TEM, rTEM, and R values obtained in terms of measurement reliability show that the results of this study are reliable. The Triangular Area (TA) between the superficial points of GON, TON, and LON was measured and analyzed according to sides and genders. The mean TA value was calculated as 952.82 ± 313.36 mm2 (approximately 10 cm2) in male cadavers and 667.55 ± 273.82 mm2 (approximately 7 cm2) in females, and a statistically significant difference was detected between the genders in this regard (p < 0.05) (Table 1).
It was reported that there is no consensus on the amount of volume of ONBs used in the treatment of headaches [26]. In previous studies in which volumes ranging from 0.5 ml to 10 ml were injected into one side, it was shown that the most commonly used volume was 3 ml [26]. Baek et al. [27] used 5 fresh cadavers (10 sides) to compare the diffusion pattern and nerve involvement of the dye at different volumes of 1 ml and 5 ml by applying ultrasound-guided GON blocks at the C2 level, and reported that the 5 mL volume injection seemed appropriate for the therapeutic purpose. They reported that 5 ml dye solution stained all GON, TON, and suboccipital nerves, as well as most of the muscles in the region. Song et al. [28] applied 2 ml, 3.5 ml, and 5 ml of methylene blue to the tissues in a 1:1:1 ratio with fluoroscopy-guided GON block in 15 formalin-fixed cadavers and obtained satisfactory results. They found that all GON and suboccipital nerves, TONs, and LONs were stained on all sides that were stained with 3.5 ml of methylene blue, as well as a predominant portion of the region’s muscles. As well as the results obtained at 3.5 ml in the amount applied as 5 ml, they also reported that the trapezius muscle was stained on 6 sides. In conclusion, they emphasized that 3.5 ml of methylene blue stained all of the GON, suboccipital nerve, TON, and LON, and a volume of 3.5 ml was sufficient for a therapeutic fluoroscopy-guided GON block.
The 3.5 ml amount, which was indicated by Song et al. [28] to have stained all of the GON, TON, and LONs, probably includes or corresponds to the average TA that lies between the GON, TON, and LONs measured in the present study. For this reason, it can be argued that a single injection is sufficient to block these three nerves. ON may occur when one of the GON, LON, and TON or different combinations of these three nerves are involved [1, 29]. In this regard, considering that an injection is sufficient for blocking GON, TON and LONs, it can be argued that the average TA value has some advantages in an application to TA. These can be listed as follows:
1) Occipital nerves can be blocked without the need for additional blocking.
2) Side effects caused by nerve block can be reduced by preventing the exposure of patients to unnecessary block applications [8, 19].
3) Complications such as Cushing’s Syndrome [30] or neck muscle weakness [4] because of repeated injections can be prevented.
5) Labor and time loss that may result from multiple injections can be reduced.
6) This can guide clinicians in applications.
In the present study, the statistically significant difference in TA value according to the genders (approximately 10 cm² in men, 7 cm2 in women) may indicate that there may be differences in the amount of blocks according to the genders. In other words, it may be possible to obtain the same result by using less amount of ONB in females than in males. No gender-dependent block application at different volumes was detected in previous studies that were conducted in Turkey or abroad in the literature review. At this point, we think that new studies are needed in this regard.
In the present paper, the center of gravity (CGT) of the triangle was approximately 5 cm inferior and 3-3.5 cm lateral to the EOP in both genders and sides. In ON, where at least two occipital nerves are involved, this landmark can be used as the injection point in invasive procedures to the TA, and the occipital nerves can be blocked with a single injection to this point without the need for a separate injection for each nerve (Fig. 2). In this sense, we predict that the average CGT value found here can guide clinicians as a reference value in block applications to the suboccipital region in OA.
In conclusion, in ON that has more than one occipital nerve involvement, one single ONB injection about 5 cm inferior and 3-3.5 cm lateral from the EOP can theoretically block all of the involved occipital nerves, and this method can be used to prevent ONB origin and to reduce side effects and complications. In addition, the statistically significant difference between the genders in the triangular area suggests that different block amounts can be applied according to the genders.
Limitations of the study
A colored block material could not be applied to the area investigated because it was not within the scope of the study. It would have been more meaningful to discuss the results obtained in the study by injecting the block material into the triangular area.