J 0lomechan;c.s.

1976. Vol. 9. pp. 669-675.

FORCE

Pergamon Press. Printed in Great tinlain

MEASUREMENTS

IN SCREW FIXATION*

DAVID M. NuNAMAKERt University of Pennsylvania, School of Veterinary Medicine, 3800 Spruce Street, Philadelphia, PA 19174, U.S.A. and STEPHAN

M.

PERREN

Swiss Research Institute, Davos, Switzerland Abstract-An entire range of screw sizes and types from one manufacturer (Synthes, Switzerland) along with two experimental sizes were tested in bovine cancellous bone to measure torque of insertion, screwdriver pressure, axial compression generated by the screw, screw breaking strength and efficacy of various head types. The results showed a gradual increase in strength as diameter increased. Maximal axial compression peaked at 5.0 mm. The hexagonal shaped imbus head proved superior to the Phillips

INTRODUCTION Internal fixation by means of interfragmentary compression permits early joint mobilization and partial weight bearing which comprises functional fracture treatment. During this treatment interfragmentary compression prevents relative motion at the fracture site and permits primary bone healing (Perren, 1969). Screws provide rigid fixation through compression at the fracture plane alone as lag screws or in combination with plates. Comparatively little information has been generated about the efficiency of various screw designs and sizes. In a small series of experiments on cortical bone Koranyi (1970) demonstrated that the pullout strength of various screw types and diameters varies only slightly and probably is related to the shear strength of bone. In an attempt to determine the best thread form, Bechtol (1959) used three commercial bone screws and five experimental thread forms in pullout tests. He concluded that there was no need to change the design of presently available screw threads because different thread forms did not markedly alter the pullout strength of screws in cortical bone. Schatzker (1975), using an in vitro testing method, also showed that the type of screw is not too important and that resistance to pushout increases after biological responses occur to unloaded screws in uioo. His studies seem to indicate that difference in screw diameter is of significance. Ansell and Scales (1968) stressed the importance of pretapping screw holes to allow the creation of

* Received 14 June 1976. t Assistant Professor of Orthopedic Surgery. $ A.S.I.F.-The Association for the Study of Internal Fixation, which is the American counterpart of A.O. (Arbeitsgemeinschaft ftir Osteosynthesefragen). This study group controls the manufacture of the implants by Synthes.

maximal holding power with reasonable torque values. The use of a torque limiting screwdriver to prevent plastic deformation or failure of the screw was also advocated. The problem most often encountered during the insertion of a screw at surgery is stripping the bone thread when working in thin cortical or soft cancellous bone. The level of screw compression that is achieved before stripping occurs determines the amount of functional load that is permitted while still preserving the rigid fixation necessary for primary bone healing. In clinical practice the pullout of screws in cortical bone during fracture healing seems to occur only after instability develops causing an abnormally highly loaded implant. It should be remembered that well placed implants are protected by the load bearing capacity of the stably fixed bone fragments. The current study was undertaken to test an entire range of screw types with various diameters from a single manufacturer (Synthes, Switzerland) to determine maximum force values during insertion.

MATERIALS Fresh adult bovine femurs were obtained from a slaughter house and stored at -20°C until used. Standard A.S.1.F.S cortical screws of 2.0, 2.7, 3.5, and 4.5 mm dia. were tested. Experimental cortical type screws of 5.0 and 6.0mm were also included. Special hexagonal (imbus) head type screws were tested in the 2.7 and 3.5 mm sizes along with the standard Phillips head screws. All screws tested were of A.I.S.I. 316L Stainless Steel, with a Vickers Hardness value of 245 kg/mm2 (819 N/mm’) (Pohler, 1975). Standard special purpose screws designed for cancellous type bone were also tested. These included the 4.0mm navicular (small cancellous), 4.5 mm mal-

670

DAVID

M. NUNAMAKERand STEPHANM.

F%RREN

Table 1. Screw specifications

2.0mm 2.7 mm 3.5 mm 4.5 mm 5.0mm 6.0mm 4.0 mm 4.5 mm 6.5 mm

Cortical screw STD A.S.I.F. Cortical screw STD A.S.I.F. Cortical screw STD A.S.I.F. Cortical screw STD A.S.I.F. Cortical screw EXP A.S.I.F. Cortical screw EXP A.S.I.F. Navicular screw STD A.S.I.F. Malleolar screw STD A.S.I.F. Large cancehous screw STD A.S.I.F.

leolar,

and 6.5 mm cancellous (large cancellous) screws. These screws were specially machined to have only 6mm of thread on the distal end of the shaft. All cortical screws had a buttress thread with the upper surface of the thread forming an 87” angle with the shaft and the lower surface of.the thread forming a 55” angle with the shaft. (Fig. Sa). The special purpose cancellous screws also had a buttress type thread with the upper surface of the thread forming an 85” angle with the shaft while the lower surface formed an angle of 65” with the shaft of the screw (Fig. 5b). The malleolar screw had threads like the cortical screw. All screws had an electro polished surface. The specifications of these screws conformed to the Swiss standards for screw design (1974)(Table 1 and Fig. 5). An instrumented strain gauge screwdriver (Rumul, Schallhausen, Switzerland) that looked and felt like a normal A.S.I.F. screwdriver was used to measure torque during screw insertion. The electrical signal from this strain gauge was amplified on a digital bridge amplifier (Huggenberger, Zurich) and recorded on a six-channel pen recorder (Watanabe, Multicorder). Force measurements were made with piezo-

Fig. 1. This diagram shows the cortical screw inserted into the bone through the piezoelectric washer and through the first 10mm of cartilage and bone. The screw threads engage only the last 6 mm of bone.

Outside thread diameter (mm)

Core diameter (mm)

Distance between threads (mm)

2.0 2.7 3.5 4.5 5.0 6.0 4.0 4.5 6.5

1.3 1.9 2.0 3.0 3.5 4.5 1.9 3.0 3.0

0.6 1.0 1.75 1.75 1.75 1.75 1.75 1.75 2.75

electric conditioning Type 568).

by a charge

Pitch angle 8” 10” 14” 11” 9” 8” 12” 11” 11”

amplifier

32 29 3’ 55’ 18’ 20 57

(Kistler

METHOD

The individual screws were inserted in bovine cancellous bone and torque applied until bone stripping occurred or until the screws broke. The experiments were carried out on saline moistened bovine bone at room temperature. All screw holes were placed perpendicular to the cartilage surface and those for the cortical screws were overdrilled to a depth of 10mm to avoid the hard and variable thick subchondral bone. A further 1Omm of cancellous bone was then tapped and the screw inserted to a depth of 16mm for the test (Fig. 1). Half of the 2.7 and 3.5mm screws tested were of the hexagonal head type while the remaining ones had the Phillips head type. The bone was prepared in a similar manner for the special purpose screws except that overdrilling was not performed and only the distal 6 mm of thread on each screw were used (Fig. 2). All screws were inserted according to the method described by

Fig. 2. This diagram shows the cancellous screw inserted into the bone through the piezoelectric washer. Note that no overdrilling has been done and that the screw only has threads on its distal 6 mm.

Force measurements in screw fixation

671

the bone was measuring the screwdriver pressure as well as the axial compression exerted by the screw itself, the screwdriver pressure reading from the upper piezoelectric washer was subtracted from the total force of the lower one, to give just the axial compression of the screw (Fig. 3). All recordings were made simultaneously on the pen recorder. Breaking strength of the screw in torsion was recorded during the testing procedure. Torque testing to failure of the screw for those screw sizes that did not fail when tested in bone, was carried out in a vise with the Rumul screwdriver. RESULTS

Fig. 3. This diagram depicts the measuring method. The strain gauge mounted in the screwdriver handle measures torque. The piezoelectric washer mounted in the screwdriver shaft measures screwdriver pressure. The piezoelectric washer between the screw head and bone measures total pressure applied. Subtraction electronically gives the difference of these measurements and results in just axial compression generated by the screw.

Mueller (1965), the only exception being that in the hrst half of the experiment, using the 6.5 mm cancellous screw, no pretapping of the hole was done. The torque application was relatively constant at three to four seconds per revolution of the screw, and it was performed on all screws by the same individual. The torque measurements were made using the Rumul screwdriver. Compression between the screwhead and the bone surface was measured with the piezoelectric washer. The screwdriver pressure was also measured with a piezoelectric washer. Since the bottom piezoelectric washer between the screw and

The individual screws were inserted in bovine cancellous bone and torque applied until bone stripping occurred or until ‘the screw broke in torsion. Measurements were taken of axial compression, torque and screwdriver pressure during insertion. The type of bone and the thread length of the screws had been selected to permit stripping of the bone in aitro. The data relating axial compression and torque for the cortical screws is shown in Table 2. As expected, there is a clear correlation between increased cortical screw size and greater axial compression in bovine cancellous bone. Good statistical differences (p < 0.001) are shown between the 2.0 and 2.7 mm screws, the 3.5 and 4.5 mm screws, and the 4.5 and 5.0mm screws; no statistical significance is evident between the 2.7 and 3.5 mm screws, as well as the 5.0 and 6.0 mm screws. The data for the special purpose screws relating torque and axial compression is presented in Table 3. A significant difference @ < 0.01) is present in the axial compression of the three screw types. The data for the 6.5 mm cancellous screw shows the average for both the tapped and the untapped holes; the mean value for the pretapped holes in 24 trials is 1286-N, for the 24 untapped holes 1204-N. There is no statistical difference for these two groups. A summary of the data of the tests to determine the failure strength of the screw resulting from torsion

Table 2. Maximal axial compression and maximum torque of cortical screws in bovine cancellous bone 2mm Axial compression in N/m

Torque at maximal axial compression in N/m

2.1 mm

3.5 mm

4.5 mm

5.0 mm

6.0 mm

Number tested Range Mean s.d. se.

50 39.2-549 211 111.8 15.79

50 157-941 395 165.7 23.44

51 49-785 390 163.8 22.56

50 37 38 177-l 147 402-2452 618-2353 1667 660 1223 256.9 523.7 389.3 36.38 86.23 63.06

Number tested Range Mean s.d. s.e.

50 0.0740.637 0.324 0.122 0.017

50 0.2941.13 0.656 0.238 0.032

51 0.3041.50 0.828 0.340 0.048

37 50 0.466-2.23 1 0.686412 1.27 2.15 0.424 0.854 0.140 0.060

38 1.42-5.49 2.92 0.918 0.149

612

DAVID

M. NUNAMAKER and STEPHANM. PERREN

Table 3. Maximal axial compression and maximum torque of special purpose screws in bovine cancellous bone

Axial compression in N/m

Number tested Range Mean s.d. s.e.

Torque at maximal axial compression in N/m Number tested Range Mean s.d. se.

4.0 mm Navicular

4.5 mm Malleolar

6.5 mm Cancellous

50 58.8-735 363 163 23

41 579-1667 1079 262 40.8

48 569-1716 1236 306 44.1

50 0.196-1.37 0.635 0.263 0.037

41 0.883-2.84 1.88 0.51 0.079

48 1.08-3.33 2.38 0.551 0.079

Table 4. Breaking strength of A.S.I.F. screws in torsion Cortical screws

2mm

2.7 mm

3.5 mm

4.5 mm

5.0 mm

6.0 mm

Failure in N/m Special purpose

0.564 4.0

0.785 4.0 mm Navicular 1.27

1.57

4.9 4.5 mm Malleolar 4.41

8.63

10.8 6.5 mm Cancellous 6.08

Failure in N/m

Table 5. Screwdriver pressures applied to Phillips and hexagonal headed screws during insertion

is shown in Table 4. The relationship between core diameter and screw failure in torsion is demonstrated by the 4.5, 5.0 and 6.Omm cortical screws in which the core diameters were 3.0, 3.5 and 4.5 mm respectively: The 6.5 mm cancellous screw has a shaft-thread junction core diameter (2.85 mm) that is smaller than that of the 4.5 mm cortical screw (3.0 mm), but still

Phillips head 2.7 mm 2.3mm

Hexagonal head

18.3 f 0.92 kp 13.5 f 1.1 kp

5.64 + 0.70 kp 3.10 f 0.54 kp

failed at a higher torque*. Table 5. demonstrates the relationship between head type and screwdriver pressure. The pressure required to insert a Phillips head screw is three to four times greater than that needed to insert the same

*The shaft thread junction of the 6.5 mm cancellous screw has now been increased by enlarging the core diameter at the point thread run-out; thereby making the screw stronger than this data suggests.

. . * .

*

.

* . . -.. . . ..*

.

.

I

I 20

I

1

40

60 Axial

I

80

I loo

compression

Fig. 4. Torque vs axial compression in the 4.5mm cortical screw, r = 0.9439.

I

I20

Force measurements in screw fixation

&AL_ r’/ \, II

/3j

35O

\

(0)

Fig. 5(a). Profile for cortical screws.

Fig. 5(b). Profile for special purpose screws (large and small cancellous).

size screw with a hexagonal head. 50% of the trials using the Phillips head failed to generate maximal axial compression of the screw before the screwdriver slipped out. In 25”,/,of these trials maximal axial compression could not be determined because of this interface failure. The problem never occurred with the hexagonal head type screw. The graph in Fig. 4 shows a linear relationship between torque and axial compression for the 4.5 mm screw. Smaller diameter screws had more scatter, and the correlation coefficients improved as screw size increased: z = 0.6394 for the 2.Omm screw and 0.9439 for the 4.5 mm screw. Maximal axial compression and maximal torque always occurred simultaneously. If the application of torque was stopped soon after maximal torque was passed some residual axial compression remained.

DISCUSSION

Maximal axial compression generated ranged from 211-N for the 2 mm cortical screw to 1216-N for the experimental 5.0mm cortical screw. The maximal axial compression for the special purpose screws ranged from 363-N for the 4.0mm navicular screw to 1236-N for the 6.5 mm cancellous screw. Torque values increased with increasing screw sizes. Screwdriver pressures were shown to be related to the type of head used on the screw and screw failures in torsion were demonstrated to be related to the core diameter of the screw.

613

Adult bovine cancellous bone was chosen for this experiment because of its “consistent density”, because many individual screw tests could be made in a single specimen, and because human cancellous bone was so variable that no reliable results could be obtained. Bovine cancellous bone is harder than human cancellous, but softer than human cortical bone. Bone was chosen and its variation accepted rather than some standard material (such as Delron) to approximate closely the clinical situation of bone failure and residual holding power of the screw. The length of the screw that engaged the bone for this experiment was determined by trial and error; it was necessary to distinguish between different screw types and sizes in relation to the axial compression and torque and yet use bone that could be stripped without breaking the tiny 2.0mm screw. The measuring screwdriver used in this experiment had the same size, shape and rigid feel during use as the standard A.S.I.F. screwdriver; therefore, a normal torque force was exerted. Although measurements were recorded during the experiment, the person using the screwdriver did not know the individual results. The 5.0 and 6.0 mm screws were tested to find the optimal screw diameter in bovine cancellous bone. Although the only apparent change in these screws’ specifications is the increasing core size with the same thread depth, it should be noted that as the diameter of the screw increases the thread pitch flattens. The 5.0mm cortical screw seemed to be the optimal size for this model. This screw has also been shown to give optimal pullout value when inserted into equine cortical bone (Pohler, 1975). The 100% increase in mean axial compression generated by this screw would seem to make the 0.5 mm larger diameter screw suitable to certain human and animal application. The Iack of statistical significance between axial compression generated by the 2.7 and 3.5 mm cortical screws and the 5.0 and 6.0mm cortical screws may be attributed either to bone variation or the hardness of the bovine cancellous bone which might conceal slight differences. In a small number of experiments on human distal fibula, which is softer bone, a clear difference between the 2.7 and 3.5 mm screws exists: the larger screws generate more axial compression and therefore hold better. No significant differences could be detected when comparing the 5.0 and 6.0mm cortical screws. The plateau reached by the 5.0 mm screw was not seen to improve by another 0.5 mm of screw diameter. No larger diameter screws were tested. The untapped 4.5 mm malleolar screw appears to hold better than the tapped 4.5 mm cortical screw; however these screws cannot be compared with each other because the method of testing was different. The cortical screw holes were overdrilled but the holes for the cancellous screws were not, since these screws had threads only on their distal 6.0 mm. Failure there-

674

DAVID M. NUNAMAKER and STEPHANM. PERREN

fore may have occurred somewhat differently; the bone above the threads may have supported the malleolar screw. One hundred holes were drilled and tapped for the 4.5 mm screw before overdrilling was used to standardize the method of testing the cortical screw. These early experiments which included subchondral bone showed a 30% increase in axial compression and brought the two screw types’ results much closer together. The second difference was again the variation in bone used. However, a small test series in another bone seemed to substantiate these differences. The third major difference between these two tests is that the cortical screw has its threads pretapped while the maleolar screw formed its own threads when it was inserted. The malleolar screw has a plain point and does not carry any flutes with which to cut a thread. The effect of pretapping on the holding power in various densities of bone is not well understood. There is disagreement regarding the pretapping of holes before the insertion of the 6.5 mm cancellous screw in cancellous bone. If the cancellous bone is so soft that it is broken rather than cut by the tap, untapped holes may be an advantage, but this experiment in relatively hard bone indicates that pretapping may help. As expected, the torsional breaking strength of a screw seems to be mainly associated with its core diameter. The one exception in this study was the 6.5 mm cancellous screw which had a smaller core diameter (2.85 mm)* than the 4.5 mm cortical screw (3.0mm) but failed at a higher torque. This is explained by the fact that the 6.5 mm cancellous screw has a larger thread diameter than the 4.5 mm cortical screw and that this increase in thread diameter adds strength to the screw as it tapers into the 4.5 mm shaft. The 6.5 mm cancellous screw always breaks at the upper thread shaft junction. The strength of this screw has been recently improved considerably by increasing the core diameter at the thread-shaft junction. It should be remembered, however, that this screw was designed to be used in soft bone where the cortical screw would not hold and should not be used because of its larger size in hard cortical or in hard animal cancellous bone for increased strength. The 5.0mm cortical screw seems to be better suited for this purpose since it generates about the same axial compression as the 6.5 mm cancellous screw but has the advantage of being stronger and would be easily removed in cortical bone, which cannot be said for any partially threaded screw since they cannot cut their way back out of healed cortical bone and therefore break. Hexagonal head type screws were tested because of the dissatisfaction with the Phillips head type on screws of the small fragment set. With the hexagonal

head type greater torque can be applied to the screw. This, in turn, allows higher axial compression forces to be generated and it is therefore possible to strip the bone and/or break the screw in torsion. The disadvantage of the Phillips head type is that maximal axial compression cannot always be achieved before the screwdriver slips out of the Phillips head causing burring and continued failure at the screwdriver head interface. It is also very difficult to remove a screw which has been inserted in such a fashion. The pressure exerted between the screw head and screwdriver, necessary to fully tighten such a screw is excessive and may lead to further fragmentation in comminuted fractures. Excessive torque application to a screw can be controlled in part by the diameter of the screwdriver handle, thereby making the hexagonal head type screws both useful and safe. Maximal torque and axial compression occurred at the same time during screw insertion. The linear results between torque and axial compression indicate a good relationship between maximal axial compression and the torque generated to achieve it. This relationship exists whenever the bone material is very uniform or whenever there is a large area of contact between the screw and the bone. These results show the importance of uniform tightening of screws to evenly distribute their loads.

CONCLUSION The results of this in oitro experiment demonstrate a. relationship between screw types and sizes; they do not attempt to duplicate conditions in human or other animal bone. Larger screws hold better and should be used whenever possible, especially in soft bone. Several small screws in relatively hard bone generate the force of a much larger screw with the advantage of spreading this force over a larger area without creating a large hole. Use of the smaller diameter screws with the standard 4.5 mm screw may allow more complete and stable reconstruction in comminuted fractures. The hexagonal head screw is far superior to the Phillips head screw in achieving maximal axial compression and reducing the screwdriver pressure necessary to insert the screw. The core diameter of the screw should be as large as conditions allow since the torque required to break a screw is largely determined by the core diameter. When comparing screws made of the same material in the same physical condition, the ratio of the breaking torques of the screws is equal to the ratio of the cubes of their respective core diameters.

REFERENCES

Ansell, R. H. and Scales, J. T. (1968) A study of some * The specifications of this screw list a 3.0 mm core dia. but the measurement of the core at the thread shaft junction after failure in this experiment revealed 2.85 mm.

factors which affect the strength of screws and their insertion and holding power in bone. J. Biomechanics 1, 279-302.

Force measurements

in screw fixation

Bechtol, C. 0.. Ferguson, A. B. Jr. and Laing, P. G. (1959) Metals and Engineering in Bone and Joint Surgery. Williams & Wilkins, Baltimore. Koranyi, E., Bowman, C. E., Knecht, C. D. and Janssen, M. (1970) Holding power of orthopedic screws in bone. Clin. Orthop. Rel. Rex 72, 283-286. Mueller, M. E., Allgoewer. M. and Willenegger, H. Technique of Internal Fixation of Fractures. Springer, NY. Perren, S. M. (1969) Cortical Bone Healing. Acta Orthop. Stand.; Supplementurn, No. 125.

675

Pohler, 0. (1975) Unpublished data. Institute Straumann Waldenburg, Switzerland. Pohler. 0. and Straumann. F. (1975) Characteristics of the Stainless Steel ASIF/AO implants. A.O. Bulletin. lnstitute Straumann Waldenburg, Switzerland. Schatzker, J.. Sanderson, R. and Murnaghan. J. P. (1975) The holding power of orthopedic screws in ciao. Clin. Orthop. Rel. Rex 108, 115-126.