Almost any injury to the hand can affect the blood vessels.  There are two primary vessels that enter the hand, one on the thumb side of the wrist and the other on the small finger side of the wrist.  These are, respectively, the radial and ulnar arteries and are the areas where pulses can be felt at the wrist.  They enter the hand and then form two "arches" as they curve around the hand.  The arteries that go to each of the fingers take off from these arches.
An injury such as a knife wound, a crush injury, a needle stick,or any other accident can cause an injury to these vessels, which may need to be repaired under the microscope.
If a finger or the hand is completely amputated, then these vessels must be repaired under the microscope to ensure survival of the part that has been severed.

Replantation Defined
Replantation refers to the reattachment of a body part that has been completely separated from the body, such as the finger or hand.
A repair of the vessels is also required when a part has been incompletely severed but circulation to it has been destroyed.  Such an operation is called revascularization.

Indications for Replantation
The indications for replantation are complex and depend largely on the requirements of the individual, the finger or part that has been amputated, and the patient's desires, as well as on the severity of the injury.
Sometimes replantation of a part is not possible because the vessels and the amputated part are too severely damaged.  At other times, the rehabilitation process would be so complex and lengthy that it does not justify replacing the part, especially if the part is a single finger.
Therefore, each situation must be thoughtfully evaluated and addressed on an individual basis by an experienced replantation surgeon.
Indications for replantation are generally as follows:
Thumb Replantation
Because the thumb provides approximately 40 percent of the function of the hand, amputated thumbs should be replanted if at all possible—that is, if the injury is not so severe that replantation must be forgone.
Multiple-Digit Replantation
Similarly, multiple-digit amputations should be replanted because of the devastating nature of losing several or all of the fingers of a hand.
Single-Digit Replantation
Single-digit amputations should seldom be replanted.  The rehabilitation effort often takes six months or more, and the function of the replanted finger may actually jeopardize the function of the entire hand.  If the amputation is farther out than the proximal interphalangeal (middle) joint of the finger and the cut is fairly clean, then a single-digit amputation very occasionally may be justified.  However, such replantation should be done only in the rare case where the individual has a specific functional need.
Replantation of the Hand or Wrist
Amputations of the hand or wrist should be replanted because the function of a replanted hand is likely to be substantially better than that of a prosthesis.  This is due to the fact that, with replantation, nerves can be reconnected and sensation can be restored.
Because useful function of a replanted part can usually be achieved, the effort involved in replantation is considered worthwhile even though that function (in terms of feeling or sensation and movement of the replanted part) will never equal the original.

What Transpires during Replantation Surgery
During replantation surgery, first the bones are reconnected and secured with wires, pins, or plates, to provide a bony framework.
Next, the tendons which move the joints are reconnected.
After that, the other structures, such as pulleys, the joint capsule, and other soft tissues, are repaired.
Once these basic structural repairs have been completed, the operating microscope is brought in, and the arteries and nerves are repaired under the microscope.  Fine sutures of approximately 17 microns (17/1000 of a millimeter) in diameter are used to repair the outer layer of the small nerves and arteries (each of which is approximately 1 to 2 millimeters in diameter) in the fingers.  This is done with the operating microscope set at a power of between 10x and 40x magnification.  To reconnect a nerve, approximately two or three stitches are placed around it.  To reconnect an artery, approximately eight stitches are placed around it, re-creating a tube that will allow the blood to flow through to the end of the finger, thereby restoring circulation to the finger.
Finally, the skin is closed and the dressings applied.

Healing after Surgery
Following replantation surgery, an essential part of the process of regaining function is an intensive and carefully monitored program of hand therapy.