Image 1: A) Caudocranial digital radiographic image of a stifle acquired with a low-power generator. 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, acquired with a higher-power generator. The exposure is improved, with no quantum mottle artefact.
Stifle radiography is not uncommonly indicated however it poses several potential challenges. Firstly, the complex anatomy means that stifle radiography is not always intuitive to the inexperienced imager, and superimposition as a result of non-ideal positioning can confound interpretation. Many horses will resent placement of the cassette, making this a potentially hazardous procedure. We recommend that chemical sedation and/or a twitch is employed. Portable generators cannot produce as much radiation as mounted units within clinics as a result of their lower power. Depending upon the size of horse being radiographed and the processing equipment, images of adequate quality cannot always be acquired in the field (Image 1). A large imaging cassette is required. A cassette holder should ideally be used, the holder must wear appropriate lead gloves, and the beam should be adequately collimated.
There are three essential views of the stifle that should be acquired routinely:
- Lateromedial (weightbearing or flexed)
- Caudo60°lateral-craniomedial oblique
For a complete evaluation, additional views include:
- Cranio40°lateral-caudomedial oblique
- Flexed cranioproximal-craniodistal oblique (skyline of the patella)
- Position the horse so that the limb to be imaged is slightly caudal to the contralateral limb and is straight (i.e. not overly turned out).
- Palpate the tibial tuberosity (tibial crest) on the cranial aspect of the stifle (Image 2). Centre the beam 1-2cm proximal to this level, and 5-10cm caudally from the cranial surface of the stifle.
Image 2: Lateromedial view. The limb to be imaged (in this case the right limb) is positioned caudal to the contralateral (left) limb. The beam is centred (cross) at a level proximal to the palpable tibial tuberosity (arrowhead) and 5-10cm caudal to the cranial surface of the stifle. The generator should be tilted so that the collimated area (indicated by the red box) is parallel to the cranial surface of the stifle, thereby reducing the required area of collimation.
TIP: Identify the correct position for beam centering and then position a square piece of masking tape on the limb to ensure that the beam is centred in the correct position!
- Minimising the area of collimation is essential radiation safety practice and will improve the quality of the resultant image. If possible, with your machine setup, tilt the generator/generator head so that the collimated area is parallel to the cranial surface of the stifle (Image 2).
- The cassette should be positioned medially to the stifle and as far proximally as possible. Try to position the plate so that it is perpendicular to the direction of the X-ray beam; this may be difficult.
With the limb in flexion, placement of the cassette is easier and sometimes better tolerated by horses. This facilitates obtaining a true lateromedial view. The resultant image may have less superimposition, thereby aiding evaluation of the trochlear ridges of the femur and the proximal aspect of the tibia, including the intercondylar eminences. However, this often requires an additional competent handler to lift and retract the limb during image acquisition. The generator will need to be tilted at a greater angle than for a weightbearing view; the collimation area should be parallel to the cranial surface of the stifle (refer to Image 2).
- Position the horse with the limb to be imaged slightly caudal to the contralateral limb as per the lateromedial view. Ensure that the limb is vertical when viewed from behind.
- Identify the ideal beam centering location by palpating the tibial tuberosity on the cranial aspect of the stifle. Using this as a landmark, identify the region on the caudal aspect of the stifle that is approximately 5cm higher than the level of the tibial tuberosity. At this height, place a marker for centering such as a small square of masking tape between the semimembranosus and semitendinosus muscles.
- With the generator angled approximately 15° downwards (in a caudoproximal-craniodistal direction, Image 3), align the beam direction with the stifle (i.e. appropriately outwards if the limb is outwardly rotated) and centre the beam over the marker.
- The imaging cassette should be placed cranial to the stifle and advanced medially until the imager can see the plate medial to the stifle. The plate should be angled by 15° so that it is perpendicular to the X-ray beam (Image 3). Take care not to touch the horse’s limb, flank, or sheath in males.
TIP: If available, a cone beam collimator attachment for the generator will further improve image quality!
This view provides useful information because it results in separation of the trochlear ridges of the femur and is generally easier to acquire than a lateromedial view.
- The horse should be standing squarely or with the limb to be imaged positioned slightly caudal to the contralateral limb.
- Angle the generator 60° caudally from the caudocranial position.
- Centre the beam at a level 1-2cm proximal to the tibial tuberosity (TIP: use the marker placed for the lateromedial view!) and approximately 10cm caudal to the cranial/craniolateral surface.
- To minimise the area of collimation, tilt the generator/generator head so that the area of collimation is parallel to the craniolateral aspect of the stifle.
- The imaging cassette should be placed craniomedial to the stifle, perpendicular to the direction of the X-ray beam.
Cranio40°lateral- caudomedial oblique
Although called a craniolateral-caudomedial oblique view, this is actually easier to acquire as a caudomedial-craniolateral oblique view.
- From the caudocranial position, angle the generator 40° medially.
- Centre at a level 1-2cm proximal to the tibial tuberosity.
- Tilt the generator/generator head if available, to minimise the required area of collimation.
- The imaging cassette should be positioned on the craniolateral aspect of the stifle, perpendicular to the direction of the X-ray beam.
Flexed cranioproximal-craniodistal oblique (skyline)
This view should be added if patella pathology is suspected from the history (e.g. trauma to the stifle from not successfully clearing a fixed fence) or as a result of abnormalities seen on other radiographic views.
- The limb should be lifted and gently retracted by a competent handler. Holding the foot towards the contralateral side of the horse will abduct the stifle and may aid centering of the beam, especially in horses with large or rounded flanks.
- The imaging cassette should be placed parallel to the tibia, under the stifle joint so that it is horizontal.
- Angle the generator 45° downwards
- Centre the beam approximately 5cm caudal to the cranial surface of the flexed stifle.
TIP: Depending upon the shape of the horse and the degree of abduction of the stifle, it may be necessary to angle the generator from lateral to medial slightly in order to centre on the stifle without the abdomen being superimposed over the stifle joint.
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