Knee Alignment Device
Sold as a pair of devices, the Knee Alignment Device (KAD) is used during static trials to independently define the alignment of the knee flexion/extension axis of the subjects knee prior to a series of walking trials. The use of the KAD based knee model enables the subject marker set to eliminate the medial knee markers which are prone to being knocked off during testing when the subject knees cross or pass closely during swing. This is primarily a problem during clinical gait trials.
The KAD is a spring-loaded metal jig that fits gently over the subjects knee while a static calibration data collection is performed. The design of the KAD is such that the absolute distance between all three of the markers on the device is identical (5.66 inches). The design of the Motion Lab Systems KAD enables the software to establish a virtual knee marker at the central joint of the Knee Alignment Device and this, combined with the KAD wand marker, enables the knee flexion axis to be measured.
The Knee Alignment Device is fully supported by the VICON Clinical Manager, Vicon Plug-in-Gait, Peak Motus and the C-Motion Visual-3D biomechanical modeling software as well as software packages by other 3D Motion Capture System manufacturers. Maintenance and spare parts for the KAD are available from Motion Lab Systems on request.
Operation
When a supported software application finds marker labels for a Knee Alignment Device defined in the 3D data file, it uses the orientation of the three markers on the Knee Alignment Device to establish the knee flexion axis. After the knee flexion axis has been established, the software calculates the relative transverse alignment of this axis, to the transverse plane orientation of the thigh and shank, as calculated using the mid-thigh and mid-shank stick markers. These relative alignments are stored as Thigh and Shank Rotations and are applied to all dynamic data within the session.
Thus the correct alignment of the thigh and shank wands becomes less critical as any minor alignment errors are measured during the static trial and can be automatically removed during the processing of the dynamic trials. Please refer directly to the software User Guide or contact the application manufacturer for information on the kinematic calculations that use the KAD data.
The design of the KAD uses a 24.8mm (1 inch) retro reflective marker - this marker diameter is part of the mechanical design of the KAD. However, many 3D motion capture systems use different marker diameters and it is common for users to replace the KAD markers. This has no effect of the functioning of the KAD device except that some software packages assume that a single marker size is used throughout the kinematic model. This can lead to problems if the Knee Joint Center calculations do not take into account the fixed 12.4mm offset that is part of the mechanics KAD design at the intersection of the three KAD marker rods. Failure to properly account for this offset may introduce a lateral offset in the Knee Joint Center calculation that is proportional to the different between the KAD marker size and the size of the actual markers used. Contact your software manufacturer if you suspect consistent errors in your Knee Joint Center calculations when using the KAD.
KAD Development History
Motion Lab Systems developed the first commercially available Knee Alignment Device (KAD) which was widely used by the Vicon Clinical Manager (VCM) software from Oxford Metrics Ltd (now Vicon Motion Systems).
The first Knee Alignment Device was built at Newington Children's Hospital in Connecticut by Dennis Tyburski and Roy Davis. This "prototype" was constructed from various materials found around the lab (principally Delrin, glue and rubber bands) and used by the Newington Software to locate the knee axis. Oxford Metrics included software support for the KAD in the Vicon Clinical Manager (VCM) software and this created a demand for the devices which, at this point, only existed in the Newington lab. Motion Lab Systems then worked with the Dennis Tyburski to create a version of the KAD that would be easy to manufacture, reliable and lightweight. These devices were then sold by Oxford Metrics with their VCM software - the MLS units are the original, red, anodized, aluminum devices that are illustrated above.
In 1994 Oxford Metrics supplied their customers with a different black plastic KAD of their own design but discontinued supporting this version after a few years. Motion Lab Systems has continued to manufacture, sell and repair the original MLS designed KAD based on the Newington design.