• View in gallery

    Diagrams of the dorsal (A), medial or lateral (B), and solar (C) aspects of an equine foot to illustrate measurements of morphometric variables of capsule shape. A—On a view of the dorsal aspect of the hoof, variables measured include lateral wall angle (LA), lateral wall length (LL), medial wall angle (MA), and medial wall length (ML). B—On a view of the medial or lateral aspect of the hoof, variables measured include lateral heel angle (LHA) and lateral heel length (LHL) or medial heel angle (MHA) and medial heel length (MHL), toe angle (TA), and toe length (TL). C—On a view of the solar aspect of the hoof, reference points include the cuneus ungula (frog), point of the frog (PF), sagittal axis (SAX), and transverse axis (TAX). Variables measured include dorsal sole length (DL), lateral heel width (LHW), lateral palmar length (LPL), lateral sole width (LSW), medial heel width (MHW), medial palmar length (MPL), medial sole width (MSW), and solar circumference (dashed line [SC]).

  • View in gallery

    Representative MRI images of the right forelimb foot of a horse to illustrate measurements of morphometric variables of capsule thickness. A—Midsagittal view of the foot. For each horse, the sagittal plane image in which the distance from the distal end to extensor process (EP) of the distal phalanx was greatest was selected as the midsagittal image for morphometric analysis. Variables measured include wall thickness at midway (a point on the dorsal aspect of the foot midway along the dorsal length of the distal phalanx [WSM]), dermis thickness at midway (DSM), wall and dermis combined thickness at midway (WDSM), wall thickness at distal border (a point on the dorsal aspect of the foot at the distal border of the distal phalanx [WSD]), dermis thickness at distal border (DSD), and wall and dermis combined thickness at distal border (WDSD). The letter S in the variable abbreviations refers to the sagittal orientation of the image used to measure these variables. B—Dorsal view of the foot. For each horse, the dorsal plane image in which the width of the distal phalanx was greatest (bracket) was selected as the dorsal image for morphometric analysis. Variables include wall thickness at medial aspect (a point on the medial aspect of the foot at the distal border of the distal phalanx [WDM]), dermis thickness at medial aspect (DDM), wall and dermis combined thickness at medial aspect (WDDM), wall thickness at lateral aspect (a point at the lateral aspect of the foot at the distal border of the distal phalanx [WDL]), dermis thickness at lateral aspect (DDL), and wall and dermis combined thickness at lateral aspect (WDDL). The letter D at the second (WDM, DDM, WDL, and DDL) or third (WDDM and WDDL) letter position in the variable abbreviations refers to the dorsal orientation of the image used to measure these variables. White dots at the level of the coronet in both views represent calibration marks.

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Changes in growth of the hoof wall and hoof morphology in response to regular periods of trotting exercise in Standardbreds

Babak Faramarzi DVM, PhD1, Jeffrey J. Thomason BSc, PhD2, and William C. Sears MSc, MS3
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  • 1 Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.
  • | 2 Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.
  • | 3 Department of Population Medicine, Ontario Veterinary College, University of Guelph, Guelph, ON N1G 2W1, Canada.

Abstract

Objective—To quantitate changes in hoof wall growth and hoof morphology induced by mild exercise in Standardbreds.

Animals—18 Standardbreds.

Procedures—Horses were exercised at approximately 6 m/s (4,200 to 5,600 m/d) on 4 d/wk for 17 weeks. Both exercise (n = 9) and nonexercise (control group; 9) groups were housed in a large paddock throughout the study. At the beginning and end of the study, right forelimb feet of all horses were digitally photographed and underwent magnetic resonance imaging. Hoof wall measurements were obtained from the images to evaluate hoof wall growth and morphometric variables. Data were compared between the groups and within each group via a quadratic model. Changes in each variable and pairwise correlations between variables were evaluated.

Results—Morphometric variables did not significantly differ between the control and exercise groups. However, differences within each group between the start and the end of the study were significant for several variables; overall, values for hoof wall variables increased and those for solar variables decreased. Between the beginning and the end of the study, the amount of variation in values of hoof capsule variables in the exercise group decreased to a greater extent, compared with control group findings. Patterns of pairwise correlations for variables differed between the groups.

Conclusions and Clinical Relevance—In Standardbreds, mild exercise for 17 weeks caused no significant changes in hoof wall growth or morphometric variables. Subtle changes may develop in equine hooves in response to loading, and mild exercise may not be a strong adaptive stimulus.

Abstract

Objective—To quantitate changes in hoof wall growth and hoof morphology induced by mild exercise in Standardbreds.

Animals—18 Standardbreds.

Procedures—Horses were exercised at approximately 6 m/s (4,200 to 5,600 m/d) on 4 d/wk for 17 weeks. Both exercise (n = 9) and nonexercise (control group; 9) groups were housed in a large paddock throughout the study. At the beginning and end of the study, right forelimb feet of all horses were digitally photographed and underwent magnetic resonance imaging. Hoof wall measurements were obtained from the images to evaluate hoof wall growth and morphometric variables. Data were compared between the groups and within each group via a quadratic model. Changes in each variable and pairwise correlations between variables were evaluated.

Results—Morphometric variables did not significantly differ between the control and exercise groups. However, differences within each group between the start and the end of the study were significant for several variables; overall, values for hoof wall variables increased and those for solar variables decreased. Between the beginning and the end of the study, the amount of variation in values of hoof capsule variables in the exercise group decreased to a greater extent, compared with control group findings. Patterns of pairwise correlations for variables differed between the groups.

Conclusions and Clinical Relevance—In Standardbreds, mild exercise for 17 weeks caused no significant changes in hoof wall growth or morphometric variables. Subtle changes may develop in equine hooves in response to loading, and mild exercise may not be a strong adaptive stimulus.

Mechanical loading is considered to be one of several stimuli that cause changes in the shape and morphology of equine hooves; however, supporting evidence for any response to this form of stress is circumstantial. The nature of such responses has not been characterized, and it is unknown whether these responses are adaptive (ie, appropriate to the stimulus). Elucidation of the role of hooves in modulating interactions between a horse and a substrate is crucial for understanding hoof biomechanics and preventing mechanically induced lameness. It has been estimated that > 30% of lameness issues in horses are associated with foot problems.1 Therefore, knowledge of the responses of hooves to mechanical stress is relevant to issues of health and welfare for horses. Failure of individual bones and soft tissues of the musculoskeletal system and lameness are major concerns of the equine industry from the perspectives of health and welfare of horses and of economic losses (ie, treatment costs and days lost for racing and training).

A concern for all sport horse disciplines (eg, racing, show jumping, dressage, and endurance riding) is conditioning the musculoskeletal system to support the loads imposed during strenuous exercise. Limbs of equids can adapt to imposed stresses. The third metacarpal bones and associated tendons and ligaments in horses were the focus of previous studies2–5; however, to our knowledge, the responses of hooves to exercise have not been evaluated. The distal phalanx acts as a stable platform, and its morphometric response to exercise is minimal.6 Differences in anatomic characteristics of the primary epidermal laminae in response to exercise have been detected and were attributed to responses to variation in the loads applied to the hooves.7,a

The hoof wall is relatively hard and insensitive and acts as a barrier to protect the structures within the hoof capsule. The shapes of the hoof capsule and the inner structures affect how forces generated during hoof contact with the ground are transmitted through the capsule to the skeleton (ie, distribution of stress in the capsule and deeper structures is partly dependent on hoof shape). Some variables of capsule shape and internal anatomic characteristics of hooves (ie, laminar morphology) are correlated.7,a Our group recently reported7 a correlation between laminar morphology and capsule shape and also detected different patterns of correlation for anatomic variables between Standardbreds and Thoroughbreds; this difference could be related to different gaits and exercise patterns between these 2 breeds.

The repeated loading that is imposed over time during activities such as regular exercise causes changes in the magnitude of the applied stress. Additionally, changes in distribution of stress may be induced. Such loading may stimulate biologic responses that alter hoof morphology. Changes in the mechanical behavior of hooves in response to alterations in loading or extrinsic or intrinsic modifiers develop over time.a There is empirical evidence that exercise influences hoof morphology and biomechanics in horses, but this has not been evaluated experimentally to our knowledge. Evaluation of potential relationships over time among capsule thickness (hoof wall thickness and dermis thickness), capsule shape (hoof shape), and growth of the hoof wall requires assessment of variables associated with these characteristics in live horses and at the same time points. The purpose of the study reported here was to evaluate the influence of mild exercise over time on growth of the hoof wall and hoof morphology in Standardbreds.

Materials and Methods

Animals—Eighteen adult (mean ± SD age, 5.8 ± 2.6 years) Standardbreds (6 mares and 12 geldings) were randomly allocated to a control group (n = 9) or an exercise (9) group. Mean weight of horses in the control group was 459 ± 39 kg, and mean weight of horses in the exercise group was 463 ± 34 kg. All horses were kept in a large (200 × 200-m) paddock throughout the 17-week study period. Both groups had free access to water. The study protocol was reviewed and approved by the University of Guelph Animal Care Committee and was performed in compliance with its guidelines and Canadian federal regulations.

Study design—Horses in the exercise group were exercised 4 d/wk for 17 weeks. These horses were exercised at a medium-speed trot (approx 6 m/s) on a straight track that was 700 m in length. The track had a crushed limestone surface over a gravel base, which is representative of harness tracks in North America. When a horse reached an end of the track, it was slowed to a walk, randomly turned left or right, and then reaccelerated to a trot. To condition each horse, the exercise regimen started at 1,400 m/d; within 2 weeks, the amount of exercise was gradually increased to 4,200 and 5,600 m/d, depending on the fitness level of the horse. Horses performed a mean of 47,000 strides (range, 21,000 to 81,000 strides) during the exercise period. Horses in the control group received no forced exercise; they could freely run in the paddock, but estimating the amount of free exercise for each horse in the control group was not feasible.

Because of higher energy expenditure for horses in the exercise group, they received a grain ration daily (mean quantity, 0.45 kg of grain/d). The grain ration contained 17% protein but did not contain dietary supplements such as biotin and zinc. To balance energy intake and expenditure for the horses, each horse was weighed approximately every 4 weeks, and the ration for each horse was revised as necessary to maintain a consistent body weight. Some horses in the control group were also fed grain to maintain their weights.

Hoof maintenance—The same farrier trimmed and shod the hooves of all horses at the beginning of the study and at 6-week intervals thereafter. The same trimming protocol was used for all horses, and no correctional trimming techniques were used. For each foot, a point (located approx 1 cm caudal to the apex of the cuneus ungula [frog] for a typical riding horse [Duckett's dot]), which was assumed to be directly distal to the center of the distal phalanx, was used as the primary reference point. An imaginary line was drawn through the center of the foot (ie, along the sagittal axis) from the center of the hoof toe, through the reference point, and over the central sulcus of the frog. The excess sole was removed, and the bars (ie, pars inflexa lateralis and medialis) that protruded beyond the plane of the sole were trimmed so they were level with the sole. The toe of the hoof was trimmed so that the long axes of the proximal and middle phalanges and the toe were aligned when the limb was viewed from the lateral side. The heels and distal aspects of the lateral and medial sides of the hoof were trimmed to preserve mediolateral symmetry. The same shoe type (flat steel, full swedge Standardbred training shoe) was used for all horses on all feet. Shoes with a swedge were used to provide traction on the stone track rather than shoes with gait-altering devices such as caulks or wedges. On the basis of the size of the hooves, 3 shoe sizes (00 to 2) were used. None of the heels of the shoes extended > 6 mm beyond the most palmar or plantar aspect of the heels of the feet.

Data collection—Data were collected at the beginning (June) and approximately 17 weeks later (October) at the end of the study. The right forelimb feet of all 18 horses were evaluated. Three sets of data (relating to growth of the hoof wall, capsule shape, and capsule thickness) were collected.

Assessment of growth of the hoof wall—At the beginning of the study, a shallow groove was made approximately 1.3 cm distal to the coronary band on the dorsal, medial, and lateral aspects of the hoof wall of each right forelimb foot. The 3 grooves were filled with an indelible paint. Three photographs (dorsal, medial, and lateral views) of each right forelimb foot, along with a scale reference, were obtained by use of a digital camerab at the beginning and end of the study. The images were obtained after removing the old shoe but before a new shoe was applied. The digital images were analyzed by use of image analysis software.c The growth of each aspect (dorsal [toe], medial, and lateral) of the hoof wall was calculated as the difference in the measurements of the position of these grooves between the beginning and the end of the study.

Assessment of capsule shape—Four photographs (solar, lateral, medial, and dorsal views) of the right forelimb feet were obtained at the beginning and end of the study by use of a standardized protocol and the aforementioned camera and scale reference. For each horse, measurements for 18 linear and angular variables (adopted from a study by Thomason et al8) were obtained from the photographs by use of the aforementioned image analysis software (Figure 1).

Figure 1—
Figure 1—

Diagrams of the dorsal (A), medial or lateral (B), and solar (C) aspects of an equine foot to illustrate measurements of morphometric variables of capsule shape. A—On a view of the dorsal aspect of the hoof, variables measured include lateral wall angle (LA), lateral wall length (LL), medial wall angle (MA), and medial wall length (ML). B—On a view of the medial or lateral aspect of the hoof, variables measured include lateral heel angle (LHA) and lateral heel length (LHL) or medial heel angle (MHA) and medial heel length (MHL), toe angle (TA), and toe length (TL). C—On a view of the solar aspect of the hoof, reference points include the cuneus ungula (frog), point of the frog (PF), sagittal axis (SAX), and transverse axis (TAX). Variables measured include dorsal sole length (DL), lateral heel width (LHW), lateral palmar length (LPL), lateral sole width (LSW), medial heel width (MHW), medial palmar length (MPL), medial sole width (MSW), and solar circumference (dashed line [SC]).

Citation: American Journal of Veterinary Research 70, 11; 10.2460/ajvr.70.11.1354

Assessment of capsule thickness—Magnetic resonance images of the right forelimb feet were obtained by use of equipmentd at the Ontario Veterinary College. Horses were anesthetized for MRI procedures. The imaging techniques and protocols were determined on the basis of a preliminary study that used feet from the cadavers of 6 horses and also 3 live horses. Each horse was positioned in right lateral recumbency, and the right forelimb was placed inside the MRI unit. Images along the sagittal and dorsal planes were obtained by use of T1-weighted, fast spoiled, gradient-recalled pulse sequences. A repetition time of 8.4 milliseconds, echo time of 3.6 milliseconds, and flip angle of 30° were used. Images were obtained within a 16-cm field of view with a 256 × 256-pixel matrix. A total of 64 slices, each 1.5 mm in thickness, were obtained with zero spacing. The same MRI sequences, procedures, and anesthetic protocol were used for all 18 horses.

Image enhancement was provided by use of a flexible array of 4 coils, which was custom built for the study. The array was wrapped around the hoof before images were obtained. Prior to obtaining MRI images, an L-shaped calibration gauge constructed from acrylic plastic was placed next to the hoof; the gauge was positioned identically for each hoof. To verify the linear calibration of the MRI images, 6 identical spherical holes within the acrylic plastic that had been filled with oil were used as calibration marks. A plastic tube (length, 15 cm; internal diameter, 2 mm) filled with mineral oil was wrapped around the coronary band as a landmark.

Images were analyzed by use of proprietary software.e Prior to obtaining the measurements, images were rotated and placed in the same plane to avoid measurement errors attributable to plane incongruity. Several measurements were obtained from dorsal and midsagittal MRI views. The views in which the width of the distal phalanx was greatest and in which the distance from the distal end to the extensor process of the distal phalanx was greatest were selected as the dorsal and midsagittal views, respectively, for morphometric analysis for each horse (Figure 2).

Figure 2—
Figure 2—

Representative MRI images of the right forelimb foot of a horse to illustrate measurements of morphometric variables of capsule thickness. A—Midsagittal view of the foot. For each horse, the sagittal plane image in which the distance from the distal end to extensor process (EP) of the distal phalanx was greatest was selected as the midsagittal image for morphometric analysis. Variables measured include wall thickness at midway (a point on the dorsal aspect of the foot midway along the dorsal length of the distal phalanx [WSM]), dermis thickness at midway (DSM), wall and dermis combined thickness at midway (WDSM), wall thickness at distal border (a point on the dorsal aspect of the foot at the distal border of the distal phalanx [WSD]), dermis thickness at distal border (DSD), and wall and dermis combined thickness at distal border (WDSD). The letter S in the variable abbreviations refers to the sagittal orientation of the image used to measure these variables. B—Dorsal view of the foot. For each horse, the dorsal plane image in which the width of the distal phalanx was greatest (bracket) was selected as the dorsal image for morphometric analysis. Variables include wall thickness at medial aspect (a point on the medial aspect of the foot at the distal border of the distal phalanx [WDM]), dermis thickness at medial aspect (DDM), wall and dermis combined thickness at medial aspect (WDDM), wall thickness at lateral aspect (a point at the lateral aspect of the foot at the distal border of the distal phalanx [WDL]), dermis thickness at lateral aspect (DDL), and wall and dermis combined thickness at lateral aspect (WDDL). The letter D at the second (WDM, DDM, WDL, and DDL) or third (WDDM and WDDL) letter position in the variable abbreviations refers to the dorsal orientation of the image used to measure these variables. White dots at the level of the coronet in both views represent calibration marks.

Citation: American Journal of Veterinary Research 70, 11; 10.2460/ajvr.70.11.1354

Measurements obtained from the midsagittal plane MRI views included the distance between the dorsal surface of the distal phalanx and the outer surface of the dorsal aspect of the foot and dermis thickness (ie, the distance from the dorsal surface of the distal phalanx to the inner surface of the dorsal aspect of the hoof wall). Measurements were obtained along a line perpendicular to the dorsal surface of the distal phalanx at 2 separate points. One point was midway along the dorsal length of the distal phalanx, and the other was at the distal border of the bone (Figure 2). From the dorsal plane MRI views, similar measurements were obtained at points on the medial and lateral sides of the distal border of the distal phalanx; the measurements were obtained along lines perpendicular to the medial and lateral surfaces of the bone. From these measurements, variables for capsule thickness, including wall thickness (mostly stratum medium) and dermis thickness (including the laminae), were calculated.

Statistical analysis—Data were analyzed by use of a general linear mixed model.f Data between the groups and within each group were compared by use of a quadratic model. To evaluate capsule shape and thickness, a completely randomized split-plot design was used with exercise effect as the whole-plot factor and month (month the study started or ended) as the split-plot factor; month was split within the factor for horse. The ANOVA assumptions were assessed via residual analyses; normality of residuals was examined by use of the 4 tests (Shapiro-Wilk, Kolmogorov-Smirnov, Cramer–von Mises, and Anderson-Darling) in the univariate procedure.g Residuals were plotted against the predicted values and explanatory variables in the model. Such plots can be used to assess equality of variances, reveal outliers, or detect any other patterns that suggest violation of the ANOVA assumptions. Data for some of the variables were transformed logarithmically to obtain a normal distribution. Values of variables between the control and exercise groups at the start and at the end of the study were compared, as were values at the start and end of the study within groups. For all statistical analyses, significance was set at a value of P < 0.05. The power of the ANOVAs for growth of the dorsal aspect of the hoof wall, toe angle, and wall thickness and dermis thickness at the distal border of the distal phalanx was determined; the power to detect 10% changes in means exceeded 99%.

A 2-sample t test was used to evaluate the differences between groups for growth of the dorsal and lateral aspects of the hoof wall. Data for growth of the medial aspect of the hoof wall were not normally distributed and were resistant to normalization via logarithmic transformation; therefore, growth of the medial aspect of the hoof wall was analyzed by use of a Wilcoxon–Mann-Whitney test.

With regard to capsule shape and thickness, data for each variable were examined for normality. Data for medial heel angle, length of the dorsal aspect of the sole, medial heel width, and width of the lateral aspect of the sole (Figure 1) were not normally distributed; therefore, they were logarithmically transformed to achieve normality. The main factors of group and month and the interactions between these factors were included in the model. When an interaction was not significant, it was removed from the model. The same procedure was used for all variables.

The Pearson product moment correlation coefficient was used to compute pairwise correlations between pairs of capsule shape variables and between pairs of capsule thickness variables. Pairwise correlations were also computed between capsule shape and capsule thickness variables. Data were evaluated for normality. For a few pairs of variables, both variables were not normally distributed and did not achieve normality via logarithmic transformation (pairwise correlations between combinations of medial heel angle, medial heel width, length of the dorsal aspect of the sole, width of the lateral aspect of the sole, and wall thickness and dermis thickness of the medial aspect of the foot). These variable pairs were evaluated via the Spearman rank correlation coefficient rather than the Pearson product moment correlation. All pairwise correlations were evaluated at the beginning (June) and end (October) of the study in both groups. On the basis of the sample size, correlation coefficients > 0.666 and values of P < 0.05 were considered significant.

Results

Growth of the hoof wall—At any of the 3 sites evaluated (dorsal [toe], medial, and lateral aspects), there were no significant differences in growth of the hoof wall between the control and exercise groups. However, there were considerable differences in growth at the toe and at the medial and lateral aspects of the hoof wall among individual horses. Mean ± SD values for growth of the hoof wall in all 18 horses during the study period were 2.58 ± 0.195 cm at the toe, 3.79 ± 0.225 cm at the medial aspect, and 3.67 ± 0.265 cm at the lateral aspect. The mean growth rate of the hoof wall in all 18 horses was 1.52 mm/wk (6.5 mm/mo) at the toe. Most values for the ratio of toe growth to toe length were 25% to 30% (range, 21% to 42%).

Capsule shape—Differences between the control and exercise groups for capsule shape variables were not significant (Table 1). However, between the beginning and the end of the study, the exercise group had more changes for these variables than the control group. Variables for the length of the hoof wall (lengths of the medial and lateral aspects of the hoof wall, toe length, medial heel length, and lateral heel length) increased over time in both groups; however, the changes in the exercise group were greater than the changes in the control group. For most of the hoof variables, the SDs in the exercise group decreased to a greater extent than did the SDs in the control group between the beginning and the end of the study.

Table 1—

Mean ± SD values of capsule shape variables in the right forelimb feet of 18 Standardbreds before and after the horses were exercised at approximately 6 m/s (4,200 to 5,600 m/d) on 4 d/wk for 17 weeks (exercise group; n = 9) or were not exercised (control group; 9).

VariableControl groupExercise groupP value
Start of studyEnd of studyStart of studyEnd of studyStart vs end of study*Control group vs exercise group
Hoof wall
   LL (cm)6.1 ± 0.316.4 ± 0.576.2 ± 0.476.4 ± 0.770.0810.904
   ML (cm)6.2 ± 0.416.3 ± 0.486.1 ± 0.686.5 ± 0.740.0550.955
   TL (cm)8.5 ± 0.388.8 ± 0.758.5 ± 0.519.2 ± 0.680.0020.442
   LHL (cm)3.7 ± 0.774.0 ± 0.863.6 ± 1.024.3 ± 0.550.0620.709
   MHL (cm)3.7 ± 0.654.2 ± 0.453.5 ± 0.574.4 ± 0.33< 0.0010.886
   LA (°)78.7 ± 4.9278.2 ± 3.6976.6 ± 3.9978.7 ± 2.430.3920.628
   MA (°)82.4 ± 2.9779.3 ± 2.2279.6 ± 5.3278.1 ± 2.510.0190.164
   TA (°)52.6 ± 3.3656.0 ± 2.0651.5 ± 2.3954.3 ± 1.23< 0.0010.120
   LHA (°)46.7 ± 6.8753.4 ± 5.5146.7 ± 7.9747.8 ± 4.850.0850.210
   MHA (°)46.5 ± 3.3848.5 ± 5.9845.8 ± 6.8747.5 ± 5.310.0150.403
Sole
   DL (cm)5.0 ± 0.275.0 ± 1.135.0 ± 0.694.7 ± 0.570.4690.557
   LPL (cm)8.1 ± 0.597.8 ± 0.988.1 ± 0.577.5 ± 0.600.0390.615
   MPL (cm)8.1 ± 0.747.8 ± 0.867.8 ± 0.357.5 ± 0.700.1900.276
   SC (cm)35.4 ± 1.2433.9 ± 2.2936.0 ± 1.9934.7 ± 2.240.0010.434
   LHW (cm)2.9 ± 0.503.1 ± 0.572.9 ± 0.553.2 ± 0.420.0160.639
   MHW (cm)3.0 ± 0.603.3 ± 0.803.2 ± 0.763.0 ± 0.530.3970.985
   LSW (cm)6.1 ± 0.546.0 ± 0.746.6 ± 0.426.2 ± 0.320.0050.233
   MSW (cm)6.0 ± 0.455.8 ± 0.666.3 ± 0.686.0 ± 0.320.0330.240

All horses were kept in a large (200 × 200-m) paddock throughout the study. The study was conducted during the months of June to October.

Represents differences in values between the start and the end of the study from all 18 horses.

Represents differences in values between the control group and the exercise group from both time points.

Between groups, values for the indicated comparisons are significantly (P < 0.05) different.

DL = Dorsal sole length. LA = Lateral wall angle. LHA = Lateral heel angle. LHL = Lateral heel length. LHW = Lateral heel width. LL = Lateral wall length. LPL = Lateral palmar length. LSW = Lateral sole width. MA = Medial wall angle. MHA = Medial heel angle. MHL = Medial heel length. MHW = Medial heel width. ML = Medial wall length. MPL = Medial palmar length. MSW = Medial sole width. SC = Solar circumference. TA = Toe angle. TL = Toe length.

Although the differences in capsule shape variables between the control and exercise groups were not significant, some variables changed significantly between the beginning and the end of the study (Table 1). Among hoof wall variables, medial heel length, medial heel angle, toe angle, and toe length increased significantly, whereas the angle of the medial aspect of the hoof wall decreased significantly between the beginning and the end of the study; no changes in variables for the lateral aspect of the hoof wall were detected. Among solar variables, length of the lateral palmar aspect, widths of the medial and lateral aspects, and circumference of the sole significantly decreased, whereas only lateral heel width significantly increased between the beginning and the end of the study; these changes (except for the width of the medial aspect of the sole) developed on the lateral side of the sole. Of the variables that changed significantly between the beginning and the end of the study, hoof wall variables (except for the angle of the medial aspect of the hoof wall) increased, whereas solar variables (except for lateral heel width) decreased.

Capsule thickness—Differences in capsule thickness variables between the control and exercise groups were not significant (Table 2). However, 3 variables (dorsal hoof wall thickness measured midway along the length of the distal phalanx, dorsal hoof wall thickness measured at the distal border of the distal phalanx, and dorsal dermis thickness measured at the distal border of the distal phalanx) changed significantly between the beginning and the end of the study in both groups. Changes in wall thickness of the dorsal aspect of the foot were identified in both groups between the beginning and the end of the study; wall thickness slightly increased at the point midway along the length of the distal phalanx but slightly decreased at the distal border of the bone. In contrast, dermis thickness decreased (albeit not significantly) at the point midway along the length of the distal phalanx but increased at the distal border of the bone between the beginning and the end of the study.

Table 2—

Mean ± SD values of capsule thickness variables in the right forelimb feet of 18 Standardbreds before and after the horses were exercised at approximately 6 m/s (4,200 to 5,600 m/d) on 4 d/wk for 17 weeks (exercise group; n = 9) or were not exercised (control group; 9).

VariableControl groupExercise groupP value
Start of studyEnd of studyStart of studyEnd of studyStart vs end of study*Control group vs exercise group
WSM (cm)9.7 ± 1.4210.0 ± 1.579.5 ± 0.5610.0 ± 10.950.0420.967
DSM (cm)6.1 ± 0.536.0 ± 0.366.0 ± 0.605.8 ± 0.560.0860.666
WDSM (cm)15.8 ± 1.4915.9 ± 1.3815.5 ± 0.9415.9 ± 1.400.1540.842
WSD (cm)9.7 ± 0.989.0 ± 1.119.3 ± 0.958.9 ± 0.970.0080.527
DSD (cm)6.8 ± 1.007.2 ± 1.026.2 ± 0.886.6 ± 0.810.0040.182
WDSD (cm)16.4 ± 1.2316.2 ± 1.6415.4 ± 1.4115.5 ± 1.270.6730.185
WDL (cm)8.5 ± 0.718.5 ± 1.148.8 ± 0.738.7 ± 1.120.7770.530
DDL (cm)6.5 ± 0.986.6 ± 0.616.7 ± 0.986.8 ± 0.990.5800.611
WDDL (cm)15.0 ± 1.4615.1 ± 1.5015.4 ± 1.3715.5 ± 1.410.8420.476
WDM (cm)8.4 ± 0.988.2 ± 0.998.3 ± 1.118.7 ± 1.320.7380.993
DDM (cm)7.1 ± 0.846.9 ± 0.917.1 ± 0.686.7 ± 1.630.4390.849
WDDM (cm)15.5 ± 1.7015.1 ± 1.8515.4 ± 1.5615.3 ± 1.110.5290.906

All horses were kept in a large (200 × 200-m) paddock throughout the study.

DDL = Dermis thickness at lateral aspect. DDM = Dermis thickness at medial aspect. DSD = Dermis thickness at distal border. DSM = Dermis thickness at midway. WDDL = Combined wall and dermis thickness at lateral aspect. WDL = Wall thickness at lateral aspect (a point on the lateral aspect of the foot at the distal border of the distal phalanx measured on a dorsal plane MRI view). WDDM = Combined wall and dermis thickness at medial aspect. WDM = Wall thickness at medial aspect (a point on the medial aspect of the foot at the distal border of the distal phalanx measured on a dorsal plane MRI view). WDSD = Combined wall and dermis thickness at distal border. WDSM = Combined wall and dermis thickness at midway. WSD = Wall thickness at distal border (a point on the dorsal aspect of the foot at the distal border of the distal phalanx measured on a sagittal plane MRI view). WSM = Wall thickness at midway (a point on the dorsal aspect of the foot midway along the dorsal length of the distal phalanx measured on a sagittal plane MRI view).

See Table 1 for remainder of key.

Pairwise correlations between pairs of capsule shape variables—With the exception of circumference of the sole, there were few correlations between solar and hoof wall variables (Table 3). This pattern was more apparent in the correlations between solar (except for circumference of the sole) and heel (medial and lateral heel lengths and angles) variables. Among the correlations that were identified, those between 2 hoof wall variables or between 2 angles of the hoof wall were positive, whereas those between a hoof wall variable and the angle at the lateral or medial aspect of the hoof wall were negative. Regarding correlations that included a heel angle variable, there were more changes in correlations that included the lateral heel angle than there were in those that included the medial heel angle. The patterns for correlations that included toe angle did not change greatly between the beginning and end of the study; however, the patterns for correlations that included toe length did change, primarily in the exercise group (Table 3).

Table 3—

Correlations between pairs of capsule shape variables assessed in the right forelimb feet of 18 Standardbreds before and after the horses were exercised at approximately 6 m/s (4,200 to 5,600 m/d) on 4 d/wk for 17 weeks (exercise group; n = 9) or were not exercised (control group; 9).

Pairwise correlationControl groupExercise group
Start of studyEnd of studyStart of studyEnd of study
TA-LHANYNN
TA-MHLNNNY
TA-MHWNNNY (−)
MA-LAYYNN
MA-LLNNY (−)N
MA-MLNY (−)NN
MA-LHWY (−)NNN
MA-LSWY (−)NNN
LA-TLNNNY (−)
LA-MLNNNY (−)
LA-LLNYNY
LA-MHLYNNN
LA-MHWNNY (−)N
LA-MSWNNNY (−)
MHA-LHANYNN
MHA-MHLNNNY (−)
MHA-DLNY (−)NN
LHA-TLNNY (−)N
LHA-MLNNY (−)N
LHA-LLNNY (−)N
LHA-MHLNNY (−)N
LHA-LHLNNY (−)N
LHA-SCNNY (−)Y (−)
TL-MLYNYY
TL-LLYNYY
TL-MHLNNYN
TL-LHLNNYN
TL-SCNNYN
ML-LLYYYY
ML-LHLNNYN
LL-MSWNNYN
MHL-LHLYNNN
MHL-DLNNYY
MHL-SCNNYN
LHL-DLNNYN
LHL-SCNNYN
DL-LSWNNNY
DL-SCNNYN
MPL-LPLNYNY
MPL-LHWYNNN
MPL-SCYNNN
LPL-MHWNYNN
LPL-MSWNYNN
LPL-SCYNNN
MHW-LHWNNYY
MHW-MSWYNNN
LHW-MSWYNNN
LHW-LSWYYNN
MSW-LSWYYYY
MSW-SCYYYY
LSW-SCNYNN

All horses were kept in a large (200 × 200-m) paddock throughout the study.

N = Correlation was not detected. Y = Correlation with coefficient > 0.666 (P < 0.05) was detected. (−) = Negative correlation.

See Table 1 for remainder of key.

The total number of correlations of pairs of capsule shape variables in the exercise group exceeded those in the control group at both the beginning and end of the study (Table 4). Between the groups, the patterns of correlations differed, as additional correlations were detected and initial ones were eliminated between the beginning and the end of the study. The difference in number of correlations that were altered over time between the 2 groups was mostly because of changes in the number of correlations that were between 2 hoof wall (wall-wall) variables and between hoof wall and solar (wall-sole) variables.

Table 4—

Number of correlations between pairs of capsule shape variables, between pairs of capsule thickness variables, and between capsule shape and capsule thickness variables in the right forelimb feet of 18 Standardbreds before and after the horses were exercised at approximately 6 m/s (4,200 to 5,600 m/d) on 4 d/wk for 17 weeks (exercise group; n = 9) or were not exercised (control group; 9).

Pairwise correlationCapsule shape vs capsule shapeCapsule thickness vs capsule thicknessCapsule shape vs capsule thickness
Control groupExercise groupControl groupExercise groupControl groupExercise group
No. at start of study162462717
No. at end of study1417251315
No. of initial correlations eliminated at the end of the study111641515
No. of additional correlations detected at the end of the study99041115

All horses were kept in a large (200 × 200-m) paddock throughout the study.

Pairwise correlations between pairs of capsule thickness variables—Between the beginning and the end of the study, the number of correlations between pairs of capsule thickness variables decreased from 6 to 2 in the control group, whereas the number increased from 2 to 5 in the exercise group (Table 4). Correlations that included a variable for wall thickness and one for dermis thickness differed between the groups. Also, there were more changes to correlations for variables of the medial aspect of the foot than there were to those for variables of the lateral aspect of the foot (Table 5).

Table 5—

Correlations between pairs of capsule thickness variables assessed in the right forelimb feet of 18 Standardbreds before and after the horses were exercised at approximately 6 m/s (4,200 to 5,600 m/d) on 4 d/wk for 17 weeks (exercise group; n = 9) or were not exercised (control group; 9).

Pairwise correlationControl groupExercise group
Start of studyEnd of studyStart of studyEnd of study
DDL-DDMYNYY
DDM-DSDNNNY
DDM-WDLYNNN
DDM-WDMYYNY (−)
DSD-DSMYNNN
DSM-WSMNNNY
WDL-WSMNNNY
WDM-WDLYNNN
WSD-WSMYYYN

All horses were kept in a large (200 × 200-m) paddock throughout the study.

See Tables 2 and 3 for remainder of key.

Pairwise correlations between capsule shape and capsule thickness variables—With few exceptions, the capsule thickness variables were not correlated with capsule shape variables related to the toe (toe angle and toe length) and heel (lateral heel length, medial heel length, lateral heel angle, and medial heel angle). Correlations that were identified between wall thickness or dermis thickness and lengths of the medial or lateral aspects of the hoof wall were positive (Table 5). Correlations that were identified between dermis thickness and the angles of the medial or lateral aspects of the hoof wall were negative. Among capsule shape variables, the solar variables for widths of the medial and lateral aspects and circumference of the sole had more correlations with capsule thickness variables. Between the beginning and the end of the study in the exercise group, the number of correlations between capsule shape and capsule thickness variables related to the hoof wall decreased from 9 to 5, and the number between capsule shape variables related to the sole and capsule thickness variables related to the hoof wall decreased from 6 to 2 (Table 4). In the control group, the numbers of the corresponding correlations did not change.

Table 6—

Correlations between variables for capsule shape (hoof wall and sole) and capsule thickness assessed in the right forelimb feet of 18 Standardbreds before and after the horses were exercised at approximately 6 m/s (4,200 to 5,600 m/d) on 4 d/wk for 17 weeks (exercise group; n = 9) or were not exercised (control group; 9).

Pairwise correlationControl groupExercise group
Start of studyEnd of studyStart of studyEnd of study
Hoof wall vs capsule thickness at the dorsal aspect (toe) of the hoof
   LL-DSDNNYN
   LL-WSDNYNN
   LL-WSMNYNN
   LHL-WSMNNYN
   LHL-WSDNNYN
   ML-WSMNNNY
   MHL-DSMNYNN
   TL-DSMNNNY
   TL-WSDNYNN
   TL-WSMNNNY
Sole vs capsule thickness at the dorsal aspect (toe) of the hoof
   DL-DSDNNYN
   DL-WSDNNYN
Hoof wall vs capsule thickness at the lateral or medial aspect of the hoof
   LA-DDLY (−)NNY (−)
   LA-DDMNNNY (−)
Sole vs capsule thickness at the lateral or medial aspect of the hoof
   LHW-DDLNYNN
   LHW-WDLNYNN

All horses were kept in a large (200 × 200-m) paddock throughout the study.

See Tables 1, 2, and 3 for remainder of key.

Discussion

The purpose of the present study was to evaluate, via quantification of changes in shape and gross anatomic structures, the ability of equine hooves to respond to repetitive loading during a period of 17 weeks. Of the many subtle changes in the morphometric variables detected between the beginning and the end of the study, 2 were potentially induced by exercise: the reduction in amount of variation in values of capsule shape variables in the exercise group and the differences in the patterns of pairwise correlations between the control and exercise groups. These changes indicated a small response at the gross level of structure. This response was smaller than expected, given the magnitude of the stimulus and the empirically observed plasticity of hooves. The stimulus per individual horse ranged from a total of 21,000 to 81,000 strides that were performed on a rigid surface during exercise sessions 4 d/wk for a period of 17 weeks.

For the purposes of our study, we proposed that assessment of growth of the hoof wall could be used to identify a relationship between exercise and response of the hooves. It was speculated that growth of the hoof wall would be accelerated by increased blood flow, and growth was expected to be higher in the exercise group. Accelerated growth could have changed the hoof shape; however, growth of the hoof wall and hoof shape variables did not change markedly between the beginning and the end of the study.

Among the factors that may influence biological responses in hooves, composition (largely dead keratinized cells) and restricted blood flow of the hoof wall have primary importance. The highly keratinized tissue of the hoof wall may need more time and stronger stimuli to respond to changes in its environment, compared with the duration and strength of stimuli required for osseous and muscular tissues to respond. Similar to other epidermal tissues, the hoof wall is an avascular structure that depends on blood flow through the dermal tissues for nutrition. Therefore, restricted blood flow may also decrease the response of hoof wall cells to stimuli. The dorsal aspect (toe) of the hoof wall grew by approximately 30% of its initial length during the 17 weeks of the present study. An increase in the duration of the study may have provided more definitive results.

The hoof wall at the toe grew slightly faster in the exercise group than in the control group, but this difference was not significant. The mean growth rate in the study reported here was 6.5 mm/mo, which is similar to the rate of 6 mm/mo reported by Kainer9 and slightly less than the rate of 8 to 10 mm/mo reported by Pollitt.10 Differences between the rates in the 3 studies may be attributable to the season of the year and breeds of horses; many studies do not report the climate and season of the year when the study was performed or breeds of horses evaluated. The shape and the length of hooves of semiferal ponies adapt to the environment.11 For horses kept on pasture during summer (wet season), lush forage appears to promote hoof growth and a soft substrate retards wear; therefore, the hoof wall at the toe becomes longer during summer, compared with its length at other times of the year.11 In the present study, all horses were kept in the same paddock; nutrition and conditions of temperature and humidity were similar for both groups. However, growth of the hoof wall was different among individual horses. In addition to nutritional and environmental factors, other factors such as genetics could be influential.

Values of capsule shape variables had less variation in the exercise group, compared with these values in the control group. Rasping of the hooves during shoe removal by the farrier at the end of the study may have interfered with the ability to detect slight differences between the groups. However, both groups had changes in values of capsule shape variables over time. In general, most of the hoof wall variables increased, whereas most of solar variables decreased. These changes indicated the dynamic structure of hooves and their ability to change but did not strongly reflect a response to the applied exercise.

In Thoroughbreds kept on pasture, hoof angle increases during summer and decreases during winter, compared with changes at other times of the year; however, these changes are not detected in show horses that are kept in stalls throughout the year.12 The authors of that report concluded that a high level of atmospheric moisture (frequent rain) during winter was causally linked to a decrease in hoof angle in free-ranging horses. Hoof angle in Thoroughbred racehorses undergoing a regimen of fast exercise (galloping) with an extended period of rest (no exercise) was also evaluated.12 Fast exercise was associated with a decrease in the hoof angle during the training period, whereas during the resting period, hoof angle increased. In the present study, mild exercise (medium-speed trotting) in Standardbreds did not significantly affect hoof angle; the influence of exercise intensity, breed of horse, stride pattern, and direction of exercise path around a racetrack on capsule shape needs further investigation.

Some capsule thickness variables changed over time. Variables for hoof wall thickness changed more at the dorsal aspect (toe) than at the medial and lateral aspects of the foot, which could be related to a higher concentration of stress at the toe. Alternatively, laminar morphology is more consistent at the toe, and it becomes more variable abaxially.7

Wall thickness and dermis thickness changed in a manner such that the combined thickness of the wall and dermis did not change significantly. Although the wall consists of keratinized tissue, detection of changes in the hoof wall provided evidence for potential remodeling in the keratinized tissues of the strata medium and internum. Results from in vitro studies13–15 suggest that laminae are capable of responding to changes in mechanical loading. The MRI resolution was not sufficient for studying the detailed morphology of the laminae in the present study. Technical improvements in MRI technology may in the future enable study of laminar morphology in live horses. However, evaluation of the capsule thickness variables that were chosen for the present study revealed changes in capsule thickness and the potential of the hoof capsule to remodel and respond to loading.

We previously reported6 that gross changes in response to exercise for variables of the distal phalanx were small and most were nonsignificant. Evaluation of radiographic images of distal phalanges in 103 Thoroughbred racehorses revealed no significant differences between left and right distal phalanges.16 In the present study, only right forelimb hooves were examined. Previous studies3,5 to evaluate the responses of other bones of equine limbs to exercise or forced rest revealed complex responses at gross and microstructural levels. In humans17 and horses,3,4,18–20 confinement and immobilization induced changes in bone resorption and formation, with the rate of bone resorption greater than the rate of bone formation. A study5 that compared the effects of stall rest versus forced exercise on metacarpal bones in horses revealed increased bone mineralization in the exercised group, compared with the amount of mineralization in the rested group. Another study19 in horses revealed that 26 weeks of exercise did not induce a significant difference in bone mineralization content; in that study, all horses were housed in a large paddock. Presumably, allowing horses in the control group to walk and run freely prevented the reduction in mineralization that was detected in stall-confined horses in other studies.5 Because the horses were not euthanatized at the end of the present study, any differences in bone density and mineralization between groups were not examined. The freedom of movement for the control horses might have reduced the differences in morphometric variables between the 2 groups.

In the present study, the number of the correlations between pairs of capsule shape variables changed in both groups over time. However, in the exercise group, there were twice as many correlations that changed between the start and the end of the study (ie, additional correlations detected or initial correlations eliminated), compared with the number of correlations that changed in the control group. It appeared that different variables for hoof shape respond differently to exercise.

The pattern of correlations between pairs of capsule thickness variables changed in both groups over time. Compared with the number of correlations detected in each group at the beginning of the study, twice as many correlations were detected at the end of the study in the exercise group and a third as many correlations were detected at the end of the study in the control group. Most of the correlations that changed included variables related to hoof wall thickness. It appeared that exercise altered the wall thickness variables in a similar manner; therefore, the number of detectable correlations increased in the exercise group and decreased in the control group over time.

There were more correlations between variables for dermis thickness than there were between variables for wall thickness. This may indicate an increased likelihood for remodeling in soft tissues of the dermis, compared with the likelihood for remodeling in the highly keratinized hoof wall. One reason for a difference in remodeling could be comparatively more blood circulation and less keratinized tissue in the dermis. In general, dermis thickness was correlated between the medial and lateral aspects of the foot; however, wall thicknesses in these areas were not correlated. This could be attributable to a difference in the stimulus to the medial and lateral aspects of the foot.

The pattern of correlations between capsule shape (hoof wall and sole) variables and capsule thickness variables changed in both groups over time. However, the nature of these changes differed between the groups. It appeared that over time, more correlations were detected between variables for capsule thickness and the hoof wall but fewer correlations were detected between variables for capsule thickness and the sole in horses that were exercised. Presumably, exercise had similar effects on variables for capsule thickness and the hoof wall but had dissimilar effects on variables for capsule thickness and the sole.

Overall, for correlations between capsule shape variables and capsule thickness variables, the number of changes between the start and the end of the study (ie, additional correlations detected or initial correlations eliminated) in the exercise group was twice that in the control group. It may be assumed that exercise affected the variables for capsule shape and capsule thickness in a similar manner; therefore, a marked number of changes in the exercise group were detected.

The findings of the study reported here indicated that over time, some morphologic changes in the hooves developed in horses that were or were not exercised but there were few differences between the groups. Further investigation is needed to determine whether exercising horses for an increased duration or with increased intensity could induce marked changes in the values of the morphometric variables for the hooves. The different alterations in the correlation patterns between pairs of variables for each group during the study indicated that exercise induced some morphologic changes over time, even though the stimulus was not sufficiently great to significantly affect values of the morphometric variables for the hooves. Although the patterns of correlations differed between the groups, this finding did not confirm adaptive remodeling of the hoof wall in response to applied stress; it did, however, provide evidence that such adaptive changes develop. A better understanding of how exercise interacts with the biomechanics and anatomic characteristics of hooves will aid in the prevention and treatment of lameness and musculoskeletal injuries that develop at the distal portions of limbs.

ABBREVIATION

MRI

Magnetic resonance imaging

a.

Faramarzi B. Morphology of the equine laminar junction correlated with external anatomy of the hoof. MSc thesis, Department of Biomedical Sciences, University of Guelph, Guelph, ON, Canada, 2003.

b.

Panasonic VDR-M30 digital camera, Panasonic Consumer Electronics Co, Elgin, Ill.

c.

Optimas 6.5 image analysis software, BioScan Inc, Edmonds, Wash.

d.

1.5 Tesla whole-body scanner, General Electric Healthcare, Milwaukee, Wis.

e.

Volume Viewer Software, GE Advantage Windows Workstation Software 4.2, General Electric Healthcare, Milwaukee, Wis.

f.

Proc Mixed, SAS, version 9.1.3, SAS Institute Inc, Cary, NC.

g.

Proc Univariate, SAS, version 9.1.3, SAS Institute Inc, Cary, NC.

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Contributor Notes

Dr. Faramarzi's present address is College of Veterinary Medicine, Western University of Health Sciences, Pomona, CA 91766.

Supported by the Ontario Horse Racing Industry Association and Ontario Veterinary College.

The authors thank Drs. Howard Dobson, Antonio Cruz, and Sukhpal Singh and Norman Konyer, Robert Cook, Warren Bignell, and Alice Daw for technical assistance.

Address correspondence to Dr. Faramarzi (bfaramarzi@westernu.edu).