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Doniel Drazin, Mir Hussain, Jonathan Harris, John Hao, Matt Phillips, Terrence T. Kim, J. Patrick Johnson and Brandon Bucklen

OBJECT

Abnormal sacral slope (SS) has shown to increase progression of spondylolisthesis, yet there exists a paucity in biomechanical studies investigating its role in the correction of adult spinal deformity, its influence on lumbosacral shear, and its impact on the instrumentation selection process. This in vitro study investigates the effect of SS on 3 anterior lumbar interbody fusion constructs in a biomechanics laboratory.

METHODS

Nine healthy, fresh-frozen, intact human lumbosacral vertebral segments were tested by applying a 550-N axial load to specimens with an initial SS of 20° on an MTS Bionix test system. Testing was repeated as SS was increased to 50°, in 10° increments, through an angulated testing fixture. Specimens were instrumented using a standalone integrated spacer with self-contained screws (SA), an interbody spacer with posterior pedicle screws (PPS), and an interbody spacer with anterior tension band plate (ATB) in a randomized order. Stiffness was calculated from the linear portion of the load-deformation curve. Ultimate strength was also recorded on the final construct of all specimens (n = 3 per construct) with SS of 40°.

RESULTS

Axial stiffness (N/mm) of the L5–S1 motion segment was measured at various angles of SS: for SA 292.9 ± 142.8 (20°), 277.2 ± 113.7 (30°), 237.0 ± 108.7 (40°), 170.3 ± 74.1 (50°); for PPS 371.2 ± 237.5 (20°), 319.8 ± 167.2 (30°), 280.4 ± 151.7 (40°), 233.0 ± 117.6 (50°); and for ATB 323.9 ± 210.4 (20°), 307.8 ± 125.4 (30°), 249.4 ± 126.7 (40°), 217.7 ± 99.4 (50°). Axial compression across the disc space decreased with increasing SS, indicating that SS beyond 40° threshold shifted L5–S1 motion into pure shear, instead of compression-shear, defining a threshold. Trends in ultimate load and displacement differed from linear stiffness with SA > PPS > ATB.

CONCLUSIONS

At larger SSs, bilateral pedicle screw constructs with spacers were the most stable; however, none of the constructs were significantly stiffer than intact segments. For load to failure, the integrated spacer performed the best; this may be due to angulations of integrated plate screws. Increasing SS significantly reduced stiffness, which indicates that surgeons need to consider using more aggressive fixation techniques.

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Patrick W. Hitchon, Jonathan M. Mahoney, Jonathan A. Harris, Mir M. Hussain, Noelle F. Klocke, John C. Hao, Doniel Drazin and Brandon S. Bucklen

OBJECTIVE

Posterior 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.

METHODS

Grade 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.

RESULTS

The 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.

CONCLUSIONS

While 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.