Biomechanical Analysis Of The Stress Along The Screw And Rod Of The Posterior Internal Fixation In The Treatment Of Lumbar Spondylolishesis

Spondylolishesis is a common degenerative disease in clinic of orthopedic surgery. It is the main cause of the chronic lower back pain. To treat the spondylolishesis using the pedicle screw fixation technology is the conventional method in orthopedic surgery. But the doctors have different opinion on the number of replacement and fixation. Some scholars consider that the intensity of the two-segment replacement and fixation can satisfy the fusion. It can decrease the number of segment to fuse and save much range of motion. It can also shorten the time of operation and reduce the injury by operation. But the deficiency of replacement strength in treatment of midrange and severe spondylolishesis is the disadvantage. It can usually not achieve the objective of replacement and have the lower replacement and fusion ratio, and have the inferior biomechanical stability. At the same time, it may induce the breakage of pedicle screw fixation system easily. Some scholars consider that three-segment replacement and fixation have the better biomechanical stability. It generate much replacement strength and have the higher replacement and fusion ratio. It can also depress the probability of breakage of pedicle screw fixation system. But it have a longer operative incision and fixate more segments. In addition to this, it cost more expensive. To analysis the clinical case, we know that the location of breakage is at the root of the screw. The system fatigue caused by intermittent loading is the primary reason of the complication. The consequence of mechanics of materials test show that the cause of metal material fatigue is stress concentration. Presently, the study of this territory concentrate on the biomechanical stability of internal fixation. It is all the qualitative study. There are fewer reports on the study of its fatigue characteristic, especially on the quantitative study of the fatigue characteristic about the pedicle screw fixation system.Objective: To compare and quantitative analysis the stress of the screw and rod in two-segment and three-segment posterior internal fixation in spondylolishesis, and provide evidence for choosing operate method and definiting the number of replacement and fixation.Method: Six fresh samples of human lumbar vertebras were used in the study. The spondylolishesis model were made by the Panjabi method, and were placed with the universal spine system of SINO. The samples are embedded and fixed in a cupuliform metal container. The vertical compression, flexion, extension, left bending and right bending loading are exerted by the INSTRON 5567 material testing machine. Stress along the screw and rod in two-segment (group A) and three-segment (group B) were measured respectively. The results are analysised by the statistics software of SPSS 11.0.Result: The rod in group A shared more stress than in group B(P<0.05); The draw-screw in group A shared more stress than in group B(P<0.05);The fixed-screw in group B shared more stress than in group A(P<0.05).Conclusion: The three-segment internal fixation can effectively decrease the stress along the rod and draw-screw, decrease the stress concentration effect, enhance the anti-fatigue effect and depress the probability of breakage of the screw and rod due to the system fatigue.