Patent Publication Number: US-11389216-B2

Title: Orthopedic fixation screw with bioresorbable layer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 15/451,521 filed Mar. 7, 2017, which is a continuation of Ser. No. 14/876,162, filed Oct. 6, 2016, now U.S. Pat. No. 9,622,803 issued Apr. 18, 2017, which is a continuation of U.S. patent application Ser. No. 13/953,095, filed Jul. 29, 2013, now U.S. Pat. No. 9,179,956 issued Nov. 10, 2015, which is a continuation of U.S. patent application Ser. No. 12/730,661, filed Mar. 24, 2010, now U.S. Pat. No. 8,506,608, issued Aug. 13, 2013, which patent claims priority to U.S. Provisional Application No. 61/162,987 filed Mar. 24, 2009, the applications all being hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to screws used with fixation devices for the treatment of bone fractures using flexible and rigid osteosynthesis. 
     BACKGROUND 
     The clinical success of plate and screw systems for internal fixation of fractures is well-documented. Current systems offer the surgeon a choice of conventional plates and screws, locking plates and screws, or various types of combination plates and screws. 
     Conventional bone plates and screws may be used for treating fractures involving severely comminuted bone or missing bone segments. These conventional systems may also be described as “flexible osteosynthesis” or “biological osteosynthesis” and are particularly well-suited to promoting healing of the fracture by compressing the fracture ends together and drawing the bone into close apposition with other fragments and the bone plate. They are particularly useful in the treatment of comminuted fractures in the diaphyseal region of bones or in regions with severe segmental bone loss. In the case of these fractures, it is imperative to maintain proper bone length while correcting fracture fragments for proper anatomic alignment. With flexible osteosynthesis, the fracture zone is not directly affixed or manipulated, and consequently, the blood circulation in this area is not inhibited. 
     Bone plates designed for flexible osteosynthesis thus operate similarly to a locking, intramedullary nail, which is anchored only in the metaphyses. Flexible osteosynthesis repair constructs allow for micromotion across the fracture site stimulating callous formation. Since the angular relationships between the plate and screws are not fixed, they can change postoperatively, leading to mal-alignment and poor clinical results. 
     The primary mechanism for the change in angular relationship is related to energy storage. Threading a bone screw into bone compresses the bone against the plate. The compression results in high strain in the bone, and, consequently, energy storage. With the dynamic loading resulting from physiological conditions, loosening of the plate and screw and loss of the stored energy can result. 
     Conventional bone screws, i.e. screws that are not secured to a plate so that a fixed angular relationship between the plate and screw is maintained (hereinafter “non-locking screws”) effectively compress bone fragments, but possess a low resistance to shear force that can lead to loosening of the screw. 
     The development of plates incorporating a fixed angular relationship between the bone plate and screws have been developed to combat this problem. Methods of securing the screw to the plate are known as so-called “locking plates”, “locking screws” or “rigid osteosynthesis”. This type of fixation is particularly useful in treating peri-articular fractures, simple shaft fractures (where nailing is impossible), as well as osteotomies. Aside from the possibility of anatomical repositioning, the bone itself supports and stabilizes the osteosynthesis, which allows for the possibility of putting stress on the extremity earlier and without pain. 
     Securing the screw in a fixed angle to the plate reduces the incidence of loosening. As the relationship between the locking screws and the plate is fixed, locking screws provide a high resistance to shear or torsional forces. 
     However, locking screws have a limited capability to compress bone fragments. Additionally, locking screws hold the construct in such a rigid position that micromotion across the fracture site may be impeded thereby inhibiting callous formation. Though used successfully for certain fractures, rigid osteosynthesis has been shown to promote the occurrence of non-unions at the fracture site. 
     A locking screw has threading on an outer surface of its head that mates with corresponding threading on the surface of a plate hole to lock the screw to the plate. Bone plates having threaded holes for accommodating locking screws are known. For example, German Patent Application No. 43 43 117 discloses a bone plate with threaded holes for locking screws. 
     Locking screws have a high resistance to shear force that ensure stability at the bone screw/plate hole interface, but possess a limited ability to compress bone fragments. 
     Since fractures cannot always be treated with both types of osteosynthesis at the same fixation point, surgeons must frequently compromise because bone plate screw holes only allow him to choose between one of these two types of continuous osteosynthesis discussed above. The ideal fracture stabilization construct would allow the surgeon to choose between continuous flexible osteosynthesis, continuous rigid osteosynthesis and temporary rigid osteosynthesis transforming to flexible osteosynthesis within a pre-defined time period. 
     By having the option to rigidly fix a fracture fragment via a known location for a pre-determined period of time and allowing that rigid fixation to transform into a region of flexible osteosynthesis, the surgeon is thus enabled to expose the fracture site to a period of stability followed by controlled micromotion thus stimulating bony healing. 
     SUMMARY 
     The invention concerns an orthopedic fixation device for connecting a first bone portion to a second bone portion. The device comprises a body for linking the first bone portion to the second bone portion. The body has a plurality of holes extending therethrough. A screw is insertable though at least one of the holes extending through the body. The screw comprises a shaft having a distal end and an oppositely disposed proximal end. External helical screw threads extend along at least a first portion of the shaft. A head is attached to the proximal end of the shaft. A layer of bioresorbable material is positioned surrounding a second portion of the shaft, either adjacent to the head or in spaced relation to the head. The layer of bioresorbable material has an outer surface engageable with the body to initially fix the screw at a desired angular position relatively to the body. 
     The screw is angularly movable with respect to the body upon resorbtion of at least a portion of the bioresorbable layer. 
     In another embodiment, an orthopedic fixation device for connecting a first bone portion to a second bone portion according to the invention comprises a body for linking the first bone portion to the second bone portion. The body has a plurality of holes extending therethrough. A layer of bioresorbable material is positioned on the body within at least one of the holes. A screw is insertable though the at least one hole. The screw comprises a shaft having a distal end and an oppositely disposed proximal end. External helical screw threads extend along at least a first portion of the shaft. A head is attached to the proximal end of the shaft. A second portion of the shaft, adjacent to the head, or in spaced relation to the head, is engageable with the bioresorbable layer to initially fix the screw at a desired angular position relatively to the body. The screw is angularly movable with respect to the body upon resorbtion of at least a portion of the bioresorbable layer. 
     In another embodiment of an orthopedic fixation device for connecting a first bone portion to a second bone portion, the device comprises a body for linking the first bone portion to the second bone portion. The body has a plurality of holes extending therethrough. 
     A screw is insertable though at least one of the holes extending through the body. The screw comprises a shaft having a distal end and an oppositely disposed proximal end. External helical screw threads extending along at least a first portion of the shaft. A head is attached to the proximal end of the shaft. The head has a surface portion contiguous with the proximal end of the shaft. A layer of bioresorbable material is positioned on the surface portion of the head contiguous with the proximal end of the shaft. The layer of bioresorbable material has an outer surface engageable with the body to initially fix the screw at a desired angular position relatively to the body. The screw is angularly movable with respect to the body upon resorbtion of at least a portion of the bioresorbable layer. 
     In another embodiment of an orthopedic fixation device for connecting a first bone portion to a second bone portion according to the invention, the device comprises a body for linking the first bone portion to the second bone portion. The body has a plurality of holes extending therethrough. 
     A layer of bioresorbable material is positioned on the body within at least one of the holes. A screw is insertable though the at least one hole. The screw comprises a shaft having a distal end and an oppositely disposed proximal end. External helical screw threads extend along at least a first portion of the shaft. A head is attached to the proximal end of the shaft. The head has a surface portion contiguous with the proximal end of the shaft. The surface portion contiguous with the proximal end of the shaft is engageable with the bioresorbable layer to initially fix the screw at a desired angular position relatively to the body. The screw is angularly movable with respect to the body upon resorbtion of at least a portion of the bioresorbable layer. 
     Another orthopedic fixation device for connecting a first bone portion to a second bone portion comprises a body having a bone contacting surface and an obverse surface arranged opposite to the bone contacting surface. A side surface extends between the bone contacting surface and the obverse surface. A plurality of holes extending through the body. At least one channel is positioned within either or both the obverse surface and the bone contacting surface and extends from one of the holes to the side surface. 
     Another embodiment of an orthopedic fixation device for connecting a first bone portion to a second bone portion comprises a body having a bone facing surface, an obverse surface arranged opposite to the bone facing surface, and a plurality of holes extending through the body between the bone facing surface and the obverse surface. A plurality of projections are positioned on the bone facing surface and extend outwardly away therefrom. 
     Another orthopedic fixation device for connecting a first bone portion to a second bone portion according to the invention comprises a body for linking the first bone portion to the second bone portion. The body has a plurality of holes extending therethrough. 
     A fastener is insertable though at least one of the holes extending through the body. The fastener comprises a shaft having a distal end and an oppositely disposed proximal end. A head is attached to the proximal end of the shaft. A layer of bioresorbable material is positioned surrounding a portion of the shaft, adjacent to the head, or in spaced relation to the head. The layer of bioresorbable material has an outer surface engageable with the body to initially fix the fastener at a desired angular position relatively to the body. The fastener is angularly movable with respect to the body upon resorbtion of at least a portion of the bioresorbable layer. 
     Another embodiment of an orthopedic fixation device for connecting a first bone portion to a second bone portion according to the invention comprises a body for linking the first bone portion to the second bone portion. The body has a plurality of holes extending therethrough. 
     A fastener is insertable though at least one of the holes extending through the body. The fastener comprises a shaft having a distal end and an oppositely disposed proximal end. A head is attached to the proximal end of the shaft. The head has a surface portion contiguous with the proximal end of the shaft. A layer of bioresorbable material is positioned on the surface portion of the head contiguous with the proximal end of the shaft. The layer of bioresorbable material has an outer surface engageable with the body to initially fix the fastener at a desired angular position relatively to the body. The fastener is angularly movable with respect to the body upon resorbtion of at least a portion of the bioresorbable layer. 
     In the example embodiments described the bioresorbable material is selected form the group consisting of polylactic acid (PLA), poly-L-lactic-co-glycolic acid (PLGA), poly-D/L-lactic acid with or without polyglycolic acid (PDLLA, PDLLA-co-PGA), poly-L-lactic acid with or without -tricalcium phosphate (PLLA, PLLA-TCP), poly-L-lactic acid with hydroxyapatite (PLLA-HA), polycaprolactone (PCL), polycaprolactone-Calcium Phosphate (PCL-CaP), poly(L-lactide-co-D,L-lactide) (PLADLA), hydroxyapatite (HA), tricalcium phosphate m-TCP), nanodiamond particles (ND) and combinations thereof. Another example includes bioresorbable material that expand upon contact with bodily fluids. In a specific example embodiment, the bioresorbable material is selected form the group consisting of copolymer lactic glycolic acid, biodegradeable self-expanding poly-L,D-lactide, PDLLA comprising D-Lactide and L-lactide and poly-L-lactide and poly-E-caprolactone homopolymers, methylmethacrylate and acrylic acid and cross linking agent allymelhacrylate, and combinations thereof. 
     The invention also encompasses a method of treating a bone fracture in a living organism having a plurality of bone fragments. The method comprises:
         attaching a body to at least two of the bone fragments using a plurality of fasteners joining the body to the fragments;   fixing an angular orientation of at least one of the fasteners in relation to the body using a bioresorbable material positioned between the fastener and the body, the bioresorbable material contacting the one fastener and the body and preventing relative rotation therebetween;   allowing the bioresorbable material to be resorbed by the living organism, thereby allowing relative rotation between the one fastener and the body.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional view of a bone plate fixation system; 
         FIG. 2  is a longitudinal sectional view of a hip screw fixation system; 
         FIG. 3  is an elevational view of an intramedullary rod fixation system; 
         FIGS. 4-9  are partial sectional views of bone screws having a layer of bioresorbable material thereon; 
         FIGS. 10, 11, 11A, 12-14, 14A, 15-20, 20A, 21 and 21A  are detailed elevational views of bone screws having features for facilitating attachment of a layer of bioresorbable material thereto; 
         FIGS. 22-27  are elevational views of pins having a layer of bioresorbable material thereon; 
         FIGS. 28-31, 31A, 32-38 and 38A  are elevational views of pins having features for facilitating attachment of a layer of bioresorbable material thereto; 
         FIGS. 39-44  are partial sectional views of bone screws having a layer of bioresorbable material thereon; 
         FIGS. 45-48, 48A, 49-53, 53A, 54 and 54A  are detailed elevational views of bone screws having features for facilitating attachment of a layer of bioresorbable material thereto; 
         FIGS. 55-60  are elevational views of pins having a layer of bioresorbable material thereon; 
         FIGS. 61-64, 64A, 65-69, 69A, 70 and 70A  are elevational views of pins having features for facilitating attachment of a layer of bioresorbable material thereto; 
         FIGS. 71, 71A, 72, 72A, 73, 73A, 74 and 74A  are partial sectional views of a portion of a fixation device having a bioresorbable layer thereon; 
         FIGS. 75-77  are elevational views of fasteners used with the fixation device; 
         FIGS. 78, 78A, 79, 79A, 80, 80A, 81, 81A, 82, 82A, 83, 83A, 83B and 83C  are partial sectional views of a portion of a fixation device illustrating rigid to flexible osteosynthesis transformation; 
         FIGS. 84 and 85  are isometric views of an example bone plate embodiment; 
         FIGS. 86 and 87  are partial isometric views of example embodiments of bone plate details; 
         FIGS. 88-91  are cross sectional views showing different embodiments of the bone plate shown in  FIGS. 84-87 ; and 
         FIG. 92  is an elevational view of an alternate embodiment of a bone plate according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 3  illustrate example orthopedic fixation devices  10  according to the invention.  FIG. 1  shows device  10  having a body  12 , in this example, a bone plate  14 . Bone plate  14  has a plurality of holes  16  which receive fasteners  18  for attaching the bone plate  14  to bone portions  20   a  and  20   b  for the repair of a fracture  22 . Example fasteners  18  include bone screws  24 , and pins  26 . The example orthopedic fixation device  10  in  FIG. 2  is a hip screw  28 , the hip screw comprising a body  12  having holes  16  which receive fasteners  18 . Again, the fasteners include bone screws  24  and pins  26  as well as other components such as compressing screw  24   a.    
       FIG. 3  illustrates an intramedullary rod  30 , the rod comprising a body  12  having holes  16  to receive fasteners  18 , such as bone screws  24  and or pins  26  for attachment of the rod to bone portions  20   a  and  20   b . The body, screws and pins are made of biocompatible materials such as stainless steel and titanium. 
     These three orthopedic fixation devices are illustrative examples of the invention disclosed herein, but are not meant to limit application of the invention, it being understood that the detailed descriptions of the various components which follow apply to the devices disclosed herein as well as similar devices used for orthopedic fixation in the treatment of bone fractures as well as other disorders. For example, the invention may be used in spinal fixation systems, in particular, to anterior cervical plating systems. 
       FIG. 4  shows an example bone screw  24 , comprising a shaft  32 , the shaft having a distal end  34  and an oppositely disposed proximal end  36  to which a head  38  is attached. Shaft  32  has external helical screw threads  39  extending along at least a portion of the shaft. Cutting flutes  40  may be positioned at the distal end  34  of the shaft  32 , and the screw  24  may be cannulated, having a duct  42  therethrough. In this embodiment, a layer of bioresorbable material  44  is positioned surrounding a portion of the shaft  32  adjacent to the head  38 . The layer of bioresorbable material may be formed on the shaft  32  by injection molding techniques for example. The layer of bioresorbable material  44  has an outer surface  46  which is engageable with the body  12  of the device  10  (see  FIGS. 1-3 ) to initially fix the screw  24  at a desired angular position relatively to the body  12 . The screw  24  becomes angularly movable relatively to the body  12  when the bioresorbable layer, or a portion thereof, is absorbed as described in detail below. 
     Outer surface  46  may be smooth, as shown in  FIGS. 4-6  and may comprise a cylindrical surface  48  ( FIG. 4 ), a conical surface  50  ( FIG. 5 ) or a spherical surface  52 , shown in  FIG. 6 . Other surface shapes are also feasible. The smooth outer surface  46  may engage the body through frictional contact to fix the angular position of the screw, or external screw threads may be cut into the outer surface  46  upon contact between the outer surface  46  and the body  12  as explained below. 
     As shown in  FIG. 7 , the outer surface  46  may have external helical screw threads  54  which are compatible with internal screw threads in holes  16  of the body  12  to effect angular fixation of the screw  24  relative to the body  12 . Threads  54  may have the same or different pitch from threads  39  on the shaft  32 . Threaded outer surface  46  may comprise a cylindrical surface  56  as shown in  FIG. 7 , a conical surface  58  as shown in  FIG. 8 , or a spherical surface  60  as shown in  FIG. 9 . 
     To facilitate attachment of the bioresorbable layer  44  to the shaft  32  of the bone screw  24 , surface features may be positioned on a portion of the shaft adjacent to head  38 . The surface features increase the surface area of the shaft to afford greater adhesion between the layer  44  and the shaft  32 , and also act as positive areas of contact which prevent relative rotation between the layer and the shaft. Examples of shaft surface features are shown in  FIGS. 10-13 .  FIG. 10  shows a screw  24  having the external threads  39  extending along the entire length of shaft  32 .  FIGS. 11 and 11A  show a screw  24  having a plurality of ribs  62  projecting radially outwardly from the shaft  32 . In this embodiment, ribs  62  extend lengthwise along the shaft  32 . In another embodiment, shown in  FIG. 12 , the ribs  62  extend circumferentially around the shaft  32 .  FIG. 13  shows an embodiment wherein the ribs are oriented helically around shaft  32 . 
     Alternately, as shown in  FIGS. 14-16 , the surface feature may comprise grooves or channels  64 . The channels  64  may extend lengthwise along the shaft  32  as shown in  FIG. 14 , also shown in cross section in  FIG. 14A , or circumferentially around the shaft as shown in  FIG. 15 , or the channels may be arranged in a helical pattern as shown in  FIG. 16 . Although shown elsewhere as well, it should be noted that each bone screw head  31  has a top side  31   y  and a bottom side  31   x , and as shown in  FIG. 48 , the channels  65  extend therebetween. 
     Additional surface features to facilitate attachment of the bioresorbable layer  44  to shaft  32  include knurling  66  as shown in  FIG. 17  or a rough textured surface  68  as shown in  FIG. 18 . The rough textured surface  68  may result from a powdered metal coating adhered to the shaft using epoxy, cyanoacrylate, or other adhesives, or may be formed by sand blasting the shaft  32 .  FIG. 19  shows a shaft  32  having a reversed tapered portion  70  adjacent to head  38 . Attachment of the bioabsorbable layer may also be facilitated by modifying the cross sectional shape of the shaft  32  over a portion of the proximal end  36  near the head  38 .  FIGS. 20 and 20A  show a shaft  32  with an oval cross section, whereas  FIGS. 21 and 21A  show a shaft having a polygonal cross section. 
       FIG. 22  shows an example pin  26 , comprising a shaft  72 , the shaft having a distal end  74  and an oppositely disposed proximal end  76  to which a head  78  is attached. The pin  26  may be cannulated, having a duct  80  therethrough. In this embodiment, a layer of bioresorbable material  44  is positioned surrounding a portion  82  of the shaft  72  adjacent to the head  78 . The layer of bioresorbable material may be formed on the shaft  72  by injection molding techniques for example. The layer of bioresorbable material  44  has an outer surface  84  which is engageable with the body  12  of the device  10  (see  FIGS. 1-3 ) to initially fix the pin  26  at a desired angular position relatively to the body  12 . The pin  26  becomes angularly movable relatively to the body  12  when the bioresorbable layer, or a portion thereof, is absorbed as described in detail below. 
     Outer surface  84  may be smooth, as shown in  FIGS. 22-24  and may comprise a cylindrical surface  88  ( FIG. 22 ), a conical surface  90  ( FIG. 23 ) or a spherical surface  92 , shown in  FIG. 24 . Other surface shapes are also feasible. The smooth outer surface  84  may engage the body through frictional contact to fix the angular position of the screw, or external screw threads may be cut into the outer surface  84  upon contact between the outer surface  84  and the body  12  as explained below. 
     As shown in  FIG. 25 , the outer surface  84  may have external helical screw threads  94  which are compatible with internal screw threads in holes  16  of the body  12  to effect angular fixation of the pin  26  relative to the body  12 . Threaded outer surface  84  may comprise a cylindrical surface  96  as shown in  FIG. 25 , a conical surface  98  as shown in  FIG. 26 , or a spherical surface  100  as shown in  FIG. 27 . 
     To facilitate attachment of the bioresorbable layer  44  to the shaft  72  of the pin  26 , surface features may be positioned on a portion of the shaft adjacent to head  78 . The surface features increase the surface area of the shaft to afford greater adhesion between the layer  44  and the shaft  72 , and also act as positive areas of contact which prevent relative rotation between the layer and the shaft. Examples of shaft surface features are shown in  FIGS. 28-30 .  FIG. 28  shows a pin  26  having a plurality of ribs  102  projecting radially outwardly from the shaft  72 . In this embodiment, ribs  102  extend lengthwise along the shaft  72 . In another embodiment, shown in  FIG. 29 , the ribs  102  extend circumferentially around the shaft  72 . 
       FIG. 30  shows an embodiment wherein the ribs are oriented helically around shaft  72 . 
     Alternately, as shown in  FIGS. 31-33 , the surface feature may comprise grooves or channels  104 . The channels  104  may extend lengthwise along the shaft  72  as shown in  FIG. 31 , also shown in cross section in  FIG. 31A , or circumferentially around the shaft as shown in  FIG. 32 , or the channels may be arranged in a helical pattern as shown in  FIG. 33 . 
     Additional surface features to facilitate attachment of the bioresorbable layer  44  to shaft  72  include knurling  106  as shown in  FIG. 34  or a rough textured surface  108  as shown in  FIG. 35 . The rough textured surface  108  may result from a powdered metal coating adhered to the shaft using epoxy, cyanoacrylate, or other adhesives, or may be formed by sand blasting the shaft  72 .  FIG. 36  shows a shaft  72  having a reversed tapered portion  110  adjacent to head  78 . Attachment of the bioabsorbable layer may also be facilitated by modifying the cross-sectional shape of the shaft  72  over a portion of the proximal end  76  near the head  78 .  FIGS. 37 and 37A  show a shaft  32  with an oval cross section, whereas  FIGS. 38 and 38A  show a shaft having a polygonal cross section. 
       FIG. 39  shows another example bone screw  25 , comprising a shaft  33 , the shaft having a distal end  35  and an oppositely disposed proximal end  37  to which a head  31  is attached. Head  31  has a surface portion  31   a  contiguous with the proximal end  37  of shaft  33 . Shaft  33  has external helical screw threads  39  extending along at least a portion of the shaft. 
     Cutting flutes  41  may be positioned at the distal end  35  of the shaft  33 , and the screw  25  may be cannulated, having a duct  43  therethrough. In this embodiment, a layer of bioresorbable material  45  is positioned surrounding the surface portion  31   a  of head  31  contiguous with the proximal end  37  of shaft  33 . The layer of bioresorbable material may be formed on the head  31  by injection molding techniques for example. The layer of bioresorbable material  45  has an outer surface  47  which is engageable with the body  12  of the device  10  (see  FIGS. 1-3 ) to initially fix the screw  25  at a desired angular position relatively to the body  12 . The screw  25  becomes angularly movable relatively to the body  12  when the bioresorbable layer, or a portion thereof, is absorbed as described in detail below. 
     Outer surface  47  may be smooth, as shown in  FIGS. 39-41  and may comprise a cylindrical surface  49  ( FIG. 39 ), a conical surface  51  ( FIG. 40 ) or a spherical surface  53 , shown in  FIG. 41 . Other surface shapes are also feasible. The smooth outer surface  47  may engage the body through frictional contact to fix the angular position of the screw, or external screw threads may be cut into the outer surface  47  upon contact between the outer surface  47  and the body  12  as explained below. 
     As shown in  FIG. 42 , the outer surface  47  may have external helical screw threads  55  which are compatible with internal screw threads in holes  16  of the body  12  to effect angular fixation of the screw  25  relative to the body  12 . Threads  55  may have the same or different pitch from threads  39  on the shaft  33 . Threaded outer surface  47  may comprise a cylindrical surface  57  as shown in  FIG. 42 , a conical surface  59  as shown in  FIG. 43 , or a spherical surface  61  as shown in  FIG. 44 . 
     To facilitate attachment of the bioresorbable layer  45  to the shaft  33  of the bone screw  25 , surface features may be positioned on the surface  31   a  of head  31  contiguous with the proximal end  37  of shaft  33 . The surface features increase the surface area of the head to afford greater adhesion between the layer  45  and the head  31 , and also act as positive areas of contact which prevent relative rotation between the layer and the head. 
     Examples of head surface features are shown in  FIGS. 45-47 .  FIG. 45  shows a screw  25  having a plurality of ribs  63  projecting radially outwardly from the head  31 . In this embodiment, ribs  63  extend toward the shaft  33 . In another embodiment, shown in  FIG. 46 , the ribs  63  extend circumferentially around the head  31 .  FIG. 47  shows an embodiment wherein the ribs are oriented helically around head  31 . 
     Alternately, as shown in  FIGS. 48-50 , the surface feature may comprise grooves or channels  65 . The channels  65  may extend toward the shaft  33  as shown in  FIG. 48 , also shown in cross section in  FIG. 48A , or circumferentially around the shaft as shown in  FIG. 49 , or the channels  65  may be arranged in a helical pattern as shown in  FIG. 50 . 
     Additional surface features to facilitate attachment of the bioresorbable layer  45  to head  31  include knurling  67  as shown in  FIG. 51  or a rough textured surface  69  as shown in  FIG. 52 . The rough textured surface  69  may result from a powdered metal coating adhered to the shaft using epoxy, cyanoacrylate, or other adhesives, or may be formed by sand blasting the shaft  33 . Attachment of the bioabsorbable layer may also be facilitated by modifying the cross sectional shape of the head  31 .  FIGS. 53 and 53A  show a head  31  with an oval cross section, whereas  FIGS. 54 and 54A  show a head  31  having a polygonal cross section. 
       FIG. 55  shows an example pin  27 , comprising a shaft  73 , the shaft having a distal end  75  and an oppositely disposed proximal end  77  to which a head  79  is attached. Head  79  has a surface portion  79   a  contiguous with the proximal end  77  of shaft  73 . The pin  27  may be cannulated, having a duct  81  therethrough. In this embodiment, a layer of bioresorbable material  45  is positioned surrounding the surface portion  79   a  of head  79  contiguous with the proximal end  77  of shaft  73 . The layer of bioresorbable material may be formed on the head  79  by injection molding techniques for example. The layer of bioresorbable material  45  has an outer surface  85  which is engageable with the body  12  of the device  10  (see  FIGS. 1-3 ) to initially fix the pin  27  at a desired angular position relatively to the body  12 . The pin  27  becomes angularly movable relatively to the body  12  when the bioresorbable layer, or a portion thereof, is absorbed as described in detail below. 
     Outer surface  85  may be smooth, as shown in  FIGS. 55-57  and may comprise a cylindrical surface  89  ( FIG. 55 ), a conical surface  91  ( FIG. 56 ) or a spherical surface  93 , shown in  FIG. 57 . Other surface shapes are also feasible. The smooth outer surface  85  may engage the body through frictional contact to fix the angular position of the screw, or external screw threads may be cut into the outer surface  85  upon contact between the outer surface  85  and the body  12  as explained below. 
     As shown in  FIG. 58 , the outer surface  85  may have external helical screw threads  95  which are compatible with internal screw threads in holes  16  of the body  12  to effect angular fixation of the pin  27  relative to the body  12 . Threaded outer surface  85  may comprise a cylindrical surface  97  as shown in  FIG. 58 , a conical surface  99  as shown in  FIG. 59 , or a spherical surface  101  as shown in  FIG. 60 . 
     To facilitate attachment of the bioresorbable layer  45  to the head  79  of the pin  27 , surface features may be positioned on the surface  79   a  of the head  79  contiguous with shaft  73 . The surface features increase the surface area of the head to afford greater adhesion between the layer  45  and the head  79 , and also act as positive areas of contact which prevent relative rotation between the layer and the head. Examples of head surface features are shown in  FIGS. 61-63 .  FIG. 61  shows a pin  27  having a plurality of ribs  103  projecting radially outwardly from the head  79 . In this embodiment, ribs  103  extend toward the shaft  73 . In another embodiment, shown in  FIG. 62 , the ribs  103  extend circumferentially around the head  79 .  FIG. 63  shows an embodiment wherein the ribs  103  are oriented helically around head  79 . 
     Alternately, as shown in  FIGS. 64-66 , the surface feature may comprise grooves or channels  105  on head  79 . The channels  105  may extend toward the shaft  73  as shown in  FIG. 64 , also shown in cross section in  FIG. 64A , or circumferentially around the head as shown in  FIG. 65 , or the channels  105  may be arranged in a helical pattern as shown in  FIG. 66 . 
     Additional surface features to facilitate attachment of the bioresorbable layer  45  to head  79  include knurling  107  as shown in  FIG. 67  or a rough textured surface  109  applied to the head as shown in  FIG. 68 . The rough textured surface  109  may result from a powdered metal coating adhered to the shaft using epoxy, cyanoacrylate, or other adhesives, or may be formed by sand blasting the head  79 . Attachment of the bioabsorbable layer may also be facilitated by modifying the cross sectional shape of the head  79 . 
       FIGS. 69 and 69A  show a head  79  with an oval cross section, whereas  FIGS. 70 and 70A  show a head having a polygonal cross section. 
       FIGS. 71-74  show detailed cross sectional views of alternate embodiments of holes  16  in body  12 , which represent, for example, the holes through bone plate  14 , shown in  FIG. 1 , hip screw  28 , shown in  FIG. 2 , and intramedullary rod  30 , shown in  FIG. 3 .  FIG. 71  shows hole  16  in body  12  having a countersink surface  112  surrounding hole  16 . The countersink hole in this example is conical.  FIG. 72  shows a spherical countersink surface  114 . The countersink surfaces  112  and  114  permit angular motion of the fasteners  18  relative to the body  12  when the fasteners are released from the body by absorbtion of the bioresorbable material as described below. The range of angular motion of the fasteners is further augmented by the use of an undercut surface  116  in conjunction with the countersink surface as shown in  FIG. 73 . Undercut surface  116  is positioned opposite to the countersink surface, meaning that the undercut surface is positioned surrounding the hole  16  on an opposite face of the body  12 .  FIG. 73  shows a conical undercut surface  116  matched with a conical countersink surface  112 , while  FIG. 74  illustrates a spherical undercut surface  118  matched with a spherical countersink surface  114 . 
     In the embodiment shown in  FIG. 71 , a layer of bioresorbable material  44  may be positioned on the body  12  within at least one of the holes  16 . The layer  44  takes the form of an annulus  120  and has an inwardly facing surface  122  which may be cylindrical and/or conical as shown in  FIGS. 71 and 73 , as well as spherical, as shown in  FIG. 72  and other shapes are also feasible. Inwardly facing surface  122  may be smooth as shown in  FIGS. 71-74 , or may have internal screw threads  124  as shown in  FIGS. 71A-74A . When surface  122  is threaded, the threads engage external threads  124  which are positioned on shaft  126  adjacent to the head of a fastener  128  as shown in  FIG. 75 , or on a head  130  of a fastener  132  as illustrated in  FIG. 76 . Note that either or both fasteners  128  and  132  may be a bone screw ( FIG. 75 ) or a pin ( FIG. 76 ). When the inwardly facing surface  122  is smooth it engages fasteners through friction, or, a fastener  134 , shown in  FIG. 77 , may have a cutting edge  136  which cuts internal screw threads into the smooth inwardly facing surface  122  as the fastener is rotated. Again, fastener  134  may be a bone screw or a pin, a bone screw being shown by way of example. It is further understood that fasteners having a layer of bioresorbable material thereon, as shown in  FIGS. 4-9, 22-27, 39-44, and 55-60  may also be used with a body having bioresorbable material as shown in  FIGS. 71-74 and 71A-74A . 
       FIGS. 78-83  illustrate operation of the fixation device according to the invention. These figures represent a body  12  having holes  16  that receive fasteners  18 . The body could be, for example, part of a bone plate as shown in  FIG. 1 , a hip screw as shown in  FIG. 2 , an intramedullary rod as shown in  FIG. 3 , or another fixation device. The fasteners are bone screws and pins as described above. 
       FIG. 78  shows bone screw  24  having the bioresorbable layer  44  on a portion of screw shaft  32  adjacent to the head  38 . When the screw  24  is inserted through the hole  16  in body  12  and tightened, the outer surface  46  of the layer  44  engages the body and rigidly fixes the angular orientation of the screw relative to the body (the threaded portion of shaft  32  engages the bone, not shown for clarity). Engagement between the layer  44  and the body  12  may be through any of the example mechanisms described above. For example, outer surface  46  may have external screw threads that engage compatible internal screw threads within hole  16 ; the outer surface  46  may be smooth and a cutting edge (not shown) positioned within hole  16  cuts external threads in the layer  44 ; or, the outer surface  46  of layer  44  may depend on friction between the it and the body portion surrounding the hole to provide the desired angular fixation. When all, or at least a portion, of the layer  44  is resorbed, as shown in  FIG. 78A , the screw  24  is free to move angularly relatively to the body  12 , as evidenced by the canted position shown, and thus the interaction between the body  12  and the bone is transformed from a region of rigid fixation to a region of flexible osteosynthesis which permits micromotion across a fracture site stimulating callous formation and bony healing. Angular rigidity of the screw may be augmented by the particular shape of the layer  44 , for example, a conical, tapered shape being advantageous for rigidity. Angular motion of the screw  24  is further controlled through the use of countersink and undercut surfaces as described above and shown in  FIGS. 71-74 . 
     In an alternate embodiment, shown in  FIG. 79 , the layer of bioresorbable material  44  is positioned on the body  12  within at least one of the holes  16 . 
     When the screw  24  is inserted through the hole  16  in body  12  and tightened, the outer surface  46  of the layer  44  engages the screw and rigidly fixes the angular orientation of the screw relative to the body. 
     Engagement between the layer  44  and the screw may be through any of the example mechanisms described above. For example, outer surface  46  may have internal screw threads that engage compatible external screw threads on the screw  24 ; the outer surface  46  may be smooth and a cutting edge (as shown at  136  in  FIG. 77 ) positioned on the screw  24  cuts internal threads in the layer  44  as the screw is rotated, the internal threads engaging external threads on the screw; or, the outer surface  46  of layer  44  may depend on friction between it and the screw shaft to provide the desired angular fixation. When all, or at least a portion, of the layer  44  is resorbed, as shown in  FIG. 79A , the screw  24  is free to move angularly relatively to the body  12  and thus transform the engagement between body and bone from a region of rigid fixation to a region of flexible osteosynthesis and permit micromotion across a fracture site stimulating callous formation and bony healing. Angular rigidity of the screw may be augmented by the particular shape of the layer  44 , for example, a conical, tapered shape being advantageous for rigidity. Angular motion of the screw  24  is further controlled through the use of countersink and undercut surfaces as described above and shown in  FIGS. 71-74 . 
       FIGS. 80 and 80A  show another embodiment wherein the bioresorbable material layer  44  is positioned on both the screw  24  and the body  12 . In this example embodiment, interaction between the outer surfaces  46  of the layers  44  on the screw  24  and on the body  12 , as shown in  FIG. 80 , initially fixes the angular orientation of the screws relatively to the body. Interaction may be through friction between the surfaces or threaded engagement. Countersink and undercut surfaces may again be used to control the limits of relative angular motion between the screw and the body. When the layers  44 , or a portion thereof, are resorbed, the screws  24  are no longer rigidly fixed and may move angularly with respect to the body  12  as shown in  FIG. 80A , thereby providing the advantages of both the rigid and flexible osteosynthesis systems. 
     In another embodiment, shown in  FIG. 81  a bone screw  25  has the bioresorbable layer  45  on a portion of the head  31 . When the screw  25  is inserted through the hole  16  in body  12  and tightened, the outer surface  47  of the layer  45  engages the body and rigidly fixes the angular orientation of the screw relative to the body. Engagement between the layer  45  and the body  12  may be through any of the example mechanisms described above. For example, outer surface  47  may have external screw threads that engage compatible internal screw threads within hole  16 ; the outer surface  47  may be smooth and a cutting edge (not shown) positioned within hole  16  cuts external threads in the layer  45 , or, the outer surface  47  of layer  45  may depend on friction between the it and the body portion surrounding the hole to provide the desired angular fixation. When all, or at least a portion, of the layer  45  is resorbed, as shown in  FIG. 81A , the screw  25  is free to move angularly relatively to the body  12  and thus transform from a region of rigid fixation to a region of flexible osteosynthesis and permit micromotion across a fracture site stimulating callous formation and bony healing. 
     Angular rigidity of the screw may be augmented by the particular shape of the layer  45 , for example, a conical, tapered shape as shown being advantageous for rigidity. Angular motion of the screw  25  is further controlled through the use of countersink and undercut surfaces as described above and shown in  FIGS. 71-74 . 
     In another alternate embodiment, shown in  FIG. 82 , the layer of bioresorbable material  45  is positioned on the body  12  within at least one of the holes  16 . 
     When the screw  25  is inserted through the hole  16  in body  12  and tightened, the outer surface  47  of the layer  45  engages the screw&#39;s head  31  and rigidly fixes the angular orientation of the screw relative to the body. Engagement between the layer  45  and the screw head  31  may be through any of the example mechanisms described above. For example, outer surface  47  may have internal screw threads that engage compatible external screw threads on the head  31 ; the outer surface  47  may be smooth and a cutting edge (not shown) positioned on the screw  25  cuts internal threads in the layer  45 , or, the outer surface  47  of layer  45  may depend on friction between it and the head to provide the desired angular fixation. 
     When all, or at least a portion, of the layer  45  is resorbed, as shown in  FIG. 82A , the screw  25  is free to move angularly relatively to the body  12  and thus transform from a region of rigid fixation to a region of flexible osteosynthesis and permit micromotion across a fracture site stimulating callous formation and bony healing. Angular rigidity of the screw may be augmented by the particular shape of the layer  45 , for example, a conical, tapered shape (shown) being advantageous for rigidity. Angular motion of the screw  25  is further controlled through the use of countersink and undercut surfaces as described above and shown in  FIGS. 71-74 . 
       FIGS. 83 and 83A  show another embodiment wherein the bioresorbable material layer  45  is positioned on both the head  31  of screw  25  and the body  12 . In this example embodiment, interaction between the outer surfaces  47  of the layers  45  on the screw  25  and on the body  12 , as shown in  FIG. 80 , initially fixes the angular orientation of the screws relatively to the body. Interaction may be through friction between the surfaces or threaded engagement. Countersink and undercut surfaces may again be used to control the limits of relative angular motion between the screw and the body. When the layers  45 , or a portion thereof, are resorbed, the screws  25  are no longer rigidly fixed and may move angularly with respect to the body  12  as shown in  FIG. 83A , thereby providing the advantages of both the rigid and flexible osteosynthesis systems. 
     Another embodiment is shown in  FIGS. 83B and 83C , wherein screw  25  has a head  31  with a substantially smooth side surface  31   a  and a layer of bioresorbable material  45  on the top of the head  31 . Bioresorbable layer  45  engages the body  12  using screw threads  95  which mate with compatible internal threads in hole  16  and initially fix the angular orientation of the screw relative to the body. Screw threads  95  may be molded into the bioresorbable layer  45  when it is applied to the screw  25 , or the layer  45  may be initially smooth and the threads cut, for example, as the screw is threaded into the hole  16 . When the bioresorbable layer  45  is resorbed, as shown in  FIG. 83C , the screw  25  no longer fixedly engages the body  12  and is free to rotate angularly relative to the body. Note that a counter sunk screw is shown by way of example, but other shapes of screw heads and bioresorbable layers, such as cylindrical, conical and spherical shapes, are equally feasible. 
     The invention also encompasses a method of treating a bone fracture in a living organism having a plurality of bone fragments. The method comprises:
         attaching a body to at least two of the bone fragments using a plurality of fasteners joining the body to the fragments;   fixing an angular orientation of at least one of the fasteners in relation to the body using a bioresorbable material positioned between the fastener and the body, the bioresorbable material contacting the one fastener and the body and preventing relative rotation therebetween;   allowing the bioresorbable material to be resorbed by the living organism, thereby allowing relative rotation between the one fastener and the body.       

     The fasteners used in the method according to the invention include bone screws and pins as described herein. The bioresorbable material may be located on the fastener, on the body, or on both the fastener and the body. The angular orientation of the fasteners relative to the body may be fixed by frictional engagement between the body and the bioresorbable layer on the fastener, by frictional engagement between the fastener and the bioresorbable layer on the body, or between bioresorbable layers on both the body and the fastener. The angular orientation of the fasteners relative to the body may be also fixed by engagement between internal screw threads on the body and external screw threads on the bioresorbable layer on the fastener, by engagement between external screw threads on the fastener and internal screw threads on the bioresorbable layer on the body, or between internal and external screw threads on the bioresorbable layers on both the body and the fastener, respectively. The body may be part of a fixation device, such as a bone plate, a hip screw, an intramedullary rod and the like. 
       FIGS. 84 and 85  show an example body  12  in the form of a bone plate  140  according to the invention. Plate  140  comprises a bone contacting surface  142  ( FIG. 85 ) and an obverse surface  144  ( FIG. 84 ) arranged opposite to the bone contacting surface  142 . Side surfaces  146  extend between the bone contacting and obverse surfaces  142  and  144 . A plurality of holes  148  extend between the bone contacting surface  142  and the obverse surface  144 . Holes  148  receive fasteners  18 , which could be bone screws as shown in  FIGS. 4-9 and 39-44 , and/or pins as shown in  FIGS. 22-27, and 55-60  (fastener  18  is shown as a bone screw by way of example). The holes  148  may be round, as well as non-round, for example oval or elliptical as shown in  FIGS. 86 and 87 . Other, more complicated shapes are also feasible. One or more cutting edges  137  may be positioned in the holes to cut threads in a bioresorbable material layer positioned on the fastener  18  as described above. The holes may also be countersunk and undercut as described above. A layer of bioabsorbable material  44  may be positioned within one or more of the holes  148  similar to the embodiments illustrated in  FIGS. 71-74 and 71A-74A . 
     As best shown in  FIGS. 84 and 85 , the plate  140  comprises a plurality of channels  150  positioned within either or both the obverse surface  144  and the bone contacting surface  142 . Each channel  150  extends from a hole  148  to a side surface  146  and facilitates the flow of bodily fluids to and from the hole. This flow of fluids allows bioresorbable layers  44 , either on the plate  140  or the fasteners  18 , or on both, to be readily resorbed to transform the plate  140  from operation as a rigid osteosynthesis device to a flexible osteosynthesis device. When the bioresorbable layers are present, the angular orientation of fasteners  18 , which could be bones screws and/or pins as described above, is fixed with respect to the plate  140 . When the layers are resorbed the fasteners are free to move angularly with respect to the plate  14  and thereby permit the micromotions conducive to callous formation and bony healing. 
     Channels  150  as shown in  FIGS. 84 and 85  comprise a concave, conical surface. Note that in the example shown, the width of the channel where it intersects side surface  146  is greater than where the channel intersects the hole  148 . Channels  150  may have different cross sectional shapes from those shown in  FIG. 85 . As shown in  FIG. 88 , the channel  150  may have a spherical shape;  FIG. 89  shows a channel  150  having a “V” cross sectional shape;  FIG. 90  shows a channel  150  having a cylindrical or “U” cross sectional shape, and  FIG. 91  shows a channel  150  having a trapezoidal cross sectional shape. Other channel shapes are also feasible. 
     As shown in  FIG. 92 , the body  12  represented by an example bone plate  152  has a plurality of projections  154  on its bone facing side  156 . 
     Projections  154  act as spacers to stand the plate  152  in spaced relation away from bone to permit bodily fluids to flow to and from holes  158  in the plate to facilitate resorbtion of the bioresorbable material on the plate and/or the fasteners use to attach the plate to the bone. Projections  154  may be integrally formed with the plate or attached thereto as separate components. 
     Further by way of example applications of the invention, the hip screw  28 , shown in  FIG. 2  may use a layer of bioabsorbable material  44  surrounding a portion of the shaft adjacent to the head of the compressing screw  24   a . As shown in  FIG. 3 , screw  24 , which secures the intramedullary rod  30  to bone portion  20   b , may have a layer of bioresorbable material  44  positioned on a portion of the screw shaft in spaced relation away from the head. Pin  26  may also have a layer of bioresorbable material positioned along its shaft as well. 
     The bioresorbable materials comprising the layers attached to the fasteners, such as the bone screws and pins, as well as the layers on the body, such as the bone plate, the plate associated with the hip screw, and the intramedullary rod may comprise polymer materials and/or polymer-glass/ceramic including (but not limited to) polylactic acid (PLA), poly-L-lactic-co-glycolic acid (PLGA), poly-D/L-lactic acid with or without polyglycolic acid (PDLLA, PDLLA-co-PGA), poly-L-lactic acid with or without -tricalcium phosphate (PLLA, PLLA-TCP), poly-L-lactic acid with hydroxyapatite (PLLA-HA), polycaprolactone (PCL), polycaprolactone-Calcium Phosphate (PCL-CaP), poly(L-lactide-co-D,L-lactide) (PLADLA), hydroxyapatite (HA), tricalcium phosphate (-TCP) and combinations thereof. Nanodiamond particles may be admixed with the bioresorbable materials to increase their strength. 
     Additionally, bioresorbable materials which expand when in contact with bodily fluids, or by the action of heat or ultrasonic waves may also be feasible for use with the fixation device according to the invention. Such materials include copolymer lactic glycolic acid (80/20), biodegradeable self-expanding poly-L,D-lactide, PDLLA comprising D-Lactide and L-lactide and poly-L-lactide and poly-E-caprolactone homopolymers. 
     Expanding or swelling polymeric materials include the monomers methylmethacrylate and acrylic acid and cross linking agent allymelhacrylate. Material layers made of these materials swell by absorbtion of body fluids and thereby produce fixation between the fastener and the bone plate, hip screw or intramedullary rod by an interference fit. 
     Selective degradation of the bioresorbable material layer may be controlled at the discretion of the surgeon or healthcare practitioner through various means including focal hydrolysis with acids, alkalis or enzymes. 
     Other means of inducing degradation include the exposure of the bioresorbable layer to UV light or radiation, oxidation, high temperatures, ultrasound and focused high intensity acoustic pulses.