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.

Materials and Their Failure Mechanisms in Total Disc Replacement

“…It is not merely a joint which connects two moving parts. The intervertebral disc, especially in the lumbar region, serves to resist motion and reduce stress concentrations in the adjacent level tissue [4,9].…”

“…Ball and socket bearings to not completely replicate the functions of the natural IVD. These bearings tend to be axially rigid and are not designed to resist bending or rotational moments, allowing motion to occur in excess of that permitted by the natural disc [9,80]. These qualities can lead to changes in range of motion (ROM), segmental lordosis, or facet stressing [3,15,26,[81][82][83].…”

“…This treatment method alleviates pain, but limits range of motion (ROM) and mobility for the patient. Total disc replacement (TDR) is another treatment method for DDD that aims to preserve motion and limit complications related to spinal fusion, such as adjacent level wear and disc degeneration [8][9][10][11][12]. This motion preservation is critical to preventing wear and injury in the area surrounding the treated disc.…”

“…This motion preservation is critical to preventing wear and injury in the area surrounding the treated disc. Through the inhibition of motion fusion creates stress concentrations at adjacent level facets and discs [9,[12][13][14]. TDR is an effort to solve these issues and help maintain normal range or motion for patients [11].…”

We Need to Talk about Lumbar Total Disc Replacement

“…654 In cases of narrowed and functionally incompetent intervertebral discs, less body weight is supported by the disc and forces are consequentially transmitted across the facet joints, 654 reflected in the observation that up to 70% of an axial load can be borne by the facet joints in cases of severe LDDD (thus predisposing to facet arthrosis). 655 Biomechanical studies have suggested that LTDR using ball-and-socket-type LADs (such as the ProDisc-L) results in increased facet loading at the surgical level, 624 especially following L4-L5 and L5-S1 LAD implantation, 590 and these observed increases in facet loading are more evident during lateral bending and axial rotation; and that posterior placement of the prosthesis provides a more physiologic load transfer to the vertebral body.…”