Stem Cells in Dentistry ?!?

Absolutely !!  Oral Surgery in general and bone grafting in particular are starting to make a transition.  More and more bone grafting scenarios employ cell-based or growth factor-enhanced grafting material.  Four items are absolutely essential in bone regeneration if predictable results are sought.

  • A matrix or scaffolding (collagen, bone mineral, synthetic grafts)
  • Cells (stem cells, platelets, osteoblasts)
  • Signaling molecules (growth factors, morphogens, adehsion molecules)
  • Time (often underestimated)

Within minutes after an “injury” to the bone structure, platelets aggregate in the area and release PDGF (Platelet Derived Growth Factor) and a variety of TGF-beta (Transforming Growth Factor – beta) molecules, to which BMPs (Bone Morphogenic Proteins) belong.  Some of the BMPs signal the Mesynchemal Stem Cells (MSCs) to “morph” into bone-precursor cells (osteoprogenitor cells).  Subsequently, PDGF signals these precursor cells to divide rapidly, in order to increase their number.  Once their number has increased (usually by an order of magnitude), a different set of BMPs will signal the precursor cells to “morph” again into mature bone-building cells (osteoblasts).

From this somewhat simplified molecular “injury cascade”, we can certainly appreciate the importance of stem cells.

A quick word on stem cells, because it has brought up some ethical and political issues in the past.  There are three types of stem cells in our body:

  • Embryonic Stem Cells (most potent, can form every tissue in our body)
  • Fetal Stem Cells (almost as potent, but somehwat more restricted in what they can become.  Often harvested from the umbilical cord and cryogenically frozen)
  • Adult Stem Cells (a.k.a. mesynchemal stem cell.  This cell is already committed to form only tissues of mesynchemal origin, i.e. bone, muscle, cartilage tissue, etc.).

The embryonic stem cells are the ones wich caused all the ethical controversy and to this day we can not perform any experiments with this cell lineage here in the U.S.  This cell line is extremely potent and can form any kind of tissue from all three primitive germ layers.

Our interest, however revolves around the Adult or Mesynchemal Stem Cells.  It stands to reason that an increased number of such stem cells during an “injury” or bone surgery can not only improve but also accelerate the bone healing.  It is now possible to use stem cell-fortified bone graft material (several thousand times the cell concentration of the human body), for various grafting procedures.  This graft material is very expensive and must be delivered within a day of surgery.  Initial results look very promising across several research studies.

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A little bit on bone grafting…

The topic of bone grafting seems to be one of the more confusing areas in implant dentistry.  When do I need it, why do I need it and why is it so darn expensive?  Generally a bone grafting procedure is needed whenever there is insufficient bone available to place a dental implant of proper dimension.  Bone will resorb over time in the areas where teeth are missing.  I still don’t believe that patients always get the best explanation as to what the graft options are and what the pros and cons are – well, hopefully this little blog will shed some light on this topic.

For starters, let’s just quickly review the different classifications of bone grafts.  All bone graft materials can essentially be classified into the following classes:

  1. Autografts
  2. Allografts
  3. Alloplasts
  4. Xenografts
  5. Growth Factor Enhanced Grafts

Now, all of these five classes can have either or both of the following properties:

  • Osteoconductive – the ability to “guide” bone cells
  • Osteoinductive – the ability to accelerate bone regeneration

    A typical autograft from the chin

    A typical autograft from the chin

Autografts are bone grafts, which come from your own body.  This type of graft is still considered the “gold standard” by many surgeons.  It offers the best of both worlds; it has very good osteoconductivity and also great osteoinductivity due to its high content of resident growth factors.  The disadvantage is the higher morbidity.  It is always necessary to conduct a secondary surgical access in a remote location of the body in order to harvest the bone.  Depending on the quantity of bone needed, this can be the hip (for larger quantities) all the way down to an intra-oral site, such as the chin or the back of the jaw.  Autogenous bone grafts have shown to be some of the most predictable grafts in surgery.

A typical allograft from a bone bank

A typical allograft from a bone bank

Allografts are a close relative of the autograft, in that it is of human origin, usually cadaver bone from a bone bank.  This always sets off a red flag with people with respect to disease transmission, however with the tight screening protocol and advanced processing technology this is virtually unheard of in the United States.  Allografts have the big advantage that they do not require a secondary surgical access site, however, they also have predominantly osteoconductive properties and very little if any osteoinductive properties.  Graft assimilation and maturation takes therefore longer than with the autografts.  Grafting success rates have also favored the autografts by a slight margin, depending on the individual application.

A typical xenograft of bovine origin

A typical xenograft of bovine origin

Xenografts are bone graft substitute from a different species all together, usually from bovine origin.  These grafts have usually only osteoconductive properties, since the organic portion has been completely removed (so no, there is no chance to contract BSE a.k.a. Mad Cow Disease).  The advantage of a xenograft is that there is a large quantity of bone available for the screening process for an exact micro architecture that is needed.  Their osteoconductive properties are therefore very good.  It can be distributed in particular form or as blocks.  Xenografts have shown to generate astonishing results, especially in sinus lift surgeries, which are often necessary for the placement of dental implants into the back areas of the upper jaw.

An example of recombinant human growth factor

An example of recombinant human growth factor

Growth Factor Enhanced Grafts are very new on the market.  These grafts capitalize on the regenerative powers of human growth factors such as Platelet Derived Growth Factors or Bone Morphogenic Proteins, both of which are generated by our own body during a bone repair and remodeling cycle, however these grafts are manufactured with recombinant DNA technology and are very concentrated.  These growth factor formulations are usually in liquid form that need to be combined with a “carrier”, such as collagen or a calcified matrix.  These grafts offer very good success rates, but their caveat is that they are VERY expensive.  This is an important point to consider, because more often than not, they just accelerate the graft assimilation and maturation time.  Research has yet to show whether the bone quality differs between all these different graft materials after a year or two.

Well, I hope this was informative for you.  I will be available for questions and comments, so don’t hesitate to post.

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