Ultrasound/photoacoustic/shear wave imaging of skin vasculature and elasticity to predict craniofacial implant exposure

Introduction: : Craniofacial implants are widely used in reconstructive surgery but are often associated with skin necrosis and subsequent implant exposure. Prior studies suggest implant-induced mechanical stress compromises skin vasculature and elasticity, leading to skin breakdown. However, quantitative assessment of both vascular and mechanical changes in the skin post-implantation remains limited. Photoacoustic (PA) imaging combines a pulsed laser excitation with ultrasound, providing optical contrast for the detection of hemoglobin. Shear wave elasticity imaging (SWEI) uses induced shear acoustic waves to probe the elasticity of tissue. In this study, we combined ultrasound/photoacoustic (US/PA) imaging, photoacoustic tomography (PAT), and SWEI to non-invasively monitor changes in skin vascularity and shear modulus over time. From these metrics, we identified several biomarkers of implant-induced skin damage which can be used to predict implant exposure.

Materials and

Methods: : Two types of 3D-printed polycaprolactone implants, block and dome (Fig. 1a), were subcutaneously implanted in the lumbar region of mice (n=8), with an additional non-implanted control group (n=4). Imaging of the skin over the implant was performed biweekly for 12 weeks using (1) 3D US/PA imaging (Vevo LAZR, 21 MHz, 700–950 nm) to calculate total hemoglobin, (2) 3D PAT (TriTom, 6 MHz, 532 nm) to examine the vessel skeleton/density, and (3) SWEI (Vantage 128, 16 MHz) to calculate skin shear modulus. Image analysis was conducted using Vevo LAB and MATLAB. In addition, skin health was visually scored (1-5) based on the extent of skin damage.

Results, Conclusions, and Discussions:: All mice in both the control and dome groups maintained healthy skin, while three mice in the block group exhibited skin breakdown and implant exposure by week 12 (Fig. 1c). This skin breakdown over blocks was preceded by vessel deterioration in PAT images (Fig. 1b, circled), while the skin over domes better preserved their vascular structure. The initial post-implantation skin stress was high for both implanted groups, but the stress was relieved faster in the dome group animals compared to the block (Fig. 1d). This heightened sustained skin stress over the block implants may have led to the observed increase in vascular ischemia. Total hemoglobin levels in the block group decreased over time, whereas no significant changes were observed in the other groups (Fig. 1e). Similarly, vascular density in the block group sharply declined prior to implant exposure despite an initial increase (Fig. 1f). Together, these findings highlight the diagnostic potential of US/PA/SWEI biomarkers to serve as early predictors of skin damage and offer critical insights for improving implant design, interventional mitigation, and reconstructive outcomes.

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