Imaging the equine stifle joint (1 of 4)

The stifle is an important potential source of pain causing lameness in horses. Imaging this region is relatively frequently performed, both as part of pre-purchase evaluations and investigation of hindlimb lameness. The reciprocal apparatus of the equine hindlimb means that lameness caused by pain localised to the stifle cannot be determined based upon gait evaluation only. Diagnostic imaging of the stifle is therefore indicated in horses with a suggestive history, for example;

  • acute lameness following trauma to the stifle after failing to clear a fixed fence
  • localising clinical signs such as femorotibial or femoropatellar joint effusion or other palpable abnormalities
  • following localisation of pain to the stifle joint using intra-articular diagnostic analgesia (most commonly)

However, it is important to note that intra-articular analgesia of the stifle joints has the potential to desensitize the distal limb in one third of horses, so additional diagnostic analgesia is required to rule out pain causing or contributing to lameness in the distal limb.1

Radiography is a widely accessible and commonly used modality for assessing the stifle joint. Radiographic evaluation of the osseous and soft tissue structures can provide important information and is required for complete assessment of the stifle. However, negative radiographic studies do not rule out clinically significant osseous pathology. Additionally, radiological findings may not always be clinically significant, and osseous abnormalities may coexist with soft tissue abnormalities.

radiographic image of a stifle

Image 1: A) Caudocranial digital radiographic image of a stifle. The image is underexposed as a result of too low mAs usage. There is quantum mottle artefact (*, an artefact seen on underexposed digital images), no trabecular detail and poor demarcation. B) Caudocranial radiographic image of the same stifle as in A. Higher exposure settings (an increased mAs) resulted in a better exposed image.

The large muscle overlying the lateral and caudal aspects of the stifle means that some portable generators cannot generate the radiation required for adequate exposure (Image 1). In addition, the large muscle mass creates large amounts of scattered radiation, causing radiographic ‘noise’, which may obscure trabecular and other fine detail. Superimposition of structures on the resultant images can make subtle lesions easy to miss.

Ultrasonography has been shown to be a more accurate predictor of the presence of osteochondrosis dissecans (OCD) lesions of the femoral trochlear ridges than radiography2-3 and is more sensitive for the identification of periarticular osteophytes and subchondral bone or articular cartilage defects.4 In addition, the majority of stifle injuries in adult horses involve the soft tissues, so radiographic studies tend to have poor sensitivity in adult horses. However, ultrasonography does not replace radiography, and the information they both provide is mutually complementary because osseous and soft-tissue pathology commonly coexist.5 Radiography and ultrasonography should therefore be used in conjunction to maximise the diagnostic yield. Although the availability of advanced imaging modalities such as computed tomography (CT) and magnetic resonance imaging (MRI) for investigating stifle conditions is increasing and may offer improved diagnostic capabilities,5 it remains impractical to perform in the majority of cases, so radiography and ultrasonography remain important techniques in the investigation of stifle pain.

In addition to providing information on the osseous structures,2-5 ultrasonography can provide important diagnostic information regarding the soft tissue structures of the stifle joint (Image 2).4,5,6,7 Ultrasonography is an important modality for assessment of the menisci and can identify intrasubstance lesions which cannot be visualised arthroscopically.4,9 In contrast, lesions exist that are only seen on arthroscopy4 and some areas of the stifle are not accessible ultrasonographically because ultrasound waves cannot penetrate bone.

middle patellar ligamentImage 2: Transverse image of the middle patellar ligament. There is a well-defined hypoechogenic lesion in the caudal one half of the ligament (arrowheads).

The quality of information gained from ultrasonography is user-dependant and pathology of multiple structures is frequently identified in horses with stifle pain.5 Therefore, an ultrasonographic evaluation of the stifle joint must always be comprehensive. Adopting a systematic and thorough approach to this anatomically complex joint is imperative. The aim of this equine stifle series is to provide a practical guide to imaging of the important structures of the stifle.  

References:

  1. Radtke A., Fortier L.A., Regan S., Kraus S., Delco M.L. (2019) Intra-articular anaesthesia of the equine stifle improves foot lameness. Equine Vet J 52: 314-319.
  2. Beccati F., Chalmers H.J., Dante S., Lotto E., Pepe M. (2013) Diagnostic sensitivity and interobserver agreement of radiography and ultrasonography for detecting trochlear ridge osteochondrosis lesions in the equine stifle. Vet Radiol Ultrasound 54: 176-84.
  3. Bourzac C., Alexander K., Rossier Y., Laverty S. (2009) Comparison of radiography and ultrasonography for the diagnosis of osteochondrosis dissecans in the equine femoropatellar joint. Equine Vet J 41: 685-692.
  4. Nelson B.B., Kawcak C.E., Goodrich L.R., Werpy N.M., Valdés-Martínez A., McIlwraith C.W. (2016) Comparison between computed tomographic arthrography, radiography, ultrasonography, and arthroscopy for the diagnosis of femorotibial joint disease in western performance horses. Vet Radiol Ultrasound 57: 387-402.
  5. Hoaglund E.L., Barrett M.F. Daglish J., Contino E.K. (2018) Intermediate patellar ligament desmopathy often occurs in conjunction with other stifle abnormalities. Vet Radiol Ultrasound 60: 416-422.
  6. Daglish J., Frisbie D.D., Selberg K.T., Barrett M.F. (2017) High field magnetic resonance imaging is comparable with gross anatomy for description of the normal appearance of soft tissues in the equine stifle. Vet Radiol Ultrasound 59: 721-736.
  7. Dyson S.J. (2002) Normal ultrasonographic anatomy and injury of the patellar ligaments in the horse. Equine Vet J 34: 258-64.
  8. Gottlieb R., Whitcomb M.B., Vaughan B., Galuppo L.D., Spriet M. (2015) Ultrasonographic appearance of normal and injured lateral patellar ligaments in the equine stifle. Equine Vet J 48: 299-306.
  9. Adrian A.M., Barrett M.F. Werpy N.M, Kawcak C.E., Chapman P.L., Goodrich L.R. (2016) A comparison of arthroscopy to ultrasonography for identification of pathology of the equine stifle. Equine Vet J 49: 314-321.

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