(From Green)Answer:
The thenar space is bounded dorsally by the musculature of the adductor pollicis, volarly by the fascia of the adductor pollicis, radially by the insertion of the adductor pollicis fascia onto the proximal phalanx of the thumb and ulnarly by the oblique or vertical septum running from the palmar fascia to the volar ridge of the third metacarpal and which separates the thenar space from the midpalmar space.
The midpalmar space lies between the flexor tendons and the fascia over the second and third volar interossei and the third and fourth metacarpals. It is bounded radially by the vertical or oblique septum and ulnarly by the fascia of the hypothenar muscles. Distally lie the vertical septa of the palmar fascia which are about 2 cm short of the webs, and proximally there is a thin fascial layer at the distal end of the carpal canal.
(From Green)
Both midpalmar and thenar space infections can result from a penetrating wound or extension of infections from elsewhere. In the case of the midpalmar space, flexor tenosynovitis of the ulnar three digits or distal palmar abscesses are possible causes. For thenar space infections, tenosynovitis of the thumb or index finger or extension from a midpalmar space or radial bursa infection can be the cause.
Midpalmar infections will result in dorsal swelling like all palmar infections, however, only midpalmar space infections will result in the loss of concavity of the palmar aspect of the hand. Motion of the middle and ring fingers is painful and limited. Thenar space infections present with a marked swelling of the thenar eminence that forces the thumb into abduction.
Midpalmar incisions (from Green)
Thenar incisions (from Green)
Treatment for both is surgical drainage with systemic antibiotics. The midpalmar space is usually approached by a transverse or oblique incision across the palm. The thenar space can be approached via a dorsal longitudinal incision perpendicular to the web, or a palmar incision parallel to the thenar crease or a combination of the two. Great care must be taken to protect the arteries and nerves passing through the areas. After evacuation of the purulence and intraoperative irrigation, the wound is closed and either a drain is placed for 48 hours or continuous postoperative irrigation via a catheter is performed for 48 hours.