Polymers used in Artificial joints

Polymers used in Artificial joints

INTRODUCTION

During the last 90 years man made materials and devices have been developed to the point at which they can be used successfully to replace parts of living system in the human body. These special material able to function in intimate contact with living tissue with minimal adverse reaction by the body.

An advances have been mode in the medical science and with the advent of antibiotics, infectious diseases have become a much smaller health threat. Because average life expectancy has increase. More organs joints and other critical body parts will wear out and must be replaced if people are to maintain a good quality of life.

Now polymers plays a major rate in replacing or improving the function of every major body system (skeletal, circulatory, nervous etc.) some common implant include cordial implants such as artificial heart valve soft tissue implant and also. Orthopedic devices such as total knee hip joint replacement. Spinal implants and bone fixators.

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OVERVIEW ON JOINTS In this section we know basic about the joints and how is a total joint replacement performed. Now

What is a joint : A joint is formed by the end of two or more bones which are connected by thick tissues. e.g. Our hip is a ball and socket joint formed by the upper end of the femur the ball and a part of pelvis called the acetabulum, the socket.

How is a total joint replacement performed ? You will be given an anesthetic and surgeon will replace the damaged parts of the joint. For example, in an arthritic knee the damaged ends of the bones and cartilage are replaced with metal and plastic surfaces that are shaped to restore knee movement and function. In an arthritic hip, the damaged ball (the upper end of the femur) is replaced by a metal ball attached to a metal stem fitted onto the femur and a plastic socket is implanted into the pelvis, replacing the damaged socket. Although hip and knee replacements are the most common, joint replacement can be performed on other joints, including the ankle, foot shoulder, elbow and fingers.

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BIO-MECHANISM OF ARTIFICIAL JOINTS

In the bio-mechanism of artificial joints we know the :

Hip joint replacement fundamentals : The basic idea of joint replacement surgery is to replace the diseased articular surface with one made from a synthetic material. This new joint surface must then be part of the artificial joint which itself is fixed to the bone near the joint. The major design issues in artificial joint replacement are 1) The geometric and material design of the articulating surfaces and 2) Design of the interface between the artificial joint and the surrounding bone. Most of the joint replacement use a polyethylene for the bearing surface and either a titanium alloy or a chrome-cobalt alloy for the rest of the joint in most cases that interfaces with the bone. There are two widely used methods for interfacing the joint with the bone. 1. Using a polymethyl methoacrylate (PMMA) cement to adhere the metal to the bone. 2. Using a porous metal surface to create a bone ingrowth interface. A Schematic hip replacement is shown below :In this big (a) we can see that both the acetabulum ( the “Socket” ) and the proximal femur ( the “ball” of the hip joint ) have been replaced. The femoral side is completely metal. The acctabular side is composed of the Polythylene bearing surface

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Polymers used in Artificial joints that may or may not have a metal backing. Both the components in this schematic are shown to be tired to bone with PMMA cement. (fig b ).

acetabulm

femur

Lateral medial

Fig. A. – Schematic hip replacement

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Fig. B. – Joint Replacement using PMMA cement

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Brief Notes on Porous Coated : Implant Design : As noted above and shown in fig. B, an alternative to cemented implant interface is a porous coated implant interface. The theory behind porous coating is that bone will grow into 300 to 500 micron. Pores on the implant surface. This will eliminate the poor mechanical properties of the cement. However, porous coated implant have not overtaken cemented implants in terms of clinical usage. That may be for two reasons. 1. Porous coated implants rely on a biological process that is not as predictable as an immediate cemented interface. The interface bond develops over time and 2. Porous coated implant interface is much more dependant on precise. Surgical fit to obtain an implant bone. In hip stems. Porous coating is most often limited to the proximal portion of the implant. This is because an implant completely covered with porous coating will most likely have ingrowth at the tip. This leads to extreme stress shielding. Finally as noted in the text, the stability of porous coated implants is more dependant on the location of ingrowth than on the depth of ingrowth. This means that the ability to control ingrowth location in perhaps the most critical aspect of the porous coated implants but the one aspect most difficult to control.

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Fig. C. - Joint Replacement Using Porous Coated Implant Design

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CHARACTERISTICS OF THE POLYMER USED IN ORTHOPEDICS :

The characteristics are as follows : 1. They must be compatible with proposed design and function. 2. It must be pure. 3. Should be capable of fabrication without damaged. 4. Physical, chemical and mechanical properties does not change during sterlization. 5. Should not affected by body fluid. 6. Polymer should not influence the blood element. 7. It should be durable. 8. Good strength.

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TYPES OF THE POLYMER USED IN ARTIFICIAL JOINTS WITH PROPERTIES AND STRUCTURE

Among the material that have been considered for the design of load bearing joints are :1. Silicon rubber. 2. UHMWPE ( ultra high molecular wt. polythylene ). 3. PTFE ( Polytetrafluroethylene . 4. PMMA ( Polymethyl methaacdylate )

1. Silicon rubber :- ( Polydimethyl siloxone ) Generally Polydimethyl siloxane is generally used in the fingure joint replacement. CH2

Structure : O

Si CH2

n

Polydimethylsiloxane.

Properties : C.O.E.&T.,Akola

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Polymers used in Artificial joints 1. Having a very good hinging property. 2. Highest flexible material in the silicon family. 3. It is having lowest Tg. 4. Having Tg about - 1270 c. 5. Mol. Wt is about 5000-9000.

2. UHMWPE (ultrahigh mol wt. polyethylene)

Structure : CH2

CH n

Properties : 1. Molecular weight 3 x 106 – 6 x 106 2. It is slippery which is important for joint motion. 3. It is very strong and wear resistant to withstand the stress of walking. 4. It is also very well accepted by the body. 5. Has tendency to flow under pressure. 6. Good corrosion resistant. 7. Low coefficient of surface friction. 8. Nonstick surface.

Density increase in UHMWPE :

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Polymers used in Artificial joints -

Prepared by ziegler. Natta process from HDPE.

-

It is low press. Process so density increases.

-

Mol. Wt. goes on increasing hence density increases.

-

Linear structure with no branching hence chains are density packed and hence density increase.

3. PTFE ( Polytetrafluro ethylene ) Structure : F

F

C

C

F

F

n

Properties : -

High heat stability

-

Better chemical resistance

-

It is tough

-

It is flexible

-

It is non – resilient

-

Resistance to heat

-

Izod Impact strength – 2.0 ( ft lb in )

4. PMMA ( Polymethyl Methoacrylate ) C.O.E.&T.,Akola

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Structure :

CH3 CH2

C COOCH3

n

Properties : 1. Minimum heat production. 2. It has a good weathering resistance. 3. It is dimensionally stable. 4. Good adhesion properties. 5. It does not have toxic effect. 6. It is nonallergic. This polymethylmethacrylate can be used as an cement. The characteristics of PMMA to be used as an cement are given in the next section.

POLYMER CEMENT USED IN ARTIFICIAL JOINT C.O.E.&T.,Akola

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Polymers used in Artificial joints Bone cements are used to fill defective porous bones to stabilize fractures and to secure implants to bone. Early work on the impregnation of porous bones with 1. Alkyl caynoacrylates 2. amine – cured epoxy resin. 3. Polyurathans. These all are unsatisfactory because of the lack of stability in the biological environment and toxic properties. Now a days a typical acrylic bone cement is used. This bone cement consist of liquid component and polymeric powder. The liquid component consist of : 

97.4 %

Methylmethaacrylate



2.6 %

N.N – dimethyl – p – toluidine (Accelerator ) hydroquinone ( Inhibitor )



Polymeric powder consist of : 

95.5 %

Polymethylmethaacrylate



2.3 %

Polystydene.



1.94 %

Benzoyl Peroxide ( Initiator )



0.28 %

residual monomer.

For used ; appropriate quantities of the monomer and polymer are mixed for up to 4 min.

PMMA used as a cement : C.O.E.&T.,Akola

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Polymers used in Artificial joints The PMMA used as a cement because of 1. Good adhesion properties. 2. Very fast curing. 3. Titely holding edges of most wounds. 4. Minimum heat production. 5. It is non auergic. 6. It is not serve as barrier to heal. 7. It does not have any toxic effect on surrounding tissue. 8. As it is amorphous shrinkage is low and hence dimensionally stable.

APPLICATION OF POLYMER IN ARTIFICIAL JOINTS : C.O.E.&T.,Akola

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1. UHMWPE

Polymer

Application In Artificial Joints 1. Acetabulum in total hip prostheses.

2. PP 3. Silicon rubber

2. Artificial Knee prostheses. 1. finger Joint prostheses 1. Finger – Joint repair

4. PTFE

2. Ear prostheses. 1. In shoulder Joints.

5. PMMA

2. In knee Joints. 1. Bone cement. 2. Bone prostheses. 3. Cranial bone replacement.

OTHER APPLICATION OF POLYMER IN BODY INPLANT : Polymer 1. PP

Application 1. Membrane for membrane oxygenator.

2. Silicon rubber

2. Unabsorable sutures. 1. Roller pump tubing (heart-lung M/c) 2. Artificial skin. 3. Plastic surgery Implant. 4. Artificial heart valve prostheses.

3. PTFE

5. Bladder prostheses. 1. Vascular graft prostheses. 2. Heart patch.

4. PMMA

3. Retinal disattachment. 1. Artificial teeth. 2. Inplanted teeth.

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Polymers used in Artificial joints 3. denture material. 4. dental filling. 5. Intraocular lens.

PP used in unabsorable sutures : -

Good abrasion resistance.

-

High tenacity.

-

Easily stearlise.

-

Excellent wear resistant.

-

Soft in touch.

-

free from stress cracking.

-

good chemical resistance.

-

It has low density.

RECENT DEVELOPMENT The MMA Tech. LTD develops a Polymer called “MP-1”. An innovative wear free polymer for medical use. Immediate applications include articulating components for orthopedic Inplants such as hip, knees, shoulder joints, bone components ( for humans and veterinary uses ) etc. Other applications include the cardiovascular area : Pacemakers, valve and stents.

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Polymers used in Artificial joints MP-1 is preferable over existing materials used for articulating Joints. However it will not cost more.

Properties :“MP.1” is 1. Tougher than Metal. 2. Wears less. 3. has fracture toughness. 4. Higher durability. 5. High density. 6. Self – lubrication. 7. Chemical inertness. The company presently tests.

The material for biocompatibility with

human bones & soft tissue. The company will possess conclusive material test results by Mid-2002.

WHAT ARE POSSIBLE COMPLICATIONS ? Joint replacement surgery is successful in more than I out of 10 people. When complications occur. Most are successfully treatable. Possible complications include :

1. Infection :

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Polymers used in Artificial joints Infection may occur in the wound or deep around the prosthesis. It may happen while in the hospital. Minor infections in the wound area are generally treated with antibiotics. Major or deep infection may require more surgery and removal of the prosthesis. Any infection in your body can spread to your joint replacement.

2. Blood Clots : Blood clots result from several factors, including your decreased mobility causing sluggish movement of the blood thro your leg veins. Blood clots may be suspected if pain and swelling develop in your thigh. Several measures may be used to reduce the possibility of blood clots including : 1. Blood thinning medications (anticoagulants). 2. elastic stockings. 3. Exercise to increase blood flow in the leg muscles.

3. Loosening : Loosening of the prosthesis within the bone may occur after a total joint replacement. This may cause pain. If the loosening is significant, a revision of the joint replacement may be needed.

4. Dislocation :

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Polymers used in Artificial joints Occasionally, after total hip replacement the ball can be dislodged from the socket. In most cases the hip can be relocated without surgery.

5. Wear : Some wear can be found in all joint replacement : Excessive wear may contribute to loosening and may require revision surgery.

6. Prosthetic breakage : Breakage of the metal or plastic joint replacement is rare, but can occur. A revision surgery is necessary if this occurs.

7. Nerve injury : Nerves in the vicinity of the total joint replacement may be damaged during the total replacement surgery, although this type of injury is infrequent. This is likely to occur when the surgery involves correction of major joint deformity or lengthening of a shortened limb due to an anthritic deformity. Overtime these nerve injuries often improve and may completely recovered.

CONCLUSION

Everyday we observe plastics in endless fields and to be near to this material technology-wise as a polymer technologist is a wonderful experience.

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Plastics today can be made much more softer than than any other material & can be made much more stronger than metal by possessing adequate knowledge of this material. Plastics have found applications not only in day to day purposes but also in special fields such as medical application, aerospace, microelectronics etc. However, there are certain hurdles for this wonder material in its applications much more is expected from plastics. Plastics can be used in artificial heart and artificial skin as a important material. In the future, plastics will be fully developed and applied will be the one after which the hurdles will be gone and polymers would be applied inadvertently in any and every field.

REFERENCES

[1] Mark Bikales, Overber berger, Menges, Encyclopedia of polymer Science & Engineering , Vol . 9, 2nd Edition , A wiley – Interscience publication( 1985 ), Page No. 486-500 C.O.E.&T.,Akola

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[2] Mark Bikales, Overber berger, Menges, Encyclopedia of polymer Science & Engineering , Vol . 6, 2nd Edition , A wiley – Interscience publication( 1985 ), Page No. 762-764

[3] Mark Bikales, Overber berger, Menges, Encyclopedia of polymer Science & Engineering , Vol . 2, 2nd Edition , A wiley – Interscience publication( 1985 ), Page No. 368 - 375, Page No. 205 – 223.

[4] J. A. Brydson , Plastic Material ,Vol.6, 6th Edition, Butterworth Heinemann,

[5] www.searchalot.com [6]

www.google.com

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Polymers used in Artificial joints

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