A Compression Force Comparison StudyAmong Three Staple Fixation Systems
Naohiro Shibuya, DPM,1 Shane N. Manning, DPM,2 Amanda Meszaros, DPM,3
Adam M. Budny, DPM,4 D. Scot Malay, DPM, FACFAS,5 and Gerard V. Yu, DPM, FACFAS6
Traditional staples have recently been redesigned with both new materials and engineering techniquesto facilitate interfragmentary compression in theory, resulting in greater friction between bone fragmentsto counteract shearing forces. In the current study, the biomechanical properties of 3 different stapleswere investigated. The interfacial force at 2 different sites within a calcaneal bone model was measuredafter insertion and activation of the OSStaple, the UNI-CLIP, and the Smith and Nephew Standard LargeStaple after precompression with the SYNTHES Small Distractor. Additionally, the ability of each stapleto maintain compression over a short period of time was investigated. In the current bench study, theOSStaple consistently generated the greatest and most uniform compression across the bone modelosteotomy and was also capable of sustaining the compression over the duration of all of the trials. (TheJournal of Foot & Ankle Surgery 46(1):7–15, 2007)
Before the broad adoption and acceptance of the AO/ASIF principles and techniques, cerclage wire, Kirschnerwires, Steinmann pins, and staples were among the mostcommon internal fixation devices. Rigid internal compres-sion fixation techniques have become more commonplace infoot and ankle surgery, resulting in improved surgical out-comes. Although screw fixation has been most commonlyused, optimal placement may require modification of tradi-tional surgical techniques, which may not be desirable;these include alteration of osteotomy design to facilitatescrew fixation, insertion of large and multiple implants, aswell as increased usage of fluoroscopy. Periosteal stripping,
Address correspondence to: Naohiro Shibuya, DPM, 2351 E. 22ndStreet, Cleveland, OH 44115. E-mail: [email protected]
1Third Year Podiatric Surgical Resident, St. Vincent Charity Hospital,Cleveland, OH.
2Private Practice, Cleveland, OH.3Third Year Podiatric Surgical Resident, St. Vincent Charity Hospital,
Cleveland, OH.4Second Year Podiatric Surgical Resident, St. Vincent Charity Hospital,
Cleveland, OH.5Private Practice, Philadelphia, PA; Director of Podiatric Research,
Penn-Presbyterian Medical Center; Diplomate, American Board of Podi-atric Surgery; Fellow, American College of Foot and Ankle Surgeons;Fellow, Center for Clinical Epidemiology and Biostatistics, School ofMedicine, University of Pennsylvania, Philadelphia, PA; and FacultyMember, The Podiatry Institute, Tucker, GA.
6Director of Podiatric Medical Education, Chief, Section of Podiatry,Division of Orthopedic Surgery, St. Vincent Charity Hospital, Cleveland,OH; Diplomate, American Board of Podiatric Surgery; Fellow, AmericanCollege of Foot and Ankle Surgeons; Faculty Member, The PodiatryInstitute; Director of Program Development, The Podiatry Institute,Tucker, GA.
retraction, and insertion of a relatively large internal fixationdevice across a bone-healing interface can also impair thecirculation to the bone dramatically and affect the bone-healing process (1).
Traditional staples have recently been redesigned withboth new materials and engineering techniques to facilitateinterfragmentary compression in theory, resulting in greaterfriction between bone fragments to counteract shearingforces. The ability of these new staples to impart compres-sion may allow the surgeon to return to more traditional, andperhaps simpler, operative techniques.
An example of a recently designed staple is the UNI-CLIP (Newdeal Inc, USA, Plano, TX), which providesreduction by mechanically altering the shape of the staple.Another example, the OSStaple (BioMedical Enterprises,Inc, San Antonio, TX), is composed of a shape-memoryalloy, which can provide both reduction and compression.In theory, these implant devices not only give the surgeon acapacity to stabilize the fusion or osteotomy, but also impartmechanical compression to aid the bone-healing process.Traditionally, the same effect was thought to be achievableby insertion of a standard bone staple after establishing“precompression” via a temporarily placed external distrac-tion and compression device. Conventional staples, theSmith and Nephew Standard Large Staple (SLS; formallyknown as the Richard staple) (Smith & Nephew Inc., Mem-phis, TN), are used to maintain compression created by anexternal device, such as the SYNTHES Small Distractor(SYNTHES USA, Paoli, PA) (Fig 1). We have presumedthat the standard staple cannot only stabilize the fusion orosteotomy, but also maintain compression via this tech-
nique.
E 46, NUMBER 1, JANUARY/FEBRUARY 2007 7
The UNI-CLIP is actuated by pliers. This staple has 2legs and a diamond-shaped bridge (Fig 2). The surgeon,using the pliers, opens the diamond-shaped metal bridge ofthe UNI-CLIP to draw or close the legs of the staple. TheUNI-CLIP is made of stainless steel and has both elastic andplastic deformation properties; therefore, the staple is antic-ipated to close, providing measurable compression forcedirectly related to the strength of the surgeon actuating thepliers.
In contrast, the OSStaple is made of nitinol (a nickel-titanium alloy) that possesses unique material thermoplasticproperties. It is capable of providing compression across aninterface without need of an external compression device.By heating the implant with bipolar electrical current, themolecular structure is altered, causing the implant to returnto a “preprogrammed” shape (Fig 3). This results in aninward movement of the legs and shortening of the bridge.Collectively, it would presumably provide uniform, residual
FIGURE 1 SYNTHES Small Distractor.
FIGURE 2 UNI-CLIP.
dynamic compression force across the healing interface.
8 THE JOURNAL OF FOOT & ANKLE SURGERY
Because of the thermoplastic nature of the staple, it wouldprovide dynamic compression force while it is in a warmenvironment, for example, at normal human body temper-ature. With physician-controlled reduction and plastic-de-forming energy stored in the implant, OSStaple is thought toimpart residual compression between the bone fragments.
Despite anecdotal evidence of the merits of these newcompression staples, there is no scientific data that supporttheir advantage over traditional noncompression staplesused to maintain precompressed bone segments. To theauthors’ knowledge, there are no published compressionforce measurements for either new compression staples ortraditional staples after precompression. In the currentstudy, the biomechanical properties of 3 different stapleswere investigated. The interfacial force at 2 different siteswithin a calcaneal bone model was measured after insertionand activation of the OSStaple, the UNI-CLIP, and theSmith and Nephew SLS after precompression with theSYNTHES Small Distractor. Additionally, the ability ofeach staple to maintain compression over a short period oftime was investigated.
Materials and Methods
A through-and-through transverse osteotomy was createdin a Sawbones calcaneal bone model (Pacific ResearchLaboratories, Inc, Vashon, WA) posterior to the posteriorfacet of the subtalar joint articular surface within the bodyof the model. This osteotomy was uniformly created in allbone models with a special jig and automated saw (Fig 4).
A single staple was then placed in the lateral cortex so asto evenly bridge the calcaneal bone model across the os-teotomy. The OSStaple OS 2020, the UNI-CLIP 2020, andthe Smith & Nephew Standard Large (14 mm � 25 mm)Staple with teeth were evaluated.
Ten trials per staple system were performed to measure
FIGURE 3 OSStaple.
the compression force across the osteotomy. The initial
compression force after activation of the staple and thecompression force at 10 minutes were recorded at the nearand far cortices of the model. If the trial resulted in gappingof the osteotomy, the gap was measured with feeler gaugesat the end of the trial. One trial of each staple system wascarried out to 12 hours.
All compression forces were measured with 2 TekscanFlexiforce 0- to 100-pound transducers (Tekscan Inc,South Boston, MA). These transducers provide real-timeforce measurement with less than 5 microseconds delayand repeatability of � 2.5% of full scale. The 2 forcetransducers were placed centrally within the osteotomysite (Fig 5). The primary force transducer (transducer A)was placed closest to the bridge of the staple at the lateralcortex of the calcaneal bone model. The secondary trans-
FIGURE 4 This osteotomy was uniformly created in all bone mod-els with a special jig and automated saw.
FIGURE 5 Placement of the 2 transducers within the osteotomysite representing the near and far cortices.
ducer (transducer B) was placed at the far cortex of the
VOLUM
calcaneal model in close proximity to the tips of thestaple legs.
The 2 fragments were then approximated and held inplace with a vice modified to accommodate the calcanealmodels (Fig 6). The transducers were fixed on the vice toensure consistent placement relative to the model osteot-omy throughout the experiment. The vice was also fittedwith 2 compression adjustment screws to provide a uni-form preloading compression force across the simulatedosteotomy. The preloading force across the osteotomywas then applied by the vice until both transducers read1 � 0.5 lb. This minimal preloading assured a standard-ized starting point for all trials of each staple system. Alltrials were performed with the construct placed in aheated chamber at 36° to 38°C to simulate normal bodytemperature.
After standardization of each model, testing was per-formed as described below, for each staple system.
Smith & Nephew SLS
1. Small Distractor was placed 1 cm superior and parallelto the planned staple insertion site with two 3/32-inchSteinmann pins (Fig 7). The device was then used incompression mode to provide compression across theosteotomy by tightening the dial until a maximum forceof 20 � 0.5 lb was measured at the near cortex. Themaximum force was predetermined by taking the aver-age maximum force (where the force measurements nolonger increase during tightening of the distractor de-vice) from 3 pilot trials. In the pilot trials, compressionforce was not detected at the far cortex immediatelyafter tightening was instituted with the distractor device.
2. Drill holes were made with the drill and guide provided
FIGURE 6 A specialized vice was used to hold the calcanealfragments throughout each trial.
with the system.
E 46, NUMBER 1, JANUARY/FEBRUARY 2007 9
3. The staple was impacted into the site until the bridgewas firmly abutting the lateral cortex. The teeth on thebridge portion of the staple were engaged in the bonemodel.
4. The construct was placed in the environment chamber at36° to 38°C for the duration of a trial.
5. The distractor and vice were slowly released.6. Force data were simultaneously recorded while oven
temperature was maintained at 36° to 38°C.
UNI-CLIP
1. Drill holes were made with the drill and guide providedwith the system.
2. The UNI-CLIP was inserted in the holes and placedflush with the cortex.
3. The construct was placed in the environment chamber,and the temperature was maintained at 36° to 38°C.
4. The environmental chamber was opened for a minimalamount of time to activate the implant with the UNI-CLIPpliers until the primary transducer read 16 � 0.5 lb. Thevalue, 16 lb, was chosen because the compression forceexerted by the UNI-CLIP leveled off at this value in all3 pilot trials. For the rest of the trials, this compressionforce was achieved by the same investigator using themodified UNI-CLIP pliers (Fig 8). The attached nut onthe modified pliers allows the investigator to precisely“dial in” the handles of the pliers closer together in asmall increment. It can also maintain the force withoutfluctuation over time to minimize human error. In thepilot trials, compression force was not detected at the farcortex immediately after actuation of the staple usingthe pliers.
5. The pliers and vice were slowly removed.6. Force data were simultaneously recorded while oven
FIGURE 7 Smith & Nephew SLS was inserted after compressionwas achieved via Synthes Small Distractor.
temperature was maintained at 36° to 38°C.
10 THE JOURNAL OF FOOT & ANKLE SURGERY
OSStaple
1. Drill holes were made with the drill and guide providedwith the system.
2. The OSStaple was inserted in the drill holes, and thebridge was placed flush with the cortex.
3. The construct was placed in the environment chamberand maintained at 36° to 38°C.
4. The implant was activated with the OSSforce Controllerin accordance with the manufacturer’s recommendedsettings while the chamber door was open for a minimalamount of time.
5. The vice was slowly released.6. Force data were simultaneously recorded while oven
temperature was maintained at 36° to 38°C.
Statistics
Sample size estimates were based on 90% power to detecta statistically significant difference at the .05 alpha level,using differences between population means (PS version2.1.31, copyright 1997, WD Dupont and WD Plummer;http://www.mc.vanderbilt.edu/prevmed/ps/index.htm). Stu-dent’s t tests, for paired and unpaired continuous data, wereused to detect statistically significant differences within andbetween each group of staples (STATA/SE 8.0; Stata Cor-poration, College Station, TX). All differences were con-sidered significant when P � .05.
Results
A summary of results is presented in Table 1.Both the SLS and UNI-CLIP immediately lost compres-
sion at the far cortex as soon as the external devices (thevice and distractor device for the SLS and the vice and pliersfor the UNI-CLIP) were released (P � .0001) (Table 2). Both
FIGURE 8 Modified UNI-CLIP pliers.
the SLS and UNI-CLIP displayed gapping at the far cortex;
hence, no difference was measured when the 2 staples werecompared relative with this variable (Table 3). The OS-Staple did not display gapping at the far cortex, and this wasstatistically significant (P � .0001) in comparison with boththe SLS and the UNI-CLIP (Tables 4 and 5).
The UNI-CLIP lost compression at the near cortex afterthe staple was actuated and the preload device was removed(P � .0001) (Table 6). The OSStaple displayed a statisti-cally significant increase in initial compression when com-pared with preload at both the near and far cortices (Tables2 and 6). However, the OSStaple displayed decreased com-
TABLE 2 Comparison of far cortex preload versus initialstaple compression
pression, at both the near and far cortices, when initial
12 THE JOURNAL OF FOOT & ANKLE SURGERY
compression was compared with compression at 10 minutes(Tables 7 and 8).
At the near cortex, the SLS provided greater compressionvalues at 0 (P � .0366) and 10 minutes (P � .0366) incomparison with the UNI-CLIP (Table 3). The OSStapleprovided greater initial compression than the SLS at thenear cortex (P � .0001) and at the far cortex (P � .0001).After 10 minutes, the compression force exerted by theOSStaple was still greater at both the near cortex (P � .02)
and far cortex (P � .0001). In comparison with the UNI-
CLIP, the OSStaple provided greater initial compression atthe near cortex (P � .0001) and the far cortex (P � .0001).It also provided greater compression at 10 minutes at thenear cortex (P � .0017) and the far cortex (P � .0001).
Discussion
The 2 most influential factors that govern bone healingare vascular supply and biomechanical conditions, namely,stability (2). Inadequate blood flow may lead to the depo-sition of fibrous connective tissue and fibrocartilage at thefracture or osteotomy ends, resulting in delayed union ornonunion. An unstable or flexible surgical construct is sub-ject to local stresses and strains that strain the healinginterface and, in turn, result in loss of reduction and, ulti-mately, nonunion.
Claes et al demonstrated that movements greater than 1mm across osteotomized metatarsals in sheep resulted in areduced amount of endosteal vessel bridging compared withbones subjected to movements (0.2 mm or less) (3). Theyproposed that increased formation of fibrocartilage was ob-served histologically when larger interfragmentary move-ment was permitted. The results suggested that attainingstability through internal fixation was necessary not only forprompt revascularization of the traumatized bone ends, butalso for proper tissue differentiation. Claes et al also inves-tigated the importance of preserving the periosteal vascula-ture. They compared traditional compression plate fixationwith less invasive bridging external fixators in the repair ofcomminuted fractures and concluded that bone healing wasnegatively impacted by excessive periosteal compressionand stripping used to achieve traditional methods of openrepair (4).
There are several biomechanical variables that influencenew bone formation in proximity to a fracture line orosteotomy. Previous investigations have shown that gapsize, interfragmentary motion, and strain impact the pro-gression of osteoneogenesis (1, 2, 5–11). It is well docu-mented that an interfragmentary gap of less than 2 mm willresult in adequate bone healing because of the ability of theHaversian system to commence internal remodeling andsubsequent progression of recanalization by the advance-ment of cutting cones (10–13). Achieving absolute stabilitythrough anatomic reduction and rigid fixation serves toreduce or eliminate both gap and strain. If the strain (changein gap length � gap length) exceeds the tolerable load of thenewly formed bone, fibrocartilage deposition and nonunionare likely to result.
Large amounts of motion between bone fragments retardbone growth because of asymmetrical revascularization.Fibrous tissue formation, focal hemorrhage, and minimalcartilage formation were observed in osteotomies with min-
imally restrained motion during healing, whereas primary
VOLUME
bone healing resulted in no visible callus formation becauseof the creation of a stable construct (1, 3, 7, 8).
Conversely, using sheep models, Park et al demonstratedthat micromotion resulted in abundant callus formation,which increased torsional strength and stiffness (14). Inter-fragmentary movements of greater than 1 mm, on the otherhand, inhibit angiogenesis and result in a large amount ofnonvascular fibrocartilaginous tissue in sheep models (3).
Interfragmentary compression, therefore, generates greaterfrictional force between the bone ends that serve to counter-act mechanical stresses, such as shear and torsional forces.Ultimately, absolute stability can be maintained only incircumstances where the frictional force exceeds the de-forming forces acting on the complex; conversely, when thedeforming force exceeds the frictional force, movement atthe bone-healing interface will result and primary bonehealing will not take place. Given that each osteotomy wascreated under uniform conditions with identical bone mod-els in the current study, the coefficient of friction at eachosteotomy site was consistent throughout the trials. There-fore, a direct relationship between the compression force(normal force) and frictional force acting on the osteotomysite may be derived by the law of physics: frictional force �coefficient of friction � normal force.
With the SLS system, compression fixation with theSYNTHES Small Distractor was engaged with greater thannormal clinical force and interfacial pressure after engage-ment at the near cortex was recorded as high as 20 lbwithout disturbance of the bone model. However, this veryhigh initial compression dropped significantly when thedistractor was removed and the vice was slowly releasedafter insertion of the staple. This finding was unexpected;however, we suspect this to be due to elastic recoil createdby the compressed bone model, where the SLS is unable tocounteract this force. This would result in lesser frictionalforce at the osteotomy site after the preload was released,thereby making the construct more susceptible to deforma-tion and failure.
With the UNI-CLIP, the force applied by the pliers re-sulted in gapping of the far cortex, and the compressionforces were not measurable in either cortex. Activation ofthese staples via the pliers, in fact, surprisingly distractedthe fragments rather than compressed them. This is mostlikely due to differential of resistance between the bridgeand legs of the staple. As the bridge of the staple shortens,the legs of the staple encounter greater resistance from thebone model fixated in the vice. As more force is appliedthrough the pliers, the diamond-shaped bridge will con-tinue to deform without further displacement of the legsin the vice. The deformation will result in angulation ofthe bridge-leg junction away from the osteotomy site.When the vice was released, the legs of the staple willnow follow the angulation of the deformed bridge-leg
junction because they are not resisted by the vice. Be-
46, NUMBER 1, JANUARY/FEBRUARY 2007 13
cause of the relatively plastic nature of stainless steel, thedeformation will be more permanent.
The gap was as large as 0.12 in (3 mm) in this series (Fig 9).Our data suggest that overall stability might be compromisedbecause of lack of frictional force in the absence of otherpotentially stabilizing factors (ligaments, tendons, adjacentbone structures). In vivo, secondary bone healing may ensue tocompensate for the gap formation if the overall construct canresist large interfragmentary movements and strain.
In the current bench study, the OSStaple consistentlygenerated the greatest and most uniform compression acrossthe bone model osteotomy (Fig 10). Theoretically, uniformcompression should enhance the frictional force and ac-tively resist mechanical stresses encountered during bonehealing. In this situation, primary bone healing should befavored. The OSStaple was also capable of sustaining thecompression over the duration of all of the trials.
For this comparison study, Sawbones were used becauseof the fact that the investigators can more precisely controland minimize the variables than they would with cadavericspecimens. Biomechanical properties of the Sawbones areconsistent throughout the trials. Consistent bone cuts werepossible with the same jig because the size and shape of themodels were identical.
However, use of Sawbones excludes the external stabi-lizing factors provided by adjacent soft tissue structures.Ligamentotaxis created by the surrounding soft tissue struc-
FIGURE 9 Gap at the far cortex: SLS versus UNI-CLIP versusOSStaple (inch).
FIGURE 10 Compression in pounds at time 0 and 10 minutes: SLSversus UNI-CLIP versus OSStaple.
tures would affect the magnitude of compression force
14 THE JOURNAL OF FOOT & ANKLE SURGERY
applied by each staple system. Even with the through-and-through osteotomy, it is less likely in a human or cadavericspecimen that gapping at the far cortex would occur, as wasdemonstrated in trials of the UNI-CLIP and SLS systems.From this experiment, it is clear that the vector of forces wasdirected outward, away from the osteotomy site in theUNI-CLIP and SLS systems at the far cortex. Again, thisloss of compression force at the far cortex would result inoverall lack of frictional force and, ultimately, less rigidity.
The effect of micromotion on each of the staple con-structs was not explored in this bench study; however, itmay serve as a foundation for future research.
Conclusion
This nonsponsored analysis, using plastic calcaneal bonemodels, showed significantly different geographic patternsand magnitudes of compression force exerted by each staplesystem. The clinical significance of these findings, if any,are yet to be determined by any clinical study; however, theresults of this study should be helpful in appreciating themechanism of action of each of the 3 different types ofstaples and can be used to guide surgeons in their choice ofstaple fixation.
References
1. Perren SM, Claes L. Biology and biomechanics in fracture manage-ment. In AO Principles of Fracture Management, edited by PRThomas, MM William, Clavadelerstrasse, Switzerland, AO Publish-ing, 2001:7–30.
2. Claes LE, Heigele CA, Neidlinger-Wilke C, Kaspar D, Seidl W,Margevicius KJ, Augat P. Effects of mechanical factors on the fracturehealing process. Clin Orthop Oct (suppl 355):S132–147, 1998.
3. Claes L, Eckert-Hubner K, Augat P. The effect of mechanical stabilityon local vascularization and tissue differentiation in callus healing.J Orthop Res Sep 20:1099–1105, 2002.
4. Claes L, Heitemeyer U, Krischak G, Braun H, Hierholzer G. Fixationtechnique influences osteogenesis of comminuted fractures. Clin Or-thop Aug:221–229, 1999.
5. Augat P, Burger J, Schorlemmer S, Henke T, Peraus M, Claes L. Shearmovement at the fracture site delays healing in a diaphyseal fracturemodel. J Orthop Res 21:1011–1017, 2003.
6. Claes L, Augat P, Suger G, Wilke HJ. Influence of size and stability ofthe osteotomy gap on the success of fracture healing. J Orthop Res15:577–584, 1997.
7. Claes LE, Heigele CA. Magnitudes of local stress and strain alongbony surfaces predict the course and type of fracture healing. J Bio-mech 32:255–266, 1999.
8. Ilizarov GA. The tension-stress effect on the genesis and growth oftissues. Part I. The influence of stability of fixation and soft-tissuepreservation. Clin Orthop Jan:249–281, 1989.
9. Ilizarov GA. The tension-stress effect on the genesis and growth oftissues: Part II. The influence of the rate and frequency of distraction.Clin Orthop Feb:263–285, 1989.
10. Mark H, Nilsson A, Nannmark U, Rydevik B. Effects of fracturefixation stability on ossification in healing fractures. Clin Orthop Relat
Res Feb:245–250, 2004.
11. Yamaji T, Ando K, Wolf S, Augat P, Claes L. The effect ofmicromovement on callus formation. J Orthop Sci 6:571–575,2001.
12. Harrison LJ, Cunningham JL, Stromberg L, Goodship AE. Controlledinduction of a pseudarthrosis: a study using a rodent model. J Orthop
Trauma 17:11–21, 2003.
VOLUME
13. Claes L, Eckert-Hubner K, Augat P. The fracture gap size influencesthe local vascularization and tissue differentiation in callus healing.Langenbecks Arch Surg 388:316–322, 2003.
14. Park SH, O’Connor K, McKellop H, Sarmiento A. The influence ofactive shear or compressive motion on fracture-healing. J Bone Joint
