On possibility of unloading control at leg orthotics

Abstract

In earlier papers the idea was formulated of programmed control of the limb unloading coefficient (CU) through change of the brace lateral compression when tightening. With regard to the shin the hypothesis was that due to circumferential compression the shin will be "pushed" out of the brace up, its contact with the sole be weakened, and the load be redistributed from the shin on the brace case.With PAC DiaSled for measuring subplantar pressure fields conducted were systematic measurements in vivo of CU of the orthosed limb using under the brace both cotton and special slippery synthetic stockings. For cotton stockings no dependence of CU on compression was found, for slippery stockings in walking – as well, while in statics recorded was a sharp rise in CU with an increase of compression on the left side of the curve and the stabilization in the right part of it. For the simulation of unloading in orthotics measured were friction coefficients of pairs of leather-stocking and stocking-brace for cotton (0.48 and 0.57, respectively) and for synthetic stockings (0.42 and 0.16).A model was considered of rough rigid wedge (the shin), covered by a conformal expandable rigid holder (brace sleeve) and loaded with vertical force (weight) and belt-like load (lateral compression). From the requirement of implementation of the regime of orthosis sliding with respect to the limb and based on their geometry the estimate of the value of the required friction coefficient between them k <~ 0.25 was obtained. For cotton stockings, taking into account friction coefficients obtained the direction of the compression force acting on the shin is within the cone of friction. Therefore, conditions are realized of limb cohesion with the orthosis and lateral compression can not affect the unloading. For slippery synthetic stocking slippage regime is realized leading to change in CU by varying the circumferential compression.Models of leg-brace system considering the deformability of soft tissues were studied numerically: conical models – based on the method of boundary integral equations, models of real forms – by finite element method using a specially made for the purpose computer tomograms of the shin in vivo. It was found that on the shin-brace contact surface slip areas arise. For a cotton stocking with a coefficient of friction between the shin and the orthosis k = 0.48 their size are strongly dependent on the geometry adopted, for synthetic stocking with k = 0.16 in all cases they are significant.