You will notice that you cannot fully extend the extremity of your finger. The last phalanx remains bent, it does not "react any longer". This is sufficient evidence to diagnose "mallet finger" or "doigt en maillet": analogous the small hammers of a piano. This accident is very frequent in sports (with balls, boxing, and other wrestling sports). It is also very common when tucking in the sheet under the mattress.
The loss of extension of the last phalanx is due to the extensor tendon rupture. When there is a forceful flexion motion of the finger (crushing of a ball, or the hooking of your finger in the inside part of the mattress), the abrupt strengthening of the tendon causes its rupture. This rupture is usually closed, painless and without ecchymosis (bruise). Diagnosis is established due to the deformation of the finger.
Postero-anterior finger radiographs are essential to see if there is bone laceration at the insertion of the tendon.
The aim of the treatment is to obtain union of the tendon at its exact length. Surgical intervention is not required for suture: it is enough for the two extremities of the tendon to be in contact during the union period. A simple splint will suffice, in order to keep the extremity of the finger in extension.
It is required to keep this position -by using a splint- during the time of consolidation, that is, a month and a half. A slight flexion of the finger during this time makes the treatment ineffective.
This treatment is therefore simple but demanding: 
very simple because it is enough to keep the splint in place.
very demanding : because you should never remove the splint , not even for hygienic matters, or if is uncomfortable, or you feel the need to remove it. It is also necessary to control the position of the splint; a slight movement may make it ineffective.
After a month and a half, you will be able to remove the splint during the day, but you should wear it at night for another one and a half month.
Surgery is indicated when the tendon laceration involves a thick bony fragment of the base of the second phalanx. This bony structure involves a joint surface that sometimes may require relocation and surgical fixation. You may also consider surgical intervention in case the orthopedic treatment failed. Various techniques can be performed, but most times a transfer of mobility is applied with a loss of full flexion of the F3.
The treatment helps regain a normal finger in most cases, provided the treatment had been strictly followed. Sometimes, a reduced extension deficit persists; when it is around 10-15°, it can be corrected with time (3-6 months).
