Tapper for forming internal threads in a precision frustoconical hole in bones

Abstract: The tapper (60;160) is used for tapping a precision hole (40) formed in a bone and obtained by using a cutter (100) in the shape of an inverted "wedding cake" and then reaming the obtained cavity with a manual reamer (140). In the tappered precision hole (40) is then inserted a screw device (10;80) for fixing prostheses to bones, the threaded shank (14) comprising a core (32,34) of overall frustoconical shape. The screw has a cylindrical neck (12) of diameter equal to or greater than the maximum diameter of the thread.

Description: This invention relates to a tapper for forming internal threads in a hole in bones. As is well known, in terms of mechanical characteristics bone tissue can be divided into two distinct regions, namely the cortical bone region with an elastic modulus of between 1000 and 1200 dN/mm 2 and the spongy region, of trabecular bone tissue containing medulla or fat, with an elastic modulus of roughly between 20 and 400 dN/mm 2 . Currently, to execute an implant in any bone, use is made of known bone screws which have a substantially cylindrical shank and are constructed of a biocompatible metal such as titanium, austenitic stainless steel, tantalum, niobium or zirconium. These screws require a cylindrical hole to be previously drilled in the bone. If the screws are of the self-tapping type they are inserted directly into the cavity thus obtained, which always has a diameter less or at most equal to the core of the screw. In the case of non self-tapping screws the relative female thread has to be formed in the side surface of the hole. For fixing purposes, known screws utilize either the mechanical characteristics of the trabecula and therefore have a thread of rather large pitch, of the type suitable for fairly soft materials, or the mechanical characteristics of the cortical bone opposite the point of penetration of the screw, which has a thread pitch significantly less than in the previous case and suitable for ensuring a good mechanical grip in hard materials, but not suitable for gripping the trabecular bone tissue. In its turn, because of its thinness the cortical region of the bone can generally only receive one large-pitch turn. In addition, because of its relative fragility the cortical bone tissue is unsuitable for receiving a large-pitch thread. This applies particularly to the cortical bone located on the same side as that from which the self-tapping screw or thread tapper is inserted. In this respect, as stated the thread of the screw or tapper has a diameter greater than that of the drilled hole. In addition the screw neck (ie the cylindrical end of the screw to which the prosthesis is fixed, and which is normally not threaded but enters the cortical bone) has a diameter less than that of the thread. As a result, on inserting the self-tapping screw or tapper into the drilled hole the most outer part, ie the cortical bone, of the bone tissue is removed. Consequently, once the screw has been inserted into the bone, an empty annular space remains around the screw neck. This means that the cortical bone is surgically damaged for a certain area around the screw neck. The damage is directly proportional to the size of the tooth of the thread on the self-tapping screw or tapper. That part of the cortical bone which has been thus removed does not form again. This represents a serious drawback as the cortical bone is the strongest region of the bone and the most suitable for supporting loads, particularly loads perpendicular to the screw axis. The cavity for receiving the screw is formed by rotary tools mounted on hand-controlled drills. The shape and dimensions of the cavity obtained depend on various factors, and in particular: a) the bone to be drilled; b) the drilling tool; c) the operator holding the drilling tool. The causes influencing these three factors will he examined in detail: a) The bone to be drilled cannot be fixed rigidly, with the result that there is a certain freedom of movement. It also has a smooth, moist and therefore slippery surface. In addition the surface is somewhat round. Again, the structure is anisotropic so that the resistance offered to the tool cutting edge varies as drilling proceeds. b) The drilling system comprises a drill bit or cutter of various shapes, such as a spade tip with a solid cylindrical body, a flared tip with a vertically grooved body to collect shavings, or a flared tip with a helically grooved body to collect shavings. No studies appear to have been carried out with the purpose of determining the best cutting angle for the bone, or conceiving a good system for discharging the bone shavings which mix with the blood and tend to coagulate. As is well known, when the drill bit is fitted into the drill it is retained by a quick-action mechanism consisting of a hollow neck which receives the relative part of the bit, which is thus locked in terms of axial movement, whereas the bit has a certain radial play. Consequently the bit does not rotate about a fixed axis but about an axis which can undergo small oscillations and movements perpendicular to itself. The mechanism which transmits movement to the drill bit can also move slightly because of intrinsic mechanical play within the mechanism. The combination of all these causes means that the drill bit undergoes a complex "wobbling" movement. c) The hand of the operator gripping the drill is subject to muscular control, which varies from operator to operator and can also vary with time for the same operator. From the aforegoing, and considering any cross-section through a drill bit when freely rotating, before it makes contact with the bone it describes a peripheral or enveloping circle which has a diameter greater than the true diameter of the bit at that cross-section, because of the effect of said wobbling. Moreover when the tip of the drill bit comes into contact with the bone, no matter how expert or attentive the operator is, the bit axis is generally not exactly perpendicular to the bone surface. Consequently, even if a starter cavity is present (previously made in the bone surface), when the operator exerts a certain pressure on the bit to drill the hole, a non-axial reaction is applied to the bit, which consists of one component perpendicular to the bit axis to flex it, and a second component along the bit axis. Said flexing force has two effects, the first being the nullifying of the radial play of the mechanism which holds the bit so that said peripheral circle becomes the maximum possible, the second effect arising when play has been nullified, to deform the bit by flexure, so further increasing the diameter of the peripheral circle. When the drill bit has initially entered the bone a cavity of previously indeterminable diameter results, this diameter being in any event certainly greater than that of the corresponding cross-section of the bit. This cavity will have a certain depth, say one or two millimetres. It is therefore apparent that the amount of play varies in practice and cannot be controlled, and it can only be stated that the drill bit will advance through this first section in a merely "prevalent" direction, being substantially that of the theoretical axis of the bit. It is however apparent from the aforegoing that the inaccuracy will be somewhat high. In practice, in its initial portion the cavity can be considered to consist of a series of probably irregular superposed circles of variable diameter, slightly mutually off-centre, to form a cavity extending prevalently in a certain direction. When further pressure is applied to the drill bit it advances through the bone. Two new substantial factors now come into play to influence the operation, namely the accumulation of shavings which tend to coagulate, and the presence of that part of the cavity which has already been drilled. The presence of shavings increases friction, to generate heat and result in further small removals of bone material from the cavity walls. The friction can increase to the extent of stopping the drill motor. The shavings must therefore be removed whatever type of drill bit is used. This is firstly to prevent the bit heating, and secondly to allow it to move forward. They are removed by extracting the bit from the hole. Each time this is done new material is inevitably removed from the walls of the already drilled hole. That part of the hole which has already been drilled performs the important function of guiding the cylindrical body of the bit. In bits with a helical groove this body has a cutting or partly cutting effect, whereas in bits without a groove or with a vertical groove it does not cut. If the bit body has a cutting or partly cutting effect each change in the bit direction results in a removal of material. The cavity therefore widens, so reducing its guide function. As stated, on termination of the operation the cavity is found to be formed from a series of superimposed circles of a diameter which varies within a certain range and slightly off centre to each other, to form a cavity which is therefore somewhat irregular. Of necessity the cavity will have a diameter which is greatest at its open end and smallest at its other end. If the bit body does not have a cutting effect, the guiding efficiency of the already drilled cavity increases with increasing depth. However this does not mean that greater accuracy is obtained in drilling the cavity. In fact all the reasons which make the initial cavity wider than required (from the wobbling of the drill bit to the non-perpendicularity between the bit and the surface of the bone) remain. In fact, a further drawback arises, and one which helically bodied bits do not possess, namely that drill bits with a cylindrical lateral surface do not have space for discharging the shavings. The bit must therefore be extracted much more frequently to clean it, this finally resulting in further widening of the cavity. The bit penetration movement is in reality helical in the direction of rotation of the drill, this movement being a combination of advancement and rotation. Thus on termination of an in-vivo bone drilling operation conducted by normal surgical methods, the result is an approximately frusto-conical cavity of unknown diameters but certainly greater than the diameters of the drill bit used. From tests carried out it has been found that this increase is in the order of some tenths of a millimetre, with wide variation. To make an initial approximate guess at the type of cavity obtained, one must think of a pile of discs with diameters gradually increasing upwards and decreasing towards the lower end. The discs will be slightly off-centre to each other and their centres will approximately form an irregular helical pattern. If an ideal axis is imagined passing through the centres of the two end discs, the centres of the intermediate discs will not generally lie on this axis but will lie within a certain helix about it. If a circle is drawn having the nominal diameter of the screw (having an overall cylindrical shape) and centered on said axis, and then on this circle a further circle is drawn having the measured diameter of a certain cross-section of the cavity obtained and with its centre in its true position eccentric to the axis, the points of contact, if there are any, and the maximum distances between the two circles can be seen. If this operation is repeated for a certain number of cross-sections the number of points of contact between the screw and cavity can be determined accurately, as can the size of the non-adhering regions and their distance from the screw. This also clarifies why even with an effective diameter which is constantly greater than the nominal diameter of the drill bit there can only be a number of points of contact distributed randomly over the surface of the cavity. If in order to verify this a drill bit is inserted into the cavity obtained, the bit may appear stable if by chance it touches the walls at a few points, but these do not ensure effective stability. What however normally happens is that the drill bit has a certain play when inserted into the cavity, showing that there is an insufficient number of points of contact. Consequently when the cavity is finally tapped, the resultant thread will be complete in terms of depth only at the said points of contact, whereas the remainder of the thread will be only partial or indeed be completely lacking. This however does not mean that sufficient information is available to ensure healing, given that it is not known how and in particular when the bone will reform. The question arises as to whether it is possible to adapt operational technology in such a manner as to obtain cavities with a precision of the order of that obtainable in the machining of the actual implants to be inserted into the cavity. The present invention shows that this is possible. In this respect it teaches that such levels of precision can be in reality obtained without having to use too complicated and very costly procedures. In this respect, with the present invention precision levels of 0.02 mm as general dimensional tolerance can be obtained, while for a series of reasons which have already been stated a precision of 0.01 mm can be obtained for surface irregularities, which can be considered optimal. The reason for this search for precision is to reduce as much as possible, and in the theoretical limit to zero, the quantity of bone tissue which has to reform about the implant. The problem of play between the drill bit and chuck and the intrinsic play within the drill head can only be solved by completely changing current technology. This however would result in very high cost. It has been seen that any movement of the screw relative to the walls of the cavity which receives it has a negative effect on the repair of the bone lesion. The object must therefore he to obtain a connection in which such relative movement is not possible. This need is currently satisfied by using a more or less forced insertion of the implant into the cavity. The bone tissue in contact with the implant is therefore compressed. This compression is the price paid by all known insertion methods which provide initial immobility of the implant. The blades or cylinders currently used for this purpose are in fact inserted with small hammer blows. In this manner a forced fit is obtained between the bone and implant by virtue of the mutual compression between certain regions of the implant and the corresponding regions of the cellular wall. Implants formed from different elements (disk implants) utilize the traction between screw elements and prismatic bodies to obtain bone-implant adhesion areas which provide the necessary initial stability. The screw has encountered considerable success because it enables an excellent and immediate rigid connection to be easily obtained between the implant and bone. This method has however certain negative aspects due to two basic reasons, namely the trauma (COMPRESSION) produced by the helical thread in the tissue, and the transmission to the bone, via the thread, of loads perpendicular to the screw axis. Attempts have been made to solve both these problems by eliminating the thread and proposing cylindrically shaped implants, but these demonstrate poor initial stability (PRIMARY), require larger holes to be made, require higher bone crests and provide a lesser lateral surface for equal dimensions. In reality the solution to the problem does not consist of attaining a stability which allows any level of bone repair, but consists of establishing best bone conditions for healing. The best healing conditions are obtained by satisfying two general conditions: 1. Reducing surgical trauma to a minimum and eliminating debris. 2. Attaining maximum initial congruence with minimum pressure. These two conditions result in an improvement in the progress of the reparative process, which normally involves: 1) Resorption of certain bone tissue; 2) Reshaping of other bone tissue; 3) Bone tissue neoformation. The reduction in surgical trauma limits the necrosis of the tissue of the cellular implant wall; the elimination of debris avoids compression, resorption and infection. The fact of obtaining maximum congruence with minimum pressure results in primary stability, no bone resorption stage, no bone to be neoformed in the cortical bone part and little in the spongy part. The female thread is currently made in the bone by two substantially different methods, namely by partial mechanical tapping (as in the Branemark method), and by self-tapping screws (as in the case of Tramonte screws). Partial mechanical tapping involves the insertion of self-tapping screws which traumatize the bone and retain all the debris. This means that with Branemark screws the bone in contact is quickly resorbed and congruence is lost. The reparative process takes place by callus formation. Self-tapping screws of Tramonte type allow maximum congruence between the thread and bone, but involve a forced compressive insertion which causes serious damage. The insertion of either a self-tapping screw or a tapper into the drilled cavity causes both local and general effects in the bone, as follows: I - Local effects These are caused by the following actions: a) Cutting action The cutting action of the thread separates the bone tissue, damaging the calcified bone matrix, the collagen, the basic substance, the cells, the vessels and the nerves. At the commencement of the tapping operation and in the case of the self-tapping screw the cutting action causes inflammation and loss of blood. The tissue lesion results in the release of inflammatory substances (H.W. HAM - Istologia, USES 1969). b) Compression action at interface As the tapping or the insertion of the self-tapping screw continues, the tissue is divaricated by the thread. On commencement of tapping or insertion of the self-tapping screw, if there is no counteracting element in contact with the bone surface breakage occurs by raising of the cortical bone surface, with consequent disruption of the architecture in the surrounding region. In particular, it is the tearing of the vascular connections which seriously prejudice the bone reparative process in this region. In the compagination of the tissue, the divarication necessary for the advancing movement of the thread is obtained by compression of the tissue at the interface. Under the advancement thrust the bone tissue volume corresponding to the volume of the tapper thread or of the thread of the self-tapping screw is fractured and pushed to the sides of the advancing thread. As the spongy trabecular bone tissue lies below the cortical bone, its disturbed solid part, formed of calcium salts, fills the entire available surrounding space, squeezing the vessels contained in the medulla and reducing the blood flow, with consequent ischemia, whereas its liquid part is thrust into the most peripheral trabecular region. If the thread has a pitch which results in superposing in the spongy bone tissue regions, which then become compressed by two successive turns of the thread, a particularly negative situation arises due to the combining of harmful effects which complicate healing. In determining the pitch of the thread of self-tapping screws or of the tapper, the size of the relative core and the size of the cavity to drill in the bone, this important aspect must be taken into account. c) Action of heat It is well known that the heat developed in the bone during the drilling of the hole into which the self-tapping screw or tapper is to be inserted is the main cause of formation of cicatricial fibrous connective tissue, rather than new bone tissue, in the subsequent reparative process which the surgical lesion undergoes. For this reason, in drilling said hole it is advisable to use known rotary instruments internally cooled by physiological solution which in addition to cooling the drill bit also removes the bone shavings by collecting them in the grooves provided. Another method for reducing the heat produced is to limit the rotational speed of the drill to the minimum rpm which allows the hole to be drilled. Likewise the tapping operation or the insertion of the self-tapping screw must also be very slow, so that all phenomena arising can be considered of static type, and the applied forces must be only just greater than equilibrium forces. It is essential to limit friction so as not to excessively increase temperature, which in practice must be maintained below 44°C. The rate of tapping or of insertion of the self-tapping screw must therefore be the lowest possible for screwing into the bone. This operation can therefore only be carried out manually. The use of motorized tappers or screwdrivers does not allow easy control of the speed or consequently of the heat produced. In conclusion, in the current state of the art, as a result of a combination of the aforesaid local effects, the damaged spongy bone tissue becomes replaced with soft cicatricial tissue, which by its nature is unable to ensure effective fixing to the screw. II - General effects As is well known, the trabecular spaces are not empty, nor is any part of the bone. The system which they form can be considered a closed hydraulic system containing a system of channels through which blood flows. Consequently the insertion of an additional volume must necessarily result in a reduction in the blood flow and an increase in the total volume of the system. Thus as in the known art the drilled hole is equal at most to the volume of the screw core, inserting the self-tapping screw or the tapper means that an additional volume is inserted into the bone tissue which is at least equal to the volume of the threads. This produces a significant increase in the internal pressure of the bone, which can easily exceed the breakage limit of the bone and cause fracture. Such fracture does not generally occur at the interface, where the aforesaid local phenomena occur, but starts from the external cortical bone, at the hole. Any excessive increase in the pressure within the system must therefore be avoided. There is a second effect which produces a pressure increase within the bone. This is generated by the insertion of a self-tapping screw or of a tapper of known type. This is because from the very commencement of their insertion these close the hole in the bone, from which the blood should emerge. This blood is therefore pushed to the base of the hole to further increase the internal pressure of the bone, so that said fracture risk increases. EP-A-0424734 of which this application is a divisional overcomes the aforesaid drawbacks of known bone screws and of their methods of application, by providing a screw device for fixing prostheses to bones, a method for applying the device, and the instrument for effecting the application, such as to result in spontaneous repair (by creeping substitution) of the lamellar bone tissue around the screw, the screw becoming thus securely and permanently fixed in the bone. EP-A-0424734 discloses a screw device comprising a neck and a threaded shank, and characterised in that the threaded shank of the screw has a core of overall frusto-conical shape, the screw core being cylindrical and having a diameter equal to or just greater than the maximum diameter of the thread on the shank of the screw, and the thread being of two different types, namely a first thread of large pitch suitable for fixing into the trabecular bone tissue and extending along that part of the shank which is designed to make contact with said trabecular tissue, and a second thread, which can be of the self-tapping type, and intended to fix into that cortical part of the bone opposite the part into which the screw is inserted, said second thread having a number of starts which is a multiple of that of the first thread. To obtain the best result from the use of the above mentioned screw device a particular method of application must be followed for the device. This method enables a cavity to be obtained having dimensions substantially more precise than that obtainable by the known art and such as to reduce the quantity of bone tissue to be reformed to a minimum (see documents DE-B-3735378 or DE-B-3622676). Specifically, the method for applying the screw device of the invention consists of: forming a precision hole in the bone in the position in which said screw device is to be inserted, the hole comprising: a first more outer cylindrical portion to receive the screw neck, this first portion having a diameter equal to or preferably slightly less than that of the non-threaded neck of the screw, or slightly greater than the maximum core diameter of the neck if this latter is threaded; a second more inner frusto-conical portion of transverse dimensions equal to or preferably slightly less than those of the core of the first screw shank part carrying said first type of large-pitch thread; and a third portion extending along the remaining length of the screw shank, this third portion being relative to said second type of screw thread and of transverse dimensions slightly greater than those of the core of that shank part with said second type of thread; tapping the said second portion of the hole to obtain in it a female thread suitable for receiving the said first screw thread; if said second screw thread is not of self-tapping type, tapping said third portion to obtain in it a female thread suitable for receiving said second screw thread; completely screwing said screw into the tapped hole. This method of application results in maximum congruence between the screw and bone. The said hole provided in the bone can also be a through hole if appropriate. A precision bore in a bone is obtained by a cutter according to a co-pending european patent application, and reamer for forming said precision hole, by means of a precision boring method using said cutter and reamer. Specifically, the cutter according to the invention is cooled by sterile liquid which also performs the function of removing the bone shavings which form, and is characterised by having its cutting part in the shape of an inverted "wedding cake". By this term, which immediately enables the shape of the cutter to be visualized, it is meant that the cutter consists of a number of coaxial cylindrical bodies rigid with each other, their diameter decreasing towards the tip of the cutter. The manual reamer is of such form and dimensions as to enable the final hole to be obtained with the required precision, ready for tapping, the reamer having a relief angle suitable for cutting bone tissue. Conveniently, the reamer comprises means for conveying isotonic liquids into the cavity formed in the bone, to facilitate the operation. The purpose of such liquids is to reduce bone necrosis. The means for conveying nutrient liquids can simply consist of a coaxial channel passing through the entire reamer, in communication with a device for feeding isotonic liquids and with lateral apertures provided between the reamer cutting edges, to enable to the isotonic liquid to make contact with the tissues concerned. The method for forming said precision hole for the insertion of a screw device consists of: forming with the inverted "wedding cake" cutter a cavity with steps having diameters less than or at most equal to those of the required precision hole; then, by means of said reamer, manually reaming the thus formed stepped cavity to obtain the required precision hole ready for tapping. To form the large-pitch female thread in the side walls of the second hole portion to receive the first type of screw thread, the tapper according to the invention is used, having a tapping thread with a maximum diameter not exceeding that of the screw neck, this tapping thread having the same number of starts and the same pitch as the first screw thread, and extending for the same length as said first screw thread, the end part of the tapper, of length substantially equal to that of the second screw thread, being free of tapping threads and having transverse dimension not exceeding those of the corresponding third portion of the hole if said second screw thread is of the self-tapping type, whereas said end part of the tapper has a tapping thread with the same number of starts and the same pitch as the second screw thread if this second thread is not self-tapping, characterized in that the discharge means (74, 174) of the tapper (60) are one or more longitudinal grooves (74) or coaxial channel (174) extending along the entire length of the tapper for discharging the organic liquid within the bone. In one embodiment of the tapper according to the present invention the discharge means can be a coaxial channel communicating with apertures which open between the tapper threads. In a modified embodiment of the tapper according to the invention the discharge means are one or more longitudinal lateral grooves extending along the entire length of the tapper to interrupt all of its threads and partly involve the core of the tapper. The outer edges of each groove are conveniently rounded to reduce damage to the bone tissue to a minimum. As stated, the tapper for forming the female thread for receiving the first screw thread cuts and laterally displaces the solid part of the spongy bone tissue, which fills the available adjacent space. As also stated, the purpose of the discharge means provided in the tapper for the liquids contained in the bone is to enable both the blood emerging from the surgical wound and that liquid fraction displaced by the formation of the female threads to escape. This enables local effects (which have already been mentioned) to be controlled to the desired degree and also inhibits the already mentioned negative general effects. It has been stated that the discharge means can be grooves provided in the tapper. It should be noted that normal tappers for mechanical use also comprise longitudinal discharge grooves which interrupt the tapping threads and also involve their core. These grooves have however a different purpose. In these, the edges of the longitudinal grooves must be properly sharp in order to cut the material in which the female thread is to be formed. The purpose of these grooves is to allow collection and removal of the shavings formed by the action of the groove cutting edges against the hole wall. In contrast in the present case, as the formation of shavings during the making of the large-pitch female thread is to be prevented and the said trabecular tissue compression is to be limited, the edges of the longitudinal groove are rounded. In tapping with the tapper according to the invention there is therefore no removal of bone tissue but only the removal of an equivalent volume of organic fluids. The trabecular tissue is therefore only cut and dislodged by the tapper threads without any pressure increase occurring. The spongy bone tissue therefore only undergoes displacement of the said solid and liquid, which does not prejudice the crawling substitution reparative process of the new lamellar bone tissue in the surrounding regions damaged by the tapping operation. In penetrating the spongy bone tissue the large-pitch thread of the tapper must damage this tissue its little as possible. In particular, the crest of the first turn of the tapping thread must be pointed to allow optimum tissue cutting action. A convenient cross-sectional shape for the other turns of the tapper thread could therefore be trapezoidal without sharp edges, this being easily obtained mechanically. The first thread of the screw can also have threads of trapezoidal cross-section. This shape enables external loads perpendicular to the tapper axis to be absorbed without any cutting action occurring, and which would in contrast occur with pointed crests. For the second screw thread involving the cortical bone, said problems are not so stringent, so that the cross-section of the relative thread can conveniently be triangular but with a rounded crest to avoid as much as possible any cutting action or dangerous load concentration should a force with a component perpendicular to the screw axis act on the screw. The same applies to the tapping thread on the end of the tapper if the second screw thread is not self-tapping. A description of the hole for the screw, and of the corresponding tapper, follows. Reference is made in this description to the accompanying drawings, in which: Figure 1 is an axial longitudinal section through the hole for receiving a screw of the above mentioned type, before the hole has been tapped; Figure 2 is a side view of a first embodiment of the tapper according to the invention for tapping the hole of Figure 1, the tapper having an unthreaded end part; Figure 3 is a cross-section therethrough on the line IV-IV of Figure 2; Figure 4 is a cross-section therethrough on the line V-V of Figure 3; Figure 5 is a side view of a second embodiment of the tapper according to the invention; Figure 6 is a cross-section therethrough on the line VII-VII of Figure 5; Supposing that a precision hole 40 shaped as in Figure 1 has already been provided in a bone (see EP-A-0424734) the first portion 42 of the hole 40 is cylindrical and has a diameter less by a few microns than the diameter of the cylindrical neck 12 (Figure 1) of the screw. The height of this first portion 42 is equal to or slightly greater than the thickness of the cortical bone 50, and in any event sufficient for receiving that part of the neck of the screw which is intended to enter the bone. The hole 40 proceeds inwards via a short frusto-conical connection portion 44, corresponding to the frusto-conical portion of the screw. It connects the first section 42 to the second frusto-conical section 46. This latter has diameters less by a few microns than the diameters of the core of the first part of the shank of the screw. The hole 40 terminates with a third portion 48, which is nothing other than the prolongation into the opposite cortical bone 54 of the directrices of the second hole portion 46. In the aforesaid case in which the screw does not have the frusto-conical connection portion, the hole will also not have said connection portion 44, the second frusto-conical portion of the hole then extending directly from the base perimeter of the first cylindrical portion 42 of the hole. Conveniently, the third portion 48 of the hole 40 is slightly longer (for example by 1 mm) than the corresponding lower part of the shank of the screw. The purpose of this is to prevent destruction of the female thread in the bone by any over-tightening of the screw, which could occur if the two said lengths are equal. In this respect any further advancement of the screw is prevented by the base 41 of the hole 40. The slightly longer length of the hole 40 results in adhesion between the conical cavity and the core of the screw. This also compensates for tolerances. In the specific case of dental screws, the upper cortical bone 50 becomes covered by the gingive 55 (see Figure 2), so that this latter has to be perforated by conventional tools before proceeding with the drilling. The hole will therefore also comprise an upper gingival portion 49. When the hole 40 has been made, a female thread (not shown in the figures) is formed in the side wall of its second portion 46 to receive the first thread of the shank of the screw. This is obtained using the tapper 60 shown in Figure 2. To increase adhesion between the screw and the new bone tissue which has to reform about the screw, the first part of the shank and the respective part of the neck are normally coated with titanium in known manner by plasma spray treatment, which slightly increases its dimensions. Consequently the dimensions of the tapping thread 62 and of the core 64 of the part 68 of the tapper 60 must be proportionally increased with respect to the dimensions of the bare screw, as must the dimensions of the parts 46, 42 and 44 of the hole 40. The lower frusto-conical part 66 of the tapper 60 is unthreaded, it has the same length as the corresponding second part of the shank of the screw, and at most has the same transverse dimensions as the core of said part of the screw. The tapper also comprises an upper part 70 substantially analogous to the neck of the screw, the part 70 upperly comprising a projection 72 having a polygonal cross-section for engagement by a suitable tool (not shown) to enable the tapper 60 to be inserted. This latter comprises a longitudinal groove 74 of substantially trapezoidal cross-section extending along the entire tapper (see also Figures 3 and 4), its purpose having already been stated. It will be noted that the edges 73 of the groove 74 are rounded, for the previously stated reasons. Figures 5 and 6 show a modification of the tapper according to the invention which has proved particularly convenient. The tapper 160 is particularly suitable for tapping holes for receiving screws without the connection portion, so that the relative precision hole will be without tile portion 44. The only true difference compared with the tapper 60 of Figures 2 to 4 is that instead of the longitudinal groove 74 (Figure 2) for discharging the organic liquids there is a coaxial circular channel 174 which passes longitudinally through the entire tapper 160. This channel communicates with the outside not only at its two ends but also via the series of apertures 176 provided in the core part 164, each aperture opening between two successive turns of the tapping thread 162.

Claims: A tapper (60) for tapping a precision hole (40) previously formed in a bone, and comprising a tapping thread (62), the latter has a maximum diameter not exceeding that of the neck of the screw to be inserted in the hole (40), said tapping thread having the same number of starts and the same pitch as the first thread of the screw and extending for the same length as said first screw thread, the tapper end part (66), of length substantially equal to that of the second thread of the screw, being free of tapping threads and transverse dimensions not exceeding those of the corresponding third portion (48) of the hole (40) if said second screw thread is of self-tapping type, whereas said end part of the tapper has a tapping thread with the same number of starts and the same pitch as the second screw thread if this second thread is not self-tapping, characterized in that the discharge means (74, 174) of the tapper (60) are one or more longitudinal grooves (74) or coaxial channel (174) extending along the entire length of the tapper for discharging the organic liquids within the bone. A tapper as claimed in claim 1, where the one or more longitudinal grooves (74) are interrupting all of its threads (62) and involving part of its core (64), the outer edges (73) of said groove being rounded. A tapper as claimed in claim 1, where the coaxial channel (174) communicates with apertures (176) provided in the core part (164) of the tapper.

Inventors: VRESPA GIUSEPPE, VRESPA, GIUSEPPE
Applicants: VRESPA GIUSEPPE, VRESPA, GIUSEPPE
Application year: 1990
Main classification: A61B17
IPC codes: A61B17, A61C8