This joint, technically termed the =distal interphalangeal articulation=
(Articulatio phalangis tertiæ), is a ginglymus formed by the junction of
the second and third phalanges and the third sesamoid bone.
=Articular Surfaces.=—The surface on the distal end of the second
phalanx is convex from before backward, concave transversely. The
articular surface of the third phalanx slopes sharply upward and
forward; its central part is prominent, and is flanked by two glenoid
cavities. It is completed behind by the articular surface of the third
sesamoid or navicular bone.
[Illustration:
FIG. 152.—LATERAL LIGAMENTS OF PASTERN JOINT AND SUSPENSORY LIGAMENTS
OF NAVICULAR BONE. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]
=Joint Capsule.=—This is attached around the margins of the articular
surfaces. In front and laterally it is tight, and is blended with the
extensor tendon and the lateral ligaments respectively. Posteriorly, it
forms a considerable pouch which extends upward to about the middle of
the second phalanx, where it is separated by a fibrous membrane from the
digital synovial sheath. Laterally small pouches project outward
(especially during volar flexion) against the lateral cartilages, just
behind the lateral ligaments.[36]
=Ligaments.=—The =lateral ligaments=, external and internal (Ligamentum
collaterale ulnare, radiale), are short strong bands which are attached
above in the depressions on either side of the lower part of the second
phalanx, under cover of the lateral cartilage. They widen below and end
in the depressions on either side of the extensor process and on the
anterior end of the lateral cartilages.
The =suspensory navicular ligaments=, external and internal (Ligamentum
sesamoideum collaterale ulnare, radiale),[37] are strong, somewhat
elastic bands, which form a sort of suspensory apparatus for the third
sesamoid. They are attached superiorly in and above the depressions on
either side of the distal end of the first phalanx and are here partly
blended with the lateral ligaments of the pastern joint. They are
directed obliquely downward and backward, and end chiefly on the ends
and proximal border of the third sesamoid, but detach a branch to the
inner surface of each lateral cartilage and wing of the third phalanx.
The =inferior navicular ligament= (Ligamentum phalangeo-sesamoideum)
reinforces the capsule interiorly. It is a strong layer of fibers which
extend from the distal border of the third sesamoid to the tendon
surface of the third phalanx, near the posterior margin of the articular
surface.
=Movements.=—The chief movements are flexion and extension. In the
standing position the joint is extended. During volar flexion a very
small amount of lateral movement and rotation can be produced by
manipulation. Dorsal flexion is very limited.
Dorsal flexion appears to be checked mainly by the deep flexor tendon,
since in cases of rupture of the latter the toe turns up. The slight
mobility of the posterior part of the socket for the second phalanx
(formed by the third sesamoid) diminishes concussion when the weight
comes on the foot.