Injury to the inferior alveolar nerve during implant placement in the posterior atrophic mandible is a rare but serious complication. Although a preoperative computerized tomography scan can help determine the distance from the alveolar ridge to the nerve canal, variables such as magnification errors, ridge anatomy, and operator technique can increase the chance for complications. The routine use of intraoperative periapical radiographs during the drilling sequence is an inexpensive and reliable tool, allowing the operator to confidently adjust the direction and depth of the implant during placement. Most important, it helps avoid the risk of injury to the inferior alveolar nerve in cases in which there is limited vertical alveolar bone. Using this technique for 21 implants placed in the posterior atrophic mandible, with less than 10 mm of vertical bone to the inferior alveolar nerve canal, the authors observed no incidents of postoperative paresthesia.
Oftentimes, as implant surgeons, we are presented with a preoperative computerized tomography (CT) scan of the posterior mandible showing adequate bone width for implant placement but limited vertical bone height. Presented with this scenario, and in light of the increasing number of lawsuits from damage to the inferior alveolar nerve, many surgeons refrain from placement of implants in compromised situations. Injury to the inferior alveolar nerve during implant placement is a serious complication. The incidence of altered nerve sensation following implant placement in the atrophic posterior mandible has been reported as 0% to 13%.1,–3 While CT scans are an excellent preoperative tool, intraoperative periapical radiographs can offer critical information to avoid nerve injury.
Many critical factors must be kept in mind when placing implants in this region. The first is understanding the anatomy of early resorption patterns in the posterior edentulous mandible, which usually produce vertical and horizontal alveolar atrophy on the lateral aspect of the ridge (Figure 1). This irregularity can introduce inaccuracies and intraoperative difficulties even for the experienced surgeon. While a preoperative CT scan is an excellent diagnostic tool, intraoperative errors can still occur when placing implants in limited vertical bone of the posterior mandible. In the presented clinical situation (Figure 2a and b), a preoperative CT scan was done; however, an implant was placed into the inferior alveolar canal, demonstrating that the preoperative CT scan does not guarantee or verify correct placement. Operator-induced error or variation in surgical anatomy may cause distortion in already limited alveolar bone height.
A second critical factor is radiographic magnification errors. In traditional CT scans of the mandible, magnification errors are reported to be from 0% to 8% and with cone beam CT as high as 3.8%.4,–6 Other diagnostic tools such as periapical radiographs have a magnification error reported at an average distortion of 14%.5 Although preoperatively, a CT scan will give a more exact distance from the alveolar ridge to the nerve canal, variables such as ridge anatomy and operator technique can lead to complications during implant placement. The routine use of intraoperative periapical radiographs during the drilling sequence, for implants placed in the atrophic posterior mandible, can help avoid the risk of injury to the inferior alveolar nerve. Periapical radiographs used intraoperatively to obtain working length measurements are similar in concept to techniques used in root canal therapy, and the method is reliable for determining the safe distance between the implant and the inferior alveolar canal, thus avoiding the risk of injury to the nerve altogether. This not only increases the accuracy of implant placement but also helps prevent nerve damage.
A third critical factor is the implant's ability to withstand occlusal load. Placing an implant that is short yet wide can overcome the height disadvantage in the molar region.7 The more surface area the implant has with the bone, the better its ability to withstand long-term occlusal load. For example, placing a 6 × 8 mm implant that has a surface area of 54 mm2 which is roughly the same amount of surface area as a 3.75 × 18 mm implant with approximately the same predictable success rate.7
The necessary equipment includes a dental x-ray machine preferably using a digital sensor. The recommended safe distance for an implant is about 2 mm above the alveolar canal.8 Periapical radiographs were taken using a long cone paralleling technique with an endodontic film holder (Figure 3) (Rinn Corporation, Elgin, Ill). Intraoperative periapical films are taken at different stages (Figures 4a to d) during implant placement in the atrophic posterior mandible. The first drill in the sequence was drilled halfway to the desired depth, followed by the placement of a guide pin within the osteotomy and a periapical radiograph. This is followed again by drilling halfway from the osteotomy site to the desired depth, placing a guide pin, and taking a second intraoperative PA. This is continued until only 2 mm exists from the osteotomy to the desired depth. This distance is then drilled with a final PA taken to confirm depth and direction. This series of radiographs is demonstrated in Figure 5a to e. Care must be taken to ensure that the patient is not allowed to bite down on the guide pins or drills while the PA is being taken. This can be accomplished by using a bite block at all times.
The magnification error using cone beam CT to evaluate alveolar bone dimensions has been reported to be as high as 3.8%.6 The reported magnification error using periapical radiographs averages 14% depending on the technique used.5 We use these results from 2 separate studies to estimate the average error in a preoperative cone beam CT scan to evaluate the distance to 2 mm above the inferior alveolar (IA) canal located 10 mm below the alveolar ridge as 0.3 mm (3.8% of 8 mm) (Table). Taking a periapical radiograph approximately 2 mm above our desired osteotomy end point (Figures 4d and 5d), the average error in measurement is about 0.3 mm (14% of 2 mm; Table). Although radiographic evaluation of the bone reaches a precision of only 0.5 mm,9 the calculation of errors in these 2 technique demonstrates how intraoperative PAs taken at different stages can be helpful and critical tools during surgery.
During the past year, we have used this technique for 21 implants placed in atrophic posterior mandibles of 20 patients with less than 10 mm of bone to the IA nerve canal. These implant sites all had adequate width to accommodate at least a 5-mm implant. There were no incidents of postoperative paresthesia.
This technique is an inexpensive and reliable method that allows 2-dimensional accuracy that can be similar to CT scans in selected cases. In addition, it allows for operator real-time data and confidence to avoid the inferior alveolar nerve during surgery. Placing 21 implants while using this technique in atrophic posterior mandibles this past year, we had no incidence of nerve injury. Although additional diagnostic data gained from a preoperative CT scan are useful, the periapical method has proven quite reliable at giving up-to-the-minute accuracy of osteotomy length. In addition, it is a critical diagnostic tool to adjust direction and depth of the implant during placement and to help avoid inferior alveolar nerve injury. The described technique might be recommended as a simple, inexpensive, and reliable method for safe implant placement in the atrophic alveolar ridge of the mandible.
The authors have no financial interest in any of the companies or any of the products mentioned in this article.
Jeffrey Burstein, DDS, MD, is the former chief resident, Chris Mastin, DMD, is the current chief resident, and Bach Le, DDS, MD, is an assistant professor at the Department of Oral & Maxillofacial Surgery, USC School of Dentistry, Los Angeles, California. Address correspondence to Dr Burstein at USC Department of Oral Surgery, Room 146, 925 West 34th Street, Los Angeles, CA 90089-0641. (email@example.com)