Underbite Correction: How Braces Reposition Teeth and Jaws

How Do Braces Fix Underbite?

Braces correct an underbite by applying continuous, precise forces to gradually reposition teeth and, in some cases, modify jaw growth, leveraging the body's natural bone remodeling processes to achieve proper dental alignment and bite relationship.

Understanding Underbite: A Biomechanical Perspective

An underbite, clinically known as a Class III malocclusion, is a dental condition where the lower front teeth overlap the upper front teeth when the mouth is closed. From a biomechanical standpoint, underbites can be primarily classified into two types, each with distinct underlying anatomical issues that dictate the approach to correction:

• Dental Underbite: This type occurs when the lower teeth are positioned too far forward relative to the upper teeth, or the upper teeth are positioned too far back, without a significant skeletal discrepancy. It's often due to misaligned teeth rather than jaw size.
• Skeletal Underbite: More complex, this involves an imbalance in jaw size or position. It can manifest as an overgrowth of the lower jaw (mandibular prognathism), an underdevelopment of the upper jaw (maxillary hypoplasia), or a combination of both. The biomechanical challenge here is not just tooth movement but also influencing jaw growth and position.

Causes can range from genetic predisposition and prolonged habits like thumb sucking (which can impact jaw development) to trauma or certain medical conditions. The goal of orthodontic intervention is to restore proper occlusal function, improve aesthetics, and prevent long-term issues like excessive tooth wear, temporomandibular joint (TMJ) dysfunction, and speech difficulties.

The Core Principles of Orthodontic Correction

The fundamental principle behind how braces work is bone remodeling. When continuous, gentle pressure is applied to a tooth, it creates areas of compression and tension on the surrounding alveolar bone (the bone that holds the teeth).

• Pressure Side (Compression): On the side where the tooth is being pushed, the blood supply is temporarily reduced, signaling osteoclasts (bone-resorbing cells) to break down bone.
• Tension Side (Tension): On the opposite side, where the periodontal ligament (PDL) is stretched, osteoblasts (bone-building cells) are activated to deposit new bone.

This synchronized process of bone resorption and deposition allows the tooth to gradually move through the bone. For underbite correction, this principle is harnessed to move specific teeth into their correct positions and, critically, to influence jaw growth in younger patients.

Components of Braces and Their Role in Underbite Correction

Traditional fixed orthodontic braces comprise several key components, each playing a vital role in applying the controlled forces necessary for tooth and jaw repositioning:

• Brackets: Small, square attachments typically made of metal or ceramic, bonded directly to the front surface of each tooth. They act as handles, transferring the force from the archwire to the tooth. Their specific design (torque, angulation, and rotation) is crucial for directing tooth movement.
• Archwires: These are the primary force-generating component. A thin, flexible wire made of nickel-titanium or stainless steel, the archwire is threaded through the slots in the brackets. It exerts continuous pressure as it tries to return to its original shape, guiding teeth into alignment. For underbites, specific bends and loops can be incorporated to push upper teeth forward or pull lower teeth back.
• Ligatures (Elastics or Wires): Small rubber bands or thin wires that secure the archwire into the bracket slots. They ensure the archwire remains engaged and effectively transmits force.
• Elastics (Rubber Bands): Often the most critical component for underbite correction, these removable rubber bands are hooked between specific brackets on the upper and lower arches. They provide inter-arch forces, pulling the lower jaw backward relative to the upper jaw, or pulling upper teeth forward and lower teeth backward, thus correcting the front-to-back discrepancy. The direction and strength of these elastics are precisely prescribed by the orthodontist.
• Coil Springs: Placed on the archwire between brackets, these springs can either push teeth apart (open coil) to create space or pull them together (closed coil) to close spaces. In underbite cases, they might be used to create space for an upper tooth to move forward or to retract a lower tooth.

Adjunctive Appliances for Underbite Treatment

For more severe skeletal underbites, particularly in growing patients, orthodontists often combine braces with specialized adjunctive appliances to modify jaw growth:

• Palatal Expander (Rapid Maxillary Expander - RME): If the upper jaw is too narrow, an RME can be used to widen it. This appliance is cemented to the upper molars and gradually widened with a key, separating the two halves of the palate. This creates space for upper teeth to move forward and helps the upper jaw catch up to the lower jaw in width.
• Facemask (Reverse-Pull Headgear): This external appliance is typically used in growing children. It consists of a frame that rests on the forehead and chin, with elastic bands attaching to an expander or hooks on the upper molars. The elastics apply forward and downward traction to the upper jaw, encouraging its forward growth and discouraging lower jaw growth.
• Chin Cap: An external appliance that fits over the chin and is connected to a headcap. It applies backward pressure to the lower jaw, aiming to restrict its forward growth in growing patients.
• Herbst Appliance: An internal, fixed appliance that connects the upper and lower jaws. It holds the lower jaw in a more backward position, encouraging the upper jaw to grow forward and inhibiting the lower jaw's forward growth. This is particularly effective in adolescents during their growth spurt.
• Surgical Intervention (Orthognathic Surgery): For severe skeletal underbites in adults where growth modification is no longer possible, orthodontic treatment is often combined with jaw surgery. Braces align the teeth within each jaw, and then surgery repositions the jaws themselves into the correct relationship.

The Treatment Process: A Step-by-Step Overview

Correcting an underbite with braces is a multi-stage process requiring careful planning and execution:

1. Diagnosis and Treatment Planning: This initial phase involves a comprehensive examination, including dental impressions, X-rays (panoramic, cephalometric), and photographs. The orthodontist determines the type of underbite (dental vs. skeletal), its severity, and the patient's growth potential. A customized treatment plan is then developed, outlining the sequence of tooth movements and the specific appliances to be used.
2. Active Treatment Phase:
   • Appliance Placement: Brackets are bonded to the teeth, and the initial archwires are placed.
   • Initial Alignment: Lighter, more flexible archwires are used first to begin leveling and aligning the teeth.
   • Underbite Correction: As teeth become more aligned, stiffer archwires are introduced, and elastics, springs, or adjunctive appliances (like facemasks or Herbst) are incorporated to address the front-to-back jaw discrepancy. This is the most crucial phase for underbite correction.
   • Detailing and Finishing: Once the underbite is corrected and teeth are in their approximate final positions, finer adjustments are made to achieve optimal bite and aesthetics.
3. Retention Phase: After the active treatment, braces are removed. This phase is critical to stabilize the corrected bite and prevent relapse. Retainers (removable clear aligners or fixed wires bonded to the back of the teeth) are prescribed to hold the teeth and jaws in their new positions while the surrounding bone and soft tissues adapt.

Considerations and Expected Outcomes

The duration and complexity of underbite treatment vary significantly based on the severity of the condition, the patient's age, and compliance with instructions.

• Age Factor: Treatment in growing children (interceptive orthodontics) is often more effective for skeletal underbites because their jaws are still developing, allowing for growth modification. Adult treatment for skeletal underbites may require orthognathic surgery in conjunction with braces.
• Compliance: Consistent wear of elastics and proper care of appliances are paramount for successful outcomes. Non-compliance can significantly prolong treatment or compromise results.
• Expected Outcomes: Successful treatment results in a harmonious bite where the upper front teeth slightly overlap the lower front teeth, improved facial aesthetics, enhanced chewing function, and reduced risk of future dental problems.

Conclusion: A Coordinated Biomechanical Approach

Fixing an underbite with braces is a testament to the sophisticated application of biomechanical principles in orthodontics. It involves a meticulously planned application of forces through brackets, wires, and elastics, often supplemented by specialized appliances, to meticulously guide teeth into proper alignment and, critically, to modulate jaw growth when possible. This coordinated approach leverages the body's natural adaptive processes, leading to a functional, healthy, and aesthetically pleasing bite that can dramatically improve a patient's quality of life.