The influence of in vitro testing method on measured intervertebral disc characteristics

Time-dependent behavior of functional spinal units, especially the anterior column, may interact with experimental methods when measuring their mechanical properties. Since they are tested using different load cycles, the comparison of results is difficult. The goal of this study was to determine the response of anterior column spinal units (body-disc-body units) to load cycles varying in waveform and cycle duration using an experimentally based mathematical model. Specimen specific transfer function models were formulated for seven ovine lumbar anterior column units by fitting high order exponential functions to axial stress-relaxation measurements. Using axial load cycles with different waveforms and cycle durations, nine simulations in the time domain with each transfer function model were performed. For each simulation, the neutral zone, peak to peak, and hysteresis were evaluated. The neutral zone and hysteresis were significantly dependent on the waveform of the load cycle. The differences of peak to peak for sinusoidal and triangular waveforms were low. All these characteristics were significantly dependent on the cycle duration. For example, comparison of 1 s to 20 s cycle durations exhibited neutral zone and hysteresis differences of 61-69% and 100%, respectively. Consequently, loading protocols should be chosen carefully and should be clearly reported.