Mandibular prognathism: interceptive correction

By dental occlusion we mean a repeatable, physiological and standard situation in which, when mandible and maxilla are clenched, the lower occlusal table interposes itself in the corresponding upper occlusal table. The situation is considered repeatable because chewing in carnivores consists of a repetitive closing-opening movement, performed by the chewing muscles, which pivot on the temporomandibular joint, a diarthrosis joint, which, together with the lengthened shape of the teeth in carnivores, does not allow lateral movements. This is a physiological condition since, like all movements that revolve around a joint, it makes the expenditure of energy minimal and the closing-opening movement of the jaws as natural as possible. The above definition is for a standard occlusion, given that each canine breed has its own ideal typical occlusion, determined by the morphofunctional peculiarities of the skull of the breed itself.

The shape of the skull is determined by the cephalic index:

skull width X 100/skull length = cephalic index

Patients are divided into three categories, based on the different shape of the skull:

• Mesocephalic breeds (cephalic index = 50): includes the majority of canine breeds (German Shepherd, Setter, Cocker Spaniel, etc.)
• Dolichocephalic breeds (cephalic index < 50): these subjects have a narrow and elongated skull (Dobermann, Greyhound, etc.)
• Brachycephalic breeds (cephalic index > 50): these subjects have a wide and short skull (Boxer, Pug, Pekingese, etc.)

With the changing of shape of the skull there is a change in the dimensions and positions of the upper and lower jawbones and, in their turn, a change in the relationship between the individual teeth: all this to obtain the most physiological and natural occlusion possible for the subject in question.

In veterinary medicine the correct occlusion is important for several reasons, not just for aesthetic reasons but also for functionality and comfort. As for the occlusion itself, the most important aspect is probably the functioning of the bite; if the bite is not functional, or if it is deficient, prehension and chewing of food will change. Comfort is important, and the job of the veterinarian is to help obtain the most natural and physiological occlusion possible for the animal, trying at the same time to attain the breed standard of the species in question.

Some malocclusions can worsen existing pathological conditions, such as periodontal disease, oronasal fistulae, chronic infections and/or cause discomfort during both mastication and rest. Dental occlusion depends on the breed standard and what is normal for one breed might be considered abnormal for another.

EVALUATION OF THE OCCLUSION

Five parameters are essential for evaluating dental occlusion:

• Scissors bite (relationship between the incisors) (Fig. 1). The scissors bite is considered by the majority of dog experts as the correct bite (the six upper incisors rostrally overlap the six lower ones), even though this does not take some breed standards into account; in all cases, dental occlusions different from the scissors type are visually obvious, therefore, since this is most likely a hereditary trait, it will be a crucial parameter in selecting animals destined for reproduction. 

• Canine interlock (Fig. 2). An adequate canine interlock is of essential importance for maintaining a constant relationship between maxilla and mandible. This parameter is already fundamental during deciduous dentition; the lower canine tooth interposes itself between third upper incisor and upper canine with a slight vestibular inclination, creating a lock that prevents the maxilla from sliding on the mandible and vice versa. Already at around 4 – 6 months of age, in puppies, a length discrepancy between maxilla and mandible or malocclusions caused by a short mandible may be ascertained: these conditions involve an inadequate canine interlock

• Interdigitation of the premolars (Fig. 3). The best parameter which establishes the relationship between mandible and maxilla is given by the interdigitation of the premolars. Normally, the cusp of the fourth upper premolar should be positioned in the diastema found between the fourth lower premolar and the first lower molar. In case of malocclusion due to abnormal development of the maxilla or mandible, this relationship between the premolars will be altered.

•  Uniform free space between the dental arches (Fig. 4). In the normal occlusion, there is a  constant, uniform left and right space between the upper and lower arches; looking at the occlusion in a caudo-rostral direction, a sort of an angle formed by the two arches may be noticed: the upper premolars and molars have a vestibular inclination, while the lower premolars and molars maintain a slight lingual inclination.

• Symmetry of the head. The symmetry of the head is instead evaluated from a rostral perspective; the midline between the upper central incisors should correspond to the midline of the lower incisors, and both the left and right sides should be a mirror image one of the other.

DIFFERENT TYPES OF ORTHODONTICS

The age of the subject and the therapeutic goals allow to divide orthodontics into:

Interceptive orthodontics. This is aimed at correcting malocclusions early on, eliminating bad habits and promoting the correct growth of the mandible and maxilla with the use of fixed or removable dental appliances: treatment should be started very early, in some cases when deciduous teeth are still present. In many cases, an early intervention allows to attain results that will no longer be possible once the animal has completed its growth.

Corrective orthodontics. This is used to correct malocclusions that were not initially treated and is undertaken when teething is final and complete. Generally speaking, fixed or removable dental appliances, including dental brackets, springs, wires of different diameter and materials (titanium, steel), elastics or actual braces, are used.

Preprosthetic orthodontics. This is not often used in veterinary medicine, because it is aimed at improving the dental occlusion in order to optimise subsequent prosthetic rehabilitation.

Based on the therapeutic appliance used, orthodontics can instead be divided into:

Removable orthodontics. This type uses individual dental appliances usually made on a plaster mould obtained from the impression of the mandible and/or maxilla of the patient. It is called removable because these appliances can be put in and taken out. In veterinary medicine, removable appliances are used for the interceptive correction of prognathism.

Fixed orthodontics. This type uses fixed dental appliances (brackets, bands, buttons, expanders, etc.) cemented to the teeth. The purpose of fixed orthodontics is to correct the relationships between the teeth, expand the palate as well as for other interventions.

TREATING PUPPIES SUFFERING FROM PROGNATHISM WITH EXTRAORAL DENTAL APPLIANCES – Personal experience

The technique subsequently described has already been adopted in around 900 subjects, within the time span of 14 years; in almost all cases the subjects responded positively to the treatment. The side effects, found in some cases, were negligible and entirely temporary.

Applicability of the system
Unlike what occurs in orthodontics practiced in humans, where orthodontic techniques envisage the frequent use of removable dental appliances, in veterinary medicine the use of fixed appliances is usually preferred. Nevertheless, the peculiarity of the appliance described below is precisely that it is a removable system, developed to correct prognathism; the mechanism of action on which the method is based is that of temporarily blocking, or better slowing down, the rostral growth of the mandible, preventing a worsening of prognathism, until the entire maxillary portion of the splanchnocranium has developed properly; this allows a correct anterior interlock with consequent scissors bite and an optimal relationship between the dental elements of the two arches.

The use of the device is recommended in the following cases:

• The patient should belong to a breed whose standard envisages the scissors or level bite or, if a crossbreed, have a dolichocephalic or mesocephalic skull structure (or at least have craniofacial lines of convergence such as to lead us to hypothesise the scissors bite as the optimal occlusion in that particular subject).
• The patient should still be in the growth phase; since this period varies depending on the breed and, above all, the size, it is important to act in time. The rapid growth that is noticed in small and toy breeds imposes an early intervention, while a relatively longer amount of time is available in larger breeds, keeping in mind, however, that the therapeutic results will also take longer. The mask (see later) application times vary considerably: the therapeutic result can be obtained in a short (2-4 weeks) or even very short (a few days [Figs. 5 and 6]) time.

• The owner must be fully willing to cooperate as the appliance will have to be worn for several hours each day and for many days or several weeks; the owner will also have to be available for follow-up visits, necessary to check the progress made and, possibly, for any constructive or dynamic modifications that the veterinarian may consider appropriate.

Making extraoral appliances
Several materials are needed to make these appliances:

• Essential
  • Turbocast®
  • Twill tape (various sizes)
  • Flat elastic (various sizes suitable for the dimensions of the tape)
  • Glue (cyanoacrylate)
  • Zinc oxide adhesive plaster or moisture resistant adhesive tape
  • Hairdryer
  • Stapler with metal staples
  • Ice spray bottle
• Optional
  • Material for impressions (alginate)
  • Impression tray (various sizes)
  • Plaster for moulds
  • Plaster vibrator

The first phase consists of constructing the chin cup: the most precise and less irritating solution for the patient is to make the chin cup out of Turbocast® on a plaster mould obtained from an alginate impression of the chin of the dog while it is sedated or put under slight anaesthesia. The impression has to involve the most rostral part of the mandible, including the canines and the chin, up to the apex of the root of the canines themselves.

In alternative, it is possible to mould the Turbocast® directly on the chin of the dog, under light sedation (Video). Over the years, we have found that this second option is less cumbersome and more practical and that it does not have a negative effect on the technical construction of the appliance.

Turbocast® is a heat mouldable synthetic resin generally used for orthopaedic bandages; it comes in finely perforated sheets of different thicknesses; it can be heated with a simple hairdryer, making it pliant and mouldable and, once let cool (a few dozen seconds at room temperature or, much more quickly, using ice spray), it regains its original stiffness. This resin is very light and slightly flexible and it can be used for chin cups of any size and thickness, adding successive layers of Turbocast® which, once heated, adhere layer on layer.

Human orthodontic tension bands have been used in larger dogs as they can be calibrated according to a predetermined traction scale. The transverse band (overhead) only serves to support and stabilise the appliance but does not exert any active force on mandibular growth.

In the treated cases, the traction force exerted on the chin cup was adjusted empirically; as mentioned previously, it should be moderate but with prolonged application times: at first, the chin cup is used for up to twenty hours per day, removing it only to allow the patient to drink and eat. The traction force line is perforce in the horizontal direction; the daily application time was gradually reduced to a few hours, with the decision based on the clinical response obtained, it being understood that in any case no damage or ulcerations to the soft tissues (lip, gums, skin of the tip of the chin) should occur, which would point to excessive compression with a consequent local ischaemia. With the growing of the skull the chin cup will then be reworked, readjusted and possibly remade and, likewise, the tension bands will be adjusted to the progressive dimensions of the head.

After having prepared the chin cup, the lateral traction bands should be fastened to it; they are made of twill tape and a flat elastic (the height of these elements should be chosen based on the size of the subject). The traction bands start laterally to the chin cup and continue in the elastic portion, which instead passes around the back of the neck. An additional overhead piece of only fabric tape joins the first two transversally, in correspondence with the temporal and frontal regions and, as mentioned previously, this band only serves to stabilise the appliance and to prevent it from assuming incorrect traction lines. In short, once applied, the system will exert moderate but continuous traction on the rostral portion of the mandible.

The various parts of the appliance are assembled using fast acting glues (cyanoacrylate) and/or metal staples applied using a common office stapler; once joined, the parts are protected and reinforced using cloth adhesive tape.

In almost all of the treated cases the patients tolerated the appliance very well; in any case, an Elizabethan collar should be used at least for the first 4-7 days: the only side effects recorded, in some dogs, were a mild case of contact dermatitis or, in the worst cases, some negligible pressure ulcers that healed quickly and spontaneously once treatment was suspended.

Approximately 900 subjects have been treated within the time span of 14 years, mainly small and toy breed dogs, but also medium and large size dogs. In almost all cases the results obtained were excellent, with complete correction of the prognathism, especially in patients treated as soon as the problem arose. In other cases, instead, an incomplete correction resulted, but in any case sufficient to guarantee a dental occlusion which although imperfect in terms of breed standards was in fact acceptable from the functional and biomechanical point of view. No damage to the arrangement of the dental elements was reported, nor to the soft tissues, the latter often arising as a complication to traditional orthodontic treatments. Some subjects underwent a Computed Tomography of the skull and no signs of articular stress and damage or iatrogenic abnormalities to the temporomandibular joint were detected by this examination, further confirming the tolerability of the procedure described above.

This technique, which is absolutely non-invasive, non-surgical, economical and immediately removable when necessary, without requiring anaesthesia or sedation, is an effective solution for improving or correcting prognathism in puppies, for reasons which go well beyond purely aesthetic factors.

Suggested readings

1. Aminoto A., Iwamoto S., TauraY.,Nakama S., Baker L.W., (1922) The influence of the forces of occlusion on the development of the   bones of the skull. Journal Orthodontic Oral Surgery Radiology : 53, pp.259-281
2. Barone R., (1981) Anatomia comparata dei Mammiferi Domestici. Edagricole, Bologna, volume terzo, pp.77-98 ; pp.164-172
3. Bonetti F., (1995) Zoognostica del cane. Editrice San Giorgio, Bologna - pp.63-69 ; pp.259-268
4. Brusa F., (1998) Correzione del prognatismo. Atti del 38° Congresso
5. Squarzoni P. (2003) Odontostomatologia del cane