Introduction
Tweed’s analysis, undertaken initially in 1954, is primarily based on the premise that the inclination of the mandibular incisors to the basal bone and the latter’s association with the vertical relation of the mandible to the cranium are significant considerations for facial aesthetics.
Tweed’s cephalometric analysis had its beginning in clinical orthodontics, where Dr. Charles Tweed found that those cases of malocclusion having a pleasing outcome, harmonious profiles and stable occlusion following orthodontic treatment had a common, consistent feature of occlusion: patients’ mandibular incisors were upright on their mandibular skeletal bases. Upright lower incisors could not be achieved in all cases of malocclusion without creating space for the uprighting of the lower incisors, which may necessitate the extraction of teeth. Angle’s orthodontic treatment philosophy assumed that once the cuspal interdigitation of teeth had achieved a healthy fit, the stimulation occasioned by orofacial function would result in the growth of the basal bone structures, that is, the maxilla and mandible. The growth achieved by establishing a normal occlusion would accommodate the full complement of teeth and result in a balanced, harmonious face.
Hence, little or no thought was given to the inclination of the mandibular incisors and the mesiodistal relationship between the teeth and their respective jaw bones. In the era of Angle, Charles Tweed, a disciple of Edward Angle, strictly adhered to the non-extraction philosophy during his practice’s initial years.
However, in the following years, Tweed became aware of his inability to create balance and harmony in the face of more than a few patients. Moreover, he noticed significant relapses in some of his patients, thus casting doubt over the long-term stability of non-extraction treatment and maintaining the full complement of teeth.
In 1934, he embarked on a critical review of treatment records, which prompted him to study the features and characteristics of occlusion, dentition and the faces of ‘normal’ people who never had orthodontic treatment. His initial impression was based on clinical examinations alone. The relationship of the teeth to the basal bone was carefully noted, especially the inclination of the incisor teeth.
Clinical observations supplemented and quantified on cephalograms led to the development of the diagnostic triangle. Tweed’s diagnostic triangle is simple and basic yet provides a definite guideline in treatment planning.
Tweed’s analysis is clinically oriented, sets the goals and outlines a plan for orthodontic treatment.
Landmarks used in constructing Tweed’s triangle were the Frankfort horizontal plane, the mandibular plane and the long axis of the mandibular incisor ( Fig. 23.1 ).
Planes and angles used in Tweed’s analysis.
FMA, Frankfort mandibular plane angle; FMIA, Frankfort mandibular incisor angle; IMPA, incisor-mandibular plane angle.
Facial triangle and clinical implications
Dr. Tweed, with his astute clinical observations, concluded that in average non-orthodontic normal subjects, the incisal inclination was approximately 90 degrees to the mandibular plane. His study showed a variation of 10 degrees in the inclination of the mandibular incisors to the mandibular plane in subjects with normal occlusion. Further, from the records, he observed that in cases showing relapse, the incisor-mandibular plane angle (IMPA) deviated significantly from the ideal of 90 degrees.
Therefore, he concluded that for an orthodontist to attain facial aesthetics and occlusion like that found in non-orthodontic normal subjects, the mandibular incisors should be positioned in the 85–95 degrees range with a mean of 90 degrees. Based on his observations of the lower IMPA and its association with the Frankfort mandibular plane angle (FMA) variation, he found a third angle of the triangle: the Frankfort mandibular incisor angle (FMIA) consistent in values hovering around 65 degrees. Tweed found that extraction of premolars was necessary for patients with an FMA of more than 30 degrees.
He observed that when the FMA is higher than 35 degrees, it was physically impossible to fully compensate for the inclination of the mandibular incisors (i.e. make them upright). The prognosis is not good in such cases, and the orthodontist is limited to creating stable results and establishing harmony and balance in facial aesthetics.
Clinical research set the norm for FMA as 25 degrees with a normal variation of 17–35 degrees. Since the sum of the three angles of a triangle is 180 degrees, in an average case with 25 degrees of FMA and 90 degrees of IMPA, the third resultant angle, FMIA, would be expected to be 65 degrees.
A cephalometric study further supported his clinical observations. The sample size consisted of 100 people, chosen based on the balance and harmony of their facial aesthetics. The average of the three angles was as follows ( Table 23.1 ) :
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FMA, 25 degrees
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IMPA, 90 degrees
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FMIA, 65 degrees
TABLE 23.1
Tweed’s cephalometric norms
| FMA ° | FMIA ° | IMPA ° | ||||||
|---|---|---|---|---|---|---|---|---|
| S. no. | Population | Age | Mean | Range | Mean | Range | Mean | Range |
| 1. | Caucasians | 100 subjects | 24.57 | 17–35 | 68.2 | 56–80 | 86.93 | 76–99 |
| 2. | North Indians | 18–26 yrs, 25M, 23F | 23.49 ± 4.23 | 13–35 | 53.87 ± 7.98 | 36–74.5 | 101.7 ± 8.02 | 81–117 |
| 3. | Indian adults | 20–48 yrs, 10M, 10F | 21.6 | 13.5–33.0 | 51.87 | 32.5–66.5 | 106.5 | 88–129.5 |
| 4. | Nepalese | 14.2 ± 3.2 yrs, 14.94 ± 3.8 yrs, 50M, 50F | 28 ± 5.9 | 15–39 | 57 ± 6.8 | 42–75 | 95 ± 5.8 | 84–105 |
| 5. | Koreans | 18–28 years, 52M, 43F | 23.65 ± 5.19 | 23.87 ± 4.19 | 59.6 ± 5.62 | 59.22 ± 7.07 | 96.75 ± 6.02 | 96.91 ± 5.12 |
| 6. | Mongoloids | 18–28 years, 35M, 39F | 22.13 ± 4.59 | 25 ± 4.21 | 60.98 ± 5.26 | 59.36 ± 6.13 | 96.89 ± 5.17 | 95.64 ± 5.75 |
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