table 1).31 The dorsal imaging plane of the navicular bone was added to our foot protocol in 2004, therefore, horses imaged before this did not have navicular bone images in this plane.

Table 1.   Phillips 1 Tesla Magnetic Resonance Imaging Parameters for the Foot of the Horse
Slice PlaneSequenceTR (ms)TE (ms)FAFOV/ rFOVMatrix SizeSlice #/ Width (mm)Gap (mm)Time (min)
  1. * Inversion time (TI) for STIR sequences is 140 ms . †T2 and PD sequences are collected together during the TSE scan . TSE, turbo spin echo; STIR, short tau inversion recovery; 3D GE, three dimensional gradient echo; TSE PD, proton density; TSE T2, T2-weighted; TR, repetition time; TE, echo time; FA, flip angle; FOV/rFOV, field of view/relative field of view; Slice #, number of slices; Width, thickness of slice; Gap, space between slices; Time, actual scanning time for each sequence.

TransverseTSE T221161009015/10.5256 × 51230/40.55:08
TransverseTSE PD2116119015/10.5256 × 51230/40.5
TransverseSTIR*1725359015/10.5192 × 25630/3.51.04:42
Transverse3D GE4792510/10192 × 25630/1.5−1.53:13
SagittalTSE T233951109014/10256 × 51222/40.52:21
SagittalTSE PD3395149014/10256 × 51222/40.52:21
SagittalSTIR*1500359014/10256 × 25622/3.50.55:48
Dorsal3D GE4792510/10192 × 25630/1.5−1.53:13

A standardized format was used to record all MR image observations and images were compared with images previously obtained from normal horses. On all horses, magic angle artifact occurring in any structure(s), most commonly tendons and ligaments, was identified by comparing PD images and T2W images from the dual echo sequences.32–35 A primary finding, the most severe finding, based on severity and lack of other observed abnormalities, was made in horses where it was clear that one principle abnormality was present. In some horses, a primary pathologic change could not be identified because multiple abnormalities judged to be of similar severity were observed in different structures. Owing to the observations of multiple abnormalities in many horses, an attempt was made to determine the primary abnormality based on the most obvious or severe finding. The association of imaging findings with the most clinically affected limb was determined by having the evaluator be unaware of results of lameness examination.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. References

The mean lameness grade was three when the horses were trotted in a straight line and circled in each direction over a smooth hard surface. Lameness was observed on both forelimbs in 54 (76%) horses when the horse was trotted in small circles. All other horses had a unilateral lameness before the palmar digital nerve block on the most lame limb.

Pathologic change within the navicular bone was seen as changes in signal intensity within the medullary cavity with or without signal abnormalities within the flexor cortex (Table 2). Abnormal hyperintensity was observed in the navicular bone on STIR sequences in 108 limbs (62 horses [86%]). Thirty of these limbs (23 horses [38%]) also had abnormal hypointensity in the medullary cavity of the navicular bone. One of these horses had a 3 mm navicular bone flexor cortex erosion just medial to the sagittal ridge that was not visible on radiographs. Focal hyperintensity was seen within the proximal, middle, and/or distal aspect of the navicular bone on the sagittal STIR images. Diffuse hyperintensity was also seen, with or without areas of focal hyperintensity, throughout the medullary cavity, which varied in both signal intensity and location (Fig. 1).

Table 2.   Magnetic Resonance Imaging Findings in Structures of the Navicular Region
Structure# of Limbs Affected# of Horses AffectedPrimary Lesion/Horse
  1. * Only 24 horses (48 limbs) were imaged with coronal slices through the navicular bone enabling complete evaluation of the distal border . NB, navicular bone; DDFT, deep digital flexor tendon; CSL, collateral sesamoidean ligament; DSIL, distal sesamoidean impar ligament; NBA, navicular bursa; DIP, distal interphalangeal joint; NA, not applicable.

NB108 (75%)62 (86%)24 (33%)
DDFT49 (34%)32 (44%)13 (18%)
CSL86 (60%)54 (75%)11 (15%)
DSIL40 (28%)26 (36%)7 (10%)
NBA58 (40%)32 (44%)0
NB Fragments15/48 (31%)*9/24 (37.5%)*0
DIP Effusion71 (49%)36 (50%)0
MultipleNA13 (18%)NA
image

Figure 1.  Sagittal short tau inversion recovery (STIR) images. (A) Note the abnormal hyperintensity in the distopalmar aspect (arrow) of the navicular bone near the origin of the distal sesamoidean impar ligament. There is focal hyperintense signal that extends proximally to the insertion of the collateral sesamoidean ligament. A small amount of synovial fluid is seen as focal hyperintensity at the palmar aspect of the deep digital flexor tendon (arrowhead). (B) The abnormal hyperintensity is more diffuse and extends proximally along the palmar half of the medullary cavity (arrows) of the navicular bone. In this image, there is also hyperintensity at the proximal aspect of the distal sesamoidean impar ligament. The focal area of hyperintensity along the flexor cortex represents the normal crescent shaped depression in this area found in many horses.

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Fifteen of 48 (31%) limbs that had the navicular bone imaged in a dorsal plane on gradient echo sequences had distal navicular margin bone fragments at the origin of the distal sesamoidean impar ligament. Six horses had these bilaterally, with five having lateral angle fragments on each foot, and one horse having medial and lateral angle fragments on each foot. Three horses had these unilaterally, with one horse having a medial angle fragment and two horses having lateral angle fragments. Two of the unilateral fragments were on the most lame limb and one was on the least lame limb.

Nineteen limbs (10 horses [14%]) had abnormalities in the distal sesamoidean impar ligament at its origin from the navicular bone which included abnormal hypointensity of the adjacent medullary cavity on PD images and entheseous new bone formation at its insertion, with or without distal margin fragmentation. Changes within the ligament included loss of distinct dorsal and palmar margins, evaluated on transverse and sagittal images, and loss of the bursal space between the distal sesamoidean impar ligament and deep digital flexor tendon.

Pathologic change in the collateral sesamoidean ligament was observed in 86 limbs (54 horses [75%]) (Fig. 2). All these horses had diffuse thickening of the body and branches with or without abnormal hyperintensity. This included loss of distinct dorsal and palmar margins, evaluated on transverse and sagittal images, and loss of the bursal space between the collateral sesamoidean ligament and deep digital flexor tendon. Pathologic change of the distal sesamoidean impar ligament was observed in 40 limbs (26 horses [36%]) (Fig. 3).

image

Figure 2.  Sagittal (A) and transverse (B) proton density (PD) images (medial is to the left) of a horse with diffuse thickening of the collateral sesamoidean ligament (arrow) and abnormal increased signal intensity abaxially within its collateral branches. (A) The distal interphalangeal joint capsule is distended due to increased synovial fluid (arrowhead). There is also a focal mild hyperintensity on the palmar aspect of the flexor cortex midway between the proximal and distal aspect of the bone that represents the normal depression in this area of the flexor cortex with a focal pocket of bursal fluid within the depression. (B) There is irregularity of the dorsal aspect of the deep digital flexor tendon (arrowheads) that is contrasted by the hyperintense signal of fluid in the navicular bursa between the tendon and the collateral sesamoidean ligament.

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image

Figure 3.  Sagittal proton density (PD) image from a horse with thickening and hyperintensity within the distal sesamoidean impar ligament (arrow). Corresponding T2-weighted images were used to rule out magic angle artifact in the distal sesamoidean impar ligament. Magic angle artifact is evident in the distal aspect of the deep digital flexor tendon (arrowhead). Enthesopathy of the palmarodistal aspect of the navicular bone is also visible at the origin of the distal sesamoidean impar ligament.

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Forty-nine limbs (32 horses [44%]) had pathologic change within the deep digital flexor tendon (Fig. 4), most commonly located in the tendon proximal to the navicular bone, at the level of the collateral sesamoidean ligament. These abnormalities were seen as thickening and/or hyperintense signal of portions of the deep digital flexor tendon. In some horses, abnormalities extended proximally as far as the horse was imaged, midway in the proximal phalanx. Lesions within the deep digital flexor tendon were divided into four locations: distal to the navicular bone (19 limbs [40%]), at the navicular bone (16 limbs [35%]), proximal to the navicular bone (46 limbs [94%]), and between the metacarpophalangeal and proximal interphalangeal joints (15 limbs [31%]). The lesions distal to the navicular bone were either core lesions (14 limbs [74%]) or (para)sagittal splits (five limbs [26%]). The lesions at the level of the navicular bone were either core lesions (three limbs [19%]) or (para)sagittal splits (13 limbs [81%]). The lesions proximal to the navicular bone were either core lesions (16 limbs [35%]), (para)sagittal splits (six limbs [13%]), or focal dorsal border defects (24 limbs [52%]). All of the lesions within the proximal phalanx region were core-type lesions (15 limbs). Twenty-four limbs (49%) had lesions in more than one region and 25 (51%) limbs had lesions in only one region. Of the 25 limbs that had lesions in only one region, 21 (84%) had lesions proximal to the navicular bone, two (8%) had lesions distal to the navicular bone, and one (4%) had a lesion at the navicular bone. Ten (31%) of the 32 horses with deep digital flexor tendon lesions had dorsal border defects proximal to the level of the navicular bone as the only tendon injury present. Twenty-seven (84%) horses with deep digital flexor tendon lesions also had enlargement of the collateral sesamoidean ligament.

image

Figure 4.  Transverse proton density (PD) images (medial is to the left) of a horse with hyperintensity in the medial lobe of the deep digital flexor tendon with corresponding enlargement of this tendon lobe (arrow) at the level of the proximal aspect of the middle phalanx (A). The area of hyperintensity extends proximally in the tendon to the level of the distal third of the proximal phalanx, as far proximally as the horse was imaged (B).

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Seventy-one limbs (36 horses [50%]) had increased synovial fluid in the distal interphalangeal joint (Fig. 5). All of these horses also had other abnormalities observed in the navicular bone or supporting soft tissue structures. Fifty-eight limbs (32 horses [44%]) had increased synovial fluid in the navicular bursa, although in most horses, a relatively small increase in fluid was observed. In 17 limbs (29%) there was severe bursitis, which included horses with increased fluid with or without hypointense tissue within the bursa; 12 of these (71%) had deep digital flexor tendonitis at the level of the navicular bursa. Although a pronounced increase in synovial fluid was observed in the navicular bursa in seventeen horses, a large amount of fluid consistent with bursitis was not considered the primary abnormality in any horse, although one with multiple other chronic lesions and an acute lameness history did have severe fluid in the navicular bursa (Fig. 6). Bursitis was determined by evaluation of the bursal fluid signal between the deep digital flexor tendon and collateral sesamoidean ligament as well as the size of the abaxial proximal bursal outpouchings, seen best on sagittal STIR images. Horses with substantial scar tissue in the proximal bursa between the deep digital flexor tendon and collateral sesamoidean ligament only had fluid visible in the abaxial proximal outpouchings of the navicular bursa.

image

Figure 5.  Sagittal (A) and transverse (B) short tau inversion recovery (STIR) images. There is increased fluid in the distal (large arrows) and proximal (small arrows) interphalangeal joints. There is also increased fluid in the digital flexor tendon sheath on the sagittal image (arrowhead).

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image

Figure 6.  Sagittal (A) and transverse (B) short tau inversion recovery (STIR) images (medial to the left). Note the increased fluid in the navicular bursa in a horse in which severe effusion (arrows) of the bursa was present. There is also moderately hypointense tissue present on the dorsolateral aspect of the lateral lobe of the deep digital flexor tendon (DDFT) indicative of scar tissue and mild, diffuse enlargement of the medial lobe.

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Pathologic change within the navicular bone was the primary abnormality in 24 (33%) horses. Pathologic change within the deep digital flexor tendon was the primary abnormality in 13 (18%) horses. Pathologic change within the collateral sesamoidean ligament was the primary abnormality in 11 (15%) horses. Pathologic change within the distal sesamoidean impar ligament was the primary abnormality in seven (10%) horses. Multiple abnormalities were observed in 13 (18%) horses where a primary abnormality could not be determined.

Abnormalities were not observed in the navicular bone and its supporting soft tissues in four (6%) horses. One horse had bilateral increased synovial fluid in the proximal interphalangeal joint, best seen on STIR images. One horse had thickening of and hyperintensity in the straight distal sesamoidean ligament proximal to its insertion, best seen on PD and T2W images. One horse had thickening of and heterogenous signal intensity in the distal digital annular ligament bilaterally, best seen on PD and STIR images. One horse had increased thickness and irregularity of the laminae over the medial and lateral palmar processes of the distal phalanx, best seen on PD images, in both front feet without increased signal intensity on T2 or STIR images.

Twenty-two (15%) limbs had increased synovial fluid in the digital flexor tendon sheath as a secondary finding, best seen on STIR and T2W images. Six limbs had thickening and irregularity of the distal digital annular ligament, best seen on PD images. Six limbs had increased synovial fluid of the proximal interphalangeal joint, best seen on STIR images. Four limbs had abnormal hyperintensity within the distal phalanx seen on STIR images. Two horses had irregularity of the subchondral bone and/or cartilage of the distal interphalangeal joint seen on transverse PD, T2, and STIR images crossing through the joint spaces. Three horses had smoothly marginated semicircular defects within the laminar tissue of the medial (1) or lateral (2) quarter, best seen on PD images, and all were found on the most lame limb. One horse had thickening, best seen on PD and T2 images, with abnormal hyperintensity, best seen on T2 and STIR images, of the medial collateral ligament of the distal interphalangeal joint.

Fifty-six horses had obvious differences in severity of lesions between limbs. In 93% (52/56) of horses, the most severe abnormalities occurred in the lamest limb. In 7% (4/56) of horses the worst findings were in the less lame limb.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. References

Contrary to expectations, the initial pathologic change was not within the navicular bursa or navicular bone, as multiple abnormalities were observed. The abnormalities seen on the MR images were not visible on radiographs, even though the majority of horses had changes involving the navicular bone. A specific diagnosis is possible in some horses after evaluation with MR imaging, and these horses can then be grouped into specific subgroups based on the primary lesion. Because of the variation observed in this study, MR imaging will have to be performed on a large number of horses with clinical signs of navicular syndrome before patterns can be recognized and related to a specific etiology and pathogenesis.

Our patient population contained a high percentage of Quarterhorses, of which the majority were western performance horses. This is a different population than reported in most other studies of foot lameness assessed with MR imaging.10,11,13,16,19–21,23,25–28

Abnormal signal intensity in the navicular bone was seen in 62 (86%) horses and was the most frequent observation. The hyperintensity in the navicular bone seen with STIR sequences could be indicative of hemorrhage, synovial fluid, bone necrosis, fibrosis, or inflammation.12,14–15,36,37 Granulation tissue can appear hyperintense, but differentiation of granulation tissue was not attempted in this study, and granulation tissue is not a component of navicular syndrome unless flexor cortex lesions are present.38 Correlation of MR images with histopathologic observations may help to determine the source of abnormal navicular bone hyperintensity in horses with recent onset of signs of navicular syndrome, but this comparison has only been made in horses with chronic navicular syndrome.13–15,36,39,40

Deep digital flexor tendonitis was recognized previously in horses with adhesion of the tendon to the navicular bone3 or observed in the area of the tendon over the flexor surface of the navicular bone on postmortem studies,7,8 and has been recognized as a primary problem in some horses with clinical signs of navicular syndrome.11,16,19 Thirteen (18%) horses had deep flexor tendonitis as the primary diagnosis. This is different than in a prior report where 33% of horses with foot lameness had deep flexor tendonitis as the most common injury.16 This may relate to the population being assessed, as in England, jumping horses are more likely to sustain tendon injury than nonjumping horses.11 The prevalence of horses with pathologic change in the deep digital flexor tendon in this study (44% [34% of limbs]) is different than in a previous report (83%).17 Lesion location within the deep digital flexor tendon was similar at the interphalangeal region (31% in this study vs. 29% elsewhere) and distal to the navicular bone (40% in this study vs. 35–37% elsewhere), although it differed at the level of the collateral sesamoidean ligament (94% in this study vs. 59% elsewhere) and the navicular bone (35% in this study vs. 59% elsewhere).17

The observation that inflammation extends proximally in the tendon above the level of the proximal interphalangeal joint affects treatment for some horses. Before MR imaging, rest, rehabilitation, and physical therapy to stretch the deep digital flexor tendon within the digit had not been used routinely to treat horses with navicular syndrome. The report of deep digital flexor tendon injury as a major contributor to lameness in 61% of 75 horses with foot lameness evaluated with MRI further emphasizes the importance of the deep digital flexor tendon as a cause of lameness in the horse's digit.11 Extension of the lesion(s) proximally into the area covered by the digital flexor tendon sheath makes injection of the sheath a possible diagnostic (local anesthetic injection) or therapeutic technique (cortisone and/or hyaluronic acid injection) for these horses (R.K. Schneider, Unpublished data).41 Horses with deep digital flexor tendon injury and inflammation associated with the navicular bursa may be more effectively treated by injection of anti-inflammatory medications into this synovial structure.42

Pathologic change in the collateral sesamoidean ligament was the primary abnormality in 15% of horses, but this has not been previously reported as a primary cause of injury in a group of horses. There are two case reports of collateral sesamoidean ligament desmitis; one diagnosed by ultrasound and one diagnosed by MRI.22,43 The prevalence of pathologic change in the collateral sesamoidean ligament in a group of horses has been reported previously (10.5% of limbs), but is much lower than was seen in this study (60% of limbs).16 Injury to the collateral sesamoidean ligament was seen as either one of multiple abnormalities within the navicular region of the foot or as a single injury.

Pathologic change in the distal sesamoidean impar ligament was a primary abnormality in seven (10%) of the horses in this study. This is similar to the prevalence of 7% in a group of horses with lameness localized to the foot.16 The overall prevalence of injury to the distal sesamoidean impar ligament in this study (28% of limbs) is also similar to a prevalence of 38% in a previous study where horses had unilateral or bilateral lameness localized to the foot in either the forelimb or hindlimb.17

Multiple structures were affected in 13 (18%) horses, and a primary abnormality could not be identified. This is similar to other horses with lameness localized to the foot in which 17% of horses had multiple abnormalities.16

The hypothesis that navicular syndrome is a continuum that starts as navicular bursitis and progresses to degeneration of the flexor cortex of the navicular bone7,44 was not supported by the MR imaging findings in our study. Increased synovial fluid in the navicular bursa as a primary entity was not observed in any horse, making it unlikely that navicular syndrome is initiated by inflammation of the bursa. Distension of the navicular bursa, most visible as outpouching of the proximal, abaxial synovial pouches, was most often seen in horses that had pathologic changes within the collateral sesamoidean or distal sesamoidean impar ligament, or deep digital flexor tendon, and rarely in horses that did not have damage to these soft tissue structures. The amount of increased distension visible on sagittal STIR images of horses ranged from 0.5 to 2.0 cm if measured in a proximal to distal direction at the most distended portion of the bursa on either side of the deep digital flexor tendon. These horses may also have variable amounts of hypointense scar tissue in the proximal aspect of the navicular bursa, which was often seen concurrently with deep flexor tendon injury in the region of the tendon covered by the bursa, and in limbs in which the tendon damage extended to the dorsal surface of the tendon covered by the navicular bursa.

Chronic strain on the palmar support structures of the distal interphalangeal joint, including the distal sesamoidean impar ligament, collateral sesamoidean ligament, and the deep digital flexor tendon, due to repetitive use or poor conformation, may cause mechanical damage and inflammation in these tissues. Hyperextension of the distal interphalangeal joint occurs late in the stride when the horse is pushing off the ground,45 and the greater the speed on an uneven surface, the more hyperextension of this joint may occur. Rough uneven surfaces and deep plowed ground can also contribute to hyperextension of the distal interphalangeal joint. Hyperextension could also result in injury to the deep digital flexor tendon from forces that cause sudden upward pressure on the toe. Horses frequently perform on deep or uneven surfaces, where increased pressure to the toe seems likely. Acute, blunt trauma to the bottom of the foot may also be a cause of navicular bone injury,46 particularly when lameness is unilateral or markedly asymmetric. In one horse in this study, acute lameness was observed on one limb 3 weeks before MR imaging. This horse had diffuse abnormal high-signal intensity in the navicular bone that was more severe than in the opposite limb, possibly due to a contusion.

Unfortunately, due to the bilateral nature of lameness associated with the palmar heel region, bilateral forelimb lameness can be difficult for owners to identify early. It is impossible to accurately determine how long these horses were experiencing pain. It is possible that the duration of disease was longer than the owners thought, even though lameness was only noticed by owners or trainers within the last 6 months.

In conclusion, inflammation of several different tissues in the heel can cause clinical signs that have historically been diagnosed as navicular syndrome. The multiple abnormalities observed in this study support the use of MR imaging for evaluating horses with navicular syndrome because a specific diagnosis can be made that can affect treatment of these horses. The presence of abnormalities seen in the MR images in this report substantiate the limited use of radiography for assessment of navicular syndrome. Abnormalities observed in the front feet in this study may be the result of use-related remodeling of the navicular bone and its supporting soft tissue structures, however, the findings in these horses are not observed in sound horses.13,47 More importantly, the most severe MR imaging abnormalities occurred in the foot in which the horse was most lame 93% of the time.

Footnotes
  1. *Philips Gyroscan, Medical Systems, Best, the Netherlands.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. References
  • 1
    Ackerman N, Johnson JH, Porn CR. Navicular disease in the horse: risk factors, radiographic changes, and response to therapy. J Am Vet Med Assoc 1977;170:183187.
  • 2
    Lowe JE. Sex, breed and age incidence of navicular disease and its treatment. Equine Pract 1982;4:29.
  • 3
    Stashak TS. Lameness. In: StashakTS (ed): Adams' lameness in horses. Philadelphia: Lippinjcott, Williams, & Wilkins, 2002;664680.
  • 4
    Colles C, Hickman J. The arterial supply of the navicular bone and its variations in navicular disease. Equine Vet J 1977;9:150154.
  • 5
    Rijkenhuizen ABM, Nemeth F, Dik KJ, et al. The arterial supply of the navicular bone in adult horses with navicular disease. Equine Vet J 1989;21:418424.
  • 6
    Fricker C, Riek W, Hugelshofer J. Occlusion of the digital arteries—A model for pathogenesis of navicular disease. Equine Vet J 1982;14:203207.
  • 7
    Pool RR, Meagher DM, Stover SM. Pathophysiology of navicular syndrome. In: YovichJV (ed): Vet Clin N Am Equine Pract, 1989;109129.
  • 8
    Wright IM, Kidd L, Thorp BH. Gross, histological and histomorphometric features of the navicular bone and related structures in the horse. Equine Vet J 1998;30:220234.
  • 9
    Doige C, Hoffer MA. Pathological changes in the navicular bone and associated structures of the horse. Can J Comp Med 1983;47:387395.
  • 10
    Dyson S, Murray R, Schramme M, et al. Magnetic resonance imaging of the equine foot: 15 horses. Equine Vet J 2003;35:1826.
  • 11
    Dyson S, Murray R, Schramme M, et al. Lameness in 46 horses associated with deep digital flexor tendinitis in the digit: diagnosis confirmed with magnetic resonance imaging. Equine Vet J 2003;35:681690.
  • 12
    Schramme MC, Murray RC, Blunden TS, et al. Title: A comparison between magnetic resonance imaging, pathology, and radiology in 34 limbs with navicular syndrome and 25 control limbs. In: Brokken TD (ed): Proceedings of the 51st Annu Conv Am Assoc Equine Pract. Lexington, KY: AAEP, 2005;51:348358.
  • 13
    Murray RC, Schramme MC, Dyson SJ, et al. Magnetic resonance imaging characteristics of the foot in horses with palmar foot pain and control horses. Vet Radiol Ultrasound 2006;47:116.
  • 14
    Murray RC, Blunden TS, Schramme MC, et al. How does magnetic resonance imaging represent histologic findings in the equine digit? Vet Radiol Ultrasound 2006;47:1731.
  • 15
    Busoni V, Heimann M, Trenteseaux J, et al. Magnetic resonance imaging findings in the equine deep digital flexor tendon and distal sesamoid bone in advanced navicular disease: an ex vivo study. Vet Radiol Ultrasound 2005;46:279286.
  • 16
    Dyson SJ, Murray RC, Schramme MC. Lameness associated with foot pain: results of magnetic resonance imaging in 199 horses (January 2001–December 2003) and response to treatment. Equine Vet J 2005;37:113121.
  • 17
    Dyson SJ, Murray R. 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 2007;39:340343.
  • 18
    Murray RC, Roberts BL, Schramme MC, et al. Quantitative evaluation of equine deep digital flexor tendon morphology using magnetic resonance imaging. Vet Radiol Ultrasound 2004;45:103111.
  • 19
    Mair TS, Kinns J, Jones RD, et al. Magnetic resonance imaging of the distal limb of the standing horse: technique and review of 40 cases of foot lameness. In: Bramlage LR (ed): Proceedings of the 49th Annu Conv Am Assoc Equine Pract. Lexington, KY: AAEP, 2003;49:2941.
  • 20
    Dyson SJ, Murray RC, Schramme MC, et al. Magnetic resonance imaging in 18 horses with palmar foot pain. In: Lenz TR (ed): Proceedings of the 48th Annu Conv Am Assoc Equine Pract. Lexington, KY: AAEP, 2002; 48:145154.
  • 21
    Mair TS, Sherlock CE. Osseous cyst-like lesions in the feet of lame horses: diagnosis by standing low-field magnetic resonance imaging. Equine Vet Educ 2008;20:4756.
  • 22
    Kofler J, Kneissl S, Malleczek D. MRI and CT diagnosis of acute desmopathy of the lateral collateral sesamoidean (navicular) ligament and long-term outcome in a horse. Vet J 2007;174:410413.
  • 23
    Smith MRW, Wright IM, Smith RKW. Endoscopic assessment and treatment of lesions of the deep digital flexor tendon in the navicular bursae of 20 lame horses. Equine Vet J 2007;39:1824.
  • 24
    Dyson S, Murray R. Use of concurrent scintigraphic and magnetic resonance imaging evaluation to improve understanding of the pathogenesis of injury of the podotrochlear apparatus. Equine Vet J 2007;39:365369.
  • 25
    Sherlock CE, Kinns J, Mair TS. Evaluation of foot pain in the standing horse by magnetic resonance imaging. Vet Record 2007;161:739744.
  • 26
    Mitchell RD, Ewards RB, Makkreel LD, et al. Standing MRI lesions identified in jumping and dressage horses with lameness isolated to the foot. In: Corey DG (ed): Proceedings of the 52nd Annu Conv Am Equine Pract. Lexington, KY : AAEP, 2006;52:422426.
  • 27
    Martinelli MJ, Rantanan NW. Relationship between nuclear scintigraphy and standing MRI in 30 horses with lameness of the foot. In: Brokken TD (ed): Proceedings of the 51st Annu Conv Am Assoc Equine Pract. Lexington, KY : AAEP, 2005;51:359365.
  • 28
    Mair TS, Kins J. Deep digital flexor tendonitis in the equine foot diagnosed by low-field magnetic resonance imaging in the standing patient: 18 cases. Vet Radiol Ultrasound 2005;46:458466.
  • 29
    Dyson S, Murray R. Verification of scintigraphic imaging for injury diagnosis in 264 horses with foot pain. Equine Vet J 2007;39:350355.
  • 30
    Dyson SJ. Radiological interpretation of the navicular bone. Equine Vet Educ 2008;20:268280.
  • 31
    Sampson SN, Schneider RK, Tucker RL. Magnetic resonance imaging of the equine distal limb. In: AuerJA, StickJA (eds): Equine surgery, 3rd ed. Philadelphia: WB Saunders Co, 2005;946963.
  • 32
    Erickson SJ, Prost RW, Timins ME. The “magic angle” effect: background physics and clinical relevance. Radiology 1993;188:2325.
  • 33
    Erickson SJ, Cox IH, Hyde JS, et al. Effect of tendon orientation on MR imaging signal intensity: a manifestation of the “magic angle” phenomenon. Radiology 1991;181:389392.
  • 34
    Spriet M, Mai W, McKnight A. Asymmetric signal intensity in normal collateral ligaments of the distal interphalangeal joint in horses with a low-field MRI system due to the magic angle effect. Vet Radiol Ultrasound 2007;48:95100.
  • 35
    Busoni V, Snaps F. Effect of deep digital flexor tendon orientation on magnetic resonance imaging signal intensity in isolated equine limbs – the magic angle effect. Vet Radiol Ultrasound 2002;43:428430.
  • 36
    Widmer WR, Buckwalter KA, Fessler JF, et al. Use of radiography, computed tomography and magnetic resonance imaging for evaluation of navicular syndrome in the horse. Vet Radiol Ultrasound 2000;41:108116.
  • 37
    Zanetti M, Bruder E, Romero J, et al. Bone marrow edema pattern in osteoarthritic knees: correlation between MR imaging and histologic findings. Radiology 2000;215:835840.
  • 38
    Schmid MR, Hodler J, Vienne P, et al. Bone marrow abnormalities of foot and ankle: STIR versus T1-weighted contrast-enhanced fat suppressed spin-echo MR imaging. Radiology 2002;224:463469.
  • 39
    Blunden A, Dyson S, Murray R, et al. Histopathology in horses with chronic palmar foot pain and age-matched controls. Part 1: navicular bone and related structures. Equine Vet J 2006;38:1522.
  • 40
    Blunden A, Dyson S, Murray R, et al. Histopathology in horses with chronic palmar foot pain and age-matched controls. Part 2: the deep digital flexor tendon. Equine Vet J 2006;38:2327.
  • 41
    Schneider RK, Sampson SN, Gavin PR. Magnetic resonance imaging evaluation of horses with lameness problems. In: Brokken TD (ed): Proceedings of the 51st Annu Conv Am Assoc Equine Pract. Lexington, KY : AAEP, 2005;51:2134.
  • 42
    Dabareiner RM, Carter GK, Honnas CM. Injection of corticosteroids, hyaluronate, and amikacin into the navicular bursae in horses with signs of navicular area pain unresponsive to other treatments: 25 cases (1999–2002). J Am Vet Med Assoc 2003;223:14691474.
  • 43
    Jacquet S, Coudry V, Denoix JM. Severe tear of the collateral sesamoidean ligament in a horse. Vet Record 2006;159:818820.
  • 44
    Adams OR. Lameness in horses, 3rd ed. Philadelphia: Lea & Febiger, 1974;260.
  • 45
    Denoix JM. Functional anatomy of the equine interphalangeal joints, in Proceedings. In: Franklin B Jr. (ed): Proceedings of the 44th Ann Conv Am Assoc Equine Pract. Lexington, KY: AAEP, 1999;44:174177.
  • 46
    Barber M, Sampson SN, Schneider RK, et al. Unilateral navicular bone injury diagnosed with magnetic resonance imaging in a horse. J Am Vet Med Assoc 2006;229:717720.
  • 47
    Busoni V, Snaps F, Trenteseaux J, et al. Magnetic resonance imaging of the palmar aspect of the equine podotrochlear apparatus: normal appearance. Vet Radiol Ultrasound 2004;45:198204.