The femoro-tibial unresisted motion follows a well-defined path [1], also called knee natural motion, that is determined by the constraints exerted by ligaments and contacts. To have unresisted motion, the constraints must produce no work against the allowed motion. Experimental evidences indeed showed that it is possible to identify some fibers in the knee ligaments that remain isometric along the natural motion [2], thus producing no elastic work. Generalizing this reasoning, the work produced by each of the passive constraints for the instantaneous act of motion, i.e. about the instantaneous helical axis (IHA), must be zero or, in terms of screw theory, the knee twist must be reciprocal to all the passive constraint wrenches [3], independently from their number or type. We hypothesized that ligaments and contact wrenches correspond to pure forces. This reasonable assumption is supported by the work presented in [4,5], where it led to the synthesis of parallel mechanisms capable to replicate the knee natural motion. In this condition, if the femoro-tibial twist corresponds to a pure rotation about an instantaneous floating axis, all the constraint forces must intersect the knee IHA in order to produce no work, as also shown in [6]. The aim of this work is to experimentally prove this property by analyzing the relation between the line of action of passive constraints and IHA in the natural motion of two in-vitro and one in-vivo knee joint.
Ligament And Contact Forces Must Intersect The Instantaneous Helical Axis During The Knee Natural Motion: An Experimental Assessment
Michele Conconi;Nicola Sancisi;Vincenzo Parenti-Castelli
2017
Abstract
The femoro-tibial unresisted motion follows a well-defined path [1], also called knee natural motion, that is determined by the constraints exerted by ligaments and contacts. To have unresisted motion, the constraints must produce no work against the allowed motion. Experimental evidences indeed showed that it is possible to identify some fibers in the knee ligaments that remain isometric along the natural motion [2], thus producing no elastic work. Generalizing this reasoning, the work produced by each of the passive constraints for the instantaneous act of motion, i.e. about the instantaneous helical axis (IHA), must be zero or, in terms of screw theory, the knee twist must be reciprocal to all the passive constraint wrenches [3], independently from their number or type. We hypothesized that ligaments and contact wrenches correspond to pure forces. This reasonable assumption is supported by the work presented in [4,5], where it led to the synthesis of parallel mechanisms capable to replicate the knee natural motion. In this condition, if the femoro-tibial twist corresponds to a pure rotation about an instantaneous floating axis, all the constraint forces must intersect the knee IHA in order to produce no work, as also shown in [6]. The aim of this work is to experimentally prove this property by analyzing the relation between the line of action of passive constraints and IHA in the natural motion of two in-vitro and one in-vivo knee joint.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
