Injury, Int. J. Care Injured 43 (2012) 205–208

A biomechanical study on variation of compressive force along theAcutrak 2 screw

Hari Kovilazhikathu Sugathan a,*, Max Kilpatrick b, Tom J. Joyce b, John W.K. Harrison a

a Queen Elizabeth Hospital, Gateshead, UKb School of Mechanical and Systems Engineering, Newcastle University, Newcastle upon Tyne, UK

A R T I C L E I N F O

Article history:

Accepted 15 July 2011

Keywords:

Acutrak 2 screw

Compressive force

Scaphoid fracture

A B S T R A C T

Introduction: Acutrak 2 screws are commonly used for scaphoid fracture fixation. To our knowledge, the

variation in compressive force along the screw has not been investigated before. The objectives of our

study were to measure variance in compression along the length of the standard Acutrak 2 screw, to

identify the region of the screw which produces the greatest compression and to discuss the clinical

relevance of this to the placement of the screw for scaphoid fractures.

Materials and methods: A laboratory model was set up to test the compressive force at 2 mm intervals

along the screw, using solid polyurethane foam (Sawbone) blocks of varying width. The Acutrak 2 screws

were introduced in the standard method. Forces were measured using a custom-made load cell washer

introduced between the Sawbone blocks and were plotted as a graph along the whole length of the

screw.

Results: Maximum compression was at the mid-point of the screw. Overall compressive forces were

higher in the proximal half of the screw by 19% when compared with the distal half. Minimum

compression was seen at 4 mm or less from either end of the screw.

Conclusions: There is variation in compression along the length of the standard Acutrak 2 screw and the

maximum compression was obtained at the mid-point of the screw. From this study, we would

recommend when using an Acutrak 2 screw for internal fixation of scaphoid fractures, to attain

maximum compressive force, place the fracture at the mid-point of the Acutrak screw. If this is not

possible, then place the fracture towards the proximal half of the screw.

� 2011 Elsevier Ltd. All rights reserved.

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Injury

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Introduction

The Acutrak 2 (Acumed, Hillsboro, OR, USA) is a headlesscompression screw commonly used for scaphoid fracture fixation.Published data by the manufacturer indicates the Acutrak 2 to havea similar compressive force to the Acutrak screw but an increasedpullout strength. The possible reasons for the increased pulloutstrength are not given. The Acutrak screw has a smooth taper witha variable pitch. It tapers only in the proximal third and has aconstant shaft diameter in the distal two-thirds. There is a constantincrease in pitch along the length of both screw types.

Internal fixation of fractures with compression screws ismechanically advantageous and it is associated with good clinicalresults.1 Even though there are several biomechanical studies1–5

showing superior compressive forces and fixation strength forAcutrak screws when compared with other variable-pitch headlessscrews (Herbert screw, HBS screw and Bold screw), we could find

* Corresponding author. Tel.: +44 7939603304; fax: +44 1914452833.

E-mail address: haks@doctors.org.uk (H.K. Sugathan).

0020–1383/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.injury.2011.07.011

only one comparison study for the Acutrak 2 screw.6 Four of thosestudies were done in cadaver bone3–6 and two were done inSawbone.1,2 Despite this, the clinical union rate of scaphoidfractures appears to be similar for various screw types and notsolely related to the compressive force.7 A particular concernrelated to the Acutrak screw is the large thread diameter and thedifficulty in using it for small bone fragments.

Previous biomechanical studies have measured the compres-sive force at the mid-point of the screw. In clinical use, the fracturemay lie in the proximal or distal half of the screw. For example, aproximal pole fracture would generally be fixed with a screwplaced proximal to distal. With a scaphoid nonunion treated withan interpositional wedge graft, the screw is required to providecompression on either side of the graft and with a large graft, thismay be both in the proximal and distal halves of the screw.

To our knowledge, the variation in compressive force along theAcutrak 2 screw has not been investigated before. The objectives ofour study were to measure variance in compression along the lengthof the Standard Acutrak 2 screw, to identify the region of the screwwhich produces the greatest compression and to discuss therelevance of this to the placement of the screw forscaphoid fractures.

Fig. 1. Experiment set up (sizes of block A and B were varied in order to simulate

different locations of the fracture line).

H.K. Sugathan et al. / Injury, Int. J. Care Injured 43 (2012) 205–208206

Materials and methods

Acutrak 2 screw

Titanium, headless, cannulated, fully threaded screw withvariable diameter and variable pitch. The screw has a threaddiameter widest at the proximal end. This tapers in the proximalthird before becoming constant in the distal two-thirds. The shaftdiameter follows a similar change but the increase in diameter inthe proximal third is more marked compared with the threaddiameter. The screw thread pitch increases in proportion to thedistance from proximal end of the screw. The wider thread pitch atthe tip of the screw penetrates the bone faster than the finertrailing threads, compressing the two fragments gradually as thescrew is advanced. Acutrak 2 screws are available in four differentsizes based on the thread diameter (micro, mini, standard and 5.5).The design parameters of all four types of screws are essentially thesame. For this study, we used Standard Acutrak 2 screws.

Sawbone blocks (Stockholm, Sweden)

Solid, rigid, polyurethane foam are widely used as standard testmaterials for mimicking human cancellous bone (Standardspecification for rigid polyurethane foam for use as a standardmaterial for testing orthopaedic devices and instruments: Ameri-can Society for Testing and Materials; 2001). It has been wellestablished that the foam materials produce relatively less intra-and inter-specimen variability when compared with cadaverbone.8 Although it cannot fully represent the mechanical proper-ties of the bone, it does provide a consistent material withmechanical properties within the range of human cancellous bone.For this investigation, the Sawbone used is grade 15 pcf(0.24 g cm�3),8 which matches the mechanical properties of thecancellous bone of the scaphoid.9–12

Load cell sensor

Flexiforce (Tekscan Inc., Boston, MA, USA) is a very thin(0.127 mm) load sensor, which can accurately measure loads up toand over 100 lb (45 kg). The sensor consists of an ultra thin, flexibleprinted circuit board comprising of an outer substrate, two thinstrips of silver, pressure-sensitive ink and a layer of adhesive.

Experiment set-up

Square Sawbone blocks with dimensions of 12 mm � 12 mmwere set in pairs with a combined length of 30 mm (to cover thelength of the 30-mm Acutrak screws). A rig was set up consisting ofa Sawbone block held in a clamp. The other Sawbone block of thepair was of X mm thickness (X representing the distance along thescrew to measure the compression load and increased in 2 mmincrements). A custom-made load cell washer consisting of twoFlexiforce sensors sandwiched between two steel plates (each1 mm thick) was placed between the Sawbone blocks (Fig. 1). Thetotal thickness of the load cell washer measured 2.12 mm and ismuch thinner than any commercially available load cell washer.

A 30-mm standard Acutrak 2 screw was inserted using the samestandard technique described by the manufacturer for fracturefixation. A 1.4-mm guide wire was drilled through the centre ofboth sections of the Sawbone blocks and through the hole in thecentre of the load cell. A second wire was drilled through bothsections of the Sawbone blocks and to the side of the load cell toprevent rotation of the blocks whilst drilling/screwing. TheSawbone blocks were then drilled with the Acutrak cannulateddrill. The standard Acutrak 2 screw was then introduced over theguide wire and screwed in manually until the blocks were secured.

This was the point when the screw head was buried just under thesurface of the Sawbone block. A similar set-up has been usedpreviously.4

The compression produced by the Acutrak screw is transferredthrough the Sawbone blocks directly onto the force plates and thensplit evenly on to the force metres on each side of the screw usingtwo small discs of dimensions identical to the sensing area of theFlexiforce. The construct was modelled using Inventor and thestress analysis software Ansys (ANSYS Inc., Canonsburg, PA, USA).

After the final step, a measurement of the load created by thescrew is made using the Flexiforce metres and the ELF software(Tekscan Inc., South Boston, MA, USA). The process was thenrepeated through 2-mm sections of the Sawbone, giving a range ofvalues for the compression produced by each of the screws alongthe length of the screw. The experiment was repeated on a secondset of identical Sawbone blocks.

Results

Table 1 gives the mean compressive forces registered along thelength of the screw by the two Flexiforce sensors for test 1 and test2 and also the mean compression of both tests. Fig. 2 displays howthe mean interfragmentary compression varies along the length ofthe Acutrak 2 screw. The graph displays a gradual increase incompression from the proximal end (0 mm) to a point around thecentre (15 mm) of the screw. The compressive forces obtainedrange from a maximum of 59.7 N (at the mid-point of the screw) toa minimum of 0.5 N (at the distal end of the screw). Compressiondecreases gradually from the centre to either end of the screw, withminimal compression obtained in the 4 mm from either end of thescrew (we have defined minimal compression as <10 N). To assessthe total compression forces attained by the proximal half and

Table 1Mean compressive forces registered along the length of the screws by the two

flexiforce sensors (test 1, test 2 and mean).

Sawbone blocks Mean compression (N)

obtained by the two flexiforce

sensors

Section A (mm) Section B (mm) Test 1 Test 2 Mean

2 28 1 0.5 0.75

4 26 11 5.1 8.05

6 24 30.2 13.7 21.95

8 22 34.2 23.5 28.85

10 20 55.3 34.6 44.95

12 18 59.1 41.9 50.5

14 16 64.3 55.1 59.7

16 14 64.3 52.5 58.4

18 12 45.2 35.9 40.55

20 10 35.2 31.2 33.2

22 8 30.1 21.5 25.8

24 6 34.1 12.4 23.25

26 4 10.1 4.9 7.5

28 2 1 0 0.5

Fig. 2. Mean compression graph along the Acutrak 2 screw.

H.K. Sugathan et al. / Injury, Int. J. Care Injured 43 (2012) 205–208 207

distal half of the screws separately, we have split the compressiongraph into two halves. The area under Section 1 represents theproximal half and the area under Section 2 represents the distalhalf of the screw, respectively. The area under section 1(372 N mm) was found to be much higher than the area undersection 2 (312 N mm) of the graph. The overall compression forceproduced by the proximal half of the screw is 19% higher than thatof the distal half.

Discussion

Previous studies have investigated the maximum compressionobtained by various headless compression screws.1–6,8 To ourknowledge, the variation in compression along a screw has notbeen investigated before. Our study confirms that there is variationin compression along the length of the Acutrak 2 screw, andmaximum compression was obtained at the mid-point of thescrew. The combined compressive forces produced in the proximalhalf of the screw are 19% higher than those in the distal half. This ispossibly due to the increase in shaft diameter towards theproximal end of the screw. It may also be due to the larger screwthread surface area in the proximal half of the screw. Further workis needed to investigate this and could be done by comparing theseresults to the Acutrak screw, which has an even taper but again avariable pitch.

The minimum compression was seen within 4 mm of eitherend of the screw. This would suggest that the mechanicaladvantage is less for bone fragments of 4 mm or less. There arealready theoretical concerns in using Acutrak screws for smallbone fragments due to the large thread diameter and the risk offracture.

This study suggests increased compressive force by insertingthe Acutrak 2 screw from the bone end nearer to the fracture.Generally, surgeons would place a screw from proximally for aproximal pole fracture and from distally for a distal waist fracture.This study would suggest a mechanical advantage to this andwould support this practice. For waist fractures, there is nomechanical advantage to the direction of placement of the screw aslong as a screw for the full length of the bone is used so that thefracture lies at the mid-point of the screw. Other factors, such asplacement of the screw along the central axis of the bone, areimportant. It was thought that only a proximal entry point allowsplacement of the screw along this axis. However, more recent workhas shown central placement is possible through a distal entrypoint, even without partial resection of the trapezium.13

It is accepted that compression of a fracture allows increasedbony contact and improved stability. This has been associated withincreased union rate and may enable accelerated fracture healing.However, the ‘ideal’ compressive force is not known, as improvedhealing results have not been seen for screws allowing greatercompression.7

Our study has several limitations. We have used Sawboneblocks that may not match the biomechanical properties of bone orthe variation in density in the scaphoid bone. This is an in vitro

study and further work is needed to confirm the clinical relevance.We did not measure for any time-dependent change in compres-sion. Beadel et al.,4 however, has shown that there is minimal lossof compression in a 5-min period after insertion for the standardAcutrak screw.

We have only studied one size and length of the Acutrak 2 screwand the biomechanical properties of the other sizes of Acutrak 2screws may differ. Hence, further lab studies comparing differentsizes and different types of Acutrak 2 screws could be of interest.We also suggest a future clinical study to compare the rate ofhealing of scaphoid fractures treated with Acutrak 2 screw, basedon the level of the fracture. In our study, we have looked at thecompression at one interface. For scaphoid nonunion surgerywhere an interpositional bone graft is used, this study wouldsuggest that the compressive force at the graft–bone interfacecrossed by the distal half of the screw would be lower than for theproximal graft–bone interface. This needs further investigation,which would require the use two load cell washers.

Conclusions

Based on our study results, we would like to make the followingrecommendations whilst using an Acutrak 2 screw for fixation ofscaphoid fractures. To attain maximum compressive force, placethe fracture of the scaphoid at the mid-point of the Acutrak 2screw. If the above-mentioned option is not possible, then placethe fracture in the proximal half of the screw. The minimumcompressive force is seen for bone fragments of 4 mm or less.

Conflict of interest statement

The authors, their immediate family, and any researchfoundation with which they are affiliated have not received anyfinancial payments or other benefits from any commercial entityrelated to the subject of this article. No funding was receivedrelated to the subject of this article.

H.K. Sugathan et al. / Injury, Int. J. Care Injured 43 (2012) 205–208208

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