The role of mandibular stabilization in orthognathic surgery

Abstract

Stability of the temporomandibular joint (TMJ) structure is a critical requirement for treatment that includes orthognathic surgery. If the condyles are not positioned properly in the articular fossae during the manual positioning of the condyle or the intermaxillary fixation, postoperative relapse can result. However, it is difficult for the orthognathic surgeon to control the positioning of the mandibular condyles during orthognathic surgery due to muscle relaxation and the harsh intraoperative environment. Well-managed presurgical orthodontic treatment does not always guarantee the proper positioning of the proximal segment either, especially if the TMJ structures are not stable in their functional area. Therefore, the mandible should be stabilized with a presurgical stabilization splint to provide proper stimulation that forms a pseudodisc in the TMJ structures before surgical procedures.

Introduction

Postsurgical proximal segment position and the stability of the surgical results are long-discussed issues, not only for oral and maxillofacial surgeons, but also for orthodontists, as dental occlusion is determined by the position of mandibular condyles in the articular fossae. Since orthodontists are usually responsible for the results of surgical orthodontics, changes in occlusion derived from an unstable condylar position can be catastrophic.

Many studies have suggested that certain characteristics related to a patient such as their gender, age, steep mandibular plane angle, and a tendency to skeletal open bite are factors that influence changes in condylar position and/or morphology. It has been also claimed, on the other hand, that adverse changes in the TMJs are related to inadequate control of the proximal segments of the condyles during orthognathic surgery or intermaxillary fixation.

Previous studies reported that the incidence of condylar resorption after orthognathic osteotomies ranged from 23 to 31%, which is strikingly higher than orthodontists might have presumed. Skeletal relapses after orthognathic surgery are known to result from displaced proximal segments that returned to their physiologically functional area. Biomechanical loading followed by autorotation of the mandible is the major component in the etiology of condylar position changes with orthognathic osteotomy. Compressive loading during the surgical procedure can cause undesirable torque and rotation of the ramus because of autorotation of the mandible. When condyles are not positioned properly in the articular fossae during autorotation of the mandible, natural seating of the condyles will occur when the intermaxillary fixation is released, which results in an early relapse. However, improperly positioned condyles during surgical procedures will eventually be seated due to focal bone resorption and apposition at the condylar bone surfaces, but this takes several months and can result in a late relapse.

Despite its importance, positioning of the mandibular condyles during orthognathic surgery is not easy for orthognathic surgeons to control, as relaxed muscles during the surgery are affected by gravitation, so spontaneous displacement of the proximal fragment can occur. Simultaneous bleeding and unstabilized joint structures combine to create a harsh intraoperative environment that makes it difficult to find the best position for the condyle.

The undesirable consequences of incorrectly positioned proximal segments include condylar sagging, TMJ pain and dysfunction, and changes in occlusion and facial esthetics. When remodeling of the condyles stays within the physiological capacity of the adaptive mechanism of the TMJ, occlusal instability is much less likely expected. However, when mandibular condyles resorb with substantial loss of surface bone in the condylar heads, significant occlusal alteration and reduction of mandibular ramus height can occur.

Well-managed presurgical orthodontic treatment does not necessarily guarantee the proper positioning of the proximal segment, if stability of the TMJ structures is not maintained in their functional area. The articular disc is attached posteriorly to a region of loose connective tissue that is highly vascularized and innervated which is known as the retrodiscal tissues. Invasion of the condyle into the retrodiscal tissues or stimulation that extends the retrodiscal tissues can produce severe pain. Blood vessels and nerve cells fill the space when the condyle moves and changes its position, which eventually causes a thickening of the retrodiscal tissues. If the articular disc is dislocated and retrodiscal tissue is thickened, it will be difficult for the surgeon to find a stable position for the condyle during the orthognathic surgery.

Therefore, it is critical to position the condyle within the functional area and the mandible should be stabilized to provide proper stimulation that forms a pseudodisc in the TMJ structures.

The role of a stabilization splint

Splint therapy can be a reliable way to stabilize the TMJ before starting orthodontic and/or orthognathic treatment in patients with temporomandibular joint disorders (TMD). Stabilization of the TMJ is a therapeutic process that allows clinicians to identify the true mandibular position can help to make an make an accurate diagnosis. Stabilization also helps surgeons position the proximal segments of the mandible in a biomechanically stable position. Splints originated in the 1860s as an intraoral appliance designed for skeletal fixation of jaw fracture patients but now they are also being used to treat musculoskeletal disorders in TMJs. With splints, clinicians can predict a patient’s response to future occlusal reconstruction with an orthodontic and/or surgical approach.

As stabilization splints eliminate protective co-contraction by reproducing the scheme of functional occlusion, a musculoskeletally stable position of the TMJ condyles can be achieved. Centric relation (CR) position refers the mandibular position in which the condyles are in their most superoanterior position in the articular fossae, resting against the posterior slopes of the articular eminences with the articular disc properly interposed. This can be regarded as a musculoskeletally stable position. When mandibular movement begins from this position, orthopedic joint stability can be maintained during heavy contraction of the elevator muscles. This position is therefore considered the most musculoskeletally stable position of the mandible.

Many attempts have been made to recapture the articular disc for patients with disc displacement or derangement, but a displaced disc cannot be recaptured or repositioned into a normal position, since the articular disc is comprised of dense fibrous connective tissue, not hyaline cartilage, which naturally changes in morphology rather than recovering under inadaptable pressure. Instead, the retrodiscal tissue undergoes adaptive and reparative changes and becomes fibrotic and avascular and if it is adequately adapted, condyles can articulate on the newly adapted retrodiscal tissue that is known as “a pseudodisc” ( Fig. 1 ). Even though the disc is still anteriorly displaced, the condyle can function on the adaptive pseudodisc without pain and discomfort but with persistent sound (click) when the condyle moves. If orthodontic surgery is performed with widened retrodiscal tissue and if the surgeon attempts to position the condyle forcefully, the condyle will stimulate the extended retrodiscal tissue, which can cause pain and delayed resorption of the condylar heads.

Figure 1
Formation of pseudodisc after stabilization: (A) fibrotic retrodiscal tissue; (B) posterior border of disc; (C) a displaced articular disc.

Under these conditions, proper stimulation should be applied to form a pseudodisc and stabilize the mandible. The use of a stabilization splint can promote formation of the pseudodisc on the posterior band of the disc. Once the condyles have been seated in the most forward and uppermost position in the articular fossae with the help of a stabilization splint, it can be considered that the patient’s condyle is in a reproducible and reliable reference position for mandibular movement. A stabilization splint that reproduces an ideal functional occlusion can eliminate protective co-contraction of the masticatory muscles and eventually lead to an orthopedically stable joint position of the mandible.

Importance of stabilization of TMJ for orthognathic surgery

Stabilization of the mandibular position is of special importance in surgical orthodontics since the use of a stabilization splint before the surgical procedures can promote postsurgical stability by reducing unexpected condylar remodeling. The mandible cannot be stabilized just because a surgeon locates the condyles forcedly in the most anterosuperior position during a surgical procedure. If the disc is displaced and the articular joint is in disorder, a small malposition of the condyle will bring about adverse compression and torque, eventually resulting in remodeling of the condylar heads.

Stabilization of the mandible does not mean to manipulate the position of the condyle to the designated location. It rather refers to an appropriate relationship of surrounding structures of the TMJ including mandibular condyles, articular discs, retrodiscal tissues, muscles and ligaments. Even for highly experienced surgeons, stabilization of the mandible cannot be guaranteed if surrounding structures of the TMJ are not properly aligned.

Patients who have an open bite, facial asymmetry and skeletal Class II high angle are frequently found to have unstable condylar positions; therefore, stabilization of the TMJ structure using a stabilization splint should precede the orthognathic surgery for postoperation stability ( Fig. 2 ).

Figure 2
Protocol of surgical orthodontic treatment: (A) presurgical orthodontic treatment is started according to the treatment plan; (B) continue presurgical orthodontic treatment avoiding the mechanics that can induce positional change of condyles; (C) a displaced articular disc can result due to premature contacts during the presurgical orthodontic treatment; (D) to confirm stabilization of the mandible, dental models are mounted on an articulator; (E) a presurgical splint is applied to stabilize the TMJ structure before surgical procedures; (F) condylar positions are stabilized; (G) orthognathic surgery is performed; (H) postsurgical orthodontic treatment can be finished with stabilized mandible.

Case reports

Case 1

An 18-year-old male had the chief complaint of anterior open bite. He explained that pain and discomfort in the TMJ area had persisted for years although his anterior open bite tendency had recently become worse. The patient showed prognathic facial profile and mild facial asymmetry with his chin point deviated to the left. Intraoral findings confirmed his anterior open bite, mild crowding in both arches with a 0 mm overjet and a 3.0 mm anterior open bite. A panoramic radiograph showed severely flattened and backwardly inclined condylar heads on both sides. A lateral cephalometric analysis indicated skeletal Class III pattern (ANB, −0.5° and Wits, −3.5 mm) and the maxillary incisors were proclined (U1-SN, 125.0°) ( Fig. 3 , Table 1 ).

Figure 3
Pretreatment diagnostic data: (A) facial photographs; (B) intraoral photographs; (C) a panoramic radiograph; (D) lateral and frontal cephalometric radiographs.

Table 1
Cephalometric measurements
Measurement Korean norm Pretreatment Poststabilization Presurgical Posttreatment
SNA (°) 82.0 81.5 81.5 81.5 80.0
SNB (°) 79.0 82.0 81.0 81.0 76.5
ANB (°) 2.5 −0.5 0.5 0.5 3.5
Saddle angle (°) 126.0 115.0 112.5 112.0 113.5
Articular angle (°) 149.0 155.0 159.0 159.0 162.0
Gonial angle (°) 118.5 130.5 130.0 130.0 125.0
Upper gonial angle (°) 45.0 44.5 43.0 42.5 41.0
Lower gonial angle (°) 74.0 86.0 87.0 87.5 84.0
SUM (°) 393.0 400.5 401.5 401.0 400.5
Facial angle (Down’s) (°) 89.0 90.5 89.5 89.5 87.0
Wits (mm) −2.5 −3.5 −2.5 −5.5 3.0
SN-MP (°) 33.5 31.0 32.0 32.0 33.0
Ramus height (mm) 51.5 49.5 50.5 52.0 45.0
Post.FH/Ant.FH (%) 66.8 62.5 63.0 64.0 61.0
U1-SN (°) 104.0 125.0 125.0 112.0 105.0
U1-FH (°) 116.0 132.5 132.5 119.5 112.5
U1-NA (°) 22.0 43.5 43.5 30.0 24.5
U1-NA (mm) 4.0 11.0 11.0 6.0 4.0
IMPA (°) 90.0 90.0 90.0 88.0 89.0
L1-NB (°) 25.0 31.0 32.0 31.0 25.5
L1-NB (mm) 4.0 9.0 9.0 9.0 8.0
U1/L1 (°) 124.0 105.0 104.0 118.0 123.0
Overjet (mm) 2.8 0.0 0.5 −4.0 3.0
Overbite (mm) 3.0 −3.0 −4.5 −1.5 3.0
Upper lip-E plane (mm) 0.0 −2.0 −2.0 −3.0 −2.5
Lower lip-E plane (mm) 0.0 3.0 3.5 2.5 −2.0
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Jan 9, 2020 | Posted by in Orthodontics | Comments Off on The role of mandibular stabilization in orthognathic surgery
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