• View in gallery

    Opposite oblique radiographic views (A and B) of a cadaveric equine metacarpophalangeal joint after ultrasound-guided injection of 2 mL of contrast medium through a 25-gauge, 16-mm needle into the subcircumneural space and outside the circumneural sheath. When injected within the subcircumneural space, contrast medium is seen following the nerve, which is outlined as a tubular filling defect. When injected outside the circumneural sheath (asterisk in both panels), the contrast medium only partially surrounds the nerve and does not extend as a uniform column. After contrast medium was injected, a hypodermic needle was inserted into tissue to mark the side on which the contrast medium was injected outside the circumneural sheath.

  • View in gallery

    Transverse-plane ultrasonographic images of the pastern region in the lame forelimb of a horse showing the technique used for injection of local anesthetic solution. A—A 12-MHz linear probe was used to image the neurovascular bundle at the site of a basisesamoid nerve block. B—A 25-gauge needle was placed with ultrasonographic guidance through the circumneural sheath into the neurovascular bundle (subcircumneural space). C—After local anesthetic solution was injected within the subcircumneural space, fluid could be seen in the pastern region surrounding the neurovascular structures.

  • View in gallery

    Mean ± SD values of VS for 6 horses with forelimb lameness evaluated with an inertial sensor–based motion analysis system while trotting. Ultrasound guidance was used to inject 2% mepivacaine hydrochloride around the palmar digital nerves of the affected forelimb at the level of the base of the proximal sesamoid bones into the subcircumneural space (dashed line) or outside the circumneural sheath (solid line) in a crossover study design. Gait was evaluated immediately before injection (0 minutes) and at 5-minute intervals thereafter for 45 minutes.

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  • 3. Boezaart AP. Sweet spot of the nerve: is the “paraneural sheath” named correctly, and does it matter? Reg Anesth Pain Med 2014;39:557558.

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    • Export Citation
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  • 6. Millesi H, Hausner T, Schmidhammer R, et al. Anatomical structures to provide passive motility of peripheral nerve trunks and fascicles. Acta Neurochir Suppl 2007;100:133135.

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  • 7. Missair A, Weisman RS, Suarez MR, et al. A 3-dimensional ultrasound study of local anesthetic spread during lateral popliteal nerve block: what is the ideal end point for needle tip position? Reg Anesth Pain Med 2012;37:627632.

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  • 13. McCracken MJ, Kramer J, Keegan KG, et al. Comparison of an inertial sensor system of lameness quantification with subjective lameness evaluation. Equine Vet J 2012;44:652656.

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  • 27. Nagy A, Bodó G, Dyson SJ, et al. Distribution of radiodense contrast medium after perineural injection of the palmar and palmar metacarpal nerves (low 4-point nerve block): an in vivo and ex vivo study in horses. Equine Vet J 2010;42:512518.

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Comparison of speed of onset and analgesic effect of 2% mepivacaine hydrochloride deposited within or outside the neurovascular bundle at the level of the proximal sesamoid bones in horses with naturally occurring forefoot-related lameness

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  • 1 1Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL 36849.
  • | 2 2Department of Agriculture, Ogden College of Science and Engineering, Western Kentucky University, Bowling Green, KY 42101.

Abstract

OBJECTIVE

To compare the speed of onset and analgesic effect of mepivacaine deposited within or immediately outside the neurovascular bundle at the base of the proximal sesamoid bones in horses.

ANIMALS

6 horses with naturally occurring forefoot-related lameness.

PROCEDURES

In a crossover study design, horses were randomly assigned to receive 1 of 2 treatments first, with the second treatment administered 3 to 7 days later. Trotting gait was analyzed with an inertial sensor–based motion analysis system immediately before treatment to determine degree of lameness. Afterward, ultrasound guidance was used to inject 2% mepivacaine hydrochloride around the palmar digital nerves of the affected forelimb at the level of the base of the proximal sesamoid bones either within the subcircumneural space or outside the circumneural sheath. After injection, gait was reevaluated at 5-minute intervals for 45 minutes.

RESULTS

Mepivacaine deposition outside the circumneural sheath did not resolve lameness in any horse; for 3 horses, the mean time to 70% reduction of initial vertical head movement was 13.3 minutes, and the remaining 3 horses had no such reduction at any point. Mepivacaine deposition within the subcircumneural space resulted in a mean time to 70% reduction of initial vertical head movement of 6.7 minutes and mean time to resolution of lameness of 21.7 minutes.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that when peripheral nerves of horses lie within a sheath, local anesthetic solution should be deposited within the sheath for an effective nerve block. If local anesthetic solution is deposited outside the sheath, the nerve block may yield erroneous results.

Abstract

OBJECTIVE

To compare the speed of onset and analgesic effect of mepivacaine deposited within or immediately outside the neurovascular bundle at the base of the proximal sesamoid bones in horses.

ANIMALS

6 horses with naturally occurring forefoot-related lameness.

PROCEDURES

In a crossover study design, horses were randomly assigned to receive 1 of 2 treatments first, with the second treatment administered 3 to 7 days later. Trotting gait was analyzed with an inertial sensor–based motion analysis system immediately before treatment to determine degree of lameness. Afterward, ultrasound guidance was used to inject 2% mepivacaine hydrochloride around the palmar digital nerves of the affected forelimb at the level of the base of the proximal sesamoid bones either within the subcircumneural space or outside the circumneural sheath. After injection, gait was reevaluated at 5-minute intervals for 45 minutes.

RESULTS

Mepivacaine deposition outside the circumneural sheath did not resolve lameness in any horse; for 3 horses, the mean time to 70% reduction of initial vertical head movement was 13.3 minutes, and the remaining 3 horses had no such reduction at any point. Mepivacaine deposition within the subcircumneural space resulted in a mean time to 70% reduction of initial vertical head movement of 6.7 minutes and mean time to resolution of lameness of 21.7 minutes.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that when peripheral nerves of horses lie within a sheath, local anesthetic solution should be deposited within the sheath for an effective nerve block. If local anesthetic solution is deposited outside the sheath, the nerve block may yield erroneous results.

Peripheral nerves often lie adjacent to blood vessels within neurovascular bundles surrounded by a sheath composed of layers of fascia.1–5 The purpose of the sheath is to allow friction-free movement of nerves against surrounding structures.6 This sheath has been termed the common epineural sheath,5 complex fascial layer,7 adventitia of the nerve,6 and gliding tissue of the nerve.8 To avoid confusion, it was suggested that the sheath be called the paraneural sheath and the space below it the subparaneural space.2 More recently, it was proposed that the sheath be called the circumneural sheath and the space below it the subcircumneural space.3,9

The subcircumneural space is sometimes referred to as the “sweet spot of the nerve” by anesthesiologists, representing the site where a nerve block provides a fast onset and satisfactory duration.3 The circumneural sheath itself has clinical importance because it may delay diffusion of local anesthetic solution into the nerve when the solution is deposited outside the sheath.2,3,8 The purpose of the study reported here was to determine whether injection of a local anesthetic solution outside the circumneural sheath of the palmar digital nerves (ie, outside the neurovascular bundle) at the base of the proximal sesamoid bones would ameliorate lameness caused by pain within the region innervated by this nerve.

Materials and Methods

Animals

Six horses with naturally occurring lameness obtained from the teaching herd of Auburn University were used for the study. Prior to study inclusion, horses were determined to have a consistent, subjectively observable forelimb lameness when trotting on a hard surface. Foot pain had previously been identified as the cause of lameness. Horses ranged in body weight from 544 to 625 kg (mean ± SD, 577 ± 29 kg) and in age from 8 to 16 years (mean ± SD, 12.5 ± 2.6 years). The study protocol was approved by the Auburn University Institutional Animal Care and Use Committee (protocol No. 2019–3448).

Experimental protocol

In a crossover study design, horses were randomly assigned by coin toss to receive 1 of 2 treatments first (n = 3 horses/treatment), with the other treatment administered 3 to 7 days later. Objective lameness evaluation was performed immediately before treatment to determine the forelimb to be treated (for the first treatment only) and degree of lameness in that limb (both treatments). Afterward, the palmar digital nerves were anesthetized by injection of 2% mepivacaine hydrochloridea into the subcircumneural space near the base of the proximal sesamoid bones (ie, a basisesamoid nerve block) or immediately outside the circumneural sheath. For both treatments, ultrasonography was used to verify needle position prior to injection and again after injection to determine whether solution had been deposited at the intended site. Lameness was reassessed at 5-minute intervals after injection for a total period of 45 minutes in both treatment sessions. Details of the specific procedures used in this protocol were as follows.

Preliminary testing—A single investigator (RCC) performed all ultrasound-guided needle placements and injections. Preliminary testing was first performed to determine the accuracy of needle placement and the location of injectate in 4 equine cadaveric forelimbs. For this testing, ultrasonography was used to identify the neurovascular bundle on the abaxial side of the proximal sesamoid bones. This bundle contains the palmar digital artery, vein, and nerve, which are surrounded by loose hypoechoic fascia (the circumneural sheath).9 A 12-MHz linear probe was oriented to produce a transverse image. A 25-gauge, 16-mm needle was then placed with ultrasound guidance through the circumneural sheath into the neurovascular bundle (subcircumneural space) or directly adjacent to the neurovascular bundle without penetrating the fascia, and 2 mL of iopamidolb was injected as contrast medium. Radiography was performed to verify that the contrast medium had been deposited at the expected site.

Injection technique—Hair was clipped from the planned site of injection, and the skin was cleaned with 70% isopropyl alcohol. Each horse was restrained with a lip twitch, and ultrasound guidance as described for the preliminary testing was used to place a 25-gauge, 16-mm needle into the subcircumneural space of the medial and lateral palmar digital nerves by inserting it over the neurovascular bundle and directing it distally so that when the needle was inserted to its hub, the needle tip lay at the distal border of the proximal sesamoid bone within the subcircumneural space. To place the needle directly outside and adjacent to the circumneural sheath, the needle was inserted abaxial to the neurovascular bundle and directed distally. Once the needle was confirmed to be in the proper site, 2 mL of mepivacaine was deposited either into the subcircumneural space or outside but adjacent to the circumneural sheath. Afterward, ultrasonography was used to verify that local anesthetic solution had been deposited at the intended site.

Objective lameness evaluation—Lameness evaluations were performed with a body-mounted inertial sensor systemc while horses were trotting in a straight line as led by a handler from the left with a lead shank attached to a head halter. For these evaluations, each horse was instrumented with head, right forelimb pastern, and pelvis sensor devices placed and secured in accordance with the manufacturer's recommendations and as described elsewhere.10–16 To keep the speed of movement consistent, the same handler was used for all evaluations.

The nature of forelimb lameness was measured as the VS of MaxHDiff and MinHDiff (ie, VS = [MaxHDiff2 + MinHDiff2]1/2), both of which are measures of asymmetric head height between right and left forelimb strides. The VS is an overall measurement of the magnitude of forelimb lameness and is given a positive value if MinHDiff is > 0 and a negative value if MinHDiff is < 0, indicating right forelimb and left forelimb lameness, respectively. An estimate of the reference range of VS between evidence of forelimb lameness and no evidence of forelimb lameness based on the 95% CIs of the y-intercepts of MaxHDiff and MinHDiff (both of which are ± 6 mm) is ± 8.5 mm.14 Because we were not concerned with hind limb lameness in this study, data collected from vertical pelvic movement were not used. The described method,16–19 measures,10,13,20,21 and thresholds11–13 for evaluation of forelimb lameness in horses have been previously described.

Statistical analyses

Vector sum data were calculated for all lameness trials and are reported as mean ± SD. Mixed linear modeling for repeated measurementsd was performed to compare VS data between treatments and among assessment points. Correlated data were accounted for by use of the following linear model22–24:

article image

where Y is the vector of observations, X is the treatment design matrix (treatment site and timing of lameness evaluations), β is the vector of fixed treatment effects, Z is the random effects design matrix (horse), μ is the vector of random block effects, and e is the vector of experimental error. To account for the non-independence of observations within horses, 5 correlation structures were tested (compound symmetric, first-order autoregressive, Toeplitz, unstructured, and variance components).22,23 Models were compared by consideration of the Akaike information criterion.22–24 The autoregressive correlation structure provided the best fit to the data and was used for the final models. In all models, horse was included as a random effect and the Kenward-Roger correction was used.22–24 P values for multiple comparisons in the final models were adjusted by use of the Tukey-Kramer method.23 Values of P ≤ 0.05 were considered significant.

Results

Injection technique

Radiographs of cadaver limbs verified the accuracy of the ultrasound-guided technique used for needle placement for injection of local anesthetic into the subcircumneural space or outside the circumneural sheath (Figure 1). During ultrasonographic examination of the neurovascular bundle after injection of local anesthetic solution, echogenic reverberation artifact and fluid could be seen in the pastern region surrounding the neurovascular structures after injection (Figure 2). When the needle was placed external to the circumneural sheath, no fluid or gas reverberation was identified around the neurovascular structures in the pastern region after mepivacaine injection.

Figure 1—
Figure 1—

Opposite oblique radiographic views (A and B) of a cadaveric equine metacarpophalangeal joint after ultrasound-guided injection of 2 mL of contrast medium through a 25-gauge, 16-mm needle into the subcircumneural space and outside the circumneural sheath. When injected within the subcircumneural space, contrast medium is seen following the nerve, which is outlined as a tubular filling defect. When injected outside the circumneural sheath (asterisk in both panels), the contrast medium only partially surrounds the nerve and does not extend as a uniform column. After contrast medium was injected, a hypodermic needle was inserted into tissue to mark the side on which the contrast medium was injected outside the circumneural sheath.

Citation: American Journal of Veterinary Research 81, 5; 10.2460/ajvr.81.5.394

Figure 2—
Figure 2—

Transverse-plane ultrasonographic images of the pastern region in the lame forelimb of a horse showing the technique used for injection of local anesthetic solution. A—A 12-MHz linear probe was used to image the neurovascular bundle at the site of a basisesamoid nerve block. B—A 25-gauge needle was placed with ultrasonographic guidance through the circumneural sheath into the neurovascular bundle (subcircumneural space). C—After local anesthetic solution was injected within the subcircumneural space, fluid could be seen in the pastern region surrounding the neurovascular structures.

Citation: American Journal of Veterinary Research 81, 5; 10.2460/ajvr.81.5.394

Objective lameness evaluation

When values for the 5- to 45-minute assessment points were modeled simultaneously, the VS of vertical head movement in the 6 horses was significantly lower for mepivacaine deposition within the subcircumneural space than for mepivacaine deposition outside the circumneural sheath of the palmar digital nerves (P < 0.001; Figure 3). In addition, the asymmetry of vertical head movement decreased significantly (P = 0.03) over time. Mepivacaine deposition within the subcircumneural space induced significantly more effective analgesia as reflected by VS values at 30 (P = 0.02), 35 (P < 0.001), 40 (P = 0.009), and 45 (P = 0.008) minutes after injection, compared with analgesia achieved when mepivacaine was injected outside the circumneural sheath. No significant (P = 0.08 to 0.99) differences in VS were identified at any other assessment point.

Figure 3—
Figure 3—

Mean ± SD values of VS for 6 horses with forelimb lameness evaluated with an inertial sensor–based motion analysis system while trotting. Ultrasound guidance was used to inject 2% mepivacaine hydrochloride around the palmar digital nerves of the affected forelimb at the level of the base of the proximal sesamoid bones into the subcircumneural space (dashed line) or outside the circumneural sheath (solid line) in a crossover study design. Gait was evaluated immediately before injection (0 minutes) and at 5-minute intervals thereafter for 45 minutes.

Citation: American Journal of Veterinary Research 81, 5; 10.2460/ajvr.81.5.394

Mepivacaine deposition within the subcircumneural space resulted in a mean ± SD time to resolution of lameness (vertical head movement < 8.5 mm) of 21.7 ± 14.0 minutes. Amelioration of lameness persisted throughout the 45-minute assessment period for this treatment. No resolution of lameness at any assessment point was observed with mepivacaine deposition outside the circumneural sheath.

Mepivacaine deposition within the subcircumneural space resulted in a mean time to achievement of a 70% reduction of initial vertical head movement (calculated from the time of injection) of 6.7 ± 2.4 minutes. On the other hand, mepivacaine deposition outside the circumneural sheath resulted in a mean time to 70% reduction of initial vertical head movement of 13.3 ± 2.4 minutes for 3 horses only; the other 3 horses did not have a 70% reduction of initial vertical head movement at any assessment point.

Discussion

Many clinicians believe that after receiving a regional nerve block, a lame horse should have a 70% improvement in symmetry of gait for the response to the block to be considered positive.25 Three horses in the present study failed to have a 70% reduction in forelimb lameness for at least 45 minutes when mepivacaine was deposited outside the circumneural sheath of the palmar digital nerves. Although 3 other horses had a 70% improvement in gait, the delay in reaching this degree of improvement was substantially longer than when mepivacaine was deposited within the subcircumneural space. Our study clearly demonstrated that the onset of desensitization of the digit is likely to be delayed substantially when local anesthetic solution is deposited outside the circumneural sheath of a neurovascular bundle containing the palmar digital nerve.

A previous study26 in horses showed that 11% of perineural injections of the palmar digital nerves with radiodense contrast medium were inadvertently deposited outside the circumneural sheath. In another study27 involving the palmar and palmar metacarpal nerves (ie, a low 4-point nerve block), 22% of injections of radiodense contrast medium were inadvertently deposited outside the sheath. Investigators in these 2 studies26,27 speculated that deposition of local anesthetic solution outside this sheath might result in delayed onset with decreased effect. One of these investigators later speculated that deposition of local anesthetic solution outside this sheath might result in no anesthetic effect.28 The results of the present study suggested that when local anesthetic solution is deposited outside the circumneural sheath containing the palmar digital nerves, the solution would likely fail to substantially desensitize the digit within 45 minutes after injection. When a peripheral nerve lies within a circumneural sheath, local anesthetic solution should be deposited within the sheath for maximum effect.

In the present study, we found it difficult to avoid placement of the needle into the subcircumneural space in both cadaveric equine limbs and live horses. In addition to ultrasonographic confirmation of needle position, we were able to discern whether the needle had been placed within or outside the subcircumneural space on the basis of the amount of pressure we needed to apply to the syringe plunger to inject the contrast medium or local anesthetic solution. This observation is in contrast to that of Nagy et al,26 who performed injections guided by palpation alone and found no differences in resistance to injection when radiodense contrast medium was identified in a tubular pattern, assumed to be within the sheath, compared with when it was identified in a patch, assumed to be outside the sheath. Most clinicians likely would not proceed with perineural injection when encountering resistance to depression of the syringe plunger, believing the needle to be embedded intradermally or in fascia other than that of the subcircumneural space. If there is no resistance to injection, however, the site of deposition of local anesthetic solution might be determined by testing the dermatome of the nerve for sensation or by examining the appearance of the subcircumneural space ultrasonographically.

A poor correlation may exist between amelioration of lameness and loss of skin sensation after nerve block administration. In a study29 in which a palmar digital nerve block was administered to horses with signs of foot pain, some horses had amelioration of lameness despite the persistence of skin sensation. When palmar digital nerve blocks were administered by use of 2% mepivacaine in another study30 involving horses with signs of foot pain, skin desensitization occurred sooner for some horses than did resolution of lameness, and use of lidocaine instead of mepivacaine caused skin desensitization but failed to resolve lameness in most horses. Nevertheless, a shortcoming of the present study was the failure to check for skin sensation in the coronary band at any assessment point to allow assessment of correlation of those results with results of gait analysis. It would have been interesting to determine whether skin could be desensitized by injection of local anesthetic solution outside the circumneural sheath.

In people, not all nerves are enclosed within a circumneural sheath for their entire length.5 Envelopment of nerves with a circumneural sheath might explain failure of some nerve blocks and indicate the need for ultrasonographic verification that local anesthetic solution has been deposited within the subcircumneural space if results of perineural anesthesia are unexpectedly negative or equivocal. It would be valuable to investigate other nerves that are commonly blocked in horses to determine whether those nerves are enveloped by a circumneural sheath at the site of injection for regional anesthesia.

Results of the study reported here drew attention to practical issues concerning response to perineural anesthesia in horses. Amelioration of lameness in response to subcircumneural injection of local anesthetic solution was initially rapid through 20 minutes after injection, gradual until 35 minutes, and then relatively steady state through 45 minutes. The study results as well as our clinical impressions suggested that if local anesthetic solution is deposited outside the sheath, the nerve block may yield erroneous results. Results of this study also indicated that gait should be reassessed at 10 minutes after perineural injection of local anesthetic solution if no substantial amelioration of lameness is evident at 5 minutes after injection. If improvement is noted at 10 minutes, a longer period of assessment may be appropriate before a sequential block is administered.

Acknowledgments

No third-party funding or support was received in connection with this study or the writing or publication of the manuscript. The authors declare that there were no conflicts of interest.

The authors thank Jessica Brown for technical assistance.

ABBREVIATIONS

MaxHDiff

Difference between the local maximum head height before right forelimb stance and the local maximum head height before left forelimb stance

MinHDiff

Difference between the local minimum head height during right forelimb stance and the local minimum head height during left forelimb stance

VS

Vector sum

Footnotes

a.

Carbocaine-V, Zoetis Inc, Kalamazoo, Mich.

b.

Isovue-370, Bracco Diagnostics Inc, Monroe, NJ.

c.

Q with Lameness Locator, Equinosis LLC, Columbia, Mo.

d.

PROC MIXED, SAS, version 9.1, SAS Institute Inc, Cary, NC.

References

  • 1. Grabinsky A. Mechanisms of neural blockade. Pain Physician 2005;8:411416.

  • 2. Andersen HL, Andersen SL, Tranum-Jensen J. Injection inside the paraneural sheath of the sciatic nerve: direct comparison among ultrasound imaging, macroscopic anatomy, and histologic analysis. Reg Anesth Pain Med 2012;37:410414.

    • Search Google Scholar
    • Export Citation
  • 3. Boezaart AP. Sweet spot of the nerve: is the “paraneural sheath” named correctly, and does it matter? Reg Anesth Pain Med 2014;39:557558.

    • Search Google Scholar
    • Export Citation
  • 4. Prasad NK, Capek S, deRuiter GCW, et al. The subparaneurial compartment: a new concept in the clinicoanatomic classification of peripheral nerve lesions. Clin Anat 2015;28:925930.

    • Search Google Scholar
    • Export Citation
  • 5. Vloka JD, Hadžić A, Lesser JB, et al. A common epineural sheath for the nerves in the popliteal fossa and its possible implications for sciatic nerve block. Anesth Analg 1997;84:387390.

    • Search Google Scholar
    • Export Citation
  • 6. Millesi H, Hausner T, Schmidhammer R, et al. Anatomical structures to provide passive motility of peripheral nerve trunks and fascicles. Acta Neurochir Suppl 2007;100:133135.

    • Search Google Scholar
    • Export Citation
  • 7. Missair A, Weisman RS, Suarez MR, et al. A 3-dimensional ultrasound study of local anesthetic spread during lateral popliteal nerve block: what is the ideal end point for needle tip position? Reg Anesth Pain Med 2012;37:627632.

    • Search Google Scholar
    • Export Citation
  • 8. Boezaart AP, Zasimovich Y, Parvataneni HK. Long-acting local anesthetic agents and additives: snake oil, voodoo, or the real deal? Pain Med 2015;16:1317.

    • Search Google Scholar
    • Export Citation
  • 9. Denoix J-M. The equine distal limb: atlas of clinical anatomy and comparative imaging. Ames, Iowa: Iowa State University Press, 2000.

    • Search Google Scholar
    • Export Citation
  • 10. Keegan KG, Yonezawa Y, Pai PF, et al. Evaluation of a sensor-based system of motion analysis for detection and quantification of forelimb and hind limb lameness in horses. Am J Vet Res 2004;65:665670.

    • Search Google Scholar
    • Export Citation
  • 11. Keegan KG, MacAllister CG, Wilson DA, et al. Comparison of an inertial sensor system with a stationary force plate for evaluation of horses with bilateral forelimb lameness. Am J Vet Res 2012;73:368374.

    • Search Google Scholar
    • Export Citation
  • 12. Keegan KG, Kramer J, Yonezawa Y, et al. Assessment of repeatability of a wireless, inertial sensor-based lameness evaluation system for horses. Am J Vet Res 2011;72:11561163.

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Contributor Notes

Address correspondence to Dr. Schumacher (schumjo@auburn.edu).