The Mandible


Chapter 19
The Mandible


Terminology


See ‘terminology’ section in Chapter 16.


Anatomy, morphology and size


Normal Anatomy and Subunits


The mandible, the bone of the lower jaw, is one of the most important bones in the craniofacial complex; its anatomy and structural relationships are thus highly significant (Figure 19.1). For descriptive purposes, the mandible may be said to be comprised of a number of subunits: the two main parts are the ramus, which is for articulation and the insertion of jaw‐moving muscles, and the body (corpus), which carries the dentition (Figure 19.2). The body is horseshoe‐shaped when viewed from above, and the two vertical rami project upwards one from each posterior end of the body. The posterior border of the ramus projects upwards as the condylar process, which consists of a neck, expanding into a condylar head or condyle. The body of the mandible projects up around the teeth as alveolar bone, which forms the walls of the tooth sockets. After loss of a tooth the living alveolar bone atrophies. Extensive tooth loss may thereby lead to a reduction in lower anterior face height due to loss of the dentoalveolar process (Figure 19.3). The inferior border of the mandible provides the demarcation between the lower face and neck. The anterior extremity of the mandibular body is the highly variable prominence of the mandibular or mental symphysis (bony chin).


Morphology


Typical morphology: The morphology of the mandible (excluding the bony chin) is relatively consistent, depending on the basic type of dentofacial deformity with which a patient may present, e.g. ‘Class II’, ‘Class III’, ‘short face’ or ‘tall face’. Conversely, the morphology of the bony chin is highly variable and will be described in detail in Chapter 20.


Gonial angle (Ar‐Go‐Me): This is a measure of the angle formed between the slope of the posterior border of the mandibular ramus and the mandibular plane. It helps to describe the morphology of the mandible, in particular the relationship between the ramus and the body (Figure 19.4). It is highly correlated with the mandibular plane angle.



  • Riolo et al. (USA, 1974)1 described normal values:

    • Males: 124° ± 6°
    • Females: 122° ± 4°.

  • Bhatia and Leighton (UK, 1993)2 described normal values:

    • Males: 126° ± 6°
    • Females: 128° ± 5°.

An increased gonial angle is associated with posterior mandibular growth rotation, and a reduced gonial angle is associated with anterior mandibular growth rotation. In order to determine the relative contribution of the ramus and body of the mandible to mandibular morphology, the gonial angle may be divided into two parts, an upper and lower component, by drawing a facial depth line (nasion‐gonion). The upper component of the gonial angle (50° ± 2°) identifies the inclination of the ramus and the lower component identifies the inclination of the body of the mandible (Figure 19.5).3

An image of human skull teeth from the right side indicates the ramus and alveolar process structures.

Figure 19.1 (A) Oblique right lateral view. (B) Superior view. (C) Oblique left posterior view.



  1. Condylar head
  2. Condylar neckCondylar process
  3. Pterygoid fovea
  4. Coronoid process
  5. Ramus
  6. Oblique line
  7. Gonial angle
  8. Alveolar process
  9. Body (corpus)
  10. Mental foramen
  11. Inferior border
  12. Mental protuberance
  13. Mandibular foramen
  14. Superior and inferior mental spines
  15. Sigmoid (mandibular) notch
An image of the upper jaw with labeled teeth and key mandibular subunits indicates dental and anatomical structures.

Figure 19.2 Mandibular subunits:



  1. Condylar subunit
  2. Coronoid subunit
  3. Ramus
  4. Gonial angle
  5. Body (corpus)
  6. Dentoalveolus (dentoalveolar subunit)
  7. Inferior border
  8. Mandibular symphysis (bony chin subunit)
An image of the original document of the first United States Constitution indicates its importance and historical context.

Figure 19.3 Extensive tooth loss results in resorption of the alveolar process and thereby a reduction in the lower anterior face height; the mandible will rotate anteriorly bringing the chin characteristically closer to the nose. (Head of an Old Man, c. 1487–90, Leonardo da Vinci, Codex Trivulzianus, Sforza Castle, Milan.)

Two skull images indicate one with a reduced acute gonial angle and the other with an increased obtuse gonial angle both missing teeth.

Figure 19.4 (A) Reduced, acute gonial angle. (B) Increased, obtuse gonial angle.

An image of the gonial angle indicates its upper and lower sections for analyzing mandible inclination.

Figure 19.5 The gonial angle may be divided into an upper and lower component in order to determine the relative contribution of the ramus and body inclination to mandibular morphology.


Mandibular growth rotations: Growth rotations of the mandible occur when there is a discrepancy in the amount of anterior and posterior lower facial heights. It is important to bear in mind that the amount of rotation is masked to some extent by periosteal remodelling and dentoalveolar adaptation. Forward (anterior) rotation of the mandible, in the direction of mouth closing, is due to increased posterior vertical facial growth relative to anterior vertical facial growth (Figure 19.6A). Backward (posterior) rotation of the mandible, in the direction of mouth opening, is due to reduced posterior vertical facial growth relative to anterior vertical facial growth (Figure 19.6B). Björk4 described seven structural signs found on a lateral cephalometric radiograph, which may provide an indication to the pattern of mandibular growth (Table 19.1 and Figure 19.7).


Inferior border of the mandibular body: The lower border of the mandible should be well‐defined, providing distinct separation of the lower face from the neck. This region will be relatively unattractive in the absence of such demarcation between the lower face and neck, which may be due to skeletal and/or soft tissue factors (Figure 19.8):



  • Skeletal factors:

    • Increased gonial angle
    • Increased mandibular plane angle
    • Reduced mandibular body length.
      Two images of a man's face with an x-ray of his skull indicate the connection between mandible rotation and facial growth.

      Figure 19.6 (A) Forward (anterior, anticlockwise) rotation of the mandible, in the direction of mouth closing, is due to increased posterior vertical facial growth relative to anterior vertical facial growth. (B) Backward (posterior, clockwise) rotation of the mandible, in the direction of mouth opening, is due to reduced posterior vertical facial growth relative to anterior vertical facial growth.


      Table 19.1 Björk’s seven structural signs indicating the pattern of mandibular growth rotation












































      Structural signs Forward (anterior) mandibular growth rotation Backward (posterior) mandibular growth rotation
      1 Inclination of the condylar head Forward Backward
      2 Curvature of the mandibular canal Increased curvature Straight canal
      3 Shape of the lower border of the mandible Absence of an antegonial notch Convex lower border and antegonial notch
      4 Inclination of the mandibular symphysis (bony chin) Forward inclination, increased chin projection Backward inclination, reduced chin projection
      5 Interincisal angle Increased Reduced
      6 Intermolar and interpremolar angle Increased Reduced
      7 Lower anterior face height Reduced Increased

      For the numbers refer to Figure 19.7.


  • Soft tissue factors:

    • Increased cervical facial adiposity
    • Redundant lower facial soft tissues/jowling.

  • Any combination of the above.

A thorough clinical evaluation of this region is required, involving clinical inspection and palpation. The lateral cephalometric radiograph, orthopantomograph (OPT) and where necessary three‐dimensional reconstruction of the mandible permits more accurate analysis of the inferior mandibular border. When skeletal and soft tissue factors coexist, surgical correction to redefine the natural demarcation in this region may involve augmentation of the lower mandibular border and gonial angles in conjunction with rhytidectomy.


Size and position



  • Absolute size: Normal values for mandibular dimensions in white Caucasians have been provided by Bhatia and Leighton2 and Riolo et al.1 (Table 19.2 and Figure 19.9).
    An image of Bjork's seven structural signs indicates mandibular growth patterns for orthodontic analysis.

    Figure 19.7 Björk’s seven structural signs, which may be used to indicate the pattern of mandibular growth (for an explanation of the numbers, refer to Table 19.1).

    An image of the inferior border of the mandible indicates the indistinct outline of the jaw's lower section.

    Figure 19.8 Indistinct inferior border of the mandible.


  • Relative position: The sagittal position of the mandible may be analysed in relation to the anterior cranial base, the facial vertical (or nasion perpendicular) or the sagittal position of the maxilla:

    • Anterior cranial base (SN) (see Figure 7.22):

      • SNB: 79° ± 3°
      • SN‐Pog: 80° ± 3°
      • SND: 76°–77°.

    • Nasion perpendicular (see Figure 7.14):

      • B‐point is 2–3 mm behind N‐perpendicular.

    • Maxilla:

Proportional relationship of body to ramus


Mandibular micrognathia (a small mandible) may have reasonable proportionality between the ramus and body. Although hypoplasia of the mandible may affect the ramus, body or both, syndromic mandibular deficiency is often due to a short mandibular ramus. The surgical technique of distraction osteogenesis may be used to differentially lengthen the ramus and/or the body of the mandible. The craniofacial dimensions of patients with craniofacial syndromes in particular may not fit into normal ranges of variability. Therefore, the use of proportional analysis rather than comparison with population ‘norms’ is a useful method of evaluating the specific area (ramus or body), direction and extent of lengthening that may be required.



  • Ratio of ramus height (Co‐Go): mandibular body length (Go‐Gn) is 5:7.
  • Ratio of maxillary length (ANS‐PNS): mandibular body length (Go‐Gn) is 2:3.
  • Ratio of anterior cranial base length (SN) to mandibular body length (Go‐Gn) is approximately 1:1.

Sagittal and vertical relationships


Mandibular deficiency


The characteristic feature of sagittal mandibular deficiency, and the most common complaint from the ‘Class II’ patient, is the retruded position of the chin relative to the rest of the face in profile view. It is important to distinguish true skeletal mandibular deficiency from relative mandibular deficiency.


True sagittal mandibular deficiency


True sagittal mandibular deficiency may be due to:



  • A small mandible (mandibular micrognathia).
  • A retropositioned mandible (mandibular retrognathism).
  • A combination of the above.

Relative mandibular deficiency


Relative mandibular deficiency may be due to:



  • Horizontal microgenia: It is important to bear in mind that the deformity may be localized to the chin, with an otherwise normal mandible. The morphology of the chin is highly variable and will be discussed in Chapter 20.

    Table 19.2 Normal mandibular dimensions (white Caucasian adults)




















































































    Parameter Normal value (mm)
    UK norms (Bhatia and Leighton, 1993)2 US norms (Riolo et al., 1974)1
    Male Female Male Female
    Total mandibular length


    Condylion‐B point 103 ± 4 97 ± 3 117 ± 5 110 ± 4
    Condylion‐pogonion 116 ± 5 108 ± 4 131 ± 6 121 ± 4
    Condylion‐gnathion 119 ± 5 110 ± 4 134 ± 5 124 ± 4
    Condylion‐menton 117 ± 5 108 ± 4 130 ± 5 122 ± 4
    Body length


    Gonion‐B point 71 ± 5 66 ± 3 80 ± 4 77 ± 4
    Gonion‐pogonion 78 ± 4 71 ± 4 87 ± 4 82 ± 4
    Gonion‐gnathion 78 ± 4 71 ± 4 86 ± 4 81 ± 4
    Gonion‐menton 75 ± 4 68 ± 4 81 ± 4 78 ± 3
    Ramus height


    Condylion‐gonion 59 ± 4 55 ± 4 66 ± 4 61 ± 2
    Articulare‐gonion 48 ± 4 44 ± 4 54 ± 4 50 ± 4

    Refer to Figure 19.9.

    An image of the jaw including major bones and muscle structures for anatomical.

    Figure 19.9 Mandibular dimensions (for normative values see Table 19.2).


  • Posterior mandibular rotation (downward and backward, in the direction of mouth opening): Secondary to vertical maxillary excess (VME).
  • Maxillary and/or midfacial prognathism: The mandible may be in a normal sagittal position, but will appear deficient as the maxilla is prognathic.
  • Any combination of the above.

The aetiology of mandibular deficiency may result from a combination of relative and true mandibular deficiency.


Diagnostic features


The diagnostic features of sagittal mandibular deficiency depend to a great extent on the lower anterior facial height (LAFH). In patients with sagittal mandibular deficiency and a normal or reduced LAFH, the diagnostic features are (Figure 19.10):



  • Chin position: The chin is retropositioned in the sagittal plane relative to the rest of the face in profile view. The chin itself may have normal morphology, but it will appear retropositioned due to a small or retropositioned mandible. If the chin is well developed in the sagittal plane (normogenia or horizontal macrogenia), it may reduce the Class II appearance of the facial profile. Conversely and less commonly, if the chin is underdeveloped in the sagittal plane (horizontal microgenia) as well as mandibular deficiency, the Class II appearance will be more pronounced.
  • Lower lip position: The lower lip is retropositioned in the sagittal plane relative to the rest of the face in profile view. This is perhaps the most important diagnostic feature, as it distinguishes the Class II profile due to horizontal microgenia (isolated small chin), where the lower lip relationship will be normal to the rest of the face, with that due to horizontal mandibular deficiency, where the lower lip is retropositioned.
    An image of a woman with a missing tooth standing next to an X-ray indicates mandibular deficiency and reduced facial height.

    Figure 19.10 Sagittal mandibular deficiency with a reduced lower anterior facial height.

    An x-ray of a man with a broken jaw indicates sagittal mandibular deficiency and increased lower anterior facial height.

    Figure 19.11 Sagittal mandibular deficiency with an increased lower anterior facial height.


  • Lower lip morphology: The lower lip is commonly everted, particularly in Class II division 1 incisor relationship, when the lower lip is caught behind the maxillary incisors leading to their proclination. The proclined maxillary incisors are interposed between the lips and in turn will evert the lower lip. This tendency is accentuated as the lower anterior face height reduces.
  • Mentolabial fold: This tends to be accentuated, with a reduced, acute mentolabial angle, particularly when the lower anterior face height is markedly reduced.
  • Gonial angles: These tend to be reduced, giving a square appearance to the mandibular angles. It is not uncommon to have hypertrophy of the masseter muscles.
  • Intraoral features: The incisor overjet tends to be increased, although the mandibular incisors may be proclined as a result of dentoalveolar compensation for the underlying Class II skeletal pattern. Proclined mandibular incisors may in turn lead to a degree of protrusion of the lower lip. The maxillary incisors are often proclined due to the lower lip being trapped behind them. The incisor overbite tends to be increased, with an increased curve of Spee due to over‐erupted mandibular incisor teeth.

In patients with sagittal mandibular deficiency and an increased LAFH, the diagnostic features are (Figure 19.11):



  • Chin position: Excessive LAFH is often accompanied by posterior vertical maxillary excess (posterior VME), i.e. there is inferior rotation of the posterior maxilla and vertical downward movement of the associated posterior maxillary dentition. Secondary to posterior VME, the mandible rotates posteriorly (downwards and backwards, in the direction of mouth opening), thereby moving the chin downwards and backwards. This rotation will make the patient ‘more Class II’, the severity depending on the original sagittal mandibular problem. A ‘tall face’ patient may be described as skeletal Class I rotated to Class II (or as skeletal Class III rotated to Class I).
  • Lip morphology and posture: One of the most significant clinical signs of excess LAFH is excessive separation of the lips at rest, termed an incomplete lip seal (or lip incompetence). Proffit6 has stated that lips held habitually apart by more than 4 mm in repose may be considered the outer limit of normal. In the Class II patient with mandibular retrognathia, the upper lip has a tendency to be short and hypoplastic. This is likely to be due to lack of function, as the patient will form an anterior oral seal by elevating the lower lip behind the maxillary incisors and contacting the protruding tongue. Correction of the underlying skeletal deformity will permit normal lip function, which may allow the upper lip to develop normally.
  • Mentolabial fold: This tends to be flat, with an increased mentolabial angle.
  • Gonial angles: These tend to be increased, due to the posterior mandibular growth rotation.
  • Intraoral features: With posterior rotation of the mandible, the mandibular incisors will often retrocline, possibly due to pressure from the lower lip. As the anterior face height increases, the tendency for the maxillary and mandibular incisors will be to separate in the vertical plane. If the incisor eruption is capable of compensating for the excess face height, the overbite may remain within normal limits; otherwise an anterior open bite may occur.

Mandibular excess


The aetiology of Class III jaw deformity, in particular mandibular excess, has a very strong genetic basis.7 The familial tendency of Class III deformity may be frequently observed in members of the same family. A famous example is of the Hapsburg dynasty, one of the German royal families. Over 23 generations, portrait painters have captured images of the Hapsburgs, and they all exhibit the same mandibular prognathism and protrusive lower lip, often termed the ‘Hapsburg jaw’ (Figure 19.12). In addition, evidence from twin studies has suggested the strongest genetic predisposition for Class III malocclusion, with concordance in identical twins six times higher than in non‐identical twins for mandibular prognathism.8 Although there is a strong familial occurrence, there seems to be no association with sex, with different modes of transmission in different families or populations.9


Edward Angle, described as the father of modern orthodontics, recognized early in the twentieth century that severe mandibular excess could only be corrected through a combination of orthodontics and mandibular surgery (Figure 19.13).10


The characteristic feature of mandibular excess, and the most common complaint from the ‘Class III’ patient, is the prominence of the chin and lower lip relative to the rest of the face in profile view (Figure 19.14). It is important to distinguish true skeletal mandibular excess from relative mandibular excess.

An image features portraits of two notable figures from the Hapsburg dynasty: Charles V, Holy Roman Emperor, and Charles II of Spain. Both portraits highlight the distinctive 'Hapsburg jaw' and protrusive lower lip characteristic of their lineage. The description emphasizes their facial features as well as aspects of their clothing.

Figure 19.12 The prominent ‘Hapsburg jaw’ and protrusive lower lip of the Hapsburg dynasty is evident in the portraits of (A) Charles V, Holy Roman Emperor (1500–58) and (B) Charles II of Spain (1661–1700).

An image features Dr. Edward H. Angle's chin retractor, designed for use with a traction bar in dental procedures. It is associated with the S. White Dental Manufacturing Company. The tags suggest that the content may include elements related to human anatomy, possibly illustrating its application on a man's face or involving related text and font details.

Figure 19.13 The ‘chin retractor’ of Edward Angle was a predecessor to the chin cup (chin cap) used by orthodontist in the attempt to correct mandibular excess in young patients by restraining mandibular condylar growth; Angle later recognized the limitations of such appliances.


True mandibular excess


True sagittal skeletal mandibular excess may be due to:



  • A large mandible (mandibular macrognathia).
  • An excessively forward positioned mandible (mandibular prognathism).
  • A combination of the above.

Relative mandibular excess


True skeletal mandibular excess must be distinguished from relative (apparent) mandibular excess. Conditions in which the mandible appears excessively prominent are:



  • Horizontal macrogenia (osseous and/or soft tissue): The chin is unusually large and prominent on an otherwise normal mandible.
  • Maxillary and/or midfacial hypoplasia: The mandible may be in a normal sagittal position, but will appear prominent as the maxilla is retrognathic. This is a very common finding in patients with operated cleft lip and palate.
    Two images present a comparative profile view of two human faces highlighting the characteristic feature of mandibular excess. The left panel showcases an attractive profile, while the right illustrates a 'Class I I I' facial presentation, characterized by the prominence of the chin and lower lip relative to other facial features.

    Figure 19.14 The characteristic feature of mandibular excess is the prominence of the chin and lower lip relative to the rest of the face in profile view. The two contrasting heads demonstrate an attractive profile (left panel) and a ‘Class III’ profile with mandibular excess and protrusive lower lip (right panel). (Michelangelo, c. 1516–24, The Ashmolean Museum, Oxford.)


  • Anterior rotation of the mandible (upward and forward, in the direction of mouth closing): This condition is often secondary to maxillary hypoplasia and vertical maxillary deficiency (VMD), as the mandible rotates forward excessively (overclosure) around the condylar hinge axis (Figure 19.15).
  • Anterior mandibular displacement: In the presence of a mild Class III incisor relationship, patients with an uncomfortable edge‐to‐edge incisor occlusion may displace and posture the mandible anteriorly in order to achieve maximum intercuspation of the posterior teeth (Figure 19.16).
  • Any combination of the above.

The aetiology of mandibular excess often results from a combination of relative and true mandibular excess.

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