The purpose of this short video is to provide the orthodontist with the information necessary to reliably and accurately superimpose longitudinal cephalograms on natural cranial and cranial base reference structures. By the end of this presentation, it should be clear to you why superimpositions are necessary, what they represent, and most important, how they should be performed. Cranial base superimpositions allow the orthodontist to assess the overall or total changes of the teeth that occur. The overall changes include both the tooth movements that occur within the maxilla and mandible, along with the displacements of the teeth due to jaw growth or treatments. To determine the skeletal changes that occur, the orthodontist must subtract the tooth movements found in the maxillary and mandibular superimpositions from the total change found in the cranial base superimposition.
As such, the first step in this process is the cranial base superimposition. In order to accurately superimpose cephalograms, You need structures that do not change over time. Most cranial and cranial base structures grow or remodel and therefore cannot be superimposed upon. For example, superimposing on the posterior cranial base is problematic due to the growth that occurs throughout childhood and adolescence at the spheno-occipital synchondrosis and the remodeling that occurs on the surfaces of the occipital and posterior sphenoid bones, including the posterior wall of cella. In the anterior and middle cranial base, there are also a number of structures that have been shown to exhibit little or no growth after seven or eight years of age, when the sphenoethmoidal synchondrosis ceases to grow.
After that time, A number of structures, especially those associated with neural tissues, remain stable and can be relied upon for superimposition. Two of the most important structures for cranial base superimposition are the anterior wall of cella tercica below the anterior clinoid processes, which is stable after five or six years, and the cribriform plate, which is stable after approximately four to five years. The planum or jugum sphenondale shows minimal growth after age 6, but bony apposition can occur in some cases up to 14 years of age. The various regions of growth can also be viewed from a different perspective.
In addition to the cribriform plate and the anterior wall of cella, it's also been shown that the ethmoidal crests which grow only minimally after six years of age, can be relied upon for superimposing, as can the cerebral surfaces of the frontal bone associated with the orbits and the greater wing of the sphenoid, both of which are relatively stable after age seven. The midline structures between the anterior wall of cella and the cribriform plate, commonly known as the planum sphenoidale, are mostly stable after age 7, but some bony apposition is possible up to age 14. All of these structures can be readily identified on most lateral cephalograms. Start with the radio-opaque surface that represents the anterior wall of cella. Move along that surface until the anterior wall intersects with the anterior clinoid process.
This point, referred to as the Walker point, is stable after 5 to 6 years of age and serves as an important landmark to superimpose on. As you continue anteriorly along the sphenoid bone, you should note the planum sphenoidale, which is relatively flat. and extends to the greater wings of the sphenoid. The greater wings demarcate the separation of the sphenoid and ethmoid bones.
Continuing beyond the planum and past the greater wings of the sphenoid, you should see two radiopaque lines that diverge. One usually diverges superiorly and the other diverges inferiorly. The lower line, which is sometimes continuous with the planum, is difficult to see because it demarcates the cribriform plate where the olfactory bulbs lie.
The more superior line represents the ethmoidal crest. These crests should always be visible. The most superior radiopaque lines visible above the ethmoidal bones are the cerebral surfaces of the orbital part of the frontal bone. They typically appear somewhat disorganized due to their bilateral nature and uneven surfaces.
In review, the structures used to perform a cranial base superimposition are the anterior wall of sella, its intersection with the inferior portion of the anterior clinite processes, the plenum sphenoidale, and the greater wings of the sphenoid. Anteriorly, we rely upon the cribriform plate and the ethmoidal crests. Superiorly, we use the cerebral surfaces of the orbital part of the frontal bone. Certain structures are more important to ensure an accurate sagittal orientation, while others are more important for vertically orienting your cranial base superimposition. The primary structures for sagittal orientation are, first, the contour of the anterior wall of sella tercica, and second, the greater wings of the sphenoid.
Both the right and left wings are usually visible. Try to use the average of the two whenever possible. The primary structures for vertical orientation are first the intersection of the anterior wall of sella and the anterior clinoid processes and second the cribriform plate.
If the cribriform plate cannot be clearly seen the ethmoidal crest should be used. If these structures are not clearly visible then the planus sphenoidale and cerebral surfaces of the frontal bone should be used. Additionally, the occipit can be used to aid in proper orientation.
While the occipit is not a stable structure during growth, it can be used as a rotational reference. In conclusion, the accurate use of the primary and the secondary cranial base structures makes it possible to to reliably represent the overall changes that take place during treatment or during growth.