Lines Planes And Curves In Prosthodontics Notes

Lines Planes And Curves In Prosthodontics Introduction

Some lines and planes are used as references and guides for various prosthodontic procedures. This chapter describes various lines, planes, and curves related to the prosthodontic specialty.

Lines In Prosthodontics

These guidelines are used as a reference during the fabrication of occlusal rims on maxillary and mandibular casts.

Maxillary Guidelines:

M line: Midline between the center of the incisive papilla and fovea palatine. It divides the cast into right and left halves.

R line: Crest line from distal of canine to the crest of maxillary tuberosity.

B line: Line between right and left second premolars.

Read and Learn  More: Preclinical Prosthodontics Notes

C line: Line between distal of canine and point X.

Mandibular Guidelines:

M line: Midline between labial frenum and midpoint between both retromolar pads.

R line: Line between the distal part of the canine and the anterior part of the retromolar pad.

B line: Line joining the right and left second premolars.

C line: Line between distal of canine and point X.

Planes In Prosthodontics

Camper’s Plane (Ala-tragus Plane/Bro mell’s Plane):

A plane established by the inferior border of the ala of the nose and the superior border of the tragus of both ears. This is also known as the ala-tragus plane or line. This plane is used to establish the occlusal plane during jaw relation.

Occlusal Plane:

The occlusal plane is one where maxillary and mandibular teeth occlude with each other. In the instance of occlusal rims plane of occlusal is established at the level of the interface when maxillary and mandibular rims are in contact with each other (the plane between maxillary and mandibular occlusal rims).

Plane of Articulator:

For mean value and articulator plane of the articulator are established by mid mid-incisal pin and posterior reference bar/rod.

Curves In Prosthodontics

Various imaginary curves exist in natural dentition. These curves are due to the anteroposterior and lateral inclination of individual teeth. During teeth arrangement, these curves should be reproduced by aligning the artificial teeth.

When posterior teeth are in occlusion, maxillary and mandibular anterior teeth. When protrusive movement is performed (bringing the mandible forward), mandibular anterior teeth come in contact with maxillary anterior teeth, resulting in disocclusion of posterior teeth. This is called a “mutually protected occlusion”. This is also known as Christensen’s phenomenon.

Note: Perform yourself: To understand Christensen’s phenomenon, close your teeth and note that only posterior teeth are in occlusion and there is a gap between anterior teeth. Now bring your mandible forward till mandible anterior teeth come in contact with maxillary anterior. Note the gap between mandibular and maxillary posteriors.

In natural dentition when the mandible is protruded, maxillary and mandibular anterior teeth come in contact with each other and there will be disocclusion (space) between the upper and lower posterior teeth. This space is triangular in shape or wedge-shaped with the apex directed toward the anterior/canine teeth.

If the same occlusion is provided in complete dentures results in instability of the denture, i.e. maxillary denture will be displaced downward in the posterior region and the mandibular denture will be displaced upward in the posterior region.

This can be prevented; if artificial teeth are arranged in such a manner that the contact between opposite posterior teeth is maintained even if the patient moves the mandible in front (protrusion), or left side or right side (left and right lateral movements).

In any position or movement, there is constant contact maintained between anterior teeth and posterior teeth. This concept of maintaining bilateral, simultaneous, anterior, and posterior occlusal contact of teeth in centric and eccentric positions is called “balanced occlusion”. Balanced occlusion gives stability to the complete denture.

Curves In Natural Dentition

Anteroposterior Curve:

The curve of Spee:

This curve was described by German anatomist Ferdinand Graf Spee in 1890.

It is defined as the anatomic curve established by the occlusal alignment of the teeth, as projected onto the mesial plane, beginning with the cusp tip of the mandibular canine and following the buccal cusp tips of the premolar and molar teeth, continuing through the anterior border of the mandibular ramus, ending with the anterior-most portion of the mandibular condyle.

Mediolateral Curves:

The curve of Wilson:

It is a mediolateral curve that contacts the buccal and lingual cusp tips on each side of the arch. It results from the lingual inclination of a mandibular molar, making the lingual cusps lower than the

buccal cusps. Accordingly, the buccal cusps of maxillary molars are placed higher than the palatal cusps resulting in buccal inclination of maxillary molar teeth. This arrangement results in a concave curve for the maxillary arch and a convex curvature for the mandibular arch.

The curve of Monson:

It is defined as the curve of occlusion in which each cusp and incisal edge touches or conforms to a segment of a sphere of 8 inches in diameter with its center in the region of the glabella. This curve is based on the spherical theory of occlusion proposed by Monson.

Compensating Curve:

Compensating curves are artificial counterparts of natural curves. They are established during the arrangement of complete denture teeth.

Definition:

It is defined as the anteroposterior curvature (in the median plane) and the mediolateral curvature in the frontal plane in alignment with the occluding surfaces and incisal edges of the artificial teeth that are used to develop balanced occlusion.

They are called compensating curves because they compensate for the space formed between the maxillary and mandibular posterior teeth during protruding and lateral mandibular movements. These curves compensate for Christensen’s phenomenon occurring in natural dentition so that maxillary and mandibular posterior teeth occlude with each other and there will not be any space between them during any eccentric movement of the mandible. They hence help in achieving balanced occlusion in complete dentures and provide stability to complete dentures.

Curves in Artificial Dentition:

• Anteroposterior compensating curve: Compensating curve for curve of Spee.
• Mediolateral compensating curves:
  • Compensating curve for the curve of Wilson
  • Compensating curve for the curve of Monson
• Pleasure curve
• Reverse curve.

Compensating Curve for Curve of Spee:

This compensating curve can be established at the distal marginal ridge of the first posterior replacement tooth and continue through the second molar. It is incorporated into artificial occlusion by raising a distal portion of the first and second molars to conform to the curve. This is usually done to maintain posterior balancing contact and to compensate for the spacing that occurs between maxillary and mandibular posterior teeth during protrusive movements of the mandible.

Mediolateral Compensating Curves:

• The compensating curve for the curve of Wilson: This compensating curve can be achieved by tilting maxillary molars such that the buccal cusps are higher than the palatal cusps and mandibular molars are tilted lingually to make lingual cusps lower than buccal cusps. This compensating curve is important for the lateral balance of occlusion.
• Compensating curve for the curve of Monson: This curve runs across the buccal and palatal cusps of maxillary molars. This is important to achieve a lateral balance of occlusion.

Note: The Wilson curve runs opposite to the direction of the Monson curve and is hence called an “anti-Monson curve”

Pleasure Curve:

It is defined as the helicoidal curve of occlusion that when viewed in the frontal plane conforms to a curve that is convex superiorly except for the last molars which reverse the pattern.

This was proposed by Maximum Pleasure to provide balanced occlusion and better stability. Maximum Pleasure proposed to modify the mandibular teeth’ occlusal surfaces to a reverse curve by tilting the mandibular teeth buccally. He proposed a reverse curve in premolars, the flat occlusal surface on the first molars, and a Monson curve on the second molar for balance.

Reverse Curve:

This curve is not a compensating curve but is established in artificial teeth arrangement. It is established in for maxillary first premolar by raising the palatal cusp higher than the buccal cusp (reverse than usual arrangement pattern, i.e. buccal cusp higher than palatal).