this invention may be accomplished by an interference screw 10 , fig1 having an elongated body 12 with a root portion 14 about its inner or minor diameter . the major or outer diameter of the screw 12 is formed by a helical thread 16 which traverses the root portion 14 . the thread 16 is shown having a crest 17 bounded by edges 18 , 20 which define between them arc &# 34 ; a &# 34 ;. proximal and distal slope portions 22 , 24 lie between the crest 17 and root portion 14 . in this construction , the root portion 14 is arcuate having a curvature of radius r as indicated in phantom by circle 19 . the curvature of the thread 16 changes at tangency point t , which is the intersection between line 21 of proximal slope 22 and line 23 of distal slope 24 . slope lines 21 , 23 are relatively straight in this construction . tangency points t occur approximately at the pitch diameter , that is , halfway between the major and minor diameters . a biocompatible coating 30 is shown in fig2 covering or enveloping the crest portion 17 and on the distal and proximal slope portions 22 , 24 . the root portion 14 lying between and intersected by the thread 16 remains uncoated . in this construction , screw 10 includes a distal tip 32 which is also covered with the coating 30 . a coating applied according to the present invention does not cover the entire root portion . preferably , the coating extends inwardly along a thread or other ridge no more than halfway from the crest of the ridge to the inwardmost point i of the root portion as indicated in fig1 . when the coating 30 is a teflon - type material or other biocompatible material having a low coefficient of friction and the elongated body 12 is stainless steel or a titanium alloy having a higher coefficient of friction , the coating 30 reduces the amount of torque required to insert the screw 10 . by leaving the root portion 14 exposed , the screw 10 provides a desirable amount of resistance during insertion to enable the surgeon to &# 34 ; feel &# 34 ; the amount of engagement between the screw and a bone into which it is being inserted . this &# 34 ; feel &# 34 ; is particularly important when the screw is being guided between the surface of a bone tunnel and a bone block to be wedged in a bone tunnel such as during acl reconstruction . one method of selectively applying the coating 30 is illustrated in fig3 . a brass wire 40 is wrapped about the screw body 12 to lie along the root portion 14 . the screw 10 is placed upright by mounting the screw 10 on a post which engages the drive socket 31 , fig2 . teflon or other desired material then is conventionally spray coated onto the upright screw 10 ; the exterior of the screw 10 which is masked by the masking wire 40 remains uncoated . after conventional curing or drying , the screw 10 is removed from the vertical post and is unscrewed from the masking wire 40 using a driver . instead of manually wrapping a straight wire about the elongated body of the fastener , a pre - formed masking spring coil 42 , fig4 a and 4b , can be used . the spring 42 includes a main body 44 formed of a wire 46 which terminates at one end in a tapered tip 48 and at the other end at an extension or tab 50 . to mask the root portion of a screw , the tip of the screw is rotatably inserted past the tapered tip 48 into the main body 44 . the tab 50 is useful during insertion to position the spring mask 42 and prevent its rotation relative to the screw being inserted . during removal of the screw , the tab 50 serves as a lever to slightly untwist and expand the spring 42 to assist removal from the now coated screw . dimensions for one type of screw and masking coil are as follows . for an interference screw having a major diameter of 9 mm ( 0 . 354 inch ), a minor diameter of 0 . 216 inch , a pitch ( distance between crests ) of 0 . 110 inch and a thread angle b of thirty degrees , and a depth ( vertical distance between root and crest ) of 0 . 069 inch , the masking spring 42 has a inner diameter c of 0 . 216 inch ( which matches the minor diameter of the screw ), a diameter of wire 46 of 0 . 080 inch , and a length d of approximately 0 . 7 to 1 . 1 inch depending on the length of the screw . similarly , a screw having a major diameter of 7 mm ( 0 . 276 inch ) and a minor diameter of 0 . 138 inch would utilize a masking spring having an inner diameter c of 0 . 138 inch . for a screw having a square thread , a masking coil formed of a square wire can be utilized . another fastener which can be coated according to the present invention is shown in fig5 during insertion into a bone 72 . a suture anchor 60 is shown interlocked with a driver shaft 62 and carrying a suture 68 . the bone 72 is formed of an outer cortical layer 74 and an inner cancellous layer 76 . the anchor 60 is shown in fig6 in cross - section to illustrate a coating 80 disposed on a portion of ridges 82 , 84 and 86 . each ridge includes a crest portion 88 , a distal slope portion 90 and a proximal slope portion 92 . root portions 94 , 96 and 98 lie between the ridges 82 , 84 and 86 and are parallel to longitudinal axis 95 . the coating 80 covers the crest portions 88 and part of the distal slope portions 90 which are the regions that slide past and generate friction against the cortical region 74 of the bone 72 . the proximal slope portions 92 and the root portion 94 , 96 , 98 and the distal tip 66 remain uncoated . the inwardmost , center point p of the root portion 98 is illustrated for reference . the anchor 60 further includes a driver engagement socket 64 at its proximal end and a rounded tip 66 at its distal end . instead of a drive socket 64 , a flat head can be provided at the proximal end to be impacted by a mallet to accomplish the linear insertion of the suture anchor or similar fastener . the driver shaft 62 can include a stop member to limit the depth of insertion . one technique by which the coating 80 could be applied utilizes bands of metal that are wrapped about the root portion 96 and 98 . suitably configured cylindrical caps are separately placed over tip 66 and proximal root portion 94 . the type of coating to be deposited according to the present invention on a fastener depends on the overall desired coefficient of friction of the fastener . if the fastener has an undesirably low coefficient of friction and inserts too easily , a coating having a higher coefficient can be selectively applied . further , the coating can have a selected dissolution or erosion rate upon contact with bodily fluids . nonabsorbable teflon is desired for lessening the overall coefficient of friction of a stainless steel or titanium alloy screw as described above . alternative nonabsorbable coatings which may be acceptable for certain applications include parylene - c available from nova tran corporation of clearlake , wis . parylene - c is marketed as a sealant and guards against moisture . other possible alternatives include silicone , hypan from kinston technologies , dayton , n . j ., and &# 34 ; diamond - like carbon &# 34 ; coating available from diamonex , inc ., allentown , pa . bioabsorbable coatings include polycaprolate and others such as disclosed in u . s . pat . no . 4 , 788 , 979 , incorporated herein by reference . one acceptable polycaprolate is a random copolymer of 85 weight percent epsilon - caprolactone and 15 weight percent glycolide . a bioabsorbable coating can be applied over a bioabsorbable bone screw such as the screw disclosed in german patent no . de 3 , 811 , 345 . other absorbable surgical devices are disclosed in u . s . pat . nos . 4 , 300 , 565 ( rosensaft et al .) and 4 , 429 , 080 ( casey et al . ), incorporated herein by reference . one consideration to be made for absorbable coatings is that , if the coating dissolves too rapidly , the coating will not be present to assist removal and repositioning of the screw if desired . the configuration of a fastener may affect the desired type of coating : a helical thread provides greater surface area in contact with a bone or other tissue than circumferential ridges of the fastener shown in fig5 and 6 . the amount and type of material in contact with the tissue will affect the &# 34 ; feel &# 34 ; of the device to the surgeon . several examples are provided below to illustrate the effects of selective coating according to the present invention . three uncoated , titanium alloy ( astm f - 136 ) screws lacking a head member and having a major diameter of 7 mm , a length 25 mm , a minor diameter of 0 . 138 inch , a thread pitch of thirty degrees , and a thread depth of 0 . 069 inch were separately inserted into separate holes drilled in bovine bone . each hole had a diameter of 0 . 125 inch . each hole and screw were maintained moist with water during insertion . the results for the three baseline screws are presented in table 1 . the average torque was 10 . 9 in . lbs . table 1______________________________________ max . observedscrew torque depth sound______________________________________baseline1a 12 in . lbs . 15 mm squeak1b 9 . 8 in . lbs . 15 mm squeak1c 11 in . lbs . 15 mm squeak100 % teflon2a 2 . 4 in . lbs . 15 mm none2b 3 . 4 in . lbs . 15 mm none2c 2 . 8 in . lbs . 15 mm nonemasked teflon3a 8 . 2 in . lbs . 15 mm slight squeak3b 6 . 8 in . lbs . 15 mm slight squeak3c 6 . 4 in . lbs . 15 mm slight squeak100 % parylene - c4a 3 . 4 in . lbs . 15 mm none4b 2 . 6 in . lbs . 15 mm none4c 3 . 4 in . lbs . 15 mm none______________________________________ three screws having a major diameter of 7 mm and a length of 20 mm were one - hundred percent coated with teflon 959 - 205 from du pont . the teflon was conventionally applied by spraying , and conventionally dried . the resulting coating had a thickness of approximately 0 . 0008 - 0 . 0010 inch . tests were conducted as presented in example 1 and the results are as presented in table 1 . the average torque was 2 . 8 in . lbs . three 7 mm by 20 mm screws identical to those of example 2 were masked with a brass wire so that only 0 . 10 inch of the distal tip and part of the threads were covered with teflon as shown in fig3 . the coating had a thickness of approximately 0 . 0008 - 0 . 0010 inch . tests were conducted as set forth in example 1 and the results are presented in table 1 . the average torque was 7 . 1 in . lbs . three screws were conventionally coated completely with parylene - c , the screws having a major diameter of 7 mm and a length of 25 mm . the coating had a thickness of approximately 0 . 0008 - 0 . 0010 inch . tests were conducted as described in example 1 and the results are presented in table 1 . the average insertion torque was 3 . 1 in . lbs . in summary , the uncoated screws of example 1 provided the most positive feel , that is , resistance , during insertion . the one - hundred percent coated teflon and parylene - c provided very little resistance during insertion and therefore potentially could be over - inserted . the masked screws provided a slight resistance and a slight &# 34 ; squeak &# 34 ; sound which provided desirable feedback during its insertion to the operator of the driver . although specific features of the invention are shown in some drawings and not in others , this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention . other embodiments will occur to those skilled in the art and are within the following claims .
a fastener including an elongated body having one or more ridges passing across its exterior surface intersecting root portions between them . each ridge has a crest portion with edges bounded by distal and proximal slope portions extending inwardly from the crest edges to the root portions . a coating is disposed on the crest portion and at least part of the distal slope portion but not on at least part of the root portion . the coating has a selected coefficient of friction different from that of the exterior surface of the elongated body . the fastener further includes an element for engaging a driver to enable insertion of the elongated body into a tissue of the patient . the coating modifies the amount of force which must be applied to the driver to insert the fastener .