TALAS™ (Torque Ankle Lever Arm System)
CubeVue’s newest feature, TALAS™, is a new hindfoot alignment measurement for weight bearing CT datasets.
A precise pre-operative measurement of hindfoot misalignment is paramount to surgical planning and post-operative follow up. Weight bearing CT imaging is evolving our understanding of how to evaluate for alignment because it overcomes numerous measurement errors encountered by standard posterior weight bearing radiographs (1).
TALAS™ (Patent No. 10/646137) is a software algorithm developed with the help of leading foot & ankle surgeons that can be used to automatically calculate hindfoot alignment based on reproducible three-dimensional anatomical landmarks. TALAS™ introduces the concept of 3D-specific biometric measurements, as opposed to 2D biometrics (ex. tibia-calcaneus angle), which are subject to projection and rotation error.
Using the TALAS™ tool in CubeVue, the user selects four anatomical landmarks: the most plantar weight bearing point of the first metatarsal (M1), the fifth metatarsal (M5), the calcaneal weight bearing area (C) and the center-most point of the ankle joint (D). The landmarks are recorded as coordinates in a three-dimensional volume. TALAS uses a mathematical algorithm as defined by Dr. Francois Lintz et al. (2) to calculate:
Foot Ankle Offset (FAO)
The FAO represents the distance between where the center of the ankle joint projects on the ground and where the center of the gravity of the foot lies. FAO is given as a percentage of foot length rather than a distance, so that the results may be compared regardless of foot size. According to the initial data, the norm for this value is 2-3%, which corresponds to a slight valgus. This is in accordance with data gathered from 2D biometric studies.
Calcaneal Offset (CO)
Calcaneal offset is defined as the distance between a theoretically neutral position of the calcaneus (where FAO = 0%) and the actual position of the calcaneus (C’). The calcaneal offset is presented as a distance measurement (mm).
Hindfoot Angle (HA)
The hindfoot angle is included as a reference measurement for surgeons who may not be accustomed to working with offset calculations. The apex of the center of the talar dome projected on the ground plane (D) forms the vertex, and the ideal position of the calcaneus (C) an the actual position of the calcaneus (C’) form the endpoints of this angle.
Hindfoot alignment is presented as a value on a scale in which the right-most value is -30 % and the left-most value is 30 %. A hindfoot with no varus or valgus will scale in the center, at 0%. A hindfoot in varus will scale between 0 and -30%. The position of the presentation reflects, in %, the degree of varus. Conversely, a hindfoot in valgus will scale between 0 and 30 %.
(1) “Measuring hindfoot alignment in weight bearing CT: A novel clinical relevant measurement method”. Burssens, A. et al. Foot and Ankle Surgery, Volume 22, Issue 2, Supplement 1, Page 24
(2) “Ground Reaction Force Calcaneal Offset: A new measurement of hindfoot alignment”. Lintz, F. et al. Foot and Ankle Surgery, Volume 18, Issue 1. Foot and Ankle Surgery, Volume 0, Issue 0.
(3) “The hindfoot alignment view”. Saltzman C. et al. Foot Ankle Int. 1995 Sep; 16(9):572-6.
Smart M1-M2 Angle
Intermetatarsal angle (IMA)
The intermetatarsal angle (IMA) is a radiographic measurement of the long axis of the 1st and 2nd metatarsals. Identification and measurement of the IMA represents a near universal tenet of determining hallux valgus (HV) deformity severity. When an increased IMA is present, surgical intervention can be indicated. Comparison of pre-operative and post-operative radiographic analysis of IMA is often the standard used to determine if HVA deformity was adequately corrected.
IMA is typically measured on two-dimensional plain radiographs. Conventional computed tomography may be utilized for complex cases.
Traditional IMA methodology
Reference points are placed on the proximal and distal midmetaphyseal regions of the 1st and 2nd metatarsals. The axis of the first metatarsal is defined as the axis of the first metatarsal head as a line connecting the articular surface of the metatarsal head and the center of the proximal articulation.
Axes are drawn through these reference points on the first and second metatarsals. The angle created by the intersection of these axes is the IMA. Normal is less than 9 degrees, mild deformity 9 – 11 degrees, moderate deformity 12 – 16 degrees, and severe deformity is greater than 16 degrees. The consistency of IMA measurement has been found to be within 5 percent or less in 96.7 percent of cases.
Limitations of Traditional Imaging Techniques
HV is a triplanar deformity comprised of valgus deformity of the proximal phalanx, a pronation deformity of the first metatarsal, and a lateral displacement of the sesamoid apparatus. The standard imaging of HV with two-dimensional plain radiographs provides limited information because of the rotational, three-dimensional nature of the deformity. Conventional computed tomography can provide the surgeon with more information in complex cases but is not weight bearing.
Advantages of M1-M2 methodology
Smart M1-M2 is an automatic process that does not require user calculation. The auto-measurements are typically completed in less than 20 seconds.
Smart M1-M2 can account for metatarsal position and length in the Z-plane, eliminating projection error that is characteristic of plain radiographic imaging.
Smart M1-M2 can account for the rotational status of the first metatarsal bone.