Abstract
Summary
Background:
Lameness, discipline, training intensity, environmental variability, and shoeing are all factors demonstrated to affect hoof loading and therefore act as adaptive stimuli to alter hoof morphology.
Objectives:
To evaluate effect of age at training initiation on hoof morphology and lameness incidence and determine if specific hoof morphology measurements correlate with lameness in juvenile American Quarter Horses.
Study design:
Prospective cohort study.
Methods:
American Quarter Horses (n=42; 29 2-year-olds, 13 3-year-olds) entering training were monitored for hoof morphology and lameness over six months (months 0,2,4,6). Hoof measurements (palmar/plantar angles, frog base width/length, toe length/angle, heel length/angle, heel and foot width, wall height/angle) from radiographs and photographs were recorded. Lameness was graded subjectively and objectively (Lameness locator®). Statistical analyses were performed with Fisher’s exact test and repeated measures ANOVA with P < 0.05.
Results:
25/42 horses developed sub-clinical lameness (16/42 forelimb, 19/42 hindlimb), with three-year-olds developing lameness more frequently compared to two-year-olds overall (p=0.04; 84.6 vs. 48.3%) and in forelimbs (p=0.05; 61.5% vs. 27.6%); no difference was noted between two- versus three-year-olds in hindlimbs (p=0.2; 61.5% vs. 37.9%). In lame versus sound forelimbs, three-year-olds had decreased foot width (p=0.03; 11.48cm (CI 10.68–12.28) vs. 12.21cm (CI 11.99–12.42)), decreased toe length (p=0.03; 6.02cm (CI 5.69–6.36) vs. 6.45cm (CI 6.32–6.58)), shorter lateral wall height (p=0.03; 4.64cm (CI 4.31–4.96) vs. 5.11cm (CI 5.03–5.2)), and shorter medial wall height (p=0.02; 4.58cm (CI 4.06–5.10) vs. 5.15cm (CI 4.99–5.30)).
In lame versus sound hindlimbs, horses overall (p = 0.05; 3.74, CI 3.53–3.96 vs 3.55, CI 3.48–3.61) and three-year-olds had longer heels p=0.01; 3.90cm (CI 3.5–4.3) vs. 3.50cm (CI 3.39–3.61)).
Main limitations:
Small sample size, lack of control group not entering training.
Conclusions:
Three-year-old American Quarter Horses entering training were more likely to develop forelimb lameness than two-year-olds. This sub-clinical lameness was associated with specific hoof morphology characteristics (decreased foot width, toe length, heel length, and lateral/medial wall height; greater toe angle).
Keywords: horse, lameness, hoof conformation, training
Introduction
Lameness originating from the foot has been reported to be the most common source of lameness in horses 1,2. Furthermore, hoof size, shape and balance have been identified as risk factors for severe musculoskeletal injuries in other locations 3–6. Hoof morphology impacts whole limb soundness by influencing limb motion and forces placed on other structures (i.e. tendons, ligaments) within a limb 3,4. Recently, a long-toed, low-heeled morphology and negative/neutral plantar angle have been associated with back pain and hindlimb lameness localised to the distal tarsal joints, proximal metatarsal region, and stifles in different sample populations 5–7. While equine veterinarians have begun to recognise the role that internal and external hoof characteristics play in lameness, this remains a component of therapy that may be over-looked when lameness is diagnosed.
A number of factors have been identified that influence hoof size and shape, including lameness, athletic discipline, training intensity, environment, and shoeing, which all affect loading of the hoof, and therefore act as adaptive stimuli to affect hoof growth and morphology 7–11. Furthermore, a significant body of evidence has supported the effect that hoof morphology and stance, which may be altered with shoeing, have on other structures proximal to the hoof 12–24. To date, there has not been an investigation examining the effect of initiation of training on hoof conformation and lameness incidence in juvenile horses not intended for racing, or whether age at training initiation affects hoof morphology and lameness incidence. As many American Quarter Horses begin training at two or three years of age to compete in aged events, this is an important group to study to better understand the effect of training and age on hoof morphological characteristics. Improved understanding of how the foot adapts to exercise may help equine veterinarians identify hoof-related factors that may result in lameness in young horses entering training.
Therefore, the overall objective of this study was to evaluate the effect of initiation of training on hoof characteristics and lameness over a six-month period in young American Quarter Horses. As a sub-aim, the influence of age (two- versus three-year-olds) at training initiation on lameness incidence and hoof morphology measurements was investigated. We hypothesised that lameness incidence would correlate with changes in hoof morphology and that lameness incidence and changes in hoof measurements would be seen more commonly in two- versus three-year-olds.
Materials and Methods
Power Analysis
Based on preliminary data in a group of ten American Quarter Horses entering training (seven two-year-olds, three three-year-olds), when comparing the change in toe angle over four to five months, we anticipated that the two-year-old horses would have an increase in toe angle of 2.3° and the three-year-old horses to have an increase in toe angle of 0.3° with a common standard deviation of 2°. To obtain power of 80% with alpha 0.05, 17 horses would be needed per group. Therefore, the goal was to recruit 20 horses per group, which would provide power of 0.87.
Horses
Fifty-three American Quarter Horses (35 two-year-olds, 18 three-year-olds; 18 geldings, 35 fillies) enrolled in the colt training program in the Equine Sciences Department at Colorado State University and all were studied over a six-month period following initiation of training over two consecutive years (October 2019 through April 2021). These young horses were bred for western performance or ranch work. At study initiation, they were approximately 1.5 or 2.5 years of age, entering their two-year-old or three-year-old season. In year one, data collection was initiated in 29 horses (16 two-year-olds and 13 three-year-olds; 10 geldings, 19 mares); in year two, 24 horses (19 two-year-olds and five three-year-olds; eight geldings and 16 fillies) were enrolled. The study overview is summarised in Figure 1.
Figure 1:
Flow diagram illustrating study methods. Fifty-three American Quarter Horses (35 two-year-olds, 18 three-year-olds; 18 geldings, 35 fillies) enrolled in the colt training program in the Equine Sciences Department at Colorado State University were studied over a six-month period following initiation of training over two consecutive years (October 2019 through April 2021). In year one, data collection was initiated in 29 horses (16 two-year-olds and 13 three-year-olds; 10 geldings, 19 mares); in year two, 24 horses (19 two-year-olds and five three-year-olds; eight geldings and 16 fillies) were enrolled. Horses were assessed for hoof morphology and lameness over six months (months 0, 2, 4, 6). At each time point, lameness was graded subjectively and objectively (inertial sensor system) and hoof measurements were recorded from radiographs and photographs. Overall, 42 horses completed the six-month study (29 two-year-olds and 13 three-year-olds).
Horse management
All horses were housed in outdoor pens for the first four months of training (October through January) with six to eight horses per pen, and then horses were stalled separately for the final three months (February to April). All horses had a two to three-week acclimation period prior to initiation of the study. Horses were maintained under the care of a single trainer throughout the study duration. All horses had routine farrier work on a six-week interval and were not shod for the duration of the study; the same farrier trimmed all horses. Horses were maintained without shoes so as not to allow shoes to influence hoof growth and to mirror industry standard for western performance horses at two to three years of age. A weekly exercise training log was kept for each horse during the study period, recording type of exercise (ground versus ridden work), number of days per week, amount, and intensity level.
Study design
Horses were assessed for lameness and allowed to continue training if they were not greater than 2/5 lame on the AAEP scale. Horses were examined for lameness and photographic images of the hooves and distal limbs were performed at four approximately 60-day intervals (months 0, 2, 4, 6). These evaluations were scheduled to be one to two weeks after they received routine farrier work. At each time point, horse heights were measured at the withers and recorded.
Musculoskeletal examination and lameness evaluation
Horses were subjectively assessed for lameness at the walk and trot over an asphalt surface in a straight line in hand, and objective lameness evaluation was performed using an inertial sensor system (Lameness locator®, Equinosis). Objective values of head movement (HDMax, HDMin, and vector sum [VS]) and pelvic movement (PDMax, PDMin, and vector sum [HS]) asymmetry were recorded. Lameness evaluations were video recorded, and each of the four limbs was assigned a subjective lameness score using the AAEP lameness scoring system (0 to 5 scale) by three blinded observers (LP, VM, LB) with experience in equine lameness evaluation and specialty board-certification (two with DACVS-LA, one with DABVP) which were averaged to achieve a final overall subjective lameness score.
Each limb (fore and hind) was determined to be either lame or sound at each time point based on VS scores (> ±17 mm for forelimbs, > ±8 mm for hindlimbs), which corresponded to approximately a grade 2/5 subjective lameness score (AAEP scale). Briefly, the threshold VS score for forelimb lameness is 8.5 mm (no reported score for HS exists). To calculate these values, the square root of (MaxDiff2 + MinDiff2) was taken using the lameness threshold values of Max/Min Diff values of ±6 mm for forelimbs and ±3 mm for hindlimbs. These values were doubled for analysis to identify horses with more than mild lameness (i.e. 17mm and 8 mm).
Distal limb photographs
Digital limb photographs at all four time points were obtained with horses standing squarely on four 3” wooden hoof blocks. Lateral, dorsal, and solar digital photographs of all four feet were acquired to assess sagittal and medial-lateral hoof balance, toe and heel angles, toe and heel lengths, lateral and medial wall heights, width at the widest part of the foot, length of foot on the weightbearing surface (‘weightbearing length’ determined on lateral image), and frog base width and length (Figure 2). A ruler was included in each photograph so that measurements could be compared across all time points. All photographs were uploaded to a computer so that measurements could be performed digitally using commercial software (IC Measure, The Imaging Source, LLC). Digital measurements were calibrated to the ruler in each image.
Figure 2:
Gross measurements obtained from photographs of the hoof. a) Lateral hoof photograph. Measurements include A) toe length (cm), B) toe angle (°), C) heel length (cm), D) heel angle (°). b) Dorsal hoof photograph. The lateral side is indicated with ‘L.’ Measurements are A) medial wall length (cm), B) medial wall angle (cm), C) lateral wall angle (cm), D) lateral wall length (cm). c) Solar hoof photograph. Measurements include A) foot length (cm), B) foot width at the widest part of the hoof (cm), C) heel width (cm), D) frog length (cm), E) frog width at base (cm).
Radiographs
Lateromedial (LM) and dorsopalmar/plantar (DP) radiographic projections were obtained of both fore and hind hooves at all four time points. Horses were asked to stand squarely with all four hooves on wooden blocks and the primary radiographic beam was centred approximately 1 cm above the ground (approximate level of the solar surface of the distal phalanx). The generator-plate distance was standardised to 61 cm. On the lateromedial radiograph, the palmar or plantar angle of the distal phalanx (PaADP or PlADP, respectively) was measured and recorded. On the dorsopalmar/plantar radiographs, medial to lateral balance was assessed. The angle of the distal phalanx and distance from the distal phalanx to the ground medially and laterally were determined using commercial software for on-screen measurements of angles and distances (IC Measure, The Imaging Source, LLC).
Data analysis
For statistical analysis, STATA (version 13.1, StataCorp LLC) was utilised. Data was assessed visually using histograms and Shapiro-Wilk tests were performed to determine normality for all measured variables. Fisher’s exact test was used to compare the incidence of lameness (fore and hind) in two- versus three-year-old horses over the course of the study, as well as the incidence of lameness at each time point. Logistic regression was used to compare incidence of lameness among each time point in all horses and divided into two- and three-year-olds. Repeated measures ANOVA was used to compare each variable (lameness, radiograph measurements, and photographic measurements) across time for all horses and separated by two- and three-year-olds. Multiple comparisons for each time point were calculated using the Sidak post-hoc test. Significance for all statistical tests was assessed at p≤0.05.
Results
Horses
A total of 42 of the 53 horses completed the entirety of the training schedule for the six-month study period during the two years, with horse attrition and incomplete training records due to the global pandemic. In year 1, only 18 horses completed the six-month study (10 two-year-olds and eight three-year-olds; six geldings and 12 fillies). All horses from year 2 completed the study (19 two-year-olds and five three-year-olds). In total, 42 horses completed the study; 29 two year-olds, 13 three year-olds (14 geldings, 28 fillies).
Horse heights
Horse heights (cm) increased significantly from baseline (month 0) to study completion (6 month) for horses overall (month 0 mean 142.2 cm, CI 141.0–143.4; month 6 mean 147.5 cm, CI 146.2–148.9) and when separated by two- (month 0 mean 141.3 cm, CI 139.8–142.8; month 6 mean 147.6 cm, CI 145.9–149.3) and three-year-olds (month 0 mean 144.0 cm, CI 141.9–146.1; month 6 mean 147.4cm, CI 145.0–149.9, p<0.001 for all three comparisons). Heights are summarised in Table S1.
Lameness
All horses completed the training schedule of the study and no horses required further diagnostics or treatment due to lameness severity over the course of the study. Horses were deemed ‘serviceably sound’ with a 2/5 AAEP lameness grade and therefore allowed to continue in training and diagnostics were not pursued. Exercise logs over the course of study duration indicated horses underwent ground-work of increasing intensity over the first four months followed by a combination of ground work and riding under saddle during the final two months of the study. Regimented exercise protocols were previously determined and standardised between all horses across years 1 and 2. Horses were exercised for two hours total, five days per week, except for month 3 between fall and spring semesters. All ground-work was performed in a 20 metre diameter round pen. Work under saddle was performed in a 30.5 × 61 metre enclosed arena on soft ground. During the two-hour period of ground-work and riding, horses were worked in the round pen for one hour followed by riding for one hour. This training regimen was performed in all horses across years of data collection and is summarised in Table S2. During the six-month period of observation, sub-clinical lameness was recognised in 25 of 42 horses. There was a significant difference in lameness incidence between two- and three-year-old horses, with three-year-old horses developing lameness more frequently (p=0.04; 84.6% versus 48.3%). Over the study time course, 16 of 42 horses developed forelimb lameness, with three-year-old horses having a higher incidence of forelimb lameness when compared to two-year-olds (p=0.05; 61.5% versus 27.6%). Nineteen of 42 horses developed hindlimb lameness, with no significant difference between lameness incidence in two- versus three-year-old horses (p=0.2; 37.9% versus 61.5%, respectively). Objective lameness scores of fore- and hindlimbs (VS and HS) (medians and ranges) determined by inertial sensor system are summarised in Table S3.
When comparing the incidence of lameness at each time point individually, there was no significant difference in lameness incidence between two- versus three-year-old horses overall (p>0.1), and no differences in forelimb (p>0.1) or hindlimb lameness (p>0.2) at any of the individual comparisons at each of four time points (p-value included lowest of four individual tests) (Table 1). When assessing the incidence of lameness in a horse over time, there was no significant difference among any of the time points when examining all horses (P=0.7), two- year-olds (P=0.7), or three-year-olds (P=0.9). When assessing the incidence of fore- and hindlimb lameness over time, there was no significant difference among any of the time points for the combined horses (fore: P=0.08, hind: P=0.9), two-year-olds (fore: P=0.4, hind: P=0.8), and three-year-olds (fore: P=0.4, hind: P=0.8).
Table 1:
Incidence (frequency and percentages) of lameness, divided into fore and hind lameness, with total combined horses and divided into lameness at months 0, 2, 4, and 6 in a group of (42) 2- and 3-year-old American Quarter Horses in the first six months of training.
| Month | Total | 2-year -old | 3-year-old | P-value | |
|---|---|---|---|---|---|
| Lame Horses | Combined | 25/42 | 14/29 | 11/13 | 0.04 |
| 59.50% | 48.30% | 84.60% | |||
| 0 | 11/42 | 6/29 | 5/13 | 0.30 | |
| 26.2% | 20.7% | 38.5% | |||
| 2 | 12/42 | 6/29 | 6/13 | 0.1 | |
| 28.5% | 20.7% | 46.2% | |||
| 4 | 9/42 | 5/29 | 4/13 | 0.4 | |
| 21.4% | 17.2% | 30.8% | |||
| 6 | 13/42 | 8/29 | 5/13 | 0.5 | |
| 31.0% | 27.6% | 38.5% | |||
| Fore lameness | Combined | 16/42 | 8/29 | 8/13 | 0.05 |
| 38.1% | 27.6% | 61.5% | |||
| 0 | 9/42 | 4/29 | 5/13 | 0.1 | |
| 21.4% | 13.8% | 38.5% | |||
| 2 | 9/42 | 5/29 | 4/13 | 0.4 | |
| 21.4% | 17.2% | 30.8% | |||
| 4 | 1/42 | 1/29 | 0/13 | >0.9 | |
| 2.4% | 3.4% | 0% | |||
| 6 | 6/42 | 4/29 | 2/13 | >0.9 | |
| 14.3% | 13.8% | 15.4% | |||
| Hind lameness | Combined | 19/42 | 11/29 | 8/13 | 0.2 |
| 45.2% | 37.9% | 61.5% | |||
| 0 | 7/42 | 5/29 | 2/13 | >0.9 | |
| 16.7% | 17.2% | 15.4% | |||
| 2 | 7/42 | 4/29 | 3/13 | 0.7 | |
| 16.7% | 13.8% | 23.1% | |||
| 4 | 8/42 | 4/29 | 4/13 | 0.2 | |
| 19.0% | 13.8% | 30.8% | |||
| 6 | 9/42 | 6/29 | 3/13 | >0.9 | |
| 21.4% | 20.70% | 23.10% |
Changes in hoof measurements by age over time
Mean palmar angle of the distal phalanx (PaADP) significantly increased in forelimbs over time in horses overall (LF p=0.01, RF p<0.001). Mean plantar angle of the distal phalanx (PlADP) significantly changed in hindlimbs over time in horses overall (LH p<0.001, RH p<0.001). In two-year-old horses, there was a significant change in PaADP over time in the right forelimb (RF p<0.001); however, there was only a significant decrease (mean decrease of 1.5°) at month 2 compared to baseline. In two-year-olds, PlADP significantly decreased in both hindlimbs (LH p<0.001, RH p<0.001). In three-year-old horses, PaADP and PlADP significantly increased by the end of the study in both forelimbs and the right hindlimb (LF p=0.01, RF p<0.001, RH p<0.001); both forelimbs were increased by a mean of 1.5°, and right hindlimb was increased by a mean of 1.2°. There was a significant decrease in mean PlADP in the left hindlimb at month 4 (P=0.002); this difference was 1.3° lower than baseline. Palmar/plantar angle of the distal phalanx measurements over time are summarised in Table S4.
Forelimb hoof measurements over time for horses overall and two- and three-year-olds analysed separately are summarised in Table S5 (left forelimb) and S6 (right forelimb). Hindlimb hoof measurements for horses overall and two- and three-year-olds analysed separately are summarised in Table S7 (left hindlimb) and Table S8 (right hindlimb).
Hoof measurements and lameness
In horses overall, lame forelimbs had decreased toe length (p=0.03), greater toe angle (p=0.01), decreased heel length (p=0.01), and decreased lateral wall height (p=0.01). In two-year-old horses, lame forelimbs had greater toe angle (p=0.01). In three-year old horses, lame forelimbs had decreased foot width (p=0.03), decreased toe length (p=0.03), decreased lateral wall height (p=0.03) and decreased medial wall height (p=0.02). Forelimb hoof morphometric measurements in lame and non-lame limbs are summarised in Table 2.
Table 2:
Forelimb hoof measurement differences in lame versus sound limbs
| Lame Limbs | Not Lame Limbs | P-value | Lame Limbs | Not Lame Limbs | P-value | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Frog Base Width (cm) | Total | Number | 25 | 311 | >0.9 | Frog length (cm) | Total | Number | 25 | 311 | 0.5 |
| mean | 5.34 | 5.48 | mean | 7.46 | 7.73 | ||||||
| 95% CI | 4.97 – 5.71 | 5.38 – 5.58 | 95% CI | 7.05 – 7.88 | 7.63 – 7.83 | ||||||
| 2-year-old | Number | 14 | 218 | 0.5 | 2-year-old | Number | 14 | 218 | 0.4 | ||
| mean | 5.33 | 5.39 | mean | 7.66 | 7.69 | ||||||
| 95% CI | 4.81 – 5.85 | 5.27 – 5.51 | 95% CI | 7.16 – 8.16 | 7.57 – 7.81 | ||||||
| 3-year-old | Number | 11 | 93 | 0.5 | 3-year-old | Number | 11 | 93 | 0.06 | ||
| mean | 5.36 | 5.69 | mean | 7.22 | 7.84 | ||||||
| 95% CI | 4.72 – 6.00 | 5.53 – 5.84 | 95% CI | 6.45 – 7.98 | 7.66 – 8.02 | ||||||
| Heel Width (cm) | Total | Number | 25 | 311 | 0.4 | Widest part of Foot (cm) | Total | Number | 25 | 311 | 0.3 |
| mean | 7.76 | 7.88 | mean | 11.69 | 11.80 | ||||||
| 95% CI | 7.34 – 8.18 | 7.77 – 7.99 | 95% CI | 11.25 – 12.13 | 11.69 – 11.91 | ||||||
| 2-year-old | Number | 14 | 218 | 0.8 | 2-year-old | Number | 14 | 218 | 0.5 | ||
| mean | 7.84 | 7.73 | mean | 11.86 | 11.62 | ||||||
| 95% CI | 7.23 – 8.46 | 7.60 – 7.85 | 95% CI | 11.30 – 12.42 | 11.50 – 11.75 | ||||||
| 3-year-old | Number | 11 | 93 | 0.07 | 3-year-old | Number | 11 | 93 | 0.03 | ||
| mean | 7.65 | 8.24 | mean | 11.48 | 12.21 | ||||||
| 95% CI | 6.99 – 8.31 | 8.06 – 8.42 | 95% CI | 10.68 – 12.28 | 11.99 – 12.42 | ||||||
| Foot length (cm) | Total | Number | 25 | 311 | 0.4 | Weight bearing length (cm) | Total | Number | 25 | 311 | 0.5 |
| mean | 10.78 | 11.04 | mean | 8.50 | 8.64 | ||||||
| 95% CI | 10.31 – 11.25 | 10.93 – 11.15 | 95% CI | 8.23 – 8.77 | 8.54 – 8.73 | ||||||
| 2-year-old | Number | 14 | 218 | 0.4 | 2-year-old | Number | 14 | 218 | 0.9 | ||
| mean | 11.06 | 10.97 | mean | 8.47 | 8.58 | ||||||
| 95% CI | 10.59 – 11.53 | 10.84 – 11.10 | 95% CI | 8.18 – 8.76 | 8.47 – 8.69 | ||||||
| 3-year-old | Number | 11 | 93 | 0.07 | 3-year-old | Number | 11 | 93 | 0.4 | ||
| mean | 10.43 | 11.21 | mean | 8.54 | 8.78 | ||||||
| 95% CI | 9.47 – 11.38 | 10.98 – 11.43 | 95% CI | 7.98 – 9.10 | 8.59 – 8.97 | ||||||
| Toe length (cm) | Total | Number | 25 | 311 | 0.03 | Heel length (cm) | Total | Number | 25 | 311 | 0.01 |
| mean | 6.11 | 6.34 | mean | 3.62 | 3.96 | ||||||
| 95% CI | 5.88 – 6.34 | 6.27 – 6.41 | 95% CI | 3.34 – 3.91 | 3.89 – 4.04 | ||||||
| 2-year-old | Number | 14 | 218 | 0.2 | 2-year-old | Number | 14 | 218 | 0.08 | ||
| mean | 6.17 | 6.30 | mean | 3.65 | 3.98 | ||||||
| 95% CI | 5.82 – 6.53 | 6.22 – 6.38 | 95% CI | 3.22 – 4.07 | 3.89 – 4.07 | ||||||
| 3-year-old | Number | 11 | 93 | 0.03 | 3-year-old | Number | 11 | 93 | 0.09 | ||
| mean | 6.02 | 6.45 | mean | 3.59 | 3.93 | ||||||
| 95% CI | 5.69 – 6.36 | 6.32 – 6.58 | 95% CI | 3.15 – 4.03 | 3.80 – 4.06 | ||||||
| Toe angle (°) | Total | Number | 25 | 311 | 0.01 | Heel angle (°) | Total | Number | 25 | 311 | 0.2 |
| mean | 57.28 | 54.93 | mean | 50.01 | 48.13 | ||||||
| 95% CI | 55.01 – 59.55 | 54.44 – 55.42 | 95% CI | 46.60 – 53.41 | 47.24 – 49.02 | ||||||
| 2-year-old | Number | 14 | 218 | 0.01 | 2-year-old | Number | 14 | 218 | 0.08 | ||
| mean | 58.43 | 55.21 | mean | 50.43 | 47.89 | ||||||
| 95% CI | 55.62 – 61.24 | 54.62 – 55.80 | 95% CI | 46.10 – 54.75 | 47.00 – 48.79 | ||||||
| 3-year-old | Number | 11 | 93 | 0.3 | 3-year-old | Number | 11 | 93 | 0.8 | ||
| mean | 55.81 | 54.28 | mean | 49.47 | 48.68 | ||||||
| 95% CI | 51.69 – 59.93 | 53.39 – 55.17 | 95% CI | 43.11 – 55.84 | 46.55 – 50.81 | ||||||
| Lateral Wall height (cm) | Total | Number | 24 | 309 | 0.01 | Medial Wall height (cm) | Total | Number | 24 | 309 | 0.06 |
| mean | 4.64 | 5.11 | mean | 4.87 | 5.22 | ||||||
| 95% CI | 4.31 – 4.96 | 5.03 – 5.20 | 95% CI | 4.55 – 5.20 | 5.13 – 5.30 | ||||||
| 2-year-old | Number | 13 | 218 | 0.09 | 2-year-old | Number | 13 | 218 | 0.6 | ||
| mean | 4.78 | 5.15 | mean | 5.12 | 5.24 | ||||||
| 95% CI | 4.37 – 5.18 | 5.05 – 5.25 | 95% CI | 4.69 – 5.55 | 5.14 – 5.35 | ||||||
| 3-year-old | Number | 11 | 91 | 0.03 | 3-year-old | Number | 11 | 91 | 0.02 | ||
| mean | 4.47 | 5.02 | mean | 4.58 | 5.15 | ||||||
| 95% CI | 3.89 – 5.05 | 4.86 – 5.18 | 95% CI | 4.06 – 5.10 | 4.99 – 5.30 | ||||||
| Lateral Wall angle (°) | Total | Number | 24 | 309 | 0.3 | Medial Wall angle (°) | Total | Number | 24 | 309 | 0.4 |
| mean | 77.96 | 77.20 | mean | 77.31 | 76.78 | ||||||
| 95% CI | 76.55 – 79.37 | 76.64 – 77.77 | 95% CI | 75.10 – 79.52 | 76.15 – 77.40 | ||||||
| 2-year-old | Number | 13 | 218 | 0.5 | 2-year-old | Number | 13 | 218 | 0.2 | ||
| mean | 78.28 | 77.60 | mean | 79.68 | 77.54 | ||||||
| 95% CI | 76.59 – 79.97 | 76.95 – 78.24 | 95% CI | 77.09 – 82.28 | 76.85 – 78.24 | ||||||
| 3-year-old | Number | 11 | 91 | 0.3 | 3-year-old | Number | 11 | 91 | >0.9 | ||
| mean | 77.59 | 76.26 | mean | 74.51 | 74.95 | ||||||
| 95% CI | 74.90 – 80.28 | 75.15 – 77.38 | 95% CI | 71.16 – 77.86 | 73.69 – 76.20 |
Lame hindlimbs were associated with an increased heel length in three-year-olds (p=0.01) and overall (p = 0.05), and greater lateral wall angle in two-year-olds (p=0.04) and horses overall (p=0.04). Hindlimb hoof morphometric measurements in lame and non-lame limbs are summarised in Table 3.
Table 3:
Hindlimb hoof measurement differences in lame versus sound limbs.
| Lame Limbs | Not Lame Limbs | P-value | Lame Limbs | Not Lame Limbs | P-value | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Frog Base Width (cm) | Total | Number | 40 | 296 | 0.07 | Frog length (cm) | Total | Number | 40 | 296 | 0.5 |
| mean | 5.93 | 5.79 | mean | 7.55 | 7.48 | ||||||
| 95% CI | 5.67 – 6.20 | 5.69 – 5.89 | 95% CI | 7.30 – 7.80 | 7.39 – 7.57 | ||||||
| 2-year-old | Number | 21 | 211 | 0.3 | 2-year-old | Number | 21 | 211 | >0.9 | ||
| mean | 5.80 | 5.69 | mean | 7.41 | 7.44 | ||||||
| 95% CI | 5.33 – 6.26 | 5.57 – 5.80 | 95% CI | 7.04 – 7.78 | 7.33 – 7.55 | ||||||
| 3-year-old | Number | 19 | 85 | 0.2 | 3-year-old | Number | 19 | 85 | 0.4 | ||
| mean | 6.08 | 6.04 | mean | 7.70 | 7.58 | ||||||
| 95% CI | 5.82 – 6.34 | 5.84 – 6.23 | 95% CI | 7.35 – 8.06 | 7.40 – 7.76 | ||||||
| Heel Width (cm) | Total | Number | 40 | 296 | 0.7 | Widest part of Foot (cm) | Total | Number | 40 | 296 | 0.8 |
| mean | 8.01 | 8.11 | mean | 11.07 | 10.95 | ||||||
| 95% CI | 7.90 – 8.12 | 7.85 – 8.36 | 95% CI | 10.83 – 11.32 | 10.85 – 11.05 | ||||||
| 2-year-old | Number | 21 | 211 | 0.8 | 2-year-old | Number | 21 | 211 | 0.9 | ||
| mean | 7.89 | 7.85 | mean | 10.75 | 10.78 | ||||||
| 95% CI | 7.47 – 8.31 | 7.73 – 7.97 | 95% CI | 10.40 – 11.11 | 10.66 – 10.90 | ||||||
| 3-year-old | Number | 19 | 85 | >0.9 | 3-year-old | Number | 19 | 85 | >0.9 | ||
| mean | 8.34 | 8.41 | mean | 11.43 | 11.36 | ||||||
| 95% CI | 8.07 – 8.62 | 8.21 – 8.61 | 95% CI | 11.13 – 11.73 | 11.19 – 11.54 | ||||||
| Foot length (cm) | Total | Number | 40 | 296 | 0.8 | Weight bearing length (cm) | Total | Number | 40 | 296 | 0.5 |
| mean | 10.87 | 10.74 | mean | 8.61 | 8.55 | ||||||
| 95% CI | 10.56 – 11.18 | 10.61 – 10.86 | 95% CI | 8.35 – 8.87 | 8.45 – 8.65 | ||||||
| 2-year-old | Number | 21 | 211 | >0.9 | 2-year-old | Number | 21 | 211 | 0.7 | ||
| mean | 10.68 | 10.66 | mean | 8.50 | 8.55 | ||||||
| 95% CI | 10.25 – 11.11 | 10.52 – 10.80 | 95% CI | 8.21 – 8.80 | 8.43 – 8.66 | ||||||
| 3-year-old | Number | 19 | 85 | 0.8 | 3-year-old | Number | 19 | 85 | 0.3 | ||
| mean | 11.08 | 10.93 | mean | 8.73 | 8.56 | ||||||
| 95% CI | 10.61 – 11.55 | 10.68 – 11.17 | 95% CI | 8.26 – 9.20 | 8.37 – 8.76 | ||||||
| Toe length (cm) | Total | Number | 40 | 296 | 0.2 | Heel length (cm) | Total | Number | 40 | 296 | 0.05 |
| mean | 6.63 | 6.49 | mean | 3.74 | 3.55 | ||||||
| 95% CI | 6.44 – 6.83 | 6.41 – 6.56 | 95% CI | 3.53 – 3.96 | 3.48 – 3.61 | ||||||
| 2-year-old | Number | 21 | 211 | 0.5 | 2-year-old | Number | 21 | 211 | 0.9 | ||
| mean | 6.65 | 6.49 | mean | 3.60 | 3.57 | ||||||
| 95% CI | 6.42 – 6.88 | 6.40 – 6.57 | 95% CI | 3.40 – 3.81 | 3.49 – 3.65 | ||||||
| 3-year-old | Number | 19 | 85 | 0.1 | 3-year-old | Number | 19 | 85 | 0.01 | ||
| mean | 6.61 | 6.48 | mean | 3.90 | 3.50 | ||||||
| 95% CI | 6.26 – 6.96 | 6.35 – 6.62 | 95% CI | 3.50 – 4.30 | 3.39 – 3.61 | ||||||
| Toe angle (°) | Total | Number | 40 | 296 | 0.1 | Heel angle (°) | Total | Number | 40 | 296 | 0.2 |
| mean | 55.45 | 56.96 | mean | 42.69 | 44.62 | ||||||
| 95% CI | 53.96 – 56.93 | 56.49 – 57.42 | 95% CI | 40.74 – 44.63 | 43.86 – 45.38 | ||||||
| 2-year-old | Number | 21 | 211 | 0.4 | 2-year-old | Number | 21 | 211 | 0.4 | ||
| mean | 56.12 | 57.22 | mean | 42.63 | 44.22 | ||||||
| 95% CI | 53.79 – 58.46 | 56.67 – 57.77 | 95% CI | 39.56 – 45.71 | 43.39 – 45.06 | ||||||
| 3-year-old | Number | 19 | 85 | 0.2 | 3-year-old | Number | 19 | 85 | 0.2 | ||
| mean | 54.69 | 56.29 | mean | 42.75 | 45.60 | ||||||
| 95% CI | 52.75 – 56.64 | 55.39 – 57.19 | 95% CI | 40.13 – 45.37 | 43.96 – 47.25 | ||||||
| Lateral Wall height (cm) | Total | Number | 40 | 295 | 0.5 | Medial Wall height (cm) | Total | Number | 40 | 295 | 0.5 |
| mean | 4.57 | 4.47 | mean | 4.77 | 4.81 | ||||||
| 95% CI | 4.37 – 4.77 | 4.40 – 4.55 | 95% CI | 4.53 – 5.00 | 4.72 – 4.89 | ||||||
| 2-year-old | Number | 21 | 211 | 0.8 | 2-year-old | Number | 21 | 211 | 0.3 | ||
| mean | 4.50 | 4.48 | mean | 4.66 | 4.82 | ||||||
| 95% CI | 4.19 – 4.82 | 4.39 – 4.57 | 95% CI | 4.37 – 4.95 | 4.72 – 4.92 | ||||||
| 3-year-old | Number | 19 | 84 | 0.2 | 3-year-old | Number | 19 | 84 | 0.8 | ||
| mean | 4.65 | 4.46 | mean | 4.88 | 4.77 | ||||||
| 95% CI | 4.38 – 4.91 | 4.32 – 4.59 | 95% CI | 4.49 – 5.28 | 4.61 – 4.93 | ||||||
| Lateral Wall angle (°) | Total | Number | 40 | 295 | 0.04 | Medial Wall angle (°) | Total | Number | 40 | 295 | 0.4 |
| mean | 77.67 | 75.84 | mean | 80.25 | 81.95 | ||||||
| 95% CI | 76.10 – 79.25 | 75.23 – 76.46 | 95% CI | 78.38 – 82.12 | 81.34 – 82.55 | ||||||
| 2-year-old | Number | 21 | 211 | 0.04 | 2-year-old | Number | 21 | 211 | >0.9 | ||
| mean | 78.33 | 75.87 | mean | 83.15 | 83.21 | ||||||
| 95% CI | 75.81 – 80.85 | 75.17 – 76.58 | 95% CI | 80.76 – 85.54 | 82.57 – 83.85 | ||||||
| 3-year-old | Number | 19 | 84 | 0.4 | 3—year-old | Number | 19 | 84 | 0.3 | ||
| mean | 76.95 | 75.78 | mean | 77.05 | 78.78 | ||||||
| 95% CI | 74.93 – 78.97 | 74.54 – 77.02 | 95% CI | 74.74 – 79.35 | 77.65 – 79.91 |
Discussion
The findings of this study partially supported our hypotheses: there were hoof morphological changes that occurred over the course of the study and there were several that were associated with lameness. Contrary to our initial hypothesis, we found a higher incidence in lameness in three-year-old horses. The higher prevalence of lameness in three-year-olds noted here could arise from a number of factors. It is possible that this group of coming three-year-old horses were started later due to pre-existing lameness or perceived immaturity as coming two-year-olds. There was a smaller number of coming three-year-old horses compared to two-year old horses so expanding the group size would provide more information on this effect. In this study, 59.5% of juvenile Quarter Horses entering training developed lameness over the course of the six-month study. For comparison, several studies have identified lame horses in sample populations of adult horses in training or competition. For example, 73% of adult horses (median age 11 years) in ‘regular work’ for competition in multiple disciplines including eventing, show jumping, and dressage exhibited mild lameness (≤ grade 2/8) 25. In another study of 841 actively competing event horses, 8.6% of horses had an overt lameness (9% forelimb, 8% hindlimb) 26. While the current prevalence of lameness is unknown for most populations of horses, these and the current study suggest that some degree of lameness may be expected in horses across multiple disciplines both during early training and athletic competition.
In this group of horses, there was also a greater incidence of lameness identified in forelimbs versus hindlimbs. Interestingly, greater changes to hoof measurements were also observed in the forelimbs. Both lameness incidence and changes in hoof characteristics could be hypothesised to be due to the addition of the weight of the rider, as this weight may be primarily distributed to the forelimbs when under saddle. Hoof size, shape and balance have been previously identified as contributing factors to lameness and external characteristics of the hoof have been noted to differ between lame and non-lame horses 28. Notably, in this study, we identified that lame forelimbs had decreased hoof width, toe length, heel length, and hoof wall height, consistent with previous reports 28. However, lame hindlimbs had increased heel length compared to sound limbs.
Hoof measurements evaluated in this study were selected individually to assess different aspects that may relate to lameness. Frog measurements (length, width at base) were performed as the frog is a weight-bearing structure. In horses with heel pain, the frog can atrophy with the width becoming less than 2/3 the length 29. In the current study, we did not identify changes in frog length or width in lame versus non-lame fore- or hindlimbs. It is possible that this group of horses did not have either chronic or severe enough lameness to result in changes to the frog. Toe and heel angle have been shown to decrease with race training and have been thought to reflect general hoof balance and affect palmar or plantar angle of the distal phalanx 8,9. In the current study, a significant decrease in toe angle was identified in lame compared to non-lame forelimbs, but this was not identified in hindlimbs; there were no significant changes in heel angles in either fore or hind hooves when comparing lame versus non-lame limbs. Hoof width and toe and heel length were evaluated as chronic lameness has been associated with hoof contraction and longer heels 7. In three-year-old horses, lame forelimbs had significantly narrower hoof width at the widest part of the foot compared to non-lame limbs; there was no significant difference in hoof width at the widest part of the hoof in lame versus non-lame hindlimbs. When looking at both two- and three-year-old horses, there was a significant decrease in heel length in lame versus non-lame forelimbs. However, in hindlimbs overall and in 3-year-old horses, there was a significant increase in heel length in lame versus non-lame limbs, which is more similar to what has been previously reported.
Lameness has previously been found to be associated with changes in hoof morphology. Verschooten (1993) described in horses with chronic forelimb lameness, the lame limb was associated with an upright hoof shape with long heels. In the contralateral limb, the heels were low, resulting in a flat foot conformation 7. In another study, lame forelimbs, with a foot-source of lameness, had higher dorsal to palmar coronary band height, indicating a long-toed, lower heeled appearance 28. These findings contrast with those of this study, where a shorter toe and heel length were found in lame forelimbs, which may be attributed to differences in age, breed and occupation of horses studied. Horses in the current study were considered serviceably sound by veterinarians based on lameness examinations at each time point and therefore were allowed to continue in training and no further diagnostics or treatment were pursued. It is unclear the level of lameness in the previous studies as well as its impact on athletic function.
Level of exercise has also been previously found to be associated with hoof morphology. In Thoroughbred racehorses, a reduction in hoof angle and decrease in proximal hoof circumference during race training have been reported 8,9. In contrast, a small group of Thoroughbred show-horses maintained by the same group of farriers as the racehorses had less variability in hoof angle 8. Furthermore, in a group of shod adult Standardbreds undergoing light exercise for 17 weeks, there were no changes in hoof wall growth or morphometric variables observed 10. The horses in our study underwent a lighter level of work compared to Thoroughbred racehorses and may have had more equivocal work level compared to show horses or Standardbreds in light training.
Environmental conditions have also been identified to be related to hoof morphology. A group of Thoroughbred racehorses had lower hoof angles when turned out on wet pasture during winter months compared to during the summer 8. In the current study, when combining lame and non-lame forelimbs, there were significant increases in medial and lateral heel heights and heel lengths, as well as a decrease in heel angle when comparing horses housed in stalls to horses in outdoor pens (shown in the supporting information). Based on the design of this study, it is possible that some of the changes to hoof morphology may be attributable to environmental changes of the horses during the study.
To the authors’ knowledge, there has not been an investigation examining the effect of initiation of training at different ages on hoof morphology and lameness incidence in non-racing horses. Horses enrolled in this study were intended for future occupations in western performance and ranching. Faramarzi et al. (2017) had previously quantified external (conformational) and internal (radiographic) anatomical characteristics of front feet of Arabian foals until one year of age, demonstrating that many changes occurred between four and eight months of age. Future investigations could examine initiation of training in other breeds and/or athletic discipline on hoof morphology.
Our findings raise several further discussion points and limitations. Both the two- and three-year-old horses grew taller over the course of the study, which may have affected hoof size parameters and was likely reflected in the increase in various hoof measurement parameters over time (e.g. palmar/plantar angle of the distal phalanx, frog length, toe length, medial and lateral wall height) as horse mass has previously been described to influence hoof morphology. Interestingly, in this population of horses, the two-year-old horses grew to approximately the same size as the three-year-olds by the end of the study; this may have been the result of the three-year-old cohort entering training later due to smaller size the previous year and therefore further investigation into populations where size differed between age groups would be warranted. An additional consideration is that while efforts were made to standardise housing and timing of training regimens over the two-year data collection period, there may have been differences between years in terms of weather, environment, and timing of farrier visits, which was particularly challenging at the beginning of the COVID19 global pandemic. Therefore, limitations of study design include lack of a control group of horses with similar environment and of the same age and mass that did not begin training. As this study was observational, diagnostics to fully characterise the source of lameness were not performed and the degree of lameness observed was not considered so severe as to warrant veterinary intervention. Standardisation of timing of the farrier schedule with data collection was attempted but not possible for every time point. All horses received approximately four weeks off study between autumn and spring semesters, and it is acknowledged that preexisting lameness in horses prior to that time period may have been exacerbated by that period of rest. While exercise protocols were generally predetermined and standardised across horses, it is possible that slight variability occurred between individual riders. Two- and three-year-olds were treated with the same regimented exercise protocol, and therefore differences in lameness between age groups were not attributed to differences in exercise programs (i.e. the higher rate of lameness in three-year-olds was not thought to be due to being worked harder). Sex (male versus female) was not considered as a statistical factor due to the relatively low animal number and as all males evaluated here were gelded. Finally, due to the COVID19 pandemic, approximately 1/3 of horses (11 out of 29) during the first year of the study were lost to study attrition at the final time points and could not be included for full analysis, which decreased total study numbers. While an association of lameness to hoof morphology changes was established, it is not possible to determine from this study whether conformational changes precipitated lameness or vice versa.
In conclusion, in this study we found that approximately 60% of horses developed non-performance limiting lameness during their first six months of training. We also identified that three-year-old horses entering training had a higher incidence of lameness and greater changes in hoof conformation over time compared to two-year-olds. The evaluation of hoof morphology in other groups of young growing horses entering training could be indicated for specific athletic disciplines and identification of lameness incidence between different age groups may help organisations make decisions about the appropriate time at which young horses should enter training. In addition, identification of hoof factors that precede development of lameness may allow equine veterinarians to screen horses and identify them prior to injury.
Supplementary Material
Table S1: Horse heights over time by age.
Table S2: Horse exercise regimen throughout study duration.
Table S3: Objective lameness fore- and hindlimb (VS and HS) scores (medians and ranges) from the inertial sensor system.
Table S4: Palmar/Plantar distal phalangeal angles (PaADP/PIADP) over time by age.
Table S5: Left forelimb hoof photograph measurements over time by age.
Table S6: Right forelimb hoof photograph measurements over time by age.
Table S7: Left hindlimb hoof photograph measurements over time by age.
Table S8: Right hindlimb hoof photograph measurements over time by age.
Acknowledgements
We thank the staff of the Colorado State University Equine Sciences Program and veterinary students who assisted with data collection.
Source of funding
This study was supported by Animal Health and Disease Grant No. 20AHDRXXXXG005150001 / Project Accession No. 1023187 from the USDA National Institute of Food and Agriculture and the Zoetis Small Grant Program. Stipend support for LP was provided by the CCTSI NIH/NCATS CTSA 5TL1TR002533-02, NIH 5T32 OD010437-19, and Carolyn Quan and Porter Bennett.
Footnotes
Authors’ declarations of interest
The authors declare no other competing interests related to this report.
Ethical animal research
This study was approved by the Institutional Animal Care and Use Committee of Colorado State University, Protocol #1457.
Informed consent
Owners provided consent for enrollment of client-owned equine patients in this clinical study.
Data availability statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Table S1: Horse heights over time by age.
Table S2: Horse exercise regimen throughout study duration.
Table S3: Objective lameness fore- and hindlimb (VS and HS) scores (medians and ranges) from the inertial sensor system.
Table S4: Palmar/Plantar distal phalangeal angles (PaADP/PIADP) over time by age.
Table S5: Left forelimb hoof photograph measurements over time by age.
Table S6: Right forelimb hoof photograph measurements over time by age.
Table S7: Left hindlimb hoof photograph measurements over time by age.
Table S8: Right hindlimb hoof photograph measurements over time by age.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.