Quantitative Assessment Protocol for Facial Soft Tissue Volumetric Changes with Stereophotogrammetry

Our protocol provides a methodology for the conduction of a semi-automated, high accuracy, and high reproducibility 3D volumetric change assessment using facial surface imaging. 3D imaging enables quantitative and comprehensive assessment of craniofacial morphology, growth, development, and pathology, surpassing the limitations of traditional 2D techniques. To begin, open the 3D facial photograph on the imaging software.

Show the axis grids in the viewport. Click Snap View to square the image to the nearest 90-degree frontal view. From the left-side menu, select the Spin Active Surfaces icon and rotate the image until it is evenly bisected by the vertical y axis.

Now use the Paint Area Selection tool and drag the brush across the frontal surface of the image to highlight the face. Once the region is selected, click Find Symmetry to automatically align the image along the vertical axis through its center. Use Clear Area to deselect the region after selection.

To correct image rotation and establish front-to-back orientation for registration, begin by displaying the image in two viewports. Use the Spin tool and Snap function in one viewport to obtain a lateral view. In the lateral viewport, use Roll Active Surfaces to adjust the image so the head is aligned vertically with the grid.

Then use Pan Active Surfaces to center the image on the main vertical axis. Save the registered image. To annotate landmarks, use the Landmark option from the left-side menu.

Annotate the medial canthus, lateral canthus, and glabella in the same order bilaterally, verifying correct placement in the frontal view. Save the landmarked image. Next, in the frontal view, select Pick Multiple Points for Closed Loop and place points along the perimeter of the region of interest.

After finishing the loop, select Add to Area. Now open both the baseline image and the subsequent image to be registered or registration image. Place the landmarks in the same sequence as before.

Select Register Surfaces, then move the registration image to fit the surface baseline using landmarks with corresponding names. Switch to a single viewport and check registration using the Color Surface by Distance tool. Select Color whole of and choose registration image from the this surface options and the baseline image from the by the distance to the surface options.

Click on Calculate distances to generate a distance-based color heat map. Return to two viewports. Toggle off the synchronization and hide the registration image.

Use Project Selected Area to highlight the corresponding region from baseline onto the registration image. Select the Reverse Surface Color from the Preferences menu. Then select Hide from the Area menu to hide the selected area from the baseline image.

Click on the registration image and then select the between two surfaces option from the Measure Volume options. Now return to one viewport view and hide the registration image. Select the Volumes option from the Surface menu and then measure the volume between the two selected areas by using the Volume of Closed Surface option from the Measure menu.

Baseline three-dimensional facial images were acquired one year after surgery and registered using anatomical landmarks, including the bilateral medial and lateral canthi and the glabella. Postoperative images captured one week after surgery were registered to the same axis grid with the baseline region of interest projected onto them. Baseline and postoperative images were superimposed to verify the accuracy of alignment.

Volumetric masks from both time points were generated based on the region of interest and used to calculate the volume in cubic millimeters. A heat map was produced to compare volume differences between the masks showing increased volume in blue and green and decreased volume in yellow and orange. Prior facial soft tissue assessment methods require extensive training and specialized image analysis, multiple imaging modalities and software, or extensive landmarking.

We present here a straightforward approach that does not require expertise in the field of image analysis. This is a practical guide for clinical assessment and research evaluating the impact of therapeutic interventions on facial aesthetics and soft tissues. This type of assessment is applicable to fields of dentistry and particularly fixed and removable prosdontics, orthodontics, and orthognathic surgery.

This methodology is expected to be used in the future for a deeper understanding of soft tissue response to different treatment modalities. This knowledge will enable customized treatment approaches and the optimization of treatment results.