Back to Basics: Surgically Assisted Rapid Maxillary Expansion (SARME)
This is another great post by Martyn Cobourne, in his Back to Basics “ series. He first published a version of this in his excellent Evidence-Based Orthodontics Facebook Group.
In this post, I will continue the maxillary expansion theme. In severe maxillary hypoplasia and transverse deficiency cases, considerable expansion is often required in the maxillary arch in adults. This may not be attainable with traditional orthodontic or skeletal expansion methods alone. As a result, we can consider other methods, one of which is surgically assisted rapid maxillary expansion (SARME).
This form of distraction osteogenesis utilises orthopaedic maxilla expansion in combination with partial osteotomy or corticotomy. The surgery facilitates expansion which is followed by bony infill during the post-surgical period. The corticotomy can involve maxillary separation from the piriform aperture to the pterygoid plates and zygomatic buttress, splitting the intermaxillary suture and separating the nasal septum from the maxilla. The orthopaedic force is applied through a conventional tooth-born, hybrid or direct bone-born appliance. It has been suggested that adult cases requiring >5 mm expansion SARME should be considered.
- The reported advantages of SARME are that it affords considerable expansion of the maxilla in adult patients and reduces the likelihood of relapse. However, compelling evidence regarding long-term stability currently needs to be improved.
- The main disadvantage is the requirement for an additional surgical intervention, which in patients requiring a definitive osteotomy will mean two separate surgical procedures. A further issue with SARME can be asymmetric expansion between the left and right sides, caused mainly by variations in the surgical cuts. In addition, this procedure can produce quite a significant midline diastema which, although temporary, can be a source of dissatisfaction for the patient.
What is the evidence base?
Several studies have compared the immediate skeletal and dental effects of SARME and the stability of this procedure. Still, like many studies on expansion, they only offer low-moderate quality evidence, suffering from small sample sizes and significant heterogeneity in the techniques used (particularly the type of surgical intervention and method of expansion) and outcomes measured:
- SARME is effective in obtaining significant transverse expansion of the maxilla. However, RCTs have demonstrated an average of 3.3 mm skeletal and 7.0 mm inter-molar dental expansion when conventional appliances are used, suggesting that a significant proportion of the change achieved remains dental rather than purely skeletal.
- Bone-anchored appliances seem to provide more predictable expansion than tooth-borne, but the evidence base is currently weaker; and
- The significant dental and skeletal expansion achieved by SARME does seem to be reasonably stable, with average losses of around 20%; however,
- High-quality data is lacking for all these outcomes.
A burning question is the role of MARPE in potentially replacing SARME in adult patients. In this domain, the evidence base is very much in its infancy. However, a recent CBCT study comparing hybrid MARPE with historical SARME data has shown greater transverse skeletal change in the midface and maxillary base with MARPE and a tendency to a more parallel expansion. In addition, there was less buccal inclination of the teeth and alveolar processes with MARPE – which, combined with the lack of sub-total maxillary osteotomy in the MARPE group, suggests a significant advantage over SARME. I suspect more data comparing these techniques will appear over the next few years – ideally based upon RCTs and more assessment of patient-related factors.
Professor of Orthodontics, Kings College, London.
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“However, a recent CBCT study comparing hybrid MARPE with historical SARME data has shown greater transverse skeletal change in the midface and maxillary base with MARPE and a tendency to a more parallel expansion. In addition, there was less buccal inclination of the teeth and alveolar processes with MARPE – which, combined with the lack of sub-total maxillary osteotomy in the MARPE group, suggests a significant advantage over SARME. ”
Understanding the lack of sub-total maxillary osteotomy with MARPE and am not at all surprised that SARME has no effect on midface and movement above the osteotomy cut. This stuff about more parallel expansion and less buccal inclination of the teeth and alveolar processes with MARPE, however, that’s a bit less understandable.
For a given intraoral expansion device (for this let’s think full bony, non-tooth included, expander), the center of resistance of the SARME produced segments is somewhere inferior to the osteotomy (perhaps modeling the soft-tissue in a finite element analysis would put the center of resistance a bit more superior but on-the-fly FEM is outside of my primitive skill set). The moment arm from the SARME osteotomy cut to the lateral forces produced by the expander is short. Now with the non-osteotomy MARPE the center of resistance is significantly superior/cephalad. The moment arm from transverse force to center of resistance significantly longer, wherever the heck that center of resistance is located. The longer the moment arm the more rotation of the segment vs translation/parallel movement.
Would you be able to explain this significant inconsistency or error in my thought process or point me to a FEM that predicts this outcome?
Else perhaps this emphasizes a probable gaping error in the studies available.