Is a collagen scaffold for a tissue engineered nucleus replacement capable of restoring disc height and stability in an animal model?

Abstract

The idea of a tissue engineered nucleus implant is to seed cells in a three-dimensional collagen matrix. This matrix may serve as a scaffold for a tissue engineered nucleus implant. The aim of this study was to investigate whether implantation of the collagen matrix into a spinal segment after nucleotomy is able to restore disc height and flexibility. The implant basically consists of condensed collagen type-I matrix. For clinical use, this matrix will be used for reinforcing and supporting the culturing of nucleus cells. In experiments, matrixes were concentrated with barium sulfate for X-ray purposes and cell seeding was disclaimed in order to evaluate the biomechanical performance of the collagen material. Six bovine lumbar functional spinal units, aging between 5 and 6 months, were used for the biomechanical in-vitro test. In each specimen, an oblique incision was performed, the nucleus was removed and replaced by a collagen-type-I matrix. Specimens were mounted in a custom-built spine tester, and subsequently exposed to pure moments of 7.5 Nm to move within the three anatomical planes. Each tested stage (intact, nucleotomy and implanted) was evaluated for range of motion, neutral zone and change in disc height. Removal of the nucleus significantly reduced disc height by 0.84 mm in respect to the intact stage and caused an instability in the segment. Through the implantation of the tissue engineered nucleus it was possible to restore this height and stability loss, and even to increase slightly the disc height of 0.07 mm compared with the intact stage. There was no statistical difference between the stability provided by the implant and intact stage. Results of movements in lateral bending and axial rotation showed the same trend compared to flexion/extension. However, implant extrusions have been observed in three of six cases during the flexibility assessment. The results of this study directly reflect the efficacy of vital nucleus replacement to restore disc height and to provide stability to intervertebral discs. However, from a biomechanical point of view, the challenge is to employ an appropriate annulus fibrosus sealing method, which is capable to keep the nucleus implant in place over a long-time period. Securing the nucleus implant inside the disc is one of the most important biomechanical prerequisites if such a tissue engineered implant shall have a chance for clinical application.

Fig. 1

Condensed collagen matrix concentrated with barium sulfate for radiographic observations. The matrix has been used as a nucleus replacement in these experiments (left). This collagen matrix may serve as a three-dimensional scaffold to allowing cell seeding inside (right)

Fig. 2

Specimens depicted at the stage of intact, without nucleus and after implantations. Nucleotomy was performed with Rongeurs from a right lateral approach. Implantation of the nucleus replacement was achieved using a guidance tube for the implantation

Fig. 3

Radiographs which show disc height (h) in the intact state (h=0), after nucleotomy, and after implantation

Fig. 4

Quantification of the implanted volume of the collagen nucleus replacement. Cranial caudal radiograph (left) and the reconstructed volume obtained from a microCT scan (right)

Fig. 5

Range of motion (ROM) and neutral zone (NZ) in degrees of the six specimens. Specimens were tested intact, after nucleotomy and after implantation of the nucleus replacement. NS Not significantly different; * significantly different with P<0.05; + statistical tendencies of a difference with 0.05<P<0.1

Fig. 6

Sequence of images from implant extrusion during the flexibility tests in the spine tester. Extrusion mostly occurred in the direction of lateral bending