Double Jaw Surgery

Introduction to the problem of sequencing bimaxillary surgery:

With traditional bimaxillary surgery techniques, the LFI operation is done prior to the sagittal osteotomy. This sequencing began out of necessity with wire fixation in the early 1970’s. With wire fixation the maxilla was easily stabilized and the mandible was then moved to the stable maxilla. The reverse was not possible with wire fixation – the mandible could not be done first. With rigid fixation either jaw can be done first. This discussion will list the advantages of completing the sagittal osteotomy prior to the LFI osteotomy during bimaxillary jaw surgery.

Several advantages exist for doing the BSSO prior to the LFI during bimaxillary surgery. Unfortunately, as a rule, surgeons don’t complete the lower jaw first and additionally are unaware there is a problem doing the upper jaw first. When surgeons are asked why the upper jaw is done first, the common response is – “I was trained to do the upper first”. Others respond that the upper cannot be done first because model surgery won’t permit doing the lower first. When doing model surgery, if the lower is advanced first, the mandibular incisors contact the upper incisor moving the articulator pin open. This is a legitimate concern but can be planned around enabling the model surgery to be done mandible first.

Inaccurate mandibular position for presurgical records:

The adverse consequences of sequencing the maxilla first in bimaxillary surgery are many. Why does doing the maxilla first during bimaxillary surgery lead to inaccurate positioning of both the upper and lower jaws at surgery? The disadvantages involve inaccurate presurgical records involving mandibular seating. In a nut shell, measures of yaw, cant, midline, and even projection of the mandible are unreliable in presurgical records. Why? All mandibular measures are subject to where the joints are positioned when measures are taken. Are the joints seated for records or is one or are both in a forward position? Where this question may seem simple to determine while taking records, it is not. As a consequence of this problem presurgical mandibular projection, midlines, yaw, and cants are unreliable. If the maxilla is therefore done first, it is aligned to a possibly and probable spurious position of the mandible. Tomography can reveal condyles which are forward positioned – most surgeons and orthodontist don’t do basic checks of condyle position for lateral headfilms and mounted models with tomograms.

Often the cephalometric occlusion and model mounting are accomplished with the mandibular condyles positioned forward. Forward displacement might be equal or perhaps one condyle might be displaced more than the other. This condyle displacement is an inadvertent result of occlusal interferences or neuromuscular posturing of the mandible when the waxbite is made. The waxbite is then utilized to obtain the planning cephalogram and mount the surgical models. In this instance, when the maxilla is operated first, the maxilla and mandible will be positioned posterior to VTO plan as the condyles will seat during surgery. We can call this the “retraction error” of doing the maxilla first. The maxilla has further inaccuracies involving midlines and cants. How? Suppose the right joint was farther forward than the left during planning. The joints seat at surgery and both jaws end with midlines to the right and cant up on right. Nothing prevents the condyles from seating during the surgery, and as a consequence, the bimaxillary complex will displace from VTO plan as the joints seat at surgery.

Planning headfilm used for VTO. Condyles are forward when headfilm taken.

The planning headfilm is depicted. The original maxilla and mandible are traced in black. The proposed advancement (VTO) of the maxilla and mandible are shown in red. The original black tracing, unbeknownst to the surgeon, was taken with the condyles forward.

Condyles seat during surgery to blue position.

The post surgical tracing of the maxilla and mandible are in blue. Both jaws are retracted from the planned red VTO. The condyles, at surgery, seated posterior to the position on the original red VTO tracings.

Conversely, if the mandible is done first, even if the condyles are grossly forward on the cephalogram and model mounting, there is no inaccuracy in the final position of the maxilla or mandible. Why does doing the mandible first avoid the “retraction error”? When the mandible is sequenced first, the intermediate splint guides the mandibular teeth into the accurate relationship to the fixed maxilla, the condyles seat and the correct length mandible undergoes rigid fixation. No retraction, midline shifts, or cants – simply a larger BSSO vertical gap on one or both sides of the BSSO.

The planning headfilm is depicted. The original maxilla and mandible are traced in black. The proposed advancement (VTO) of the maxilla and mandible are shown in red. The original black tracing, unbeknownst to the surgeon, was taken with the condyles forward.

The post surgical tracing of the maxilla and mandible are in red. Both jaws are in the planned red VTO position as planned on the left. The condyles, at surgery, seated posterior to the position on the original black and red VTO tracings but because the BSSO was done first, jaw positions are correct. The gap at the osteotomy merely was larger than expected.

Additionally, accurate measurements of the dental midlines are enhanced when the mandible is operated first. Both midlines of the upper and lower are measured relative to the middle of the philtrum. The upper dental midline is next to the philtrum – easy to measure without parallax error. Contrary to this, the lower dental midline is distant to the philtrum and accordingly difficult to measure do to parallax errors. The assessment of the mandibular midline becomes more difficult the greater the anteroposterior skeletal problem.

Additionally, when the upper jaw is operated first on model surgery it is moved relative to the mandible. Again, if the joints are not seated the maxilla will be moved to the variable jaw.

Ease of measure upper vs lower incisors to philtrum

An additional advantage of operating the mandible first is that this requires the difficult joint seating first. All surgeons have more difficulty accurately seating the joints during BSSO surgery. by doing the lower the accuracy of positioning the joints is tested. Release of intermaxillary fixation reveals the mandibular teeth to the maxillary teeth. This relationship should match the model surgery. the relationship is easy to assess – do the lower teeth relate to the fixed maxilla as planned? If yes, go to the maxillary surgery. if no, reinstitute intermaxillary fixation over the intermediate splint and redo fixation.

When the maxilla is opertated first however the intermediate bite check is unavailable. Why? The maxilla will have the correct relationship to the mandible but the bimaxillary complex may not be properly oriented to the remainder of the face depending on how the condyles changed from presurgery to intraoperative.

Able measure accuracy of plated mandible to stationary maxilla

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