BSSO: repositioning the lower jaw

The bilateral sagittal split osteotomy, always shortened to BSSO, is the standard operation for repositioning the lower jaw. It has been in use since the 1950s and refined continuously since, and it is one of the most predictable procedures in facial surgery.

What the operation does

The surgeon splits each side of the lower jaw lengthwise, in the region behind the molars, so that the tooth-bearing front portion can slide independently of the portion carrying the jaw joint. Because the split is lengthwise rather than straight across, the two pieces overlap over a broad surface, which is why it heals well and holds its position. Plates and screws fix the result. The incisions are inside the mouth; there is no external scar.

Advancement and setback

Advancement moves the lower jaw forward. This is the more common direction, used for a retruded lower jaw where the chin sits back and the overjet is large. It opens the space behind the tongue, which is why the same movement appears in sleep apnea surgery.

Setback moves the lower jaw back, for a skeletal underbite where the lower jaw has outgrown the upper. In practice many underbites are corrected by advancing the upper jaw instead, or by moving both, because a large setback can crowd the airway. That is a decision the surgeon and orthodontist make together, and it is one worth asking about directly if you have any history of snoring or disturbed sleep.

Genioplasty, and why it is a separate question

The chin point can be cut and repositioned independently of the jaw, in an operation called a genioplasty or sliding genioplasty. It changes the profile without changing the bite. It is often added to a BSSO because the amount of chin projection that looks balanced is not always the amount that follows from correcting the bite. It is worth understanding that these are two different goals — one functional, one aesthetic — being addressed in the same anaesthetic.

The nerve conversation

The inferior alveolar nerve, which supplies sensation to the lower lip, chin and lower teeth, runs through the lower jaw directly across the surgical field. Some altered sensation afterwards is expected rather than exceptional. Most of it resolves over weeks to months. A minority of patients — the figure varies by study, by the size of the movement and by patient age — retain some permanent numbness or altered feeling in the lower lip or chin.

This is not a complication to be glossed over, and it is the single thing we most want patients to have understood before consenting. It does not affect movement of the lip, only sensation. Ask your surgeon for their own figures.

Recovery

One to two nights in hospital. Swelling peaks in the first few days. A liquid then soft diet for several weeks, guided elastics rather than wired jaws, and typically two to four weeks off work or school. Chewing strength returns gradually over months. Jaw physiotherapy is sometimes prescribed.

Relapse, honestly

Large advancements, open bite corrections and cases with jaw joint problems carry more relapse risk than straightforward setbacks. Modern rigid fixation has reduced this considerably compared with older techniques. Wearing retainers and following the elastic instructions afterwards genuinely matters — see why retainers matter.

Where orthodontics fits

Braces before, to position the teeth over their own jaw bases. Braces after, to settle the bite. The pre-surgical phase can make the bite look temporarily worse, and knowing that in advance makes it much easier to live with. See orthognathic surgery for the full sequence, and Le Fort I for the upper jaw operation that BSSO is often combined with.