Orbit and orbital trauma

The paired orbits housing the globes are highly evolved and specialized structures that maintain the structural integrity of the globe and the highly complex coordinated movements that facilitate binocular vision. The neuromuscular mechanisms are complex and require considerable neurological investment, in that, of the twelve cranial nerves, three nerves are required for eye movement and one for visual information, with one contributing to the control of lacrimation.

The eyes are also the focal point for an observer to fix on during conversation. Asymmetry is obvious and often disfiguring.

Core procedures

Surgical access to the orbit
Orbital dissection
Deep orbital dissection

Surgical surface anatomy

The influence of the orbit on the surface anatomy of the globe is profound. Clinical assessment looks at the width of the palpebral aperture, which is typically 10 mm from upper to lower eyelid. This is subdivided into two measurements. The margin reflex distance (MRD) one is the light reflection of the camera to the upper eyelid (MRD 1 at 4 mm), and the margin reflex distance two (MRD 2 at 6 mm) is the distance between the light reflection of the camera to the lower eyelid.

The lower eyelid typically crosses the cornea at 5 and 7 o'clock and should be symmetrical with the contralateral orbit. In some individuals there is increased scleral show, and the sclera is visible below the iris.

The thickness of the upper eyelid can be increased in retropositioning of the globe, in which case the major contribution is that of the palpebral part (the white double-ended arrows in Fig. 6.1 ).

Fig. 6.1, The complex surface anatomy of the globe and periorbita. Note the position of the iris as it crosses the upper and lower eyelids (black arrows) and the position of the light reflection related to the upper and lower eyelids (blue and yellow arrows). The shape of the upper lid and distance from the margin to the eyebrow are influenced by the globe position, and subdivided into palpebral (white arrow) and orbital parts (green arrow). The shape of the lateral and medial canthus (red lines) is dictated by the underlying tendons, the medial ligament opening out of the angle and exposing the caruncle (C). Skin creases caused by muscle skin attachments (orange arrows) vary according to age and race.

The appearance of the orbit is also dependent on the position of the soft tissue attachments in the form of the medial and lateral canthal tendons, the position of the eyebrow, and the condition of the overlying skin.

Clinical anatomy

The orbit is classically described as a pyramidal structure that is made up of four bony walls (floor, medial and lateral walls, roof) and a fifth wall, closed by the upper and lower eyelids on their fibrous soft tissue skeleton, the tarsal plates, which are rigidly attached both medially and laterally. This essentially closed system is analogous to that of the lower limb, which is at risk of a compartment syndrome caused by any volume-occupying medium, usually blood in the form of a retrobulbar haematoma, but also air as a tension pneumo-orbit, or even cerebrospinal fluid ( Fig. 6.2 ).

$Fig. 6.2, An axial CT scan demonstrating a left orbital retrobulbar haematoma (blue arrows) following fracture of the left medial orbital wall (red arrows).$

Surgical pathology

Historically, the mechanism of failure of the orbit in response to trauma has been described in terms of either transfer of energy to the infraorbital margin with secondary dissipation along the floor (buckling theory), or a direct blow to the globe with onward propagation through the floor.

Each orbit is made up of seven constituent bones that have differing thicknesses and strengths, and whose individual physical characteristics influence the lines of failure and subsequent fracture configurations ( Fig. 6.3 ). These have distinct surgical pathologies and can be separated functionally into a number of different functional units.

Fig. 6.3, The orbit divided into five regions. These regions behave in stereotypical ways in response to energy transfer. See text for colour key.

The floor (blue on Fig. 6.3 ) is composed of the orbital process of the maxilla anteriorly, and the orbital process of the palatine bone posteriorly.

The medial wall (purple) is composed of very thin bones, the ethmoid and lacrimal bones. Because the bone in this region is thin, it is prone to fracture with differing degrees of comminution, depending on the volume of energy transfer running across it; the pathway of energy propagation has long been disputed but it is, in fact, largely academic and clinically irrelevant.

The frontal bone (red) forms the roof of the orbit. It separates the anterior cranial fossa from the orbit and so any fracture propagation is part of a craniofacial sequence with all of the attendant sequelae.

The lateral wall (yellow), composed of the zygomatic bone and the greater wing of the sphenoid, is thick and fracture separation at this part represents a fracture of the zygomatic bone.

The orbital apex (green) is composed of the lesser and greater wings of the sphenoid, which are separated by the superior and inferior orbital fissures. The robust bone is implicated in only the most severe mechanisms.

Enophthalmos

Fractures of the orbital floor, the medial wall and the roof present clinically according to the size, position and surface area of the wall deficits. The fundamental physical sign is that of enophthalmos, which is defined as a relative difference in the anterior position of the globe compared to the contralateral side. This manifests in a smaller palpebral aperture, resulting in a smaller eye and a relative increase in the amount of upper lid visible. The globe may sit lower, which is called hypoglobus.

There are a number of theories as to the aetiopathogenesis of this condition.

Orbital volume

The soft tissue volume constrained by the orbital septum acts as a closed box. If this box is opened, with tearing of the orbital septum and herniation of orbital contents, then there is a relative increase in the volume of the orbit, which will alter the relative position of the globe ( Fig. 6.4 ).

Geometry of the orbital walls

The advent of modern CT scanning technology, with the ability to render planes of view in three dimensions, has led to an increased understanding of orbital fracture configuration patterns, as well as refining surgical reconstruction ( Fig. 6.5 ). Recent advances and refinements in surgical technique allow for full exposure and visualization of extensive orbital defects and have made accurate reconstruction possible. Greater surgical understanding has led to the identification of key internal orbital shapes and regions, which has subsequently refined reconstruction. Key examples are recognition of the sinusoidal shape of the floor on sagittal section, the posteromedial bulge of Hammer, and the importance of reconstructing the orbital floor.

Fig. 6.5, Volumetric analysis of the case shown in Fig. 6.4 . A , Sagittal view. B , Coronal view. The yellow and purple lines indicate the volume object measurements. The green line represents the shape of the normal orbit. The red line represents the mirrored normal left orbit, which clearly shows an alteration in shape and, in particular, loss of the sinusoidal S-shape in the right orbit.

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