Non-weight bearing lameness

What can I do with my imaging equipment in this situation?

Introduction

The non-weight bearing lame horse is a common emergency encountered by equine practitioners. Obtaining the history (such as recent activities, farriery work, or known trauma) and performing a physical examination (a methodical evaluation for wounds, heat, swelling, pain on palpation or manipulation and increased amplitude of digital pulses) will usually serve to localize the source of pain and narrow down the list of differential diagnoses. However, without the use of additional diagnostic tools, a definitive diagnosis cannot always be reached. Even in situations where the problem is obvious, for example a penetrating injury to the solar surface of the foot, the extent of damage cannot always be fully appreciated from physical evaluation alone. Rapidly obtaining a definitive diagnosis is essential to ensure optimal management, an accurate prognosis and the best chance of a successful outcome and so further diagnostic tests are often required. The aim of this article is to discuss several common clinical presentations of the non-weight bearing lame horse, and how portable imaging (ultrasound and X-ray) can contribute to achieving a full and definitive diagnosis.

Solar foot penetration

Penetration of the foot by a foreign body such as a nail is a potentially serious injury which can threaten athletic function and life.1 The prognosis and treatment required both depend upon the depth of penetration, involvement of internal structures and chronicity. With the foreign body in situ, the acquisition of at least two orthogonal radiographic views (e.g. lateromedial and dorsopalmar) will determine the depth and direction of penetration. Knowledge of the internal anatomy of the foot is essential for determining the likelihood of involvement of important structures. If the penetrating foreign body is not present or was removed, contrast radiography or placement of a radiodense malleable probe to identify the trajectory and depth of penetration should be performed.

Dorsopalmar

Foot pain

Severe lameness in association with foot pain is relatively common and is usually straightforward to identify. Localising clinical signs include heat in the hoof, increased amplitude of digital pulses, swelling around the coronary band or pastern, a painful response to the application of hoof testers, and reluctance to bear weight through the entire solar surface of the foot e.g. toe touching. Unless there is an obvious discoloured track of infection or a history of trauma immediately preceding the onset of lameness, it can sometimes be difficult to differentiate subsolar abscessation from fracture of the distal phalanx as both may display similar clinical signs. Radiography can be used to obtain a definitive diagnosis quickly and direct the treatment and prognosis. Foot preparation (shoe removal, paring away loose sole and packing the sulci) is very important because artefacts from an improperly prepared foot can both mask and mimic the radiological signs of subsolar abscess or fracture. Radiolucent regions within the horn indicate gas in association with an abscess. Irregular radiolucent lines consistent with a fracture of the distal phalanx can be seen in several locations. The location and involvement with the joint surface (i.e. fracture type) will influence prognosis.2 ‘Skyline’ (palmaroproximal-palmarodistal oblique) and flexed 45° oblique views are particularly useful for identification of fractures of the palmar processes.

Flexed dorsopalmar

Dorsoproximal

Joints

Septic arthritis is a common emergency, usually presenting as severe lameness with heat, joint capsule effusion, periarticular swelling or cellulitis and pain on manipulation of the affected joint with or without evidence of a wound. Arthrocentesis and synovial fluid analysis are usually diagnostic, however laboratory facilities are not always readily available in first-opinion practice and rapid diagnosis and instigation of aggressive therapy (usually emergency referral for arthroscopic joint lavage) are essential. If it is unclear whether septic arthritis is present in addition to a periarticular cellulitis or wound, performing arthrocentesis through affected skin risks causing iatrogenic septic synovitis in a normal joint. Ultrasonography can be used to help distinguish articular from periarticular involvement. Common ultrasonographic findings in horses with septic arthritis or tenosynovitis include effusion (usually marked), capsule thickening, focal hyperechoic spots and loculations.3 Synovial fluid can be echoic or anechoic. There appears to be a temporal and causative correlation with the detection of ultrasonographic findings, and so false negatives may occur, especially in the early acute stage.3 Performing radiography is useful to assess osseous involvement, for example, fractures involving the joint may cause similar clinical signs, and in the presence of a wound traumatic involvement of the underlying bone in addition to penetration and sepsis of the joint is possible. Radiography is also useful to assess the response following treatment because joint sepsis will predispose to future osteoarthritis, which is characterised radiologically by joint remodelling (including increased and/or decreased mineralisation), osteophytosis and joint space narrowing. With chronic sepsis, osteomyelitis is readily identified radiologically.

Wounds

Non-weight bearing or severe lameness in conjunction with even a small wound to the affected limb can be a result of blunt trauma (e.g. kick wound) and should always be investigated radiologically for concurrent fracture. Multiple orthogonal views should be acquired and assessed critically. Non-displaced fissure fractures may not be evident on initial radiographs and so should not be ruled out. Fractures may become more visible after 10-14 days due to increased osteoclastic activity and fissure widening. Using the correct exposure values is important so that the soft tissues can also be evaluated. Air in the soft tissue will appear more radiolucent than the normal soft tissues and can be useful for identifying the depth of the wound, especially if wound size precludes digital exploration. Foreign bodies such as metal and grit are usually visible radiologically. Ultrasonography can also be useful to assess wounds. Air in the soft tissues will create a high difference in acoustic impedance and cause reverberation artefacts. Whilst the presence of air will preclude ultrasonographic evaluation of deeper structures, the location of reverberation artefacts can aid with the determination of wound depth and trajectory. Involvement of specific structures (e.g. tendons) or the presence of foreign material may be identified ultrasonographically. Ultrasonography can be particularly useful in the investigation of non-healing or chronic draining wounds; abscessation, foreign material and sequestration can all be readily identified.4,5

Wound       

                    Dorsopalmar

Difficult to localise

The localisation of pain causing severe or non-weight bearing lameness can sometimes be difficult, either due to diffuse swelling precluding palpation of the underlying structures, or a lack of any localising clinical signs. Where there is diffuse swelling, a combination of radiography and ultrasonography to assess the underlying osseous and soft tissues is recommended. Cellulitis, lymphangitis and oedema have the ultrasonographic appearance of a thickened, hypoechogenic subcutaneous layer because of fluid accumulation. With cellulitis and lymphangitis, the underlying tendons/ligaments should appear normal. Oedema may occur secondary to injury and so all the soft tissue structures in the affected region should be evaluated in two planes (longitudinal and transverse). In addition to the limitations of using radiography in the early stages of a fissure fracture, radiography of the proximal limb (femur and pelvis) is limited. Ultrasonography is a useful imaging modality for the detection of pelvic fractures, which will appear as a discontinuity to the usually smooth, hyperechogenic line of the bone.6

Conclusions

Radiography and ultrasonography are important imaging modalities for the investigation of several common causes of non-weight bearing or severe lameness in horses. They are readily available and increasingly portable meaning that many systems can be used in virtually any situation in first opinion equine practice. This is of particular importance because the non-weight bearing lame horse can often not easily be moved to facilities such as a yard with power and being able to image the horse in the field offers many practical advantages.

References:

  1. Smith M.R.W. (2013) Penetrating injuries of the foot. Equine Veterinary Education 25: 422431.
  2. Rijkenhuizen A.B.M., de Graaf K., Hak A., Furst A., ter Braake F., Stanek C., Greet T.R.C. (2012) Management and outcome of fractures of the distal phalanx: a retrospective study of 285 horses with a long term outcome in 223 cases. The Veterinary Journal 192: 176-182.
  3. Beccati F., Gialletti R., Passamonti F., Nannarone S., Di Meo A., Pepe M. (2015) Ultrasonographic findings in 38 horses with septic arthritis/tenosynovitis. Veterinary Radiology and Ultrasound 56: 68-76.
  4. Armbrust L.J., Biller D.S., Radlinsky M.G., Hoskinson J.J. (2005) Ultrasonographic diagnosis of foreign bodies associated with chronic draining tracts and abscessed in dogs. Veterinary Radiology and Ultrasound. 44: 66-70.
  5. Smith R.K.W. (2002) Equine musculoskeletal ultrasonography. BMUS Bulletin 10: 34-40.
  6. Shepherd M.C., Pilsworth R.C. (1994) The use of ultrasound in the diagnosis of pelvic fractures. Equine Veterinary Education 6: 223-227.

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