Ultrasound-guided out-of-plane approach to the sciatic nerve in dogs: a cadaveric comparison with the caudal in-plane approach

Abstract

Objectives: To assess the feasibility [defined as successful circumferential staining of the sciatic nerve (SN) over > 2 cm] and incidence of needle-nerve contact of a novel ultrasound-guided out-of-plane approach targeting the subepimysial perineural compartment cranial to the SN, compared with the conventional caudocranial in-plane technique.

Study design: Prospective, randomized, experimental anatomical study.

Animals: Twelve canine cadavers (24 pelvic limbs).

Methods: Bilateral injections of 0.1 mL kg^-1 of a lidocaine-dye mixture were performed at the midthigh level in each canine cadaver, with one limb randomly assigned to the in-plane (caudocranial) approach and the contralateral limb to the out-of-plane (lateromedial) approach. Ultrasound recordings were analyzed for needle-nerve contact and number of attempts to reach the injection site. Anatomical dissections assessed the length of circumferential staining. The incidence of needle-nerve contact between approaches was analyzed using McNemar's test, with statistical significance set at p < 0.05. Data not submitted to statistical testing are presented as median and 25th and 75th percentiles (Q1-Q3).

Results: Needle-nerve contact occurred in 9/12 (75%) in-plane injections but in 0/12 (0%) out-of-plane injections (p = 0.003). The out-of-plane approach required fewer attempts to reach the injection site [1 (1-2)] than the in-plane approach [2.5 (2-3.5)] and a shorter needle insertion depth [2.4 (2.1-3.1)] versus 4.61 (3.6-5.2) cm]. Circumferential staining was achieved in all specimens, with staining lengths of 3.5 (2.8-6.6) cm for the in-plane and 4.3 (3.5-4.9) cm for the out-of-plane approach.

Conclusions and clinical relevance: The ultrasound-guided out-of-plane approach targeting the subepimysial perineural compartment cranial to the SN in dogs consistently achieved circumferential staining without observable needle contact, requiring fewer attempts and a shorter needle path compared with the traditional in-plane technique.

Keywords: dogs; ischiatic nerve; nerve block; nerve injury; paraneural sheath; ultrasonography.