Measuring Relative Motion to Develop an Interface That Off-Loads the Lower Limb

Abstract This investigation seeks to understand how the interfaces between fracture orthoses and the body affect the relative motion between the user and device. Excessive relative motion can inadvertently load the fracture and prevent healing. Identifying interfaces that minimize relative motion will inform the designs of future fracture orthoses. Three types of interfaces are used to off-load the injured limb: a thigh corset, a sling that loaded the ischium, and a sling that loaded the patellar tendon. However, little is known how these interfaces interact or with one another. Eighteen able-bodied participants were fitted with an adjustable interface testing system (ITS) that redirected loads away from the foot and lower leg to more proximal parts of the body using different combinations of the three interfaces. A video motion capture system measured the relative motion between the ITS and the participants while varying the type of interface, load through the ITS, and tightness of the thigh corset. Average relative motion varied from 1.6 to 6.5 cm across all conditions. The thigh corset combined with either the ischial sling or patellar tendon-bearing sling allowed the least relative motion of the interface conditions tested. However, the combinations differ in the sagittal profile they create for the user as well as the breadth of injuries they can treat. All these factors need to be considered in the design of future fracture orthoses.