Linear Occlusion Concept – A Review

INTRODUCTION

Linear occlusion is an intriguing concept because of its simplicity and its remarkable success in practical usage. The eighth edition of The glossary of prosthodontic terms defines linear occlusion as “the occlusal arrangement of artificial teeth, as viewed in the horizontal plane, wherein the masticatory surfaces of the mandibular posterior artificial teeth have a straight, long, narrow occlusal form resembling that of a line, usually articulating with opposing monoplane teeth.¹” The concept has also been described as lineal occlusion or non interceptive occlusion.

Linear occlusion definitely provides a stabilizing effect on the denture bases during function. Literature has supported the use of linear occlusion to provide stability for complete denture prosthesis. By incorporating an extremely effective chewing mechanism with linear stability in the occlusion, an entirely new combination of prosthetic advantages can be achieved.²

Linear occlusion requires that there is no interference to mandibular movement during lateral and protrusive movements.³ As originally conceived this long straight, narrow blade was positioned over the crest of the mandibular residual ridge for mechanical stability during function although it functions well if used for maxillary arch as well depending on the posterior ridge relations.⁴

It is beneficial when treating combination syndrome where eliminating anterior tooth contact is paramount and this is made possible through the use of linear occlusion with a noninterceptive arrangement and bilateral fulcrum of protrusive stability.⁵ A more predictable treatment outcome is possible when treating the patients who require multiple fixed restorations in conjunction with a removable partial and complete denture, by incorporating linear occlusal concepts to prevent anterior hyperfunction.⁶

A recent study investigated clinical feasibility and technical features of immediate loading with linear occlusion on 2- implant-supported bar retained overdenture and evaluated the short term effect of the treatment. The results were predictable in some cases and achieved satisfaction in the short term service.⁷

The linear and anatomic occlusion complete dentures were compared in 15 edentulous patients with severe resorption after wearing for three months, linear occlusal denture was similar in masticatory efficiency to the anatomic occlusion denture, but there were statistically significant increased masticatory strokes and times before swallowing. The comfort and stability of linear occlusion denture were significantly  improved.⁸

The analysis and comparison of the distribution of high and low vertical stress bearing areas in the mandibular alveolar mucosa under linear occlusal complete dentures (LOCD) and anatomic occlusal complete dentures (AOCD) at lateral excursion showed larger medium vertical stress (less in intensity) bearing area under a mandibular LOCD and the high vertical stress (more in intensity) bearing area was smaller than that under an AOCD. Therefore, the vertical stress under the LOCD was distributed more evenly and over a wide area than that under the AOCD, thereby improving denture stability.⁹

CONCEPT OF LINEAR OCCLUSION

The concept of linear occlusion was introduced by Dr. John P. Frush in 1966. He described occlusion in geometric terms which describes the dimensional contact (one, two or three dimensional) between the occluding surfaces of any two opposing teeth.²

1) Cusp Occlusion: In this type of cusp occlusion, the contact between the opposing surfaces of any two teeth occurs in three dimensions. These dimensions are the width, the length and the depth of the occluding surface (Fig 1A).

2) Flat Plane Occlusion: In this type of flat plane occlusion, the contact between two opposing posterior teeth occurs in two dimensions. These dimensions are the width and the length of the occluding surface (Fig 1B).

3) Linear occlusion: In linear occlusion, a one dimensional contact occurs between two opposing posterior teeth, that is, the length of the contacting blade (not surface). This blade, being always in the form of a straight line, geometrically constitutes “length” in occlusal contact without either “width” or “depth” of occlusal contacts. Because of the geometric simplicity, one-dimensional occlusion has certain advantages over either two or three-dimensional occlusion of the occlusal contact. There is reduction in the amount of occluding surface and also accomplished by one-dimensional occlusion reduces the potential occlusal deflections which may occur with broader contacting surfaces. The one dimensional occlusion includes a crushing surface lingual to the linear contact. (Fig 1C).

The rational for the reduction of forces was explained diagrammatically by Gronas and Stout.³

In a cusp-type occlusion, the multidirectional force vectors are equalized (Fig 2A) but there is significant lateral force during eccentric movements which may result in instability of denture base (Fig 2B).

Zero degree teeth may establish a flat plane or monoplane occlusion.¹⁰ This nonanatomic (flat plane) occlusion eliminates most lateral components of force (Fig 3A). However, because of the width of the occlusal table the contacts shift on the occlusal table during mandibular movements, there is a change in the direction of forces applied to the denture base. The frictional resistances of buccal and lingual contacts that occur during this shift on the occlusal table and also change in the direction of forces probably produce undue leverage on the denture base resulting in denture base movement ³(Fig 3B).

A line of occlusal contacts in one dental arch opposing a flat occlusal table in the other dental arch has the potential of creating the smallest lateral component of force against the denture bases (Fig 4A). Since the area of contact is minimal, the frictional resistance is reduced. Also, in the dental arch with the line of occlusal contacts, there is no change in the location of the contact during lateral movements. Therefore, force in that dental arch remains fairly constant (Fig 4B).

The linear (non interceptive) occlusion has the following parameters.¹¹

• Zero degree teeth opposed by bladed teeth in which blade forms a straight line over the crest.
• Mandibular teeth set in flat occlusal plane.
• The arch which requires maximum stability receives bladed teeth.
• There is no anterior interference to protrusive and lateral movements.
• Non interceptive occlusion provides a vertical and seating force in both centric and eccentric movements.

The linear ridge of occlusal contacts may be located in maxillary or mandibular arch and this decision depends on the factors of denture stability and esthetics.³ Denture stability: Since the mandibular dentures are usually less stable than maxillary dentures, the line of occlusal contacts is usually placed on the lower denture. With the ridge of occlusal contacts located in the mandibular arch, occlusal forces in any jaw position will be applied to the mandibular dentures at the same point. The result will be increased stability for the mandibular denture. If for some reason, the maxillary denture needs more stabilization than is required by the mandibular denture, the line of occlusal contacts would be placed on the maxillary denture.

Denture esthetics: Non anatomic maxillary first premolars can usually be contoured to be anatomically and esthetically pleasing. However, where esthetics of the maxillary posterior teeth is a prime consideration, the line of occlusal contacts is developed using anatomic maxillary posterior teeth to occlude with nonanatomic mandibular teeth.

According to this concept there is no need of traditional interocclusal rest space of 2-3 mm and the centric is recorded at vertical dimension of rest.¹² Minimum clearance can be created by establishing a horizontal plane of occlusion. After processing of the maxillary prosthesis, the occlusal one third of the first premolar can be reduced at a 45-degree angle to form a ridge that acts as a point of posterior contact for the mandibular blades in a protrusive position of the mandible (bilateral fulcrum of protrusive stability).

Denture Techniques: The clinical and laboratory steps used for fabrication of denture with linear occlusion are essentially the same with certain modifications.

To reduce the influence of muscle activity a myostatic outline is used for the peripheral extent of prosthesis. Myostatic is a term defined by Frush to describe an area or location on the mandible that remains static or immobile regardless of muscle activity.^13,14

For linear occlusion there is a definite requirement, so any articulator with an incisal guide pin, a flat incisal guide plate, and an adjustable condylar inclination can be used. The Hanau articulator is ideal for this purpose.²

• The mandibular anterior teeth are arranged with as much horizontal overlap as necessary, but with zero vertical overlap between the anterior teeth. Vertical overlap of anterior teeth which usually occurs in conventional dentures causes contact of anterior teeth during protrusion resulting in rotational movement of prosthesis. Linear occlusion allows mesial third of premolar to provide an edge with which the opposing blade makes contact.¹⁵ Anterior contact is thus prevented and denture stability is enhanced.
• The lower posterior teeth are arranged first and centered over the crest of the residual ridge. The buccal line of contacts should be set in a straight line anteroposteriorly. The lingual part of the mandibular posterior teeth is positioned 0.5 mm below a plane contacting the right and left lines of contact.
• All lower teeth are arranged on a flat occlusal plane extending from the lower anteriors towards the top of the retromolar pads. The occlusal plane should be kept as high posteriorly as practical, usually the top one third of the retromolar pad to aid in developing protrusive balancing contacts with a flat plane of occlusion.
• The combined buccal blades of the lower posterior teeth should form a perfect straight blade to support onedimensional contact against the opposing occlusion.
• The maxillary posterior teeth are arranged with the occlusal surface level and with central fossa of the maxillary teeth against the blade of mandibular posterior teeth so that the line of contacts of the lower teeth is centered buccolingually.
• The crushing table lingual to the buccal blade on the lowers should be adjusted to create an inter-occlusal crushing space of approximately one millimeter.
• Cusp rise is removed from the upper occlusion before the dentures are placed in the patient's mouth. For this purpose a flat incisal guide plate must be used on the articulator.

Jameson⁴ has suggested the use of two maxillary and one mandibular record base for fabrication of denture with linear occlusion concept. One record base is used along with mandibular record base for centric records and the other record base is the esthetic control base (ECB) used solely for the purpose of evaluating certain esthetic criteria and arrangement of maxillary prosthetic teeth.

The prosthesis may be processed using injection moulding  technique to produce minimum pin opening.¹⁶ Occlusal equilibration should be done on articulator before placing the prosthesis.^3,17

OCCLUSAL FORMS USED AND OCCLUSAL ADJUSTMENTS FOR LINEAR OCCLUSION:

A) Nonanatomic maxillary porcelain teeth opposing mandibular porcelain linear teeth:³

Since this combination shows the least occlusal wear, it is recommended for young and healthy patients with good residual ridges. They are recommended for patients who have adequate interarch space for porcelain teeth because, porcelain teeth are difficult to grind. The main drawback of this porcelain-to-porcelain occlusion is that it exhibits occlusal disharmonies earlier than an occlusion formed with other materials, because the porcelain does not wear fast enough to keep up with changes that occur in the residual ridges after denture placement. Also, the increased clicking sound during mastication may be disturbing to the patient.

Occlusal adjustments: If the nonanatomic maxillary or the mandibular line of contacts becomes uneven during processing, No.180 grit silicone carbide abrasive can be used to correct the error. If the mandibular line of contacts is broadened by the abrasive paper, a porcelain stone is used to sharpen it. A rubber polishing wheel is used to polish the adjusted surfaces. The final intraoral occlusal adjustment is done on the nonanatomic maxillary teeth by using porcelain grinding wheel.³ Abrasive strips can also be used for the occlusal adjustment.¹⁸

B) Nonanatomic maxillary acrylic teeth opposing mandibular acrylic linear teeth:

The modified maxillary and mandibular posterior teeth are used in this combination. The maxillary posterior teeth are modified to a flat surface by rubbing them against abrasive paper on a flat surface. The mandibular teeth are modified by reducing marginal ridges and lingual cusps with an abrasive wheel so that the only occlusal contacts are made along the line of the buccal cusps.

This combination of posterior teeth is the easiest to fabricate and adjust as well. But, the main disadvantage of this combination of teeth is the susceptibility of the teeth to wear. As a result of this wear, the line of occlusal contacts becomes broader and the occlusion gradually changes from a linear type to a flat-plane occlusion. Therefore, it is contraindicated for patients having bruxism, who use abrasive diets and for young patients with good ridges. It is recommended for patients with severely resorbed residual ridges and poor muscle function.

Occlusal adjustments: This combination is most easily adjusted with a help of carborundum strips by reducing only the linear teeth.

C) Nonanatomic maxillary acrylic teeth opposing mandibular porcelain linear teeth:

When modified nonanatomic maxillary teeth opposing mandibular porcelain linear teeth are used in the denture, there is less wear of the teeth in this combination than all acrylic combination.^19,20 The linear occlusion is maintained for a longer period because, the porcelain teeth in occlusal contact will wear very little when compared to all acrylic teeth. It is also more self adjusting as residual ridge changes occur in comparison to all-porcelain posterior teeth.

Occlusal adjustments: No adjustments are done on the porcelain linear teeth because maintenance of the original glaze on these teeth will minimize wear of the opposing acrylic nonanatomic teeth. Interceptive contacts are marked in the mouth with articulating paper and adjustments are made on the maxillary acrylic teeth using a large diamond stone or an arbor band. The nonanatomic acrylic teeth are reduced evenly in order to keep the plane flat.

D) Anatomic maxillary porcelain teeth opposing non anatomic mandibular plastic teeth:

When esthetics is a prime consideration, the linear variation is preferable to a completely anatomic occlusion or to the nonanatomic combinations. In maxillary arch, 20 or 30 degree porcelain teeth may be used. Teeth are arranged in such a way that a line of contact is developed between the lingual cusps of maxillary porcelain teeth and the center of the nonanatomic mandibular acrylic teeth.

Occlusal adjustments: The occlusal corrections should be done on the nonanatomic mandibular acrylic teeth, because the glaze on the lingual cusps of the maxillary porcelain teeth should be maintained. The buccal cusps of the anatomic maxillary porcelain teeth should be out of contact by approximately 0.5 mm in all occluding positions.

SUMMARY

The rationale for using linear occlusion for maximizing mandibular denture stability has been discussed. Linear occlusion utilizes straight line of points or knife-edge contacts on artificial teeth in one arch occluding with flat non anatomic teeth in the opposing thereby reducing unfavorable occlusal forces and simplifying occlusal adjustment in complete dentures. By using linear occlusion concept and alternative tooth forms a functional and esthetically pleasing prosthesis can be fabricated. It has been acknowledged by many authors that the use of this concept provides exceptional stability to mandibular prosthesis and patient satisfaction due to lack of movement of denture base and fewer post insertion adjustments for irritated tissue. This occlusion concept can also be used successfully in treating combination syndrome and implant supported bar retained over denture.