Interarch Jaw Registration Devices for Oral Sleep Appliance

Part 2: Shift Happens

by Allen J. Moses, DDS, DABCP, D.ABDSM

Obstructive Sleep Apnea (OSA) is an intermittent collapse of the tongue into the uniquely at-risk human pharynx during sleep that disrupts breathing and contributes to numerous morbid long-term health consequences. OSA has been found amenable to treatment by dental prostheses that reposition the anatomic parts of the head, neck and pharynx. By initially registering a specific stable multidimensional mandibular interarch relationship in the mouth, an intraoral mechanical stent fabricated as close as possible to a position of optimal pharyngeal dilation serves to prevent collapse of the tongue and preserve patency in the upper airway.

There are few scientific correlates for simultaneous jaw comfort and pharyngeal airway size. Numerous techniques have been developed and are being used in clinical dental sleep practice to register the specific mandibular interarch relationship of optimal airway dilation, however many of them are incompatible with anatomic function. The first paper in this series¹ discussed ”slant” in terms of differences between anterior and posterior stabilization devices. This second in the series discusses the effect of condylar asymmetry on interarch jaw registration devices. This article will discuss the interarch jaw registration devices that mechanically direct mandibular movement in a midline sagittal plane and expect to achieve optimal airway dilation and stenting.

Asymmetry

Human beings as a species may be considered bilaterally symmetrical. The reality is they are quite asymmetrical. Dentists doing craniofacial evaluations carefully note higher eye, higher ear, larger nostril and higher shoulder. Faces are universally asymmetric. A study by Moses, et. al.² demonstrated that 95% of all human condyles are asymmetric relative to a sagittal plane, defined by the contact between teeth numbers 8 and 9 and the middle of the anterior border of foramen magnum. Moses’ study also demonstrated that anterior condylar protrusion at the level of the maxillary incisors is asymmetric by 5-5.5 millimeters. Intercondylar asymmetry is the normal state in human beings.

Nonlinear Jaw Movement

According to Sicher and DuBrul,³ the translatory component of joint movement is completely dependent on the shape of the articular eminence. This means that all movements of condyles with their discs must follow the curved surfaces of the articular eminentia. Normal human jaw movement functions smoothly on asymmetric curves and with asymmetric neuromuscular movement. Within human biologic systems, joints never move in a straight line, either vertically, antero-posteriorly or laterally. The temporomandibular joints from the closed mouth position to the open position rotate forward and simultaneously glide along the articular disc and eminentia of the temporal bones out of the glenoid fossa. This is an anterior and downward movement guided by the neuromusculature, restricted by the rigid nature of the bony architecture, the hardness of the articular disc and the asymmetry of the condyles and limited by fibrous capsules. It is not a simple unidirectional pull-forward mechanism (Figure 2).

The mandible is the only bone with free-moving bilateral joints in the body. Smooth mandibular movement requires simultaneous complex neuromuscular coordination of both sides. The mandible of humans is not a simple hinge like other animals but has an extended range of motion and function, such as facilitating speech and separating food from airflow to prevent aspiration.

Normal interarch jaw position varies depending on the biological function being carried out, such as mastication, breathing, swallowing, incising, speaking, and screaming. Each different biological function utilizes different combinations of muscle activity and different muscle pressures. Each person has a unique combination of facial features and facial muscle alignment.

Pharyngeal airway dilation and stenting is accomplished by the complex inter-reaction between at least seven neurological/anatomic factors:

1. Create optimal space for the tongue in the mouth
2. Advance the tongue anteriorly
3. Advance the mandible anteriorly
4. Stretch the mylohyoid muscle
5. Lower the mandibular position
6. Advance the hyoid bone anteriorly
7. Change the alignment of the masticatory muscles

No one factor acting independently can accomplish unidirectional movement. All seven factors working in synchrony never causes unidirectional movement. Jaw movement to create an open airway is always multidimensional.

The Consequence of Condylar Asymmetry

Since condylar movement rarely, if ever, occurs in a straight midline linear plane, interarch jaw registration devices that mechanically restrict protrusive movement to a midline cranial plane do not have a sound anatomic or scientific basis.

Protrusive or pull-forward interarch jaw positioners not only introduce an effect on the positions of the temporomandibular joints that is not desirable for patients having TMD problems, but they also misdirect protrusive movement as often as 95% of the time. This can carry over to the custom intraoral mechanical stent. Anterior push or pull mechanisms and appliance material in the palate can restrict proper tongue positioning and/or lip closure. Lateral adjustment mechanisms by either screw, strap or tube all direct jaw movement in a straight linear trajectory.

There is little or no substantiating scientific literature:

• That optimal airway dilation occurs at a position arbitrarily determined as upper midline over lower midline
• On how far forward to protrude the mandible to establish optimal posterior airway dilation
• On how much vertical component is necessary in the interarch jaw registration to establish optimal posterior airway dilation
• On what combination of vertical and protrusive components is necessary or indicated scientifically to establish optimal posterior airway dilation for an oral sleep appliance.

Many dentists may fail to recognize that the jaw becomes trapped in an anatomy-required deflection coupled with a rigid-position pull-forward registration device and take a bite record in a forced position. Given that the milled and printed MADs out there are similarly constraining in jaw movement, clinicians should think multidirectionally about what they are affecting with their interarch jaw registration device and choice of MAD.

Purely mechanical adjustment devices such as screws, tubes, straps, hinges, rubber bands and springs bring about unidirectional structural changes that are a mere guess at achieving optimal posterior oral airway stenting, and they are capable of bringing about deleterious changes and pain.

Registering Three Dimensional Interarch Relationship

X-Slant™, a new disposable posterior/molar stabilization system makes it possible for a dental sleep clinician to factor the parameters of vertical, lateral, protrusive and slant into the design process for effective oral sleep appliances. Posterior/molar stabilization makes possible the capture of multidimensional movement, eliminates slant by aligning the dental arches parallel to each other and enables normal shift to be registered.

X-Slant™ is comprised of the following parts: stackable shims and a shim coupler.  The shims, are essentially square-shaped discs about 15 mm per side, having curved corners for patient comfort. The molar shim system is comprised of square-shaped, rigid plastic stackable shims of either 1.5 mm or 2.0mm thickness and having two alignment pegs on one side and two matching apertures on the other side. Said pegs and apertures allow the shims to be stacked in a stable, accurate, vertical, friction-fit manner. The essentially smooth underside of the shims also has a slot to receive a rectangular arch-wire that connects two sets of stacked shims for bilateral positioning inside the mouth.

Procedure to Register the Interarch Position

The patient closes on shim sets of varying heights placed bilaterally in the molar area until the clinician feels the proper vertical dimension is established. The intent of this paper is not how a clinician should arrive at the optimal vertical dimension. Consideration of the seven neurologic / anatomic factors mentioned earlier, as well as selection of lateral and protrusive dimensions are based on clinical judgment. Bite registration putty is then placed on the peg upside of both shim stacks, which is then placed in mouth. Patient is instructed to close in a comfortable bite until set. Patient is then instructed to slide along lower shim surfaces to where clinician determines is the best protrusive and comfortable lateral position and then instructed to hold that position for the clinician to scan the bite. Bite registration putty can be added if stone or printed models are used.

Discussion

There are few scientific correlates for simultaneous jaw comfort and pharyngeal airway size. There are scientific correlates for establishing the jaw position for an intraoral appliance. Establishing the initial position to dilate and stent the airway during sleep is possible based on embracing anatomic characteristics. Two factors of biologic movement have been introduced for consideration: slant and shift. Establishing the optimal position to dilate and stent the airway during sleep is not based on unidirectional movement of hard tissue components. The position of optimal pharyngeal airway dilation and stenting is best arrived at by posterior stabilization to establish a position of parallel upper and lower dental arches.

Humans have asymmetric translatory jaw movement, and the intracondylar asymmetry of human beings averages 5 degrees. Shift, as smooth multidimensional functioning over asymmetric curved systems is normal and healthy. However, within human biologic systems jaw joints never move naturally in a straight linear plane. Unidirectional mechanical adjustment devices are based on illogical, biologically flawed perceptions of jaw movement.  Bite registration devices that mechanically direct protrusive movement to a midline cranial plane also do not have a sound anatomic or scientific basis.

Conclusion

Slant happens in oral sleep devices. It is bad. It can be eliminated.

Shift happens normally. It is okay. It should be facilitated in oral sleep devices.

Anterior Stabilization Systems cause slant, restrict normal anatomic shift and can cause pain.

Available now is a new posterior stabilization system for interarch jaw registration that both eliminates slant and facilitates normal shift.