The hierarchy of stability and predictability in orthognathic surgery with rigid fixation: an update and extension
© Proffit et al; licensee BioMed Central Ltd. 2007
Received: 27 March 2007
Accepted: 30 April 2007
Published: 30 April 2007
A hierarchy of stability exists among the types of surgical movements that are possible with orthognathic surgery. This report updates the hierarchy, focusing on comparison of the stability of procedures when rigid fixation is used. Two procedures not previously placed in the hierarchy now are included: correction of asymmetry is stable with rigid fixation and repositioning of the chin also is very stable. During the first post-surgical year, surgical movements in patients treated for Class II/long face problems tend to be more stable than those treated for Class III problems. Clinically relevant changes (more than 2 mm) occur in a surprisingly large percentage of orthognathic surgery patients from one to five years post-treatment, after surgical healing is complete. During the first post-surgical year, patients treated for Class II/long face problems are more stable than those treated for Class III problems; from one to five years post-treatment, some patients in both groups experience skeletal change, but the Class III patients then are more stable than the Class II/long face patients. Fewer patients exhibit long-term changes in the dental occlusion than skeletal changes, because the dentition usually adapts to the skeletal change.
The Dentofacial Program at the University of North Carolina was begun in 1975 as a way to coordinate the evaluation and treatment of patients who needed orthodontics and orthognathic surgery, and as a way to facilitate research in this area. A research grant focused on the outcomes of orthognathic surgery at UNC, funded by the National Institute of Dental and Craniofacial Research, enters its 28th year in June 2007.
This research project has resulted in more than 100 research papers in peer-reviewed journals, and about half that many invited contributions and book chapters. It became obvious by the 1990s that a major influence on the outcomes of orthognathic surgery was the amount and direction of surgical movement. A series of research papers that focused specifically on stability as related to the different surgical movements was summarized in 1996 in a paper outlining a hierarchy of stability related to surgical movements . The purpose of this paper is to update the hierarchy by extending it to include treatment of asymmetries and provide further information with regard to long-term stability.
The data base created through this project currently (February 2007) has records on 2264 patients who have had orthognathic surgery. Nearly twice that many have had initial records through the Dentofacial Program after they were referred for evaluation. Many of these were judged not to need surgery; the remainder did not accept it if it was recommended [2, 3]. As of February 2007, at least one year follow-up is available for 1475 patients who did receive surgery, and five year or longer postsurgical follow-up is available for 507 patients.
Stability has been evaluated primarily from lateral cephalometric radiographs, which for all our studies have been oriented with the SN line rotated down 6° anteriorly, a position that approximates natural head position and is more reproducible than the Frankfort plane. This horizontal line is used as the x axis, and a vertical plane perpendicular to it through sella as the y axis, so that changes in landmark locations can be registered as x, y coordinate changes.
When stability is considered, it is important to keep in mind that there is not a normal distribution of post-surgical or post-treatment change. Instead, most of the changes occur in a few of the patients. Mean changes and standard deviations, therefore, can be misleading. The error in locating most cephalometric landmarks is less than 1 mm, and does not exceed 2 mm for any landmark. The hierarchy of procedures presented in this paper is primarily based on the number (percentage) of patients who experienced changes of at least 2 mm. We consider changes of <2 mm within the range of method error and clinically insignificant; 2–4 mm outside the range of method error and potentially clinically significant; and >4 mm as often beyond the range of orthodontic compensation and clinically highly significant.
The results presented below represent a compilation of stability data from the UNC database that have been reported previously in separate publications.
For the purposes of this extension of the hierarchy, it is important to differentiate post-surgical stability (changes in the first post-surgical year, which relate directly to the surgical healing, post-treatment orthodontics and short-term physiologic adaptation) from post-treatment stability (changes beyond one year post-surgery, which relate to long-term adaptation and for some patients, to post-treatment growth).
The First Post-Surgical Year
It is interesting that the two single-jaw procedures used to correct skeletal Class II problems, superior repositioning of the maxilla and advancement of the mandible, fall into the highly stable category [4–11]. This was also true with wire fixation. It must be kept in mind, however, that mandibular advancement at UNC has been restricted to patients with short or normal face height. Early experience showed a lack of stability with ramus surgery to rotate the mandible at the osteotomy site so that the chin was moved up to close an anterior open bite, and we have used superior repositioning of the maxilla (with or without mandibular surgery) for these long face patients, so that the rotation occurred at the condyle instead.
Stable only with rigid fixation
Three procedures fall into this category: combined maxillary and mandibular surgery for correction of either Class II (maxilla up + mandible forward) or Class III (maxilla forward + mandible back) problems, and correction of facial asymmetry [8, 16, 17].
Long-term Stability (Beyond One Year Post-Surgery)
A different pattern of stability exists when long-term post-treatment changes (changes between one and five years post-surgery) are considered [22–28]. After the first post-surgical year, when healing is complete, four interesting phenomena are observed: (1) in about 20% of the patients who had mandibular advancement (with or without simultaneous maxillary surgery), mandibular length decreases between 1 and 5 years post-treatment; (2) after superior repositioning of the maxilla, downward movement of the maxilla, in what appears to be a resumption of the original growth pattern, leads to >2 mm change in about one-third of the patients; (3) clinically significant changes in the position or dimensions of the maxilla and mandible occur in about twice as many patients as similar changes in overjet or overbite; and (4) the Class III patients who tended to be less stable than Class II patients in the first post-surgical year show less change thereafter. Considering these in turn:
Changes in mandibular length: long-term condylar remodeling
Problematic Post-surgical Stability: Why?
Widening the maxilla with a segmental osteotomy stretches the palatal soft tissues, and this tissue elasticity provides a force to decrease the expansion post-surgically (see Figure 6). Surgically-assisted expansion (SARPE), with a jackscrew in place across the palate to provide somewhat slower expansion and (perhaps more importantly) rigid retention, is a reasonable alternative if only transverse changes are needed. Are two surgical procedures, first SARPE and then a later one-piece LeFort I osteotmy, indicated instead of a one-stage segmental LeFort I when three-dimensional movements are needed ? The major reason for 2-stage surgery would be presumed better stability for expansion with SARPE, and a current study with better methodology than previous publications shows no significant differences between long term stability of expansion with osteotomy or SARPE . Significant differences have not been documented between the outcomes of two-stage and one-stage approaches, but good data for this comparison do not yet exist.
Long-term Post-treatment Stability
Beyond one year, changes are only indirectly related to surgery. Skeletal changes over a 5 year period can be shown in patients who did not have orthognathic surgery,  but in post-treatment orthognathic surgery patients, the changes tend to be larger . In this time period, changes reflect adaptive bone remodeling and/or a resumption of growth, and adaptive changes in the dentition.
The data show that after Class II surgery, in patients who have long-term changes, there usually is a smaller increase in overjet than the decrease in mandibular length. Adaptation of the dentition to skeletal change, primarily proclination of the lower incisors, largely prevents the same degree of change in overjet. The same thing is seen in long face patients, many of whom had an anterior open bite, in whom long-term downward movement of the maxilla occurred. There was not the same degree of bite opening, because of compensatory eruption of the anterior teeth in both arches.
It is surprising that a smaller percentage of patients treated surgically for Class III problems have long-term changes than those treated for Class II problems. Because mandibular prognathic patients often have mandibular growth until an older age than individuals who do not have this problem, it would seem reasonable that continued mandibular growth long-term after surgery might occur, and that this would be more likely in those who had mandibular setback surgery at a younger age. The data do not support either of those ideas [35, 36]. Beyond one year post-surgery, very few patients have forward growth of the mandible. Girls who had setback surgery before age 18, and boys who had it before age 20, were no more likely to have long-term mandibular growth than those treated at later ages.
Data now exist to document the stability of changes in jaw position from orthognathic surgery. From the perspective of stability during the first post-surgical year, the surgical movements can be placed in four groups ranging from highly stable to problematic. The procedures typically used to treat Class II/long face problems are quite stable in the first year, the procedures typically used to treat Class III problems less so. A surprisingly large number of patients experience skeletal changes from one to five years post-surgery, when healing is complete, and in that time frame clinically relevant (>2 mm) changes are more likely in Class II/long face patients than in Class III patients. Fewer patients exhibit long-term changes in the dental occlusion than skeletal changes, because adaptive changes often occur in the dentition as skeletal changes occur. In both the post-surgical and post-treatment periods, almost all the changes occur in a minority of patients, so it is better to consider the percentage of patients with clinically significant changes than the mean changes. The database makes it clear that clinically satisfactory results can be obtained and maintained long-term in the great majority of orthognathic surgery patients, but the differences among various directions of movement must be taken into account when treatment is planned.
This work was supported in part by NIH grant DE-05221 from the National Institute of Dental and Craniofacial Research. We thank Ms. Debora Price for her long-time efforts in developing the data base and for her application of SAS programs for various statistical analyses. We also thank other orthodontic and surgery faculty who have worked with the Dentofacial Program, and the many orthodontic and surgery residents who participated in stability-related research projects while at UNC.
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