FORCE DISTRIBUTION ON IMPLANTS SUPPORTING OVERDENTURES - THE EFFECT OF DISTAL BAR EXTENSIONS

Force distribution on mandibular implants supporting overdentures was registered in vivo by means of piezo-electric transducers that allow f or simultaneous force measurements in 3 dimensions. The anchorage devi ce for connecting the overdenture to the implants was a U-shaped bar t o which distal extensions were soldered bilaterally. Force patterns we re analyzed under different test conditions such as maximum force when biting in centric occlusion, maximum biting with the unilateral use o f a bite plate, parafunction and chewing bread. Maximum force measured in centric occlusion and on the ipsilateral implant with the use of b ite plate was increased in the vertical dimension, compared to transve rse dimensions. On the contralateral implant, equally low values were found, in all 3 dimensions. Transverse force components reached 5 to 3 5% of the vertical magnitudes. With the use of the bite plate on the i psilateral 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 grindin g resulted in lower vertical forces compared to maximum biting, while transverse forces in the backward forward direction reached force magn itudes that resembled the vertical component (50 to 100%). The prevale nt (>95%) or exclusive force direction in the vertical dimension, regi stered 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 tra nsverse dimensions, the specific influence of bar extensions was recog nised in backward directions on the contralateral implant. In comparis on with previous results, it was concluded that, in vivo, the effect o f distal bar extensions was of much lesser influence regarding force m agnitudes and force directions than was expected.