**2. Anatomical basics**

The canine elbow joint is composed of the humerus proximally and the radius and ulna distally, and can be divided into three joint compartments: the humeroulnar, humero-radial and proximal radio-ulnar joint [1, 2]. The humero-ulnar joint is formed by the humeral trochlea and intercondylar region of the condyle and the ulnar trochlear notch, which extends from the anconeal process to the radial incisure, and continues to the medial coronoid process of the ulna. The humero-radial joint is formed by the capitulum of the humeral condyle and the radial head. The radial incisure and the medial aspect of the radial head form the proximal radio-ulnar joint. Altogether the elbow joint acts as a hinge joint (ginglymus) with extension and flexion being the main motion pattern and some amount of pronation and supination, mainly taken over by the radio-ulnar joint [1].

In healthy canine elbows the radio-ulnar joint shows a congruent shape without any step formation between the ulnar and radial joint surface, at least under static conditions. However, the humero-ulnar joint is not perfectly congruent even in healthy dogs [3–6]. The radii of curvature of the humeral condyle and ulnar trochlear notch show different values along their curvilinear course, resulting in reduced contact in the central notch region [3–7]. The trochlear notch shows a slightly elliptical shape, so that the anconeal process and distal aspect of the notch as well as the coronoid process are in contact with the humeral condyle. This kind of physiological humero-ulnar incongruence was first described in humans and could be detected in the canine elbow joint, too [4–6, 8, 9].

The maximum range of motion (ROM) varies between 110 to 150 degrees, with breed-specific maximum flexion of 25 to 49 degrees and maximum extension of 155 to 175 degrees [10–14]. The main extensor muscle of the elbow joint is the triceps brachii muscle [1]. Further this muscle prevents flexion of the elbow during the stance phase. The anconeal and tensor fasciae antebrachii muscles are additional extensors of the elbow joint. Flexion is performed by the biceps brachii and brachial muscles. The extensor carpi radialis muscle contributes to flexor function to some amount. The canine antebrachium can be pronated 17 to 50 degrees and supinated 31 to 70 degrees [10, 15]. The supinator and brachioradial muscles are responsible for supination of the antebrachium. The latter contributes only minimal to supination and is missing in some individuals [16]. The pronator teres and pronator quadratus muscles are responsible for pronation and the pronator teres muscle is supposed to contribute to elbow joint flexion as well [1, 2].

Four ligaments support the elbow joint: the medial and lateral collateral ligament, the annular ligament and interosseous ligament/interosseous membrane [1, 2]. The medial and lateral collateral ligaments origin from the medial and lateral humeral epicondyle. The medial collateral divides into two crura. The cranial one is weaker and attaches at the radius, while the stronger caudal one attaches mainly at the ulna and to some amount at the radius. The lateral collateral ligament consists of two crura as well. The cranial part attaches to the radius, and the caudal part attaches to the ulna and colligates with the annular ligament, which can contain a sesamoid bone [2]. The annular ligament runs transversely around the radial head spanning from the lateral to the medial aspect of the radial incisure of the ulna. It runs underneath the medial and lateral collateral ligaments. The radius and ulna are further attached to each other by the interosseous ligament and interosseous membrane, which spans the interosseous space. Distally the radius and ulna are connected to each other by the radioulnar ligament.
