Dental implants are widely used in modern restorative dentistry. A typical dental implant consists of a metal screw, a metal abutment, and a ceramic crown (Fig. 1). The screw is inserted into the jawbone, and the abutment is attached to the screw using a retaining bolt approximately 2 mm in diameter. To provide access to this bolt, a narrow channel slightly larger than 2 mm in diameter is drilled through the abutment (Fig. 1, bottom). The channel and the abutment surface are coated with an adhesive gel, and the ceramic crown is bonded to the abutment surface (Fig. 2, top).

Despite their widespread use and durability, dental implants may require removal or replacement due to complications such as jawbone inflammation, screw loosening, gum inflammation, or peri-implant infection.

To disassemble an implant, a dentist must first locate the hidden access channel (Fig. 2, bottom), drill through the ceramic crown at the precise location, expose the channel within the abutment, and unscrew the retaining bolt. Current dental imaging systems rely primarily on X-ray. However, these methods are incapable of accurately imaging the hidden channel required for implant disassembly. Penetrating both a ceramic crown and a metal abutment with X-rays would require radiation doses that are unsafe for human tissue.

We propose a novel method for subsurface infrared (IR) imaging based on compact volume holographic elements (VHEs). A VHE is a thick holographic grating recorded within semitransparent photosensitive materials, such as photopolymers or photo-thermal glass, that diffracts broadband light into spectrally selective patterns. A key feature of VHEs is their high angular (spatial) and spectral Bragg selectivity, which enables layerby-layer subsurface imaging with high precision.

As a proof-of-principle demonstration, we investigated under-crown imaging of the metal abutment surface with the goal of locating the precise position of the abutment screw access channel. We tested this approach in vitro using a dental implant model consisting of a metal abutment and a removable dielectric crown. In the experimental model, the access channel within the metal abutment was inclined and approximately 2 mm in diameter.

Figure 3 shows the spectral image of the exposed metal abutment without the crown, revealing the inclined access channel and screw hole. A red spot inside the channel spatially correlates with a corresponding spot observed on the crown. The lower portion of Figure 3 presents the spectral image of the dielectric crown covering the metal abutment. A localized light spot within the image accurately indicates the position of the hidden access channel beneath the crown.

Experimental testing of our compact VHE method demonstrated a spatial resolution of less than 2mm , indicating strong potential for highly detailed subsurface imaging applications. This technology enables dentists to precisely identify the hidden access channel beneath a ceramic crown, significantly accelerating implant removal procedures.