Can cervical disc herniations be provoked in vitro?

INTRODUCTION: Neck pain, one of the leading causes of physical disability, can be caused by cervical disc herniation. The purpose of this in vitro study was to provoke cervical disc herniations under complex mechanical loading to understand the failure mechanisms and to identify risk factors caused by daily-life activities. METHODS: Six cervical motion segments (3x C4-C5, 3x C6-C7) from four human donors (19-58 years) with Miyazaki degeneration grades II-IV were included. A novel test method was developed to simulate complex neck motions occurring during typical daily-life activities using a dynamic disc loading machine. Long-term effects were simulated by exaggerating and combining these motion patterns. The specimens were subjected to the dynamic loading protocol (n = 3,000 cycles) in the intact state, after 1 mm incision in the posterior longitudinal ligament (PLL), and after complete transection of the PLL. Quasi-static flexibility testing was performed before and after each testing step. For microstructural investigations, histological analysis was performed by cryosectioning three C6-C7 discs. RESULTS: Using the dynamic loading protocol alone, no herniation could be provoked in intact specimens. Only after the PLL was injured (1 mm incision), a clear prolapse with nucleus extrusion through the defect (n = 1) as well as a protrusion (n = 1) were identified, while complete transection of the PLL did not cause further herniations. Both specimens were less degenerated and from young donors. The extrusion led to a slight increase in range of motion by about 1° in each motion direction. Histological analysis revealed microstructural damage in nucleus pulposus and posterior annulus fibrosus fibers, along with nucleus material flowing through the ruptured annulus in both herniated specimens. DISCUSSION: Using this novel test method, the simulation of physiological, long-term loading of human cervical discs is feasible. In general, an intact cervical disc does not seem to herniate due to daily-life activities, even under long-term complex motions. The PLL appears to have a main protective effect on the cervical disc, because a herniation only happened when an artificial defect was created. Moreover, the risk of cervical disc herniation might be highest in young patients with low disc degeneration.