Background <p>The internal fit of the implant-abutment connection plays a crucial role in implant success. This study aims to assess the impact of various connection designs and materials of customized implant abutments on the gap distance at the implant-abutment interface.</p> Methods <p>Two internal connection implant systems, star-shaped tube-in-tube and hybrid Morse taper with internal hex anti-rotation feature (<i>n</i> = 24 each, total <i>n</i> = 48) were evaluated. Each group was subdivided into four material groups: zirconia (Zr), titanium (Ti), cobalt-chromium (Co-Cr), and soft-milled cobalt-chromium-molybdenum (Co-Cr-Mo). The internal fit was assessed via micro-computed tomography (µCT).</p> Results <p>All specimens demonstrated clinically acceptable microgap values (≤ 150&#xa0;μm). The star-shaped tube-in-tube connection exhibited significantly narrower gaps compared to the hybrid connection. Zr showed the greatest gaps among materials, followed by Ti, Co-Cr, and soft-milled Co-Cr-Mo, which had the smallest gaps.</p> Conclusions <p>The star-shaped tube-in-tube connection demonstrated superior internal fit to the hybrid connection. Zirconia abutments demonstrated inferior internal adaptability. Dimensional changes during sintering of soft milled materials may have influenced these outcomes and should be considered when interpreting the data. All tested abutments fell within clinically acceptable misfit ranges.</p>