Abstract:
The invention contemplates a fixation pin for retaining small bone fragments in an osteosynthesis procedure. The pin comprises a smooth-walled shank portion and an adjoining threaded portion, wherein a step-down conical shoulder is formed between the shank portion and the threaded portion.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a fixation pin for use in retaining small-bone fragments in an osteosynthesis procedure. 
     It is generally known in osteosynthesis to fix bone fragments by screws or pins. In the case of small-bone splinterings, the screws or pins available in the prior art are, however, much too large to fix a small splintered portion of a given bone to another portion of the same bone without damaging these parts. The fixing of such small bone fragments by means of simple pins or nails is therefore problematical, since there is no abutment to hold the parts together, and the bone may shift on the outer wall of the pin or nail. 
     BRIEF STATEMENT OF THE INVENTION 
     The object of the present invention is to provide a fixation pin for bone fragments which, despite its necessarily small size, will produce a dependable fixation of small-bone fragments. 
     The invention achieves this object by providing a small-bone pin with an elongate smooth-walled shank, the distal end portion of the pin having step-down transition to a smaller-diameter threaded portion. The step-down transition is effectively a conical shoulder which can abut a bone splinter while the threaded portion is anchored to the main part of the same bone. 
     Stated in other words, a small-bone pin of the invention has a smooth-walled shank portion along a relatively large part of the overall length of the pin, and a relatively short threaded part adjoining the lower end of said shank portion. In use of this pin, the threaded portion is screwed into the bone fragment and the remaining or main fraction of the same small bone; the upper side of the bone fragment is abutted by the conical step formed between the threaded portion and the shank portion, so that it is no longer possible for the bone fragment to slide on the pin. 
     When relying solely upon the small-bone pin of the invention the shank portion may be located within surrounding muscular tissue. The shank portion also projects externally, for easy cut-off to desired length, using a suitable tool; in that event, it would seem appropriate to refer to the bone pin of the invention as a compression wire. 
     Optionally, and depending upon the shape of the proximal face of the bone fragment to be secured, a special washer is applicable over the threaded portion of the pin (a) with a counterbore adapted to locate on the conical step, (b) with a larger diameter than that of the shank portion of the pin, and (c) with a concave distal face adapted for relatively large-area retaining engagement with the bone fragment. 
    
    
     DETAILED DESCRIPTION 
     The preferred embodiment will be described in conjunction with the accompanying drawings, in which: 
     FIG. 1 is an enlarged view in elevation of a small-bone pin of the invention; 
     FIG. 2 is a simplified section, on a reduced scale, through a small bone, fractured at X, to show use of the invention; and 
     FIG. 3 is a greatly enlarged fragmentary view of coacting elements of the invention in an installed condition, pursuant to an optional employment of kit components of the invention. 
    
    
     In FIG. 1, a small-bone fixation pin or compression wire 1 is seen to comprise essentially a smooth-walled shank portion 2, with an adjoining distal threaded portion 3. The outside diameter of the threaded portion 3 is smaller than the outside diameter of the shank portion 2; the diameter of the threaded portion is preferably constant, except for a sharp distal end 5, for self-tapping entry into bone tissue. Between the threaded portion 3 and the shank portion 2, as seen in FIG. 2, a frusto-conical shoulder or step 2 serves as an abutment for a bone fragment A which is to be fixed onto the remainder of the main bone B, by advancing the threaded portion 3 into the main bone. 
     The half-angle of conical convergence, i.e., with respect to the central axis of pin 1, is suitably in the range 30° to 60°, and is preferably about 45°. 
     To obtain an idea of the sizes involved, it is pointed out that a fixation pin 1 can, for example, have a length of 100-mm, with the threaded portion 3 having a length of 15-mm and the shank portion 2 a length of 85-mm. The diameter of the threaded portion can be 1.4 to 1.6-mm when, for example, the diameter of the shank portion 2 is 2-mm. Stated in other words, the diameter of the threaded portion is in the range of approximately 70 to 80 percent of the diameter of the shank portion. 
     With the indicated orders of magnitude, the shank portion 2 can easily be cut off by suitable nippers so that protruding regions of the threaded portion or of the shank portion can easily be removed. 
     Depending upon the externally exposed shape of a particular bone fragment to be secured to the remainder of the same bone, the invention is shown in FIG. 3 to permit optional employment of a washer 6, which, as in the case of pin 1, is also preferably and suitably of stainless steel. In FIG. 3, diametric dimensions D 1  and D 2  identify the sizes and size relationships discussed above for the shank and threaded portions (2, 3) of the pin 1. 
     Washer 6 features a bore of diameter D 3  to clear the threads of portion 3 and a conical counterbore in its proximal face, adapted for centering engagement with the frusto-conical step 4. Washer 6 has an outer diameter D 4  in the range 1.5 to 2.5 times the shank diameter D 1 , and its distal face 7 is concave, being suitably a circular or parabolic arc of revolution about the axis of the bone pin. The axial depth d of the concavity 7 may suitably be about one third of the axial extent of washer 6; the bore diameter D 3  may be approximately another third of the axial extent, and the remaining axial extent is the depth of the conical counterbore. 
     In FIG. 3, the small bone fragment B is of course greatly enlarged but is seen to present a convex shape for washer (6) engagement, with the convex bone shape in close conformance with the concave depth and curvature of the distal face 7 of the washer. This is as it should be for the fracture X as shown in FIG. 3, wherein bone-engagement via distal face 7 provides an enlarged area of bone-fragment support and compression surrounding the region of threaded-portion (2) engagement to bone at A and at B. 
     It is realized, however, that not all bone fragments will present a convex shape that is so well accommodated by the distal face 7 that has been described. To best equip the orthopedic surgeon who must deal with whatever confronts him, the invention is to be understood as being available in kit form, wherein at least one and preferably several washers 6 are provided for each bone screw, pin, or wire 1, and wherein the several washers 6 differ as to axial depth d of the concavity 7. The surgeon has further opportunity to adapt the described washer 6 to particular circumstances of small bone fragment contour, in that the washer 6 may be bent as necessary by pliers or other tools which are standard equipment for the orthopedic surgeon. Thus, if need be, a washer 6, of preselected axial depth d of its otherwise spherical distal face 7 may be bent to distort the distal face 7 into a more complex curvature wherein the curvature is, for example, (i) of relatively short-focus parabolic nature in a first longitudinal section which includes the pin axis and (ii) of longer-focus parabolic nature in a second longitudinal section, taken 90 degrees from the first longitudinal plane. 
     A kit of the nature indicated preferably includes a set of small-bone pins or compression wires 1, wherein there is at least one pin or wire 1 of each of several shank (2) diameters, illustratively of 3-mm, 2-mm, and/or 1.6-mm diameter, with thread (3) diameters of 2.2-mm, 1.6-mm, and/or 1.2-mm, respectively. Such a kit would include a set of washers 6 at least to fit the 3-mm and 2-mm shank sizes indicated, and with at least two different axial depths d for each of these shank sizes. In the case of the 3-mm shank size, washers 6 are suitably of 6-mm diameter D 4  ; and in the case of the 2-mm shank size, washers 6 are suitably of 4-mm diameter D 4 .