Effect of length variations in 3D-printed middle-ear prostheses on ossicular chain mobility

Patient-specific 3D printing may improve ossiculoplasty, yet the mechanical tolerance to prosthesis length remains poorly quantified. We evaluated how systematic lengthening of a custom 3D-printed incus replacement affects middle-ear mobility in human temporal bones.

Five fresh-frozen human temporal bones were prepared through a posterior tympanotomy. Cone-beam CT was used to reconstruct specimen-specific ossicular geometry; the incus was digitally removed and a malleus-stapes prosthesis was designed and printed by stereolithography. For each specimen, three prostheses were fabricated: nominal length (P0), +10% length (P10), and +20% length (P20). Under probe-microphone-monitored ear-canal acoustic stimulation (10-80 dB SPL; analysis focused on 80 dB SPL), laser Doppler vibrometry (LDV) measured velocity at the incudostapedial joint region in the intact chain and at the corresponding point on the prosthesis after reconstruction.

Across frequencies, reconstruction reduced velocity relative to the intact chain. The largest reduction occurred at 2 kHz (mean about -15 dB for P0) and was smaller at 4 kHz (about -6 dB for all reconstructed conditions). Lengthening produced a frequency-dependent effect: relative to P0, P10 tended to improve velocity at 1-2 kHz, whereas P20 reduced low-frequency (0.5 kHz) velocity. Repeated-measures ANOVA showed a strong main effect of frequency (p = 4.8 × 10^-5) but no statistically significant main effect of prosthesis length (p = 0.28).

Within this cadaveric ex vivo model, small deviations in prosthesis length meaningfully altered middle-ear mobility in a frequency-dependent manner, consistent with changes in effective preload. Patient-specific design should therefore be complemented by intraoperative sizing and seating strategies that avoid excessive tension while maintaining stable coupling.