Biomechanical Study Of A New Lumbar Dynamic Internal Fixation System

ObjectiveLumbar degenerative diseases have become common illness in the field of orthopaedics due to the development of social aging problem. Traditional surgical method is lumbar fusion with internal fixation and bone graft. But lumbar fusion failed to obtain good clinical results. By further researching, lumbar dynamic fixation become more and more popular. The dynamic fixation includes transpedicular system and interspinous system, which have some limitations at materials and biomechanical properties. In ous study, we utilize a new titanium alloy with super-low elastic modulus to design a new lumbar dynamic fixation system. The bridged segmental stability, intradiscal pressure, stress distribution and fatigue test are performed to identify the biomechanical properties for further clinical applications.Materials and MethodsWe utilized a new titanium alloy (Ti-24Nb-4Zr-7.9Sn) to design and construct the lumbar dynamic internal fixation system (DIFS) and simi-rigid fixator comparing with SINO rigid fixator. In the DIFS and simi-rigid fixator, pedicle screws are 6.0X 45mm sized, made of 75GPa new metal. The rods of DIFS have a diameter of 4.0mm made of 43GPa metal, and the rods of simi-rigid have a diameter of 4.8mm made of 43GPa metal. The pedicle screws of SINO is also 6.0 X 45mm sized with the rods of 6.0mm diameter made of normal medical titanium alloy (Ti-6A1-4V, 110GPa).Eight calf lumbar spine specimens were used for stability and intradiscal pressure test, keeping the bone, disc, ligament and zygapophysial joint intact.. The half of cranial and caudal segment were embedded in PMMA to make the superior and inferior parallel plane, and inserted the markers from the anterior, lateral side of vertebra and spinous process to mark the motions of segments. The instruments of stability test include MTS tester, pulley block system and MotionAnalysis system. Before testing, the makers were recognized by the MotionAnalysis system. The 8Nm pure moment was loaded to specimens in three motion planes. Preload of 2 cycles was performed to eliminate the viscosity. The motion of segments were recorded from the third cycle. The ROM and NZ are calculated, and the load-displacement curve is drawn. The method of loading in the measurment of intradiscal pressure is similar to stability test. Before testing, a hole was made adjacant to intervertebral disc for placing the sensor. We use the KYOWA sensor system to measure the pressure through the cartilage end plate. When loading, the pressure is recorded for load-pressure curve. The randomized blocks analysis of variance is applied to analyze the data of ROM, NZ and intradiscal pressure, with two-two correction by post hoc test.The Ansys11.0 was utilized to build finite element model of DIFS and normal medical titanium alloy fixator following loading and calculation. The compression test was performed to evaluate the validity of the model. By the postprocessing function, the angular displacement and internal stress were compared between DIFS and normal medical titanium alloy fixator.Fatigue test is performed according to the ASTM standard. The test instruments include the test block and clamping apparatus, which can rotate with each other around an axis. DIFS was fixed in test block, and connect to tester through clamping apparatus. The displacement-controlled mode is used to perform the cycle motion in the range of 15°flexion/extension. The test will not stop until 10 million cycles.ResultsThe results of stability test showed that the defect significantly increased the ROM and NZ in the three motion planes. ROM of the defect increased 25%,38%and 38%in flexion/extension, lateral bending and rotations, and NZ increased 55%,38%and 65% in these three planes. All the three fixator showed stabilizing effect to the defect, limiting the ROM and NZ below the level of the intact. DIFS restored the ROM to 78%, 60%,62%and 69%of the intact level in flexion, extension, lateral bending and rotation. Simi-rigid fixator restored the ROM to 35%,28%,46%and 37%of the intact level in flexion, extension, lateral bending and rotation. SINO restored the ROM to 10%,25% and 33%of the intact level in flexion/extension, lateral bending and rotation. Compared with intact state, DIFS almost ideally restored the ROM to>60%of the intact. Compared with the DIFS, the simi-rigid and rigid fixator significantly limited the bridged segmental motion. The ROM and NZ in adjacent segments are not affected by internal fixation.The defect does not significantly affect the intradiscal pressure of bridged and adjacent segments. All three fixators bore the intradiscal pressure. DIFS bore the pressure of 43%,30%and 39%of the intact level in lateral bending, rotation and flexion, simi-rigid fixator bore 58%,72%and 66%, SINO bore 76%,88%and 83%. In the extension direction,100%pressure was born by the fixator. Adjacent segmental intradiscal pressure was not significantly affected by fixations.The results of finite element analysis demonstrated that the load-displacement curves are similar between the finite element calculation and compression test. The 100N load produced 9°angualr displacement, and high stress area was present at the conjunctions of the screws and rods. The high stress zone is consisted with the common location of system breaking. Compared with the normal medical titanium alloy fixator, DIFS has better motion property (2.3 times) and lesser maximum internal stress (57%).Conclusion1. DIFS is able to dynamically stabilized the defect segments, and share the intradiscal pressure with the segments in the most directions.2. DIFS has lesser internal stress during angular motion and better motion property indicating relatively good anti-fatigue performance, which needs to confirm by cycling fatigue test. 3. With simple structures, DIFS is convenient to utilize with good potential of clinical application.