Abstract
Summary
Lumbar spinal specimens exhibited high fatigue strength. The cycles to failure are not only dependent on the maximum peak load, but also on the load offset or the amplitude, respectably.
Introduction
Spinal injury might be caused by whole body vibrations. The permitted exposure to vibration in the workplace is therefore limited. However, there is a lack in knowledge how external vibrations might cause internal damages. Numerical whole body models might provide the potential to estimate the dynamic spinal loading during different daily activities, but depends on knowledge about the corresponding fatigue strength. This study is aiming to determine the in vitro fatigue strength of spinal specimens from donors of working age.
Patients & Methods
Lumbar functional spinal units (L2/L3 and L4/L5) from midlife donors (45–65 yrs, n = 24) and young donors (20–45 yrs, n = 6) were collected and stored deep frozen. CT scans were obtained to determine the endplate area and the bone mineral density of the vertebrae. Their product is referred to as vertebral capacity (VC). Muscles were removed from the thawed specimens, but apart from the transversal ligaments, all ligaments and the intervertebral disc were left intact. During the experiments, the specimens were immersed in saline solution (37°C) containing antibiotics (PAA, Austria) to reduce biological degeneration. After preconditioning (2.5 h) the specimens were exposed to continuous sinusoidal axial compression (5Hz, <300,000 cycles). Distinct changes in the characteristic creep curve of specimens’ height indicated fatigue failure. Specimens of midlife donors were equally assigned to three groups with different peak-to-peak loads (NORM: 0–2 kN; HIGH: 0–3 kN; OFFSET: 1–3 kN), while specimens from young donors were solely assigned to the HIGH group, since a previous study [1] had shown that young specimens hardly failed for NORM loading conditions. Findings from that previous study (midlife, n = 6; young, n = 6) were merged to NORM for analyses.
Results
Within the NORM group, specimens only failed within 300,000 cycles when VC was below 2,000 cm2 mg K2HPO4/ml (8 of 20). Within the HIGH group, endplate failure occurred frequently within the test duration (10 of 13; 1 excluded). For the OFFSET group, specimen failure was occasionally observed (4 of 7; 1 excluded). Exponential regression of cycles to failure dependent on VC showed significant correlations for the specimen loaded in the NORM and HIGH group (r2NORM = 0.57, p = 0.029; r2HIGH = 0.47, p = 0.029; r2OFFSET = 0.83, p = 0.091).
Discussion/Conclusion
Specimens’ fatigue failure strength depends on load offset and amplitude. The group with higher loading amplitudes (HIGH: 1.5 kN) resisted fewer loading cycles than those with the smaller amplitude (OFFSET: 1 kN), even though the maximum peak was the same (3 kN). The exponential regression is conservative, since several specimens did not fail within the predicted loading cycles. Vertebral capacity might suitable predict the fatigue strength of specimens. Together with numerical modelling, these findings might promote the appraisal of occupational diseases and might help to determine the duty cycles for new implants. The funding of FIOSH, Germany is thankfully acknowledged (project F2059 and F2069).
