Longitudinal and cross-sectional studies have shown that the growth of the face occurs in all three dimensions, at different rates and various sites, with its structures changing their relative spatial positions in space with time. Some cranial structures establish their relative size and form earlier in life than others, while others continue to actively grow till adulthood. ^,

Traditionally, cranial base structures were used to superimpose the lateral cephalogram taken at an interval (or at intervals) to either study the growth of the face and dental arches or evaluate changes due to treatment. The cranial base is considered stable since the majority of the development of the middle cranial base (90%–95%) is completed at 7 years of age.

The anatomical cranial base has three major components: the smaller and most anterior from the foramen caecum to the nasion, the middle cranial base from the foramen caecum to the hypophyseal fossa and the posterior cranial base from the hypophyseal fossa to the foramen magnum ( Fig. 30.1 A and B).

The cranial base.

(A) The internal bone structures as viewed from the top and (B) the mid-sagittal section of the skull. The anterior part of the cranial base extends from the foramen caecum to nasion ( red-dashed line ), the middle part from the foramen caecum to hypophyseal fossa ( yellow dashes ) and the posterior part from the hypophyseal fossa to the foramen magnum ( blue-dashed line ).

The anterior segment of the cranial base, which extends to a relatively short distance from foramen caecum to nasion, continues to grow into adulthood as a result of growth at bony and frontal sinuses. The posterior segment of the cranial base that extends from the hypophyseal fossa to the foramen magnum continues growth into early adulthood due to the activity at sphenoid-occipital synchondrosis (SOS).

The middle segment of the cranial base extends from the hypophyseal fossa’s anterior boundary to the foramen caecum anteriorly, which is the most stable region. It completes its growth earliest in life, reaching 62% of the adult size at birth, 94% between 4 and 7 years of age and 98% of the adult size between 8 and 13 years of age.

The anatomical cranial base structures are internally stable by about 8 years of age after the fusion of the spheno-ethmoid, fronto-ethmoid, and inter-sphenoid synchondroses. ^, Therefore, these structures were considered a stable reference for cephalometric superimposition. The points are easily identifiable on radiographs and reproducible with accuracy were chosen for superimposition.

De Coster used bony anatomy of the anterior contour of the cranial base from the anterior half of the sella turcica to the region of the foramen caecum and the internal outline of the frontal bone as a sufficiently stable support for meaningful superimposition of the cephalograms ( Fig. 30.2 A).

Cranial base superimposition.

(A) As depicted here in a heavier outline, only the cranial base structures between the dashed vertical lines are employed for superimposition. Emphasis is placed on the elements of De Coster’s ‘basal line’; the posterior half of sella turcica and structures in the region of nasion are ignored. W denotes ‘wing point’, the point at which the averaged outline of the greater wings crosses the jugum. A long arbitrary line with crosses on each end ‘fiducial line’ is drawn above the orbital plates of one tracing. The cranial base superimposition is then used to pass this line through to the other tracings in the series. (B) The drawings of the basal lines of a family of four children. The first one has a short basal line, while the lines of the three other children are longer and are the same. The first girl was slightly retarded and probably had an encephalic disease in early childhood or before birth.

Source: Based on the concept of De Coster L. A new line of reference for the study of lateral facial teleradiographs. Am J Orthod 1953 Apr;39(4):304–306 .

Later, De Coster, in a study of 200 families, reported that all the children of the same family have similar cranial baselines. He re-emphasised the known research findings on the baseline being fixed from 7 years of age onwards with a relatively fixed frame of the facial bones, the variations in facial bones being only metric ( Fig. 30.2 B). He proposed that the study of the shape of the cranial base anatomy is of great importance and gives valuable indications both in treatment and prognosis.

Later, Ford, in another study, confirmed that the growth of the anterior part of the cranial base was distant from sella turcica to foramen caecum and did not increase after the eruption of the first permanent molars. Ford also found that the cribriform plate completes its growth as early as the end of the second year. Therefore, the cribriform plate and the nasion-sella line are recommended as baselines for serial radiographic studies of the developing skull.

The stable landmarks on the cranial base are:

• 1.

  The anterior lip of the sella turcica in its midst

• 2.

  The sphenoid ethmoidal suture

• 3.

  The planum or upper surface of the sphenoid body

• 4.

  The roof of the ethmoid masses

• 5.

  The endocranial side of the frontal bone

Richardson found that De Coster’s cribriform plane was only moderately successful as far as reproducibility was concerned. Applying the results of his investigation into the reproducibility of cephalometric landmarks did not favour the usage of this line, which, however, might find preference on grounds other than reproducibility.

The structural method of superimposition proposed by Bjork was used to describe and study the pattern of overall growth of the face in reference to the stable structures of the cranial base growth of the maxilla and mandible, their direction, quantification of displacement and mandibular rotation. This superimposition method uses cranial base reference structures to superimpose three or more lateral cephalograms taken at sufficient intervals to discern growth changes.

Tracing superimpositions are seen with pre-treatment tracings in black and post-treatment in red colour.

The first and foremost step involves marking the landmarks (S and N) to draw a nasion-sella line (NSL) and a perpendicular line through the sella point (NSP) with a pencil directly on the first/initial cephalogram. The sella point and the cross lines are then transferred from the initial to the subsequent/stage cephalograms. When nasion is displaced vertically during growth, it is projected onto the transferred NSL. The serial cephalograms are then superimposed according to the structures on the first cephalogram, using natural stable reference landmarks/bony structures ( Fig. 30.3 A–D).

Structural method of superimposition according to Bjork.

(A) Landmarks by A. Bjork on cranial base. 1, The inner contour of the anterior wall of sella turcica; 2, the mean intersection point of the lower contours of the anterior clinoid processes and the contour of the anterior wall of sella, Walkers’ point; 3, the anterior contours of the middle cranial fossae; 4, the contour of the cribriform plate; 5, details in the trabecular system in the anterior cranial base; 6, the contours of the bilateral fronto-ethmoidal crests; 7, the cerebral surfaces of the orbital roofs. (B) The principles of superimposition of profile radiographs using structures in the anterior cranial base (ACB) as reference. (C) The sella point is marked on the first profile radiograph in a series. It is found by bisecting the sagittal diameter of the sella turcica and the height from tuberculum sellae to the floor. It has been reported that during growth, sella turcica increases in size by apposition on tuberculum sellae and by resorption at the posterior wall and the floor. (D) By superimposition of profile radiographs should depict a logical sequence of growth changes from three or more stages of untreated individuals. The points, nasion (n), articulare (ar), pterygomaxillare (pm), the anterior nasal spine (ANS) (sp) and pogonion (pg) should follow a logical path during normal growth.

Source: Cited from www.angle-society.com/case/guide.pdf .; kind courtesy of Per Rank; reproduced with permission from Björk A, Skieller V. Normal and abnormal growth of the mandible. A synthesis of longitudinal cephalometric implant studies over a period of 25 years . Eur J Orthod 1983 Feb;5(1):1–46. PubMed PMID: 6572593 .

Mandibular superimposition

The profile superimposition of the cephalogram does not provide accurate information on mandible growth due to known remodelling of the mandibular body, more so at its lower border. The mandibular superimposition is considered separately on the following stable anatomical structures ( Fig. 30.4 A and B):

• 1.

  The anterior contour of the chin.

• 2.

  The inner cortical structure at the inferior border of the symphysis.

• 3.

  Trabecular structures in the symphysis.

• 4.

  Trabecular structures related to the mandibular canal.

• 5.

  The lower contour of a molar germ from the time mineralisation of the crown is visible until the roots begin to form.

Mandibular and maxillary superimposition.

(A) The anterior contour of the chin is made to coincide on two radiographs. 1, Anteriorly, the radiographs are orientated in a vertical direction by 2, the inner contour of the cortical plate at the lower border of the symphysis and by 3, any distinct trabecular structure in the symphysis. Posteriorly, the radiographs are vertically orientated by 4, the contour of the mandibular canal and 5, the lower contour of a mineralised molar germ before root development begins, and possibly also of a premolar germ; 6, the anterior contour of the ramus. (B) A superimposition of the mandible using landmarks. (C) Maxillary superimposition is done on the anterior contour of the zygomatic process of the maxilla.

Source: Reproduced with permission from Björk A, Skieller V. Normal and abnormal growth of the mandible. A synthesis of longitudinal cephalometric implant studies over a period of 25 years. Eur J Orthod 1983 Feb;5(1):1–46. PubMed PMID: 6572593; cited from www.angle-society.com/case/guide.pdf . Reproduced with permission. Kind courtesy of Per Rank.

Bjork has suggested some practical steps for the superimposition of the mandible. On the first or initial cephalogram, a reference line is drawn in the mandible with a pencil from the anterior contour of the chin to the contour of the mandibular canal. If the mandibular canal is not pre-eminent, the lower contour of a molar germ can be used.

The subsequent cephalograms are then superimposed on the naturally stable five reference structures listed above, and the constructed reference line is transferred with a pencil to the remaining radiographs. The cephalograms are first orientated sagittally by the anterior contour of the chin, which is made to coincide on two X-rays. Anteriorly, the radiographs are orientated in a vertical direction by the inner contour of the cortical plate at the lower border of the symphysis and by any distinct trabecular structure in the symphysis. Posteriorly, the radiographs are vertically orientated by the contour of the mandibular canal and by the lower contour of a mineralised molar germ before root development begins, and possibly also of a premolar germ ( Fig. 30.4 B).

Bolton, in his early studies on the growth of the face, proved that in early infancy and childhood, before the growth of the braincase is complete, certain cranial areas change in size at one time while the other regions of the head remain fixed during the same period.

He used a plane at the base of the cranium as the ‘stable’ reference to study the growth of the face. According to Broadbent, his 7-year study data have substantiated the stability of Bolton’s nasion plane of orientation and its registration point in the sphenoidal area as the most fixed point in the head. The Bolton’s plane is drawn connecting the following points ( Fig. 30.5 ):

• 1.

  Nasion defines the anterior limit. Nasion is defined as the junction of frontal and nasal bones in the midplane.

• 2.

  Bolton’s point defines the posterior limit. The highest point in the profile of the notches at the posterior end of the occipital bones, behind the foramen magnum, is known as the Bolton’s point . The right and left occipital condyles (OCs) are in proximity and may appear as a single shadow on a lateral cephalogram.

Lateral cephalometric superimposition utilising Bolton’s plane for the assessment of growth changes.

Tracings of 9, 14 and 1/2 developmental years. Note the relative stability of the Bolton’s plane during the age range when most orthodontic cases are treated.

Source: Reproduced with permission from Broadbent BH. The face of the normal child. Angle Orthod 1937 Oct;7(4):183–208.

Broadbent’s registration point is the midpoint of the perpendicular from the centre of the sella turcica to the nasion-post-condylar plane or Bolton’s plane. This point has been conveniently called registration point ‘R’, which is used to register the tracings of the same individual taken at different times or of two individuals for comparison.

The extension down from the plane of registration usually passes obliquely through the face at an acute angle to the plane of occlusion. Therefore, Broadbent suggested the use of two additional reference planes; he suggested the use of FH plane and vertical orbital plane to record the changes in the face from more constant Bolton’s landmarks.

Brodie used the sella-nasion line registered at ‘S’ as a method for orientation and superimposition of lateral cephalograms, ^, which is the most popular and practical method used for this purpose. The ‘sella’ point is considered stable as it distinguishes the cranial base from the face, and it is presumed that the major cranial base growth is completed by the age of 7.

Two or more hand-drawn tracings or computerised images are superimposed on SN plane registration at ‘S’. Resultant changes are seen in the location of teeth and structures consequent to growth or orthodontic treatment, including growth. Since most patients are beyond 7 years of age, we commonly use S-N plane for orientation to evaluate changes in growth or by treatment in the face and dental structures.

Both S and N planes are located in the mid-sagittal planes, which makes it an easily identifiable reference point ( Fig. 30.6 A and B). The use of the S-N plane was hugely popularised by Steiner, and therefore, this method of superimposition is more popularly called Steiner’s method.

Lateral cephalometric superimposition of various stages of treatment and the recommended colour scheme.

(A) Superimposition on the S-N plane with registration at S shows treatment changes in a class II division 1 malocclusion patient treated with two-phase appliance therapy. Black , pre-treatment; blue , after completion of a phase of functional appliance therapy; red , at the completion of treatment. (B) Superimposition on the S-N plane with registration at S shows treatment changes in a case of class I bimaxillary protrusion treated with fixed appliance therapy after all first premolar extractions. Black , pre-treatment, blue , after completion of space closure, red , post-treatment.

Steiner suggested the superimposition of lateral tracing on S-N plane registration at ‘N’, thus causing the lines NA to superimpose. The record of anteroposterior growth is thus expressed posterior to these lines.

To evaluate changes in the maxilla, the tracing is moved vertically parallel to the S-N plane till drawings of the maxilla, like the palatal contour, are superimposed as the best fit. The movement of maxillary teeth can now be visible.

The movement of teeth within the mandible is assessed by superimposition of the mandible over the best-fit cross-section of symphysis of the mandible, keeping the lower border parallel.

For registering the changes in the location of the mandible and its relationship to craniofacial structures, the tracings are superimposed on the line S-N, registered at ‘S’.

Distances between points ‘L’ and ‘E’ represent the anteroposterior lengths of the mandible ( Fig. 24.3 ). This measurement is an indication and not an accurate measurement of change in the length of the mandible because these measurements are affected by a change in the inclination of the occlusal plane with the opening of the bite.

The use of the S-N plane should be viewed with limitations on remaining growth at sella. There is a change in the relative position of the sella consequent to the remaining active growth at the posterior cranial base at synchondroses and enlargement of the pituitary gland, enlargements of sphenoidal air sinuses and minor remodelling of the fossa. Sella may move posteriorly and inferiorly while there is an anterosuperior (sometimes anteroinferior relocation) relocation of the nasion with the growth of the frontal sinuses.

Stramrud advocated the use of sella ethmoidale (SE) to overcome the variation associated with growth direction at nasion; however, research studies have indicated that SN and SE were closely correlated from adult to 3 years of age.

Several authors have used the FH plane as a reference plane for superimposition. However, when compared with the S-N plane, the FH plane is found to be more variable. Wei has studied and tested the variability of five commonly used cephalometric planes. The S-N plane was the least variable, and the FH plane was reasonably stable.