Fundamentals of Biomedical Engineering

142

FA

FC

FUNDAMENTALS OF BIOMEDICAL ENGINEERING

D1

FB

D2

Shear force	Bending mom ent
diagram	diagram

ORTHOSIS FOR GAIT DISABILITY

1.A very throughtful orthosis device can be prescribed to a person with neurologic involvenment so that he may access more efficient movement patterns and may reduce residual gait disability. Three basic functional tasks are essential for lower limbs for efficient and successful ambulation. They are (1) weight acceptance (2) one leg stance support (3) swing phase (limb advancement). The above process must happen in smooth manner and the limb must remain stable. It must be able to absorb the impact of superincumbent body weight and

stand all moments developed during forward movements of the limb and the body. The function of single limb support involves that the body can carryout forward movement on the weight bearing limb. The limb has to perform this task over a reduced area of support on the ground. The swing advancement involves removal of weight from the limb and its forward progression until next step is initiated. The ground clearance is an essential element of the functional task of the limb during swing limb advancement. The funtional task, gait subphase and critical events for a normal gait cycle are as tabled below :

Function task	Gait subphase		Critical events
1. Weight acceptance	1.	Initial contact	1.	Heel first contact
	2.	Loading response	2.	(a) Hip stability
				(b) Controlled knee with 15°
				flexion
				(c) 10° Ankle plantar flexion
2. Single limb stance	1.	Mid distance	1.	Controlled tibial progression
	2.	Terminal stance	2.	(a) 10° dorsiflexion of ankle joint
				(b) Heel rise from ground
				(c) Trailing limb position
3. Swing phase	1.	Preswing	1.	40° knee flexion
	2.	Initial swing	2. (a) 15° hip flexion
				(b) 60° knee flexion
	3.	Mid swing	3.	(a) Hip flexion to 25°
				(b) Zero ankle dosiflexion
	4.	Terminal swing	4.	Knee extention to neutral

ORTHOSIS

143

2.The goals of orthotic prescriptions are to improve the biomechanics of gait. Primary emphasis is on the more commonly prescribed lower limb braces. The orthosis aims to achieve biomechanical alteration of human movement in upright function. A wide range of ankle-foot orthosis (AFO) designs are used in treating the person with neurologic involvement. Braces are capable of providing some degree of control during stance, swing or both phases of gait. The use of knee braces, taping or foot orthosis can be beneficial treatment during strengthening programme to achieve dynamic control and balance at the patelloferroral joint at knee. Supportive wrapping and bandaging techniques are beneficially used for athletics. Adhesive strapping and protective padding techniques are commonly accepted as orthotic treatment to orthopaedic patients.

PARAPLEGIC ORTHOTIC WALKING SYSTEM

1.Spinal cord disease or injury causes paraplegia. This results in the loss of physical function like standing and walking. The ability to stand and walk is considered most important in the individuals’s potential to return to a normal life style. There has been an increase in the research and development of rehabilitation technology, enabling more paraplegic patients to stand and ambulate. The ability of a person with paraplegia to stand upright imparts several physiological benefits. They include improvement in blood circulation, reduction in spasticity and retardation of osteoporosis. The joint contractures and kidney malfunction are also prevented. The ability to stand and walk also imparts psychological benefits as person

Knee Brace

Knee Taping

144	FUNDAMENTALS OF BIOMEDICAL ENGINEERING

with paraplegia may feel himself a normal person having standing and walking ability. The most commonly used orthotic systems for the paraplegic patients are (1) bilateral KAFO (knee-ankle-foot orthosis) (2) HKAFO (Hip-knee-ankle-foot orthosis). KAFO requires a large energy expenditure for the patients to walk. HKAFO for paraplegic gait allows ambulation at a lower energy cost. Two major designs of HKAFO are hip guidance orthosis (HGO) and reeiprocating gait orthosis (RGO) as shown

in the figure. These orthotic systems are similar as the patient is braced from the mid trunk to the feet and the knee and ankles immobilized in a neutral position. These orthosis permit hip flexion and extention but prevent hip adduction. The RGO is designed to be worn inside the patients clothes. The patient requries assistance of roller or reciprocating roller while walking wearing RGO. However HGO is worn outside the patient’s clothes and the patient walks with the assistance of crutches.

Trunk Band

Pelvic Band Dual

cable

system

Hip Joint

Pelvic Band

Thigh cuff

Knee cuff

Knee cuff

Ankle cuff

M etal foot plate

HGO

RGO

ORTHOSIS

145

OBJECTIVE TYPE QUESTIONS

Fill in the gaps.

1.Orthosis means --------- of maladjustment. (a) correction (b) reduction

2.--------- is a science that deals with making and fitting of orthopaedic appliances.

(a) prostheses (b) orthosis

3.We try to correct one or two of the --------

potential degree of freedom of motion through the application of orthotic devices. (a) four (b) six

4.The goals from orthosis can be achieved through selected application of ---------

developed by the orthopic device. (a) forces

(b) movements

5. The biomechanics of gait can be improved by ---------. (a) orthosis (b) prostheses

6.Suppertive wrapping and bandaging techniques are used by ---------

(a) orthopaedic patients (b) athletics

7.Addesive strapping and protective padding techniquel are used by ---------

(a) orthopaedic patient (b) athletics.

8.Three basic functional tasks are essential for lower limbs for efficient and successful

---------. (a) ambulation (b) stance

ANSWERS

1. (a)

2. (b)

3. (b)

4. (a)

5. (a)

6. (b)

7. (a)

8.(a)

146	FUNDAMENTALS OF BIOMEDICAL ENGINEERING

	METALLIC	#
	BIOMATERIALS

You spend the first two years of your kids' lives teaching them to walk and talk and the next 16 years telling them to sit down and be quiet.

INTRODUCTION

1.Biomaterial is a synthetic material used to replace a part of a living system or to function in intimate contact with living tissues. Therefore biocompatibility is acceptance of an artificial implant by the surrounding tissues and by the body as a whole. The success of implant depends upon–

(a) Acceptance of the implant by the surrounding tissues.

(b)	Implant is nontoxic and non
	carcinogenic.
(c)	The material of implant must have high
	mechanical strength
(d)	Implant faces varying loads and hence
	it should have high fatigue life
(e)	The material should be chemically stable
	and inert
(f)	Implant should have sound engineering
	design

(g)Appropriate molecular weight and molecular weight distribution (weight of implant & its density)

(h) Material should be easy to be fabricated and processed for large scale production.

2.Biomaterials can be (1) Polymers (2) Metals

(3)Ceramics and (4) Composites. Nylon, silicone rubber, polyster and poly tetrafuoroethylene are polymers commonly being used as biomaterials. Polymers are resilent and easy to be fabricated but they deform and degrade with time. Titanium & its alloys, Cobalt and Chromium alloys, stainless steel, Gold, Silver and Platinum are common biocompatible metals. Metals are strong, tough and ductile. However, metallic implants are difficult to be produced and they are like to corrode. Aluminium oxide, Calcium phosphates and carbon are common bioconspatible ceramics. Ceramics are inert, strong and highly biocompatible. However ceramics are brittle and they are not resilent. Carbon fibers reinforced polymers and bone

cement are biocompatible composite materials. They are tailor made and strong but difficult to be made.

3.The uses of biomaterials are :–

(a) Replacement of damaged or diseased parts like hip joint, knee joint & heart valves etc.

(b) Assistance in healing as done by sutures, bone plates and intramedullary rod etc.

METALLIC BIOMATERIALS

(c) Assistance in functioning as by cardiac pacemaker and intraocular lens etc.

(d) Correction of functional abnormality. Example : Cardiac pacemaker.

(e) Cosmetic correction. Example: chin augmentation, augmentation mamo plasty.

(f) Assistance in diagnosis: Probes and catheters are made of biomaterials.

(g) Assistance in treatment eg. catheters and drains of biomaterial.

4.As explained above, the success of implant depends upon various factors. Also if 'f' is failure and 'r' is reliability, then r = l – f. The total reliability of implant having

multiple modes of failure (f1 f2 ... to fn) can be given as:

r = (l – f1) (l – f2) ... x (l – fn) = r1 × r2 ... x rn

Most pure metals generate a severe tissue reaction. The reason is that metals have high free energy and they tend to lower their free energy by oxidation/corrosion. This is the reason that vanadium steel can not be used for implants.

5.Stainless steel : The austenite stainless steel such as 316 stainless steel (molybdenum in small percentage) and 316 L stainless steel (carbon up to 0.08% only) are most commonly used for implants. These steels are non magnetic and corrosion resistant. The austenite stainless steel is prone to work hardening and it has to be heat treated after cold working. 316 L stainless steel has 17 to 20% chromium, 12 to 14% nickel & 2 to 4% molybdenum. The inclusion of

molybdenum enhances resistance to pitting corrosion. Nickel stabilizes the austenite (α) phase at room temperature. It also enhances corrosion resistance. The austenite phase stability of steel is influenced by both Nickel & Chromium contents as shown in the figure. The implants of these steels are suitable as temporary implants such as fracture plates, screws and hip nails.

147

25
20
kel
15
ic			Austenite phase
N
↑ 10
5	Ferite phase
0	5	10	15	20	25
		Chronium

Effects of Ni and Cr on Phase of Steel

6.Co Cr alloys : These are mainly two types of cobalt - chronium alloys viz (1) Castable Co Cr Mo alloy (2) Wroughtable by forging Co Cr Mo alloy. The castable is used for dentistry and artificial joints. The wrought Co Cr Mo alloy is used for making prostheses suitable for heavy loaded joints like hip and knee. The wrought Co Cr Mo alloy has good fatigue and ultimate strength and it is used where we require long service life. The cast Co Cr Mo alloy (F – 75) has mainly 22 to 33% Cr, 5 to 7% Mo, 2.5% Ni and balance Co while wrought Co Cr Mo alloy (F 562) has mainly 19 to 21% Cr, 9 to 10.5% Mo, 33 to 37% Ni and balance Co.

7.Titanium and Alloys : Titanium is used for implant as it has low density and good mechanical properties. Titanium (also aluminium) evokes minimum amount of tissue reaction as it forms a tancious oxide layer which resists further diffusion of metal ions to oxygen gas at the interface. While making implant, titanium has to be processed in an inert atmosphere. Titanium is an

allotropic material. It can exist as a hexagonal close-packed structure (α phase) upto 82ºC and as a body - centered cubic structure (β phase) above 82ºC. The transformation temperature changes on addition of alloying elements which enables the titanium alloys to have a wide range of properties. The main alloying elements of the alloy are aluminium (5.5 to 6.5%) and vanadium (3.5 to 4.5%).

148	FUNDAMENTALS OF BIOMEDICAL ENGINEERING

The titanuim nickel alloys have properties of "shape memory effect (SME)". It means that the material can gain its original shape on heating. Therefore these alloys having SME are used for orthopedic implants, contractile muscles for artificial heart, filter for vena cava, orthodontic dental archwires and intracranial aneurysm clips.

8.Dental metals : Dental amalgam is an alloy obtained by mixing silver-tin alloy with mercury. Since mercury is in liquid form, the silver - tin alloy can be mixed in it. The resultant paste is packed into a prepared cavity of tooth. The final composition of dental amalgam contains 45 to 55% mercury, 35 to 45% silver and 15% tin. The amalgam sets in solid form in one day.

9.Gold : Gold being base metal has high corrosion resistance but poor mechanical properties. Gold and its alloys are used as dental materials as they have durability, stability and corrosion resistance. Dental fillings of gold can be carried out by two methods viz. casting and maletting. Mechanical properties of gold can be improved by adding not more than 25% copper or 4% of platinum.

10.Corrosion of metallic implants : The main reason of metallic corrosion is its oxidation. A metal in pure form stays in metastable equilibrium. Tissue fluid in the human body contains water, dissolved oxygen, chloride

and hydroxide ions etc. Hence metallic implants have to face a very corrosive environment. Corrosion is unwanted chemical reaction of metallic implant with environment. The metals tend to lower their energy state by electrochemical reaction with environment. Oxidation and reduction are two electrochemical reactions. Oxidation is a reaction in which electrons are consumed. Oxidation reaction is :–

M → M+n + ne		...(I)
Similarly reduction reactions are –
M+n + ne → M		...(II)
2H2O + O2 + 4e → 4.0H		...(III)
4H+ + O	+ 4e → H O	...(IV)
2	2
2H+ + 2e → H2		...(V)

Equation (III) gives corrosion reaction occurring at neutral PH solution and metallic implants are generally corroded according to this. It is also seen that variation of oxygen on metal surface leads to corrosion with sites with lower oxygen concentration (cracks, dirt or along screw of implants) become anodes and exposed sites with higher oxygen concentration become cathodes. These anodes and cathodes with body solution connecting them to form electro chemical cells leading to deterioration of metallic implants.The tendency of metals to corrode is given by the standard electrochemical series of Nernst potentials as given below :

				Dirt
e	e	e	e	e
e				e
M etal				M etal
Screw in M etal		Cracks in M etal		Dirt on M etal

Deterioration Due to Oxygen Concentration Gradient

METALLIC BIOMATERIALS

149

S.N.	Metal	Potential CV	Remarks
1	L +	+ 2.96	Anode
	i
2	K+	+ 2.92
3	Ca2+	+ 2.90
4	Na+	+ 2.71
5	Mg2+	+ 2.40
6	Ti2+	+ 2.00
7	Al2+	+ 1.70
8	Zn2+	+ 0.76
9	Cr2+	+ 0.56
10	Fe2+	+0.414
11	Ni2+	+0.23
12	Sn2+	+0.14
13	Pb2+	+ 0.12
14	Fe3+	+0.045
15	H	0.000	Reference
16	Cu2+	– 0.34
17	Cu+	– 0.47
18	Ag+	– 0.80
19	Pt2+	– 0.86
20	Au+	– 1.50	Cathode

These protentials are found out in electrochemical measurement in which one electode is metal and other is a hydrogen electrode consisting of porous platinum tube through which hydrogen is passed. The protential of hydrogen electrode is taken as reference i.e. Zero potential. Metal having higher protential than hydrogen electrode are known as noble metals while those having lower protential are known as base metals. If two dissimilar metals are present in a

solution, then the metal having higher protential will become anode and galvanic corrosion starts which is much rapid than corrosion of a single metal. Hence metallic implant should be made of a single metal without any impurity. Any region of stress will become anode with respect to unstressed region of the same material as stressed region has higher energy level. The corrosion of stressed region starts at the earliest opportunity. Also base metals are less prone to corrosion.

150

M etal		Fe++

FUNDAMENTALS OF BIOMEDICAL ENGINEERING

	+0.44v	Porous
		platinum
	V	tube
		H2 gas

Fe++

Fe++

H+	H+
H+	Solution

Measurement of Potential Against Standard Hydrogen Electrode

OBJECTIVE TYPE QUESTIONS

Fill in the gap

1. ------- is a synthetic material to replace a part of a living system. (a) Biomaterial (b) Biometal.

2.The reliability of an implant is ------- if it depends upon two factors having probability

of failures as f1 and f2. (a) f1 × f2 (b) (l – f1) (l – f2)

3.The acceptance of an implant by surrounding living tissues is called -------. (a)bioacceptance (b) biocompatibility

4.Metals have high ------- energy. (a) latent (b) free

5.Vanadium steel is no longer used for implants as it is pron to -------. (a) break (b) corrode

6.Titanium has ------- density. (a) low (b) high

7.Shape memory metals can gain ------- shape on heating. (a) original (b) small

8.Titanium alloys show -------. (a) shape memory effect (b) good mechanical properties

9.Dental amalgam is an alloy obtained by mixing ------- with mercury. (a) silvercopper alloy (b) silver-tin alloy

10.Dental amalgam can set in -------. (a) one week (b) one day

11.Mechanical properties of Gold can be improved by mixing copper not more than -

------ percent. (a) 30 (b) 25

12.Oxidation is a process in which electrones are -------. (a) liberated (b) consumed

13.Reduction is process in which electrons are

-------. (a) liberated (b) consumed

14.------- protential is given by the standard electro chemical series. (a) Faraday (b) Nernst

15.Hydrogen electrode has ------- protential. (a) one (b) zero

16.Gold and silver are ------- metals. (a) noble (b) base

						ANSWERS
1.	(a)	2.	(b)	3.	(b)	4.	(b)	5.	(b)	6.	(a)	7.	(a)	8.	(a)
9.	(b)	10.	(b)	11.	(b)	12.	(a)	13.	(b)	14.	(b)	15.	(b)	16.	(b)

POLYMERIC $ BIOMATERIALS

Human beings dream of life everlasting. But most of them want it on earth and not in heaven.

INTRODUCTION

1.Polymer as name suggests is many mers (small molecules or repeating units) joining together under suitable condition to form a long chain (a heavy molecule). The process of forming a long chain of a heavy molecule from small molecules is called polymerization. The polymerization can be done by condensation (water is condensed out) or by addition (by rearranging bonds within each monomer). Polymer can be linear, branched or cross linked as shown in table below .

Polymer can be obtained by linking of one type of mers (monomers) or more than two types of mers. Hence copolymers are polymers made form two of more types of mers. The degree of polymerization (DP) is defined as average number of mers (repeating units) per molecule (long chain)i.e., Molecular weight of polymerization = DP × molecular weight of mers. The possible arrangements of copolymers can be :–

		M er
S.N.	Type	or Repeating unit	Linking
1	Linear polym er
2	Branched polym er
3	Crossed Linked