As hearing aid manufacturers became experienced in the fitting of deep-canal instruments where a precise fit (or lack of it) determined the success or failure of the fitting, research pointed to the flexibility of ear canal cartilage as the source of feedback-producing movement. This is true for high-gain, closed-mold fittings as well. Pirzanski (1996) recommended taking an open-jaw impression if any of these conditions exist:
- Significant mandibular displacement is seen externally during jaw movements.
- Changes in the auditory canal can be detected during the otoscopic examination.
- The patient complains of feedback or loose fit problems related to TMJ movement.
- The earmold or hearing aid lacks retention and slides out of the ear.
- The patient reports a noticeable loss of hearing aid power caused by mandibular movement.
A viscous, or not overly soft, silicone impression material is recommended for these impressions, as it will properly expand cartilage. Earmolds or hearing aids will fit comfortably because with the jaw closed, the cartilage relaxes and rests comfortably against them.
For best results, open-jaw impressions should extend beyond the second bend of the canal, even though the finished product may not fit that deep in the ear. This is so because a deep impression will permit adequate expansion of the cartilage at all depths, while a shorter impression is likely to result in inadequate expansion of the canal by the tip of the earmold.