A twin block appliance is a type of removable orthodontic device used to correct Class II malocclusion, where the lower jaw is positioned too far back compared to the upper jaw.
Introduction
Malocclusion Malocclusion often involves misalignments between the upper and lower jaws, leading to poor tooth-to-tooth contact and improper biting function. This can hinder the natural growth and development of the jaw.
In Angle's classification system for malocclusion, a Class II bite (Figure 1) occurs when the lower jaw (mandible) is positioned more behind relative to the upper jaw (maxilla). This misalignment means that when the teeth come together, the lower teeth bite significantly behind the upper teeth creating a large horizontal gap between upper and lower teeth. It is defined as increased overjet, which is measured from the labial surface of the tips of the upper incisors to the labial surface of the lower incisors horizontally.
Twin Block Appliance Functional appliance therapy is a treatment approach designed to correct these issues. By addressing factors that impede jaw growth and enhancing the muscle function surrounding the teeth. Functional appliances aim to improve the functional relationship of the tooth and facial structures by eliminating unfavourable developmental factors and improving the muscle environment.
By making changes to the position of the teeth and supporting tissues, functional appliance therapy can establish a new, healthier biting pattern that promotes optimal jaw growth and development.
Twin block appliances are simple, full-time bite blocks designed to treat skeletal Class II malocclusion by encouraging the lower jaw to move forward. The goal of this treatment approach is to maximise the growth response of the lower jaw while ensuring patient comfort and aesthetic appeal.
These appliances such as seen in Figure 3 work by enhancing forward growth of the mandible, which helps to correct the misalignment between the upper and lower jaws. This functional approach to treatment is designed to be efficient and effective in addressing Class II malocclusion. Reverse Twin Block appliance can be used in addressing Class 3 malocclusion.
History Twin block appliance was introduced by Dr William Clark. It is composed of interlocking upper and lower bite blocks which position the mandible forward for overjet correction. Overjet is measured from the labial surface of the tips of the upper incisors to the labial surface of the lower incisors horizontally.
The Twin Block appliance was developed due to an incident involving a young patient, the son of a dental colleague, who had completely knocked out the upper central incisor because of a fall such as seen in Figure 5. Luckily, the tooth was preserved as he was treated within a few hours after the incident. Reimplantation of the tooth was done and a temporary splint was constructed to hold the tooth.
The occlusal relationship was classified as Class II division 1, with a 9mm overjet, and the lower lip was positioned behind the upper incisors. This unfavourable lip positioning lead to mobility and root damage of the reimplanted incisor. To prevent the lip from getting trapped in the overjet as seen in Figure 6, it was essential to create an appliance that could be worn full-time to hold the mandible in a forward position. Since no such appliance existed at the time, simple bite blocks were developed to fulfil this purpose.
The appliance is designed to utilise occlusal forces to correct the distal occlusion and reduce the overjet without exerting direct pressure on the upper incisors. The upper and lower bite blocks made contact mesial to the first permanent molars at a 90° angle to the occlusal plane when the mandible is positioned forward. This alignment brought the incisors edge-to-edge with a 2mm vertical gap to keep them out of occlusion (interlocking). The patient was required to actively move the mandible forward to occlude the bite blocks in a protrusive bite.
The first Twin Block appliances were fitted on September 7, 1977, when the patient was 8 years and 4 months old. The bite blocks were comfortable to wear, and treatment progressed smoothly, with the distal occlusion improving and the overjet decreasing from 9mm to 4mm within 9 months.
Evolution in Angulation of the Inclined Planes The earliest Twin Block appliances were designed with bite blocks that angulated at 90° to the occlusal plane, requiring the patient to consciously occlude in a forward position. However, some patients struggled to maintain this forward posture and would often return to move the mandible back to its original distal occlusal position, causing the bite blocks to stack on top of each other due to their flat occlusal surfaces. This issue became apparent early in treatment when it was observed that the patient was not consistently adopting a forward posture. Biting on the blocks in this manner resulted in a significant posterior open bite such as seen in Figure 9, a complication that occurred in about 30% of the initial Twin Block cases. To address this, the angulation of the bite blocks was modified to 45° to the occlusal plane, effectively guiding the mandible forward. This adjustment immediately resolved the issue.
As the technique developed, the angle of the inclined plane was adjusted from 90° to 45° to the occlusal plane, eventually settling at 70° to the occlusal plane. For patients who struggle to maintain a forward mandibular posture, a 45° angle can still be utilised.
An angle of 45° to the occlusal plane provides an equal downward and forward force to the lower dentition. The occlusal force on the inclined planes promotes a corresponding downward and forward growth stimulus. After utilising a 45° angle on the blocks for 8 years, the angulation was ultimately adjusted to a steeper 70° to introduce a more horizontal force component. It was believed that this change might encourage greater forward growth of the mandible. However, if the patient experiences difficulty in maintaining a forward posture, it is advisable to decrease the angulation of the inclined planes back to 45° to help guide the mandible forward and facilitate easier maintenance of the forward position.
Design of twin block The design of dental appliances must prioritise comfort and aesthetics to encourage patient cooperation and compliance throughout treatment. "Patient-friendly" designs can motivate patients to actively participate in their care and follow their treatment plan.
Twin block appliances offer versatility in design, making them suitable for treating a wide range of malocclusion cases in growing patients. The separate upper and lower arch components allow for individualised adjustments using screws, springs, or bows to address specific dental issues like crowding or irregularity.
Standard twin block Standard Twin Block appliances are a widely used method in orthodontics for treating Class II division 1 malocclusions. These appliances are particularly effective when the dental arches are well-formed and there is enough overjet (horizontal distance between upper incisors and lower incisors) to allow the lower jaw to move forward freely. The goal of the treatment is to correct the bite by bringing the jaws into a Class I relationship, which ensures proper alignment of the back teeth.
Components
The Delta Clasp
The Delta clasp was developed to improve retention in Twin Block appliances. It is based on the Adams clasp shown in Figure 10 A) but includes features that increase durability and reduce the likelihood of breakage. The Delta clasp in Figure 10 B) has closed loops, which can be triangular, circular, or oval in shape. These closed loops maintain their form better than the open U-shaped loops of the Adams clasp, resulting in fewer adjustments and a reduced risk of breakage.
This clasp design is particularly effective in securing the Twin Block appliance on lower premolars and other posterior teeth. Its improved durability allows it to retain its shape through repeated insertion and removal, contributing to a stable and long-lasting fit. In a study by Clark & Stirrups (1979–1993), the failure rates of the delta clasp and the Adams clasp were compared through statistical analysis of two patient groups (69 and 72 patients, respectively) treated with Twin Block appliances. The findings revealed that the delta clasp demonstrated a significantly lower breakage rate (1%) compared to the modified arrowhead (Adams) clasp, which had a breakage rate of 10%. The delta clasp's design can be modified based on the best area of retention, either by following the curvature of the tooth into mesial and distal undercuts, making it suitable for teeth with favourable shapes and well-defined undercuts.
Clasp Construction Methods
The Delta clasp can be constructed in different ways, depending on the shape of the patient's teeth. If the teeth have adequate undercuts (spaces that the clasp can grip), the clasp loop is angled to follow the contour of the tooth and fit into the mesial (front) and distal (back) undercuts. If the teeth do not have sufficient undercuts, the clasp is constructed with the loop directed between the teeth (interdentally) to gain retention from adjacent teeth.
In permanent dentition, Delta clasps are commonly placed on the upper first molars and lower first premolars. They can also be used on deciduous (baby) molars. Additional clasps, such as ball-ended, or C-shaped clasps, may be added for extra retention and to prevent unwanted tooth movements, such as tipping.
Ball-ended clasp
The ball-end clasp in Figure 11 is a type of wire component used in removable orthodontic appliances, including the Twin Block appliance, to help keep the device securely in place. It consists of a thin metal wire that ends in a small, rounded ball, which rests against a tooth to provide gentle but firm retention. In the Twin Block appliance, the ball-end clasp is usually placed between the lower canines and premolars, where it grips onto the undercuts of the teeth without causing discomfort. Its smooth, rounded shape prevents irritation to the soft tissues while ensuring the appliance stays in position during wear. This clasp is particularly useful because it allows for easy insertion and removal of the appliance while still offering reliable retention, making it an effective choice for maintaining stability throughout treatment.
Base Plate
The base plate of the Twin Block appliance can be made from either heat-cured or cold-cured acrylic. Heat-cured acrylic offers better strength and precision, which is essential for maintaining the appliance's effectiveness during treatment. By modelling the appliance in wax first, the bite blocks can be shaped with greater accuracy.
Cold-cured acrylic is quicker and more convenient to use but tends to be less durable. This can become an issue in the later stages of treatment, especially when the bite blocks are trimmed to allow for tooth eruption. The inclined planes (sloped surfaces) of the bite blocks may wear down if a softer, cold-cured acrylic is used.
To overcome the limitations of cold-cured acrylic, some orthodontists opt for preformed bite blocks made from high-quality heat-cured acrylic. These preformed blocks ensure a consistent angle for the bite planes and offer greater durability, making the appliance more effective over the long term.
Lower incisor capping