Part 1. Surgical Anatomy and General Considerations


■ Part 1. Surgical Anatomy and General Considerations

The Palatal Platform

The hard palate is the platform upon which the seven craniomaxillofacial buttresses, three posterior and four anterior, reside.1 , 2 Each buttress is launched from the thicker periphery (alveolus) of the palate, then rises to engage the cranial base ( Fig. 4.1 ).

The base and ascent of each buttress from the alveolus is readily apparent in the edentulous maxilla, particularly when the thin bone of the maxillary walls has been drilled away. The architecture is also demonstrated by three-dimensional reformats of high-resolution computed tomography (HRCT) ( Fig. 4.2 ).

The palate is a by-product of two conjoined bones: the (palatine portion of the) maxilla and the (horizontal part of the) palatine. The palatal shelves define the dimensions of the base of the maxilla and are capped by an arch of dentition that is open posteriorly ( Figs. 4.3 and 4.4 ).

The parabola-shaped alveolar process of the maxilla is more delicate than that of the mandible. The maxilla lacks a substantial cortical margin, but strength is instead conferred by the palatal shelves.3

The width and depth of the hard palate and the width and height of the mandible combine to determine the three-dimensional architecture of the lower third of the face2 , 4 ( Fig. 4.5A,B ).

The bone of the palate is thick anteriorly, but it thins progressively in its approach to the soft palate.2 This anatomical feature is most apparent in sagittal and parasagittal sections.

Fig. 4.1
Fig. 4.2
Fig. 4.3

Coronal cuts, however, reveal a different perspective: ignoring the palatal crest at the midline, the hard palate is relatively thin off-midline, but the bone thickens as the alveolus is approached. As the tooth-bearing periphery of the palate (alveolus) is reached, thicknesses of 12.0 to 14.0 mm are not uncommon.5 8 Screws some 4 to 5 mm in length are chosen medially to avoid penetrating the floor of the nose when repairing the palate, but screws of greater length can be chosen in the para-alveolar area without fear of penetration into the maxillary sinus ( Figs. 4.6 and 4.7 ).

The incisive foramina are near the midline of the palate anteriorly (behind the medial incisors) and the palatine foramina are far off the midline posteriorly (near the distal molars). The foramina and canals within the palate are diminutive and do little to compromise the structural integrity of the palatal platform and its ability to bear impact2 ( Fig. 4.8 ).

Fig. 4.4
Fig. 4.5 (A, B)
Fig. 4.6

Ossification of the median palatal suture lags behind other craniomaxillofacial sutures. The ossification is, therefore, seldom complete before the third decade,5 , 9 , 10 in part explaining the higher incidence of split-palatal fractures in adolescence and early adulthood.2

Ossification of the palatal suture line begins posteriorly and progresses more rapidly on the oral side of the suture than on the nasal.5 Histologic and microradiographic studies further reveal that the initially broad and Y-shaped palatal suture becomes increasingly tortuous with age as suture ossification advances.11

The periosteum adheres more intimately to the mucous membrane of the roof of the mouth than it does to the bone of the palatal shelves. Hence, the two are referred to as mucoperiosteum ( Figs. 4.9 and 4.10 ).

The maxillary alveolus is thickened both to accommodate the launch of the buttress above and to house the lateral incisor, canine, and molar dentition below. The increased density of bone to buffer this select dentition is apparent by structural analysis and digital photography.

Fig. 4.7

The Craniomaxillofacial Buttresses

Seven buttresses arise from the palatal platform, as previously noted.

The Anterior Buttresses

By removing the thin, delicate midfacial bone dismissed by Le Fort,12 Testut,13 and Cryer,1 as a fragile “curtain,” the four anterior buttresses ascending from the palatal platform are revealed. Two of the anterior buttresses are anteromedial, and two are anterolateral.

Fig. 4.8
Fig. 4.9
Fig. 4.10

Each anteromedial midfacial buttress (nasomaxillary buttress1 ) begins at the anteromedial border of the palatal platform, briefly cants outwardly, then curves toward the nasal bones. The outward cant creates the tapered, lower piriform margin of the nasal vault, and the subsequent inward cant creates the upper margin of the nasal vault. The anteromedial buttress, joined by the nasal bone, ascends the sidewall of the nose, as the frontal process of the maxilla, to reach the nasofrontal suture. The buttress gives off a process laterally to form the medial portion of the inferior orbital rim. Although the anteromedial buttress is contoured, its ascent is near-vertical, as readily apparent in an oblique view.

Fractures of the four anterior buttresses tend to occur just above the palatal platform and along the piriform edge, where the bone is relatively thin. The buttress thickens progressively as the join with the nasal bones is secured and, even more superiorly, as the inferior and medial orbital frame and lacrimal bone are reached. The anteromedial buttress is substantial at the level of the nasofrontal suture. Horizontal sections through the anteromedial maxillary buttress, from inferior to superior, demonstrate the progressive depth and thickness of the buttress. This virtual change in density and shape is dramatized by removal of the nasal bones to allow unrestricted view of the anteromedial buttress ( Figs. 4.11 and 4.12 ).

Fig. 4.11
Fig. 4.12
Fig. 4.13

In the presence of nasoethmoid injuries of the central upper face, the nasal bones are often removed to gain exposure and first realign the splayed medial orbital frame with heavy wire (see Chapter 7).

Progressive thickening occurs in the nasal bones from distal to proximal and from anterior to posterior. Fractures of the nasal bones usually extend to involve the subjacent portion of one or both anteromedial buttresses. Thus, seldom is injury restricted to the nasal bones per se, particularly after high-velocity impact.

Fracture and comminution of the thicker portions of the anteromedial buttress trigger a more devastating “cascade of injury,” leading to complex nasal and septal disruption and grades of nasomaxillary and orbitoethmoid involvement (see Chapter 7). The nasal fragments and components of each buttress may be displaced inwardly by extreme impact, disrupting the underlying agger nasi cells, narrowing the nasofrontal recess (through which the frontal sinus drains), and distorting the cribriform plate. The middle ethmoid cross-struts (microbuttresses) collapse, and one or both middle turbinates are displaced inwardly as part of the nasal-nasoseptal, nasomaxillary, orbitoethmoid “cascade of injury”14 , 15 ( Fig. 4.13 ).

The anterolateral midfacial buttress (zygomaticomaxillary buttress1 ) begins at the anterolateral aspect of the palatal platform, over the thickened alveolus housing the distal molars. For approximately a centimeter above the alveolus, the buttress is relatively thin and delicate, as is the case with the anteromedial buttresses. As it cants outwardly by some 45 to 50 degrees, in its ascent to engage the malar prominence, the anterolateral buttress progressively thickens. The body of the zygoma is an aggregate that defines the contour of the cheek and (with the zygomatic arch) establishes the projection of the upper face. The zygoma has five processes, one inframalar and four supra-malar, and is a major contributor to the lateral and inferolateral orbit and orbital frame; thus, it is structurally more orbitozygomatic than zygomaticomaxillary, as emphasized by Clark16 and Kelley and associates.17 The zygoma articulates with the zygomatic process of the frontal bone and the greater wing of the sphenoid, maxilla, and the zygomatic process of the temporal bone (see Chapter 8) ( Fig. 4.14 ).

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Jul 2, 2020 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Part 1. Surgical Anatomy and General Considerations
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