Normal Radiographic Anatomy of the Canine Elbow

The elbow joint has a complex articulation and knowledge of normal anatomy is important. In the mediolateral view, the normal elbow appears as two concentric, parallel rings. The larger ring is a continuous arc formed by the trochlear notch, lateral coronoid process, and articular surface of the radial head, whereas the smaller circle represents the narrowest part of the trochlea humeri articulating with the trochlear notch. The width of the humeroradial joint and humeroulnar joint should be nearly equal. The medial coronoid process (MCP) articulates with the widest part of the trochlea humeri. The MCP is superimposed with the radial head and proximal ulna and tapers cranio proximally at 45° from the ulna with a slightly concave contour. The apex of the MCP should appear as a sharp point superimposed with the proximal physeal scar of the radius. Trabecular detail of the trochlear notch should be clearly visible. The anconeal process of the ulna has a slightly concave margin and partially obscured by the medial epicondyle on extended and standard mediolateral views. Uni- or bilateral lateral sesamoid bones within the tendon of origin of the supinator muscle may be present on craniocaudal views.

Radiography of the Elbow

Radiographs are the routine imaging modality practitioners use to diagnose elbow dysplasia. As early osteophytic changes and pathology associated with medial coronoid disease may be subtle, optimal imaging techniques are essential to improver diagnostic accuracy. Standard film screen techniques should use slow (detail) screens and short scale contrast techniques. High quality, properly positioned radiographs remain the most cost-effective method of diagnosing elbow dysplasia. Radiographs should be obtained with a nongrid tabletop technique, low kVp and high mAs exposure, with detailed film screen combination and tight collimation. Digital imaging is more forgiving regarding image quality assuming the correct look up tables are used and standard exposure principles are applied. Remember to collimate to the joint and not to over collimate on digital systems.

Radiographs are usually taken in lateral or sternal recumbency. They may also be made in dorsal recumbency or with horizontal beam radiography, but these are not described here. Standard radiographic projections of the elbow include the 90° flexed mediolateral and craniocaudal views. True lateral positioning is important with the x-ray beam centered at the medial epicondyle. For the craniocaudal view, the x-ray beam should be angled 10–15° toward the humerus to best display joint surfaces. The standard mediolateral view (90° flexion) provides good overall evaluation of the MCP and joint congruence but the anconeal process is often obscured by the medial epicondyle. Maximally flexed mediolateral projections (45° inside angle) have the highest sensitivity for UAP and early osteophyte formation and are also recommended for routine screening. However, this view does not contribute to the evaluation of fragmented medial coronoid process (FMCP) when compared to other views. The craniocaudal view is useful for evaluating the medial aspect of the joint for osteochondrosis (OC) or kissing lesions of the trochlea humeri, but fragments of the MCP are rarely demonstrated in this view. A supplemental view that may be useful to highlight OC or FCP lesions is the craniolateral-caudomedial oblique projection because it reduces superimposition with the ulna.

Standard Views

Mediolateral Extended

For a mediolateral extended (ML extended) view, the patient is positioned in lateral recumbency lying on the affected limb. The upper limb is retracted caudally and the head and neck are slightly extended. The angle between the humerus and radius and ulna is 120 degrees. The beam is centered on the medial epicondyle. This view optimizes evaluation of elbow incongruity osteophytes on the cranial aspect of the joint and lateral epicondylar crest medial coronoid process that is superimposed on the radial head.

Craniocaudal

For a craniocaudal (CrCd) view, the patient is positioned in sternal recumbency ensuring the humerus, radius and ulna are in a straight line. The head is elevated and retracted away from the affected limb. A thin foam pad under the elbow may prevent rotation. The beam is centered on the joint space just distal to the prominent medial epicondyle. This view optimizes medial humeral condyle osteochondral defects (kissing lesion) osteophytes on the medial humeral epicondyle distinguishing the supinator long tendon sesamoid from a FMCP mediolateral maximally flexed.

For a mediolateral maximally flexed (ML flexed) view, the patient is positioned in lateral recumbency lying on the affected limb. The upper limb is retracted. The distal antebrachium is pulled towards the neck so that the angle between the humerus and radius and ulna is < 45 degrees. The carpus should not be elevated to maintain the elbow in a true lateral position. The beam is centered on the medial epicondyle. This view optimizes osteophytes on the anconeal process ununited anconeal process flexor enthesopathy.

Extended Supinated Mediolateral

For an extended supinated mediolateral (Cd75°MCrLO) view, the patient is positioned in lateral recumbency lying on the affected limb. The upper limb is retracted. The joint is maximally extended and the limb supinated about 15 degrees. The beam is centered on the medial epicondyle. This view optimizes the cranial border of the medial coronoid process and increases the possibilities of detecting FMCP, as the primary beam is more likely to be in line with the fragment edge

Craniolateral-Caudomedial Oblique (Pronated View)

For a craniolateral-caudomedial oblique (Cr15°LCdMO) view, the patient is positioned in sternal recumbency ensuring the humerus, radius and ulna are in a straight line and the limb is pronated 15 degrees (15–50 degrees is the range in the literature). The beam is centred on the joint. This view optimizes medial humeral condyle osteochondral defects elbow incongruity, but extended ML view is more reliable of the MCP as it is isolated from other structures, improving visibility of fragments.

Craniomedial-Caudolateral Oblique (Supinated View)

For a craniomedial-caudolateral oblique (Cr45°MCdLO) view, the patient is positioned in sternal recumbency ensuring the humerus, radius and ulna are in a straight line and the limb is supinated 45–50 degrees. The beam is centred on the joint. This is not a standard elbow dysplasia view but is useful to optimize visibility of the lateral humeral condyle visibility of the supinator longus tendon sesamoid which could be confused with a medial coronoid fragment on ML views incomplete ossification of the humeral condyle; best seen on 15 degree supination.

Distomedial-Proximolateral Oblique

Distomedial-proximolateral oblique (Di35°MPrLO) view is also known as the medlap view. The patient is positioned in lateral recumbency lying on the affected limb. The upper limb is retracted. The joint is flexed to 90 degrees, the antebrachium elevated 35 degrees and the extremity supinated 40 degrees. A foam wedge may be used for this. The beam is centered on the medial epicondyle. This view optimizes the medial coronoid process, which is now seen proximal to or superimposed on the humero-radial joint.

Normal Anatomy

The elbow is a composite joint consisting of three bones, resulting in a structurally complex joint with superimposition of several clinically significant structures. Several views may be needed to identify the various components.

Canine elbow

Incidental Findings

In up to 15% of large-breed dogs a sesamoid may be seen in the origin of the supinator muscle on CrCd or Cr45°MCdLO views. The sesamoid is located laterally or craniolaterally to the radial head and may have a distinct articulation with the radius. It should not be confused with joint mice, chip fractures or a medial coronoid process fragment which lies medially. It is rarely seen on ML views and if seen lies slightly more proximal than a fragmented medial coronoid process with which it can be confused on this view. It may also be seen in the cat where it often is located more cranially than laterally.

Pathology

Developmental elbow abnormalities included in the term elbow dysplasia are FMCP or medial coronoid disease osteochondritis dissecans of the medial humeral condyle ununited anconeal process elbow incongruity. In affected dogs a skeletal survey should be considered to rule out concomitant hip dysplasia and other potential OCD lesions. Osteoarthrosis, the end result of elbow dysplasia, is a common finding and is seen as osteophyte formation at the following locations:

ML views on the dorsal border of the anconeal process on the cranioproximal edge of the radius and craniodistal aspect of the humeral condyle on the cranial edge of the medial coronoid process on the proximal edge of the lateral epicondylar crest subtrochlear sclerosis at the base of the medial coronoid process CrCd and Cr15°LCdMO views distal aspect of medial humeral condyle medial aspect of medial coronoid process.

References

1.  Cook CR, Cook JL. Diagnostic imaging of canine elbow dysplasia: a review. Vet Surg. 2009;38:173–184.

2.  Haudiquet PR, Marcellin-Little DJ, Stebbins ME. Use of the distomedial-proximolateral oblique radiographic view of the elbow joint for examination of the medial coronoid process in dogs. Am J Vet Res. 2002;63:1000–1005.

3.  Kirberger RM, Fourie SL. Elbow dysplasia in the dog: pathophysiology, diagnosis and control. J S Afr Vet Assoc. 1998;69:43–54.

4.  Samoy Y, Gielen I, Saunders J, van Bree H, van Ryssen B. Sensitivity and specificity of radiography for detection of elbow incongruity in clinical patients. Vet Radiol Ultrasound. 2012;53:236–244.