Revision strategies for single- and two-level total disc arthroplasty procedures: a biomechanical perspective.

BACKGROUND CONTEXT

The utilization of motion-preserving implants versus conventional instrumentation systems, which stabilize the operative segments, necessitates improved understanding of their comparative biomechanical properties and optimal biomechanical method for surgical revision.

PURPOSE

Using an in vitro human cadaveric model, the primary objective was to compare the multidirectional flexibility properties of single- versus two-level total disc arthroplasty procedures and determine the acute in vitro biomechanical characteristics of two methods of surgical revision-posterior transpedicular instrumentation alone or circumferential spinal arthrodesis.

STUDY DESIGN

This in vitro biomechanical study was undertaken to compare the multidirectional flexibility kinematics of single- versus two-level lumbar total disc arthroplasty reconstructions using an in vitro model.

METHODS

A total of seven human cadaveric lumbosacral spines (L1-sacrum) were biomechanically evaluated under the following L4-L5 reconstruction conditions: intact spine; discectomy alone; Charité total disc replacement; Charité with pedicle screws; two-level Charité (L4-S1); two-level Charité with pedicle screws (L4-S1); Charité L4-L5 with pedicle screws and femoral ring allograft (FRA) (L5-S1); and pedicle screws with FRA (L4-S1). Multidirectional flexibility testing used the Panjabi Hybrid Testing protocol, which includes pure moments for the intact condition with the overall spinal motion replicated under displacement control for subsequent reconstructions. Hence, changes in adjacent level kinematics can be obtained compared with pure moment testing strategies. Unconstrained intact moments of +/-7.5Nm were used for axial rotation, flexion-extension, and lateral bending testing with quantification of the operative- and adjacent-level range of motion (ROM). All data were normalized to the intact spine condition (intact=100%).

RESULTS

In axial rotation, single- and two-level Charité reconstructions produced significantly more motion than pedicle screw constructs combined with the Charité or FRA (p<.05). There were no differences between the Charité augmented with pedicle screws or pedicle screws with FRA (p>.05). The two-level annulus lumbar resection required for multilevel Charité implantation had an added destabilizing effect, resulting in a 140% to 160% ROM increase over the intact condition. Under two-level reconstructions, rotational motion at the L4-L5 level increased from 160+/-26% to 263+/-65% with the implantation of the second Charité at L5-S1. Flexion-extension and lateral bending conditions with the Charité reconstructions in this group of seven spines demonstrated no significant differences compared with the intact spine (p>.05). The Charité combined with pedicle screws or pedicle screws with FRA significantly reduced motion at the operative level compared with the Charité reconstruction (p<.05). The most pronounced changes in adjacent level kinematics and intradiscal pressures were observed under flexion-extension loading. The addition of pedicle screw fixation increased segmental motion and intradiscal pressures at the proximal and distal adjacent levels compared with the intact and Charité reconstruction groups (p<.05).

CONCLUSIONS

The findings highlight a variety of important trends at the operative and adjacent levels. In terms of revision strategies, posterior pedicle screw reconstruction combined with an existing Charité was not found acutely to be statistically different from pedicle screws combined with FRA.