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The Tweed-Merrifield Philosophy James L. Vaden

Charles Tweed's concepts have been simplified, enhanced, and expanded by Levern Merrifield. Merrifield's ideas have augmented Tweed's to give orth- odontics the Tweed-Merrifield philosophy. Adherence to the philosophy allows the orthodontic specialist to define objectives for the face, the skeletal pattern, and the teeth, and to diagnose and treat a malocclusion to efficiently reach these predetermined objectives. (Semin Orthod 1996;2:237- 240.) Copyright © 1996 by W.B. Saunders Company

T he 1950s we re" growing years" for or thodon- tics in America. Many orthodontists at that

t ime had precep tor training, and graduate pro- grams were just getting started in many areas of the country. Orthodontis ts were looking for a bet ter way to diagnose and treat their patients. Because of Charles Tweed's lectures and pub- lished writings he developed a considerable repu- tation as a clinical orthodontist . Most American orthodontists, both preceptees and those with university training, went to Tucson to participate in the Tweed Study Course in order to learn how to use the edgewise appliance.

As the Tweed Study Course grew, it became more time consuming for Dr Tweed. To help him present the material, he selected many of his fo rmer students to serve as instructors. A list of these instructors would read like a who's who in orthodontics in the 1950s and 1960s. In the late 1950s Tweed had some health problems so he began to seriously consider the selection of a young person with the desire, talent, and "single- ness of purpose" to carry on his work and the Tweed Study Course. He chose Levern Merri- field (Fig 1).

After graduate training at UMKC, Levern Merrifield went to Tucson in 1953 to take Tweed's course. At Tweed's invitation, he immediately jo ined the teaching staff. In 1960 Tweed named

From the Graduate Department of Orthodontics, University of Tennessee, Center for the Health Sciences, Memphis, TN; and the Department of Orthodontics and Pediatric Dentistry; University of Michigan, Ann Arbor, MI.

Address correspondence to James L. Vaden, DDS, MS, 308 East First St, Cookeville, TN38501.

Copyright © 1996 by W.B. Saunders Company 1073-8746/96/0204-000355.00/0

him Tweed Study Course codirector. Merrifield remained codirector until Tweed's death in 1970, at which time he became the director.

Merr i f ie ld has e n h a n c e d and e x p a n d e d Tweed's concepts. He has simplified the treat- m e n t mechanics. Instead of 12 sets of archwires, as in Tweed's day, each malocclusion correct ion now requires a m a x i m u m of four to five sets of archwires. The manipula t ion of the appliance is simple and straightforward. The diagnosis is sophisticated and focuses on the area of the denti t ion with the greatest n u m b e r of problems.

Merrifield's concepts that have been popular- ized over the last 35 years are: the fundamenta l and all-encompassing concept of dimensions of the dentition, 1 the diagnostic concept of dimen- sions of the lower face , 2 the t rea tment concepts of directional control dur ing treatment, 3 sequen- tial tooth movement , and sequential anchorage prepara t ion . 4 Merrifield 's concepts have en- hanced and augmen ted Tweed's to give orth- odontics the Tweed-Merrifield philosophy. Adher- ence to the phi losophy allows the clinician to define objectives for the face, the skeletal pat- tern, and the teeth, and to diagnose and treat a malocclusion to efficiently reach these predeter- mined objectives.

The stated objectives of the Tweed-Merrifield phi losophy of t rea tment are: (1) position and ar range the teeth for m a x i m u m facial balance and harmony, (2) position and arrange the teeth for m a x i m u m health of the teeth, the jaws, the joints and the sur rounding tissues, (3) position and arrange the teeth for m a x i m u m functional efficiency, (4) position and arrange the teeth for m a x i m u m stability and esthetics, (5) position and arrange the teeth on the immature pat ient

Seminars in Orthodontics, Vol 2, No 4 (December), 1996: pp 237-240 2 3 7

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238 James L. Vaden

Figure 1. Levern Merrifield.

to harmonize the correct ion with normal growth processes and maximize the compensa t ion for the less than normal pat tern, and (6) position the denture and arrange the teeth so that bo th are in a continual state of m a x i m u m environmen- tal harmony. (This objective can be realized only if the first five objectives have been successfully achieved.) (7) These clinical objectives must be pursued in an ethical, moral, and compassionate m a n n e r with an overriding concern for the public 's welfare. The all impor tan t seventh objec- tive is the crux of the philosophy.

The basic and fundamenta l diagnostic con- cept of dimensions of the denti t ion was Merri- field's first, and one of his most impor tan t contr ibutions to the philosophy. Both the diag- nostic and t rea tment philosophy have been devel- oped within the f ramework of this impor tan t concept. Dimensions of the denti t ion includes four basic premises, providing the muscular env i ronment is normal .

Premise no. 1. There is an anter ior limit of the dentition. Teeth should not be pushed forward off basal bone. I f the teeth are pushed too far for- ward, all the objectives of treat- men t are compromised. Tweed defined the anter ior limit with the diagnostic facial triangle. 5

Premise no. 2. There is a poster ior limit of the dentition. Teeth can be pushed

distally off the maxillary tuberos- ity and impacted into the area beh ind the mandibular first mo- lar in the lower arch just as easily as they can be pushed too far forward.

Premise no. 3. There is a lateral limit of the dent i t ion. Tee th canno t be pushed laterally into the masse- ter and buccinator muscles and be expected to remain in place over time.

Premise no. 4. There is a vertical limit of the dentition. Vertical expansion is disastrous to both facial balance and to stability.

In summation, the concept of dimensions of the dent i t ion is the founda t ion of the Tweed- Merrifield diagnostic and t rea tment philoso- phies. Orthodontis ts should recognize and treat within the dimensions of the denti t ion if the pat ient has normal muscular balance.

The Diagnostic Philosophy

The Tweed-Merrifield diagnostic philosophy 6-° can be outl ined as follows: (1) Recognize and treat within the dimensions of the dentition. Patients who have normal muscular balance should be treated without expansion. (2) Recog- nize the dimensions of the lower face and treat for m a x i m u m facial h a r m o n y and balance. (3) Recognize and unders tand the skeletal pattern. Diagnose and treat in ha rmony with normal growth and developmental pat terns and en- hance the less than normal pat tern.

The Treatment Philosophy

A t rea tment phi losophy must c o m p l e m e n t a diagnostic philosophy. Tweed-Merrifield Direc- tional Force technology is simple, straightfor- ward, and modern . It complements the previ- ously described diagnostic philosophy. It is the most con temporary orthodontic technology avail- able today for the or thodont is t who desires a control led or thodont ic response.

Th roughou t the Tweed-Merrifield era, the key to quality individualized or thodont ic service has been direct ional ly cont ro l led precis ion archwire manipulat ion. Archwire manipulat ion,

The Tweed-Merrifield Philosophy 239

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Figure 2. Upward and forward force system.

and not bracket manipulat ion, enables the clini- cian to reach his or her objectives by placing the p roper force on the p rope r tooth or teeth at the p roper time. There are essentially five concepts which make up the Tweed-Merrifield t rea tment philosophy. These are: (1) sequential appliance placement; (2) sequential tooth movement ; (3) sequential mandibular anchorage preparat ion; (4) directional force (augments control of the vertical dimension which, in turn, enhances mandibular response); and (5) p roper t iming of treatment.

Sequential Appliance Placement

Sequential appliance p lacement allows the opera- tor to start a pat ient 's t rea tment with small edgewise archwires. Edgewise archwires enhance control of the teeth f rom the outset of t reatment.

Sequential Tooth Movement

Tooth movemen t is sequential, not en masse. Teeth are moved rapidly and with precision because they are moved individually or in small units while the remaining teeth in the arch act as stabilizing units.

Sequential Mandibular Anchorage Preparation

Sequential mandibular anchorage prepara t ion was developed by Levern Merrifield as a re-

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Figure 3. Maxillary incisor movement must be distal and superior.

sponse to technical problems with en masse anchorage preparat ion. ]°J1 Mandibular anchor- age is p repa red quickly and easily by t ipping two teeth at a t ime (one on each side of the arch) to their anchorage p repared positions.

Directional Force

A hal lmark of Tweed-Merrifield t rea tment strat- egy is a directional force system that controls the mandibular anter ior and poster ior teeth and the

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Figure 4. Mandibular plane, occlusal plane, palatal plane.

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240 James L. Vaden

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Figure 5. Downward a n d backward force system se- quelae.

maxillary anter ior teeth. The resultant vector of all or thodont ic forces should be counterclock- wise so that the oppor tuni ty for a favorable skeletal change is enhanced (Fig 2). Such a force system requires that the mandibular incisors be upr ight over basal bone so the maxillary incisors can be moved distally and superiorly (Fig 3). For the counterclockwise force system to be a reality, vertical control is critical. To control the vertical dimension, one must control the mandibular plane, the occlusal plane, and the palatal plane (Fig 4). If Point B drops down and back, the face becomes lengthened, the mandibula r incisor is t ipped forward off basal bone, and the maxillary incisor drops down and back instead of being moved up and back (Fig 5).

T/m/.g Timing of t rea tment is an integral par t of the t rea tment philosophy. Trea tment should be initi- ated at a t ime when p rede te rmined objectives can be most readily accomplished. This might mean interceptive t rea tment in the mixed denti- tion, selected extractions in the mixed dentition, or waiting for second molar erupt ion before initiating active t reatment .

References

1. Merrifield LL. The dimensions of the denture: Back to basics. AmJ Orthod Dentofacial Orthoped 1994;106:535- 542.

2. Merrifield LL. The profile line as an aid in critically evaluating facial esthetics. AmJ Orthod 1966;11:804~822.

3. Merrifield LL, Cross JJ. Directional force. Am J Orthod 1970;15:435-464.

4. Merrifield LL. Edgewise sequential directional force technology. J Charles Tweed Foundation 1986;14:22-37.

5. Tweed CH. The Frankfort-Mandibular plane angle in orthodontic diagnosis, classification, treatment plan- ning, and prognosis. AmJ Orthod Oral Surg 1946;32:175- 230.

6. Merrifield LL, Klontz HA, VadenJL. Differential diagnos- tic analysis systems. AmJ Orthodo Dentofacial Orthoped 1994;106:641-648.

7. Merrifield LL. Differential diagnosis with total space analysis. J Charles Tweed Foundation 1978;6:10-15.

8. Ledyard BC. A study of the mandibular third molar area. AmJ Orthod 1953;39:366-374.

9. Richardson ME. Development of the lower third molar from ten to fifteen years. Angle Orthod 1973;43:191-193.

10. Merrifield LL. The systems of directional force.J Charles Tweed Foundation 1982;10:15-29.

11. Vaden JL, Dale JG, Klontz HA. The Tweed-Merrifield Edgewise Appliance: Philosophy, Diagnosis, and Treat- ment. Orthodont ics--Current Principles and Tech- niques, (Ed 2): St. Louis, MO. Mosby, 1994:627-684.