Martin S. Spiller, DMD, Family Dentistry in Townsend, MA
Why can't Granny wear her lower denture?
The image above shows just what the inside of granny's mouth looks like when looking down at her lower jaw. Her tongue is located at the top of the picture, and the lower lip is pulled out at the bottom to so you can see her "mandibular ridge". The mandible is the bone inside the lower jaw, and it is shaped like a horseshoe. The ridge is the the subtle looking arch of pink tissue sticking up from the floor of the mouth, and it conforms to the shape of the top of the mandible. Notice that the level of the tissue on the tongue side of the ridge (the floor of the mouth) is at about the same level as the tissue on the lip side of the mouth (the vestibule). The only vertical structure sticking out above this level which might stabilize a denture is the ridge itself.
Lower dentures are built in the form of an arch, and they are made to fit over the patient's existing ridge. The only thing that keeps a lower denture from sliding around in the mouth is the vertical height of the bony ridge underlying the soft tissue ridge. The bony ridge is what is left of the jawbone after the teeth are extracted. Unfortunately, much of the ridge you are looking at in the image above is simply flabby soft tissue. This is because over a period of years, the alveolar bone (the bone that used to surround the teeth) simply melts away (resorbes) after the teeth are extracted . Later, we will look at x-rays showing the outline of dentures and their underlying ridges.
The lower jaw on the right shows the extent of the bone in a younger person who has all of his or her teeth. As long as that person keeps the teeth in healthy bone, the distance between the tip of the chinbone and the top edge of the incisor teeth (the four front teeth) remains about the same throughout life. I have a large number of 70 year old patients with jaws that look like this.
Remember that the soft tissue on the floor of the mouth (inside the curve of the ridge) lies at about the same vertical level as the place where the lips and cheeks attach to the gums on the outside (the vestibule). These attachments remain constant throughout life.
I have marked the approximate level of attachment of the floor of the mouth and the vestibule on the image to the left with a dotted line.
On the outside, (the vestibule, where the cheeks and lips fold over to become the gums) the level of soft tissue attachment coincides approximately with the level of the external oblique ridge and the mental foramen. The mental foramen is a hole in the bone where the mandibular nerve exits to provide feeling to the lips and cheeks.
On the inside, (the floor of the mouth) the level of soft tissue attachment runs below the internal oblique ridge and coincides with the genial tubercle anteriorly. The genial tubercle is the little bump of bone you can feel with your tongue on the inside of the lower jawbone. You can feel it in front, below the central incisors. Notice that the dotted line on the outside of the mandible lies at about the same level as the dotted line on the inside.
Each of these landmarks marks the highest extent of the cortical bone, which lies below the dotted lines. The cortical bone remains stable over the patient's lifetime. It remodels and changes shape slightly throughout life depending on the stresses placed upon it, but it retains most of its original dimensions. All the muscles that attach to the lower jaw attach only to the cortical bone.
The bone that supports the teeth is called alveolar bone. Everything above the dotted lines in the image above represents alveolar bone, and once the teeth are extracted, the alveolar bone begins first to remodel, and then to resorb (melt) away. The only permanent way to preserve the alveolar bone once a tooth is removed is the immediate placement of a dental implant.
Assume that the patient above lost his teeth at about the age of 30. The ridge would look something like this image by the time the patient reached the age of forty. Since the alveolar bone is no longer needed to hold the teeth, the body simply removes it. When it comes to alveolar bone, the body's motto is "use it or lose it".
This image illustrates the general position of the attachments of the floor of the mouth and the lips. Since these are the soft tissue structures that limit the extent of the vertical flanges of the denture, the less vertical ridge that remains sticking up above the soft tissue attachments, the shorter the flanges of the denture have to be. The length of the flanges determines the general stability of the denture in the mouth. The shorter the flanges, the more difficult it is to get the denture to work properly.
The actual extent of the loss of the alveolar bone over a ten year period varies from patient to patient depending on lots of factors including bone density, the level of stress, nutrition etc. The example above represents a best case scenario, and for the time being, this patient has a reasonably stable lower denture.
This image shows the extent of the remaining ridge about 20 years after extractions. By this time, the patient is having problems wearing the denture as it no longer wants to stay in one place. If the patient has not gotten a new denture since the first one was built, the flanges are over-extended and causing soreness, since they are extending below the soft tissue attachments of the floor of the mouth and the vestibule. The anterior bony ridge is shown below.
The same patient will reach the stage represented in the image to the left between the ages of 60 and 70. Some unfortunates reach this stage much sooner. I have seen people with ridges like this in their early 50's. Bone resorption in an edentulous patient accelerates as a patient ages, but also due to other conditions such as type II diabetes or osteoporosis. By this stage, not only has the alveolar bone disappeared, but the cortical bone has become more dense. Although implants can be done at this stage, and most work out quite well, the denseness of the bone makes their placement more difficult, and the reduced blood supply found in very dense bone makes their prognosis a bit more shaky. Note also the position of the mental foramen in this mandible. Its location on, or close to the top of the ridge may cause problems since the foramen is the location where the mandibular nerve exits the bone. Its new location can lead to pain as a result of pressure placed upon it by the denture base.
By the time the patient has reached this stage, the top of the bony ridge is essentially flat. Surprisingly, the floor of the mouth may actually contain a raised ridge like the one in the image below. Unfortunately, the ridge is composed entirely of soft tissue which, though attached to the top of the underlying bone, tends to be somewhat flabby and offers little real resistance to the movement of the denture
X-rays of the symphysis
When we plan to do implants on edentulous patients (patients without teeth), we frequently take x-rays from the side through the chin so we can measure the amount of bone available to do the implant. Above you can see the extent through the chinbone of two patients. The dentures have been outlined in lead foil so that they can be seen in profile on the x-ray. The x-ray on the left shows a patient with at least some bony ridge left after the resorption of most of his alveolar bone. His old denture was made about 10 years before this x-ray was taken, and actually, the flanges which project down over the bone are somewhat over-extended. In other words, if I were to make him a denture today, the flanges would have to be shorter because the floor of the mouth and the vestibule are now at a higher level of attachment on the bone.
The x-ray on the right was from a woman in her early 50's who has been wearing dentures for about 30 years. The denture seen in the x-ray is relatively new and reflects the very poor ridge quality that was available to the dentist when making the denture. This patient has NO ridge to place a denture on. The bony point sticking up toward the left side of the x-ray is actually the genial tuberosity, which is an attachment point for muscles. A denture cannot cover the genial tuberosity, since the pressure on the muscle attachments would cause intense pain.
Both patients were unable to wear their dentures at the time these x-rays were taken. Both now have implants to hold their dentures and are able to wear them.