Letter to the EditorMechanical Testing of Fixation Techniques for Scaffold-BasedTissue-Engineered Grafts

Sir,I read with great interest the article on mechanical test-

ing of various fixation methods for scaffold-based cartilage

repair. I must commend the authors for their innovative

approach to this question. I do have a number of comments

regarding this study:

1. One of the scaffolds tested was the collagen mem-

brane, Bio-Gide1 (Geistlich Biomaterials, Wolhusen,

Switzerland). Whilst Bio-Gide1 certainly is a collagen

membrane; it is specifically indicated for use in Dental

\Guided Bone Regeneration". In dental and Maxillo-

facial practise it is almost always placed under a soft

tissue flap to prevent the ingrowth of soft tissue into a

healing bony defect, which may or may not be filled

with bone graft. Thus there is no real need for any

great tensile strength. Chondro-Gide1 on the other

hand, also manufactured by Geistlich Biomaterials is

indicated for the repair and regeneration of chondral

defects was not tested in this study. It has a Collagen

I/III bilayer configuration which is cell occlusive on

one side.1,2 Although Chondro-Gide1 is produced by a

proprietary method; it is formulated especially to cope

with the unique demands of the regenerating cartilage

surface and thus has indications completely different to

those of Bio-Gide1.

2. The paper describes the rigid clamping of the experi-

mental materials between two defined rough surfaces,

and later between the jaws of a surgical clip. I believe

this method of fixation may damage the scaffolds,

which could cause premature weakening and thus

detachment. Additionally the interposition of the lower

component of the clamping device between the scaf-

fold and subchondral bone would act to weaken the

friction and form locking forces acting between the

scaffold and defect base.

3. All testing was carried out using a unxial tensile test-

ing apparatus with constant force. This is contrary to

what occurs in vivo where the forces tend to be more

cyclical in nature. The accepted post-operative regime

includes a period of non-weightbearing and CPM.3,4

These activities will yield shear stress forces far below

the estimated 11N quoted in the paper.

As our experience with chondral regeneration increases

and new and potentially unproven products and techniques

flood the market, I believe it is vital that we carefully and

thoroughly investigate all aspects of the various techniques

and for this I applaud the authors. It is however vital that

when testing any technique or product, we bear in mind

how and at what magnitude the forces in the body will be

acting on it and tailor or testing accordingly.

REFERENCES

1. Ehlers EM, Fub M, Rohwedel J, Russlies M, Kuhnel W, Beh-rens P. Development of a biocomposite to fill out articular car-tilage lesions. Light, scanning and transmission electronmicroscopy of sheep chondrocytes cultured on a collagen I/IIIsponge. Ann Anat 1999;181:513–518.

2. Fub M, Ehlers EM, Russlies M, Rohwedel J, Behrens P. Char-acteristics of human chondrocytes, osteoblasts and fibroblastsseeded onto a type I/III collagen sponge under different cultureconditions. Ann Anat 2000;182:303–310.

3. Jones DG, Peterson L. Autologous chondrocyte implantationJBJS-A 2006;88:2502–2520.

4. O’Driscoll SW, Salter RB. The repair of major osteochondraldefects in joint surfaces by neochondrogenesis with autogenousosteoperiosteal grafts stimulated by continuous passive motion.An experimental investigation in the rabbit. Clin Orthop RelatRes 1986;208:131–140.

Sven Kili

Middle Farm, Habberley, Shrewsbury, SY5 0TP,United Kingdom

Correspondence to: S. Kili (e-mail: sven.kili@geistlich.co.uk)Received 29 May 2007; accepted 15 June 2007Published online 1 October 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30935

Response to Dr. Sven Kili

We thank Dr. Kili for taking interest in our article titled

\Fixation Stability of Scaffold-Based Tissue Engineered

Grafts: Mechanical Testing of Fixation Techniques for

Scaffold-Based Tissue Engineered Grafts" by Sven Knecht,

Christoph Erggelet, Michaela Endres, Michael Sittinger,

Christian Kaps, and Edgar Stussi, and we appreciate the

comments on our study. We clarify some of the issues

raised in Dr. Kili’s letter.

Journal of Biomedical Materials Research Part B: Applied Biomaterials, Vol. 84,298–299 (2008)' 2007 Wiley Periodicals, Inc.

298

The aim of our study was not to perform product testing

on various scaffold materials, but to obtain quantitative

measures of the primary fixation strength of commonly

used biomaterials for autologous chondrocyte implantation

(ACI). To assess this, we have chosen a representative col-

lagen membrane, two three-dimensional scaffolds, and a

gel-like matrix.

One major criticism in the letter was that the loads

occurring in the test do not resemble the in vivo situation.

However, as we have explicitly outlined in the discussion

of the work, we are well aware of the limitations of the

testing approach. Regarding the possibility of weakening

during fixation, only the three-dimensional scaffold mate-

rial was slightly affected. Incidentally, such weakening

might well be happening during the implantation procedure.

Moreover, the frictional and form-locking forces acting par-

ticularly on the three-dimensional scaffold materials when

implanted in vivo may increase failure loads. In the sim-

plest case, we estimated in vivo shear forces of roughly 4

N acting on the implant. Thus, except for the gel-like

matrix, the unfixed and fibrin-glued scaffolds, all sutured

materials might withstand this load in an in vivo situation.

To date, there are no data available that would allow a

comparison and, thus, it remains speculative whether the

observed loads are below or above in vivo conditions.

Although pointing out these limitations and difficulties,

we have finally presented a procedure according to the

standard of knowledge to test the postoperative stability of

biomaterials for ACI in situ.

Sven Knecht,1 Christoph Erggelet,2 Michaela Endres,3

Michael Sittinger,3 Christian Kaps,3 Edgar Stussi1

1Institute for Biomechanics, ETH Zurich, Departmentof Mechanical and Process Engineering

2University of Freiburg, Department of OrthopaedicSurgery and Traumatology

3Tissue Engineering Laboratory, Charite-Universitatsmedizin,Department of Rheumatology

Correspondence to: S. Knecht (e-mail: sven.knecht@gmail.com)Received 12 June 2007; accepted 15 June 2007Published online 1 October 2007 in Wiley InterScience(www.interscience.wiley. com). DOI: 10.1002/jbm.b.30945

Journal of Biomedical Materials Research Part B: Applied BiomaterialsDOI 10.1002/jbmb

299LETTER TO THE EDITOR