Published 13 October 2026 · Reviewed by Dr Sin Yong
SMAS stands for the superficial musculoaponeurotic system, the continuous fibrous and muscular layer beneath the skin and subcutaneous fat of the face that connects the facial muscles to the dermis and transmits their movement. The SMAS sits at roughly 4.5 mm below the surface over the cheek and jawline and is the layer repositioned in a surgical facelift. Non-surgical lifting devices either heat focal points within it or work on the layers above it.
Key facts
Full name
Superficial musculoaponeurotic system
Depth
Approximately 4.5 mm below the skin surface over the cheek and jaw; continuous with the platysma in the neck
Why it matters
The layer a surgical facelift repositions; the 4.5 mm target of HIFU and Ultherapy
Not reached by
Monopolar radiofrequency, which heats the dermis and subcutaneous layer volumetrically rather than placing focal heat in the SMAS
Where the SMAS matters on this site
The SMAS is the reference layer for every lifting page on this site. Whenever a page explains why one device suits a face and another does not, the answer usually comes back to whether the SMAS, the dermis or the fat above it is the layer that has loosened.
Before I choose a lifting device I ask which layer has loosened; the SMAS is a depth to aim at, not a word to sell.— Dr Sin Yong
Ultherapy: focused ultrasound with real-time imaging of the layers, so the SMAS depth is confirmed before each line.
What HIFU cannot do: why heating the SMAS does not replace volume, remove heavy skin or treat jowl fat.
VF Lift – Vertical Facelift: Dr Sin Yong's radiofrequency protocol on Volnewmer, which heats the dermis and subcutaneous layer along vertical vectors rather than placing focal heat in the SMAS.
The Bliss Lift: threads placed in the subcutaneous fat above the SMAS, engaging tissue mechanically.
On each page the same question is asked at consultation: is the SMAS lax, is volume missing, or both? The answer decides the tool.
Not to be confused with
The SMAS is one layer in a stack, and several neighbouring structures are easily mistaken for it.
The dermis. The skin's own collagen layer, much closer to the surface. Monopolar radiofrequency heats the dermis and subcutaneous layer as a volume; it does not place focal heat in the SMAS.
Facial fat compartments. The fat lies in separate compartments above and below the SMAS, and fat that has descended or thickened looks like laxity. The jowl fat vs sagging page explains how the two are told apart.
The platysma. The thin neck muscle continuous with the SMAS below the jaw. Neck bands and neck laxity involve it, and the HIFU for the neck page covers that region.
Retaining ligaments. Fibrous anchors running from bone to skin, such as the ligament beneath the tear trough. They tether layers rather than forming one.
Deep-plane surgery. A surgical facelift repositions the SMAS and removes skin; no energy device does either.
A device marketed as reaching the SMAS should be able to say at what depth, and how that depth is confirmed.
What decides whether the SMAS matters for you
The SMAS is only the right target when it is the layer that has changed. Dr Sin Yong works through the following before any lifting device is chosen.
Where the descent is. Jawline, submental and brow descent from a lax SMAS and platysma are the classic focused-ultrasound targets.
Whether volume is missing. Heating a hollow face can accentuate the hollow, so fat and bone support are mapped with ultrasound first.
Whether fat is the problem. A full jowl fat pad is not reduced by heating the SMAS beneath it.
Skin thickness and type. Thin skin over bone changes the depth chosen and the line placement.
Previous treatments. Threads, filler and biostimulators in the planned zone are imaged, because they change what is heated.
When the SMAS is lax and volume is intact, focused ultrasound is the logical tool. When the dermis or the fat is the main problem, the HIFU suitability page explains why another route is chosen.
Where it fits
The SMAS is the reference depth for every lifting treatment on this site. HIFU converges ultrasound so that a point of tissue under 1 mm³ is heated to roughly 60 to 70 °C at 4.5 mm, where the SMAS and platysma are lax along the jawline and beneath the chin, with 3.0 mm for the dermis and 1.5 mm for the superficial dermis; Ultherapy does the same with real-time imaging of the layers to about 8 mm before each line is delivered. Dr Sin Yong's VF Lift – Vertical Facelift works differently: Volnewmer monopolar radiofrequency heats the dermis and subcutaneous layer along vertical vectors and does not place focal heat in the SMAS. At consultation he maps where the SMAS is lax against where volume is missing, because the two need different tools.
Frequently Asked Questions
Not directly. Threads are placed in the subcutaneous fat above the SMAS, where barbed threads engage the fat and fibrous septae they pass through and carry that tissue along a vector. They do not heat or reposition the SMAS itself, which is why a thread suits early to moderate descent rather than heavy laxity, and why focused ultrasound and threads are sometimes combined rather than swapped.
Because laxity of the SMAS and platysma is one of several contributors to jowl and neck descent, alongside fat compartment change, bone remodelling and skin laxity. Each needs a different tool. When the SMAS is the main contributor, focused ultrasound at that depth is the logical option; when fat or lost support dominates, heating the SMAS addresses the wrong layer. The assessment separates them before anything is chosen.
It is a fibrous and muscular sheet rather than one muscle: a continuous layer that envelops and connects the mimetic muscles of the face to the overlying skin, continuous with the platysma in the neck. Surgeons lift it; focused ultrasound heats points within it.
High-intensity focused ultrasound at the 4.5 mm depth, including Ultherapy and Ultraformer MPT, places focal heat in the SMAS. Monopolar radiofrequency such as Volnewmer heats the dermis and subcutaneous layer and does not target the SMAS.
References
US FDA 510(k) K260618 — Ulthera System (Merz), 2026: indicated to lift the eyebrow, lift lax submental and neck tissue, improve lines and wrinkles of the décolleté, and improve skin laxity on the abdomen, arms and knees. source
Amiri M, et al. Microfocused ultrasound with visualization (MFU-V) effectiveness and safety: a systematic review and meta-analysis. Aesthet Surg J. 2025;45(3):NP86-NP94. doi:10.1093/asj/sjae228. source
Alhaddad M, et al. Randomised split-face comparison of monopolar radiofrequency and microfocused ultrasound with visualisation for laxity (n=20). Dermatol Surg. 2019;45(1):131-139. doi:10.1097/DSS.0000000000001653. source
A Systematic Review of the Efficacy of Microfocused Ultrasound for Facial Skin Tightening. International Journal of Environmental Research and Public Health (PMC), 2023. pmc.ncbi.nlm.nih.gov
The fat compartments of the face: anatomy and clinical implications for cosmetic surgery. Plastic and Reconstructive Surgery (PubMed), 2007. pubmed.ncbi.nlm.nih.gov
Customized Treatment Using Microfocused Ultrasound with Visualization: Pan-Asian Adaptation of the Expert Panel's Consensus. PMC. pmc.ncbi.nlm.nih.gov
