Sl

AO/ASIF Self-tapping screws (STS)

Frank W. Baumgart, J. Cordey, K. Morikawa, S.M. Perren, B. A. Rahn, R. Schavan, S. Snyder

AO/ASIF Research and Development Institute Clavadelerstrasse Davos, Switzerland

1. Introduction

The term self-tapping screw refers to screws which are inserted into a predrilled hole directly without first tapping a thread. A self-drilling screw would refer to a screw inserted without predrilling which would function similar to a threaded K-wire. Self- tapping screws are further subdivided into thread- farming and thread-cutting screws. The former forms its thread by elastic-plastic deformation or by local destruction of the bone, the latter by cutting through the bone and simultaneously performing the function of a tap. The AO/ASIF has opposed the use of self- tapping screws in cortical bone for many years and had good reason to do so.

The “Manual of Internal Fixation” (Miller et al., 1991, pp. 179-180) addresses the subject as follows:

“....Because the screw has to cut its own thread as it is inserted, it encounters considerable resistance, particularly in thick cortical bone. At times the resistance may be such that the torque required to drive the screw in is greater than the tolerance of the screw and the screw may break. In addition, the resistance to screw insertion may interfere with the accuracy of insertion, particularly if one is trying to insert the screw obliquely into bone to lag two fragments together. It used to be thought that self- tapping screws had a weaker hold in bone. Experimental investigation (Schatzker et al., 1975) has shown that a self-tapping screw can be removed or inserted without weakening its hold in bone provided it is carefully inserted. However, if inadvertently angled it will cut a new path and destroy the already cut thread, which is a

disadvantage. Self-tapping screws should therefore not be used as lag screws’.”

11 . . . Recent investigative work has shown that in extremely thin lay&s of cortical bone, such as facial bones, self-tapping screws appear to have a better holding power than the non-self-tapping screws of corresponding size (Phillips and Rahn,. 1989). The non-self-tapping screw has clear superiority2 except in extremely thin cortical bone, cancellous bone, in flat bones such as those of face, the skull, and the pelvis.”

However, from time to time it is necessary to investigate the problem anew on the basis of technical advances. The AO/ASIF has developed and has always used some self-tapping screws designed for special applications only (malleolar screws,. locking bolts, Schanz screws, maxillofacial screws). Most of these screws have a sharp edged or “trocar” shaped tip and have not generally been used as lag screws.

Specific questions must be answered before a self- tapping screw for general application can be developed.

1 If the self-tapping tip of the screw is located in the cortex, the holding power is reduced. This argument against self- tapping lag screws is true but not releiant if the screw tip protrudes through the cortex.

2 “superior” refers to the degree of delamination at the screw exit. This is negligibl6 in thin cortices, but can be detected in thicker cd&x. This effect is caused by the pushing effect of the tip driven by the part of the screw in the cut thread.

s2

These are: Technique:

The exact alignment of the instruments by the surgeon also influences the successful insertion of screws. This starts with the drilling of the hole and ends with the screw insertion process. In the case of inexact control of the drill bit, tap or screwdriver axis, additional bending moments may be applied to the device too. This may lead to deviations from the straight track for both the drill bit and the tap and therefore to misplacement of a hole in the second cortex.

l What is the scientific rationale for the application of self-tapping screws ?

l What is the optimal design of a self-tapping screw ?

l What could the potential problems be ?

l Which special techniques are necessary for the successful use of self-tapping screws ?

l Why should the AO/ASIF wish to introduce self- tapping screws ?

The following describes some considerations re- garding the optimal characteristics of self-tapping screws.

Flute design:

It is mechanically evident that the geometry of the cutting flutes in bone screws has a considerable effect on the insertion torque, on the axial force necessary for insertion and on the movement or clearing of bone debris. For cutting tools in general, the sharpness of the cutting edges is an important parameter. This depends on machining and surface treatment procedures (mechanical and/or electro-polishing) (Fuchsberger, 1989). The volume of the cutting flute is important for the management of bone debris. The volume of bone debris caused by screw insertion is limited depending on the thickness of the cortex (see appendix).

Insertion characteristics:

The insertion torque and the axial insertion force are important parameters controlled by the surgeon. These two variables are independent of each other. The insertion torque is determined mainly by the design of the screw and the strength3 of the bone. The surgeon feels it in two different ways when turning the screwdriver:

l First by the contact forces felt at the finger tips. The magnitude of these forces times the lever arm (screwdriver handle diameter) results in the insertion torque. This feeling depends on the screwdriver handle diameter.

l Secondly the direct feeling of the torque in the forearm. This feeling does not depend on the diameter of the screwdriver handle. This torque is also perceived during machine insertion.

The axial force can be optionally applied by the surgeon, independent of the torque.

3 The strength of the bone correlates with the density. Therefore, data on bone density allow a rough estimation of strength.

The tolerance between the hexagonal screw head recess and the screwdriver allows a certain small angulation of the screwdriver axis against the theoretical axis of the hole without applying any bending moment. The screw can adjust its position along the axis of the hole as long as these tolerances are not exceeded*. Therefore, this method of insertion is not as sensitive to malalignment as compared to drilling holes with a drill bit fixed in the drill which immediately leads to angulation and to bending moments (therefore to axial deviations) in the device.

In osteoporotic bone of low strength, the tendency to malalignment is greater since it is harder to detect unintentional angulation in softer bone.

The use of self-tapping screws has one main clinical advantage: To decrease the operation time for internal fixation. The number of steps and necessary instruments may also be reduced.

Insertion torque and axial force should be kept as low as possible to avoid additional load on the bone thread, excessive heat generation and fatigue of the surgeon who is inserting numerous screws. Furthermore, “tip catch” (the initial “bite” of the screw at the “entrance” to the hole) must be possible without application of axial force.

A self-tapping screw should also maintain sufficient holding force (pull-out force) and should not damage the bone more than a regular tap. It should be possible to insert the screw obliquely with ease and to find the drilled hole in the far cortex.

4 The standard tolerances of IS0 5835 for 4.5 mm screws and IS0 8319 for screwdriver tips permit (in the case of full insertion of the screwdriver tip) a minimum angle of 0 degrees and a maximum angle of 1.5 degrees. This angle is different for each screw combined with an ar&rary screwdriver. If the screwdriver is not fully inserted, the angle may exceed the maximum value causing the additional risk of stripping the recess.

Baumgart: Self-tapping screws (SE) s3

2. History

2.1 An overview

When summarising the development of self-tapping screws from the very beginning (Sherman, 19121, it must be taken into consideration that scientific findings related to materials, in biomechanics and medicine, have led to remarkable progress. Some of the findings (Wagner, 1963) can only be regarded in the context of their time and have to be rediscussed today in relation to present knowledge of self-tapping screws. The reduction in the pull-out force of a screw when the cutting flutes are positioned in the cortex (Lamarque and Cordey, 1988) is now an established fact and the existence of microfractures in certain areas of the cortex due to screw insertion has been proven. The influence of the self-tapping design on the pull-out force is small and depends on the degree of damage caused by the cutting tip. Since screw designs vary, the results of testing one type of screw cannot be generalised. The results of individual tests will depend on the test conditions, the type of screw, techniques and test materials and these must be compared in detail before any general conclusion can be reacheds.

2.2 The A0 designs for special application

Trocar tip

The trocar tip for the following existing self-tapping AO/ASIF devices is part of conventional design. Most of these devices (Schanz screws, malleolar screws) were developed many years ago and found to be optimal in terms of their intended use and the state of the technical art at this time. Manufacturing advantages are obvious, but in some cases (locking bolts) certain additional advantages, e.g. self- centering, play a role.

Malleolar screws (Fig. 1)

These 4.5 mm screws are designed mainly for use in the cancellous bone of the malleolus. The cutting tip is a trocar tip, angle 25”. The core diameter and the shaft diameter is 3.0 mm for stainless steel screws. The shaft diameter which is smaller than the outer diameter of the screw may be difficult to remove if strong bone covers the screw shaft in a thick near cortex after healing.

Fig. 1: Malleolar screws.

Locking bolts (Fig. 2)

Locking bolts are used for the locking procedure in medullary nailing. They have a trocar tip with sharp edges. A pilot hole must be predrilled for the locking procedure of universal femoral and tibia1 nails. The trocar tip eases insertion of the locking bolt into the locking hole in the nail. The diameters are as follows:

locking bolt outer dia. locking bolt core dia. drill bit

4.9 mm 4.3 mm 4.0 mm

Fig. 2: Locking bolt for AO/ASIF universal nails.

Schanz screws (Fig. 3a-c)

The Schanz screws are available in 4.0 mm, 4.5 mm, 5.0 mm and 6.0 mm diameter. Most of them have rounded trocar tips except for the 4 and 6 mm Schanz screws which have a flattened tip. A recently developed Schanz screw with radial preload has a slightly larger outer diameter compared to the core diameter. Therefore, only a very small volume of bone chips is produced during insertion and the cutting property of the tip is less important.

5 For historical details, see Schatzker, 1975.

injury 1993, Supplement 1

Fig. 3a: AO/ASIF 4.0 mm Schanz screw. Fig. 4: Thread-forming maxillofacial screw.

Fig. 3b: AO/ASIF 5.0 mm Schanz screw.

2.3 Other technical designs

There are a wide variety of technical designs of self- tapping screws which will not be described here. Self- tapping screws have to be distinguished according to the materials for which they are designed. It is evident that screws designed for use in metal are not as appropriate for use in bone as screws designed for soft materials like wood or polymers. The mechanical properties of bone are much closer to the properties of wood than to those of metal.

Thread-forming screws are usually suitable for use in soft materials such as polymers and cancellous bone, but inadequate for cortical bone. In cortical bone, an oversize of 1% of the diameter of the hole is tolerated before microcracks and fractures occur, e.g. for a 4.5 mm diameter bolt, a maximum of 0.05 mm oversize will be tolerated. This was demonstrated by the investigations on radial preload carried out by Biliouris et al. (1989).

Thread-forming screws for polymers with self- tapping tips, trocar tips and cylindrical tips are commercially available.

3. The AO/ASIF self-tapping screw (Fig. 5)

The characteristics of the screw are:

Fig. 3c: AO/ASIF 6.0 mm Schanz screw.

Maxillofacial screws (Fig. 4)

Some AO/ASIF maxillofacial screws have a self- tapping trocar tip which has been recently optimised by Rahn, Risch, Filoso (internal reports) and by Kuhn (1991).

- three flutes - positive rake angle - short, large cutting flute - tapered tip

Baumgati: Self-tapping sc~czus (STSJ

Fig. 5: The AO/ASIF self-tapping screw.

4. Studies and tests

4.1 Insertion torque testing

Initial comparative tests in Celcon6 cubes were carried out using the torque testing equipment of Synthes in Paoli/USA (Fig. 6). A computer controlled testing machine developed in the A0 Research Institute (ARI), Davos, Switzerland, was used for functional insertion tests in cadaveric bone specimens (Fig. 7).

Fig. 6: Testing device for screws (Synthes, USA).

6 Celcon is an ASTM standard material for comparative tests.

Fig. 7: AO/ASIF test rig for functional screw test with data processing by PC.

4.2 Pull-out testing

The pull-out tests on the 10 kN Instron machine of the A0 Research Institute (AR11 in Davos were done

a) in vitro (human cadaveric femora)

b) on standardised animal bone specimens

The A0 Research Institute uses standardised cortical bovine bone specimens of constant thickness. For the pull-out tests, 4 and 5 mm specimens were used. The screws tested were the A0 self-tapping screw and three other commercially available self-tapping screws. The results confirmed the findings of the randomised tests on cadaveric bone. The values obtained from tests on specimens fluctuate less; however, the cadaveric bone tests simulate the clinical situation more closely.

The following devices were taken into consideration:

- AO/ASIF tap - AO/ASIF 4.5 mm cortex screw (standard) - AO/ASIF 4.5 mm self-tapping screw - A0 prototype, trocar tip - Zimmer cortex screw 4.5 mm - Howmedica cortex screw 4.5 mm - Richards cortex screw 4.5 mm (3.5 mm drill bit)

The AO/ASIF 4.5 mm cortex screw was used under two conditions: pretapped and untapped. The mid- shaft of 10 human cadaveric femora were cut into 10 annular rings of 20-22 mm thickness. The tests were performed as statistically planned comparative tests. X-rays of each femur were made to estimate the bone density. The annular rings were predrilled radially

injury 1993, Supphnent 2

S6

with small radial pilot holes of 1.5 mm diameter and 1-3 mm depth. The radial screw holes were drilled using the appropriate drill bit (3.2 mm in general, the Richards screw with a 3.5 mm drill bit). Then the screws were inserted in a self-tapping procedure or if appropriate in a pretapped mode. The axial force was set to about 40 N applied by a 4 kg weight. The actual values for torque and axial load were recorded by the PC. The temperature of the screw tip once it protruded through the second cortex was recorded manually by a contact thermocouple. The thickness of the cortex of each sample was measured and recorded. The screws were partially unscrewed so that there was no contact with the far cortex. The specimens were placed into a fixture of the lnstron tensile tester for pull-out tests. The screws were then pulled out of the near (lateral) cortex, thus destroying the hole and making histological evaluation irrelevant. The empty screw hole in the far cortex was not involved in the pull-out tests and could subsequently be prepared for histological evaluation of the damage to the bone.

Separate tests on standardised bovine bone samples were performed using an infrared camera to observe the surface terrzperuture of the bone close to the inserted device. Drill bits, taps and AO/ASIF screws were compared using various drill speeds. Two holes in each test sample (25 by 25 mm in size and 4 mm thick) and two such samples per device were used, i.e. 4 tests per device were performed. The drill speed varied between 50 and 420 rpm.

4.3 Evaluation of mechanical test results

The insertion torque of the 4.5 mm self-tapping screw is in the range of 1 to 1.5 Nm (for human cortical bone of regular thickness and quality). This is about twice the value for a tap, but it is in the same range as the insertion torque of a pretapped screw. Inserting a standard screw without tapping (thread-forming) leads to a value of 2.5 to 4 Nm (Fig. 8).

Insertion torque of STS Human cadaveric femora

Torque [Nm]

STS tapped tap untapped J

Fig. 8: Insertion torque for a self-tapping screw, a pretapped screw, tap and an untapped screw in human cadaveric femora (n=lO).

The pull-out force is between 400 and 550 N/mm (per mm cortex) for the self-tapping screw (full thread engaged, cutting flutes not positioned in the cortex) which is in the same range as for the standard screw. The low pull-out force of the untapped screw is due to local micro damage to the bone’s structure caused during screw insertion (Fig. 9).

Pull-out force of STS Human cadaveric femora

Pull-out force [N/mm] So0

T

STS tapped Device

untapped

Fig. 9: Pull-out force of a self-tapping screw, a pretapped screw and an untapped screw in human cadaveric femora (n=lO).

The heat generated during introduction of the self- tapping screw causes an increase in temperature (Fig. 10) at the screw tip. The surrounding bone does not reach this temperature. Starting at body temperature (37 “C) instead of room temperature (22 ‘C) shifts the values linearly by 15 “C.

Baumgart: Self-tapping SCOWS (93) s7

Temperature of STS after insertion Human cadaveric femora

Temperature [“C] 70

5w

50.

40- +

30- 20-

10 ., * ,.’ 0

STS tapped tap untapped

Device

Fig. 10: Heat generation of a self-tapping screw, a pretapped screw, a tap, and an untapped screw measured at the metal tip of the screw (n=lO).

An infrared camera was used to measure the increase in surface temperature of the bone (S=self-tapping screw, T=Tap). The diagramme (Fig. 11) shows that the increase in surface temperature in relation to the initial temperature does not depend on the rate of insertion, provided the screw advances unhindered. Starting at body temperature instead of room temperature does not affect the results.

Heat vs speed, insertion of STS (S) and tap (T) Standardised bone samples, 4 mm, n=4

Two holes each

Maximum increase of temparature [“cl 20 , 1

o! I 0 100 200 SO0 400 500

Spaad of insertion [rpm]

Key f4 S average I S limita

i T average

1 T limits

less damage and the Howmedica, Zimmer and untapped screws somewhat more.

_

The screw advancement into the material is inherent to each threaded device and the displacement per revolution is given by the pitch of the thread. If there is any resistance to the screw, for example, due to bone chips, a pushing effect occurs. When the screw tip exits through the surface of the cortex, a slight delamination effect can sometimes be observed. This effect depends on the strength and the brittleness of the bone, but also on the cutting properties of the tip. If the cutting tip geometry is able to cut and transport the bone chips as quickly as the prescribed displacement requires, no effect occurs. Otherwise irregular breakage of the bone takes place to allow the tip to move forward. This phenomenon can also be observed when using a drill bit with insufficient cutting properties and when exerting a higher axial

Fig. 11: Heat generation of a self-tapping screw and a tap vs speed of machine insertion measured at the bone surface (bovine bone samples, n=4).

4.4 Histological evaluation

The histological evaluation of the damage caused by the different screws and instruments was completed by B. A. Rahn, K. Morikawa and E. Rampoldi in Davos. Rahn and Morikawa classified the damage in detail with reference to 3 zones of cortical bone: entrance, middle and exit (Kuhn, 1991) (Fig. 12). The results show no remarkable differences between the different screws in terms of damage to the bone (average over all three zones). The standard deviation is quite high. Nonetheless, the general tendency is that the A0 and Richards screws cause

force.

Classification of damage *R&n, Kuhn, Morikaws

Humsn wdsvsricfanors

Fig. 12: Evaluation of microdamage in bones by K. Morikawa (unpublished data).

cadaveric

5. Clinical observations 7

The application of self-tapping screws has one main clinical advantage. The number of steps and instruments necessary is reduced, thus decreasing the operation time. Furthermore, a very tight fit of screw thread to bone is ensured as the screw cuts its own thread. Clinical experience with the 4.5 mm cortex screw shows that it performs comparably to the standard screw, with the additional convenience of eliminating the tapping step.

7 Clinical observations described by Prof. Dr. med. R. Ganz, lnselspital, Berne, Switzerland

Injury 1993, Suppletne?lt 1

St?

The use of the small air drill for screw insertion provides improved coaxial alignment and precision.

The drill should be stopped before the screwhead is completely seated on the plate.

It is recommended that the self-tapping screws selected be 2 mm longer than measured. This will ensure that the cutting flutes extend beyond the opposite cortex. This is especially important when the screws are inserted oblique to the axis of the plate.

It is recommended that the direction of the screw be adjusted exactly to the axis of the pilot hole. The accuracy of machine insertion is better than that of manual insertion.

In hard and brittle bone, the screw may be inserted by hand in the same way as a tap.

In the case of reinsertion into a pretapped hole, the screw should be inserted by hand to avoid tapping a new thread.

6. Conclusion

From the results recently obtained, the conclusion can be drawn that the new design of the 4.5 mm self- tapping screw does not cause more damage to the bone than other self-tapping screws or the tap. Insertion torque and pull-out forces are improved for these screws and are similar to pre-tapped screws.

Removal of a self-tapping screw may be difficult if bone ingrowth into the tapping flutes has taken place. In this case, it is recommended that the screw be inserted by turning it one third of a revolution and thus breaking off the bone chips. The screw is then easily removed.

8. Questions and comments on AO/ASIF self- tapping smews

7. Guidelines for the use of self-tapping screws

The only change in technique required for the AO/ASlF self-tapping screw is the omission of tapping.

The recommended technique for insertion of a self- tapping screw is as follows:

8.1 General questions and answers

What is the scientific rationale for the application of self- tapping screws ?

The self-tapping screw fits exactly into the self- tapped thread (provided bone chip transport and/or storage is perfect).

The number of steps to achieve fixation is 1. Drill a pilot hole using the 3.2 mm drill bit. reduced

2. The self-tapping screw is normally inserted using the AO/ASIF small air drill. Tests show that the heat generated during insertion is independent of the machine speed (Fig. 11). This screw may also be inserted by hand in the conventional way.

What is the practical rationale for the application of self- tapping screws ?

Less instruments decreases risks, operation time and costs.

It is advantageous to cool the bone and the screw by applying saline solution during the insertion procedure in order to keep the bone temperature as low as possible.

Insert the screw using the AO/ASIF large hexagonal screwdriver shaft 314.15 for the small air drill when using machine insertion.

The three-fluted drill bit is recommended for exact drilling of the pilot hole.

If a self-tapping screw is used as a lag screw, especially in oblique insertion through a plate, the next available longer screw should be ,used (add 2 mm to the measured length).

What is the optimal design of u self-tappin screw ?

The AO/ASIF design provides a minimum of insertion torque, maximum pull-out force and an acceptable increase in bone temperature even during machine insertion.

Which special techniques ure necessary for the successful use of self-tapping screws ?

None. Machine insertion is recommended. However, in the case of a very strong bone, the screw may be handled similar to a tap during insertion.

Baumgart: Self-tapping screws (STS) s9

8.2 Technical questions

Is the guidance by the screw tip sufficient ?

by tests in standardised bone samples (Baumgart, 1992).

This question is directed at the fact that the AO/ASIF tap has a cylindrical part at the tip which centres the tap in the axis of the predrilled hole exactly. The screw tip is conical in shape. All tests have shown that there is no problem if the “hole quality” is sufficient (wall thickness and strength of the bone). The screw centres itself. Experimental modification of the tip of the AO/ASIF STS to resemble that of the tap offered no improvement and the tip did not catch anymore. The optimised design of the self- tapping screw tip reacts very sensitively to design changes.

Torque comparison 4.5 mm self-tapping acrew

Bovine bone samples, 5 mm

rl 1

IS it difficult to find the hole in the second cortex ?

0 ‘ t-

Insertion torque Thread strip A Thread strip B

I plate Key

1

Torque

El cortex Owerage I SD.

All tests showed that it was not difficult to find the second hole. In cases involving a thin cortex and osteoporotic bone, there was negligible difficulty in finding the hole.

Fig. 13: Stripping torque (cutting) and thread

What is the stripping torque when the tip is located in the cortex and the screwhead is prevented from moving forwards by the position of the plate ?

stripping torque (pull-out) compared to insertion torque, bone samples, 5 mm thick, for 4.5 mm STS (Baumgart, 1992). Similar results are available from other tests on cadaveric bones (Cordey, 1986).

Is it easy to find the second hole even during oblique insertion ?

and

What is the thread stripping torque during pull-out when the tip is not located in the cortex 1

Tests on 4.5 mm STS in 5 mm bovine bone specimens produced the following results (Fig. 13).

Preliminary model tests in plexiglass tubes with 30” insertion were carried out in Davos. Comparing the tap with the screw shows that the degree of difficulty in finding the second hole is equal for both. The problems, if there are any, have existed for many years with the tap and have not been cause for complaint.

insertion torque: stripping torque: thread stripping torque (pull-out):

1 Nm 2.3 Nm 4.5 Nm

What is the recommended speed of machine insertion Crprn)?

Is there an additional risk of twisting off a screwhead 7 There is no speed limitation using the AO/ASlF standard machines.

This is not the case. The screw is loaded when it is tightened. At this time, the tip has passed the far cortex and the screw acts as a standard screw. Therefore, the question is not relevant because if there was a problem it would have become apparent with the non-self-tapping screw already. Furthermore, the insertion torque of the AO/ASlF self-tapping screw is as low as the insertion torque of the AO/ASlF tap. It is only about 20% of the stripping torque (pull-out torque) of the thread in the bone. This means, the regular forces during tightening against the bone thread (failure at 4.5 to 5 Nm) can be much higher than the insertion torque of the self- tapping screw (1 Nm) or the stripping torque of the tip (2.3 Nm). These values were determined

What temperatures may be produced by machine insertion of self-tapping screws ?

The tests showed (Fig. 11) that the temperature on the surface of the bone does not depend on the drill speed. The increase in bone temperature when inserting a 4.5 mm self- tapping AO/ASlF screw should not exceed 15°C. A clinical pilot study includes machine insertion to assess this issue clinically.

What is the pull-out force of a 4.5 mm cortex screw (in cortical bone) (strong bone, 5 mm thick) ?

About 2000 to 3000 N (Breuing et al., 1986).

Injury 2993, Supplement 1

SlO

What is the pull-out force of a 4.5 mm self-tapping screw in cortical bone ?

The same as for a non-self-tapping screw in a similar application. If the cutting tip does not protrude through the cortex, the pull-out force is reduced by more than 10%.

why nut use a self-tapping shaft screw ?

The axial force of a shaft screw is only transferred to the bone via one cortex. If the cutting flutes of the tip do not protrude through the cortex, the maximal value of axial force transferable via the screw is reduced. The reduction is more than 10%.

What is the length recommendation for a STS to be used as a lag screw ?

The surgeon has to decide what length is necessary regarding bone quality, type of fracture, type of internal fixation and functional load. In general, the self-tapping tip should pierce the second cortex completely. The surgeon has also to decide whether there is any risk of damage to a vessel or a nerve or of irritation after insertion. If a self-tapping screw is used as a lag screw, especially during oblique insertion through a plate, the next available longer screw should be used (add 2 mm to the measured length).

Does a STS damage the soft tissue during insertion ?

During insertion the use of a drill sleeve prevents damage to the soft tissue as usual. If there is a problem, then this was also the case for the tap and the clinician was obviously able to deal with it.

Does the self-tapping screw damage the bone ?

The self-tapping screw does not damage the bone more than the pre-tapping procedure does. The results of S. Snyder, B. Rahn and K. Morikawa (unpublished data) show no statistically significant difference compared with pretapped screws and the AO/ASIF self-tapping screws. In brittle bones, a delamination effect at the exit of the screw tip may occur.

Was a delamination effect observed in the clinic ?

The delamination effect is not a specific phenomenon of self-tapping screws. The effect can be produced by drilling and tapping with a sufficiently high axial force and also dramatically by “insertion” of a standard screw

without tapping. Appropriate application of axial force is always the responsibility of the experienced surgeon. However, if at all relevant, the effect is difficult to detect on X-rays. The tests on cadaveric bone show no statistically significant difference compared with standard screws in pretapped holes. Whether clinical observation will produce different results is being investigated in an ongoing pilot study. From in vitro tests, it is obvious that the periosteum plays a key role in preventing or allowing this effect.

Does the delamination effect decrease the pull-out force ?

This effect only exists in cortices of a certain thickness. In thin cortices, it cannot occur because the screw needs a certain thread length in the bone to cause the pushing effect and consequently delamination. The pull-out force in thin cortices is not affected. In thick cortices, the pull-out force may theoretically be decreased, but not dramatically.

The screw is guided better by machine insertion (laboratory test equipment) than by the surgeon. What are the results of tests with machine insertion by hand ?

Machine insertion in a clinical pilot study has shown positive initial results with the self- tapping screw.

What are the general characteristics $ a self-tapping tip ?

The depth of cutting flute, the rake angle, the number of cutting flutes, the volume of the cutting flutes, the length of the cutting flute. The sharpness of the cutting edges is important and may be influenced by manufacturing processes (electropolishing, etc.). Quality control takes care of the specified quality of the approved prototype design.

What are the characteristic properties of the tip of the AOIASIF self-tapping screw ?

Positive changing rake angle, three cutting flutes, sufficient volume of the cutting flute. The sharpness is achieved by a very specific electro- polishing procedure. Good “tip catch’ has been confirmed by various tests.

Is there a real difierence between tap and STS regarding insertion torque ?

The insertion torque for both tap and screw is about the same.

Baumgart: Self-tapping screws (STS) Sll

Repeated insertion of a self-tapping screw may destroy the thread and lead to loss of holding power. is this a problem ?

Schatzker et al. (1975) demonstrated in in vivo tests that 12 reinsertions of a self-tapping screw into the same threaded hole do not cause a problem. It is easy to find the thread again. A reduction of pull-out force was not observed. However, in the case of a self-tapping screw, machine reinsertion should be avoided.

A “mixing” of self-tapping and conventional screws in the operating theatre calz lead to the risk of instin of a self- tapping screw into a pre-tapped hole. Is this a problem ?

This should be avoided by organisation, teaching and even labelling the screws. The danger of cutting a new thread is minimal as shown by Schatzker et al. (1975).

Is the tap still necessay if there will only be self-tapping screws in the fiture ?

As long as shaft screws are used as lag screws and a self-tapping shaft screw has not been recommended and approved by the AO/ASIF, a tap will be needed to insert the conventional shaft screw. As soon as extensive clinical experience with the self-tapping screws is available, other solutions may be discussed.

9. Comments on the references

Sherman (1912) investigated self-tapping screws and came to the conclusion that machine type screws have advantages over wood type screws.

Danis (1949) used pretapped screws with a buttress thread in bone surgery. He concluded that the shape of the buttress thread is compatible with the strength of the bone and with the strength of the metal. The AO/ASIF later modified this thread to the existing AO/ASIF thread.

Bechtol (1959) tested several commercial and experimental screws of the same overall dimensions and with different threads. He reported that reduction in holding power of the fluted portion was 20-30%. The use of a separate tapping device offers little advantage in surgery.

Wagner (1963) compared self-tapping and pretapped screws and did a histological comparison after nine months. He reported that the pretapped screws were surrounded by hard reconstructed bone and that the self-tapping screws led to microfractures and

necrosis and tended to loosen after 23 days. These results must be seen today in the light of the early state of the art of self-tapping screws in 1963 and have to be rediscussed in detail.

Koranyi et al. (1970) tested the holding power of orthopaedic screws in vitro on cattle and dog femora. He used Sherman V-threaded self-tapping screws, pretapped screws and AO/ASIF screws. The conclusion was that there was no difference in holding forces between Sherman and AO/ASIF screws, but significant differences (17-24s less holding force) between self-tapping and pretapped screws in monocortical application.

Eichler and Berg (1972) estimated the temperature in the cortex during drilling, tapping and insertion of screws. They used AO/ASIF 4.5 mm cortex screws, 4.5 mm and 3.2 mm drill bits, 4.5 mm taps, K-wires, Steinmann pins and Schanz screws in in vitro tests on fresh human cadaveric bones and calf bones. The conclusion was that insertion of drill bits, screws, pins, and wires by machine can increase the temperature in the surrounding tissue dramatically. Predrilling and pretapping reduce the temperature considerably.

Schatzker et al. (1975) investigated the holding power of orthopaedic screws. In vivo tests on the femora of mongrel dogs over 12 weeks with pretapped AO/ASIF 4.5 mm and 3.5 mm cortex screws, Howmedica 4.0 mm non-self-tapping screws and self- tapping 3.6 mm Vitallium screws were performed. Push-out tests were included. The 4.5 mm A0 screws provided the greatest resistance to push-out loading over the test period. Self-tapping and non-self- tapping screws of similar material and shape showed comparable holding forces in vivo in unloaded systems for all test periods. No histological differentiation could be made with regard to the reaction of the surrounding tissues between materials or between self-tapping and non-self-tapping screws. Self-tapping screws inserted into the same hole up to 12 times at 80% of their stripping torque showed no difference in push-out resistance.

Schatzker et al. (1975) did a study on the effect of movement on the holding power of screws in bone. They used a special in vivo model on the radius and ulna of mongrel dogs to achieve relative movement of the screws in the bone. They detected that movement between screw threads and bone inhibits bone formation. Movement causes the screw to become enveloped by fibrous tissue. Self-tapping screws were not investigated. The results are in agreement with Perren’s report (1975) on the influence of instability on bone resorption and growth of fibrous tissue around a screw.

lnju y 1993, Supplement 1

s 12

Vangsness, Carter and Frankel (1981) evaluated the loosening characteristics of self-tapping and non-self- tapping cortical bone screws in vitro. They used 4.5 mm AO/ASIF screws, 4.5 mm self-tapping Zimmer screws, 4.0 mm Wright Dow-Coming screws, all made from stainless steel, to test the pull- out strength and the transverse shear load in mongrel dog femora (14 pairs). Microfractures were detected, but no significant differences in pull-out forces. The time scale is not mentioned in the paper. Extensive microcracking after cyclic loading of the screwhead was reported. Cyclic shear loading with 110 N for 200 cycles produced measurable loosening. The pull-out strength was then significantly reduced.

Cordey (1983) evaluated the literature on self-tapping cortical screws and compared the data with those for standard AO/ASIF screws. He concluded that laboratory experiments, which are performed under unrealistic conditions (fluted part of the self-tapping screw protruding through the far cortex) disguise the disadvantages of self-tapping screws. This should be addressed clearly in the case of shaft screws.

Breuing, Gotzen, Haas and Hammer (1986) tested AO/ASIF 4.5 mm and 3.5 mm cortex screws in fresh human cadaveric bones from the radius, ulna and tibia. For the 3.5 mm cortex screws, pull-out forces of 394 N/mm in the radius and ulna and of 554 N/mm in tibia1 bone were determined. For the 4.5 mm cortex screws holding forces of 424 N/mm in the radius and ulna and of 597 N/mm in tibia1 bones were determined. Self-tapping effects were not taken into account.

Xu, Cordey, Rahn, Ziegler and Perren (1986) published a paper on stripping of threads in bone for commercial cortical self-tapping and pretapped screws. In 20 human tibiae, standard AO/ASIF 3.5 mm cortex screws (stainless steel), Zimmer 3.5 mm self-tapping cortex screws (stainless steel) and Howmedica 3.6 mm cortex screws (Vitallium) were inserted using the appropriate drill bits and taps. Although the pretapping procedure took up a few minutes of operating time, the tapped hole for screw insertion was found to be far better and more accurate. The tap should be cleaned before each use. After tapping, the screw can easily be inserted and a higher holding power can be expected.

Xu, Ziegler, Cordey, Perren and Rahn (1986) tested the stripping force in bone of commercially available self-tapping and non-self-tapping cortical screws in 20 human cadaveric tibiae using A0 3.5 mm cortex screws, Zimmer 3.5 mm self-tapping screws and Howmedica 3.6 mm self-tapping screws. The investigators concluded that in clinical practice tapping is more accurate. After tapping, easier insertion and higher holding force can be expected.

Fuchsberger (1987) tried to optimise drill bits for medical use. Different design parameters of drill bits were tested at varying speeds. Torque, temperature and protrusion were recorded. Fuchsberger recommends a new geometry of drill bit rith special angles (twist angle lo-14 , tip angle 70-75 , free angle 18-24’) and a core diameter of 10% of the outer diameter. Screws are not addressed here, but the results may be interesting for designing the cutting flutes of screws too. The question of whether this small core diameter meets the practical requirements of hospital use is not discussed.

Schmelzeisen (1987) investigated mechanical and thermal effects during drilling in cortical bone. Various drill bit shapes, 3.2 mm dia., were tested and the axial force measured for constant axial displacement rate (0.1 mm/round) by time unit. The drill speed and the shape of the drill bit tip were varied. The sharpness of the cutting edges and the bone density have a remarkable influence on the heat generation process. The Jip geometry is important. Smaller tip angles (60-70 ), i.e. sharper tips, lead to smaller axial forces for all tested speeds (500/700/900 rpm) and axial displacement rates.

Lamarque and Cordey (1988) investigated the stripping strength of pretapped and self-tapping 4.5 mm screws of the same thread shape in human tibiae. They found that self-tapping screws showed reduced holding forces if the fluted tip was located in the second cortex. If the tip protruded, then the holding force increased.

Phillips and Rahn (1988) compared the compression and torque of self-tapping and pretapped screws. Standardised bone specimens of 1, 2, 3 and 4 mm thickness were used. Pretapped A0 1.5, 2.0, 2.7 and 3.5 mm rescue screws, self-tapping Luhr, Champy, and A0 1.5 and 2.0 mm screws were measured. They found that for 1 and 2 mm bone thicknesses self- tapping screws resulted in the highest compression values. In 3 and 4 mm thick samples, the pretapped screws resulted in the highest compression values.

Fuchsberger (1989) tested different reaming tools for bone reaming. High speed and appropriate axial forces are recommended to achieve low temperature in bone. For smooth cut surfaces, low axial forces and low speed are recommended.

Xu, Rahn, Cordey, Moor, Tepic and Perren (1990) worked on a photoelastic model and evaluated morphological changes in bone following insertion of self-tapping and pretapped screws using AO/ASIF 3.5 mm cortex screws, Zimmer 3.5 mm self-tapping screws (one cutting flute), Howmedica 3.6 mm self- tapping screws (three cutting flutes). The bone chips fill the flute during insertion because of the narrow

Baumgart: Self-tapping screws CSTSJ s 13

and short cutting flutes of the self-tapping screws. Stress concentrations and cracks in bone could be observed after insertion in all cases, but not in the case of tapped screws. Therefore, the holding strength of the self-tapping screws investigated is significantly lower.

From Perren’s results (pers. comm., 1991), it is known that the second cortex contributes only lo-20% to the axial screw force in the case of a fully threaded screw used as a non-lag screw in a plate (which is in evident agreement with biomechanical considerations). For a shaft screw, all the holding force is located in the far cortex. For a self-tapping shaft screw with the tip located only in one cortex, a drastic decrease of holding force can be expected.

Hess et al. (1991) compared pretapped screws and a special self-tapping screw. They found similar values for insertion torque, stripping torque and pull-out force for both types.

Snyder S. (Internal report, May, 1992) reported basic work on the development of 4.5 mm self-tapping screws. The optimal design of this screw in terms of insertion torque, pull-out force, delamination effect and temperature development was determined by mechanical and histological tests on human cadaveric femora.

Acknowledgement: The authors wish to express their thanks to K. Carouge, R. Christensen, T. Mciff, K. Ito, P. Matter, M.E. Muller, E. Rampoldi, P. Regazzoni, S. Tepic, and the members of the AOTK and LBTK for all the support, comments and criticisms they received during tests and presentations, and to Mrs. Joy Buchanan for editing the manuscript.

10. References

Bechtol C.O. (1959) Internal Fixation with plates and screws. Bechtol, Ferguson and Laing feds) Metal and Engineering in bone and joint surgery, Williams & Wilkins, Baltimore.

Biliouris T. L., Rahn B. A., Gasser B. et al. (1989) The effect of radial preload on the implant-bone interface: a cadaveric study, I. Orthup. Trauma, 323 - 332.

Breuing K.H., Gotzen L., Haas N. et al. (1986) Biomechanical Investigations of holding force of the new 3.5 mm ASIF Cortical Screw. Hefte Unfallheilkunde (Part I), 181,40 - 46.

Cordey J. (1983) Self-tapping cortical screws against standard ASIF screws. Internal report from the lab. for Exp. Surgery, Davos.

Danis R. (1949) Theorie et pratique de l’osteosynthese, Masson, Paris.

Eichler J. and Berg R. (1972) Temperatureinwirkung auf die Kompakta beim Bohren, Gewindeschneiden und Eindrehen von Schrauben. Z. f. Orthop. 110,909 - 913.

Fuchsberger A. (1986) Untersuchung der spanenden Bearbeitung von Knochen. iwb-Furschungsberichte. Vol. 2. Springer Verlag, Berlin-Heidelberg.

Fuchsberger A. (1987) Spiralbohreroptimierung fur den Einsatz in der Medizin. Z. f. Orthop. 125, 290 - 297.

Fuchsberger A. (1989) Die spanende Bearbeitung von Knochen mit Fraswerkzeugen, Unfallchirurgie 15 (2), 59 - 72.

Hess Th., Hopf Th., Fritsch E. et al. (1991) Biomechanische Vergleichsuntersuchungen iiber herkommliche und selbstschneidende Kortikalis- schrauben Z. Orthop. 129,278 - 282.

Koranyi E., Bowman C. E., Knecht C. D. et al. (1970) Holding power of orthopedic screws in bone, Clin. Orthop., 72,283 - 286.

Kuhn A. H. (1991) Knochendeformationen durch selbstschneidende und verdrtingende Kortikalis- schrauben. lnaug. Diss. Basle.

Lamarque M. and Cordey J. (1988) Stripping strength of pretapped and self-tapping 4.5 mm screws in human tibiae, Internal Project Report, Labor fir experimentelle Chirurgie, Davos, Switzerland.

Muller M. E., Allgower M., Schneider R. et al. (1991) Manual of Internal Fixation, 3rd ed., Springer Verlag.

Perren S. M., Cordey J., Baumgart F. et al. (1992) Technical and biomechanical aspects of screws used for bone surgery. ht. j. Orthop. Trauma, 2,31 - 48.

Perren S.M. (1991) pers. comm.

Phillips J. H. and Rahn B. A. (1988) Comparison of compression and torque measurements of self- tapping and pretapped screws. Plast. Reconstr. Surg. 83,3.

Schatzker J., Sanderson R. and Murnaghan J. P. (1975) The holding power of orthopedic screws in vivo, Clin. Orthop., 108,115 - 126.

Injury 1993, Supplement 1

s 14

Schatzker J., Home J. G. and Sumner-Smith G. (1975) The effect of movement on the holding power of screws in bone, Clin. Orthop., 111,257 - 262.

Schmelzeisen H. (1987) Mechanische und thermo- metrische Befunde beim Bohren in der Corticalis. Hefte Unfallheilkunde, 189,33 - 39.

Sherman, W.O. (1912) Vanadium steel bone plates and screws, Surg. Gynec. Obst., 629 - 634.

Snyder S. (1992) 4.5 mm self-tapping screw - Mechanical and histological test study in human cadaveric femora, internal report.

Vangsness C. T., Carter R. D. and Frankel V. H. (1981) In vitro evaluation of the loosening characteristics of self-tapped and non-self-tapped cortical bone screws, Clin. Orthop., 157,279 - 286.

Wagner H. (1963) Die Einbettung der Metall- schrauben im Knochen und die Heilungsvorgange des Knochengewebes unter dem Einfluss der stabilen Osteosynthese. Lungenb. Arch. klin. Chir. 305,28 - 40.

Xu X. X., Cordey J., Rahn B. A. et al. (1986) Stripping of thread in bone by commercial cortical self-tapping and pretapped screws. Fifth meeting of the Europ. Sot. of Biomechanics, September, Berlin, Germany, 531 - 535.

Xu X. X., Ziegler W. J., Cordey J. et al. (1986) Stripping force in bone of commercially available self- tapped and non-self-tapped cortical screws. Fifth Meeting of the Europ. Sot. of Biomechanics, September, Berlin, Germany, 294.

Xu X. X., Rahn B. A., Cordey J. et al. (1990) Photoelastic model and morphological changes of bone following insertion of self-tapping and pretapped screws. 1. Biomech., 23 (4), 389.

Baumgart: Self-tapping screws (STS)

Appendix

Biomechanical considerations

There are some basic considerations involved in the investigation of the principles of self-cutting (or self- tapping) screws.

Direction of local cutting force

The sharp edges of the cutting flute at the tip of the screw cut through the material, e.g. bone. This is the result of their rotation and additional axial force applied by the hand of the user.

The cutting force is related to the plastic deformation of the material and the energy required to cut it. The direction of the cutting force depends mainly on the direction of the cutting edge. Frictional forces have to be added to this force and may influence the resultant force. This resultant force has a circumferential component which contributes to the insertion torque.

I local friction

al cutting force

Fig. Al: Local forces.

The radial component of the resultant force acts perpendicular to the screw axis and is compensated by the lateral material support on the opposite side (this may be important to start the cutting procedure, when material support is still weak). Lateral displacement of the screw tip may occur.

Finally, there is an axial component of the resultant force which exerts an axial force on the screw and will therefore move the screw in a longitudinal direction.

s 15

Forces at the cutting flute, definition of cutting flute angle

Conclusion

1)

2)

3)

The direction of the cutting edge influences the axial force acting directly on the screw. This will ease or obstruct the process of screw insertion.

This conclusion is valid if no external axial load is applied by the screwdriver, but only torque.

Therefore, it should be possible to design a cutting flute which acts as a driving force for insertion. The user just has to turn the screw and not to apply an additional axial force to it.

This may be important for the “bite” of the screw at the beginning of the process (“tip catch”). To equilibrate the radial cutting force, two or more cutting flutes are adequate.

According to the optimal external parameters (possibly no axial force, minimum cutting torque, perfect initial “bite” etc.), an optimal cutting edge direction can be estimated.

When removing the screw, all forces change their signs. In addition, there may be resistance due to bony ingrowth. The opposite edge of the cutting flute now acts as the cutting edge. There is also an optimal direction of this cutting edge, which is the same as that estimated for insertion, but with reference to a left-handed coordinate system.

The recommended procedure is to advance the screw by one third of a revolution in order to cut the bone in the cutting flute. After that the resistance against removal is broken.

Transportation of bone debris by cutting flutes - volume of the chips

In the case of taps, the cutting flutes act as channels for transportation of material chips. The length of the cutting flutes is in general the same as the threaded part of the tap.

The cutting flutes of a self-tapping screw have much less volume than those of a tap. Therefore, it has to be discussed whether the bone chips can be stored in the flute, transported towards the tip or compressed between the screw and the bone. The volume of the material to be transported depends on the thickness of the material, i.e. the depth of the bore hole. Therefore, a formula can be developed for the volume of material chips which are produced by cutting a thread into a hole of a certain depth.

InjuryZ993,Suppkmentl

S 16

Cross section and volume of bone chips

Looking at a longitudinal axial section of the screw where A is the area cut by one tooth of the thread (depending on the thread profile and the border line of the bore hole), rs is the radial distance to the centre of gravity of this area, d is the depth of the bore hole and p is the pitch of the thread [mm], the volume V of the material produced by complete thread cutting is

V=2nrsAd/p.

(n: = 3.14.J.

Fig. A2: Thread-cutting tooth. where:

In the case of a 4.5 mm cortex screw with a the velocity vector v represents the direction in which corresponding 3.2 mm bore hole, the volume of the cutting edge moves. The angle j3 is the cutting material cut per revolution is approximately 1 mm3 angle between the tangential plane to the cutting flute (for the standardised asymmetric thread of an surface and the direction perpendicular to the AO/ASIF cortex screw). relative speed v.

The pitch of a 4.5 mm screw is 1.75 mm which means that any one point on the outer surface of the screw needs approximately 3 revolutions to pass through 6 mm thick material producing a total volume of 3 mm3.

In this plane, the cutting of a chip is clearly visible. Different materials will have optimal cutting angles. In the case of metallic materials, a wealth of information on material properties is available. For biological materials such as bone, other values will be optimal.

4.6mm Cortex screw 3 turns = thickness of cortex

Fig. A3: Volume of chips.

Conclusion:

1)

2)

The volume of chips (cut by a self-tapping A0 screw) is limited and depends linearly on the thickness of the bone.

The discussion about transportation of chips becomes irrelevant if the volume of the cutting flute is able to accommodate the whole chip volume from the tapping process.

Local cutting process and cutting force

Both the direction of the cutting edge seen in a tangential plane of the bore hole cylinder and the cutting angle seen in a plane perpendicular to the cutting edge play an important role in the cutting process.

Fig. A4: Cutting edge geometry definition

For this reason, calculations using the theory of plasticity may be useful. Practical tests are required to confirm the theoretical values.

Conclusion

There is a need for a basic study which investigates the effect of variation of edge geometry and relative displacement between cutting edge and material in a direction parallel to the cutting edge line (knife effect). The cutting force depends on the strength of the material to be cut. An upper limit for this force

BaumRati: Self-tapping screzus (STSJ s17

can be estimated if it is assumed that the chip material reaches a full plastic stage by pure compression stress od. Then the cuttingforce is

F=odA

per cutting flute. This cutting force F is directed perpendicular to the cutting surface, if friction is neglected. If this surface forms an angle 01 (flute angle) with the axis of the screw, the circumferential component is F cosa. The radial component of the force is not discussed here because of the approximate character of the theory. If k is the number of teeth in the cutting flutes and the force F is applied to a radius rs, the torque Mt for inserting the screw is approximately

Mt = k rs od A COW.

This theory only offers a qualitative statement for the influence of the important parameters. It is not at all dependent upon the cutting angle.

The activated axial force Fx is then

Fx=kodAsina

This formula reflects the change in force orientation if the angle a changes its sign (as discussed above). The formula does not take into consideration the friction between chip and cutting flute. Furthermore, the friction generated between thread and material in the cut channel is neglected. The activated axial force depends approximately linearly on the active length of the thread. The above considerations have not yet been fully evaluated in mechanical tests on screws, but more detailed investigations will be undertaken in the future to obtain this valuable data.

injury 1993, Swpplemnt 1