A Comparative Mechanical Analysis Of The Pointe Shoe Toe Box

1y ago
373.42 KB
7 Pages
Last View : 9d ago
Last Download : n/a
Upload by : Oscar Steel

0363-5465/98/2626-0555 02.00/0THE AMERICAN JOURNAL OF SPORTS MEDICINE, Vol. 26, No. 4 1998 American Orthopaedic Society for Sports MedicineA Comparative Mechanical Analysis of thePointe Shoe Toe BoxAn In Vitro StudyBryan W. Cunningham,* MSc, Andrea F. DiStefano,† PT, Natasha A. Kirjanov,‡Stuart E. Levine,* MD, and Lew C. Schon,*§ MDFrom *The Orthopaedic Biomechanics Laboratory, Department of Orthopaedic Surgery, TheUnion Memorial Hospital, Baltimore, †Health South Spine Center, St. Joseph Hospital,Towson, and the ‡Ballet Theatre of Annapolis, Annapolis, Marylandical demands on the body while requiring the productionof aesthetic and graceful movements. From the 1581 introduction of ballet at the French Court (attributed toCatherine de Medici), the popularization of this art formby Louis XIV, and the 1661 creation of the AcademieRoyale de Danse,2 the technique of ballet has become moredemanding, requiring refinement of the dancer’s strength,technique, and tools. This was highlighted by MarieTaglioni, who, in 1832, was the first to dance en pointe.This was originally done with soft satin slippers containing a leather sole. As pointe technique developed, the shoealso evolved to allow the ballerina to perform more exacting maneuvers. In time, the shanks became harder, theboxes became stronger, and the toe platform becamewider.When dancing en pointe, today’s ballerina stands on hertoes with little more than a papier-mâché or cardboardshell to protect her forefoot. The pointe shoe, made up ofthe toe box, shank, and the outer material, is one of themost important tools of the dancer. The outer material isusually a soft, cotton-backed cloth called corset satin. Thepointe material provides a relatively low-friction surfaceto permit spins while allowing for sufficient “grip” duringstanding or jumping. Although it is fairly durable, thiscovering will wear out, usually over the toe platform, andreplacement will be necessary. The conical toe box consistsof layers of burlap, cardboard, or paper, or a combinationthereof, that have been saturated with glue.3 It tightlysurrounds the toes so that the dancer’s weight rests on theplatform. The shank, made up of cardboard, leather, or acombination of the two, also helps to support the foot whileen pointe by providing a certain degree of stiffness. Thebreaking-in process of the toe box softens the cardboardand conforms the shoe to the foot, but destroys the gluebonds. The optimum toe box shape lasts for only a shortABSTRACTDancing en pointe requires the ballerina to stand onher toes, which are protected only by the pointe shoetoe box. This protection diminishes when the toe boxloses its structural integrity. The objectives of this studywere 1) to quantify the comparative structural staticand fatigue properties of the pointe shoe toe box, and2) to evaluate the preferred shoe characteristics asdetermined by a survey of local dancers. Five differentpointe shoes (Capezio, Freed, Gaynor Minden, Leo’s,and Grishko) were evaluated to quantify the static stiffness, static strength, and fatigue properties (cycles tofailure) of the shoes. Under axial loading conditions,the Leo’s shoe demonstrated the highest stiffnesslevel, and the Freed shoe exhibited the least strength.Under vertical loading conditions, the Leo’s and Freedshoes demonstrated the highest stiffness levels, andthe Gaynor Minden and Freed shoes exhibited thehighest strength. Fatigue testing highlighted the greatest differences among the five shoes, with the GaynorMinden demonstrating the highest fatigue life. Dancersrated the top five shoe characteristics, in order of importance, as fit, comfort, box/platform shape, vampshape, and durability and indicated that the “best” shoeis one that “feels right” and permits artistic maneuvers,not necessarily the strongest or most durable shoe.Appreciated for the artistry, grace, and elegance of thedancers, ballet is an art form that makes immense phys-§ Address correspondence and reprint requests to Lew C. Schon, MD, c/oElaine P. Bulson, Editor, Union Memorial Orthopaedics, The Johnston Professional Bldg., No. 400, 3333 N. Calvert Street, Baltimore, MD 21218.No author or related institution has received any financial benefit fromresearch in this study.555

556Cunningham et al.period because of the rigors of the performance. Once thetoe box loses its structural integrity and becomes too softto adequately support and protect the foot, the shoes areusually discarded.4To date, most of the research interest surrounding thepointe shoe is directed toward the prevention of danceinjuries1, 5 and augmentation of pointe shoes with shoeorthoses,7 as well as determining the toe pressures generated while dancing en pointe.9, 10 However, no studieshave quantitatively assessed the comparative intrinsicmechanical properties of different pointe shoes. Therefore,the objective of our current research project was twofold:1) to compare and evaluate the structural static and fatigue properties of five different types of pointe shoes, and2) to evaluate the preferred shoe characteristics as determined by a survey of the local dance population to compare the relationship between mechanical properties andchoice of shoe.American Journal of Sports MedicineCalculation of Toe Box DimensionsBefore mechanical testing, the toe box outer dimensionsand inner volume were calculated for correlation to themechanical properties of peak toe box stiffness (in kilonewtons per meter) and strength (in newtons). Using Vernier calipers, the outer dimensions of toe box depth,height, and width were measured as follows: depth, thedistance from the vamp edge to the tip of the toe box;height, the distance from the sole of the toe box to the topof the vamp; width, the side-to-side distance across the toebox (Fig. 2). For toe box volume calculation, we poured aknown quantity of polymethyl methacrylate beads(200-!m diameter, Polysciences, Inc., Warrington, Pennsylvania) from a 250-mm graduated cylinder into thepointe shoe, leveled the beads at the vamp edge, andcalculated the difference in cylinder volume.Mechanical AnalysisMATERIALS AND METHODSShoe Types and SizesA total of five different brands of pointe shoes were evaluated: the Ariel (Capezio, Totowa, New Jersey), Chacott(Freed, New York, New York), Gaynor Minden (GaynorMinden, Inc., New York, New York), Leo’s (LeoDancewear, Chicago, Illinois), and Fouetté (Grishko Ltd.,Villanova, Pennsylvania). The shoes were obtained fromlocal distributors with the understanding that they wouldbe used for research purposes. The shoe size requested fortesting was standardized and equal to a size 61 2 to 7 streetshoe (Fig. 1). The shoes were delivered in their normalpackaging materials, and any shoes found damaged ordefective on receipt were returned to the manufacturerwith a request for replacement. The shoes were carefullyremoved from the packaging, separated, and randomlylabeled according to the mechanical test to be performed.Figure 1. Five different brands of pointe shoes were evaluated: Capezio, Freed, Gaynor Minden, Leo’s, and Grishko.All shoes were unused before mechanical testing and werenot retested.Mechanical analysis of the pointe shoes was performedusing a servohydraulic MTS 858 Bionix testing device(MTS Systems Inc., Minneapolis, Minnesota). Using anMTS interface cable, load-displacement data acquisitionwas performed through an analog-to-digital DAS16G Metrabyte board (Metrabyte Corp., Taunton, Massachusetts)interfaced with an IBM 486 PS/2 (IBM Inc., Armonk, NewYork). All data files were downloaded into Lotus 1 ! 2 ! 3(Lotus Development Corp., Cambridge, Massachusetts)for spreadsheet computational data analysis. Testing ofthe pointe shoes was performed under both static anddynamic testing conditions. All shoes were unused beforetesting and were not retested.Static Testing ConditionsStatic testing quantified the ultimate compressivestrength and stiffness of the pointe shoe toe box withrespect to the applied load under conditions of vertical andFigure 2. Before mechanical testing, the toe box outer dimensions and inner volume were calculated for relationshipto the mechanical properties of peak toe box stiffness andstrength.

Vol. 26, No. 4, 1998axial shoe alignment. Static analysis for both axial andvertical tests was performed under displacement controlat a constant rate of 0.5 mm/sec. Using a 2-inch-diametercylindrical steel ram with a flattened bottom, the load wasapplied to the toe box until failure occurred. Peak failurewas defined as a significant and consistent decrease in theregistered load or collapse of the toe box by 7 mm (anarbitrarily chosen point), whichever came first. For axialloading conditions (N ! 5), the toe box was positionedusing C-clamps so that the applied load originated at thetip of the toe box and was directed along the shank axis, asoccurs when the ballerina is en pointe. The cylindrical ramcompletely covered the tip of the toe box. For verticalloading (N ! 5), the shoe was mounted on the MTS loadcell horizontally so that the applied load was delivered tothe distal plantar surface of the toe box, as occurs whenthe ballerina is in the demi-pointe position (Fig. 3).Cyclical Fatigue Testing ConditionsDynamic testing of the pointe shoes highlights the fatigueproperties (cycles to failure) of the toe box under repetitiveloading conditions. Setup of the pointe shoe toe box forfatigue testing was identical to that used for static axialloading conditions. Testing was performed using a cyclicloading rate of 10 Hz, a load level of 2 kN, and an R ratio(minimum stress:maximum stress)6 of 0.1, so that theapplied load cycled between "200 and "2000 N. Fatiguefailure was defined as collapse of the toe box by 7 mm.Pointe Shoe Cost Analysis and Dancer QuestionnaireThe local suppliers of pointe shoes provided retail costinformation for each of the five shoe types. A dancer questionnaire was distributed to 200 ballet dancers fromnearby colleges and dance programs. Those participatingin the study (average age, 19.3 # 0.25 years) averaged11.27 # 0.66 years of ballet experience. Most of the students surveyed were primarily interested in ballet withsecondary interests in jazz. The questionnaire was de-In Vitro Mechanics of the Pointe Shoe Toe Box557signed to evaluate the medical history of each dancer, thedancer’s training in all forms of dance, and the preferredcharacteristics of the pointe shoe selected (Table 1).Data and Statistical AnalysesStiffness calculations represent the peak load divided bythe corresponding displacement within the first 2 mm foraxial loading and within the first 5 mm for vertical loading. Strength values represent the peak load (in newtons)within the first 7 mm of displacement for both axial andvertical loading. Statistical analyses of the mechanicaldata included descriptive statistics, a one-way analysis ofvariance (ANOVA), and a post hoc Student-NeumanKeuls procedure for multiple comparisons between groups(statistical significance was indicated at P 0.05). Unlessotherwise noted, data are represented as mean valuesplus or minus the standard deviation. The pointe shoe toebox dimensions were considered in relation to the mechanical parameters using linear regression analysis.RESULTSPointe Shoe DimensionsArea computations calculated from the toe box height,width, and depth of the five pointe shoes were similar, butvolume computations were somewhat different, particularly when comparing the Gaynor Minden with the Leo’spointe shoes. The thickness of the layers in the toe boxmay contribute to the volume variation (Table 2).Mechanical AnalysisStatic Analysis. Axial compressive stiffness comparisons indicated that the Leo’s pointe shoe was the stiffestand was statistically different (P 0.05) compared withthe four other shoes. There were no differences betweenthe Capezio and Freed or the Gaynor Minden and Grishko(P % 0.05) pointe shoes in axial compressive stiffness, butthe differences in all remaining comparisons were statistically significant (Fig. 4). The peak axial compressivestrengths exhibited by the pointe shoes were not as sigTABLE 1Pointe Shoe Selection Criteria: Top 12 Characteristics in Orderof Preference Based on Dancer QuestionnaireFigure 3. A view of the pointe shoe oriented for verticalcompressive loading. The applied load was delivered to thedistal plantar surface of the toe /platform shapeVamp shapeDurabilityShank styleBreaks in quicklyHeel depthPriceAvailabilityDrawstring locationColor

558Cunningham et al.American Journal of Sports MedicineTABLE 2Pointe Shoes: Area and Volume CalculationsShoe type and sizeWidth (mm)(Mean # SD)Height (mm)(Mean # SD)Depth (mm)(Mean # SD)Volume (ml)(Mean # SD)Capezio 4-1 2 CFreed 4 XGaynor Minden 7 WLeo’s 4-1 2 DGrishko 4 M70.60 # 0.5570.00 # 2.4577.00 # 0.0064.40 # 1.3473.80 # 1.1051.68 # 0.6449.55 # 2.3750.00 # 1.4153.54 # 1.5755.80 # 0.8451.50 # 0.8744.13 # 5.0260.00 # 2.8342.04 # 2.6752.80 # 2.0569.20 # 5.0252.75 # 13.7968.00 # 0.0045.80 # 3.7775.80 # 2.86Figure 4. Axial compressive stiffness levels of the five toeboxes demonstrated that the Leo’s shoe had the greateststiffness and was significantly different from the other fourshoes (P 0.05). There was no statistically significantdifference between bars with an equal number of asterisks(*). All other comparisons were statistically significant at P 0.05 (one-way ANOVA, F ! 12.74, P 0.001). The error barsignifies 1 SD.Figure 5. The axial compressive strength levels of the fiveshoe types were compared. The Freed shoe demonstratedsignificantly lower strength levels than the other four shoes(P 0.05). The Capezio demonstrated the greatest axialstrength, but it was not significantly different from that of theGaynor Minden, Leo’s, or Grishko shoes (one-way ANOVA,F ! 17.90, P 0.001). The error bar signifies 1 SD. Anasterisk (*) indicates statistical significance (P 0.05).nificantly different as the peak axial compressive stiffnesses. The Freed shoe demonstrated strength levels thatwere significantly less than the four remaining shoes (Fig.5). The Capezio demonstrated the highest level of axialstrength, but the difference compared with the GaynorMinden, Leo’s, or Grishko shoe was not significant.Vertical loading of the pointe shoes resulted in stiffnesses and strength levels much lower than the corresponding axial tests. The Freed shoe exhibited the highestvertical stiffness level, and was significantly differentfrom the remaining four shoes (P 0.05). The Leo’s shoewas significantly different from the Gaynor Minden, butneither was significantly different from the Grishko orCapezio (Fig. 6). Peak vertical strengths exhibited by thefive shoes demonstrated trends opposite to those in thestiffness levels. For example, the Gaynor Minden, whichhad the lowest vertical stiffness, demonstrated the highest level of vertical strength. Both the Freed and GaynorMinden shoes, although not significantly different fromeach other, were significantly different from the Capezio,Leo’s, and Grishko pointe shoes (P 0.05), which demonstrated similar peak strengths under this vertical loadingcondition (Fig. 7).The predominant failure mechanism, exhibited by allpointe shoes under both axial and vertical static testing,was buckling of the toe box tip in on itself (Fig. 8). Whencompared with pointe shoe failure mechanisms observedin actual ballet shoes, the failure patterns generated under benchtop laboratory conditions appeared grossly similar, despite the in vitro loading condition.Cyclical Fatigue Analysis. Fatigue testing of the pointeshoes at the 2-kN load level demonstrated highly significant differences among the five shoe types, particularlywhen comparing the Gaynor Minden with the remainingshoes (P 0.05). As indicated by the number of cyclesrequired for shoe failure, the Gaynor Minden demonstrated more resiliency under conditions of repetitive loading; the Capezio, Freed, Leo’s and Grishko shoes exhibitedlower thresholds of elastic deformation, resulting in relatively quick plastic (permanent) shoe deformation underthe applied load. The mean cycles at failure for the GaynorMinden was statistically higher (P 0.05) than those forthe other shoes, which were not different from each other(Fig. 9).Regression Analysis. The purpose of the volume andstiffness calculations was to relate the dimensional andmechanical properties of the shoes with the peak failurestrengths exhibited by the different shoes. For axial loading, significant predictive value was found when the Leo’s

Vol. 26, No. 4, 1998Figure 6. The Freed shoe demonstrated greater verticalcompressive stiffness levels than the other four shoes (P 0.05). The Leo’s shoe was significantly different from theGaynor Minden (P 0.05), but neither was significantlydifferent from the Grishko or Capezio (one-way ANOVA, F !21.46, P 0.001). The error bar signifies 1 SD. The doubleasterisk (**) indicates a statistically significant difference incomparison with all the other shoe types. The single asterisk(*) indicates statistical significance (P 0.05) in comparisonof the Leo’s shoe with the Gaynor Minden shoe.Figure 7. Vertical strength levels for the five shoes demonstrated trends opposite to those for stiffness levels. TheGaynor Minden and Freed shoes demonstrated the greatestvertical strength and were significantly different from theCapezio, Leo’s, and Grishko shoes (P 0.05; one-wayANOVA, F ! 26.49, P 0.001). The error bar signifies 1 SD.An asterisk (*) indicates statistical significance (P 0.05).peak strengths (failure loads) were compared withthe stiffnesses exhibited by the same toe box (r2 ! 0.94)(Table 3). This significance in predictive value was foundonly in the Leo’s shoe under axial loading conditions.In Vitro Mechanics of the Pointe Shoe Toe Box559Figure 8. A lateral view of the secured pointe shoe toe boxat the endpoint of the destructive axial compressive loadingtest using a cylindrical steel ram.Figure 9. Cyclical fatigue testing at the 2-kN load level demonstrated differences between one shoe type and the otherfour. The mean number of cycles to failure for the GaynorMinden shoe was significantly higher (P 0.05) than for theother four types, which were not significantly different fromeach other (one-way ANOVA, F ! 14.30, P 0.001). Theerror bar signifies 1 SD. An asterisk (*) indicates statisticalsignificance (P 0.05).of pointe work, jazz training averaged 4.5 # 0.64 years,and modern dance training averaged 3.5 # 0.48 years. Thedancers spent an average of 59% (#0.035 SEM) of theirtraining time in ballet, 31% (#3.5 SEM) in modern dance,and 4% (#1.4 SEM) in jazz. The pointe shoes worn included Capezio (33%), Chacott (32%), Bloch (26.5%), Freed(10%), and Gaynor Minden (6%). The top five preferredcharacteristics of pointe shoes selected were fit, comfort,box/platform shape, vamp shape, and durability (Table 1).DISCUSSIONDancer QuestionnaireAccording to questionnaire responses, ballet training averaged 11 # 0.66 years with an average of 6 # 0.44 yearsThe modern pointe toe shoe must be able to provide support and protection whether the dancer is en pointe, in ademi-pointe position, or performing a jump.2 The shoe

560Cunningham et al.American Journal of Sports MedicineTABLE 3Linear Regression Analysis (r2) Comparing Pointe Shoe Dimensional and Mechanical Properties with Peak Failure StrengthsLoading modeShoe typeAxialStiffness vs strengthVolume vs strengthStiffness vs dGaynor MindenLeo’sGrishkoaVerticalVolume vs strengthSignificant predictive value.must add to the aesthetics of dance by enhancing theconical shape of the leg while providing for a quiet landing. The distal portion of the pointe shoe box, composed oflayers of burlap, cardboard, or papier-mâché, contains thetoes by bunching them into an oval platform. The hardmaterial in the box

A Comparative Mechanical Analysis of the Pointe Shoe Toe Box . ness, static strength, and fatigue properties (cycles to failure) of the shoes. Under axial loading conditions, the Leo’s shoe demonstrated the highest stiffness level, and the Freed shoe exhibited the least strength.