Significant difference was found between certain screw-angle configurations; however, when compared with simply placing the screws straight in, the difference was never more than 8%. This implies that there is greater freedom in the angle and placement of screws than previously thought. Our results show that there is little change in fixation strength when placing the screw in a different direction.

BackgroundScrew length pertains to stability in various orthopedic fixation devices. There is little or no information on the relationship between plate pullout strength and screw length in anterior lumbar interbody fusion (ALIF) plate constructs in the literature. Such a description may prove useful, especially in the treatment of osteoporotic patients where maximizing construct stability is of utmost importance. Our purpose is to describe the influence of screw length on ALIF plate stability in severely and mildly osteoporotic bone foam models.MethodsTesting was performed on polyurethane foam blocks with densities of 0.08 g/cm3 and 0.16 g/cm3. Four-screw, single-level ALIF plate constructs were secured to the polyurethane foam blocks by use of sets of self-tapping cancellous bone screws that were 20, 24, 28, 32, and 36 mm in length and 6.0 mm in diameter. Plates were pulled out at 1 mm/min to failure, as defined by consistently decreasing load despite increasing displacement.ResultsPullout loads in 0.08-g/cm3 foam for 20-, 24-, 28-, 32-, and 36-mm screws averaged 303, 388, 479, 586, and 708 N, respectively, increasing at a mean of 25.2 N/mm. In 0.16-g/cm3 foam, pullout loads for 20-, 24-, 28-, 32-, and 36-mm screws averaged 1004, 1335, 1569, 1907, and 2162 N, respectively, increasing at a mean of 72.2 N/mm.ConclusionsThe use of longer screws in ALIF plate installation is expected to increase construct stability. Stabilization from screw length in osteoporotic patients, however, is limited.

Background: Screw length pertains to stability in various orthopedic fixation devices. There is little or no information on the relationship between plate pullout strength and screw length in anterior lumbar interbody fusion (ALIF) plate constructs in the literature. Such a description may prove useful, especially in the treatment of osteoporotic patients where maximizing construct stability is of utmost importance. Our purpose is to describe the influence of screw length on ALIF plate stability in severely and mildly osteoporotic bone foam models. Methods: Testing was performed on polyurethane foam blocks with densities of 0.08 g/cm 3 and 0.16 g/cm 3 . Four-screw, single-level ALIF plate constructs were secured to the polyurethane foam blocks by use of sets of self-tapping cancellous bone screws that were 20, 24, 28, 32, and 36 mm in length and 6.0 mm in diameter. Plates were pulled out at 1 mm/min to failure, as defined by consistently decreasing load despite increasing displacement. Results: Pullout loads in 0.08-g/cm 3 foam for 20-, 24-, 28-, 32-, and 36-mm screws averaged 303, 388, 479, 586, and 708 N, respectively, increasing at a mean of 25.2 N/mm. In 0.16-g/cm 3 foam, pullout loads for 20-, 24-, 28-, 32-, and 36-mm screws averaged 1004, 1335, 1569, 1907, and 2162 N, respectively, increasing at a mean of 72.2 N/mm. Conclusions: The use of longer screws in ALIF plate installation is expected to increase construct stability. Stabilization from screw length in osteoporotic patients, however, is limited.

Introduction: Previous studies demonstrated increases in single screw pullout strength with increases in material density. Recent anterior cervical interbody fusion plate pullout studies utilizing a polyurethane foam block model have shown that alterations in screw insertion angle from straight-in are not associated with an increase in pullout strength. The purpose of this study is to characterize the pullout strength of an anterior lumber interbody fusion (ALIF) plate when installed at various screw angles in different simulated bone densities. Materials and methods:Ninety ALIF plate pullout tests were performed using three common screw insertion angles in polyurethane (PU) foam blocks of three densities: 0.08 g/cm 3 , 0.16 g/cm 3 and 0.24 g/cm 3 , simulating severely, mildly and nonosteoporotic cancellous bone, respectively. Plates were pulled out axially at 1 mm/min and pullout strength and stiffness compared.Results: Doubling foam density yielded 2.6-fold and 3.0-fold increases (p < 0.05) in mean pullout strength and stiffness, respectively. Tripling foam density yielded 4.5-fold and 5.3-fold increases (p < 0.05) in mean pullout strength and stiffness, respectively. Screw angle placement contributed relatively less to pullout strength and stiffness compared to PU foam density.Conclusion: In our model, ALIF plate pullout strength and stiffness appear to be more associated with increased foam block density than screw trajectory. Vertebral bone density should be a strong consideration in preoperative planning for ALIF with plating. Screw trajectory should be based on vascular anatomy and screw placement safety, rather than the classic lateral-to-medial trajectory.