This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
By the term elbow-joint is meant the articulation between the humerus above and the ulna and upper surface of the radius below. The articulation between the upper end of the radius and the ulna forms the superior radio-ulnar articulation and does not belong to the elbow-joint proper. As has already been pointed out, the ulna articulates with the trochlea and the radius with the capitellum.
The elbow-joint is a pure hinge-joint. The articulation between the trochlea and ulna is so shaped as to allow no lateral motion, but only an anteroposterior one. The articulation between the capitellum and upper surface of the head of the radius is, on the contrary, a ball-and-socket joint. The socket, it is true, is shallow, but it is perfectly spherical, made so by the rotary movement of the radius in pronation and supination. Hence it follows that the shape and continuity of the upper extremity depends upon the articulation of the ulna with the humerus: it further follows that if the radius be removed from the elbow-joint the forearm would still be held in its proper relation to the arm, but if the ulna be removed the stability of the joint would be lost and the forearm would move in any direction, laterally as well as anteroposteriorly. It is for this reason that injuries involving the medial condyle and trochlea are more liable to be followed by serious disability than are those of the lateral condyle and capitellum.
The movement of the joint takes place around a transverse axis, which passes from side to side below and in front of the condyles. The forearm can be extended to an angle of 180 degrees, or a straight line, with the arm. It can be flexed to an angle of 30 to 40 degrees. Sometimes it cannot be flexed so much, so that if after an injury to the joint the patient can flex the elbow to half a right angle, or 45 degrees, he may be regarded as having regained a normal amount of motion.
The axis of motion of the joint is not exactly transverse, but slopes slightly from the outside downward and inward. The effect of this is to give a slight obliquity to the motions of flexion and extension. This obliquity is not noticeable except in extreme extension and flexion. When the forearm is completely extended it is seen to lie not in the axis of the arm but to bend outward from the elbow at an angle of 170 degrees. This angle is called the " carrying angle," because by resting the elbow against the side, any article which is carried in the extended hand is kept away from the body. Sometimes the line of the forearm is almost straight with that of the arm, at others the deflection may amount to 15 degrees. It may vary on the two sides and 10 degrees may be considered an average; Woolsey. gives 6 degrees as the average. This carrying angle becomes lost in certain cases of fracture of the elbow, as will be pointed out later. As the elbow is flexed the carrying angle disappears (see Fig. 294).
When flexion is complete the ulna instead of coming up toward the head of the humerus inclines inward at an angle of 10 degrees. Morris states that the hand has a tendency to point to the middle of the clavicle, which would make an angle of 20 degrees. This we believe to be too great.
Fig. 294. - The carrying angle; formed by the deviation outward of the axis of the forearm from the axis of the arm.