MAGNETIC RESONANCE IMAGING,GROSS POSTMORTEM,AND HISTOLOGICAL FINDINGS FOR SOFT TISSUES OF THE PLANTAR ASPECT OF THE TARSUS AND PROXIMAL METATARSAL REGION IN NON‐LAME HORSES

Injuries of the plantar soft tissues of the tarsus and proximal metatarsus can be a source of lameness in horses, however published information is lacking on high field MRI characteristics of these tissues. Objectives of the current anatomic study were to (1) describe high‐field MRI features of the plantar tarsal and proximal metatarsal soft tissues; and (2) compare MRI findings with gross and histological appearances of selected structures for a sample of cadaver limbs from non‐lame horses. Single hindlimbs for 42 horses, and right and left hindlimbs for eight horses were scanned using high‐field MRI. The MRI findings were described for the 50 single limbs; and the MRI, gross postmortem and histological findings were compared for the eight pairs of hindlimbs. The superficial digital flexor tendon had uniform low signal intensity, surrounded by the flexor retinaculum of intermediate to high signal intensity on all sequences. The lateral digital flexor tendon had slightly higher signal intensity, enclosed on the plantaromedial aspects by the low signal intensity metatarsocalcaneal ligament. The accessory ligament of the deep digital flexor tendon varied in size and signal intensity. The proximal and distal plantar ligaments, accessory ligament of the suspensory ligament, and calcaneoquartal ligament had low signal intensity. The long plantar ligament comprised a number of related parts, separated by lines of high signal intensity corresponding with fibrous septae seen in gross anatomical specimens. The plantar aspect of the ligament had uniform low signal intensity in all sequences, but the dorsal half was more heterogeneous with multifocal spots or lines of higher signal intensity.     

CONGENITAL DISLOCATION OF THE DEEP DIGITAL FLEXOR TENDON ASSOCIATED WITH HYPOPLASIA OF THE SUSTENTACULUM TALI IN A THOROUGHBRED COLT

An 11-month-old Thoroughbred colt was presented with a hard swelling at the proximal third of the right 4th metatarsal bone. A medial dislocation of the deep digital flexor tendon ( flexor digitorum profundus ) was also observed on the same leg. On the plantaroproximal-plantarodistal projection of the calcaneus, there was flattening and shortening of the sustentaculum tali. The smooth bony proliferation at the proximal third of the right 4th metatarus was compatible with a chronic splint bone fracture. This report describes a medial deep digital flexor dislocation associated with hypoplasia of the sustentaculum tali .     

COMPUTED TOMOGRAPHY AND POSITIVE CONTRAST COMPUTED TOMOGRAPHIC ARTHROGRAPHY OF THE CANINE SHOULDER: NORMAL ANATOMY AND EFFECTS OF LIMB POSITION ON VISIBILITY OF SOFT TISSUE STRUCTURES

Soft tissue injuries of the shoulder are an important cause of forelimb lameness in dogs. The objectives of this canine cadaver study were to describe normal anatomy of shoulder soft tissue structures using computed tomography (CT) and computed tomographic arthrography (CTA) and to determine the effects of positioning on visualization of shoulder soft tissue structures. Thirteen forelimbs were removed from eight canine cadavers. Two forelimbs were used for contrast dose optimization. For the remaining 11 forelimbs, shoulder CT and CTA were performed using three defined joint angles (140°, 90°, and 70°). For three forelimbs, CT and CTA images were compared with frozen anatomic sections to describe normal anatomy. Ten forelimbs were used for analysis of positioning effects. Soft tissue structures evaluated were the joint capsule, cartilage, ligaments, tendons, and muscles. A visual assessment score was assigned to each structure using a consensus of two observers. The range and mode of scores were calculated and compared for each modality and limb position. The shoulder joint capsule and medial and lateral glenohumeral ligaments were completely visible with CTA. All tendons and muscles were visualized in all the examinations except for the teres minor muscle tendon and the coracobrachialis muscle, which were not visible on all scans. Positioning the limb in an extended position significantly improved visualization of most soft tissue shoulder structures. Shoulder cartilage was best seen with CTA and with neutral or flexed positioning of the shoulder. Findings indicated that both CT and CTA are feasible imaging techniques for visualization of soft tissue structures of the canine shoulder.     

CHARACTERIZATION OF THE MAGIC ANGLE EFFECT IN THE EQUINE DEEP DIGITAL FLEXOR TENDON USING A LOW-FIELD MAGNETIC RESONANCE SYSTEM

Three isolated equine limbs were imaged with a low-field magnetic resonance system with a vertical magnetic field. Each limb was scanned in multiple positions with mild variation of the angle between the magnetic field and the long axis of the limb. When the long axis of the limb was not perpendicular to the magnetic field, a linear hyperintense signal was present at the palmar aspect of one of the deep digital flexor tendon lobes, at the level of the navicular bone and collateral sesamoidean ligaments, in proton density and T1-weighted pulse sequences. With increased angulation of the limb, the palmar hyperintense signal extended farther distally and proximally and additional signal hyperintensity was present at the dorsal aspect of the distal part of the other lobe of the deep digital flexor tendon. Increased signal intensity was also present in the collateral ligament of the distal interphalangeal joint on the same side as the palmar hyperintense signal in the tendon. The changes in the deep digital flexor tendon are due to the specific orientation of fibers at the palmar and dorsal aspect of the tendon, which is responsible for focal manifestation of the magic angle effect. Careful positioning of the limb perpendicular to the magnetic field can prevent this phenomenon. The association of palmar increased signal intensity in the deep digital flexor tendon with increased signal in the collateral ligament of the distal interphalangeal joint on the same side should be recognized as manifestations of the magic angle effect.     

COMMINUTED FRACTURE OF THE DISTAL SESAMOID BONE AND DISTAL RUPTURE OF THE DEEP DIGITAL FLEXOR TENDON

A 10-year-old show jumper was evaluated for an acute severe lameness (grade 4 of 5) of the right foreleg. During weight bearing, the toe of the affected limb rotated dorsally suggesting rupture of the deep digital flexor tendon (DDFT). Upon radiographic examination of the hoof, a severe erosion at the flexor surface and a parasagittal fracture of the distal sesamoid bone were found. Ultrasonographic examination confirmed rupture of the DDFT. These findings were confirmed on post-mortem examination. Prior to the acute lameness, the horse was treated with corticosteroid injections into the podotrochlear bursa. Repeated intra-bursal injections of corticosteroids as a possible cause for DDFT rupture are discussed as well as the possible association between a degenerative distal sesamoid bone, a distal sesamoid bone fracture and a DDFT rupture.     

DEEP DIGITAL FLEXOR TENDONITIS IN THE EQUINE FOOT DIAGNOSED BY LOW-FIELD MAGNETIC RESONANCE IMAGING IN THE STANDING PATIENT: 18 CASES

Injury to the distal aspects of the deep digital flexor tendon (DDFT) is an important cause of lameness in horses. The purpose of this study was to review the magnetic resonance imaging (MRI) findings of 18 horses affected by DDFT injuries in the foot. The MRI was performed with the horses standing using an open low-field (0.21 T) MRI scanner. The results were compared with those previously reported for horses using high-field MRI. Eighteen of 84 horses (21%) with undiagnosed forefoot pain were found to have lesions affecting the DDFT. The history, clinical findings and results of radiography, diagnostic ultrasonography and nuclear scintigraphy of these horses were reviewed. The duration of lameness ranged from 1 to 12 months, and the severity varied from 1/10 to 6/10. Fifteen horses had unilateral lameness (right fore in nine, left fore in six), whereas three horses were bilaterally foreleg lame. Radiological changes, considered of equivocal significance, were found in six of 18 horses. Ultrasonographic changes involving the DDFT were identified in only one of nine horses. DDFT lesions were detected in both T1- and T2-weighted MRI sequences. Four different types of lesions were identified: core lesions, sagittal splits, dorsal border lesions, and insertional lesions. Combinations of different lesion types within the same horse were common. The types and locations of the DDFT lesions were similar to those previously reported using high-field MRI. The use of low-field standing MRI avoids the necessity for general anesthesia and access to conventional high-field MRI scanners. However, studies comparing the results of standing low-field MRI with high-field MRI (and other imaging procedures) are required before the sensitivity and specificity of the technique can be assessed.     

CLINICAL SIGNIFICANCE AND PROGNOSIS OF DEEP DIGITAL FLEXOR TENDINOPATHY ASSESSED OVER TIME USING MRI

Deep digital flexor (DDF) tendinopathy is one of the most frequent causes of foot lameness and the prognosis is guarded. The progress of lesion healing may be followed by magnetic resonance (MR) imaging to formulate a prognosis and to adapt the rehabilitation program. We assessed the correlation of outcome with total tendon damage and temporal resolution of MR abnormalities. Images from 34 horses with DDF tendinopathy that had undergone at least two low‐field standing MR examinations of the foot (mean 2.5 ± 1.3 times) were reviewed. No horse having a T1‐GRE hyperintense lesion over 30 mm in length or over 10% tendon cross‐sectional area returned to its previous activity level. Horses with concomitant lesions had worse outcome than horses with DDF tendinopathy only (P = 0.005). In all horses including those with excellent outcome, the lesion persisted, even mildly, in T1‐GRE and PD images. Horses with tendon lesion resolution on STIR‐FSE and T2‐FSE images on recheck examination had a better outcome (P = 0.0004 and P = 0.002, respectively), and all horses that returned to their previous level of performance had complete resolution of signal hyperintensity on the STIR‐FSE sequence. Although rehabilitation remains multifactorial, characteristics of DDF tendinopathy and concomitant lesions on first and recheck MR examinations allow refining the prognosis.     

LOW-FIELD MAGNETIC RESONANCE IMAGING OF THE EQUINE TARSUS: NORMAL ANATOMY

The objective of this study was to define the normal gross anatomic appearance of the adult equine tarsus on a low-field magnetic resonance (MR) image. Six radiographically normal, adult, equine tarsal cadavers were utilized. Using a scanner with a 0.064 Tesla magnet, images were acquired in the sagittal, transverse and dorsal planes for T1-weighted and the sagittal plane for T2-weighted imaging sequences. Anatomic structures on the MR images were identified and compared with cryosections of the imaged limbs. Optimal image planes were identified for the evaluation of articular cartilage, subchondral bone, flexor and extensor tendons, tarsal ligaments, and synovial structures. MR images provide a thorough evaluation of the anatomic relationships of the structures of the equine tarsus.     

Ultrasonographic examination of the caudal structures of the distal antebrachium in the horse

Evaluation of the caudal distal antebrachium using ultrasound is a very useful diagnostic procedure for identification of soft tissue abnormalities in flexor structures of the equine limb. In this article we describe how ultrasonographic imaging may be used to complement radiography of the carpus in evaluating horses that present with peri‐carpal swelling and/or lameness localised to the carpal region. Ultrasonographic examination assists with the identification or exclusion of significant injuries to muscular, tendonous and ligamentous structures within the distal antebrachium and enables practitioners to proceed with appropriate therapeutic and rehabilitation plans for the immediate benefit for their patients while also recognising pathology that could progress to cause long‐term, chronic lameness issues without aggressive and effective intervention.     

Short-term temporal alterations in magnetic resonance signal occur in primary lesions identified in the deep digital flexor tendon of the equine digit

Reasons for performing study: Primary lesions of the deep digital flexor tendon (DDFT) within the digit are an important cause of lameness diagnosed using magnetic resonance imaging (MRI) but appearance of these lesions over time has not been documented. Objectives: To determine whether the magnetic resonance (MR) appearance of different primary DDFT lesions alter over a 6 month period and whether lesion type is a determinant of these changes. Methods: Cases included had lameness attributable to a primary lesion involving the DDFT in the digit diagnosed on MRI. Lesions were typed into parasagittal, dorsal border and core lesions. Approximate volumes and intensities were quantified for each lesion type using T2* scan sequences. Follow‐up examinations and measurements were repeated at 3 and 6 month periods following conservative management. Results: Twenty‐three horses fitted the inclusion criteria. Lesion distribution included: parasagittal (n = 7), dorsal border (n = 11) and core lesions (n = 5). No association was found between age of horse, degree of lameness and lesion type. Only dorsal border lesions showed statistically significant reduction both in volume (initial scan: 0.18 ± 0.14 cm³) at 3 months (0.11 ± 0.10 cm³, P<0.05) and 6 months (0.05 ± 0.05 cm³, P<0.01) and ratiometric intensity (initial scan: 4.06 ± 1.54) at 6 months (2.00 ± 0.43; P<0.01). Parasagittal and core lesions showed no difference in lesion volume or ratiometric intensity. Lameness improved in all lesion types following conservative management. Conclusions: Dorsal border lesions of the DDFT show reduction in both volume and intensity whereas parasagittal and core lesions do not. Potential relevance: Lesion typing may be important in predicting lesion behaviour and short‐term outcome using MR imaging.     

RADIOGRAPHIC, COMPUTED TOMOGRAPHIC AND MAGNETIC RESONANCE IMAGING EVALUATION OF A CHRONIC LONG DIGITAL EXTENSOR TENDON AVULSION IN A DOG

Long digital extensor tendon avulsion is reported in a 5 month old Great Dane. Clinically the dog presented with a unilateral weight-bearing pelvic limb lameness. Joint effusion was present and there was pain and crepitance associated with flexion of the stifle. Orthopedic evaluation and radiographs were suggestive of a long digital extensor tendon injury which was confirmed by computed tomography and magnetic resonance imaging.The injury was surgically repaired with screw and spiked washer fixation.     

CLINICAL AND DIAGNOSTIC IMAGING CHARACTERISTICS OF LATERAL DIGITAL FLEXOR TENDINITIS WITHIN THE TARSAL SHEATH IN FOUR HORSES

Lateral digital flexor tendonitis is a rarely reported cause of hind limb lameness in performance horses. The purpose of this retrospective study was to describe clinical and diagnostic imaging findings for a group of horses with lateral digital flexor tendinitis within the tarsal sheath. Equine cases with a diagnosis of lateral digital flexor tendonitis and magnetic resonance imaging (MRI) studies of the affected region were retrieved from North Carolina University's medical record database. Recorded data for included horses were signalment; findings from history, physical examination, lameness examination, and all diagnostic imaging studies; treatment administered; and outcome. Four horses met inclusion criteria. Lameness was mild/moderate in severity and insidious in onset in all patients. Responses to flexion tests were variable. All horses showed positive improvement (70–90%) in lameness after tarsal sheath analgesia. Radiographic, scintigraphic, and ultrasonographic findings were inconclusive. For all horses, MRI characteristics included increased T2, PD, and STIR signal intensity within the lateral digital flexor tendon in the area of the tarsal sheath. Tarsal sheath effusion was slight in three horses, and mild/moderate in one horse. With medical treatment, two horses were sound at 6‐month followup, one horse was sound at 1‐year followup, and one horse had a slight persistent lameness (grade 1/5) at 9‐month followup. Findings supported the use of MRI for diagnosing lateral digital flexor tendonitis within the tarsal sheath in horses. Affected horses may have a good prognosis for return to athletic performance following appropriate medical treatment.     

EFFECT OF DEEP DIGITAL FLEXOR TENDON ORIENTATION ON MAGNETIC RESONANCE IMAGING SIGNAL INTENSITY IN ISOLATED EQUINE LIMBS—THE MAGIC ANGLE EFFECT

Ten normal equine isolated limbs were imaged using a knee coil in a 1.5 Tesla magnetic field, with short echo time sequences (TE < 15 ms). Magnetic resonance imaging was performed on each isolated limb in different positions, with and without extension of the metacarpophalangeal joint. Deep digital flexor tendon orientation ranged from 20 to 60 degrees in relation to the static magnetic field. Increased intratendinous signal intensity was observed when the angle between the deep digital flexor tendon and the constant magnetic field approached 55 degrees ("magic angle"). The increased signal intensity was independent from extension of the metacarpophalangeal joint. Recognition of the magic angle phenomenon is essential for proper evaluation of magnetic resonance imaging studies of the equine foot.     

MAGNETIC RESONANCE IMAGING FEATURES OF PROXIMAL METACARPAL AND METATARSAL INJURIES IN THE HORSE

Magnetic resonance (MR) imaging abnormalities in horses with lameness localized to the proximal metacarpal or metatarsal region have not been described. To accomplish that, the medical records of 45 horses evaluated with MR imaging that had lameness localized to either the proximal metacarpal or metatarsal region were reviewed. Abnormalities observed in the proximal suspensory ligament or the accessory ligament of the deep digital flexor tendon included abnormal high signal, enlargement, or alteration in shape. Twenty-three horses had proximal suspensory ligament desmitis (13 hindlimb, 10 forelimb). Sixteen horses had desmitis of the accessory ligament of the deep digital flexor tendon. One horse had desmitis of the proximal suspensory ligament and the accessory ligament of the deep digital flexor tendon on the same limb and one horse had desmitis of the proximal suspensory ligament on one forelimb and desmitis of the accessory ligament of the deep digital flexor tendon on the other forelimb. Four horses did not have abnormalities in the proximal suspensory ligament or accessory ligament of the deep digital flexor tendon. Eighty percent of horses with forelimb proximal suspensory ligament desmitis and 69% of horses with hindlimb proximal suspensory ligament desmitis returned to their intended use. Sixty-three percent of horses with desmitis of the accessory ligament of the deep digital flexor tendon were able to return to their intended use. MR imaging is a valuable diagnostic modality that allows diagnosis of injury in horses with lameness localized to the proximal metacarpal and metatarsal regions. The ability to accurately diagnose the source of lameness is important in selecting treatment that will maximize the chance to return to performance.     

FEASIBILITY AND REPEATABILITY FOR IN VIVO MEASUREMENTS OF STIFFNESS GRADIENTS IN THE CANINE GASTROCNEMIUS TENDON USING AN ACOUSTOELASTIC STRAIN GAUGE

B‐mode ultrasound is an established imaging modality for evaluating canine tendon injury. However, full extent of tendon injury often remains difficult to estimate, as small changes in sonographic appearance are associated with large changes in biomechanical strength. The acoustoelastic strain gauge (ASG) is an ultrasound‐based tissue evaluation technique that relates the change in echo intensity observed during relaxation or stretching of tendons to the tissue's mechanical properties. This technique deduces stiffness gradient (the rate of change of normalized stiffness as a function of tissue strain) by analyzing the ultrasound dynamic images captured from gradually deforming tissue. ASG has been proven to accurately model strain and stiffness within tendons in vitro. To determine the feasibility and repeatability for in vivo ASG measurements of canine tendon function, stiffness gradients for the gastrocnemius tendons of 10 clinically normal dogs were recorded by two nonindependent observers at three sites (musculotendinous junction, mid tendon, and insertion). Average stiffness gradient indices (0.0132, 0.0141, 0.0136) and dispersion values (0.0053, 0.0054, 0.0057) for each site, respectively, were consistent with published mechanical properties for normal canine tendon. Mean differences of the average stiffness gradient index and dispersion value between observers and between limbs for each site were less than 16%. Using interclass coefficients (ICC), intra‐observer (ICC 0.79–0.98) and interobserver (ICC 0.77–0.95) reproducibility was good to excellent. Right and left limb values were symmetric (ICC 0.74–0.92). Findings from this study indicated that ASG is a feasible and repeatable technique for measuring stiffness gradients in canine tendons.