Fracture toughness of bovine claw horn from cattle with and without vertical fissures

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The Veterinary Journal 173 (2007) 541–547

TheVeterinary Journal

Fracture toughness of bovine claw horn from cattlewith and without vertical fissures

Chris Clark *, Lyall Petrie

Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Canada SK S7N 0A5

Accepted 1 February 2006

Abstract

The study was undertaken to determine whether the claw horn from cows suffering from vertical fissures (sandcracks) is weaker withregard to fracture formation than claw horn from cows without vertical fissures. Fracture toughness was determined using a compositetesting procedure previously developed for use in equine hoof horn. Twenty cows with vertical fissures of the lateral front claw werecompared with 20 case controls. Due to the calibration technique needed as part of the testing method results were obtained for onlyfive animals in each group; cows with and without vertical fissures had median J-integrals of 8182 J/m2 and 8483 J/m2, respectively,but the difference was not statistically significant. Although the numbers are small it is suggested that since no difference existed inthe fracture toughness of the claw horn from the two groups of cattle, vertical fissure formation is not directly dependent on the biome-chanical property of the horn itself.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Vertical fissures; Sandcracks; Biomechanics; Bovine lameness

1. Introduction

A vertical fissure or sandcrack is a split aligned in a ver-tical orientation on the dorsal wall of the claw. The fissuretypically extends through the more superficial layers of theclaw horn, although it may expand through to the sensitivelaminae resulting in lameness. The crack may originate atany level of the dorsal wall and extend distally for a vari-able distance (Greenough, 1997). The lesion is relativelycommon in western Canada, where approximately 20%of adult beef cattle are affected and there have been severalobservational studies reported (Westra, 1981; Hand et al.,1992; Goonewardene and Hand, 1995; Greenough et al.,1998; Petrie et al., 1998). The lesions are most commonlyseen in the lateral fore claws (Clark et al., 2004) of older(Goonewardene and Hand, 1995; Petrie et al., 1998), hea-vier (Goonewardene and Hand, 1995) cattle. The lesion isalso more common in larger claws although the role of

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doi:10.1016/j.tvjl.2006.02.002

* Corresponding author. Tel.: +1 306 966 7165; fax: +1 306 966 7152.E-mail address: [email protected] (C. Clark).

claw horn hydration in the formation of fissures has notyet been determined (Clark et al., 2004).

The possible association between diet and the formationof vertical fissures has received considerable attention(Hand et al., 1992; Campbell et al., 2000; Greenough,1999) and it would seem that if diet is a factor then it mustinfluence the physical properties of the claw horn. How-ever, no studies have attempted to link the physical proper-ties of bovine claw horn with diet and, in particular, thephysical properties of horn from fissured and non-fissuredhooves have not been compared, although there is evidencethat supplementation of the diet with biotin reduces theincidence of vertical fissures (Campbell et al., 2000).

The field of biomechanics has expanded considerably inthe past 15 years. The equine hoof has been the focus ofmuch research due to its apparent resistance to the extremeforces to which it is subjected when the animal is galloping.Indeed, it has been estimated that the pressures may exceed10 Megapascals (MPa) (Bertram and Gosline, 1986). Mate-rials may need to be subjected to a large number of tests inorder to determine their physical characteristics. In the field

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542 C. Clark, L. Petrie / The Veterinary Journal 173 (2007) 541–547

of engineering, the tests applied to materials are dependenton their final function. Several such tests have been appliedto the hoof horn of both cattle and horses, including tensilestrength (Albarano, 1993), compressive strength (Tranteret al., 1993) and impact toughness testing (Mulling, 1993)and from the compressive stress–strain curves it is possibleto calculate the elastic modulus (Bertram and Gosline,1986).

Fracture mechanics (Gordon, 1988) help to describehow cracks form in materials. The basis of this science isthat materials naturally contain large numbers of micro-scopic defects or cracks and when energy is imparted tothese materials, the cracks may grow, until they reach acritical length at which point the material breaks. Two fac-tors largely unrelated to tensile strength are particularlyimportant, the number of defects within the material andthe ability of the material to resist the progressive growthof these defects into cracks. For example, the tensilestrength of wet wood and glass is essentially the samebut, the two materials react very differently to impact: aglass that has very low fracture toughness shatters whereaswood absorbs the blow and the surface may simply dent.The fracture toughness of metals and some other materialsis measured under very controlled conditions using preciseprocedures outlined by the American Society for Testingand Materials (ASTM).1

Bertram and Gosline (1986) adapted composite materialstesting procedures for use in horse hoof horn and were ableto measure the fracture toughness using a value called the J-integral, which is a measure of the energy required to pro-duce a new surface area through cracking at a defined criti-cal point during crack formation. The value for equine hoofhorn was determined to be approximately 10,000 J/m2,approximately 2000 times greater than glass and equivalentto most man-made, fibre-reinforced composite materials.The same authors further showed that pigmentation hadno effect on the strength of hoof horn at least in respect tothe various parameters that they examined (Bertram andGosline, 1986). Compared with the proximal hoof horn,the fracture toughness of the most distal horn was signifi-cantly reduced, which was attributed to the horn accumulat-ing fatigue damage with age, resulting in the formation ofmultiple microcracks (Gordon, 1988).

Bertram and Gosline (1986) also showed that hoof hornbehaved differently when subjected to cracks in differentdirections. The greatest fracture toughness was observedwhen the crack ran perpendicular to the orientation ofthe intertubular elements. Conversely, the lowest fracturetoughness occurred when the crack ran parallel to the inter-tubular elements. It was concluded that the tubular ele-ments of the hoof wall reinforce the horn against fracturein its weakest plane.

1 ASTM Standard E-813-89 Standard test method for Jic, a measure offracture toughness; Annual Book of ASTM Standards, part 10.

The aim of the present case–control study was to test thehypothesis that the physical properties of claw horn fromcattle with vertical fissures make it inherently more proneto fracture formation than claw horn from cows withoutfissures. This change in physical characteristics may ormay not be due to the diet of the individual animal.

2. Materials and methods

2.1. Feet

Both front feet from 20 mature beef cows with no evi-dence of vertical fissures (group 1) and from 20 mature beefcows with a vertical fissure in at least one of the four claws(group 2), were collected from a local abattoir. The ana-tomical location of the feet was identified as they cameoff the slaughter line by placing a staple in the toe of thelateral claw. Each set of two front feet were then placedin uniquely identified plastic bags. The feet were thenbrought back to the Western College of Veterinary Medi-cine and stored in sealed plastic bags at 4 �C untilprocessing.

2.2. Processing

Each individual lateral claw was identified with thecow’s project number and its position. The claws were care-fully examined for vertical fissures and their presencerecorded. In group 2, vertical fissures were present in 34/40 lateral claws. There were no vertical fissures in any ofthe 80 front claws from group 1 cows.

The claws were separated from the distal limb using aband saw at the level of the horn–skin junction. Compacttest (CT) specimens of claw horn were produced withdimensions similar to those described by Bertram and Gos-line (1986). The lateral claw was sawn in a saggital plane2.5 cm from the axial wall and the abaxial section of theclaw wall was further trimmed to produce a 3 · 3 cm sec-tion of claw wall. This sample was produced from the cen-tre of the abaxial claw wall approximately 3 cm from thesaggital saw cut and 2 cm from the coronary band(Fig. 1). The attachments between the horn and the under-lying bone were severed with a large bladed sharp knife.The sample was then stored in 0.01% sodium azide solutionat 4 �C until required. A maximum of 40 samples was avail-able in each study group.

Each 3 · 3 cm sample was trimmed with a water-cooled,precision diamond saw (Hamco Machines Inc.) to producea specimen size of 10 · 10 mm. These 10 · 10 mm sampleswere machined on a water-cooled sanding machine to pro-duce a specimen 3 mm thick, which is smaller than thatdescribed in ASTM E-399-80, but retains the same relativedimensions (Fig. 2).

The CT sample was finally prepared by drilling two2 mm holes, the position of which was ensured by the useof a drilling template made from aluminium. A notch ofknown length (a) was produced on the appropriate side

Fig. 1. Diagram to illustrate the production of a compact test specimenfrom the abaxial wall of the bovine hoof.

Fig. 2. Diagram to illustrate the dimensions of a compact test specimen,based on Bertram and Gosline (1986) and ASTM E-813-89.

C. Clark, L. Petrie / The Veterinary Journal 173 (2007) 541–547 543

with a razor blade mounted in a cutting press. The sampleswere randomly allocated a notch depth such that thelength/width (a/w) ratio ranged from 0.2 to 0.8. Such arange of a/w ratios was required in order to calibrate theassay but only samples with an a/w ratio of 0.45–0.55 wereeligible for inclusion in the final analysis of the J-integral(ASTM 399-80). All specimens were manufactured suchthat the notch was aligned with the orientation of the horntubules.

At all times, the samples remained immersed in 0.01%sodium azide to ensure 100% hydration. The resulting spec-imens were then reviewed as part of a quality control pro-cess, using a micrometer calibrated to 0.01 mm. At thispoint it became apparent that the thickness of the diamondcutting wheel had not been correctly accounted for so thespecimens used in the later part of the study were deter-mined to have the dimensions, 9.75 mm · 9.75 mm · 3 mm.In all dimensions the variation was restricted to a maxi-mum of 5%.

Horn samples could not be removed from the wall in theregion of the fissure for two main reasons: firstly, in manyof the samples the horn was already affected by a fissure.Secondly, even if a fissure was absent, the horn from thisregion was highly curved preventing the machining of asuitable sample.

The diamond saw worked extremely well for cutting theinitial sample, but it was difficult to clamp the sample inposition and to ensure that the cuts were made perpendic-ular to each other. The final machining was done manuallybut, again, securing the specimens was extremely difficultand a number of samples were lost during this part ofthe processing. All samples that were not completely squarewere also removed from the study. When these criteria hadbeen applied, only 20 (50%) specimens remained in each ofthe two groups.

2.3. Fracture testing

Fracture testing was performed by placing two 1.5 mmpins through the drilled holes and mounting the specimenin two specially designed clevis mounts which could beattached to the Lloyd MK5 5KN (JJ Lloyd InstrumentsLtd.). The test consisted of loading the specimen perpen-dicular to the notch at an extension rate of 5 mm/min.The arrangement of the specimen, pins and clevis mountsensured that the sample would align itself with the direc-tion of the applied force (Fig. 3). The results were plottedon a force versus extension graph for each sample(Fig. 4). The graph may be divided into several regions:

1. An initial region, in which the slack within the system isbeing removed.

2. A linear region, in which the stiffness of the specimen isdetermined by the length of the prepared notch (Gaggarand Broutman, 1975; Wright and Hayes, 1977).

3. A region in which there is an apparent decrease in stiff-ness, thought to be associated with plastic deformationand stable crack growth at the crack tip (Wright andHayes, 1977).

4. In the final section of the curve the stiffness decreasesstill further as the crack progresses through the materialin an unstable manner.

The critical failure point in this test was defined by Ber-tram and Gosline (1986) as (Pq). This is determined byplotting a line equivalent to 95% of linear portion of the

Fig. 3. Compact test specimen undergoing testing. The sample is held inthe two clevis mounts using steel pins. The arrow indicates the opening ofthe notch as the force is applied.

Fig. 4. A typical force/extension curve produced on testing a compact testspecimen (for explanation of regions see text).

Fig. 5. Diagram to indicate the addition of a line drawn at 95% of theinitial slope of the force/extension curve, and the position of the interceptof this line and the trace. The intercept is termed Pq.

Fig. 6. For each point on the results curve, the length of the crack may becalculated from compliance. It is therefore possible to determine the rateof crack growth at the point of critical failure (Pq). The energy imparted tothe system may be calculated from the area under the curve at the samepoint. The shaded portion of the graph represents the energy imparted atPq.


graph and recording the force value at the intersect (Fig. 5).Energy is known to be equal to the product of force andextension; it is therefore possible to calculate the energythat has been imparted to the specimen from the area underthe curve integrated up to the critical failure point (Fig. 6).

The analysis of fracture formation requires that thelength of the crack that grows from the tip of the notchbe precisely known. In metal specimens, as the crack grows,its length can be measured in a number of ways, but thesetechniques are not applicable to composite materials suchas claw horn. It is therefore necessary to use the methodof compliance calibration (Gaggar and Broutman, 1975;Wright and Hayes, 1977), which assumes that physicalcharacteristics of the test material are uniform in the linearportion of the curve (region 2) and that the sample basi-cally acts as a spring. The constant in this case is known

as the compliance (which is the reciprocal of stiffness)and is also dependent on the width of the specimen.

Compliance ¼ ðdisplacement ðqÞ=force ðpÞÞ � thickness ðtÞ

The compliance versus initial notch lengths was plottedfor all specimens. A regression analysis was performed witha/w as the independent variable and compliance as thedependent variable in order to determine the mathematicalnature of the relationship. Since at any stage during the testprocedure, the extension and the force applied to the spec-imen are always known, it is possible to calculate the cracklength at any time during the test. The calculated cracklength is denoted as (A).

The fracture toughness of the samples was calculatedbased on the method of Rice (1968), to give the J-integral


as a measure of the amount of energy required to extendthe length of the crack by a known distance at a criticalpoint of sample failure. In order to calculate this value itis necessary to first transform each of the sample resultgraphs so that calculated energy (area under the curve) isplotted against the calculated crack length A (Fig. 7).The point of failure (Pq) was identified on the graph andthe gradient at this point determined. The J-integral maythen be calculated (Broek, 1982) as

J ¼ ð�1=tÞ � ðdU=dAÞqwhere J is the J-integral, t is the specimen thickness, U isthe energy, A is the calculated crack length at the criticalfailure point (Pq).

All testing was carried out using samples with a relativehydration of 100% (water content equivalent to 37.5% byweight (data not shown)). Previous studies have shown thatfracture toughness is extremely dependent on hydrationstatus (Bertram and Gosline, 1987). Using 100% relativehydration enabled us to control for any variation in hydra-tion status in the samples and was easier to maintain than apartial hydration status.

2.4. Statistics

Data were collected and entered into a spreadsheet pro-gram (Corel Quatro Pro 8.0 (Corel Corp.)), which was usedto perform all necessary transformations. Curve fitting wasachieved using SPSS 9.0 for Windows (SPSS Inc.). Thefracture toughness of the two sample groups was comparedusing the Mann–Whitney non-parametric test, with signif-icance assumed at P < 0.05. A non-parametric test was

Fig. 7. The figure indicates a typical graph depicting energy against cracklength (A). The point Pq is indicated. The gradient of the curve at thispoint is calculated and the J-integral may then be calculated using themethod of Broek (1982). Sample 14.1: Least Squares Regression modelusing a quadratic equation y = 0.1835 � 101.24X + 13710.6X2 (R2 =0.962); differential (dy/dx = 27421x � 101.2).

used because the data was not normally distributed. Curvefitting was performed using SPSS 9.0 for Windows (SPSSInc.).

3. Results

Compliance calibration curves were produced for bothgroups (Fig. 8). In each case the curve was best fitted bya cubic equation and the best fit line was drawn and usedto determine the crack length in all further calculations.The compliance curve calibration for group 1 was extre-mely good with an R2 value of 0.83, compared with thecurve for group 2 which is noticeably shallower and hasmuch greater variability (R2 = 0.65).

ASTM 399-80 states that only samples with an a/w ratiobetween 0.45 and 0.55 are eligible for inclusion in this formof materials testing. Since 75% of test specimens were usedto produce the calibration curve outside of this narrowrange, only five specimens fulfilled these requirements.For each of these samples graphs were produced in whichenergy (equivalent to the area under the test curve) wasplotted against the calculated length of the crack (A)(Fig. 7). In all cases, the resulting curve was best fitted bya simple quadratic equation which could be easily differen-tiated to determine the gradient. The value of A at thepoint equivalent to Pq was determined and the J-integralcalculated.

The summary results of the test specimens are presentedin Table 1. The mean fracture toughness for horn fromnon-fissured and fissured claws was 8483 J/m2 and8182 J/m2 respectively. The Mann–Whitney test was usedto test for potential statistical significance. No significantdifference was found (P = 0.69).

Fig. 8. (a) Compliance calibration curve for group 1 cows (withoutvertical fissures) Least Squares Regression model using a cubic equation:y = 2.6 · 10�06X � 2 · 10�07 � 7 · 10�06X2 + 7.8 · 10�06X3 (R2 = 0.827).

Table 1Results of fracture testing between the two groups of cattle

Group J-integral (J/m2)

median (inter-quartile range)

Group 1 (no fissures) (n = 5) 8483 (1332)Group 2 (fissured) (n = 5) 8182 (6480)

(n = 5) Refers to the number of specimens fulfilling the compact testspecimen requirements.


4. Discussion

The manufacture of the compact test specimens for frac-ture testing was much more difficult than anticipated. TheASTM E-813-89 guidelines call for a sample with error mar-gins <0.5% but this level of accuracy was not possible andwe opted to use 5% as our error margin. Out of a potential40 specimens from each group, only 20 specimens that ful-filled the specifications could be produced. Since 75% ofsamples are required to produce the compliance calibrationcurve and cannot be included in the final test, the resultantnumber of suitable test specimens (n = 5) is small so increas-ing the chances of a type 2 error (that a true difference in thegroups will be missed). A larger number of specimens wouldbe required to increase the power of the study.

The testing procedure was straightforward but in only 1sample from each group did the samples fracture perpen-dicular to the axis of the force. This effect has been notedby previous researchers in the field (Bertram and Gosline,1986; Kasapi and Gosline, 1997; Kitchener, 1987) andhas been uniformly attributed to a Cook–Gordon crackstopping mechanism (Gordon, 1988); the material containsplanes of weakness, which divert an advancing crack,resulting in an overall increase in material strength. Thedetails of this mechanism have been described in detailby Kasapi and Gosline (1997) who present the results ofdetailed histological investigations and biochemical testingof horse hoof horn. Histologically, the hoof wall could bedivided into six distinct layers and differences in both thenumber and structure of the tubules were found in eachof the layers. In addition, these workers determined differ-ences within the intertubular horn within the layers. Whenthis information was correlated with the fracture toughnessdeterminations it proved possible to develop a novel modelof the horse hoof wall in which the tubular and intertubu-lar horn combined to form a crack diverting system, whichfunctions to divert cracks away from the dermis and to pre-vent cracks from progressing in a proximal direction. Themiddle region of the stratum medium is of considerableimportance in this function and was named the ‘‘mid-wallcrack diversion mechanism’’. It is the intertubular horn ofthis region that appears to be of greatest importance.

Similar histological studies have now been performed onbovine claw horn (Baillie and Fiford, 1996). Their descrip-tion of the microscopic structure is basically similar to thehorse (Kasapi and Gosline, 1997) with the bovine hoofhorn consisting of tightly ordered tubules surrounded byintertubular horn in which the cells are aligned in a uni-

form direction. The intertubular horn has the appearanceof having flowed around the tubules but there are severalimportant differences between bovine and equine hoofhorn (Baillie and Fiford, 1996):

(a) The tubule wall of equine hoof horn is thicker.(b) The tubules in bovine claw horn are more oval in

shape, although this may purely reflect the level atwhich the samples were taken (Kasapi and Gosline,1997).

(c) There appears to be much more interaction betweenthe tubular and intertubular horn in bovine clawhorn.

(d) The keratin cells in the intertubular horn are orien-tated differently in bovine claw horn where the cellsappear to be orientated parallel to the long axis ofthe tubules, whereas in the horse the cells of the inter-tubular horn are orientated almost perpendicular tothe long axis of the tubules.

Baillie and Fiford (1996) concluded that a vertical fis-sure in bovine claw horn would not be expected to bediverted in a manner similar to that seen in equine hoofhorn, but would be expected to advance in a direction per-pendicular to the plane of the sole.

The median fracture toughness of the 100% hydratedclaw horn from our control cows (group 1) was 8182 J/m2,which is approximately two-thirds of the value of12,000 J/m2 for horse hoof horn reported by Bertram andGosline (1986) and is consistent with the theory outlinedabove (Baillie and Fiford, 1996). It could also explain whycattle and horses that are kept in the same environmentand on similar feed do not suffer equally from vertical fis-sures. However, it is surprising that all cracks did notadvance in a linear manner in our studies and, unfortu-nately, this may indicate that the samples were of insufficientquality, resulting in unequal stresses within the test speci-men and a non-uniform failure.

The fracture toughness for the horn from cows with ver-tical fissures did not differ significantly from the controlcows. The testing of samples from cows with sandcracks(group 2), proved to be more problematical. The compli-ance calibration curve was noticeably shallower and alsohad a much poorer fit. The reason for this difference isunknown, but it may have contributed to the very muchgreater variability observed in this group.

On the basis of these admittedly limited findings, thehypothesis that the claw horn of cattle with vertical fissuresis inherently more prone to fracture formation is unproven.If true, it would imply that nutritional supplementationaimed at improving the biomechanical properties of thehorn is not the most promising means of reducing the prob-lem. The samples were tested at a relative humidity (RH) of100% and Bertram and Gosline (1987) demonstrated thatthe fracture toughness of horse hoof horn is highly depen-dent on hydration status. Hoof samples conditioned at RH0% and RH 50% (moisture content of 5.5% and 11%,


respectively) had very low fracture toughness values andbasically acted as brittle materials. Similarly, studies ofbovine claw horn have shown that its mechanical proper-ties are very dependent on moisture content (Baillie et al.,2000; Winkler et al., 2004; Hinterhofer et al., 2005), withincreasing hydration resulting in lower stiffness values.Testing of all samples was performed at 100% RH bothto ensure uniformity and also because it is easy to maintainsamples at this level of hydration by ensuring that they arekept soaked in sodium azide solution.

It is our contention that the low environmental relativehumidity, especially during the winter, in the Canadianprairies region influences the mechanical properties of clawhorn in beef cattle by increasing its brittleness increasingthe likelihood of vertical fissures. Hinterhofer et al.(2005) found that claw horn from the dorsal wall regionof the hind feet has the highest stiffness and lower moisturecontent of any region of the claw horn capsule. This mayexplain why vertical fissures are invariably found on thedorsal wall. The marked predominance of lesions on thelateral claws of the fore feet is less easily explained.

This study has shown that the technique of hoof hornfracture toughness measurement described by Bertramand Gosline (1986) can be applied to cattle. Our resultslend some support to the theoretical properties of thebovine claw wall described by Baillie and Fiford (1996).The technique has potential applications in shorter termfeeding trials, in which researchers attempt to nutritionallymodify the properties of the claw horn. The advantagebeing that the properties of the horn may be analyseddirectly, rather than waiting for failure to occur.

Acknowledgements

We would like to thank Brad Novolobski of the Depart-ment of Geology, University of Saskatchewan for his assis-tance in producing the compact test specimens. We wouldalso like to thank Professor S. Yannacopoulos and P. Sim-minoff of the Department of Mechanical Engineering, Uni-versity of Saskatchewan for making the departmentalfacilities available and for assistance with specimen produc-tion and testing.

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Fracture toughness of bovine claw horn from cattle with and without vertical fissures