Design of spinous process hooks for flexible fixation of the lumbar spine

Abstract

A prototype flexible fixation system for the lumbar spine was subjected to tensile testing to failure and cyclic tensile testing in order to determine any regions of weakness. The system consisted of a spinous process hook and two laminar hooks made of stainless steel (316L). Each laminar hook was attached to the spinous process hook by a loop of polyester braid secured by a crimped metal sleeve. In five tensile tests, the system failed by irreversible deformation of the spinous process hook at 2.5 ° 0.3 kN (mean ° standard deviation). In three cyclic tests, in which the applied tension varied sinusoidally between 0.04 and 0.4 kN at a frequency of 5 Hz, failure occurred after less than 400000 loading cycles. This occurred as a result of fatigue crack initiation and propagation in the spinous process hook. A finite element model showed a stress concentration in the region where the crack occurred, which raised the applied stress above the tensile fatigue strength of this stainless steel. The spinous process hook was redesigned for manufacture in a titanium alloy (Ti-6Al-4V ELI) to minimize artefacts in magnetic resonance imaging. Further finite element models showed no unacceptable stress concentrations.

Keywords

finite element modelling spine flexible stabilization mechanical testing

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References

Graf

Lumbar instability surgical treatment without fusion: Soft stabilisation system. Rachis, 1992, 4, 123–137.

Fischgrund

J. S.

Montgomery

D. M.

Diagnosis and treatment of lower back pain. Orthop. Rev., 1993, 22, 311–318.

Hanley

E. N.

The indications for lumbar spine fusion with and without instrumentation. Spine, 1995, 20 (Suppl.), 143–153.

Sonntag

V. K.

Mariciano

F. F.

Is fusion indicated for lumbar spine disorders?

Spine, 1995, 20 (Suppl.), 138–142.

Fraser

R. D.

Interbody, posterior, and combined lumbar fusions. Spine, 1995, 20 (Suppl.), 167–177.

Vaccaro

A. R.

Garfin

S. R.

Internal fixation (pedicle screw fixation) for fusions of the lumbar spine. Spine, 1995, 20 (Suppl.), 157–165.

Gardner

A. D. H.

Lumbo-sacral and spondyl-pelvic arthrodesis. In Lumbar Spine Disorders: Current Concepts (Eds Aspden

R. M.

Porter

R. W.

), 1995, pp. 185–208 (World Scientific, Singapore).

Grevitt

M. P.

Gardner

A. D. H.

Spilsbury

Shackleford

I. M.

Baskerville

Pursell

Hassaan

Mulholland

R. C.

The Graf stabilisation system: Early results in 50 patients. Eur. Spine J., 1995, 4, 169–175.

Markwalder

T. M.

Dubach

Braun

Soft stabilisation system of the lumbar spine as an alternative surgical modality to lumbar arthodesis in the facet syndrome. Preliminary results. Acta Neurochir. (Wien), 1995, 134, 1–4.

10.

Markwalder

T. M.

Merat

The lumbar and lumbo-sacral facet-syndrome. Diagnostic measures, surgical treatment and results in 19 patients. Acta Neurochir. (Wien), 1994, 128, 40–46.

11.

Strauss

P. J.

Novotny

J. E.

Wilder

D. G.

Grobler

L. J.

Pope

M. H.

Multidirectional stability of the Graf system. Spine, 1994, 19, 965–972.

12.

Papp

Porter

R. W.

Aspden

R. M.

Shepherd

J. A. N.

An in vitro study of the biomechanical effects of flexible stabilisation on the lumbar spine. Spine, 1997, 22, 151–155.

13.

BS 7252: Part 1; 1997 Metallic Materials for Surgical Implants. Specification for Stainless Steel, 1997 (British Standards Institution, London).

14.

BS 7252: Part 3: 1997 Metallic Materials for Surgical Implants. Specification for Wrought Titanium 6-Aluminium 4-Vanadium Alloy, 1997 (British Standards Institution, London).

15.

Rudisch

Kremser

Peer

Kathrein

Judmaier

Daniaux

Metallic artifacts in magnetic resonance imaging of patients with spinal fusion. A comparison of implant materials and imaging sequences. Spine, 1998, 23, 692–699.

16.

BS EN 552: 1994 Sterilisation of Medical Devices. Validation and Routine Control of Sterilisation by Irradiation, 1994 (British Standards Institution, London).

17.

ASTM F1717 Standard Test Methods for Static and Fatigue for Spinal Implant Constructs in a Corpectomy Model, 1996 (American Society for Testing and Materials, Philadelphia, Pennsylvania).

18.

NAFEMS. A Finite Element Primer, 1991 (National Agency for Finite Element Methods and Standards Limited, UK).

19.

Gibbons

D. F.

Materials for orthopaedic joint prostheses. In Biocompatibility of Orthopaedic Implants (Ed. Williams

D. F.

), 1982 (CRC Press, Florida).

20.

Wenger

Miller

Wilkerson

Evaluation of fixation sites for segmental instrumentation of the human vertebra. Orthop. Trans., 1982, 6, 23–24.

21.

Coe

J. D.

Warden

K. E.

Herzig

M. A.

McAfee

P. C.

Influence of bone mineral density on the fixation of thoraco-lumbar implants. A comparative study of transpendicular screws, laminar hooks and spinous process wires. Spine, 1990, 15, 902–907.

22.

Shepherd

D. E. T.

Leahy

J. C.

Mathias

K. J.

Wilkinson

Hukins

D. W. L.

Spinous process strength. Spine, 2000, 25, 319–323.

23.

Armco Inc.

Product Data Bulletin No. S-22 for Stainless Steel, 1986 (Armco Inc., USA).

24.

Boyer

Welsch

Collings

E. W.

Materials Properties Handbook: Titanium Alloys, 1994 (ASM International, USA).

25.

Van Brussel

Vander Sloten

Van Audekercke

Fabry

Internal fixation of the spine in traumatic and scoliotic cases. The potential of pedicle screws. Technology Health Care, 1996, 4, 365–384.

26.

Ransom

La Rocca

S. H.

Thalgott

The case for pedicle fixation of the lumbar spine. Spine, 1994, 19, 2702–2706.

27.

Faraj

A. A.

Webb

J. K.

Early complications of spinal pedicle screws. European Spine Journal, 1998, 6, 324–326.

28.

Robertson

P. A.

Novotny

J. E.

Grobler

L. J.

Agbai

J. U.

Reliability of axial landmarks for pedicle screw placement in the lower lumbar spine. Spine, 1998, 23, 60–66.

29.

Leahy

J. C.

Mathias

K. J.

Hukins

D. W. L.

Shepherd

D. E.T.

Deans

W. F.

Mechanical testing of a flexible fixation device for the lumbar spine. Proc. Instn Mech. Engrs, Part H, Journal of Engineering in Medicine, 2000, 214 (H5), 489–495.

30.

Rohlmann

Bergmann

Graichen

Loads on an internal spinal device during walking. J. Biomechanics, 1997, 30, 41–47.