Micromotion in knee arthroplasty

Micromotion:and bone.Divided into two typesmigration and inducible displacement. Migration:original position to a new resting position. Inducible displacement:the prosthesis is detachable from its resting position. Rigid body:Rigid-body model (RBM): A configuration of three or more non-collinear markers representing the bodies of interest (prostheses or bone segments), and used for calculation of movements. Mean error of rigid-body fitting (ME):The degree of conformity of an object to the rigid-body concept.The magnitude of ME is a function of errors of measurements and true movements of the markers within the object.Unicompartmental prosthesis: A prosthesis that replaces one femoro-tibia1 compartment in the knee.In this study only one compartment was replaced in each knee, either the medial or the lateral one. Total prosthesis:Coordinate system: The micromotion is expressed in a laboratory coordinate system defined by the calibration cage approximately parallelling the major planes of the body.The X-axis is transverse, the Y-axis is vertical and the Z-axis is sagittal (Fig. 1).For rotations, movements are expressed with a positive (+) sign if the anterior part of the prosthesis moves downwards (x-axis), peripherally or laterally for unicompartmental and total prostheses, respectively (y-axis) and if the central or medial side for unicompartmental and total prostheses, respectively, moves downwards (z-axis).Translations are expressed with a positive sign if peripheral or lateral (x-axis), upwards (y-axis) or forwards (z-axis).Small movements between the tibia1 component (called "prosthesis") Gradual micromotion over time; the prosthesis is detached from its Instant micromotion occurring in response to external forces;A body in which distances remain constant in time.A prosthesis that replaces both femoro-tibia1 compartments.