Onlay augmentation is a
special technique used to build up bone, in which the alveolar ridge is
raised.
The animation shows how bone recession sets in due to a lack of chewing
pressure. Our body is very efficient - use it or lose it! After a while
there is not enough bone matter left for an implant, the implant would
stick out (as shown by the blinking light).
Onlay surgery is used to replace the missing bone, a chunk of bone
(either from a donor or from the patient’s body, for example the chin)
is screwed into place, evening out the difference in height to the rest
of the alveolar ridge. After a few weeks, the transplant heals and an
implant can be done.
The sinus lift operation is an example of inlay surgery – the exact
opposite because in this case the replacement bone is inserted between
the patient’s own bone and the mucous membrane in the jaw. The level of
the alveolar ridge stays the same.
This is the main difference between inlay and onlay surgery. In inlay
surgery, the level of the alveolar ridge remains the same, in onlay
surgery it changes. If there are enough teeth remaining and only some
bone and a few teeth missing in the back, then onlay surgery is better,
because it makes for a more pleasing appearance. If there are no teeth
left and the alveolar bones have receded evenly, then inlay surgery,
meaning sinus lift, is better. Sometimes the two techniques are
combined.
Nowadays there are many surgical techniques which can be used to build
up jaw bone, using either human bone, or bone from other animals, such
as cattle, or synthetic bone material.
Human bone heals best, while synthetic materials heal worst. With
respect to human bone, it is possible to use either purified donor bones
– or the patient’s own, by transplantation.
If bone is taken from the patient’s own body, the chunk of bone should
not exceed a certain size, because otherwise it would recede. This is
because the transplanted chunk of bone contains various types of cells.
The moment that the chunk is removed, the cells it contains are cut off
from the supply of nutrients. If a chunk is small enough, then soon
after transplantation it is once again connected to the supply of
nutrients by the process of diffusion. If the bone is too big, then the
innermost cells cannot be reached by diffusion, and it takes days for
new blood vessels to grow into the bone. The cells inside the block are
left without nutrients for too long – and they die. The immune system
reacts by removing the dead cells, which causes the bone to recede. This
is why larger bone chunks are transplanted on the micro-vascular level,
along with the vessels which are then connected to other vessels in the
area. This ensures the supply of nutrients to the transplant.
Micro-vascular transplantation is usually not necessary, smaller chunks
of bone are enough. Nowadays there are expensive techniques for growing
bone (both synthetic and natural) in laboratories using a patient’s own
cells. This is supposed to speed up the transplantation process, but
once the bone is removed from the optimum conditions of this „incubator“
– and transplanted into the patient, after a while the bone starts
„melting away“ due to a lack of nutrients. This method is interesting
and sounds very nice, but it is expensive and ineffective.