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The Twin Block technique - Dr. W.J. Clark

The Twin Block technique - Dr. W.J. Clark

The fundamental concept of functional therapy has changed very little since the development of the monobloc by Pierre Robin (1902). Over the years, however, functional appliance design has been progressively modified for day and night time wear. There have been improvements in the design of functional appliances in recent years that have led to more consistent results in functional orthopedic treatment. A significant advance is the introduction of appliances for full time wear, including when eating, in order to maximize the functional forces in the developing dentition.

Twin Blocks are simple bite-blocks that effectively modify the occlusal inclined plane using upper and lower bite blocks which engage on occlusal inclined planes. Twin Block appliances achieve rapid functional correction of malocclusion by the transmission of favorable occlusal forces to occlusal inclined planes covering the posterior teeth. (Fig 1 - Twin Blocks modify the occlusal inclined plane to guide the mandible forward into correct occlusion)


The Twin Block Technique has two stages :

1. Active Phase Twin Blocks use posterior inclined planes to correct the malocclusion by functional mandibular protrusion, and to adjust the vertical dimension.

2. Support Phase An anterior inclined plane is used to retain the corrected incisor relationship until the buccal segment occlusion is fully established.


The cant of the Occlusal Plane

The cant of the occlusal plane is the fundamental functional mechanism of the natural dentition. In normal development, inclined cuspal planes play an important part in determining the relationship of the teeth as they erupt into occlusion.

Occlusal forces transmitted through the dentition provide a constant proprioceptive stimulus to influence the rate of growth and adaptation of the trabecular structure of the supporting bone.

Fixed occlusal inclined planes have been used to alter the distribution of occlusal forces in animal experiments investigating the effects of functional mandibular displacement on mandibular growth and on adaptive changes in the temporomandibular joint. (McNamara, 1980)

The proprioceptive sensory feedback mechanism controls muscular activity and provides a functional stimulus or deterrent to the full expression of mandibular growth. When a distal occlusion develops, the occlusion of the teeth represents a servo-mechanism, which locks the mandible in a distally occluding functional position.


Twin Blocks are designed for full time wear to correct the maxillo-mandibular relationship by functional mandibular displacement. Twin Blocks achieve rapid functional correction of malocclusion by modifying the occlusal inclined plane, to guide the mandible forward into correct occlusion. The forces of occlusion are used to correct the malocclusion.

Upper and lower bite-blocks interlock at a 70° angle and are designed for full-time wear to take advantage of all functional forces applied to the dentition including the forces of mastication. Wearing bite-blocks is rather like wearing dentures and patients can eat comfortably with the appliances in place. With twin blocks, full functional correction of occlusal relationships can be achieved in most cases without the addition of any orthopedic or traction forces.

Twin Blocks are designed for full-time wear to take advantage of all functional forces applied to the dentition, including the forces of mastication. The bite blocks interlock at a 70° angle, usually covering the upper and lower teeth in the buccal segments. By causing a functional mandibular displacement, the interlocking occlusal bite blocks alter the distribution of occlusal forces acting on the dentition to correct a malocclusion during the development of the dentition.

Muscle behavior is immediately influenced by placing inclined planes between the teeth. The muscles of mastication must adapt to the altered balance of occlusal forces by guiding the mandible into protrusive function. This results in rapid soft tissue adaptation to achieve a new position of equilibrium in muscle behavior.

This produces rapid improvements in facial appearance during the first few weeks and months of treatment.

Bite Registration

In Class II Division 1 malocclusion, a protrusive bite is registered to reduce the overjet and the distal occlusion by up to 10 mm on initial activation of twin blocks, depending on the freedom of movement in protrusive function. This degree of activation allows an overjet of up to 10 mm to be corrected without further activation of the twin blocks. Larger overjets invariably require partial correction, followed by reactivation after the initial correction is complete.

The amount of vertical activation is also important, and is determined by two factors. First, there must be adequate vertical clearance between the cusps of the upper and lower first premolars, or deciduous molars to accommodate blocks of sufficient thickness to activate the appliance. The blocks are normally 5 to 6 mm thick between the first premolars.

Secondly, the vertical activation must open the bite beyond the free-way space to ensure that the patient cannot drop the mandible into rest position and negate the proprioceptive functional response of the inclined planes. For the same reason, opening the bite beyond the free-way space may be an important factor in ensuring that the appliance is active when the patient is asleep


A normal period of retention follows treatment after the occlusion is fully established. During the retention period, appliance wear can be gradually reduced to night time wear.

1) Vertical Activation - Treatment of Deep Overbite

Vertical control in treatment of deep overbite associated with a brachyfacial growth pattern aims to increase lower facial height by correcting the incisors to an edge-to-edge relationship, while at the same time, adjusting the height of the upper bite-block in the molar region to encourage molar eruption. The aim is to increase the vertical dimension, and at the same time to improve the profile by increasing lower facial height.

Overbite reduction is achieved by trimming the occlusal cover on the upper twin block occluso-distally to encourage eruption of the lower molars. It is important that the inclined plane remains intact, however, in order to maintain the activation to propel the mandible downwards and forwards.

2) Vertical Activation - Treatment of Reduced Overbite

It is essential that all posterior teeth are in occlusal contact with the opposing bite blocks to prevent over-eruption, which would increase the anterior open bite and accentuate the vertical growth tendency.


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