3D CT stereoscopic imaging: observations of the frontal and anterior ethmoid sinuses development from birth to early adulthood*

Objective : Our objective is to provide observations demonstrated with 3Dimensional Computed x-ray Stereoscopic Imaging (3DCTSI) in the evaluation of the anterior ethmoid and frontal sinus development from birth to age 18. Methods : This is a retrospective evaluation of patient’s CT studies performed over a fifteen-year period, reported as normal studies, and included 53 patients (142 sides) from birth to age 18. Results : At birth, there are two spaces covered by folds, the uncinate and bulla lamellae. The spaces communicate with the Middle Meatus (MM) through the emerging ethmoid infundibulum (EI) and the retrobulbar recess space (RBRS). In the first month after birth, an expansile and breakdown developmental phase blend and continue throughout the growth into the teenage years. The 3D images reveal dark lamellar structures, on the surface of the medial lamina papyracea as well as bridging the broken spatial outlines. The dark lamellae represent the mucosal lamina propria, in unossified lamellae and are the origin of permanent spatial walls. From ages 4 to 18 years, initially, the frontal recess (FR) and later the MM penetrate into the cancellous frontal bone creating the frontal Sinus (FS), the frontal septum (FS), Inter-Frontal Sinus Septal Cell (IFSSC), as well as the Fronto-Ethmoidal and Frontal Bulla Spaces. Conclusion : 3DCTSI is the first intuitive imaging modality to reveal the microanatomical development of the anterior ethmoid and frontal sinus anatomy.

However, despite the advances made in endoscopic and imaging technology, the highly compartmentalized nature of the ethmoid and frontal sinus development remains unexplained, largely due to the lack of anatomical specimens, as well as the lack of indications for the use of imaging to study the sinus development at various ages.
Our study focuses on patients ranging in age from birth to 18 years of age, a dynamic period in sinus development (22) . The detailed anatomy is illustrated and findings are compared to previous reports in the literature. See Table 1 for annotations.

Imaging equipment
Axial CT scans were performed on a Siemens Flash or Force 125 slice CT scanner using the following parameters: slice thickness less than 0.75 mm, field of view 14cm, 140mA, 120 kV and a pitch of 0. 9.
Studies were performed without administration of intravenous contrast material.

The CT scanners provide density measurements in Hounsfield
Units (HU). The range of the density measurements extend from -1000HU for air; 0HU for water; and +3000HU for solid bone.
On 3D images, low density membranes (< 0HU) appear as dark lamellae, as described below.
An advanced evolution of the Dextroscope imaging device was used to create 3D CT Stereoscopic Imaging displays (3DCTSI) from the original CT data. The device provides "en bloc" 3D display of the imaging data, with the capability of a scrolling removal of the image data in the axial, coronal and sagittal planes. It also has the ability to scroll into the imaging volume in a planar view, which is adjustable to view the image volume at any angle and reveal the anatomy from any obliquity. The device has a virtual surgery capability to remove structures, which obstruct the view of specific anatomic detail. It also has a "replace function" tool to restore inadvertently removed anatomy.

Source of information
The following study, approved by The Johns Hopkins Medical Institutions IRB, is a retrospective study performed on deidentified CT data (IRB00224384). All CT scans were ordered by the patient's physicians. Access to personal information which included the reason for ordering the scans was denied to the current investigators, due to US HIPAA laws.
Only patients with normal sinuses were included: without evidence of trauma, congenital abnormalities, inflammatory or neoplastic pathology. CT data of 53 patients (142 sides) were evaluated. 42 patients had single studies (84 sides); and 11 patients had 2 or more studies, for a total of 29 studies (58 sides). Table 2 for developmental summary) Three phases are recognized in the development of the anterior ethmoid sinus: 1) the presentation at birth ( Figure 1); 2) the first month through 4th year, i.e., "breakdown phase"; and 3) the 4th

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year to early teenage years, the "rebuilding phase" (Figures 2 -9). The FS development begins at the age of 3-4 years, with the anterior expansion of the frontal recess (FR) into the cancellous frontal bone (Figures 4-10).

Birth to 1 month old
The middle meatus (MM), which is demarcated posteriorly by the Basal Lamella (BaLa) of the middle turbinate (MT), covers two "hockey sticks shaped folds", from anterior to posterior: the Uncinate Process (UP) having two components: "pars ascendens" and "pars descendens" and the Ethmoid Bulla (EB) (3,12) . The MM extends from the middle turbinate medially to the lamina papyracea (LP) laterally, penetrating the furrows between the uncinate lamella (UL) and bulla lamella (BL), anteriorly; as well as between the BL and the middle turbinate basal lamella (BaLa), posteriorly. The anterior furrow is the precursor of the hiatus semilunaris and the ethmoid infundibulum (EI) (4,5) . It communicates with the space being enclosed anteriorly by the ethmoid uncinate process ("pars ascendens"/EUP) (previously called the Agger Nasi Cell) or in an alternative designation the Infundibular Space of the Ethmoid Uncinate process (ISEUP) (33,34) . The posterior furrow is the precursor of the retro-bulla recess space, which will eventually communicate with the emerging EB space.
The horizontal portion of the UP is in close spatial relationship with the lacrimal bone and the inferior turbinate, and it visually extends posteriorly to the Posterior Fontanelle ( Figure 1) (33)(34) .

to 2 months old
The EI furrow expands superiorly and creates an anteriorsuperior space. A third "fold" is created, which covers this newly created space between the UP and EB.

months to 1 year old
Expansion with individual breakdown of the outlines of anterior ethmoidal spaces continues. Well defined dark outlines appear on the medial wall of the LP, which when compared to nonhuman mammalian species correspond with the lamina propria Table 2. Summary of 3DCTSI developmental observations.

3DCTSI Development of the anterior Ethmoid Sinus birth to early teenage years.
Birth -1 month. The middle meatus space covers the uncinate and ethmoid bulla folds. It extends through the hiatus semilunaris and retrobulla recess to the lamina papyracea, creating two distinct spaces, the ethmoid infundibulum (EI), and the retrobullar recess (RBR) (Figure 1).
1 to 2 months. The EI groove expands superiorly creating an anterior-superior space covered by a third fold between the uncinate process (UP) and ethmoid bulla (EB). At this point two bony ridges separate the three spaces. In the 2nd month, the two ridges undergo a resorption, the beginning of the breakdown stage, and the beginning of the frontal recess (FR). The space covered by the turbinal uncinate process (TUP), is the turbinal infundibulum (TI) which expands inferiorly into the maxilla, the early maxillary sinus (MS) (Figure 1).
3month -1 year. The expansile process increases as does the breakdown of spatial boundaries. Well defined dark outlines appear on the medial wall of the lamina papyracea (LP), which when compared to non-human mammalian species correspond with the mucosal lamina propria, the origin of spatial bony barriers /walls. A similar dark outline is seen along the evolving lamellae of the UP and BL, as well as within the posterior ethmoid sinus (PES) (Figures 2-4).
The "rebuilding" process is more prolonged, with evidence seen at three years (  d.-f. 3D left sided sagittal images of a 7day old patient, viewed progressively from medial to lateral. They reveal the UP (gold) and the EB (blue) "folds", as well as the anterior rim of the MT Basal Lamella (yellow-green). The Hiatus Semilunaris and EI "groove" in red is between the UP & EB. The RBS is visible between the EB and the MTBL (BaLa). The lateral extension of the MM above the EB and UP creates the respective "pits" (white asterisks) and also the "pit" above the RBS (red asterisk). The space within the outline of the UP is the emerging Infundibular Space, which will become enclosed by the Ethmoid Uncinate Process (IS-EUP) (gold asterisk).
g.-i. 34 d old Infant. Sequential 3D obliqued, right sided sagittal images from medial to lateral reveal the UP (gold) and EB (blue) folds, as well as the anterior ridge of the BaLa (yellow-green). The ridges (black arrows) separate the MM evaginations into the LNW, which create the "pits" or "furrows", that become apparent with progressive medial scrolling into the AES (white asterisks). Note the gradual balloon like expansion of the superior EI (red arrows, yellow 2), as well as the appearance of the IS-EUP (red 1), the space above the EUP (gold asterisk) and the space above the EB (3). Note also, the remodeling dissolution of the "dark" cartilaginous ridges, most pronounced in fig. (i.), between the spaces above EUP (gold asterisk), the superior EI (yellow 2) and above EB (3), which is highlighted by the red doted lines. This reveals the early phase in the creation of a single superior antero-lateral space within the AES, above both IS-EUP and Ethmoid Bulla.
of developing ridges, the origin of spatial bony walls or barriers.
A similar dark outline is seen along the margins of the evolving EUP and EB, as well as within the PES.
The expansile process is more evident in the AES.  b.-f. Sagittal 3D images of the right nasal cavity and ethmoid sinus at various oblique angles. Note the "folds" of the UP (U, in gold), the Ethmoid Bulla (in blue) and the MT Basal Lamella (yellow-green). In figs (b., c.) the cartilaginous "ridge" (black arrow), which separates the spaces above the uncinate and ethmoid bulla, is undergoing dissolution as it is remodeled. Also note that the remodeled ridge has a dark contour. The dark density is also evident in figs. (d., e.) at the yellow arrow, which marks the separation of the superior IS-EUP space (gold arrow) from the space above the EI (red outline, red arrow) and the EB (blue outlined red arrow). In figs. (c.-f.) the superior space above the uncinate and bulla folds is a united singular space (white asterisk). The Maxillary sinus opening (black outlined red arrow).

to 4 years old
The expansile breakdown of spatial boundaries continues.
However, in this age range new structures emerge: indicating a "rebuilding process" whereby new laminae are formed. This rebuilding process is prolonged, with evidence seen at three years ( Figure 3) and extending into teenage years (Figures 3-7).
In this process, new spatial boundaries/lamellae are created within parallel layers of mucous membranes that we recognize as dark lamellae.
They are composed of soft tissue with a low (negative) HU density. With aging, we see a progressive transition to higher HU densities, finally reaching HU densities in the positive range i.e., the transition to bony density ( Figure 7). Specifically, the dark membrane density ranged from (-700 HU to -400 HU) in younger patients, and (-400 HU to 0 HU) in older patients. The HU measurements indicate that the density of the dark outline represents a transition from soft tissue to bone formation.

years to adulthood
The appearance of supernumerary FS and supraorbital Ethmoid Spaces (cells) were not present in our pediatric population. This is likely due to the fact that additional enlargement of the FS is a prerequisite, and noted to occur beyond the age of 18 years and observed in the adult population (22)(23)(24)(25)31) .

Discussion
Reports on the development of the ES and FS date back to the early 1900s (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) . Disagreement is reported with respect to time of origin of the FS: Davis stated that the FS is demonstrable near the end of the first year, as pneumatization gradually extends from the anterior ethmoid sinus into the frontal bone as cancellous bone is resorbed (9) . Shaeffer stated that a frontal sinus is first recognized "at the end of the first year, however, one is certain of its presence by the third year" (4) . There appears to be disagreement regarding end of FS development: Koch reports the end of FS growth to be at age 20 (24) ; Stern reports the end of the FS expansion to be at age 40 (25) , while Finby and Kraft Thus, bone is produced on the external surfaces of existing bone, by osteoblasts that migrate to the bone's surface from the periosteum, analogous to intramembranous bone formation. The bony lamellae that separate ethmoid air cells originate from the existent ethmoid surface and invade existing membranous septa (i.e., the "dark membranes").
The two studies indicate that the "sandwiching dark lamella" with progressively diminishing negative HU measurements suggests that these structures start of as entirely soft tissue and transition into a bony structure as peripheral existing bone invades and replaces the midline soft tissue.
report that FS growth extends beyond age 40 (26) . We limited our evaluation to the "generally accepted" pediatric age i.e., birth to age 18 (22) .
The literature presents several concepts regarding the origin of the FR. Onodi and Killian refer to the FS as a space in the antero-superior MM containing the ostium of the FS (1,2) . Schaeffer and Kasper considered the FR to be the origin of the FS (3,4,6) . Van Alyea considered that the FS developed from the frontal recess in the ascending ramus of the middle meatus or the ethmoidal Infundibulum, or from a cell located in or around the infundi-bular groove (8) . Our observations on 3DCTSI reveals that the  cartilage is more or less 350HU; soft tissue is at or above 0HU; air is -1,000 HU and the faint dark membranous outline, described above, range between -330HU and -670HU.
h., i. 3D Sagittal images focused on the AES and FS in a 12 yr, 8 month old patient, reveal the "dark lamella like" membranous structure (white asterisks) noted to arise from the junction of the bulla lamella (blue outline) and uncinate lamella (gold outline). Note that, posteriorly, the dark lamella is fused with a cartilaginous structure (red asterisk), which adheres to the bulla lamella. In the FS, the irregular outline of the "dark lamella" appears to initiate the creation of a "bullous space" within the FS and has the characteristics of an evolving fronto-ethmoidal space or a frontal bulla.
Since human pediatric sinuses cannot be examined systematically, non-human primates, offer tractable models of sinus development (33)(34)(35)(36)(37) . Previously, Smith et al. found that primates developing maxillary and frontal sinuses have a more fragmented lateral nasal capsule, than primates without the frontal and maxillary sinuses. At earlier stages of development, the lateral wall of the cartilaginous nasal capsule is more complete. This suggests that cartilage may be considered to be the "gate-keeper" that determines the timing of secondary pneumatization (35) . Histological studies of tamarin monkeys (e.g., Saguinus spp) reveal details which are currently precluded from radiological display. Notably, cartilage breakdown by resorption (e.g., evidence of chondroclasts) or other mechanisms is seen prior to pneumatic expansion of sinuses (37,38) .
Based on these observations, we hypothesize that the human fronto-ethmoid sinus system develops in a similar manner, following localized cartilage resorption or breakdown. Further work with animal models might elucidate radiological properties of cartilage, which is known to undergo tissue-level changes as the nasal cavity develops (38) , to enhance our ability to track human sinus development.
Bone forms by direct or indirect routes. In "indirect" forms of ossification, bone replaces an existing tissue, such as cartilage (39) . Intramembranous bone forms directly within mesenchymal tissue in association with a "membrane" (i.e., a collagenous fascia). Mesenchymal cells differentiate into bone-producing cells (osteoblasts) at these sites, producing bones of the superficial facial skeleton, clavicle, and distal phalangeal tips.
Our observation regarding the 'dark membranes' is discussed in this context. Pertinent to the present study is the fact that additional elaboration of bones occurs even after bones are first formed. Generally, this bone is produced on the external surfaces of existing bone, and the process is termed "appositional growth. " Bone is produced by osteoblasts that migrate to the bone's surface from the periosteum, and it is laid down on external surfaces; this is considered analogous to intramembranous bone formation ( Figure 6) (40) . Such bone has long been acknowledged to make endochondral bones more elaborate after they are ossified, and the new bone is known to follow membranes such as fontanelles of the skull (41) . Similarly, we suggest that the bony lamellae that separate ethmoid air cells originate from existing ethmoid surface and invade existing membranous CT images. Given the information available from the studies of non-human mammals it is our belief that the dark membrane is comprised of 2 parallel mucosal epithelium layers "sandwiching" a core of lamina propria between them (Figures 4-7), "appositional bony growth" and when evaluated with HU density measures reveals a transition from soft tissue to bone (Figure 7).
Our observations also allow an extended discussion about the development of the frontal sinus with its structural components: -The frontal sinus drainage has been the topic of recent publications (42)(43)(44) . On 3DCTSI, the frontal sinus drainage pathway is primarily accomplished through the FR and subsequently the EI, and/or through the MM. Less frequently the FR has a direct communication with the ISEUP or the Supra Bullar Recess space to then communicate with the EI and/or the MM (31) .
-The frontal sinus septum is a medial/paramedial complete separation within the frontal sinus arising from the bonding of the medial surfaces of the expanding lateral frontal sinus cavities  septation(s) the midline space is noted to communicate with the MM bilaterally (Figures 9, 10). To date the origin of the IFSSC has been considered to be the result of a diverticular process (17,18) .
However, 3DCTSI reveals that the IFSSC is created by a developmental process with potential openings within the paracentral septations as these fuses with bony ridges within the outline of the FS. Thus, the central space will communicate with the peripheral FS (es) as well as the MM bilaterally and therefore the central space cannot be the result of a diverticular process (45) .

Conclusions
Since the introduction of FESS in the 1980s, we have witnessed numerous advances in surgical techniques, strategies and instrumentation accompanied by advances in imaging. Each advance in imaging from plain X-ray imaging to X-ray polytomography to single plane CT imaging to CT MPR improved the display and understanding of the nasal cavity and paranasal sinus anatomy.
which steadily replace the midline cancellous frontal bone (Figure 8b). This entity is not to be confused with IFSSC (discussed below) (Figure 9), and the incomplete septations resulting from bony grooves arising from the antero-inferior surfaces of the frontal sinuses ( Figure 10). The ability to decrease the CT slice thickness from 2mm (late 1980s) to 0.7 to 0.1mm (recently) significantly improves the 3D CT volumetric display with a markedly improved perceptive understanding of this regional anatomy.

Authorship contribution
SJZ, FAK, SM, and WH contributed to the writing and edited the anatomic detail; MS assisted in creating the needed images.