Fig. 129.—The three regions used as points of resistance to upward pressure.
Fig. 130.—The three regions at which rotation of the apparatus may be prevented.
1. The Concavity of the Armlet.—At the elbow joint the pinching of the anterior soft parts on flexion is liable to take place in just the same way as occurs at the back of the knee in amputations through the leg. To avoid this it is necessary—
(1) That the axis of the joint should lie in a prolongation of a line passing through the epicondyle and the epitrochlea.
(2) That the armlet and the forearm socket should be cut away in front in crescent-shaped concavities.
Fig. 131.—Limb for amputation in the middle third of the forearm.
The depth of these concavities is estimated when the limb is fitted. Both the arm and the forearm may be cut away freely without any resulting inconvenience, provided that the stump is long; but if the stump is short and includes only the upper third of the forearm, it is impossible to cut away the forearm socket sufficiently without depriving the stump of a proper hold in the socket, so that movements are not transmitted to the forearm lever with their proper force. Consequently the socket for the forearm must be cut away very little, and must be carried up to the level of the fold of the elbow when the joint is flexed. The flesh in front of the elbow will not be pinched if, the forearm being fitted very accurately, the muscles of the upper arm are allowed free play, by cutting away the front of the armlet to half its height, but in this case an indirect attachment to the shoulder is essential.[12]
[12] Another difficulty in fitting a short forearm stump arises from the fact that the antero-posterior diameter of the forearm immediately below the elbow increases considerably when the joint is flexed, because of the contraction of the muscles arising from the condyles. If the forearm socket is made to fit closely when the elbow is extended it will be too small when the joint is flexed and will prevent full flexion. If it was made to fit with the elbow flexed, there is risk of the stump slipping out of the socket when the joint is extended. (Ed.)