The influence of cancellous bone density on load sharing in human lumbar spine: a comparison between an intact and a surgically altered motion segment

Abstract

IntroductionLittle is known about the exact distribution of forces within healthy lumbar spine due to its complex geometry and physiology. Furthermore, defects by trauma or tumorous destruction may influence spinal biomechanics to a great extent. Surgery also changes the biomechanics of the spine, and the true way in which the spine behaves following, for instance, decompression and stabilisation can only be estimated.Loads applied onto the spine are shared among spinal components: this is called spinal load sharing. We know that lumbar spinal load sharing takes place, passing 80-96% of the applied load through the anterior part of the spine, and the remainder through the posterior ele- AbstractThe aim of the current study is twofold: first, to compare load sharing in compression between an intact and a surgically repaired lumbar spine motion segment L3/4 using a biomechanically validated finite element approach; second, to analyse the influence of bone mineral density on load sharing. Six cadaveric human lumbar spine segments (three segments L2/3 and three segments L4/5) were taken from fresh human cadavers. The intact segments were tested under axial compression of 600 N, first without preload and then following instrumented stabilisation. These results were compared to a finite element model simulating the effect of identical force on the intact segments and the segments with constructs. The predictions of both the intact and the surgically altered finite element model were always within one standard deviation of the mean stiffness as analysed by the biomechanical study. Thus, the finite element model was used to analyse load sharing under compression in an intact and a surgically repaired human lumbar spine segment model, using a variety of E moduli for cancellous bone of the vertebral bodies. In both the intact and the surgically altered model, 89% of the applied load passed through the vertebral bodies and the disc if an E modulus of 25 MPa was used for cancellous bone density. Using 10 MPa -representing soft, osteoporotic bone -this percentage decreased, but it increased using 100 MPa in both the intact and the altered segment. Thus, it is concluded that reconstruction of both the disc and the posterior elements with the implants used in the study recreates the ability of the spine to act as a load-sharing construction in compression. The similarity in load sharing between normal and instrumented spines appears to depend on assumed bone density, and it may also depend on applied load and loading history.

Biomechanical comparison between fusion of two vertebrae and implantation of an artificial intervertebral disc

“…The thickness of the lateral cortical wall was 0.35 mm and the thickness of the vertebral endplates was 0.5 mm (Silva et al, 1994). It has been shown that the center of the vertebral endplates is weaker and less stiff Baroud et al (2003), Goel et al (1995), Homminga et al (2002), Pitzen et al (2001), Shirazi-Adl et al (1984) Hexahedral Cancellous bone 100 0.2 Dooris et al (2001), Goel et al (1993), Lavaste et al (1992), Linde (1994), Sharma et al (1995), Shirazi-Adl et al (1984) Tetrahedral Posterior vertebra 3000 0.3 Smit et al (1997) Hexahedral Annulus ground substance 4.2 0.45 Goel et al (1993Goel et al ( , 1995, Pitzen et al (2001), Sharma et al (1995), Shirazi-Adl et al Goel et al (1993Goel et al ( , 1995, Pitzen et al (2001), Sharma et al (1995), Smit et al (1997) Hexahedral (Grant et al, 2001;Polikeit et al, 2003). As a consequence, the endplates were partitioned into three areas with variable mechanical characteristics ranging from 2000 to 12,000 MPa as shown in Fig.…”

“…A natural ligament exhibits strong nonlinear load/deformation behavior (White and Panjabi, 1990). As with most of the reported FEM studies (Dooris et al, 2001;Goel et al, 1993Goel et al, , 1995Pitzen et al, 2001;Smit et al, 1997), the current model has adopted a bilinear model, described in Table 2.…”

“…The fibers had an inclination of between 151 and 301 with respect to the transverse plane. The natural nonlinear behavior of the fibers was approximated using different linear models for each layer considering their respective range of deformation (Baroud et al, 2003;Pitzen et al, 2001;Smit et al, 1997). The fiber content of the modeled annulus was approximately 19%, appropriate to the natural collagen content of the annulus (Sharma et al, 1995;Shirazi-Adl et al, 1984).…”

Primary stability of anterior lumbar stabilization: interdependence of implant type and endplate retention or removal

“…It is known from previous studies that ceramic material sometimes has tends to fracture, however, with no reported change in the position of the treated segment [9,60]. It is also known from the literature that the average elasticity module of the lumbar cancellous bone of a 60-year old individual is about 25 MPa [46]. The required compressive strength of the bone substitute of 30 MPa was thus judged sufficient for the size of the blocks tested in order to avoid fractures during testing.…”

Influence of ligament stiffness on the mechanical behavior of a functional spinal unit

“…Several groups have analytically examined spinal segments using the finite element method Lavaste et al, 1992;Pitzen et al, 2001;Shirazi-Adl, 1991;Zander et al, 2001Zander et al, , 2002. To calculate intersegmental rotation as well as stresses and strains in their components, this method requires data on the geometry, loads, and material properties of the different components.…”