The response of the human periodontal ligament to torsional loading—I. Experimental methods

Abstract

A test technique is described in which torsional loads are applied to human maxillary central incisors in vivo. The central axis of the incisor is located by a stereoscopic X-ray method and the tooth loading device is adjusted by means of a setting jig so that torque is applied about this central axis. The maximum torque which can be applied is ± 0·05 Nm and the maximum rotation of the tooth is ± 0·02 radian (± 1·2). A servo-control system allows one to apply any desired torque or deflection history. e.g. a creep or stress relaxation test or a cyclic load of any waveform. Initial results obtained with this system are presented and show that the torque vs rotation response is initially linear but becomes highly nonlinear at higher torques. Creep tests and cyclic loading tests indicate that the periodontal ligament is viscoelastic in nature. Periods of cyclic loading separated by rest periods of 1–20 min show that the commonly observed decrease in tissue stiffness during the initial few load cycles is not a permanent effect in this in vivo test. Recovery to the initial stiffness takes place rapidly. e.g. 50 per cent recovery in 5 min at zero load.