Abstract
Aim
Sella turcica bridging and ossified carotico-clinoid ligament are two variants of the sella turcica, the origin of which is partially unknown. These variations should be properly recognised, as they may hamper the removal of the anterior clinoid process in surgical procedures. Therefore, our aim was to determine the prevalence of these two anatomical variants and to investigate their prevalence according to patient sex and age in a series of maxilla computed tomography scans.
Materials and methods
We revised 300 computed tomography scans of the head from northern Italian patients, stratified into three age groups (18–40 years, 41–60 years, >60 years): a logistic regression analysis was used to explore an association of sella turcica bridging with age and sex through Matlab software, also including a test for the extracted model (P < 0.05).
Results
The mean prevalence of sella turcica bridging and ossified carotico-clinoid ligament were 0.16 ± 0.06 (48/300, 16.0%) and 0.09 ± 0.03 (26/300, 8.7%), respectively. Statistically significant differences according to sex were found neither for sella turcica bridging (P = 0.345) nor for ossified carotico-clinoid ligament (P = 0.412). Only sella turcica bridging showed a correlation with age (P = 0.007). In addition, the two variants were often associated, as patients without sella turcica bridging usually did not show ossified carotico-clinoid ligament (P < 0.001).
Discussion
Our results suggest an association between the two variants, and provide a novel contribution to the debate around their origin.
Keywords: Anatomical variants, sphenoid bone, sella turcica bridging, caroticoclinoid ligament ossification, radiology, CT scan
Introduction
‘Sella turcica’ indicates the pituitary fossa on the upper surface of the sphenoid body, defined anteriorly by two anterior clinoid processes and posteriorly by two posterior clinoid processes, the former being parts of lesser wings of the sphenoid bone and the latter the posterior boundary of the sella turcica of the sphenoid bone.1 Variations in size of the clinoid processes may occur, and the condition when the anterior and posterior clinoid processes are merged is called ‘sella turcica bridge’. The frequency of sella turcica bridging is 6% at autopsy,2 whereas 4.6% has been reported by radiological studies,3 with a general prevalence ranging between 1.1% and 13%.4
A high frequency of this variant has been reported in patients affected by severe craniofacial deviations, accounting for up to 18.6%,5 and in patients with inherited developmental conditions, such as nevoid basal cell carcinoma (NBCCS), Williams syndrome and Axenfeld–Rieger syndrome,6–9 in patients presenting with dental anomalies and in skeletal class III individuals.10 The origins of sella turcica bridging are still unclear: some authors have stated that it may derive from a prenatal malformation involving a cartilage primordium,11 whereas other studies have hypothesised an association with the path of the internal carotid artery.12 Other authors have even proposed an ossification of the dura mater between the anterior and posterior clinoid processes,13 or have suggested a possible correlation with focal infections of the pituitary gland.6,14 In addition, a distinction between a real bone fusion and an overlapping between the anterior and posterior clinoid processes is difficult to identify.13
As sella turcica bridging occurs early during the developmental period, it has been suggested as a screening test for possible genetic diseases and dental alterations.9,11
Another abnormal ossification of the sella turcica perimeter has been reported in the literature regarding the carotico-clinoid ligament, which links the anterior clinoid process to the middle clinoid processes; a complete ossification of this ligament was reported in five out of 27 cases on skulls;15 however, more precise epidemiological data concerning radiological examinations are still missing. The ossified carotico-clinoid ligament may have a developmental origin, deriving from the ossification of a ligament linking the anterior and middle clinoid processes, or of a fold of the dura mater.16
Also, this variant has clinical importance as it may involve risks for compression of the internal carotid artery14 and its possible injuries in neurosurgical procedures involving mobilisation of the anterior clinoid process.16
Both variants have a strong clinical importance, as they may complicate surgical procedures on the skull base; in particular, the exposure of the cavernous sinus that is a very delicate procedure due to the strict neuronal and vascular relationships, necessitating the removal of the anterior clinoid process.14 The presence of variants of sella turcica may make this step difficult and increase the risk of surgical complications, especially in cases of aneurysm of the internal carotid artery and tuberculum sella meningiomas.15,17
Very little information is available concerning the origin and distribution of such variants in the general population; with regard to sella turcica bridging, a general prevalence ranging between 1.1% and 13.0% has been reported.2–4 On the other hand, the ossified carotico-clinoid ligament is mainly described in case reports,18 whereas a few systematic studies have taken dry skulls into consideration,12,19–21 and have yet to consider radiological scans. The literature suggests a general prevalence of this variant between 0.9% and 27%.14 The concurrent description of sella turcica bridging and ossified carotico-clinoid process is very rare, with a general prevalence ranging between 0.8% and 3.5% in Indian populations.12,19–21 In addition, the same combination has been the topic of a few case reports.22,23
The lack of epidemiological data concerning these variations is a severe deficiency in modern anatomy textbooks.14,24 A detailed knowledge of the type of ossification of sella turcica can be useful to increase the success rates of surgery.14
In order to improve the existing data on these specific variations, we undertook a study to determine the occurrence of bridging of sella turcica and ossification of the carotico-clinoid ligament in an Italian cohort, and to explore associations between these two anatomical variants with sex and age. In addition, the present study analysed the prevalence of both the variants in a series of computed tomography (CT) scans, whereas the literature concerning this topic has so far dealt with cephalograms.4,6,7,10,11 This may add some information for a more precise knowledge of these variations in the morphology of sella turcica.
Materials and methods
Sample recruitment
Institutional review board approval was granted for this study with a waiver of the requirement to obtain written informed consent from each patient.
A sample of 300 head CT scans acquired in a hospital in northern Italy of patients aged between 18 and 99 years (150 men and 150 women; mean age 51.5 ± 19.7 years in men and 58.4 ± 22.3 years in women) was revised by two experienced radiologists in consensus (MC and AGO with 6 and 25 years of experience, respectively). The sample size was automatically calculated in 278 subjects, stating a confidence interval of 5% and a confidence level of 5% on a CT scan database of 1000 patients.24 The chosen group was more numerous than the required sample size.
The most frequent reasons for CT scan were: screening for cranial fractures in cases of trauma (172/300, 57.3%), cephalalgy (60/300, 20.0%) or neurological symptoms (38/300, 12.7%), with no differences between men and women. All CT scan images were anonymised for the analysis.
Exclusion criteria were as follows: subjects with a diagnosis of craniofacial deformities, congenital alterations of sphenoid bones, trauma involving sella turcica, inherited developmental conditions linked to sella variants (such as nevoid basal cell carcinoma, Williams syndrome and Axenfeld–Rieger syndrome),6–9 gross dental anomalies and skeletal class III patients.
CT scan acquisition
All CT scans were performed on a second-generation dual-source scanner: Somatom Definition Flash (Siemens, Forchheim, Germany); parameters of acquisition: kV 120; reference mAs: 320 (Care Dose); collimation: 128 × 0.6 mm; tube rotation: 1 s; pitch value: 0.55; scan direction: caudocranial; reconstruction thickness: 5 mm; reconstruction filters: H30 for parenchyma and H60 sharp for bone.
Data acquisition
The frequency of sella turcica bridging and ossified carotico-clinoid ligament (Figure 1) was assessed in men and women and in three arbitrarily chosen age groups in order to divide the population into young adults (18–40 years), middle-aged subjects (41–60 years) and old people (older than 60 years).
Figure 1.
Examples of anatomical variants of sella turcica: (a) sella turcica bridging; (b) ossified carotico-clinoid ligament; (c) concurrent presence of sella turcica bridging and ossified carotico-clinoid ligament.
The sella turcica bridge can be identified as a continuous band of bony tissue extending from the anterior cranial fossa to the posterior cranial fossa. The carotico-clinoid ligaments connect the anterior clinoid process to the middle clinoid processes; when ossified, these are visible as a linear ossification on CT scan.
The assessment of anatomical variants was verified through visualisation on sagittal and transverse planes. The condition of complete ossification was considered as positive evidence.
Statistical analyses
Statistical analysis was performed using SPSS (IBM SPSS Statistics for Windows, Version 22.0; IBM Corp., Armonk, NY, USA).
We first compared differences according to sex and age in patients presenting with sella turcica bridging and ossified carotico-clinoid ligament using Pearson chi-square test for categorical variables (P < 0.05).
We then sought to determine predictive factors of sella turcica bridging and ossified carotico-clinoid ligament in our cohort performing a multivariate model including variables with a P value lower than 0.200 in order to understand better the interplay between the different factors. Adjusted odds ratios with 95% confidence intervals were estimated for each risk factor.
Results
The prevalence of different anatomical variants is shown in Figures 2 and 3. The mean proportion of sella turcica bridging was 0.14 ± 0.06 of men (21/150, 14.0%) and 0.18 ± 0.06 (27/150, 18.0%) of women, with a slight prevalence in women. In the entire population the frequency of sella turcica bridging was 0.16 ± 0.06 (48/300, 16.0%). In 0.81 ± 0.04 (39/48, 81.3%) bridging was bilateral.
Figure 2.
The prevalence of sella turcica bridging in men and women in different age groups.
Figure 3.
The prevalence of ossified carotico-clinoid ligament in men and women in different age groups.
Ossified carotico-clinoid ligament was observed with a proportion of 0.09 ± 0.03 (26/300, 8.7%) of the entire population: the variant was found with a proportion of 0.07 ± 0.04 in men (11/150, 7.3%) and 0.10 ± 0.05 in women (5/150, 10.0%). In 0.92 ± 0.03 (24/26, 92.3%) of patients the ossification was observed on both sides.
The prevalence of ossified carotico-clinoid ligament was different according to the presence of sella turcica bridging: in the general population without other sellar variants the prevalence was 0.09 ± 0.04 (22/252, 8.7%), whereas among subjects with lateral bridging the prevalence of ossified carotico-clinoid ligament was more than three times higher (0.29 ± 0.13, 14/48, 29.2%). On average, 0.05 ± 0.02 (14/300, 4.7%) of subjects showed both variants.
We did not observe a statistically significant relation between sella turcica bridging and sex (P = 0.345), nor with ossified carotico-clinoid ligament and sex (P = 0.412). Sella turcica bridging showed a relation with age (P = 0.007), with a significant association with the oldest group of subjects (P = 0.006).
We did not find a statistically significant correlation between age with ossified carotico-clinoid ligament and age (P = 0.740), nor with the oldest age group (P = 0.520). We observed a statistically significant correlation between the two variants, as patients without sella turcica bridging usually did not show ossified carotico-clinoid ligament (P < 0.001).
At multivariate analysis (Tables 1 and 2) we observed a significant correlation only between sella turcica bridging and age (P = 0.04, odds ratio 3.966, 95% confidence interval 1.558–10.097).
Table 1.
Multivariate analysis of the correlation between sex, age and sella turcica bridging.
| P value | Odds ratio | 95% CI |
||
|---|---|---|---|---|
| Inferior | Superior | |||
| Sex | 0.573 | 0.831 | 0.436 | 1.583 |
| Age | 0.040 | 3.966 | 1.558 | 10.097 |
CI: confidence interval.
Table 2.
Multivariate analysis of the correlation between sex, age and ossified carotico-clinoid ligament.
| P value | Odds ratio | 95% CI |
||
|---|---|---|---|---|
| Inferior | Superior | |||
| Sex | 0.454 | 0.728 | 0.317 | 1.671 |
| Age | 0.501 | 1.424 | 0.509 | 3.979 |
CI: confidence interval.
Discussion
The association between sella turcica bridging and ossified carotico-clinoid ligament with anatomical anomalies and variants has previously been analysed in the literature.25,26 However, information about their origin and the possible mutual relations between the two variants is still scanty and incomplete. The present study reports for the first time the prevalence of both sella turcica bridging and ossified carotico-clinoid process in a cohort of patients undergoing CT scan, without any possible predisposition towards this specific set of variants.
In the present population, sella turcica bridging was found in 16.0%, more frequently than reported by the literature (1.1–13%).4 Ossified carotico-clinoid ligament was observed in 8.7%; unfortunately, reference epidemiological data on radiological samples are still missing.
The current results provide a contribution to the existing debate about these anatomical variants, especially regarding what concerns their relation with age. In fact, both variants are reported as developmental disorders, and some authors describe them as the same phenomenon affecting the anterior and posterior clinoid processes rather than the anterior and the middle clinoid processes.27 Sella turcica bridging is considered an abnormal embryologic development of the sphenoid bone resulting in an irregular bridge formation,4,12 or it is interpreted as possible sequelae of ossification in dura mater extending between the anterior and the posterior clinoid process.27 Possible evidence supporting this theory is the association with abnormal morphology of other anatomical structures with similar embryologic origin from the neural crests, such as palate and teeth in canine impaction, palatal displacement of canines, dental displacement and cleft palate,4,12,14,26,28 and neck skeletal elements in cases of ponticulus posticus.26 In addition, sella turcica bridging was also reported as an additional anatomical abnormality in specific syndromes.6–9
Although there is a wide consensus towards the embryological origin of sella turcica bridging, some authors report that the same phenomenon may be the result of focal infections of the pituitary gland, which have not yet become clinically evident.3 In addition, ossification of dura mater folds is a phenomenon often related to age, as also shown by hyperostosis frontalis interna, a bone proliferation on the inner table of the frontal bone, commonly observed in women aged over 60 years.29 This type of anomaly is interpreted as a calcification of dura mater triggered by an enlargement of the intradural vasculature.30 These indications may explain the present data, which highlighted a higher frequency of sella turcica bridging with age; from this point of view, this study first suggests that sella turcica bridging may also be caused by aging, in a possible alternative type of the same condition besides the above-mentioned embryological disorder, sharing the same morphological features.
On the other hand, no relation was found between sella turcica bridging and sex, but the literature is divided on this point; in fact, some authors found an association with age,27 whereas other authors did not.12
Even fewer data are available in the literature about ossified carotico-clinoid ligament; Ozdogmus et al. report that, although ossification of dura mater is considered a normal age-dependant physiological condition, it is not the case of this anatomical variant.17 Other authors postulated that it originates from a developmental anomaly, highlighting cases of ossified carotico-clinoid ligament in fetal and infant skulls.31 In general, this type of ossification is not reported to be age related,17 as also shown by the present study. In addition, no statistically significant differences were found according to sex.
However, the most interesting result concerns the verified association between sella turcica bridging and ossified carotico-clinoid ligament; in fact, the probability of being affected by anterior-posterior clinoid process bridging passes from 8.7% in the general population to 29.2% in subjects affected by sella turcica bridging. The existing literature was so far limited to the statement that the concurrent description of sella turcica bridging and ossified carotico-clinoid process is very rare, with a general prevalence ranging between 0.8% and 3.5% in Indian populations;15,19,21 in addition, the same combination has been the topic of few case reports.16,23 From this point of view, the present study is the first to highlight a possible association between the two variables.
The main limitation of our study is the possible discordance between the sample analysed and the general population; however, CT scan was requested for reasons not related to the presence of sella turcica variants.
In conclusion, this study provides additional information about the variants of sella turcica; first, it showed that physiological conditions caused by aging may increase the frequency of sella turcica bridging together with the embryological disorders already ascertained by the literature. Secondarily, a possible association between sella turcica bridging and ossified carotico-clinoid ligament was observed and reported for the first time. These results will provide an important contribution to further research on this incompletely known field of clinical anatomy.
Conflict of interest
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
References
- 1.Jones RM, Faqir A, Millett DT, et al. Bridging and dimensions of sella turcica in subjects treated by surgical-orthodontic means or orthodontics only. Angle Orthod 2005; 75: 714–718. [DOI] [PubMed] [Google Scholar]
- 2.Bergland RM, Ray BS, Torack RM. Anatomical variations in the pituitary gland and adjacent structures in 225 human autopsy cases. J Neurosurg 1968; 28: 93–99. [DOI] [PubMed] [Google Scholar]
- 3.Carstens M. Die selladiagnostik. Fortschr Geb Rontgenostrahlen 1949; 71: 257–272. [Google Scholar]
- 4.Leonardi R, Farella M, Cobourne MT. An association between sella turcica bridging and dental transposition. Eur J Orthod 2011; 33: 461–465. [DOI] [PubMed] [Google Scholar]
- 5.Becktor JP, Einersen S, Kjaer I. A sella turcica bridge in subjects with severe craniofacial deviations. Eur J Orthod 2000; 22: 69–74. [DOI] [PubMed] [Google Scholar]
- 6.Axelsson S, Storhaug K, Kjaer I. Post-natal size and morphology of the sella turcica in Williams syndrome. Eur J Orthod 2004; 26: 613–621. [DOI] [PubMed] [Google Scholar]
- 7.Kimonis VE, Goldstein AM, Pastakia B, et al. Clinical manifestations in 105 persons with nevoid basal cell carcinoma syndrome. Am J Med Genet 1997; 69: 299–308. [PubMed] [Google Scholar]
- 8.McLachlan MS, Wright AD, Doyle FH. Plain film and tomographic assessment of the pituitary fossa in 140 acromegalic patients. Br J Radiol 1970; 43: 360–369. [DOI] [PubMed] [Google Scholar]
- 9.Meyer-Marcotty P, Weisschuh N, Dressler P, et al. Morphology of the sella turcica in Axenfeld–Rieger syndrome with PITX2 mutation. J Oral Pathol Med 2008; 37: 504–510. [DOI] [PubMed] [Google Scholar]
- 10.Muller F. Die bedeutung der sellabruecke fur das auge. Klin Monatsbl Augenheikd 1952; 120: 298–302. [PubMed] [Google Scholar]
- 11.Platzer W. Zur anatomie der “sellabrucke” und uhrer bezeihung zur A. carotis interna. Fortsch Geb Roentgen Nuklear 1957; 87: 613–616. [PubMed] [Google Scholar]
- 12.Ali B, Shaikh A, Mubassar F. Association between sella turcica bridging and palatal canine impaction. Am J Orthod Dentofacial Orthop 2014; 146: 437–441. [DOI] [PubMed] [Google Scholar]
- 13.Meyer-Marcotty P, Reuther T, Stellzig-Eisenhauser A. Bridging of the sella turcica in skeletal class III subjects. Eur J Orthod 2010; 32: 148–153. [DOI] [PubMed] [Google Scholar]
- 14.Leonardi R, Barbato E, Vichi M, et al. A sella turcica bridge in subjects with dental anomalies. Eur J Orthod 2006; 28: 580–585. [DOI] [PubMed] [Google Scholar]
- 15.Souza AD, Ankolekar VH, Nayak N, et al. Morphometric study of anterior clinoid process and optic strut and the ossification of carotico-clinoid ligament with their clinical importance. J Clin Diagnostic Res 2016; 10: AC05–AC07. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Das S, Suri R, Kapur V. Ossification of caroticoclinoid ligament and its clinical importance in skull-based surgery. Sao Paulo Med J 2007; 125: 351–353. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.Ozdogmus O, Saka E, Tulay C, et al. The anatomy of the carotico-clinoid foramen and its relation with the internal carotid artery. Surg Radiol Anat 2003; 25: 241–246. [DOI] [PubMed] [Google Scholar]
- 18.Inoue T, Rhoton AL, Jr, Theele D, et al. Surgical approaches to a cavernous sinus: a microsurgical study. Neurosurgery 1990; 26: 903–932. [DOI] [PubMed] [Google Scholar]
- 19.Archana R, Anita R, Jyoti C, et al. Incidence of osseous interclinoid bars in Indian population. Surg Radiol Anat 2010; 32: 383–327. [DOI] [PubMed] [Google Scholar]
- 20.Kanjiya D, Tandel M, Patel S, et al. Incidence of ossified interclinoid bars in dry human skulls of Gujarat state. Int J Biomed Adv Res 2012; 3: 874–880. [Google Scholar]
- 21.Kolagi S, Herur A, Patil G, et al. Complete sella turcica bridges prevalence and dimensions. J Anat Soc India 2011; 60: 22–25. [Google Scholar]
- 22.Mrinmoy P, Sangma GTN. The ossified caroticoclinoid ligament and interclinoid ligament in specimen of sphenoid bone: a case report. Schol J Appl Med Sci 2014; 2: 633–635. [Google Scholar]
- 23.Paraskevas G, Nitsa Z, Koutsouflianotis K. Bilateral osseous interclinoid bridges associated with foramina of Vesalius: a case report. J Clin Diagn Res 2015; 9: AD03–AD04. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 24.Creative Research Systems. The Survey System. Sample Size Calculator, 2012. https://www.surveysystem.com/sscalc.htm (accessed 3 October 2017).
- 25.Scribante A, Sfondrini MF, Cassani M, et al. Sella turcica and dental anomalies. Int J Paediatr Dent 2017; 27(6): 568–573. [DOI] [PubMed]
- 26.Haji Ghadimi M, Amini F, Hamedi S. Associations among sella turcica bridging, atlas arcuate foramen (ponticulus posticus) development, atlas posterior arch deficiency, and the occurrence of palatally displaced canine impaction. Am J Orthod Dentofacial Orthop 2017; 151: 513–520. [DOI] [PubMed] [Google Scholar]
- 27.Brahmbhatt RJ, Bansal M, Mehta C, et al. Prevalence and dimensions of complete sella turcica bridges and its clinical significance. Indian J Surg 2015; 77: S299–S301. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 28.Sundareswaran S, Nipum CA. Bridging the gap: sella turcica in unilateral cleft lip and palate patients. Cleft Palate Craniofac 2015; 52: 597–604. [DOI] [PubMed] [Google Scholar]
- 29.Standring S (ed). Overview of the development of the head and neck head: skull and mandible. In: Gray’s Anatomy: the anatomical basis of clinical practice. New York: Elsevier, 2005.
- 30.Hershkovitz I, Greenwald C, Rothschild BM, et al. Hyperostosis frontalis interna: an anthropological perspective. Am J Phys Anthropol 1999; 109: 303–325. [DOI] [PubMed] [Google Scholar]
- 31.Kier EL. Embryology of the normal optic canal and its anomalies. An anatomic and roentgenographic study. Invest Radiol 1966; 1: 346–362. [DOI] [PubMed] [Google Scholar]