in Weight Bearing Computed Tomography

, Francois Lintz², Cesar de Cesar Netto³, Alexej Barg⁴, Arne Burssens⁵ and Scott Ellis⁶

(1)

Department for Foot and Ankle Surgery, Hospital Rummelsberg, Schwarzenbruck, Germany

(2)

Foot and Ankle Surgery Centre, Clinique de l’Union, Toulouse, France

(3)

Department of Orthopedics and Rehab, University of Iowa, Iowa City, IA, USA

(4)

University Orthopedic Center, University of Utah, Salt Lake City, UT, USA

(5)

Department of Orthopedics and Trauma, University Hospital of Ghent, Ghent, OVL, Belgium

(6)

Department of Orthopedic Surgery, Hospital for Special Surgery, New York, NY, USA

Keywords

Weight bearing CTWBCTMeasurementsAdult-acquired flatfoot deformityHallux valgusHindfoot alignmentDistal tibiofibular jointSyndesmosis

The literature on weight bearing computed tomography (WBCT) measurements in the assessment of complex foot and ankle deformities has been steadily growing during the last 4–5 years [1–4]. The main subjects of interest have been adult-acquired flatfoot deformity [5–12], hindfoot alignment [10, 13–20], alignment of the subtalar joint [5, 10, 11, 21–25], the distal tibiofibular (syndesmotic) joint [26–31], first ray hypermobility [32–34], hallux rigidus (HR) [35], and hallux valgus (HV) [33, 36–40]. A summary of some of the most important measurements reported in the literature is presented in this chapter, along with references to figures and photos in earlier chapters that depict examples of the measurements.

Adult-Acquired Flatfoot Measurements

Talus-first metatarsal angle [8, 41]: measured in the axial and sagittal planes, by intersection of the lines representing the axis of the first metatarsal and the axis of the talus (Figs. 2.1, 2.2, and 4.3a)
Talonavicular coverage angle [8, 42]: measured in the axial plane by the relationship between a line connecting, respectively, the medial and lateral edges of the articular surface of the talar head and a line connecting the medial and lateral edges of the articular surface of the navicular at the talonavicular joint (Fig. 15.2)
Forefoot arch angle [43]: angle between the floor and a line drawn from the inferior aspect of the medial cuneiform to the inferior aspect of the fifth metatarsal (Figs. 2.4 and 15.3)
Navicular to skin distance [43]: distance between the most inferior point of the navicular to the plantar skin, perpendicular to the floor (Fig. 2.4)
Navicular to floor distance [8]: distance between the most inferior point of the navicular to the floor (Fig. 2.4)
Medial cuneiform to skin distance [8]: distance between the most inferior point of the medial cuneiform to the plantar skin, perpendicular to the floor (Fig. 2.4)
Medial cuneiform to floor distance [8]: distance between the most inferior point of the navicular to the floor (Fig. 2.4)
Cuboid to skin distance [44]: distance between the most inferior point of the cuboid to the plantar skin, perpendicular to the floor (Fig. 2.4)
Cuboid to floor distance [9]: distance between the most inferior point of the cuboid to the floor (Fig. 2.4)
Calcaneofibular distance [6, 44]: measured in the coronal plane by shortest distance between the distal aspect of the fibula and the lateral-superior surface of the calcaneus (Fig. 2.4)
Calcaneal inclination angle [8, 45]: measured in the sagittal plane by the intersection of the floor line and a line connecting the most inferior point of the calcaneal tuberosity and the bottom surface of the anterior process of the calcaneus (Figs. 2.2, 2.4 and 4.3b)
Medial cuneiform-first metatarsal angle [9]: measured in the sagittal plane by the intersection of the axes of the first metatarsal and medial cuneiform (Fig. 2.4).
Navicular-medial cuneiform angle [44]: also measured in the sagittal plane, by the angulation between the lines of the axes of the navicular and medial cuneiform (Fig. 2.4)

Hindfoot Alignment Measurements

Foot and ankle offset [14, 18, 19]: semiautomatic three-dimensional (3D) offset measurement of the torque acting in the ankle joint as result of body weight and ground reaction forces. It takes into consideration the relationship between the center of gravity of the tripod of the foot and the center of the ankle joint, represented by the apex of the talar dome. Negative measurements indicate that the center of the ankle joint lies laterally to the bisecting line of the foot tripod (varus alignment). Positive values implicate that center of the ankle joint is positioned medially to the bisecting line of the foot tripod (valgus alignment). The 3D coordinates for calculation of FAO are harvested using the multiplanar reconstruction (MPR) images. The first point marked is the most distal voxel of the first metatarsal head, followed by the most distal voxel of the fifth metatarsal head and next the most distal voxel of the calcaneal tuberosity. Finally, the most central and proximal aspect of the talar dome was marked, and the automatic calculation of the FAO is given by a dedicated software (TALAS™, CubeVue™, CurveBeam, LLC, Warrington, PA, USA) (Figs. 5.2b, 5.4a, 5.4c, and 8.1).
Hindfoot alignment angle (CT, Novel) [13]: measured in the coronal plane, considering the inclination of the tibia (anatomical axis) as well as the inclination of the talus and calcaneum (talocalcaneal angle) (Figs. 2.1, 2.3, 9.1, 9.2, 9.3, 9.4, 10.1, 10.2, 10.3, 10.4, and 10.5). Represented by the sum of talocalcaneal angle with the angle of the anatomical axis of the tibia.
WBCT clinical hindfoot alignment angle [10]: obtained using 3D reconstruction images where the windowing is set to maintain the surface anatomical landmarks, including the skin. Represented by the angle formed between a line bisecting of the Achilles tendon and a line bisecting the calcaneal tuberosity/heel (Fig. 9.1).
Achilles tendon/calcaneal tuberosity angle [10]: obtained using 3D reconstruction images where the windowing is set to remove the skin and subcutaneous tissue but maintaining the overlying soft tissue structures including the Achilles tendon, also allowing a better evaluation of the calcaneal tuberosity. The angle is measured between the lines of the longitudinal axis of the Achilles tendon and the longitudinal axis of the calcaneal tuberosity.
Tibial axis/calcaneal tuberosity angle [10]: obtained using 3D reconstruction images maintaining only the bony anatomy. Angle was formed by the intersection of axes of calcaneal tuberosity and the tibia.
Tibial axis/subtalar joint angle [10]: obtained using 3D reconstruction images with the windowing set to remove all the soft tissue structures, maintaining only the bony anatomy. Formed by the intersection of tibial axis and the line connecting midpoint of the posterior facet of the subtalar joint and most inferior point of the calcaneal tuberosity.
Hindfoot alignment angle (HAA) [10, 46]: also obtained using 3D reconstruction images with the windowing maintaining only the bony anatomy. Defined as the angle between the tibial axis and calcaneal axis lines. The calcaneal axis is determined by the bisecting line of the angle formed by two lines representing the lateral and medial osseous contours of the calcaneus. The line for the lateral osseous contour is drawn between the most lateral aspect of the lateral process on the calcaneal tuberosity and the most superior and lateral discernable aspect of the calcaneus. The line for the medial osseous contour is drawn from the most medial aspect of the medial process of the calcaneal tuberosity to the most inferomedial discernable aspect of the sustentaculum tali.
Hindfoot moment arm [10, 47]: obtained using 3D reconstruction images maintaining only the bony anatomy. Defined as the distance connecting the most inferior aspect of the calcaneus to the tibial axis line.

Subtalar Joint Alignment Measurements

Subtalar horizontal angle or inferior talus-horizontal angle (inftal-hor) [5, 8, 24, 25]: measured in the coronal plane, angle between the articular surface of posterior facet of the talus and the floor, measured at three different points in the longitudinal length of the articular facet, 25% (posterior aspect), 50% (midpoint), and 75% (anterior aspect) (Fig. 15.4)
Subtalar vertical angle [23]: measured in the coronal plane, angle between the articular surface of posterior facet of the talus and a vertical line to the floor
Inferior talus-superior talus angle (inftal-suptal) [24, 25]: measured in the coronal plane by the angulation between the inferior and superior aspects of the talus, inferiorly at the posterior facet of the subtalar joint
Inferior talus-superior calcaneal angle (inftal-supcal): measured in the coronal plane by the angulation between the inferior aspect of the talar and superior aspect of the calcaneal articular surfaces at the posterior facet of the subtalar joint
Subtalar joint subluxation at the posterior facet [43]: measured in the coronal plane at the level of the most posterior aspect of the fibula. Represented by the distance from the lateral margin of the calcaneal articular surface to the lateral margin of the talar articular surface, at the posterior facet of the subtalar joint
Percentage of uncoverage of the middle facet of the subtalar joint [48]: measured in the coronal plane, at the midpoint of the anterior-to-posterior dimension of the talar middle facet, by the percentage of the talar articular surface of middle facet that is not opposed by the calcaneal articular surface
Incongruence angle of the middle facet of the subtalar joint [48]: measured in the coronal plane, at the midpoint of the anterior-to-posterior dimension of the talar middle facet. Represented by the angulation between the articular surfaces of the talar and calcaneal middle facets of the subtalar joint

Distal Tibiofibular Syndesmotic Measurements

Measurements are usually performed in the axial plane, at two different levels (10 mm above the tibial plafond and/or 2.5 mm below the talar dome) [27, 31]:

Anterior tibiofibular distance [31, 49, 50]: shortest distance between the tip of the anterior tibial tubercle and the nearest point on the anterior border of the fibula (Figs. 12.1, 12.2, and 12.5).
Posterior tibiofibular distance [31, 49, 50]: obtained by measuring the distance between the tip of the posterior tibial tubercle and the nearest point on the medial border of the fibula (Figs. 12.1, 12.2, and 12.5).
Tibiofibular clear space [31]: the tibial incisura length line is first marked by a line connecting the tips of the anterior and posterior tibial tubercles. Two parallel lines to the line of the incisura are then marked, one tangential to the deepest point of the incisura and the other tangential to the medial border of the fibula. The distance between these two lines represents the tibiofibular clear space (Figs. 12.1, 12.2, and 12.5).
Syndesmotic diastasis [31, 51]: central points of the tibia and fibula are marked by determining the midpoints of the lines connecting the anterior and posterior borders of each bone. Then, a line is marked connecting the midpoints of both bones. Diastasis measurement is represented by the distance between the medial cortex of the fibula and the lateral cortex of the tibia along the marked line (Figs. 12.1, 12.2, and 12.5).
Angular measurement of syndesmotic diastasis [31, 52]: angle created between two tangential lines over the bony surface of the anterior and posterior aspects of the distal tibiofibular joint (Figs. 12.1, 12.2, and 12.5).
DELTAFIB measurement [31, 53]: the tibial incisura length line is first marked by a line connecting the tips of the anterior and posterior tibial tubercles. The DELTAFIB measurement represents the angle between the incisura line and a line connecting the anterior and posterior tubercles of the fibula (Figs. 12.1, 12.2, and 12.5).
Zwipp rotation [31, 54]: angle formed by the connection of a line across the anterior and posterior tubercles of the fibula and a line tangential line to the most anterior point of the tibia.
Tang rotation [31, 55]: multiple bisecting lines are marked over tibial surface to determine the summation of these lines as the center of the tibia. Following that, the distances between this central point of the tibia and the most anterior and posterior points of the fibula are measured. The Tang rotation is represented by the ratio between the anterior and posterior distances.
Phisitkul translation [31, 56]: the tibial incisura length line is first marked by a line connecting the tips of the anterior and posterior tibial tubercles. A perpendicular line to the incisura line is marked at the level of the anterior tibial tubercle. The shortest distance from this perpendicular line to the most anterior point of the distal fibula represents the Phisitkul translation.
Davidovitch translation [31, 57]: first, a line connecting the widest anteroposterior dimension of fibula is marked. Then, a perpendicular to this line is marked at most anterior point of the distal fibula. A parallel line to this perpendicular line is marked at the level of the anterior tibial tubercle. The distance between these two parallel lines represents the Davidovitch translation.
Lateral clear space [31, 58]: measured at a level 2.5 mm below the talar dome by the shortest distance between the distal fibula and the talus (Figs. 12.1, 12.2, and 12.5).
Medial clear space [31, 58]: also measured at a level 2.5 mm below the talar dome, represented by the shortest distance between the medial malleolus and talus (Figs. 12.1, 12.2, and 12.5).