REDUCTION OF TRICEPS MUSCLE FORCE AFTER SHORTENING OF THE DISTAL HUMERUS - A COMPUTATIONAL MODEL

Bone deficiency resulting in shortening of the distal humerus may occu r after fractures, treatment of nonunions, or revision of fetal elbow arthroplasty. A biomechanical model of the triceps muscle and tendon w as used to investigate the effect of distal humeral shortening on tric eps force production. The analysis indicated that shortening of the di stal humerus primarily influences the media head of the triceps, which contributes most to elbow extension strength in extended elbow positi ons. In a posture of 30 degrees elbow flexion, shortening the distal h umerus by 1, 2, and 3 cm reduced the extension strength by 17%, 40%, a nd 63%, respectively. At 90 degrees of flexion, strength was reduced b y 11%, 15%, and 21%, respectively. This result suggests that shortenin g the humerus by 1 cm may be well tolerated, but shortening by 2 or mo re cm may cause a significant reduction in triceps force. This would b e particularly important in patients requiring terminal extension stre ngth For weight bearing.