Jaw Deformities

 

Overview

A jaw deformity is a condition that affects the formation, shape, and size of the jaw. In general, abnormalities in the jaw occur when there is a disruption or flaw in the fusion of the mandibular processes. 

The mandible, more than any other bone in the human skeleton, has the most differential typical growth anomalies. This is due to variations in the mandible's complex symmetrical growth pattern, Because it is the only moving part of the facial skeleton, the mandible in particular plays an important role in appearance. 

This has a significant impact on an individual's ability to speak and masticate, as well as their overall aesthetic and expressive features of the face. If there are any abnormalities in size or position, the maxilla will face the same problems. 

 

What is Jaw Deformities?

A deformity is described as an abnormal shape, disfigurement, or lack of natural arrangement. Humans have two jaws, one upper and one lower.

Disfigurements of one or both jaws are referred to as jaw deformities. The mandible is a single bone in the lower jaw, The upper jaw is a functional unit composed of four different bones: the right and left maxillae and the right and left palatine bones, with the latter referring to the sections of these bones positioned below the zygoma.

Clinically, the upper jaw is sometimes known as the 'maxilla,' which can be confusing because it also refers to a bone.  Some jaw malformations develop in utero and are apparent at birth, while the others develop later in adulthood.

They are caused by a variety of factors, including: defects in genetics, deformities, intrauterine disturbances, infections, traumas, or improper function.

Jaw abnormalities alter at least one of the jaw's geometric properties:

• Size 
• Position
• Orientation 
• Shape 
• Symmetry

A jaw deformity may be the primary problem for a given patient, or it may be secondary to disease, injury, or functional impairment. 

A woman with a family history of mandibular prognathism who developed the condition during puberty is one example of a patient whose primary problem is a deformity. 

A young man with an anterior open-bite due to condylar destruction caused by juvenile arthritis (a disease), a teenager with retrognathia and facial asymmetry caused by condylar fracture and Temporomandibular Joint (TMJ) ankylosis during childhood (an injury), and a patient with anterior open-bite due to mouth-breathing are all examples of secondary deformities.

 

Classification of Jaw Deformities

The jawbones are classified as having six geometric attributes: size, position, orientation, shape, symmetry, and completeness. Jaw deformities are classified based on the attribute that they affect.

• Size

Size deformities occur when the jaw is either too large or too small. Pathological enlargement is referred to as hyperplasia, whereas failure to attain normal size is referred to as hypoplasia.

Micrognathia is synonymous with mandibular hypoplasia, whereas macrognathia is synonymous with mandibular hyperplasia. 

The terms macrogenia and microgenia also refer to size, with macrogenia referring to a large chin and microgenia referring to a small chin.

• Position

Abnormal jaw positions can be found in all four cardinal directions. Prognathism and retrognathism are anteroposterior positions that are abnormal.

Anteroposterior position is typically measured in relation to the cranial base. When a jaw is too far forward, it is called prognathism, when it is too far backward, it is called retrognathism. 

Laterognathia is a deformity in which a jaw is displaced away from the median plane in either direction in the transverse direction. 

Vertically, a jaw can be too far down, resulting in excessive downward displacement, or too far up, resulting in insufficient downward displacement.

• Orientation

Malrotations occur when a jaw is oriented incorrectly, the axis on which the abnormal rotation occurs is used to classify these malrotations. 

A jaw is said to have abnormal pitch when it is malrotated around the transverse facial axis, when the jaw is malrotated around the anteroposterior axis, it has an abnormal roll, a condition known as can't. Finally, abnormal yaw occurs when a jaw is malrotated around the vertical axis.

• Shape

Shape is the geometric feature of an object that is not size, position, or orientation. A distorted jaw is one that has an abnormal shape.

• Symmetry

The human face has symmetry in reflection around one plane, the median. 

Two conditions must be met for facial symmetry to exist. 

First, each unit of the face must be symmetrical, a condition known as object symmetry. 

Second, each unit must be symmetrically aligned to the median plane, which is known as symmetric alignment. 

Jaws can develop symmetry deformities as a result of object asymmetry or misalignment. 

Mandibular asymmetry and maxillary asymmetry refer to abnormalities in object symmetry, whereas asymmetric alignment refers to abnormal alignment that causes asymmetry.

• Completeness

The term "completeness" refers to the completeness of the jaw. A jaw can be incomplete because one of its processes did not fully develop, such as agenesis of the mandibular condylar process, which can be seen in hemifacial microsomia. Completeness can also fail due to embryological processes in the jaw failing to fuse or an acquired defect.

Jaw deformities of various types (size, position, orientation, shape, symmetry, and completeness) are frequently associated. Asymmetric alignment, for example, cannot occur in the absence of at least one other deformity.

 

What is the effect of jaw deformities on the teeth?

Jaw deformities can also affect the teeth. Malocclusion can occur when one or more teeth in the dental arch are misaligned or when the upper and lower dental arches are not coordinated.

Deformity within a dental arch may affect tooth alignment, leveling, or spacing. The arrangement of teeth in an arch is referred to as alignment.

The incisal edges of the incisors and the buccal-cuspal ridges of the canines, premolars, and molars form an arch in ideal alignment.

• Dental displacement, dental tipping, and dental rotations can all cause misalignment. 
• A tooth is physically moved outside the arch during displacement.
• A tooth is abnormally inclined in tipping.
• A tooth is misaligned in rotations due to abnormal rotation around its long axis.
• When a tooth is infraocclusion or supraocclusion, it is located below or above its occlusal plane.
• Dental leveling is judged for the entire dental arch by gauging the Spee curve. 

The cusps of all teeth should inscribe either a flat plane or a curved plane with a slight upward concavity from the central incisor to the last molar. A deep or reverse Spee curve can be caused by a dental deformity. When the cusps of the teeth trace a plane with sharp upward curvature, the curve of Spee is deep. When the plane's curvature is downward concavit, the curve is reversed. Teeth within a dental arch should be normally spaced;that is, adjacent teeth should touch without crowding. When diastemas exist or the arch cannot accommodate the teeth, spacing is abnormal. Excessive dental spacing is the first condition, and dental crowding is the second. Furthermore, dental deformities can occur when the upper and lower arches are not in sync.  It is not enough for the upper and lower teeth to be arranged in an arch for normal occlusion to occur. The position, shape, and tooth size of the upper and lower dental arches must also be coordinated. Malocclusion is caused by discordant dental arch positions. This discordance can occur in all three cardinal planes: anteroposterior, vertical, and transverse.

Finally, transverse discordance between the maxillary and mandibular dental arches can occur. The buccal cusps of the maxillary posterior teeth are normally lateral to those of the mandibular teeth. 

A posterior crossbite occurs when the opposite occurs. In severe cases, all of the lower teeth may be trapped inside the upper teeth, a condition known as Brodie bite. In contrast, scissor bite occurs when the upper teeth are inside the lower teeth.

 

Symptoms of Jaw Deformities

A person with a deformed jaw suffers both physically and psychologically. Even when not in use, it impairs eating, breathing, sleeping, talking, and jaw movement. These issues differ depending on the disorder type, the patient's pain threshold and age, and the severity of the disorder. 

Doctors and experts in this field, on the other hand, have identified three significant disorders, which are described below:

• Difficulty chewing

Jaw abnormalities cause the upper and lower jaws to not overlap properly when chewing food, resulting in pain and discomfort as well as incomplete chewing, which can lead to a variety of digestive problems and other diseases.

• Abnormal breathing

Patients with jaw abnormalities breathe through their mouths, which causes health problems because nasal breathing removes a significant amount of air pollution. Mouth breathing causes a variety of jaw problems, including the small jaw described elsewhere in this section.

• Abnormal appearance

Patients with jaw abnormalities have the most visible facial deformities. Malocclusions typically appear at a young age in babies who have used pacifiers for an extended period of time or who are accustomed to thumb sucking. 

Not only does it deform the face, but it also causes shyness and lack of self-confidence.

 

Management of Jaw Deformities

Different operations can be used to correct jaw deformities. Orthognathic surgery or distraction osteogenesis can be used to correct jaw size, position, orientation, shape, or symmetry deformities. Jaw completeness deformities necessitate reconstructive surgery.

 

Planning Orthognatic Surgery

The term orthognathic is a compound word that means "straight jaw." As a result, orthognathic surgery refers to jaw straightening surgery. It entails the removal of a jaw and the relocation of at least one of its segments.

Presurgical orthodontics, surgery, and postsurgical orthodontics are the three distinct stages of orthognathic surgical treatment.

An orthodontist aligns and levels the teeth, removes unwanted compensations, and coordinates the dental arches in the first stage. Surgery is performed in the second stage. An orthodontist completes the orthodontic movements in the final stage.

Treatment planning is the process of determining the specifics of treatment. Formal treatment planning is required twice, once before orthodontic treatment (the initial treatment plan) and once before surgery.

• Initial treatment plan

Before beginning orthodontic treatment, the initial treatment plan is completed. The primary goal of preliminary planning is to create an orthodontic plan. A preliminary surgical plan should be agreed upon by the orthodontist and surgeon. This plan is critical because it influences important orthodontic decisions such as dental extractions, dental compensation removal, and the creation of interdental spaces for osteotomies.

• Surgical treatment plan

Before surgery can be scheduled, the surgeon must determine whether the patient is ready.

This includes confirming that the presurgical orthodontic goals were met and that the patient's health was optimized to ensure the lowest possible surgical risk. Surgeons obtain progress-dental-models to ensure that the presurgical orthodontics goals have been met.

They hand-articulate the models in Class I occlusion to ensure proper occlusion. When the following conditions are met, good occlusion can be achieved:

• Dental benefits are no longer available.
• The teeth are properly aligned, resulting in a smooth arch.
• The upper and lower dental arches are the same shape and size.
• The adjacent marginal ridges have been leveled.
• Interproximal spaces have been closed.
• Spee's curve is flat or minimal.
• The posterior teeth's labiolingual inclination is normal.
• Normal incisal overjet and overbite
• Occlusal contacts are maximized because tooth size discrepancies (Bolton) have been addressed.

The patient is ready for surgery if good intercuspation is observed and the risks of surgery are acceptable. Because of the presence of an apical base deformity, good intercuspation is not always possible. 

The apical base is a section of the jawbone that is located around the apices of teeth and determines the position of the dental roots.  Maximum intercuspation cannot be achieved when the apical bases are deformed because dental roots should not be moved outside the bone.

For example, despite adequate presurgical orthodontics, when the maxillary apical base is narrow, the posterior teeth will end up in crossbite. In such cases, the maxilla must be segmented (divided into two or more tooth-bearing bone segments) in order to be expanded.

If good intercuspation is not possible due to an apical-base problem, the surgeon should segment the dental models to see if good occlusion is possible.  When dental models are cut into segments, each piece is manually articulated into occlusion before being reassembled and glued. If the surgeon confirms that the operation can be performed safely on the patient, he or she is deemed ready for surgery.

 

Modeling

During the modeling phase, a 3D virtual model of the craniofacial complex is created. This model should include:

1. Have a centric mandible, accurately render the skeleton, teeth, and facial soft-tissue, 
2. Have a correct frame of reference

CASS 3D virtual models should include a mandible in a centric relationship.

Centric relationship (CR) refers to the position of the condyles within the glenoid fossa.

It is an important reference position in orthognathic surgery because it is the only reproducible tooth-independent mandibular position. Furthermore, the condyles can rotate for about 20 degrees around an axis that passes near the center of both condyles in this position. 

Autorotation is the rotation of the mandible around the hinge axis.

• Planning

Surgery in CASS is planned using a VTO approach, which means that surgery is simulated until the desired final outcome is achieved. Surgical simulation is carried out on three-dimensional composite models with the aid of specialized software. These programs can do three basic things: cut and move bones, articulate teeth, and morph soft tissue.

• Cutting and moving bones

A computer operation that simulates an osteotomy is known as bone cutting. A simple plane or a three-dimensional array of adjacent planes can be selected as the cutting tool. 

Position, orientation, size, and thickness are all adjustable in both options. An operator makes a cut by first inserting the cutting tool into the planned osteotomy and then activating the cutting command. 

This operation divides an object into two new objects that can be differentiated by recoloring or renaming. When bones move, they undergo two types of transformations: translation and rotation. 

• Translation refers to movement without rotation (sliding)
• Rotation refers to turning around a point.

Both types of transformations are required during planning. 

Translation can be performed in the direction of the coordinate system axes, whereas rotation can be performed around any pivot point.  The software allows the user to select the center of rotation.

• Dental articulating

Traditional planning involves hand-articulating stone dental models to determine final occlusion. This maneuver is quick and dependable, early contacts are easily identified, making occlusal adjustments easier. However, establishing final occlusion digitally is difficult. 

Upper and lower digital dental models are overlapping images. Furthermore, there is no tactile sensation in CASS, nor are there real-time collision constraints. Because of these factors, putting two dental models into occlusion takes time. Final occlusion is first established on stone models in the current CASS routine.

After that, the models are scanned in final occlusion to create a digital-final-occlusion-template. This template is a computer-generated object that depicts upper and lower teeth in their final occlusion.

It is divided into two sections: 

1. Top (upper teeth) 
2. Bottom (lower teeth). 

Once created, the template is imported into the CASS software and used to align the jaws of the composite model into final occlusion. The alignment is a two-step procedure. The template is first aligned with one of the jaws. The other jaw is then aligned to the template. 

The upper and lower teeth are in final occlusion, as in the template; aligning one part of the template to one jaw and then the opposite jaw to the template automatically places the jaws in final occlusion.

• Soft-tissue morphing

Current software packages can simulate soft-tissue changes caused by the movement of osseous or dento-osseous segments, and they use various strategies to do so. The simulation methods must be precise and quick. 

However, achieving both is challenging because these characteristics are inversely related; the more accurate the model, the longer it takes to prepare and run. The facial soft-tissue envelope is a heterogeneous structure made up of different types of tissue, each with its own mechanical properties: skin, fat, connective tissue, muscle, and mucosa. 

Furthermore, the properties are complicated because they are non-linear and anisotropic.

• Planning algorithms

Orthognathic surgery is used to correct deformities in one or both jaws. A single-jaw operation is easier to plan than a double-jaw operation. The sections that follow present planning algorithms for single- and double-jaw surgery, starting with the simplest scenario and progressing to the most complex.

• Single-jaw maxillary surgery

The simplest surgery to plan in CASS is single-jaw maxillary surgery, which is performed when the maxilla is deformed but the mandible is normal.

The planner will make three decisions in this scenario: final occlusion, vertical maxillary position (i.e., the position of the upper dental midpoint), and an assessment to determine the need for complementary genioplasty.

• Single-jaw mandibular surgery

The next most difficult procedure is single-jaw mandibular surgery, which is performed when the mandible is deformed but the maxilla is normal.  Assuming it involves mandibular ramus osteotomies (sagittal, vertical, or inverted L osteotomies).

Four decisions must be made:

1. Final occlusion,
2. Right proximal segment alignment
3. Left proximal segment alignment
4. Final symmetry.

 

• Double-jaw surgery 

When both jaws are deformed or the gap between the jaws is so large that both jaws must be moved, even if one is normal, double-jaw surgery is required. A double-jaw operation is a complicated, multi-step process.

Planning without a strategy wastes time, leads to mistakes, and yields unsatisfactory results. The authors created a planning algorithm to assist surgeons in this process.

• Preparing for plan execution 

Planning is useless if it cannot be implemented during surgery. The ultimate goal is to achieve the same surgical outcome as planned. This is achieved in orthognathic surgery when the bone segments are precisely moved to their intended location.

Several procedures and appliances have been developed for this purpose, and they all necessitate preparation prior to surgery. Dentate and non-dentate movable bone segments can result from jaw osteotomies. The location of the osteotomies determines the type and number of segments produced.

In a genioplasty, for example, one movable non-dentate segment is created. A single dentate segment is produced in a standard LeFort I osteotomy. Three segments are created in mandibular ramus osteotomies: one distal and two proximal; the distal is dentate, but the proximals are not.

 

Conclusion

Jaw deformities are a common condition that can range from mild to severe defects that can be surgically corrected. In some cases, the upper or lower jaw, or both, may grow too slowly or too quickly, resulting in malocclusion, or improper tooth alignment in relation to the first molars.

Jaw deformities can be caused by genetic factors, trauma, and certain birth defects, in addition to growth differences between your upper and lower jaws.