Elsevier

The Spine Journal

Volume 4, Issue 3, May–June 2004, Pages 265-274
The Spine Journal

Clinical Studies
Stiffness of prosthetic nucleus determines stiffness of reconstructed lumbar calf disc

https://doi.org/10.1016/j.spinee.2003.11.003Get rights and content

Abstract

Background context

Currently, artificial spinal discs require transection or partial removal of the annulus fibrosis in order to excise the nucleus and implant a prosthetic nucleus or implant a total disc device, respectively. Preservation of the annulus for prosthetic disc replacement maintains the function of the annulus and may improve annulus load sharing with the prosthesis.

Purpose

To quantify the biomechanical characteristics of an annular sparing intervertebral prosthetic disc (IPD) in a lumbar calf spine model. The aim of the study was to determine whether altering the stiffness of the elastic component of this unique prosthesis would correspond to changes of the overall reconstructed disc.

Study design/setting

A biomechanical study was conducted in vitro using cadaveric calf spines such that each specimen served as its own control. Investigations were performed at the Minneapolis Medical Research Foundation, Orthopaedic Biomechanics Laboratory.

Methods

Six L45 or L56 motion segments (from which the posterior elements had been removed) were studied in axial compression, sagittal and lateral bending and torsion. These load states were applied to the intact, denucleated and prosthetically reconstructed disc using four IPDs of differing stiffness.

Results

Load-displacement testing demonstrated that stiffer IPDs resulted in a decreased range of motion and neutral zone, and greater stiffness of the reconstructed disc. Disc reconstruction with the stiffest IPD approximated the behavior of the intact disc.

Conclusions

The overall biomechanical characteristics of a reconstructed disc are related to the stiffness of a nucleus prosthesis. The similarities in the mechanical behavior of reconstructed and intact discs suggest that additional feasibility studies for the annulus-sparing IPD are warranted.

Introduction

The current treatment for end-stage lumbar degenerative disc disease causing severe, incapacitating low back pain is often a spinal fusion procedure. Because spinal fusion may result in decreased range of motion and flexibility of the lumbar spine and might aggravate degeneration of the adjacent remaining segments, interest in spine arthroplasty has increased [1].

Two major categories of spine arthroplasty currently exist. One category involves replacement of the nucleus. These replacement devices require the creation of a small annulotomy through which the nucleus is removed and the prosthesis implanted. The prosthesis typically is manufactured from an elastic material, such as a polyvinyl alcohol hydrogel (PDN, RayMedica, Bloomington, MN; Aquarelle, Stryker Spine, Allendale, NJ; Aquacryl, Replication Medical, New Brunswick, NJ) or polyurethane (Disc Dynamics, Minneapolis, MN; NewCleus, Spinetech, Minneapolis, MN) [2], [3], [4]. With these devices, spinal motion is obtained either by deformation of the implant (similar to motion through a normal disc) or by rocking of the vertebra over the implant, in which motion is constrained by the annulus and facet joints or capsules. Some of these nucleus implants also restore disc height.

The second major category of spine arthroplasty is “total” disc replacement. Such a replacement device typically is placed after removal of at least the anterior one-third of the annulus and excision of the nucleus. The disc space then is filled with a mechanical device. Most of these devices allow spinal motion by means of an articulating joint that is constrained by the remaining annulus, surrounding soft tissues and facet joints (SB Charité, Link, Hamburg, Germany; Pro-Disc, Aesculap, Germany; Flexicore, Spinecore, Summit, NJ; and Maverick, Medtronic/Sofamor/Danek, Memphis, TN) [5], [6]. Other total disc replacements are designed to allow spinal motion through deformation of polymeric material between rigid end plates (Acroflex, Johnson and Johnson/Depuy/Acromed, Raynham, MA, and C-Flex, Concept Polymer Technologies, Clearwater, FL) [7], [8], [9]. These latter devices also allow axial compression similar to that which occurs in the normal, intact disc. All of the currently proposed total disc replacements have metallic end plates with porous ingrowth surfaces for fixation to the bony vertebral end plates. Common to both nucleus prostheses and total disc replacements is the need to alter (i.e., excise or transect) the annulus fibrosus of the disc being treated. Consequently, the biomechanical function of the annulus, which requires intact circumferential annular fibers, cannot be restored.

In our study, we evaluated biomechanically a modular intervertebral prosthetic disc (IPD; Dynamic Spine, Mahtomedi, MN) that is completely annulus sparing and has unique (intravertebral) fixation to the vertebral bodies. Implantation of the IPD is by means of one or two cavities made in the vertebra(e) adjacent to the indicated disc through which the central vertebral end plate and nucleus are excised—hence the concept of an annulus-sparing prosthesis. The IPD comprises an elastic component, which is placed in the excised nuclear space, and one or two fixation components, which may be elongated to apply a force, or preload, on the elastic component, resulting in restoration of disc height and providing immediate fixation of the device to the vertebra (Fig. 1). Clinically, the bone removed during creation of the vertebral cavities is replaced, because the fixation components occupy only a small portion of the cavity. This bone is expected to heal rigidly back to the vertebral body bone and to the porous ingrowth surface of the fixation components, thereby giving long-term stability to the IPD.

Conceptually, the elastic component, which in this study consisted of multiple metallic springs, is designed to occupy only a small portion of the excised nuclear space. This allows fibrous tissue to grow between the spring coils. Thus, when an axial load is applied to the motion segment, the fibrous tissue will bulge outward as it is compressed. It is predicted that the outward bulge of the fibrous tissue will apply a radial force on the inner surface of the spared annulus. The expected outcome is that the annulus will experience circumferential stress, as does the normal intact annulus, instead of axial shear stresses, as are seen in a degenerated disc or in competing disc prostheses.

The purpose of this study was to determine whether altering the stiffness of the elastic component of this unique prosthetic device would result in corresponding changes of the overall reconstructed disc. Biomechanical properties of the isolated and implanted IPD were measured in response to applied external loads. These measured properties were compared with those of the intact disc and of the isolated annulus of a denucleated disc. The specific properties measured in response to altering the spring stiffness of the elastic component were axial displacement under compressive load, disc height restoration, range of motion and neutral zone in multiple planes and initial stiffness in multiple planes.

Section snippets

Methods

The IPD has an elastic component that is connected to one or two intravertebral body fixation components (see Fig. 1). The two-fixation-component design was used in this study to allow subsequent monitoring of intrinsic device loading by means of an intravertebral body load transducer; these data will be discussed in an upcoming report [10]. The elastic component is itself modular and consists of four cobalt chrome springs connected to metallic plates. Four different spring types were tested in

Results

The change in disc height from the intact specimen to the reconstructed IPD specimen was an increase of 1.5±1.2 mm (mean±SD). The disc height of the denucleated specimen after removal of the IPD was 2.6±1.2 mm less than that of the reconstructed disc and 1.1±0.8 mm less than that of the intact disc.

The axial compression load-displacement curve for discs reconstructed with varying implant stiffness is demonstrated in Fig. 4. At higher loads, a plateau was seen at approximately 2.5 to 3.0 mm of

Discussion

In this study, we used a modular, annulus-sparing nucleus prosthesis (IPD) to reconstruct the lumbar calf spine disc. Calf spines were used because they simulate the human lumbar spine in both anatomical size and biomechanical properties [15]. The purpose of this study was to assess the possible influence of the mechanical properties of the IPD on the load–displacement relationship of the reconstructed calf disc. Initial loading studies tested the IPD alone using four types of springs of

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    FDA device/drug status: investigational/not approved: Prosthetic nucleus.

    Author GRB acknowledges a financial relationship (grant research support from and board member for Dynamic Spine LLC) that may indirectly relate to the subject of this research.

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