P24. Rod attachment in long segment pedicle screw-rod constructs results in high lumbosacral instrumentation strain

Abstract

BACKGROUND CONTEXT Surface strain on instrumentation spanning the lumbosacral junction is often recorded during in vitro studies evaluating the biomechanical effects of different pedicle screw and rod (PSR) configurations. However, little is known about the strains that the instrumentation is subjected to during attachment of the rods prior to testing, after rods have been contoured to fit the specimens to the best of the surgeon's ability. PURPOSE The purpose of this study was to analyze lumbosacral rod strains and sacral screw bending moments during PSR L2-IL rod attachment, and to compare these to strains observed during sagittal plane bending of the same constructs immediately following attachment. STUDY DESIGN/SETTING In vitro biomechanical study using human cadaveric specimens. PATIENT SAMPLE 12 (L1-Pelvis) cadaveric spines (3F/9M, 53.1±8.1 yrs, DEXA 0.835 g/cm2) were studied. OUTCOME MEASURES L5-S rod strains and resultant sacral screw bending loads. METHODS A total of 12 cadaveric specimens were instrumented with L2-IL PSR instrumentation. CoCr rods were manually contoured with a French bender to fit inside all poly-axial screw heads without significant reduction required and then instrumented with 2 uni-axial strain gauges (CEA-06-062UW-350) facing anteriorly and posteriorly on the rods at L5-S. Sacral screws were instrumented with 4 uni-axial gauges (EA-06-031CE-350) placed circumferentially near the screw head and calibrated before insertion. Rod and sacral screw strains were recorded during rod-to-screw tightening, performed sequentially level by level cranial to caudal. Instrumented constructs were then tested in a 6DOF robot under continuous dynamic loading to 7.5 Nm in flexion (FL) and extension (EX) with the same constructs in place. Absolute values of rod bending strains (RS) for right and left side rods and sacral screw bending moments (SS), averages of right and left sides, during initial rod attachment and during loading (FL and EX) were compared using one-way RM-ANOVA followed by Holms-Sidak paired analysis (p RESULTS The smallest mean (±SD) RS occurred while securing the first (right side) rod (171±192 uE) and the greatest mean RS occurred while securing the second (left side) rod (322±269 uE). Mean RSs during loading (in FL (264±109 uE) and EX (315±125 uE) were less than during rod attachment of the second rod, however, differences in RS among conditions (ie, rod attachment and loading) were not statistically significant (p>0.14). The mean SS during rod attachment (1.21±0.56 Nm) was significantly greater than during loading in both FL (0.27±0.10 Nm) and EX (0.24±0.10 Nm). CONCLUSIONS Securing contoured rods to pedicle screws while instrumenting cadaveric spines with L2-IL PSR instrumentation result in considerable lumbosacral instrumentation strains, with magnitudes comparable to, and in some cases even greater than, strains recorded during sagittal plane loading of the same constructs. Strains experienced during activities of daily living are additive to pre-strains observed during attachment, with high overall strains increasing the risk of 1) rod fracture through fatigue mechanisms and 2) pedicle screw loosening. These results thus have clinical implications with regard to rod fit and contouring. FDA DEVICE/DRUG STATUS Unavailable from authors at time of publication.