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Compendium December 2010 (Vol 32, No 12)

Surgical Views — Navicular Syndrome in Equine Patients Anatomy, Causes, and Diagnosis

by R. Wayne Waguespack, DVM, MS, DACVS, R. Reid Hanson, DVM, DACVS, DACVECC

    CETEST This course is approved for 3.0 CE credits

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    Abstract

    Navicular syndrome is a chronic and often progressive disease affecting the navicular bone and bursa, deep digital flexor tendon (DDFT), and associated soft tissue structures composing the navicular apparatus. This syndrome has long been considered one of the most common causes of forelimb lameness in horses. Diagnosis of navicular syndrome is based on history, physical examination, lameness examination, and peripheral and/or intraarticular diagnostic anesthesia. Several imaging techniques (e.g., radiography, ultrasonography, nuclear scintigraphy, thermography, computed tomography [CT], magnetic resonance imaging [MRI]) are used to identify pathologic alterations associated with navicular syndrome. Radiographic changes of the navicular bone are not pathognomonic for navicular syndrome. Additionally, not all horses with clinical signs of navicular syndrome have radiographic changes associated with the navicular bone. Therefore, newer imaging modalities, including CT and especially MRI, can play an important role in identifying lesions that were not observed on radiographs. Navicular bursoscopy may be necessary if the clinical findings suggest that lameness originates from the navicular region of the foot and if other imaging modalities are nondiagnostic. With new diagnostic imaging techniques, clinicians are learning that anatomic structures other than the navicular bursa, navicular bone, and DDFT may play an important role in navicular syndrome.

    Navicular syndrome is chronic and often progressive, affecting the navicular bone and bursa as well as the associated deep digital flexor tendon (DDFT) and soft tissue structures composing the navicular apparatus.1 Navicular syndrome has long been considered one of the most common causes of forelimb lameness in horses. As early as 1752, it was recognized that changes in the distal sesamoid bone (navicular bone) correlated with clinical lameness.2 In one study, navicular syndrome was found to be responsible for approximately one-third of cases of chronic lameness affecting horses.3 This syndrome is responsible for loss of function in many equine athletes. Although this syndrome has been present for centuries, considerable controversy regarding its pathogenesis, diagnosis, and treatment persists. This article reviews the anatomy of the navicular region and the pathogenesis and diagnosis of navicular syndrome.

    Anatomy of the Equine Digit

    The navicular bone is boat shaped and lies at the palmar aspect of the distal interphalangeal joint (DIPJ; FIGURE 1 ). The bone develops by endochondral ossification from a single center of ossification4 and is completely ossified by approximately day 325 of gestation. Dorsally, its articular surface, which is covered with hyaline cartilage, articulates with the distal-palmar aspect of the middle phalanx. The palmar flexor surface is characterized by a prominent sagittal ridge and is covered by fibrocartilage, providing a smooth surface on which the apposing DDFT can glide during loading of the limb. The distal border contains a small articular facet of hyaline cartilage for articulation with the distal phalanx.5 In addition, the distal border contains numerous foramina lined with synovium.4,6

    Three ligaments support the navicular bone. A pair of collateral sesamoidean ligaments arises from depressions on either side of the distal end of the proximal phalanx. They extend in a distal-palmar direction to insert on the extremities and proximal border of the navicular bone, thereby “suspending” the navicular bone.7,8 A branch from each collateral sesamoidean ligament originates at the palmar process (angle) of the distal phalanx and inserts on the axial surface of the ipsilateral cartilage of the foot.8 The distal sesamoidean impar ligament originates from the distal margin of the navicular bone, extending to the flexor surface of the distal phalanx and inserting deep to the DDFT.7

    The synovial structures associated with the navicular bone are somewhat complex (FIGURE 2). The navicular bursa is located between the flexor surface of the navicular bone and the DDFT. It extends proximally 1 to 1.5 cm from the navicular bone and distally to the insertion of the DDFT on the distal phalanx.9 Several studies using positive-contrast radiography have confirmed that no direct anatomic communication exists between the navicular bursa and the DIPJ.10,11 The DIPJ is a complex synovial structure near the navicular bone. It consists of dorsal and palmar extensions along the respective surfaces of the middle phalanx, with a small additional recess between the navicular bone and the distal phalanx.12

    Sensory innervation to the navicular bone is supplied by the digital nerves.7 Nerve fibers run distally through the collateral sesamoidean ligaments and are present within the distal sesamoidean impar ligament.13 Innervation to the synovial structures (i.e., the navicular bursa and the DIPJ) has been the subject of recent studies.14,15 The digital nerves supply sensory innervation to these structures; however, there is considerable controversy over the possibility of an indirect, functional communication between the DIPJ and the navicular bursa, which may confuse intrasynovial anesthesia results.14

    The arterial supply to the navicular bone is formed from an anastomosing network between the medial and lateral palmar digital arteries 7,16,17 Proximally, a transverse plexus joining the lateral and medial palmar digital arteries gives rise to several small arteries that enter foramina of the navicular bone along its proximal border. Distally, branches connecting the medial and lateral palmar digital arteries form a distal navicular plexus, which gives off additional small arteries that enter foramina along the distal border. These distal arteries supply most of the blood to the navicular bone. Anastomoses between the proximal and distal blood supply have been demonstrated in adult horses. Venous drainage occurs via the medial and lateral palmar digital veins. 

    Biomechanical Considerations

    The primary function of the navicular bone is to provide a constant angle of insertion for the DDFT.18 Alignment of the trabecula (i.e., Wolf’s law states that the trabecula will align in the direction of stress) within the cancellous bone of the navicular bone suggests that the principal force experienced by the navicular bone is compression by the DDFT.19 The forces acting on the navicular bone include forward compression by the DDFT, compressive forces downward from the middle phalanx, and tension from the suspensory apparatus of the navicular bone.20 The relative contribution of each of these forces varies from horse to horse, depending on factors such as conformation and weight.

    Pathogenesis

    Three major theories regarding the pathogenesis of navicular syndrome have evolved.21 The first one proposes a vascular etiology. This theory, supported by Colles,22 cites thrombosis of the arterioles supplying the distal aspect of the navicular bone, causing pain and ischemic necrosis of the bone. Others23,24 have found evidence of hyperemia (increased vascularity) rather than ischemia in navicular bones from clinically affected horses.

    The second theory involves biomechanical causes. Thompson et al25 suggested that continual pressure between the DDFT and the flexor surface of the navicular bone leads to degenerative changes of these structures. Other investigators26 have shown that structural changes within the distal sesamoid bone were due to remodeling of the spongiosa underlying the flexor fibrocartilage in response to increased pressure between the DDFT and the flexor surface of the navicular bone. Additionally, factors such as upright conformation, small hoof size, poor shoeing technique (short heel and long toe), and large body size were considered to contribute to this syndrome.27

    The third and most accepted theory suggests that navicular syndrome is a process similar to osteoarthritis (degenerative joint disease). Numerous researchers21,23,28–32 have demonstrated that changes in the fibrocartilage of the flexor surface of the navicular bone, subchondral bone, medullary cavity, and bursal synovium are similar to changes observed in the hyaline cartilage and synovial membranes of joints with osteoarthritis.28,32 Additionally, surface fraying of the DDFT, core lesions, and adhesions between the navicular bone and the DDFT have been observed.21,28,32

    Evaluation of these theories supports a mechanical cause of navicular syndrome that involves either abnormal loads applied to normal structures or abnormal structures subjected to normal loads. Either cause leads to an imbalance between applied forces and the tissues’ ability to compensate and withstand the loads.20 The various theories of etiology reflect, in part, the lack of understanding of the temporal relationship of the pathologic changes found in the navicular bone and its surrounding structures.

    Diagnosis

    Navicular syndrome can affect horses of many breeds and ages. In North America, Quarter horses and Thoroughbreds have a higher incidence than other breeds.33 Geldings also appear to be overrepresented.33 Whether these are true breed and sex predilections is unclear; these findings may be related to the workload of the horses in this study. Affected horses are typically 4 to 15 years of age. A hereditary component of navicular syndrome has been investigated; however, this may be related to certain conformational factors.34

    Horses with navicular syndrome often present with a history of forelimb lameness of insidious onset—often mild at first, with the lameness resolving with work. In time, the lameness may become exacerbated by exercise. The gait is often characterized by short, choppy strides, and the lameness is usually bilateral, with one limb more predominant than the other. Hard surfaces and trotting tight circles (which typically worsens the lameness when the lame leg is on the inside) also exacerbate the lameness. Severely affected horses may demonstrate a characteristic stance at rest, pointing the affected forelimb(s). Although hoof capsule shape can change in many cases of chronic lameness, longstanding cases of navicular syndrome may involve alterations in hoof conformation, including a small, narrow hoof with high heels.1

    Horses with navicular syndrome often show a pain response (i.e., limb withdrawal) to pressure applied with hoof testers over the middle one-third of the frog (FIGURE 3). To reduce the likelihood of false-positive results, it is important to compare the horse’s pain response over the navicular region to the pain response in other regions on the same foot, in the opposite foot, and in the hind feet. False-negative results can occur with improper application of hoof testers, with the use of poor-quality hoof testers, or when inadequate pressure is achieved in feet with pads and/or thick, tough, keratinized frogs.

    Regional perineural anesthesia of the palmar digital nerves is an important diagnostic tool for evaluating suspected cases of navicular syndrome. Careful technique must be used to ensure that the dorsal branches of the palmar digital nerves are not inadvertently included during injection and that only the palmar aspect of the hoof is anesthetized. One to 2 mL of 2% mepivacaine or lidocaine is administered perineurally at the level of the cartilage of the foot just proximal to the heel bulbs (FIGURE 4). Adequate anesthesia of the region, both cutaneous and deep, should be confirmed before evaluating the lameness. For confirmation, a blunted object (e.g., ballpoint pen cap, hoof pick) should be applied just proximal to the coronary band (both the dorsal and palmar aspects) and heel. Deep sensation in the navicular region, heel, and sole should be tested by reapplication of hoof testers approximately 5 to 10 minutes after injection. Frequently, the entire foot can become anesthetized after a palmar digital block. This usually results from inadvertent anesthesia of the dorsal branches of the palmar digital nerves due to inaccurate location of the palmar digital block or excessive use of local anesthetic and subsequent diffusion. Additionally, it must be recognized that deep sensation may persist although cutaneous sensation is lost. A probable explanation for this phenomenon is that variant nerve branches may escape anesthesia with routine palmar digital nerve blocks. With the possible variations in performing palmar digital nerve blocks, it is important to determine the area of desensitization.

    After resolution of the lameness secondary to diagnostic anesthesia, unilaterally lame horses often exhibit a similar lameness in the contralateral limb. Horses with bilateral forelimb lameness usually show a dramatic improvement in gait after palmar digital nerve anesthesia of both forelimbs. Although useful in diagnosing navicular syndrome, palmar digital nerve blocks are not specific for navicular syndrome, and horses with navicular syndrome may not always show significant improvement after blocking. This can be due to factors such as sole bruising, mechanical restrictions involving adhesions between the navicular bone and the DDFT, and accessory nerve supply to the region.1 The results of regional anesthesia should be interpreted with the results of other clinical findings, such as those from the history, physical examination, and imaging.

    Anesthesia of the synovial structures surrounding the navicular region has also been used in an attempt to localize pain in clinically affected horses.35–37 It is thought that injection of local anesthetic into the DIPJ (FIGURE 5) can distinguish navicular pain from DIPJ pain; however, the specificity of this technique has recently been questioned. One group of investigators35 demonstrated that a positive response to anesthesia of the DIPJ did not imply that pain arose solely from the joint; rather, the response could be associated with significant abnormalities (e.g., navicular bursitis, deep digital flexor tendonitis, impar ligament desmitis) of the navicular region. A negative response to anesthesia of the DIPJ did not preclude the presence of navicular bone pathology.35 Additional studies support the theory that intraarticular anesthesia of the DIPJ is not specific for lameness originating from the DIPJ. In these studies, a positive response to intraarticular anesthesia of the DIPJ alleviated solar pain38 and pain associated with the palmar processes of the distal phalanx, the distal sesamoidean impar ligament, the DDFT, and the joint itself.35–37 To better interpret intraarticular anesthesia of the DIPJ, several studies have evaluated the specific volume to use and when to evaluate the response to this block.35,39,40 Intraarticular anesthesia of the DIPJ using 6 mL of mepivacaine can alleviate or improve pain associated with the navicular bone within 5 minutes of injection.35,39 Schumacher et al40 showed that misleading results are more likely to occur with larger volumes of local anesthetic solution and with evaluation of the response more than 10 minutes after injection.

    Intrabursal anesthesia (3 to 4 mL of 2% mepivacaine hydrochloride (Carbocaine-V, Pfizer Animal Health) of the navicular bursa has been recommended for the evaluation of navicular pain (FIGURE 6). Some researchers39 believe this technique is useful only in distinguishing horses with true navicular bursitis. Improvement in lameness is usually appreciated within 5 minutes of injection of local anesthetic solution. A positive response to bursal anesthesia most likely indicates that pathology of the navicular bursa, bone, and/or supporting ligaments is the source of pain.37 Interpretation of coffin joint and navicular bursa anesthesia should be approached judiciously, as the specificity of each has not been clearly delineated. However, a negative response to analgesia of both the DIPJ and the navicular bursa makes it unlikely that the horse has navicular syndrome.

    Diagnostic Imaging

    Radiography

    Radiography is the most commonly used modality for supporting a clinical diagnosis of navicular syndrome. To fully evaluate the navicular bone, five views are recommended,41 but at least three views (i.e., lateromedial, 60° dorsoproximal–palmarodistal oblique, 45° palmaroproximal–palmarodistal oblique) should be obtained (FIGURE 7). High-quality, correctly positioned radiographs are necessary for an accurate diagnosis. Artifacts associated with improper positioning, technique, and hoof preparation can lead to erroneous information. After removal of the shoes, adequate preparation of the hoof is imperative for obtaining high-quality radiographs. Radiographic artifacts associated with air accumulation within the crevices of the frog appear as lucencies on the radiograph and can mimic fracture lines or cyst-like lesions on the navicular bone. To eliminate air pockets and prevent these artifacts, the hoof should be well packed with a compound having radiographic soft tissue density (e.g., Play-Doh) or should be placed in a plastic container filled with water (the cassette is placed beneath the container). A grid (minimum of 5:1 ratio, with 80 lines/inch) is recommended for dorsopalmar views when using conventional radiography.

    Several radiographic changes of the navicular bone have been reported to be compatible with navicular syndrome.42–44 These changes include increase in size and number of the distal border synovial invaginations, remodeling of the proximal border and extremities of the navicular bone (enthesiophyte formation), flexor cortex erosions, flexor cortex irregularity and thickness, decreased flexor corticomedullary definition (medullary sclerosis), distal border irregularity and fragmentation, mineralization within the collateral ligament of the navicular bone, and cyst-like lesions within the navicular bone (FIGURE 7). (Several grading systems exist for evaluating the navicular bone.45,46) A great deal of controversy surrounds the interpretation of these various radiographic findings. In particular, the importance of changes in the number and shape of distal border synovial invaginations has been questioned.44 There appears to be some variation between breeds with regard to distal border invaginations, and these and other radiographic changes have been recognized in sound horses.47,48 This suggests that radiographic changes of the navicular bone are not pathognomonic for navicular syndrome. Additionally, not all horses with clinical signs of navicular syndrome have radiographic changes associated with the navicular bone. The degree to which changes in the navicular bone (i.e., osseous pathology) contribute to clinical lameness can vary; consequently, other imaging modalities have been used to elucidate what role structures such as soft tissues and vascular supply play in the lameness associated with navicular syndrome.

    Ultrasonography

    Ultrasonography has been used in a limited capacity in diagnosing navicular syndrome. Ultrasonographic evaluation of the navicular region is difficult because of the hoof capsule and the difficulty in maintaining a 90° angle between the sound beam and the objects of interest when imaging through the heel bulb. However, ultrasonography has been used to assess the distal aspect of the DDFT and the navicular bursa.49 Normal anatomy has been described for the proximal navicular bone, navicular bursa, and proximal palmar pouch of the DIPJ.49 Imaging through the sole of the hoof is possible if the sole and frog are adequately prepared. The distal impar ligament and distal margin of the navicular bone have been imaged in this manner.50 This modality shows promise for clinical application for evaluating the navicular bursa and the DDFT as it passes over the bursa.

    Nuclear Scintigraphy

    Nuclear scintigraphy has been successful in evaluating horses for navicular syndrome (FIGURE 8). Scintigraphic imaging depends on the distribution and uptake of a radiopharmaceutical that is injected into the patient. Studies indicate that many horses with pain attributed to the navicular region have increased scintigraphic uptake within the navicular bone.51,52 Compared with radiography, scintigraphy is much more sensitive in detecting lesions of the bone because radiography requires the loss of at least 50% of the bone’s mineral content before lysis is visualized.53,54 There is a relatively high incidence of horses that have clinical signs compatible with navicular syndrome, a positive response to intraarticular anesthesia of the DIPJ or intrathecal anesthesia of the navicular bursa, no detectable radiographic abnormalities of the navicular bone, and increased radiopharmaceutical uptake associated with the navicular bone.55

    For evaluation of navicular syndrome, the radionuclide technetium 99m (99mTc) is labeled with a bone-seeking molecule, a diphosphonate, the uptake of which depends on blood flow and osteoblastic/osteoclastic activity.56 Two phases of the study are usually performed: (1) soft tissue–phase images are usually acquired 5 to 15 minutes after injection, and (2) bone-phase images are acquired after a 2- to 4-hour delay after injection to allow adequate soft tissue clearance of the radionuclide. Bone-phase views of the lateral and palmar region (solar view) of the affected hoof in horses with clinical navicular syndrome often demonstrate increased scintigraphic uptake. Navicular syndrome is often bilateral; therefore, comparison of uptake in one limb with the contralateral limb is often not helpful because uptake is increased in both navicular bones. Lateral views of these structures appear less sensitive than solar views for demonstrating increased uptake within the navicular region.52 This is most likely due to the greater distance and tissue thickness between the navicular region and the face of the gamma camera, decreasing the number of detectable gamma rays emitted from the navicular region.52 False-positive lateral views usually result from increased uptake within the distal phalanx, most notably the wings and cartilages of the foot.52 The solar view can be used to determine the extent and distribution of uptake within the wings and cartilages of the foot, helping eliminate false-positive lateral views. Therefore, both views should be used in combination to obtain the most useful information, especially when evaluating horses with mild or early navicular bony lesions.

    In a study of 264 horses with foot pain, increased radiopharmaceutical uptake was detected in the navicular bone (36.6%), in pool-phase images in the DDFT (13.0%), and at the insertion of the DDFT on the distal phalanx (14.3%).57 There was focal increased radiopharmaceutical uptake at the insertion of the medial or lateral collateral ligament of the DIPJ in 9.4% and 1.5% of limbs, respectively. This study showed that positive nuclear scintigraphic results are good predictors of injury or disease of the navicular bone, the DDFT, and the collateral ligaments of the DIPJ.57 Overall, nuclear scintigraphy offers a more sensitive diagnostic modality when diagnosing navicular syndrome, but its specificity is lower than that of radiography. When used together, the two modalities can provide useful and complementary information.

    Thermography

    Thermography is a noninvasive method for measuring heat emitted from a subject.58 For medical purposes, it represents the surface temperature of the skin and, therefore, is useful in detecting changes in local circulation and blood flow and, in turn, in detecting inflammation.59 Thermography has been used as an adjunct diagnostic modality to evaluate navicular syndrome. In one study, the thermal pattern demonstrated by horses with navicular syndrome was characterized not by increased blood flow but by decreased blood flow.59 Although thermography is not commonly used to diagnose navicular syndrome, it can provide additional, potentially useful information.

    Computed Tomography

    Computed tomography (CT) generates cross-sectional images of an anatomic site by using x-rays and computer integration. The internal anatomy of a structure is reconstructed by a computer from numerous x-ray projections through the object.60 This produces an image that lacks superimposition of overlying structures and has superior soft tissue contrast compared with conventional radiography.61 The normal anatomy of the equine digit62 has been used in the postmortem evaluation of lameness attributable to the foot and in clinical cases, including patients with suspected navicular syndrome.63,64 Both reports found resolution of bony detail and soft tissue contrast to be superior on CT than on conventional radiography. In one report,63 investigators visualized pathology within the DDFT, along with numerous bony changes affecting the navicular bone. The significance of these findings is that, for the first time, a single imaging modality was able to define lesions within the soft tissue and bone. The limitations of this modality include the high cost of setup and upkeep, the relative lack of availability for veterinary use, the need for general anesthesia, and machine design (namely, the gantry, which can be small and cumbersome for equine patients).

    Magnetic Resonance Imaging

    Magnetic resonance imaging (MRI) has recently been used to evaluate navicular syndrome. This technique does not use ionizing radiation but depends on the resonance of protons within the body’s tissues (namely, fat and water) in response to magnetic fields and radiofrequency waves to produce an image.65 The difference in the chemical composition of tissues and their proton density determines the signal intensity and, therefore, the image.65 MRI produces the most superior soft tissue contrast of all imaging methods mentioned in this article and allows reconstruction of the object being imaged in numerous planes. The combination of these factors makes MRI ideal for imaging complex structures with soft tissue components, such as joints.66

    Only recently has MRI become available to equine patients67; before this availability, MRI had been performed only on cadaver limbs.63,68,69 MRI of the digit in horses with clinical lameness associated with navicular syndrome has revealed osseous and soft tissue changes, including enlarged synovial fossae, fragmentation of the distal navicular bone, and cyst formation on the midsagittal ridge, as well as areas of attenuation within the DDFT63,67 (FIGURE 9). Murray et al70 directly compared histology and MRI, revealing that signal alterations on MRI represent tissue-level changes, including structural damage, fibroplasia, fibrocartilaginous metaplasia, and hemosiderosis in ligaments and tendons; trabecular damage; osteonecrosis; cortical defects; increased vascularity in bone; and fibrocartilage defects. MRI had high specificity for lesions of the DDFT, collateral sesamoidean ligament, navicular bursa, and navicular bone.70 In another study71 of 264 horses with unilateral or bilateral foot pain, MRI indicated that lesions of the DDFT were most common (82.6% of feet). These lesions were most commonly located at the level of the collateral sesamoidean ligament (59.4%) and the navicular bone (59.0%). Core lesions in the DDFT predominated at the level of the proximal phalanx (90.3%). At the level of the collateral sesamoidean ligament and navicular bone, the most common lesions in the DDFT were core lesions, sagittal splits, and dorsal abrasions.71 Lesions of the distal sesamoidean impar ligament (38.2%) were more common than those of the collateral sesamoidean ligament (10.5%), but the presence of either was associated with abnormalities of the navicular bone.71

    Drawbacks of this imaging modality include its limited availability to equine patients due to the high cost of setup and upkeep, the design of some systems (which is cumbersome for equine patients), and the need for general anesthesia. Standing MRI equipment is available; however, the ability to obtain a consistently high-quality image proximal to the foot is complicated by motion.

    Arthroscopic Examination

    Arthroscopic examination of the navicular bursa permits evaluation of the fibrocartilage on the flexor surface of the navicular bone, the navicular bursa itself, and the overlying dorsal surface of the DDFT as well as limited visualization of the distal sesamoidean impar ligament72,73 (FIGURE 10). Therefore, it is possible to definitively identify adhesions between the DDFT and the palmar aspect of the navicular bone, thinning or full-thickness erosion of the flexor fibrocartilage of the bone, fibrillation of the dorsal aspect of the DDFT, and synovitis of the bursa. In a retrospective study by Smith et al,74 23 bursae were examined arthroscopically in 20 horses. Tears in the DDFT were seen in all horses (22 bursae). In eight bursae, cartilage lesions were also present; in one bursa, this was the only abnormal finding. CT and low-field MRI predicted tendon lesions in most cases but failed to identify the cartilage damage.74 Based on this study, navicular bursoscopy allows comprehensive diagnostic evaluation of the navicular bursa, a significant portion of the navicular bone, and surrounding anatomic structures; this can provide additional useful information in some horses with navicular syndrome.

    Conclusion

    The diagnosis of navicular syndrome is often complex because the pathogenesis is still unclear and the exact source of the pain is neither well defined nor consistent among horses. With new diagnostic imaging and techniques, veterinarians are learning that anatomic structures other than the navicular bursa, navicular bone, and DDFT may play an important role in lameness associated with navicular syndrome. Often, these horses present late in the course of the condition, when destructive changes have already occurred; thus, treatment often addresses pain modification rather than inhibition of the pathologic processes.

    For more on this topic, read the companion article titled “Treating Navicular Syndrome in Equine Patients.”

    Downloadable PDF

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