Patent ID: 8645106
Filing Date: 2014-02-04
Classification: A61F

Abstract:
1. A method of determining a shape of an artificial cement-less hip prosthesis stem, adapted to be implanted into a deep hollow extending from an epiphysis of a femur to a diaphysis, so as to be gradually united with the femur by bone growth, comprising; detecting a limitary thickness of a bony tissue of each part of the femur destined to receive the prosthesis stem, which is necessary for forming a wall of the deep hollow, by applying a threshold bold density value to three dimensional graphics data based on computed tomography (CT) data and magnetic resonance imaging (MRI) data obtained from the femur, calculating a shape of an imaginary three dimensional deep hollow extending to a medullary cavity, which is formed inside the bony tissue, and also calculating coordinates of a crotch side line and a counter-crotch side line for regulating a shape of a lateral section of the deep hollow, and coordinates of an anterior side line and a posterior side line for regulating a shape of a section perpendicular to the lateral section of the deep hollow; calculating a shape of an epiphysis opening of the imaginary three dimensional deep hollow and a position of a center of a figure of the shape, from a position and an angle of the epiphysis opening determined based on a predetermined reference line which is parallel to a longitudinal direction of the femur, and also calculating a shape of a diaphysis opening of the imaginary three dimensional deep hollow and a position of a center of a figure of the shape, from a length of the stem to be applied to the femur and an angle of the diaphysis opening; calculating a reference arc on a plane including the crotch-side line and the counter-crotch line which passes through the center of the figure of the epiphysis opening, the center of the figure of the diaphysis opening, and a reference point in a lateral direction, which lies halfway between the two centers of the two figures, and a distance to said crotch side line corresponding to a distance to the counter-crotch side line; obtaining a pattern of a provisional deep hollow on the plane including the crotch and counter-crotch side lines, which is formed inside said imaginary three dimensional deep hollow surrounded by a small arc on the plane including the crotch and counter-crotch side lines which is concentric with the reference arc on the plane including the crotch and counter-crotch side lines and has a radius which is shorter than a radius of the reference arc and longer than a distance to an end point of the epiphysis lying on said crotch side line, a large arc on the plane including the crotch and counter-crotch side lines which is concentric with said reference arc and has a radius which is longer than the radius of said reference arc and shorter than the distance to the end point of the epiphysis lying on said counter-crotch side line, the crotch side line which extends from a terminal point of the small arc where said small arc intersects the crotch side line to an end point of the diaphysis lying on the crotch side line, and the counter-crotch side line which extends from a terminal point of the large arc where said large arc intersects the counter-crotch side line to an end point of the diaphysis lying on the counter-crotch side line; calculating a reference arc on a plane including the anterior and posterior side lines, the reference arc passing through the center of the figure of the epiphysis opening, the center of the figure of the diaphysis opening, and a reference point in a direction perpendicular to a lateral direction, which lies halfway between the two centers of the two figures, and a distance to said anterior side line corresponding to a distance to the posterior side line, obtaining a pattern of the provisional deep hollow on the plane including the anterior and posterior side lines, which is formed inside said imaginary three dimensional deep hollow surrounded by a small arc on the plane including the anterior and posterior side lines which is concentric with the reference arc on the plane including the anterior and posterior side lines and has a radius which is shorter than a radius of the reference arc and longer than a distance to an end point of the epiphysis lying on said anterior side line, a large arc on the plane including the anterior and posterior side lines which is concentric with said reference arc and has a radius which is longer than a radius of said reference arc and shorter than a distance to an end point of the epiphysis lying on said posterior side line, the anterior side line extending from a terminal point of the small arc where said small arc intersects the anterior side line to an end point of the diaphysis lying on the anterior side line, and the posterior side line extending from a terminal point of the large arc where said large arc intersects the posterior side line to an end point of the diaphysis on the posterior side line; making a provisional three dimensional reference stem containing the reference arc on the plane including the crotch and counter-crotch side lines and the reference arc on the plane including the anterior and posterior side lines, which consists of a body surrounded by four side planes formed by arranging said patterns of the provisional deep hollow on the plane including the crotch and counter-crotch side lines on the anterior side line and the posterior side line respectively so as to face each other and said patterns of the provisional deep hollow on the plane including the anterior and posterior side lines on the crotch side line and the counter-crotch side line respectively so as to face each other, and a quasi-circular cylinder having a same shape as said imaginary three dimensional deep hollow corresponding to a region extending from each terminal point of the small arc and the large arc of the body surrounded by the four planes to the diaphysis opening; making a provisional three dimensional reference deep hollow which has a same shape as said provisional three dimensional reference stem, in order to calculate a position and a posture of the provisional three dimensional reference stem on each step as the stem is gradually pulled out of the deep hollow, obtaining a residual portion of the provisional three dimensional reference deep hollow whose diaphysis portion is removed, where the deep hollow does not overlap every step of the stem, calculating a center of gravity and a principal axis of inertia thereof, and obtaining a residual portion of the provisional three dimensional reference stem whose epiphysis portion is removed, where the stem does not overlap every step of the deep hollow, calculating a center of gravity and a principal axis of inertia thereof, shifting the center of gravity of the residual portion of the provisional three dimensional reference stem to the principal axis of the residual portion of the deep hollow in a direction for pulling the stem with a condition that a posture of the stem is kept in the step, rotating the residual portion of the provisional three dimensional reference stem around the center of gravity shifted in a last step so as to coincide the principal axis of inertia of the residual portion of provisional three dimensional reference stem in the direction of pulling the stem with the principal axis of inertia of the residual portion of provisional three dimensional reference deep hollow in the direction of pulling the stem, calculating positions of the end points of the diaphysis lying on the crotch side line and the anterior side line of the residual portion of the provisional three dimensional reference stem, and the end points of the diaphysis lying on the counter-crotch side line and the posterior side line which occupy every step; making an imaginary three dimensional stem which has a same shape as said imaginary three dimensional deep hollow, moving the imaginary three dimensional stem in the imaginary three dimensional deep hollow by applying transitional values to said occupied positions to the imaginary stem, calculating interference portions where the imaginary three dimensional stem is disturbed when moving in the imaginary three dimensional deep hollow while the stem is pulled out of the deep hollow and saving the calculated interference portions as data; calculating an outer surface of the shape of the imaginary three dimensional stem whose interference portions are entirely removed based on the stored data, obtaining a reference stem whose final shape is inscribed to an inner surface of the shape for the purpose of smoothing an outer surface of the shape of the imaginary three dimensional stem, taking an alternative reference point in a lateral direction on a region between a straight segment from the center of the figure of the epiphysis opening to the center of the figure of the diaphysis opening and said reference point in the lateral direction, taking an alternative reference point in a direction perpendicular to a lateral direction on a region between said straight segment and said reference point in a direction perpendicular to the lateral direction, the alternative reference point in the lateral direction being regarded as the reference point in the lateral direction, and the alternative reference point in the direction perpendicular to the lateral direction being regarded as the reference point in the direction perpendicular to the lateral direction, obtaining an alternative provisional three dimensional reference stem having an alternative reference arc on the plane including the crotch and counter-crotch side lines whose curvature is different from that of the reference arc on the plane including the crotch and counter-crotch lines and an alternative reference arc on the plane including the anterior and posterior side lines whose curvature is different from that of the reference arc on the plane including the anterior and posterior side lines, calculating said occupied positions for every step, obtaining a final shape of the alternative stem by following the procedure for calculating the interference portions of the stem where the imaginary three dimensional stem is disturbed when moving in the imaginary three dimensional deep hollow by using the occupied position, and storing the data; and, computing data on positions and postures of the reference stem and the alternative stem which have said final shape while putting these stems gradually into said imaginary three dimensional deep hollow, by following the above steps in reverse.