Ankle and Foot 47 - Department of Radiology - University of ...

2208 VII Imaging of the Musculoskeletal System
over, displaying the talar and calcaneal articular surfaces of
the posterior, middle, and anterior facets of the subtalar
joint in red, blue, and green, respectively.
The posterior facet is the largest and is the primary
weight-bearing portion of the subtalar joint. At the anterolateral
corner of the posterior facet, the talus comes to an
acutely angled corner, the lateral process of the talus. When
the subtalar joint experiences an extreme axial load, such
as when a person falls from a height or undergoes a deceleration
injury in a motor vehicle collision, the pointy
lateral process of the talus acts like a wedge, splitting
and fracturing the calcaneus. 13 Calcaneal fractures tend to
extend into the posterior facet, and when imaging calcaneal
fractures we obliquely angle our coronally reformatted
CT slices to be perpendicular to the posterior facet.
The middle facet is defined by the sustentaculum tali,
a shelflike projection from the anteromedial portion of the
calcaneus that supports the middle of the talus. Sustentaculum
in Latin means “a supporting structure.” The flexor
hallucis longus tendon passes under the sustentaculum
tali. The middle facet of the subtalar joint is a completely
separate articulation from the posterior facet. When injecting
contrast (often mixed with anesthetic) into the posterior
facet of the subtalar joint, we do not expect it to
communicate with the middle facet. Across the middle
facet of the subtalar joint is one of the two most common
locations for tarsal coalitions to occur, the other being
between the anterior process of the calcaneus and the
lateral pole of the navicular.
Unlike the posterior and middle facets, the anterior
facet is not well defined and may even be absent. When
present, the anterior facet is a smooth continuation of the
middle facet, extending under the head of the talus. Directly
lateral to the anterior and middle facet is the sinus tarsi, an
area devoid of bone and filled primarily with fat.
• Anatomic Divisions
Figure 47-5 is a three-dimensionally reformatted CT image
showing the anatomic divisions between the tarsals and
metatarsals. The hindfoot consists of the talus and the calcaneus
and is separated from the midfoot by the Chopart*
joint, a smooth continuation between the talonavicular
and calcaneocuboid joints. The midfoot consists of the
other five tarsal bones, the navicular, the cuboid, and the
three cuneiforms. The forefoot consists of the metatarsals
and phalanges and is separated from the midfoot by the
tarsometatarsal joint, also known as the Lisfranc † joint. Along
Figure 47-1. Gross anatomy of the tarsals and surrounding bones.
Ti, tibia; Fi; fibula; Ta, talus; Ca, calcaneus; ST, sustentaculum tali;
N, navicular; Cu, cuboid; 1, 2, and 3, refer respectively to the first,
second, and third cuneiforms (sometimes referred to as the medial,
intermediate, and lateral cuneiforms, respectively); I, II, III, IV, and V
refer to the first through fifth metatarsals, respectively.
*François Chopart (1743-1795), a pioneer in urology, was known for the particular
attention he gave to recording his numerous clinical observations. Thus, it
is somewhat surprising that he never wrote about the midtarsal amputation that
bears his name almost three centuries later. He performed this surgery only once,
on August 21, 1791, to resect a presumed liposarcoma of the foot. The approach
was based on Chopart’s knowledge of the anatomy of the midfoot and was published
by his student, Laffiteau, in 1792.
† Jacques Lisfranc (1790-1847) was a very aggressive surgeon who wrote
extensively and described many new procedures, including disarticulation of the
shoulder, excision of the rectum, and amputation of the cervix. At age 23 he joined
Napoleon’s army as a battlefront surgeon, a setting where amputations were the
norm. Military surgeons (of the period) were not given the calm and unhurried
atmosphere necessary for the task of laboriously picking out bone splinters and
bits of clothing from gaping wounds. Locating the open ends of severed arteries
and tying them off in the smoke of battle or by flickering candlelight was an enormous
problem. Although some wounds did not themselves dictate amputation, it
often had to be done because the patient could not otherwise survive the rigors
of transport to the rear. The mind did not have time to reason. Experience and
cold-bloodedness counted for more than talent. Everything had to be done with
prompt and decisive action. In 1815, the final year of the war, Lisfranc wrote a 50-
page paper describing his technique for performing a partial amputation of the
foot at the tarsometatarsal joint, with the sole being preserved to make the flap.
The technique was used to treat forefoot gangrene from frostbite. Lisfranc was
widely known for his ability to amputate a foot in less than a minute, an important
skill in that preanesthesia era.
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