Introduction of a bone-centered three-dimensional coordinate system enables computed tomographic canine femoral angle measurements independent of positioning

Abstract

IntroductionMeasurement of torsional deformities and varus alignment in the canine femur is clinically and surgically important but difficult. Computed tomography (CT) generates true three-dimensional (3D) information and is used to overcome the limitations of radiography. The 3D CT images can be rotated freely, but the final view for angle measurements remains a subjective variable decision, especially in severe and complex angular and torsional deformities. The aim of this study was the development of a technique to measure femoral angles in a truly three-dimensional way, independent of femoral positioning.MethodsTo be able to set reference points in any image and at arbitrary positions of the CT series, the 3D coordinates of the reference points were used for mathematical calculation of the angle measurements using the 3D medical imaging Software VoXim®. Anatomical reference points were described in multiplanar reconstructions and volume rendering CT. A 3D bone-centered coordinate system was introduced and aligned with the anatomical planes of the femur. For torsion angle measurements, the transverse projection plane was mathematically defined by orthogonality to the longitudinal diaphyseal axis. For varus angle measurements, the dorsal plane was defined by a femoral retrocondylar axis. Independence positioning was tested by comparison of angle measurement results in repeated scans of 13 femur bones in different parallel and two double oblique (15/45°) positions in the gantry. Femoralvarus (or valgus), neck version (torsion), and inclination angles were measured, each in two variations.ResultsResulting mean differences ranged between –0.9° and 1.3° for all six determined types of angles and in a difference of <1° for 17 out of 18 comparisons by subtraction of the mean angles between different positions, with one outlier of 1.3°. Intra- and inter-observer agreements determined by repeated measurements resulted in coefficients of variation for repeated measurements between 0.2 and 13.5%.DiscussionThe introduction of a bone-centered 3D coordinate system and mathematical definition of projection planes enabled 3D CT measurements of canine femoral varus and neck version and inclination angles. Agreement between angular measurements results of bones scanned in different positions on the CT table demonstrated that the technique is independent of femoral positioning.

Computed tomographic angular measurements using a bone-centered three-dimensional coordinate system are accurate in a femoral torsional deformity model and precise in clinical canine patients

“…The software was designed for 3D image-guided surgery, had medical device approval, and was used in prior research. MPR, VR, bone segmentation, and a 3D coordinate system were the main features used for angular measurements as previously described for normal canine femurs ( 58 ).…”

“…When setting the measurement points in the CT images, the result of the measurement could not be seen. Femoral torsion angles were calculated using a three-dimensional method with a bone-centered 3D coordinate system and projection of the femoral neck and retrocondylar axis into a plane transverse to the femoral longitudinal axis as described earlier ( 58 ). The femoral neck axis was defined by two reference points.…”

“…The femoral head center was the midpoint of a 3D semiautomatic femoral head fitting sphere along its bearing surface. The femoral neck base center was set directly using a circular crossline tool at the midpoint of the femoral metaphysis at the level of the lesser trochanter ( 58 ). The retrocondylar axis was determined by the lateral and the medial femoral condyle center at the most caudal aspect of the subchondral bone surface ( 58 ).…”

“…The femoral neck base center was set directly using a circular crossline tool at the midpoint of the femoral metaphysis at the level of the lesser trochanter ( 58 ). The retrocondylar axis was determined by the lateral and the medial femoral condyle center at the most caudal aspect of the subchondral bone surface ( 58 ). The software measured the torsion angles in the transverse plane using 3D coordinates of the anatomical reference points.…”

“…Angular measurement techniques heavily rely on standardized positioning, projection, or perspective that can be easily achieved in normal bones or normal cadavers but can be difficult in animals that suffer from an orthopedic disease with deformed bones or restricted articular ranges of motion. Many radiographic and computed tomographic techniques were established in normal individual bones or normal cadavers only ( 2 , 25 – 27 , 30 , 32 – 34 , 39 , 40 , 42 , 44 , 52 – 58 ). Accuracy and precision of measurements in normal cadaver bones do not necessarily prove the validity of a measurement technique in deformed bones and in patients in a clinical setting.…”