The aim of this study was to assess the impact of incorporating a fourth osteotomy along the lower border of the mandible on the lingual split pattern during BSSO, using 3D CBCT. The results of this study showed that the original Obewgeser/Dal Pont complete lingual horizontal fracture extending toward the posterior border of the mandible, as well as the Hunsuck fracture pattern that runs vertically toward the lower border of the mandible, behind the mandibular foramen, are favourably affected by the addition of a fourth osteotomy cut at the lower border of the mandible. It is noteworthy that no undesirable fracture patterns were observed on the sides with a horizontal lower border osteotomy, in contrast to three cases of unfavourable fractures patterns on the opposite side without an inferior border osteotomy.

Sagittal split ramus osteotomy remains the established standard for correcting mandibular skeletal deformities. Since its introduction by Obwegeser in the mid-50 s, numerous modifications have been introduced to facilitate the splitting process, reduce complications, expedite bone healing, and prevent relapse. The addition of a fourth osteotomy cut along the lower border serves to further weaken the mandibular body and streamline the splitting process. Access to the lower border can be challenging and may necessitate extensive soft tissue manipulation and potential trauma; however, the utilization of a piezotome can mitigate the level of trauma involved.

The lingual split pattern is inherently concealed and not clinically visible. However, with the advent of CBCT technology, predicting and visualizing the fracture pattern has become more accessible. In a study focused on BSSO for mandibular advancement involving 40 patients with mandibular hypoplasia, Plooij et al. [19] introduced a new scale to categorize the path of the lingual cortical fracture line. Their findings revealed that 51.25% of splits followed Hunsuck’s description (LSS1), 13.75% of fractures extended horizontally to the posterior border (LSS2), 32.5% of lingual fractures occurred along the inferior alveolar canal (LSS3), and 2.5% were buccal or categorized as other unfavourable fracture types (LSS4).

In the current study, the distribution of lingual split patterns was as follows: LSS1 occurred in 40%, LSS2 in 20%, LSS3 in 32.5%, and LSS4 in 7.5%. The inclusion of a lower border osteotomy cut has advantages in terms of the lingual fracture pattern. Specifically, there was a tendency toward LSS1 and LSS2 fracture patterns compared to the conventional technique without an inferior border cut, which showed a propensity for LSS3 fracture patterns. Unfavourable splits (LSS4) were exclusively observed in the traditional technique without the inferior border cut.

Notably, the LSS1 and LSS2 split patterns are located away from the mandibular canal, potentially reducing the risk of inferior alveolar nerve damage. These fracture patterns also increase the bone contact surface area, which proves advantageous in cases involving mandibular advancement. Our findings align with previous studies conducted on cadaveric animals [20, 21] and a cadaveric human study [22]. However, our results diverge from the clinical studies of Houppermans et al. [23] and Mohlhenrich et al. [24], who reported no significant association between the inferior border cut and the lingual split pattern.

The variation in outcomes among different studies can be attributed to several factors, including differences in anatomy between human and animal models, the expertise of the surgeon, the sample size, and the influence of factors like ethnicity, age, sex, the presence or absence of third molars, the degree of mandibular divergence, and variations in the size and shape of the mandible. The size and shape of the mandible can vary significantly among individuals due to a complex interplay of genetic factors [25], environmental infleunces [26], sex-related variations [27], and geographic and ethnic differences [28]. Additionally, variations in the tools used for osteotomy and splitting procedures may contribute to this diversity.

Dal Pont proposed a pattern that enhances the common surface area between the split bone segments, thereby promoting bone integrity. This modification allows for further displacement of the distal segment and has been shown to be effective [29]. The Hunsuck and Epker modification, on the other hand, involves an incomplete lingual osteotomy that terminates just behind the lingula and inferior alveolar canal. This modification is notably easier compared to the conventional lingual osteotomy, which traditionally extends to the posterior border of the ramus and may occasionally result in unfavourable split fractures [30].

The occurrence of unfavorable splits is genuinely possible with both Hunsuck and Dal Pont modifications. Therefore, selecting a technique that carries the least risk of encountering such complications can undoubtedly lead to a more satisfactory surgical outcome. A study conducted by Zamiri et al. [31] involved an evaluation and comparison of fracture patterns within the medial cortex resulting from both medial long-cut and medial short-cut techniques during BSSO. In their investigation, they identified three distinct fracture patterns but found no significant correlation between the type of medial cut and the resulting fracture pattern. Consequently, it was concluded that the length of the medial cut does not significantly influence the occurrence of unfavorable split fractures. Zeynalzadeh et al. [32] reported that when using the Hunsuck approach, both osteotomies and splitting procedure require significantly less time, and there are also fewer instances of unfavorable fractures compared to when employing the Dal Pont osteotomy technique.

Cortical bone thickness has also been implicated as a risk factor for bad splits. In the study of Arabi et al. [33], they observed that the average buccolingual thickness of the retromandibular area measured 14.98 mm in the group of patients who experienced poor split outcomes. This measurement significantly differed from the average thickness observed in their control group, which stood at 11.21 mm. Additionally, the buccolingual thickness of the ramus at the lingula level was found to be associated with unfavorable split results. The occurrence of unfavorable intraoperative splitting has been linked to a limited gap between the inferior alveolar nerve canal and the buccal cortex, as well as a reduction in the thickness of both the buccal cancelous bone and the overall cancelous bone along the path of the splitting [34].

The height of the lingual osteotomy and the amount of cancelous bone between the ramus cortices may also be contributing factors to bad splits. Numerous studies have indicated that when the lingula is positioned high on the mandibular ramus, the medial horizontal osteotomy needs to be performed at a higher level on the mandibular ramus, specifically in a thin area where there is minimal or no cancelous bone present [35]. In skeletal class III malocclusions, the lingula tends to be situated higher compared to class I and class II malocclusions, and the ramus often displays limited marrow. Consequently, class III malocclusions are commonly associated with the highest risk of experiencing unfavourable fractures [36].

This study does have certain limitations, including the challenge of controlling the inferior border osteotomy cut due to limited access. Furthermore, despite employing a split-mouth design, it;s important to acknowledge the presence of anatomical variations between the two sides, which may impact the results.