Cephalometric analysis

Cephalometric analysis is the clinical application of cephalometry. It is analysis of the dental and skeletal relationships in the head.^[1] It is frequently used by dentists, orthodontists, and oral and maxillofacial surgeons as a treatment planning tool.^[2]

Two of the more popular methods of analysis used in orthodontology are the Steiner analysis, named after Cecil C. Steiner, and the McNamara analysis, named after James A. McNamara.^[3] There are other methods as well, including the Ricketts analysis.^[4]

Cephalometric radiographs

Cephalometric analysis depends on cephalometric radiography to study relationships between bony and soft tissue landmarks and can be used to diagnose facial growth abnormalities prior to treatment, in the middle of treatment to evaluate progress or at the conclusion of treatment to ascertain that the goals of treatment have been met.^[5] A Cephalometric radiograph is a radiograph of the head taken in a Cephalometer (Cephalostat) that is a head-holding device introduced in 1931 by Birdsall Holly Broadbent Sr. in USA^[6] and by H. Hofrath in Germany. The Cephalometer is used to obtain standardized and comparable craniofacial images on radiograghic films.

Lateral cephalometric radiographs

Lateral cephalometric radiograph, used for skull analysis

Lateral cephalometric radiograph is a radiograph of the head taken with the x-ray beam perpendicular to the patient's sagittal plane. Natural head position is a standardized orientation of the head that is reproducible for each individual and is used as a means of standardization during analysis of dentofacial morphology both for photos and radiographs. The concept of natural head position was introduced by C. F. A. Moorrees and M. R Kean in 1958 and now is a common method of head orientation for cephalometric radiography.

Registration of the head in its natural position while obtaining a cephalogram has the advantage that an extracranial line (the true vertical or a line perpendicular to that) can be used as a reference line for cephalometric analysis, thus bypassing the difficulties imposed by the biologic variation of intracranial reference lines. True vertical is an external reference line, commonly provided by the image of a free-hanging metal chain on the cephalostat registering on the film or digital cassette during exposure. The true vertical line offers the advantage of no variation (since it is generated by gravity) and is used with radiographs obtained in natural head position.

Posteroanterior (P-A) cephalometric radiograph

A radiograph of the head taken with the x-ray beam perpendicular to the patient’s coronal plane with the x-ray source behind the head and the film cassette in front of the patient’s face.

Cephalometric tracing

A Cephalometric tracing is an overlay drawing produced from a cephalometric radiograph by digital means and a computer program or by copying specific outlines from it with a lead pencil onto acetate paper, using an illuminated view-box. Tracings are used to facilitate cephalometric analysis, as well as in superimpositions, to evaluate treatment and growth changes.

Cephalometric landmarks

The following are important cephalometric landmarks. (Sources: Proffit;^[7] others.)

Landmark points can be joined by lines to form axes, vectors, angles, and planes (a line between 2 points can define a plane by projection). For example, the sella (S) and the nasion (N) together form the sella-nasion line (SN or S-N). A prime symbol (′) usually indicates the point on the skin's surface that corresponds to a given bony landmark (for example, nasion (N) versus skin nasion (N′).

Classification of Analyses

The basic elements of analysis are angles and distances. Measurements (in degrees or millimetres) may be treated as absolute or relative, or they may be related to each other to express proportional correlations. The various analyses may be grouped into the following:

1. Angular – dealing with angles,
2. Linear – dealing with distances and lengths,
3. Coordinate – involving the Cartesian (X, Y) or even 3-D planes,
4. Arcial – involving the construction of arcs to perform relational analyses.

These in turn may be grouped according to the following concepts on which normal values have been based:

1. Mononormative analyses: averages serve as the norms for these and may be arithmetical (average figures) or geometrical (average tracings). E.g. Bolton Standards.
2. Multinormative: for these a whole series of norms are used, with age and sex taken into account, e.g. Bolton Standards.
3. Correlative: used to assess individual variations of facial structure to establish their mutual relationships, e.g. Sassouni’s arcial analysis.

Cephalometric angles

According to the Steiner's analysis:

• ANB (A point, nasion, B point) indicates whether the skeletal relationship between the maxilla and mandible is a normal skeletal class I (+2 degrees), a skeletal Class II (+4 degrees or more), or skeletal class III (0 or negative) relationship.
• SNA (sella, nasion, A point) indicates whether or not the maxilla is normal, prognathic, or retrognathic.
• SNB (sella, nasion, B point) indicates whether or not the mandible is normal, prognathic, or retrognathic.

SNA and SNB is important to determine what type of intervention (on maxilla, mandible or both) is appropriate. These angles, however are influenced also by the vertical height of the face and a possible abnormal positioning of nasion.^[7] By using a comparative set of angles and distances, measurements can be related to one another and to normative values to determine variations in a patient’s facial structure.^[8]

Computerised Cephalometrics

Computerised cephalometrics is the process of entering cephalometric data in digital format into a computer for cephalometric analysis. Digitization (of radiographs) is the conversion of landmarks on a radiograph or tracing to numerical values on a two- (or three-) dimensional coordinate system, usually for the purpose of computerized cephalometric analysis. The process allows for automatic measurement of landmark relationships. Depending on the software and hardware available, the incorporation of data can be performed by digitizing points on a tracing, by scanning a tracing or a conventional radiograph, or by originally obtaining computerized radiographic images that are already in digital format, instead of conventional radiographs. Computerized cephalometrics offers the advantages of instant analysis; readily available race-, sex- and age-related norms for comparison; as well as ease of soft tissue change and surgical predictions.

Digitization

Computer processing of cephalometric radiographs uses a digitizer. Digitization refers to the process of expressing analog information in a digital form. A digitizer is a computer input device which converts analog information into an electronic equivalent in the computer’s memory. In this treatise and its application to computerized cephalometrics, digitization refers to the resolving of headfilm landmarks into two numeric or digital entities – the X and Y coordinate. 3D analysis would have third quantity - Z coordinate.

References