Special Article - Dental Implants
J Dent & Oral Disord. 2016; 2(6): 1030.
Safety Zone to Mandibular Canal for Posterior Mandible Implant Surgeries
Garcia Blanco M* and Puia SA
Department of Oral and Maxillofacial Surgery, University of Buenos Aires, Argentina
*Corresponding author: Garcia Blanco M, Department of Oral and Maxillofacial Surgery, School of Dentistry, University of Buenos Aires, Buenos Aires, Argentina
Received: July 04, 2016; Accepted: August 04, 2016; Published: August 05, 2016
Dental implant surgeries are widely used in dentistry due to their numerous advantages. Many articles have focused on local accidents and complications of this treatment. One of the major complications is the injury of the inferior alveolar nerve, which could potentially be damaged permanently. In order to protect clinicians and patients, some authors recommend leaving a safety zone around this nerve. The aim of this article is to review guidelines proposed to avoid this complication, and discuss critical clinical situations when this recommendation could be reappraised.
Keywords: Dental implants; Diagnosis; Nerve injury; Safety zone; Short implants
IAN: Inferior Alveolar Nerve; MC: Mandibular Canal; CT: Computed Tomography; CBCT: Cone Beam CT
Nerve injuries and safety zone
Replacement of teeth through dental implants is a widely accepted technique due to its numerous advantages [1-3]. Although it has many benefits, there may be some unfavorable outcomes. Damage to the Inferior Alveolar Nerve (IAN) is a potential major complication in mandibular dental implant surgeries. Physical harm can occur during anesthesia, flap elevation or advancement, bone graft harvest, osteotomy preparation or implant placement [4-7]. The prevalence of nerve damage that results in altered lip sensations ranges from 0% to 40% in old literature [8-10]. Currently, midcrestal incisions and Computed Tomography (CT) help prevent this type of injury, and recent studies report prevalence below 3% [11,12]. Patients’ symptoms include complete absence of sensation, diminished or increased sensitivity, abnormal sensations which may not be unpleasant and spontaneous or mechanically evoked painful symptoms [13-15]. The IAN enters the mandible in the internal side of the ramus, lies beside the lingual plate and makes a sudden turn in direction towards the buccal plate in the first molar area to the mental foramen, which size and frequency are still controversial [16-19]. Although Cone Beam CT (CBCT) seems to have the best potential efficiency in the identification of the Mandibular Canal (MC) , nerve detection maybe difficult in some cases .
Recommendations have been proposed to avoid injuring the inferior alveolar nerve. It is especially important to set guidelines in the dental community about how to act prudently to avoid nerve damage, and it is essential for beginners in the discipline. Lack of experience can cause injuries due to not recognizing drill length marks, confusing these marks through poor visualization, extending drilling time and overheating the bone, and other deficiencies in surgical technique (such as vertical incision next to mandibular foramen, or excessive traction of the flap). As some manufacturing companies make implant drills slightly longer to improve drilling efficiency [22,23] drill stops could be used to prevent over-drilling [4,22,24]. It would also be useful to take intra-operative radiographs during implant bed preparation in atrophic mandibles to confirm distance to MC [22,24].
The safety zone to the MC that most authors have proposed is 2 mm [25,26]. Implant length is chosen by making vertical linear measurements, from the top of the alveolar crest to the upper border of the mandibular canal, subtracting 2 mm as a safety zone to the MC. Since the advent of CT, clinicians have begun to reduce this limit, setting a safety zone in the range of 1 to 2 mm to MC [27-29]. This is especially important in some clinical situations, when bone height is reduced, and the safety zone needs to be reappraised. The aim of this article is to review the guidelines proposed to avoid nerve injuries and discuss different clinical situations.
Bone height 12 mm or greater
When patient’s bone height is 12 mm, the clinical situation has a simple, predictable resolution by placing a 10mm long dental implant, maintaining a 2 mm safety zone to the nerve (Figures 1-6). If there is greater bone height, implant length could be increased, improving bone implant contact, although some authors recommend not placing implants longer than 12 mm in posterior mandible, because it would increase the possibility of complications .
Figure 1: Screenshot of treatment planning for implant length and position in good clinical conditions.
Figure 2: Posterior mandible flapless approach.
Figure 3: Cortical bone drilling in left first molar area.
Figure 4: Implant placement leaving 2 mm of safety zone or more.
Figure 5: Periapicalpost-surgical radiograph.
Figure 6: Screw-retained single crown in good clinical condition.
Bone height 11 mm or less
Problems begin when bone height is 11 mm or less, as the clinician may decide to use bone regeneration, IAN lateralization, placing the implant buccal or lingual to the IAN, placing short implants (less than 10 mm) , or reducing the safety zone by placing a standard 10 mm long implant.
In this situation, an experienced surgeon may consider drilling the bone with controlled drill pressure to 10 mm and placing a 10 mm long implant (Figures 7-10). This resolution would provide greater predictability over time than a shorter implant would. The safety zone set to MC is a general guideline to avoid nerve injuries, but sometimes 1 or 2 mm can make the difference between performing a short, predictable surgery with low morbidity or a long, complex, less predictable bone regeneration surgery.
Figure 7: 10.5 mm bone height for right second molar implant placement.
Figure 8: Intra operative picture leaving external hex dental implant platform located slightly above the bone limit.
Figure 9: Less than 2 mm safety zone for replacement of a right second molar.
Figure 10: Prosthetic rehabilitation of standard length implant in 10.5 mm bone height.
IAN lateralization is a complex surgery that requires more research to determine whether the technique can be recommended or not, as postoperative permanent nerve injury of 22% has been reported .
Placement of an implant buccal or lingual to the MC is a highly sensitive and blinded surgery. From the prosthetic point of view, the implant emergence profile is usually far from the occlusal surface. Currently, there is no long-term evidence to support its use .
Bone height less than 10 mm
The most challenging situation is to have less than 10mm, as the clinician may decide either to place a short implant or to perform bone regeneration.
Mandibular bone regeneration includes procedures ranging from simple guided bone regeneration to microvascular free flaps in extreme situations. Other procedures as distraction osteogenesis, onlay and inlay grafts may also be performed. Guided bone regeneration is the most frequently used regeneration technique due to its simplicity and predictable results for horizontal regenerations of exposed implant threads, and some articles report vertical regenerations of 3 to 6 mm, especially when titanium membranes were used. The key factor in these surgeries is to avoid exposure of the membrane, which my lead to infection and resorption of the graft . Onlay bone grafts are unlikely to cause IAN injury, involve relatively simple surgery and provide immediate bone regeneration. Their main disadvantage is that the resorption rate is often high, with reports of 17 to 41% of the total bone regeneration achieved. Alternatively, regeneration by inter positional or inlay grafts have been proposed. The main advantage of this technique is the lower bone resorption because of its better vascularization. Its disadvantages are the complexity of the surgery, increased possibility of IAN injury, and the requirement of at least 4 mm of basal bone . Osteogenic distraction is similar to inlay grafts, but regeneration performed also gradually recovers soft tissue. Its main disadvantage is the complexity of surgery for posterior mandible and discomfort of the device for the patient, though the latest intraosseous distractors may reduce these problems . Microvascular bone grafts are highly sensitive surgeries performed in extreme cases. Their main advantage is immediate internal vascularization of the graft resulting in an almost complete absence of loss of graft volume .
When augmentation procedures were compared with the placement of short implants, the latter were considered more successful, since the treatment is faster, cheaper and associated to less morbidity . Reducing this safety zone carefully, and considering the simplicity, speed, and predictable placement of short dental implants (shorter than 10 mm) an alternative to vertical bone regeneration in posterior mandible could be proposed (Figures 11- 14). Although many papers have highlighted the advantages of short implants in the posterior mandible; the studies do not exceed 10 years of clinical follow-up, so these implants should still be used with some caution.
Figure 11: Pre surgical CT with 7.07 mm bone height for a left second molar replacement.
Figure 12: Reduced interocclusal space for placing a crown.
Figure 13: Less than 1 mm safety zone for replacement of a left second molar.
Figure 14: Prosthetic rehabilitation of short implant in 7 mm bone height.
Dental implant surgeries in posterior mandible need to be planned carefully to avoid any permanent undesirable complications. When an acute dental intervention is performed, the clinician must clarify all possible outcomes and specify them in the informed consent form. Any high risk surgeries should be evaluated in accordance with patient’s decision. Reducing the safety zone carefully, and considering the placement of short dental implants (evaluating occlusal loads and crown to implant ratio), appears to be a straightforward, quick, predictable method for resolving intermediate atrophied mandibles. Extremely reabsorbed mandibles (less than 6 mm) should still be treated with bone regeneration surgeries, using the procedures described above.
Many research papers on morse taper implants currently report better stability and less bone resorption, recommending an infrabone position for implantation . This treatment leads to more dangerous situations through closer approach to the MC in the posterior region of the mandible. As this technique still does not position the implant in the apical-occlusal dimension  (1 or 2 mm below crest) and no randomized controlled clinical trial has been conducted to compare it to conventional implant placement, it should be used with some caution.
Finally, if a nerve injury develop, it could be treated physiologically or pharmacologically. Physiological treatment includes removal of the implant within 36 hours post-surgery when it is in contact with or causing pressure on the MC, in order to prevent permanent damage . If the implant causing the problem is already osseointegrated, it can be removed using trephine drill; or alternatively, an apicoectomy of the implant can be performed. Pharmacologic therapies for acute nerve injuries include the use of corticosteroids (administered in high doses within 1 week of the injury) and non-steroidal antiinflammatory drugs. To control inflammatory reactions in the injured nerve after the first week, a course of oral steroids can be prescribed. Oral prednisolone 1 mg per kg per day (maximum 80 mg) for the first week, stepping down by 10 mg daily over the following week, can be prescribed. As an alternative or adjunct therapy, non-steroidal antiinflammatory drugs can be prescribed (800 mg of ibuprofen 3 times daily for 3 weeks). If the situation improves, the clinician can prescribe another course of anti-inflammatory drugs . The combination of high-dose steroids and non-steroidal anti- inflammatory medication can be associated with significant complications including upper gastrointestinal ulceration if prescribed long term. Even for a few weeks, prescription of these drugs should be undertaken with consideration to the patient’s medical history and caution . In some complicated cases, additional pharmacologic agents can be prescribed, such as antidepressants, anticonvulsants, antisympathetic agents, and topical medications .
When placing implants in the posterior mandible, a safety zone is recommended to avoid IAN injuries. A standard implant length of 10 mm provides predictable rehabilitation, and implants longer than 12 mm are not recommended because of the greater likelihood of complications. As a general rule, when working with panoramic radiographies, a distance of 2 mm to the MC would be recommended; and this safety zone could be reduced to 1 mm with high quality CT o CBCT. Short implants appear to be a reasonable alternative to avoid performing bone regeneration surgeries which may have major complications. Each clinical situation should be analyzed individually to choose the safest, most predictable resolution for each patient.
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Citation:Garcia Blanco M and Puia SA. Safety Zone to Mandibular Canal for Posterior Mandible Implant Surgeries. J Dent & Oral Disord. 2016; 2(6): 1030. ISSN:2572-7710