The Twin-Block appliance evolved from European orthodontists in the early twentieth century treating patients with trapped skeletal Class II mandibles using posterior bite ramps. Dr Bill Clark developed a protocol in the United Kingdom for adapting the appliance for use in the National Health System and termed the appliance the Twin-Block appliance. (See. Fig.#1) He was assisted in the development of the appliance by Mr. Clive Hudson who was a noted laboratory technician in Sheffield, England.

Dr. William J. Clark

Fig . #1

The Twin Block appliance basically an upper Schwarz appliance with occlusl coverage of acrylic with a posterior bite ramp, and a lower Schwarz appliance with a posterior bite ramp to distract the mandible forward into a skeletal Class I relationship. (See fig.#2)

Clark Twin Block Fig.#2

There are numerous modifications for the design of the Twin-Block and for its use in maxillofacial orthopedic therspy. This article is presented as a practical approach to incorperating the Twin Block appliance into a general maxillofacial orthopedic practice.

Our use for the Twin-Block appliance is to distract the mandible from a trapped skeletal Class II into a sleletal Class I relationship in the prebubertal patient in the pemanent dentition. It can also be used in the adult patient as a repositioning splint for TMD patients.

The Clark Twin-Block appliance was modified by Dr. Ian Walters in Sydney Australia. (See fig.#2)

Dr. Ian P.Wlters

(Fig.#2)

Dr. Walters modifivstions included moving the posterior bite ramps forward to the cuspids which allows the patient to chew in a rotary motion. He closed the anterior open bite created by the appliance with an anterior cap. This modification allows the patient to bite on their front teeth and prevents the patient from develipong an anterior tongue thrust. (See fig.#3)

Fig.#3

There are a number of options for the design of the upper block. Most skeletal Class II patients have a narrow maxilla as part of the maxillary entrapment on the mandible. The width of the maxilla is developed prior to the placement of the Twin-Block appliance. I no additional development is required the upper block has a trans palatal bar for stabality. This makes it easier for the patient to eat and speak in the appliance. If there is any additional maxillary development required the upper block has a midline screw. The screw is adjusted once a week as the mandible is distracyed into the sleketal Class I relationship. (See fig.#4)

Fig.#4

The lower block almost always has a midline scerw to insure the case has the correctly key stoned intercaine width. The correct intercaine width is critical for long term stabality. This screw is adjusted once a week as well. (See fig.#5)

Fig.#5

Twin-Block Registration

The Twin-Block appliance is the final step in the maxillofacial orthopedic therapy. This means the maxilla is fully developed and the dental alveolar base is a dental Division One prior to making the bite registration.

The Twin-Block is one in a number of damand position appliances that requires the patient to occlude down and forward position from their current occlusion. The first step in making a bite registration for any demand position appliance is to be sure that the the mandibular condyles are correctly positionen on the biconcave articular disks. This is accomplished by having the patient open as wide as posible prior to taking the registration.

The patient should be positioned vertically for the registration. The mandible is moved forward into an end to end relation of the incisors with 4mm vertically between the incisors. The skeletal midlines are correcrly aligned. A bite jig is used to hold the mandible in this demand position. Registration paste is used to record the demand position bite. (See fig.#6)

Fig.#6

Horizontal Forces in the Twin-Block Appliance

Any distraction appliance initially has horizontal forces within the appliance created by the muscles of mastication. The greater the mandibular the distraction the higher the level of this horizontal. This force remains active for approximately the first 90 days of treatment. (See fig.#7)

Fig. #7

This horizonal force in the Twin-Block must be anchored to prevent it from forcing the lower incisors into excess proclination and from distalizing the maxillary buccal segments. This anchorage must be maintained for approximately 90 days until the muscle attachments begin to migrate and the horizontal force becomes passive. (See fig.#8)

Fig. #8

Inserting the Twin-Block Appliance

The insertion adjustment for the upper block is to engage the labia bow into the gingival one third of the maxillary incisors to engage maxillary cross-arch anchorage. The adjustment is made symmetrically with the 3-Jaw pliers on the superior and inferior loops in the Hawley labial bow. This prevents the upper block from distalizing the maxillary buccal segments. (See fig.#9)

Fig.#9

Marking paper is used to insure there is even occlusal contact within the Twin-Block. Groves are cut in the occlusal surface of the acrylic that open to the the papatal surface to facalite the patient eating in the appliance. (See fig.#10)

Fig. #10

The insertion adjust for the lower block is to releive all acrylic contacting the lingual surface of the lower incisors. This adjustment isolates the horizonal force on the lower buccal segments engaging mandibular cross-arch anchorage. This prevents the lower incisors frombeing dumped forward into excess proclination.(See fig.#11)

Fig. #11

The clinician should monitor the inclination of the lower incisors every four weeks. If the lower incisors begin to move into excess proclination the lingual acrylic of the lower block has moved back into contact and needs to immediately be releived. This is caused by the cranial sutures responding the the mandibular distraction. (See fig.#12)

Fig.#12

The patient must wear the Twin-Block appliance at night and as much during the day as possible. Eating in the appliance will reduce the treatment time by around 50%.

The skeletal Class II mandible is one of the major causes for air way obstruction. As a result, the patient mouth breathes when sleeping and drops back of the ramps in the Twin-Block. It is important to have the patient sleep with class II elastics which hold the mandible forward into the distracted position. The elastics are 3/16” 4.5oz. (See fig.#13)

Fig.#13

Opening a deep bite

It is posible to adjust the postreior acrylic on the upper block to allow the lower second bicuspids and the lower molars to erupt which will open a deep bite. See fig.#14)

Fig.#14

This adjustment to open the deep bite is used only if the patient is not going to be finished with fixed appliance therapy. The adjustment requires removing the Adams clasp and and elastic hook from the lower first molars. It also crears a step in the occlusal plane at the lower first bicuspid. The Twin-Block is followed by a Stge Two appliance that is a Hawley retainer modified with an anterior bite ramp. This allows the remaining lower posterior to function into occlusion. (See fig.#15)

Fig.#15

Once the Twin-Block appliance distracts the mandible into a Skeletal Class I relationship the treatment is usually followed with fixed appliance therapy. The Rick-a-nator appliance is used inconjunction with the fixed wire for the final bite opening and repositioning of the mandible. This technique also allows the clinician to actively stabalize the occlusion using vertical elastics. (See fig.#16)

Fig.#16

Open Bite Skeletal Class II

The Twin-Block appliance is not effective in treating a patient that is skeletal Class II and has an open bite due to the anterior tongue thrust.The best option for treating these patients is full time wearing of the maxillary arch development appliance as a retainer, and using a Bionaror to close the bite at night. (See fig. #17)

Fig.#17