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Journal of the Anatomical Society of India

Anatomy of The Bony Orbit - Some Applied Aspects

Author(s): 1Patnaik V.V.G., 2Bala Sanju, 3Singla Rajan K.

Vol. 50, No. 1 (2001-01 - 2001-06)

For Reprints, request the first author.

Abstract

The bony orbit which lodges the visual apparatus is important not only for anatomists but also for ophthalmologists, oral & maxillo facial surgeons, neurosurgeons & forensic experts. In the present paper, its bony boundaries, walls & various openings are delineated. Various weak areas in its different walls are discussed which guide the fracture lines. A mention is made of distances of the globe from important bony land marks which are to be born in mind doing surgery in the region. Last but not least, are discussed the various age changes & sex differences to help forensic experts.

Key words : Bony orbit, Anatomy, Age changes, Senile changes.

Introduction :

The two orbital cavities are situated on either side of the sagittal plane of the skull between the cranium & the skeleton of the face. Thus situated, they encroach about equally on these two regions. (Last, 1968). Each orbital cavity essentially is intended as a socket for eye-ball & also contains associated muscles, vessels, nerves, Iacrimal apparatus, fascial strata & soft pad. In nutshell it lodges the visual apparatus. (Williams et al, 1999) This is an anatomical region which is of clinical & surgical interest to many disciplines like ophthalmology, oral & maxillofocial surgery & neurosurgery. The present paper is designed to discuss the bony orbit in detail which will help not only the anatomists but also the students of allied disciplines.

Each bony orbit is made up of 7 bones namely maxilla, palatine, frontal, zygomatic, sphenoid ethmoid & lacrimal so arranged to enclose a roughly quadrilateral pyramidal cavity with base directed forwards, laterally & slightly downwards corresponding to orbital margin & apex directed postero medially & situated at medial end of superior orbital fissure, optic foramen or at the bar of the bone between these 2 apertures (Whitnall 1921) Its comparison with a quadrilateral pyramid is a rough one because the floor doesn’t reach the apex & so cavity is triangular in section in this region. (Last, 1968)

Also, since the orbit is developed around the eye, it has a tendency towards being spheroidal in form, and its widest part is not at the orbital margin but about 1.5 cm. behind this. Moreover, this results in the fact that its four walls are for the most part separated from each other by ill-defined rounded borders, so that Whitnall, 1921 compares the shape of the orbit to a pear whose stalk is the optic canal. It is important to note that the medial walls of the two orbits are almost parallel, whereas the lateral walls make an angle of about 90° with each other. The direction of each orbit is given by its axis which runs from behind forwards, laterally and slightly downwards. (Last 1968)

Applied: The fact that the greatest diameter lies within 1.5cm of the orbital margin is important when elevating the periosteum from the margins of the rim, since it is necessary to change the angulation of the blade of the elevator accordingly when entering the orbit if a strictly sub-periosteal plane of dissection is to be maintained and penetration of the orbital septum to be avoided. (Williams 1994)

Walls of the Orbit :

1. Superior Wall or roof is triangular in shape. It is formed in great part by the triangular orbital plate of the frontal bone and behind this by the lesser wing of the sphenoid. It is markedly concave anteriorly and more or less flat posteriorly. The anterior concavity is greatest about 1.5 cm from the orbital margin and corresponds to the equator of the globe.

It presents :

  • (a) The fossa for the lacrimal gland. This lies behind the zygomatic process of the frontal bone. It is simply a slight increase in the general concavity of the anterior and lateral part of the roof, and is better appreciated by touch than by sight. It contains not only the lacrimal gland but also some orbital fat found principally at its posterior part (accessory fossa of Rochon-Duvigneaud). It is bounded below by the ridge corresponding to the zygomatico-frontal suture, at the junction of roof and lateral wall of the orbit.
  • (b) The fovea for the pulley of the superior oblique is a small depression situated close to the fronto-lacrimal suture some 4 mm. from the orbital margin. In 10% cases, the ligaments which attach the U-shaped cartilage of the pulley to it are ossified. Then the fossa is surmounted most often posteriorly by a spicule of bone (the Spina trochlearis). Extremely rarely a ring of bone, representing the trochlea completely ossified, may be seen. Above the fovea the frontal sinus separates the two plates of the frontal bone; the cavity extends lateral and posterior from the fovea to a very variable extent.
  • (c) The fronto-sphenoidal suture, which is usually obliterated in the adult, lies here between the orbital plate of the frontal bone and the lesser wing of the sphenoid.

The roof of the orbit is separated from the medial wall by fine sutures between the frontal bone above and the ethmoid, lacrimal, and frontal process of the maxilla below. It is separated from the lateral wall posteriorly by the superior orbital fissure, anteriorly by the slight ridge that marks the fronto-zygomatic suture. The orbital aspect of the roof is usually quite smooth, but may be marked by certain small apertures and depressions. The apertures known as the Cribra orbitalia of Welcker are found most commonly to the medial side of the anterior portion of the lacrimal fossa. They are not always present and are best marked in the foetus and infant. They give the bone a porous appearance, and, are for veins which pass from the diploe to the orbit. (Last, 1958)

In the posterior part of the orbit, in or around the lateral part of the lesser wing of the sphenoid, small orifices may also be found which serve as communications between the orbit and the cranial dura mater and contain vessels during life. Numerous small grooves may be seen in the roof of the orbit. These lead to the above orifices and are made by vessels or nerves.

Structure : The roof of the orbit is very thin, translucent, and fragile except where it is formed by the lesser wing of the sphenoid, which is 3 mm. thick. If the bone be held up to the light, one can make out the ridges and depressions on the cranial aspect formed by the sulci and gyri of the frontal lobe of the brain. This is especially true of the posterior two-thirds. The translucency of the anterior third enables the outline of the orbital extension of the frontal sinus to be seen.

Occasionally in old age portions of the bone may be absorbed, and then the periorbita is in direct contact with the dura mater of the anterior cranial fossa. It is quite easy, in the disarticulated skull, to break the roof of the orbit by slight pressure with the finger.

The roof of the orbit is invaded to a varying extent by the frontal sinus and sometimes by the ethmoidal air-cells. The frontal sinus may extend laterally to the zygomatic process and backwards close to the optic foramen. The sphenoidal sinus or the posterior ethmoidal air-cells not infrequently invade the lesser wing of the sphenoid. (Last 1968),

Applied Aspect : We have seen that the roof is thin but reinforced laterally by the greater wing of sphenoid & anteriorly by superior orbital margin so the fractures which involve frontal bone tend to pass towards the medial side. At junction of the roof & medial wall the suture line lies in close proximity to the cribriform plate of ethmoid which may become fragmented. Rupture of dura mater causes CSF to escape into either orbit or nose or both. However permanent anosmia is unlikely because only lateral fibres of olfactory nerve on one side are involved. (Williams, 1994)

2. Medial Wall : It is about ½ the height of lateral wall since the floor of the orbit inclines upwards to meet it at about 45° (Williams 1994) It is the only wall which is not obviously triangular. It is roughly oblong, either quite flat or slightly convex towards the orbital cavity. It runs parallel with the sagittal plane, and consists from before backwards of four bones united by vertical sutures.

  • (a) The frontal process of the maxilla.
  • (b) The lacrimal bone.
  • (c) The orbital plate of the ethmoid.
  • (d) A small part of the body of the sphenoid. Of these the orbital plate of the ethmoid takes by far the largest portion. It often shows a characteristic mosaic of light and dark areas. The dark areas correspond to the ethmoidal air-cells, while the light lines between them correspond to the partitions between the cells. In the anterior part of this wall is the lacrimal fossa, formed by the frontal process of the maxilla and the lacrimal bone. It is bounded in front and behind by the anterior and posterior lacrimal crests. Above there is no definite boundary, while below the fossa is continuous with the bony naso-lacrimal canal. At their point of junction the hamulus of the lacrimal bone curves round from the posterior to the anterior lacrimal crest and bounds the fossa to the lateral side. At this point the fossa is some 5 mm. deep, while it gradually gets shallower as we trace it upwards. It is about 14 mm. in height. The lacrimal bone and frontal process of the maxilla take varying parts in the formation of the fossa; and so the position of the vertical suture between them varies also.

The anterior lacrimal crest on the frontal process of the maxilla is ill-defined above but well marked below, where it becomes continuous with the lower orbital margin and here often presents a lacrimal tubercle. The lacrimal bone separates the upper half of the fossa from the anterior ethmoidal air-cells, and the lower part from the middle meatus of the nose (Last, 1968)

Structure : The medial wall is by far the thinnest orbital wall (0.2-0.4mm). It is translucent, so that if held up to the light, the ethmoidal air-cells can be easily seen.

The orbital plate of the ethmoid, (lamina papyracea) is, in fact, as thin as paper, and infection from the ethmoidal air-cells can easily get into the orbit. This is the reason why ethmoiditis is the commonest cause of orbital cellulitis.

Despite its thinness, the orbital plate rarely shows senile absorptive changes, whereas the thicker lacrimal bone, especially that portion which forms the lacrimal fossa, is often absorbed.

Variations : (i) The lacrimal bone may be divided by accessory sutures into several parts (ii) A Wormian bone may develop in its upper and fore part. (iii) An accessory lacrimal bone, such as is found in many lower animals, may be split off the front of the ethmoid. (iv) The hamulus may be absent, may exist as a separate bone, or may be double. (Last 1968)

Applied Aspect : The medial wall is aligned parallel to the anteroposterior axis or median plane of the skull and is extremely fragile becuase of the presence of the adjacent ethmoid air cells and, more anteriorly, the nasal cavity. Gross disruption usually accompanies the more severe type of nasoethmoidal fracture and lateral displacement, or splaying apart, of the medial walls gives rise to the condition known as traumatic hypertelorism. Lateral displacement of the frontal process of the maxillae, to which the medial palpebral (canthal) ligaments are attached, results in traumatic telecanthus. The two conditions are often combined. Medial displacement of the orbital plate of the ethmoid bone is occasionally a sequel to an increase in intraorbital pressure associated with a sudden posterior displacement of the globe.

3. Floor of the Orbit : It is of particular interest because of its frequent involvement either in isolation as a so called ‘pure’ type of blow out fracture or more commonly as an impure fracture in association with other fractures in zygomatic area. Its shape is almost triangular with rouded corners being narrow posteriorly. Contrary to the common belief, the floor is not horizontal but slopes upwards & medially at 45° & ascends posteriorly at about 30°, to terminate as the anterior margin of inferior orbital fissure. At this point the bone curves smoothly but abruptly downwards into infratemporal fossa to form posterior wall of maxillary antrum. The floor is formed by three bones :

  1. The orbital plate of the maxilla forms the largest part.
  2. The orbital surface of the zygomatic forms the antero lateral part.
  3. The orbital process of the palatine bone forms a small area behind the maxilla.

The floor of the orbit is traversed by the infraorbital sulcus, which runs almost straight forwards from the inferior orbital (spheno-maxillary) fissure. At a variable distance (usually about halfway). it is converted into a canal by a plate of bone which grows over it from its lateral side to meet the medial in a suture (the infraorbital suture), which is but rarely obliterated. The suture can be traced over the lower orbital margin to the medial side of, and into, the infra-orbital foramen. It sometimes cuts across the zygomatico-maxillary suture.

Applied aspect : The infra orbital groove & canal weaken the already thin (0.5mm thickness) floor further. It is immediately medial to this line that most blow out fractures of orbital floor occur & for this reason, infraorbital nerves & vessels are almost invariably involved either by compression or contusion or by direct penetration from spicules of bone. Complete division is uncommon. (Williams, 1994).

The infra orbital canal, formed as described above, sinks anteriorly into the orbital floor and opens at infraorbital foramen some 4 mm. from the orbital margin. it transmits the infraorbital vessels and nerve. Along its course it gives off the middle and anterior superior alveolar (dental) canals, for the corresponding nerves and vessles. (Last, 1968) The infra orbital foramen is sometimes double or even multiple, accessory foramina being usually smaller & recorded at incidences of 2-18 % in various populations (Harris, 1993).

Lateral to the opening of the naso-lacrimal canal a small pit or roughness marking the origin of the inferior oblique muscle may (rarely) be found. The floor of the orbit is separated from the medial wall only by a fine suture; the lateral wall is separated from it posteriorly by the inferior orbital (spheno-maxillary) fissure, while anteriorly it is continuous with it.

Variations : (a) Not infrequently the roof of the infraorbital canal and sometimes its floor may be incomplete, but otherwise only very rarely does floor of the orbit show holes, the result of senile absorption. (b) Rarely infraorbital canal may run in the suture between the maxilla and the zygomatic bone.

Relations and Structures : Below the floor of the orbit for nearly its whole extent is the maxillary sinus, a most important practical relation. For as the bone between them is only 0.5-1mm, thick, tumours of the antrum can easily invade the orbita causing proptosis. It is in fact thinnest at the inferior orbital groove and canal.

More posteriorly is the air-cell inside the orbital process of the palatine bone, and sometimes extensions from the ethmoidal air-cells may invade the floor.

(4) The Lateral wall of the orbit is triangular in shape, the base being anterior. It makes an angle of 45° with the median sagittal plane and faces medially, forwards and slightly upwards in its lower part., It is slightly convex posteriorly, flat at its centre, while anteriorly the orbital surface of the zygomatic bone 1cm. behind the oribital margin is concave.

The lateral wall of the orbit is formed by two bones : (a) Posteriorly by the orbital surface of the greater wing of the sphenoid. (b) Anteriorly by the orbital surface of the zygomatic bone.

The sphenoidal portion passes imperceptibly into the floor, and is separated from the roof by the fronto-zygomatic suture, which is roughly horizontal and often marked by a slight ridge. The suture between the two portions of the lateral wall is vertical.

The lateral wall presents :

  1. The Spina recti lateralis : This is a small bony projection situated on the inferior margin of the superior orbital fissure at the junction of its wide and narrow portions. It may be pointed, rounded, or grooved, and gives origin to a part of the lateral rectus muscle, but it is produced mainly by a groove which lodges the superior ophthalmic vein. This groove is prolonged upwards, then runs anterior to the spine. Not infrequently the spine is duplicated.
  2. The Zygomatic Groove and Foramen : The groove which lodges the nerve and vessels of the same name runs from the anterior end of the inferior orbital fissure to a foramen in the zygomatic bone. This leads into a canal which divides into two, one branch opening on the cheek, the other in the temporal fossa. Thus the branches of the zygomatic nerve reach their destination. If the nerve divides before entering its canal, there may be two or even three grooves and foramina in the orbit.
  3. The Lateral Orbital Tubercle (Whitnall) : This is a small elevation on the orbital surface of the zygomatic bone just within the lateral orbital margin and about 11mm. below the fronto-zygomatic suture. It gives attachment to : (a) The check ligament of the lateral ractus muscle. (b) The suspensory ligament of the eyeball. (c) The aponeurosis of the levator palpebrae superioris.
  4. Not infrequently there is a foramen in or near the suture between the greater wing of the sphenoid with the frontal, near the lateral end of the superior orbital fissure. This leads from the orbit to the middle cranial fossa, and transmits a branch of the meningeal artery and a small vein (Testut).
  5. Infra orbital sulcus : It was described Ist by Royle (1973) in 22 out of 64 skulls extending from supero lateral end of superior orbital fissure towards orbital floor associated sometimes with an anastomosis beetween MMA & infra orbital artery. Later on Santo Neto et al (1984) confirmed it in 45% of 100 orbits.

Structure : Being the one most exposed to injury, the lateral is the thickest of the orbital walls, and It is especially strong at the orbital margin. Behind this is a relatively weaker part, then comes a thicker portion, and the most posterior portion, walling in the middle cranial fossa, is thinner again. The most posterior is, in fact, the feeblest portion. Here on either side of the spheno-zygomatic suture it is only 1 mm. thick and its lamellar structure makes it translucent. In 30 percent of cases, there exist in this area supplementary fissures which represent the extensive primitive communication between the orbit and the temporal fossa. (Last, 1968)

Applied Aspect : The junction of this wall with the roof and floor of the orbit are smooth and rounded anteriorly but weakened for about half the distance by the superior orbital fissure and for some two-thirds of the distance by the inferior orbital fissure. At the anterior limit of the latter, the gap becomes wider and its superior margin turns upwards slightly to join the less dense sutural interface between the zygomatic bone and the greater wing of the sphenoid. This line of relative weakness extends as far as the frontozygomatic suture and is consistently involved in fractures of the zygomatic bone. By contrast, the anterior limit of the superior orbital fissure is strongly reinforced by the cerebral surface of the greater wing of the sphenoid bone as it curves outwards to merge with the squamous portion of the temporal bone. Fractures passing through this region are rare and are often complicated by an adjacent fracture of the skull.

It should be noted that the orbit may easily be penetrated through the inferior orbital fissure by the point of a circumzygomatic awl if this is allowed to deviate medially, an error which can easily be made when the normal anatomy is distorted by displacement and communication of the fragments. (Williams 1994)

Fissures & Canals between walls of orbit : Following fissures & canals lie between various orbital walls.

  1. Superior orbital (sphenoidal) fissure.
  2. Inferior orbital (Sphenomaxillary) fissure.
  3. Anterior & posterior ethmoidal canals.
  4. Optic canal or foramen.

1. Superior Orbital fissure : It is also known as sphenoidal fissure because it is the gap between lesser & greater wings of sphenoid closed laterally by frontal bone. It lies between roof & lateral wall of the orbit. It is wider at the medial end, where it lies below the optic foramen, and is often described as comma-or retort-shaped. Sometimes there is gradual reduction in size towards the lateral extremity, but usually it is composed of two limbs, a narrow lateral portion and a wider medial part. At the junction of the two limbs is the Spina recti lateralis.

The superior orbital fissure is some 22 mm. long, and is the largest communication between the orbit and the middle cranial fossa. Its tip is 30 to 40 mm. from the fronto-zygomatic suture. Its medial end is separated from the optic foramen by the posterior root of the lesser wing of the sphenoid on which is found the infra-optic tubercle. This lies below and lateral to the optic foramen on the middle of the vertical part of the medial border of the wide part of the superior orbital fissure.

The common tendinous ring (anulus tendineus communis) spans the superior orbital fissure between the wide medial and narrow lateral parts. The lateral rectus arises here, from both margins of the fissure.

One or more fronto-shenoidal foramina may be present in the fronto-sphenoidal suture and transmit an anastomosis between the middle meaningeal and the lacrimal arteries.

Passing within the anulus or between the two heads of the lateral rectus are the superior division of the 3rd nerve, the naso-ciliary and sympathetic root of the ciliary ganglion, the inferior division of the 3rd, then the 6th (and then sometimes the ophthalmic vein or veins)—in that order from above downwards. The 6th nerve is actually passing from below the inferior division of the 3rd to lie lateral and between the two divisions.

As a rule, inferior ophthalmic vein passes below the annulus.

(2) The inferior orbital (spheno-maxillary) fissure lies between the lateral wall and floor of the orbit. Through it, the orbit communicates with the pterygo-palatine and infratemporal fossa. It commences below and lateral to the optic foramen, close to the medial end of the superior orbital fissure. It runs forwards and laterally for some 20 mm, its anterior extremity reaching to about 2 cm. from the inferior orbital margin.

The inferior orbital fissure is bounded anteriorly by the maxilla and the orbital process of the palatine bone; posteriorly by the whole of the lower margin of the orbital surface of the greater wing of the sphenoid. In the majority of cases it is closed anteriorly by the zygomatic bone. However in 3540% skulls, maxilla & sphenoid meet at the anterior end of fissure to exclude zygomatic. (Williams et al, 1999).

The fissure is narrower at its centre than at its two extremities, the anterior end sometimes being markedly expanded. The width of the inferior orbital fissure depends on the development of the maxillary sinus and thus is relatively wide in the foetus and infant.

The lateral border is sharp and may have grooves above and below it; it is higher than the medial border anteriorly, but lower posteriorly. It is closed in the living by periorbita and muscle of Muller. The inferior orbital fissure is near the openings of the foramen rotundum and the sphenopalatine foramen.

The inferior orbital fissure transmits the infraorbital nerve,the zygomatic nerve, branches to the orbital periosteum from the pterygo-palatine ganglion and a communication between the inferior ophthalmic vein and the pterygoid plexus.

(3) The ethmoidal foramina lie between the roof and medial wall of the orbit either in the frontoethmoidal suture or actually in the frontal bone. They are the openings of canals which are formed in greater part by the frontal but are completed by the ethmoid.

  1. (a) The anterior ethmoidal canal looks backwards as well as laterally. Its posterior border is ill-defined and continuous with a groove on the orbital plate of the ethmoid. It opens in the anterior cranial fossa at the side of the cribriform plate of the ethmoid, and transmits the anterior ethmoidal nerve and artery. Analysis of racial & seasonal variation in position & incidence of ethmoidal canals in 580 crania from several populations showed the anterior foramen to lie outside the fronto-ethmoidal suture in 10-20% of several modern races & 62% out of 53 Pervian crania. (Williams et al, 1999).
  2. (b) The posterior ethmoidal canal transmits the posterior ethmoidal nerve and artery. Supplementary foramina are common.
  3. (c) The Middle ethmoidal formen—Downie et al (1995) encountered a middle ethmoidal foramen in 28% of skulls examined by them.
  4. (4) The optic foramen, or rather the optic canal, leads from the middle cranial fossa to the apex of the orbit, and it is formed by the two roots of the lesser wing of the sphenoid. It is directed forwards, laterally, and somewhat downwards, its axis making an angle of about 36° with the median sagittal plane. If produced forwards, the axis passes approximately through the middle of the infero lateral quadrant of the orbital opening. Hence it is neither in the axis of orbit nor of its lateral wall. If produced backwards it would meet its fellow at the dorsum sellae of the sphenoid. The canal is funnel-shaped, the mouth of the funnel being the anterior opening. This is oval in shape,with the greatest diameter vertical. The cranial opening, on the other hand, is flattened from above down, while in its middle portion the canal is circular on section. With regard to the intracranial opening, the upper and lower borders are sharp, the medial and lateral rounded. The inter-optic groove is thus continuous with the medial wall without line of demarcation.

The lateral border of the orbital opening is more or less well defined. It is formed by the anterior border of the posterior root of the lesser wing of the sphenoid. The medial border is less well defined. The distance between the intracranial openings of the two canals in 25mm. The distance between the orbital openings is 30mm. The roof of the canal reaches farther forwards than the floor, while posteriorly the floor projects beyond the roof. This gap in the roof is filled in by a fold of dura mater with a free posterior edge (the falciform fold).

We have seen that optic canal is formed medially by the body and laterally by the lesser wing of the sphenoid bone. The walls of this canal thus have an important but varying relationship to the sphenoid sinus and to posterior ethmoid sinuses, depending upon the extent to which these sinuses may have invaded the lesser wing and the anterolateral aspect of the body of the sphenoid. The medial wall of the optic canal is rather regularly adjacent to the sphenoid sinus, unless this is particularly poorly developed, or partially replaced by an ethmoid cell; Van Alyea (1941) in a study of 100 sphenoid sinuses, found the wall of the optic canal actually projecting into the sinus in 40, and Dixon (1937) found it projecting deeply, so as to be sometimes almost completely surrounded, in 7 percent of 1600 skulls. Whitnall (1921) quotes another worker as having found the sphenoid sinus projecting into the lesser wing of the sphenoid, above the optic canal,in about one third of skulls examined, and refers to the fact that the optic canal may be completely surrounded by the sphenoid sinus, so that it forms a very thin-walled tube passing through the sinus. A similar relationship to a posterior ethmoid cell has been described by Goodyear (1948), who stated that he has seen complete blindness follow surgical opening of the sphenoid sinus and of posterior ethmoid cells. Less extensive growth of a posterior ethmoid cell may bring it into relation with the optic canal only medially, or, through invasion of the bone between the canal and the superior orbital fissure, inferiorly and laterally; Van Alyea (1941) found the canal projecting into an ethmoid cell in 5 per cent of 100 cases. In any case the bony wall between the cavities of the canal and sinus may be quite thin (0.5 mm or less), or may even present dehiscences. Vail (1931) has pointed out that this wall may also vary in character, as it may be quite dense or may present large sponge like marrow spaces.

The optic canal is separated from the medial end of the superior orbital fissure by a bar of bone, on which there is a tubercle or roughness for the anulus tendineus. The optic canal transmits the optic nerve and its coverings of dura, arachnoid, and piamater; the ophthalmic artery which lies here below than lateral to the nerve and embedded in its dural sheath and a few twigs from the sympathetic which accompany the artery. Separating artery and nerve is a layer of fibrous tissue which may (rarely) be ossified.

The orbital margin

It has the form of a quadrilateral with rounded corners. The orbital margin usually has the form of a spiral; the inferior orbital margin is continuous with the anterior lacrimal crest, while the superior is continued down into the posterior lacrimal crest. The lacrimal fossa thus lies in the orbital margin.

Each side measures some 40mm., but usually the width is greater than the height; the relation between the two is given by the orbital index, which varies in the different races of mankind.

The orbital index = (Height of orbit x 100)/Width of orbit

Taking the orbital index as the standard, three classes of orbit are recognised :

  1. Megaseme (large) : The orbital index is 89 or over. This type is characteristic of the yellow races, except the Esquimaux. The orbital opening is round.
  2. Mesoseme (intermediate) : Orbital index between 89 and 83. This type is found in the white races (European 87, English 88.4).
  3. Microseme (small) : Orbital index 83 or less. This type is characteristic of the black races. The orbital opening is rectangular.

The opening is directed forwards and slighly laterally, and is tilted so that the upper and lower margins slope gentle down-wards from the medial to the lateral side.

The orbital margin is made up of three bones, the frontal, zygomatic, and maxilla.

(a) The superior orbital margin is formed entirely of the frontal bone, i. e. by its orbital arch. It is generally concave downwards, convex forwards, sharp in its lateral two-thirds, and rounded in the medial third. At the juction of the two portions, some 25 mm. from the mid-line and situated at the highest part of the arch, is the supraorbital notch, whose lateral border is usually sharper than the medial. Not in-frequently it is converted into a foramen by the ossification of the ligament which closes it below. The posterior opening then is 3 to 6 mm. from the orbital margin. It transmits the supraorbital nerve and vessels. Notch and foramen are easily palpable in the living.

Sometimes medial to this a second notch (of Arnold) or foramen is found. This transmits the medial branches of the supraorbital nerves and vessels where these have divided inside the orbit. Supraorbital grooves leading from these notches or foramina are sometimes seen. A groove may also be present some 10 mm. medial to the supraorbital notch for the supratrochlear nerve and artery.

A supraciliary canal (Ward) is found in about half the cases. It is a small opening near the supraorbital notch, and transmits a nutrient artery and a branch of the supraorbital nerve to the frontal air sinus.

(b) The lateral orbital margin being the most exposed to injury, is the strongest portion of the orbital outlet. It is formed by the zygomatic process of the frontal and by the zygomatic bone. If looked at from the side it appears to be concave forwards and not to reach as far forward as the medial margin.

Applied Aspects : The lateral orbital rim is recessed on its deep aspect approximately 0.75 cm above the rim margin to accommodate the lacrimal gland and this occasionally leads to a segmental fracture in this region. The insertion of a screw pin into the external angular process requires considerable care with its orientation if penetration into the orbit, or more significantly into the anterior fossa, is to be prevented. The pilot hole should be commenced 1.5 cm above the frontozygomatic suture and 0.5 cm behind the rim. The angulation should be posteriorly at 45° to the long axis of the skull and inferiorly at 30° to the horizontal axis, limiting the penetration to 0.75 cm.

The narrowest and weakest part of the lateral rim corresponds to the frontozygomatic suture and, as previously pointed out,this is in continuity with the line of least resistance in the lateral wall which is situated at the junction of the greater wing of the sphenoid and the zygomatic bone. Consequently, separation at the frontozygomatic suture to a varying degree is a frequent finding following trauma in this region. (Williams, 1994).

(c) The inferior orbital margin is raised slightly above the floor of the orbit. It is formed by the zygomatic bone and the maxilla, usually in equal portions.

The zygomatic portion forms a long thin spur (the maxillary or marginal process) which lies on the maxilla. The suture between the two, which is not infrequently marked by a tubercle, can be felt lying usually about half-way along the margin just above the infraorbital foramen. Sometimes, however, the zygomatic (malar) may reach the anterior lacrimal crest, thus excluding the maxilla, or may take only a very small part itself in the formation of this margin. (Last, 1968).

Applied Aspect : The inferior orbital margin is clearly defined on its outer aspect and is readily palpated but the inner third, like the superior rim of the orbit, is more rounded. It is, however, not able to withstand direct force in the central and medial areas because of the proximity of the underlying maxillary antrum and the closely related infraorbital canal. Just within the rim, at the junction of the outer two-thirds and the inner one-third, there is a small depression which marks the origin of the inferior oblique muscle, the only extra ocular muscle which does not arise from the back of the orbit. This part of the inferior orbital rim is often fractured producing associated disruption of the muscle and subsequent diplopia. Penetrating wounds in this area may involve the lacrimal fossa and sac and severe the lacrimal passages. (Williams 1994)

(d) The medial margin is formed by the anterior lacrimal crest on the frontal process of the maxilla and the posterior lacrimal crest on the lacrimal bone. These crests overlap; the medial margin is thus not a continuous ridge, but runs up from the anterior lacrimal crest across the frontal process of the maxilla to the superior margin.

Rontal et al. (1979) studied 48 orbits in 24 skulls to determine the relationship of important structures to well-defined landmarks in the walls. From their observations they were able to arrive at mean values for their measurements of which the following are of particular interest.

  1. Infraorbital foramen to the midpoint of the inferior orbital fissure : 24 mm.
  2. Anterior lacrimal crest to anterior ethmoidal foramen : 24 mm.
  3. Anterior lacrimal crest to medial aspect of optic canal : 42 mm.
  4. Frontozygomatic suture to the superior orbital fissure; 35 mm.
  5. Supraorbital notch to the superior orbital fissure: 40 mm.
  6. supraorbital notch to the superior aspect of the optic canal : 45 mm.

These authors advice that dissection within the orbit should be limited to certain measurements and suggest that the lateral portion of the orbital floor is a safer area for commensing an exploration in this region. They consider that the anterior margin of the inferior orbital fissure, found 25 mm from the infraorbital foramen, should limit the operative field in this part of the orbit. They further advise that dissection should be restricted to 25 mm posterior to the frontozygomatic suture. The periosteum may safely be elevated along a line extending from the frontozygomatic suture laterally, to the frontoethmoid suture medially, as far back as 30 mm from the superior orbital rim without risking injury to the important structures passing through the superior orbital fissure.

To summarise the useful measurements, it may be stated that a subperiosteal dissection may safely be extended for a distance of 25 mm posterior to the inferior and lateral rim and for a distance of 30 mm from the superior rim and the anterior lacrimal crest. Due care must, however, be taken to avoid damage to the medial palpebral (canthal) ligament, lacrimal apparatus pulley of the superior obique muscle, supraorbital nerves and vessels, the structures attached to the orbital (Whitnall’s) tubercle and the origin of the inferior oblique muscle.

Age Changes in the Orbit :

The changes in the orbit during the period of growth depend partly on the development of the cranium and skeleton of the face, between which the orbit is placed, and also on the growth of the neighbouring air sinuses.

(i) The orbital margin is sharp and well ossified at birth. The eyeball is therefore well protected from stress and injury during parturition. At seven years,except at its upper part, it is less sharp, and as the supero-medial and infero-lateral angles are better marked than the others the orbital opening tends to be triangular.

(ii) The form of the orbit on coronal section behind the orbital margin is that of a quadrilateral with rounded corners. In the newborn it has the form of an ellipse higher on the lateral than on the medial side.

(iii) The infantile orbits look much more laterally than the adult, i.e. their axes,or the lines drawn from the middle of the orbital opening to the optic foramen, make an angle of 115°, and, if produced backwards, meet in the middle at the nasal septum. In the adult the axes make an angle of 40°-45° with each other, and, if produced backwards, meet at the upper part of the clivus of the sphenoid, These axes, too, lie in the horizontal plane in the infant, whereas in the adult they slope downwards from 15° to 20°.

(iv) The orbital fissures are relatively large in the child owing to the narrowness of the orbital surface of the greater wing of the sphenoid, and the wide and narrow portions are not well differentiated.

(v) The orbital index is high in the child, the vertical diameter of the orbital opening being practically the same as the horizontal, but later the transverse increases more than the vertical.

(vi) The interorbital distance is small. This is of some practical importance. Children are not infrequently brought to the ophthalmic surgeon because they are thought to squint when the strabismus is apparent only. This appearance is due to the narrow interorbital distance,which makes the eyes look too close together. With the growth of the frontal and ethmoidal air-cells the interorbital distance increases, and so causes the squint to disappear.

(vii) The infraorbital foramen is usually present at birth; but at times it may be represented by the terminal notch of an infraorbital groove whose roof has not grown over to convert it into a canal and which thus reaches to the orbital margin.

(viii) The orbital process of the zygomatic (malar) bone may almost reach the lacrimal fossa, and this condition may persist to ten years.

(ix) The roof of the orbit is relatively much larger than the floor at birth compared with large the adult proportions. The foetal skull has a large cranium (orbital roof) and a small face (orbital floor). The fossa for the lacrimal gland is shallow. But the accessory fossa is well marked.

(x) The optic canal has no length at birth, so that it is actually a foramen : at one year it measures 4 mm. The axis also changes with age ; essentially while facing forwards and laterally it looks much more downwards at birth than in the adult.

(xi) The periosteum or periorbita is much thicker and stronger at birth than in the adult.

(xii) In old age, the changes are due to absorption of the bony walls

  1. Thus in the skulls of old people holes are sometimes found in the roof of the orbit. In such cases the periorbita is in direct contact with the dura mater.
  2. The medial wall, although normally very thin, rarely shows senile holes in its ethmoidal portion. Parts of the lacrimal bone are, however, commonly absorbed.
  3. The lateral wall not uncommonly shows holes or such marked thinning that it becomes very fragile in these places.
  4. As regards the floor, senile changes very rarely produce holes apart from those in the roof or floor of the infraorbital canal.
  5. In old people, too, the orbital fissures, especially the inferior, become wider owing to absorption of their margins.

(xiii) In longheaded (dolichocephalic) skulls the orbits tend to look more laterally than in the shortheaded (brachycephalic).

Sex differences : Up to puberty there is little difference between the orbits and, in fact, the skulls of male and female.

After this the male skull takes on its secondary sexual characters, seen especially in the formation of the lower jaw and in the forehead region.

The female remains more infantile in form. The orbits tend to be rounder and the upper margin sharper than in the male. The glabella and supercilliary ridges are less marked or almost absent. The forehead is more vertical and the frontal eminences more marked. The contours of the region are rounder and the bones smoother. The zygomatic process of the frontal bone is more slender and pointed.

The female orbit is more elongated and relatively larger than the male.

References :

  1. Dixon, F. W. (1937) : A comparative study of the sphenoid sinus : A study of 1600 skulls. Annales of otology. Rhinology Laryngology: 46: p. 687
  2. Downie, I. P.: Swans, B. T; Mitchell, B. (1995) : The middle ethmoidal foramen & its contents. an anatomical study. Clinical Anatomy 8 : 149.
  3. Good year, H.M. (1948) : Ophthalmic conditions referable to diseases of the paranasal sinuses. Archives of otolaryngology 48 : p. 202.
  4. Haris, H. A. : Bone Growth in Health & Disease.Oxford University Press London (1933).
  5. Hollinshead, W. H.: Anatomy for surgeons In : The Orbit,Vol. 1, Hoebner Harper Bork New York.pp 109-113. (1958).
  6. Last, R. J.: Eugene Wolff’s Anatomy of the Eye & Orbit in : The orbit & paranasal sinuses. 6th Edn, H. K. Lewis & Co. Ltd, London pp. 1-29 (1968).
  7. Rontal, N. L; Rontal, M; Guilford, F. T. (1979) : Surgical anatomy of the orbit. Annals of otology, Rhinology & Laryngology 88 : p 382.
  8. Royle, G (1973) : A groove in lateral wall of orbit. Journal of Anatomy 115 : 99. 461-5.
  9. Santo Neto, H; Penteado, C. V.; de Carvath, V. C. (1985): Presence of a groove in the lateral wall of human orbit. Journal of Anatomy 138: 631-3
  10. Vail, H. H. (1931): Retrobulbar optic neuritis originating in the nasal sinuses : A new method of demonstrating the relation between the sphenoid sinus & the optic nerve. Archives of otolaryngology. 13 p. 846
  11. Van Alyea, O. E. (1941) : Sphenoid sinus: anatomic study with consideration of the clinical significance of the structural characteristics of the sphenoid sinus. Archives of Otolaryngology. 34 : p. 225.
  12. Whitnall S. E. : The anatomy of the human orbit & accessory organs of vision. OXford Press, New York (1921).
  13. Williams, J. L. : Rowe & Williams Maxillo facial Injuries In : Fractures of the zygomatic complex & orbit. 2nd Edn. Vol.I, Churchill Livingestons Edinburgh, London : pp: 475-9. (1994).
  14. Williams, P. L; Bannister, L.H; Berry M. M; Colllins, P; Dyson, M; Dussek, J. E; & Ferguson, M.W. J: Gray’s Anatomy In : Skeletal system. Editd by Saoemes R. W. Churchill Livingstone, Edinburgh, London. pp. 555-560 (1999).

J. Anat. Soc. India 50(1) 68 (2001)

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