Abstract
Background: The anterior ethmoidal region, including the bulla ethmoidalis, is the most common area addressed during functional endoscopic sinus surgery. Therefore, a detailed understanding of the bulla is essential for safe and effective surgery.
Hypothesis: Based on a review of historical articles on sinus anatomy and review of the current understanding of sinonasal embryology, it is suggested that the ethmoidal bulla is a "lamella" structure rather than a "cell," as it is widely accepted to be.
Objective: To analyze the anatomic conformation and nature of the ethmoidal bulla.
Methods: Detailed gross anatomic sagittal dissection of 14 sinonasal complexes with special attention to the ethmoidal bulla and surrounding structures and pneumatization tracts.
Results: The ethmoidal bulla consisted of a distinct bony lamella in all cases. The degree of development and pneumatization was variable, ranging from a rudimentary torus to a relatively well-pneumatized "bulla"-like structure. A pneumatization tract originating from the retrobullar recess was present in all specimens. This pneumatization excavated into the lamella, creating the bulla-like appearance as viewed from the middle meatus. However, the bulla was not a discrete individual ethmoid cell as it did not have a complete or discrete posterior bony wall. Rather, the posterior wall of this pneumatization tract was formed by the basal lamella.
Conclusion: The ethmoidal bulla lacks a distinct posterior wall and therefore is not a separate cell but rather a bony lamella with an air space behind it. From an anatomic perspective, bulla is perhaps not the best term for this structure.
Key words: chronic rhinosinusitis, endoscopic sinus surgery, ethmoid sinus
Sommaire
Introduction: La region des ethmeides anterieures incluant la bulle ethmoidale est certainement la region la plus souvent manipulee durant la chirurgie endoscopique fonctionnelle du sinus. C'est pour cette raison que la comprehension de cette region anatomique est essentielle a une chirurgie securitaire et efficace.
Hypotheses: En revoyant les articles sur l'anatomie du sinus et en analysant notre comprehension de l'embryologie nasosinusienne, nous suggerons que la bulle ethmoidale est en fait une structure ,lamellaire- plutot qu'une veritable cellule comme ii est generalement accepts.
Objectifs: Analyser l'anatomie et la nature de la bulle ethmoidale.
Methode: Nous avons revu l'anatomie de quatorze complexes naso-sinusiens disseques en coupes sagittales en portant une attention speciale a la region de la bulle ethm6idale et des structures avoisinantes.
Resultats: La bulle ethmeidale consiste en fait en une lamelle osseuse distincte dans tous les cas. Le degre de developpement et de pneumatisation est variable s'etalant d'un simple torus rudimentaire jusqu'a une bulle bien pneumatisee. Un trajet de pneumatisation originant du recessus retrobullaire est note dans tous les specimens. Cette pneumatisation excave a travers la lamelle ce qui cree l'apparence de bulle quand on la regarde depuis le meat moyen. Cependant, cette bulle nest pas une cellule ethmoidale individuelle puisqu'elle n'a pas de mur posterieur complet et independant. On remarque plutot que le mur posterieur de ce trajet de pneumatisation est forme par la lame basale.
Conclusion: La bulle ethmofdale ne presente pas de mur posterieur distinct et nest donc pas une cellule independante. Elle est plutot une lamelle osseuse avec un espace aerien derriere elle. D'une perspective anatomique, le terme bulle nest donc peut-etre pas le meilleur choix pour decrire cette structure.
In the last 15 years, there has been a tremendous change in the management of chronic sinus disease. The advent of rigid endoscopy permitted a more detailed understanding of the pathophysiology of chronic sinusitis and provided the ability to surgically manage this disease in a precise and more complete fashion. The initial descriptions of functional endoscopic sinus surgery (FESS)1,2 emphasized the ostiomeatal complex and the anterior ethmoidal region in the etiology and treatment of chronic sinusitis. Therefore, a detailed understanding of the anterior ethmoidal anatomy and its components, such as the ethmoid bulls, is critical.
The ethmoidal bulls has been referred to as the most constant and largest cell of the anterior ethmoid, sitting like a bleb attached to the lamina papyracea.3 Its origin is described as occurring due to pneumatization of the bulls lamella. This pneumatization can be variable, with poorly developed or absent pneumatization in anywhere from 8 to 40% of cases.3-5 The ethmoidal bulls has been described as draining into the retrobullar space of the sinus lateralis.3
During embryologic development of the lateral wall of the nose, rudimentary structures, termed ethmoturbinals, give rise to the more permanent structures such as the uncinate process (first ethmoturbinal), middle turbinate (second ethmoturbinal), and superior turbinate (third ethmoturbinal).6,7 As development progresses, secondary invaginations and evaginations emerge from the lateral nasal wall.3 These evaginations have been termed secondary or accessory conchae, whereas the invaginations have been termed secondary furrows or accessory meati. The ethmoidal bulls is thought to arise as a secondary lateral nasal wall evagination with the sinus lateralis arising from a portion of the corresponding secondary furrow.8 As mentioned above, it is pneumatization of this structure that results in formation of the ethmoidal bulls.
From an embryologic and anatomic perspective, it is difficult to reconcile the consistent finding of a posterior opening for the ethmoidal bulls with its development from pneumatization of the accessory concha found between the first and second ethmoturbinal. There is no clear explanation of the mechanism of pneumatization of the bulls lamella or for its consistently posterior drainage pattern. From a clinical perspective, the endoscopic sinus surgeon will frequently open the anterior-most aspect of the bulls lamella and find himself or herself viewing the basal lamella. These two findings led us to question our concept of the origin and anatomic structure of what is probably the most pathologically and surgically important structure in the paranasal sinuses.
It was the aim of this investigation to evaluate the adult anatomy of the anterior ethmoidal region with specific attention to the ethmoidal bulls. We hoped to clarify our understanding of the true nature of this structure.
Methods
Eight fresh human cadaver heads were used for this study. The specimens were not previously dissected, nor were they chemically treated or fixed. They were chosen in a random fashion and sectioned in the midline sagittal plane to provide exposure of the lateral nasal wall. If a sinonasal complex was violated during the midline sectioning process, that side of the specimen was excluded from dissection and analysis for the purposes of the study. Each side of a cadaver head was treated separately and counted in the total (14 sides).
Dissections were carried out in a standardized fashion beginning with superior and posterior reflection of the middle turbinate. This permitted exposure of the uncinate process and the ethmoidal bulla. To facilitate exposure and documentation, the reflected portion of the middle turbinate was trimmed along its attachment to the lateral nasal wall, and the line of resection was marked using an acrylic blue paint to maintain its identity as a key point of reference.
After obtaining exposure of the middle meatus as outlined above, documentation of the specimen was made using an SLR camera with a macro lens and ring flash. Each specimen was numbered for later reference. Photo documentation was made from lateral and several oblique views and varying depths of field.
Based on the varying degrees of pneumatization anticipated, a coding scheme was developed for description and recording of the development of the ethmoid bulla. A rudimentary lamellar structure without pneumatization was designated as a Class I bulla. This corresponds to the torus ethmoidalis of Grunwald (as opposed to the bulla ethmoidalis). Minimal pneumatization excavating into the bulla lamella was designated as a Class II bulla. Extensive or near-complete pneumatization was designated as a Class IV bulla. The Class III bulla was intermediate between Classes II and IV.
The assignment of bulla class type was made based on the dissection/photodocumentation and was reviewed by the other investigator for consistency. In all cases, there was agreement between evaluators as to the assignment of bulla class.
Results
The bulla lamella was identified in all specimens, as was a hiatus semilunaris superioris and retrobullar recess. It appeared that the bulla lamella was pneumatized from the retrobullar recess. The ethmoid bulla was not a discrete air cell with an anterior and posterior wall, as is pictured in some anatomy texts. Rather, the variable pneumatization tract that arises from the retrobullar recess excavates and shapes the lamella into a bulla or blister-like structure (Figs. 1-5). The pneumatization can extend into the orbit and the skull base. The posterior limit of the space is formed by the basal lamella of the middle turbinate. Traversing the posterior wall of the bulla will therefore place one within the posterior ethmoid sinus. In no case did the ethmoid bulls have a complete posterior wall that was distinct from the basal lamella. The distribution of the specimens within the classification system outlined in the methods section is present in Table 1.
In six instances, bilateral specimens were preserved and available for comparison of congruence. Congruence was defined as two sides of the specimens that had an identical bulla class (the degree of pneumatization). In four cases, there was congruence. In two instances where there was no congruence, there was only one bulla class separating the two sides.
Discussion
It is of some interest to follow the evolution of our understanding of sinus anatomy. The early otolaryngologists/anatomists did their dissections from a lateral approach and had a remarkably detailed understanding of the pneumatization of the complex region of the anterior ethmoid.4,5,9,10 This was ironic since they lacked the technology (that we currently enjoy) to put this understanding to good clinical use in the operating room. It is also regrettable to note that this remarkable and valuable fund of knowledge remained largely unrecognized and underused by modern surgeons, until recently.
Prior to the development of endoscopic diagnosis and surgery, this finely detailed understanding of the clefts of the ethmoid region was of less clinical utility. However, with the advent of endoscopic microsurgery and endoscopic diagnosis, precise description and understanding of the sinus anatomy have assumed great clinical relevance. We are now able to use the existing literature concerning the anatomy of the paranasal sinuses to perform safe and complete endoscopic sinus surgery with maximal preservation of normal structures/mucosa.
In addition to endoscopic microsurgery, another recent advance in the treatment of rhinosinusitis has been high-resolution computed tomography scanning. Computed tomography scans are primarily used to visualize the paranasal sinus anatomy via the coronal plane, the result of which is that the endoscopic sinus surgeon tends to view the ethmoid bulla from this coronal perspective. This results in the concept of the bulla being a round bleb (implying a complete air cell) sitting on the lamina papyracea (Fig. 6). Further, in the case of nasal endoscopy, the view of the surgeon is primarily from this same anterior to posterior perspective. These two situations may therefore have altered our understanding of and appreciation for the anatomy of the ethmoid bulla.
In the present study, we have tried to examine the true nature of the ethmoid bulla. Our questioning of the anatomy of the ethmoid bulla is derived from the surgical experience of opening the anterior face of the bulla and visualizing the basal lamella. Further, the finding of rudimentary or poorly pneumatized bullae also raised questions about the anatomy of the ethmoid bulla. We have found that, in all cases, the bulla has an incomplete posterior wall and drains into the middle meatus via a pneumatization tract from the retrobullar space (see Fig. 6), irrespective of the degree of pneumatization or the presence of transverse septae.
Our finding of general agreement between two sides of a given specimen with respect to their bulla class (degree of pneumatization) supports our anecdotal experience from intraoperative findings. Generally, a patient's anatomy will be consistent between the two sides.
The clinical relevance of these findings is threefold. The first, as mentioned previously, is the clinical significance of the anterior ethmoid region in the etiopathogenesis of chronic rhinosinusitis and its importance in FESS. Additionally, when performing endoscopic sinus surgery, as we pass through the bulls lamella we will frequently encounter the basal lamella rather than a posterior wall to the bulls. Recognizing this fact will prevent unnecessary trauma to sinus mucosa in the posterior ethmoid. Third, to achieve complete exenteration of the ethmoid bulls, the surgeon must recognize the presence, in the more extensively pneumatized bulls region, of the medial portion of the bulls lamella. This medial wall must be taken down to achieve complete exenteration of the anterior ethmoid, and failure to do so will likely result in persistent inflammation in this critical ostiomeatal region.
A frequent teaching point in endoscopic sinus surgery is the sequential removal of the various significant lamellae as the surgeon progresses through the ethmoid and sphenoid regions. These lamellae include the uncinate process, bulls lamella, basal lamella, and anterior face of the sphenoid. An improved understanding of the anatomy of the ethmoid bulls, including the presence of its medial wall and lack of a complete posterior wall, will permit a safer performance of this lamellar, depth-related surgery.
Conclusion
In conclusion, we have shown that, in all cases, the ethmoid bulls has an incomplete back wall. In more than 25% of our cases, the bulls was poorly or not at all pneumatized. The drainage pattern of the bulls was always posteriorly into the retrobullar space. Thus, it would seem that the bulls might actually be more of an air space created behind a variably developed lamella rather than a true bleb or air cell. From an anatomic perspective, bulls is perhaps not the best term for this structure.
Received 13/06/00. Received revised 20/11/00. Accepted for publication 08/12/00.
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Erin D. Wright: Department of Otolaryngology, The University of Western Ontario, London, Ontario; William E. Bolger: Department of Otorhinolaryngology, The University of Pennsylvania, Philadelphia, Pennsylvania.
Address reprint requests to: Dr. Erin D. Wright, Department of Otolaryngology, London Rhinosinology Centre, 900 Richmond Street, 3rd Floor, London, ON N6A 5B3.
Copyright Decker Periodicals, Inc. Jun 2001