"Absent Middle Facet": A Sign on Unenhanced Radiography of Subtalar Joint Coalition
Patrick T. Liu1,
Catherine C. Roberts1,
F. Spencer Chivers1,
Todd A. Kile2,
Richard J. Claridge2,
Joseph R. DeMartini1,
Robert E. Kenrich2 and
Lewis H. Freed3
1 Department of Diagnostic Radiology, Mayo Clinic Scottsdale, 13400 E Shea
Blvd., Scottsdale, AZ 85259.
2 Department of Orthopedic Surgery, Foot and Ankle Division, Mayo Clinic
Scottsdale, Scottsdale, AZ 85259.
3 East Valley Foot and Ankle, Ste. 114, 215 S Power Rd., Mesa, AZ 85206.
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Fig. 1A. —Volume-rendered 3D images from CT scan of normal calcaneus in
39-year-old man show three facets of subtalar joint. View from medial to
lateral shows parallel orientation of posterior facet (thick arrow)
and middle facet (thin arrow).
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Fig. 1B. —Volume-rendered 3D images from CT scan of normal calcaneus in
39-year-old man show three facets of subtalar joint. View from superior shows
anterior aspect of calcaneus angled slightly in caudal direction. Thick arrow
= posterior facet, thin arrow = middle facet.
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Fig. 2A. —11-year-old male control subject with normal subtalar joint
motion and score of 1 for "absent middle facet" sign, "talar
beak" sign, and C sign. Standing lateral radiograph from middle
facet (short solid arrow) shows posterior facet joint spaces
(long solid arrow) of subtalar joint are clearly visible. Lateral
process of talus (open arrow) marks anterior extent of posterior
facets.
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Fig. 2B. —11-year-old male control subject with normal subtalar joint
motion and score of 1 for "absent middle facet" sign, "talar
beak" sign, and C sign. Coronal CT scan that corresponds to
A shows normal joint space at middle (short arrow) and
posterior (long arrow) subtalar joints.
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Fig. 3. —39-year-old man. Diagram depicting the creation of
radiolucent subtalar joint middle facet joint space on standing lateral
radiograph (left) by passage of central X-ray beam (arrows)
through horizontally oriented middle facet joint space, shown on coronal CT
image (right).
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Fig. 4A. —64-year-old woman with surgically proven subtalar coalition.
Standing lateral radiograph was used to assign score of 3 for "absent
middle facet" sign, "talar beak" sign, and C sign.
Long straight arrow points to expected location of middle facet joint space.
Curved arrow marks grade 3 talar beak. Short straight arrows outline contours
of C sign. Note that posterior facets (open arrows) are also
obscured.
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Fig. 5A. —67-year-old woman with surgically proven subtalar coalition.
Standing lateral radiograph was used to assign score of 3 for "absent
middle facet" sign, "talar beak" sign, and C sign.
Long straight arrow points to expected location of middle facets. Curved arrow
points to grade 3 talar beak. Short straight arrows outline contours of
C sign.
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Fig. 4B. —64-year-old woman with surgically proven subtalar coalition.
Coronal CT scan of same foot confirms bony coalition of middle facet
(solid arrow) and posterior facets (open arrow).
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Fig. 5B. —67-year-old woman with surgically proven subtalar coalition.
Coronal T2-weighted image shows cartilaginous or fibrous coalition of middle
facet (arrow).
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Fig. 6. —61-year-old woman with surgically proven coalition of
subtalar middle facets. Standing lateral radiograph was found to have scores
of 3, 1, and 1 for "absent middle facet" sign, "talar
beak" sign, and C sign, respectively. True-positive absent middle
facet sign (arrow) was present. Neither talar beak sign nor C
sign (false-negative signs) was detected.
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Fig. 7. —Standing lateral radiograph of 27-year-old woman with
surgically proven subtalar coalition of middle and posterior facets.
True-positive "absent middle facet" sign (arrow) but
false-negative "talar beak" sign and C sign were seen.
Scores for absent middle facet, talar beak, and C signs were 3, 1, and
2, respectively.
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Fig. 8. —Bar graph shows sensitivity (black), specificity
(gray), and accuracy (white) achieved by three observers
using composite criteria (score of 3 for either "absent middle
facet" sign or C sign) and individual criteria at different
thresholds for positive diagnosis of subtalar middle facet coalition. Note
highest accuracy was achieved when score of 3 for either absent middle facet
sign or C sign was considered positive for subtalar middle facet
coalition. AMF = absent middle facet sign, TB = "talar beak"
sign.
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Fig. 9A. —52-year-old man with false-positive "absent middle
facet" sign. Nonstanding lateral radiograph shows false-positive absent
middle facet sign (straight arrow) due to overlap of middle facet by
anterior process of calcaneus. Note also superiorly directed talar beak
(curved arrow).
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Fig. 9B. —52-year-old man with false-positive "absent middle
facet" sign. Standing lateral radiograph of same foot obtained 3 months
after A shows normal subtalar joint middle facet (arrow).
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Fig. 9C. —52-year-old man with false-positive "absent middle
facet" sign. Sagittal reformatted image from CT scan of same foot
obtained on same day as B confirms that middle facet (arrow)
is normal. This case illustrates need to assess middle facet on only standing
lateral radiographs.
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Copyright © 2003 by the American Roentgen Ray Society.
