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Mandibular Anatomic Implications for Dental Implant Surgery
Introduction
A strong understanding of surgical anatomy and its variations is crucial for clinicians placing implants in the mandible.
A detailed evaluation of mandibular vital structures is necessary before surgery, including clinical evaluation, visual examination, and palpation.
Clinicians should have a clear 3D vision of anatomic structures in relation to the intended implant procedure.
A thorough radiographic examination is needed to provide information on the location and topography of the 3D anatomy.
Mandibular Anterior Region
Mandibular Anterior Hourglass Anterior Mandibles
The anterior region has been considered safe and predictable for implant placement due to favorable bone quality (thick cortical and dense trabecular bone).
Mandibular anterior shapes are classified as hourglass, ovoid, pear, sickle, and triangular; the pear shape is most common in edentulous and dentate patients.
Narrow alveolar width or osseous constriction can compromise the area, creating an "hourglass effect," indicative of a developmental abnormality.
Hourglass mandibles, with an incidence rate of approximately 4%, require careful consideration to avoid perforations during implant placement.
The position of the alveolar constriction may vary (high, low, or variable within the alveolus).
A thorough 3D cone beam computed tomographic (CBCT) examination is recommended to prevent complications, and guided surgery is advised to minimize perforation risks.
Butura et al. classified mandibular anterior constrictions as facial, lingual, and hourglass.
Treatment options include alveoplasty, staged bone graft reconstruction, angled implants, and extra-long implants to bypass the constriction.
Clinical Relevance
Treatment strategies are based on the location and extent of the undercut in hourglass mandibles.
Implant placement may be contraindicated in some cases.
Less severe constrictions may undergo osteoplasty with implant placement; however, this may increase crown height space, leading to biomechanical issues.
Other constrictions may require grafting to increase bone volume.
Perforation of bony mandibular plates can lead to life-threatening hemorrhage, especially in the sublingual region, potentially traumatizing the sublingual or submental artery.
Arterial bleeding requires immediate identification and aggressive treatment; the origin may be the lingual artery, facial artery, or their branches.
Perforation of the submental or sublingual artery can cause an expanding ecchymosis (sublingual hematoma) compromising the airway.
If this occurs, reposition the patient upright, apply bimanual pressure, and summon immediate emergency assistance if the airway is compromised.
Median Vascular Canal
Radiographic examination often reveals a radiolucent median vascular canal in the mandibular midline, housing the bilateral sublingual arteries that enter the lingual foramen.
The lingual foramen is seen as a radiopacity below the genial tubercles, visible on approximately 52% of CBCT scans.
This arterial anastomosis may travel anteriorly, inferiorly, or superiorly within the anterior mandible, sometimes exiting the facial aspect of the symphysis.
Median vascular canals are present in 100% of cases detected on CBCT examinations, but only 4.2% of the time on 2D panoramic radiographs.
Gahleitner et al. reported one to five canals per patient with an average diameter of mm, ranging from to mm.
The presence and size of the sublingual anastomosis and the median vascular canal are easily seen on CBCT scans.
In approximately 31% of lingual vascular canals, the diameter exceeds at least 1 mm.
The sublingual artery, originating from the external carotid artery, has four main branches: suprahyoid, dorsal lingual, deep lingual, and sublingual.
Clinical Relevance
When planning implants in the anterior mandible, modify the position to avoid encroaching on a large anastomosis.
Violation of this area may result in excessive bleeding, which is usually controlled by placing an implant, direction indicator, or surgical bur in the osteotomy site.
Encroaching on this area will not cause neurosensory issues because there are no sensory fibers within the canal.
Severely Angled Anterior Mandible
Division C-a refers to Division C bone with excessive angulation, greater than 30 degrees, most often found in the anterior mandible.
Bone augmentation is usually required for ideal implant placement, but a diagnostic wax-up should be completed first.
Division C-a mandibles are usually associated with skeletal Class III patients.
Clinical Relevance
Root form implants placed in Division C-a will lead to poorly positioned implants that will most likely be nonrestorable for a fixed prosthesis.
This will likely result in an overcontoured prosthesis, speech difficulty, compromised tongue space, and inability to obtain an ideal occlusion.
A staged bone graft and implant treatment plan should be formulated in most cases.
Lack of Keratinized Tissue
As the mandibular osseous process progresses, the presence of keratinized tissue becomes more compromised.
Implants are healthiest when sufficient keratinized tissue exists; its absence may contribute to implant failure.
Mobile, nonkeratinized mucosa exhibits greater probing depths and increases the susceptibility of peri-implant regions to plaque-induced destruction.
Mobile mucosa may disrupt the implant-epithelial attachment zone, increasing the risk for inflammation from plaque.
Attached tissue on the facial flap (mandible) provides greater resistance for the sutures against tension from the mentalis and buccinator muscles.
An incision made facial to the attached tissue may cause partial ischemia to some of the crestal tissue.
Incision in unkeratinized facial tissue may sever larger blood vessels, increasing bleeding and potentially complicating final suturing.
Clinical Relevance
Evaluate the quality and quantity of keratinized tissue for implant sites.
If insufficient attached tissue is present, complete tissue augmentation procedures before implant placement.
For larger edentulous sites, especially in the mandible, modify the incision to maintain the attached tissue in some cases.
If the crest of the ridge is above the floor of the mouth and there exists greater than 3 mm of attached, keratinized gingiva on the crest of the ridge, a full-thickness incision is made, bisecting the attached tissue.
If less than 3 mm of attached gingiva exists on the ridge, the full-thickness incision is made more to the lingual so that at least 1.5 mm of the attached tissue is to the facial aspect of the incision line.
Another treatment option includes the use of an acellular dermis (e.g., OraCell; Salvin Dental Corp.) at the time of implant placement, increasing tissue thickness and quality during implant integration.
Inadequate Width of Bone
A common consequence of tooth loss and bone remodeling in the mandibular anterior region is the resultant narrowing and knife-edge configuration of the mandibular bony ridges (i.e., Division B available bone).
Pietrokovski et al. evaluated edentulous ridges in human jaws and found that 43% of mandibular anterior ridges were knife-edge and 38% in the premolar region.
Nishimura et al. reported a higher incidence of mandibular knife-edge ridges in females compared with males, mainly because of increased osteopenia changes with unfavorable bone mineral density values.
Clinical Relevance
Before implant placement, it is often necessary to reduce the bone width (i.e., osteoplasty), which results in an increased horizontal width of available bone.
An osteoplasty may be carried out with various methods, including osteoplasty burs, barrel burs (i.e., acrylic burs), rongeurs, bone chisels, and Piezosurgery units.
By increasing the width of bone, dental implants may be placed with sufficient bone on the buccal (∼ mm) and on the lingual (∼ mm).
Although an osteoplasty increases available bone for implant placement, many detrimental effects may result.
Reduction of a knifelike ridge will decrease the amount of cortical bone present.
Cortical bone is a crucial component for primary stability of the implant and is responsible for a greater stress distribution.
As the bony height is reduced, the crown height space increases.
The increased crown/implant ratio results in greater strain at the peri-implant interface, which predisposes the implant to biomechanical complications.
The amount of osteoplasty required should be determined before surgery via the use of CBCT treatment planning.
In general, if a fixed prosthesis (e.g., FP-1, FP-2, FP-3) is indicated, a minimal osteoplasty is recommended.
By minimizing the reduction of bone height, the possibility of a crown/implant ratio problem resulting is decreased.
In some cases bone augmentation may be required to maintain the height of bone and increase the bone width, rather than reducing the ridge by means of an osteoplasty.
If a removable prosthesis (e.g. RP-4, RP-5) is indicated, a more aggressive osteoplasty is recommended, because this will allow for increased space for the removable prosthesis (i.e., thickness of acrylic, attachment space).
In general, the greater the interocclusal space, the less likely a prosthesis or attachment fracture can occur.
Mandibular Posterior
The posterior mandible is a predictable anatomic area for implant placement; however, there are many drawbacks that include compromised available bone in height and width, significant undercuts, difficult access, and numerous vital structures that may be damaged (e.g., inferior alveolar nerve [IAN], mental nerve, submandibular gland).
Iatrogenic violation of these vital structures may result in neurosensory disturbance, pain, infection, excessive bleeding, and compromised implant positioning.
Lack of Bone Height
The posterior mandible resorbs from buccal to lingual, transforming from a Division A to a Division B rather rapidly.
Implant placement in an ideal position for prosthetic rehabilitation may be difficult because of the trajectory of the posterior mandible.
When limited alveolar ridge height exists, four options are usually available: (1) no treatment, (2) vertical ridge augmentation with delayed implant placement, (3) vertical bone augmentation with simultaneous implant placement, and (4) the use of short implants.
Vertical Bone Augmentation
With severely resorbed alveolar ridges in the posterior mandible, the available bone height for standard implant placement is often limited by the proximity of the mandibular canal.
Vertical bone augmentation is an option for increasing the ridge dimensions, thereby allowing for placement of standard-length implants.
By increasing the bone height, esthetics and biomechanical complications are less likely to complicate the longevity of the implant prosthesis.
Increasing bone height in the posterior mandible is one of the most challenging procedures in implant dentistry.
To increase the available bone, various techniques, including autogenous block grafts, guided bone regeneration, and distraction osteogenesis, have been discussed in the literature.
An increased rate of surgical complications and enhanced patient morbidity have been associated with these types of procedures.
Shorter Implants
Recently, the use of short implants (∼8 mm) in the atrophic posterior mandible has been introduced to circumvent the need for vertical bone augmentation.
The loss of vertical bone height is often associated with inadequate available bone for implant placement, and the safety zone (2-mm distance between the implant and nerve canal) is sometimes compromised with conventional length implants.
The use of shorter length implants offers several advantages in comparison with vertical bone augmentation: less invasive surgery, less surgical experience required, less expensive, and faster treatment time.
Shorter implants do have drawbacks: they may result in an increased crown height space, less surface area in comparison with standard length implants, and a possible higher rate of biological and technical complications from occlusal overload.
Studies on the use of short implants (∼8 mm) are promising.
Evaluate force factors (e.g., opposing occlusion, parafunction) and adhere to an implant-protected occlusion.
With short implants, the widest diameter implant possible should be selected, along with an increased number of implants.
The final prosthesis involving multiple implants should always be splinted for greater force distribution.
Mandibular Deformation (Flexure of the Mandible)
Full-arch implant-supported prostheses with a rigid substructure have become controversial because of the associated increased strain at the bone-implant interface.
Jaw deformation may transmit excessive stress, which can result in complications.
Pain has been associated with full-arch rigidly splinted prostheses.
Inaccuracies from deformation of impression material may result in ill-fitting or nonpassive superstructures in different jaw positions.
Mandibular deformation has been associated with loosening of full-arch implant-supported prostheses and possible fractures of prostheses during mastication.
Etiology
Flexure: The body of the mandible flexes distal to the foramen on opening and has torsion during heavy biting, with potential clinical significance for full-arch implant prostheses.
Medial convergence is the movement most commonly addressed.
The mandible between the mental foramina is stable relative to flexure and torsion; however, distal to the foramina, the mandible exhibits considerable movement toward the midline on opening due to the attachment of the internal pterygoid muscles.
The distortion of the mandible occurs early in the opening cycle, and the maximum changes may occur with as little as 28% opening (or about 12 mm).
The greater the active opening and protrusive movements, the greater the amplitude of mandibular flexion.
The amount of movement varies among individuals and depends on the density and volume of bone and the location of the site in question.
The more distal the sites, the more medial is flexure.
The amplitude of the mandibular body flexure toward the midline has been measured to be as much as μm in the first molar-to-first molar region to as much as μm in the ramus-to-ramus sites.
Torsion: Torsion of the mandibular body distal to the foramina has also been documented.
Hylander evaluated larger members of the rhesus monkey family (macaque) and found the mandible twisted on the working side and bent in the parasagittal plane on the balancing side during the power stroke of mastication and unilateral molar biting.
Torsion during parafunction is caused primarily by forceful contraction of the masseter muscle attachments.
Parafunctional bruxism and clenching may cause torsion-related problems in the implant support system and prosthesis when the mandibular teeth are splinted from the molar-to-molar regions.
Implants placed in front of the foramina and splinted together or implants in one posterior quadrant joined to anterior implants have not shown these complications related to the flexure or torsion of the mandible.
Complete implant-supported fixed restorations can halt the posterior bone loss associated with edentulism, improve psychological health, and produce fewer prosthetic complications than removable restorations.
Prevention
The concept of flexure and torsion does not affect the maxilla, where all implants are often splinted together, regardless of their positions in the arch.
If implants are positioned bilaterally in the premolar/molar regions of the mandible, the final prosthesis should be fabricated with two sections. This will minimize the possibility of flexure/torsion issues. Usually, the prosthesis is. splint in the premolar area.
With implant support on only one posterior side, full-arch splinted prostheses will not be subject to the flexure/torsion problems.
With no posterior implant support, full-arch splinted prostheses may be fabricated without concern regarding flexure/torsion problems.
Treatment
If a full-arch splinted prosthesis is fabricated and the patient exhibits complications (e.g. pain, difficulty opening, posterior bone loss) related to the flexure/torsion of the mandible, the prosthesis should ideally be re-fabricated to allow for stress relieve for flexure and torsion forces. This is most likely completed by making the prosthesis in more than one piece.
Bony Anatomic Areas
Fixed implant prosthesis (FP-1, FP-2, FP-3)
Porcelain fused to metal: 10 mm
Zirconia: 8 mm
Removable implant prosthesis (RP-4, RP-5)
Attachments (no bar): 9 mm (e.g., Locator)
Bar + attachment: 15 mm
Posterior Lingual Undercut
detailed knowledge of the three-dimensional anatomy is important, a lingual undercut is often present, this may lead to complications with life-threatening consequences.
Parnia et al. classified posterior lingual concavities into three types: type 1 (20%): flat depressions less than 2 mm in depth, type 2 (52%) occur with 2 to 3 mm in depth, type 3 (28%) showed significant concavities of more than 3 mm.
Nickenig et al. classified posterior mandible morphology to be U-shaped (undercut), P-shaped (parallel), and C-shaped (convex). Lingual undercuts had a prevalence rate of 68% in the molar region, with the prevalence rate far greater in the second 24.91
molar region (90%) than in the first molar region (56%). Other studies have shown that lingual undercuts occur in approximately 66% of the population, with a mean undercut of 2.4 mm (Fig. 30.15).
Clinical Relevance
If perforation of the lingual plate is made with either the surgical drill or implant placement, life-threatening situations may result from sublingual bleeding.
Within the lingual undercut area, the sublingual and submental arteries are present. Trauma to either of these arteries can result in a sublingual hematoma and airway compromise.
If the perforation were to occur above the mylohyoid muscle, damage to the lingual nerve may result in a neurosensory impairment.
If an implant is inserted in this area that extends into the undercut, constant irritation from the extruded implant in the soft tissue may cause the patient chronic pain.
In some cases violation may predispose the patient to infection.
Accurate measurements must be determined to prevent over-preparation of the osteotomy site in the posterior mandible.
This is most easily completed with a CBCT examination. A clinical examination and palpation of the bone ridge at the proposed implant sites should also be completed.
Osteotomy angulation should be carefully evaluated because improper drilling angulation may also lead to perforations.
Shorter implants with a tapered design have been shown to be beneficial in avoiding lingual bone perforations.
de Souza et al. has shown that the submandibular fossa has a direct influence on implant placement (i.e., implant size, position, and angulation) 20% of the time (Fig. 30.16).
Vascular Considerations
Incisive Canal Vessels
The incisive artery is the second terminal branch of the inferior alveolar artery, which is a branch of the maxillary artery. The incisal branch continues anteriorly after supplying the mandibular first molar area, where it innervates the incisor teeth and anastomoses with the contralateral incisal artery. In rare cases the incisive canal is large, lending to greater bleeding during osteotomy preparation or bone-grafting procedures. The exact location of the incisive canal is easily determined via a CBCT evaluation in the panoramic or sagittal views.
Clinical Relevance
Clinicians often confuse the incisive canal with an anterior loop of the mental nerve. The nerve, artery, and vein within this canal may cause bleeding episodes if traumatized. Usually, the placement of the implant, a direction indicator, or surgical bur can be placed into the osteotomy to apply pressure to allow for the clotting process.
Inferior Alveolar Artery
The inferior alveolar artery is a branch of the maxillary artery, one of the two terminal branches of the external carotid. Before entering the mandibular foramen, it gives off the mylohyoid artery. In approximately the first molar region, it divides into the mental and incisal branches. The mental branch exits the mental foramen and supplies the chin and lower lip, where it eventually will anastomose with the submental and inferior labial arteries. The exact location of the inferior alveolar artery is easily determined via a CBCT evaluation in the panoramic or sagittal views.
Clinical Relevance
Normally the inferior alveolar artery is located superiorly to the Inferior alveolar nerve (IAN) IAN within the bony mandibular canal. Drilling or placing an implant into the inferior alveolar canal may predispose to significant bleeding. Some authors have recommended the placement of an implant or direction indicator short of the canal to control the bleeding; however, this may lead to possible neurosensory disturbances from hematoma or local irritation to the IAN canal. A 2.0-mm safety zone should be established to prevent complications in this area. If bleeding does occur, follow-up postoperative care is essential because hematoma formation within the canal may lead to a neurosensory impairment. This condition should be monitored because it may progress to respiratory depression via a dissecting hematoma in the floor of the mouth.
Buccal Artery
A common donor site for autogenous grafting is the lateral ramus area in the posterior mandible. When making the incision lateral to the retromolar pad, a common blood vessel to sever is the buccal artery. The buccal artery is a branch of the maxillary artery and will most likely cause a significant bleeding episode. This artery runs obliquely between the internal pterygoid and the insertion of the temporalis on the outer surface of the buccinator.
Clinical Relevance
In most cases, damage to the buccal artery is very difficult to avoid. Incision and reflection will usually encompass the area of buccal artery location. When performing surgery in this area, a hemostat should always be available for immediate access to clamp the vessel. A curved hemostat should be used to clamp the vessel, thus decreasing the bleeding. It should be left in place for 3 to 5 minutes. If bleeding persists, a ligature may be placed with Vicryl suture material (i.e., resorbable) (Fig. 30.18).
Facial Artery
The facial artery is a branch of the external carotid artery, lying superior to the lingual artery and medial to the ramus of the mandible. It courses below the digastric and stylohyoid muscles, and passes through a groove in the submandibular gland before it becomes superficial around the inferior border of the mandible. There are two main branches of the facial artery: the facial and cervical. The facial branch encompasses five branches, which supply the eye, nose, and lips. There are four branches of the cervical region, supplying the pharynx, soft palate, auditory tube, and submandibular gland.
Clinical Relevance
Excessive retraction in this area may lead to trauma to the facial artery. If bleeding from the facial artery exists, pressure should immediately be applied to the angle of the mandible over the vessel. Usually, immediate medical assistance will need to be summoned.
Neural Considerations
Lingual Nerve
The lingual nerve is a branch of the trigeminal nerve that provides sensory innervation to the mandibular lingual tissue and the anterior two-thirds of the tongue. The lingual nerve is a concern for implant clinicians because it may be damaged during reflection of the lingual flap. The lingual nerve is most commonly found 3 mm apical to the alveolar crest and 2 mm horizontal from the lingual cortical plate. However, 22% of the time, it may contact the lingual cortical plate. Variations of this nerve have reported it to be located lingual to the third molar area, at or above the crest of the bone.
Clinical Relevance
When elevating tissue in the posterior mandible, always maintain the retractor on the bone and minimize stretching of the tissue on the lingual aspect of the mandible. The lingual nerve is very susceptible to neuropraxia types of nerve impairments. In addition, no lingual vertical release incisions should be used because of the variant lingual nerve anatomy. In addition, in the posterior ramus area, incisions should always be lateral lateral to the retromolar pad because the lingual nerve transverses this area in 10% of cases.
Inferior Alveolar Nerve
Prevention of iatrogenic injuries to the third division of the trigeminal nerve is paramount in implant dentistry today. A resultant neurosensory impairment in the head and neck region may affect the patient’s quality of life and may present potentially significant medicolegal problems for the clinician. To prevent damage to these vital nerve structures, it is imperative for the implant dentist to have a comprehensive radiographic survey of the region, thorough knowledge of the normal versus variant anatomy, and awareness of intraoperative surgical techniques to minimize the possibility of nerve impairment.
Radiographic Considerations
Two-Dimensional Radiography
Today the use of 2D radiography is becoming less common for dental implant treatment planning. Two-dimensional radiographs, mainly panoramic, have many inherent disadvantages in evaluating potential implant sites. All panoramic (2D) radiographs exhibit some degree of distortion, nonuniform magnification, and