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 Simulation of skin-SMAS single-component facelift without anchoring sutures. Horizontal vector (left) and oblique cephalad vector with displacement of 15 mm (right). Warm colors indicate volume reduction, while cold colors indicate volume increase. Shear stress is visible between SMAS and skin. (Photo credit: Maurice Mommaerts, M.D., D.M.D., Ph.D.) |
An initial model was created using a mannequin head covered with a layer of chamois glued at the virtual boundaries of the dissection. However, this approach had several limitations.
"Chamois is a stiff material compared to skin, which has elasticity, and the model also did not simulate that facelift surgery may involve several tissue layers and even use deep-anchoring sutures at distant sites," Dr. Mommaerts says.DEVELOPING THE MODEL Engineers from the ETH (Swiss Federal Institute of Technology) in Zurich created the 3-D finite model, which includes both the face and neck. It uses the Rubin and Bodner material model, which represents nonlinear time-dependent changes in the soft tissues, and incorporates findings from traction tests performed on samples of skin and SMAS, taken from facelift patients. It also incorporates MRI data to account for other properties of skin and SMAS and for the anatomy and properties of bone, muscle and subcutaneous fat. Additional testing also allowed integration of the reaction forces of stretched skin (the relaxation effect), and the model integrates gravimetric effects.
"Without the force of gravity, there would be no skin or soft tissue descent and no need for facelift surgery. Therefore, it was critical to include this element in the predictive model," Dr. Mommaerts says.
The model features both skin and SMAS, but the analysis of incision effects can be based on each of the layers separately or combined. In addition, it allows simulation of the consequences of applying fibrin glue between the layers, he says.
The effects of surgical incisions on tissue displacement and volumetric changes are displayed with a colorimetric code. Dr. Mommaerts presented images highlighting the changes resulting from different surgical incisions that exert vertical, horizontal or oblique vector components.
In testing so far, the model has successfully verified several classical concepts about rhytidectomy incisions. In addition, Dr. Mommaerts demonstrated how the model was used to solve the problem of redundancy in front of and below the earlobe after a short-scar facelift procedure.
"Based on the findings from this model, we propose creating an extra, preferably zigzag, incision in the mastoid area to undermine the skin and apply another oblique force that would eliminate vertical pleating in front of the ear," he says.
MODEL LIMITATIONS Currently, due to the borders of the finite element model, it can only be used to predict outcomes from incisions placed in a sub- or intra-sideburn location.
"A high vertical temporal incision, such as the vertical preauricular incision described by Stuzin and Marchac, is positioned too close to the border to simulate its effects, and the results generated are unreliable due to distortions," Dr. Mommaerts notes.
Future research will focus on how skin quality, atrophy and acne may influence the results predicted by the model. In addition, it is hoped that a personalized model can be developed by integrating information on hairline and individual skin features characterized from analysis of tissue taken in a preoperative biopsy.
Disclosures:
Drs. Loomans and Mommaerts report no relevant financial interests. The basic 3-D finite element model has been developed with the help of a research grant from Johnson & Johnson MEDICAL GmbH, Norderstedt, Germany.
