Determining Thickness of the Alveolar Ridge Without a CT Scan
Radiographs provide 2-dimensional assessments that can be used to assess bone height in the maxilla and mandible. However, without a CT scan, clinical determinations of bone width can be misleading. To determine alveolar bone width in the maxilla or mandible, a caliper (eg, Vernier) can be employed to map the width of the ridge (combined soft tissue and bone thickness) at the crest and then every 3 mm up to the vestibule. After local anesthesia, the bone is sounded with a No. 15 endodontic file, which has a rubber endodontic stopper. This is done buccally and lingually, at the same locations as the above readings. The distance that the rubber stopper is displaced reflects soft tissue thickness. The soft-tissue widths of the buccal and lingual tissues are combined and subtracted from the ridge width to determine bone thickness.
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Figure 5. Ridge mapper used to measure ridge thickness. It penetrates through the soft tissue. |
Figure 6. Mandibular ridge gets wider apically. |
Tissue mapping provides an accurate assessment of bone thickness at different levels of the alveolar ridge and often precludes the need for a CT scan. This information can also be transposed to a model of the ridge. Cut the cast in cross section and draw the measurements on the cast to visualize the relationship between the soft tissue and alveolar bone. Bone thickness can also be evaluated using and an instrument called a ridge mapper (Figure 5) (Salvin Dental Specialties). The above techniques are particularly useful when evaluating a ridge that visually, and upon palpation, appears to be too narrow at the crest to receive implants. However, sounding the bone or a CT scan assessment may reveal that the width of the alveolar ridge expands as you proceed apically (Figure 6).
Orientation
In the mandible, follow the lingual cortex for orientation, because the labial bone may be resorbed. This is done for safety, but not always for a prosthetic point of reference. The submandibular area should be palpated to assess the degree of lingual undercut. Furthermore, during surgical procedures, the precise degree of undercut can be determined without extensive lingual flap reflection by gently reflecting apically under the full thickness of tissue with a Prichard curette and observing its angle of penetration. A Naber’s 2N probe can also do this with less trauma.
Detecting and Avoiding Injury to the Mental Nerve
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Figure 7. Measuring a safety zone for implant placement: alveolar crest to mental foramen. |
Figure 8a. If placement of the probe into the mental foramen on the distal side reveals that the mental canal is patent, then the anterior loop is not present. 8b. If placement of a probe into the mental foramen on the distal side reveals that the mental canal is not patent, then an anterior loop of the mental nerve exists. The nerve must have traversed inferiorly and looped back to the foramen creating an anterior loop. (Reprinted with permission from J Periodontol.) |
When in doubt regarding the position of the mental nerve, the mental foramen should be exposed prior to implant placement to ascertain its position. First, take a measurement on the radiograph with respect to how far the mental foramen is from the adjacent teeth. If it is located in the bicuspid area, make a vertical releasing incision mesial to the canine and after the flap is elevated past the mucogingival junction, use wet gauze to push the tissue apically to expose the coronal aspect of the mental foramen. The gauze protects the nerve from being injured, and the periosteal elevator can be used to gently push the gauze apically. A measurement is taken from the alveolar crest to the roof of the foramen with a periodontal probe to determine the height of bone over the mental nerve (Figure 7).
Selection of an implant length should provide for a safety distance of 2 mm from the nerve. This distance minus 2 mm can also be used to safely place an implant anterior, over and posterior to the mental foramen up to the mesial half of the first molar area. Note: markings on an implant drill do not reflect the true length that the tip of the drill penetrates. The drill tip point usually adds 0.4 mm to 1 mm to the depth of the osteotomy. Therefore, it is essential to be familiar with each manufacturer’s equipment.
When it is necessary to determine if there is an anterior loop to the mental nerve (eg, desire to place an implant mesial to the foramen that is deeper than the safety distance determined above), gently probe the foramen with a curved Nabers 2N probe to assess whether the distal aspect of the foramen is patent. When it is unblocked, there is no anterior loop (Figure 8). If the distal is closed, then there is an anterior loop. When the mesial is patent, it may reflect the presence of the incisive canal or an anterior loop, and it is not possible to differentiate between the 2 structures by probing. When there is uncertainty, regarding the presence of an anterior loop, it may be prudent to follow Solar’s recommendation of staying at least 6 mm anterior to the mental foramen when placing an implant that is deeper than the determined safety distance.
If a guided bone regeneration procedure is done in the foraminal area and the flap needs to be advanced, buccal periosteal fenestration should be limited in depth to one mm, the width of the bevel on a No. 15 blade, to facilitate release of the flap. Avoid dissecting deeply into the tissue over or anterior to the mental foramen, because branches of the mental nerve are within the tissue. If additional flap advancement is needed in the foraminal area, locate the nerve, and then elevate a full thickness flap that contains the nerve fibers, posterior, anterior and apical to the mental foramen to facilitate flap advancement.
Surgical Templates
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| Figure 9. Surgical guide to replace tooth No. 30. Lingual contour is present and brought buccally to the center of the tooth. A groove is placed to guide mesiodistal osteotomy development. Teeth Nos. 19 to 21 are also being replaced. |
They can be constructed numerous ways. A simple method is to design one that engages teeth adjacent to the edentulous area for retention and incorporates only the buccal or lingual contour of the future restoration at the site to receive the implant. At this site, the buccal or lingual contours could be widened in acrylic to demarcate the precise buccolingual location of the osteotomy. In addition, a groove should be placed in the acrylic denoting the mesiodistal position of the future implant (Figure 9).
Sterilize Pencils
Sterilize No. 2 pencils and use then to mark the alveolar ridge. It saves a lot of time when initiating an osteotomy. It is also efficient to outline on bone the location of the osteotomy for a lateral window sinus lift. In this regard, drawing the root of the tooth anterior to the lateral window on the bone may provide additional orientation, denoting the mesiodistal position of the future implant.
CLOSING COMMENTS
Numerous studies have provided biologic rationales for procedures associated with implant dentistry. In addition, many techniques, and subsequently their modifications, were developed to enhance implant placement. Some of these ideas have appeared in the literature and others have not been published.