Effects of Maxillary Skeletal Expansion on Craniofacial Structures and Temporomandibular Joints in Adults with Skeletal Class III Malocclusion: A Retrospective Cone-Beam Computed Tomography Study
This retrospective CBCT study demonstrates that maxillary skeletal expansion in adults with skeletal Class III malocclusion induces a coordinated three-dimensional craniofacial response, characterized by lateral and anteroinferior displacement of the zygomaticomaxillary complex, posterior mandibular rotation, and significant increases in temporomandibular joint spaces and condylar cross-sectional areas.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
For many adults, the shape of their face and the way their teeth fit together are set in stone, literally. The bones of the skull, particularly the upper jaw, fuse together over time, making it difficult to change their width without major surgery. When the upper jaw is too narrow, it can force the lower jaw to shift forward to find a comfortable bite, creating a "Class III" malocclusion. This condition often results in a concave facial profile and a bite where the lower teeth sit in front of the upper ones. While traditional braces can move teeth, they cannot easily widen the fused bone of an adult skull. For decades, the only way to truly widen the upper jaw in an adult was to cut the bone surgically, a procedure that carries significant risks and recovery time.
A newer approach, known as maxillary skeletal expansion, offers a middle ground. Instead of cutting the bone, doctors use a device anchored directly into the jawbone with tiny screws to apply steady pressure. This pressure is strong enough to gently pull the fused sutures—the seams where the skull bones meet—apart, creating new space. While this technique has been known to widen the upper jaw, scientists have been uncertain about what happens to the rest of the face when this force is applied. Does the widening stop at the teeth, or does it ripple through the entire skull, affecting the cheekbones, the jaw joint, and the overall shape of the face?
Researchers at Lanzhou University in China set out to map these hidden movements. They focused on a group of twenty-six adults who had both a narrow upper jaw and a lower jaw that sat too far forward. Using a specialized 3D imaging technique called cone-beam computed tomography, which creates a detailed digital model of the skull, they scanned the patients before treatment and again six months after the expansion was complete and the device was removed. This allowed them to measure changes in bone position with millimeter precision, tracking not just the width of the jaw, but the movement of the cheekbones, the rotation of the lower jaw, and the condition of the jaw joints.
The study revealed that the effects of the expansion were far more extensive than a simple widening of the mouth. When the upper jaw was pulled apart, the entire midface responded. The cheekbones, which form a complex structure with the upper jaw, did not just move sideways; they shifted outward, forward, and slightly downward. This movement created a pyramidal shape, where the base of the face widened more than the top, effectively pushing the cheekbones out to create a broader, more balanced facial structure. The researchers observed that the angle of the cheekbone complex changed by nearly three degrees, indicating a significant rotation of the bone rather than just a straight push.
This skeletal shift triggered a chain reaction in the lower jaw. As the upper jaw widened, the lower jaw was no longer forced to sit forward to make contact with the teeth. Instead, it rotated backward and downward, settling into a more natural position. This backward movement was small but measurable, with the angle of the lower jaw changing by more than one degree. Crucially, this repositioning happened without the need for the upper front teeth to tilt forward, a common side effect of other treatments. In fact, the upper teeth actually stood straighter, and for most of the patients, the crossbite was corrected simply by creating more room, without needing to move the teeth themselves.
The changes extended deep into the jaw joints, the hinges where the lower jaw connects to the skull. As the lower jaw moved back, the position of the condyle—the rounded end of the jaw that sits in the joint socket—shifted. The space behind the condyle increased, suggesting the jaw was settling into a more relaxed, centered position within the socket. Perhaps most surprisingly, the bone of the condyle itself began to change shape. The cross-sectional area of the bone increased significantly on both sides, growing by nearly ten square millimeters on the left and nearly seven on the right. This indicates that the adult jaw joint is not static; it can remodel and adapt its shape in response to the new mechanical forces created by the expansion.
The researchers noted that while the changes were generally symmetrical, there were slight differences between the left and right sides of the face. The cheekbones moved slightly different distances on each side, and the bone growth in the jaw joints was not identical. This asymmetry likely stems from the natural variations in how tightly the skull bones are fused in different people and the slight differences in how the pressure was applied. Despite these minor variations, the overall pattern was clear: the force applied to the upper jaw traveled through the entire facial skeleton, causing a coordinated, three-dimensional reorganization of the face.
These findings suggest that for adults with mild to moderate cases of a forward-sitting lower jaw and a narrow upper jaw, this non-surgical approach offers a viable alternative to major surgery. By widening the upper jaw, the treatment unlocks the lower jaw, allowing it to rotate back into a better position and prompting the jaw joints to adapt to this new alignment. The study confirms that the adult face is a connected system, where changing one part can lead to beneficial adjustments throughout the structure. While the researchers caution that more long-term studies are needed to ensure these changes remain stable over many years, the results provide strong evidence that the adult skull retains a surprising capacity for change when guided by the right forces.
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