|Occlusion lec 5
Dr Suzan Hattar
A relationship between lower and upper teeth of a patient in eccentric movements (protrusive and lateral movements) and it is different amongst patients.
It is a dynamic (during movement) relationship between the lower anterior teeth against the upper anterior teeth through all ranges of function.
It is determined not only by the position of the anterior teeth, but also by the contour of the palatal surfaces of the upper teeth and inter and intra arch relationship, i.e. since the incisal edges of lower anterior teeth lie on the palatal surfaces of the uppers, then the morphology of the palatal surfaces also plays a role.
The anterior guidance is linked to vertical and horizontal overlaps of anterior teeth.
The envelope of motion
The path that the condyles travel which dictates the outer limit to which the mandible moves, this is known as the envelope of motion.
Outer limit: it is the maximum movement the person can perform i.e. maximum opening, maximum protrusion. It is determined by the envelope of motion and by the presence of muscles, bones and ligaments.
The upper superior posterior part of the envelope of motion is considered as the anterior guidance.
The envelope of function
The three dimensional space contained within the envelope of motion that defines mandibular movement during masticatory function, including phonation.
The path that the front end of the mandible travels is dictated by the functional movement of the muscles as it relates the lower teeth to the upper teeth in chewing cycle. The outer limit of these functional movements is called the envelope of function.
The envelope of function is smaller than the envelope of motion i.e. you do not open your mouth to the maximum during mastication or talking.
Nonexistent anterior guidance
The guidance is different or even nonexistent in the following situations:
During large horizontal overlap (over jet).
It is when the patient is performing a protrusive movement and he still does not have a contact between upper and lower teeth; meaning that the patient does not have an anterior guidance.
Edge to edge incisal relation; since it represents the end of the anterior guidance.
The anterior guidance starts from the maximum intercuspation, sliding on the palatal surfaces of upper anterior teeth until it achieves an edge to edge contact; if the patient already has an edge to edge contact when he bites, then he does not have an anterior guidance (when you ask him to protrude, no teeth will be in contact with each other anteriorly).
Protrusive incisal path
The track of the incisal edges of the mandibular teeth from maximum intercuspation to edge to edge occlusion is termed protrusive incisal path.
During protrusive movement of the mandible, the incisal edges of the mandibular teeth move forward downward along the lingual concavities of the maxillary teeth. Their movement is like the condyler movement during protrusion; the condyles move downward forward during protrusion.
Protrusive incisal path inclination
The angle formed by the protrusive incisal path and the horizontal plane, it ranges between 50 and 70 degrees.
The relationship between anterior and posterior guidance
Condyler paths do not dictate the anterior guidance; meaning that there is no equation that relates the anterior and posterior guidance to each other.
While conventionally regarded as independent factors, in healthy occlusion it was found that the anterior guidance is approximately 5-10˚ steeper than the condyler guidance.
They are considered independent because one of them is related to the condyles and bones (posterior guidance) and the other is related to the teeth (anterior guidance).
The anterior guidance -most often- is a mirror image of the posterior guidance; in the anterior guidance, the teeth move forward downward and in the posterior guidance the teeth move downward forward and this is in order to maintain a disocclusion on posterior teeth. I.e. if both guidances move parallel to each other (e.g. they both move forward downward), there is a possibility to have contact between posterior teeth.
The anterior and the posterior guidance have a complementary relationship.
The role of the anterior guidance
Next to the centric relation, the anterior guidance is the most important determinant in representing occlusion, meaning that if we knew the centric relation of a patient and examined the anterior guidance and the shape of the teeth then we nearly finished the job, working on the posterior teeth is much easier.
The anterior guidance has a role in:
Separation of posterior teeth during mandibular movement.I.e.no interferences between posterior teeth during protrusion and lateral movements of the mandible.
Posterior teeth are not designed to withstand lateral forces; some problems may arise due to these forces.
Esthetics; it is provided by the form and position of anterior teeth and it is related to the labial surfaces. (The functional part of the anterior guidance is related to the palatal surfaces of anterior teeth).
Determining the envelope of function.
Protection of posterior teeth during eccentric mandibular movements
This is the most important role of the anterior guidance, and the posterior teeth, in turn protect the anterior teeth during maximum intercuspation.
The impact of anterior guidance on occlusal morphology
The occlusal contours of all posterior teeth and their morphology are dictated by the anterior guidance as well as by the condyler guidance.
The anterior guidance is the principle determinant of posterior occlusal form.
Effect of Overbite
The greater the vertical overlap (overbite) between anterior teeth, the higher the posterior cusps can be.
The flatter the anterior guidance is, the lower the posterior cusps can be.
When the anterior guidance is flat then the posterior cusps must be flatter and shorter, otherwise there will be posterior interferences during protrusive movement.
Effect of over jet
The greater the horizontal overlap (over jet), the shorter the cusp height should be.
The lesser the horizontal overlap, the higher the posterior cusps should be.
The occlusal plane and the curve of spee also play a role; the steeper the anterior guidance the steeper the curve of spee can be.
Factors influencing the anterior guidance
Position of mandibular incisors
Level of incisal edge.
Axial angle in relation to the lower border of the mandible and to the occlusal plane. (It is the angle of the lower incisors either with the lower border of the mandible or with the occlusal plane).
Position of maxillary incisors
Over jet (horizontal plane).
Overbite (vertical plane).
Envelope of function; depends on the palatal surfaces of maxillary anterior teeth as well as the angle between the superior and the inferior incisors.
Requirements of a harmonized/ideal anterior guidance
When the patient has a missing anterior tooth, whether upper, lower or both or the patient is edentulous and the dentist is asked to construct him a prosthesis, the dentist must first establish an ideal anterior guidance.
One of the requirements of an ideal occlusion is to have no contact (interferences) on posterior teeth during eccentric movements, because any interferences on posterior teeth will transmit forces that are oblique to the surfaces of the teeth (these forces are not transmitted along the long axis of the teeth), and these forces -most of time- may result in some problems in the muscles, TMJ or cause wearing of the teeth.
In maximum intercuspation:
When closing into terminal hinge position of the mandible until the front teeth contact, if all lower anterior teeth contact simultaneously against stable centric stops at the correct vertical dimension, the first requirement of the good anterior relationship has been fulfilled.
Explanation: in maximum intercuspation, if all lower anterior teeth hit stop contact the uppers simultaneously at the same time of the same intensity, at stable posterior contact, at the proper vertical dimension, the first requirement of the good anterior guidance is fulfilled.
There should be an equal, balanced bilateral contact between all posterior teeth; this is a harmonized maximum intercuspation.
In protrusive movement:
There should be no contact posteriorly.
There should be a balanced contact between the upper six anterior teeth and the lower six anterior teeth and this contact must be equal in distribution.
By balanced we mean that there should not be for example, three contacts on the right side and one contact on the left. The load should be distributed between the teeth.
Example: if you ask the patient to perform a protrusive movement, and he has only one contact on the lower central incisor, then this is an overload on this tooth and this is not considered a balanced contact.
Establish coordinated centric relation stops on all anterior teeth.
Establish a group function in straight protrusion.
Establish a group function or establish a group contact on anterior teeth in protrusion.
Establish an ideal lateral function (preferably a canine guidance).
Having maximum comfort and stability; the patient does not have any problem.
Centric relation contact occurring simultaneously and with equal intensity (maximum intercuspation contact).
Position and contour of anterior teeth in harmony with the envelope of motion.
Immediate disocclusion of all posterior teeth once the mandible leaves the centric relation (lateral and protrusive movements on both sides).
Maximum comfort and stability may be achieved when:
The position and contour of all anterior teeth are in harmony with the lip contour and the neutral zone; meaning that the position of the anterior teeth is in harmony with the lips and tongue and this is added to the requirements of a harmonized anterior guidance.
Question: When the doctor says immediate disocclusion of all posterior teeth on the right and left sides in lateral movements, what is the name of this relationship?
Answer: Canine guidance (where the canines are the only teeth that are in contact).
Most of us will have a contact on the canines when performing a lateral movement, but some people may have a contact on other teeth on the same side of the canine (they are helping the canine in the contact), they are named a group function. In the case of a group function, it is preferable to have no contact on the nonworking side (the opposite side) because these contacts are interferences.
The benefit of posterior disocclusion
It has been found that posterior disocclusion during protrusive and lateral excursion reduces the muscle contraction in two of three elevators (masseter and medial ptergoid).
Once you have a posterior interference, it has a bad effect on the teeth and on the neuromuscular system (causes hyperactivity in the muscles leading eventually to problems).
How can anterior teeth reduce stresses transmitted on them?
The position of the anterior teeth in relation to the fulcrum and the muscular position (they are located far away from the muscles), so if there is hyperactivity in the masseter for example, the posterior teeth will be more affected.
Anterior teeth, especially canine have a strong proprioception; once the load is high, they send signals to the muscles to relieve the load, and thus the teeth can balance themselves.
The canine can withstand protrusive and lateral loads; this is due to its location at the corner of the mouth, having a long, well anchored root and having the strongest proprioception among all anterior teeth, so it can stimulate reflexes in the muscles to reduce the tone once the load is high.
Bilaterally balanced occlusion.
It is a prosthodontic concept which dictates that a maximum number of teeth should contact in all excursive positions of the mandible, i.e. when the patient moves his mandible to the right, the working side and the non-working side are contacting.
It is only applied for edentulous patients when constructing a removable complete denture (not fixed), so if we want to construct crowns and bridges for dentate patient, this scheme must not be applied.
Extreme stresses can be transmitted on the teeth causing teeth wear and neuromuscular disturbances if this scheme is applied to natural dentition.
Unilaterally balanced occlusion (group function).
When the patient performs a lateral movement, all teeth on the working side will be in contact, the teeth on the non-working side are contoured to be free of contact.
In protrusive movement, there will be no contact posteriorly.
Mutually protected occlusion (canine guidance).
When the patient moves his mandible laterally, the canines on the working side only will be in contact with each other. No contact will be present on the non-working side.
In protrusive movement, the contact will be on the anterior guidance.
It is named mutually protected occlusion because the anterior teeth provide protection to the posterior teeth during movement.