Morphological and functional interface between palmar plates of metacarpophalangeal joints and intrinsic muscles of the hand

Abstract

Summary. After a ten-year period of intensively dealing with the hand as a Medical Artist, it became quite obvious that the palmar section of MP joints is described in the literature partly different from what we found in our cadaver studies. This especially applies to the proximal continuation and fixation of palmar plates and the deep transverse metacarpal ligament. Thus we tried to understand and explain the complex character of interaction between locomotion apparatus and structures of the hand's "connective-tissue body". Cadaveric operation microscope-assisted dissection of the palmar ligament (plate), deep transverse metacarpal ligament, interosseous muscles as well as mechanically stable sections of the "connective tissue body" and fascia allowed a clear view of morphology, interaction, and character of these structures during passive finger motion. 1. Proximally, the palmar ligament (plate) of a metacarpophalangeal joint ranges to the tendinous sheets of origin of the interosseous muscles and in extension of these it is attached to the palmar carpal ligaments. In the following, these tendinous structures will be referred to as "deep longitudinal metacarpal fibres". This highly sophisticated proximal attachment is formed bilaterally from each palmar plate in several cascades. Thus, a widely ranging distribution of forces implied by MP-extension is guaranteed. The deep transverse metacarpal ligament executes the function of a "force dissipating center": Extension strain is passed on from the fingers via the "lateral digital sheet" and the "spiral ligament" to the palmar fascia and further on into a proximal and "deep" direction by the vertical sheets (Legueu and Juvara 1892/1974). On the next central level, a similar flow of forces reaches the palmar carpal ligaments from the osseous phalangeal base via the phalangoglenoidal ligaments and the palmar plate. Junction of these forces is the "soft tissue confluence" (Zancolli 1992), whereas the described "deep longitudinal metacarpal fibres" can be understood as a final common route. Overstretching forces at the index finger are transferred to the palmar carpal ligaments by a ligamentous "Arcus adductorius".2. Contrary to published reports, the palmar ligament (plate) does not have a determined and constant length but does adapt its extension in longitudinal direction to the joints' excursions, more than doubling in length in extended position compared to its minimal length in a maximally flexed MP joint. This is performed by criss-crossing fibers throughout the palmar ligament.3. Distally, the palmar ligament is attached to the base of the proximal phalanx on both sides with no mechanically relevant fixation in the middle. This predominant lateral attachment is additionally stabilized by the proximal section of the phalangoglenoidal ligaments, thus extending the area of osseous fixation. The biomechanical principle of restraining extension in the MP joints can be compared to the architecture of Gothic cathedrals or modern bridges. Here too, simple algorithms create complex systems. One major force (extension of MP joint) is split up into many single components to spread out the strain over a wide area. This allows high endurance with minimal expension of materials and simultaneously a well-distinguished mobility of the fingers. Operative treatment in this area (Zancolli's capsuloplasty for treatment of paralytic claw fingers) could be modified by considering these functional and morphological criteria, thus lowering frequency of relapse in palmar plate surgery.