Reproducibility and feasibility of acoustoelastography in the superficial digital flexor tendons of clinically normal horses Michelle E. Ellison, VMD; Sarah Duenwald-Kuehl, PhD; Lisa J. Forrest, VMD; Ray Vanderby Jr, PhD; Sabrina H. Brounts, DVM, MS

Objective—To evaluate the feasibility and repeatability of in vivo measurement of stiffness gradients by means of acoustoelastography in the superficial digital flexor tendons (SDFTs) of clinically normal horses. Animals—15 clinically normal horses. Procedures—For each horse, stiffness gradient index and dispersion values for SDFTs in both forelimbs were evaluated in longitudinal orientation by use of acoustoelastography at 3 sites (5, 10, and 15 cm distal to the accessory carpal bone) by 2 observers; for each observer, data were acquired twice per site. The left forelimb was always scanned before the right forelimb. Lifting of the contralateral forelimb with the carpus flexed during image acquisition resulted in the required SDFT deformation in the evaluated limb. Interobserver repeatability, intraobserver repeatability, and right-to-left limb symmetry for stiffness gradient index and dispersion values were evaluated. Results—Stiffness gradient index and dispersion values for SDFTs at different locations as well as effects of age or sex did not differ significantly among the 15 horses. Interclass correlation coefficients for interobserver repeatability, intraobserver repeatability, and limb symmetry revealed good to excellent agreement (intraclass correlation coefficients, > 0.74). Conclusions and Clinical Relevance—Results indicated that acoustoelastography is a feasible and repeatable technique for measuring stiffness gradients in SDFTs in clinically normal horses, and could potentially be used to compare healthy and diseased tendon states. (Am J Vet Res 2014;75:581–587)

I

n performance horses, the SDFT is the most commonly injured soft tissue structure; the incidence of SDFT injury in Thoroughbred racehorses is 8% to 43%.1–4 During maximal exercise, the SDFT operates close to its biomechanical limits.5 Injury can result following an accumulation of microdamage, age-induced degeneration, damage caused by repetitive cycling, or Received November 26, 2013. Accepted January 31, 2014. From the Department of Surgical Sciences, School of Veterinary Medicine (Ellison, Forrest, Brounts), and the Department of Orthopedics and Rehabilitation, School of Medicine and Public Health (Duenwald-Kuehl, Vanderby Jr), University of Wisconsin, Madison, WI 53705. Dr. Ellison’s present address is Department of Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803. Presented in part as abstracts at the 2013 American College of Veterinary Surgeons Symposium, 2013 Annual Scientific Conference of the American College of Veterinary Radiology, and 2013 Annual Meeting of the British Equine Veterinary Association. Supported by a Companion Animal Grant through the University of Wisconsin School of Veterinary Medicine. The authors thank Hirohito Kobayashi of Echometrix LLC for technical assistance. Dr. Vanderby Jr holds a patent associated with some aspects of this concept for ultrasound analysis. No other authors have any conflicts of interest with the present study. Address correspondence to Dr. Brounts ([email protected]. edu). AJVR, Vol 75, No. 6, June 2014

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AEG DV ICC SDFT SGI

ABBREVIATIONS

Acoustoelastography Dispersion value Intraclass correlation coefficient Superficial digital flexor tendon Stiffness gradient index

a single acute overload.3,4,6,7 Heat damage as a consequence of the viscoelastic nature and poor blood supply of the tendon may also have a role in injuries.4 For affected horses, prognosis for return to racing is guarded to poor, and up to 23% to 80% of horses sustain reinjury; however, prognosis for return to other athletic disciplines is more optimistic.2–4 Traditional B-mode sonography is the current standard test for diagnosis of tendon injury in horses. Sonographic characteristics of tendon injury include changes in cross-sectional area, shape, fiber pattern, and pixel intensity.8–11 Assessment of tendon function can only be determined by extrapolation of this anatomic information combined with physical examination findings. Because small changes in B-mode sonographic appearance are associated with large changes in biomechanical strength,8 it is difficult to determine the degree of activity that the tissue can sustain.8,10 For example, 581

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slight and moderate superficial digital flexor tendonitis may appear healed at 4 to 5 months after injury on the basis of findings of clinical and sonographic examination.3 However, a minimum period of 6 to 8 months and as long as 18 months of rehabilitation is needed before scar tissue in the SDFT will have the biomechanical properties required to support full work.2,3,6,7 Because of the importance of determining the effect of injury on the function of tendons and ligaments, multiple methods for measuring strain (defined as the change in tendon or ligament length in response to applied force or deformation normalized by the length before manipulation) have been examined.12–15 If the strain of a tendon remains low, the tendon can return to its original length and function as a result of the elastic manner in which it behaves. When the strain in a tendon increases, first microscopic damage and eventually macroscopic failure will develop, thereby affecting the function of the tendon.16 Previous strain measurement techniques are poorly adaptable to veterinary medicine because of invasiveness, necessity for unrealistic levels of patient cooperation, expense, and specialized equipment requirements. Preliminary studies of noninvasive, sonographic methods for evaluation of tendon function in veterinary species have been based on elastography,17 measurement of the axial speed of sound,18–22 or acoustoelasticity.23,24 These techniques currently offer the greatest potential for practical clinical application in horses with tendon injury. Elastography is an ultrasonographic technique that evaluates motion of tendon tissue by comparing ultrasound echoes before and after compression with the ultrasound transducer. If the compression is uniform, the strain of the tissue can then be estimated.17 This technique works best when strain increments are small (< 1%) because the model assumes that the mechanical properties of a material and the ultrasound wave velocity within that material do not change with deformation.23 Larger strain changes relating to movements of the horse during examination could impact the technique. Furthermore, during activity, tendons undergo a relatively large amount of deformation. For example, strains from 2.2% to 4.5% when walking and up to 16.6% when galloping have been measured within the SDFTs of horses.5 Thus, any future application of the technique to assess stiffness during locomotion would be precluded by its small strain assumptions. In addition, elastography indirectly assesses tissue stiffness by examining compression in the transverse plane, whereas tendons are physiologically loaded in a longitudinal plane. Although both of these shortcomings could be addressed by measurement of axial speed of sound, the specialized transducers required for this technique limit the potential for widespread clinical use.19,20 Acoustoelasticity is a mathematical theory that relates changes in innate acoustic characteristics, including the speed and amplitude of propagating sound waves, to deformation applied to a target material. Acoustoelastography is a postprocessing ultrasoundbased tissue evaluation technique derived from this theory. Specifically, AEG relates the change in echo intensity (numerical grayscale brightness) observed during deformation of a tendon from an unloaded to 582

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loaded state to the tissue’s mechanical properties.23,25,26 In vitro, AEG has been proven to accurately model strain and stiffness within porcine flexor tendons.23 To the authors’ knowledge, AEG has yet to be evaluated for use in vivo in equine tendons. Because SDFT injuries are a prevalent problem in horses competing in all disciplines, the study reported here was undertaken to evaluate the feasibility and repeatability of in vivo measurement of stiffness gradients by means of AEG in the SDFTs of clinically normal horses. Materials and Methods Horses—Fifteen clinically normal horses were included in the study. Nine horses were selected from the University of Wisconsin teaching herd; the remaining 6 were recruited with informed owner consent from the population of client-owned animals brought to the University of Wisconsin Veterinary Medical Teaching Hospital for a procedure unrelated to orthopedic disease. Horses were defined as clinically normal on the basis of history (obtained from the owner) and results of a physical examination and complete orthopedic and lameness examination performed by a board-certified equine surgeon (SHB). Lack of tendon abnormalities was confirmed during the subsequent sonographic evaluations. All animal protocols were approved by and in accordance with the University of Wisconsin Research Animal Resources Center. Experimental procedures—Each horse was sedated with a combination of detomidine hydrochloride (10 to 20 mg/kg, IV) and butorphanol tartrate (0.01 to 0.02 mg/kg, IV). The palmar portion of both forelimbs from the proximal to distal aspect of the metacarpus was clipped and washed with water in preparation for sonographic evaluation. To establish that no B-mode sonographic abnormalities were evident, longitudinal and transverse images of the right and left forelimb SDFTs at 5, 10, and 15 cm distal to the accessory carpal bone were obtained with a 12-MHz linear transducer (range of transducer, 7.5 to 12 MHz). A standardized frequency setting of 12 MHz was maintained during each scan. Focal zone position was set at the level of the SDFT, and gain (maintained between 54% and 62% for all horses) was subjectively set by the sonographer to provide the optimal image for each acquisition. Cine videos were obtained in longitudinal orientation at each image site as the tendon was weight-loaded from a baseline square stance to a full weight-bearing stance and then placed back to a baseline square stance. This was accomplished by having an assistant lift the contralateral forelimb into a non–weight-bearing position with the carpus flexed, so that additional weight was shifted onto the limb being scanned, and replacing the contralateral forelimb back down on the ground. This motion was performed as fluently as possible. At each site along each SDFT, the cine video acquisition was performed 2 times by each sonographer (MEE and SHB) to analyze intraobserver repeatability. At the time of image acquisitions, each sonographer acted as the assistant to the other person when not performing the sonographic scans. For all horses, the left forelimb was always scanned before the right limb; however, which AJVR, Vol 75, No. 6, June 2014

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sonographer acquired data first was alternated between examined horses. Data analysis—Uncompressed audio video interleave format files for each of the cine videos were then used for SGI analyses with special analytic softwarea by 1 person (SDK) who was unaware of which horse and which sonographer contributed to each cine video. A region of interest consistent in size and location (approx the middle half of the tendon section) was selected in each cine video. To maintain this same region of interest throughout the cine video, a region-based optical flow tracking technique was used to estimate the movement of speckles between consecutive frames.23 The changes in relative pixel intensity and location over time were analyzed via AEG for calculation of a stiffness gradient (change in stiffness in relation to the change in strain) for each pixel. To quantify stiffness gradients, SGI values for each cine video were calculated by averaging the stiffness gradients for each pixel within a 50 X 300-pixel area located centrally within the original region of interest. Inhomogeneity in the tissues was quantified by use of DVs. Dispersion values were calculated by averaging the SD of stiffness gradients within the defined area. This metric allowed assessment of the distribution of the stiffness gradients. Mean values for SGI and DV for each site on both forelimbs across both observers, mean difference of SGI and DV between observers for each site on both forelimbs, and mean difference of SGI and DV for each site between all right limbs and all left limbs were calculated. Statistical analysis—All statistical tests were selected by a statistician and performed by 1 person (SDK) using statistical software.b,c Regional differences in SGI and DV were compared across all sites for each limb by use of a repeated-measures ANOVA and post hoc analysis with a Tukey honestly significant difference test. Effects of age and sex (male [sexually intact and castrated] vs female) on SGI and DV were tested with a Pearson correlation and Student t test, respectively. For all comparison tests, a significant difference was defined as a value of P < 0.05. Intraobserver repeatability, interobserver repeatability, and symmetry beTable 1—Mean SGI and DV for the SDFTs of both forelimbs in 15 clinically normal horses derived by use of AEG. Distance distal to the accessory carpal bone (cm) Mean

SGI 95% CI

Mean

DV 95% CI

5 10 15 Overall

0.0172–0.0214 0.0175–0.0218 0.0178–0.0220 0.0176–0.022

0.0024 0.0024 0.0025 0.0024

0.0022–0.0026 0.0022–0.0026 0.0022–0.0028 0.0022–0.0027

0.0193 0.0196 0.0199 0.0196

For each horse, SGI and DV for SDFTs in both forelimbs were evaluated in longitudinal orientation by use of AEG at 3 sites (5, 10, and 15 cm distal to the accessory carpal bone) by 2 observers; for each observer, data were acquired twice per site. The left forelimb was always scanned before the right forelimb. Lifting of the contralateral forelimb with the carpus flexed during image acquisition resulted in the required SDFT deformation in the evaluated limb. The overall value of SGI and DV were calculated as the mean of all values obtained from the 3 sites. CI = Confidence interval. AJVR, Vol 75, No. 6, June 2014

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tween right and left limbs at each of the 3 sites for both the SGI and DV were measured with ICCs. Intraclass correlation coefficients cutoffs were defined as follows: poor agreement, < 0.4; moderate agreement, 0.41 to 0.6; good agreement, 0.61 to 0.79; and excellent agreement, > 0.8.27 Any horse that had an error for a site was removed from the intraobserver repeatability analysis for that site. In addition, 95% confidence intervals were calculated for mean values for SGI and DV for each site on both forelimbs across both observers, mean difference of SGI and DV between observers for each site on both forelimbs, and mean difference of SGI and DV for each site between all right limbs and all left limbs. Results Horses included in the study ranged in age from 1 to 22 years (mean age, 8.3 years; median age, 9 years). Among the 15 horses, there were 9 mares, 1 stallion, and 5 geldings. All of the geldings had been brought to the University of Wisconsin Veterinary Medical Teaching Hospital for elective castration, which was performed the day before the sonographic examination. Five breeds were represented in the group: Quarter Horse (n = 9), Thoroughbred (2), Arabian (2), Saddlebred (1), and Tennessee Walking Horse (1). Four of the 360 cine videos acquired failed to capture a strain of at least 2% during loading (1 each for 4 horses; errors occurred at 15 cm [n = 2], 10 cm [1], and 5 cm [1] distal to the accessory carpal bone). Information from these 4 cine videos was not used in subsequent analyses. The SGIs for the SDFT at sites 5, 10, and 15 cm distal to the accessory carpal bone as well as overall SGI values for the imaged regions were summarized (Table 1). There was no significant difference in SGI at the examined sites (ANOVA, P = 0.907; post hoc, P = 0.818, 0.835, and 0.661 for comparisons between the sites at 5 and 10 cm, 10 and 15 cm, or 15 and 5 cm distal to the accessory carpal bone, respectively). Similarly, no significant effects of age (P = 0.265) or sex (P = 0.322) on SGI were observed. The DVs for the SDFT at sites 5, 10, and 15 cm distal to the accessory carpal bone as well as overall DVs for the imaged regions were summarized (Table 1). There was no significant difference in DV at the examined sites (ANOVA, P = 0.751; post hoc, P = 0.995, 0.818, and 0.766 for comparisons between the sites 5 and 10 cm, 10 and 15 cm, or 15 and 5 cm distal to the accessory carpal bone, respectively). Similarly, no significant effects of age (P = 0.66) or sex (P = 0.352) were observed. Mean difference in SGIs and DVs between the 2 observers (Table 2) and between right and left forelimbs (Table 3) for each site as well as overall values were summarized. All differences between observers and between limbs were < 13% of the mean for SGI and < 23% for DV. All 95% confidence intervals contained zero for these comparisons. Repeatability, as assessed by ICC values, of SGI and DV within the same observer, between observers, and between right and left limbs was summarized (Table 4). Intraobserver (ICC, 0.80 to 0.88) and interobserver (ICC, 0.82 to 0.89) reproducibilities were excellent. 583

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Table 2—Mean difference in SI and DV for the SDFTs of both forelimbs in 15 clinically normal horses derived by use of AEG performed by 2 sonographers (sonographer 1 – sonographer 2). Distance distal to the accessory carpal bone (cm) 5 10 15 Overall

Difference in SGI Mean 95% CI 6.35 X 10–4 2.75 X 10–4 6.20 X 10–4 5.10 X 10–4

Difference in DV Mean 95% CI

–0.00182 to 0.00309 –0.00311 to 0.00366 –0.00282 to 0.00406 –0.00258 to 0.00360

–1.64 X 10–4 –5.46 X 10–4 9.21 X 10–5 –2.28 X 10–5

–0.0005 to 0.0002 –0.0003 to 0.0003 –0.0003 to 0.0005 –0.0004 to 0.0003

See Table 1 for key. Table 3—Mean difference in SGI and DV for the SDFTs of the left and right forelimbs (left – right) in 15 clinically normal horses derived by use of AEG. Distance distal to the accessory carpal bone (cm) 5 10 15 Overall

Difference in SGI Mean 95% CI 0.00170 –0.00242 8.48 X 10–4 4.49 X 10–5

Mean

–0.00395 to 0.00054 –0.00416 to –0.00067 –0.00126 to 0.00296 –0.00215 to 0.00224

Difference in DV 95% CI

–1.10 X 10–4 –1.59 X 10–4 1.18 X 10–4 –5.03 X 10–5

–0.0004 to –0.0002 –0.0004 to 0.0001 –0.0002 to 0.0004 –0.0003 to 0.0002

See Table 1 for key. Table 4—Interclass correlations for SGI and DV for the SDFTs of both forelimbs in 15 clinically normal horses derived by use of AEG performed by 2 sonographers. SGI Comparison Intraobserver reliability: sonographer 1 Intraobserver reliability: sonographer 2 Interobserver reliability Left-to-right limb symmetry

DV

ICC

95% CI

ICC

95% CI

0.875 0.819 0.822 0.918

0.805–0.945 0.744–0.895 0.772–0.087 0.869–0.966

0.830 0.805 0.891 0.740

0.770–0.889 0.694–0.915 0.855–0.928 0.639–0.841

See Table 1 for key.

There was symmetry between right and left limbs (ICC, 0.74 to 0.92). Discussion In the present study, we evaluated the feasibility and repeatability of AEG to measure stiffness gradients of SDFTs in clinically normal horses. Excellent (ICC, > 0.8) intraobserver repeatability and interobserver repeatability indicated reproducibility in application of AEG by the same or different sonographers. The mean difference between observers relative to the measured mean for SGI was < 13%, and the mean difference between observers relative to the measured mean for DV was < 23%, which further established that there was good repeatability and precision in the use of AEG for normal equine SDFTs in vivo. Because the 95% confidence intervals for the differences between observers all contained zero, there was no statistical difference in SGIs or DVs obtained by the 2 sonographers. Although previous research has revealed great changes in biomechanical behavior in injured tendons,28,29 additional studies must be done to determine whether SGI values for injured tendons are outside the confidence intervals for SGI values of noninjured tendons. The small SGIs and low DVs derived by AEG evaluation of SDFTs in clinical normal horses were indicative of relatively stiff tissue with minimal internal variation. 584

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In addition, there was good to excellent right-to-left symmetry between limbs (ICC, > 0.74). These results were consistent with expected findings because the SDFT is composed of a dense, homogeneous, parallel arrangement of fibers with a high elastic modulus and low homotypic variation between limbs.10,30 There were no significant differences in SGI or DV at 5, 10, or 15 cm distal to the accessory carpal bone. Decreased elastic modulus and stiffness of the sesamoidodigital portion, compared with findings for the metacarpal region, of the equine SDFT have been reported.26 Evaluation of the sesamoidodigital portion of the tendon was not included in the present study because most SDFT injuries occur in the mid-metacarpal region.3,7,31 No age or sex effect (male [sexually intact and castrated] vs female) on SGI and DV for the SDFTs in the horses of the present study was evident. This result differs from findings of previous studies32–37 in both humans and horses. However, the small number of horses in the present study and their variation in age, sex, and breed limited the likelihood of finding any effect of sex or age on equine tendon biomechanics. In 1 study,32 stiffness of the SDFT in horses > 2 years of age was greater than that in 2-year-old horses. The increased stiffness of mature tendon is attributed to an increase in nonreducible collagen cross-linking, a decrease in fascicle size, and selective reductions in collagen crimp angle and length over time.5,7 In human quadriceps AJVR, Vol 75, No. 6, June 2014

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and Achilles tendons, decreased stiffness has been correlated with age.33,34 Similarly, an apparent decrease in elastic modulus has been in observed horses > 15 years of age, compared with horses ≤ 15 years of age; however, that finding was not significant, possibly because of the small sample size.32 This age-related decrease in stiffness is most likely a result of tendon matrix degeneration secondary to an accumulation of microdamage. The cause of microdamage is unknown but may involve some combination of repetitive trauma, hyperthermia, and hypoxia.4,7 In humans, gender-specific differences have also been reported and may have a role determining tissue strength because tendons in females have lower stiffness and elasticity, compared with findings for tendons in males.35,37 Analysis of SDFT crosssectional area and tendon biomechanics in horses has not revealed any sex-specific differences; however, the previous reports also had small sample sizes.32,36 For horses, it is considered unlikely that the potential effect of either age or sex would result in change in the mechanical properties of tendons similar in magnitude to that caused by acute tendon injury.38 Additional research would be required to determine whether any potential age- or sex-related changes could affect the interpretation of AEG findings with regard to evaluation of more chronic lesions, risk of clinically relevant tendon injury, or degree of healing. None of the horses in the present study were used for high-level athletic activity; therefore, the effect of exercise on AEG evaluation was not determined. Exercise has been shown to affect the collagen fibers (reduced crimp angle and decreased fibril diameter) as well as the extracellular matrix (decreased cartilage oligomeric matrix protein) in both mature and immature SDFTs.5,39 Attempts to sonographically identify the effect of exercise on equine tendons by monitoring their cross-sectional area have led to mixed results; in some studies, no effect was detected, whereas in others, training-induced adaptive hypertrophy was apparent.36,40–42 Most studies did not include histologic examination of tendon tissues to confirm that the small increases in cross-sectional area were not a result of subclinical injury. Additional research will be required to assess the potential effect of exercise on AEG evaluation of equine tendons. The present study had some limitations. One of the limitations was the small sample size, which reduced the likelihood of finding any effect of the horses’ age or sex on the variables of interest. Future studies with a larger sample size are needed to evaluate the influence of age and sex on the AEG technique. Also, differences in breed and exercise regimens may be factors that could influence AEG findings; the effects of such differences need to be further explored. Another limitation was that the study horses were presumed to have normal SDFTs on the basis of a combination of history (obtained from the owner) and findings of physical examination, complete orthopedic and lameness evaluation, and sonographic assessment. No histopathologic confirmation of normal tendon status was obtained. Horses with any clinical abnormalities or abnormalities relating to the SDFT were excluded from the study because the sensitivity of the AEG technique for deAJVR, Vol 75, No. 6, June 2014

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tection of subtle compensatory changes due to lameness is unknown. Despite these restrictions, chronic or subclinical tendon injury could have been present in some of the horses. In addition, the use of AEG requires tissue deformation. Thus, static images or video images, which fail to capture tendon stretch, cannot be used for analysis. Only a minimal number of cine videos (4/360) could not be analyzed in the present study because of inadequate strain; this was likely a result of weight being distributed toward the hind limbs rather than shifting solely to the evaluated forelimb when the contralateral forelimb was lifted or because the entirety of the limb movement was not captured in the cine image. Given that stiffness is a strain-dependent property, analysis of stiffness gradient relies on creating a consistent motion within a tendon. All horses included in the present study were able to comply with the requirements to generate adequate strain within the SDFT when the contralateral limb was lifted in a controlled manner. Despite some of the limitations of the present study, AEG is a new technique that has potential benefits for tendon health in horses. Evaluation of a tendon’s mechanical properties with AEG could possibly allow earlier detection of injury and more accurate monitoring of healing.43,44 Rehabilitation programs could be individually tailored during the healing process of a tendon, thereby reducing the rate of reinjury. Degenerative changes in horses at high risk for tendon injury may be detected prospectively before a career-ending injury occurs. Functional information derived by such a new technique has the potential to quickly provide more accurate prognoses to clients. Additional research is needed to determine whether evaluation of injured and healing equine tendons with AEG would reveal injury-induced alterations in the mechanical properties of tendon tissues. Tendons with acute injury have been previously described as having decreased elastic modulus, stiffness, and echogenicity, compared with findings for normal noninjured tissue.5,38,45,46 In clinical pilot studies47–49 in humans with tendon injury, sonoelastography has most commonly detected decreased tissue stiffness and increased heterogeneity within the elastogram, compared with findings for tendons of unaffected humans. However, some conflicting results have also been published.50 During the healing process, echogenicity of a tendon may return to normal; however, the mechanical properties of the healed tendon may remain inferior to that of uninjured tissue.46,51 Both axial speed of sound analysis and kinematic evaluation have revealed a gradual return of variables to values expected for noninjured tendons during healing of SDFTs in horses; however, only the initial course of the healing process was examined.19,45 Of the 3 methods of sonographic evaluation of tendon function currently being investigated in veterinary species (elastography, measurement of the axial speed of sound, and AEG), only the measurement of the axial speed of sound has been successfully used to identify surgically induced tendon injury and monitor healing.19,20 Unfortunately, this measurement technique requires the use of specialized transducers designed to fit the anatomic region of interest.19,20 585

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Multiple specialized transducers would be needed to evaluate different locations along a limb or equids with large variation in size, such as draft or pony breeds. Widespread adoption of functional ultrasonography in veterinary clinical practice requires an inexpensive, uncomplicated, easily repeatable, noninvasive method of analysis. As software-based techniques that use equipment that are used in most veterinary practices, only AEG and elastography currently meet these requirements. Acoustoelastography provides more mechanical metrics (both stress and strain) as well as longitudinal plane assessment of tendon mechanics, compared with elastography.23 However, its application is limited to those tendons for which a sonographic window can be maintained while being moved from a stressed to relaxed state.23 Experiments to compare the use of AEG and elastography for a variety of tendons and injuries are required to determine whether there are clinically important differences in the functional analysis provided by the 2 techniques. Results of the present study indicated that the stiffness gradients of SDFTs measured with AEG in clinically normal horses were consistent with a dense, homogeneous, parallel arrangement of fibers with a high elastic modulus and low homotypic variation between forelimbs. The AEG technique was found to have high repeatability and precision for measuring stiffness gradients in a clinical setting. Studies to determine the ability of the AEG to detect tendon injury and monitor tendon healing in clinically affected horses are warranted. a. b. c.

Echosoft by Echometrix LLC, Madison, Wis. Kaleidagraph, version 4.03, Synergy Software, Reading, Pa. Microsoft Excel 2010, Microsoft, Redmond, Wash.

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Reproducibility and feasibility of acoustoelastography in the superficial digital flexor tendons of clinically normal horses.

To evaluate the feasibility and repeatability of in vivo measurement of stiffness gradients by means of acoustoelastography in the superficial digital...
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