Abstract:
A surgical joint implant has a cap in the form of at least two intersecting circles of the same diameter having an articular outer surface and an inner surface for bone adhesion. At the center of each circle a peg, for bone insertion into a hole of a nominal diameter, extends. One peg is slightly larger than said nominal diameter, to achieve an interference fit and the other peg is slightly thinner, to achieve a slide fit. A tubular drill rig open at both ends and having an interior circumference corresponding to the outer shape of the implant, can be mounted over the bone surface to be repaired. It accommodates a double drill for drilling, at the same time, a shallow hole of the diameter of the intersecting circle and a deeper narrow hole at the center of the circle of said nominal diameter.

Description:
[0001]    The present invention relates to a surgical joint implant and a bone-mountable rig for correct drilling and insertion of such a surgical joint implant. The implant according to the invention is intended for repair of the surface of a joint of a human or animal. 
         [0002]    It is intended that the implants of the present invention may be tailor-made to the patient and the damage to her joint to be repaired. This individually shaped implant can be made by the method described in Application No. PCT/EP2014/064749, reference No. IPQ6028, filed by the same applicant and having the same filing date. This co-pending application filed together herewith is hereby incorporated by reference. 
         [0003]    The advantages of implants over knee replacement have stimulated a further development of smaller implants that can be implanted with less invasive surgery. In this development there has also been an effort to achieve small joint implants, suitable for repair of a small cartilage injury that have a minimal influence on the surrounding parts of the joint. In the current development, such small implants are designed with an implant body that may be formed as a mushroom cap with a hard surface to face the articulating side of the joint and a bone contacting surface engaging the bone below the damaged part of cartilage. The shape and the curvature of the articulating surface of the implant may be designed to be a reconstitution of the shape and the curvature of the part of the joint when it was undamaged. Such implants are usually designed as mushrooms with an implant body or head and with a peg or a rod projecting from the bone contacting side of the implant body for anchoring the implant into the bone. 
         [0004]    WO2007/014164 A2 describes a kit comprising a plurality of small joint implants having different predetermined shapes described as circle, oval, L-shape and triangular shape and tools for placing the implants and a method for placing the implant in a joint, e.g. in the knee or other joints where there is a need for repair of a cartilage and/or bone damage. In this piece of prior art each implant shape has a specific guide tool which corresponds to the shape of the implant. 
         [0005]    The cartilage damage is repaired by choosing the most suitable implant from the different shapes mentioned above. The corresponding guide tool is selected and is used for faster reaming of the area where the implant is to be placed. A drill is used for drilling a hole to accept the post extending from the bone contacting side of the implant. In the end, the implant is placed on the area reamed or drilled out for the implant. Although it is the intention that the guide tool shall be used for the preparation of the placement of the implant it is also said that the use of the guide tool is optional, see passage sections [019, 020]. 
       THE ADVANTAGES OF THE INVENTION 
       [0006]    The aim of the present invention is to solve a complex of difficulties encountered when attempting to repair damaged joints using surgical implants. For a number of different types of joint damage, a circular implant mushroom cap with a central anchoring stem or peg of smaller diameter is preferably used. The deeper small diameter central hole for the central anchoring peg and the shallower larger diameter hole for the implant cap having the new joint repair surface can be accurately drilled at the same time where a circular double drill is preferably used. A double drill has a central small diameter bit and further up a larger diameter drill cutting surface. An example of such a drill used together with the drilling rig of the invention is shown in  FIG. 4 . Such drills are commonly used in other areas for inlaid discs and for countersinking screws. 
         [0007]    But the area of the joint damage may not be easily covered by a single circular implant if the damaged area is elongate or is irregular or large in shape. Instead of using a number of separate implants or an implant requiring complicated bone removal techniques, using several different drills and tools, the surgical implant and the rig according to the present invention provide an exceptionally simple solution which also utilizes a single rig anchored in place for the entire pre-drilling and drilling operation. The same double-drill, the same pre drilling guide socket and the same depth adjustment socket is used for all drillings. This is made possible by a rig which permits shifting of the guide socket or adjustment socket from one side to the other side (or the other sides) of the hollow shell interior between drillings. A shiftable interior arcuate wall can also be inserted in each position to provide a complete circular cylinder for holding the pre-drilling guide socket for each drilling. 
         [0008]    According to one embodiment, this will simply create two identical peg holes and an exactly excavated cavity to fit an implant in the form of two intersecting circles of the same diameter. Merely removing the insert wall in the cylindrical interior then creates a shell, already securely rigged in location, for a gauge for the oblong implant with at least two pegs. A handled gauge in the shape of the implant is inserted after drilling to check that the proper drilling depth has been reached. After all drillings have been made and depth checked, the drilling rig is removed. 
         [0009]    The implant should comprise a biocompatible metal, metal alloy, ceramic or polymeric material. More specifically it may comprise any metal or metal alloy used for structural applications in the human or animal body, such as stainless steel, cobalt-based alloys, chrome-based alloys, titanium-based alloys, pure titanium, zirconium-based alloys, tantalum, niobium and precious metals and their alloys. If a ceramic is used as the biocompatible material, it can be a biocompatible ceramic such as aluminium oxide, silicon nitride or yttria-stabilized zirconia. Preferably the articulate surface comprises a cobalt chrome alloy (CoCr) or stainless steel, diamond-like carbon or a ceramic. 
         [0010]    The implant according to one embodiment of the present invention has two parallel pegs of the same nominal diameter, but with one being slightly larger than the diameter of the hole to provide an interference fit. The other peg of the same nominal diameter is very slightly smaller than the diameter of the hole to provide a slide fit. This relationship will provide secure anchoring of the implant by virtue of the interference fit. The slide fit peg will prevent rotation of the implant and will not give rise to problematic stresses between the pegs which might be the case with two interference fits. This implant and rig will also make is easier to insert and make sure that the implant cap seats securely in place against the bottom of the shallow wide hole drilled into the bone. This is very important in making sure that the implant is held securely by bone growth without cavities. 
         [0011]    The present invention also contemplates as a first alternative a surgical implant having two pegs, but it is also contemplated according to the invention an implant having three or more pegs and an implant form comprising three or more intersecting circles. In this case the drill guide insert wall is shifted between three or more different arcuate depressions in the interior of the rig. 
     
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
         [0012]    The implant and rig of the invention will be described below with reference to a non-limiting example shown in the accompanying drawings of which, 
           [0013]      FIG. 1  shows a two-pegged implant, 
           [0014]      FIG. 2  shows a rig according to the invention for a two pegged implant. The rig is mounted in place on a femoral condyle. 
           [0015]      FIG. 3 a    shows a three-pegged implant having the form of three identical intersecting circles. 
           [0016]      FIG. 3 b    shows from above a rig with wall insert for a three-peg implant. 
           [0017]      FIG. 4  shows a double drill for use with the drilling rig according to the invention. 
           [0018]      FIG. 5 a    shows a pre-drilling guide socket. 
           [0019]      FIG. 5 b    shows a drilling depth adjustment socket. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  shows one exemplary implant according to the present invention, in this case for use in the repair of a damaged condylar surface of the human femur. It is contemplated that in certain applications of the invention the outer surface of cap  3  of the implant  1  will be shaped to conform to the undamaged shape of the patient&#39;s condyle. Standard sized and shaped implants will also be covered by the scope of the main claim. Such implants can also be used for many different joint surfaces in, for example, the joints of the hip, knee, toe and shoulder. 
         [0021]    The implant  1  has a cap  3  with on its outside  41  a new joint surface and on its inside, in this particular embodiment, a ridge  47  which lodge in a drilled groove as will be explained below. The implant cap has the shape of two intersecting circles of the same diameter. Typical implants according to the invention may have a cap with two intersecting circles of diameter 15 mm. Other shapes which may be suitable are 17+17 mm, 20+20 mm and 25+25 mm. At the center of each circle there extends a peg  48 ,  49 . Each peg has, in this particular embodiment, a narrower end  48   a,    49   a  to aid in directing the pegs correctly into drilled holes in the condyle, as will be explained in more detail below. 
         [0022]    In this case, the first peg  48  is longer than the second peg  49 , but they can also be of the same length. According to the invention, both pegs are of the same nominal diameter, but the first peg  48  is slightly larger than the nominal diameter, providing an interference fit shaft of said nominal diameter. An anchoring interference fit between hard metal and living bone requires a greater differential than an interference fit between two metal elements. How much larger than the nominal diameter the first peg is will be a matter of clinical testing and revision. In this context involving a metal shaft in a hole in living bone and in the appended claims the term interference fit in relation to a nominal hole diameter is deemed to include positive differences up to and including approximately +11% increase in diameter over the nominal diameter. To get a very secure grip between a hole of a diameter of 4 mm in living bone and a peg of one of the materials described in the paragraph above, the peg should have a diameter of between ca 4.1 and 4.4 mm. An interference fit between hard metal and living bone requires a significantly larger difference than between a shaft and a hole of hard metal for example. The differential between the first peg diameter and the hole should not be so great as to require excessive force to put it in place with the risk of cracking in the bone. The second peg  49  has a diameter of the same nominal diameter but falling within the standard definitional boundaries for a clearance fit, i.e. almost of the same diameter but very slightly smaller. This relationship will ensure that the implant is securely anchored, is fairly easy to install, and will not give rise to problematic stresses between the pegs, either during implantation or thereafter. 
         [0023]      FIG. 2  shows an example of a rig according to the present invention which is used for all of the hole preparation. The rig comprises an elongated hollow shell  51  having the form of two intersecting (overlapping) right circular cylinders  52 ,  53  of the same diameter. The rig can be formed to conform to the shape of the bone and cartilage area of the patient to be repaired or can be a standard rig. The rig is held securely in place on the condylar surface in this case by pins (not shown) driven in through holes  61 , to hold the rig securely in place throughout the entire drilling process. 
         [0024]    After the pins have been driven in, the cutting and drilling process can begin, with a wall insert  55  inserted in one end of the hollow shell, leaving an entire first right circular cylinder  52  at one end of the hollow tubular shell. At this time the surgeon may insert into the first right circular cylinder a depth adjustment socket  505  ( FIG. 5 b   ) and then a sharp cylindrical hand knife, sized exactly to the interior of the adjustment socket  505 , make a preliminary circular sharp edged cut through the cartilage down to the bone. A circular bare bone area is left after this cartilage removal. 
         [0025]    In one embodiment, the surgeon uses a 17/4 mm double drill as shown schematically in  FIG. 4 . It has a central narrow 4 mm diameter bit  401 , and a wider 17 mm diameter cutting bit  402 . The outer lateral surface  403  of the double drill conforms to a height adjustment socket placed inside the wall insert, which securely holds the double drill to drill, in the same operation, a central 4 mm hole for the first peg  48  and a much shallower surrounding bore  17  mm in diameter in this example. A pre-drilling of the initial part of the peg hole in the bone can be made using a guide socket  501  ( FIG. 5 a   ). This improves the exact placement of the simultaneous drilling of the peg hole and the circular bare-bone area with the double drill ( FIG. 3 ). After removing the drill, and flushing out organic matter, the surgeon then slides the wall insert  53  out and inserts it in on the other side of the hollow shell, creating a complete right circular cylindrical guide hole on the opposite side of the hollow shell. 
         [0026]    The surgeon then inserts the adjustment socket and uses the same cylindrical knife in the newly created guide hole, to make a circular excision of the cartilage (not a complete circle since the intersecting portion has already been removed in the previous step). The in this embodiment 17/4 mm double drill is then used again first with the guide socket  501  to pre-drill the peg hole and then with the adjustment socket  505  to double-drill the peg hole to its full depth and create the bare-bone circle , i.e. the 4 mm hole for the second peg and a second surrounding shallow bore which is of course also 17 mm in diameter. 
         [0027]    These two drilling operations have created 4 mm peg holes and a space in the bone to exactly accommodate in this case a 17+17 implant of the invention. The wall insert  53  is then completely removed. A handle-equipped gauge corresponding to the intersecting circular forms making up the implant, is used to make sure that the holes have been drilled to the proper depth in the bone. The rig is then removed and the implant pegs are inserted into their holes. For the cap of the implant to lodge exactly in the in this case 17+17 shallow cavity removed from the surface of the bone it is usually necessary to carefully tap the cap, preferably on top of the first peg, with interference fit, with a hammer via a special mandrel. The first, slightly thicker peg, is tapped down into its hole while the second peg, slightly narrower, slides easily into its hole. The larger diameter part of the 17/4 mm drill in this example has a rim to excavate a peripheral slot slightly deeper than the 17 mm shallow cavity, to accommodate the peripheral ridge  47  of the implant, helping to hold the implant securely in place during healing and subsequent loading during use. 
         [0028]    Thus the rig, which can be form-fitted to the shape of the individual patient&#39;s condyle in this example, is placed over the damaged area of the condyle and is anchored securely in place, in this particular non-limiting example, by driving in four pins (not shown) into holes  61  in the condyle shaped lower end of the rig  50 . It is now securely in place for the entire drilling operation, which be simplified greatly and made much more exact and less dependent on the artistry of the surgeon, which may vary from day to day. 
         [0029]    After drilling of the holes, the pins are pulled out and the rig is removed from the site, for implantation of the implant and reconstitution of the joint with the new implant. 
         [0030]    It will be understood by the person skilled in the art that the rig as claimed can be supplemented with for example an insert sleeve to make one of the right circular cylinders of a small diameter, e.g. from 17 to 15 mm in diameter, to accommodate an implant having the form of two intersecting circles of slightly different diameters, for example 15+17 millimeters. 
         [0031]    It will of course also be possible, within the scope of the invention to create an implant in the form of three, or more, intersecting circles, to cover bone damage of more irregular shape. 
         [0032]    One such three-circle implant  101  is shown from below in  FIG. 3 a    showing three pegs  148 ,  149  and  150 . In this example peg  148  has an interference fit diameter in relation to the common nominal diameter of all three pegs and the other two pegs  149  and  150  have clearance fit diameters in relation to the common nominal diameter. 
         [0033]    The rig for this three-circle implant is shown from above in  FIG. 3 b   . The rig is held in place on the bone by pins (not shown) inserted through holes  161 . The wall insert  155 , completes the first right circular cylinder  152  covering the remaining portions of the other two right circular cylinders. When the first circular drilling has been made the wall insert  155  is pulled out, rotated 120 degrees and is inserted again to provide a drill guide for the next circle drilling with the same double drill, which in one embodiment can be the same 17/4 drill used together with the two-circle rig. After rotation 120 degrees again and drilling, a three pegged implant is inserted. As stated above, this insert has one peg which is of interference fit dimension in relation to its nominal diameter (in this case 4 mm) and the other two pegs are of clearance fit. 
         [0034]    The implant has a bone contact surface on the underside, on the sides of the cap and on the pegs, which will be in direct contact with the bone tissue when the implant is in place. In one embodiment the bone contact surface comprises a biocompatible metal, metal alloy or ceramic, such as any of the metals, metal alloys or ceramic described above for the articulate surface. Preferably the bone contact surface comprises a cobalt chrome alloy (CoCr), a titanium alloy, titanium or stainless steel. 
         [0035]    In one specific non-limiting embodiment the bone contact surface comprises, or in one specific non-limiting embodiment is coated with, a material that promotes osseointegration. In an alternative embodiment of the invention the bone contact surface does not comprise such a material and/or is uncoated. 
         [0036]    The bioactive material or the material that promotes osseointegration of the bone contact surface, if present, preferably stimulates bone to grow into or onto the implant surface. Several materials that have a stimulating effect on bone growth are known and have been used to promote adherence between implants and bone. Examples of such prior art materials include bioactive glass, bioactive ceramics and biomolecules such as collagens, fibronectin, osteonectin and various growth factors. A commonly used material in the field of implant technology is hydroxyapatite (HA), chemical formula Ca 10 (PO 4 ) 6 (OH) 2 . HA is the major mineral constituent of bone and is able to slowly bond with bone in vivo. HA coatings have been developed for medical implants to promote bone attachment. Another bioactive material commonly used in prior art is bioactive glass. Bioactive glasses, generally comprising SiO 2 , CaSiO 3 , P 2 O 5 , Na 2 O and/or CaO and possibly other metal oxides or fluorides, are able to stimulate bone growth faster than HA. 
         [0037]    The fixation of the implant can also be improved by decreasing the catabolic processes i.e. decrease the amount of bone resorption next to the implant. The bone contact surface and/or the extending post can also be modified with bisphosphonates. 
         [0038]    In one embodiment the bone contact surface is coated with a double coating. Such double coating may for instance comprise an inner coating comprising titanium (Ti). The second, outer coating, that is configured to contact the cartilage and or bone, is preferably a hydroxyapatite and/or beta tricalcium phosphate (TCP) coating. By this design even more long-term fixation of the implant is achieved, since bone in- or on-growth to the implant is further stimulated by the titanium, even if the more brittle hyroxyapatite would eventually shed/dissolve. 
         [0039]    The bone contact surface may also be further modified with fluoro compounds or acid etching to enhance the bioactivity and the osseointegration of the surface. Another method to facilitate osseointegration is blasting of the bone contact surface. 
         [0040]      FIG. 4  shows an exemplary 4/17 double drill for use with the multiple circle rigs described above (or with a previously known single circle rig). The double drill has a 4 mm central bit  401  for creating the hole for the peg and a wider cutting surface  402  for creating the 17 mm shallow hole. One of the advantages of the invention is that the same double drill can be used for single, double or triple (or more) intersecting circle shaped implants, used twice or three times as the case may be for the two embodiments shown here.