Whenever a tooth is extracted, nature will remove the bone that used to surround it.

When the body removes any tissue, we say that it has been resorbed.  Think of resorption as the "melting" away of the bone after a tooth is extracted.  The longer the tooth is missing, the less bone that remains behind.  Thus, when a tooth is extracted from a young person, by the time that person is middle aged, a great deal of bone will be missing.  If our friend in the image on the left above had all his teeth extracted when he was 30, by the time he reached the age of 75, his skull might look like the image below which I have Photoshopped.  Note that without changing the relationship of the upper and lower jaws, he now looks just like the toothless image above, on the right:

Bone that surrounds a natural tooth is called alveolar bone.  The job of the alveolar bone is to support the teeth.  Once a tooth is extracted, the alveolar bone no longer has a purpose, and the body resorbs it.  Eventually, the resorption slows down and stops.  What is left behind is the cortical bone, a part of the skeleton which, like the rest of the skull, may change shape during life, but never entirely resorbs without being rebuilt.   The cortical bone is like the main beam that supports the house. 

The red ellipse highlights the symphysis of the lower jaw.  The symphysis is made of the densest bone in the human body.  Thus, it generally remains thicker than much of the other cortical bone in the jaw.  The symphysis and its surrounding bone is very important to dentists who make dentures and who do implants.  It is often the only bone in the lower jaw that remains high enough to present a ridge to support a denture. 

The image below shows what the floor of the mouth may look like in a person who has been toothless for twenty or thirty years.  The low "hill" in the form of an arch is called the residual ridge.  The ridge is composed of firm gums overlying the bone of the lower jaw.  The "gums" in the anterior part of the ridge (at the bottom of the picture) overlay the symphysis.  When we build a denture, it must gain support and stability from the vertical height of the residual ridge.  You can see that there isn't much vertical ridge here to help stabilize the denture.  And this is by no means the worst lower jaw we see on a regular basis. 

 

 

The reason that I have marked the symphysis in the skull image above is to illustrate what happens even to dense, cortical bone if there are no teeth to maintain it.  You are looking at illustrations of a cross section through the middle of the symphysis.  The tongue would be to the left, and the tip of the chin would be at the lowest point of the gray outline of the bone.  The prominent point on the left of each stage illustration is the genial tubercle, which you can feel with the tip of your tongue in the front of the floor of your mouth.  The genial tubercle is a landmark which never resorbs, since it represents an important muscle attachment point.  It also represents the level of the soft tissue floor of the mouth.  For a full explanation of the way the lower jaw changes after the teeth are extracted, including lots of images, click the icon on the right.

The illustration labeled stage 1 shows the general shape of the bone when teeth are present.  Once the teeth are removed, the alveolar bone above the genial tubercle begins to resorb, and over the years, the shape of the symphysis progresses through the stages you see here.  This process happens all over both jaws, but it is most pronounced in the lower jaw.  This is the reason that so many people cannot wear their lower dentures.

Lest anyone think that the stage diagram above is some theoretical figment of an anatomist's imagination, the two x-rays above show x-rays of two of my own patient's edentulous symphyses.  This is what an x-ray looks like when you shoot broadside to a patient's chin.  The white stringy things that you see above each ridge are their dentures, which have been outlined with lead foil.  The shape of the dentures give a fair idea of the extent of the bone that sticks up and is available as a ridge upon which to rest the denture.  The one on the left has retained a reasonable amount of its vertical height and represents about a stage II symphysis.  The one on the right has lost virtually all of its alveolar bone and has suffered extensive cortical remodeling since the teeth were extracted.  It represents a stage IV.  The point of bone sticking up on the left side of the symphysis of the stage IV is the genial tubercle. 

How fast does bone resorb once a tooth is extracted?

Whenever a tooth is extracted, and no interventions are planned to preserve the bone, approximately 25% of the bone height above the base of the socket may be lost within the first year.  Within the first three years, as much as 63% of the bone height will be resorbed.  The final height of the remaining ridge depends upon the depth of the original socket, and the presence of adjacent teeth.  If there are adjacent teeth present, less bone will be lost.  On the other hand, if multiple teeth are lost, then, over a period of years, bone will be lost down to the depth of the of the original socket, and even beyond, since the cortical bone will eventually remodel.

Real world consequences of bone resorption.

The images above are drawn by hand, but they show the real effect of the loss of the teeth.  The image to the left shows the profile of a middle age woman with a full set of teeth.  The center image shows what the patient would look like immediately after the extraction of her teeth.  The image to the right shows the what the patient would look like at the same age if the teeth had been removed about ten years before.  If you have ever ridden the subway in any large city, you have seen people with this type of deformity.  They were not born that way.  They have simply lost all their teeth.  Visit my page on dentures to see several more images of patients who have lost their teeth.  Click on the image above to go to the website of the International Congress of Oral Implantologists for more on this subject.

Socket Preservation--How the dentist can prevent the loss of bone after extractions

 

(For dental professionals and students) Guided tissue regeneration--Technically, the term "guided tissue regeneration" applies to the use of resorbable or non resorbable membranes to allow for the rebuilding of bone around periodontally involved teeth.  The same term can be applied to the use of resorbable or non resorbable membranes with a bone graft material to prevent epithelial migration into a socket during any form of socket preservation procedure.  Both the bone graft and the membrane act as barriers to epithelial migration, however, the bone graft is secondary to the membrane in this respect, and in cases in which the membrane is sufficiently supported by the patient's surrounding natural bone, the bone graft material may not even be necessary.  This applies mainly to small residual spaces surrounding an implant that is placed directly into a socket  immediately after an extraction. The reason that guided tissue regeneration works is outlined below.

After a tooth is extracted, the socket fills with blood.  The blood clots, and acts as a kind of scaffold for somatic (from the body) cells to begin the work of healing the wound.  There are essentially three types of cells that concern us here.  Epithelial cells from the gingiva (the gums),  begin to creep down over and into the clot, or over the exposed bone of the socket if the clot is not well adhered to the socket bone.  These epithelial cells come from the top down, and begin creating a new "skin" to heal over the socket.  From the bone deep inside the socket, two other types of cells begin working their way into the deep layers of the clot to reshape the remaining bone, and to build new bone within the clot.  Osteoclasts are cells who's job is to break down existing bone so that it can be rebuilt to better conform to the newly toothless environment that the bone will occupy when healed.  Osteoblasts are cells which build new bone in the socket.

Thus, when a tooth is extracted, a sort of race begins to see which process "wins".  The osteoblasts and osteoclasts work from the bottom up to reshape and rebuild bone in the socket, while the epithelial cells work their way from the top down into the socket displacing the clot and producing a soft tissue "scar".  Bone building is called osteogenesis, while the process of epithelial cells migrating down the walls of the socket is called epithelialization.  Under the epithelialized layer, another process begins to form tiny blood vessels and collagen fibers throughout the blood clot.  This granulation tissue then becomes a soft tissue scar which prevents bone from fully filling the extraction socket.  Because the body builds soft tissue much faster than bone (about a mm per day as opposed to a mm per month), the process of epithelialization and granulation often wins out, filling the socket from half to two thirds full of epithelialized collagen scar tissue.  If the patient gets a dry socket, the socket may end up as an epithelialized hole in the surrounding bone.  Some patients are lucky and build more bone in their sockets, but many do not.

Dentists have discovered that they can prevent the epithelialization process by filling the socket with a material which can prevent epithelial cells from migrating into the socket, and then covering the socket with a membrane.  Ideally, these materials should be resorbable themselves, and replaced by the body's own bone.  There are essentially three ways of doing this.  These three techniques have the added advantage of preventing dry sockets after the extraction.

Socket preservation--three ways to prevent bone loss

1. Rootform Implants

   When a tooth is extracted, it is possible to replace it with an artificial tooth root called an implant.  Implants are generally (though not always) made from titanium and if properly placed, bone will grow around it and actually attach to it, a process called osseointegration.  An implant is the most expensive form of socket preservation, but it is always considered the best thing to do after extracting a functioning tooth since it is the closest thing to a natural tooth replacement offered by dental science today.  The implant may be placed at the time a tooth is extracted (or if the socket bone has been preserved, it can be placed later).  The dentist drills a perfectly shaped and sized hole in the empty socket, and screws a titanium "root" into it.  This implant is then covered by suturing the gums over it, and allowed to heal for about six months.  Implants are the only permanent way to prevent bone loss after an extraction. 

   Sometimes the dentist will fill in any remaining space around the implant with bone grafting material, and then cover the implant and the bone graft with a collagen membrane.  Between the implant itself, and the bone graft material, epithelial cells are prevented from migrating into the socket.  During the healing process, the bone surrounding the titanium implant osseointegrates with the titanium, and the bone graft material is removed by osteoclasts and replaced with the patient's own bone by osteoblasts.  At the end of the healing period, the dentist uncovers the implant and attaches an abutment to it.  The abutment sticks up out of the gums and serves as an anchor for a crown.  This combination of implant, abutment and crown serves as a very firm and permanent tooth.  With good hygiene, a crown/abutment placed on an  implant can last as long as a healthy natural tooth. 

   The popularity of rootform implants is growing at an exponential rate.  It is beginning to become popular to extract seriously damaged teeth that were formerly restorable and replace them immediately with implants which have better long term prognoses.  Implants have the additional benefit of not being susceptible to decay like a natural tooth. 

   Click here to learn how implants are done, how long they take to do, and how much they cost.

    

2. Bone Grafts

   Bone grafts are the best non-implant form of socket preservation.  Bone grafts are very effective at preserving bone height, and they also create more bone for an implant later on.  There are three types of bone graft material.

   • Xenogenic grafts are made from animal bone, most frequently bovine (cattle) or porcine bone. Xenografts are processed in such a way that all organic material is removed leaving only the hydroxyapatite component.  (hydroxyapatite is the mineral that makes natural bone and teeth hard.) The bone structure remaining is very porous and has about the same structure as natural bone (see image to the right).  When you look at an x-ray of normal human bone, you can see a weblike pattern in the marrow spaces.  The weblike pattern is called trabeculation, and it has the same general pattern as the demineralized bovine bone that you can see on the right.  Xenogenic grafting has been shown to be one of the most effective methods of creating bone in areas where there is none.  Xegenogenic grafts are known to be osteoconductive, which means that it supports the formation of new bone by acting as a matrix or scaffolding for extension or apposition of new bone from existing bone (i.e. the patient's own bone).  Xenografts also may have varying degrees of osteoinductive potential, which means that in addition to acting as a simple scaffolding, the graft material may actually stimulate the patient's own mesenchymal cells to transform into osteoblasts (bone-forming cells) hastening the replacement of the graft material with the patient's own bone.  Xenograft grafting materials are generally resorbed and replaced entirely with the pathien's own bone.

   • Alloplastic grafts are made from synthetic material such as ceramic material (bioactive glass), tricalcium phosphate, calcium sulfate (plaster) and hydroxyapatite, (the hard mineral that makes up teeth and bones).  The most popular brand of alloplast today is called Bioplant (a highly magnified microsphere is seen in the image to the right).  It is made of very thin microspheres of methyl methacrylate (plastic) which are perforated, and coated inside and outside with Ca(OH)2.   During healing, the osteoblasts and osteoclasts migrate inside and between the spheres and form new bone within and around them.  Alloplastic graft material constitutes the second of the most popular forms of bone grafting material in dentistry.  Alloplastic grafts are known to be osteoconductive and have varying degrees of osteoinductive potential.  Alloplastic materials may, or may not be resorbed and replaced by the patient's own bone.  Plaster always resorbs, but bioactive glass does not.  When not resorbed, the material remains behind as an implant acting as a sort of scaffolding that is surrounded by the patient's own bone.  Non-resorbable alloplastic bone grafting materials, can be used in most oral applications, but they are especially good for permanent ridge augmentation procedures because the resulting bone is unlikely to further resorb over time.

   •  Allogeneic grafts-- Allografts are derived from human sources and are obtained from tissue banks.  They are made from freeze dried human cadaver bone, or bone from living donors such as people undergoing total hip replacement.  The allograft is prepared by treating a section of cadaver bone to remove all soft tissue, then texturing the bone surface to produce a pattern of holes of selected size, density, and depth.  It is processed in such a way that it is well cleaned, sterile, and free of viruses. Allogenic grafting material is osteoconductive and has a higher osteoinductive potential than xenografts or alloplastic grafts.   

   • Autografts are made from the patient's own bone.  Bone is taken from a donor site, such as the crest of the pelvic bone and transferred  to the surgical site where bone is needed.  An autograft is considered the gold standard in bone grafting because in addition to being osteoconductive and osteoinductive, it is known to be osteogenic, which means that it supports the formation of new bone by direct interaction with and stimulation of osteoblasts (bone-forming cells). This phenomenon is based on the contribution of the patient's own living cells that are contained in the graft. Autogenous bone can promote osteogenesis, with the new bone being generated from endosteal osteoblasts and marrow stem cells that are contained within the graft material. An autograft is the most predictable grafting technique available, however it leaves a second surgical site in need of healing which causes extra discomfort after the surgical procedure.  In dentistry, bone can sometimes be scavenged from areas adjacent to the primary surgical site.  However, since the advent of the artificial bone substitutes, this is rarely done today.  On the other hand, whenever an implant is placed, there is generally some bone "sawdust" in the flutes of the drills used to create the space for the implant, and this material is often scraped out of the drills and added to the xenograft or alloplastic grafting material that the dentist plans to use in the grafting procedure.

   The dentist mixes the bone graft granules with the patient's blood and forces it into the socket immediately after the tooth is extracted.  The mixture is held in place either by tightly suturing the gums over the socket, or by suturing a collagen membrane over it.  Over the course of four to six months, the patient's body resorbs the artificial bone and replaces it with his or her own.  A bone graft is nearly 100% effective at preserving bone height.

    

3. Collagen Plugs

Collagen is a component of connective tissue.    The collagen used in dental procedures is derived from bovine Achilles tendon.  Collagen is a connective tissue protein which forms fibers.  It is the elastic material underlying your skin that makes it tough and rubbery.  In its pure form, collagen is not species specific.  Cattle have about the same collagen as humans.  Since all other bovine organic material is removed from it during processing, the  human body does not reject it as it would for foreign tissues.  The material is supplied in the form of a soft, fiberous "plug" in a single use sterile vial .  After an extraction, the dentist places a collagen plug into the socket and sutures it in place.  The sutures are removed in a week.

A collagen plug is a good deal less expensive than a bone graft, and the procedure for placing it is easier.    This procedure may preserve between 50% and 70% of the original bone height.  Unfortunately, it is a much less predictable method of socket preservation than bone grafting.

Finally, note that the only way to permanently preserve bone after a dental extraction is by placing a titanium implant, or by using non-resorbable alloplastic graft materials in the site.  Even well preserved socket bone will eventually resorb over a period of many years if it is not kept in function.  An implant signals to the body that the bone is in use, and therefore necessary.  This is the body's way of saying "use it or lose it".  Alloplastic graft materials remain in the socket as a permanent implant material and act as a scaffolding to maintain the intervening natural bone that infiltrates between the alloplastic particles.

Click the image above to go to the companion page of this one.  It shows the stages of resorption of the lower jaw, and explains why granny can't wear her lower denture.