Reminder of the fundamental bases of bone anatomy and physiology
Introduction: Implant placement is a potentially dangerous procedure. Indeed, certain vascular and neurovascular elements can be damaged during implant surgery. These iatrogenic lesions can be benign or cause an unfavorable prognosis for the patient’s life in the absence of treatment. It is therefore essential to have a good anatomical knowledge of these structures.
1-Maxillary bone
A-Anatomical description: the maxillary bones constitute the skeleton of the upper jaw. The maxilla articulates with all the other bones of the face. It is hollowed out by a large cavity, the maxillary sinus, and contributes to limiting the nasal fossae on the outside, the orbital cavities below and the oral cavity above.
It supports the maxillary teeth.
B-Vascularization: the vascularization of the maxilla depends on certain branches of the maxillary artery:
-the infra orbital artery: provides vascularization of the upper orbital surface and the anterior surface of the maxilla and anterior teeth
-the posterior and superior alveolar artery which provides vascularization to the posterior surface of the maxilla and the posterior teeth
The infraorbital artery and the posterior superior alveolar artery are anastomosed by an artery located between the sinus mucosa and the lateral part of the maxillary sinus.
This can constitute an anatomical obstacle in the case of a lateral approach to the maxillary sinus for elevation of the maxillary sinus floor.
-The greater palatine artery, branch of the descending palatine artery, which provides vascularization of the posterior part of the palatine mucosa.
-the nasopalatine artery, branch of the sphenopalatine artery, which vascularizes the anterior part of the palatine mucosa
The greater palatine artery is anastomosed with the nasopalatine artery.
C-Innervation: innervation is dependent on the maxillary nerve, second branch of the trigeminal nerve (V2) which is the sensory nerve of the face.
It gives:
-the zygomatic nerve, which anastomoses with the lacrimal nerve from the ophthalmic nerve V1 at the level of the lateral wall of the orbit to provide the sensory branches for the skin of the cheekbone.
- The pterygopalatine nerve, which gives off the palatine nerves that enter the greater palatine canal to provide sensory innervation to the soft palate and the posterior part of the hard palate.
- The superior and posterior alveolar branches which descend, applied against the maxillary tuberosity, and penetrate into canals dug in the bone to ensure the sensitive innervation of the maxillary molars.
- Then he goes through the infraorbital canal and gives:
- -the superior and middle alveolar branch, which descends into the lateral wall of the maxillary sinus.
- -the superior and anterior alveolar branches, which detach from the nerve at the end of the infraorbital canal and run in the anterior wall of the maxillary sinus to provide sensory innervation of the maxillary anterior teeth.
- -the infraorbital nerve (the terminal branch of the maxillary nerve), originating from the infraorbital foramen, which gives off sensory branches intended for the lower eyelid, cheek, nose and upper lip.
- Any trauma to this nerve at the level of the infraorbital foramen during subperiosteal detachment of the anterior surface of the maxilla results in hypoesthesia of the upper lip and the ipsilateral nasal ala.
2-The mandible
The mandible is an odd and symmetrical bone that alone constitutes the skeleton of the lower level of the face. It is the only mobile bone of the face; it articulates with the temporal muscles. This bone supports the mandibular teeth.
A- Anatomical description
The mandible is composed of a body and two lateral ascending branches. The body is curved, like a horseshoe open at the back. The ascending branches detach themselves on each side of the posterior end of the body. They present, on their internal face, the entrance to the inferior dental canal or Spix canal which provides passage to the inferior dental nerves and vessels. The upper edge corresponds to the coronoid process and the condyle around which the temporomandibular joint capsule is inserted.
B-Vascularization: the external vascular supply includes an external and internal periosteal network formed by the submental, masseterine, pterygoid, facial and lingual arteries.
The internal, endosteal vascular supply includes the condylar artery and the inferior dental artery.
C-Innervation: the mandibular nerve is the largest branch of the trigeminal nerve. It is located in the mandibular canal, usually on the apices of the teeth and it mainly innervates the mandible. The mandibular canal starts at the mandibular foramen located approximately 2 cm behind and 1 cm above the crown of the lower wisdom tooth. It follows a curved path from the mandibular foramen, equidistant from the cortices of the vestibular and lingual compact bone. Between the first and second premolars, the canal curves towards the vestibular edge. Its emergence is called the mental foramen. At this level begins the incisive canal which is the extension of the mandibular canal. Located in the center of the lamellar bone, it contains the nerves and vessels of the canines and incisors.
D-Clinical implications:
-Anterior region : in the symphyseal region, the anatomical elements at risk are the incisive nerve and the sublingual and submental arteries.
Nervous accidents that may affect the incisive nerve are hyperesthesia.
The majority of hemorrhagic accidents are caused by damage to the sublingual artery after perforation of the lingual cortex.
Laceration of the submental artery following perforation of the mandibular lingual cortex during drilling is also responsible for hemorrhagic accidents.
-Middle region : the mental foramen corresponds to the anterior end of the mandibular canal. At this level, the inferior alveolar nerve divides into two terminal branches: the mental nerve and the incisive nerve. They are accompanied by their corresponding vessels. The incision path must therefore take this anatomical structure into account. In addition, only a full-thickness detachment will be performed in order to avoid injury to a terminal branch.
-Posterior region : injury to the lingual nerve during drilling is possible due to perforation of the lingual cortex, as well as injury to the inferior alveolar nerve.
Laceration of the inferior alveolar artery and the submental artery following bone perforation is the cause of hemorrhagic accidents.
Reminder of the fundamental bases of bone anatomy and physiology
3-Anatomical variations due to toothlessness
Maxillary and mandibular resorption:
The new anatomy (size and shape of the bone ridges after tooth loss) will dictate the position of the dental implants.
Indeed, alveolar remodeling always occurs after dental extraction. It combines osteoclastic resorption of the alveolar bone with bone apposition in the extraction socket.
Resorption mainly affects the alveolar portion of the bone (it is very active during the first months of healing). Age-related resorption occurs at the level of the basal bone.
Anatomical modifications induced by edentulism : certain modifications must be considered with regard to their impact on the surgical technique or on the length of the implants (Schroeder et al 1996)
At the mandibular level :
Anterior crestal resorption is 4 times faster than that of the maxilla. In addition, resorption is faster in the lingual region (centrifugal resorption). The crest thus gradually loses its height and mesio-distal width. Vertical resorption of the mandible brings the mandibular canal closer to the crestal edge.
At the maxillary level : resorption in the vertical direction is associated with a more significant resorption in the vestibular direction (centripetal resorption movement). Vertical resorption often limits the bone volume available under the maxillary sinuses.
The maxillomandibular relationship is then modified.
Depending on the areas , tooth loss changes the anatomical relationships:
- The mandibular canal is usually located under the dental apices, it approaches the crest as mandibular resorption occurs. In the presence of advanced resorption, the placement of implants behind the mental foramens is contraindicated.
- Regarding the floor of the mouth and the mylohyoid line, the body of the mandible appears flat at the level of the crest of the molar regions. This is due to a lingual bony convexity often present. The placement of implants in a too lingual position can then lead to a breach of the floor of the mouth .
- For the maxillary sinuses, after tooth loss, crestal resorption of the maxilla is associated with their pneumatization, which limits the subsinus bone volume available for implant placement . Sometimes several years after tooth loss, only a thin bone lamella remains under these sinuses.
- The tuberosity and pterygomaxillary regions undergo less bone resorption than other parts of the maxilla. Implant placement at this level is sometimes indicated when bone volume is insufficient in the maxillary molar region . This is a risky procedure due to the many anatomical obstacles it contains (descending palatine artery).
- Muscle insertions become more superficial due to bone resorption and thus limit the space available for the prosthesis.
In the vestibular regions, this is the case of the insertion of the buccinator muscle. When the bone is resorbed, this insertion and that of the other muscles can be located near the crestal edge. The mylohyoid and genioglossus muscles have an impact on the space available on the lingual surface of the mandible . The floor of the mouth approaches the mandibular crest, leading to a superficialization of the lingual nerve laterally and of the genioglossus muscles behind the mandibular symphysis.
4-Surgical risks :
Maxillary:
-sinus penetration
-Perforation of the lower nasal wall
-Section of the greater palatine artery (risk of bleeding)
Mandible:
-Risk of breakage of the lower dental canal
-Injury to the mental pedicle (labio-mental anesthesia)
-Injury to the sublingual artery or submental artery (risk of bleeding).
5-Bone remodeling : Bone is a living material that constantly adapts to its environment. Physiological bone remodeling occurs through the alternation of a resorption phase and a bone formation phase. This process is essential for maintaining skeletal integrity and calcium-phosphorus homeostasis. Many systemic (mainly hormonal) or local factors effectively regulate bone remodeling and ensure a balance between resorption and bone formation.
The remodeling cycle lasts about 4 months in adults. During this cycle, the formation phase is significantly longer (4 to 6 months) than the resorption phase (2 to 4 weeks). Bone remodeling occurs in 4 distinct phases:
a-activation phase : access of osteoclasts to the bone surface is blocked by the bordering osteoblasts. These bordering cells retract, under the effect of so-called “osteoresorbing” factors such as parathyroid hormone, thus freeing access to osteoclast cells which can then adhere to the bone matrix.
b-Resorption phase : osteoclasts flatten along the bone borders and attach themselves to them by a cementing zone, with proteins that create a watertight seal. The central zone of these osteoclasts, called the “brush border”, synthesizes lysosomal enzymes leading to a significant lowering of the pH, thus solubilization of the hydroxyapatite crystals and the creation of a “resorption chamber”.
c-reversal phase : the action of the osteoclast eventually stops and the cell migrates to the bone or dies by apoptosis, a process by which cells trigger their self-destruction in response to a signal. Macrophages then intervene and smooth the bottom of the gap.
d-bone formation phase : this phase includes two times induced by osteoblasts: production of collagen fibers forming the bone matrix, then mineralization of this matrix. Once bone formation is complete, the osteoblasts lose their activity, take on a flattened shape and become bordering osteoblasts.
6-Bone repair around an implant : this process will differ depending on the nature of the bone in contact with the implant: trabecular cancellous bone, highly vascularized, or cortical bone, poorly vascularized. Jan Lindhe (2002) clearly indicates that “the biological reaction when the implant is placed will cause a different reaction at the level of the cortical bone and the cancellous bone”
- Cancellous bone : Marco et al (2005) indicate that the first biological response to the insertion of an implant is bleeding by vascular rupture putting the blood in direct contact with the surface of the implant. This contact will allow the calcium and phosphorus ions present in the blood to be adsorbed by this surface made of an oxide, ultimately leading to the biological bonding of the bone to the titanium forming a calcified fibrillar layer. From the first days, mesenchymal cells, preosteoblasts and osteoblasts adhere to this calcified layer and produce collagen fibers characteristic of a “woven bone” then of an osteoid tissue. After a few days, this osteoid tissue transforms into a lamellar bone with trabeculations delimiting large medullary spaces rich in vessels, mesenchymal cells and osteoblasts. After a few months this bone will undergo a remodeling depending on the loads that the implant undergoes and will show Haversian trabeculae.
Reminder of the fundamental bases of bone anatomy and physiology
Reminder of the fundamental bases of bone anatomy and physiology
- Cortical bone : cortical bone is poorly vascularized. Lindhé has shown that its reaction to drilling will be much more complex, and that its healing will be preceded by necrosis and resorption, explaining the poor results of short implants placed only in this bone. Bone preparation and implant placement have the effect of damaging the collagen of which the bone is composed. This tissue will undergo hyalinization (result of the destruction of collagen), normally resorbed by osteoclasts and macrophages. Since cortical bone is only very poorly vascularized, this hyaline tissue (a type of bone necrosis) is maintained and can increase in thickness over time, leading to mobility of the implant and its fibrointegration. The repair process begins with the formation of a vascular network within the cortical bone, a network leading to the neo-alveolus created. The appearance of osteoclasts and macrophages allows the elimination of damaged bone, then osteoblasts intervene to ensure the reconstitution of new bone.
Reminder of the fundamental bases of bone anatomy and physiology
Conclusion : knowledge of bone, its remodeling and its healing will allow us to adopt reasoned surgical protocols, which vary according to the clinical situation and no longer standardized protocols, which are unsuitable in certain cases.
Reminder of the fundamental bases of bone anatomy and physiology
Bibliography
-G.STEPHAN, R.NOHARET, MVBERTERETCHE, P.MARIANI Anatomical risks in the mandible in implant surgery implant-volume 12-Number 1- 2006
-MITHRIDADE DAVARPANAH, BORIS JAKUBOXCZ-KOHEN,MIHAELA CARAMAN, MYRIAM KEBIR-QUELIN Implants in dentistry Cdp edition
-M. DAVARPANAH,S. SSZMUKLER-MONCLER PMKHOURI, B. JAKUBOWICS-KOHEN, H. MARTINEZ Manual of clinical implantology concepts, protocols and recent innovations 2nd edition Edition Cdp Accredited collection Continuing education JPIO
-M.DAVARPANAH, H.MARTINEZ, M.KEBIR, J -F. TECCUCIANU Clinical Implantology Manual
-implantology: fundamental bases, clinical consequences
Reminder of the fundamental bases of bone anatomy and physiology
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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.
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Early consultation helps detect dental problems in time.