Equine MRI

Rachel C. Murray, Rachel C. Murray

• Publisher: Wiley-Blackwell
• Year: 2010
• ISBN:
  9781444329209
• Edition: 1
• Topic: Médecine
• Subtopic: Sciences vétérinaires équines

Equine MRI is a unique, comprehensive guide to MRI in the horse. Edited by Rachel Murray, a leading authority and researcher in the field with over ten years of equine clinical MRI experience, the book also includescontributions from worldwide experts in the subject.

Divided into the following four sections, the book presents key information based on previous validation work and clinical practice:

• Principles of MRI, including the practicalities of image acquisition and interpretation
• Normal MRI anatomy and normal variations
• Different types of pathological change
• Options for clinical management and prognosis for different conditions

MRI is a rapidly expanding area in veterinary medicine that confers detailed, three-dimensional information on both bone and soft tissue. Expanding clinical knowledge, improvements in technology, and practical application of MRI to the standing and recumbent horse means this useful imaging modality has become an integral and essential part of the diagnostic evaluation in lameness and is a realistic option for investigation of ophthalmological, neurological and cranial pathology.

Equine MRI enables readers to understand the best ways to achieve good quality images, and provides a detailed explanation of the problems that may occur.

With close to 950 normal and abnormal images, this book offers considerable detail and examples of both common and uncommon problems, making it a great reference for equine veterinarians, veterinary students, specialists in equine surgery, and specialists in veterinary imaging.

Part C: Pathology

12 THE FOOT AND PASTERN 271

Sue Dyson and Rachel Murray

13 THE FETLOCK REGION 315

Sarah Powell

14 THE METACARPAL/METATARSAL REGION 361

Matthew Brokken, Russell Tucker and Rachel Murray

15 THE CARPAL REGION 385

Sarah Powell and Rachel Murray

16 THE DISTAL TARSAL REGION 405

Sue Dyson

17 THE PROXIMAL TARSAL REGION 421

Rachel Murray, Natasha Werpy, Fabrice Audigié, Jean-Marie Denoix, Matthew Brokken and Thorben Schulze

18 THE STIFLE 451

Carter Judy

19 THE HEAD 467

Russell Tucker, Katherine Garrett, Stephen Reed and Rachel Murray

Chapter 12

The foot and pastern

Sue Dyson and Rachel Murray

INTRODUCTION

The aims of this chapter are to discuss briefly the indications for magnetic resonance imaging (MRI) of the foot and pastern, to review the current literature, to give an overview of the spectrum of injuries identified at the Animal Health Trust between January 2001 and December 2007 in horses undergoing either low-field or high-field MRI and to provide detailed descriptions of the lesions identified. Where applicable the current knowledge about the nature of these lesions is reviewed.

INDICATIONS FOR MAGNETIC RESONANCE IMAGING

Prior to consideration of MRI, all horses should be subjected to an in-depth lameness evaluation, including local analgesic techniques, high-quality radiography and, where appropriate, ultrasonography and nuclear scintigraphy. Magnetic resonance imaging is an expensive technique and is not required for the diagnosis of all causes of foot and pastern-related pain. However, in the absence of significant imaging findings, MRI is indicated even in the early stages of lameness. Early recognition of the cause of lameness may permit the most appropriate, targeted treatment. A reliable history of a recent, acute puncture wound to the foot is an exception, when MRI may be considered without previous in-depth evaluation.

Local analgesia should be used to define as objectively as possible the source or sources of pain, bearing in mind the current knowledge about the limitations of the majority of local analgesic techniques. Local analgesia is particularly important when acquiring low-field MR images, because the field of view is relatively small and if lameness is not abolished by palmar digital analgesia, then the pastern must be included in the examination. A number of abnormalities may be identified using MRI and interpretation of their relative significance can be facilitated by accurate localization of pain. Likewise the use of nuclear scintigraphy can help to determine the likely clinical significance of lesions identified using MRI, and in some horses may give information about both the nature of the pathological process and the prognosis.

If a flexor cortex defect in the navicular bone is identified radiologically, it is likely that there is associated pathological change in at least the deep digital flexor tendon (DDFT) and such horses may not require MRI to give a prognosis. While MRI could give additional information, the prognosis for such lesions is extremely guarded, and the additional information obtained may have to be balanced against the cost.

SPECTRUM OF INJURY

Five hundred and eighty-four horses were examined at the Animal Health Trust during the study period using low- or high-field MRI, 568 with forelimb lameness and 16 with hind limb lameness. Pain was localized to the digit using local analgesic techniques. In addition to horses with negative imaging findings, horses with the following findings underwent MRI in order to determine better the nature and extent of pathological change:

• ill-defined osseous cyst-like lesions (OCLLs) in the spongiosa of the navicular bone without apparent communication with the flexor (palmar) cortex, or with equivocal radiolucent zones in the flexor cortex of the navicular bone associated with focal, moderate or intense increased radiopharmaceutical uptake (IRU)
• punctate focal core lesions in the DDFT in the pastern, with or without IRU in the region of the DDFT in pool phase scintigraphic images, or at its insertion in the distal phalanx in bone phase images
• enlargement of a collateral ligament (CL) of the distal interphalangeal (DIP) joint with or without change in echogenicity, or with focal IRU in the distal phalanx at the site of insertion of a CL of the DIP joint.

Horses with focal lesions in an oblique sesamoidean ligament (OSL), or a branch of the superficial digital flexor tendon (SDFT), which were considered insufficient to cause the degree of lameness, were also examined. Horses with a previously identified fracture of the distal phalanx or cartilage of the foot were evaluated if concurrent soft tissue pathology, which may influence treatment and prognosis, was suspected. Horses with periarticular new bone around the dorsal aspect of the proximal interphalangeal (PIP) joint which did not respond to intra-articular analgesia were also examined, because other lesions were thought likely be responsible for lameness.

The distribution of injuries is outlined in Table 12.1. This table reflects the cause or causes of lameness considered likely to be of most clinical importance based on the interpretation of the MR images in conjunction with the results of local analgesia (e.g. intra-articular analgesia of the DIP joint, intrathecal analgesia of the navicular bursa) and other imaging modalities (e.g. nuclear scintigraphy). Multiple injuries (i.e. several lesions likely to be contributing to pain and lameness), desmitis of a CL of the DIP joint and DDF tendonitis were the predominant injury categories.

Table 12.1 Distribution of injuries considered to be the primary cause of lameness based on magnetic resonance imaging, clinical findings, response to local analgesia and other imaging modalities in 584 horses with foot pain, January 2001–December 2007

Injured structure*	Number of horses (%)	Comments
Navicular bone	21 (3.6)	+ 31 horses with multiple injuries, 4 CL DIP joint
Navicular bone and DDFT	60 (10.3)	+ 60 horses with multiple injuries, 1 DSL
DSIL	20 (3.4)	+ 53 horses with multiple injuries, 5 navicular bone and DDFT, 1 navicular bone, 4 CL DIP joint, 1 distal phalanx
CSL	3 (0.5)	+ 57 with multiple injuries, 1 CL DIP joint
DDFT	89 (15.2)	+ 52 horses with multiple injuries, 3 primary distal phalanx pathology, 12 CL DIP joint
CL DIP joint	179 (30.1)	+ 106 horses with multiple injuries, 15 DDFT, 11 navicular bone and& DDFT, 4 primary distal phalanx pathology
SSL	2 (0.5)
Multiple injuries	176 (30.1%)
DIP joint	8 (1.4)	+ 10 horses with multiple injuries, 6 CL DIP joint, 1 DSIL
Primary injury of the middle and/or distal phalanges	25 (4.3)	Includes 5 with evidence of previous penetrating injury + 23 horses with multiple injuries, 10 CL DIP joint, 1 navicular bone and DDFT

DDFT, deep digital flexor tendon; CL, collateral ligament; CSL, collateral sesamoidean ligament; DSIL, distal sesamoidean impar ligament; DIP, distal interphalangeal; SSL, straight sesamoidean ligament.

All horses with a suspected lesion of the navicular bone, DDFT or a CL of the DIP joint based on conventional imaging techniques were determined to have much more severe lesions on MR images, often with lesions to other structures. A large proportion of lesions of the DDFT that extended proximal to the PIP joint were not detected ultrasonographically. Similarly, ultrasonography failed to detect many lesions of the CLs of the DIP joint, the straight sesamoidean ligament (SSL) and the OSLs.

TYPES OF PATHOLOGICAL CHANGE

Navicular bone

Lesions of the navicular bone are seen alone, in conjunction with injuries of the DDFT, distal sesamoidean impar ligament (DSIL) or collateral sesamoidean ligament (CSL), or as a complex of injuries to multiple structures [1–7]. A comparison of MRI findings in control horses with no history of foot-related pain and horses with chronic palmar foot pain showed significant alterations of the podotrochlear (navicular) apparatus in the lame horses [8]. A comparative MRI and postmortem study showed good correlation between the lesions identified using MRI, gross [9] and histopathological findings [10]. The typical types of lesion identified are summarized in Table 12.2 and illustrated in Figures 12.1–12.8. We believe that there are a number of different pathological processes which can take place within the navicular bone and other components of the podotrochlear apparatus and the DDFT.

Table 12.2 Summary of magnetic resonance imaging abnormalities of the navicular bone

Distal border	Smooth extension of the distal border into the DSIL Entheseophyte formation Irregular thickness of distal cortex with mineralization extending proximally Enlargement of synovial invaginations Distal border fragment with no reaction in parent bone Distal border fragment with change in contour and signal intensity in adjacent navicular bone
Proximal border	Entheseophyte formation Endosteal mineralization Proximal border fragment
Flexor (palmar) border	Endosteal irregularity Increased thickness of flexor cortex Focal increased signal in flexor cortex in all image sequences Focal fluid accumulation palmar to bone consistent with fibrocartilage loss More extensive loss of fibrocartilage Linear increase in signal intensity through flexor cortex in STIR images Focal disruption of flexor cortex, with reaction (abnormal fluid and mineralisation) extending dorsally into the spongiosa Adhesions of DDFT
Dorsal border	Periarticular osteophyte formation Endosteal mineralization
Spongiosa/medulla	Discrete osseous cyst-like lesions in the distal third of the bone Diffuse increased signal intensity on STIR images Focal increased signal on STIR images at insertion of CSL and/or origin of DSIL Linear increased signal on STIR images between insertion of CSL and origin of DSIL Focal or diffuse decreased signal intensity on T1- and T2-weighted images consistent with mineralization

Figure 12.1 (a) Sagittal 3D T2* GRE high-field MR image of a navicular bone of a 4-year-old Warmblood. The dorsal, distal and palmar cortices of the navicular bone are thick. There is distal extension of the flexor cortex of the bone and a small entheseophyte on the proximal border of the bone. (b) Transverse 3D T2* GRE high-field MR image of the same navicular bone as in (a). The dorsal cortex of the navicular bone is thickened. (c) Transverse 3D T2* GRE high-field MR image of the same foot as in (a) and (b). The normal architecture of the distal sesamoidean impar ligament is disrupted with large areas of increased signal intensity (arrows).

Figure 12.2 (a) Parasagittal 3D T2* GRE high-field MR image of a navicular bone. There is a partial thickness defect in the palmar cortex of the bone with focal fluid accumulation, reflecting loss of fibrocartilage (black arrow). There is irregular endosteal mineralization (open arrow). There was no significant radiological abnormality and radiopharmaceutical uptake was normal. (b) Transverse 3D T2* GRE high-field MR image of the navicular bone in (a). There is focal fluid accumulation palmar to the bone (arrow) reflecting fibrocartilage loss. Abaxial to this the deep digital flexor tendon is in close apposition to the bone and probably adherent. Dorsal to the fluid accumulation there is focal endosteal mineralization (open arrow).

Figure 12.3 (a) Transverse 3D T2* GRE high-field MR image of a navicular bone. There is focal disruption of the palmar cortex of the bone just lateral to the sagittal ridge (white arrow). There is also mild endosteal irregularity laterally (arrow heads). In a palmaro 45° proximal–palmarodistal oblique radiographic view there was an extremely subtle area of reduced opacity within the palmar cortex coincident with this lesion, but it did not appear to penetrate the palmar aspect of the cortex. No other radiological abnormality was detected. There was focal moderately increased radiopharmaceutical uptake in the navicular bone. (b) Transverse 3D T2* GRE high-field MR image of the foot in (a). Medial is to the right. There is complete loss of separation between the deep digital flexor tendon and the distal sesamoidean impar ligament. There is a linear region of increased signal intensity through both structures (black arrow). Dorsal to this there is mild endosteal irregularity of the palmar cortex of the distal phalanx (open arrow). There is entheseophyte formation on the axial aspect of the medial palmar process of the distal phalanx (white arrow)....