Interbody Carbon Fiber

Dorn Spinal Therapy

Spine Healing Therapy

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Rudolf Bertagnoli

Klinikum St. Elisabeth, Spinal Department, Straubing, Germany

The fusion of spinal segments is one of the major goals in surgical treatment of degenerative disc disease (DDD). To obtain the best biomechanical support and fusion rates, interbody fusion is the preferred method. The main advantages of implants made of carbon fiber reinforced plastic (CFRP) are the radiolucency and the fact that there is no distortion on CT and MRI. The surgeon can chronologically follow the biological reaction in the fusion section and can clearly detect bony fusion. Regions which are hidden using metal implants could now be analyzed on standard radiographs. In implants made of CFRP matrix materials such as thermoset epoxy resin systems (EPN/DDS) or thermoplastic systems PEAK (PEKEKK™, PEEK™, ULTRAPEK™) are used. The first ones have been clinically used since 1988 following detailed in vitro and in vivo tests starting 1975 according to ISO 10993-1 [1]. In the spine these implant materials have been used since 1993 [2].

These CFRP implants are manufactured using a special fiber winding process of carbon fiber roving which is impregnated with resin and laid on a rotating rod. Due to that and to the design-adapted machining processes the properties of the material could be chosen for each kind of implant design. Hardening, tempering, and machining lead to the final implant geometry and must be controlled.

Interbody fusion with cage support is an ideal situation for the application of resorbable biomaterials [3, 4]. To monitor the production of the new bone masses and to determine the degree of bone fusion, cages made from carbon composite materials are superior to metallic cages due to their radiolucent characteristics.

In the literature some mechanical and in vitro tests performed on CFRP implants could be found: Brantigan et al. [5], Ciapetta et al. [6] (vertebra replacement), Jost et al. [7], Kandziora et al. [8] (80 cervical spines of sheep with different cage designs), Shono et al. [9] (18 calf spines in compression and rotation), and Steinhauser et al. [10] (static and dynamic compression and shear

Fiber Matrix

Fig. 1. Cross section and SEM (artificial fracture) of CFRP implants: carbon fiber with a diameter of «5 ^m surrounded by matrix material.

Fiber Matrix

Fig. 1. Cross section and SEM (artificial fracture) of CFRP implants: carbon fiber with a diameter of «5 ^m surrounded by matrix material.

testing) all showed very good results. The first animal tests of CFRP implants as interbody lumbar fusion devices were done by Brantigan et al. [5]; they reported 100% fusion after 6, 12 and 24 months in 27 Spanish goats. A summary of clinical experience with CFRP cages is shown in table 1. In all cases high fusion rates were detected with no device-related complications. One case report showed infection and a broken CFRP cage (see table 1) [17].

Material and Methods

Mechanical Tests

To test the axial compression and shear behavior the spinal implants were loaded between two parallel stainless steel plates according to ASTM F2077-00. An axial force rate of 500N/min was used for quasistatic testing. The loading was stopped when a permanent failure of the specimen occurred or a displacement of 3.0 mm was reached.

For dynamic testing the specimens were tested under cyclic fatigue using a sinusoidal loading waveform at a constant frequency of 5 Hz (lumbar) and 12 Hz (cervical), with an R ratio (Pmin/Pmax) of 0.1. The maximum load cycle was 5,000,000 cycles and a displacement limit error of 3 mm was established.

To determine the pull-out strength, the implants were placed in between two blocks made of Rohacell RC 300WL with mechanical properties comparable to cancellous bone. An axial preload of 100 N was applied. The instrument intended for clinical use was attached

Table 1. Summary of the clinical experience with CFRP cages

Location

Cases Follow-up

Results

Lumbar (PLIF) 71

Lumbar (PLIF) 11

Lumbar (PLIF) 221

Lumbar (PLIF) 51

Lumbar (L4-L5)

Bilateral 83

Unilateral 80 Lumbar (ALIF and PLIF), 70 cervical

Cervical 19

Lumbar (PLIF) 1

28 months 6-48 months 24 months 12 months

12-24 months 3-12 months 12-21 months

90% fusion rate, 11

66% overall satisfaction rate 90% fusion rate after 48 months 98.9% fusion rate,

86% clinical success 86% fusion rate (89% of levels)

>97% fusion rate 100% fusion rate >98% fusion rate

100% fusion rate

4 week to 25 months Case report of infection and broken CFRP cage

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