Establishment And Application Of The Ligament-based Mechanical Instability Models Of Lumbar Spine

Intervertebral disc degeneration(IVDD)is a common disease and also one of the main factors causing such spinal diseases as chronic low back pain,disc herniation and cervical spondylosis.Studies have shown that 70% of people in their lifetime are suffered from lumbar disease and the incidence is increasing.At present,there is still a lack of uniform and clear understanding of the pathogenesis and a lack of long-term effective treatment,leading to frequent recurrence of the disease.Therefore,it's necessary to establish effective disease models for understanding the mechanism of IVDD and facilitating the development of effective treatment and intervention?The animal model of IVDD plays an important role in elucidating its pathogenesis and evaluating new treatment methods.The current animal models are mainly based on the physiological and biochemical changes in the process of intervertebral disc degeneration.For instance,classic annulus fibroin puncture model and nucleus pulposus dissolution model simulate the typical behaviors during the degeneration process including the degradation of the extra nuclear matrix of the nucleus pulposus and the disorder or rupture of annulus fibrosus.However,these models cannot tell the initial factor of IVDD.Considering that vertebral joints are an integrated mechanical system and there are different understanding on the disc degeneration in Chinese and Western medicine,we speculate that spinal ligament injury and the resulting intervertebral instability should be the initiating factor responsible for IVDD.At present,some scholars have successfully induced the disc degeneration by excising the ligaments.Despite,in clinical conditions,ligaments are rarely defected;moreover,after ligament resection,the reconstruction of the mechanical environment of the intervertebral disc would become very difficult.Therefore,the animal models based on ligament resection have limited contribution for guiding the treatment or intervention of IVDD.Herein,in order to verify our hypothesis that ligament injury and the resulting intervertebral instability should be one of the initiating factors for IVDD,a lumbar spine instability model was constructed by axially and transversally tearing the ligaments of rabbits to simulate the in vivo ligament injury and induce the degeneration of intervertebral discs.Thereafter,the constructed model was used to evaluate the efficacy of a drug in restoring the mechanical properties of the injured ligaments so as to intervene the progress of IVDD.The main contents and results of the study are listed as follows:Firstly,the in situ test method for the mechanical properties of ligaments was established by clamping the spinous process with a towel clamp.The test procedure was set as:(1)pre-condition: tensile displacement range = 0-2 mm,tensile rate = 2 mm/min,number of cycles = 3;(2)test: tensile rate = 10 mm/min.It was found that the obtained force-displacement curves of the normal ligaments are consistent with the typical curves of soft tissues,and the curve is linear between the displacements of 4-8 mm range.The stiffness of ligaments is 6.66 N/mm,and the average failure displacement and load are 8.58 mm and 39.37 N,respectively.Subsequently,to establish the instability model,the ligament was injured and disabled using three types of mechanical loading: constant magnitude of force(The clamped spinous processes were loaded with a constant 3 kg towel clamp for 1 min and then unloaded.The loading and unloading were repeated until the produced displacement was 8-10 mm.),constant displacement(The ligament was stretched to its failure point.),and transversal resection.It was found that,after suffered from the stretching of constant force or constant displacement,the ligament became relaxed,the force required to separate the spinous processes significantly decreased,and the stiffness of the ligament was reduced,indicating that the instability model is successfully constructed.Regarding the transversal resection,the supraspinous ligament was separated from the deep fascia,and the supraspinous and interspinous ligaments were resected to form a defect of "?" shape.The mechanical test indicates that the supraspinous(SSL)and interspinous(ISL)ligaments are responsible for constraining the movement of spinous processes between 0-6 mm displacements.Therefore,injuring ISL/SSL may induce lumber instability.To investigate the feasibility of the injured ligament to induce intervertebral disc degeneration,the images(X-ray and MRI)and pathological observations were conducted.The images indicate a decrease in both the height index of the disc and the signal intensity of nucleus pulposus.The pathological observation indicates that the nucleus pulposus gradually breaks through the dorsal lateral fiber loop and moves toward the dorsal side,which is consistent with the clinical results of the lumbar disc herniation.These results verify that the ligament injury-induced instability can result in the degeneration of intervertebral disc to some extent.In addition,the posterior herniation of disc in IVDD might be caused by the injury of posterior ligamentous complex.Finally,the constructed instability model was employed to investigate the role of a commercialized Chinese medicine Shu Jin Jian Yao Wan and a self-formulated compound Chinese medicine in intervening the progress of intervertebral disc degeneration.It was found that the drugs can restore the mechanical function of the injured ligament.Moreover,the imaging and pathological examinations indicate that the posterior herniation of the nucleus pulposus was partially prevented and the degeneration of the intervertebral disc was inhibited.These results verify that the drugs are effective in slowing the progress of the degeneration of the intervertebral disc by restoring the mechanical properties of ligament.More importantly,these results further confirm our hypothesis that ligament injury and the resulting instability of vertebral joints is the initiating factor causing intervertebral disc degeneration.In summary,this study successfully established an animal model of lumbar instability through ligament injury,and the degenerated intervertebral disc in this instability model was partially reconstructed by drugs through restoring the mechanical properties of the injured ligaments,which verifies our hypothesis that ligament injury and the resulting intervertebral instability is the initiating factor for disc degeneration.Therefore,it is recommended that the mechanical function of the spinal ligament should be repaired during the treatment of spinal diseases.