Abstract:
A fracture fixation system in which a plate is secured to stable bone and posts are inserted at varying angles into an unstable bone fragment by engaging in rotatable bearings which are fixedly secured in the plate when the posts are fully engaged in the bone fragment. The bearings are formed as truncated spherical members having a number of longitudinal slots extending partway along the length of the bearing to form petals which are expanded outwardly when the posts are advanced in the bearing to produce non-uniform distribution of forces between the bearing and the plate which generate force couples to resist angulation of the posts and loss of fracture fixation. Various other ways of producing non-uniform force distribution are described.

Description:
FIELD OF THE INVENTION 
     The invention relates to a fracture fixation system in which a post is secured in a bone fragment and is locked in an adjustably angulated position in a plate secured to stable bone. 
     The invention further relates to the construction of a bearing which is disposed in the plate and which locks the post in the plate as the post is inserted in the bearing. 
     The invention further relates to a method for securing a post in an angulated position in a bearing in turn secured in a hole in a plate. 
     BACKGROUND AND PRIOR ART 
     Fixation of fractures near an articular surface are often fixed with devices, such as a buttress plate in order to support the articular surface from displacing. These devices share several common features: nearly all are manufactured as a plate which has fixation of the central fragment with one or more bone screws and provide fixation of the peri-articular fragment with one or more supporting tines, posts, or locked screws. 
     One such device is shown in published U.S. application 20030105461 (Putnam) and comprises a plate with fixed tines that extend at an angle to the plane of the plate. These tines are contiguous with the plate and extend at a fixed angle. A disadvantage of this construction is that the tines are located at fixed positions and the surgeon must implant all of them. Often the geometry of the fracture is such that the location of the tines is not optimally positioned and may enter a fracture site or protrude into the joint. 
     Another device includes plates that have fixed screws or posts that are screwed into a threaded hole in the plate. Also known, as shown in Orbay U.S. Pat. No. 6,358,250, are plates in which the posts or screws are inserted at varying angles. These constructions require that the head of the post or screw have enough thread purchase on the threads in the hole of the plate; this requires the plate to have sufficient thickness to allow an adequate number of threads for fixation of the post or screw. In addition, these plates require the post or screw to be inserted at a predefined angle. 
     A further device is shown in Orbay U.S. Pat. No. 6,364,882 and U.S. Publ. Appln. 2002/0143338 wherein the post or screw is provided with a partial spherical head so that it can be placed at a variable angle in the plate, and thereafter secured with a set screw. The spherical head is provided with slots to facilitate its expansion into the hole when it is secured by the set screw. Although this construction allows greater flexibility for inserting the post at a variable angle, it requires even more thickness for the plate in order to compensate for the thickness of the post or screw head in the plate hole as well as providing a sufficient number of threads for purchase by the set screw. In addition, since the set screw only compresses the head of the post or screw over a fairly small surface area, the frictional forces generated are relatively small whereby there is a risk of inadequate fixation of the post or screw and subsequent postoperative angulation and loss of reduction. In addition, this system is cumbersome to use because of the need for the surgeon to manipulate a small set screw behind the post or screw head, resulting in increased difficulty in surgical technique. Finally, because the head of the fixation post is slotted, it creates an area of considerable weakness where the shaft of the post connects to the slotted head. This is subject to considerable torque and is prone to breakage. 
     Another device is shown in U.S. Pat. No. 5,954,722 (Bono) wherein a split bushing is engaged in a plate and receives a bone screw which expands the bushing and frictionally locks the bushing in the plate. Although this configuration is useful to compress the head of the post to prevent it from backing out, it does not produce a large surface area of contact between the bushing and the surface of the hole. The bushing is narrow and has a spherical radius that is slightly smaller than that of the hole in order to allow initial rotation of the bushing in the hole to orient the screw. As the bone screw head expands the bushing cylindrically, contact of the bushing with the surface of the hole in the plate is primarily restricted to a narrow zone of contact along a single equatorial plane of the bushing since the curvature of the bushing along its principal axis is more pronounced than that of the hole. Although this configuration generates compressive forces on the head of the screw to prevent it from backing out, if forces on the bone fragment generate torque on the post, the resistance to torque is limited to the frictional forces generated from this limited plane of contact of the bushing along the equatorial zone. The device is intended for use in stabilizing multiple vertebrae in the spinal column and the frictional forces are intended to prevent the screws from backing out of the plate but they are not intended to nor are they sufficient to resist torque imposed on the screws by shifting of an unstable bone fragment of a fracture. 
     Another limitation of the device in U.S. Pat. No. 5,954,722 (Bono) is that the bushing allows variation of the insertion angles within a predefined conical range. However, there are clinical situations in which it is desirable to limited the range of insertion angles primarily along a single plane. For instance, in some situations it may be desirable to allow variation along an axis from proximal to distal, but limit variation from side to side so that the post will not penetrate the joint or other important structures. 
     Another limitation of the device in U.S. Pat. No. 5,954,722 (Bono) is that the bushing tends to spin as the post head is inserted. As the size of the bearing is reduced, it becomes increasingly difficult to insert the post fully since the bearing spins before sufficient expansion has occurred to generate large frictional forces. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a fracture fixation system which avoids the problems described above and by which reliable and simple securing of the post or screw in the plate is obtained. 
     The fixation system comprises an implant based on a concept of providing an intermediate fixation element or bearing between the post or screw and the hole in the plate. The post is threaded into this intermediate element to cause threads on the post to produce expansion or displacement of the intermediate element against the hole in the plate. This intermediate element can be inserted into the plate hole before the plate is applied to the bone whereby no manipulation of cumbersome small set screws is required. Moreover, since the compression of the intermediate element is through the periphery or surface of the plate hole, a wider surface area for compression is obtained resulting in greater resistance to torque. Since the threads are distributed within the intermediate element, the plate does not have to be made excessively thick to allow enough thread purchase for fixation. Additionally, the construction of the invention allows the capability of placing the post or screw in a range of angles, either along two separate degrees of freedom (in a variety of conical positions) or varying primarily along a single degree of freedom (varying along a single plane as directed by the shape of the hole and the bearing). 
     Finally, since the head of the post is not split with a thin zone of connection to the shaft of the post, the risk of breakage of the post or screw from torque is greatly reduced. 
     The invention is characterized by forming the bearing in such a way to produce more than one zone of contact when the bearing is expanded, which provides greater resistance to torque applied to the post or screw by tendency of displacement of the bone fragment, after its fixation. By providing more than one zone of contact, a non-uniform distribution of radial forces is produced between the bearing and the plate to resist the applied torque in addition to friction forces. In addition, certain variations allow production of a force couple, providing improved resistance to applied torque. 
     More particularly, the bearing has an outer surface of spherical contour which can be angularly rotated in a spherical hole in the plate and the bearing is provided with one or more longitudinal slots extending from one pole of the bearing partially or completely along the length of the bearing to cause expansion to occur when the post or screw is engaged in the bearing which creates multiple areas of contact at the top and bottom of the bearing to improve resistance in applied torque on the post or screw by the bone fragment. 
     Another object of the invention is to restrict the range of insertion angles to a single plane, or a limited variation to either side of a single plane, in order to improve the resistance to torque applied to the post at right angles to the plane as well as restrict the possible insertion angles to a predefined range. 
     Another object of the invention is to provide means to overcome the problem of spinning of the bearing as the post is inserted into the bone in the bearing. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING 
         FIG. 1  is a diagrammatic sectional view illustrating the fracture fixation system of the invention. 
         FIG. 2  is a top view thereof. 
         FIG. 3  shows a portion of the system in  FIG. 1  in a first stage of installation thereof. 
         FIG. 4  shows the system in a completed stage of installation. 
         FIG. 5  is a side elevational view of a bearing of the fracture fixation system. 
         FIG. 5   a  shows a modification of the bearing of  FIG. 5 . 
         FIG. 5   b  shows another modification of the bearing of  FIG. 5 . 
         FIG. 6  is a top, plan view of the bearing. 
         FIG. 7  is similar to  FIG. 4  and shows the forces developed when the bearing is expanded. 
         FIG. 7   a  is similar to  FIG. 5  and shows a modification of the bearing therein. 
         FIG. 7   b  is similar to  FIG. 5  and shows another modification of the bearing therein. 
         FIG. 8  is a side elevational view of another embodiment of the bearing. 
         FIG. 9  is a view of the bearing of  FIG. 8  from below. 
         FIG. 10  is a diagrammatic perspective view of another embodiment of the bearing. 
         FIG. 11  shows the bearing of  FIG. 8  in its initial loosely supported state in the plate. 
         FIG. 12  shows the bearing of  FIG. 8  after its rotation in the plate. 
         FIG. 13  shows a portion of the plate in which the hole has been modified to prevent rotation of the bearing when the post is being installed. 
         FIG. 14  shows the bearing installed in the plate hole in  FIG. 13 . 
         FIG. 15  is an elevational view of another embodiment of the bearing according to the invention. 
         FIG. 16  is a sectional view taken along line  16 — 16  in  FIG. 15 . 
         FIG. 17  is an elevational view showing the bearing of  FIG. 15  installed in a hole in the plate. 
         FIG. 18  is a sectional view taken along line  18 — 18  in  FIG. 17 . 
         FIG. 19  shows the bearing of  FIG. 17  in an angularly adjusted position. 
         FIG. 20  is a sectional view taken on line  20 — 20  in  FIG. 19 . 
         FIG. 21  is an elevation view of another embodiment of the bearing. 
         FIG. 22  is a transverse sectional view of the bearing of  FIG. 21  shown installed in a hole in the plate. 
         FIG. 23  shows the bearing of  FIG. 22  in a slightly rotated limited position in the hole in the plate. 
         FIG. 24  is a side view of another embodiment of the bearing. 
         FIG. 25  shows the embodiment in  FIG. 24  after it has been prestressed. 
         FIG. 26  shows the superimposed outline of the bearing shown in  FIGS. 24 and 25 . 
         FIG. 27  shows the bearing of  FIG. 25  after it has been inserted into a bone fixation plate and before seating of a post in the bearing. 
         FIG. 28  shows the bearing of  FIG. 27  after it has been inserted into the plate and after seating of a post in the bearing. 
         FIG. 29  is a diagrammatic elevation view of another embodiment according to the invention, in which the post has not yet been inserted into the bearing. 
         FIG. 30  shows the embodiment of  FIG. 29  in which the post has been inserted into the bearing 
         FIG. 31  is a diagrammatic elevational view of another embodiment according to the invention in which the bearing is free for rotation. 
         FIG. 32  shows the embodiment of  FIG. 31  in which the bearing has been rotated and locked in position in the plate. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will be described hereafter with reference to fixation of a fracture of the radius of the wrist utilizing a fixation system which enables fixation of the fracture and which secures the fracture and resists forces tending to produce fracture displacement. As will be evident to those skilled in the art, the invention is applicable to other fractures as well, such as fractures of the fibula, the medial malleolus of the ankle, the distal end of the ulna, the humerus and the tibia. 
     Referring to  FIGS. 1 and 2 , therein is shown, on enlarged scale, the distal end portion of the radius  1  of the wrist in which a fracture  2  is formed near the distal end  3 . The fracture  2  defines an unstable distal bone fragment  4  and a stable proximal bone fragment  5 . 
     Fixation of the fracture  2  is achieved with a fracture fixation system  10  which includes a plate  11  having a proximal portion  12  fixed to the stable bone fragment  5  by bone screws  13 . The bone screws  13  may have smooth heads or threaded heads that lock into a threaded hole in the plate, the term screw being used to refer to either type of fixation. The plate  11  has a distal portion  14  having a number of spherical holes  15  in which are secured fasteners  16  which enter and are secured in the distal unstable bone fragment  4 . The fasteners  16  can be in the form of pins, rods, wires or screws and hereafter will be referred to as posts. The posts  16  are supported in bearings  17  which are adjustably secured in the holes  15  in the plate to enable the posts  16  to be positioned at different angles  18  in the unstable fragment  4 . 
       FIG. 3  shows the post  16  in a partially inserted state in the bearing  17  and the unstable fragment  4 .  FIG. 4  shows the post in a fully inserted position in which the bearing  17  is locked in the plate  11 . 
     The bearing  17  has an outer surface  18  of spherical shape which fits in a respective hole  15  in plate  11 . The hole  15  is of a corresponding spherical shape as the outer surface of the bearing to allow the capability of the bearing to turn in the hole. Accordingly, the bearing is freely turnable in all directions within the hole  15  in order to adjust the angle at which the post  16  is to be inserted into the fragment  4 . The bearing  17  is provided with slots (as will be explained in detail later) so as to be outwardly expandable and become locked in hole  15 . When the post is fully seated in the bearing, the bearing will be locked in the hole  15 . 
     In order to obtain the expansion capability of the bearing  17  to lock the bearing in the hole  15 , the post has a threaded portion  20  which engages a threaded bore  21  centrally located in the bearing which advances the post into the bearing. In one embodiment, the threaded portion  20  of the post is oversized so that when the threaded portion  20  of the post is threadably advanced in the threaded bore in the bearing, it will produce a radial expansion of the bearing to lock the bearing securely within the hole  15  in the plate  11 . The threaded portion  20  may be tapered over its length in order to produce increasing amounts of expansion as the post is advanced. Alternatively, the threaded portion may have a uniform diameter over most of its length so that a predetermined amount of expansion will occur. In either embodiment, the threaded portion  20  may have a tapered region at the leading threads in order to make it easier to get started and prevent cross threading as the threaded portion  20  is inserted into the threaded bore  21 . 
     In another embodiment, a head  22  at the end of the post is oversized relative to the threaded bore  21  such that the head of the post causes expansion at the upper open end of the bearing as the head is seated into the bearing. The head  22  enables the post to be turned in order to threadably advance the post in the bore in the bearing. The head  22  also limits the advance of the post in the bearing. The post  16  can be formed without head  22  and a slot can be formed at the top of the post for engagement by a tool to rotate the post and advance it into the bearing. In another embodiment in which the slots in the bearing extend from the bottom of the bearing either partially or completely to the top, the expansion mechanism can be formed by a reduced diameter of the lower end of the threaded bore  21  which corresponds to a shoulder at the lower end of the threaded region  20  of the post so as to cause expansion at the lower end of the bearing. 
     In  FIGS. 3 and 4  the post is inserted initially into the open end of the bearing and the upper end of the post causes the expansion of the bearing as it is fully seated. Since the expansion occurs primarily at the open end  23  of the bearing rather than the closed end  24 , the outer surface  18  becomes pear shaped rather than spherical as the expansion occurs. As will be shown subsequently, this creates two zones of contact at different levels of the plate hole  15  and improves torque resistance when forces are applied to the distal end of the post due to drift of bone fragment  4 . 
     Moreover, the design in  FIGS. 3 and 4  shows insertion in the bearing at the open end  23 . The bearing could also be inverted in which case the post will be inserted into the closed end  24  and expansion generated by a conical thread on the lower end of the bore in the bearing. 
       FIGS. 5 and 6  illustrate a preferred embodiment of the invention respectively in elevation and plan views. The bearing  17  is formed as a body  30  of truncated spherical form. In order to confer radial expandability of the bearing  17 , a plurality of slots  32  extend longitudinally along the length of the bearing, at equally spaced circumferential intervals, from the upper edge  33  of the bearing to a position more than halfway along the length of the bearing. Preferably, the slot extends *about* three-quarters of the length of the bearing. Although four slots have been shown in the embodiment of  FIGS. 5 and 6 , a greater or lesser number can be provided. The slots  32  extend from the upper end  23  of the bearing to provide a means by which the bearing will be locked securely in the plate as will be explained more fully later. 
     At the lower end of each slot  32 , is an enlargement to provide stress relief to prevent cracking at the bottom of the slot. The enlargement can take numerous forms, such as a horizontal cut to form a T-shape enlargement. Preferably, the enlargement is in the form a hole  34  of diameter greater than the width of the slot which provides stress relief for the bearing at the bottom of the slot to prevent cracking when the bearing is expanded. The slots  32  define lever-like petals  35  circumferentially spaced around the bearing, which act as cantilever beams, to undergo opening as the post is threadably advanced in the threaded bore  21  of the bearing. The stress relief holes  34  also promote the expandability of the lever-like petals  35 . In a particular embodiment, the post has a diameter of 0.117″, and the truncated bearing has a diameter of 0.165″ and a height of 0.100″. The diameter of the bore in the bearing is substantially equal to the diameter of the post. The slots have a width of 0.032″ and a length of 0.073″. The stress relief hole at the end of each slot has a diameter of 0.040″. The bearing is made of stainless steel or titanium. The dimensions expressed herein are only exemplary and will vary depending on particular use. 
       FIG. 7  shows the forces acting between the bearing  17  and the plate  11  which serve to lock the bearing in the plate, when the post is threadably advanced in the bore of the bearing. Namely, as the post  16  is advanced in the bearing  17 , the upper ends  33  of the lever-like petals  35  expand outwardly in the hole  15 . This causes the bearing to be pushed down vertically trying to force it out of the bottom of the hole  15 . When a load A is applied to the distal end of the post, the combination of the vertical displacement of the bearing in the hole and the pear shaped geometry of the expanded bearing produce a cam-like mechanism binding rotation of the bearing in the hole and generating reactive forces F and T which produce a resistant force couple. The forces F and T are non-co-linear and develop the force couple which tightly and securely locks the bearing in the plate hole  15 . Conventional bearings which rely solely on frictional fit without the force couple may not adequately resist the torque applied to the posts by migration of the bone fragments after fracture fixation. When a solid or fully split spherical bearing is expanded cylindrically by a post, the main contact between the bearing and the surrounding hole is at the equator of the bearing which provides a limited frictional force generated from the relatively limited zone of contact between the bearing against the hole to resist the applied torque. Since the area of frictional contact is relatively small, the frictional forces are a fraction of the compressive forces F and T developed at the upper and lower surfaces as shown in  FIG. 7 , and relatively poor torque resistance is obtained. In the bearing of the present invention, the force couple developed by the two forces F and T, which are not co-linear, provides substantially higher resistance to the torque applied by loads from the unstable bone fragment into which the post has been inserted. 
     By virtue of the shape of the force applied by the post when introduced into the hole (along a cylindrical front) and the asymmetrical expansion of the petals of the bearing as cantilever beams, the contact between the petals of the bearing produces the non-colinear compression forces. The ensuing metal deformation between the bearing and the plate hole adds to the non-colinearity of the forces. The absence of contact in the central or equatorial region of the bearing further promotes the separation and non-colinearity of the developed compression forces. As a result, when torque is applied to the bearing due to torque applied to the post by drift of the unstable bone fragment, to which the post is secured, a reliable force couple is developed between the bearing and the plate hole to resist the torque applied to the post. 
       FIGS. 7   a  and  7   b  show two alternative embodiments that improve the surface contact and torque resistance between the bearing and plate hole  15  as the bearing expands cylindrically. In order to assure that the zone of contact between the bearing and the plate hole  15  is not limited to a single equatorial plane, the hole in the plate can be formed so that the central or equatorial region of the bearing will not contact the plate, whereby contact at the plate hole  15  will occur at two planar zones of contact at both the upper end and the lower end of the bearing, greatly increasing the surface area of contact and resistance to applied torque. In  FIG. 7   a  the central or equatorial region  17   a  of the bearing is flattened all around so that contact of the bearing with hole  15  is achieved in contact regions  17   b  and  17   c . In  FIG. 7   b  a central or equatorial region  15   a  of hole  15  is recessed so that only upper and lower portions  15   b  and  15   c  of the hole  15  make contact with the bearing  17 . 
     The arrangements in  FIGS. 7   a  and  7   b  develop two rings of contact above and below the equatorial region of the bearing, increasing the surface area of contact as well as generating forces that are distributed non-uniformly across the thickness of the plate to resist torque. 
     While the sides of the slots  32  have been shown straight and parallel in vertical planes, it is also possible to make the slots curved or angled as shown at  32   a  in  FIG. 5   a  and to make the slots widen as shown at  32   b  in  FIG. 5   b . The curved and widened slots  32   a  and  32   b  provide edges of acute angle shape as shown at  36   a  and  36   b  which tend to deform more as the post is installed in the bearing and can bite into the wall of the plate hole to enhance the compressive force and the engagement of the top of the petals in the bearing hole. 
       FIGS. 8 and 9  show another embodiment of the bearing in elevation and plan views. The body  30  of bearing  17  has a head  31  at its upper end. The head  31  has a hexagonal shape in order to be engaged by a tool such as a wrench so as to be held against spinning when the post is threadably advanced in the threaded bore  21  (see  FIG. 3 ) in the bearing. The head  31  is optional and the bearing may be held against spinning by friction with the wall of the plate hole  15  or tabs as will be explained later. 
     In  FIGS. 8 and 9 , two slots  32 A are shown which extend from the lower end  24  of the bearing upwardly along the length of the bearing and terminate at relief holes  34 . The expansion mechanism is the same as in  FIGS. 5 and 6  except that it is the lower ends of the petals  35  which are forced outwards when the bearing is expanded. A similar force couple is produced as shown in  FIG. 7 . 
     Referring to  FIG. 10 , therein is seen another embodiment of the bearing designated by numeral  40 . In this embodiment, the bearing is provided with two sets of slots  32 ,  32 A extending alternately from opposite ends of the bearing. In this way, the lever-like petals  35  extend from the top and bottom ends of the bearing and produce two zones of contact force couples which develop to lock the bearing in the hole  15  in the plate when the post is inserted into the bore  21  of the bearing. 
       FIG. 11  shows the bearing  17  of  FIG. 8  in its initially loosely supported state in the plate  11  whereas  FIG. 12  shows the bearing  17  after rotation in the plate  11  to its desired position. 
     Instead of using threads to engage the post in the bearing, the tapered end of the post can be made smooth and the post can be axially driven into the bore in the bearing to produce the expansion of the bearing. 
       FIGS. 13 and 14  illustrate a modification which employs means to prevent the bearing from spinning in the plate hole  15  when the post  16  is threadably advanced in the bore  21  in the bearing  17 . In this regard, the plate is provided with one or more grooves or slots  50  extending from the hole  15 . The bearing  17  is provided with radially extending tabs  51  at its upper end which fit into grooves  50 . Thereafter, when the post  16  is advanced into the bore in the bearing, the bearing will be held fast in the plate hole. Although two diametrically opposed grooves  50  and tabs  51  have been shown, only a single groove and tab may be necessary. It is to be noted that although this drawing shows the grooves and locking tab at the top of the bearing and plate, these can also be placed at the lower portion of the bearing and plate without departing from the invention. The bearing can also be provided with a countersink  52  at the upper end of the bore so that when the post is fully advanced in the bore, the head of the post will be recessed in the bearing. 
     Another way of providing resistance of the bearing  17  from spinning in the plate hole  15 , when the post  16  is advanced in the bore  21  in the bearing is to provide microtexturing in the form of microgrooves on the surface of the bearing. The microgrooves also improve torque resistance. Additionally, microgrooves can be provided on the surface of the hole  15 . 
     Since the bearing and the plate hole have corresponding spherical surfaces closely conforming to one another, there is a problem of installing the bearing in the plate hole. 
     To avoid this problem, the slots in the bearing can be made sufficiently wide so that the bearing can be compressed to enable it to be inserted into the plate hole and thereafter resume its original spherical contour. The slots confer the flexibility of the petals of the bearing to enable the insertion to be made. This feature will be discussed in more detail later with reference to  FIGS. 24–27 . 
     Another way of inserting the bearing in the plate hole is to initially form the hole with a large cylindrical diameter at its top to enable insertion of the bearing. Thereafter a crimping or crushing pressure is applied to the plate around the top of the hole so that the hole deforms and retains the bearing in the hole while still permitting the bearing to be freely rotatable. 
     Still another way of installing the bearing is to cool the bearing in dry ice or liquid nitrogen and/or to heat the plate which will cause the bearing to shrink and the hole to expand allowing the bearing to be inserted into the hole. After the bearing and plate return to room temperature, the bearing will be rotatably retained in the hole. 
     Another problem to be resolved is to prevent the bearing from rotating 180° upside-down in the hole preventing proper installation of the post. In order to prevent this, the bearing is provided with a slight extension similar to tab  51  at one side to prevent it from turning upside down. 
     The embodiment illustrated in  FIGS. 15-20  is constructed to restrain the bearing from spinning around a vertical axis during insertion of the post into the bore of the bearing and also to restrict angular travel of the bearing predominantly to a single plane. 
     Referring to  FIGS. 15 and 16  therein is shown a radial tab  60  extending outwardly from the surface of the bearing  61 . The bearing  61  is similar to the bearings previously described except that it is provided with a single longitudinal slot  62  extending along the entire length of the bearing to form a split bearing which can be outwardly expanded when the post (not shown) is inserted into the hole  63  in the bearing  61 . The bearing can also be constructed with slots as shown in any of the previous FIGS. or as the simple truncated spherical bearing with a single longitudinal slot as shown. The feature disclosed in this embodiment is the means to prevent spinning and restrict angular travel of the bearing. This means is comprised of the tab  60  which engages in a groove  64  which extends outwardly from the hole  65  in plate  66  along the thickness of the plate. As seen in  FIG. 18 , the arrangement of the tab  60  and groove  64  forms a key and keyway type of engagement which limits angular travel around the vertical axis  67  of the bearing and permits angular rotation of the bearing in plane A—A with capability of a small amount of side-to-side movement based on an oversize of the groove  64  relative to tab  60  as seen in  FIG. 18 . 
       FIGS. 19 and 20  show the bearing in an angularly rotated position in the hole  65  in plate  66  where the bearing has been turned to the side (clockwise) to the extent permitted by the travel of the tab  60  in the oversize groove  64 . 
     The use of a single tab as shown in  FIGS. 15 and 16  has the disadvantage that the bearing tends to rotate around the tab as the post is inserted, generating a large shearing force across the tab. This can be overcome by placing a second tab on the bearing and a second groove in the hole on the opposite side (not shown). 
       FIGS. 21 and 22  show a bearing  71  according to a modified embodiment which limits angular movement of the bearing  71  in the plate  11 . The bearing  71  is provided with opposite flat sides  72  extending from top to bottom of the bearing, although any non-circular cross-sectional outline would serve an equivalent function. The plate  11  has a hole  73  with a shape conforming to the outer surface of the bearing. Namely, the hole  73  has flat sides  74  facing the flat sides  72  of the bearing and the remainder of the surface of the hole is spherical and conforms to the surface of the bearing. Accordingly, angular rotation of the bearing is limited as shown in  FIG. 23 . The magnitude of angular movement is a function of the clearance at the flat sides of the bearing and the hole. The embodiment also provides resistance of the bearing from spinning when the post is screwed into the hole in the bearing. In the embodiment illustrated in  FIG. 22  the bearing is free to rotate angularly in a vertical plane C—C. 
       FIGS. 24–28  show another embodiment designed to reduce stress and increase the resistance to fatigue failure and cracking of the bearing. 
     Referring to  FIGS. 24–27  therein is shown a truncated bearing  100  in which the slot  101  is wider at the lower end  102  than the upper end  103 . The slot ends in an area of enlarged radius  104  in order to provide stress relief from cracking at the top of the slot. The bearing has a spherical outer surface in its at rest state and has a central threaded bore  105  for insertion of a post with a threaded shank. The outer diameter of the bearing  100  is slightly larger than the diameter of the hole  201  in the plate  200 . 
       FIG. 25  shows the bearing after it has been compressed. The outer surface of the bearing  102  has been reduced in size allowing it to be inserted in the hole  201  in plate  200 . In addition, the compression has caused the central threaded bore  105  to become inclined, with a smaller width at the bottom than at the top of the hole. Finally, the compression has pre-stressed a bridge or solid region  106  of the bearing above the slot  101  by bending it up causing slight compression on the lower surface of the bridge and tension on the upper surface of the bridge. 
       FIG. 26  shows superimposed the unstressed bearing  100  over the pre-stressed bearing  107  with the reduction in cross-sectional diameter. 
       FIG. 27  shows the post  108  before it is seated into the pre-stressed bearing  107  which has been placed in the hole  201  in plate  200 . The upper portion of the post has a lower tapered zone  111  and an upper threaded zone  110  that matches the width of the upper portion of the threaded bore  105  of bearing  107 . Prior to expansion, the bridge  106  above the slot is pre-stressed. 
       FIG. 28  shows expansion of the bearing  100  towards its original shape as the post  108  is inserted. The bend in bridge  106  above the slot is close to its original at-rest state when the bearing is expanded, reducing the stresses in this region when the bearing is expanded. This pre-stressed bearing design can be used with any of the other previously described slotted bearing designs. 
       FIGS. 29 and 30  show another embodiment in which the bearing  107  is similar to that shown in  FIGS. 24–28 . The plate  11  in which the bearing is seated is shown in  FIGS. 29 and 30 . The post  108  has a smooth cylindrical portion  112  adapted for insertion into a bone fragment (not shown). The cylindrical portion  112  can also be threaded to increase engagement with the bone fragment. The post is provided with the threaded portion  110  extending upwardly from the cylindrical portion  112 . The threaded portion  110  is tapered to facilitate entry into a widened end  114  at the top of the threaded bore  113  in bearing  107 . The tapered section  111  merges with a cylindrical threaded portion  110  which threadably engages in the threaded bore  113  in the bearing  107 . A head  109  is formed at the top of the upper threaded portion  110  of the post and the head  109  has a lower bevel surface  116  which conforms to a bevel surface  117  at the upper end of the bore  113 . When the post  108  is fully seated in the bearing, the head of the post causes the open ends  33  of the petals  35  to expand and bite into the wall of the hole in the plate (not shown). This arrangement for expansion of the bearing can be used with any of the previously described embodiments in which the post is inserted from the open end of the bearing. The bevel surface  116  can be formed with a taper having a slight mismatch with the taper of the bevel surface  117  at the top of the bearing  107  to form a Morse taper to lock the post in the bearing when fully seated. 
     Referring to  FIGS. 31 and 32 , therein is seen plate  300  with bearing  301  in hole  302 . In  FIG. 29 , the bearing is rotatable in the hole  302  in plate  300  and the post is not shown. The plate  300  is formed with a horizontal slot  303 , that is partially cut horizontally in the plate to form upper plate segment  304 , and lower plate segment  305 , with a separation between the two. The plate segments  304 ,  305  are formed with aligned holes  306 ,  307  respectively. The hole  306  is smooth and the hole  307  is threaded. In this embodiment, the bearing  301  does not have to be split or expandible. After the post (not shown) has been inserted into the angularly adjusted bearing and into the associated bone fragment, the bearing  301  is locked in place by inserting threaded fastener  308  in holes  306  and  307  to clamp and tighten the plate segments  304 ,  305  against the bearing  301  as shown in  FIG. 32 . This will produce compressive forces acting on the bearing in the upper hemisphere at the right and the lower hemisphere at the left to produce non-uniformly acting forces on the bearing to develop torque resistance to forces applied to the post. 
     Instead of being horizontal, the slot  303  can be oblique or even vertical. Instead of extending partially through the plate, the slot can extend completely through the plate and the segments of the plate can be hinged or screwed together. Instead of using a threaded fastener  308  to compress the two segments of the plate, any connecting fastener such as a clip or rivet can be used. 
     In addition, the bearing  301  may have one or more partial or complete slots so that as the two segments of the plate are compressed against the bearing, the bearing compresses against the post so as to lock the post to the bearing.