Morphometric Variables

Measurements were performed by the primary investigator (SCK) on CT images. Static and dynamic parameters were measured for the IVD spaces C4–C5, C5–C6, and C6–C7. Static variables were defined as measurements that were not changed by vertebral motion, whereas dynamic variables were defined as measurements that changed throughout vertebral motion.

Static parameters included the width and height of the cranial and caudal VEPs (Figure 2) and an estimation of the VEP surface. References to a cranial or caudal VEP were made with respect to the cranial and caudal aspects of a particular vertebra and not in relation to the IVD space (e.g., the cranial VEP for the C4–C5 space was the cranial VEP of the vertebral body of C5, and the caudal VEP was the caudal VEP of the vertebral body of C4).

Sagittal (A,B) and dorsal (C,D) CT images of the segment C5–C6 in a cat (A,C) and a dog (B,D). Cranial is to the left (A,B) and to the top (C,D). Images originate from the midsagittal (C,D) and middorsal planes (A,B). Measurement of height (1 and 2) and width (3 and 4) of the caudal (1 and 3) and cranial (2 and 4) vertebral endplates and definition of the intervertebral disc (IVD) wedge angle (*) are shown as well.

VEP height was defined as the maximum distance from the dorsal to the ventral tip of the VEP as measured in the midsagittal plane (line c in Figure 3A) for both the cranial and caudal VEPs.

Dorsal (A) and sagittal (B) CT images of a feline vertebral specimen. (A) The image is at the level of the dorsal tip of the caudal vertebral endplate. Lines a through e are equidistant and parallel and divide the intervertebral disc (IVD) space into five sagittal planes. (B) Sagittal view corresponding to line c (midline on the dorsal view). Five lines (labeled 1 through 5 from dorsal to ventral) were drawn equidistant and parallel to each other. Measurement of the IVD width was determined for these lines.

VEP width was defined as the left-to-right margin of the VEP in the middorsal plane (line 3 in Figure 3B).

VEP height and width were used to approximate VEP surface area, assuming the VEP to have the form of a rectangle. Size was calculated by multiplying height and width. To compare VEP size between the two species, the body weight of the animal was normalized against the VEP surface (body weight ^* 9.81/VEP surface). The factor 9.81 was used to convert body weight (kg) into N by the standard gravity. The N/mm² value however must not be misinterpreted as a pressure. The ratio was calculated simply to be able to compare the area in specimens of different body weights. Ratios for medium-sized dogs were similarly calculated from previously unpublished data from a previous study (20).

Dynamic parameters included the IVD space width and IVD wedge angle measured in neutral position as well as flexion, extension, and lateral bending. Each of the landmarks was set manually for each CT reconstruction:

The IVD wedge angle was defined as the angle between two lines that intersected the cranial and caudal VEPs (20). These two lines were defined in the midsagittal plane of the IVD space by the use of the reconstructed CT images. Bony protrusions surrounding the region of the nucleus pulposus of both cranial and caudal VEPs were identified. A line connecting these landmarks was drawn for each VEP. The angle between these two lines was defined as the IVD wedge angle (Figure 2). A positive value indicated that the angle opened ventrally, whereas a negative value indicated that the angle opened dorsally. When these lines were parallel, the angle was recorded as 0°.

IVD space width was defined as the distance between the cranial and caudal VEPs relying on a grid of 25 landmarks on the VEP as reported previously (20): to create this grid, the first step was to define a dorsal plane at the dorsal tip of the VEP of each segment in the multiplanar CT reconstruction. In this dorsal plane, five equidistant lines were created from the cranial to caudal VEPs parallel to the long axis of the vertebrae from the right lateral to left lateral border of the caudal VEP (lines were labeled a through e from left to right to define five sagittal planes; Figure 3). Each sagittal plane image was exported in DICOM format to enable the measurements using a free DICOM-viewer (see Footnote 3). The IVD space width was measured at five locations and defined separately for each of the five sagittal planes. Line 1 was located between the craniodorsal tip of the cranial VEP and the caudodorsal tip of the caudal VEP. Similarly, line 5 was initiated by drawing a line from the most cranioventral tip of the cranial VEP and the caudoventral tip of the caudal VEP (Figure 3). The additional three lines were added between lines 1 and 5 such that the lines were equidistant. These five lines were labeled from 1 to 5 (dorsal to ventral). By use of lines 1 through 5, IVD width was measured at five dorsoventral levels for all sagittal slices (planes a through e), which resulted in a map of the IVD width at 25 locations. However, because the height of the VEP differed among the sagittal planes (e.g., the VEP height at sagittal plane a was smaller than the VEP height at sagittal plane c), the 25 points on the IVD map were not evenly distributed.

The range of motion (ROM) was calculated as the difference between the IVD wedge angle in flexion and extension.