This blog series explores orthognathic surgery. Part 1 discusses orthodontic preparation, detailing the stages of levelling, decompensation, and arch coordination, both pre-and post-surgery. Part 2 reviews contemporary evidence of the stability of orthognathic surgery, while Part 3 examines the concept of surgery first, its evidence, and its place in current clinical practice.
If no vertical face height changes are planned, both arches are levelled to achieve a flat occlusal plane and level marginal ridges, which are the majority of orthognathic cases. By achieving levelling before surgery, the intra-operative and post-operative occlusion will be stable without occlusal interferences. Occlusal interferences can result in occlusal instability, resulting in a displacement and potential relapse. Utilising rigid archwires is usually sufficient to level the arches, or segmental mechanics. The decision of which mechanics to use is based on achieving the ideal lower incisor inclination, with a continuous archwire resulting in proclination to the lower incisors, and segmental mechanics maintaining the lower incisor inclination. Our previous blog explores the mechanics of levelling the Curve of Spee – click here.
If an increase or decrease in vertical face height is planned, post-surgery orthodontic levelling occurs.
Reduced anterior face height and increased overbite: To increase the lower anterior face height, the lower arch is purposefully not-levelled before surgery, and can be accentuated to 5 mm Naini 2017, which will extrude the lower An interesting effect occurs with surgery, the extruded lower incisors create a clockwise rotation, increasing the lower anterior face height. The occlusion achieved is a ‘tripod’, a stable occlusion with 3 occlusal contacts (2 posterior contacts on molars right and left, and 1 contact on the incisors). Post-surgery orthodontic levelling occurs through posterior extrusion and an overall increase in the lower anterior face height is achieved Proffit 2010.
REDUCED FACE HEIGHT: Increase curve of Spee pre-surgery to achieve ‘tripod’ occlusion post-surgery (increase in face height) Image Farooq Ahmed
Increased anterior face height with anterior openbite: To reduce the lower anterior face height, the upper arch occlusal curve is accentuated, or a step in the occlusal plane is orthodontically created, increasing the anterior openbite (principle 3 of orthognathic planning, for those who did not read the introduction, don’t worry Farooq doesn’t read them either). The planned surgical movement involves a Le Fort 1 posterior maxillary impaction, the mandible rotates in a counterclockwise direction until anterior contact is established, and in doing so the lower anterior face height is reduced. The posterior teeth will not be in occlusion due to the accentuated occlusal curve, which is levelled post-surgery.
INCREASED FACE HEIGHT: Increase curve of Spee pre-surgery upper arch, posterior maxillary impaction with counterclockwise rotation of mandible (decreases face height) Image Farooq Ahmed
Skeletal discrepancies frequently result in dental compensation – a process of dental tipping through soft tissue influence to reduce the underlying skeletal discrepancy. Decompensation refers to the ‘undoing’ of dental compensation, idealising the dental inclinations relative to their skeletal bases in anteroposterior and transverse planes, which usually worsen the occlusal relationships.
Clockwise and counterclockwise surgical rotation of the maxilla or mandible results in changes to the dental inclinations, for example a clockwise rotation of the maxilla retrocline the upper incisors, and a counterclockwise rotation of the maxilla procline the upper incisors. At this point, it’s worth mentioning cephalometric assessments of dental inclination relative to their skeletal bases becomes a false measure of inclination position relative to other facial structures, therefore the use of external measures such as SN or the facial tangent line are reliable measures of assessment Naini 2018.
Coordination refers to the transverse relationship between maxillary and mandibular arches, and achieves occlusal stability post-surgery. The pre-surgical orthodontic preparation is based on the planned post-operative occlusion, and usually entails worsening of the pre-surgical transverse relationship.
To assess the coordination (post-surgical), one can simply place the dental casts into a class 1 relationship and evaluate the coordination. The next decision regarding either arch constriction or expansion of the maxilla or mandible is based on the dental inclinations as well as the patients’ gingival phenotype, The instability of transverse changes associated with conventional orthodontics may not apply as the zone of equilibrium will be altered after surgery, which although a perceived liberation to the orthodontist, may result in an unstable occlusion. Fortunately, through measures of assessment such as the WALA-line, the dental inclination can be placed in a stable position relative to the dental base and in the most neutral position in the basal bone, therefore limiting future instability. Where orthodontic coordination cannot be achieved, surgical or miniscrew assisted expansion can be used.
Surgical expansion or constriction can be achieved through segmental surgery. Expansion of the maxilla can occur through either a 2-part or 3-part segment, and mandibular expansion or constriction through a midline osteotomy. Orthodontic preparation for segmental surgery entails creating inter-radicular space on either side of the planned osteotomy cuts. Inter-radicular space can be created through inter-proximal reduction, open-coil, or selection of the appropriate bracket prescription to diverge the roots.
Below is a list of orthodontic preparations for surgery. The list is not exhaustive, and individual adaptation is required, however we have included common case types and aims of pre-surgical orthodontics
Skeletal II & Skeletal III | average vertical relationship |
Level ✅ Decompensate ✅ Coordinate ✅ |
Skeletal II & Skeletal III | reduced vertical relationship |
Level ❌ Decompensate ✅ Coordinate ✅ |
Lower arch is not levelled – the curve of Spee is maintained for ‘tripod’ occlusion to increase the face height |
|
Skeletal I, II & Skeletal III | increased vertical relationship (AOB) |
Level ❌ Decompensate ❌ Coordinate ✅ |
Upper arch is not levelled – the curve of Spee is increased for posterior maxillary impaction to reduce the face height through counterclockwise rotation of the mandible |
Upper incisors are not decompensated – Upper incisors are proclined, and are decompensated through surgical posterior impaction |
Segmental expansion surgery |
Level ✅/❌ Decompensate ✅ Coordinate ❌ |
Segmental arch can be levelled before surgery or levelled with surgery |
Arches are not coordinated – arches are coordinated through surgery |
Contents: Kerolos AlHakeem 
Contents: James Andrews
Edited/contents: Farooq Ahmed
We hope you have enjoyed this blog, This blog started with a conversation between Kerolos AlHakeem, Farooq Ahmed and James Andrews, to follow more conversations, clinical tips and commentaries on orthodontics please check us out on Instagram / facebo
1 comment
worth mentioning that AOB ( increase in vertical dimension) accompanied with a gummy smile , the presurgicsl prepration is different from what mentioned above as there will be total impaction