Compressive strength of interbody cages in the lumbar spine: the effect of cage shape, posterior instrumentation and bone density

Abstract

IntroductionPosterior lumbar intervertebral fusion (PLIF) was introduced to clinical practice in the mid-1940s independently by Jaslow [33] and by Cloward [14][15][16]. The theoretical bases of this procedure were outlined: mechanical stability is provided by the intervertebral fusion, the original disc height is restored and the intervertebral foramina are distracted. Lin et al. [38] postulated four biomechanical principles to get a high rate of fusion: preservation of the integrity of the posterior portion of the motion segment; partial preservation of the integrity of the cortical endplates; attempted maximal removal of disc material, especially the nucleus pulposus, as a potential source of chronic low back pain; and the use of several "unicortical peg grafts" as opposed to smaller "chips" or "strips" of bone [36]. These variables and others impact the clinical results Abstract One goal of interbody fusion is to increase the height of the degenerated disc space. Interbody cages in particular have been promoted with the claim that they can maintain the disc space better than other methods. There are many factors that can affect the disc height maintenance, including graft or cage design, the quality of the surrounding bone and the presence of supplementary posterior fixation. The present study is an in vitro biomechanical investigation of the compressive behaviour of three different interbody cage designs in a human cadaveric model. The effect of bone density and posterior instrumentation were assessed. Thirty-six lumbar functional spinal units were instrumented with one of three interbody cages: (1) a porous titanium implant with endplate fit (Stratec), (2) a porous, rectangular carbon-fibre implant (Brantigan) and (3) a porous, cylindrical threaded implant (Ray). Posterior instrumentation (USS) was applied to half of the specimens. All specimens were subjected to axial compression displacement until failure. Correlations between both the failure load and the load at 3 mm displacement with the bone density measurements were observed. Neither the cage design nor the presence of posterior instrumentation had a significant effect on the failure load. The loads at 3 mm were slightly less for the Stratec cage, implying lower axial stiffness, but were not different with posterior instrumentation. The large range of observed failure loads overlaps the potential in vivo compressive loads, implying that failure of the bone-implant interface may occur clinically. Preoperative measurements of bone density may be an effective tool to predict settling around interbody cages.

Anterior lumbar interbody fusion with threaded fusion cages and autologous bone grafts

“…Secondly, the discongruency between the cylindrical shape of the cage and the wedge-shaped intervertebral disc space in the sagittal plane could result in destruction of the posterior part of the L5 endplate. If so, the posterior part of the cage, which is supposed to resist a considerable amount of compressive force, will rest on the softer cancellous vertebral body bone [2,6,16]. The decrease of posterior intervertebral disc height during this time is a result of the subsidence of the cage into the posterior part of the vertebral body.…”

The use of a quadruped as an in vivo model for the study of the spine – biomechanical considerations

“…In order to mimic the clinical situation as much as possible, spinal implants have often been tested in vitro on human cadaver segments [6,17,20]. One problem with the use of human specimens, however, is the large variation in geometry and mechanical properties.…”

Biomechanical compression tests with a new implant for thoracolumbar vertebral body replacement

“…Hansson et al [14] found a linear correlation between BMD and maximum compression force, calculating a significant (P<0.01) correlation coefficient of 0.81-0.90. In compression tests with "interbody cages" between vertebral bodies, Jost et al [18] observed a significant (P<0.0005) linear correlation between the above variables.…”

“…The literature offers no uniform recommendations for the test conditions in compression trials of this type, and test speeds of 5 mm/min [14,30], 0.4 mm/s [18], 35 mm/ min [38] and 2.54 mm/s [16] are described in the literature. Since no group of authors offers any rationale for their respective test speed, we assumed that the various speeds were determined by the test machine employed in each case.…”