| Objectives:The purpose of this study was to investigate the effects of different degrees of annulus fibrosus(AF)defect on the mechanical stability of bovine intervertebral disc(IVD).Methods:Forty Bovine caudal IVD motion segments(bone-disc-bone)were obtained from the local abattoir.Placed the prepared specimen on the carrier of the biomechanical torsion material testing machine and fixed it with screws.Then the specimen was given a 30minute preload before testing with 100 Newtons(N)of compressive force at 0.1 Hertz(Hz)to reduce the adhesion of the specimens.After preloading,the following mechanical tests were performed on the intact specimens in turn:(1)axial compression at 0-0.5 mm;(2)axial torsion at ±5°;(3)flexion-extension at ±1°;and(4)lateral bending at ±2°.All tests were conducted for 15 cycles at a rate of 0.1 Hz,with the 12th cycle used for further biomechanical analysis.After the intact specimen test was completed,we used a surgical scalpel to make a cruciform incision with a transverse length of 0.8 mm and a longitudinal length of 0.5 mm at the right side of the specimen’s AF.We then performed the second round of biomechanical testing.After the second round of mechanical testing was completed,we used a surgical scalpel to make a square incision of 0.8×0.5×0.5 mm on the original incision to remove part of the AF.We then performed the third round of biomechanical testing.Force,displacement,torque,and angle were recorded synchronously by the built-in sensor on the dedicated MTS computer.Axial compression stiffness was calculated as the axial maximum compression force divided by the axial maximum displacement;torsional stiffness was calculated as the maximum torque divided by the maximum rotation angle.Then the samples were observed by scanning electron microscope.P-value<0.05 was considered statistically significant.Results:1)The axial compressive stiffness value of IVD through right incision were(187.39±132.88)N/mm,(108.31±118.01)N/mm and(63.44±101.76)N/mm and of IVD through left incision were(152.43±56.26)N/mm,(32.73±24.22)N/mm and(14.14±14.81)N/mm in the intact specimens,cruciform incision specimens and square incision specimens,respectively,which was significantly lower in cruciform and square incisions specimens than that in the intact specimens(P<0.05).Both cruciform and square incisions significantly reduced the axial compressive stiffness of the IVD.2)The left-side axial torsional stiffness value of IVD through right incision were(418.03±194.77)N·mm/deg,(342.93±134.79)N·mm/deg and(279.04±108.48)N·mm/deg in the intact specimens,cruciform incision specimens and square incision specimens,respectively;The right-side axial torsional stiffness value of IVD through right incision were(346.75±181.06)N·mm/deg,(305.59±161.59)N-mm/deg and(271.91±123.30)N·mm/deg in the intact specimens,cruciform incision specimens and square incision specimens,respectively;which was significantly lower in cruciform and square incisions specimens than that in the intact specimens(P<0.05).The left-side axial torsional stiffness value of IVD through left incision were(294.36±89.17)N·mm/deg,(245.58±54.28)N.mm/deg and(230.67±54.35)N·mm/deg in the intact specimens,cruciform incision specimens and square incision specimens,respectively;The right-side axial torsional stiffness value of IVD through left incision were(383.32±80.42)N·mm/deg,(312.82±56.99)N.mm/deg and(272.46±55.98)N·mm/deg in the intact specimens,cruciform incision specimens and square incision specimens,respectively;which was significantly lower in cruciform and square incisions specimens than that in the intact specimens(P<0.05).Both cruciform and square incisions significantly reduced the axial torsional stiffness of the IVD,and AF defect could significantly reduce the torsional stiffness of the contralateral side of the IVD.3)The Extensional stiffness value of IVD through right incision were(7480.14±2913.73)N·mm/deg,(6774.43±2678.02)N·mm/deg and(6574.54±2820.12)N·mm/deg and of IVD through left incision were(9853.74±2226.60)N·mm/deg,(8526.52±2694.96)N·mm/deg and(8210.468±2093.88)N·mm/deg in the intact specimens,cruciform incision specimens and square incision specimens,respectively,which was significantly lower in cruciform and square incisions specimens than that in the intact specimens(P<0.05).Both cruciform and square incisions significantly reduced the extensional stiffness of the IVD.4)Additionally,both the cruciform and square incisions did not impair the flexional stiffness and lateral-bending stiffness of the IVDs.5)The results of the scanning electron microscope showed that the fibers of AF were swollen and disordered after the mechanical test,and the gap between adjacent fiber layers was wide,in addition,the cartilage endplate was broken.Conclusions:The cruciform and square incisions of the AF reduced its axial compression and axial rotation,but they did not change the flexion-extension and lateral-bending stiffness of the bovine caudal IVD.In addition,the mechanical loading after AF defect can lead to the fracture of cartilage endplate and destroy the structural integrity of IVD.This mechanical study will be meaningful for the development of new approaches to AF repair and the rehabilitation of the patients after receiving a discectomy. |
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