Abstract
Knowledge of the precise location of anatomical landmarks such as the anterior (AEC) and posterior ethmoid (PEC) canals facilitates medial orbital wall surgery and is of major importance for the protection of the orbital nerve. The aim of this study was to identify these anatomical structures in 100 consecutive CT scans and measure the distance between them. The authors investigated whether a predictable symmetry existed between the left and right side. The AEC was not identified unilaterally in one patient, the PEC was not identified unilaterally in six patients and not bilaterally in one patient. An additional PEC was found unilaterally in 12 and bilaterally in five patients. If an anatomical structure was found bilaterally, the authors obtained a strong Pearson’s correlation between the sides ( r = 0.798–0.903, p < 0.001). An anatomical variation was found in nearly every fourth patient. The authors think that these data call into question the use of the PEC and AEC as reliable surgical landmarks in medial orbital surgery.
The medial orbital wall is affected in 38–54% of orbital trauma, which is higher than formerly reported. Operative indications for orbital fractures remain an area of great controversy. Extraocular muscle entrapment, symptomatic diplopia, and enophthalmus are seen as strong indications for surgery. The most delicate anatomical structure in the orbit is the optic nerve, which if impaired can lead to partial or complete loss of sight. It is enclosed by the annulus of Zinn which is created by the origin of the superior medial, inferior and lateral rectus muscles. Other important structures (origin of the levator palpebrae superior, superior oblique muscles; inferior and superior ophthalmic vein; lacrimal, frontal and trochlear nerves) are found outside it but have very close relationships to this tendinous ring. Dissection which does not respect these anatomical structures will cause severe morbidity.
If reconstruction of the orbital walls is necessary further anatomical landmarks are necessary to protect the above named structures. Those anatomical landmarks, from anterior to posterior, are the orbital margin, the anterior lacrimal crest, the anterior ethmoid artery (AEA) entering the anterior ethmoid canal (AEC) and the posterior ethmoid artery (PEA) penetrating the medial orbital wall through the posterior ethmoid canal (PEC). The latter has the closest anatomical relationship to the optic nerve. Some authors regard the PEA as a border that should not be crossed in trauma surgery to minimize the risk of optic nerve damage. These anatomical structures are difficult to identify especially in trauma patients with a fractured medial wall. In cases of comminuted fractures, a scan of the contralateral side sometimes gives the only reference for finding these structures. With respect to that, knowledge of computed tomography (CT) based anatomy and morphology of the orbit is of great importance. Few CT data and cadaver based standards of the distances and bilateral symmetries of these anatomical landmarks are available. Most of them concentrate on only some of those landmarks and do not investigate the complete medial wall from a surgeon’s point of view. The authors compared their data with that in the literature and focused on the symmetry of these important surgical landmarks, which is helpful in orbital surgery.
The aim of this study was primarily to identify the medial orbital margin, the AEC, the PEC and the medial margin of the optic foramen in coronal and axial CT scans of the orbit and to measure the distances between these structures and their average distribution. The authors also investigated the degree of symmetry between the left and right side.
Materials and methods
CT scans of 100 consecutive patients (24 women and 76 men, aged 18–79 years, mean age 44.4 years) who underwent a CT scan of the head but had no former operations or acute traumatic injuries in the orbital area or the base of the skull were included. All scans had an effective slice thickness of 1 mm reconstructed with 50% overlap and were retrospectively investigated. The scans were obtained with a multi-detector CT (Brilliance 64, Philips, Best, The Netherlands) with a tube voltage of 120 kV and a tube current-time product of 180 mAs according to a bone protocol recommended by the manufacturer’s application guide. All the scans were examined according to a pre-arranged protocol in consensus by two observers (SW and CB) (Osirix © , Pixmeo, Switzerland). First sequential coronal, axial and sagittal images were viewed in a split screen to identify and mark the anatomical points of interest (nasion, AEC, PEC, OF) confidently ( Figs. 1 and 2 ). The AEC and PEC were marked at the points of perforation of the medial orbital wall. The optic foramen was defined as the medial eminence of the sphenoid bone. All marks could then be retrieved in every plane. The nasal point and the AEC were identified and connected with a virtually drawn line. A paraxial plane was aligned to this virtual line. The medial orbital rim seen on this angulated axial slice was defined as the medial orbital wall (MOR) and marked. Every distance to measure the axial plane of the scans was angulated so that the two points of interest were seen in the same paraxial plane ( Fig. 3 a and b ). This technique allowed measurement (in mm) of the smallest distance between those structures. The non-existence of structures under investigation was also noted.
The authors measured the distance between the MOR to the AEC, the PEC and the orbital foramen (OF), respectively. The distances from the AEC to the PEC and OF were also measured. The distances between the two ethmoidal canals and the distance between the PEC and the OF were also measured.
Statistical analysis
Expectations between two unrelated variables were tested for equality using a Mann–Whitney U -test (e.g. when testing for differences between men and women). The Wilcoxon signed rank test was used to detect differences in the distributions of the parameters from paired samples (left side compared to right side). To indicate the strength and direction of a linear relationship between two random variables, Spearman’s rho correlation coefficient was measured. Statistical analyses were performed with SPSS (version 18, SPSS Inc., Chicago, IL, USA).
Results
The authors were not able to identify the AEC in the orbit of one patient, for whom the structure was only present unilaterally. The PEC was not observed in seven patients, in six of whom it was unilaterally and in one of whom it was bilaterally absent. The authors never identified an additional AEC but observed multiple PECs in 17 patients ( Fig. 4 ); a unilateral additional PEC in 12 of these patients and a bilateral additional PEC in 5. Altogether an anatomical variation was found in 25 patients.
Mean distances (left/right) between the anatomical structures ( Fig. 5 ) were: rim of the MOR to the AEC 22.1 ± 2.9/22.2 ± 3.0 mm; MOR to the PEC 35.5 ± 3.3/3.56 ± 3.2 mm; MOR to the optic foramen 40.3 ± 3.6/40.6 ± 3.5 mm; AEC to the PEC 13.7 ± 2.5/13.6 ± 2.5 mm; AEC to the optic foramen 19.1 ± 2.7/19.3 ± 2.5 mm; and the PEC to the optic foramen 5.9 ± 0.18/6.3 ± 0.19 mm.
Increasing patient age was significantly correlated with a decreasing distance from the MOR to the optic foramen on either side (left side: r = −0.248, p = 0.013; right side: r = −0.209, p = 0.038).
Significant differences between men and women were found between the following points of measurements on either side: the MOR and the AEC; the MOR and the PEC; the MOR and the optic foramen; and between the AEC and the optic foramen. The distances between the points were consistently larger in men compared to women ( Table 1 ).
| Measure points | Men ( n = 70) | Women ( n = 30) | p -Value |
|---|---|---|---|
| MOR–AEC, left | 2.28 ± 0.28 | 2.06 ± 0.22 | p < 0.001 |
| MOR–AEC, right | 2.28 ± 0.27 | 2.09 ± 0.30 | p = 0.003 |
| MOR–PEC, left | 3.64 ± 0.31 | 3.33 ± 0.27 | p < 0.001 |
| MOR–PEC, right | 3.66 ± 0.28 | 3.34 ± 0.27 | p < 0.001 |
| MOR–OF, left | 4.14 ± 0.34 | 3.77 ± 0.30 | p < 0.001 |
| MOR–OF, right | 4.17 ± 0.32 | 3.82 ± 0.29 | p < 0.001 |
| AEC–OF, left | 1.96 ± 0.25 | 1.80 ± 0.26 | p = 0.006 |
| AEC–OF, right | 1.97 ± 0.26 | 1.83 ± 0.21 | p = 0.017 |
If an anatomical landmark was identified bilaterally the authors found a strong correlation of symmetry shown by Pearson’s correlation test between the left and right side ( r = 0.798–0.903) ( Table 2 ). The remaining results of the CT scan measurements for the 100 patients are summarized in Tables 1–3 and Fig. 6 .
| Correlation coefficient | p -Value of correlation | Mean value ± standard deviation of difference | p -Value of difference | |
|---|---|---|---|---|
| MOR–AEC left | r = 0.798 | p < 0.001 | −0.011 ± 0.186 | p = 0.560 |
| MOR–AEC right | ||||
| MOR–PEC left | r = 0.745 | p < 0.001 | 0.003 ± 0.219 | p = 0.888 |
| MOR–PEC right | ||||
| MOR–OF left | r = 0.903 | p < 0.001 | −0.040 ± 0.156 | p = 0.011 |
| MOR–OF right | ||||
| AEC–OF left | r = 0.715 | p < 0.001 | −0.016 ± 0.198 | p = 0.417 |
| AEC–OF right | ||||
| AEC–PEC left | r = 0.681 | p < 0.001 | 0.016 ± 0.193 | p = 0.424 |
| AEC–PEC right | ||||
| PEC–OF left | r = 0.558 | p < 0.001 | −0.048 ± 0.168 | p = 0.007 |
| PEC–OF right |
| Author | Patient group | Most anterior landmark (MAL) | MAL–AEC | MAL–PEC | AEC–PEC | PEC–OF | AEC–OF | MAL–OF |
|---|---|---|---|---|---|---|---|---|
| This study | 100 CT scans | Medial orbital rim | 22.8 (left) 22.8 (right) |
36.4 (left) 36.6 (right) |
13.7 (left) 13.6 (right) |
5.9 (left) 6.3 (right) |
19.1 (left) 19.3 (right) |
40.3 (left) 40.6 (right) |
| Kirchner et al. | 40 dried skulls and 35 cadavers | Sutura frontomaxillo lacrimalis | 16.5 (left) 17.3 (right) |
No data | 9.9 (left) 10.5 (right) |
5.6 (left) 5.9 (right) |
No data | No data |
| Mc Donald et al. | 50 CT scans | Ant. lacrimal crest | 22.4 (left) 23.1 (right) |
No data | No data | No data | No data | No data |
| Harrsion et al. | 40 exenteratios and 45 cadavers | Ant. lacrimal crest | 22 | 37 | No data | 2–9 | No data | 40 |
| Cailot et al. | 100 dried skulls | Medial orbital rim | 20 | 31 | No data | 7 | No data | No data |
| Han et al. | 24 cadavers | No data | No data | No data | 14.9 | No data | No data | No data |
| Cankal et al. | 150 CT scans | Sutura frontomaxillo lacrimalis | 29.8 | 46.3 | 13.7 | 6.7 | 19.7 | No data |
| Akdemir et al. | 6 cadavers and 25 CT scans | Sutura frontomaxillo lacrimalis | 19 | 32 | 13 | 5.1 | No data | 37.5 |
| Erdogmus et al. | 19 cadavers | No data | No data | No data | No data | 7.2 | No data | No data |
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