CORRECTION OF MANDIBULAR ANTERIOR-POSTERIOR EXCESS

• Mandibular setback can be accomplished by either an Intraoral Vertical Ramus Osteotomy (IVRO) or BSSO with mandibular setback. Post-operative stability, while clinically acceptable in both cases, varies depending on which surgical technique is used. 

HISTORY 

Efforts to shorten the mandible to correct mandibular excess and/or mandibular asymmetry have produced a variety of osteotomy designs and surgical instrumentation. Limberg described the oblique subcondylar osteotomy in 1925. Subsequently, Moose and others described intraoral procedures for mandibular reduction. The vertical ramus osteotomy popularized in 1954 by Caldwell and Letterman required an extraoral approach. The sagittal split ramus osteotomy (SSO) described by Trauner and Obwegeser in 1957 was the first intraoral ramus osteotomy that permitted mandibular reduction. In 1968, Winstanley reported the first intraoral vertical ramus osteotomy (IVRO), performed with a dental drill. A significant advance in the IVRO technique was reported by Herbert et al in 1970 with the use of the motorized oscillating saw. The work of Hall et al and Hall and McKenna in the 1970s further popularized the procedure, and Hall’s work in the 1980s helped quantify clinical outcomes and proposed technique refinements to minimize “condylar sag caused by stripping of the masseter and medial pterygoid muscles, which led to a 14% incidence of open bite at the time of maxillomandibular fixation (MMF) release.”

IVRO

• One year following IVRO, there is a chance of either forward or backward movement of the mandible. 
• Intraoral vertical ramus osteotomy (IVRO) is a useful surgical procedure for mandibular setback in patients with mandibular prognathism or mandibular asymmetry. 
• Major complication of IVRO- Medial trapping of the proximal segment. The proximal fragment, pulled by the medial pterygoid muscle, is frequently trapped medially when VRO is achieved.
• TECHNIQUE- 
• A mucosal incision is made along the anterior border of the ramus from the base of the coronoid laterally to the buccal vestibule of the mandible in the first molar area. 
• Dissection exposes the coronoid process, the lateral aspect of the ramus, the antilingula prominence, the sigmoid notch, and the inferior and posterior borders of the ramus. 
• The pterygo-masseteric sling is removed from the inferior border and gonial angle. 
• An oscillating saw, angled at 105 degree, is used to perform the osteotomy cut of the ramus, posterior to the antilingula prominence, directed superiorly to the sigmoid notch and inferiorly to the mandibular angle. 
• Osteotomy is completed in the mandibular angle, the muscular fibres of the medial pterygoid that are still inserted in the proximal fragment are released. 
• This is followed by intermaxillary fixation. After that rigid fixation of the osteotomized segments are done. In the absence of internal fixation, a period of maxillomandibular fixation is required.
• In general, up to 10 mm of mandibular setback is possible with IVRO. Because it is important to preserve as much medial pterygoid muscle attachment as possible, the magnitude of the setback should not exceed the width of the proximal segment medial pterygoid attachment. 
• If the posterior movement of the distal segment exceeds the width of the medial pterygoid attachment, there will be little or no remaining medial pterygoid attachment to the proximal segment. 
• This promotes condylar sags and even condylar subluxation due to unopposed lateral pterygoid activity. 
• Conversely, if insufficient medial pterygoid muscle is stripped from the proximal segment, backward rotation of the segment occurs as the distal segment moves posteriorly, predisposing to postoperative forward relapse. 
• Internal fixation is technically more difficult with IVRO because of limited visibility and access for fixation instru­mentation. 
• However, right-angled drilling and screwdriving instruments have expanded the opportunity for transoral “rigid” internal fixation. In the absence of internal fixation, a period of maxillomandibular fixation is required.

• Because IVRO generally is used in the management of horizontal mandibular excess, it is prudent to consider any history of obstructive sleep apnea (OSA) before surgical mandibular reduction. If OSA is suspected, polysomnography should be obtained before mandibular reduction surgery. The potential for mandibular reduction to exacerbate OSA should be considered.

Finally, with two-jaw surgery in which the surgeon prefers to perform mandibular surgery first, Sagittal Split Osteotomy generally provides superior internal fixation and osteotomy stability to allow mandible-first surgery.

Following the initial description of IVRO by WINSTANLEY, several modifications have been introduced to ameliorate the visibility and reduce the difficulty of the procedure. HERBERT et al. introduced the use of the Stryker oscillating saw, the Bauer sigmoid notch retractor and the Levasseur-Merill posterior border retractor to provide good visibility during surgery. MANOR et al. proposed placing a Bauer retractor in the sigmoid notch and in the pre-angular area to improve visibility.

                                         

BSSO

With a BSSO there is no post-surgical backward movement, but forward relapse is more frequent. Regardless of surgical technique, up to 50% of patients experience more than two millimeters of post-operative change following a mandibular setback, with 20% of these patients experiencing change of more than four millimeters. For patients experiencing significant post-surgical relapse following a mandibular setback, the cause may be the result of a technical problem. During surgery, the position of the ramus (proximal segment with condyle) can inadvertently be pushed posteriorly into the condylar fossa. Following surgery, the ramus will return back to its original orientation.

Detailed explanation of BSSO technique will be explained elsewhere in this website.