Rehabilitation of severely resorbed posterior mandibles with dental implants can be performed using two different strategies: vertical bone augmentation in order to gain alveolar height for longer implants, or utilizing the residual native bone by placing either short implants or longer implants simultaneously with IAN repositioning.
The success of bone augmentation largely depends on the blood supply to the recipient site, quality of the soft tissues, availability of autogenous bone, and/or growth factors, age, and health status of the patient. The amount of osteoprogenitor cells in the bone marrow, and as a consequence, the osteogenic potential of the autogenous bone declines dramatically with age [16,17,18]. Moreover, many patients with severe bone defects have a previous negative dental implant experience. Therefore, the blood supply and quality of soft tissues in these areas are impaired, and the availability of the bone for harvesting is limited. All the abovementioned factors can negatively affect bone regeneration. In addition, the total cost of bone augmentation could be significantly higher than IAN repositioning, especially when growth factors or additional autogenous bone-graft donor sites are utilized. As a result, IAN repositioning with dental implant placement is a valuable option for patients whose medical condition or physiology (age, etc.) necessitates a reduction in the number or extensiveness of surgical steps in order to achieve stable, functional results following treatment.
Short implants are an acceptable alternative in patients with lowered bone height in the posterior mandibles. However, it is often challenging to place them without damaging the IAN when the bone height is less than 8 mm above the canal. In our study, the mean residual bone height above the IAN was 4.3 mm (range, 0.5 to 7.0 mm). Another limiting factor is poor bone quality in the posterior segments of severely resorbed mandibles due to osteoporosis. Therefore, it may be extremely difficult to achieve monocortical primary stability of short dental implants at the time of installation, which is one of the most important criteria for successful osseointegration [19].
In contrast, poor bone quality in combination with extensive resorption poses a serious risk of rare potential complications such as mandibular fracture, as IAN repositioning with implant placement does not restore the alveolar ridge anatomy, but rather weakens the basal bone [4, 20, 21].
One patient in our study experienced a spontaneous fracture of the mandible while yawning 10 days after surgery. The implant in the fracture line was removed at the time of open reduction and fixation. After 2 months, two implants on both sides of the fracture line were used for final prosthetic rehabilitation, which also acted as reinforcement to fix the mandibular fracture (Fig. 21a–c).
Some authors believe that surgery of the neurovascular bundle changes the blood supply to the lateral mandible and consider it as the main reason for delayed bone fractures. However, as the inferior alveolar artery mainly supplies the teeth of the mandible, gingivae, and the skin over the chin and lower lip, its role in the blood supply to the lateral mandible is not critical [22, 23]. Moreover, with the increase of age and with the loss of teeth, the importance of the inferior alveolar artery in blood perfusion of the lower jaw decreases, and the blood supply to the mandible mostly depends on the periosteum [22, 24, 25]. We suppose that the fracture on tenth day after surgery occurred due to loss of bone structural integrity after surgical trauma. Our assumptions coincide with other reports in the literature on the same subject [26,27,28,29,30].
Regardless of the potential risks and in light of the aforementioned discussion, IAN transposition or lateralization in combination with dental implant placement is an attractive treatment modality in cases of severely resorbed posterior mandible. Although postoperative neurosensory disturbances are the most common adverse effects of this procedure, they resolved over time, and none of the patients developed neuropathic pain after the procedure. The diagnosis of neurosensory dysfunction of the IAN is based on subjective clinical sensory testing and objective sensory tests. According to Loescher et al. [31], a patient’s subjective report is the most sensitive measure of sensory abnormalities, as objective testing may not diagnose minor ND. Moreover, patient complaints are the main incentive for seeking further recommendations or treatment of their postoperative altered IAN sensation. Therefore, in the present study, the postoperative function of IAN was evaluated subjectively using a modified questionnaire. In our patients, ND after IAN transposition was higher and remained longer than after IAN lateralization, as IAN transposition appeared to be a more invasive procedure. This assumption is supported by systematic reviews of the same subject [4, 10].