Pullout of a lumbar plate with varying screw lengths

Abstract

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.

Impact of bone density and integrated screw configuration on standalone anterior lumbar interbody construct strength

“…While screw length proved to be an important consideration, the density of the bone surrogates played an even larger role in some studies [ 23 , 24 ]. Hitchon et al instrumented cadaveric spines with two of either 12, 14, or 16 mm screws.…”

“…[ 23 ]. Palmer et al found that doubling screw length for ALIF plate constructs translated into approximately twice the pullout strength, while doubling the density of the bone surrogate approximately tripled the pullout load [ 24 ]. Our study demonstrated similar results, suggesting that although longer screw length confers significant ALIF stability, patient bone density may be more important in determining failure.…”

“…There were several limitations to this study. Use of bone surrogates allowed for reproducibility of testing; similar materials were used by Palmer et al and Amirouche et al [ 11 , 24 ]. However, the homogenous structure of the surrogates did not precisely represent the more heterogenous nature of human bone.…”

“…Our mechanism for testing, although providing standardized values for experimentation and comparison, differs from the in vivo mode of construct failure, which involves cyclic loading mechanisms [ 24 ]. Additional parameters may also affect ALIF construct stability, including screw insertion angle and screw diameter.…”