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Magnetic Resonance Imaging and Scintigraphic Findings in Five Horses with Obscure Foot Lameness Associated with Penetrating Injuries
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Magnetic resonance imaging (MRI) has diagnostic and prognostic value in evaluating acute and chronic penetrating injuries to the foot. The structures involved can be imaged in detail, and therapy can be planned and monitored.
1. Introduction
Penetrating wounds of the sole and frog in the hooves of the horse are common and potentially serious injuries [1,2]. The exact location of penetration of the foot is a major prognostic indicator. Puncture wounds to the frog (Cuneus ungulae) and the collateral sulci have a much graver prognosis than similar wounds to the sole [2], because they put the deep digital flexor tendon (DDFT), the Bursa podotrochlearis, the distal sesamoid bone (DSB), the distal sesamoidean impar ligament (DSIL), the distal interphalangeal (DIP) joint, the digital flexor tendon sheath, and the distal phalanx at risk.
If an injury is acute and the site of penetration is known, diagnosis is relatively uncomplicated. Radiographic examination combined with the use of a probe inserted into the site of penetration and/or the use of radiographic contrast studies may help to determine the structures involved [3-5]. Transcuneal ultrasonography may yield additional information [6,7] Endoscopic evaluation of the B. podotrochlearis [8,9] and the proximal palmar recess of the DIP joint [10] may facilitate diagnosis and surgical therapy.
However, if the injury is chronic or if a horse is presented for lameness evaluation with no history of a penetrating wound, diagnosis becomes much more challenging. Nuclear scintigraphy [11-13] and magnetic resonance imaging (MRI) [14-17] have the potential to yield more information.
The purpose of this paper is to describe the clinical, scintigraphic, and MRI findings in five horses with chronic lameness after a presumed penetrating injury to the foot. The advantages of MRI over other imaging techniques are discussed.
2. Materials and Methods
Between January 2001 and December 2003, 199 horses were examined at the Centre for Equine Studies of the Animal Health Trust (AHT) using magnetic resonance (MR) images. All had lameness localized to the foot, and a conclusive diagnosis for the pain causing lameness could not be achieved using local analgesic techniques, radiography, ultrasonography, or nuclear scintigraphy. Of these, five horses were included in the current study. Four of the horses (Cases 1-4) had evidence on MR images of a penetrating injury. One horse (Case 5) had an intraosseous abscess in the distal phalanx assumed to be the result of a known prior penetrating injury.
Clinical Examination
The feet were examined thoroughly. Lameness was assessed in straight lines on a hard surface (all cases) and in circles of 10- to 15-m diameter on both hard and soft surfaces (not Cases 1 and 2 because of the severity of the lameness in walk and straight lines). A lameness grading scale from 0 to 8 was used (0, sound; 2, mild; 4, moderate; 6, severe; 8, non-weight-bearing). Flexion tests of the distal limbs were performed in Cases 3-5 but not in Cases 1 and 2 because of the severity of the lameness in walk and straight lines.
Diagnostic analgesia of the distal limb was performed at the AHT in Cases 1-3 and by the referring veterinary surgeon in Cases 4 and 5. The sequence of nerve blocks varied according to the investigating veterinarian and the lameness characteristics (Table 1).
Table 1. Signalment, Lameness Characteristics, and Results of Diagnostic Analgesia. | |||||
| Case 1 | Case 2 | Case 3 | Case 4 | Case 5 |
Signalment | |||||
Age (yr) | 8 | 14 | 12 | 5 | 7 |
Breed | TB cross | TB | TB cross Welsh Cob | Irish Draught | Warmblood |
Purpose | GP | Hunter | GP | GP | Showjumper |
Lameness Characteristics* | |||||
Duration of lameness (mo) | 3 | 3 | 3 | 14 | 6 |
Lame limb | LF | LF | LH | RH | RF >LF ‡ |
Lame in walk | 3 - 4 | 3 | 0 | 0 | 0 |
Straight lines | 3 - 4 | 3 | 4 | 0 | 2 LF |
Circle: lame limb on the inside | NP | NP | 5 | 0 | § |
Circle: lame limb on the outside | NP | NP | 5 | 2 | 4 RF/2 LF § |
Results of Diagnostic Analgesia † Palmar digital nerve block | 100% | 100% | NP | NP | RF 50% ** LF 50% ** |
Palmar/plantar (abaxial sesamoid) nerve blocks | NP | NP | 75% | 100% ¶ ** | RF 100% ** LF 100% ** |
Distal interphalangeal joint | NP | NP | 50 - 100% †† | NP | LF 0% ** |
* Lameness graded on a scale from 0 to 8 (0, not lame; 2, mild; 4, moderate; 6, severe; 8, non-weight-bearing). † Improvement of lameness after diagnostic analgesia is shown as a relative percentage of improvement compared to baseline lameness. ‡ More lame RF than LF in circles; lame LF in straight lines. § Bilaterally front limb lame with the lame limb on the outside of the circle. ¶ Only analgesia of the lateral plantar nerve. ** Performed by the referring veterinary surgeon. †† 50% immediately after injection, 100% after 5 min. TB, Thoroughbred; GP, general purpose; NP, not performed. |
Radiography
Radiography was performed at the AHT in Cases 1-3 and by the referring veterinarians in Cases 4 and 5. Lateromedial, dorsoproximal-palmarodistal oblique, palmaroproximal-palmarodistal oblique, weight-bearing dorsopalmar, and flexed oblique radiographic views of the foot were obtained in Cases 1-3 [18].
Ultrasonography
Ultrasonography was performed at the AHT in Cases 1-3. The palmar aspect of the pastern region was imaged using a 10-MHz linear transducer with and without a stand off. The area between the heel bulbs was imaged with a 6.5-MHz convex array transducer. The collateral ligaments of the DIP joint were also assessed. Case 3 had transcuneal ultrasonography performed; images were obtained with a 7.5- to 10-MHz linear transducer without a stand-off.
Nuclear Scintigraphy
Nuclear scintigraphy of both front feet was performed at the AHT in Cases 1-3 and by the referring veterinarians in Cases 4 and 5. Lateral, dorsal/plantar, and solar views were obtained using vascular, pool, and bone phase images [11]. The referring veterinarian in Case 4 obtained no solar images.
MRI
MRI was performed using a 1.5-T GE Signa Echospeed magnet [a] following the procedure previously described [16]. Sagittal, dorsal, and transverse MR images were obtained using three-dimensional (3D) T1-weighted spoiled gradient echo (SPGR), 3D T2* gradient echo (GRE), and 2D short-inversion recovery (STIR). Slice thickness was 1.5 mm for the SPGR and GRE images and 4 mm for the STIR images.
3. Results
Clinical History
Five horses with occult lameness and pain localized to the foot were examined. Cases 1 and 4 had no history of a penetrating injury. Cases 2 and 3 had a history of a penetrating injury to the frog; however, in Case 2, the injury had a presumed depth of only 1 cm, and lameness had resolved a few days after the injury only to return when the horse resumed work. In Case 3, a left hindlimb (LH) lameness had occurred after a penetrating injury had allegedly been resolved. The horse was presented for investigation of longer-standing right hindlimb (RH) lameness, but it actually showed (LH) lameness. Case 5 had a history of left forelimb (LF) lameness followed by a penetration of the sole of the right forelimb (RF). It was presented with bilateral forelimb lameness that was more severe on the LF. The interval of time between when the lameness was detected and referral was ~3 mo for Cases 1-3, 6 mo for Case 5, and >1 yr for Case 4.
Clinical Features
Two horses (Cases 1 and 2) had unilateral forelimb lameness, two horses (Cases 3 and 4) had unilateral hindlimb lameness, and one horse (Case 5) had bilateral forelimb lameness. The severity and characteristics of the lamenesses are summarized in Table 1. The digital pulse amplitudes were increased in both forelimbs in Cases 1 and 2 but were greater in the lame limb. Flexion tests of the distal limb were performed in Cases 3-5. They were only positive in Case 3 with the lameness accentuated in the lame limb after flexion of the contralateral limb.
Radiography
Case 1 had a round, mineralized opacity close to the lateral tuberosity of the left middle phalanx. It was thought that this opacity had little clinical significance. Case 4 had an irregular margin of the plantar aspect of the lateral palmar process of the distal phalanx. No significant radiological abnormalities were detected in Cases 2, 3, or 5.
Ultrasonography
No ultrasonographic abnormalities were detected in Case 1. Case 2 had enlargement of and central hypoechoic areas in the lateral branch of the superficial digital flexor tendon. The lateral abaxial margin was poorly defined. Transcuneal examination in Case 3 revealed increased thickness of both the DDFT and the DSIL distal to the navicular bone compared with the non-lame limb. There was loss of fiber pattern in the DDFT, especially adjacent to the insertion on the distal phalanx.
Nuclear Scintigraphy
Cases 1-3 had increased radiopharmaceutical uptake (IRU) in the lateral pool phase image of the foot in the region of the DDFT (Fig. 1A). Case 1 had focal mild IRU in the distal phalanx in the solar bone phase image at the site of the medial insertion of the DDFT. Case 2 had intense focal IRU in the region of the DDFT’s insertion on the distal phalanx (LF) in both pool and bone phase lateral (Fig. 2A) and solar images (Fig. 2B). Case 3 had diffuse IRU in the distal phalanx (LH) in both pool and bone phases that was visible in dorsal, lateral (Fig. 1A), and solar images (Fig. 1B). Case 4 had IRU in the lateral aspect of the foot (RH) in both pool and bone phases seen in dorsal images. The referring veterinarian did not obtain solar images. Case 5 had focal intense IRU in the medial part of the distal phalanx (RF) seen in the dorsal view in the pool phase and both dorsal and solar images in the bone phase.
Figure 1. (A) Lateral pool phase scintigraphic images of the hind feet of Case 3. There is IRU in the region of the distal aspect of the DDFT and its insertion on the left distal phalanx (arrow). (B) Solar bone phase scintigraphic images of the hind feet in Case 3. There is diffuse IRU in the left distal phalanx. It is most intense in the region of insertion of the DDFT. M, medial.
Figure 2. (A) Lateral and (B) solar scintigraphic images of the front feet of Case 2. There is generalized greater radiopharmaceutical uptake (RU) in the left forelimb compared with the right forelimb. There is diffuse intense IRU in the left front distal phalanx in A and in the region of insertion of the DDFT in B. M, medial.
MRI
Cases 1-4 had evidence of a penetrating tract through the frog or its collateral sulci that was most clearly seen in Cases 3 and 4 (Fig. 3 and Fig. 4). Several areas of hypointense signal, compatible with hemosiderin deposition, gas accumulation, or mineralization, were seen in a straight line from the dermal tissue toward the deeper structures (Fig. 3 and Fig. 4).
Figure 3. Identical sagittal MRIs of Case 4 obtained using (A) SPGR and (B) 3D T2* GRE sequences. There is enlargement and disruption of the normal DDFT architecture and loss of separation from the DSIL. There is a hypointense signal representing hemosiderin deposition along the penetration tract through the digital cushion, the DDFT, and the DSIL (small white arrow). There is disruption of the DSIL and endosteal irregularity of the plantar cortex of the distal phalanx (black open arrow). There is proliferation of soft tissue in the proximal recess of the B. podotrochlearis (white open arrow). The increased signal in the distal aspect of the DDFT in A is caused by the magic-angle effect.
Figure 4. (A) Sagittal, (B) transverse, and (C) dorsal SPGR images of Case 3. There is disruption of the DDFT and the DSIL architecture in A and B. There is a hypointense signal representing hemosiderin deposition along the penetration tract through the digital cushion, the DDFT, and the DSIL (black open arrow). The increased signal in the distal aspect of the DDFT is caused by the magic-angle effect.
Cases 1-4 had lesions in the DDFT (Fig. 3, Fig. 4 and Fig. 5). These were areas of hyperintense signal in SPGR, 3D T2*GRE, and STIR sequences. The DDFT was enlarged, and there was disruption of the normal tendon architecture. The hyperintense lesion traversed the tendon thickness and was closely related to lesions in the laminae and digital cushion in the same plane. Cases 1-4 had severe disruption of the DSIL, adhesion formation to the DDFT, and loss of normal ligament architecture visible in both transverse and sagittal sequences (Fig. 3 and Fig. 4). These findings were compatible with a penetrating injury, resulting in disruption of both tendon and ligament architecture.
Figure 5. (A) Sagittal SPGR and (B) sagittal STIR MRIs of Case 2. There is a decreased signal in the distal phalanx in the SPGR image that is consistent with fluid or mineralization. There is also an increased signal in the STIR image consistent with bone pathology (white open arrow). There is a hypointense signal in both sequences that reflect hemosiderin along the penetration tract (black open arrow). There is cortical irregularity at the insertion of the DDFT (small white arrow). The DDFT is markedly enlarged at its insertion with an increased signal in the STIR image. There is effusion in the DIP joint in both image sequences.
Cases 2 and 5 had major changes in the distal phalanx. Case 2 had an abnormal signal in T1, T2-weighted, and fat-suppressed images in the palmar aspect of the distal phalanx where the DDFT and DSIL insert. This abnormality was consistent with both fluid and mineralization (Fig. 5). The palmar cortex of the distal phalanx had an irregular outline (Fig. 5). Case 5 had abnormalities of the medial aspect of the distal phalanx. A focal defect in the solar margin extended into an intraosseous lesion with high signal (Fig. 6) in SPGR, 3D T2*GRE, and STIR sequences, and it was surrounded by a region of reduced signal in all three sequences, which reflects mineralization. The high signal in all three sequences was compatible with fluid with high cell content, consistent with an intraosseous abscess (Fig. 6). Cases 1, 3, and 4 had endosteal and/or cortical irregularity of the distal phalanx at the insertion of the DDFT and DSIL (Fig. 3 and Fig. 5). Cases 3 and 4 had low-grade endosteal mineralization and thickening of the flexor cortex of the DSB and focal edema in the spongiosa. Case 1 had effusion in the B. podotrochlearis. Cases 3 and 4 had extensive proliferation of tissue in the proximal recess of the B. podotrochlearis (Fig. 3). Case 2 had effusion in the DIP joint (Fig. 5).
Case 5, lame bilaterally, had evidence of a low-grade insertional injury of the DDFT and desmopathy of the DSIL in the contralateral lamer limb.
Figure 6. (A) Sagittal SPGR, (B) dorsal T2* GRE, and (C) transverse STIR MRIs of Case 5. Medial is to the right in B and C. There is a hyperintense signal in the distal phalanx (small white arrow) in all three image sequences. This indicates that there is fluid present with a high cellular content, which is consistent with an intraosseous abscess. There is a decreased signal in the cancellous bone of the distal phalanx that is the result of increased mineralization (large open arrow).
Outcome
Cases 1-4 were not treated because of the severity and chronicity of the injuries. Case 1 was humanely destroyed. Cases 2-4 had persistent lameness. Case 5 was rested and treated with systemic antimicrobial drugs. The horse improved and resumed full work, but it experienced recurrent lameness several months later.
A post-mortem examination was performed in Case 1. There was a triangular area of hemorrhage extending from the frog and digital cushion into the lesion of the medial lobe of the DDFT. Histological examination confirmed the presence of hemosiderin coincident with the focal areas of hypointense signal seen in the MR images. Macroscopic examination revealed a focal hemorrhagic lesion in the medial lobe of the left DDFT and degenerative changes on the flexor surface of the DSB.
4. Discussion
The five horses presented in this study were believed to have sustained trauma because of a penetrating injury based either on the history (Cases 3 and 5) or the MRI findings (Cases 1, 2, and 4). This shows the limitations of an owner’s history. The presence of multiple foci of hypointense signal traversing the tissues in the line of the postulated penetration seen in Cases 1-4 (Fig. 3 and Fig. 4) is consistent with findings in a previously documented case series of known penetrating injuries [17].
A detailed clinical examination did not suggest previous penetrating injuries in any of the horses. However, given the chronicity of the lesions, this is not surprising. The severity of lameness varied between horses. Cases 1 and 2 showed obvious lameness at a walk. A similar degree of lameness was obvious in Case 3 at the trot. However, Cases 4 and 5 only showed lameness on a circle. These differences cannot be explained by lesion severity. However, the horses that were not as severely impaired had a much longer duration of lameness. Case 3 showed increased lameness after flexion of the contralateral limb, a feature that has been seen in other horses with strain-induced DDFT injuries.
The diagnosis of a penetrating injury was made on the basis of the findings on the MRIs. In Cases 1-4, these included well-defined linear areas of hypointense signal in the palmar aspect of the DDFT, digital cushion, and laminae visible in all three image sequences (Fig. 3 and Fig. 4). There was major disruption of the architecture of the DDFT and DSIL in all horses with lesions traversing the full thickness of the tissues. Case 2 had severe disruption of the flexor cortex of the distal phalanx (Fig. 5). The position of the intraosseous abscess seen in the medial aspect of distal phalanx of Case 5 (Fig. 6) correlated with the site of a previous penetrating injury. However, in this horse, there was no disruption of the DDFT or the DSIL nor were there foci of hypointense signal.
Hypointense signals in soft tissues seen in all three image sequences may be a manifestation of mineralization, gas accumulation, or deposits of hemosiderin [19]. In Cases 1-4, mineralization seemed unlikely, because no changes were detected ultrasonographically or radiographically and their anatomical location was such that the same area should have been visible in a lateromedial radiographic view. Gas accumulation could have been the result of a septic process, but there was no evidence of an active infectious process in the post-mortem performed in Case 1. Septic core lesions in the DDFT in the metacarpal and metatarsal regions have been reported [20]. The rarity of these lesions, the clinical findings, the localization, and the fast progression of the lesions differ from the chronic cases described here.
Hemosiderin is a breakdown product of hemoglobin, and its magnetic properties create a hypointense signal in MR images in SPGR, GRE, and STIR sequences [19]. In Case 1, the post-mortem confirmed deposits of hemosiderin in the region of the hypointense signals. It has been previously described that areas of hypointense signal can lead to hemosiderin deposition in horses with known penetrating injuries of the foot; however, this was not verified by histopathology [17]. The absence of a hypointense signal in Case 5 is curious, and it remains speculative whether or not the intraosseous lesion was truly the result of a previous penetrating injury.
The DDFT lesions seen in Cases 1-4 differed from previously described strain-type injuries to the DDFT [21]. The major lesions in Cases 1-4 were in close proximity to the focal areas of hypointense signal. They did not respect the borders of the tissues. The abnormal architecture of the flexor cortex of the distal phalanx in Case 2 was not typical of a strain injury. These findings support a diagnosis of traumatic damage to the DDFT and the closely related structures, probably the result of a penetrating injury. The findings in Case 3, a horse with a known history of a deep penetrating injury, were similar to the findings in Cases 1, 2, and 4. It, therefore, seems reasonable that the injuries were of similar etiology.
No significant radiographic changes were found. Major lesions of the distal phalanx were identified with MRI in Cases 2 and 5 (Fig. 5 and Fig. 6. Radiographic detection requires a loss of bone density of at least 30% [22]. The osteitis in Case 2 was related to the insertion of the DDFT and DSIL. In a lateromedial view, the area is superimposed by the palmar processes. In the dorsoproximal-palmarodistal view, the area is superimposed by the full thickness of the distal phalanx. Therefore, changes in this area are difficult to detect radiographically. In Case 5, the referring veterinarian had not detected a radiological abnormality; additional radiographs were not obtained at the AHT, but we suspect that a lesion may have been detectable.
MR images have previously been reported to be more sensitive in detecting changes of bone and to have a higher resolution than radiography in the horse [23]. In human medicine, MRI is considered the best diagnostic imaging technique to diagnose acute osteomyelitis [24]. Several studies have shown sensitivity between 82% and 100% and a specificity of 75 - 100% [24,25]. The nuclear scintigraphic findings correlated well with MRI, but MRI yielded more specific findings. Cases 1-3 had focal IRU in the distal DDFT in the lateral pool phase image (Fig. 1A), reflecting a tendon injury that was confirmed using MRI. Case 4, with lameness of a 14-mo duration, had a DDFT injury diagnosed with MRI but no associated IRU. This is consistent with false negative scintigraphic results reported in horses with MRI evidence of deep digital flexor (DDF) tendonitis with lameness of >3 mo duration [21]. The solar bone phase view gave the most information (Fig. 1B and Fig. 2B); localization and intensity of the IRU was most easily established. Case 2 had focal-intense IRU in the region of insertion of the medial part of the DDFT and DSIL on the distal phalanx (Fig. 2B). This was more intense than the findings previously reported in insertional DDF tendonopathies [21] and supports the need for an alternative etiopathogenesis. Case 5 had a focal intense IRU in the medial part of the distal phalanx seen in the solar view. The highly intense focal area of IRU seen in both Cases 2 and 5 could be the result of bone trauma, fracture, and aseptic or septic osteitis. In Case 2, bone edema and mineralization were seen in MR images as generalized decreased signal intensity in T1-weighted images and mixed signal intensity in T2-weighted images (Fig. 5). In Case 5, mineralization surrounded an intraosseous abscess with high cellular fluid content in the medial aspect of the distal phalanx. Mineralization of the cancellous bone was seen as a decreased signal in T1- and T2-weighted images. Fluid with a high cellular content was seen as an increased signal in all three sequences (Fig. 6).
Transcuneal ultrasonography may reveal some information after a penetrating injury to the frog and its sulci. The technique can be used to image the DDFT, the DSIL, the B. podotrochlearis, the flexor surface of the distal phalanx, and the DSB, but the technique is limited to the sagittal midline [6,7]. Transcuneal ultrasonography was performed in Case 3, and it revealed an enlarged DDFT with a loss of fiber structure, especially at the insertion on the distal phalanx. However, the full extent of the lesions could not be determined, and a complete pathoanatomic diagnosis could not be established.
Kinns and Mair [17] described five horses with a recent history of a penetrating injury to the foot in which MRI was used to diagnose the extent of the injury. Four horses (80%) returned to their previous function with conservative management. This contrasts with the chronic cases described in this study; only one of five horses resumed work, and it suffered from recurrent lamenesses. In both studies, it is notable that ongoing infection did not accompany most penetrating injuries.
In our opinion, the lesions and their severity described in this case report could not have been diagnosed without MRI. Nuclear scintigraphy yielded valuable information that correlated well with the MRI findings, but it was not specific. MRI is a very valuable tool in the diagnostic investigation of obscure foot lameness. It has the best anatomical detail of all the diagnostic imaging techniques. It may also have a prognostic value in evaluating acute to chronic penetrating injuries. Structures involved can be imaged in detail, and therapy can be planned and monitored.
We thank Katherine Whitwell for performing the post-mortem examination.
Footnote
- General Electric, Milwaukee, WI.
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