Comparison of intradiscal pressures and spinal fixator loads for different body positions and exercises

Abstract

Loading of the spine is still not well understood. The most reliable results seemed to come from the intradiscal pressure measurements from studies by Nachemson, 1966. A new similar study by Wilke et al. (1999) complemented the present study and confirmed some of the earlier data, although it contradicted others. The new data did not confirm that the load on the spine is higher in sitting compared with standing and did not find distinct differences between positions in which subjects were lying down. The objective of this paper was to compare results from two independent in vivo studies (applying different methods) to provide information about spinal loading. In one of these studies (Wilke 1999), intradiscal pressure was measured in one volunteer in different postures and exercises, and in the other study (Rohlmann et al. 1994) the loads on an internal spinal fixation device (an implant for stabilising unstable spines) were determined in 10 patients. The absolute values of the results from both studies were normalized and compared for many body positions and dynamic exercises. The relative differences in intradiscal pressure and flexion bending moments in the fixators corresponded in most cases. Both studies showed slightly lower loads for sitting than for standing and comparatively low loads in all lying positions. High loads were measured for jogging, jumping on a trampoline and skipping. Differences between trends for intradiscal pressure and for flexion bending moments in the fixators were found when the load was predominantly carried by the anterior spinal column, as during flexion of the upper part of the body or when lifting and carrying weights. The combination of the results from these two methods may improve the understanding of the biomechanical behaviour of the lumbar spine and may be used to validate models and theories of spinal loading.

Comparison of the effects of bilateral posterior dynamic and rigid fixation devices on the loads in the lumbar spine: a finite element analysis

“…The measured values were also much higher in cases where the bridged level was distracted. The calculated intradiscal pressures are also very similar to the values measured in vivo by Wilke et al [22,36,37]. They measured a pressure of about 0.5 MPa for standing, 0.6 MPa for 19°extension, and 1.1 MPa for 36°flexion.…”

“…The implant forces and moments calculated for the rigid fixator agree well with those measured in patients [20][21][22][23]. The measured values were also much higher in cases where the bridged level was distracted.…”

Load‐transfer in the human vertebral body following lumbar total disc arthroplasty: Effects of implant size and stiffness in axial compression and forward flexion

“…The 800 N force we applied in both compression and bending (approximately 1× body weight) facilitated comparison between models since it enabled us to isolate the interaction between size and loading mode. However, the forces on the vertebral body generated in vivo during flexion can be two to three times body weight, since the moment arm caused by the weight of the trunk must be balanced by increasing forces in the erector spinae muscles, which increases the reaction force at the vertebra. Scaling the values of stress in flexion 2‐ to 3‐fold to those better representing the in vivo environment (permitted by the linear elastic nature of our study) would generate tissue‐level stresses high enough to be of concern for both monotonic and fatigue‐related tissue failure .…”

“…To simulate flexion of an intact disc, a displacement boundary condition was used to rotate the disc in the mid‐sagittal plane about the far posterior‐superior point, simulating flexion over a single motion segment (Figure 1B). Results were then scaled linearly to produce an overall reaction force of 800 N. While flexion can increase loads on the spine 2‐ to 3‐fold compared to what was modeled here, a reaction force of 800 N was maintained in order to facilitate comparison across models. Compression of an implanted segment was modeled by applying a uniform force of 800 N to the superior implant footplate (Figure 1C).…”

Monitoring the load on a telemeterised vertebral body replacement for a period of up to 65 months

“…Instrumented spinal implants allow the in vivo load measurement in many sessions and can, therefore, be used for a differentiated analysis of the factors that influence spinal loading. Although the transfer of the results to healthy persons has to be handled with care, its conformity with short-term measurements in healthy subjects has been shown several times [6,7]. In addition to basic science, in vivo measurements from implants are a unique way to obtain clinically relevant information within a patient during the fusion process.…”