Treatment planning should only be undertaken after a thorough history and clinical examination of the patient, together with an analysis of the radiographs and study casts.
Planning the Maxillary Move
The key to successful surgery is to place the maxilla and the decom-pensated maxillary incisors in the optimum anteroposterior and vertical position in relation to the upper lip and face. The mandible is then placed in a Class I incisor relationship to the maxilla.
The profile of the seated patient is assessed with the Frankfort plane horizontal to decide the horizontal anteroposterior movement of the maxilla.
Changes of the naso-labial angle can be predicted with a moist cotton roll or soft pink wax. The frontal view is assessed by:
i) The ratio of the cephalometric upper to lower facial height — 45:55%.
ii) The incisor exposure with the lips parted at rest — will decide the vertical movement of the maxilla. Aesthetic exposure may vary from 1 to 4 mm. This is inversely proportional to the upper lip length which ranges from 18-24 mms.
iii) Excessive or unaesthetic incisor exposure is corrected with appropriate maxillary impaction.
iv) Where the upper lip is unduly short, the patient can show a greater amount of incisor. If not, the resulting midfacial appearance will be disproportionately small in the vertical dimension.
v) Rarely the patient has marked dento-alveolar hypoplasia and shows little or no incisor with a normal lip length. This is corrected with an inferior movement of the maxilla.
vi) Horizontal as well as vertical maxillary movements will affect the incisor exposure. Advancing the maxilla will lead to greater incisor exposure which will need to be adjusted for when considering the vertical move.
vii) Coronal occlusal cants and midline rotations must also be corrected.
viii) Moving the maxilla will also affect the nose. Vertical impaction widens the alar base and forward movements will elevate the nasal tip. Depending upon the initial appearance these changes may or not be desirable. If not, then a record should be made to provide a “cinch suture” across the lateral alar cartilages or to reduce the anterior nasal spine at the time of surgery.
ix) The inherent inaccuracy of the planning and surgical technique and the eye's inability to perceive small anatomical changes, determine that units of horizontal advancement should be no less than 3 mm. This also facilitates planning as a 3 mm minor advancement; a 6 mm intermediate; and a 9 mm major move. Cleft cases usually require 9 mm or more.
x) Similarly vertical moves of 2 mm for minor; 4 mm intermediate and 6 mm for major impactions are appropriate for all cases. These three categories also simplify the decision making process.
Planning the Mandibular Move
· Having planned where the maxilla is to be placed, the final step is to place the mandible in a Class I incisor relationship. This is built into the final wafer.
· If the definitive occlusion is not immediately possible because of the need for further orthodontics or restorative treatment, The wafer maintains the jaw relationship until orthodontics or restorative treatment can be commenced.
· The mandible will require autorotation, antero-posterior, vertical, rotational or rarely transverse movements.
Autorotation
Any changes in the vertical and horizontal position of the maxilla will necessitate a change in the vertical position of the mandible. This is mediated naturally through neuromuscular feedback mechanisms and the mandibular elevator muscles. Impacting the maxilla will lead to a shortening of the elevator muscles and will re-establish the freeway space. This closing autorotation of the mandible will also lead to a greater anterior prominence of the lower jaw.
Anterior Movements
Forward movement of the mandible to establish a Class I incisor relationship in Class II cases, will also increase lower face height. This is particularly the case with a deep overbite, when advancing the mandible without levelling the curve of Spee. The vertical facial height will increase and the everted lip will unroll and upright. If this change is desirable, the consequent lateral open bites need to be closed with postsurgical orthodontics to a stable position. If too severe for orthodontic closure, then surgery must incorporate a levelling of the occlusal plane with an anterior mandibulotomy procedure.
Posterior Movements
Mandibular setbacks will reduce the anterior facial height and evert the lip. Occasionally this may correct the occlusion but reduce the chin prominence which will require a paradoxical advancement.
Horizontal Rotation
These are required in asymmetry cases, for example hemimandibular elongation where the need is arch coordination especially with an adequate maxillary intercanine width.
Chin Position
Both anteroposterior and vertical movements of the mandible will affect the position of the chin. It is important that the chin be carefully assessed to avoid further surgery.
A common example is the need to correct a Class II division 1 malocclusion by a mandubular advancement which invariably leaves the patient with a deficient chin requiring a simultaneous advancement genioplasty.
With rare excess chin depth a combination of a vertical reduction and anterior sliding genioplasty helps provide a harmonious and balanced appearance.
Planning based on cephalometric assessment and the study model analysis.
Cephalometric Tracing Planning
The movements of the maxilla based on the clinical prediction of the incisor position, can then be repeated on a digital image or tracing of the patient's lateral cephalometric radiograph. Many still prefer to undertake this tracing procedure by hand although cephalometric software packages are available. When planning using hand tracing it is important to trace all the teeth in order to avoid missing potential premature contacts which may preclude the planned movements.
Having traced the revised maxillary position, the next step is to autorotate the mandibular tracing into the initial contact position. In reality, mandibular autorotation is not a simple hinge but a 3-dimensional movement with an envelope of adjustment within the temporomandibular joint. Despite this, it is useful to use as a 2-dimensional tracing despite its uncertainty as to the true centre of rotation. Some use the centre of the condyle, others use the point “condylion” whilst geometric constructions suggest that the point of rotation lies in the region of the mastoid air cells. Despite these differences the mandible in life appears to have an adequate envelope of condylar movement to adapt to crude planning geometry.
Assessment of the incisor position of the autorotated mandible is also important in determining if further adjustment of the maxillary position is required in order to establish a positive overbite.
1. With an anterior open bite autorotation leads to initial buccal segment contact. Closure of the residual anterior open bite by (anticlockwise) rotation of the mandible around this posterior pivot will lead to an elongation of the pterygo-masseteric sling and relapse. In such cases it is necessary to impact the posterior part of the maxilla differentially to that of the anterior maxilla. The extent of the differential impaction can be ascertained from the tracing.
2. With impactions for vertical maxillary excess, any minor incisor discrepancy on simple autorotation can be overcome by forward or backward adjusted movement of the maxilla. Asignificant discrepancy will require a bimaxillary procedure to ensure the incisor Class I relationship without compromising the upper lip incisor relationship.
3. Finally, the predicted profile can then be drawn on the composite planning tracing although this requires an estimate of the facial soft to hard tissue movements.
Imaging Simulations
The use of computerised software packages and the morphing of facial images as a means of planning the dental and surgical moves, together with the display of the predicted surgical outcome have been covered in Chapter 3.