In today’s cosmopolitan society, it is not uncommon to have patients from different ethnic backgrounds. Even though variation in orbital anatomy is well recognised clinically, published data on ethnic variation is limited.
In our earlier study on lateral orbital wall anatomy [19], we have demonstrated that Indians, and to a lesser extent Caucasians, have smaller lateral wall trigones as compared to Chinese and Malays. This explains why lateral orbital decompression is technically more difficult in Chinese and Malay patients and often requires powered instruments. Indians also have shallower orbits which may limit the efficacy of lateral orbital decompression alone in reducing proptosis.
For patients with DON or severe proptosis in which lateral orbital decompression alone is ineffective or insufficient, removal of the medial orbital wall and/or orbital floor is required. In this study, we evaluate the medial orbital wall anatomy and its ethnic variations.
We recorded ethmoid sinus lengths ranging from 29.2-45.8mm in males and 27.0-44.0mm in females (Table 2), which were comparable to the existing literature (38.8-42.5mm in males and 36.4-40.8mm in females) [20]. There was no statistically significant difference between the ethnic groups. As a surrogate for the medial orbital wall, the mean ethmoid sinus length is rather constant and provides a reliable estimate on how deep a surgeon needs to decompress to relieve the apical crowding.
For the ethmoid sinus width, we were unable to compare our results (total mean width 10.9mm, 7.5-16.1mm in females, 8.2-15.5mm in males) with the published literature (15.1-17.5mm in males, 13.4-16.0mm in females) due to differences in measurement methods [20]. Males were noted to have wider mean ethmoid sinuses than females in our study and in existing literature [20]. Additionally, Caucasians were noted to have narrower ethmoid sinuses in our study as compared to the Chinese.
With regard to ethmoid sinus volume, our average measurement was 3.6cm3 (3.7cm3 in males and 3.3cm3 in females) and was most similar to the total mean of 4.51cm3 found in a Korean population by Park et al. [17]. There were published data of bigger ethmoid sinus volumes of 5.5cm3 (females) and 6.3cm3 (males) in a Turkish population [16] and total mean volume of 5.5cm3 in a Spanish population [21]. However, our study focused on the clinically decompressible ethmoid sinus volume from a retrocarcuncular approach instead of total ethmoidal volume and thus had smaller measured volumes.
Although the coronal slide we chose for measuring the medial orbital wall and floor angle may seem anterior to the true orbital apex, our measuring technique reduces the impact of sphenoid sinus size and location, which occasionally forms the medial wall of the optic canal and is not routinely removed in decompression surgery. Our mean angle value of 133.5° was close to Kang et al.’s 136.88° which was derived from 276 Asian orbits [22]. It is also similar to the 122° measured by Keast et al. in 36 Polynesian and 119° in 144 Caucasians [23]. This relative consistency in medial orbital wall and floor angle, reassured us of a minimum 4 clock hours wide apical relief in adequate posterior decompression surgery.
The olfactory fossa depth and Keros classification, guides us with the extent of superior bone removal in medial wall decompression. In our study, we noted an inclination towards Keros 2 in Caucasians as compared to the Chinese, differing from Badia, et al., who found no significant differences in olfactory depth between 100 Caucasians and 100 Chinese subjects [24]. This discrepancy may be due to our small sample size. However, our results concurred with Alazzawai, et al., where no significant differences were found between the Chinese, Malay and Indians in their Malaysian population with 80 % Keros 1 classification in 300 subjects, with none fulfilling Keros 3 criteria [25]. In Keros 3 patients, their ethmoidal roofs lie significantly higher than the cribriform plate, and thus bear the greatest risk of inadvertent intracranial entry during medial wall decompression [26].
Before the age of fine-cut CT scan and image-guided surgery, age-old wisdom suggested that the posterior ethmoidal wall (i.e. anterior sphenoid face) is about 1 cm behind the posterior maxillary wall. To prevent accidental entry into the sphenoid sinus and damaging the carotid syphon, one must sound out the posterior maxillary wall as a guide to see how far back decompression along the medial wall is needed (i.e. removing all the posterior ethmoid sinus until the anterior sphenoid face). In our study, we noted only Caucasians had their mean posterior maxillary wall anterior to the posterior ethmoidal wall / anterior sphenoid face. To our knowledge, there is only one other study, with 11 cadavers, where the anterior face of the sphenoid was noted to be about 2-4mm more posterior than the posterior maxillary wall [27]. For the Chinese, Malays and Indians in our study, their posterior ethmoidal wall is anterior to the posterior maxillary wall. This finding is similar to another Korean radiological study [28] of 115 CT scans, albeit with different measurement methods. This knowledge of ethnic variation in relative position of anterior sphenoid face to posterior maxillary wall is important in defining the safe zone of decompression. In reality, the sphenoidal wall is often thicker than the posterior ethmoidal wall, surgeons should think twice if they find the posterior medial orbital wall more difficult to break during decompression and when they are about 3-4 cm beyond the posterior lacrimal crest (i.e. mean ethmoid length) or 3-4mm from the posterior maxillary wall.
There were a few limitations in our study. Firstly, the sample size was relatively small and could have benefited from a larger number of subjects to allow for more accurate results. For example, we noted that Caucasians had larger ethmoid sinus volumes as compared to the Chinese but it did not reach statistical significance.
Secondly, the different ethnic groups may not be a completely homogenous sample, as there might have been Malays of Arab or of mixed Arab heritage, Chinese of Northern and Chinese of Southern descent for example.
Lastly, ethmoidal sinus anatomy is highly complex, despite the various methods used to demarcate and measure its volume, there remains much difficulty and variability in defining the boundaries of these intricate air cells [29]. We had strict and easily recognizable limits for demarcating the ethmoid sinus area for calculation and ensured that measurements were repeated twice by one observer and repeated again by a second observer to safeguard the reproducibility of the ethmoid sinus volume. On analysis, we found good inter and intra-user agreement with all our medial orbital wall measurements.
The strengths of our study include building on the same platform as earlier study on lateral orbital wall anatomy. Besides providing ethnic specific anatomical data for 4 different races, we explored the potential inter-ethnic variations in orbital wall anatomy, for which there is a dearth of literature on.
Future research in this area could expand on increasing the sample size and by involving other ethnic groups. Alternatively, the focus could also be shifted towards soft tissues measurements in normal or patients with pre-existing DON. This would allow for potential discovery of inter-ethnic pathological changes.