Characteristics of immediate and fatigue strength of a dual-threaded pedicle screw in cadaveric spines.

BACKGROUND CONTEXT: Novel dual-threaded screws are configured with overlapping (doubled) threads only in the proximal shaft to improve proximal cortical fixation. PURPOSE: Tests were run to determine whether dual-threaded pedicle screws improve pullout resistance and increase fatigue endurance compared with standard pedicle screws. STUDY DESIGN/SETTING: In vitro strength and fatigue tests were performed in human cadaveric vertebrae and in polyurethane foam test blocks. PATIENT SAMPLE: Seventeen cadaveric lumbar vertebrae (14 pedicles) and 40 test sites in foam blocks were tested. OUTCOME MEASURES: Measures for comparison between standard and dual-threaded screws were bone mineral density (BMD), screw insertion torque, ultimate pullout force, peak load at cyclic failure, and pedicular side of first cyclic failure. METHODS: For each vertebral sample, dual-threaded screws were inserted in one pedicle and single-threaded screws were inserted in the opposite pedicle while recording insertion torque. In seven vertebrae, axial pullout tests were performed. In 10 vertebrae, orthogonal loads were cycled at increasing peak values until toggle exceeded threshold for failure. Insertion torque and pullout force were also recorded for screws placed in foam blocks representing healthy or osteoporotic bone porosity. RESULTS: In bone, screw insertion torque was 183% greater with dual-threaded than with standard screws (p<.001). Standard screws pulled out at 93% of the force required to pull out dual-threaded screws (p=.42). Of 10 screws, five reached toggle failure first on the standard screw side, two screws failed first on the dual-threaded side, and three screws failed on both sides during the same round of cycling. In the high-porosity foam, screw insertion torque was 60% greater with the dual-threaded screw than with the standard screw (p=.005), but 14% less with the low-porosity foam (p=.07). Pullout force was 19% less with the dual-threaded screw than with the standard screw in the high-porosity foam (p=.115), but 6% greater with the dual-threaded screw in the low-porosity foam (p=.156). CONCLUSIONS: Although dual-threaded screws required higher insertion torque than standard screws in bone and low density foam, dual-threaded and standard pedicle screws exhibited equivalent axial pullout and cyclic fatigue endurance. Unlike single-threaded screws, the mechanical performance of dual-threaded screws in bone was relatively independent of BMD. In foam, the mechanical performance of both types of screws was highly dependent on porosity.

The mechanical effect of commercially pure titanium and polyetheretherketone rods on spinal implants at the operative and adjacent levels.

STUDY DESIGN: Single-level cadaveric lumbar constructs were instrumented with either polyetheretherketone (PEEK) or commercially pure (CP) titanium (Ti) rods and biomechanically evaluated. Strain from gauged bone screws and interbody (IB) spacers, kinematic motion, and caudal disc pressure measurements were recorded during testing. OBJECTIVE: The objective of this study was to determine the biomechanical differences in CP Ti rods and PEEK rods in conjunction with PEEK interbody spacers. SUMMARY OF BACKGROUND DATA: Very little biomechanical data exist substantiating the performance of PEEK as a spinal rod material. This study is unique, because it combines strain, motion, and pressure measurement techniques to evaluate cadaveric constructs. METHODS: Twelve human cadaveric lumbar spine segments (T12-L3 and L4-S1) were tested in compression, flexion-extension, bilateral lateral bending, and bilateral axial torsion. Bending, axial, and shear strains were recorded from a gauged bone screw; axial and shear strains were also recorded from a gauged PEEK interbody spacer. Planar motion data and subadjacent disc pressure measurements were also collected. RESULTS: Highest screw strains were in bending; the lowest screw strains derived from the shear and axial gauges. Spacer strain was high to medium in some cases, especially in compression and flexion. PEEK constructs attained higher interbody strains than Ti constructs. Conversely, Ti construct screw strains were higher in most tests. Planar motion showed no differences at any level in almost every test. There was a trend toward decreased caudal intradiscal pressure for Ti constructs in compression. CONCLUSION: Rigid CP Ti rods resulted in increased screw strain (bone-screw interface forces) and less interbody spacer compression (higher stress shielding). Furthermore, there was a trend toward decreased intradiscal pressure with Ti rods at the caudal segment. These trends suggest that segments instrumented with PEEK more closely mimicked intact physiologic loading in the subadjacent level, which may reduce the likelihood of adjacent level disease.

Evaluation of a lumbar intervertebral spacer with integrated screws as a stand-alone fixation device.

STUDY DESIGN: An in vitro cadaveric biomechanics study. OBJECTIVE: To evaluate the biomechanical contribution of the integration of screws into a polyether-ether-ketone (PEEK) spacer, and to compare the resulting construct to standard anterior lumbar interbody fusion constructs. SUMMARY OF BACKGROUND DATA: Pedicular fixation is often performed to increase the rigidity of anterior lumbar interbody fusion constructs but also increases the risk of morbidity. Integration of screws into a PEEK spacer (investigational device, ID) may increase construct rigidity and prevent migration without the drawbacks associated with supplementary fixation. METHODS: Twenty cadaveric motion segments were potted and tested under pure moments of + or - 7.5 Nm in flexion-extension, lateral bending, and axial torsion. Discectomies were performed and specimens were instrumented with the ID + or - screws, dual threaded cages, structural graft+anterior plate, and graft+pedicle screws. The ID+screws and threaded cage constructs were then pulled out at a constant rate. RESULTS: All constructs reduced the mean range of motion (ROM) below the intact level in flexion-extension and lateral bending (P<0.001) and for all devices except cages in torsion (P < or = 0.05). The median flexion-extension/bending/torsion ROM was 9.5/9.4/4.1 degrees for the intact segments, 6.1/5.1/1.8 degrees with the ID+screws, 4.9/5.2/2.4 degrees with threaded cages, 3.3/4.4/1.1 degrees with plates and 1.4/1.6/1.7 degrees with pedicle screws, respectively. The addition of the integrated screws decreased the mean ROM of the ID constructs by 0.9 degrees in flexion-extension, 1.8 degrees in bending, and 2.8 degrees in flexion-extension. The peak pullout force was 962 N for the ID and 337 N for threaded cages. CONCLUSIONS: The incorporation of screws into a PEEK interbody device was evaluated alongside traditional constructs in terms of construct rigidity and resistance to pullout. The greatest contributions of the integrated screws are in limiting torsional motion and in the containment of the interbody device. Pedicular fixation produces the most rigid constructs, but integrating screws into a PEEK spacer produces a stand-alone construct that is similar to threaded cages and provides more resistance to anterior displacement.

Shilla growing rods in a caprine animal model: a pilot study.

There are few good surgical options that allow for continued spinal growth in patients with early-onset scoliosis. The "Shilla" is a growth guidance system that does not require repeated surgical lengthenings. The Shilla system guides growth at the ends of dual rods with the apex of the curve corrected, fused, and fixed to the rods. The growth occurs through the extraperiosteally implanted pedicle screws that slide along the rods at either end of the construct. We implanted 11 2-month-old immature goats with the dual rod system and euthanized all 11 goats 6 months postoperatively. We evaluated plain radiographs, regular computed tomography, microcomputed tomography, physical and histologic examinations, and a microscopic wear analysis. All of the goat spines grew with the implants in place; growth occurred in both the thoracic and lumbar ends of the rods for a total average of 48 mm. None of the implants failed, although we observed minor wear at the rod/screw interface. Growth guidance with the Shilla rod system allowed for continued growth in this goat model.