Immediate implant placement of mandibular molars proved to be a viable surgical treatment with a high success rate [3, 14]. The results of this study may be beneficial for surgeons who are considering immediate implantation in the mandibular molar region.
Implant primary stability is the main factor determining the successful osseointegration of dental implants . However, enough primary stability in molar post-extraction sites is often difficult to achieve. Ultra-wide or wide diameter implant increases the engagement and contact area with the inter-radicular septum and socket walls, and therefore, is frequently used in post-extraction molar sites to overcome the lack of primary stability [5, 16,17,18]. Nonetheless, the long-term prognosis of an ultra-wide diameter implant remains controversial. Some studies reported a higher failure rate of ultra-wide implants than implants of diameter 4 to 6 mm . Recent study demonstrated that the use of implants with < 5 mm diameter is considered to be predictable and successful for immediate implantation in posterior areas . Regular or wide implants also have advantages, such as the low probability of bone dehiscence, lingual perforation, and IAN injury. In this study, therefore, a 4.8 mm diameter virtual implant was used.
An alternative to wide-diameter implants is the placement of implants into the inter-radicular septum . Immediate implant placement into the inter-radicular septum of mandibular molar sockets is a viable treatment option providing favorable outcomes [1, 3, 19, 20]. It was reported that a minimum 3 mm width of the inter-radicular septum was needed to provide initial implant stability [3, 21]. Based on the results presented in Table 1, the implants in the first molar region should be placed 2–3 mm subcrestally to produce better bone-to-implant contact area and obtain adequate primary stability, and less bone resorption may be expected . However, in the second molar region, the inter-radicular septum is often thin or absent, indicating a risk of inadequate primary stability of immediate implant placement.
To identify the most optimal surgical protocol, in this study, we classified the anatomic relationship between the virtual implant and inter-radicular septum into three types: M (mesial), S–M (septal–mesial), and S (septal). The results showed that type S, followed by type S–M, was the most common in the first molar. Therefore, we suggest immediate implant placement in the center of the inter-radicular septum should be considered in the first molar region, which provided favorable outcomes in former studies [1, 3, 19, 20]. However, in the mandibular second molar region, type M had the highest prevalence rate, and the inter-radicular septum is often thin or absent. In this case, the mesial root socket could be considered for immediate implant placement.
Placing an immediate implant in the inter-radicular septum is a complex procedure and that requires a learning curve. During the site preparation, the drill may slip down the slope continually because of the morphology of the inter-radicular septum, leading to the loss of the inter-radicular bone and an inaccurate implant position [21, 23]. Various surgical approaches have been described to place implants in mandibular molar sockets, and using the remaining roots as a guide for implant orientation is the most documented one [1, 20, 21, 24,25,26]. Implant site preparation guided by the remaining roots ensures predictable ideal implant positioning and orientation [1, 20, 21, 24,25,26]. Besides, it is crucial to preserve the integrity of the inter-radicular septum and the walls of the extraction socket with an atraumatic tooth extraction [1, 2].
Previous studies have predicted a higher risk of lingual perforation in U type mandible . In this study, a tendency to be classified as U type in more posterior regions was found, which is similar to the results obtained in former studies . Based on the data of the distance between the lingual outer plate surface and the implant surface at the apex in the simulation in this study, the immediate implantation in the mandibular second molar region presents a higher risk of lingual perforation than the first molar region.
To ensure the integrity of IAN, a proper distance between implant and IAN is of utmost importance. Though the results of our study showed that the mean distance from the alveolar crest to the IAN was 16.22 mm, offering sufficient bone height for immediate implant placement. The available bone height in the second molar region was significantly less than the first molar region. This result is similar to previous studies, which indicated that the distance of the IAN is minimal on the distal sections of the second molar . Therefore, to avoid lingual perforation and IAN injury, the maximal implant length in the second molar region should be carefully decided. Besides, the mean distance from the level where the virtual implant was completely surrounded by bone to IAN was 7.06 mm. Considering a safe distance of 1.5–2 mm is recommended between implant and IAN , a mean implant anchorage of 5 mm could be obtained in the apical basal bone.
The horizontal gap between the implant and the socket wall is a common finding. In this study, the horizontal widths from the implant surface to the mesial and distal socket wall were 1.59 mm and 1.89 mm. Even though some studies claimed that a large gap (4–5 mm) may heal spontaneously, without the application of regenerative materials . Most authors recommended that augmentation procedures should be considered at the time of implant placement to reduce bone resorption [18, 26].
This study still has some limitations. Firstly, this is a “virtual” study. The inter-radicular septum could be cancellous and of poor bone quality. Placing an immediate implant in a post-extraction molar site is, therefore, a complex and challenging procedure. Moreover, during the measurements, though the inter- and intra-observer variations were high, minor discrepancies may occur in determining the position of the virtual implant. For these reasons, the findings of this study require careful interpretation and should be supported by well-designed, prospective controlled trials with long-term follow-up.
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