Tweed’s philosophy, principles of treatment and standard edgewise biomechanics

‘Let us all remember this fact, clinical findings and scientific research should be closely wedded, only then many of our complex problems in orthodontics be solved, not before’. Charles H. Tweed Jr

Tweed’s journey as angle’s student to an accomplished orthodontist

Dr. Charles Tweed ( Fig. 51.1 ) has known Edward H. Angle as a best friend and benefactor since 1927. Angle and Tweed worked closely during the last 2 years of Angle’s life. During Angle’s last years, he discovered and designed the edgewise appliance; later, Charles Tweed practised and taught us how to achieve the best clinical outcomes using the ‘latest and best’ appliance of that era.

Tweed practised edgewise appliance in Phoenix, made progress records every 4 months, packed them in suitcases and carried them to Pasadena at Angle’s home. Angle, the advisor, studied the records and outlined a treatment plan for the next 4 months. Tweed held to Angle’s firm conviction adhering to the non-extraction philosophy and line of occlusion concept, which was synonymous with ‘harmony, balance, symmetry, beauty, art and permanence of normal tooth positions’. This conviction lasted for 3 years after Angle’s death.

Tweed became disheartened with his clinical outcomes after 6.5 years of orthodontic practice. The two most important reasons were:

1.
Non-extraction treatment created protrusive faces
2.
Post-treatment occlusion was unstable.

Many of his treated patients showed a lack of face balance and beauty, which was, to some extent, compromised with strict adherence to the non-extraction principle. Tweed noticed significant relapse in some cases, thus casting doubt on the long-term stability of the results.

Tweed (1934) subsequently started to analyse the treatment records the results of which prompted him to conduct a study on the features and characteristics of occlusion, dentition and faces of ‘normal’ people who never had orthodontic treatment. His initial impression was based on clinical examinations alone. The relationship of teeth to the basal bone was carefully noted, especially the inclination of the incisors.

He found that, on average, non-orthodontic normal, the incisal inclination was approximately 90 degrees when related to the mandibular plane. Further, from his records of treated cases, he also noted that in a majority of cases which showed relapse, the incisor-mandibular plane angle deviated significantly from the ideal of 90 degrees. Thus, he realised the importance of placing incisors upright on the basal bone, and to correct the incisor proclination, the creation of space in the arch was required, which could only be attained by extracting some teeth. He instituted retreatment in patients with relapse.

He presented a tremendous exhibit of records of his 100 patients treated first without extractions and then retreated with extractions at the American Association of Orthodontists meeting in 1940. Tweed’s presentation was not readily accepted, and he was severely criticised by many orthodontists of the era!

For his views contradicting non-extraction therapy, Tweed was considered Angle’s unfaithful student who defied the philosophy of his GURU.

Criticism did not deter him from his conviction and treatment planning on a case-to-case basis. Charlie, as his friends called him, never attended a meeting without plaster (Tweed), to show how he treated the cases even in his 70s!

Human face as related to orthodontic diagnosis

Charles H. Tweed focused his clinical observations on normal facial aesthetics, including their deviations and the position of the mandibular incisors as related to the medullary bone of the body of the mandible. A conviction was built, later proved through his research, leading to the development of the diagnostic facial triangle. The rule included cases with normal growth patterns and those with diminished growth of both the maxilla and mandible but where the directional growth of facial triangle the jaws was not markedly disturbed towards vertical or horizontal.

Tweed observed that true class III cases, or those cases where the growth vector of the jaws was perverted, from being downward and forward to too much forward and less downward, did not withstand the rule. The more procumbent the mandibular incisors, the less the mandibular prominence was observed; inversely, the more upright the mandibular incisors, the more pronounced the mandibular prominence was observed.

The diagnostic facial triangle

Tweed’s cephalometric analysis began in clinical orthodontics, where he found that cases of malocclusion with a pleasing outcome, harmonious profiles and stable occlusion following orthodontic treatment had a common, consistent feature of occlusion: their mandibular incisors were upright on their skeletal bases.

The 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 ( Fig. 51.2 ; Chapter 23 ).

Extraction or non-extraction treatment

Tweed concluded that if an orthodontist was to attain facial aesthetics and occlusion like that found in non-orthodontic normal, the mandibular incisors should be positioned in a range of 85–95 degrees with a mean of 90 degrees to the mandibular plane. Based on his observations on the lower incisor mandibular plane angle (IMPA) and its association with variation in Frankfort mandibular plane angle (FMA), he found that a consistent finding was a resultant ‘third’ angle of the triangle, which was Frankfort mandibular incisor angle (FMIA). The inclination of the mandibular incisors to the basal bone and its association with the vertical relation of the mandible to the skull is fundamental to Tweed’s diagnostic facial triangle.

•
Tweed found that extraction of teeth was necessary for patients with FMA of more than 30 degrees.
•
He observed that when the FMA ranges upward from 35 degrees, it is physically impossible to correct and compensate for the inclination of the mandibular incisors, that is, to make them less than upright. The prognosis is not good in such cases, and the orthodontist is limited in his efforts to create stable results and establish harmony and balance of facial aesthetics.
•
The results of this clinical research established the norm for FMA as 25 degrees with a normal variation of 16–35 degrees. This resulted in the development of the second angle of the diagnostic facial triangle. As the sum of the three angles of the triangle is 180 degrees, it is expected that in a normal case with 25 degrees of FMA and 90 degrees of IMPA, the third angle, which is FMIA, would be 65 degrees.

A cephalometric study further supported his clinical observations. The sample size consisted of 100 people, chosen based on balance and harmony of facial aesthetics. The averages of the three angles were as follows ( Table 23.1 ):

1.
FMA—25 degrees
2.
IMPA—90 degrees
3.
FMIA—65 degrees

FMA and its relationship with IMPA:

Tweed observed that the patients in whom FMA was more than 30 degrees demonstrated nature’s compensation of the inclination of mandibular incisors when related to the mandibular plane. IMPA was found to be as little as 77 degrees. The FMIA angle was around 65 degrees. The occlusal plane converged towards the mandibular plane posteriorly because of the excessive height of mandibular incisors as compared with the molar height.

In patients with FMA of 25 ± 4 degrees (21–29 degrees), FMIA was found to be 65–70 degrees. The occlusal plane did not converge posteriorly as sharply towards the mandibular plane as in patients with large FMA. The patients in whom FMA was below 20 degrees rarely demonstrated IMPA greater than 94 degrees. Their FMIA reading ranged from 68 to 85 degrees. The occlusal plane converged less sharply towards the mandibular plane. In some cases, it was parallel to the mandibular plane ( Fig. 51.3 ).

Therefore, he postulated that FMIA is critical: while planning orthodontic treatment, IMPA should be compensated for a minimum of 77 degrees for higher FMA and a maximum of 105 degrees for lower FMA. Dr. Tweed used white Indian ink to mark the estimated IMPA directly on the cephalogram, which he called the head plate correction. The three angles were measured and recorded in white ink on the lower left corner of the cephalogram. The size of angle ANB was then measured, and the value was written below the previously mentioned angular measurements.

The ‘cephalometric eye’ : Tweed emphasised training the eyes for a keen observation to match clinical profile with a visual examination of the cephalogram. He practised viewing the cephalogram on the shadow box, and then a long look was taken at the patient seated in the chair, with his head aligned in the Frankfort plane. Face proportions were compared with the bony framework simultaneously.

Growth trends of the face

In my opinion, a thorough concept of the normal growth pattern of the child’s face or any face is as important to orthodontists, if not more so, as complete mastery of the science of occlusion. Charles H. Tweed

Tweed followed a routine to observe the growth trend of the subject before any mechanical treatment, which made it possible to determine the type of facial growth. He included lateral cephalograms in the records of all young children undergoing pre-orthodontic guidance programmes. After 12–18 months, a second cephalogram would be taken, and the tracings made from the cephalograms were superimposed on the SN plane and registered at S point. Observations were then made to conclude the growth trend.

Essentially, Tweed described the growth of the face as normal when the mandible and face grow in unison in the downward and forward direction with no change in angle ANB. He called this Type A. When the mandible grows more rapidly than the maxilla with a decrease in angle ANB, he calls it Type C.

The most undesirable face growth type is Type B growth, which is downward and forward, with the middle face growing forward more rapidly than the lower face. The ANB angle is slightly larger, ranging from 6 to 12 degrees, and increases with age. The prognosis and distribution of different trends are given in charts ( Fig. 51.4 A–C).

Head plate correction or cephalogram correction

Tweed also utilised IMPA correction on a cephalogram according to his treatment objectives of head plate correction. He accordingly calculated the space requirements in the arch based on the amount of change required to place the lower incisors correctly over the basal arch. Orthodontists across America and Europe treated cases according to the IMPA goals of Tweed’s triangle. In India, during the 1970s, treatment planning was based on using Tweed’s norms of IMPA as a guideline ( Table 51.1 ).

TABLE 51.1

Tweed’s norms of IMPA according to FMA

FMA ≥ 30 degrees	Mandibular incisors are compensated so that FMIA ranges from 65 to 70 degrees (average 65 degrees) Prognosis—fair extraction is usually indicated
FMA = 25 ± 4 degrees	Efforts should be made to attain FMIA of 65–70 degrees (average 68 degrees) FMIA varied from 65 to 70 degrees with FMA of 21 degrees and FMIA of 65 degrees when the FMA approached 30 degrees
FMA ≤ 20 degrees	IMPA should not exceed 94 degrees FMIA ranged from 66 degrees to as much as 85 degrees

In his analysis, Tweed stressed the importance of FMIA and recommended that it be maintained at 65–70 degrees. For example, in the case of FMA-21 degrees, FMIA-51 degrees and IMPA-108 degrees, IMPA should be corrected to 90 degrees. With this change in IMPA, FMIA would become 69 degrees, which is within the recommended range. This would necessitate removing dental units.

Total arch length discrepancy

Tweed described a method of arch length discrepancy calculation, which accounted for the goals of IMPA. The available arch perimeter is measured from the mesial surfaces of the mandibular first molar on either side, around the dental arch. The arch perimeter is noted on the analysis chart ( Fig. 51.5 ).

In the case of mixed dentition, the total tooth material was then obtained from the measurements of the widths of erupted permanent teeth from orthodontic study models and the widths of unerupted mandibular cuspids and two premolars from intraoral radiographs.

Tweed used this method to calculate crowding and spacing. He used a formula of head plate correction to calculate the total arch length needed. The goals of IMPA are outlined, and an estimated long axis of the mandibular incisor position is drawn. The linear distance from the original incisor edge position to the incisor edge was doubled. This value was added to the crowding or subtracted from spacing, accounting for a total arch length discrepancy.

The goals of the IMPA are essential here, and they will vary from race to race and also from individual to individual according to FMA and soft tissue thickness ( Fig. 51.5 ). His treatment philosophy and objectives involved healthy oral tissues, best balance and harmony of the face, stability of the dentition and an efficient chewing mechanism ( Fig. 51.6 ).

Treatment planning: Tweed’s objectives of treatment

Tweed considered face type, growth trend, arch length discrepancy cephalometric and soft tissue parameters in formulating treatment and outlined the treatment objectives in a methodical manner.

Early treatment or mixed dentition treatment guidance

Tweed advocated the principle of early treatment guidance and mixed dentition treatment. Treatment in mixed dentition differs from that in permanent dentition, where growth is either completed or almost complete, and the malocclusion has developed to a static form. Young children with their faces, jaws and teeth continuously undergo growth changes and adjustments that should be taken into consideration during diagnosis and treatment.

Tweed’s serial extraction sequence

Serial extraction should begin if the diagnosis revealed that a discrepancy existed between teeth size and the basal bone structures; Tweed adopted the following sequence:

•
At approximately 8 years of age, all four first deciduous molars were extracted. After 4–10 months, the first premolar teeth were seen erupted to the gum level.
•
The first premolars were extracted after their crowns were seen through the alveolar bone. Deciduous cuspids were removed at the same time.
•
This procedure should be completed 4–6 months before the eruption of the permanent cuspids. When the cuspids erupt, they migrate posteriorly into good positions, and minor irregularities of the mandibular incisors correct themselves as these teeth tip lingually or posteriorly, as directed by the subjected functional forces.

Tweed warned against removing deciduous cuspids to self-correct irregularities of incisors, particularly the mandibular incisors. As the permanent cuspid would then erupt more rapidly than the first premolar, this undesirable movement would make the removal of the first premolar much more difficult. In addition, the retention of the first premolar would force the cuspid eruption into mesial positions that would require mechanical treatment later.

Treatment approach fully developed malocclusion

Dr. Tweed focussed on enhancing anchorage prior to retraction of the anterior teeth, which he called anchorage preparation. The purpose was to prepare the anchor teeth in the most efficient manner for retraction of the anterior teeth without losing the original position of the anchor teeth.

He prepared the anchorage by tilting the buccal segment distally without moving the roots forward into a mesial tip. He prepared anchorage in one arch at a time while using the other arch and headgear for support. The principle of anchorage preparation on molars is analogous to the pegs used for tying the rope of the tents at a favourable angle.

Similarly, he performed consolidation of the spaces in one arch at a time using the opposite arch to support the anchorage. Regarding his concerns on anchorage control, he evolved mechanics and treatment stages so as not to let the anchorage loss happen under any circumstances.

The standard edgewise appliance

The bracket system used in a standard edgewise system includes twin brackets of 0.022 × 0.028-in. slot size. Four types of stainless steel (SS) twin brackets are used ( Fig. 51.7 ).

•
Junior Siamese for mandibular incisors
•
Medium Siamese for maxillary lateral incisors, canines and premolars
•
Wide Siamese maxillary central incisors and first molars
•
Extra wide for large first molars

Buccal tubes with hooks on the terminal molars: can be the second molars (if erupted) or the first molars.

The maxillary molar tube is a double buccal tube with a round tube for housing the face bow or headgear of 0.045-in. internal diameter.

The brackets’ slots are parallel to the incisal edge of the teeth or parallel to the occlusal plane at a right angle to the long axis of the teeth.

To maintain the normal labiolingual position of the teeth in the arch and create a favourable mesiodistal inclination of the roots, the first and second order bends are incorporated into the arch wire.

The third order or torque bend is given for the optimum labiolingual placement of the roots in relation to the alveolar bone.

Bonwill–Hawley chart

Tweed recommended the use of an individualised arch form. He created one using a Bonwill–Hawley chart for each patient, measuring the mesiodistal widths of the maxillary central and lateral incisors and the mesiodistal width of the canine (refer Fig. 45.9 in Chapter 45 ). The sum of six anterior teeth dimensions was used to create the first circle, which determined the anterior arch form. The inner circle has a radius of the combined dimensions of centrals, laterals and canines in part (from up to the mesial edge of the contact point to the distal margin of the bracket wing). The chart provides an individualised inter-cuspid width, which would be maintained throughout the treatment.

To conform to the natural arch form, attain anchorage and move the teeth to the correct labiolingual inclination, the arch wires require the incorporation of certain bends. These bends are grouped as:

•
First order bends
•
Second order bends
•
Third order bends

First-order bends

The first-order bends are required to compensate for the variation in the labiolingual thickness of the central incisors, lateral incisors and canines in the anterior segment ( Figs 51.8 and 51.9 ).

In the buccal segment, the first-order bends are required to accommodate the prominence of the mesiobuccal cusp of the first molars and a change in the path of the buccal curve distal to the mesiobuccal cusp towards the midline of the palate, which determines a parabolic curve of the arch.

In the mandibular arch, the first-order bends in the incisor region are not required for the mandibular lateral incisors since the labiolingual thickness of the central and lateral incisors is nearly the same.

The extent of each bend is dictated by the variation in the labiolingual thickness of each tooth so that a smooth curve of incisors and canines on their lingual surface can be achieved. Suppose first-order bends are not provided in the arch wire. In that case, the labial contours of the teeth in the anterior region will confirm the arch shape, which is not anatomically and aesthetically normal.

Lack of the first-order bends in the molar region will result in distobuccal rotation of the first molars ( Fig. 51.9 F and G).

First-order bends are: lateral inset, canine eminence, molar offset and anti-rotation bends. A first-order bend between a premolar and a molar is also known as an anti-rotation or toe-in bend, and its size varies with the anatomical contour of the molars.

The upper and lower arch wires should always be coordinated for the arch shape and arch widths in the canine and molar region ( Fig. 51.10 ). Good quality Tweed’s pliers are necessary when placing the bends. The arch wire is held firmly with the pliers while bends are placed with thumb pressure. All first-order bends are placed as in-out bends without disturbing the plane of the wire. The first-order bends are provided bilaterally using 0, 1, 3, 5, 6 as guides suggesting quantification of bending movement of the free end of the arch wire in labiolingual deflection on the Bonwill’s chart.

Second-order bends or up and down bends

These bends are second in order of placement, hence the name. The second-order bends are of great significance in standard edgewise technique biomechanics.

The second-order bends are placed in the occluso-gingival direction to maintain the final angulation of teeth ( Fig. 51.10 ). In the incisal area, second-order bends (artistic bends) provide the ideal root angulation to these teeth in mesiodistal direction. In the posterior region, second-order bends maintain the distal tipping of the premolars and molars and cause bite opening (pseudo bite opening).

Types of second-order bend ( Fig. 51.11 ):

•
Short second-order bends (step up/step down bend)
•
Long second-order bends
•
V bend (neutral, step up, step down)
•
Artistic positioning bends, long V bend
•
Tip back bends

Short second-order bends given in the buccal segment are more vertical than long second-order bends.

A step-up or step-down bend can be independent or a component of a ‘V’ bend or a second-order bend.

V bend: A V bend is often provided in the canine/incisor region next to the lateral incisor. The apex of the V bend is always gingival. The heights of the mesial and distal arms of the V bend determine its action in the occluso-gingival plane, which could be the same between two legs or different.

a.
Neutral V bend: When the mesial and distal arms of the V bend are of the same height,
b.
Step up when the mesial arm is shorter in height (to maintain or open the bite) and
c.
Step down when the mesial arm is of greater height (to close open bite).

The length of the legs of the V bend could be altered (fabricated dissimilar) to produce intrusion or extrusion in the segment lying anterior and posterior to the V bend. If fabricated with equal heights of the legs, the V bend shall not create any intrusive or extrusive forces and moments into the segments lying anterior and posterior to the V bend. When introduced in any part of the wire, such a V bend shall provide a neutral area bearing zero forces and moments.

The V bend also serves as a neutral zone or differentiates between anterior and posterior segments of the arch wire. Therefore, it is possible to incorporate lingual root torque in the incisal segment and progressive buccal root torque in the premolar molar segment.

V bend also gives a reference location for the soldering attachment.

The second-order bends, that is, one in the vertical direction, located in the buccal segment, can be activated to create the distal tip of the crowns of the teeth to enhance anchorage. The activated second-order bends are called tip-back bends. Second-order bends are sometimes considered synonyms of tip-back bends.

Tip-back bends are second-order bends. However, all second-order bends are not tip-back bends.

Artistic positioning bend or long V bend is a fly bend given in the incisal segment to place the roots of the teeth in a correct mesiodistal tip (second order). These bends can be incorporated into the arch wire during the incisor retraction and finishing stages.

How to place second-order bends

Placing the second-order bends requires holding the arch wire firmly with good quality Tweed’s pliers. Wire bending is done with fingers in an occlusal-gingival direction. The steps for making second-order bends are given in Fig. 51.12 . The second-order bends are converted into tip-back bends by creating a gentle sweep in the reverse curve of Spee in the lower arch and accentuated curve of Spee in the maxillary arch.

Tags: Kharbanda's Orthodontics: Diagnosis and Management of Malocclusion and Dentofacial Deformities, Mugdha Mankar, OP Kharbanda

May 10, 2026 | Posted by drzezo in Orthodontics | 0 comments

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Tweed’s philosophy, principles of treatment and standard edgewise biomechanics

Tweed’s journey as angle’s student to an accomplished orthodontist