This, the =proximal interphalangeal articulation= (Articulatio phalangis
secundæ), is a ginglymus formed by the junction of the distal end of the
first phalanx and the proximal end of the second phalanx.
The =articular surfaces= are: (1) On the first phalanx, two slightly
unequal convex areas with an intermediate shallow groove; (2) on the
second phalanx, a corresponding surface, completed behind by a plate of
fibro-cartilage.
The =joint capsule= is close fitting in front and laterally, where it
blends with the extensor tendon and the lateral ligaments respectively.
Behind it pouches upward a little and is reinforced by the straight
sesamoidean ligament and the branches of the superficial flexor tendon.
There are two lateral and four volar ligaments.
The =lateral ligaments=, internal and external (Ligamentum collaterale
radiale, ulnare) are very short and strong bands which are attached
above on the eminence and depression on each side of the distal end of
the first phalanx, and below on the eminence on either side of the
proximal end of the second phalanx. The direction of the ligaments is
about vertical and, therefore, does not correspond to the digital axis.
The =volar= or =posterior ligaments= consist of central and lateral
pairs of bands which are attached below to the posterior margin of the
proximal end of the second phalanx and its complementary
fibro-cartilage. The lateral pair is attached above to the middle of the
borders of the first phalanx, the central pair lower down and on the
margin of the triangular rough area.
These ligaments are very commonly thickened as a result of chronic
inflammation, and then are not well defined. The central ones blend
below with the branches of the superficial flexor tendon and with the
straight sesamoidean ligament.
=Movements.=—These are very limited, and consist of flexion and
extension. The axis of motion passes transversely through the distal end
of the first phalanx. In the standing position the joint is extended. A
small amount of volar flexion is possible, and in this position slight
lateral flexion and rotation can be produced by manipulation. Dorsal
flexion is prevented by the lateral, volar, and straight sesamoidean
ligaments.