First Metatarsophalangeal JointArthrodesis: Quantitative MechanicalTesting of Six-Hole Dorsal Plate VersusCrossed Screw Fixation in CadavericSpecimens

Dale J. Buranosky, DPM,1 David T. Taylor, DPM,2 Ronald A. Sage, DPM, FACFAS,3 MarkSartori, BS, Avinash Patwardhan, PhD, Maureen Phelan, MS, and Anh T. Lam, DPM

Quantitative strength analysis of first metatarsophalangeal joint arthrodesis was performed using twofixation techniques: a small 6-ho/e plate with an interfragmentary screw or two crossed lag screws.Twelve matched-pair fresh-frozen cadaveric specimens (24 trials) were used for direct comparisonof each of the two fixation techniques. All joint surfaces were prepared with power conical reamersutilizing a standard technique. The fixation construct was stressed to failure on each specimen using acompute r-integrated materials tester. Fixation stiffness defined as force (load) over displacement andpoint of ultimate failure was evaluated. The six-hole plate and interfragmentary screw fixation methodwas a statistically stiffer form of fixation (p > .01) and displayed a greater point of ultimate failure(p » .002 ) under the laboratory conditions. (The Journal of Foot & Ankle Surgery 40(4):208-213, 2001)

Key words: first metatarsophalangeal joint arthrodesis, mechan ical test ing

Clutton first described first metatarsophalangeal (MTP)joint arthrodes is in 1894 and since then various methodsand techn iques for fixation have been described in theliterature ( 1-30). Arthrodesis of the first MTP jo int isindicated for a variety of pathologies, including severehallux valgus, hallu x rigidus, deformit y seco ndary torheum atoid arthritis, previously fai led joint procedure ,traum atic arthritis, and neuromuscular instability.

Cou ghlin developed a technique for first MTP joi ntfusion in 1990, utilizing power cannulated reamers toprep are convex -concave jo int surfaces and reported a100% success rate using a Kirschner wire (K-wire)and a dorsa l mini-fragment com pression plate for fixa­tion (\3) . In 1994 , Co ughlin and Abdo used a low-profile

From the Department of Onhropaedic Surgery, Loyola UniversityMedical Center, Maywood, IL. Presented at the 55th Annual Meetingand Scien tific Seminar of the America n College of Foot and AnkleSurgeons, Feb. 5-8, 1997, Palm Springs, CA. -Address of correspon­dence 10: Ronald A. Sage, DPM. Departm ent of Orthropaedic Surgery,Loyola Univers ity Med ical Cente r, 2160 South First Avenue, Maywood,IL 60153.

I Research completed while second year resident, Loyola UniversityMedical Center.

2 Second year resident , Hines VAMC/Loyola University.:1 Professor , Department of Orthropaedic Surgery , Loyola University,

Chicago, IL; Staff Podiatric Surgeon, Edw ard Hines VAMC. Hines, IL.Received for publication May 2, 2000; accepted in revised form for

publication March 20, 200 1.The Journal of Foot & Ankle Surgery 1067-2516/01/4004-0208$4.00/0Copyri ght © 200 I by the American College of Foot and Ankle Surgeons

208 THE JOURNAL OF FOOT & ANKLE SURGERY

mandibular plate with convex-co ncave joint surface prepa­ration and reported a 98% successful fusion rate (14) .Curtis et al. compare d various fixation techniques withtwo methods of joint surface preparation for first MTPjo int arthrodesis: planar resection and convex-concavepower conical reaming of the joint surfaces (26) . Theydemonstrated that the surfaces prepared with convex­concave reaming in combination with a single lag screwproduced a more stable fixation than planar resectionwith K-wires, lag screws, or a dorsal plate and screws.They concluded that the incre ased stability was due to thegreater surface contact at the fusion site as well as theinherent stability of the ove rlapping convex -co ncave jo intsurfaces (26).

Sage et al. publ ished their series of nine patient s(12 feet) who underwent first MT P joint arthrodesisutilizing Co ughlin's techn ique (18). The authors reporteda 100% fusion rate without cas ting, with an averag efollow-up time of 6.9 months. The jo ints were fused usingthe Small Joint Reamer System" and utilized a dorsal platewith single interfragmentary lag screw or two crossed lagscrews for rigid internal fixation (Fig . I). Imm ediate pos t­opera tive weightbeari ng was allowed in a surgical shoewith a walker or crutches . Patients were subsequentlyadvanced into a soft athletic shoe in 3-4 wee ks. Theyconcl uded the technique was effec tive in achiev ing first

~ Howmedica Inc.. Rutherford . NJ.

FIGURE 1 Radiographic examples ot the two fixation methodstested during this study. These represent actual postoperativepatient radiographs, not study trials. Left, two crossed screws.Right, six-hole dorsal plate with a single interfragmentary screw.

MTP joint fusion, and may serve as a primary procedurefor first MTP joint pathology (18).

Numerous methods of fixation have been reportedsince 1950. Examples include Kwires, Steinmann pins,screws, plates, compression clamps, staples, wires, andsutures (1- 10, 12-30). It was hypothesized for this studythat the dorsal plate and screws would be a statisticallystronger form of fixation than crossed screws. Therefore,the purpose of this study in a reamed model was tocompare the stability between two types of internal fixa­tion: six-hole dorsal plate with a single interfragmentarylag screw and two crossed lag screws, in cadaver speci­mens in a matched-pair design.

Materials and Methods

Twelve matched pairs (24 feet) from fresh-frozencadaver specimens were utilized for the trials. There wereeight male and four female specimens with an averageage of 69.8 years (range 51-95 years). The specimenswere sealed in freezer bags and stored at - 200 e untilthe day of testing. The sealed specimens were thawedin warm water and room air. After the specimens werefully thawed, all soft tissue was dissected from the firstmetatarsal and proximal phalanx. The experiments wereperformed on each matched pair on the same given day.None of the specimens displayed evidence of previousfoot surgery or trauma upon dissection.

FIGURE 2 The Small Joint Reamer System (Howmedica). Fromleft to right: convex phalangeal reamer, concave metatarsal reamer,and metatarsal barrel reamer.

The Small Joint Reamer System (Fig. 2), Luhr Vital­lium AHoy six-hole plates, and Luhr 2.7-mm self-tappingcortical screws were used to prepare and fixate the MTPjoints. The articular cartilage was resected with a cutperpendicular to the respective shaft of the first metatarsaland phalanx with a power saw. A O.062-inch K-wire wasdriven centrally into the medullary canal of the metatarsalapproximately 1.5 cm. A cannulated barrel reamer wasused to create a cylinder of constant dimension fromthe metatarsal head. Excess bone was removed using arongeur. The concave metatarsal cannulated power reamerwas used to produce a convex surface. The guide K-wirewas removed from the metatarsal and driven centrallyinto the proximal phalanx approximately 1.5 ern. Next, theconvex phalangeal reamer was used to fashion a concavesurface from the base of the proximal phalanx. The sametechnique was repeated for each matched-pair specimenby the same surgeon (DJ.B.).

The base of each metatarsal was potted in a metalholding device with polymethylmethacrylate (PMMA)bone cement. A goniometer was used to align the MTPjoint in 20° dorsiflexion and ISO abduction in relation tothe long axes of the shafts of the first metatarsal andproximal phalanx. This position was manually compressedwhile temporary fixation was applied. Temporary fixationwas either a O.062-inch K-wire or a Luhr 2.0-mm drill bitplaced from distal medial to proximal lateral across theMTP joint. The same investigator (DJ.B.) positioned eachspecimen; the alignment was reconfirmed after temporaryfixation was applied.

In one specimen of the matched pair, a six-hole platewas applied to the dorsal surface of the MTP joint. TheLuhr plate was prebent to 20° to contour the dorsal surfaceof the MTP joint. Six Luhr 2.7-mm self-tapping corticalscrews were applied, three on each side of the joint. The

VOLUME 40, NUMBER 4, JULY/AUGUST 2001 209

central holes were ecce ntrically drilled . The temporaryfixation applied across the MTP joint was removed andrepl aced with an appropriate length 2.7-mm cortical screwin lag technique (Fig . 3).

In the accompanying matched-pair specimen, a Luhr2.0-mm drill was passed from distal lateral to proximalmedi al after temporary fixation was applied. After over­drill ing the proximal cortex with a 2.7-mm drill , a Luhr2.7-mm cort ical screw was inserted in lag fashion . Thetemp orary fixation was removed and replaced with aseco nd appropriate length screw in a lag fashion (Fig. 4).

A computer-integrated Instronf" materials testingmachine (model 1122) was used to apply a force necessaryto produce the ultimate load failure. A linear variabledifferential transducer (LVDT)6 was used to measuredispl acement across the MTP jo int while the load was

FIGURE 3 An example specimen used in the study. First metatar­sophalangeal arthrodesis fixated with a six-hol e dorsal plate and asingle interfragmentary screw potted in PMMA bone cement.

FIGURE 4 An example specimen used in the study. First metatar­sophalangeal arthrod esis fixated with two crossed screws potted inPMMA bone cement.

5 1nstron, Inc., Canton , MA, model 1122.6 Sensotec, Columbus, OH, mode l S5, Ultra Precision.

210 THE JOURNAL OF FOOT&ANKLESURGERY

FIGURE 5 An sample specimen secured in the vice on the Instronmaterials testing machine. Note the LVOT is mounted on the plantaraspect of the proximal phalanx and the receiving plate is mountedon the first metatarsal. Also note the wire cable around the proximalphalanx used to apply a dorsally directed load.

applied. In addition, the LVDT was used to dete rminethe stiffness of the fixation. The resolution of the LVDTis approximately 1 uu: with a range of +/ - 5 mm. AnIBM-compatible 486 computer via an analog to digitalboard? translated force versus displacement data.

The PMMA-potted matched-pair specimens weresecured with a vice on the Instron machine. The LVDTwas mounted to the proximal phalanx utilizing a holdingdevice and the recei ving plate was fixated to the PMM A­potted first metatarsal (Fig. 5). Thus, the first metatarsalserved as a stable reference point relative to the proximalphalanx that held the LVDT transducer. The loading forcewas applied to the proximal phalanx and the displacementacro ss the first MTP joi nt was measured. The LVDTs wereapplied in the same fashion to eac h specimen by a singleinvestigator (DJ.B.).

Affixed to the Instron was a testing jig that consistedof a looped wire cable at the free end. The loop encircledthe proximal phalanx (Fig. 5) and a 2.7-mm screw wasinsetted on the plantar aspec t of the phalanx, 12 mm distalto the MTP joint. This served to prevent the cable fromsliding distally durin g load testing. The Instron machineapplied a constant vertical force to the plant ar aspect ofthe proximal phalanx at the rate of 2 mm/min. The LVDTmeasured the displacement acro ss the MTP joi nt as aresult of the load placed by the Instron testing machineacross the joint.

All tested specimens demonstrated rigid bone contactand stability upon gentle manual manipulation. Force anddisplacement data was collec ted by the computer andtranslated to the analog to digital board. The data werecollected at 100 cycles per seco nd (Hertz). Each spec­imen was loaded until the point of ultimate failure. The

7 Data Translation Inc., Marl boro , MA, model 282 1G.

TABLE 2 Stiffness (Newtons/mm) 1.0 to 2.0 mm displacement

TABLE 1 Stiffness (Newtons/mm) 0 to 1.0 mm displacement

Mean Range S.D.

Plate/screw 37 N/mm 23-59 10Crossed screws 31 N/mm 5-56 16t-Test p < .09

5864

5633

S.D.

S.D.Range

43-2336-113

Range

71-27522-246

Mean

180 N130 N

P < .002'

Mean

121 N/mm72 N/mmp < .01'

TABLE 3 Ultimate load to failure (Newtons)

Plate/screwCrossed screwsr-Test

Plate/screwCrossed screwst-Test

FIGURE 6 Ultimate load graph from specimen D31667. Thesix-hole plate with a single interfragmentary screw had an ultimateload failure of 141 N. The two crossed screws had an ultimate loadfailure of 103 N.

'Statistically significant.

FIGURE 7 Stiffness (load/displacement) from 0 to 1 mm ofdisplacement graph from specimen D31509. The six-hole platewith a single interfragmentary screw was statistically stiffer during0-1 mm of displacement than the two crossed screws.

ultimate load to failure of each fixation system was iden­tified graphically by determining the point at which thelinear progression of applied force peaked and then felloff (Fig. 6). Internal fixation stiffness, defined as force inNewtons (kg.m/s") over displacement in millimeters, wasevaluated between 0-1 mm and 1-2 mm of displacement(Fig. 7). The internal fixation stiffness was also deter­mined at the point of ultimate failure. Stiffness for eachfixation technique was quantified utilizing the paired z-test,

Two matched pairs were disregarded due to poor bonequality and not included in the experiment. The powerreamers destroyed the bone and the attempts at fixationwere not possible. As a result, the experiment consistedof 12 matched pairs.

Results

Table I summarizes data on displacement between thefirst metatarsal and proximal phalanx from 0 to I mm. Thesix-hole plate with a single interfragmentary screw wasstatistically stiffer from 0 to I mm of displacement (p <.01) than the two crossed screws (mean, 121 N/mm). Themean stiffness from 0 to 1 mm for the crossed screws was72 N/mm.

Table 2 summarizes data on displacement between thefirst metatarsal and proximal phalanx from 1-2 mm. Therewas no significant difference in stiffness between the twodifferent fixation techniques from 1-2 mm of displacement(p < .09) (Table 2). The mean stiffness between I and2 mm of displacement for the dorsal plate and screwsversus the crossed screws was 37 N/mm and 31 N/mm,respectively.

Ultimate load to failure was also compared utilizingthe paired r-test and summarized in Table 3. The six-holeplate and interfragmentary screw was a statistically stiffer(p < .002) than the two crossed screws at the point ofultimate failure (mean, 180 N/mm). The mean ultimateload to failure for the crossed screws was 130 N/mm. Thespecimens were observed during loading and examined

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after ultimate failure. Failure was noted to involve thehardw are as well as the cadaveric bone specimens. Screwsbent in one specimen represent ing each type of fixation.Specimens fixated with a dorsal plate showed increaseddeformation of the plate at the point of failure . No failurepatterns were observed involving the distal screw in theproximal phalanx used for application of the wire looptesting jig.

A variety of failure pattern s involved the bone of thefirst metatarsal and proximal phalan x. The screw headsoccas ionally broke the phalangeal cortex. Screw thread swere also found to pull through the cancellous bone. Themost common finding was channeling of the cancellousbone by both the interfragmentary screw and the crossedscrews.

The channeling was more apparent with the two crossedlag screws. As the dorsal load to the phalanx increased,deeper channels were evident. The phalanx rolled dorsall ywith the crossed screw techn ique durin g the initial loading.The cortex would fractur e and/or the phalanx would pulloff of the screws, which led to gapping across the MTPjoint. Although channeling and occas ional breaks in thecortex occurred with the dorsal plate and single lag screwtechnique, the amount of gapping across the jo int wasless than the two crossed lag screws . It appeared thatthe dorsal plate helped maint ain the bone contact despitechanneling by the inter fragmentary screw. At failu re,the phalanx would roll dorsally on the conical surfaceof the metatarsal and the plate would bend. Complica­tions occurred in two specimens utilizing two crossedlag screws . Upon insertion, the screws came into contactwith the other requiring redrilling and possible weakeningof the surr ounding bone and fixation. This complicationdid not occur with the dorsal plate and single lag screwtechnique.

Discussion

Thi s study was designed to compare quantitatively thestability of two methods of internal fixation used in firstMTP joint arthrodesis achieved with convex-concave jo intsurface preparation. A six-hole dorsal plate with a singleinterfragmentary lag screw and two crossed lag screwswere utilized in matched pair cadavers in our laboratory.Numerous other methods of fixation have been evaluatedelsewhere and successful results have been reported witha variety of techniques (1-30).

Both fusion techniques eva luated in our laborat orymaintained rigid bone contact and stability upon manualmanipulation. The two crosse d lag screw technique wasquicker and easier to apply and used fewer materials.Th is may translate to shorter operating room time, lowerincidence of technical error, and less cost.

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The dorsal plate with a single interfragmentary screwwas statistically stiffer in the first 0-1 mm of displace­ment under stress. This sugges ts that this more elaborateform of fixation may be appropriate in clinical situa­tions where greater weightbearing demands are anticipat edafter surgery, such as in heavier patient s, or those whoseactivity may be difficult to restrict. Either technique shouldprovide satisfactory initial stability for healing when usedwith Coughlin'S conical arthrodesis technique. The extentof postoperative protection from full propulsive weight­bearing needs to be eva luated for each patient, but basedon other clinical studies, cas ting is probably not essen­tial (12- 14, 18). There is no obvious explanation for thestatistical difference in stiffness at 0-1 mm of displace­ment and not at 1-2 mm. Both constructs are weak enedafter the MTP joint is displaced by 1 mm; thus the initialconstruct strength is diminished. The two fixation methodsmay tend to lose their strength in unique patterns. Further­more, the study is limited by a small sample, and a statis­tically significant difference at 1-2 mm of displacementmay be apparent with a larger sample size.

Data for this study were obtained by applying a one­time load until the ultimate point of failure was achievedfor each form of fixation. Therefore, this study doesnot addre ss the stability of the two con structs underrepetitive load conditions. Assessment of repetiti ve stressstrength of the fixation methods used in this study requiresfurther investigation and represents a limitation to thisproject.

Variation in orientation and size of the crossed screwshas been described in the literature for first MTP joi ntarthrodesis (30). Screws with greater head diameter mayreduce channeling and larger screws may impro ve fixationstrength. In addition, changes in screw orientation mayprovide a more stable construct. These variables warrantfurther investigation to compare different constructs ofcrossed screw fixation for first MTP joint arthrodesis.

Conclusion

The dorsal six-hole dorsal plate with single screwfixation, and crossed screw fixation for first metat arsopha­langeal joint arthrodesis may be utilized with convex­concave preparation of the joint surfaces to achieveinitial stability of the fusion site. The plate and screwconstruct provides stiffer fixation under stress withinthe first 0-1 mm of motion in cadavers. This tech­nique also demon strated a greater ultimate point offailure, suggesting greater overall strength compared to thecrossed screw technique in cadavers. In clinical situationwhere considerable postoperative stress on the arthrodesisis anticipated, the plate and screw fixation may be moredesirable.

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