Page 179 - Canine Lameness

12.2 Normal Anatomy 151

and proximal interphalangeal joints of digits II–V. The transversely and dorsally oriented base of
the third phalanx results in it forming an almost 90° dorsal angle with the second phalanx. On
their palmar surfaces, broad, low tubercles receive one of the five branches of the deep digital
flexor tendons. The tendon of this muscle first sends a small medial branch to the first phalanx at
the level of the proximal metacarpus before splitting into its four main branches. The principle DISTAL LIMB REGION
tendon is surrounded by a synovial sheath from the level of the distal radius to proximal metacar-
pus, and the individual branches are similar enveloped. On the dorsal surface just distal to the
joint, the extensor processes of digits II–V receive one of the four tendons of the common digital
extensor muscle. Distal to this, the conical, ventrally curved, and tapered ungula process receives
the claw (nail). The ungula crest surrounds the process dorsally and laterally.
The proximal interphalangeal joints, formed between the first and second phalanx in digits II–V,
are stabilized laterally and medially by stout collateral ligaments. The extensor tendons dorsally
and the flexor tendons on the palmar aspect are attached to the joint capsule. The joint capsule of
the distal interphalangeal joint is thickened to form collateral ligaments. Each of digits II–V has
two dorsal ligaments which extend from the base of the second phalanx to the ungula crest. These
passive extensors keep the claws elevated off the ground and counter the tension of the deep digital
flexors. The digital flexor tendons are held in place by annular ligaments at each phalanx. In the
metacarpophalangeal region, the fascia forms the transverse metacarpal ligament which holds the
digits together and anchors the metacarpal footpad.
Five of the six palmar paw pads occur in this region; whereas the sixth, the carpal pad, is located
adjacent to the accessory carpal bone (Figure 12.2). Palmar pads include the heart-shaped metacar-
pal pad as well as four digital pads which lie under the distal interphalangeal joints of digits II–V.
The digital pads are anchored in place by fibroelastic strands from the digital fascia. Deep to the
dermis and heavily keratinized epidermis, the pad is composed of fat, laced with reticular, colla-
genous, and elastic fibers, along with a scattering of eccrine sweat glands. This arrangement allows
the pads to deform under load yet return to their original shape and size. Pads also provide trac-
tion, load transfer, shock absorption, sensory perception, insulation, plus an additional cooling
mechanism.

12.2.2 Pelvic Limb
Metatarsals II–V are similar to the corresponding metacarpals although they are longer
(Figure 12.4), and their bases are flattened and compressed transversely, resulting in them being
more crowded together and having smaller intermetatarsal spaces compared to the thoracic limb.
Metatarsal I is significantly reduced in size (Figure 12.4) and sometimes fused to the first tarsal
bone (Figure 18.2). Although not always present, the associated phalanx, the dewclaw, is often
vestigial and supernumerary in some breeds. Phalanges II–V are very similar to those of the tho-
racic limb, as are the joints and ligaments.
The superficial digital flexor muscle divides into four branches at the level of the distal row of
tarsal bones. These branches insert on the plantar surface of the proximal base of phalanges II–V.
The lateral and medial heads of the deep digital flexor muscle converge and at the level of the proxi-
mal metatarsus, the common tendon splits into four branches which insert on the plantar flexor
tubercles of the third phalanx of digits II–V. The synovial and ligamentous structures of both these
muscles are similar to the flexor muscles of the thoracic limb. The tendon of cranial tibial extensor
muscle runs obliquely across the tarsus and inserts on the palmer base of metatarsals I and II. The
long digital extensor tendon branches into four at the proximal metatarsus; these synovial-sheathed
tendons run along the dorsal surfaces to phalanges II–V and receive a single sesamoid at the proxi-
mal interphalangeal joint, before inserting on the dorsal surface of the ungula process.