Keywords
Extremity CT - flatfoot - pes planus - weight-bearing CT
Introduction
Flatfoot deformity is a complex foot deformity with various components like medial
longitudinal arch collapse, hindfoot valgus, and forefoot abduction.[[1]] The deformities typically occur in the orthostatic posture[[2]] due to failure of the various static and dynamic stabilizers of the foot.
Thus, the assessment of these feet in weight-bearing position is a mandate to analyze
the biomechanical alterations and plan meticulous correction of the same. Till date,
weight-bearing radiographs have been the cornerstone for planning surgeries on flatfoot.
It is however always challenging to measure the various angles on the radiographs
due to superimposition of the bones. Another, even bigger challenge is the lack of
reproducibility[[3]] of these radiographs and the associated rotational and fan distortions.[[4]]
Conventional cross-sectional techniques are therefore often needed to complement the
information on the radiographs. These conventional cross-sectional techniques, though
are reproducible and provide a multiplanar detailed assessment of the joints and bones,
have a scanner design limitation of being able to scan only in supine/prone position.
Scans cannot be done in the natural weight-bearing position, needed to assess the
exact biomechanical alterations in the foot in that position.
A few researchers tried to simulate body weight support on the ankle and foot, passively
in the supine position using various strategies.[[5]] These systems, however, had multiple lacunae including the low reproducibility
of these techniques and the nonutilization of the active muscle forces that act during
orthostatic physiological positioning.[[6],[7],[8]] It was thus realized that these systems did not resolve the limitation of conventional
computed tomography (CT).
The weight-bearing cone-beam CT [WBCT], with its unique and compact design, overcomes
these limitations and enables cross-sectional imaging of the foot and ankle, to be
done in the natural weight-bearing position. This enables meticulous assessment of
the dynamics of the foot in the orthostatic position.[[9],[10],[11]] The use of weight-bearing CT [WBCT] is thus expected to improve precision and accuracy
in characterization of the adult acquired flatfoot [AAFD].[[12]] The same technique can also be used to assess the knee joint in the weight-bearing
position.
Apart from the advantage of multiplanar capabilities, the acknowledged high-resolution
images[[3],[13]] and the advantage of obtaining scans in weight-bearing position, these scanners
also have the advantage of reduced radiation exposure[[3],[10],[14],[15],[16]] and shorter scan time. Radiation doses for a standard extremity scan with weight
bearing CT, ranges from 0.01 to 0.03 mSv per scan.[[3],[17]] The total scan cost with this system is also less or similar to the other available
imaging technologies.
The weight-bearing CT scanner uses the technique of the clinically established dental
cone-beam scanners.[[14]] The cone--beam technology enables a smaller and compact gantry size with a wider
range of gantry movements, which includes tilting the gantry to the horizontal orientation
and lowering it close to floor level, making weight-bearing imaging, with the patient
in a standing position possible.[[17]] In orthopedics, the use of CBCT is rather new and was first published by Zbijewski
et al.,[[18]] in 2011. The first mention of CBCT in orthopedics was in a paper published by Tuominen
et al., in 2013.[[17]]
In this paper, we report our initial experience in weight-bearing cross-sectional
imaging of the foot and ankle for assessment of flatfoot deformity.
Materials and Methods
18 patients [15 females and 3 males], aged 13 to 69 years, and known to have flatfeet
were scanned on the weight-bearing CT. Only the symptomatic foot was scanned. A total
of 19 feet were scanned, which included 7right and 12 left feet.
All scans were done on the CBCT [Carestream, Onsight 3D extremity system]. The scans
were done in the standing weight-bearing position, with the foot to be scanned placed
inside the gantry and the other leg outside, and bent at the knee to provide adequate
weight bearing on the foot being scanned.
The acquired images were then reconstructed in soft tissue and bone algorithms and
sent digitally to the workstation [Osirix, IMAC, Apple, Inc, USA] for further assessment.
An MRI was also done for 8 out of 19 feet scanned on the CBCT.
Each foot was then assessed for the lateral talar i.e., the first metatarsal [Meary]
angle. The calcaneal pitch was measured in all except one patient, where the heel
was not covered. Heel valgus angle was measured. A note was made of presence or absence
of forefoot abduction. A specific note was also made for the presence/absence of extra-articular
talocalcaneal impingement and subfibular impingement. Other associated abnormalities,
which could be a cause of the patient’s complaints/symptoms were noted.
The presence of osteoarthritic changes and their distribution pattern were noted.
The status of the muscles surrounding the ankle was also assessed on the scan.
The lateral talar i.e., first metatarsal angle, also known, as the Meary angle is
the angle formed between the long axis of the talus and the first metatarsal on a
weight-bearing lateral view [[Figure 1]]. This is one of the most often used measurements for assessing medial longitudinal
arch collapse. In a normal foot, the talar axis is in line with the first metatarsal
axis. An angle that is greater than 4° convex downward is considered flatfoot.[[19],[20]]
Figure 1: Sagittal reformatted thick slab CBCT showing normal lateral talar-1st metatarsal angle (The lateral talar axis [solid arrow] is parallel to the 1st metatarsal axis [dashed arrow])
The calcaneal pitch is defined as the angle between the horizontal plane and the line
drawn along the plantar-most surface of the calcaneus to the inferior border of its
distal articular surface[[21],[22]] [[Figure 2]]. There have been differing opinions concerning the normal range of calcaneal pitch,
some consider 18 to 20° as normal,[[22]] while few consider 17 to 32° as the normal range.[[21]]
Figure 2: Sagittal reformatted thick slab CBCT showing calcaneal pitch (Normal range 18–20°/17–32°)
Heel valgus angle was measured as the angle between the medial calcaneal cortex and
the long axis of the tibia. This is measured just posterior to the sustentaculum,
at the level of the posterior talus and tibia[[23]] [[Figure 3]]. The measurement is performed in the most posterior coronal image that includes
both the tibia and calcaneus. The normal hindfoot angle is estimated between 2° and
6° of valgus, in the general population.[[24]]
Figure 3 (A and B): Coronal reformatted images (A) Tibial axis (B) normal heel valgus angle (between
the tibial axis and the medial calcaneal cortex)
Forefoot abduction was assessed on the AP view of the foot and was said to be present
when the line drawn through the mid-axis of the talus was seen to be angled medially
to the first metatarsal shaft axis[[12]] [[Figure 4]].
Figure 4: Axial reformatted thick slab CBCT showing normal AP talar-1st metatarsal angle (The AP talar axis [solid arrow] is parallel to the 1st metatarsal axis [dashed arrow])
Extra-articular talocalcaneal impingement was said to be present when there was direct
contact between the inferior aspect of the lateral talar process and the calcaneum
at the calcaneal angle. In advanced stages, sclerosis and cystic changes were also
seen in the apposing surfaces.[[23],[25]] In normal individuals, a gap is seen between these bony surfaces even on weight-bearing
positions [[Figure 5]]. A note was also made of the associated presence of subfibular impingement, described
as direct contact between the tip of fibula and the lateral calcaneal process, with
bony changes in the apposing surfaces. This is said to occur with progression of the
talocalcaneal impingement.[[23],[25]]
Figure 5 (A and B): Normal appearance of the extra-articular talocalcaneal space on weight-bearing scans.
Note the normal gap between the apposing bony surfaces on the sagittal (A) as well
as coronal (B) reformatted thick slab images (arrows)
Results
The lateral talar i.e., first metatarsal angle was more than 4° [[Figure 6]], in all but one foot. In this one foot, however, the calcaneal angle was significantly
reduced [[Figure 7]] and was thus consistent with flatfoot.
Figure 6: Sagittal reformatted thick slab CBCT showing increased lateral talar-1st metatarsal angle (There is plantarward angulation of lateral talar axis [solid arrow]
with respect to the 1st metatarsal axis [dashed arrow], with the angle between them measuring 13.47°)
Figure 7: Sagittal reformatted thick slab CBCT showing reduced calcaneal pitch
The calcaneal angle was also less than 17° in all but one foot, where also the angle
was only borderline increased and measured 17.6°.
Forefoot abduction was seen in 7 of the 19 feet [[Figure 8]]. Forefoot adduction was seen in only one foot, where the metatarsal axis was angled
medially to the AP talar axis, with the angle measuring 29°. The AP talar axis was
parallel to the first metatarsal axis in all the remaining 11 feet.
Figure 8: Axial reformatted CBCT, showing forefoot abduction, with lateral angulation of the
1st metatarsal axis [dashed arrow], with respect to the AP talar axis [solid arrow]
The hindfoot valgus angle was greater than 10° in all feet [[Figure 9]].
Figure 9: Coronal reformatted images with increased heel valgus angle (between the tibial axis
and the medial calcaneal cortex)
Extra-articular talocalcaneal impingement was seen in 13 of the 19 feet scanned. Among
these 13 feet, an MRI was also done in 5 feet and the MRI did not show any contact
between the talus and the calcaneum, at the calcaneal angle, in any of the 5 feet.
The MRI, however, showed cystic changes with edema and sclerosis of the apposing bony
surfaces. There was also edema of the sinus tarsi fat. The direct contact between
the bony surfaces was very well appreciated on the WBCT in all of these 5 cases [[Figures 10] and [11]].
Figure 10 (A and B): Extra-articular talocalcaneal impingement in patient with flatfoot (A) sagittal MRI
in neutral position do not show apposition between the bony surfaces with intervening
soft tissue seen on the MRI (arrow), (B) The sagittal reformatted images of the WBCT
clearly shows the direct contact between the bony surfaces and associated bony changes
as well (arrow)
Figure 11 (A and B): Extra-articular talocalcaneal impingement in patient with flat foot (A) coronal MR
images in neutral position do not show apposition between the bony surfaces with intervening
soft tissue seen on the MRI (arrow), (B) coronal reformatted images of the WBCT clearly
shows the direct contact between the bony surfaces and associated bony changes as
well (arrow)
Yet another patient [different from the above mentioned 5 patients], where an MRI
was done, also had an MRI appearance quite similar to that seen in the above-described
cases of talocalcaneal impingement. This patient had edema of the apposing surfaces
of the talus and the calcaneum at the calcaneal angle and edema of the sinus tarsi.
There were however no cystic changes or sclerosis in the bones. Interestingly, this
patient showed no narrowing of the distance between the bony surfaces or talocalcaneal
impingement on the WBCT [[Figures 12] and [13]]. Thus, in this patient the pattern on MRI was attributed to sinus tarsi syndrome,
rather than talocalcaneal impingement. Also this was the only case of pes planus with
normal Meary angle but with a severely reduced calcaneal pitch. Mild heel valgus was
seen but without any forefoot abduction.
Figure 12: Sinus tarsi syndrome in patient with the flatfoot: The MRI show ill-defined edematous
soft tissue in the sinus tarsi (arrow) with edema of the apposing bony surfaces
Figure 13 (A and B): Same patient as Figure 12: Sagittal MRI in neutral position (A) do not show apposition
between the bony surfaces with intervening edematous soft tissue seen similar to that
seen in cases with extra-articular talocalcaneal impingement (arrow), (B) The sagittal
reformatted images of the WBCT shows persistence of the distance between the bony
surfaces without any collapse or direct contact between the bony surfaces as seen
in the cases with extra-articular talocalcaneal impingement
Overt subfibular impingement with direct contact of the apposing fibular and calcaneal
surfaces and associated bony changes was seen in only 3 feet [[Figure 14]].
Figure 14 (A and B): Subfibular impingement in a patient with flat foot (A) coronal MRI in neutral position
show mildly reduced distance between the tip of the lateral malleolus and the lateral
calcaneal process. (arrow), (B) coronal reformatted images of the WBCT shows further
reduction of the distance with almost apposition of the bony surfaces (arrow)
Osteoarthritic changes were seen involving the intertarsal joints in 14 feet, of which,
10 feet had a predominant talonavicular osteoarthritis. Dorsal talar spurs [[Figure 15]] were seen in 9 of these 10 patients. Joint effusion and ganglion cyst [[Figure 16]] was also seen in few patients.
Figure 15 (A and B): Dorsal spur at a talonavicular joint in patient with flatfoot (A) sagittal reformatted
and (B) sagittal reformatted thick slab images of the WBCT, showing a prominent dorsal
spur at the talonavicular joint without any evidence of tarsal coalition. (arrows)
Figure 16 (A and B): Dorsal spur at tibionavicular joint with ganglion cyst in a patient with flatfoot
(A) sagittal reformatted WBCT image and (B) sagittal MRI showing a prominent dorsal
spur at the talonavicular joint (arrows) with a tuft of ganglion cyst (better appreciated
on the MRI) without any evidence tarsal coalition
Isolated atrophy of the abductor digiti mini muscle, often attributed to Baxter neuropathy,[[26]] was seen in 10 of the 19 patients.
Discussion
Flatfoot, predominantly characterized by medial longitudinal arch collapse, is also
closely associated with other mal-alignments about the ankle, like hindfoot valgus,
and forefoot abduction. The assessment of the extent of alteration in these relationships
is essential for proper surgical planning. Weight-bearing CT scan provides a very
close-to-accurate assessment of the relationships of the various bones in the orthostatic
position, thus enabling meticulous surgical planning and monitoring on follow-up as
well. It enhances our understanding of the complex three-dimensional deformities in
the foot, which is rather difficult with the conventional two-dimensional radiographic
images.[[12],[27]]
In all the patients scanned, the measurements on the WBCT were consistent with flatfoot.
Associated angle deviations consistent with forefoot abduction and heel valgus are
also conveniently assessed. The concomitant presence of these abnormalities was clearly
seen with forefoot abduction in 7 of 19 flatfeet and heel valgus seen in all patients.
The concomitant presence of and the close association of flatfoot with extra-articular
talocalcaneal impingement was also clearly depicted with extra-articular talocalcaneal
impingement seen in approximately 70% [13/19] of the feet scanned. The extra edge
of weight-bearing CBCT over MRI, in diagnosing this entity was also clearly emphasized
by the rather lack of overt impingement seen in the five cases of extra-articular
talocalcaneal impingement who also underwent an MRI. WBCT was thus helpful in differentiating
extra-articular talocalcaneal impingement from sinus tarsi syndrome, which are close
mimics both clinically as well as radiologically.
This was well depicted in one of the cases described above. Moreover, WBCT also helped
in picking up the impingement before the development of overt bony changes.
An interesting note was also made of predominant talonavicular osteoarthritis in 10
of 19 cases and of dorsal talar spurs in 9 of 10 cases with talonavicular osteoarthritis.
Seven of these 10 patients also had a presenting complain of pain over the dorsal
aspect of the talonavicular joint. This could be related to the altered biomechanics
of the foot due to the flatfoot deformity leading to unusual stress on the talonavicular
joint and resultant changes as described.
An association with selective atrophy of the abductor digiti minimi was also seen
with the atrophy of the muscle seen in more than 50% of the feet scanned.
Conclusion
Weight-bearing CT scan is a very useful and reproducible technique for evaluation
of flatfoot and associated complications. It is being increasingly used by the foot
and ankle surgeons in the western countries for assessment of patients with AAFD.[[5],[25],[28],[29],[30],[31],[32],[33]]
The superiority of this modality over the existing methods of assessing flatfoot is
evident by the fact that it not only overcomes the limitations of the radiographs
but also those of the conventional cross-sectional scanners. It overcomes the limitations
of the radiographs by providing multiplanar three-dimensional assessment of the foot
in the natural weight-bearing position. It is at the same time easily reproducible
and more accurate and consistent for the measurements around the foot. The definite
advantage over the conventional cross-sectional scanners is the weight-bearing capability.
