Are Marginal Ridges Marginalised in your Restorations?

Marginal ridges in restorations: quick clinical answer

Marginal ridges help maintain proximal form, contact relationships, occlusal continuity and load distribution in posterior teeth. During restorative treatment, ridge height and contour should be rebuilt in harmony with the adjacent tooth, contact area, embrasures and occlusion. Overcontouring, undercontouring or an uneven ridge may encourage food impaction, plaque retention or adverse occlusal contacts.

Everything about marginal ridges and their importance in restorative dentistry.

What are Marginal Ridges?

A marginal ridge is -

1. An elevation of enamel that forms the proximal boundaries of the occlusal surface of premolars and molars.
2. An elevation on the mesial and distal portions of the lingual surface and, occasionally, the labial surface of incisors.

Clear and well defined marginal ridges find an importance place in the success of minimally invasive dentistry.

Minimally invasive dentistry is the application of systematic respect for the original tissue. From an endodontic point of view, minimal invasive endodontics (MIE) ranges from diagnosis to decision making, minimally but purposefully crafted access opening based on anatomical challenges, to minimal removal of dentin during access opening, enlarging and shaping of the root canal to retain as much sound dentin as possible, to retention of tooth structure during dissembly and retreatment or considering apical surgical intervention, to performing a crown lengthening procedure to establish sound tooth margins for core/crown restorations as opposed to tooth extraction and implant or bridge placement.

Why is Marginal Ridge Important?

The thickness of enamel at the marginal ridge is high and loss of tooth structure in this area weakens the tooth structure. Both clinical and experimental studies have highlighted the importance of preserving the marginal ridge.

Their essential feature is to prevent food lodgement which causes damage to the periodontium. Don’t forget that your floss slides between the marginal ridges of your posterior teeth.

As the age advances the dimensions of the marginal ridges and occlusal embrasures are reduced due to vertical occlusion attrition and proximal flattening of the contact areas, and this further compromises the tooth structure.

Ideal Marginal Ridge in Restorations

It is imperative to have a marginal ridge of proper dimension, compatible to the occlusal anatomy, creating a pronounced adjacent triangular fossae and occlusal embrasure. In restorations, marginal ridge should always be formed in two planes bucco-lingually meeting at a very obtuse angle. This feature is essential when an opposing functional cusp occludes with the marginal ridge as it prevents food lodgement which causes damage to the periodontium.

There are two forces acting on two marginal ridges, force 1 and 2 which have their horizontal components, 1H and 2H which drive the two teeth towards each other, thus preventing any proximal impaction of food. The vertical forces 1V and 2V which are acting vertically are resolved normally by the underlying tissue . The marginal ridge portion of the restoration should be compatible with the adjacent marginal ridge, both the ridges should be approximating at the same level and display correct occlusal embrasure for the passage of the food to facial and lingual surface.

Marginal Ridges in Restorations - Different Situations

• Absence of marginal ridge in the restoration: The force 1 will be directed towards the proximal ridge of the adjacent tooth and force 2 is directed on to the same tooth, so the horizontal forces 1H and 2H will act on the same tooth, which will tend to drive the two teethaway from each other. The vertical component 1V and 2V can impact the food intraorally .

• A marginal ridge with an exaggerated occlusal embrasure: Forces 1 and 2 directed toward the adjacent proximal surfaces, with the horizontal components, 1 H and 2H, will separate the teeth with the vertical components, 1V and 2V driving interproximally.

• Adjacent marginal ridge not compatible with height: Restoring the marginal ridge higher than the adjacent one will allow force 1 (A) on the proximal surface of the restoration, the horizontal component 1H (AH) will drive the restored tooth away from the contactingtooth and the vertical component will push debris interproximally even in the presence of force 2 (B) with its horizontal component (BH) acting on the adjacent marginal ridge there is some separation of the teeth as force 2(B) is too small as compared to force 1 (A). By restoring the marginal ridge lower than the adjacent one we can expect the same movement of the tooth but the major movement will be non restored tooth.

• Marginal ridge with no triangular fossae: In this situation there are no occlusal planes in the marginal ridges, so there are no occlusal forces acting 1 and 2, so there are no horizontal component 1H and 2H to drive the tooth toward each other and vertical force1V and 2V will impact the food interproximally.

• Marginal ridge with no occlusal embrasure: Two adjacent marginal ridges will act like a pair of tweezers grasping food substance passing over it. Although debris may not be forced interproximally, it will be very difficult to remove once it is thus trapped.

• Single or one- planed marginal ridge in bucco-lingual direction: This can create a premature contact during both function and static occlusion, one plane marginal ridge increases the depth of adjacent triangular fossa by increasing the stress in this area and also increases the height of the marginal ridge at the centre, and this will deflect the food away from the spillway.

• Thin marginal ridge in mesio-distal bulk: Will be susceptible to fracture or deformation leading to the problems of previously mentioned faulty marginal ridges (discussed in point 1, 2, 3 and 4).

• Marginal ridges not compatible in dimension or location with the rest of the occluding surface components predisposes to similar problems. As the marginal ridge is frequently a part of occluding anatomy, it should have the same occlusal pattern that dictates the shapes, locations, and inter-relationships of the rest of the occluding components.. Studies have shown that regardless of loading directions and location, the marginal ridges and proximal contact area were sites of low strain.

Marginal Ridges & Force Dissipation

Marginal ridges of the posterior tooth are considered to be of primary importance in providing structural strength to crown. In posterior teeth, the occlusal morphology is reproduced in a fashion that during mandibular closure, there are even simultaneous contacts of maximum possible teeth achieving (a) maximum stability of the teeth (b) minimal amount of force placed on each tooth in function. The contact pattern of the teeth should be such that it directs the occlusal forces favorably for their adequate dissipation. When the occlusal morphology is such that the cusp tip is contacted on a flat surface, i.e. bottom of a fossa or crest of the marginal ridge, the resultant occlusal force gets directed vertically along the long axis of the tooth. The fibres of periodontal ligament are arranged in a manner that such forces are well accepted and dissipated .