Fig. 2.1
Middle wall of right orbit. PLC posterior lacrimal crest, ALC anterior lacrimal crest, H hammulus, LMS lacrimo maxillar suture, FES fronto ethmoid suture, FP frontal process, LP lamina papyracea, LB lacrimal bone, SN sutura nota, AF anterior foramen (anterior ethmoid artery), PF posterior foramen (posterior ethmoid artery), OC optic canal (optic nerve), SB sphenoid bone, MB maxilla EMS ethmoidomaxillary suture
Both lateral walls form a 90° angle, while each of them is situated at 45° from the medial wall. Orbital walls are curved in order to maintain the projection of the ocular globe while cushioning trauma to the eye.
In an adult, the height of the orbit is approximately 35 mm and the width 40 mm. The volume of the orbit is 30 mL, including 7 mL corresponding to the ocular globe [2, 3].
Medial Wall (Paries Medialis)
It separates the orbit from the ethmoid sinus and the nasal cavity. From anterior to posterior, the medial wall is constituted by the frontal process of the maxilla (processus frontalis), the lacrimal bone (os lacrimale), the lamina papyracea of the ethmoid bone (lamina orbitalis), and the sphenoid bone (os sphenoidale) (Fig. 2.1).
The lamina papyracea comprises the largest portion of the medial wall. It is an extremely thin layer of bone (0.2–0.4 mm) [4] that becomes thicker in its posterior part, where it inserts in the sphenoid body. In this area it conforms the medial wall of the optic canal (canalis opticus) [1]. Superiorly, the lamina papyracea articulates with the roof of the orbit at the frontoethmoid suture. The foramina of the anterior and posterior ethmoidal canals can be found at this level (Fig. 2.1). Through these canals, branches of the ophtalmic artery and the nasociliary nerve exit the orbit towards the nasal cavity.
The rule 24–12–6 has been suggested to remember the distance in millimeters from the anterior lacrimal crest to the anterior ethmoidal foramen (24 mm), from anterior to posterior foramina (12 mm), and from the posterior foramen to the optic canal (6 mm) [5]. The medial wall articulates with the orbital floor at the ethmoidomaxillary suture.
Lacrimal Fossa (Fossa Sacci Lacrimalis)
The lacrimal sac is contained in a groove excavated in the inferomedial region of the medial orbital wall called the lacrimal fossa. It is limited by the anterior lacrimal crest (crista lacrimalis anterior) of the frontal process of the maxilla and the posterior lacrimal crest (crista lacrimalis posterior) of the lacrimal bone. The distance between both lacrimal crests is approximately 8–9 mm [6, 7] (Fig. 2.1).
The articulation between the frontal process of the maxillary bone (margo lacrimalis) and the lacrimal bone is a vertical crest called lacrimomaxillary suture. Endonasally, this suture corresponds to the maxillary line [8] which is a very important landmark, easy to identify by endoscopic approach. For external DCR, however, the most important landmark is the anterior lacrimal crest. Anterior to this crest lies a fine vascular groove termed sutura nota that conveys a small branch of the infraorbital artery that may cause significant bleeding during dissection of this area [9].
The distance between the anterior lacrimal crest and the lacrimomaxillary suture is 4 mm, representing roughly the midpoint of the lacrimal fossa. Vertically, the lacrimal fossa measures 10–17 mm [7, 10, 11].
The palpebral portion of the orbicularis muscle (pars palpebralis) inserts in the anterior lacrimal crest by the medial palpebral ligament. This portion has a deeper part that originates in the posterior lacrimal crest (pars lacrimalis) that runs behind the lacrimal sac and helps to its dilatation [1]. This portion was first described by Professor W.E. Horner in 1824 and has been named Horner’s muscle [12].
The Lacrimal Bone
The lacrimal bone is a quadrilateral sheet of bone divided in two regions by the posterior lacrimal crest. The posterior part articulates with the lamina papyracea of the ethmoid bone, which lies at the same level. The anterior part forms the posterior boundary of the lacrimal fossa. The lacrimal bone has a thickness of 106 μm [13]. This minimal thickness allows the osteotomies to be done with laser during endocanalicular DCR.
Superiorly, the lacrimal bone articulates with the internal orbitary process of the frontal bone, forming the frontolacrimal suture (Fig. 2.1).
Nasolacrimal Canal (Canalis Nasolacrimalis)
The nasolacrimal canal opens at the base of the lacrimal fossa. It is limited laterally by the maxillary bone and medially by the lacrimal bone and the inferior turbinate.
The superior orifice is formed by the articulation of a small hook-like projection of the lacrimal bone (hamulus lacrimalis) with the upper portion of the lacrimal notch (incisura lacrimalis) of the maxilla (Fig. 2.2). The lacrimal process of the inferior turbinate (processus lacrimalis) and the inferior margin of the lacrimal bone close the canal inferiorly. The mean length of the bony canal is about 11 mm. The mean transverse diameter is approximately 3.5–4.6 mm, and the anteroposterior diameter is 5.6–6.8 mm. A narrowing is usually found at entrance of the canal that has an oblique inferior and posterior course, forming a 15–25° angle posterior to the frontal plane [14–17] (Figs. 2.3 and 2.4).
Fig. 2.2
Maxilla and palatine bone. FP frontal process, ML margo lacrimalis, LG lacrimal groove, MS maxillary sinus, PB palatine bone, CE crista ethmoidalis, CC crista conchalis, LN lacrimal notch
Fig. 2.3
Cranial CT scan of cadaver specimen showing the bony portion of the lacrimal system (nasolacrimal canal)
Fig. 2.4
Right nasolacrimal canal. View from the lacrimal fossa. LMS lacrimo maxillar suture, LB lacrimal bone, LO lacrimal orifice
Inferior Orifice of the Nasolacrimal Canal (Ostium Canalis Nasolacrimalis)
The orifice of the nasolacrimal canal is located at the roof of the inferior nasal meatus. It can be located approximately 1.5 cm superior to the nasal floor, 1.5 cm posterior to the anterior attachment of the inferior nasal turbinate to the lateral nasal wall, and 2.4 cm from the anterior nasal spine [10, 18]. This orifice is usually covered by a mucosal fold called Hasner’s valve [19] (Figs. 2.5 and 2.6).
Fig. 2.5
Projection of the lacrimal system canal in the lateral wall of the right nasal cavity. L projection of the lacrimal system, MT middle turbinate, IT inferior turbinate, ST superior turbinate
Fig. 2.6
Endoscopic view of the inferior meatus. 45° angled endoscope. IM inferior meatus, HV Hasner’s valve, IT inferior turbinate
Lateral Wall of the Nasal Cavity
The Maxilla (Maxila)
The maxilla is a paired bone that takes part in both the facial massif and the lateral wall of the nasal cavity. It is composed of a central body and four processes: zygomatic, frontal, alveolar, and palatine. The body of the maxilla contains the maxillary sinus that opens into the nasal cavity through the hiatus maxillaris. The palatine process articulates with its contralateral counterpart to form the anterior segment of the hard palate (Fig. 2.2). The frontal process grows superiorly from the anterior part of the body, to articulate cranially with the frontal bone, in the posterior margin with the lacrimal bone, in the medial aspect with the middle turbinate, and in the inferior margin with the inferior turbinate (Fig. 2.7).
Fig. 2.7
Lateral wall of right nasal cavity: maxilla. IT inferior turbinate, LB lacrimal bone, MS maxillary sinus, PB palatine bone, M maxilla
The lacrimal groove (sulcus lacrimalis) is excavated in the body of the maxilla posterior to the frontal process. In most cases it is an open or partially covered groove, but eventually a complete conduct can be found. The lacrimal groove lodges part of the lacrimal sac and the membranous duct to its outlet in the inferior meatus.
The nasolacrimal canal is completed medially by the lacrimal bone in the uppermost part and the lacrimal process of the inferior turbinate inferiorly (Fig. 2.8).
Fig. 2.8
Lateral wall of right nasal cavity. FS frontal sinus, LC lamina cribosa, PEC posterior ethmoidal cell, MT middle turbinate, ST superior turbinate, IT inferior turbinate, PB palatine bone, SS sphenoid sinus, l projection of the lacrimal system
The maxilla articulates dorsally with the palatine bone, which in turn articulates with the pterygoid process of the sphenoid bone, serving as boundary for the pterygopalatine fossa. At the same time, they compose the lateral wall of the nasal cavity and give support to the ethmoidal air cells, and the middle, superior, and supreme turbinates.
The Palatine Bone (Os Palatinum)
The palatine bone is located between the maxilla and the pterygoid process of the sphenoid. It has horizontal and perpendicular plates. The horizontal plate articulates with the horizontal plate of the maxilla forming the hard palate. The perpendicular plate has two processes, orbital and sphenoidal, and a notch that is converted into a foramen by the apposition of the pterygoid plate of the sphenoid. This foramen serves as passage for the sphenopalatine vessels and nerves to the nasal cavity. The nasal surface of the perpendicular plate has two crests. The superior crest (crista ethmoidalis) gives insertion to the middle turbinate and the inferior (crista conchalis) to the inferior turbinate (Figs. 2.2, 2.7, and 2.9).
Fig. 2.9
Endoscopic view of the right nasal cavity. L projection of the lacrimal system, UP uncinate process, MT middle turbinate, IT inferior turbinate, S septum, AN agger nasi, Ax axila
The Ethmoid Bone (Os Ethmoidale)
The ethmoid bone sits in the middle of the sinonasal structures and is part of both the lateral and the middle walls of the nasal cavity. On its superior face, the lamina cribosa separates the anterior cranial fossa from the nasal cavity. The perpendicular plate (lamina perpendicularis) hangs on a sagittal plane that forms the upper part of the nasal septum. The ethmoid bone has two lateral masses that contain the ethmoidal cells (labyrinthus ethmoidalis). The external wall of the lateral mass is called lamina papyracea and contributes to the medial wall of the orbit, together with the lacrimal bone and the lateral wall of the sphenoid bone. In its superior border there are two small grooves that house the anterior and posterior ethmoidal arteries. The medial surface of the lateral mass is part of the lateral wall of the nasal cavity. The middle, superior, and sometimes, the supreme turbinates are the main structures in this medial surface (Fig. 2.9). Each turbinate limits it corresponding meatus (middle, superior, or supreme). The lacrimal canal is partially located in the anterior part of the middle meatus in close relationship with the middle turbinate (Figs. 2.9 and 2.10a, b).
Fig. 2.10
(a) Lacrimal system. Endoscopic view. FP frontal process of maxilla, ML maxillary line, LB lacrimal bone. (b) Dissection of the lacrimal system. Endoscopic view. FP frontal process of maxilla, IT inferior turbinate, LD lacrimal duct, LP lamina papyracea, UP uncinate process, MT middle turbinate, S septum
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