Elbow Biomechanics Humeroulnar joint Flexion 100 degrees possible

Elbow Biomechanics

Humeroulnar joint Flexion > 100 degrees possible as a result of: • Trochlea and trochlear notch projecting 45 degrees forward • Coronoid fossa accomodating the coronoid process This results in additional space between humerus and forearm for the flexor muscles

Carrying Angle 7 -12 degree angulation, caused by: Inclination of the trochlea to the humerus as the medial aspect projects more medially The trochlear groove runs obliquely distally and laterally, creating a spiral effect

Humeroradial joint Head of radius is slightly oval, greates diameter AP During flexion, the concave radial head glides anterior on capitulum In full flexion the radial head moves into the radial fossa of the humerus During extension the radial head glides posterior on the capitulum The radial head is compressed at endrange, as a result of the carrying angle, and moves slightly distally

Proximal radioulnar joint Between the radial head and radial notch of ulna The anular ligament stabilizes the head of the radius and connects it to the ulna Inner fibers are lined with hyaline cartilage. Superficial layers blend with radial collateral ligament Quadrate ligament strengthens the joint capsule and functions to limit rotation

Distal radioulnar joint Between the concave ulnar notch of the radius and the convex lower end of the ulna Enclosed by joint capsule and connected by the TFCC The radius and the ulna are connected by the oblique cord and the interosseous membrane The membrane tightens up in supination

TFCC (triangular fibro-cartilage complex) Functions of the TFCC • Primary stabilizer of the distal radio-ulnar and ulnar wrist joints • Reinforces the ulnar side of the wrist • Forms part of the concavity of the radiocarpal joint • Helps transfer compression forces that cross the hand to the forearm

Pronation and supination Pronation: ulna and radius cross each other, and limited by bone on bone contact Supination: radius and ulna are parallel, limited by tightening of interosseous membrane, quadrate ligament and anterior ligament of distal radio ulnar joint During pro/supination, the radial head rotates in the anular ligament, and the distal radius describes an arc of motion This arc is due to the elliptical shape of the radial head

Pronation and supination Muscles of supination Supinator, which is coiled around the neck Biceps, which inserts at the apex of the supinator bend (radial tuberosity) Muscles of pronation Pronator quadratus, coiled around the distal ulna Pronator teres, inserted at apex of pronator bend To understand the action of the muscles, you have to look at the shape of the radius, which is roughly in the shape of a crank. To unwind this “crank”, 2 mechanisms exist: • To pull on the apex of one of the bends • To unwind a cord coiled around one of the arms

Joint Positions

ROM Elbow Flexion • 145 degrees active • 160 degrees passive Extension • 0 degrees , but 5 -10 degrees hyperextension is possible Pronation • 85 degrees Supination • 90 degrees