**3. Elbow joint kinematics**

### **3.1 Kinematic analysis**

Kinematics describe the motion of body segments without measuring the forces acting onto that segments. Kinematic analysis allows evaluation of the range of motion, angular velocities, segmental velocities of each portion of the limb, stride frequency and stride length [17]. Depending on the technique used for the kinematic analysis, motion of bones and joints can be measured with a submillimeter accuracy [18–20].

Generally two forms of kinematic analysis can be differentiated: the videokinematography, based on a video motion capture system, and the radiostereometric kinematic analysis (RSA), based on a radiographic system, coupled with high speed video cameras. Video motion capture kinematic systems use skin markers, attached to specific body areas, which are tracked in the generated videos of the moving animal and allow for calculation of the aforementioned parameters. Radiostereometric analysis can be marker based or performed without bone markers [21–30]. Furthermore, both kinematic analysis systems can be used to evaluate motion in the two or three dimensional (2D, 3D) space, depending on the technical setup [17].

The most commonly used technique is a video motion capture system based analysis. This technique is non-invasive and allows for evaluation of overall limb, limb segment or body segment motion. However, skin mounted markers do not match exactly the movement of the underlying bones. Movement of the soft tissues results in skin motion artifacts [21, 28, 31–35], with a difference of 0.4 to 1.2 cm between the skin marker and respective underlying bony landmark in small animals [33]. Especially in the proximal joints of the forelimb skin marker based data differ significantly from fluoroscopically gained kinematic data [28]. Comparison of biplanar fluoroscopy and video-kinematography in hindlimb kinematics revealed significant differences between both techniques, too [21]. Skin marker based data tend to project different trajectories and smaller amplitudes compared to fluoroscopic kinematography with particularly contradictory results, especially in proximal joints, where increased soft tissues can be found [21].

Radiostereometric analysis, also called fluoroscopic kinematography, allows for the most accurate kinematic data acquisition [19, 21–24, 28, 30]. One or two fluoroscopic units, coupled with high speed video cameras, take x-ray movies of the moving object. Based on these x-ray movies bone movement can be calculated and transferred onto 3D bone models generated from CT scans of the individual animal. Bone motion analysis can be performed using implanted bone markers, which are tracked in one (uniplanar, 2D evaluation) or both (biplanar, 3D evaluation) x-ray movies and 3D coordinates of each marker are then transferred onto the 3D bone models. Alternatively, scientific rotoscoping or autoscoping techniques can be used to track bone movement and transfer this in vivo bone motion from the fluoroscopic images onto 3D bone models [18, 20, 36]. These techniques do not rely on bone markers, rather the shape and edges of each bone are used to project digitally reconstructed radiographs (DRR), generated from the CT scans of each bone, onto the respective bone in the fluoroscopic image. By that the 3D bone model is aligned and animated along the x-ray movies. Scientific rotoscoping is performed manually, while autoscoping is a completely computerized process. Both techniques can be described as morphology based methods of motion analysis. Marker based tacking is the gold standard of kinematic analysis with an accuracy of 0.1 mm and 0.1 degrees [20].

However, scientific rotoscoping and autoscoping show a high accuracy as well, with values ranging from 0.16 to 0.66 mm in translation and 0.43 to 2.78 degrees rotation for scientific rotoscoping and 0.07 to 1.13 mm translation and 0.01 to 3.0 degrees rotation for autoscoping [18, 37–42]. Therefore, both techniques result in a highly precise evaluation of bone and joint motion with a substantially reduced invasiveness compared to a bone marker based analysis.

Multiple studies have investigated elbow joint kinematics in healthy dogs and dogs with different joint pathologies. Results have to be interpreted cautiously due to varying breeds, different technical setups and varying gaits and gait velocities, e.g. the walk or the trot, all of which influencing the kinematic pattern. **Table 1** gives an overview of previous studies on canine forelimb and elbow joint kinematics.

