Mechanical nociceptive assessment of the equine hoof after navicular bursa anesthetic infiltration validated by bursography

Abstract

The analgesic specificity of navicular bursa (NB) anesthetic infiltration is still questionable. The study aimed to determine the mechanical nociceptive threshold of non-specific analgesia in the dorsal lamellar stratum, as well as in the sole, coronary band, and heel bulbs of the hoof, after navicular bursa anesthetic infiltration. Six healthy horses with no clinical or radiographic changes of the digits and no communication between the NB and the distal interphalangeal joint, were used. After random selection, the NB of one of the forelimbs was infiltrated with 2% lidocaine and the contralateral one with lactated ringer's solution. Contrast was added to confirm radiographic infiltration. The mechanical nociceptive threshold was determined using a portable pressure dynamometer, before and at various times after the infiltration, in 10 points of the hoof. The effects of time and treatment were verified by ANOVA (P<0.05). There was no statistical difference in the values of the mechanical nociceptive threshold (P>0.05) in all regions evaluated. However, in one of the six hooves that receives lidocaine, complete absence of response to the painful stimulus (maximum force of 6 Kg over an area of 38.46 mm2, for a maximum of 4 seconds) was observed in the dorsal lamellae between 30 and 60 min after infiltration. In conclusion, lidocaine infiltration of NB did not promote significant increases in the nociceptive threshold of the sole, coronary band, bulbs of the heel and dorsal lamellae clinically healthy horses. However, the occurrence of analgesia in one of the six hooves subjected to NB anesthesia indicates that the technique may not be fully specific in few horses.

Fig 1. Equine forelimb bursography images.

(A) Lateral aspect of bursocentesis performed as described by Verschooten et al. [20]. A 20G 3½ spinal needle, inserted in the sagittal plane proximal to the coronary edge of the bulb, crossing the skin, digital cushion, and deep digital flexor tendon, towards the proximal third of the navicular bone, until it is significant. (B) Latero-medial radiography of the hoof during bursocentesis of the left forelimb, confirming the position of the spinal needle on the palmar-proximal surface of the navicular bone. (C) Navicular bursa bursography. Bursocentesis confirmation without leakage and without communication with the distal interphalangeal joint. It guarantees the reliability of the measurements of the mechanical nociceptive threshold.

Fig 2. Graphical representation of mechanical nociceptive threshold (MNT) values, with mean values and median in kilograms (kg).

Evaluation of coronary band: (A) dorsal, (B) medial, (C) lateral. Evaluation of sole borders: (D) dorsal, (E) palmaromedial, (F) palmarolateral, in healthy equine forelimbs, injected into navicular bursa with lactated Ringer’s solution or 2% lidocaine.

Fig 3. Graphical representation of mechanical nociceptive threshold (MNT) values, with mean values and median in kilograms (kg).

Evaluation of heel bulbs: (A) medial, (B) lateral. Evaluation of dorsal lamellar regions (L): (C) L2 cm, and (D) L 4 cm, distal to the coronary band of hooves in healthy equine forelimbs, injected into navicular bursa with lactated Ringer’s solution or 2% lidocaine.