The role of soft tissues in plate fixation of proximal phalanx fractures.

The tension band effect of plate fixation and the contribution of soft tissues to that effect was examined biomechanically in human proximal phalanges. Forty-six proximal phalanges in whole cadaver hands with all soft tissues in place (intact) and 43 proximal phalanges stripped of soft tissues (denuded) were tested. After midshaft osteotomy, each proximal phalanx was fixed internally with a dorsal minicondylar plate, a lateral minicondylar plate, a dorsal straight plate, or a lateral straight plate. Specimens were tested in three-point apex dorsal bending to clinical failure, defined as 30 degrees angulation. Ultimate moment (stability) at this angulation was similar among the four fixation methods in the specimens with all soft tissues intact. Stability also was similar among these methods in the denuded specimens. There were no significant differences in stability between minicondylar and straight plates or between dorsal and lateral plates in the specimens with soft tissues, nor were there significant differences between these groups in the denuded specimens. The stability of the four fixation methods was significantly greater in the specimens with soft tissues than in the denuded specimens. Soft tissues increased the stability of lateral minicondylar plates by 163%, lateral straight plates by 157%, dorsal minicondylar plates by 126%, and dorsal straight plates by 104%, providing a dorsal tension band effect that counteracted the buttress (compression) of the volar fracture surfaces of the phalanx. The results suggest that in the clinical setting a laterally placed straight or minicondylar plate may provide as much stability to a phalanx with a midshaft fracture as does the traditional, more invasive dorsally placed minicondylar or straight plate. These findings must be evaluated with caution, however, because all specimens were from embalmed cadavers, and the formalin fixation may have augmented the stability and stiffness of the soft tissues in the intact specimens. A subsequent pilot study comparing intact proximal phalangeal specimens that were formalin-fixed with those that were fresh-frozen showed a significant increase in stability and stiffness of formalin-fixed specimens.