An Injectable Nucleus Pulposus Implant to Restore Spinal Range of Motion in Compression +Malhotra, NR2; Han, W1; Beckstein, J1; Cloyd, J1; Chen, W3; Elliott DM1;

+1Department of Orthopaedic Surgery, 2Department of Neurosurgery, University of Pennsylvania, Philadelphia, PA, 3Department of Engineering, State University of New York, Stony Brook, NY

neil.malhotra@uphs.upenn.edu INTRODUCTION Surgical treatment for low back pain commonly includes discectomy, decompression, and spinal fusion. While surgical intervention is effective for some patients, treated discs undergo altered biomechanics and adjacent levels are at increased risk for accelerated degeneration [1,2]. One potential treatment as an adjunct or alternative to surgery for degenerative disc disease includes the percutaneous delivery of agents to support nucleus pulposus (NP) functional mechanics. These implants are delivered in a minimally invasive fashion, potentially on an outpatient basis, and do not preclude later surgical options. Replacement of native NP with an implant, in the setting of healthy annulus fibrosus (AF), may simultaneously reduce pain, slow progression of degeneration and restore non-degenerate spinal mobility. One of the challenges in designing such implants include the need to match key NP mechanical behavior and mimic the role of native non-degenerate NP in spinal motion. The goal of this study was to investigate the impact of a recently developed injectable NP implant [3] on disc mechanics before and after discectomy in an ovine model. METHODS Implant material: The oxidized hyaluronic acid (oHA)-gelatin implant material was prepared as follows. One gram of hyaluronan (Mw 1.5×106 Engelhard, Inc.) was dissolved in 80 ml of water in a shaded flask and sodium periodate, dissolved in 20 ml water (pH 5.4), was added dropwise. At ambient temperature, 10 ml of ethylene glycol was added to stop the reaction followed by stirring for one hour. The product was dialyzed for 3 days and lyophilized to obtain oxidized hyaluronic acid oHA (yield: 50-67%). Gelatin (Bloom 300, Type A, Mw 100,000) solution of 20% (w/v) concentration (in pH 9.4, 0.1 M borax) was prepared. oHA and gelatin solutions were mixed (weight: 7:3) and stirred for 1 min at 37oC. Mechanical testing: Repeated testing following three stages (intact, discectomy, experimental) was evaluated for range of motion (ROM), compressive stiffness, and tensile stiffness. The first stage, intact control, consisted of a cyclic tension-compression protocol performed on intact bone-disc-bone segments (n = 14). Stage 2, discectomy, consisted of the same mechanical testing protocol as stage 1 but all discs had undergone discectomy (n=14). For Stage 3, experimental, all discs were subjected to the same mechanical testing protocol as stage 1 and stage 2, however, discs were randomized to treatment by injection with the hydrogel implant material using 21-gauge syringe (n = 10) or untreated controls (n = 4). Data Analysis: A single factor repeated measures ANOVA with post-hoc Bonferoni test was used to compare effect of discectomy and treatment on ROM, compression stiffness, and tensile stiffness for treated and untreated groups, with significance set at p<0.05. In addition, tested motion segments were cut axially for qualitative structural observations. RESULTS No NP material or hydrogel was ejected from the NP cavity during injection or during mechanical testing and the material remained within the NP cavity (Fig 1). Discectomy increased the compressive range of motion 22% (p<0.05, Fig 2). In the treated group, hydrogel implantation following discectomy reduced ROM 17% (p < 0.05) to the intact values (pre-discectomy 0.71mm, post-implantation 0.72mm; Fig 2). In contrast, the ROM in untreated discs remained 36.7% higher than the intact (p<0.05). The compressive and tensile stiffness decreased following and unchanged for both the treated and untreated groups (Table 1).

DISCUSSION Restoration of ROM is a key parameter to treatment strategies designed for disc disease [4,5]. Since much of disc degeneration begins in the NP, subsequently altering ROM, a critical goal of implant development is the ability to restore ROM. A significant increase in ROM resulting from discectomy supports the notion that ROM depends on the quantity of functional NP [6]. ROM, which increased with discectomy, returned to normal after treatment with the NP implant. Baseline values of ROM were achieved after delivery of implant while motion segments without implant remained 33% greater than intact baseline values (Fig 2). Return to normal ROM following an NP implant demonstrates that an injectable implant can serve as a proxy for healthy NP in this crucial mechanical role. Discectomy, and the associated annulotomy, degrades disc mechanics. The mechanical and structural changes that occur with discectomy are similar to changes that occur in early degeneration via decreased pressure, increased deformation and flexibility, and increased annulus bulging [4,6]. In this study, compressive and tensile response of the disc, heavily reliant on AF function, declined after annulotomy and this decline was not improved after implant injection (Table 1). Decline in traits related to AF function would be expected in this model because the annular opening created during discectomy was not treated (Fig 1). A perceived limitation of this study is that it was performed in a non-human in vitro model; however, this method permits collection of baseline mechanical data in a proof of concept approach in the animal model as intended for future in vivo testing. An additional perceived limitation is that by injuring the annulus but not treating it subjects the implant to increased strains. However, this suggests the ability of the implant to reinstitute baseline ROM under a variety of conditions (i.e. without annular injury). The conclusion of this work is that an injectable implant will return ROM, a crucial parameter of NP mechanical function, to baseline. The immediate impact of this study supports the approach of stepwise development of injectable implants from ex vivo to in vitro work. ACKNOWLEDGEMENTS Work was funded by the NIH & NREF. REFERENCES [1] Min+J Spinal Disord Tech 2008 [2] Yang+Spine 2008 [3] Cloyd+Eur Spine J 2007 [4] Seroussi+J Orthop Res 1989 [5] Johannessen+Ann Biomed Eng 2006 [6] Wilke+Eur Spine J 2006

Treated Untreated Intact Disc-

ectomy Expt. Intact Disc-

ectomy Expt.

Compr. 1561 (384)

1344 (182)

1220 (142)*

1658 (256)

1481 (151)

1390 (396)

Tension 718 (131)

555 (85)*

539 (95)*

811 (91)

682 (51)*

626 (83)*

Fig. 1. Representative images of axially sectioned discs. Left: Gross morphology of a healthy, intact ovine disc. Right: Gross morphology of an ovine disc treated with discectomy, followed by NP implant injection.

Fig. 2. Discectomy alone significantly increased ROM for both groups. ROM following NP implant treatment significantly decreased, back to its intact state. * p<0.05.

Table 1. Mean (Std Dev) stiffness in Compression and Tension, N/mm. * p<0.05 compared to Intact.

Poster No. 759 • ORS 2011 Annual Meeting