The palmar aponeurosis invests muscles and tendons of the palm and consists of central,
lateral and medial bundles.
The apex of central portion triangular in shape continues the lower margin of the transverse carpal ligament.
The base of central bundle divides into four slips,
one for each finger,
with expansion for proximal phalanx bones,
flexor tendon sheaths,
and skin.
The lateral and medial bundles cover the thumb and little finger muscles,
on the radial and the ulnar side,
respectively.
Anatomically the palmar aponeurosis has two components: 1) a thick superficial part of longitudinal bundles,
continuous with the palmaris longus tendon,
and 2) a thinner layer of transverse fibers.
1) The longitudinal fibers form four bands extending towards the base of the medial four fingers and cover the long flexor tendons.
In the distal part of the palm a large number of these fibers insert into the skin,
others continue in the superficial transverse metacarpal ligament,
while a smaller number continue distally to the fingers where they terminate in the skin and in the fibrous flexor sheaths above the proximal phalanx.
About in the middle of the palm some fibers descend through the transverse fibers of the aponeurosis and between the flexor tendons and the lumbrical muscles,
to insert on the deeper fasciae and ligaments of the hand.
2) The transverse fibers in the proximal part of the palm are completely covered by the longitudinal fibers,
and lie between these and the flexor retinaculum,
with which they exchange fibers.
Distally these transverse fibers form a strong ligament,
the transverse ligament,
and radiate into the skin,
into the fasciae of the muscles and down to the metacarpal bones.
The distal border of the transverse ligament is at the same level as the distal border of the deep transverse metacarpal ligament,
and approximately opposite the joint line in the metacarpophalangeal articulation
The fibrous sheaths of the flexor tendons extend from the fingers up between the deep transverse metacarpal ligament and the transverse ligament of the palmar aponeurosis.
The deep transverse metacarpal ligament occupies a central position in the function of the hand.
It extends across the hand and is attached to the metacarpal bones and is connected to the longitudinal and transverse fibers of the palmar aponeurosis.
Fig. 1: Anatomical scheme of palmari longus (PL) and palmar aponeurosis (A). TML, transverse metacarpal ligament; TCL, transverse carpal ligament; dashed triangle, central bundle; *, slip for the fingers.
Palmaris longus (PL),
is generally considered important morphologically rather than functionally in fact it is phylogenetically degenerated flexor muscle of metacarpophalangeal joint.
PL muscle takes origin from medial epicondyle of humerus with the common flexor tendons and runs distally as a long tendon witch crosses the flexor retinaculum superficially and fans out as a flat sheet which continues as palmar aponeurosis.
So the palmar aponeurosis represents the degenerated distal part of the tendon of palmaris longus which marks the probable evidence of retrogression of this muscle.
The median nerve at the wrist lies partly under cover of the tendon of PL.