Finger prosthesis
BACKGROUND OF THE INVENTION 
     The present invention relates to a finger prosthesis with double piston for pivotal and sliding movement. 
     It is a well-known practice to substitute a finger joint damaged by rheumatism by surgical implantation of an artificial joint i.e. prosthesis which allows recovery of the patient and practical use of the damaged finger. 
     However, it is rather difficult to implant the prostheses applied nowadays and they allow only a limited angular motion of the phalanges with respect to the original one. 
     The known artificial joints generally consist of two parts connected by a joint and are fixed in the bones by osteointegration or by the use of bone cement. These prostheses are generally loosening, luxating or are damage in short time due to the loading. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to offer means for avoiding the above inconveniences. This invention, as it is characterized solves the problem of providing an improved finger prosthesis which allows a free motion of the phalanges and keeps one part of the joint perfectly safe against rotation with respect to the other part in order to avoid the risk of accidental unscrewing thereof. 
     The finger prosthesis comprises a joint for pivotal movement between two bones, including intramedullary stems adapted to be secured in the intramedullary canals of the two bones after resection. The prosthesis is essentially characterized in that the joint includes a convex semi-cylindrical part and a concave semi-cylindrical part in the form of a cushion having the same radius of curvature as the convex part has and rotating on the surface of the convex part, and that the stems connecting the joint and the bones are pistons slidably received in longitudinal bores provided in the hollow heads of self-cutting screws secured in the bones. 
     According to a non limiting embodiment of the invention, the convex part of the joint is provided with a central transverse slot and a bore receiving a pin being the axis of rotation of the piston bearing the concave part in form of a cushion which in turn is supported by the hollow head of the corresponding self-cutting screw. 
     The connection between a stem and a self-cutting screw may contain at least one flat part in order to prevent rotation. 
     The central transverse slot in the convex part has a back wall which contains the geometrical axis of the other stem and a bottom wall forming an angle of about 60° with said axis. 
     The bore in the convex part receiving the pin is arranged in a way that the distance between said bore and the bottom wall as well as the distance between said bore and the geometrical axis of the other piston is greater then the radius of the piston. The saw-tooth section of the thread of the self-cutting screws is arranged inversely with respect to the conventional orientation. 
     The advantages of the present invention are essentially that the prosthesis is fixed firmly in the bones and at the same time enables certain longitudinal movement in accordance with the pivotal movement of the phalanges even in the extreme positions and in this way allows a free and natural movement thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of the prosthesis, partly in section. 
     FIG. 2 is a sectional view taken along line II--II of FIG. 1. 
     FIG. 3 is a front view of the prosthesis according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The prosthesis shown FIGS. 1-3 comprises a joint 10 fixed in the free ends of the phalanges by pistons 11 and 20 received in the holes 42, 52 of the hollow head 41, 51 of respective self-cutting screws 40 and 50, wherein a parallelogram based cushion 30 with a semi-cylindrical concave surface 31 is connected to a second semi-cylindrical part 12 provided with a convex surface. 
     The joint 10 includes the semi-cylindrical convex part (first joint part) 12 provided with a central transverse slot 13. This part contains a bore receiving a pin 14 as the axis of rotation of piston 20. The cushion 30 (concave semi-cylindrical part or second joint part) is provided with a semi-cylindrical concave surface 31 of the same radius as the convex semi-cylindric surface 12 of the other part which is supported by the first one in order to provide a pivot and a plane on its external surface 32 which in turn is supported by the head of the corresponding screw. 
     It is to be noted that pistons 11, 20 and self-cutting screws 40, 50 are connected to each other in a way which prevents rotation between the elements by flattenings (flat parts) 111 and 201 provided on the outer surfaces of the pistons 11 and 20 and by similar flat parts 421, 521 provided in the holes 42, 52 within the hollow heads 41, 51 of screws 40 and 50. The flat parts prevent accidental unscrewing of one or the other screw. 
     The central transverse slot 13 in semi-cylindrical convex part 12 has a back wall 15 the plane of which includes the geometrical axis of piston 11 connected to said semi-cylindrical convex part 12 and a bottom wall 16 which forms an angle of about 60° with the same axis. 
     The distance between the pin 14 of the piston 20 and the base 17 of semi-cylindrical convex part 12 is greater than the radius of the pistons 11 and 20. 
     Accordingly, it is sufficient to screw self-cutting screws 40 and 50 in the medullary canals of both bones after carrying out the necessary resection of the bones. Then, piston 11 of the joint 10 is introduced in the hole 42 of screw 40 and piston 20 together with cushion 30 is introduced in the hole 52 of screw 50 to establish the artificial joint between the phalanges. 
     The pivotal movement of the phalanges is limited by wall 15, along the geometrical axis of piston 11, meanwhile there is a free pivotal movement in the other direction, where the transverse bottom wall 16 is arranged. 
     Preferably the cross-section of threads 43 of 53 self-cutting screws 40 and 50 of a saw-tooth shape is arranged inversely with respect to the conventional orientation in order to obtain the maximum resistance to external axial loads on the hollow heads 41, 51. The screws may be of cylindrical or at least partially conical shape. 
     Undesired stresses and loosening of the joint due to such stresses may be avoided within the system according to the invention. 
     The sliding surfaces are preferably treated in order to improve service life and the coefficient of friction. 
     Due to the flat parts or hexagonal cross sections the pistons prevent accidental unscrewing of the screws.