fig1 a and b show a basal screw 1 in accordance with prior art systems . it comprises a screw shaft 2 extending along a longitudinal axis 5 . at its distal end the shaft 2 comprises a threaded section 3 , the size and pitch of the thread being chosen for engagement with the internal threaded bore of an implant . at its opposing end shaft 2 joins to the screw head 4 . head 4 contains a hollow 6 in its coronal end which is shaped to allow insertion of a drive tool such as a screw driver . hollow 6 has a non - circular symmetric outline such that torque can be transmitted from the drive tool to the screw 1 . the bottom , or apical , end of head 4 has a larger maximum radius r 2 than the maximum radius r 1 of the shaft 2 . this results in an annular contact surface being formed by the underside 8 of the screw head 4 . as the abutment or other component with which the screw engages must comprise a screw channel dimensioned to allow passage of the screw shaft 2 the maximum contact area possible between the screw head 4 and screw seat is π ( r 2 2 − r 1 2 ). it is worth noting that the maximum radius r 2 of the bottom end is less than the overall maximum radius of the screw head 4 . this is because a bevelled surface 9 links the bottom end to the outer circumference of the screw head 4 . this improves the fit of the screw 1 within the screw channel , as will be demonstrated later . despite this bevel , underside 8 provides a relatively large surface area with which the head 4 can contact the screw seat of the abutment or other secondary component . further , this surface area extends from the maximum radius r 1 of the screw shaft 2 outwards . fig2 a shows a screw 10 in accordance with the present invention . the screw shaft 12 is identical to that of the prior art screw shown in fig1 and comprises a threaded section 13 at its distal end and joins to a screw head 14 at the opposite end . once again , head 14 comprises a hollow 16 shaped to allow engagement with a drive tool . although shown at the distal end threaded section 13 could alternatively be positioned at a different axial location on the shaft 2 . in contrast to the prior art , the underside 18 of screw head 14 is not planar but instead comprises a downwardly extending protrusion 17 . this protrusion 17 tapers downwards to a flat distal surface , which defines the bottom , or apical , end 15 of the screw head 14 . the taper is formed on the radially outer side by bevelled surface 19 and on the radially inner side by a concave surface . the shape of the underside 18 results in the creation of an annular contact surface having a width less than r 2 − r 1 and which is located towards the outer radius of the screw head 14 . this shape of screw head increases the friction radius of the screw and hence increases the torque required in order to overcome the frictional resistance of the screw head . this is demonstrated with reference to fig3 a and b . fig3 a shows schematic view of a partial cross section of the screw 1 of fig1 within a 3 - part implant system . abutment 20 comprises a screw channel 21 having a coronal part and an apical part , separated by a step change in diameter which forms screw seat 22 . screw seat 22 is planar and perpendicular to the longitudinal axis 5 of the system . due to manufacturing methods the transition from the outer wall of the coronal part of the screw channel 21 to the seat 22 is curved . abutment 20 is seated in an internal bore 31 of implant 30 . the bore is shaped to snugly accommodate the abutment 20 and comprises a threaded section 33 . in order to connect the abutment 20 to the implant 30 screw 1 is passed through screw channel 21 until the threaded section 3 of the screw 1 can engage with the threaded section 33 of the implant . by tightening screw 1 the head 4 is forced down onto the screw seat 22 and clamps the abutment 20 within the implant 30 . the bevelled edge 9 of screw head 4 prevents any interference with the curved transition area of the screw channel 21 . the planar surface of the underside 8 creates a broad contact region c 1 between the screw head 4 and screw seat 22 . the friction radius of the system shown in fig3 a is the mean radius of this contact region r f1 . when torque is applied to the screw 1 via the hollow 6 ( not shown in fig3 a ) a part of this torque will be used to overcome the frictional resistance under the screw head 4 , another part will be used overcoming the frictional resistance of the screw threads and the remainder will tighten the screw and increase the tension in the screw body . too much applied torque will over tension the screw 1 and cause this to fracture and break . fig3 b shows the same implant system as fig3 a , however this time screw 10 is used to connect the abutment 20 to the implant 30 . as can be seen , the downwardly extending protrusion 17 significantly reduces the contact region c 2 between the screw head 14 and the screw seat 22 . significantly , no contact between the surfaces exists at the radially innermost area of the screw seat 22 because the contact surface of the screw head has a greater minimum radius than the minimum radius r 3 of the screw seat 22 . this radius is similar to that of the maximum radius r 1 of the screw shaft 12 , as the screw shaft 12 must be able to pass through the screw seat 22 into the apical part of the screw channel 21 . the lack of contact at the radially inner area of the screw seat 22 increases the friction radius r f2 of the system and consequently the torque needed to overcome the frictional resistance under the screw head 14 . by using a screw in accordance with the present invention therefore a smaller percentage of the applied torque will be used to tension the screw body and hence the screw 10 can withstand more torque before over tensioning occurs . protrusion 17 is located as close to the outer radial edge of the screw head 14 as possible , in order to increase the friction radius r f2 . in addition the distal surface of protrusion 17 is made as narrow as possible . the protrusion 17 of the screw 10 , shown in fig2 and 3b has a tapered surface on its radially inner side . this has an additional benefit as it allows the screw head 14 to flex . as the screw head 14 is drawn downwards onto the screw seat 22 , the tapered surface enables the protrusion to pivot slightly . the screw head 14 thus acts as a loaded spring . during use of the abutment 20 , this settles or sinks further into the implant bore 31 . in prior art systems this lessens the torque required to remove the screw 1 . using a screw according to a preferred embodiment of the present invention however , as the abutment 20 settles lower in the implant 30 the tapered protrusion 17 unflexes and hence maintains a better contact with the screw seat . this leads to a higher removal torque even after prolonged use of the abutment 20 . in order to increase the length of this taper , the screw 10 comprises an undercut 11 at the transition from the screw shaft 12 to the screw head 14 . this increases the spring effect of the protrusion 17 and in addition increases the tolerance between the screw head 14 and screw seat 22 . in this embodiment it is the curve of the taper which continues into and forms a part of the undercut 11 . comparative tests have been run on screws having the designs shown in fig1 and fig2 . it was found that the average breakage torque was increased from 51 . 8 ncm , in the case of screw 1 , to 57 . 8 ncm in the case of screw 10 . in addition , after fatigue testing with a load of 280 n , the removal torque of screw 10 was 26 . 1 ncm compared with 17 . 9 ncm in respect of screw 1 . fig4 - 6 show some further embodiments of the present invention . fig4 shows a screw head 44 having a tapered protrusion 47 that tapers to an end point which forms the apical end 45 of the head 44 , located at the outer radius of the screw head . this screw 40 provides the optimum friction radius possible for a screw of a given outer radius and can be used in situations in which manufacturing tolerances permit , for example , when the screw seat 22 is formed by the outer surface of the seating component and / or extends radially beyond the screw head . this embodiment features an undercut 41 which increases the spring effect of the tapered protrusion 47 . this undercut 41 is located in the screw head 44 . fig5 shows an alternative screw head 54 in which protrusion 57 extends at right angles from the underside 58 of the head 54 . providing a flat apical end 55 and contact surface enables better consistency and predictability of the screw . in this embodiment no undercut is present at the transition between the shaft and head . fig6 shows a screw head 64 having an undulating underside 68 that results in a curved protrusion 67 . here , the undercut 61 extends into both the screw shaft 62 and screw head 64 . fig7 shows a generalised bottom view of a screw according to the present invention . screw head 74 has a greater radius than screw shaft 72 and so extends outwards from this forming an underside 78 . bevelled surface 79 extends between the radial edge of the underside 78 and the circumferential edge of screw head 74 . the underside 78 of the screw head 74 comprises a protrusion which extends in the apical direction to a distal end surface which forms a continuous annular contact surface c . the location , width and shape of the protrusion can vary , as shown in fig2 and 4 - 6 . in fig7 a single protrusion extends 360 ° about the longitudinal axis of the screw to form a uniform , continuous contact surface . it is also possible for the contact surface to be formed by a plurality of protrusions . this is shown in fig8 . here it can be seen that the underside 88 of the screw head 84 comprises multiple protrusions 87 , which again can have any of the shapes shown in previous embodiments , each extending apically to a distal surface , these distal surfaces in combination forming a broken or discontinuous annular contact surface c . the invention has mainly been described above in relation to a separate screw component , which can be used to connect a secondary component such as an abutment to an implant . however , it is also possible for the secondary component itself to form the screw component of the present invention . when it is not necessary to know with certainty the exact angular position of the component with respect to the implant the secondary component is often directly screwed into the implant . this direct connection is common for example , when the implant is intended for supporting a bridge , i . e . a single prosthesis which replaces multiple teeth . in such situations the bridge is attached to two or more implants and the angular orientation of the bridge is thus defined by these multiple connection points . other secondary components , such as healing caps , which are only used on a temporary basis and do not support a prosthesis , may also be directly screwed to the implant . fig9 shows a secondary component designed for direct connection to an implant . the component 90 comprises shaft 92 having a threaded section 93 for threaded connection to the implant . the component 90 further comprises a head 94 , which in use protrudes from the implant into and / or through the soft tissue . the bottom or apical end the head 94 has a larger radius than the shaft 92 , such that an underside 98 is created . the underside 98 comprises an apically extending protrusion 97 which extends 360 ° about the longitudinal axis of the component such that an annular contact surface is formed . the detail circled in fig9 is very similar in configuration to the screw 10 shown in fig2 a . the annular contact surface of component 90 has a larger minimum radius than the maximum radius of the shaft 92 and the minimum radius of the screw seat of the implant . therefore , the friction radius of the screw head is increased in relation to prior art components . the above described embodiments are for illustrative purposes only and the skilled man will realize that many alternative arrangements are possible which fall within the scope of the claims . where technical features mentioned in any claim are followed by reference signs , those reference signs have been included just for the sole purpose of increasing intelligibility of the claims and accordingly , such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs .
fastening system including a screw component and a seating component . the screw component has a shaft extending along a longitudinal axis and having a maximum radius r 1 , said shaft comprising a threaded section , said screw component further including , at one end of the screw shaft , a head , the head comprising a bottom end having a larger maximum radius r 2 than the screw shaft such that an exposed underside of the screw head extends radially beyond the maximum radius of the screw shaft . the seating component includes a screw channel having a planar screw seat having a minimum radius r 3 which is equal to or greater than r 1 . the bottom end of the screw head has an annular contact surface for abutment against said screw seat , the contact surface being formed by the distal end of at least one downwardly extending protrusion on the underside of the screw head , wherein the annular contact surface has a minimum radius greater than the minimum radius of the screw seat .