Elbow CT case study – What can you see?
A skeletally immature, female, neutered Labrador Retriever presented with chronic lameness of the right forelimb. Gait evaluation revealed grade 3/10 (Sumner-Smith 1993) right forelimb lameness at walk. On physical examination there was right elbow joint effusion and pain. Computed tomography (CT) was performed using a GE Revolution ACT Scanner to reach a diagnosis of the cause of pain, lameness and to guide treatment.
When performing elbow CT, it is common practice to image both the left and right forelimbs to allow comparison with the contralateral limb. As well as potentially providing a control, it is not uncommon for subclinical disease or mild pathology to be present in the non-lame limb. It is important to identify abnormalities of the contralateral limb which may influence the treatment, long-term prognosis and outcome.
Each CT scan will usually be formatted and displayed as multiple collections of images called reconstructions. For elbow scans this will usually include a soft tissue reconstruction, which is optimised for assessment of the soft tissue structures, and a bone reconstruction, which is optimised for assessment of bony structures. Intravenous administration of a radiopaque contrast medium is not usually performed for orthopaedic imaging.
In this dog a number of pathological changes of the right elbow joint were identified, consistent with marked medial coronoid disease with a loose coronoid fragment. Surgical intervention and removal of the coronoid fragment was indicated to provide benefit to the patient as part of ongoing management.
How is this conclusion determined from the CT images?
CT images are acquired in a transverse plane (as slices) and are then reconstructed to produce further slices in two additional orthogonal planes; a dorsal and a sagittal plane. To describe CT images words are used that refer to how attenuating an area is in relation to a reference structure. This may be likened to use of the term opacity in radiography Reference structures include bone, grey matter, white matter, CSF, fat, air etc. Attenuation is (among other things) related to bone density and so reference to density can also be used.
Hypoattenuating (hypodense): the described area is less attenuating/dense (darker) than expected or in relation to the reference structure.
Isoattenuating (isodense) the described area is similarly attenuating/dense as the surrounding structure or reference structure.
Hyperattenuating (hyperdense): the described area is more attenuating/dense (brighter or whiter) than expected or in relation to the reference structure.
Axial fragment and malacia of cortical bone
A transverse image (bone reconstruction) at the level of the medial coronoid process of the ulna (corresponding to the blue line in image on the right), lateral is to the left of the image. There is a fragment of bone separated from the axial aspect of the medial coronoid process (arrow) and two small focal rounded regions of hypoattenuation (hypodensity; arrowhead), indicating erosions or malacia of the cortical bone along the radial incisure. The radial incisure is the concavity on the lateral aspect of the coronoid process of the ulna that articulates with the head of the radius.
Medial coronoid fragment and densification of the adjacent bone
A reconstructed dorsal image at the level of the medial coronoid process of the ulna (bone reconstruction). Densification of the medial coronoid adjacent to the fragment bed and of the adjacent medial humeral condyle are seen as areas of hyperattenuation (arrows) resulting a relatively brighter white area in the image. Note that the coronoid process fragment is on the weight bearing portion of the ulna (red arrowhead) and that there is a narrowed joint space (grey arrowhead).
There is already evidence of osteoarthritis, with modelling present on the anconeal process of the ulna (arrowheads).
Humeroulnar joint incongruity
This sagittal image (bone reconstruction) allows visualisation of the subtle joint incongruity with the humeroulnar joint space not articulating neatly with the ulna; there is a wider humeroulnar joint space (red arrow heads) compared with the parallel humeroradial joint space (blue arrow head).
Elbow joint effusion
A sagittal image (soft tissue reconstruction) showing marked elbow joint effusion (arrowheads). Note that in this reconstruction the bone in this image is very bright but the subtle soft tissue abnormality is more easily visualised.
In conclusion there is evidence of medial coronoid disease with fragmentation, sclerosis, malacia with erosive joint lesions, joint incongruity and joint effusion. The detail and extent of the changes would be difficult to ascertain by radiography alone.
Sumner-Smith G. (1993) Gait Analysis and Orthopedic Examination. In: Textbook of Small Animal Surgery, 2nd ed. p. 1578.
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