Le Fort I maxillary and sagittal split mandibular osteotomies are performed to correct dental malocclusion and improve facial harmony. Skeletal movements of large magnitude, particularly those for correction of preexisting facial asymmetries, may create new contour irregularities and disharmony. Both the midface and lower face can manifest characteristic postorthognathic contour irregularities that are amenable to alloplastic implant improvement.
Midface Contour Irregularity
Because the Le Fort I maxillary osteotomy and advancement is performed beneath the infraorbital foramen, the midface skeleton above the foramen remains deficient in its sagittal projection. The midface skeleton is now more prominent in its lower half after Le Fort I advancement surgery and, as a result, the cheeks appear low ( Fig. 15.1 ). This imbalance is in proportion to the magnitude of sagittal skeletal advancement.
To improve midface contour some surgeons have combined zygomatic osteotomies with the Le Fort I osteotomy. More often, malar augmentation with alloplastic implants is performed at the time of the maxillary advancement procedure. Malar augmentation alone widens the midface, which tends to exaggerate an upper midface parasagittal deficiency potentially creating another imbalance. Hence, increasing the projection of the infraorbital rim is considered the critical element in creating midface balance after Le Fort I advancement, with malar augmentation often an appropriate adjunct.
Lower Face Contour Irregularity
Sagittal split osteotomy of the deficient mandible with advancement in the sagittal plane may result in ramus asymmetry and border irregularities ( Figs. 15.1 and 15.2 ). Ramus asymmetry immediately after surgery occurs when the surgeon fails to create similar osteotomies and maintain similar osteotomy gaps (with rigid fixation) on both sides. The surgeon loses control over the relation of the proximal and distal fragments because the condyle must be positioned in its fossa and the body must be positioned to correct the occlusion. The positioning of these two areas dictates ramus and angle position. In the orthognathic literature, the concern for the improper positioning of the proximal segments and condyle relates to its potential for condylar resorption and postoperative relapse. In the pre-rigid fixation era this was almost always due to counterclockwise rotation of the ramus of the mandible because wire osteosynthesis did not provide enough stability. Rigid fixation has virtually eliminated these problems related to inadequate fixation.
However, contour abnormalities may still occur due to incorrect positioning with resultant counterclockwise rotation or after very large movements. Clinical experience has revealed that this is due to not only the positional restraints of condylar positioning and osteotomy variability as described above, but also ramus shape changes resulting from bone atrophy ( Fig. 15.3 ). Depending on the circumstance, this atrophy may result from devascularization and/or remodeling from altered mechanotransduction forces in accordance with Wolff’s Law.
Gaps at the osteotomy sites after sagittal osteotomy of the mandible or horizontal osteotomies of the chin with advancement can result in border irregularities. Correction of contour irregularities after skeletal rearrangement have focused on border defects at the sliding genioplasty osteotomy site, prompting some surgeons to fill the defect with hydroxyapatite. Surgical technique and the native mandible anatomy determine the presence and severity of these irregularities.
Patients with “high-angle” mandibular deficiency, which is seen in association with antegonial notching and vertically deficient rami, are predisposed to visible notching after bilateral sagittal split osteotomy (BSSO). In patients with “low-angle” mandibular deficiency, where the ramus height is adequate and there is no antegonial notching, post-BSSO border irregularities reflect technical deficiencies in creating and maintaining (assuming rigid fixation) the correct osteotomy gap geometry of the buccal cortical plates. This geometry should be the same on both sides.
The extent of parasymphyseal notching after horizontal osteotomy of the chin with advancement is also influenced by surgical technique. By making the osteotomy oblique, as the artist has depicted in Fig. 15.1 , the distal segment is shortened as it is advanced, which not only accentuates border irregularities but also causes the bone cut to exit the inferior mandibular border where the soft tissue coverage is more tenuous and exposes the notching. By lowering the bone cut anteriorly, one can eliminate or minimize the obliquity of the osteotomy and, therefore, any shortening with a more proximal inferior border exit.
Contour irregularities can be addressed at the time of orthognathic surgery or, most often, at a later, secondary surgery.
The result obtained with readily available “off-the-shelf” implants can be limited due to the challenge of hand carving three-dimensional implant contours to correct iatrogenic as well as intrinsic facial asymmetries. Computer-aided design and manufacture (CAD/CAM) implants provide added sophistication to the procedure and are preferred by these authors.
When implants are placed at the time of orthognathic surgery, they are usually off-the-shelf implants modified at that time to address the observed skeletal deficiency. Orthognathic procedures are now routinely planned with computer technology and the surgery facilitated with computer-designed cutting guides and fixation devices. Obligatory skeletal imbalances and contour irregularities can be predicted. This can provide implants that are computer designed to correct these imbalances and irregularities at the time of the orthognathic procedure.