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Relationship between Nuclear Scintigraphy and Standing MRI in 30 Horses with Lameness of the Foot
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Regional anesthesia and radiography have comprised the standards for evaluating the foot for lameness, but they often fall short of providing a conclusive diagnosis. Nuclear scintigraphy has added sensitivity to the diagnostic process, but it still lacks the specificity often needed to convey a definitive diagnosis. Magnetic resonance imaging (MRI) combines anatomical and physiological imaging, which brings a much higher level of specificity to the diagnosis of lameness in the equine foot.
1. Introduction
Nuclear scintigraphy is a metabolic imaging modality that has been used for over 25 yr in the diagnosis of lameness, particularly for those originating in the foot. Nuclear scintigraphy has been considered by researchers and clinicians to be a very sensitive, but not specific, imaging modality. This means that subtle alterations in bone remodeling can be effectively detected. This remodeling can be caused by either direct bone change, such as bone resorption or deposition, or alterations of bone associated with the attachment of soft-tissue structures. Some of the early gamma cameras used for nuclear scintigraphy did not possess good spatial resolution. With these cameras, it was relatively easy to observe increased radiopharmaceutical uptake (IRU) in the foot, but it was very difficult to determine whether a specific anatomical structure was involved. When IRU was detected in the foot, radiography was required to determine whether any anatomical/structural defects could be seen. In the mid-1990s, gamma-camera technology and image-acquisition software improved dramatically. The resolution of the image improved, providing more spatial localization within the bone scan. This enabled clinicians to determine whether the IRU was related strictly to the navicular bone, the insertion of the deep digital flexor tendon (DDFT), or the coffin bone, and therefore, it also helped them to make a more specific diagnosis with relation to the anatomical structures involved in the disease process [1]. When nuclear scintigraphy reached this level of technical competence, radiography was used solely as an adjunct imaging modality when looking for obvious abnormalities, such as fractures, or subtle changes in navicular bone density.
With the advent of magnetic resonance imaging (MRI) with diagnostic capabilities for the equine foot, a whole new level of imaging has been reached. Both the bony and soft-tissue structures of the foot can now be visualized in ways previously limited to post-mortem examinations. Additionally, MRI has the ability to convey both anatomical and physiologic information about the structures being imaged by combining different sequences in any desired orthogonal plane [2]. In this series of cases, MRI was combined with nuclear scintigraphy to determine whether it improved the diagnostic and prognostic yield of each modality separately.
2. Materials and Methods
The records of all horses undergoing a standing MRI were reviewed. Those horses that had also undergone nuclear scintigraphy were included in the study. Nuclear scintigraphy was carried out with a refurbished Technicare Omega 500 mounted on an Equistand and running Mirage acquisition software. Tc99m-MDP was injected intravenously, and the soft-tissue phase imaging commenced immediately. Dorsal and lateral images of the front feet were obtained. The horse's front feet were covered with a plastic bag to prevent contamination with urine; then, the horse was returned to the holding stall. After ~2 h, the plastic bags were removed, and the bone phase imaging commenced. The bone phase images consisted of dorsal, lateral, and solar views of each front foot. In some cases, additional images of other bodily regions were acquired to complete the scan.
The MRIs were acquired with a Hallmarq MRI Limb Scanner. The horses were tranquilized, and the foot was positioned in the magnet with the coil surrounding the area to be imaged. Imaging consisted of T1-weighted and short inversion time inversion recovery (STIR) sequences taken in at least the sagittal and transverse planes. For the purpose of imaging the collateral ligaments, the frontal plane was also included.
3. Results
Thirty horses were included in the study. There were 10 used for showjumping, 9 for hunter-type activities, 5 for Western performance, 2 for dressage, 2 for eventing, and 2 for general riding. The history for all of the horses in the study involved lameness isolated to the foot as the presenting complaint. All of the horses had shown significant improvement to a palmar digital nerve block. Two horses had also blocked sound to intra-articular anesthesia of the coffin joint.
The interpretation of the bone scans revealed IRU in three general patterns:
- Focal navicular bone IRU (15 horses);
- IRU at the insertion of the DDFT (2 horses);
- IRU on the wing of P3 (1 horse).
Ten horses showed a combination of one or more of these patterns within the same foot, although all of these horses showed IRU associated with the navicular bone. In two horses, IRU was only noted in the soft-tissue phase, and no abnormal IRU was noted in the bone phase.
The interpretation of the MRIs fell into six general categories.
- There were six horses with obvious changes in the navicular bone present on both T1-weighted images and the STIR sequence (Fig. 1).
- Three horses showed a generalized increase in signal intensity on the STIR sequence throughout the medullary cavity of the navicular bone with no changes evident on the T1-weighted images.
- Four horses showed focal increases in signal intensity in the STIR sequence at the insertion of the impar ligament onto the distal border of the navicular bone only.
- Three horses had an increase in signal intensity in the navicular bone on the STIR sequence but also had evidence of a tear in the DDFT on the T1-weighted images (Fig. 2).
- Six horses had abnormal signal intensity in a collateral ligament of the coffin joint, prompting a diagnosis of collateral ligament desmitis.
- Six horses had slight increases in signal intensity on the STIR sequence at the insertion of the DDFT.
Figure 1. T1-weighted image on the left and STIR on the right show obvious navicular disease. The black arrow in the T1-weighted image shows a decrease in signal intensity (more black) within the medullary cavity of the navicular bone. On this particular sequence, the decrease in signal indicates an increase in bone density within the navicular bone. In the STIR sequence, the white arrow points to the increase in signal (more white) in the medullary cavity of the navicular bone, indicating fluid or inflammation. Navicular disease is diagnosed when these changes are present on both sequences in the same horse.
Figure 2. T1-weighted images showing a split in the DDFT. In the sagittal view on the left, the black arrows mark the region of the split from proximal to distal. The tendon abnormality can be seen on the sagittal view as a "wave" in the normal black signal of the DDFT. In the transverse view on the right, the black arrow points to the increase in signal intensity within the lateral lobe of the DDFT, indicating a split in the tendon.
One horse had a defect in the articular surface of P3 that was visible on the T1-weighted images, and one horse had a desmitis of the chondrosesamoidean ligament of the navicular bone.
The results of the bone scans were compared directly with the MRIs to determine whether the two imaging modalities were always in direct agreement or whether the combination improved the overall diagnostic potential. Of the 15 horses with focal and intense IRU associated only with the navicular bone, 13 had an MRI diagnosis that also involved only the navicular bone. The other two horses had some degree of navicular bone pathology, but the main lesion was a tear in the DDFT. In the six horses with a final diagnosis of collateral ligament desmitis of the coffin joint, five had IRU associated with the corresponding wing of P3 on the bone scan (Fig. 3). One had IRU along the medial aspect of the foot as seen on the dorsal view that was only present on the soft-tissue phase images. Four of these six horses also had some degree of IRU associated with the navicular bone. In the two horses that had IRU at the insertion of the DDFT, the MRI only showed increased signal intensity at the insertion of the DDFT on the STIR sequence. There were five horses that had IRU associated with the navicular bone and IRU associated with a region of P3 dorsal to the navicular bone as seen on the solar view (Fig. 4). Three of these five horses had evidence of navicular bone pathology on MRI, but they also showed an additional lesion on MRI. One horse had a tear in the DDFT as it crossed the navicular bone, one horse showed inflammation through the insertion of the DDFT and the medial collateral ligament, and one horse had an articular defect in P3. Two of the five horses with this mixed pattern of IRU did not show any indication of navicular pathology on the MRI. Both showed evidence of soft-tissue inflammation at the insertion of the DDFT. Finally, there were two horses that did not follow any of the previously mentioned patterns. One horse had focal IRU associated with the navicular bone and a focal and more intense spot of IRU just lateral to that as seen on the solar view. This horse showed evidence of mild navicular pathology on the STIR sequence, but it also showed evidence of desmitis along the lateral collateral ligament of the navicular bone where it attaches to P3. Finally, there was one horse that had IRU through the palmar soft tissues of the navicular region on the pool phase only. This horse showed an increased signal intensity in the STIR sequence only on the lateral border of the DDFT as it crossed the navicular bone.
Figure 3. This is a bone scan of a foot showing IRU associated with the navicular bone (white arrow) and with the soft-tissue insertions dorsal to it (black arrow) on both the lateral and solar views. Such a combined pattern of IRU was a common finding in this paper, and it is common in general among performance horses.
Figure 4. This is a bone scan showing IRU associated with the insertion of the medial collateral ligament. On the lateral view on the left, the IRU is seen over the palmar aspect of the coffin bone. In the solar view on the right, there is focal and relatively intense IRU in the medial wing of the coffin bone at the insertion site of the medial collateral ligament.
Radiographs were available for most of the horses in the study. Of the six horses with obvious navicular changes on all MRI sequences, five had radiographs available for viewing; all showed some degree of navicular pathology, most commonly a thickening or loss of definition at the corticomedullary junction. There were no radiographic abnormalities detected for the three horses with increased signal intensity in the navicular bone on the STIR sequence only or for the four horses with a diagnosis of insertional desmopathy or enthesopathy at the origin of the impar ligament. Two of the three horses with a DDFT tear had radiographs available, and they also showed evidence of mild to moderate navicular degeneration. Of the seven horses that had a soft-tissue abnormality diagnosed on MRI, none showed any obvious radiographic lesions. Of the six horses with a diagnosis of collateral ligament desmitis, three showed mild to moderate changes in the corticomedullary junction of the navicular bone, and one showed mild bone proliferation at the insertion of the collateral ligament onto P3. For the horse with an articular defect in P3, the radiographs showed moderate loss of definition of the corticomedullary junction of the navicular bone, but the lesion in P3 could not be identified radiographically.
An ultrasound examination was also performed on the six horses with collateral ligament desmitis. Three had evidence of mild fiber damage or disruption at the level of the coronary band. One had a measurable increase in size of the ligament compared with the contralateral side, and two had no changes detectable with ultrasound.
The treatment and outcome for the horses in the study was recorded. For the three horses with increased signal intensity in the navicular bone on the STIR sequence, a neurectomy was performed on one and recommended for the other two. The horse that was neurectomized is sound and in work. Neurectomy was not performed in the other two horses, and they have remained lame. For the four horses with a diagnosis of impar ligament enthesopathy, rest was recommended. One horse was also treated with extracorporeal shock waves through the bulbs of the heel and through the frog. Three of the four horses are sound and in full work, and one was sound at the recheck but is still convalescing. Of the six horses with obvious navicular changes on MRI, two underwent a neurectomy, and two were retired. The coffin joint of one horse was injected with hyaluronic acid, although a neurectomy has been planned. The horse with a flexor cortex defect underwent bursoscopy and bursal stem-cell injection and is still convalescing. For the three horses with a DDFT tear, one was treated with extracorporeal shock waves, one was rested, and one was retired. All three are still convalescing. Of the seven horses with MRI evidence of a soft-tissue abnormality, four are sound and back in full work, and three are still convalescing. Of the six horses with a diagnosis of collateral ligament desmitis, two received extracorporeal shock-wave treatment. One is sound in light work, and one was sound initially but went lame again. The other four received rest only and are still convalescing. For the horse with an articular defect in P3, a neurectomy was performed; the horse is sound and has returned to jumping.
4. Discussion
Several interesting concepts came out of this study. To the authors' knowledge, this is the first study that compares the pattern of IRU on the bone scan with findings on MRI in a series of horses with various lamenesses of the foot. In this series of cases, we believe that MRI combined with nuclear scintigraphy improved the diagnostic yield of each modality separately.
Most of the horses in this study (23 of 30) showed focal IRU associated with the navicular bone on the bone phase of the nuclear scan. For years, this finding has been associated with navicular remodeling and possibly even navicular disease. There were 15 horses in this study that showed IRU associated solely with the navicular bone. In 6 of these 15 horses, there was signal changes on the T1-weighted as well as STIR images. In all six of these horses, radiography was useful in determining the presence of navicular remodeling or pathology as well. More importantly, in 7 of these 15 horses, the abnormal signal in the navicular bone was only present on the STIR sequence, indicating a physiological abnormality such as inflammation or edema. None of these horses had any evidence of navicular pathology on the radiographs. The abnormal signal on the STIR sequence could be further divided into two general categories. Three horses had increased signal throughout the medullary cavity, whereas four horses had an increased signal located specifically at the insertion site of the impar ligament. The latter pattern was most readily seen in the sagittal plane. These two categories may represent two distinct pathologies, and each is associated with a different prognostic picture.
Most important to this study is that these two groups of horses could not be differentiated from each other with physical examination, radiograph, or bone scan. MRI provided the information needed to make a specific diagnosis. An increase in signal intensity throughout the medullary cavity of the navicular bone may well represent increased fluid or inflammation throughout the medullary space. This signal also seems to be very different from the very bright signal in the medullary cavity of some horses where navicular changes were also obvious on the T1-weighted images. The latter may represent granulation tissue or fibrosis within the medullary cavity rather than just edema fluid. The generalized increase in signal intensity seen in three horses in this study may have been consistent with the concept of pain associated with navicular disease being caused by increased intra-medullary pressure. This finding on the STIR sequence of the MRI has been relatively common in horses with a history of chronic lameness in the forelimbs [3]. If the history includes a lameness of >6 mo in duration that seems to be unresponsive to conservative treatment regimes such as shoeing, intra-articular or intrabursal injections, and systemic medication, then a neurectomy is currently recommended in our clinic for horses with generalized increase in signal intensity on STIR images (navicular edema). Such a treatment may actually provide more than just relief from the discomfort of the condition. If the generalized signal increase in the medullary cavity of the navicular bone on the STIR sequence does indicate early and irreversible navicular pathology, then an early neurectomy may be indicated; there is a chance that continued pain-free loading would preserve foot shape and function by restoring normal weight bearing quickly. It may also prevent the negative feedback mechanism seen in "bad"-footed horses [4].
There was a history of relatively acute lameness in all four horses in this study with a diagnosis of impar ligament enthesopathy. It was postulated that the etiology for the lameness in these cases was injury related. Further in support of this hypothesis is the fact that three of four horses have returned to soundness after a period of rest. The fourth one is still convalescing. Two other horses with similar injuries diagnosed by MRI that were not included in this study have also returned to full soundness. Because of the assertion that such a lesion is caused by trauma to the insertion of the impar ligament on the navicular bone, treating this lesion in an attempt to return the horse to work quickly seems inappropriate. In fact, a neurectomy would seem to be contraindicated in these cases without a convalescent period, although it could be considered in cases that were refractory to rest.
There were three horses with focal IRU and moderate radiographic changes associated with the navicular bone in which MRI also diagnosed a tear in the DDFT. These three horses all had a history that could be considered consistent with what we know about degenerative navicular disease, although the lameness had become worse within the last several months before presentation. Two of the horses had a tear in the lateral lobe of the DDFT, and one had the tear in the medial lobe. In all three horses, there was evidence of navicular bone involvement because of increased signal intensity seen on the STIR sequence; at least part of the tendon tear was in the region of the navicular bone itself. In one horse, the tear extended all the way to the insertion on P3, and in another horse, the tear extended well into the pastern region. In that horse, palpation of the DDFT did not elicit any painful response, and no difference could be detected in the profile of the tendon. An ultrasound examination revealed some asymmetry to the lobes, but no obvious tear could be found. One of these horses was retired, one received shock-wave treatment, and one received rest only. The prognosis for horses with combined lesions of the DDFT and abnormalities of the navicular bone as diagnosed on MRI is grave for return to soundness, and 95% of such cases have been reported to suffer persistent lameness [5]. Even so, it seems inappropriate to recommend a neurectomy for any patients with damage to the DDFT, and, in fact, the presence and subsequent exacerbation of a DDFT could have been responsible for some of the reported failures of neurectomy in the past. Without MRI, however, it seems likely that all three of these cases would have been diagnosed with navicular disease and possibly treated with neurectomy.
Two of 23 horses in this study with some degree of focal IRU associated with the navicular bone did not have any abnormalities observed in the navicular bone on MRI. In both horses, a diagnosis of collateral ligament desmitis was eventually made. One had a history consistent with acute trauma, whereas the other seemed to have a more chronic presentation. No explanation can be given for the normal signal on MRI, particularly on the STIR sequence.
Seven horses had a diagnosis of soft-tissue inflammation or injury near the insertion of the DDFT, which was prompted by the presence of a diffuse signal increase on STIR sequences. Five of seven horses have returned to soundness, and two horses are still convalescing. Although this study is based on limited numbers, these findings suggest an encouraging prognosis for insertional enthesopathy of the DDFT.
The diagnosis of collateral ligament desmitis seems to be a difficult one to make. The anatomical location of the ligament, two-thirds of which is below and distal to the coronary band, makes ultrasonographic evaluation of no value for evaluating its insertion on the distal phalanx. Nuclear scintigraphy in these cases showed IRU associated with the insertion onto the distal phalanx and not with the origin, thereby implying that pathology is more likely to be located deep within the foot. Radiology is also of limited use when making this diagnosis, because the lucency noted at the insertion of the ligament onto the distal phalanx is not a common finding and usually implies a more chronic history. MRIs acquired in both the frontal and transverse planes show promise for the diagnosis of collateral ligament desmitis of the foot. Of more concern is defining an effective treatment for this problem. According to a recent study, only 5 of 17 horses (29%) with >6 mo follow-up were sound and in full work [6].
This study was carried out in a series of 30 horses that underwent both scintigraphy and MRI to further identify and define lameness conditions within the foot. MRI has revolutionized musculoskeletal imaging in the human field since the mid-1980s. In the equine arena, it has the potential to do the same. However, the current technology of the standing magnet used for MRI in the horse limits the size of the imaging field. For this reason, the imaging study needs to be focused directly on the area of abnormality; otherwise, it could be missed. Additionally, until clinicians become skilled in the interpretation of MRIs, subtle lesions may be overlooked. Nuclear scintigraphy has been of great benefit in identifying the area of abnormality. In this study, nuclear scintigraphy improved the sensitivity of MRI, whereas MRI improved the specificity of nuclear scintigraphy (Fig. 1, Fig. 2, Fig. 3, Fig. 4 and Fig. 5; Table 1).
Figure 5. These MRIs are from the same horse as the bone scan depicted in figure 4. They are STIR-sequence images showing increased signal intensity associated with the medial collateral ligament on both the transverse (left) and frontal (right) views. The white arrow on the frontal image on the right highlights obvious "bowing" of the medial collateral ligament with an increase in signal intensity. The signal on a STIR image indicates fluid or inflammation.
Table 1. Comparison between MRI and Bone Scan | ||
MRI Diagnosis | Number of Horses | Bone Scan Findings |
Navicular changes T1/STIR | 6 | IRU navicular bone (6/6) |
Navicular changes STIR only | 3 | IRU navicular bone (3/3) |
Navicular changes impar insertion | 4 | IRU navicular bone (4/4) |
Navicular changes STIR/DDFT lesion T1 | 3 | IRU navicular bone (2/3) |
Collateral ligament desmitis | 6 | IRU wing P3 (5/6); navicular bone (4/6) |
DDFT insertional desmopathy/STIR | 6 | IRU DDFT insertion (2/6); IRU navicular bone and DDFT insertion (4/6) |
Other | 2 | Other |
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