Introduction The unacceptably low fusion rate with stand-alone ALIF cages led to the practice of combining ALIF with posterior instrumentation. Recently ALIF combined with anterior lumbar plate fixation has been promoted to obviate the need for additional posterior surgery. The purpose of this study is to compare the multidirectional flexibility of ALIF combined with posterior instrumentation (either translaminar facet screws or pedicle screws) to that combined with anterior plate fixation.

Methods Ten human lumbar cadaveric motion segments were tested in the following sequence: (i) intact, (ii) stand alone ALIF cages, (iii) ALIF and anterior lumbar plate, (iv) ALIF with translaminar facet screws, (v) ALIF with pedicle screws. In each condition, the specimens were tested under bending moments of 0–8 Nm flexion, 0–6 Nm extension, 0–6 Nm lateral bending and 0–5 Nm axial rotation. 3D motions were measured using an optoelectronic motion monitoring system.

Results The ALIF cages decreased the ROM in the sagittal and frontal planes (p< 0.05). Their effectiveness improved in the sagittal plane with the combination of either anterior plate or posterior fixation (p< 0.05). There was no statistical difference between the ALIF/ Plate and translaminar screws or pedicle screws in flexion-extension and axial rotation ROM. There was a difference in the lateral bending ROM between the ALIF with anterior plate and ALIF with either translaminar facet screws or ALIF with pedicle screws (p< 0.05) favouring posterior fixation. However there was no statistical difference between the combined ROM for ALIF/Plate and translaminar screws or pedicle screws.

Discussion This study shows that the stability achieved with the combination of ALIF with an anterior plate is comparable to that achieved with posterior instrumentation with translaminar facet screws or pedicle screws. This suggests that sufficient segmental stability may be provided by anterior plating, obviating the need for a concomitant posterior approach.

Introduction This in-vitro biomechanical study was undertaken to compare the multi-directional flexibility kinematics of single versus multi-level lumbar Charité reconstructions and determine the optimal biomechanical method for surgical revision – posterior instrumentation alone or circumferential spinal arthrodesis.

Methods A total of seven human cadaveric lumbosacral spines (L1 to Sacrum) were utilized in this investigation and biomechanically evaluated under the following L4-L5 reconstruction conditions: 1) Intact Spine; 2) Diskectomy Alone, 3) Charité, 4) Charité + Pedicle Screws, 5) Two Level Charité (L4-S1), 6) Two Level Charité + Pedicle Screws (L4-S1), 7) Charité L4-L5 with Pedicle Screws and Femoral Ring Allograft (L5-S1) and 8) Pedicle Screws and Femoral Ring Allograft (L4-S1). Multi-directional flexibility testing utilized 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 to pure moment testing strategies. Unconstrained intact moments of ±7Nm were used for axial rotation, flexion-extension and lateral bending testing, with quantification of the operative and adjacent level range of motion (ROM) and neutral zone (NZ). All data was normalized to the intact spine condition.

Results In axial rotation, single and two level Charité reconstructions produced significantly more motion than pedicle screw constructs combined with the Charité or femoral ring allograft (p< 0.05). There were no differences between the Charité augmented with pedicle screws or pedicle screws with femoral ring allograft (p> 0.05). Similar trends were observed under flexion-extension and lateral bending conditions with the Charité reconstructions demonstrating no significant differences compared to the intact spine (p> 0.05). However, the Charité combined with pedicle screws or pedicle screws with femoral ring allograft significantly reduced motion at the operative level compared to the Charité reconstruction (p< 0.05). The most pronounced changes in adjacent level kinematics were observed at the inferior level. The addition of pedicle screw fixation, in all cases, increased segmental motion at the inferior adjacent level (L5-S1) compared to the intact and Charité reconstruction groups (p< 0.05).

Discussion Single and two level total disc arthroplasty using the Charité device preserved segmental motion at the operative and adjacent levels compared to pedicle screw stabilization constructs. In terms of revision strategies, posterior pedicle screw reconstruction combined with an existing Charité is not statistically different from pedicle screws combined with femoral ring allograft. As we enter an era of total disc replacement and the impending necessity for surgical revision, the current study provides a biomechanical basis for posterior re-stabilization alone in lieu of combined anteroposterior revision.

Introduction: The artificial disc consists of proprietary polyolefin rubber core bonded between two titanium endplates. It has been developed for the treatment of symptomatic disc degeneration with the aim of providing segmental stability and motion following wide disc space clearance. It was designed to have similar properties to a normal adult human intervertebral disc when working in conjunction with the retained anulo-vertebral tissues and the supporting musculoligamentous system.

Methods: Over 120 discs were used to biomechanically characterize the Device. Range of motion tests were designed and performed to measure the axial compression, torsional, and shear stiffness of the artificial disc and to compare this with the known values for the human lumbar disc. Pullout test was performed to evaluate the immediate and short-term stability of the inserted device by assessing the mechanical resistance to pullout or expulsion. To assess the ability of the implant to withstand average daily living loads throughout its predicted life, compression and compressive shear fatigue testing were performed.

Discussion: The device was found to replicate many of the physiologic characteristics of the in-vivo FSU. The quasi-static testing showed the device to have higher strength values than the highest in-vivo loads and displacements. Fatigue testing showed the smallest device endurance limit of 3,500N at ten million cycles.

The results demonstrate that the failure modes of the device contain sufficient safety margins to support the use of the device in a prospective clinical study.