Detailed anatomy of the trochlear nerve in the superior oblique muscle

Purpose: The purpose of this study was to elucidate the detailed anatomy of the trochlear nerve in the superior oblique muscle (SOM) and the intramuscular innervation pattern using Sihler staining. Methods: SOMs were dissected from their origin to the insertion in 28 eyes of 14 cadavers. The following distances were determined: from the SOM insertion to the trochlear, from the trochlear to the entry site of the anterior branch or posterior branch, and the widths of the main trunk and anterior and posterior branches. Sihler staining was then performed. Results: The trochlear nerve traveled straight ahead medially and divided. Eighteen of 28 (64.3%) orbits showed two anterior and posterior branches, six (21.4%) showed three branches, and four (14.3%) showed no branching. The most distally located intramuscular nerve ending was observed at 62.4 ± 2.4% of the length of each muscle (35.8 mm from insertion when considering that the length of the SOM was 57.4 mm) and at 29.9 ± 3.2% of the length of each muscle (17.2 mm from the trochlear). Additionally, the length of the intramuscular arborization part was 9.4 ± 1.1% of the length of the SOM (5.4 mm when considering that the length of the SOM was 57.4 mm). Nonoverlap between two intramuscular arborizations of the nerve was detected in 20 of 28 cases (71.4%). Eight cases (28.6%) showed a definite overlap of two zones. Conclusions: This study provided a good understanding of the anatomy of the trochlear nerve in the SOM.