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Keywords:

  • horse;
  • accelerometry;
  • symmetry scores;
  • lameness scores;
  • fetlock joint distension

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturer's addresses
  10. References

Reasons for performing study: Equine lameness examination is based on subjective visual scoring of lameness. Instrumented objective methods for lameness examinations may be complicated to perform and the equipment is often stationary. Accelerometry has a potential clinical use; however, the reduction and interpretation of equine accelerometric data are not yet routine and the value of accelerometry in equine lameness examination is unclear.

Objectives: To use accelerometric data to calculate 2 different accelerometric symmetry scores and to evaluate the agreement of these with traditional lameness scores done by experienced equine practitioners.

Methods: Six sound horses were equipped with a 3 axis 10G piezoresistant accelerometer at the lowest point of the back. Horses were trotted and video recorded at 0, 3, 15, 30, 45 and 60 min after injection of saline into one metacarpophalangeal joint. Video recordings were scored in a blind manner according to the AAEP scale by 2 experienced practitioners. Interobserver agreements and 2 symmetry scores S and A, developed on the basis of Fourier transformation of the obtained accelerometric data, were calculated and regression analysis between AAEP scores and symmetry scores was performed.

Results: Interobserver agreements were 70%. There was a statistically significant relationship between AAEP lameness scores and both symmetry scores.

Conclusions: Both symmetry scores showed a significant relationship with the AAEP scores and can be a valuable tool in the detection and quantification of lameness. While the S score was able to detect changes in degree of lameness, the A score was capable of detecting the lame diagonal. However, more research is needed for the development of a combined accelerometric score to take advantage of the strengths of each of the symmetry scores.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturer's addresses
  10. References

Reliable assessment of lameness may be a clinical challenge, because the subjective visual evaluation of biomechanical changes is the clinician's primary diagnostic tool. The main aims of the lameness examination are to identify any lame limbs and subsequently to grade the severity of the lameness (Stashak 2002). For the latter task, equine lameness scales and scores are widely used; however, such data remain subjective and qualitative. It is obvious that experience and training play an important role in the assessment and quantification of lameness. Arkell et al. (2006) showed that equine orthopaedic experts were more consistent when scoring horses with variable degrees of lameness when compared to nonexperts and final year veterinary students. Keegan et al. (1998) also showed that clinical training has an influence on the assessment of lameness scores as intra-observer agreement was higher for clinicians than for residents or interns. However, the interobserver agreement was poor for both clinicians and residents or interns regarding level of exact agreement, identifying lameness in the same forelimb or identifying soundness. Another study evaluated the agreement between 2 scoring systems; a numerical scale ranging from 0–10 and a global scale which describes the change in lameness as worse, same, improved or sound (Fuller et al. 2006). In this study both scales had a good intra-observer agreement, but only the global scale had a good inter-observer agreement, while the inter-observer agreement for the numerical scale was just above the acceptable limit. Consequently, attempts to support the clinical judgement by application of objective instrumented methods are relevant. Such methods include the use of high speed cameras and force plates (Buchner et al. 1996; Weishaupt et al. 2004, 2006; Ishihara et al. 2005), which are state of the art methods, but unfortunately also very costly, complex and stationary methods. More user-friendly methods for the analysis of the equine gait in both sound and lame horses include 3D accelerometry, which is a relatively simple and noninvasive procedure, involving low cost instrumentation allowing data to be logged in a regular trot-up area.

A 3D accelerometer placed on the trunk of the horse will capture the accelerations and decelerations of the trunk within each stride. In the sound, trotting horse, the movements of the trunk are regular and symmetric. In the lame, trotting horse, the movement will be asymmetric and thus change the symmetry pattern of the accelerations and decelerations (Barrey and Desbrosse 1996; Buchner et al. 1996). Human trunk accelerometric gait analysis with sensors located close to the centre of body mass has shown good test-retest reliability (Henriksen et al. 2004). However, the reduction and interpretation of equine accelerometric data is far from routine and the clinical value of accelerometry in equine lameness examination is therefore not clear.

The objective of this study was therefore to use a new mathematical approach to reduce accelerometric data into 2 different accelerometric symmetry scores in sound and lame horses and to evaluate the agreement of these with traditional lameness scores done by 2 experienced equine practitioners. In order to obtain blinded and independent observations of lameness, an experimental model based on joint distension, which provides changing lameness scores, a minimum of discomfort to the horse, and total reversibility within 1–2 h, was used to obtain the symmetry and lameness scores.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturer's addresses
  10. References

Horses

Six sound riding horses, mares, age 4–11 years (median age 8 years), bodyweight 455–657 kg (median weight 553 kg), and height 151–163 cm (median height, 156 cm) from the university teaching herd were included after passing a clinical lameness examination.

Ethical standard

The study was approved by the Danish Animal Experimentation Board and the study was performed according to the Danish Animal Testing Act.

Instrumentation and data acquisition

Horses were equipped with a 3-axis 10G piezoresistant accelerometer (Mega Electronics Ltd)1 held firmly in place by an elastic girth at the lowest point of the back in the midline (above T13) and connected to a data logger (Mega Electronics Ltd)1 mounted on another girth. The trot-up area was a 25 m hard surfaced concrete lane, delimited by photoelectric sensors with a wireless connection to the data logger and a video camera (Panasonic NV-GS400-EN camcorder)2. Horses were trotted by hand to obtain baseline accelerometric data and video recordings. Immediately after baseline measurements, 35 ml sterile saline was injected aseptically into the distopalmar joint pouch of the metacarpophalangeal joint. Accelerometric data were obtained during the trot at 3, 15, 30, 45 and 60 min post injection and all sessions were video recorded. Two hours after injection, swelling, heat and pain were determined in the region of the fetlock joint and the horse was trotted. A sterile bandage was applied over the fetlock joint for 24 h, after which the horse and the fetlock joint were examined again.

Lameness scores

Video sequences (without sound) containing laterofrontal and laterocaudal views from all trotting sessions were cut, mixed and blinded for lameness scoring.

Two experienced equine practitioners independently scored lameness based on these video recordings using the AAEP scale (Judy and Galuppo 2005). Observers were instructed only to use absolute numbers and not, for example, include 0.5 increments. Video recordings were without sound and could be watched as many times as wanted.

Symmetry scores

Eight regular strides were cut from each trotting session and handled in a user-friendly custom-made computer program (Matlab)3 as described by Halling Thomsen et al. (2010). The 8 strides started in the suspension phase before the stance phase of the right diagonal, as determined by the video recordings.

The dorsoventral acceleration signal had a sinusoid pattern and was therefore suitable for expansion into Fourier series. By this, the signal was decomposed into sine and cosine curves and could be described as:

  • image

By choosing w= 2π/(1/8) the repetitious part of the signal related to the oscillations of the trunk in trot was extracted, thereby removing noise related to extraneous movements. The Fourier smoothed signal was based on the first 10 harmonics (K = 10) and consisted of 8 identical periods representing the 8 strides. Two symmetry scores were calculated from one of the 8 identical periods of the Fourier smoothed signal.

The even harmonics of the Fourier smoothed signal describe the symmetric part of the signal, while the odd harmonics describe the asymmetric part of the signal. The Fourier coefficients, ai and bi, determine the magnitude of the harmonics. Thus, the even-numbered coefficients reflect the symmetric contribution to the signal and odd-numbered coefficients reflect the asymmetric contribution to the signal.

A symmetry score S was calculated as the natural logarithmic quotient of the squared odd-numbered and even-numbered Fourier coefficients.

  • image

Sound horses were expected to have a high degree of symmetry in trunk accelerations and therefore smaller values of the S score. In lame horses trunk accelerations were expected to be more asymmetric resulting in higher values of the S score.

Symmetry score A was calculated as the natural logarithmic quotient of the total positive (upwards) accelerations during stance of the right and the left diagonals.

  • image

The positive accelerations during stance were reflecting the vertical ground reaction forces acting on the weightbearing limbs according to Newton's second law. Lame horses were expected to reduce the load on the lame limb resulting in lower vertical ground reaction forces. Sound horses loading the limbs in the 2 diagonals equally were expected to have A scores close to 0. Horses lame on the right diagonal were expected to have negative values of the A score, while horses lame on the left diagonal were expected to have positive values of the A score.

Statistical analysis

Interobserver agreement: The percentages of exact agreements, one-point disagreements, and 2-point disagreements were calculated as:

  • image

Where X is the number of exact agreements, one-point disagreements, and 2-point disagreements (Hewetson et al. 2006). The interobserver agreement was determined by calculating Cohen's kappa. The weighted kappa (Landis and Koch 1977) was chosen for this study. Kappa was calculated for all scores and for scores ≤2.

Symmetry score S: The S score was analysed using an analysis of variance for repeated measurements with horse as random effect. Due to the low number of horses no attempt to model the serial correlation within horse was made. The estimates at 3, 15, 30, 45 and 60 min after injection were compared to the baseline measurement at time 0.

The relationship between the mean AAEP score of the 2 observers and the S score was examined using linear regression.

Level of significance was ≤0.05.

Symmetry score A: The A score was analysed using an analysis of variance for repeated measurements with horse as random effect. When interpreting the A score, it is assumed that the A score has a value of 0 at time 0 before the induction of lameness. When lameness is induced in the left limb, A scores will attain positive values and negative values when lameness is induced in the right limb. Therefore, the AAEP scores on right limbs were converted to their corresponding negative values while the AAEP scores on left limbs kept their positive values. The relationship between the mean of the converted AAEP score of the 2 observers and the A score was examined using linear regression.

Level of significance was ≤0.05.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturer's addresses
  10. References

Horses

On the day of the experiment, one of the horses developed hindlimb lameness, and was therefore excluded from the study. The data set consisted of 30 measurements, obtained from 5 horses measured 6 times.

Lameness model

The intra-articular saline injection induced varying and transient lameness ranging from 1–4 on the AAEP scale. All horses were sound 2 h after injection and remained sound.

Interobserver agreement

The percentage of agreements was 70%, while the percentages of one- and 2-point disagreements were 23.3 and 6.7%, respectively. In 3 cases the observers disagreed on whether the horse was lame or not (all cases at 60 min post injection) and in one of these cases one observer scored the horse grade 2 while the other observer scored the same horse grade 0. In another case both observers claimed that one horse was lame in the left forelimb, but at 45 min post injection both observers had the horse being lame in the right forelimb despite the fact that lameness had been induced in the left forelimb. The results of the weighted kappa values are given in Table 1.

Table 1. Interobserver agreement
 All scoresScores ≤2
Observed agreement21 (70%)15 (75%)
Expected by chance7.4 (24.56%)8.7 (43.25%)
Weighted kappa0.7660.577
AgreementGoodModerate

Symmetry scores S and A

Calculation of the symmetry scores at 3 min for Horse 6 was not possible as the horse was not able to trot regularly in the test area because of the high degree of lameness. Therefore, these values are missing from the data.

The S score was significantly higher at 3 and 15 min post injection (P<0.001) but not at 30, 45 or 60 min. The linear correlation between mean visual score and S score was 0.79. Since the visual score is measured on a discrete scale it might be more appropriate to report instead the polyserial correlation, which amounts to 0.86. There was a significant relationship between mean visual scores of both observers and the S scores, r2= 0.63, P<0.0001 (Fig 1). The standard errors of the prediction of S from the AAEP score varied from 0.85–0.90 for the different AAEP scores.

image

Figure 1. The relationship between observers and the S score.

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There was a significant difference of the A score from 0 at 3 min post injection (P<0.001) and at 15 and 30 min post injection (P<0.01). Any differences of the A score from 0 at 45 min and 60 min post injection could not be detected. The A score attained a positive value in all measurements of left forelimb lameness, and a negative value in all measurements of right forelimb lameness. The linear correlation as well as the polyserial correlation between mean visual score and A score equalled 0.78. The intercept of the fitted linear relationship was not significantly different from zero (P = 0.152). This demonstrates that the offset of the visual score and the symmetry score A coincide. There was a significant relationship between mean visual scores of both observers and the A scores, r2= 0.606, P<0.0001 (Fig 2). The standard errors of the prediction of A from the AAEP score varied from 0.15–0.16 for the different AAEP scores.

image

Figure 2. The relationship between observers and the A score.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturer's addresses
  10. References

A gold standard for lameness evaluation methods does not exist. However, by inducing lameness in otherwise sound horses, control is gained regarding the lame limb. Experimental joint distension proved useful as an experimental method, because the degree of lameness changed from sound to lame and back to sound over a short time, thus resembling the traditional lameness examination situation. Therefore, this experimental model might have improved the inter-observer agreement, since the visual perception of lameness may be influenced by the horse, its gait and type. The Kappa coefficients in the interobserver agreement study showed moderate to good agreement between the 2 observers. These conclusions are only suggestive; however, since a sample size of 30 (from 6 horses) is small compared to the required values computed in the sample size study of Sim and Wright (2005). Despite this, these results confirm the study of Keegan et al. (1998) stating that inter-observer agreements decrease with the decreasing lameness scores, even in experienced clinicians.

In a few cases, the clinicians disagreed by >2 points on the AAEP scale and even disagreed on whether a horse was lame or not. In one case both clinicians erroneously assigned the lameness to the wrong diagonal. Complicated lameness cases or cases of legal interest are most often examined during a period of days or even weeks and different equine practitioners are often involved in the examination. Accurate and precise lameness scores and documentation of the examination is required in these cases. This study shows that even experienced equine practitioners in some cases fail to detect the correct degree of lameness and the correct diagonal. In this study the observers only had access to video recordings without sound and the sound of the hoof landing on the ground is also used in regular lameness examinations to distinguish between the sound and the lame limb. However, it is doubtful whether the lack of sound would have had any significant influence on the assessment and quantification of the lameness.

Symmetry score S showed a strong statistically significant relationship with the visually scored degree of lameness (Fig 1). Very low S scores were consistent with high degrees of symmetry in the movement, which was perceived as soundness by the observers. Asymmetric movements resulted in S scores that moved towards zero. However, the ability of the visual scores to predict the accelerometric scores is limited, due in part to uncertainty in the visual scores. In other words, considerable information in the accelerometric scores is not contained in the visual scores, hence the accelerometric scores can be useful as supplements to the visual scores. Nevertheless, the S score does not explain the anatomical origin of the asymmetry to the extent that the A score does. While the exact S score seems to be of less value in terms of correlation compared to the AAEP lameness scale for example, the changes over time observed in the score seem to be of greater value, in terms of changes in degree of lameness, i.e. symmetry. The S score was for all horses highest at 3 min post injection (S moved towards zero) and declined rapidly and almost returned to the start-value at 60 min post injection. The observers gave the highest lameness scores at 3 min, but in several cases both observers gave the same scores at 3 and 15 min and hence did not observe an improvement in the lameness, even though there was a marked decline in the S score from 3 min to 15 min. This indicates that the S score is valuable, especially when evaluating improvement or worsening of lameness as this score can measure even slight changes in lameness where the observers would tend to give the same score. The S score may, therefore, prove valuable in documentation of lameness degree, for example during a treatment course or in forensic cases.

The A score has the ability to indicate the diagonal involved in the lameness. In all instances the A score indicated the correct diagonal, in contrast to the observers, who disagreed in some cases of mild lameness. Determining the correct diagonal in cases of severe lameness is usually not a problem. However, in cases with mild lameness, determination of the correct lame limb can be a challenge. The A score has proven useful as this score was capable of determining the correct diagonal even in cases with mild lameness. Since the lameness was induced only in front limbs, the described performance of the A score only relates to front limb lameness.

Barrey and Desbrosse (1996) used the mediolateral trunk accelerations for side detection and were able to detect the correct side of lameness in 13 out of 17 cases. Keegan et al. (2004) determined the lame side based on data from gyroscopic sensors attached to the distal limb. In hindlimb lameness data from sensors at the 2 tuber coxae have been used for detection of the lame side (Pfau et al. 2007; Church et al. 2009).

The symmetry scores were based on a Fourier analysis of the dorsoventral acceleration signal. By using Fourier analysis to extract the repetitive part of the acceleration signal for the calculation of the symmetry indices, the variation within horses was expected to decrease. A good repeatability of the symmetry scores had been described as a lower within-horse variation than the between-horses variation in sound horses (Halling Thomsen et al. 2010).

Fourier analysis has been used in many other studies (Audigiéet al. 2002; Keegan et al. 2004; Pfau et al. 2007; Church et al. 2009) for assessment of lameness. In a study of hindlimb lameness the energy ratio of the Fourier coefficients from vertical pelvis displacement was shown to be a more useful parameter than the difference in vertical movement of the 2 tuber coxae and the tuber sacrale for the assessment of the degree of lameness (Church et al. 2009). Also, Keegan et al. (2004) used the energy ratio of Fourier coefficients for detection of lameness and found a high correlation with a video-based kinematic gait analysis system.

Several studies have focused on the use of accelerometers to detect and quantify lameness in horses and various anatomical locations of accelerometers have been used. Weishaupt et al. (1993) used vertical acceleration of the head to quantify lameness and found that the degree of symmetry of the amplitude of the acceleration correlated with the degree of lameness. Keegan et al. (2004) used accelerometers attached to the head and the skin over the tuber sacrale in combination with gyroscopic transducers attached to the right forelimb and the right hindlimb. The gyroscopic transducers on the limbs were connected to a transmitter on the dorsum of the back with wires, application of which is time-consuming, apart from possibly irritating the horse and even influencing its gait.

For the detection of hindlimb lameness data from sensors attached to the skin over the 2 tuber coxae and over the tuber sacrale have proven useful (Keegan et al. 2004; Pfau et al. 2007; Church et al. 2009). For attachment of sensors to the skin in the pelvic region, however, clipping the coat and glueing is necessary, which might be displeasing to some horse owners. In the studies of Keegan et al. (2004), Pfau et al. (2007) and Church et al. (2009) quantification of lameness was based on vertical displacement data derived by a double integration of the acceleration data.

In this study, the placement of the accelerometer in the saddle area by an elastic girth disturbed the horses to a minimal extent only, as most horses are trained to wear a saddle or girth. Also, the instrumentation was simple and very easy to mount on the horse.

The accelerometric method proved most useful for low or medium grade forelimb lameness evaluations. One horse attained a severe lameness (grade 4 on the AAEP scale), making it impossible for the horse to trot in a regular manner, which was a prerequisite for the use of the Fourier analysis. As a result of this, the accelerometric measurement for one horse at 3 min post injection had to be discarded. The method, therefore, appears of only little clinical value in severely lame horses. Furthermore, evaluation of the method in hindlimb lameness and in clinical lameness cases is needed.

In conclusion this study shows that symmetry scores based on accelerometric data may be an aid in the objective assessment and documentation of lameness. Future attempts to use the scores in clinical cases should aim at the development of a combined scoring system to take advantage of the strengths of each of the symmetry scores.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturer's addresses
  10. References

The study was supported by grants from Intervet Denmark A/S and Foreningen KUSTOS of 1881.

Manufacturer's addresses

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturer's addresses
  10. References

1 Mega Electronics Ltd. FI-70211 KUOPIO.

2 Panasonic, Osaka, Japan.

3 Matlab, The MathWorks, Natick, USA.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturer's addresses
  10. References
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