## 1 Introduction  
Bilateral sagittal split osteotomy (BSSO) is the most often utilized surgical technique for treating mandibular abnormalities. It is a highly stable and predictable surgical orthodontic procedure for mandibular advancement [1]. Since [PERSON] and [PERSON] introduced the surgical method in 1957, the buccal horizontal osteotomy has developed into a vertical osteotomy by being positioned slightly toward the mandibular angle. Thus, this method needs to be improved to increase the area of bone contact [2, 3]. Other writers have updated the surgical procedure. [PERSON] increased the region of bone contact with the buccal cortex by creating a true vertical cut between the first and second molar [3]. [PERSON] changed the procedure in 1968 and recommended making a shorter horizontal medial cut slightly past the lingual to reduce the amount of soft tissue dissection, but his buccal vertical cut was same as [PERSON]'s [4]. Based on [PERSON]'s modification, [PERSON]'s is even more delicate, using a medial-horizontal osteotomy to preserve the neurovascular bundle. He ended this cut just posterior to the lingula [2]. Since then, reports of BSSOmodifications have focused on the attempts to reduce intra or post-operative complications such as neurosensory disturbance, unfavorable splits, relapse, condylar resorption, and post-operative infection [5; 6]. Recently in Thailand, a BSSO modification was reported as the Novel Modification of Low Z plasty (NM-Low Z plasty) in 2021 [7]. Compared to the [PERSON] and [PERSON] modification, this technique provides many advantages [7; 8]. The osteotomy line starts at the most superior point of the retromolar triangle called the horizontal cut, then extends approximately 6-7 mm posteroinferiorly toward the buccal shelf of the mandible. Continuing from the tip of the triangle to the lateral external oblique ridge, the sagittal cut is made. After that, at the buccal alveolar bone between the first and second mandibular molars, the vertical cut is created [7]. The purpose of this modification is to avoid identification of the lingula; therefore, temporalis tendon muscular detachment is not required. This leads to the shortening of the operative period and less post-operative swelling [7]. After mandibular positional setting, the proximal and distal segments are regularly stabilized by internal fixation using miniplates and screws [9]. Clinical features demonstrate the improved quality of life following surgery [10; 11]. The study on the quality of life in patients who underwent surgery through the NM-Low Z technique in Thammasat University Hospital showed improvements in facial esthetics and awareness of facial deformities 1 month after the surgery [11]. Otherwise, one of the most commonly reported undesirable side effects of the mandibular advancement procedure was skeletal instability. This means that the mandible was unable to be retained in the post-operative position, resulting in relapse after surgery, whether in a horizontal or vertical direction [12]. An increased propensity to relapse is linked to an advancement of more than 7 mm [13]. The range for the percentage of horizontal skeletal relapse was 6 to 30% [12; 13; 14]. Many factors influence skeletal stability, such as intraoperative change in the mandibular plane angle [14], the magnitude of advancement [12; 13; 15], and skeletal muscle detachment [16]. According to the previous systematic review, the group in which the medial pterygoid muscle and the stylomandibular ligament were stripped exhibited reduced skeletal relapse [16]. Although the suprahyroid muscles had an impact on skeletal relapse, if these muscles were stretched to less than 30% of their resting length, they did not substantially contribute to relapse [17; 18]. A condylar-ascending ramus position was another factor. Following the surgical displacement of the mandibular condyles from their functional site in the glenoid fossae, a noticeable skeletal relapse occurred after intermaximillary fixation was withdrawn [17].