Abstract:
Bone-adaptive surface structures are provided for lateral jaw implants. Micromechanical and/or macromechanical surface structures having various body forms are incorporated into selected segments of the surface of a base and of a bar that connects a shaft with the base to accelerate the healing process after the implant is inserted into the jaw bone and to make a critical improvement in holding the implant firmly in place without rotation. A process of manufacturing the surface structures includes forming a macromechanical surface on the implant to increase its surface area; and superimposing micromechanical canals on the macromechanical surface, the canals having a size smaller than the minimum size of the osteons in the bone into which the implant is to be inserted.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of pending U.S. application Ser. No. 10/714,200 filed Nov. 14, 2003, and claims priority of EP 02090379.5 filed Nov. 15, 2002, which claims priority to DE 20208975 U filed Jun. 10, 2002. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The invention concerns bone-adaptive surface structures for orthopedic implants, particularly lateral jaw implants (basal osseointegration); that is, for implants which are inserted into the jaw from the side, and which are also known as “disk implants.” 
       DESCRIPTION OF THE RELATED ART  
       [0003]     Various industrially produced implants have been used for decades in dental implantology. These include cylinder implants, screw implants, leaf implants, and disk implants. Individually produced subperiostal implants also play a part.  
         [0004]     These implants developed from a kind of box shape. They generally have a disk-shaped implant base. U.S. Pat. No. 3,925,892 (Juillet) describes an implant with a rectangular base and a post screwed onto it. Then the crown or the prosthesis is fastened to the post. French patent 2 302 715 (Clune-Cost) describes a one-piece implant. According to U.S. Pat. No. 4,722,687 (Scortecci) a congruent osteotomy instrument was developed later, as well as an osteotomy tool which simultaneously served as its own implant, and so was left in the bone (U.S. Pat. No. 4,815,974). Later, U.S. Pat. No. 4,344,757 described other possible forms, as did U.S. Pat. No. 4,964,801 (Kawahara), EP 0 935 484 A1 (Ihde), etc.  
         [0005]     All the developments mentioned above had in common the fact that they essentially improved the geometry, or the external shape, of the implants. That contributed to a broader range of possible uses, lower fracturability, and better retention.  
       SUMMARY OF THE INVENTION  
       [0006]     The strongest possible hold in the jawbone is an essential objective of any implant. Aside from the variously shaped bases intended to help the implant grow into the jaw bone and improve its firm seating, surface structures intended to improve the holding of the implant have been discussed quite recently. These involve combinations of air abrasion (“sandblasting”) processes with etching processes. Etching has the additional objective of removing residue from mechanical processing and abrading from the surface of the implant. These processes treat the surface at the microscopic level.  
         [0007]     Although at least some of the prior art implants have worked well, further acceleration of the healing process of the implant into the jawbone after its insertion is desirable. Critically improving the anti-rotational retention of the implant is also desirable. The present invention is directed to providing a lateral implant which meets one or more of these goals.  
       SUMMARY OF THE INVENTION  
       [0008]     According to the present invention there is provided a lateral jaw implant that has a surface enlargement including macromechanical bone adaptive surface structures worked into selected sections of the base. In one preferred arrangement the bone adaptive surface structures are periodic.  
         [0009]     It has been found that significant improvements of the retention of the implant can be achieved by lateral implants, by means of macromechanically acting surface structures and even macromechanical surface forming, independent of the geometry of the implant body. At first glance, it appears impossible to apply macromechanical surface structures to lateral implants. It was determined that the implant bed prepared in the jawbone from laterally must be very thin, so that the blood supply is not too seriously impaired and so that primary bone healing is possible. Surprisingly, it was later found that having a surface structure in the form of grooves with profiles which are corrugated, pointed, or sawtooth-like, and which are arranged in parallel, concentric with the shaft, or in the form of a spiral form on the upper and/or the lower surface of the base, and which can at least in part extend onto the bar of the implant, can be inserted relatively easily if the height of the osteotomy slot does not exceed the core height h2 of the surface structure. After insertion of the implant, the surface structures force themselves into the jawbone. This is promoted by the force of chewing, because the surface structures are perpendicular to the masticatory force. These basal osseo implantations differ from the situation with crestal (screw) implants, in which the threads, etc., lie transversally to the direction of chewing.  
         [0010]     It has been determined from histological sections that small particles of bone are scraped off the jawbone during the insertion into the depressions of the surface structures. That avoids formation of voids, and the bone particles that are scraped off substantially accelerate the healing process.  
         [0011]     Histologic examinations have also shown that the jawbones optimally tolerate contact of the basal structures of implants having a macromechanical surface structure. If the depth F of the macromechanical surface structure is from 0.05 to 2 μm, direct bone contact with the implant occurs (osseointegration). It is therefore unnecessary to apply an additional microstructure, such as, for example, titanium coatings applied by plasma spraying. Dispensing with such measures not only reduces cost, but also prevents the layers applied from producing supplemental microstructure separating from the surface during insertion of the implant, or later, thus causing inflammation and symptoms resembling polymetallosis.  
         [0012]     According to an advantageous further development, the surface structures are worked into the base of the implant only in selected annular regions. Such implants present a situation in which only the annular regions with the enlarged surfaces are osseointegrated, while all the other regions experience only moderate osseointegration or are overgrown with connective tissue. This implant configuration is particularly advantageous for use in the vicinity of the maxillary sinuses. In this case the surface of the part of the implant in the vicinity of the maxillary sinus is smooth, resulting in very good self-cleaning, while the cortically anchored regions, i.e., the annular regions, have enlarged surfaces and so are integrated better. In this embodiment, supplemental microstructure of the surfaces by etching, air-abrading, etc., is again not necessarily required with this embodiment of lateral implants.  
         [0013]     The advantageous effects noted above also appear if the surface structure consists of bowl-like depressions. The surface structures can be formed by variously shaped depressions and milled-out regions worked into the marginal zones of the base. In this variant embodiment, the geometry of the depressions, which differ from each other, should advantageously be selected such that the forms of the depressions in contact produce a configuration with reentrant angles. Furthermore, the substrate can in turn be formed so that it ends in the peripheral region of the base with a core height of h2.  
         [0014]     Aside from the advantages noted which are also attainable with this solution, the peripheral depressions in the base substantially improve the in-growth because the jaw bones tend to close, reaching and filling out the depressions with smaller configurations sooner than those with somewhat larger configurations. In this way, a high initial strength can be attained considerably sooner.  
         [0015]     If lateral implants also have elastic properties, then the threaded parts sink into the spongy bones under stress. That results in a tension directed centrally on the threaded parts. In this situation, the surfaces of the previously known lateral implants can tear out of the bone combination. However, due to the surface structures according to the invention, it is possible to maintain the combination with the bone even under large stresses. Thus individual implants can be more heavily loaded, with the result that fewer implants per jaw are needed to reproduce the ability to chew. In this way, even severely atrophied jaws can be cared for without bone grafting.  
         [0016]     With proper selection of microdimensions and macrodimensions, it is possible to provide an implant which fits biomechanically into the bone but is still easily insertable, and which has a surface structure that offers the implant optimal macromechanical hold in the jawbone. Then the vertically acting masticatory forces are absorbed in an astonishing manner by the likewise vertical structures such as, for example, the grooves made in the surface of the implant base and/or bar.  
         [0017]     It is also possible to elevate the surface structure by sintering one or more layers of balls of titanium or a titanium alloy having a diameter of 120 to 220 μm onto the desired surfaces of the implant. This likewise provides higher surface roughness without additional micromechanical measures such as those described above.  
         [0018]     From extensive series of histological examinations and cracking tests, it has been found that the best results for adhesion are attained with the dimensions shown below for the bases and surface structures: 
        Implant diameter D/implant width: 8-15 mm     Height h3 of the implant base where it joints the shaft: 0.7-1.2 mm     Distance E between the peaks of the surface structure: 0.2-0.7 mm     Width G of the ring for the implant base: 1.0-2.4 mm     Core height h2: 0.6-0.9 mm     Depth F of the surface structure: 0.05-0.25 mm.        
 
         [0025]     Lathe-turning or milling processes, or a combination of those two processes, are particularly suitable for producing implants according to the invention. The well-known air-abrading or etching processes are used advantageously for final working.  
         [0026]     In further tests it was found that bonding strength to bone can be even further increased if a microstructure overlays the above described macromechanical structure. This microstructure comprises a net of longitudinal grooves or canals over the whole endosteal surface of the implant. Contrary to the beliefs of the dental profession, further histological investigation has shown that a strong, long-lasting, bone-to-implant adhesion can be achieved not only by large areas of bone to implant contact, but also through a second expedient. The expedient is that small implant-to-bone areas may serve as well for transmitting large forces, if the bone which is in contact to the implant is well nourished from all sides. This nourishment is performed through canals which underlie the macrostructure. It is especially advantageous if those canals, which are confluent in all directions, are equipped with a very smooth surface. The surface is preferably smooth or polished to avoid or reduce the threat of bacterial contamination.  
         [0027]     The very smooth surface structure can be produced in a particularly elegant manner using ablative laser processes. This type of production also favors production of bowl-like surface structures. Hence milling or turning procedures or even laser molding of the surface is used to create the macrostructure of the implant, whereas the confluent and continuing canals, which cover the whole endosseous surface, are carved out by means of laser energy. The surface creates the possibility of passive nutrient diffusion by creating a 3-D surface which allows flow perfusion underneath the integrated bone area virtually from “inside” the implant.  
         [0028]     It is especially advantageous to apply the surface as per onto the rings and peaks of the lateral implants, because the rings are placed in cortical areas of the jaw bone. Those areas will be again turned into cortical bone after healing. Since cortical bone does not contain any blood vessels, good circulation of nutritients over the new, canal-bearing surface, will allow easy microcirculation underneath the osseointegrated surface areas. It is true and it was found in histological tests that the microcirculation may break down during the later phase of use of the implant. This is owed to the fact that bone shows creep and plastic deformation over the time and hence some of the wells may be stuffed later with particles of bone by functional forces or events creating plastic deformation rather than by active ingrowth. Especially advantageous are canals, which show an undercut and being smaller than the size of an osteon of the relevant bone area, because bone will not be able to colonize those areas by active growth at all.  
         [0029]     The optimum size of the canals or undercuts is not nominated in mm; however the size of the undercuts and canals is smaller than the size of osteons in the bone into which the surface is implanted. This is important, because it is known that secondary osteons are considerable larger in the human femur than in human ribs. The physician should determine the minimum size of secondary osteons from experience or literature, or from investigating into the actual patient&#39;s microanatomical situation.  
         [0030]     The micromechanical surface is hence not intended to keep a distance forever between the osseointegrated parts of the implant and the core of the implant. The intention is to allow microcirculation in the initial phase of healing which may last several weeks to months. During later phase of the mentioned healing period, masticatory loads may be applied to the implant.  
         [0031]     One advantageous way of creating the desired surface properties is a two step procedure: during the first operation peaks are milled out or cut out or melted out, preferably leaving round bases without corners, and second the peaks are melted to become round and having undercuts by electrically driven power, namely laser forces or microplasma soldering devices. By this process the sintering of several layers of balls made from implantable material, which would achieve the same intentional result, can be avoided.  
         [0032]     Another advantageous way of achieving the invented surface is that the second operation comprises applying mechanical pressure, also in combination with a rotation procedure. This will bend and/or deform the peaks of the structures which have been created by the first operation.  
         [0033]     The above-described surface production technique is not limited to lateral dental implants; it may be used also for crestal implants, screw, and blade implants. And the surface can be applied to orthopedic implants which are intended to integrate onto bone with the same beneficial results.  
         [0034]     Applying the new surface to lateral dental implants, however, brings additional benefits compared to massive, crestal types of implants (i.e. screws, blades). Due to their design and due to the fact that the base plate hangs between the two corticals of the jaw bone, lateral implants show some possibilities of elastic movement within the bone. The movements are induced by masticatory forces, especially by chewing. Rhythmical motions of the jaw bones support the fluctuation of liquids underneath the integrated surface by creating a repeated pumping effect. Just as this effect could enhance fluctuation within osteons, the new surface allows fluctuation between the implants and the integrating bone surface. Implants, which show structural elasticity, distribute forces over an increasing area with the force increasing and they distribute forces over time. Strong, sharp impact of forces on small bone areas is thus avoided. Elasticity can be by design through choice of materials, thickness, and design of struts and bars. The elastic motions increase fluctuations and through this effect increase nutrition of bone which leads to fast bone growth and to stronger integration in shorter time.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]     Reference is now made more particularly to the drawings which illustrate the best presently known mode of carrying out the invention and wherein similar reference characters indicate the same parts throughout the several views.  
         [0036]      FIG. 1   a  is a longitudinal section taken generally along line L-L of  FIG. 1   b.    
         [0037]      FIG. 1   b  is a plan view of a bone-adaptive lateral jaw implant having surface structures embodying the invention and with a partial rectangular base.  
         [0038]      FIG. 1   c  is a plan view of a lateral jaw implant having surface structures with a spiral design and arrangement.  
         [0039]      FIG. 2  is a plan view of a lateral jaw implant having a round base.  
         [0040]      FIG. 3  is a section taken generally along line M-M and through the bar of the implant shown in  FIG. 2 , and showing an arrangement of the surface structure.  
         [0041]      FIG. 4  is a cross section of a base embodying a surface structure having points and valleys.  
         [0042]      FIG. 5  is a cross section of a base embodying a corrugated surface structure, and with a pointed surface structure provided in the inner wall area of the base.  
         [0043]      FIG. 6  is a partial perspective view of a base having a surface structure with bowl-like indentations.  
         [0044]      FIG. 7  is a cross section of a base with a sawtooth-like surface structure.  
         [0045]      FIG. 8  is a front view of the lateral jaw implant shown in  FIG. 9 .  
         [0046]      FIG. 9  is a plan view of a lateral jaw implant having elongate depressions worked into the peripheral area of its base.  
         [0047]      FIG. 10  is a side view of the lateral jaw implant of  FIG. 9  as seen from the left thereof. 
     
    
     DETAILED DESCRIPTION  
       [0048]     As can be seen in  FIG. 1   a , a screw thread  1  is provided at a distal end of a shaft  3  for connecting with a prosthesis (not shown). The thread can be an internal or external thread. In appropriate circumstances the prosthesis can be adhesively secured, as by an epoxy.  FIG. 1   b  shows a base  2  in the form of an annular body of a lateral dental implant. A bar  4  connects the shaft  3  to the base  2 . It has been determined that implants of this type transfer the masticatory force essentially into the peripheral annular section of the base  2 , indicated here by 5, which is anchored in the cortical jaw bone. As shown in  FIGS. 4, 5  and  7 , numerals  6   a ,  6   b  and  6   c  indicate various surface structures embodying the invention that may be on the base  2 . In  FIG. 1   a , the letter G indicates the width of an outer ring of the base  2  or of a base  8  (see  FIG. 2 ).  
         [0049]     In the embodiment shown in  FIG. 1   b , the surface structures are formed by periodic grooves  6  which can run parallel with each other, as shown at the left of the shaft  3 , or concentrically, as shown at the right of the shaft  3 . The term “periodic” means that the grooves  6  occur at regular intervals, or intermittently. It should be understood that some irregularity of occurrence is intended also to be included within the meaning of the term.  
         [0050]     In the embodiment shown in  FIG. 1   c , the periodic surface structures  6  (which can have a profile such as shown in  FIGS. 4, 5  or  7 ) have a spiral shape and extend essentially over the entire upper and/or lower surface of the base  2 , with the center of the spiral placed in the region where the shaft  3  connects to a bar  4 , advantageously a cross bar.  
         [0051]      FIG. 2  shows another variant in the form of a lateral dental implant having a round base  8 . Pointed surface structures  6   a , or  6   c , or corrugated surface structures (see  FIG. 1   a  or  5 ) are formed by the grooves  6  which may be located on the outer ring and on the bar  4  as far as the shaft  3  in the upper part B of  FIG. 2 . One alternative arrangement is shown in the lower part C of  FIG. 2  where the grooves  6  extend only to the junction of bar  4  with the base  8 . The latter of the two embodiments has the advantage that this design increases the breaking strength of the base  2  in the vicinity of the bar  4 . The letter D indicates the diameter of the round base  8 . Thus round implants are symmetrical; while there can be numerous dimensions for elongate implants or different diameters for round implants with multiple force-transferring bases.  
         [0052]     In  FIG. 3 , the top surface structures continue up to the shaft  3  at the left part B; while in the right part C they terminate about midway between the periphery of the base and the shaft  3 . Therefore, a central height h3 (or thickness) of the base  8  near its juncture with shaft  3 , and a core height h2 (or inner thickness) resulting from the corrugations are arranged so that the surface structures embodying the invention have either apexes which maintain the central height h3 or, as shown in the right part C, diminish outwardly and are less high in the peripheral region of the base  2 . It is also possible for the surface structure in the peripheral region to have a height which exceeds the central height h3 of the base  8  or  2 , or the surface structure which terminates in the vicinity of the junction of the shaft  3  and bar  4  at the implant height h3. In this event, there may be a continuous decrease of the depth of the selected profile shape from the periphery to the shaft  3 .  
         [0053]     In  FIGS. 3 and 8 , RI indicates the radius of the transition from the shaft  3  to the bar  4 . The letter K in  FIG. 3  indicates the direction of the principal masticatory force. However, forces that occur with laterotrusion motions during the act of chewing also act at an angle, even perpendicular (as indicated by the letter A), to the main masticatory force.  
         [0054]      FIG. 4  shows a bone-adaptive surface structure with a profile in which the apexes terminate in points, while the substrate or the wave valleys  7   a  have essentially a round concave shape.  
         [0055]      FIG. 5  shows another optimized variant for the shaping of the surface structure which is also inserted in the vertical sidewall surface of the force-transferring base  2  or  8 . The advantageous shaping of such structures  6   b  on the outer side of the implant in particular is being proven clinically. The letter A′ indicates a direction opposite to the path of inserting the implant. The distance between corrugations or points of the surface structure is indicated by the letter E; while the depth of the macromechanical surface structure is indicated by the letter F. According to the results of the investigations on which this application is based, the distance E is optimally 0.2-0.7 mm.  
         [0056]      FIG. 6  shows a portion of the base  4  in which the surface structure embodying the invention is formed by bowl-like depressions  16  made by milling, by lasers, or a similar process. Investigations have determined that the depth of these depressions is preferably 0.05-0.25 mm. Greater depths retard osseointegration, while lesser depths do not give good results for bone adherence of the implant. The bowl-like depressions  16  result in circumjacent individual elevations around the depressions which form the crest of the surface structure.  
         [0057]     As shown in  FIG. 7 , surface structures with edges  10  perpendicular or approximately perpendicular to the direction of insertion and slowly diminishing concave sides  11  also give particularly good adhesion values. These sawtooth-like structures can be produced either just over the annular region of the base or over the entire implant. However, for reasons of production technology, it can be difficult to provide truly perpendicular edges because the initially vertical edges formed by lathe-turning or machining can be flattened by ablative abrading. In production, the result is that the angles are between 80 and 90 degrees, but they all give good clinical results.  
         [0058]     In the embodiment shown in FIGS.  8  to  10 , elongate recesses or depressions  12 ,  13  and  14 ,  15  are made in the base  8  of the lateral dental implant. They each have a different configuration, in that the geometries that deviate from each other are selected so that there is a reentrant angle between the forms of the adjacent depressions  12 ,  13  and  14 ,  15 . Following insertion of the implant into the ground osteotomy slot, the jaw bone, which has the tendency to draw together, will reach into the lesser depressions  12  and  14  more quickly than into the depressions  13 ,  15  with the greater shaping. In this way, high initial strength is attained relatively rapidly, with the bone particles scraped off during the insertion and collected in the depressions  13 ,  15  promoting bone formation and healing of the implant. The depression  17 , directed toward the insertion side of the implant and preferably placed obliquely, makes it easier to insert the implant into the osteotomy slot. The insertion is made in the direction of arrow A′.  
         [0059]     As indicated above, the surface structure is, in the first instance a macromechanical surface structure as illustrated in  FIGS. 4, 5  and  7 , for example. Milling or turning procedures or even laser molding of the surface is used to create the macrostructure of the implant. In the second instance a micromechanical surface structure may be superimposed or overlaid. This microstructure comprises a net of longitudinal grooves or canals over the whole endosteal surface of the implant. It is especially advantageous if these canals, which are confluent in all directions, are equipped with a very smooth surface to abate bacterial contamination and allow perfusion of the bone adjacent the implant. The very smooth surface structure is produced using ablative laser processes. Hence, the confluent and continuing canals, which cover the whole endosseous surface, are carved out by means of laser energy.  
         [0060]     The optimum size of the canals or undercuts is smaller than the size of osteons in the bone into which the surface is implanted. This is important, because it is known that secondary osteons are considerable larger in the human femur than in human ribs. The physician should determine the minimum size of secondary osteons from experience or literature, or from investigating into the actual patient&#39;s microanatomical situation. This is not limited to lateral dental implants; but may be used also for crestal implants, screw, and blade implants. Further, the micromechanical surface structure can be applied to orthopedic implants which are intended to integrate onto bone with the same beneficial results.  
         [0061]     One advantageous way of creating the desired surface properties is a two step procedure: during the first operation peaks are milled out or cut out or melted out, preferably leaving round bases without corners, and second the peaks are melted to become round and having undercuts by electrically driven power, namely laser forces or microplasma soldering devices.  
         [0062]     Another advantageous way of achieving the desired surface is that the second operation comprises applying mechanical pressure, also in combination with a rotation procedure. This will bend and/or deform the peaks of the structures which have been created by the first operation and form the canals or undercuts.  
         [0063]     The invention in its broader aspects is not limited to the specific steps, processes and apparatuses shown and described, but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its advantages.