The three-dimensional movement of joints is the result of a very complex interaction between physics (gravitation, acceleration, inertia), active muscle forces and passive constraints (joint surfaces, ligaments, menisci). [1, 2] The specific combination of these parameters for the different joints still remains to be determined. Mathematical models try to replicate the true situation, based on anatomical studies [3, 4], a realistic picture is still not available. The traditional movement analysis is based on the tracking of surface markers attached to the object of interest. Most of the time the markers are attached to the skin and are thought to represent the underlying skeletal anatomy. With the help of high-frequency videography the target of the markers can easily be captured and using dedicated software the 2D or even 3D position of the markers in space can be calculated automatically. With all skin based markers, skin movement artifacts are a problem. Skin movement over the underlying skeleton results in a mismatch between the marker position and the true position of the bony protuberance defined by the marker. These errors can be in the range of centimeters [4] and are more pronounced when the animal is moving fast or when there are acceleration or deceleration forces, such as at time of heel strike or lift of.[5, 6]

For the study of stifle kinematics in the light of a torn cranial cruciate ligament, the pathological movements, which is predominately cranio-caudal instability, has to be expected in the range of 10 to 15 mm. With a skin based error of about 1 to 2 centimeters in that region, it becomes clear that skin based markers systems may fail at detecting these subtle movements.[7] Eventhough it is possible to compensate some of the skin movement errors [8, 9], the resulting error especially for abduction/adduction as well as endo- and exorotation still remains significant.[10] One solution would be intraosseous fixation of K-wires, with the markers on their tip.[11, 12] This would allow rigid fixation oft he markers to the underlying bone. However, because of the invasiveness of the method, broad application is not possible. With the installation of a biplanar fluroscopic high speed unit at the Veterinary Faculty of Leipzig, precise estimation of 2D and 3D kinamtics will be possible. Fluoroscopic cinematography allows a precision of <1 mm und <1°.[13] The inconvenient aspect of using radiation can be limited by thorough application of all measure of safety. Our project is special in that respect, that we aim at investigating small (rat, cat, mouse) and large objects (horses, cows) using the same equipment. This is why a dedicated prototype is currently in construction, allowing for the most versatile application of fluoroscopic cinematography possible.


Literature
1.Markolf, K.L., et al., The role of joint load in knee stability. J Bone Joint Surg Am, 1981. 63(4): p. 570-85.
 
2.Schipplein, O.D. and T.P. Andriacchi, Interaction between active and passive knee stabilizers during level walking. J Orthop Res, 1991. 9(1): p. 113-9.
 
3.Shahar, R. and L. Banks-Sills, A quasi-static three-dimensional, mathematical, three-body segment model of the canine knee. J Biomech, 2004. 37(12): p. 1849-59.
 
4.Saeglitz, J., Morphologische Grundlagen für ein Forward-Dynamik-Modell der Schultergliedmaße des Deutschen Schäferhundes und invers dynamische Untersuchungen zu den gelenkresultierenden Kräften der großen Gliedmaßengelenke, in Institut für Tieranatomie I. 2003, LMU-München: München.

5.Reinschmidt, C., et al., Effect of skin movement on the analysis of skeletal knee joint motion during running. J Biomech, 1997. 30(7): p. 729-32.
6.Benoit, D., et al., Effect of skin movement artifact on knee kinematics during gait and cutting motions measured in vivo. Gait Posture, 2006. 24(2): p. 152-64.
 
7.Tashman, S. and W. Anderst, In-vivo measurement of dynamic joint motion using high speed biplane radiography and CT: application to canine ACL deficiency. J Biomech Eng, 2003. 125(2): p. 238-45.
 
8.Lucchetti, L., et al., Skin movement artefact assessment and compensation in the estimation of knee-joint kinematics. J Biomech, 1998. 31(11): p. 977-84.
9.van den Bogert, A., P. van Weeren, and H. Schamhardt, Correction for skin displacement errors in movement analysis of the horse. J Biomech, 1990. 2(31): p. 97-101.
 
10.Stagni, R., et al., Quantification of soft tissue artefact in motion analysis by combining 3D fluoroscopy and stereophotogrammetry: a study on two subjects. Clin Biomech (Bristol, Avon), 2005. 20(3): p. 320-9.
 
11.Korvick, D.L., G.J. Pijanowski, and D.J. Schaeffer, Three-dimensional kinematics of the intact and cranial cruciate ligament-deficient stifle of dogs. J Biomech, 1994. 27(1): p. 77-87.
 
12.Lafortune, M., et al., Three-dimensional kinematics of the human knee during walking. J Biomech, 1992. 25(4): p. 347-57.
 
13.Brainerd, E.L., et al., X-ray reconstruction of moving morphology (XROMM): precision, accuracy and applications in comparative biomechanics research. J Exp Zool A Ecol Genet Physiol, 2010. 313(5): p. 262-79.