Magnetic Resonance Imaging Evaluation of Horses with Lameness Problems

MRI is a valuable diagnostic tool for horses with performance limiting lameness problems. Installation of high field strength magnets in equine veterinary hospitals is increasing the availability of this technology to practitioners and their clients. It is important for practitioners to gain knowledge about the technology and the information that can be obtained by performing MRI on lame horses.

1. Introduction

"The progress of magnetic resonance imaging as a clinical tool has been extraordinary, out-stripping the rate of development of any other imaging technique. This speed of growth is a testimony to its clinical significance" [1].

Magnetic resonance imaging (MRI) was first performed on live horses in 1997 at Washington State University. Since that time, the use of MRI on horses with lameness problems in the distal limbs has proven this imaging modality to be a powerful diagnostic tool. MRI allows diagnosis of both bone and tendon/ligament problems that were not previously possible in horses [2-7]. However, MRI is not routinely used to evaluate horses with performance-limiting lameness problems because of the current economic limitations involved with purchasing and maintaining a high-field strength magnet. The purpose of this paper is to discuss the application of MRI to clinical cases and its importance in making an accurate diagnosis in horses with lameness problems.

2. Physics of MRI

MRI uses the hydrogen protons within the horse's limbs to produce detailed images of bone and soft-tissue structures [1]. The physics behind MRI are beyond the scope of this paper; information on this subject has been described previously [1,8,9]. The field strength of the magnet is important, and high-field strength magnets >1.0 T are currently necessary to obtain high-quality images. Different imaging sequences are used to highlight specific tissues. Short tau inversion recovery (STIR) sequences are best for finding fluid within bone and soft-tissue structures. This is invaluable when looking for small areas of inflammation in the horse's limb that are the cause of pain and lameness. Proton density sequences are best for clearly showing anatomic structure and therefore, are best for evaluating ligaments and tendons. This sequence allows assessment of size and symmetry, but it also can assess chemical changes within the soft-tissue structure. Any change in the density of the protons results in a change in signal intensity that is observed with MRI.

MRI is an evolving technology. Refinement of imaging techniques and slice planes improve the ability to image structures in the equine distal limb. There are many different imaging sequences. The images in this presentation are from proton density or STIR sequences, because these have currently proven to be the most useful for detecting pathologic abnormalities. Slice planes can be oriented in any direction through the leg. The images shown are either sagittal or axial sections, because these orthogonal slices allow imaging in two standard planes through the limb. Orienting slice planes through specific anatomic structures has increased our ability to see some pathologic abnormalities (Fig. 1). Improvements in imaging will come as different slice planes through the limb increase our ability to observe abnormalities in specific anatomic locations.

Figure 1. This slice was oriented through the proximal to distal axis of the NB in a coronal (frontal) plane. This is an example of a new slice orientation that we are using to better evaluate the distal border of the NB. The long arrow points to an avulsion fracture, and the short arrow points to an area of fluid in the NB.

3. Mechanics of MRI

General anesthesia is currently required to perform MRI in a high-field magnet. Improvements in anesthetic techniques and use of reasonably short scan times have minimized any complications. No horses have suffered adverse consequences from anesthesia for MRI to date. By performing complete lameness examinations that have localized the problem to an anatomic area, the time that the horse is in the magnet has been kept <90 min in most cases. This is an important aspect of MRI. Imaging the area where the horse's problem is located is critical in making an accurate diagnosis.

High-field magnets used for humans have been modified with flared ends on short-bore magnets to allow the horse's limbs to be positioned in the isocenter of the magnet. In most horses, the carpus and tarsus can be imaged. Image quality deteriorates when the area being imaged is not in the isocenter of the magnet. Imaging of the limb above the carpus and tarsus is limited by the distance the horse is pulled into the magnet and the length of the horse's legs.

4. Indications for MRI

Based on our experience, there is no doubt that MRI can provide useful information in many clinical conditions. MRI has been primarily used in horses with lameness problems that cannot be diagnosed from radiographs or other imaging techniques. As our experience with MRI increases, the indications for MRI continue to expand. When diagnosing horses with lameness problems, there are many imaging options available. MRI is increasingly selected over scintigraphy or ultrasound, because it allows us to diagnose both bone and soft-tissue problems. Basically, MRI is indicated in horses that cannot be diagnosed from clinical examination and radiographs. MRI also has specific indications for some clinical problems (e.g., in a horse with a splint exostosis, it can determine if suspensory ligament desmitis is present and if there is adhesion between the suspensory ligament and the exostosis) [4].

5. MRI of the Foot

MRI has proven particularly valuable for horses with lameness problems in the foot, because it has allowed us to see inside the hoof capsule for the first time. As a result, diagnoses are being made that were not recognized or accurately diagnosed before the use of MRI [2,3,7,9].

Osteochondral abnormalities, whether from osteochondrosis or trauma, can be diagnosed with MRI. [3] Bone contusions are important to find, because it is important to rest the area and to allow time for it to heal (Fig. 2). Attempts to continue exercise in these horses can result in further damage to the subchondral bone and failure of the overlying articular cartilage, which results in a joint lesion that can affect joint function. Articular cartilage abnormalities are important to recognize, because they affect joint function, and treatment for these may involve arthroscopic surgery. Articular cartilage lesions are an example of the value of MRI, because arthroscopy is not a treatment routinely used in most horses with osteoarthritis of the distal interphalangeal joint. Small articular cartilage lesions have been observed in both the distal and proximal interphalangeal joints (Fig. 3). Some of these appear to be osteochondrosis lesions based on the MRI, but trauma to the articular cartilage and subchondral bone cannot be ruled out. With a few exceptions, most of these abnormalities cannot be observed on radiographs.

Figure 2. These are sagittal STIR images of the front feet from a horse that has a contusion in the subchondral bone of the middle phalanx (P-2) in the lame left forelimb (arrow). Arthroscopic evaluation of the joint revealed a normal articular cartilage surface over this area.

Figure 3. These are axial STIR images from a horse that has bilateral rear limb lameness that blocked out in the feet. The horse has osteochondral lesions in the subchondral bone of the distal P-2 (arrows) that could not be seen on radiographs. The sagittal STIR of the left hindlimb is included to locate the osteochondral damage in the distal P-2. Arthroscopic debridement of the lesions allowed this horse to successfully return to barrel racing.

Deep digital flexor (DDF) tendonitis is a relatively common cause of heel pain in performance horses. [2,9] MRI with a high-field magnet allows an accurate diagnosis of DDF tendonitis (Fig. 4). Until MRI was available, the high incidence of this problem in horses with heel pain was unrecognized. Accurate diagnosis is important, because horses with DDF tendonitis have treatment options that are different from horses that have degeneration of their navicular bone (NB). Rest and rehabilitation allow some horses with DDF tendonitis to return to use but does not improve most horses with navicular disease. Some horses have both DDF tendonitis and NB degeneration. More experience with accurate diagnoses and long-term follow-up is necessary to determine the inter-relationship of these two problems and the effectiveness of current treatments.

Figure 4. DDF tendonitis was initially one of the more obvious and exciting findings on MRI of horses with clinical signs of navicular syndrome. These are axial PD images from three different horses that show the locations where injury to the tendon has been observed: distal to the NB, over the palmar surface of the NB, and proximal to the NB. Arrows point to the abnormalities in the tendon in all three images.

MRI is proving to be an invaluable tool for evaluating the front feet of horses with navicular syndrome, because both the NB and supporting soft tissues can be assessed. [2] Enlargement of the proximal suspensory ligament of the NB, thickening of the impar ligament, and abnormally high signal in the ligaments or their insertions are all observed in horses with clinical signs of navicular syndrome. Chronic repetitive strain on these structures may be an initiating cause of problems in some horses with navicular disease (Fig. 5, Fig. 6, Fig. 7). Fluid in the NB is also a frequent observation in these horses (Fig. 8). MRI allows assessment of the amount of fluid in synovial spaces like the distal interphalangeal joint and navicular bursa (Fig. 9). Increased fluid can be correlated with inflammation. Fibrous tissue is observed between the proximal suspensory ligament and the DDF tendon in the proximal aspect of the navicular bursa in some horses. Many of these abnormalities, including DDF tendonitis, are observed in the same horse; this makes determining the clinical significance of specific abnormalities more difficult [2]. MRI can be useful in detecting flexor cortical bone degeneration and adhesion between the DDF tendon and the NB in horses with chronic navicular disease. MRI evaluation of a large number of horses with bilateral heel pain will be necessary to correlate MRI observations with treatment and prognosis. This is an important direction for future research in horses with navicular disease.

Figure 5. These are sagittal PD images from a horse with a thickened proximal suspensory ligament of the NB. An image from a normal horse is on the right for comparison. Arrows point to the ligament.

Figure 6. These are sagittal PD images from a horse that has clinical signs of navicular disease. There is a thickening and an abnormally high signal in the impar ligament bilaterally and increased bone density (seen as a darker area in the distal NB) at the insertion of the impar ligament to the NB. The increased bone density is marked by the upper arrow in the figure.

Figure 7. This is a sagittal STIR section that shows a high signal in the insertion of the impar ligament to the distal phalanx.

Figure 8. These are two sagittal STIR images from two different horses with fluid in the NB. The horse on the left had diffuse high signal in the medullary cavity, whereas the horse on the right had a localized area of fluid in the distal one-third of the NB. Both horses had clinical signs of navicular disease.

Figure 9. These are sagittal STIR images from two horses; one horse had increased fluid in the distal interphalangeal joint (left), and another horse had increased fluid in the navicular bursa.

Collateral ligament desmitis of the distal interphalangeal joint can be accurately diagnosed with MRI evaluation of the foot (Fig. 10). This problem has been recognized and suspected as a cause of lameness, but definitive diagnosis was difficult to confirm before MRI. MRI allows assessment of the collateral ligaments; enlargement and abnormally high signal in one of the collateral ligaments indicates that the ligament has been injured [7]. This is important, because lameness in affected horses usually improves after a palmar digital nerve block and can be clinically confused with problems in the heel. MRI allows an accurate diagnosis to be made; as a result, rest and rehabilitation has allowed many of these horses to return to their intended use.

Figure 10. These are PD images from a horse that has an enlargement and an abnormally high signal in the medial collateral ligament of the distal interphalangeal joint bilaterally.

6. MRI of the Fetlock

There is a relatively large number of horses with lameness problems localized to the fetlock area of the limb by diagnostic local anesthetic blocks that have no abnormalities observed on radiographs. These horses have been a diagnostic challenge for veterinarians because of the difficulty in detecting the source of the inflammation and pain. MRI has improved our ability to make an accurate diagnosis in these cases, allowing us to find both bone and soft-tissue injuries.

MRI has allowed subchondral bone injuries and osteochondral lesions to be diagnosed that would have gone unrecognized with other imaging techniques [3]. Bone contusions have been observed in both the distal third metacarpus or metatarsus (Fig. 11) and the proximal first phalanx (Fig. 12). STIR sequences are necessary to find fluid accumulation in the subchondral bone that can go unobserved on other imaging sequences. Chronic bone injury results in a healing response that causes increased bone density; this is observed on proton density (PD) sequences (Fig. 13). Bone is limited in its ability to respond to injury and reacts by producing more bone around the area that has been damaged. The resulting increased bone density is observed as increased low signal (blackness) in the bone (Fig. 13). MRI is also capable of detecting small osteochondral lesions that cannot be diagnosed from radiographs (Fig. 14). Diagnosing osteochondral lesions is important, because arthroscopic surgery can be used to debride the joint defect, allowing some horses to return to function.

Figure 11. This is a coronal STIR image of a horse with a bone contusion of the distal third metatarsus; there is an area of high signal in the lateral condyle.

Figure 12. This is a coronal STIR image of a Thoroughbred jumper with an acute lameness that has a small area of abnormal high-fluid signal in the proximal phalanx.

Figure 13. This is a coronal PD image of a 3-yr-old Thoroughbred with a chronic history of severe lameness after speed work. There is an abnormally low signal in the lateral condyle of the third metatarsus because of chronic bone injury.

Figure 14. This is a sagittal and an axial STIR image from a horse with an osteochrondral lesion in the proximal P-1. There is an abnormally high signal in both sections.

MRI allows precise evaluation of soft-tissue structures. Even slight enlargement of a ligament or tendon can be detected when the imaging sequences are compared with those from the contralateral limb. Small areas of inflammation in a ligament or tendon affect the chemical composition and are observed as an increased high signal on both STIR and proton-density sequences. Oblique distal sesamoidean ligament desmitis has been found in a fairly large number of horses (Fig. 15). Accurately diagnosing this problem early in its clinical course has allowed more horses to return to use than previously reported for this injury [5,8]. Small areas of inflammation in the suspensory ligament branches and their insertion on the proximal sesamoid bones have been diagnosed with MRI that, even with retrospective evaluation, have been difficult to detect with high resolution ultrasound (Fig. 16). Injuries to the collateral ligaments of the metacarpophalangeal joint, straight distal sesamoidean ligament, inter-sesamoidean ligament, and joint capsule have all been identified with MRI. Sufficient cases have been evaluated to show the value of MRI for detecting soft-tissue injuries in the distal limbs of performance horses. Finding the specific soft-tissue structure that is injured is important, because it affects treatment. For example, horses with injury to the oblique distal sesamoidean ligament can be treated by injecting anti-inflammatory medications into the digital flexor tendon sheath; this allows medication to reach the injured ligament through the synovial fluid in the sheath. This is a treatment option that would not benefit a horse with a suspensory branch desmitis. Accurate diagnosis also allows horses to receive appropriate treatment (e.g., a rest and rehabilitation program can be instituted before more severe damage occurs to the injured structure).

Figure 15. This is an axial PD image from a horse with an injury to the oblique distal sesamoidean ligament. There is an abnormal high signal in the branch of the ligament.

Figure 16. These are two axial PD images of a 3-yr-old Thoroughbred racehorse that has a small area of abnormally high signal in the medial branch of the suspensory ligament (arrow). The suspensory branch palpated normally.

MRI evaluation of horses with tendon and tendon sheath injuries may prove to be valuable in the future, but few horses with these problems have been evaluated with MRI. This is because these injuries can be diagnosed and evaluated by clinical examination and high resolution ultrasound.

7. MRI of the Metacarpus/Metatarsus

Many horses with lameness localized to the proximal metacarpus or metatarsus improve with the use of diagnostic local anesthesia, which desensitizes the area of the proximal insertion of suspensory ligament. MRI is improving our ability to evaluate this area. It is again an area where an accurate diagnosis of the problem helps to determine the most appropriate treatment.

MRI of the proximal metacarpal area can determine the source of the pain. The most common finding in the horses we have evaluated to date has been inferior check-ligament desmitis. There is mild enlargement of the check ligament and abnormally high signal (Fig. 17). Because the change in signal can be subtle, it is important to compare the check ligament with the contralateral limb. Proximal suspensory ligament desmitis also occurs when there is an enlargement or abnormally high signal in the ligament. Because the proximal suspensory ligament has an irregular appearance on MRI, it is again important to compare it with the contralateral limb. Subtle variations in the appearance of the proximal suspensory ligament can be easily missed if the images are not compared side by side (Fig. 18). This is especially important, because enlargement of the proximal suspensory ligament is not always observed with injury. Injury to the bone at the insertion of the proximal suspensory ligament can also be diagnosed with MRI; in acute injuries, there is fluid in the bone at the insertion site (Fig. 19). MRI is valuable here, because it helps to localize the specific source of the horse's problem in a location that is frequently diagnosed as a "high suspensory" when the lameness improves after the injection of local anesthetic into the region. Many horses have few definitive abnormalities observed when the area is evaluated with ultrasound, especially in the rear limb where vessels and edge enhancement from the overlying flexor tendons affect the evaluation. MRI does allow clear imaging of the proximal suspensory ligament in the pelvic limb.

Figure 17. These are axial PD images from both forelimbs of a horse with desmitis of the inferior check ligament. There is an enlargement and an increase in high signal in the check ligament of the lame left forelimb compared with the right forelimb.

Figure 18. These are axial PD images from a horse with lameness localized to the area of the proximal suspensory ligament. There is an abnormally high signal in the proximal suspensory ligament compared with the appearance of the normal ligament in the contralateral limb.

Figure 19. These are axial STIR images from a Warmblood that had injured the insertion of the proximal suspensory ligament to the third metacarpus. There is high (fluid) signal in the bone on the lame left forelimb (arrow).

A relationship between splint exostosis and suspensory ligament desmitis has been recognized in some horses using a cross-section view of the MRI. Some horses with splint exostosis and even some horses with relatively small splint bone reactions have an adhesion between the reaction around the splint and the suspensory ligament [4]. Affected horses that have been evaluated with MRI have suspensory ligament desmitis at and distal to the area of attachment between the two structures (Fig. 20). Removal of the splint exostosis eliminates the abnormal attachment and allows some horses to return to performance after rehabilitation of the suspensory ligament desmitis. Again, MRI is important in making an accurate diagnosis of the involvement of the splint exostosis so that appropriate treatment can be selected for the horse.

Figure 20. This is an axial PD image from a horse with adhesion of the suspensory ligament to a medial splint exostosis. There is an enlargement of the suspensory ligament and an abnormally high signal in the medial body of the suspensory ligament.

8. Summary

An accurate diagnosis of a horse's lameness problem is important in selecting treatment that will optimize the horse's chances of returning to performance. The development of imaging technology like nuclear scintigraphy and MRI continues to improve the understanding of lameness problems in performance horses. Combining imaging with accurate evaluation of the lameness gait and response to diagnostic local anesthesia improves the veterinarian's ability to accurately determine the source of the horse's problem. Lameness diagnosis is an important area that is rapidly changing, and it will continue to evolve in the future with more experience using MRI.