**4. Conclusion**

324 Thermoplastic Elastomers

From a design perspective the greatest advantage thermoplastics possess over other materials for the construction of spinal implants is the ability to tailor material properties to suit the requirements of the given application. The choice of polymer and specific manufacturing parameters allow for a broad range of material and bioreactive properties. PGA polymers degrade more rapidly than do PLA polymers while copolymers of the two compounds degrade at intermediate rates. Due to the relatively long time period required for successful spinal fusion (6-12 months), PLA has emerged as the material of choice for resorbable spinal implants. The ratio of L- and D-lactides comprising the PLA polymer influences the mechanical and biochemical properties of the material. Poly (L-lactic acid) (PLLA) is semi-crystalline and is characterized by high strength and long resorption time while the racemic poly (D,L- lactic acid) (PDLLA) is amorphous and characterized by lower strength and shorter resorption time. Amorphous polymers are generally preferred for bioresorbable implants because crystalline portions of the polymer degrade much more slowly and may remain in the host much longer, potentially leading to long term complications. As a result most materials used in spinal implants consist of a mixture of L-lactide and racemic D,L-lactide. The most common among these is 70:30 poly(L-lacticeco-D,L-lactice) (PLDLLA) which consists of 70% molar ratio of L-lactide and a 30% molar ratio of D,L-lactide (Smit et al. 2008). This material has been chosen for interbody fusion devices because of its long resorption time (18-36 months), relatively high elastic modulus (3.15 GPa) and high compressive strength (100 MPa) (Alexander et al. 2002; Toth et al.

While the conceptual advantages outlined above have to some extent successfully been reduced to practice, the materials described are not without their limitations. While significant clinical success in terms of fusion and appropriately timed resorption has been achieved in animals studies (Toth et al. 2002; Thomas et al. 2008), subsequent research in humans has indicated poor clinical outcomes (Smith et al. 2010; Jiya et al. 2011). Particularly, in a prospective cohort study including 81 patients who underwent Transforaminal Lumbar Interbody Fusion (TLIF) surgery with either a resorbable PDLLA cage or a similar nonresorbable carbon fiber cage, Smith et al. found significantly higher incidence of non-union and cage migration in PDLLA cages compared to the carbon fiber cages. Four of the eight patients that exhibited cage migration experienced the adverse event within six months of implantation. Cages that were explanted exhibited moderate plastic deformation, a finding consistent with insufficient material strength discussed previously (Smith et al. 2010). In a randomized prospective human study, Jiya et al. compared PDLLA lumbar interbody fusion cages to similar non-resorbable PEEK implants and found no significant improvement in clinical outcomes upon 2-year follow-up when compared to preoperative values in the PDLLA group. Conversely, significant improvements in all clinical parameters were found in the PEEK group indicating the potential inferiority of the PDLLA implant (Jiya et al.

Other studies have found anterior cervical plate systems to be inadequate or potentially problematic in providing fixation in both animal and clinical studies. Lyons et al. recently investigated a PDLLA cervical RPSS in an ovine model. Results showed a fusion rate of 25% after 3 months, and plate or screw fracture in 50% of specimens (Lyons et al. 2011). Bindal et al. clinically evaluated a PLDLLA RPSS, supplemental to anterior cervical discectomy and

2002).

2011).

Thermoplastics have demonstrated a track record of success in medical device applications. The prevalent use of PEEK is evidence of widespread acceptance of thermoplastic materials in spinal fixation procedures. The ability to tailor the properties of the device suitable for each design application continues to present thermoplastics as attractive design materials. Furthermore, the existence of biodegradable thermoplastic polymers has provided an opportunity to develop resorbable medical implants. These materials have exhibited widespread success in applications in which they are subjected to loads of relatively low magnitude and/or short duration, such as in sutures, sheaths and in extremity fixation. However, in the case of biodegradable spine implants, it is important to understand the balance between the benefits of biodegradation and potential strength limitations. In spite of investigations into the use of these materials in a wide array of spinal fixation devices, they have yet to achieve wide clinical acceptance in the field, ostensibly due to their poor performance under the sustained loads borne by the spine.

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The Performance Envelope of Spinal Implants Utilizing Thermoplastic Materials 327

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**Application of Thermoplastics** 

 **in Protection of Natural Fibres** 

Mohammadreza Samadi Tavana and Ahmad Shukri Yahaya

Nowadays thermoplastics are widely used in industries; many products used different specifications of the thermoplastics that are adapted to their requirement. The importance of consideration to the environmental protection leads the mankind to thinking about producing environment-friendly material, and also controlling the waste material by recycling or reducing them. In the meantime the natural materials such as cellulose based material are one of the major resources that can be used to replace with many manufactured materials. However, cellulosed based material such as natural fibres, because of their degradability, need protection from any circumferential agents. The protection may require a special condition in order to utilise in soil, due to water absorption, soil organisms, and minerals. Natural fibres are amenable to modifications as they bear hydroxyl groups from cellulose and lignin. In addition coating the fibres with any chemical materials reduce their water absorptions and protect them from any bacteria and fungi attack. The hydroxyl groups may be involved in the hydrogen bonding within the cellulose molecules. This weakness of the natural material and good characteristics of natural fibres is the basis of bio-

> Lignin (%)

OPEFB\* 65 - 19 248 14 2,000

Coir 32-43 0.15-0.25 40-45 140 25.0 3,200

Sisal 66-72 12 10-14 580 4.3 1,250

Banana 63-64 19 5 540 3.0 816

Pineapple 81.5 - 12.7 640 2.4 970

Table 1. Chemical and mechanical properties of some important natural fibres

Tensile Strength (%)

**1. Introduction** 

composites invention.

Fibres Cellulose

(%)

\*OPEFB : Oil Palm Empty Fruit Bunch

Hemicelluloses (%)

Fauziah Ahmad, Farshid Bateni,

Elongation (%)

Toughness (MPa)

*Universiti Sains Malaysia* 

*Malaysia* 

Yong-Hing, K., J. Reilly, et al. (1980). "Prevention of nerve root adhesions after laminectomy." Spine (Phila Pa 1976) 5(1): 59-64. **16** 
