Why Full Coverage Restorations Are Common for Second Molars

A discussion about crowns and full coverage restorations in a recent Spear Talk thread prompted our Study Club to discuss which tooth most commonly requires these types of restorations. Study Club members overwhelmingly answered second molars which led to the question — why?

How does excessive tooth wear or tooth fracture play into this discussion? To fully understand second molars and restorative dentistry, we must start by looking at occlusal force distribution and how our patients get here.

Why do teeth wear or fracture?

In most cases, tooth wear and tooth fracture result from mechanical overload at the tooth level because of uneven occlusal force distribution. Different patients will have varying risk factors for tooth wear or fracture based on how the teeth fit together and how much force they generate. For optimal occlusal management, the goal is to distribute the load evenly over as many teeth as possible in vertical and horizontal mandibular movements.

Why do second molars require full coverage restorations more than other teeth in the arch?

Based upon the discussion of why teeth fracture or wear, it is logical to infer that second molars have more force being applied compared to other teeth. So, the real question is the load distribution greater at the second molar compared to other teeth?

Surprisingly, for many patients, the answer relates to the condition of their temporomandibular joints (TMJ). When discussing force distribution at the tooth level, the force is generated by the contraction of the masticatory muscles. When the anatomy of the masticatory system is looked at, the muscles are positioned between the teeth and the TMJs. Optimal muscle force distribution at the tooth level results when as many teeth as possible make contact at the same time with the same intensity in vertical mandibular movements. Optimal force distribution at the joint level results when both joints are loaded equally with the disks in the normal position.

Force distribution at the joint level is usually not discussed when considering why second molars require full coverage restorations. Begin with evaluating the position of the disk to assess the effectiveness of TMJs dissipating the forces generated by the muscles.

While the posterior attachment of the disk is normally referenced at 12:00, a careful reading of the literature reveals normal disk position being defined as 12:00 +/- 10⁰ in 1990 and as 12:00 +/- 30⁰ in 1997.

The flaw in early MRI studies discussing normal disk position was assessing the posterior attachment instead of the intermediate zone of the disk. Normal disk position is determined by the location of the intermediate zone of the disk because it is the load-bearing portion of the disk. It is at approximately the 11:00 position in normal TMJs. An 11:00 normal disk position for the intermediate or load-bearing portion of the disk results in an approximate 1:00 position for the posterior attachment.

If the attachment of the disk to the condyle becomes compromised, the disk tends to displace/herniate anteriorly relative to the condyle. A mild displacement results in a 12:00 position of the posterior attachment of the disk. A moderate displacement results in an 11:00 disk position, and an advanced displacement results in a 10:00 disk position. Each of these displacements can change the condylar position, which will change how the teeth contact.

• A mild disk displacement (12:00) positions the thickened posterior band against the condyle. This can result in an increased vertical dimension at the joint level and is likely to cause a Class III bite shift.
• A moderate disk displacement (11:00) positions the disk anterior to the condyle. This tends to position the condyle posteriorly in the joint socket and can result in a decreased vertical dimension and is likely to cause a Class II bite shift.
• An advanced disk displacement (10:00) positions the disk anterior to the condyle. This tends to position the condyle superiorly in the joint socket and can result in a decreased vertical dimension at the joint level and is likely to cause a Class II bite shift.

From a clinical perspective, the frequency of 11:00 and 10:00 displaced disks are far greater than the frequency of 12:00 displaced disks. And as a result, disk displacements are likely to be a Class II bite shift. The Class II bite shift typically causes increased force being applied to the tooth closest to the jaw joints, which is the second molar. This results in an anterior open bite unless there is adaptation at the tooth level at the second molars.

The most common forms of tooth adaptation to increased forces at the second molar region is tooth wear or tooth fracture. This helps explain why lower second molars require full coverage restorations more often than other teeth in the arch.

From a treatment planning perspective, it would be helpful to consider a patient's TMJ condition before prepping second molars. If structural alterations at the TMJ level are suspect, it's easy to assess using MRI/CBCT imaging to understand the joint anatomy before beginning treatment.

Jim McKee, D.D.S., is a member of Spear Resident Faculty.

References

1. Drace, J. E., & Enzmann, D. R. (1990). Defining the normal temporomandibular joint: closed-, partially open-, and open-mouth MR imaging of asymptomatic subjects. Radiology, 177(1), 67-71.
2. Rammelsberg, P., Pospiech, P. R., Jäger, L., Duc, J. M. P., Böhm, A. O., & Gernet, W. (1997). Variability of disk position in asymptomatic volunteers and patients with internal derangements of the TMJ. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology, and Endodontology, 83(3), 393-399.
3. de Mello Provenzano, M., Chilvarquer, I., & Fenyo-Pereira, M. (2012). How should the articular disk position be analyzed? Journal of Oral and Maxillofacial Surgery, 70(7), 1534-1539.
4. Piper, M. (2020). Temporomandibular Joint Imaging. Handbook of Research on Clinical Applications of Computerized Occlusal Analysis in Dental Medicine. Vol. 2 Chapter 9. Hershey.