Elsevier

The Spine Journal

Volume 10, Issue 11, November 2010, Pages 987-993
The Spine Journal

Technical Report
Biomechanical evaluation of an expandable meshed bag augmented with pedicle or facet screws for percutaneous lumbar interbody fusion

https://doi.org/10.1016/j.spinee.2010.08.016Get rights and content

Abstract

Objective

To evaluate the biomechanics of lumbar motion segments instrumented with stand-alone OptiMesh system augmented with posterior fixation using facet or pedicle screws and the efficacy of discectomy and disc distraction.

Background context

OptiMesh bone graft containment system has been used for vertebral compression fractures and percutaneous lumbar interbody fusion. The filled mesh bag serves as the interbody device providing structural support to the motion segment being fused. No biomechanical data of this new device are available in the literature.

Methods

Twenty-four fresh human cadaveric lumbar motion segments were divided into two groups. In the control group, multidirectional flexibility testing was conducted after an intact condition and standard transforaminal lumbar interbody fusion (TLIF) procedure. In the OptiMesh group, testing was performed following intact, stand-alone OptiMesh procedure, OptiMesh with facet screws (placed using the transfacet approach), and OptiMesh with pedicle screws and rods. Range of motion (ROM) was calculated for each surgical treatment. The lordosis and disc height change of intact and instrumented specimens were measured in the lateral radiographs to evaluate the disc space distraction. In the OptiMesh group, cyclic loading in flexion extension (FE) was applied to measure cage subsidence or collapse (10,000 cycles at 6 Nm). After biomechanical testing, all the specimens were dissected to inspect the discectomy and end plate preparation. The area of discectomy was measured.

Results

The mean ROM of the intact specimens was 2.7°, 7.4°, and 7.2° in axial torsion (AT), lateral bending (LB), and FE, respectively. There was no difference between the control group and OptiMesh group. The mean ROM of the stand-alone OptiMesh system decreased to 2.4°, 5.1°, and 4.3° in AT, LB, and FE. The ROM decreased to 0.9° in AT, 2.2° in LB, and 0.9° in FE with OptiMesh system and facet screws. On average, OptiMesh system with pedicle screws and rods reduced the ROM to 1.3° in AT, 1.6° in LB, and 1.1° in FE. Compared with the intact condition and stand-alone OptiMesh system, both posterior fixation options had significant statistical difference (p<.001). In AT, ROM of facet screws was lower than that of pedicle screws (p<.05). There was no statistical difference between the facet and pedicle screws in LB and FE (p>.05). The mean volume of bone graft packed into each bag was 8.3±1.5 cc. The average increase of lordosis was 0.6°±1.0° after meshed bag was deployed. The average distraction achieved by the OptiMesh system was 1.0±0.6 mm. The average prepared area of discectomy was 42% of the total disc. The disc height change after cyclic loading was 0.2 mm. No subsidence or collapse was noticed.

Conclusions

The OptiMesh system offers large volume of bone graft in the disc space with small access portals. The OptiMesh system had similar construct stability to that of standard TLIF procedure when posterior fixation was applied. However, the amount of distraction was limited without additional distraction tools. With the anterior support provided by the expandable meshed bag, facet screws had comparable construct stability to that of pedicle screws. Slightly higher stability was observed in facet screws in AT.

Introduction

Posterior lumbar interbody fusion and transforaminal lumbar interbody fusion (TLIF) have been widely used for the treatment of degenerative disc disease. Interbody implants such as structural bone grafts and interbody spacers may be biomechanically beneficial because of their proximity to the instantaneous axis of rotation, where they are loaded mainly in axial compression [1]. But either structural bone grafts or interbody cages need to be inserted into the disc space through a standard open approach or a minimally invasive tube that is much larger than the implant. The open posterior approach requires extensive dissection and retraction of paraspinous muscles and nerve roots. There may be complications associated with a greater blood loss, nerve injury, and persistent sequelae, such as muscular denervation, atrophy, and pain [2], [3]. To minimize the approach-related morbidities, numerous minimally invasive techniques for the lumbar spine have been developed and have become more popular. Minimally invasive techniques significantly reduce blood loss, postoperative pain, hospital stay, and narcotic usage [4], [5], [6], [7], [8], [9], [10].

Recently, a new device, referred to as the OptiMesh deployable grafting system (Spineology Inc., Saint Paul, MN, USA), has been developed to perform percutaneous lumbar interbody fusion using an expandable meshed bag filled with compacted granular bone graft. The expandable meshed bag is shown in Fig. 1. Compared with other minimally invasive techniques, the expandable mesh system requires smaller working channel and yet provides greater final graft size. It has been previously used in the treatment of vertebral compression fractures as an alternative to vertebroplasty or kyphoplasty [11], [12], [13]. For lumbar interbody fusion, the meshed bag serves as the interbody device providing structural support to the motion segment being fused. Limited clinical trials and early clinical results have shown that this technique is safe and effective [14]. However, no biomechanical data are available for a spinal segment treated with this new device. The objective of the present study was to evaluate the biomechanics of lumbar motion segments instrumented with stand-alone expandable meshed bag or meshed bag with posterior fixation using facet or pedicle screws and the efficacy of discectomy and disc distraction using the OptiMesh system. The spinal stability of the various surgical treatments and cage settling inside the disc space were measured.

Section snippets

Materials and methods

Twenty-four fresh-frozen human lumbar motion segments were harvested from nine human donors (five men and four women) with the age ranging from 54 to 70 years (average, 64±6.2 years). Specimens were dissected leaving all ligamentous structures intact. All specimens were screened via fluoroscopy to rule out any major anatomical abnormality (eg, fracture, pars defects, deformity, dysplasia, excessive osteophytes around the annulus, or congenital anomaly). Flexion extension (FE) moment was applied

Results

There was no bony defect or deformity in the specimen. The mean bone mineral density of the segments was 1.14±0.15 (standard deviation) g/cm2, which suggested moderate bone quality compared with the bone mineral density scores of young American adults. These results were common for patients older than 50 years.

The average ROM in AT, LB, and FE of the intact specimens and subsequent treatments are presented in Fig. 3. In the OptiMesh group, the mean ROM of all the intact specimens was 2.9°±1.8°,

Discussion

The OptiMesh bag knitted with polyethylene terephthalate thread is a hollow, seamless, deployable mesh container. After the bag is filled with granular bone graft material through a small access portal, further compaction of the graft can cause the granular material to change its phase from fluidic paste to rigid solid. The graft pack can serve as a structural support and withstand physiological compression loads without being squeezed back out through the access portal [13]. The access portal

Conclusions

In conclusion, stand-alone OptiMesh cannot provide adequate stability. Supplementary fixations including facet and pedicle screws are required to achieve higher construct stability for successful fusion. With anterior support by the expandable meshed bag, facet screws placed using the transfacet approach had comparable construct stability to that of pedicle screws. The distraction of the expandable meshed bag was limited. No noticeable meshed bag subsidence or collapse was measured after cyclic

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    FDA device/drug status: not applicable.

    Author disclosures: AAM (stock ownership, including options and warrants, Stryker, Medtronic; consulting, Stryker; research support: staff/materials, Medtronic; fellowship support, Medtronic, Synthes); EET (royalties, Medtronic; consulting, Medtronic; speaking/teaching arrangements, Stryker, Medtronic; scientific advisory board, United Health Care; fellowship support, Medtronic, Synthes Spine, Zimmer Spine).

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