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.
In these amputations it is particularly necessary to be able to accurately locate the joints. The distal phalanx when flexed always passes under the proximal one. When the flexor and extensor tendons are cut they should be sewed either to their sheaths or united to one another over the ends of the bone.
In removing the distal phalanx the joint is opened by an incision across the dorsum in a line with the middle of the side of the proximal phalanx. A long flap is to be cut from the palmar surface. As the flexor and extensor tendons are inserted into the base of the distal phalanx, it will be an advantage to retain it if possible. The digital arteries may even here require ligation.
Lateral flaps are usually used. They are often made too short because the joint is thought to be higher than it really is. By flexing the thumb to a right angle the joint can be felt on the dorsum about 8 mm. (1/3 in.) below the top of the knuckle. The flaps must be cut as far forward as the middle of the phalanx. The two digital arteries on the palmar surface will require torsion or ligation. If the base of the phalanx can be retained the attachments of the short muscles of the thumb are preserved and additional control is given to the stump.
The upper limit of the metacarpal bone may often be difficult to recognize. The best way to locate it is to feel for the snuffbox and then feel for the joint a centimetre (say a half inch) in front of it. The dorsalis pollicis artery running on the dorsum of the bone and the princeps pollicis on its palmar aspect may require ligation. In disarticulating, it should be remembered that the joint is curved with its convexity toward the wrist.
In amputating the fingers, although it is easier to amputate through the joints, it is better to cut through the bone and save part of the phalanx, because much better control over the movements is obtained on account of the insertion of the tendons into the base and sides of the phalanges. Into the base of the distal phalanx is inserted the common extensor and flexor profundus digitorum. Into the base of the middle phalanx on its dorsal surface is inserted the extensor communis digitorum, which is reinforced by the lumbricales and interossei; on its palmar surface is inserted the flexor sublimis digitorum. Into the bases of the proximal phalanges are inserted the interossei muscles. The lines of the joints are to be recognized by remembering that the distal phalanx always flexes beneath the proximal one, therefore the prominence is always formed by the head of the proximal bone.
Fig. 383. - A transverse section of the proximal phalanx.
Fig. 384. - Lines of incision for amputations at the carpometacarpal joint of the thumb, the metacarpophalangeal joint of the index finger and between the proximal and middle phalanges of the middle finger.
The joint is to be opened by an incision across its anterior surface when flexed, and not on its dorsal surface. Anterior or palmar flaps are always used, except at the metacarpal joints. The digital arteries lie on the lateral palmar surface on each side of the flexor tendons and may require torsion or ligation. The finger-joints have lateral ligaments and a palmar or glenoid ligament. On the dorsal surface there is no ligament, its place being filled by the extensor tendon (Fig. 383).
Lateral flaps are used in disarticulating at the metacarpal joints. In a well-developed hand the line of the joint will be 1.25 cm. (1/2 in.) below the dorsal surface of the metacarpal bone (Fig. 384).
In consequence of not first recognizing the position of the joint the flaps are often cut too short. The incision must not involve the webs of the fingers but should reach as far forward as the middle of the phalanx. If this is not done it will 24 necessitate resection of the head of the metacarpal bone, which will materially weaken the hand. The two palmar digital arteries will require ligation, and the tendons should be sutured over the face of the bone or to their sheaths, closing them.