Prevalence and importance of the anterior loop of the inferior alveolar nerve in fully edentulous mandibles during dental implant placement.
THE ANTERIOR LOOP
The purpose of this review was to demonstrate actual instances and challenges facing practitioners in the field of implantology. This article reviews the prevalence of the inferior alveolar nerve (IAN) anterior loops in edentulous patients and the importance of their physical location. The study involves >1600 patients with mandibular dental implants over a 14-year period. The mental foramen of all subjects who received more than two mandibular implants were examined and probed to identify the presence of the IAN anterior loop. None of the patients reported permanent paraesthesia; however, some mild and transient paraesthesia was reported by some patients and lasted no more than four months. The etiology of this paraesthesia is unknown but potential contributing factors may include: probing, stretching of IAN or post-operative inflammation.
FIGURE 1. Distance and relationship between mental foramen, anterior loop, safety margin and implant position.
In patients who have been edentulous for several years, mandibular implants are placed between the mental foramina given the sufficient quantity, as well as quality of bone matter in this region. Practitioners familiar with biomechanical laws will seek the optimal distribution of the implants.
RELATIONSHIP BETWEEN THE MENTAL FORAMEN AND THE ANTERIOR LOOP
In the literature, the length of the anterior loop and sites varies, ranging from 3 mm (Misch CE, Crawford E, 1990) to 6.95 mm (Arzouman MJ, Otis L, Kip Levine D nis V,1993).
FIGURE 2. Implants placed far from the mental foramina with no anterior loop present.
FIGURE 3. Modified explorer to probe the foramen.
FIGURE 4. Mental foramen and neurovascular bundle exposed.
FIGURE 5. Probing the foramen.
FIGURE 6. Marker at 4 mm from mesial lateral wall of the foramen.
However, all agree that the presence and the physical assessment of the anterior loop are more accurate than the radiographic measurement using either periapical or panorex films. (Computer-assisted scans are considered to be more accurate than films.)
As mentioned in the literature, and assuming that patients have an anterior loop of <5 mm, the placement of implants A and E should be 9 mm anterior to the foramen to prevent nerve injury. In this case, the implant has a 4 mm diameter and its center would be 2 mm from the safety zone.
Without regard to the anatomy and position of the anterior loop, the center of the implant would be placed 7 mm from the mental foramen. As a result, the distal wall of the implant would be in contact with the nerve, running the risk of permanent trauma to the nerve and surrounding tissue. There is also an increased potential for formation of fibrous tissue around the implants if in contact with the alveolar or incisive nerve. The diameter of the latter can vary, some having even the same size as the actual mental nerve.
FIGURE 7. Distal wall of osteotomy at 2 mm from the lateral wall of the foramen.
FIGURE 8. Maximum distribution of the implants between the foramina.
FIGURE 9. Radiological view, maximum distribution of implants between the foramina.
FIGURE 10A. Possible cantilever in the posterior region (y) = 1.5 A-P Distance (x) with no anterior loop.
FIGURE 10B. Possible cantilever in the
posterior region (y) = 1.5 AP when assuming the presence of the anterior loop.
FIGURE 11. Cantilever will change the stress on the implants.
It is difficult to accurately measure via probing an anterior loop longer than 4 mm without a risk of trauma. This is why patients with anterior loops of >4 mm were considered as having a loop measurement of 5 mm and implants were placed 9 mm anterior to the mental foramen.
The presence of an anterior loop can have a clear benefit. The more the loop is pronounced, the further it descends towards the base of the mandible, which leaves bone all the way to the distal edge of the foramen, above the nerve. It should be noted that the alveolar nerve rarely runs more than 6 mm inferior to the foramen (Ritter EF, Moelleken BR, Mathes SJ, et al,1992). Since the mandibular ridge is wider in this area, one can substitute the length of the implant for the width of the implant. Hence, shorter implants with wider diameters can be placed with an excellent prognosis.
RATIONALE FOR PROBING THE MENTAL FORAMEN
Probing the mental foramen to locate the presence or absence of the anterior loop allows clinicians to optimize implants distribution in the mandible to support biomechanical forces. Ultimately, this will enhance osseointegration and longevity of the implants.
This review assessed 1,651 edentulous patients (3,246 foramina) of all races and genders. The length of their edentulous stages ranged from <1 to 45 years.
The prevalence of the anterior loop was 10.84 percent. Furthermore, the presence of a unilateral or bilateral loop was 38.79 percent and 61.21 percent, respectively.
Several other studies with limited sample sizes have demonstrated the prevalence of the anterior loop. Rosenquist dissected 58 lower alveolar nerve bundles to show the prevalence of a 1 mm anterior loop in 43 cases (74 percent). Another study used the dry skull method and found the prevalence of an anterior loop >2 mm in 92-96 percent of specimens (Arouzman MJ Otis L, Kipnis V. 1993).
Clinicians who do not probe foramina carefully are limiting both the ideal distribution of implants and the longevity of the mechanical system, by putting more stress and fatigue on the different components.
TECHNIQUE TO PROBE THE MENTAL FORAMEN FOR THE ANTERIOR LOOP:
1.Choose a dull and rounded tip of an explorer (or create it).
2.Choose an explorer of where metal is not too thick and to allow changing shape, as required.
3.Loosen the nerve capsule very gently, without breaking it.
NB: It is important to have a flap that will make it possible for you to free the posterior portion from the foramen and do the final osteoplasty on the distal portion of the mandible and have ridges of soft tissue that will be easier to close.
4.Probe distally towards distal aspect of the foramen while maintaining contact with the bone at all times.
5.If the probe follows a canal longer than 3 or 4 mm, the nerve will likely enter the foramen from the distal – absence of the anterior loop.
6.If no nerve canal is detected, move the probe towards the inferior or apical aspect of the foramen. If there is a canal, the nerve is coming from the inferior aspect – absence of the anterior loop or measurable loop (1-3 mm).
7.If the nerve canal is absent, there is certainly an anterior canal due to the incisive branch of the alveolar nerve – presence of an anterior loop (with an estimated length of 5 mm as described in the literature).
Consequently, the center of the implant will be placed 9 mm anterior to the foramen, thereby reducing the A-P distance and hence, the final prosthesis overhang. It may be necessary to review the parameters of the prosthetic aspects.
8.In cases where foramina that are difficult to probe (hemorrhage, highly resistant capsule, lack of patient cooperation, etc.), the implant should be placed carefully, as though an anterior loop was present.
The presence of an anterior loop on one side does not guarantee its presence bilaterally (only 61.21 percent are bilateral). It is then necessary to probe each side.
FACTORS INFLUENCING THE NUMBER AND DISTRIBUTION OF IMPLANTS IN FULLY EDENTULOUS MANIBLES:
1. Type of prosthesis indicated in the treatment plan: Fixed vs. Removable
• porcelain versus acrylic
• resilient or non-resilient
2. Position of major anatomical structures:
a. Inferior alveolar nerve – height in bone, anterior loop
b. Sublingual gland fossa
c. Mental foramina–position in the arcade
3. Anterior edge of the mandible (bone height available)
4. Shape of the arcade; square, triangle or ovoid
5. Vertical dimension
6. Surgeon’s experience
8. Flexibility of the mandible
9. Pecuniary aspect
MANDIBULAR IMPLANT PLACEMENT IN FULLY EDENTULOUS PATIENTS
When four or five implants are placed as anchors and connected with a bar over the edentulous areas on either side of the mandible, the biomechanical forces applied to the implants will be proportional to the length of the extension or cantilever and the height of the final prosthesis, as well as the anteroposterior (A-P) distance of the implants. The bar is not considered as the cantilever but rather as the occlusal table of the teeth, since the latter is the component of the non-resilient prosthetic device. Accordingly, the height and length of the prosthesis are decisive factors of the final treatment plan. The distribution of implants is also a key factor in determining the length of the extension in order to minimize potential damage to the implants, bone and mechanical system. (For further details, see Dental Implant Prosthetic, Carl E. Misch.)
Carl Misch’s widely accepted nomenclature (A,B,C,D,E) in the mandible as well as the A-P distance is used to determine the optimal and adequate length of the prosthesis that will bear the functional load and be supported by the bone reinforcing the dental implants.
• Trace an initial horizontal line from the center of the anterior implant(s).
• Trace a second line connecting the distal edge of the posterior implants.
• Measure the vertical distance between the two lines (x).
The distance of the bar (y) will be 1.5 times (x) for four implants and 2.5 times (x) for five implants.
For non-resilient prosthesis, implant distribution should always be at the maximum distance, so that the distal implants are as far poster
ior as possible (if placed anterior to the foramen). This will effectively handle masticatory forces in the axis along the implants and will reduce the length of the extension.
In these instances, the presence of an actual anterior loop becomes important since the anterior loop is absent in approximately 89.16 percent of cases. Practitioners who wrongly assume the presence of a loop and place implants 9 mm anterior to the mental foramen lose 7.5 mm (1 premolar) in a four-implant supported final prosthesis or 12.5 mm (greater than 1 molar) in a five-implant supported final prosthesis.
The restorative clinician who ignores the A-P distance measurement also compromises the osseointegration of the implants and the longevity of the mechanical system, as the forces applied to the distal and anterior implants will be significant due to the extension.
When parafunction exists or significant forces are applied on different axes, resulting forces on the implants will be multiplied. This pressure, applied prematurely or even later in the bone regeneration process, may result in bone liquefaction around the implants.
In conclusion, the long-term benefit of correctly and carefully probing all foramina greatly outweighs assuming the presence or absence of the anterior loop of the inferior alveolar canal. Moreover, probing techniques have not caused permanent damage to the nerve; any temporary paraesthesia was resolved within four months and cannot be attributed to probing with any certainty. Furthermore, implant distribution is of paramount importance, allowing for an overhang of an appropriate dimension, thereby reducing the stress put on the implants and increasing the maintenance of stable bone levels around the implants during mastication and function. OH
Dr. Eric Chatelain is a dentist with 16 years of experience in general dentistry. Over the past 11 years, his practice has focused exclusively on implantology. To date, he has inserted over 12,000 implants.
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