Biomechanical And Clinical Research Of The Influence Of Long Segmental Fixation On Lumbosacral Spine

[Backgrounds and Objectives]Long segmental fixation to the sacrum is indicated in various spinal disorders, such as,degenerative scoliosis and neuromuscular scoliosis. It has achieved good surgical resultsfor spinal deformities with long-fixation construct. For the special regional anatomicstructure and complex biomechanical properties of lumbosacral and sacroiliac junctions,extension of long fixation to lumbosacral junction remains a difficult clinical challengedespite the recent knowledge and technology. Significant postoperative complications likethe loosening or pullout of the sacral screws are not uncommon following the lumbosacralfixation extending from the upper lumbar or thoracic spine to the sacrum. The loosening ofscrew is correlated with the loading stresses. When carrying larger stresses, with long termcyclic loading, the screws will be more likely to loose or pull out. Clinical study hasreported that as the number of fixation levels increase, the risk of lumbosacral fixationfailure becomes higher. However, there are no studies characterized how the number offixation levels affected the sacral stresses. Because the number of fixation levels is animportant factor in considering additional fixations options to protect the sacral screws, theeffect of different fixation levels on sacral screw stresse must be understood.The objectives of the present project were there-fold. First, we establishedlumbosacral long fixation model with human cadaveric specimens, the number of fixationlevels increased from sacrum to upper L1vertebrae. The lumbosacral spines werebiomechanically evaluated to quantify of S1screw stresses and to identify the fixationlevel that affect the S1screw stresses significantly. Second, to evaluate the effect ofdifferent upper instrumentation vertebrae, lowest instrumentation vertebrae, and additionalanterior lumbar interbody fusion on S1screw stresses. Third, to compare surgicaloutcomes of posterior decompression with short fusion or long fusion in a group ofdegenerative lumbar scoliosis patients undergoing pedicle screws-only constructs. Basedon this study, we expected to provide with some experimental basis for lumbosacralarthrodesis surgeries.[Materials and methods]1. Biomechanical effect of number of fixation levels on sacrum screws stresses inlumbosacral long fixation construct1.1Specimens preparation. Six fresh-frozen cadaveric specimens (T9to pelvis) were thawed to room temperature and cleaned of all residual musculature, with care taken topreserve the bony anatomy and all ligamentous attachments and maintain segmentalintegrity.1.2Specimens lumbosacral fixation. The specimens underwent a series of fivetranspedicular lumbosacral instrumentation conditions using appropriate size pediclescrews (Expedium, Depuy Spine):(1) L5-S1,(2) L4-S1,(3) L3-S1,(4) L2-S1,(5) L1-S1.Two titanium alloy rods were secured to these pedicle screws.1.3Strain gages attachment. A combination of8strain gages were attached to the surfaceof rods between the L5and S1screws. The stresses on the sacral screws, including pulloutforce, transverse load, and bending moment were indirectly measured from these straingages attached on both rods.1.4Biomechanical test. The specimens were potted and mounted on DDL20hydraulicmaterial testing machine. The six degree of freedom pure moment of8Nm includingflexion and extension, lateral bending, and axial rotation were applied to the superior end(T10) of the specimens. The S1screws strains of different fixation levels were recorded forstresses computational analysis.2. Biomechanical effect of fixation extending past the thoracolumbar junction onsacral screws stresses in long fixation construct2.1Specimens fixation and experimental grouping. Six fresh-frozen cadaveric specimensunderwent transpedicular lumbosacral instrumentation using appropriate size pediclescrews (Expedium, Depuy Spine) according to experimental grouping method:(1) L1-S1,(2) T10-S1. Two titanium alloy rods were secured to these pedicle screws.2.2Biomechanical test. The six degree of freedom pure moment of8Nm were applied tothe superior end (T10) of the specimens. The S1screws stresses of these two differentfixation levels were compared.3. Biomechanical effect of fixation to L5vs S1on lowest instrumented screws stressesin lumbosacral long fixation construct3.1Specimens fixation and experimental grouping. Six fresh-frozen cadaveric specimensunderwent transpedicular lumbosacral instrumentation using appropriate size pediclescrews according to experimental grouping method:(1) L1-S1,(2) L1-L5. Two titaniumalloy rods were secured to these pedicle screws.3.2Biomechanical test. The six degree of freedom pure moment of8Nm were applied tothe superior end of the specimens. The lowest instrumented S1screws stresses in L1-S1 fixation constructs and L5screws stresses in L1-L5fixation constructs were compared.4. Biomechanical effect of anterior lumbar interbody fusion on sacral screws stressesin lumbosacral long fixation construct4.1Specimens fixation and experimental grouping. Six fresh-frozen cadaveric specimensunderwent transpedicular lumbosacral instrumentation using appropriate size pediclescrews in the two different fixation constructs:(1) L1-S1,(2) L1-S1+ALIF. An anteriorlumbar interbody fusion Cage (Cougar, Depuy Spine) was implanted into the L5/S1junction in L1-S1+ALIF fixation constructs.4.2Biomechanical test. The six degree of freedom pure moment of8Nm were applied tothe superior end (T10) of the specimens. The lowest instrumented S1screws stresses in thetwo fixation constructs were compared.5. Comparison of posterior decompression with short fusion or long fusion fordegenerative scoliosisForty-four degenerative scoliosis cases who received posterior decompression andshort fusion or long fusion were followed-up and reviewed. All of them underwentposterior decompression with short fusion or long fusion using pedicle screw-onlyconstructs. The radiological parameters were measured and the Oswestry disability indexscores were recorded before surgery, postoperative, and at final follow-up.[Results]1. In flexion-extension loading, S1screws were mainly loaded with pullout force,transverse load and bending moment. In lateral bending loading, the screws were mainlyloaded with transverse load and bending moment. In axial rotation loading, the screwswere mainly loaded with bending moment.2. In flexion loading, the direction of the pullout force was oriented dorsal and thetransverse load was distraction. In extension loading, the direction of the pullout force wasoriented ventral and the transverse load was compression. In left lateral bending, thetransverse load on left S1screw was compression. In right lateral bending, the transverseload on left S1screw was distraction.3. In multidirectional pure moments loading, the pullout force, transverse load and bendingmoment on S1screws all increased as instrumentation extended to L3(P<0.05), andachieved to the maximum magnitude at L1(P<0.01).4. Compared to L1-S1instrumentation constructs, the pullout force, transverse load andbending moment on S1screws in T10-S1constructs were all higher (P<0.05). 5. In multidirectional pure moments loading, the stresses on lowest instrumented L5screwsin L1-L5fixation constructs were smaller than those on the S1screws in L1-S1fixationconstructs(P<0.05).6. Compared to L1-S1instrumentation constructs, stresses on S1screws in L1-S1+ALIFconstructs in all multidirectional pure moments loading were smaller (P<0.05).7. The correction of Cobb angle in the short fusion group and long fusion group were of astatistical difference (P<0.01). Coronal imbalance improved significantly in the long fusiongroup more than in the short fusion group (P<0.01). The correction of lateral listhesis wasbetter in the long fusion group (P<0.01).[Conclusions]1. In this study, we successfully established the lumbosacral long fixation model, andmeasured all three force or moment components including pullout force, transverse loadand bending moment, which must all be considered in studying the biomechanics of longfixation construct.2. In all the three stresses, the magnitude of pullout force was relatively lower than thetransverse load, and compared to the applied pure loading, the bending moment on thesacral screw was small.3. For instrumented fusions extending above L3, the stresses on S1screw significantlyincreased.4. Long fixation extending past the thoracolumbar junction increased the sacral screwsstresses, and sacral screws should be protected with supplemental fixation.5. In L1-L5instrumentation constructs, the applied loading passed through the constructsto the L5-S1joint, thus it alleviate the L5screws stresses, however, to increase the risk ofL5-S1disk degeneration.6. Posterior long pedicle fixation construct combined with anterior lumbar interbody fusionincreased the stability of lumbosacral long instrumentation constructs. The anterior Cageeffectively decreased the sacral screws stresses.7. Short fusion is sufficient for patients with small Cobb angle and good spinal balance.For patients with severe Cobb angle and lateral listhesis, long fusion should be carried outto achieve better balance.