Abstract
Objective
To compare the effect of 2.5 mL 2% mepivacaine to the effect of 0.5 mL 2% mepivacaine administered over each palmar digital nerve, as an abaxial sesamoid nerve block (ASNB), to 6 horses lame because of naturally occurring digital pain.
Methods
In a crossover study design with 3 horses in each group, the trotting gait of the horses was analyzed with an inertial sensor–based, motion-analysis system (Q with Lameness Locator; Equinosis LLC) immediately before and after treatment with either 0.5 or 2.5 mL mepivacaine administered over each nerve near the base of the proximal sesamoid bone of the lame forelimb. The gait was reevaluated objectively at 5-minute intervals for 20 minutes. Treatments were administered 48 hours apart.
Results
Both treatments significantly reduced lameness scores at all evaluations, and the lameness scores did not differ significantly between the treatment groups.
Conclusions
The effect of 0.5 mL of 2% mepivacaine HCl administered over each palmar digital nerve as an ASNB on lameness caused by digital pain did not differ significantly from the effect of 2.5 mL.
Clinical Relevance
The volume of a local anesthetic for an ASNB, adequate to ameliorate digital pain, is less than that commonly recommended. A lower volume might more specifically localize the site of pain to the digit, but this supposition remains to be investigated.
Volumes of local anesthetic (eg, 2% mepivacaine) commonly recommended to deposit adjacent to each palmar digital nerve for an abaxial sesamoid nerve block (ASNB) typically range between 2 and 3 mL,1–5 but as little as 1 mL6 and as much as 5 mL7,8 has been recommended for injection over each nerve. A positive response (ie, resolution or amelioration of lameness) to anesthesia of the palmar digital nerves abaxial to the proximal sesamoid bones is generally considered to localize the site of pain causing lameness to the digit. An ASNB has been shown, however, to desensitize structures proximal to the digit, such as all or a portion of the metacarpophalangeal joint and the distal portion of the branches of the suspensory ligament,2,9–11 making a positive response to an abaxial nerve block difficult to interpret. Interpretation of a positive response to an ASNB might be ambiguous because varying, and often large, volumes of local anesthetic are recommended for performing this nerve block. To investigate the premise that a large volume of local anesthetic confuses the interpretation of this nerve block, we first wanted to determine if a lower volume of local anesthetic than volumes previously recommended for an ASNB could effectively resolve lameness caused by digital pain. In a previous study,12 we determined that a volume of 0.75 mL of 2% mepivacaine deposited over each palmar nerve at the proximal border of the lateral cartilages of the foot (ie, a palmar digital nerve block [PDNB]) was sufficient to alleviate digital pain. For that study, we regretted not using an even smaller volume of mepivacaine for the PDNBs. For the current study, we postulated that the effect of 0.5 mL 2% mepivacaine would be similar to that of 2.5 mL administered over each palmar nerve, as an ASNB, in ameliorating lameness caused by digital pain.
Methods
Animals
Six naturally lame horses (4 Thoroughbreds and 2 American Quarter Horses), ranging in age from 11 to 25 years (15.8 ± 4.78, mean ± SD) and in body weight from 545 to 603 kg (543 ± 41.9, mean ± SD), from the Auburn University’s horse herd were enrolled in the study. Each of these horses had previously been determined, in lameness laboratories conducted by different clinicians using visual assessment before and after performing PDNB, to be lame because of pain in 1 forefoot (1 horse) or both forefeet (5 horses). Volumes of local anesthetic used in these laboratories were not recorded and pathological conditions causing lameness were not investigated. Study procedures were reviewed and approved by the Auburn University’s IACUC (protocol No. 2024-5465).
Experimental protocol
The study had a crossover design. Horses were randomly assigned to 2 groups, with 3 horses in each group. Each group was subjected to each of the 2 treatments, after allowing 48 hours between treatments. The horses were trotted in a straight line on an asphalt surface for 30 to 40 strides. Lameness was identified and quantified by using an inertial sensor–based, motion-analysis system (Lameness Locator; Equinosis LLC). After scoring the baseline lameness, an ASNB was administered to the lame forelimb, while restraining the horse with a nose twitch. After the application of 70% isopropyl alcohol to the sites of injection, an ASNB was administered using 2.5 mL 2% mepivacaine or 0.5 mL 2% mepivacaine injected through a 25-gauge, 16-mm needle to deposit the anesthetic perineurally near the base of each proximal sesamoid bone. A 3-mL syringe was used to administer the 2.5-mL volume of local anesthetic, and a tuberculin syringe was used to administer the 0.5-mL volume. One clinician (JS) administered all nerve blocks. Gait was reevaluated at 5-minute intervals for 20 minutes, beginning 5 minutes after the nerve block. Accuracy of the nerve block was determined by a substantial change in gait by 15 minutes indicated by switching of lameness to the opposite forelimb or by a 70% decrease in lameness or lack thereof. Lack of or minimal improvement in gait prompted the investigators to determine the skin sensation at the bulbs of the heel. The presence of skin sensation and no or minimal improvement in gait indicated the nerve block was inaccurate; the horse was returned to pasture, and the nerve block and subsequent gait analysis were performed the next day. When there was a substantial but delayed amelioration of lameness using the lower volume of anesthetic, the horse was reexamined 48 hours later by performing a PDNB using 1 mL of mepivacaine. The results of this procedure were evaluated to determine if the larger volume of anesthetic that anesthetized unintended structures proximal to the site of injection could explain the delay in improvement when the lower dose was administered.
Objective lameness evaluation—Gait was analyzed by using a body-mounted inertial-sensor system (Q with Lameness Locator; Equinosis LLC). The horses were trotted in a straight line on an asphalt surface for 30 to 40 strides, led from the left by a handler with a lead shank attached to a halter. For these evaluations, each horse was instrumented with sensor devices placed and secured to the head, pastern of the right forelimb, and the pelvis, following the manufacturer’s recommendations and as described elsewhere.13–17 To keep the speed of movement consistent, the same handler was used for all evaluations.
The nature of the forelimb lameness was measured as the vector sum (VS) of MaxHDiff (difference between the local maximum head height before right forelimb stance and the local maximum head height before left forelimb stance) and MinHDiff (difference between the local minimum head height during right forelimb stance and the local minimum head height during left forelimb stance; ie, VS = [MaxHDiff2 + MinHDiff2]1/2), both of which are measures of asymmetric head height during strides of the right and left forelimbs. The VS is an overall measurement of the magnitude of forelimb lameness and is given a positive value when the MinHDiff is > 0, indicating the site pain causing lameness is in the right forelimb, or a negative value when the MinHDiff is < 0, indicating the site of pain causing lameness is in the left forelimb. 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 the MaxHDiff and the MinHDiff (both of which are ±6 mm), is ±8.5 mm.15 The described methodology,17–20 measures,14,21–23 and thresholds13,14, 24 used to evaluate forelimb lameness of horses have been described previously.
Statistical analysis
where Y was the vector of observations, X was the treatment design matrix (treatment with 2.5 mL mepivacaine and 0.5 mL mepivacaine; treatment sequence; lameness evaluations at pretreatment and at 5, 10, 15, and 20 minutes posttreatment); β was the vector of fixed treatment effects; Z was the random effects design matrix (horse); μ was the vector of random block effects; and e was the vector of experimental error. A first-order, autoregressive covariance structure was used to account for the nonindependence of observations within horses repeatedly measured.25,26 Models were compared using the Akaike information criterion.25–27 Horse was included in models as a random effect.25–27 The Kenward-Roger correction was used for all models.25–27 The level of significance was set at P ≤ .05. All statistics were performed using commercially available software (PROC MIXED; SAS, version 9.4; SAS Institute Inc).
Results
Four of the 12 ASNBs performed were inaccurate, as indicated by minimal improvement in gait and lack of skin sensation at 1 or both bulbs of the heel. Treatments (0. 5 mL mepivacaine and 2.5 mL mepivacaine) were compared by using a repeated measures mixed linear model, as described in Methods. The model was not improved by treatment sequence or treatment X time of lameness evaluation interaction and was eliminated from the final model. Time of lameness evaluation (P = .0001) was included in the final model. Treatment was forced into the final model as the primary interest of the study. The final model indicated that the effects between treatments were not significant (P = .8238).
The effect of treatments on VS was then compared to pretreatment VS for each treatment. Treatment with 0. 5 mL mepivacaine significantly reduced VS at all posttreatment time points (5 minutes, P = .0001; 10 minutes, P = .0001; 15 minutes, P = .0001; 20 minutes P = .0009; 5 to 20 minutes, P = .0001). Treatment with 2.5 mL mepivacaine also significantly reduced VS at all posttreatment time points (5 minutes, P = .0001; 10 minutes, P = .0001; 15 minutes, P = .0001; 20 minutes, P = .0001; 5 to 20 minutes, P = .0001). The mean VS ± SD for each treatment and time of lameness evaluation are presented in Figure 1. For 1 horse, lameness was slower to resolve with the 0.5-mL dose of anesthetic. Because our reason for performing this study was based on the supposition that high doses unintentionally desensitize more proximal structures, a PDNB was performed (using a low dose [1 mL] of mepivacaine) to rule out the possibility that the 2.5-mL dose used for the ASNB was alleviating pain more proximal in the limb. Lameness scores with the PDNB were similar to those achieved with the high-dose ASNB.
Mean ± SD decrease in the vector sum (VS) from baseline (0 minutes) for 6 horses with naturally occurring lameness localized to a forefoot at 5-minute intervals after administration of an abaxial sesamoid nerve block with 2.5 mL 2% mepivacaine (red line) and 0.5 mL 2% mepivacaine (black line).
Citation: American Journal of Veterinary Research 2025; 10.2460/ajvr.25.02.0042
Discussion
For the time points used in this study, the effect on lameness caused by digital pain of 0.5 mL mepivacaine deposited adjacent to each palmar digital nerve at the base of the proximal sesamoid bones did not differ significantly from that of 2.5 mL mepivacaine. To effectively anesthetize a nerve, a local anesthetic must be deposited through the circumneural sheath into the subcircumneural space28–30 and contact 2 to 3 nodes of Ranvier (gaps in the myelin sheath that are exposed to the extracellular space for exchange of ions involved in generation of the action potential).31,32 Apparently, 0.5 mL of local anesthetic deposited within the subcircumneural space of the digital nerves at the base of the proximal sesamoid bones is sufficient to effect digital anesthesia. Large volumes of anesthetic forced proximally through the circumneural space can anesthetize portions of the nerve more proximal to those portions intended to be anesthetized, resulting in desensitization of structures not intended to be desensitized, which, in turn, may result in misinterpretation of the region of the limb actually desensitized.
Although a small number of horses were used in this study, based on numerous previously published studies of similar design,12,33,34 6 horses proved sufficient to achieve a significant P value of ≤ .05 with 80% power. A shortcoming of the study is that the cause of digital pain in these horses was not determined. It is possible that digital pain caused by some types of pathology may not be alleviated with a small volume of mepivacaine.
For 1 horse, lameness was slower to resolve with the 0.5-mL volume of local anesthetic. Because our reason for performing this study was based on the supposition that high volumes may unintentionally desensitize structures more proximal to the portion of the limb intended to be desensitized, we performed a PDNB [using a small volume (1 mL) of mepivacaine] to rule out the possibility that the 2.5-mL volume used for the ASNB was alleviating pain more proximal in the limb. The effect of the low volume of mepivacaine used for the PDNB in amelioration of lameness was similar to that of the high volume of mepivacaine used for the ASNB, indicating that the pain causing the lameness was in the digit. We are not suggesting that 0.5 mL of mepivacaine be used for an ASNB, but the results of this study and a previous study12 indicate that the lower volume of currently suggested volumes of mepivacaine be administered when performing a PDNB or an ASNB. Investigations should determine the lowest effective volume of local anesthetic for other regional nerve blocks. Studies are indicated to determine if a lower volume of mepivacaine administered as an abaxial nerve block and other nerve blocks may more accurately identify the region of pain causing lameness.
Acknowledgments
None reported.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the composition of this manuscript.
Funding
The authors have nothing to disclose.
References
- 1.↑
Baxter G, Stashak T. Perineural and intrasynovial anesthesia, In: Baxter G, ed. Adams and Stashak’s Lameness in Horses. 6th ed. Wiley-Blackwell; 2011:173–202.
- 3.
Fürst A. Diagnostic anaesthesia. In: Auer JA, Stick JA, eds. Equine Surgery. 4th ed. Saunders Elsevier; 2012:998–1015.
- 4.
Carter G, Hogan P. Use of diagnostic nerve blocks in lameness evaluation. In: Proceedings 42nd Annual Meeting of American Association of Equine Practitioners. Vol 42. American Association of Equine Practitioners; 1996:26–32.
- 5.↑
Kaneps A: Diagnosis of lameness. In: Hinchcliffe KW, Kaneps AJ, Geor RJ, eds. Equine Sports Medicine and Surgery. 2nd ed. Saunders Elsevier; 2014:339–396.
- 6.↑
Bassage LH II, Ross MW. Diagnostic analgesia. In: Ross MW, Dyson SJ, eds. Diagnosis and Management of Lameness in the Horse. 2nd ed. Saunders Elsevier; 2011:100–1353.
- 7.↑
Wheat J, Jones K. Selected techniques of regional anesthesia. Vet Clin North Am Large Anim Pract. 1981;3:223–246. doi:10.1016/S0196-9846(17)30154-4
- 8.↑
Skarda RT, Muir WW, Hubbell JAE. Local anesthetic drugs and techniques. In: Muir WW, Hubbell JAE, eds. Equine Anesthesia: Monitoring and Emergency Therapy. 2nd ed. Saunders Elsevier; 2009:210–242.
- 9.↑
Kainer R, Fails A. Functional anatomy of equine musculoskeletal system. In: Baxter G, ed. Adams and Stashak’s Lameness in Horses. 6th ed. Wiley-Blackwell; 2011:3–72.
- 10.
Ross MW. Observations in horses with lameness abolished by palmar digital analgesia. In: Proceedings of the 44th Annual Meeting of the American Association of Equine Practitioners. Vol 44. American Association of Equine Practitioners; 1998:230–232.
- 11.↑
Daniel AJ, Judy CE, Saveraid T. Magnetic resonance imaging of the metacarpo(tarso)phalangeal region in clinically lame horses responding to diagnostic analgesia of the palmar nerves at the base of the proximal sesamoid bones: five cases. Equine Vet Educ. 2013;25(5):222–228. doi:10.1111/j.2042-3292.2011.00246.x
- 12.↑
Cole R, DeGraves F, Schumacher J, Brown J. Lameness is effectively ameliorated with a low volume of mepivacaine administered as a palmar digital nerve block to horses lame because of foot pain. Am J Vet Res. 2024;84(12):ajvr.24.04.0107. doi:10.2460/ajvr.24.04.0107
- 13.↑
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(9):1156–1163. doi:10.2460/ajvr.72.9.1156
- 14.↑
McCracken M, Kramer J, Keegan J, et al. Comparison of an inertial sensor system of lameness quantification with subjective lameness evaluation. Equine Vet J. 2012;44(6):652–656. doi:10.1111/j.2042-3306.2012.00571.x
- 15.↑
Keegan KG, Wilson D, Kramer J, et al. Comparison of a body-mounted inertial sensor system-based method with subjective evaluation for detection of lameness in horses. Am J Vet Res. 2013;74(1):17–24. doi:10.2460/ajvr.74.1.17
- 16.
Keegan KG. Objective assessment of lameness. In: Baxter GM, ed. Adams and Stashak’s Lameness in Horses. 6th ed. Wiley-Blackwell; 2011:154–164.
- 17.↑
Keegan K, Pai PF, Wilson DA, Smith BK. Signal decomposition method of evaluating head movement to measure induced forelimb lameness in horses trotting on a treadmill. Equine Vet J. 2001;33(1):446–451. doi:10.2746/042516401776254781
- 18.
Audigié F, Pourcelot P, Degueurce C, Geiger D, Denoix JM. Fourier analysis of trunk displacements: a method to identify the lame limb in trotting horses. J Biomech. 2002;35(9):1173–1182. doi:10.1016/S0021-9290(02)00089-1
- 19.
Keegan K, Arafat S, Skubic M, Wilson DA, Kramer J. Detection of lameness and determination of the affected forelimb in horses by use of continuous wavelet transformation and neural network classification of kinematic data. Am J Vet Res. 2003;64(11):1376–1381. doi:10.2460/ajvr.2003.64.1376
- 20.↑
Schumacher J, Taintor J, Schumacher J, Degraves F, Schramme M, Wilhite R. Function of the ramus communicans of the medial and lateral palmar nerves of the horse. Equine Vet J. 2013;45(1):31–35.
- 21.↑
Keegan K, Yonezawa Y, Pai P, Wilson DA, Kramer J. 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(5):665–670. doi:10.2460/ajvr.2004.65.665
- 22.
Donnell JR, Frisbie D, King MR, Goodrich LR, Haussler KK. Comparison of subjective lameness evaluation, force platforms and an inertial sensor system to identify mild lameness in an equine osteoarthritis model. Vet J. 2015;206(2):136–142. doi:10.1016/j.tvjl.2015.08.004
- 23.↑
Pfau T, Boultbee H, Davis H, Walker A, Rhodin M. Agreement between two inertial sensor gait analysis systems for lameness examinations in horses. Equine Vet Educ. 2016;28(4):203–208. doi:10.1111/eve.12400
- 24.↑
Keegan KG, MacAllister CG, Wilson DA, et al. Comparison of an inertial sensor system to the stationary force plate for evaluation of horses with bilateral forelimb lameness. Am J Vet Res. 2012;73(3):368–374. doi:10.2460/ajvr.73.3.368
- 25.↑
Littell RC, Henry PR, Ammerman CB. Statistical analysis of repeated measures data using SAS procedures. J Anim Sci. 1998;76(4):1216–1231. doi:10.2527/1998.7641216x
- 26.↑
Littell RC, Milliken GA, Stroup WW, Wolfinger RD, Schabenberger O. SAS for Mixed Models. 2nd ed. SAS Institute Inc; 2006.
- 27.↑
Littell RC, Pendergast J, Natarajan R. Modelling covariance structure in the analysis of repeated measures data. Stat Med. 2000;19(13):1793–1819. doi:10.1002/1097-0258(20000715)19:13<1793::AID-SIM482>3.0.CO;2-Q
- 28.↑
Boezaart AP. Sweet spot of the nerve: is the “paraneural sheath” named correctly, and does it matter? Reg Anesth Pain Med. 2014;39(6):557–558.
- 29.
Boezaart AP, Zasimovich Y, Parvataneni H. Long-acting local anesthetic agents and additives: snake oil, voodoo, or the real deal? Pain Med. 2015;16(1):13–17. doi:10.1111/pme.12614
- 30.↑
Schumacher J, Cole RC, DeGraves FJ, Cofield LG. Comparison of speed of onset and analgesic effect of 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. Am J Vet Res. 2020;81(5):394–399. doi:10.2460/ajvr.81.5.394
- 32.↑
Catterall WA, Mackie K. Local anesthetics. In: Goodman and Gilman’s The Pharmacological Basis of Therapeutics. 11th ed. In: Brunton LL, Lazo JS, Parker KL, eds. McGraw-Hill; 2006:369–386.
- 33.↑
Velloso Alvarez A, Schumacher J, DeGraves F. Effect of the addition of epinephrine to a lidocaine solution on the efficacy and duration of palmar digital nerve blocks in horses with naturally occurring forefoot lameness. Am J Vet Res. 2018;79(10):1028–1034. doi:10.2460/ajvr.79.10.1028
- 34.↑
Boone LH, DeGraves FJ, Klein CE, Cole RC, Schumacher J. Effect of 3% chloroprocaine hydrochloride when used for median and ulnar regional nerve blocks in lame horses. Am J Vet Res. 2020;81(1):13–16. doi:10.2460/ajvr.81.1.13
