Biomechanical evaluation of traditional posterior versus anterior spondylolisthesis reduction in a cadaveric grade I slip model.OBJECTIVEPosterior reduction with pedicle screws is often used for stabilization of unstable spondylolisthesis to directly reduce misalignment or protect against micromotion while fusion of the affected level occurs.  Optimal treatment of spondylolisthesis combines consistent reduction with a reduced risk of construct failure.  The authors compared the reduction achieved with a novel anterior integrated spacer with a built-in reduction mechanism (ISR) to the reduction achieved with pedicle screws alone, or in combination with an anterior lumbar interbody fusion (ALIF) spacer, in a cadaveric grade I spondylolisthesis model.METHODSGrade I slip was modeled in 6 cadaveric L5-S1 segments by creation of a partial nucleotomy and facetectomy and application of dynamic cyclic loading.  Following the creation of spondylolisthesis, reduction was performed under increasing axial loads, simulating muscle trunk forces between 50 and 157.5 lbs, in the following order: bilateral pedicle screws (BPS), BPS with an anterior spacer (BPS+S), and ISR.  Percent reduction and reduction failure load-the axial load at which successful reduction (≥ 50% correction) was not achieved-were recorded along with the failure mechanism.  Corrections were evaluated using lateral fluoroscopic images.RESULTSThe average loads at which BPS and BPS+S failed were 92.5 ± 6.1 and 94.2 ± 13.9 lbs, respectively.  The ISR construct failed at a statistically higher load of 140.0 ± 27.1 lbs.  Reduction at the largest axial load (157.5 lbs) by the ISR device was tested in 67% (4 of 6) of the specimens, was successful in 33% (2 of 6), and achieved 68.3 ± 37.4% of the available reduction.  For the BPS and BPS+S constructs, the largest axial load was 105.0 lbs, with average reductions of 21.3 ± 0.0% (1 of 6) and 32.4 ± 5.7% (3 of 6) respectively.CONCLUSIONSWhile both posterior and anterior reduction devices maintained reduction under gravimetric loading, the reduction capacity of the novel anterior ISR device was more effective at greater loads than traditional pedicle screw techniques.  Full correction was achieved with pedicle screws, with or without ALIF, but under significantly lower axial loads.  The anterior ISR may prove useful when higher reduction forces are required; however, additional clinical studies will be needed to evaluate the effectiveness of anterior devices with built-in reduction mechanisms.
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