Reminders on the fundamental bases of bone anatomy and physiology
1-Introduction:
Implantology is the science that deals with the placement of dental implants, artificial roots replacing missing teeth, which is highly dependent on osseointegration, which is currently based on solid and scientifically proven experimental bases.
Implant placement is a crucial surgical moment, its success depends as much on the practitioner’s experience, as on his knowledge of the implant anatomical environment, and of bone physiology.
In this course we will see an anatomical reminder of the maxillae, their innervation, vascularization, then we will pay attention to the morphological changes linked to bone resorption, and we will describe the different implantation zones.
2- Anatomical-histological reminder:
2-1-Anatomy:
A-The maxillary bones: constitute the skeleton of the upper jaw. With their shape of
triangular pyramid with a truncated lateral summit, they articulate with all the other bones of the
face. Each maxilla is hollowed out with a cavity, the maxillary sinus. The innervation is under the
dependence of the maxillary nerve, second branch of the trigeminal nerve, from the ganglion
trigeminal.
B-The mandible: is an odd and symmetrical bone which constitutes by itself the skeleton of the lower level of the face. The only mobile bone of the face, it is made up of a body and two
branches. The sensory innervation of the mandible and mandibular teeth is ensured
by the inferior alveolar nerve which is located under the dental roots, in contact with the internal cortex.
2-2-Histology:
Bone tissue is made of cells (osteoblasts, osteocytes, and osteoclasts), and bone matrix (protein framework, and mineral framework).
The bone is organized into four compartments. From the outside to the inside, we distinguish: the
periosteum, cortical bone, endosteum (covers the inner surface of compact bone), and spongy bone.
2-3-Vascularization:
A-The maxillary bones:
The vascularization depends on certain branches of the maxillary artery:
– The infraorbital artery provides vascularization to the upper orbital surface as well as the anterior surface of the maxilla and the anterior teeth;
– The posterior superior alveolar artery provides vascularization to the posterior surface of the maxilla as well as the posterior teeth;
– The greater palatine artery provides vascularization to the posterior part of the palatine mucosa;
– The nasopalatine artery supplies the anterior part of the palatine mucosa
B-The mandible:
Vascularization is provided by an external periosteal network and an internal endosteal network.
– The external network is formed by the facial artery, the submental artery (originating from the facial artery), the sublingual artery, the masseteric and pterygoid arteries (branches of the maxillary artery) and the mylohyoid artery (branch of the inferior alveolar artery)
– The internal network depends on the inferior alveolar artery (branch of the maxillary artery) from which branches detach to each dental apex. It divides into two: an incisive artery which gives branches to the canine and the incisors, and a mental artery which anastomoses with the submental artery.
C-Alveolar processes:
Vascularization is mainly ensured by:
– Branches of the superior and inferior alveolar arteries as well as the arteries
interalveolar septa
– The periosteal arterioles of the corticals
– Ligamentous arterioles (branches of the dental arteries and branches of the
alveolar arteries).
2-4-Innervation:
The maxillofacial nervous system is a complex system, involving several cranial nerves, twelve pairs in number. What interests us is the 5th pair (the trigeminal nerve), whose sensory fibers originate from the Gasserian ganglion also called the semilunar ganglion, while its motor fibers originate from the two masticatory nuclei, one of which is located in the pons and the second just above in the mesencephalon. It will give three branches: the ophthalmic nerve, the superior maxillary nerve, the inferior maxillary nerve.
3-Anatomical modifications linked to post-avulsion resorption:
Tooth extraction causes bone changes for 12 consecutive months, but two-thirds of the dimensional changes are obtained during the first 3 months of healing.
3-1-In the mandible: The reaction of the mandible to tooth loss is directly linked to its osteo-architecture. Indeed, it is the place of insertion of very powerful masticatory muscles, and the place of integration of masticatory pressures. The significant forces exerted by these constraints (axial and lateral) induce a dense bony cortex around the entire mandible.
After tooth loss, the organization of the mandible evolves according to the only muscular constraints that remain: the mylohyoid, the genioglossus and the geniohyoid, which pull the bony crest in a lingual direction.
3-2-In the maxilla:
The post-extraction changes will be different from those described in the mandible. Indeed, the upper jaw does not have any significant muscle insertion. In addition, the external bone table of the maxillary teeth is much thinner, particularly at the level of the incisors and canines.
3-3-Modification of the vestibular vascular supply:
Several factors make the vestibular table more vulnerable and greatly exposed to horizontal and vertical resorption, including:
-The intracrestal source does not vascularize the two coronal millimeters of the crest;
-The fascicular bone is present in greater proportion in the vestibular table. However, this bone is only kept alive if it is attached to the periodontal ligament of the tooth;
-Disappearance of the desmodontal vascularization pathway.
Reminders on the fundamental bases of bone anatomy and physiology
4- Implant areas:
4-1-Mandibular implant areas:
4-1-1- Symphyseal implant zone:
The symphyseal area is a region that does not pose too many implant difficulties since there are no major anatomical traps. Its easy surgical approach makes it a preferred site for taking small bone grafts under local anesthesia.
However, three points must be emphasized:
– The morphological variation of the bone due to resorption will lingualize the drilling axis of our implants.
– We are often in the presence of dense and poorly vascularized bone.
– Anatomical variations sometimes lead us to encounter a large incisive nerve, a terminal branch of the mandibular nerve that is difficult to anesthetize.
4-1-2- Implant zone of the horizontal branches:
This implant area presents a significant anatomical obstacle, the mandibular canal,
The content of this canal: The nerve and the artery appear united, surrounded by a loose, whitish tissue, veiling the underlying structures in places and thicker at the back. The nerve presents bifurcated or trifurcated variants.
In the case of old edentulism, reactive bone lysis reduces the height available for our implantation, thus bringing us closer to this canal. Faced with this risk, it is imperative to impose three rules that must never be transgressed:
– In case of anatomical doubt, only a computed tomographic radiological examination will be able to give us a real image of the position of this canal.
– Care must be taken to keep a safety distance of 2 mm above the canal in order to avoid any per-operative or post-operative complications.
– Never implant by attempting to drill further outward or further inward from the presumed canal path.
4-2-Maxillary implant areas:
4-2-1- Subnasal implant area:
This area is marked by the projections of the two central and lateral incisors.
Frankly concave in its upper third, its upper edge will form the base of the nasal notch. The bone quality of this site is generally good, the subnasal implant area is easy to surgically approach. It presents only one difficulty, the presence of the anterior palatine hole and adjacent teeth (aesthetic difficulty).
4-2-2- Canine corridor implant zone:
It is limited at the bottom by the implantation of the canine which will mark its imprint by a curvature, the canine boss, the premolars and the mesio-vestibular root of the first molar. This site has a pyramidal shape with three faces. The teeth, the sinus and the nasal fossae constitute major obstacles.
The canine corridor area offers an interesting bone volume both in height and width for implant placement. It is therefore a preferred area that should be sought to be exploited.
4-2-3- Lateral sub-sinus implant zone:
During tooth loss, bone resorption combined with sinus expansion will very quickly reduce the available bone height.
The sinus floor descends by the following phenomena:
-physiological pneumatization of the sinus or when intra-sinus pressure increases.
– loss of dental roots, causing the sinus floor to lose some mechanical support.
– the loss of the mandibular posterior teeth causes the upper teeth to egress, which pull the sinus floor with them
– the presence of chronic inflammation or infection
– with age and the progressive involution of the maxillary bone.
Reminders on the fundamental bases of bone anatomy and physiology
5-Classification of bone loss (Martinez and Renault):
5-1-Horizontal or vestibulo-lingual direction:
This type of alveolar loss is most common in the external or vestibular cortex. It can cause a poor aesthetic result in the anterior maxillary sector.
Class I: The bone conditions allow an implant protocol to be carried out without tissue input, recent crest.
Class II: This bone loss is the cause of a vestibular concavity because it is thinner than the buccal. The percentage of horizontal bone loss is less than 20% compared to the width of the alveolar crest. The bone conditions present frequently allow a conventional implant protocol to be carried out; in a pinch, an addition of soft tissues will be considered.
Class III: Intermediate bone loss of the alveolar crest. The vestibular concavity is more accentuated. The percentage of horizontal bone loss is on average 20 to 30% compared to the original width of the alveolar crest.
Two surgical approaches are considered:
-Sculpture of the alveolar crest by osteotomy (abandoned technique);
-Simplified bone filling of exposed implant coils.
Class IV: significant bone loss of the residual alveolar crest. The vestibular concavity is very pronounced. It is more common in the maxilla than in the mandible.
The percentage of horizontal bone loss is on average greater than 30% relative to the width of the alveolar crest.
Reconstruction of the crestal morphology is required before implant placement:
– By guided bone regeneration; – By apposition graft.
5-2-Vertical or corono-apical direction: The inter-arch distance is frequently increased. The residual bone height is often reduced in the posterior sectors. The clinical crown/implant ratio may therefore be unfavorable.
The assessment of the residual context for a reasoned therapeutic choice depends on 03 parameters:
-The crestal bone level of the edentulous site relative to the crestal level of adjacent teeth;
-The available inter-arch prosthetic space;
-The residual vertical bone height.
Class I: In the presence of a recent edentulous site, the vertical level is often preserved where the loss is minimal.
Class II: The vertical level varies depending on the healing period, the bone context and the temporization conditions. Bone loss is slight, approximately 1 or 2 mm.
Class III: Long-term partial distal edentulism frequently characterizes this category of bone loss.
-In the mandible: short implant or reconstruction of the crestal morphology.
-In the maxilla: osteotome technique (piezosurgery).
In the anterior sector, an aesthetic analysis is essential; a vertical apposition graft may be necessary.
Class IV: Long-term partial edentulism and some long-term total edentulism characterize this significant bone loss.
– In the mandible: vertical apposition graft, lateralization of the dental canal or implantation associated with guided bone regeneration at the level of the exposed spirals.
– In the maxilla: sinus filling via the lateral route; apposition graft.
NB: Bone loss often occurs in both dimensions.
Reminders on the fundamental bases of bone anatomy and physiology
6-The quality of the residual bone:
The type of bone is important to know because when placing an implant, the bone response will be different depending on whether it occurs in cortical or cancellous bone.
The classification of bone quality encountered during implant placement is that proposed by Lekholm and Zarb (1985) , which takes into account the distribution between cortical and cancellous bone:
– Type I bone , the jaw is composed almost entirely of homogeneous compact bone
– Type II bone , a thick layer of compact bone surrounds a core of dense trabecular bone
– Type III bone , a thin layer of cortical bone surrounds a core of dense trabecular bone
– Type IV bone , a thin layer of cortical bone surrounds a core of low-density trabecular bone
This histological classification is difficult to apply in practice, which is why the classification of Tsiri and Rao appears more clinical . The bone is:
– Dense, the clinician does not feel the sensitive delimitation between a cortical part and a spongy part
– Normal, the clinician clearly feels the transition from the cortex to a less resistant bone
– Of low density, the cortex and the spongy part offer little resistance, they are easily passed.
Reminders on the fundamental bases of bone anatomy and physiology
7-Conclusion:
Post-extraction dimensional alterations in the maxilla are different from the mandible due to the absence of significant muscular constraints during mastication.
The assessment of bone capital and the anatomical environment is a crucial step.
Several therapeutic procedures are available to the practitioner to manage delicate anatomical situations, in addition several types of implants can be used to spare surgical management of the implant site.
Reminders on the fundamental bases of bone anatomy and physiology
Wisdom teeth may need to be extracted if they are too small.
Sealing the grooves protects children’s molars from cavities.
Bad breath can be linked to dental or gum problems.
Bad breath can be linked to dental or gum problems.
Dental veneers improve the appearance of stained or damaged teeth.
Regular scaling prevents the build-up of plaque.
Sensitive teeth can be treated with specific toothpastes.
Early consultation helps detect dental problems in time.