Screw device for fixing prostheses to bones, a method for applying the device, and the relative instrument.

TITLE SCREW DEVICE FOR ANCHORING OF

BONE PROSTHESIS, METHOD FOR THE APPLICATION OF SUCH DEVICE AND RELATED EQUIPMENT

INVENTOR

VRESPA GIUSEPPE

Rome, June 6, 1993

THE MANAGER (ATTILIO RONCACCI)

AdelV

TO THE MINISTRY) OF THE TRADE AND CRAFTS INDUSTRY

CENTRAL OFFICE ^ BREVÉTTI · ROME,

PATENT APPLICATION FOR INDUSTRIAL INVENTION A. APPLICANT | l): applicant code 111 retireiuo code I ili Si

1) surname, first name / company, type I.VRESpA GIUSEPPE_____,.

city. {prov.) / nation. LÌEGNANO_J_Mil Ft no) __.___ j, The codes

2) surname, first name / companies. type I __________, __ L city, (prov.J / nation l___ ..

B. REPRESENTATIVE: representative code

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,; , 1 idontilicotivo code J; l_ I, l J^{1 1 1} 1 '

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n. albo £ i3ól surname first name LGIAMBROCONQ ..Alfonso denomination etudio of belonging viaI_Ri__Pi I or ___—

I ING. A. GIAMBROCONO '&amp; C, Srl end. focus

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C. ELECTIVE DOMICILE: name -_L.L ......: ______ via I________I n. L i .1..1 ... 1 city L «ρ Ri 01) .¾¾ ~ &lt;pfyj IMlJj

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0. TITLE proposed class [sez./cl/scl):

SCREW DEVICE FOR ANCHORING Ól

BONE PROSTHESIS,

LLLAP_P_LlJ * L

E. DESIGNATED INVENTORS:

surname name

1)

2) LF. PRIORITY:

country υ I none 2) I ---------- iQ type priority code

Μ name, name _J 1 | L ~ J «| L

-4-date

J | | | / | | ί / i Δ J-lG. CULTURE COLLECTION CENTER ENABLED. FOR MICROBIOLÒGICAL PROCEDURES denomination: L

J II l / l 1 L / l 1 1 JI

H. SPECIAL NOTES:

THE_______________

ATTACHED OOCUMENTATION

<td> 1)</td><td>LiSlIJ</td><td>RIS / SI</td><td>t., 154 '</td><td>description with references and claims</td><td> ·- . . *</td>

<td> «</td><td>L.iSII</td><td>RIS / SI</td><td>104</td><td>draw tables</td><td></td>

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<td> 5)</td><td>1 iNiCI</td><td>RIS / St / NO</td><td>η. 1 1 .1</td><td>own age documents with Italian translation -</td><td></td>

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<td>'THE</td><td>UNFI</td><td>RIS / YES / NO</td><td></td><td>_p '' · / full name of the applicant</td><td>^: '' r</td>

<td>YES</td><td colspan="3">lira payment certificate</td><td>r6T8.OOO =</td><td></td>

bubble marks n. fòli di lire I5.00Q COMPLETED ON L26 / jjlQ / lli9Sg CONTINUE YES / NO iNiOl

Of the present act, the request, copy authentic $ | / ND

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of the month of l.vTTOBKE.

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PROVINCIAL OFFICE INO. COMM. ART. DEPOSIT MINUTES: APPLICATION NUMBER {._221 &amp; R

The year one thousand nine hundred L OTIANTANl ^ fii.'5ri = _^=:T_1. on the day I_VEfriTISEI the | i) applicant (s) copraindicate (s) halhannol pressed or but underwritten the orotento dortwtda. cWiooUa do n. CQ I (ogll.aggluntivl for the eenco; eléne of the eopteriporigto patent. I. MISCELLANEOUS NOTES OF THE FLOGING OFFICE:

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CIME ROGANTE

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Description of an invention with title:

SCREW DEVICE FOR ANCHORING BONE PROSTHESES, METHOD FOR THE APPLICATION OF SUCH DEVICE AND RELATED EQUIPMENT on behalf: VRESPA GIUSEPPE of Italian nationality, residing in 20025 LEGNANO MI and elective-

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GE / ml

A25818 domiciled at the agent Eng.

A. GIAMBROCONO Via Rosolino Pilo 19 / b Milan.

Filed on 26 011.1989 221 3 9A / 89

SUMMARY

In a screw device (10; 80) for anchoring prostheses to the bones, the threaded stem (14) has a core (32, 34) in the truncated cone assembly. The neck of the screw is cylindrical and has a diameter equal to or greater than the maximum diameter of the thread. The latter is of two different types: a first thread (26) with rapid pitch for anchoring in the trabecular bone tissue (52) and a second thread (28) which can also be of the self-jetting type for anchoring in the cortex of the bone on the opposite side to that of the introduction. The second thread (28) has a number of principles that is multiple of that of the first thread (26). In the case of use in orthopedics, the screw (80) can also provide a third thread (128) at a part (23) of the neck (112), for anchoring in the relative cortex.

One method of applying the screw device is to drill a precision hole in the bone, tap this hole with a tap (60; 160), and screw the screw (10; 80) into it. Precision drilling is obtained by using an upside down wedding cake cutter (100), and boring the cavity obtained with a manual reamer (140).

DESCRIPTION

The present invention relates to the means of anchoring prostheses to the bones and more particularly to the screw devices for realizing such anchoring.

The invention also relates to a method for applying the aforementioned screw device, as well as the relative equipment for carrying out this application.

As is known, from the point of view of the mechanical characteristics, the bone tissue can be divided into two distinct zones: the cortical zone with an elastic modulus from 1000 to 1200 dN / mm and the zone

<img file="IT1237496B_D0008.tif" />

spongy, trabecular bone, containing marrow or fat, with elastic modulus roughly between 20 and 400 dN / min ^.

Having to perform an implant in any bone at present, the known bone screws having a substantially cylindrical shank and consisting of a biocompatible metal, such as titanium, austenitic stainless steel, 11 tantalum, niobium or zirconium are used. These screws require the prior execution of a relative drilling, also substantially cylindrical, to be made in the bone. If the screws are self-tapping, they are inserted directly into the cavity thus obtained, which always has a diameter smaller than or at most equal to that of the core of the screw. In the case of non-self-tapping screws1, it is necessary to form the relevant nut in the side wall of the hole by means of a tap.

The screws known up to now either exploit the mechanical characteristics of the trabeculation for anchoring, so they have a rather wide thread (rapid pitch), of the type suitable for rather soft materials, or they exploit the mechanical characteristics of the cortex from the opposite side to the point of screw penetration, thanks to a fi

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Injection having a significantly smaller pitch than that of the previous case and suitable for ensuring a good mechanical grip In hard materials, but not suitable for setting in trabecular bone tissue.

In turn, the cortical part of the bone, due to its subtlety, can generally accommodate only a turn of a fast-paced fillet. Furthermore, due to its relative fragility, this fabric is suitable for accommodating a thick thread.

This is especially true for the cortex located on the side from which the self-tapping screw or tapper is inserted. In fact, as already said, the thread of the screw or of the tapper has a larger diameter than that of the already prepared drilling. Furthermore, the neck of the screw (i.e. the cylindrical end of the screw to which the prosthesis is to be fixed, which is normally not threaded but enters the cortex) has a diameter smaller than that of the thread. As a result, the insertion of the self-jetting screw, or of the tapper, into the previously prepared hole, involves an initial bone removal of the cortex around the neck of the screw, in particular of its outermost part. To the bone cortex

<img file="IT1237496B_D0010.tif" />

superficial, for a certain part around the implant, surgical damage is inflicted. The damage is directly proportional to the size of the tooth of the thread of the self-tapping screw or of the tapper. The part of the cortex that detaches does not reform. This represents a serious drawback, since the cortex is the most resistant area of ​​the bone and the most suitable for bearing loads, especially loads perpendicular to the axis of the screw.

The cavity intended to accommodate the system is performed with rotating tools mounted on manually controlled drills.

The shape and size of the cavity obtained depend on several factors, and in particular on the characteristics:

a) the bone to be drilled;

b) drilling equipment;

c) of the operator's hand checking the drilling equipment.

Let's examine these three factors in detail:

a) The bone to be drilled is free to move. It has a smooth, moist, and therefore slippery surface. Furthermore, the surface is rounded. Again, the bone structure is anisotropic,

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therefore the resistance offered to the surface of the cutting edge is different during the drilling operation.

b) The perforating system consists of a drill bit, or cutter, of different shape: spearhead with full cylindrical body, countersunk tip with vertically milled body for chip collection, countersunk tip with helical grooved body for collection chip.

It does not appear that studies have been carried out to determine the best cutting angle suitable for the bone, nor to obtain a good discharge system for the bone chip which mixes with the blood and tends to coagulate. As is known, when the drill bit is inserted into the drill it is retained by a fast-acting mechanism, consisting of a hollow neck which houses the relative part of the drill, which is thus blocked in axial movements, while radially the drill has a certain game. Therefore the rotary movement of the cutter does not take place around a fixed axis, but around an axis which can undergo small oscillations and displacements perpendicular to itself.

The mechanism that gives the movement to the tip can also move slightly due to

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who mechanics intrinsic to the mechanism itself. For the combination of all these causes a milling movement which we will call flickering is obtained.

c) The hand of the operator holding the drill is subject to muscle control which varies from operator to operator and which, in the same operator, may vary over time.

Before coming into contact with the bone, the tip turns free, describing at each of its cross sections a peripheral circle with a diameter greater than that which the drill has in that same cross section, and this due to the aforementioned flickering.

Furthermore, when the apex of the drill is in contact with the bone, however much the operator pays attention, the axis of the drill will not generally be exactly perpendicular to the surface of the bone, therefore, despite the presence of an invitation ( previously performed on the surface of the bone), when the operator exerts a certain force on the drill to perform drilling, a non-axial reaction will appear on the cutter which can be broken down into a force component directed perpendicularly to the axis of the cutter, which will

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causes bending, and in a second direct component according to the axis of the cutter itself. The aforementioned bending force will have two effects: the first consists in canceling the radial clearances of the mechanism that holds the tip, so that the aforesaid peripheral circle will become the maximum possible; the second effect takes over after clearances and consists in deformation by bending the tip, with further increase in the diameter of the peripheral circle.

After the first initial sinking of the tip in the bone we will therefore have a cavity of an Indeterminate diameter, certainly greater than that of the drill, and of a certain depth, let's say a millimeter or two. As s1 understands, the variables involved are different and in practice cannot be controlled, so the advancement of the cutter in this first section will take place in a prevalent direction which is substantially that of the theoretical axis of the cutter. It is however clear that, given the premises, the inaccuracy will be quite high. In practice, in its first section, the cavity can be considered as a series of overlapping circles of variable diameter, and probably irregular, slightly offset from each other, forming a cavity.

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extending mainly in a certain direction.

By applying additional pressure to the drill, we will advance it into the bone. Now two new factors that influence the operation become sensitive: the accumulation of chips that tends to coagulate, and the presence of the part of the cavity already excavated.

The presence of chips increases the friction, which generates heat, and causes new small removal of bone material from the cavity walls. Friction can increase until the drill motor stops.

The chip must therefore be removed from any type of cutter, firstly to avoid heating of the cutter, secondly to allow the advancement of the same. To do this, however, it is necessary to extract the cutter from the hole. Each time this operation is carried out, new material is inevitably removed from the walls of the hole already dug.

The part of the hole that has already been excavated actually performs the important function of acting as a guide for the cylindrical body of the cutter, this body being able to be cutting or partially cutting in the cutters with helical discharge, or not cutting in the cutters

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If the cutter body is sharp or partially sharp »any change in direction of the cutter will result in material removal. As a result, the cavity is enlarged, thus reducing its guiding function. As said, at the end of the operation the cavity will be formed by a series of overlapping circles of variable diameter within a certain field and slightly offset from each other, to form a cavity which is therefore rather irregular. By necessity, the cavity will have a larger diameter at the mouth and a smaller one at the other end.

If the body of the cutter is not sharp, the cavity already dug has a guiding function whose effectiveness increases as you go deeper. However, it is not believed that this allows to obtain a greater precision in the execution of the cavity. In fact, all the reasons that make the initial cavity wider than desired (from the flickering of the drill to the non-orthogonality between the drill and the bone plane) remain. On the contrary, they are joined by one that the helical body cutters do not have. In fact, the cylindrical side milling cutters have no chip discharge spaces. It is therefore necessary to extract the cutter much more often, to clean it, ultimately causing a greater enlargement of the cavity. The penetration movement of the cutter is actually of the helical type in the direction of rotation of the drill, since this movement is a combination of feed and rotation.

At the end of an in vivo bone drilling operation, carried out with the usual surgical techniques, we will therefore have a roughly truncated conical cavity of unknown diameter but certainly greater than that of the used drill.

From checks carried out it was possible to ascertain that the increase is in the order of tenths of a millimeter, with a wide variability.

If we want, in a first approximation, to imagine the type of cavity obtained, we must think of a pile of discs with slightly larger diameters at the top and smaller towards the apex. The discs will not be coaxial, their centers roughly forming a very irregular helix design. If we imagine an ideal axis passing through the center of the two extreme disks, the centers of the intermediate disks will generally not be arranged on this axis, but will be located in a certain area around the axis.

If we draw a circle having the nominal diameter of the implant and centered on the aforementioned axis, e

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X '*'

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XgJAMgiSP 'when the diameter, measured, of a certain cross section of the cavity obtained, with its center in its real eccentric position with respect to the axis, we can see both 1 contact points, if any, and the maximum distances between 1 two circles . By repeating the operation for a certain number of cross sections, we can precisely determine how many are the points of contact between the implant and cavities, as well as the surface and depth of the non-adherent areas. This allows us to understand that even with an effective diameter constantly greater than the nominal diameter of the cutter c1 there can be a series of contact points randomly distributed on the surface of the cavity.

If, to carry out a check, we insert a cutter into the cavity obtained, the cutter can appear stable even if it touches the walls in a few points which do not ensure its effective stability. If on the other hand, ^ as in reality it normally happens, the cutter has a certain play, it means that surely there is not a sufficient number of contact points. Therefore, by tapping the cavity, the thread will be complete, as depth, only in the aforementioned contact points, while the rest of the

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- 13 threads will be partially or only completely missing.

At this point c1 we can ask: if we had to make a hole 1n a wall, to insert an expansion plug, would we use a drill with a tip not blocked in the spindle and flickering? Well it is done in the bone easily. Not only that, but we pretend to make certain predictions about healing, starting from these random bases. But nature helps us, and fortunately, despite everything, the bone manages to heal sufficiently in a good 80% of cases.

However, this does not mean that we have sufficient information to guarantee healing, since we do not know how and especially when the bone will rebuild.

Is it possible to intervene on operating technology in order to obtain cavities with precision of the order that can be obtained with the mechanical processing of the same implants to be inserted in the cavity? Yes of course!

What are these levels of precision? Without resorting to too complex and very expensive procedures, the present invention allows to obtain accuracies of 0.02 mm as tolerances on di14 · Ι * 0 * ΜΟ

J URLANO Ι 4 JjX Μββ &lt;0 IMILIA / £ * general dimensions, while the irregularities of the su- w \.

for a number of reasons we have already seen, precision is possible, indeed optimal

Around O.Olmm.

The reason for all this precision research is to reduce as much as possible, in theory to zero, the quantity of bone tissue that must reform around the implant.

The problem of play, both between the cutter and the spindle, and intrinsic to the drill head, cannot be solved except by completely changing the current technology. However, this entails very high costs.

We have seen that the mobility of the implant with respect to the walls of the cavity that houses it, is a negative occurrence for the repair of the bone lesion.

The aim must therefore be to obtain a coupling without mobility. This need is currently satisfied by resorting to more or less forced insertions of the implant into the cavity.

This results in the compression of the contact bone tissue. This compression represents the price paid by all the known insertion methods, r &lt;iy mimmo \\ X '0 I MH.AHO | 0 \ 5, \ HMO IMUA 7P which provide an initial immobility of the system. In particular, the blades or cylinders currently used for this purpose are inserted with small hammer blows. In this way a joint is made between bone and implant, due to the mutual compression of some areas of the implant with corresponding areas of the alveolar wall.

Implants made up of different elements (d1sk-implant) use traction between screw elements and prismatic bodies to obtain areas of adhesion bone implant that provide the initial necessary stability.

The screw has had considerable success as it allows to obtain an excellent immediate rigid connection between implant and bone with ease.

However, this solution has some negative aspects which can be attributed to two fundamental causes: the trauma produced by the helical thread in the tissue, and the transmission to the bone, through the thread, of loads perpendicular to the axis of the screw.

Of course there are those who have tried to solve both of these problems by eliminating the thread and offering cylindrical shape implants which in turn have poor initial stability, co16 fK / «AGAMO £ / X *^- fi | MILAN I * l'iV MOGIO tulli. / P tighten to larger holes, require higher bone crests, have a smaller lateral surface for the same size.

In reality, the solution to the problem is not to achieve stability that allows any level of bone repair, but is to put the bone in the best conditions for healing.

Currently the nut in the s1 bone obtains 1n two substantially different ways: for mechanical tapping (see Branemark); with self-tapping systems (see Tramonte).

Mechanical tapping is possible precisely for screws that require slightly wider nuts to allow screwing; this means that with Branemark screws it is impossible to reach the maximum congruence between thread and bone.

The self-tapping screws of the Tramonte model can reach the situation of maximum congruence between the thread and the bone, but through a compressive insertion which in many cases causes damage such as to nullify the advantages.

The insertion of the self-erecting screw or of the tapper in the prepared cavity causes local effects and general effects in the bone, which we will illustrate below:

I- Effects - local

They are provoked by the following actions:

(a) Shear-action

The cutting action of the fillet separates the tissue, damaging the calcified bone matrix, collagen, the fundamental substance, the cells, the vessels and the nerves. At the beginning of the tapping operation, as well as in the case of self-tapping screw, the cutting action causes inflammation and blood loss. The damaging action on the tissues determines the release of substances

Inflammatory (HW HAM - Histology, USES 1969). (b) Compression-action-at the interface The continuation of the tapping or insertion of the self-adjusting screw causes the thread to spread apart. If the tapping starts on the surface and there is no contrasting element in contact with this surface, therefore the breakage occurs by lifting the superficial cortex, with consequent disruption of the architecture in the surrounding area. In particular, the tearing of the vascular connections seriously affects bone repair in this area. In the structure of the tissue, the divin-

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- 18 rication necessary for the advancement of the thread is obtained by compressing the fabric at the interface. The volume of bone tissue corresponding to the volume of the tapper thread or the self-tapping screw, under the forward thrust, is fractured and dislocated on the sides of the advancing thread. Since below the cortex the bone is spongy, its solid part formed by calcium salts fills all the available surrounding space, while its liquid part is pushed into the more peripheral trabecular area.

If the fillet has such a pitch that the compressed spongy bone areas interact, there is a particularly negative situation due to a summation of harmful effects that complicate healing. This important aspect must therefore be taken into account when establishing the pitch of the self-tapping screw or tapping screw and the dimensions of the relative core, as well as the dimensions of the cavity to be made in the bone.

(c) Heat ion

As is known, the heat that develops in the bone during the perforation of the hole 1n to which subsequently introduce the self-erecting screw or the tapper, is the main cause of formation of scar fibrous connective, rather than new bone tissue, in the subsequent phase of repair of the surgical lesion. For this reason, to make said perforation it is advisable to use the known rotating instruments with internal cooling by means of physiological solution which, in addition to cooling the cutter, also removes the bone chips, lodging them in the appropriate grooves. Another method of reducing the heat produced is to limit the rate of rotation of the yield to the minimum number of revolutions that allows drilling.

Similarly, the tapping or introduction of the self-tapping screw must also be very slow, so that the phenomena that take place must be considered static and the forces applied must be just greater than those of equilibrium. It is essential to limit friction in order not to excessively increase the temperature which must be kept practically below 44 ° C.

<img file="IT1237496B_D0020.tif" />

The tapping or insertion speed of the self-tapping screw will be the minimum that allows screwing into the bone, so this operation can only be done manually.

The use of motorized tappers or screwdrivers does not allow easy control of the speed and therefore of the induced heat.

In conclusion, in the current state of the art, as a result of the summation of the local effects described above, s1 has the result that the damaged spongy bone tissue is replaced with soft scar tissue, which is not suitable for ensuring effective anchoring to the screw.

II- Effects-generaii

As is well known, trabecular spaces are not empty, like none of the bones. The system they create can be considered as a closed hydraulic system, so the insertion of an additional volume is possible only with an increase in the overall volume. Therefore, if according to the prior art the aforesaid perforation corresponds at most to the volume of the screw core, the insertion of the self-tapping screw or of the tapper causes a

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/ &lt;VVA &lt;&lt; * / «CRGAMO \« *

Ι MILAN | «Increase in volume which is at least equal to the volume

XgiÀBS ^ of fillets. This produces an increase in the internal pressure of the bone which can easily be higher than the breaking limit of the bone itself and cause fractures that generally do not occur in the interface, where the local phenomena described above occur, but start from the external cortex, in drilling correspondence. Excessive internal pressure build-up in the system should therefore be avoided.

There is a second effect that produces an increase in pressure. It is generated by inserting the self-tapping screw or a known type of tap. In fact, from the beginning of their introduction they occlude the hole made in the bone, from which blood flows.

This blood is then pushed towards the bottom of the hole, contributing to further increase the internal pressure of the bone, so the aforementioned risk of fractures increases.

The present invention aims to overcome the aforementioned drawbacks of known bone screws, providing both a screw device for anchoring prostheses to the bones, and a method for applying this device, as well as

<img file="IT1237496B_D0022.tif" />

- 22 the relative equipment for carrying out this application, so as to allow the repair of the bone tissue around the screw (by sliding replacement), the screw thus being firmly and permanently fixed to the bone. To achieve healing by creeping replacement, a bone repair method essentially identical to bone reworking, a minimum amount of blood clot must be present at the screw implant surface according to the present invention. In fact, the blood clot transforms into mature lamellar bone very slowly (6-12 months in humans), with a self-limiting process. This latter characteristic means that the ossification of the clot may not take place completely, resulting in the formation of fibrous tissue unsuitable for relating the loads.

It is also necessary, to allow the creeping replacement, that the vascular canalizations of the necrotic lamellar bone are not destroyed, therefore the pressures that s1 will exert in the screwing must be minimal.

The present invention is proposed first

<img file="IT1237496B_D0023.tif" />

- 23 to obtain the substantial elimination of the blood clot between the calcified bone and the implant, by achieving the maximum congruence or adhesion possible between the bone and the various parts of the screw, without having pressures such as to irreparably damage the lamellar bone.

In particular, it is essential to obtain a cavity having a degree of precision substantially higher (of an order of magnitude) than that currently obtainable with the known techniques, so as to minimize the bone tissue that must reform. The screw must also have a conformation such as to minimize the bone tissue that s1 must reform.

For healing to take place by transformation of the necrotic lamellar bone tissue by means of the creeping replacement method, which allows the preservation of the peculiar mechanical characteristics of the lamellar bone, and which takes place in 6-12 weeks, it is also essential that the phenomena mentioned above are avoided . In this case, even during the healing period, during which for obvious reasons we will try not to load the screw, the latter will be able to bear small loads that accidentally, but almost inevitably, can act on the same, without negative consequences .

The screw device according to the present invention comprises a neck and a threaded stem, and is characterized in that the threaded stem of the screw has a core in the truncated cone assembly; that the neck of the screw is cylindrical and has a diameter equal to or just greater than the maximum diameter of the aforementioned thread; and that this thread is of two different types: a first rapid pitch thread suitable for anchoring in the trabecular bone tissue and extending along the part of the shank destined to come into contact with such trabecular tissue, and a second threading, which can also be of the self-jetting type, intended to anchor in the cortical part opposite to the screw insertion part, this second thread having a number of principles that is multiple of that of the first thread.

The vine with a truncated cone-shaped core or stem, thanks to its geometric shape, if combined with a cavity also in the truncated complex

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ώ / \ ft I MH.ANO Ι 0 Y% \ "ιβββ luiLit i§ nica, of adequate precision, allows to reduce practically to zero the quantity of bone tissue that must reform, and therefore to obtain the maximum congruence between lives and cavities.

Furthermore, thanks to the double type of threading, the screw described above can be anchored effectively both in the trabecular bone tissue and in the cortex.

The fact that the neck of the screw has a diameter greater than its thread makes it necessary to provide a drilling having a first section, which in practice affects only the cortex, having a diameter substantially equal to that of said neck. Therefore, when inserting the self-tapping screw or the tapper, the cortex is not damaged.

In the event that in contact with the surface of the cortex, where the screw is inserted, there is a fixation means or the like, which acts as a contrast medium (for example, in the case of screws for orthopedic use, a prosthesis or a bone synthesis medium resting against the surface), the screw according to the invention may have, at the lateral surface of the screw neck, &lt;/ icacawo Υ, λ 'λ · gf (MILAN I * \ &lt;Λ «loco ιμιιο ÌQ a third thread, of the same type as the aforementioned second thread.

In this way an anchorage is obtained also in correspondence of the cortex crossed by the neck of the screw, obtaining the best possible anchorage of the screw to the bone.

It has been possible to verify that the existence of the aforementioned contrast medium in contact with the surfaces of the said cortex contrasts the lifting and destruction of the outer part of the cortex itself, which would have occurred when the aforementioned third penetrated self-tapping thread in the absence of such a contrast medium. A situation of this type occurs for example when a plate for the synthesis of bone fractures must be applied.

To obtain the best result from the use of the screw device according to the invention, it is necessary to follow a particular method of application of the same device, a method which allows to obtain a cavity having dimensions substantially more precise than that obtainable with known techniques and such as to minimize the amount of bone tissue that needs to be reformed. The tapping of the cavity obtained will then be performed by means of a tap according to the invention itself.

In particular, the method of application of the screw device according to the invention consists in:

make a precision hole in the bone, in the position in which the aforementioned screw device is inserted, this hole comprising: a first outermost cylindrical section, suitable for accommodating the neck of the screw, with a diameter equal to or preferably slightly smaller than that of the non-threaded neck of the screw, or slightly larger than the maximum diameter of the neck if the latter is threaded; a second internal truncated cone section of transverse dimensions equal to or preferably smaller than the core ones of the first part of the stem of the screw having the aforesaid first type of rapid pitch threading; and a third section extending for the entire remaining length of the stem of the screw, said third section being relative to the aforementioned second type of screw thread and having transverse dimensions slightly larger than those of the core of the part of the stem of the screw having the aforementioned second type of thread;

tap the aforesaid second section of the hole so as to obtain a suitable nut in it

<img file="IT1237496B_D0025.tif" />

to accept the first thread of the screw;

in the event that the aforesaid second thread of the screw is not of the self-tapping type, tap the said third section to obtain a nut in it suitable for receiving this second thread of the screw;

screw the screw completely into the already tapped hole.

Thanks to the application method described above, it is possible to obtain the maximum congruence between screw and bone.

The aforesaid drilling, made in a bone, may also, if necessary, be pass-through.

The present invention also relates to a milling cutter and a reamer for obtaining the aforesaid precision drilling, as well as a precision drilling method which uses the aforementioned milling cutter and reamer.

In particular, the cutter according to the invention is of the type cooled by sterile liquid, which also has the function of removing the formed bone shavings, and is characterized in that it has an inverted wedding cake shape.

With this term, which immediately displays

<img file="IT1237496B_D0026.tif" />

The shape of the cutter is intended to mean that it is made up of several coaxial cylindrical bodies, joined together and having a decreasing diameter going towards the apex of the cutter itself.

The manual reamer according to the invention has a shape and dimensions which allow to obtain the desired final drilling, ready to be tapped, and is characterized by the fact that it has a rake angle suitable for cutting bone tissue, and has means for to convey nutrients for the bone into the cavity in the bone. These liquids have the purpose of reducing bone necrosis.

The means for conveying nutrient liquids may simply consist of a coaxial channel which crosses the entire reamer and which communicates with the lateral openings provided between the cutting edges of the reamer.

The method for performing the aforesaid precision drilling for the insertion of a screw device according to the present invention consisting of:

with the upside-down wedding cake cutter, make a step cavity with diameters overall smaller than the desired final cavity; and bore ma-

<img file="IT1237496B_D0027.tif" />

GO i&gt; / \ r *

Q ί MILAN I «, &lt;\« eee IMILIA jQ the cavity \ ° V ZW stepped to obtain the desired precision drilling, ready to be tapped, by means of the above reamer.

It has been possible to verify that the best results are obtained when both the neck (not threaded) and the core of the first part of the stem of the screw have slightly smaller diameters, of a few microns, than those of the relative drilling. In this case, screwing the screw will be slightly compressive. In this way, maximum congruence between the neck and thread of the screw on one side and bone tissue on the other is obtained, also producing minimal damage to the bone.

It has also proved to be convenient to exert a slight pressure of overstanding when the screw reaches the final position in the cavity obtained. In this way, the maximum congruence between the stem or core of the vine and the cavity is also obtained.

To obtain in the side walls of the second section of drilling the fast screw nut, suitable for accommodating the first type of screw thread, a niascher according to the invention is used, characterized in that it has a male thread with a maximum diameter not exceeding that of the screw neck, and the same number of principles and the same pitch as the first type of screw thread, said tapping thread extending for the same length as said first screw thread, the end part of the tapping screw corresponding to the part of the screw having the second type of thread being free of tapping threads and having transverse dimensions not exceeding those of the corresponding third section of the drilling if the said second screw thread is of the self-tapping type, while the said end part of the tapper has a tapping thread having the same number of principles and the same pitch as the second thread of the screw in the event that said second thread is not self-tapping; and by the fact that the tap has at least one discharge means to allow the escape of the liquids contained in the bone. In particular, the discharge means may be a coaxial hole in communication, through transverse channels, with the lateral core surface of the tap. A variant of the tapper according to the invention has discharge means consisting of one or more longitudinal grooves which extend for all

<img file="IT1237496B_D0028.tif" />

ta the length of the tapper, Interrupting all the threads and also affecting the core of the tapper. The outer edges of each groove will be conveniently rounded to minimize damage to bone tissue.

The frusto-conical lateral surface of the core of the second thread of the screw is parallel and coaxial, but preferably Internal, with respect to the frusto-conical lateral surface of the core of the first thread.

Considering a generic cross section of the end part of the stem of the screw having the second type of thread, and the corresponding cross section of the third drilling section, it can be seen that, with the screw inserted, an annular space exists between the core of the screw and the side wall of the drilling. This space acts as a compensation space that is filled at least in part by the cortical bone plastically deformed at the time of the introduction in the third tract of drilling of the screw, in the case that the second thread is self-tapping, or of the threaded terminal part of the tapper, in the case of a second thread of the non-self-tapping screw.

<img file="IT1237496B_D0029.tif" />

&lt;£ * / «AGAMO \\ λ^Λ3 / \ ** p I MILAN 1 φ VÌA MOGIO Emilia / Ρ

This compensates for the volume of the \ oV Xcy fillet that enters the cortex, so that dangerous pressure increases are not created in the bone.

Preferably, in the relative part of the shank of the second thread of the screw, at least one longitudinal groove is provided which has the double function both of additional compensation space for any other pressure increases that may occur, and of an area in which to accommodate any bone chips. These increases in pressure can be generated by the fluids present under the apex of the screw and which, having no way out, would be compressed during screwing.

The said vertical groove in the terminal part of the screw also functions as an anti-unscrewing device, since in it s1 will reform new cortical bone tissue which hinders the unscrewing.

Therefore, in the event that the screw must be removed after a certain period of time, this groove must not be provided.

For the first type of screw thread, an annular compensation space is not indispensable as for the second self-tapping thread, and this is due to the different nature of the bone tissue involved, which is trabecular. As already mentioned, the ma34 \

φ f MILAN | φ

ΛίΧ woeo («ILI * / P schiatura for the formation of the nuts suitable for wV / $} accepting the first thread of the screw causes the cutting and lateral dislocation of the solid part of the spongy bone tissue, which fills the adjacent available space.

As also said, the discharge means of the liquids contained in the bone has the purpose of allowing the discharge both of the blood leaking from the surgical wound, and of the liquid part dislocated due to the formation of the threads of the nut screw.

This allows the local effects to be controlled within the desired limits and avoids the general negative effects illustrated above.

It has been seen that the discharge means can consist of grooves provided in the tapper.

It should be noted that also the common tappers for mechanical use have longitudinal discharge grooves which interrupt the tapping threads and also affect the core part of the same. In this case the edges of the longitudinal grooves must be well affiliated in order to cut the material in which the relative nut is to be made. These grooves have the purpose of allowing the collection and removal of the shavings formed by the action exerted by the edge cutters of the aforesaid groove on the hole wall.

On the contrary, in the present case, since it is desired to avoid the formation of shavings in the execution of the fast screw nut, and to limit the above described compression effect of the trabecular tissue, the edges of the longitudinal groove will be rounded. In performing the tapping with the tapper according to the invention, therefore, there is no removal of bone tissue, but only the elimination of an equivalent volume of organic fluids. The trabecular tissue will then only be cut and dislocated without pressure by the tapping threads. In the spongy bone tissue, therefore, the phenomena of dislocation of the solid and liquid part described above will manifest purely, which do not affect the repair by creeping replacement of the new lamellar bone tissue in the surrounding areas damaged by tapping.

In turn, the tap's rapid pitch thread, in penetrating the spongy bone tissue, will have to damage it as little as possible. In particular, the crest of the first turn of the tapping thread must be pointed to allow a

<img file="IT1237496B_D0030.tif" />

£&gt;/

I ΜΧΑΝΟ 1 · V3, \ »160« ΙΜΙΙΙΑ / Ρ 'excellent cutting action of the fabric. A form of Ψ77 χ £ Ϊλϊλ «§χ convenient cross section for the other Spines of the screw thread may be that trapezoid without sharp edges, which can be obtained in a mechanically simple way. The trapezoidal shape allows the other spines of the screw thread to absorb part of the external loads perpendicular to the axis of the screw, without cutting actions, which would occur instead in the case of pointed crests.

For the second threading of the screw, which affects the cortex, the aforementioned problems are not so stringent, so the cross section of the relative thread can also be conveniently triangular, but with the rounded crest, to avoid cutting or dangerous actions as much as possible load concentrations.

The same can be said for any tapping thread of the end part of the tapper, if the second thread of the screw is not self-tapping.

The invention will be more clearly understood from the following description of two embodiments of the screw according to the invention itself, of the drilling for this screw of the cutter and of the reamer to obtain the desired precision of the drilling, and of the corresponding tapper. In this description, reference will be made to the attached drawings, in which:

fig. 1 is a side view of a screw according to the invention, particularly suitable for dentistry, of the type with second self-tapping thread;

fig. 2 is an axial longitudinal section of the hole suitable for receiving the screw of fig. 1, before this puncture is tapped;

fig. 3 is a side view of a first embodiment of the tapper according to the invention, for tapping the perforation of fig. 2, the tapper having the end part not threaded;

the f1g. 4 is a cross section of it made along the line IV-IV of fig. 3;

fig. 5 is a cross section of this carried out along the line VV of fig. 4;

fig. 6 is a side view of a second embodiment of the tapper according to the invention;

fig. 7 is a cross section of the same taken on line VII-VII of fig. 6;

<img file="IT1237496B_D0031.tif" />

&lt;&lt; iy MtCAMO

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0I ΜΚΑΗΰ 1 0 ίΛ »100« IMIU * / Ρ la f 1 g. 8 is a side view of a screw particularly suitable for orthopedics;

fig. 9 is a side view of the upside down wedding cake cutter according to the present invention;

fig. 10 is a side view thereof taken along the line XX of fig. 9;

fig. 11 is a top-down view thereof along the line XI-XI of fig. 9;

the f1g. 12 is a side view of the reamer according to the present invention; and fig. 13 is a bottom-up view of it along line XIII-XIII of f1g. 12.

From figure 1 it can be seen that the screw 10 is made up of two very distinct fundamental parts, namely a top cylindrical neck 12, a threaded stem 14.

The upper portion of the cylindrical neck 12 is destined to protrude outside the bone, the rest of the neck 12 being received in a special seat provided in the cortex.

A cylindrical rather than frusto-conical shape has been chosen for the neck 12 of the screw to ensure that the neck itself, when loaded, does not transmit axial loads to the relative cortex, while it can transmit transverse loads through its lateral surface 13 which is surrounded by the cortical.

In the free upper surface of the cylindrical neck 12 there is an axial prismatic cavity 16 (indicated dashed in fig. 1) suitable for joining with a special tool (Allen key or similar), not shown in the drawings, to allow the screw to be screwed into the bone and to subsequently allow the application of dental prostheses, for example a pin-stump to be prosthesized according to the method taught by dr. Vrespa (Last Supper Italian Implant Studies Group, Bologna November 1987). At the bottom of the prismatic cavity 16 there is a coaxial hole 18 threaded or not threaded (also shown dashed in fig. 1), which serves to anchor a known healing plug (not shown) or anything else necessary to the screw.

The presence of these two cavities 16 and 18 allows the application of a screwed or cemented mesostructure according to the choices made and the needs of the case.

The threaded shank 14 is coaxial to the neck 12 and in one piece with it, and is connected to the neck by means of a short truncated cone fitting 20. The

<img file="IT1237496B_D0032.tif" />

f &lt;&lt; 7 «AGAMO 0 | MILAN | φ &lt;5, \ M0C &lt;O IMItl * JQ shank 14 is in turn formed by two coaxial parts 22 and 24 obtained in one piece and having two different types of threading. In particular, the upper part 22 of the shank 14 has a first cylindrical thread with a rapid-pitch principle,

26, suitable for anchoring in spongy bone tissue, the relative thread having a trapezoidal cross section with rounded edges.

The lower part 24 of the shank 14 has a second thread 28 having the same pitch as the first thread 26, but with three principles, and of the self-threading type. The fillet has a triangular cross section with a rounded crest.

The second thread 28, due to having three principles, behaves, from the point of view of the anchoring, substantially like a thread having a pitch equal to 1/3 of the actual pitch, so that a thread of the type suitable for the anchoring in the cortex, in particular in the cortex opposite the screw introduction side.

In the embodiment shown in fig.

the cylindrical neck 12 of the screw has a diameter equal to that of the first thread 26.

Obviously the lengths of the various parts compo41 * 5 / γ * 'φ I MILAN I φ

.... .... . . . Via · «&lt;!» "Mini IQ, the screw will be such that, once the neck 12 of the screw is inserted, it will affect the cortex from the infix side, the intermediate part 22 of the stem 14, presenting the first thread 26, will affect the trabecular bone the terminal part 24 of the shank 14, having the second thread 28, will affect the opposite cortex.

As can be seen from fig. 1, the part 24 of the shank 14 has a vertical groove 30 which interrupts the thread 28 and also partially affects the core 32.

The function of this groove 30 has already been illustrated previously.

From figure 1 it can be seen that both the core or stem 34 of the upper part 22 of the stem 14 and the core (32) of the lower part 24 are truncated, (the relative lateral surfaces being parallel), between 1 two there is a small step

36.

We will now briefly describe the method of applying the screw in fig. 1 and the tools that serve this purpose, with particular reference to the execution of the drilling in which to insert this screw.

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MOGO «IH * jQ.

The first operation consists in making a precision perforation in the bone shaped as in fig. 2.

This is achieved by the use of cutters according to the invention, of the type we have called upside down wedding cake. One of these cutters is shown in figures 9, 10 and 11. The cutter 100 is constituted by a stem 102 of conventional shape, by a spacer section, or extension, 104, and by a milling section 106, The stem 102 is suitable for mate with the quick-action connection mechanism of the drill. The extension 104, of suitable length, has the only function of allowing the milling part 106 to arrive at the desired point, for example when a hole must be drilled between two teeth adjacent to a missing tooth. If there is no such requirement, the extension 104 may be missing.

As can be seen in Figures 9 to 11, the actual milling part is substantially made up of three milling bodies designed to produce three perforating sections of circular section and of different diameters, decreasing from top to bottom.

The cutter 100 has an axial hole 108 which is in communication with the openings 110 and 112. This is Vz &lt;&lt;&lt; // «CACAMO _ά | MKANO »100« CMltlA allows the discharge of organic liquids during \ q \. &gt; the drilling operation.

Once a step drilling of the type has been obtained in the bone by means of a single feeding of the cutter 100, this drilling is enlarged by means of a manual reamer according to the present invention, to obtain the truncated cone-shaped drilling of the desired precision. A reamer of this type is shown in figs. 12 and 13.

As mentioned above, to obtain the desired results, this reamer 140 must necessarily be manually operated.

The reamer 140 comprises a part of the shank 142 of hexagonal section suitable for engaging with a special tool for boring the aforementioned step cavity, and of a reaming part 144, with a rake angle suitable for 11 cutting bone tissue. The boring part 144 is in turn divided into two portions: a first portion 143 adapted to produce a cylindrical boring portion, and a second frustoconical position 145.

In figure 12 the proportions of the lengths of the two aforesaid portions are purely indicative. In particular, the reamer shown is not exactly what it takes to obtain the hole.

<img file="IT1237496B_D0033.tif" />

- 44 rature illustrated in fig. 2, for which the reaming part should be substantially longer. The same applies also to the milling cutter of figures 9 to 11.

As already said, the reamer 140 also has an axial channel 146 which crosses it completely and which communicates with lateral openings 148 provided between the cutting edges. In the specific case of Figures 12 and 13 there are four lateral openings 148, two in the groove 147 and two in the opposite groove 149. In the channel 146 the nutrient liquid for the bone is introduced, in order to reduce bone necrosis.

Once the aforementioned reaming has been carried out, a drilling of the type shown in fig. 2.

The first section 42 of the hole 40 is cylindrical, and of a diameter smaller than a few microns that of the cylindrical neck 12 (fig. 1) of the screw. The height of this first section 42 will roughly be parts of the thickness of the cortex 50, sufficient to accommodate that neck part 12 of the screw 10 destined to enter the cortex 50.

The drilling 40 continues with a short truncated cone section 44, which connects the first section 42 to the second truncated section 46, having diameters smaller than a few microns than those of the core 34 of the first part 22 of the stem 14 of the screw 10.

The hole 40 ends with a third section 48 which is nothing more than the extension, for the whole thickness of the opposite cortex 54, of the lines of the second section 46 of the hole.

In the specific case of dental screws, the upper cortex 50 will be covered by the gingiva 55 (see Figure 2), so that the latter will also be perforated. The perforation will therefore include in this case also a gingival tract 49.

Once the aforesaid drilling 40 has been carried out, a nut screw (not shown in the figures) will be made in the side wall of its second section 46, suitable for receiving the thread 26 of the second part 22 of the screw shank. This is achieved by means of the tapper 40, shown in Figure 3. In order to increase the adhesion between the screw and new bone tissue, the first part 22 of the shank 14 of the screw 10 and part of the neck 12 are normally coated, in a known way, with titanium by means of a plasma-spray treatment, which increases their slightly the size.

<img file="IT1237496B_D0034.tif" />

Therefore the dimensions of the tapping thread 62 and the core 64 of the part 68 of the tapper 60, such as that from the parts 46, 42 and 44 of the hole 40, will be slightly greater than those of the bare screw.

The lower frusto-conical part 66 of the tapper does not have threads, has the same length as the corresponding second part 24 of the shank 14 of the screw 10, and has at most the same transverse dimensions as the core 32 of this part 24 of the screw.

The beater 60 also has an upper part 70 substantially similar to the neck 12 of the screw 10, this part 70 having a prismatic protrusion 72 at the top which is suitable for engaging with a special tool (not shown) for inserting the tapper 60.

The tapper has a longitudinal groove 74 of substantially trapezoidal shape, which extends throughout the tapper (see also Figures 4 and 5) and whose function has already been illustrated. It should be noted that the edges 73 of the groove are rounded, as previously said.

<img file="IT1237496B_D0035.tif" />

Figures 6 and 7 show a variant,

<img file="IT1237496B_D0036.tif" />

which has proved to be particularly convenient, of the tapper according to the present invention / In particular the only substantial difference with respect to the tapper 60 of figures 3 to 5 is that, instead of the longitudinal groove 74 (fig. 3) for the discharge of organic liquids , an axial circular channel 174 is provided which crosses the entire tap. This channel communicates with the outside, as well as at its two ends, also through the openings 176 provided in the core part, between the successive turns of the tapping thread 162.

Once the tapping operation has been carried out, it is sufficient to insert the screw 10 into the hole, already tapped, the second self-tapping thread 24 of which will penetrate permanently into the opposite cortex 54 (fig. 3).

After an appropriate period of time, due to the phenomenon of creeping replacement, it will reform in contact with the screw of the new bone tissue which will ensure its stability over time.

Figure 8 shows a variant of the screw according to the invention, particularly suitable for orthopedics, for example for fixing a plate to a femur. The screw 80, shown in fig, 8 already

<img file="IT1237496B_D0037.tif" />

inserted in the bone, differs from screw 10 d1 fig. 1 only due to the presence of a third self-threading thread 23 at the lateral surface of the neck 112 of the screw. This is possible due to the presence of a contrasting element 82 constituted by the plate resting on the surface of the cortex 50 and which prevents the surface layer of the cortices 50 from being raised and destroyed when the self-threaded thread 23 is inserted into the cortex 50.

Obviously, in the event that the third thread 23 is not self-tapping, a relative nut screw must be made in the first section 42 of the hole 40 by means of a special tapper (not shown).

In this case the drilling at the neck 112 of the screw 80 will have a slightly larger diameter than that of the corresponding core 132 of the thread 23, but less than that of the cylindrical surface that envelops this thread, and this for the same criteria followed for the drilling of the second thread 28.

As can be easily understood, the screw 80 allows the best possible anchoring to the bone.