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An Injectable Nucleus Pulposus Implant Restores Compressive Range of Motion in the Ovine Disc

Malhotra, Neil R., MD*,†; Han, Woojin M., MSE†,‡; Beckstein, Jesse, MS; Cloyd, Jordan, MD; Chen, Weiliam, PhD§; Elliott, Dawn M., PhD†,‡

doi: 10.1097/BRS.0b013e31825cdfb7
Basic Science

Study Design. Investigation of injectable nucleus pulposus (NP) implant.

Objective. To assess the ability of a recently developed injectable hydrogel implant to restore nondegenerative disc mechanics through support of NP functional mechanics.

Summary of Background Data. Although surgical intervention for low back pain is effective for some patients, treated discs undergo altered biomechanics and adjacent levels are at increased risk for accelerated degeneration. One potential treatment as an alternative to surgery for degenerated disc includes the percutaneous delivery of agents to support NP functional mechanics. The implants are delivered in a minimally invasive fashion, potentially on an outpatient basis, and do not preclude later surgical options. One of the challenges in designing such implants includes the need to match key NP mechanical behavior and mimic the role of native nondegenerate NP in spinal motion.

Methods. The oxidized hyaluronic acid gelatin implant material was prepared. In vitro mechanical testing was performed in mature ovine bone-disc-bone units in 3 stages: intact, discectomy, and implantation versus sham. Tested samples were cut axially for qualitative structural observations.

Results. Discectomy increased axial range of motion (ROM) significantly compared with intact. Hydrogel implantation reduced ROM 17% (P < 0.05) compared with discectomy and returned ROM to intact levels (ROM intact 0.71 mm, discectomy 0.87 mm, postimplantation 0.72 mm). Although ROM for the hydrogel implant group was statistically unchanged compared with the intact disc, ROM for sham discs, which received a discectomy and no implant, was significantly increased compared with intact. The compression and tension stiffness were decreased with discectomy and remained unchanged for both implant and sham groups as expected because the annulus fibrosus was not repaired. Gross morphology images confirmed no ejection of NP implant.

Conclusion. An injectable implant that mimics nondegenerate NP has the potential to return motion segment ROM to normal subsequent to injury.

This study investigated an oxidized hyaluronic acid gelatin implant material as an injectable nucleus pulposus implant. Discectomy significantly increased range of motion (ROM). Hydrogel implant injection returned ROM to intact level. An injectable implant that mimics nondegenerate nucleus pulposus has the potential to return disc ROM to normal subsequent to injury.

*Neurological Surgery

Orthopaedic Surgery

Bioengineering, University of Pennsylvania, Philadelphia PA

§Department of Surgery, New York University, New York, NY.

Address correspondence and reprint requests to Neil R. Malhotra MD, Department of Neurological Surgery, University of Pennsylvania, Penn Neurological Institute, Washington Square West Bldg, 235 S. Eighth St., Philadelphia, PA 19106; E-mail:

Acknowledgment date: August 16, 2011. First revision date: January 14, 2012. Second revision date: March 2, 2012. Acceptance date: April 20, 2012.

The manuscript submitted does not contain information about medical device(s)/drug(s).

Neurosurgery Research and Education Foundation funds were received to support this work.

No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

© 2012 Lippincott Williams & Wilkins, Inc.