The hierarchy of stability and predictability in orthognathic surgery with rigid fixationan update and extension

Let’s start with a simple question that’s never been simple in practice: after jaw surgery, which movements stay put and which ones drift? Surgeons have had rules of thumb for decades, but Proffit, Turvey, and Phillips did something better. They built a movement-by-movement map of stability and then updated it to include two things that weren’t in the original playbook: fixing facial asymmetry and repositioning the chin. The headline is reassuring. With rigid fixation, both of those additions behave as highly stable moves. And the broader point lands too: stability depends on what you move and when you look. The first year is one story. Years one to five can be a different one. All of this rests on a remarkable archive from the University of North Carolina’s Dentofacial Program. By early two thousand seven, it held records on two thousand two hundred sixty-four patients who’d had orthognathic surgery; one thousand four hundred seventy-five had at least a year of follow-up, and five hundred seven were tracked for five years or more. That kind of depth lets you parse short-term settling from long-term remodeling. The measurements come from lateral cephalometric radiographs oriented in a consistent way: the sella–nasion line—think of it as the cranial base reference—is tilted slightly forward to approximate natural head posture. That line becomes the horizontal axis; a perpendicular through sella is the vertical. Landmarks move as shifts in x and y over time. Importantly, the method’s noise is small—typically under one millimeter and never exceeding two—so they set a clinical yardstick: changes under two millimeters live in the gray zone of measurement error, two to four millimeters can matter clinically, and more than four almost always does. And instead of averaging everything away, they ask the right question: what fraction of patients cross that two-millimeter line? In the first post-surgical year, one move stands out as rock solid. When the maxilla is moved up—superior repositioning—with rigid fixation, clinically important relapse is vanishingly rare. The mandible, on average, doesn’t slide forward or back during that window either. But it does remodel in shape. The gonial angle, that corner at the back of the jaw, tends to drift upward; more than half of patients show more than two millimeters of upward movement at that point. Surgeons see the same thing clinically: north of ninety percent of these patients are judged to have excellent results at one year. The bone heals, the bite settles, and the plan holds. What about moving forward—bringing the maxilla forward or advancing the mandible? Those are good bets too, with a caveat. For both isolated maxillary advancement and mandibular advancement, the pattern is consistent: about eighty percent of patients show little to no horizontal change in the first year, about twenty percent fall into that two to four millimeter relapse band, and slipping beyond four millimeters is rare. That “rare” matters because it’s the difference between minor orthodontic touch-ups and a bite that just doesn’t meet where you put it. But the takeaway is still positive. With rigid fixation and good planning, forward moves tend to stick. Two-jaw procedures—moving both the upper and lower jaws—live in the stable core too, as long as you bolt them well. In Class II cases, where the lower jaw is short or the face is long, rigid fixation keeps the extra, unplanned movement after splint removal to a minority. Only about one in five patients shows more than two millimeters beyond the rotation you expect when the splint comes off. And the global clinical read is strong: roughly ninety percent excellent outcomes with rigid fixation, compared to about sixty percent without it. Class III two-jaw cases, the ones with an oversized mandible or underdeveloped maxilla, follow a similar pattern. Even biodegradable plates and screws stack up comparably to metal in this window. Where first-year risk climbs is at the edges of the movement map. Mandibular setback—the classic move for a big lower jaw—carries relapse risk if the ramus, the vertical part of the mandible, is allowed to swing back toward its preoperative angle. Composite tracings show the chin marching forward again as the ramus rights itself. Asymmetric movements of the mandible can be even touchier; nearly half of those patients see more than two millimeters of change. And certain directions in the maxilla are just harder to hold. Moving the maxilla down invites early occlusal forces to push it back up before the bone has locked in, so without special tactics, as many as half will cross that two-millimeter line and up to a fifth will exceed four. Transverse widening by segmental osteotomy has a distinctive pattern too: relapse is greatest across the molars, with about thirty percent losing more than three millimeters there. Now, take a breath, because this is where the story flips. If you fast-forward past healing—to that one to five year window—the stability leaderboard changes. The Class III patients who looked a little shakier in the first year often quiet down. The Class II and long-face patients, by contrast, are more likely to drift. The bones keep remodeling. Some people keep growing. And the teeth, ever adaptable, camouflage a lot of it. Let’s put numbers to that. After mandibular advancement, about one in five patients loses mandibular length between one and five years. You can see the mechanism at the joint. Roughly a quarter show remodeling at the condyles that shortens the ramus. Yet fewer than half of that subgroup ends up with an increased overjet—the front-to-back relationship of the incisors—because the lower front teeth tend to tip forward and the bite compensates. After moving the maxilla up, a different long-term phenomenon appears: about a third of patients show more than two millimeters of downward drift, which looks like a resumption of the original growth pattern. Again, the teeth soften the blow; overjet and overbite change in fewer patients than the skeletal landmarks do. Two-jaw Class II corrections underline the point. When you look between one and five years, about a third of those patients show more than two millimeters of backward movement at skeletal points B and pogonion. And for about half of that group, the retreat passes four millimeters. Other long-term metrics are more modest but still real; for example, the distance from the condyle to pogonion shrinks by more than two millimeters in roughly twelve percent of cases. Put differently, meaningful skeletal changes are roughly twice as common as bite changes in this period, because dentoalveolar adaptation—especially proclination of the lower incisors—keeps the occlusion serviceable in many patients. It’s a reminder that teeth and bone don’t always tell the same story. Why do some movements behave and others misbehave? Think biomechanics. In the weeks after surgery, bone is healing, but muscles, occlusal forces, and soft tissues are already tugging. If those forces align with the old position, they’ll try to drag you back before the scaffolding has set. In mandibular setback, the classic trap is the ramus. If it’s allowed to re-incline toward its original orientation, muscle pull drags the chin forward and you read that as relapse. The fix is as much geometric as it is mechanical: control the ramus inclination at the time of surgery, and you cut relapse dramatically. On paper the operated line can look “stable” while the shape of the bone is changing underneath it. That’s why surgeons care about the tracing, not just the numbers. Downward movement of the maxilla illustrates the other side of the coin: load before lock. The occlusion pushes up every time you chew. If the bone doesn’t yet have the lattice to resist it, the maxilla sneaks north. Three tactics help. One is belt-and-suspenders fixation—heavier plates than you’d grab for other moves. Another is to fill the bony gap with a rigid interpositional graft, like hydroxyapatite, so the face of the maxilla doesn’t have empty space to collapse into. The third is to do simultaneous mandibular surgery that reduces occlusal loading while the maxilla heals. All three can work. None is magic. And the long-term data warn that downward moves are simply less stable unless you pair them with that kind of reinforcement. Transverse expansion adds a different wrinkle. Segmental Le Fort I can widen the arch, but the molars love to drift back; that’s the thirty percent with more than three millimeters of relapse in the molar region. Surgically assisted rapid palatal expansion, or SARPE—the jackscrew approach with rigid retention—spreads the change out over time and can be useful when you also need three-dimensional movements. But here’s the sober part. In comparable series, there isn’t a clear long-term stability advantage for segmental osteotomy versus surgically assisted rapid palatal expansion. Two-stage plans, surgically assisted rapid palatal expansion followed by Le Fort I, are common-sense for complex cases, yet the evidence so far doesn’t show a decisive edge over a single-stage approach. So how should you think about the hierarchy today? Proffit and colleagues group the first-year moves into a practical spectrum. At the “very stable” end sit superior repositioning of the maxilla, mandibular advancement, and lower-border osteotomy for chin repositioning. Two-jaw corrections for Class II and Class III are solid too with rigid fixation; the outcome numbers—about ninety percent excellent with rigid hardware versus roughly sixty percent without in two-jaw Class II—tell you why. Forward movement of the maxilla is a step down but still dependable for most; roughly four out of five patients stay within that “no significant relapse” band, and slipping past four millimeters is uncommon. At the less stable end are mandibular setback, downward movements of the maxilla unless reinforced, and asymmetric mandibular changes, where roughly half of patients show more than two millimeters of drift. After a year, reset your expectations. Skeletal landmarks keep moving in a notable minority, particularly in Class II and long-face patterns, while the bite often looks better than the bones do because the dentition adapts. The arithmetic is simple but important: one in five losing mandibular length after advancement, about a third showing downward drift after maxillary impaction, and a third of two-jaw Class II cases nudging back at key mandibular points—with about half of that subgroup exceeding four millimeters. Meanwhile, Class III patients tend to calm down and look more stable in this long view than they did early on. There are limits to keep in mind. The changes aren’t normally distributed; a small subset accounts for most of the action. The cephalometric method is precise but not perfect, and that two-millimeter threshold is designed to sit just outside measurement error. And the database reflects one program’s practice up through two thousand seven, albeit at a scale—over two thousand cases—that few centers can match. If you’re a planner or a patient, the message is both technical and practical. Choose movements that live in the stable zones when you can. When you can’t, stack the deck—control the ramus in setbacks, overbuild fixation for downward maxillary moves, consider interpositional grafts when there’s a gap to collapse into, and use two-jaw strategies with rigid fixation when rotation and load-sharing will help you hold what you’ve built. And counsel honestly about the timelines: year one is about surgical stability; years one to five are about biology. A quick look forward, and then we’ll land the plane. The tools are improving. Three-dimensional planning and patient-specific plates should make it easier to control ramus inclination and distribute forces the way you intend. Better imaging of the condyles could help flag the patients most at risk for long-term ramus shortening after advancement. But even as the hardware gets smarter, the hierarchy holds. Stability follows the movement, and time tells you the rest.