Orthodontic Diagnosis and Treatment Planning
Master the INBDE with Dental Panda: Your go-to resource for expert practice exams and tailored study resources!
A malocclusion is anything other than what’s considered the “optimal occlusion”. Edward Angle attempted to treat his patients’ occlusion with reference to the occlusion of a skull displayed on a shelf in his surgery. This skull was affectionately called ‘Old Glory’. A malocclusion can have a dental component, skeletal component, or a combination of the two. A dentist needs to be able to make basic measurements and determine the source of the malocclusion in order to develop the best treatment plan.
The first step in orthodontic treatment is to accumulate all necessary information from:
- a verbal patient interview.
- a thorough medical and dental history, including medications that inhibit bone remodeling possibly affecting treatment.
- growth history/charting.
- a social and behavioral assessment. Will the patient comply with the proposed treatment? Any functional habits that could interfere? How well do they maintain proper oral hygiene?
- an oral and facial assessment.
- a cephalometric analysis to determine skeletal relationships.
ORTHODONTIC EXAM
This examination looks for any pathology (caries, periodontal disease, cancer etc.) or functional issues (any TMJ issues, speech, functional shifts, occlusal interference etc). Pay attention to both present and missing teeth, their size and arrangement, and arch shape. The anterior-posterior arrangement of the patient’s dentition is categorized according to the Angle classification system. Overbite and overjet is documented, and any crossbite noted. There are a few ways to predict future malocclusion:
- lack of spacing in primary arches (crowding of baby teeth).
- crowding of permanent incisors in mixed dentition stage.
- the premature loss of lower deciduous canines.
The Cephalocaudal gradient of growth follows a trend from the brain down: the further away from the brain, the later and longer it grows (in general). A fetus’ head takes up 50% of the total body length. By birth, it’s 30%, in adults about 12%. Different parts of the body grow and accelerate at different periods (See Scammon’s growth curve). There are generalized differences for the sexes, and great individual variation. Predicting growth spurts for certain areas (e.g. maxilla, mandible) is advantageous for orthodontists.
Mandibular growth usually follows the pubertal growth spurt. Because of the individual variation, chronological age is not a good predictor of mandibular growth. Neither is dental age (i.e. the eruption of teeth). Mandibular growth generally does, however, correspond to the patient’s skeletal age/development.
Wrist and hand radiographs
Orthodontic treatment is partially determined by skeletal growth. Radiographs of certain parts of the skeleton can be used to assess growth and development, and predict when certain phases of growth will start or stop. Physiological/developmental age can be judged by looking at the development and ossification of the carpal and metacarpal bones and the phalanges of the fingers.
Clinical facial assessment
A clinician would note any vertical and left-right asymmetry, knowing that certain proportions are considered to be the most aesthetically pleasing. With the teeth together at rest the lips should passively rest against each other. This is proper lip posture/lip competence. A gap of more than 3 or 4 mm indicates lip incompetence. When the jaw is at rest, there should be 2-4mm of incisor teeth visible beyond lip margin. 1-2mm of gingiva should be visible during a smile, any more is considered a “gummy smile”.
The Rickett’s Esthetic Line (E-line) extends from the tip of the nose to the chin. The lips should sit just behind this line. The position of the incisors will affect the lip position, either full protrusive, normal, or flat/retrusive. The nasolabial angle, made by connecting the inferior edge of the nose and upper lip, should be close to or just over 90 degrees. An acute nasolabial angle could indicate full/protrusive lips, and obtuse angle usually long/flat lips. The anteroposterior positions of the jaws, vertical facial proportions and inclination of the mandibular plane angle can also be gauged. The patient’s face can be viewed side on to determine the outline, either convex, straight, or concave. This may be reflective of a Class II, Class I, and Class III profile respectively.
MIXED DENTITION ANALYSIS
Arch length is measured from a point midway between the central incisors to the distal surface of the deciduous second molar/mesial surface of the adult first molar. Arch circumference/perimeter is measured across the midline, from the distal surface of the second deciduous molar/mesial surface of the adult first molar, around the arch to the same mark on the other side. There is a little more room in the maxilla (arch circumference roughly 128mm) than the mandible (roughly 126mm).
Measurements can be made during the mixed dentition phase to predict whether there will be enough space to accommodate the developing teeth. There is a size correlation between posterior teeth and anterior teeth. For the assessment you need a prediction table, study models and a Boley gauge. This analysis is most important in patients aged 7-10.
Moyer’s mixed dentition analysis is used in dentistry to predict the size of the permanent teeth by measuring the combined mesio-distal widths of the four adult mandibular incisors. You can use a Moyer’s mixed dentition analysis to make an assessment of the lower anterior region. First measure the mesiodistal diameter of the lower incisors. This is “A”. Then measure the space available in the arch of the lower incisors (arch circumference). This is “B”. If B minus A is a negative number, there will be crowding. If B minus A is zero or is a positive number, there will be enough space for teeth.
To make an assessment of the mandibular posterior regions you would need a prediction table. First measure the space available for canines and premolars on each side. Then use the measurement of the incisors (“A”) to predict how much space will be required. Again, after subtracting the two numbers, a negative number indicates future crowding, and a positive number indicates adequate space.
You can make similar predictions for the maxillary posterior regions using prediction tables, but you always use the mesiodistal widths of the mandibular incisors. The Moyers’ mixed dentition analysis uses a prediction table/probability chart. The Tanaka and Johnston prediction formula calculates the same end result, but uses half of the mesiodistal widths of the four lower incisors, and adds 10.5mm to estimate the width of the mandibular canine and premolars in one quadrant, and adds 11.0mm to estimate the width of the maxillary canine and premolars in one quadrant.
CEPHALOMETRIC ANALYSIS
Cephalometry is a method of making measurements from radiographs. The linear and angular measurements are made and compared to “normal limits” for proper diagnosis and treatment planning. They describe the relationships of tooth to tooth, tooth to bone, and bone to bone. You will need to be able to identify some of the anatomical landmarks and angle measurements, and be able to compare them with established normal limits, to make a determination about the diagnosis and treatment. A series of films can show growth and development. Soft tissue points can be used as well (nose, lips, chin).
Landmarks for Cephalometric tracing
- Ba – Basion: The most forward and highest point of the anterior margin of foramen magnum.
- Bo – Bolton: The highest point in the concavity behind the occipital condyle.
- Ar – Articulare: The point of intersection of the contour of the posterior cranial base and the posterior contour of the condylar process.
- S – Sella: The midpoint of sella turcica.
- Po – Porion: Outer superior margin of the external auditory canal.
- SO– Spheno-occipital synchondrosis: Junction between the occipital and basisphenoid bones.
- Ptm – Pterygomaxillary fissure: Point at base of fissure where anterior and posterior walls meet.
- Or – Orbitale: Lowest point on the inferior margin of the orbit.
- ANS – Anterior nasal spine: Tip of anterior nasal spine.
- PNS – Posterior nasal spine: The tip of the posterior spine of the palatine bone.
- Point A – Subspinale: Innermost point on contour of the premaxilla between the anterior nasal spine and the incisor tooth.
- Point B – Supramentale: Innermost point on contour of the mandible between incisor tooth and the bony chin.
- Me – Menton: Most inferior point on the mandibular symphysis, the button of the chin.
- Go – Gonion: Lowest most posterior point on the mandible with the teeth in occlusion.
- Pog – Pogonion: Most anterior point of the contour of chin.
- N – Nasion: anterior point of the junction between the nasal and frontal bones.
Cephalometric plane
These are straight lines between two points. There are few you need to know:
- Palatal plane (ANS-PNS).
- S-N plane – represents the anterior cranial base which is often used as a frame of reference for growth because of the early cessation of growth.
- Frankfort horizontal plane/FH plane (P-Or) – habitual horizontal postural position. Often understood as being parallel to the floor. This is regularly used as a reference.
- Functional occlusal plane (FOP) – formed by drawing a line that touches the posterior premolars and molars.
- Mandibular plane (Go-Gn or Go-Me) – connecting the point gonion to gnathion at the inferior border of the mandible.
Angular measurements
The angle measurements are established by using 3 landmarks. For example, SNA is taken by drawing a line from Sella (S), to Nasion (N), to Point A (subspinale), and measuring the angle. Significant variation exists between different races. There are many angular measurements used in orthodontics, but these are likely the ones you need to understand (including what values are normal and what variation will mean clinically).
- SNA – normal 82° ± 2. >84° indicates a prognathic maxilla. <80° indicates a retrognathic maxilla.
- SNB – normal 80° ± 2. > 82° indicates a prognathic mandible. <78° indicates a retrognathic mandible.
- ANB – normal 2° ± 2. This would suggest Class I skeletal relationship. >4° could mean either a retrognathic mandible or prognathic maxilla, and suggests a Class II malocclusion. <0° could either mean a retrognathic maxilla or prognathic mandible, and suggests a Class III malocclusion.
- Mandibular plane angle – Gonion to menton is the mandibular plane. It can be visualized externally by placing a straight object on the inferior border of the mandible. When compared to S-N (MP-SN), it gives you the mandibular plane angle. Normal is 14°. >14° suggests a vertical growth pattern with a long lower face and a tendency to Class II. <14° suggests a flat growth pattern with a short face and a tendency towards Class III.
- Maxillary Mandibular Plane Angle (MMPA) – the angle between the mandibular plane and the maxillary plane (ANS to PNS line). Normal value 27° ± 4. The greater the MMPA, the longer the anterior facial height and tendency to Class II. The smaller the MMPA, the shorter the facial height and tendency to Class III.
- Y-axis – S-N to S-Gn: a larger angle indicates more vertical development, long face and anterior open bite tendency.
- 1/-FH – Upper incisor angulation in relation to the Frankfort horizontal plane, usually 60°. A smaller angle indicates flared incisors.
- /1-MP – Lower incisor angulation in relation to the cranial base, usually 90-100°. A large angle indicates flared incisors.
- 1/-/1 – Interincisal angle, average value 130°.
