Key Topics in Restorative Dentistry

Occlusion is one of those funny things, you either obey every rule to the nth level, you are sensible with how you manage it or you are just a rogue and fly in the face of occlusal disaster….or so it may seem. Interestingly occlusion or ‘malocclusion’ has many varieties and forms. From the completely attritted decimated occlusal surfaces of bruxists to the more weird and wonderful anterior open bites with no contact 6 to 6. Indeed both of these groups have something in common…they have no canine guidance.

Canine guidance may be considered an idealistic concept that results in the disclusion of all other teeth apart from the canine when going into lateral excursion. This, theoretically, results in the avoidance of lateral forces on teeth that could potentially result in their mechanical failure. Fantastic. All we need is canine guidance and teeth won’t crack or ‘fail’. Canine guidance itself can be ‘created’ by way of restoring or additive modifying teeth, potentially electively, to provide this disclusion. Have a click of the video below.

In essence if you firmly believe in all manner of occlusal philosophies then you would probably want perfectly formed cusp fossa relationships with appropriately designed anterior guidance. This can potentially equate to a destructive full mouth rehabilitation. There are a number of exponents who claim that ‘occlusal disease’ can develop in the absence of appropriate ‘guidance’. There are the obsessive among us that may aim for the perfect occlusal relationships or indeed creating them through orthodontics or restorative dentistry. The modification of occlusal relationships can be created through non-invasive techniques OR utilised opportunistically when teeth require restoration. Indeed patients such as the one below can function, pain free without any pathology per se without canine or anterior guidance.

Canines are unique teeth. We generally, similar to Noah’s arc, have two or more of each unit. Incisors, premolars and molars. Whereas canines are the lone ‘cornerstone’ of the mouth. It’s almost like we describe them as the star of the oral show. Big roots, big flat guiding surface, thick in mass and cross section, able to take lateral loads of the whole oral apparatus. The sentiment here is correct. Lateral loading of any apparatus be it a building or a castle of cards will result in unfavourable forces and potential destruction. The classic example is a premolar with a large MOD amalgam. Egg or chicken time. Is it the presence of the large MOD that causes the buccal/palatal cusp fracture or the lateral loading or a combination there of?

Indeed there are plenty that theorise that the lack of canine guidance can result in all manner of pathology or occlusal disease. Parafunction, sensitive teeth, periodontal disease, cracked teeth and recession have all been quoted as being caused by lack of canine guidance. It may be difficult to grasp that a ‘poor’ occlusal set up can actually ‘trigger’ or cause parafunction. Further to this a systematic review recently supported the notion that lateral guidance schemes (canine vs. group function) was not related to the development of TMD. Working and living in a stressful city such as London has a lot to do with the aetiology. Mental health issues are on the increase and we are likely to see a rise in TMD problems as a result. There is a lot of research going into TMD and its correlation with stress has been a feature of this trend.

Hypodontia patients are seen on a routine basis with canines in the lateral incisor position with no ‘canine guidance’ – this can be requested as an option to close spaces with the plan to disguise canines as laterals and the premolars as canines. This has been debated at length in the American Journal of Orthodontics (Article link here) and is an ongoing talking point in those that manage this patient cohort. Members of the multidisciplinary clinic don’t tend to worry about the lack of canine guidance in these cases and patients don’t tend to mind as long as they are aware of all the risks and benefits of each option. Retracting the canine into the canine position can result in an increase of orthodontic risks and time for treatment and potentially committing the patient to a restoration in the lateral space, be it a bridge or implant.

What does the ‘evidence’ say about canine guidance ? A systematic review by Abdou and Tenant 2015 in the Journal of Prosthetic Dentistry no less revealed interesting results. After reviewing 680 studies 26 were suitable for analysis. They found that, when comparing canine guidance and group function, there was no difference in electromyography activity (a measure of the level of contraction of the muscle). They also found no difference in patient comfort and, somewhat importantly in restorative dentistry, any difference in restoration longevity.  They concluded that patients have the ability to successfully adapt to either lateral occlusal scheme. The usual caveats about the evidence base apply here of course.

The canine itself is more likely to have a large root, a flat surface and a pointy tip for pure mastication much in the same way that other carnivorous mammals utilise these teeth. It’s doubtful that mother natures aim with canines was to provide disclusion on lateral excursion to avoid fracture of a heavily restored premolar and more likely needed a set of gnashers to trap prey and pierce the skin prior to mastication.

There needs to be a more holistic approach to the management of occlusal issues. Strict rules such as the need to achieve canine guidance is unlikely to be the answer. If we were to really consider contemporary restorative techniques then the use of the Dahl approach to tooth wear completely removes any posterior contact whatsoever. If anything this approach ‘increases’ the load on the anterior teeth constantly until posterior teeth regain contact.  Are these cases ‘canine guided’ if there is no posterior contact to be discluded? In most cases the way in which individual teeth are loaded is the key and not the lateral excursive movements. Cuspal coverage where teeth are weakened trumps that for me.

The concept of lateral guidance is convenient but not necessarily always achievable…our teeth and jaws move in a multitude of different directions, angles and trajectories when we chew and when we parafunction. Is it conceivable that all our lateral movements will coincide with one plane on one tooth on the working side of the mouth ? Do we expect our patients to find that canine surface every time they chew ? And do we expect the canine tooth to remain unchanged, both positionally and morphologically indefinitely whilst it is being loaded more often than the other teeth ? These questions can be partially answered by Abdou and Tenant who showed that the type of lateral guidance depended on the age of the patient. The younger the patient the more likely canine guidance the older the patient the more likely group function, potentially signifying progressive wear of the canine over time. They also showed that canine guidance depended on the whether the patient presented with Class II when it was more likely or Class III where group function was more likely.

My good friend and colleague Mahul Patel and I wrote a pair of papers in the Singaporean Dental Journal fairly recently. If you are interested to learn more I would suggest giving them some time as they provide a good introduction to occlusal problems that can arise and also a great resume of taking records, modifiying guidance and the inter-relationship with fixed prosthodontic provision. AA

Clinical issues in occlusion – Part I

Clinical issues in occlusion – Part II