Use of a personalized hybrid biomechanical model to assess change in lumbar spine function with a TDR compared to an intact spine

Abstract

Total disc replacements (TDRs) have been employed with increasing frequency in recent years with the intention of restoring natural motion to the spine and reducing adjacent level trauma. Previous assessments of the TDRs have subjectively measured patient satisfaction, evaluated sagittal range of motion via static imaging, or examined biomechanical loading in vitro. This study examined the kinematics and biomechanical loading of the lumbar spine with an intact spine compared to a TDR inserted at L5/S1 in the same spine. A validated biologically driven personalized dynamic biomechanical model was used to assess range of motion (ROM) and lumbar spine tissue forces while a subject performed a series of bending and lifting exertions representative of normal life activities. This analysis concluded that with the insertion of a TDR, forces are of much greater magnitude in all three directions of loading and are concentrated at both the endplates and the posterior element structures compared to an intact spine. A significant difference is seen between the intact spine and the TDR spine at levels above the TDR insertion level as a function of supporting an external load (lifting). While ROM within the TDR joint was larger than in the intact spine (yet within the normal ranges under the unloaded bending conditions), the differences between spines were far greater in all three planes of motion under loaded lifting conditions. At levels above the TDR insertion, larger ROM was present during the lifting conditions. Sagittal motions were often greater at the higher lumbar levels, but there appeared to be less lateral and twisting motion. Collectively, this analysis indicates that the insertion of a TDR significantly alters the function of the spine.

Fig. 1

Biomechanical model structure and component interactions

Fig. 2

Model representations of the TDR spine

Fig. 3

Average of the peak compression loads on the lumbar spine endplates as a function of intact spine versus TDR spine and external loading condition (bending while unloaded 0 kg, lifting 9.5 kg, lifting 19 kg)

Fig. 4

Average of the peak anterior/posterior (A/P) shear loads on the lumbar spine endplates as a function of intact spine versus TDR spine and external loading condition (bending while unloaded 0 kg, lifting 9.5 kg, lifting 19 kg)

Fig. 5

Average of the peak lateral shear load magnitudes on the lumbar spine endplates as a function of intact spine versus TDR spine and external loading condition (bending while unloaded 0 kg, lifting 9.5 kg, lifting 19 kg)

Fig. 6

Average of the peak facet contact forces as a function of intact spine versus TDR spine and external loading condition (bending while unloaded 0 kg, lifting 9.5 kg, lifting 19 kg)

Fig. 7

Average of the peak ligament tensions at the L5/S1 level as a function of intact spine versus TDR spine and external loading condition (bending while unloaded 0 kg, lifting 9.5 kg, lifting 19 kg)

Fig. 8

Mean sagittal plane range of motion (ROM) at each lumbar level as a function of intact spine versus TDR spine and external loading condition (bending while unloaded 0 kg, lifting 9.5 kg, lifting 19 kg)

Fig. 9

Mean lateral plane range of motion (ROM) at each lumbar level as a function of intact spine versus TDR spine and external loading condition (bending while unloaded 0 kg, lifting 9.5 kg, lifting 19 kg)

Fig. 10

Mean twisting (transverse) plane range of motion (ROM) at each lumbar level as a function of intact spine versus TDR spine and external loading condition (bending while unloaded 0 kg, lifting 9.5 kg, lifting 19 kg)