Evaluation of the greater occipital nerve location regarding its relation to intermastoid and external occipital protuberance to mastoid process lines

BACKGROUND

Localisation of the greater occipital nerve (GON) is essential for the achievement of several procedures performed in the occipital region especially the treatment of occipital neuralgia. This study proposed to investigate the location of GON subcutaneous (Sc) and semispinalis capitis (SSC) piercing points related to the intermastoid and external occipital protuberance (EOP) to mastoid process (MP) lines.

MATERIALS AND METHODS

The Sc piercing point, relation to SSC and obliquus capitis inferior (OCI) muscles of 100 GONs from 50 cadaveric heads (23 males, 27 females) were dissected. Distances from EOP to MP (EM line) on both sides and between MPs (MM line) were measured. Perpendicular lines from Sc and SSC piercing points to EM and MM lines were created and measured. Distances from EOP to the perpendicular lines of SSC piercing point and from MP to the perpendicular lines of Sc piercing point were measured and calculated into percentage of EM and MM length, respectively.

RESULTS

Three types of Sc piercing points (I, II and III) were obtained. The percentage of GON piercing trapezius muscle (TP) (type I), aponeurosis of TP (type II) and aponeurosis between TP and sternocleidomastoid muscle (SCM) (type III) were 2, 67 and 31, respectively. In addition, 95% of GON pierced SSC, 2% pierced its tendinous band and 3% travelled between its medial fibres and the nuchal ligament. 94% of the GON turned around the lower edge of the OCI, while 6% pierced the lower edge of this muscle. Sc piercing point was always located above the MM line, but it could be above, below or on the EM line. In contrast, all of the SSC piercing points were located below the EM line except in one specimen, but it could be above, below or on the MM line. Therefore, the MM and EM lines were used as reference lines for locating the Sc and SSC piercing points, respectively. The mean EM line length was 81.26 ± 5.26 mm with statistically significant differences between genders and sides in female. The mean MM line length was 121.77 ± 8.54 mm with a statistically significant difference between genders. Sc piercing point could be located at 44% of MM line length from ipsilateral MP with a mean vertical distance of 18 mm. No statistically significant difference was found between genders and sides in these parameters, but a statistically significant difference was found in the percentage of MB to MM line between type III and type I (p = 0.02). SSC piercing point of all types could be located at the point of 25% of EM line length from EOP with a vertical distance of 18 mm below EM line. No statistically significant difference was found between genders, sides and types of both piercing points.

CONCLUSIONS

MM and EM lines are potential reference lines for locating the Sc and SSC piercing points of GON, respectively.