Terms of jaw position in the sagittal plane

• Prognathic: A term used to indicate abnormal forward projection (too far forward in position) of the maxilla or mandible in the sagittal plane in relation to the rest of the craniofacial complex; the mandibular condyles are in their normal rest relationship to the glenoid fossae of the temporomandibular joints. This may be due to a relatively larger size (hyperplasia or macrognathia) and/or a more anterior position of the basal bone of the respective jaw. Prognathism or prognathia refers to the condition in which one or both jaws are prognathic.
  
• Retrognathic: A term used to indicate lack of projection (too far back in position) of the maxilla or mandible in the sagittal plane in relation to the rest of the craniofacial complex; the mandibular condyles are in their normal rest relationship to the glenoid fossae of the temporomandibular joints. This may be due to a relatively smaller size (hypoplasia or micrognathia) and/or a more posterior position (retroposition) of the basal bone of the respective jaw. Retrognathism or retrognathia refers to the condition in which one or both jaws are retrognathic.
  
• ‘Relative’ prognathism: one jaw appears too far forward due to the opposing jaw being positioned too far back in relation to the craniofacial complex; e.g. relative mandibular prognathism means that the mandible may be in the correct sagittal position in relation to the craniofacial complex, but appears too far forward because the maxilla is too far back.
  
• ‘Relative’ retrognathism: one jaw appears too far back due to the opposing jaw being too far forward in relation to the craniofacial complex.
  
• Protrusion: A situation in which the jaws or teeth protrude farther forward than normal; the term ‘prognathism’ is preferred when referring to the jaws. Protrusion also refers to the state of being thrust forward, as in masticatory movements of the mandible.
  
• Retrusion: The state of being located posterior to the normal position.
  
• Dentoalveolar protrusion: Anterior position (too far forward) of the maxillary or mandibular dentoalveolus relative to the supporting skeletal base and craniofacial complex; e.g. maxillary dentoalveolar protrusion refers to the excessive anterior position of the maxillary dentoalveolus relative to the maxillary skeletal base and the rest of the facial profile.
  
• Dentoalveolar retrusion: Posterior position (too far back) of the maxillary or mandibular dentoalveolus relative to the supporting skeletal base and craniofacial complex; e.g. bimaxillary dentoalveolar retrusion refers to the excessive posterior position of the maxillary and mandibular dentoalveolar segments relative to the maxillary and mandibular skeletal bases and the rest of the facial profile.

Terms of maxillary rotation around the three axes of rotation

Maxillary rotation around the three axes of rotation is important mostly in relation to the effect on the maxillary occlusal plane (Figure 16.1A):

• Rotation around the vertical axis: This is termed maxillary arch rotation (or yaw), leading to transverse movement of the dental midline.
  
• Rotation around the sagittal axis: Rotation of the maxillary occlusal plane around the sagittal axis leads to a transverse cant (or roll), i.e. the left or right posterior dentoalveolus will be at a higher or lower level in relation to the contralateral dentoalveolus.
  
• Rotation around the transverse (horizontal) axis: Rotation of the maxillary occlusal plane around the transverse axis leads to a change in inclination (sagittal cant or pitch), i.e. incisors will be at a higher or lower level in relation to the molars.

Anatomy

The ‘maxilla’ is the term used to define the upper jaw, which in fact is made up of the two maxillae, each consisting of a body and four processes. The body is roughly pyramidal in shape (Figure 16.2). Its interior is hollowed out by the maxillary paranasal air sinus, alternatively termed the maxillary antrum. The upper (orbital) surface of the body occupies the floor of the orbit; the posterior surface provides the anterior wall of the infratemporal fossa, the medial surface is a major structural component in the wall of the nasal cavity and the anterior surface forms the curved external surface of the upper jaw. Above the incisor teeth, the anterior surface has a shallow depression termed the incisive fossa. Lateral to this is a ridge termed the canine eminence, formed by the root of the canine tooth, which separates the incisive fossa from the further lateral and deeper canine fossa. Above the canine fossa is the infraorbital foramen. The anterior surface ends medially at the pear‐shaped piriform (anterior nasal) aperture, inferior to which the maxillae form a median projection, the anterior nasal spine (ANS).

The four maxillary processes are (Figure 16.3):

• The zygomatic process: This projects laterally from the body, forming the anterior part of the zygomatic arch.
  
• The frontal process: This projects upwards to articulate with the frontal bone, and forms the lateral wall of the nose, articulating medially with the nasal bone.

  

  
  
• The palatine process: This projects medially to articulate with that of the opposite side, together forming the anterior three‐quarters of the bony palate; the posterior one‐quarter is formed by the horizontal plates of the palatine bones. The bony palate provides the roof of the oral cavity and the floor of the nasal cavity. The right and left sides of the bony palate are separated by the median suture. The posterior border of the bony palate has a median projection, termed the posterior nasal spine (PNS). The bony palate, together with its covering mucous membrane, is termed the hard palate. The hard palate is arched more by the downward projecting alveolar processes of the maxillae than by any upward concavity of the palatine processes.
  
• The alveolar process: This projects downwards, forming the alveoli (sockets) for the roots of the maxillary teeth. The alveolar process ends posteriorly at the maxillary tuberosity, a prominent rounded eminence located behind the last molar tooth.

Clinical evaluation

The patient should be examined in natural head position (NHP), with the mandible in the rest position and the facial soft tissues in repose. The clinical evaluation is essentially in three parts:

• Evaluation of the upper and lower midface: It is important to evaluate the soft tissue and skeletal components of the upper midface (malar eminence, inferior orbital margin, projection of the cheek and subpupil region) and the lower midface (paranasal region, nasal‐alar base and upper lip).
  
• Evaluation of maxillary position in the three planes of space: It is necessary to evaluate the maxillary position in the:
  
  
  
  • Sagittal (anteroposterior) plane
    
  • Vertical plane
    
  • Transverse plane.
  
  
• Evaluation of maxillary rotation around the three axes of rotation: It is necessary to evaluate the rotation of the maxillary corpus andmaxillary occlusal plane around the:
  
  
  
  • Sagittal axis
    
  • Vertical axis
    
  • Transverse/horizontal axis.

Sagittal midfacial‐maxillary evaluation

Soft tissue evaluation

It is often useful to mask the lower lip and mandibular region when evaluating the midface, particularly in profile view, in order to reduce the visual influence of the relative position of these lower facial structures (Figure 16.4).

Scleral exposure: In a patient with normal midfacial morphology and in NHP there should be no sclera exposed either above or below the irides in a relaxed eyelid position and forward gaze. Increased scleral exposure above the lower eyelid and below the iris of the eye is a sign of sagittal upper midfacial deficiency due to retrusion of the inferior orbital rim (Figure 16.5).

Inferior orbital rim projection: Reduced sagittal projection of the inferior orbital rim is an important indicator of midfacial retrusion. In profile view, the most anterior part of the globe of the eye is approximately 2–3 mm anterior to the soft tissue inferior orbital rim, equating to 3–4 mm anterior to the bony inferior orbital rim. The lateral orbital rim will be approximately 12–16 mm behind the most anterior projection of the globe (Figure 16.6) (see Chapter 12, section on ‘Relationship of bony orbit and globe’).

Cheek morphology and contour: The soft tissue cheek and its supporting skeletal framework form an important ‘facial aesthetic unit’ of the midfacial region. The borders of the cheek are (Figure 16.7):

• Superiorly: inferior orbital rim and zygomatic arch.
  
• Inferiorly: just above the inferior border of the mandible.
  
• Laterally: just anterior to the preauricular crease.
  
• Medially: middle and lower third of the lateral border of the nose and nasolabial fold.

  

  

The soft tissue cheek is supported by the maxilla and malar bones, the associated muscles, subcutaneous tissues, the malar fat pad and the buccal fat pad. The buccal fat pad is a component of the face that is essentially independent of the degree of generalized body fat. When excessive, removal of the pad may help to ‘skeletonize’ the midface, but age‐related changes must be taken into account. Ptosis of the pad may occur with advancing age, increasing the inferior cheek volume lateral to the mandibular body and amplifying the appearance of jowling. Combined with the generalized atrophy of midfacial fat and descent of midfacial soft tissues that occurs with age, a resultant submalar concavity may occur, which may be accentuated if the buccal fat pad has been removed previously.

Midfacial soft tissue deficiencies tend to be located within an area described as the submalar triangle (Figure 16.8).¹ This inverted triangular area of midfacial depression is bordered medially by the nasolabial fold, superiorly by the malar eminence and laterally by the masseter muscle. If the midfacial soft tissue descent and volume loss associated with ageing occurs in a patient with underlying zygomaticomaxillary skeletal deficiency, the result will be an excessively hollow overall midfacial appearance, with deep folds and wrinkles. In patients with malar prominence, the result will be depressions in the submalar region. Accurate diagnosis of such conditions is required and management tends to centre on a combination of midfacial lifting, augmentation and rhytidectomy.

Midfacial curvilinear relationship: A midfacial curvilinear contour line may be seen, originating in the soft tissues overlying the zygomatic arch, extending anteriorly along the arch then anteromedially and subsequently inferiorly to the subpupil region (located in line with the pupil of the eye, midway between the inferior orbital rim and the nasal base) over the soft tissues of the cheek, down to the paranasal region (Figure 16.9). From here the line extends inferolaterally along the lateral aspect of the upper lip, ending just lateral to the oral commissure. The line should form a smooth, uninterrupted convex curve. An interruption of this line constitutes a contour defect, which may signify an underlying skeletal deformity, e.g. a contour defect in the paranasal region is a sign of sagittal maxillary deficiency.

Paranasal area: The paranasal region of interest is that just lateral to the nasal alae. The paranasal region ideally should be slightly convex. Sagittal maxillary deficiency often results in flattening or even concavity of this region, described as paranasal hollowing (Figure 16.10).

Nasal base support: The nasal base is supported by the maxilla. Therefore, flatness or concavity of the nasal base region is an indication or maxillary deficiency (Figure 16.11).

Nasal projection: In profile view, the ratio of the horizontal linear distance from nasal tip (pronasale) to subnasale and from subnasale to alar base crease is approximately 2:1 (Figure 16.12). Assuming that the nose itself is of normal size, a reduction in this ratio may indicate sagittal maxillary deficiency.

Upper lip prominence: The prominence of the upper lip is related to the sagittal position of the maxilla and maxillary dentoalveolus, albeit variable depending on the upper lip thickness. A retrusive upper lip may be a sign of sagittal maxillary deficiency, and a protrusive upper lip a sign of sagittal maxillary excess.

Upper lip inclination: The inclination of the upper lip provides an indication of the relative sagittal prominence of the maxillary dentoalveolus.

• Upper lip inclination to nasion‐perpendicular (McNamara) (Figure 16.13): The inclination of the upper lip may be evaluated using a line tangent to the upper lip (labrale superius to subnasale) extended to intersect the nasion‐perpendicular. The angle should be:²

  

  
  
  
  
  • Male: 8° ± 8°
    
  • Female: 14° ± 8°.
  
  
• Nasolabial angle (lower component) (Figure 16.14): To assess the inclination of the upper lip, the nasolabial angle may be separated into upper and lower component parts using a true horizontal line through subnasale. This will allow the columella tangent‐true horizontal plane (upper component) and the upper lip tangent‐true horizontal plane (lower component) angles to be assessed separately, as they vary independently. Average values for upper lip tangent to true horizontal plane (lower component) are:
  
  
  
  • 85°³
    
  • 88°–92° (indirectly calculated from original data provided by Farkas⁴).

Upper lip support: The support for the upper lip is provided by the anterior maxillary dentoalveolus, but also depends to a great extent on the upper lip thickness and the amount of space in the upper labial sulcus between the inner surface of the upper lip and the anterior surface of the maxillary dentoalveolus.

The maxillary canine root is the longest of any tooth. The canine eminence is a bulge on the alveolar bone overlying the roots of the canine teeth, separating the incisive fossa anteriorly from the deeper canine fossa posteriorly (Figure 16.2). The canine eminence provides a degree of support to the upper lip, though this depends on the thickness of the upper lip. Therefore, in patients with relatively thin lips, extraction of the maxillary canine is best avoided.