Force distribution on implants supporting overdentures: the effect of distal bar extensions. A 3-D in vivo study.

Force distribution on mandibular implants supporting overdentures was registered in vivo by means of piezo-electric transducers that allow for simultaneous force measurements in 3 dimensions. The anchorage device for connecting the overdenture to the implants was a U-shaped bar to which distal extensions were soldered bilaterally. Force patterns were analyzed under different test conditions such as maximum force when biting in centric occlusion, maximum biting with the unilateral use of a bite plate, parafunction and chewing bread. Maximum force measured in centric occlusion and on the ipsilateral implant with the use of bite plate was increased in the vertical dimension, compared to transverse dimensions. On the contralateral implant, equally low values were found, in all 3 dimensions. Transverse force components reached 5 to 35% of the vertical magnitudes. With the use of the bite plate on the ipsilateral implant, force magnitudes in the vertical direction and in the backward-forward direction were significantly higher (P < 0.01, P < 0.00) compared to measurements in centric occlusion. Chewing and grinding resulted in lower vertical forces compared to maximum biting, while transverse forces in the backward forward direction reached force magnitudes that resembled the vertical component (50 to 100%). The prevalent (> 95%) or exclusive force direction in the vertical dimension, registered on both implants was downward. However, with the unilateral use of the bite plate, upward directions were found on the contralateral implant as an effect of distal bar extensions. This was in contrast to previous results where upward force directions were not found. In transverse dimensions, the specific influence of bar extensions was recognised in backward directions on the contralateral implant. In comparison with previous results, it was concluded that, in vivo, the effect of distal bar extensions was of much lesser influence regarding force magnitudes and force directions than was expected.