Range of Motion (RoM)

The range of motion (RoM) is defined to be the change in angle of a motion segment (e.g. two vertebrae) during movement. The range of motion is either captured between the two maximum positions (flexion and extension) or between one maximum position and neutral standing position.

Center of Rotation (CoR)

The center of rotation (CoR) is a point in a two-dimensional plane about which the superior vertebra rotates with respect to the inferior vertebra.
It is known that the location of the center of rotation in a spinal segment shifts slightly during motion. While increasing the number of intermediate images can capture this shift, the precise evaluation of the CoR becomes difficult for incremental angular changes below 2°. For this reason, we typically analyze the mean center of rotation (mCoR) between flexion and extension, or between flexion, neutral and extension, which results in a flexion center of rotation (fCoR) and one for extension (eCoR). If no further specification is given, CoR refers always to the mCoR.
The calculation of the CoR is conducted according to the method described first by the German engineer Franz Reuleaux.

Translation - Landmarks

Several landmarks can be freely placed in each registered object. Translation data are derived from these landmarks and expressed in two directions (X- and Y-). The landmarks are usually defined in the unloaded image and automatically transformed into the following motion image. Due to the already known transformation matrix between the objects of interest, the landmarks can be evaluated and displayed without any errors in the different images.

Translation - Vertebral Body

A landmark is placed at the inferior posterior corner of the superior vertebra. Translation of this landmark is read out in the anteroposterior direction with respect to the upper endplate of the inferior vertebra. This parameter is an indication for spinal slip according to the Meyerding classification system.

Translation - Implant

In some cases it is of interest to determine the relative motion, displacement or deformation of an implant. For this, landmarks are assigned to the implant and subsequently referred to a desired structure as a reference. This feature supports e.g. the determination of the motion of a prosthesis, the displacement of an elastic implant or the relative motion between the bone-implant-interface.

Lordosis Angle

Utilizing a special tool developed within the FXA software, it is possible to semi-automatically detect the endplates of vertebral bodies without the need for placing landmarks. The different orientations of the endplates form an angle, which is expressed as the lordosis angle.
The lordosis angle can be measured for different postures, changes due to motion and for single or multiple segments.

Intervertebral Disc Height

The intervertebral disc height can be measured at three locations: anterior, posterior and in the middle of the disc. For this, the Euclidian distances are computed between landmarks. Additionally it is possible to measure the change in disc height, which might be caused by spinal movements or progressing degeneration.