Mechanical Safety Lecture 6

8/9/2019 Mechanical Safety Lecture 6

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EXERCISE 1

A receptacle tester has the circuit shown

in Fig. Indicate which LEDs are ON or

OFF for fault conditions indicated in the

table. LED1 2 3

Hot open

Neutral open

No possible wiring

Ground open

Hot and ground

reversed

Hot and neutral

reversed

Hot open and neutralhot


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EXERCISE 2

List the switch positions in the block

diagram of a safety analyzer necessary to

measure the following quantities on a

piece of equipment under test (EUT).Switch positions

S1 S2 S3 S4 S5

Chassis leakage with

ground fault

Patient lead leakage normalequipment

Driven lead-to-lead leakage

Chassis leakage, hot to

neutral reversed


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KUEU 4132

Lecture 6: Mechanical Safety


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THE EFFECTS OF LOADING

When an external force is applied to

the human body, several factors

influence whether an injury occursy Magnitude and direction of force

y Area over which force is distributed

y Load-deformation curve

y Yield point (elastic limit)

y Failure


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Usually, dual-wall design fabricated from

lightweight plastic or graphite composite

materials.y Rigid inner socket providing total contact fit + outer wall

matching the length and the contour of the opposite, sound limb.

y Rigid frame, flexible liner approach. Inner socket is fabricated

from flexible plastic materials + rigid frame for structural support

and for attaching necessary cables/joint.

PROSTHETIC SOCKET (INTERFACE)


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Volume fluctuationy Lead to decreased comfort, increased shear forces, increased pressure on

bony prominences, pistoning and skin breakdown, as well as a poor gaitpattern.

y Caused by weight gain or loss, or childhood growth.

Pistoning

y Create greater shear forces between the limb and the socket and higherpeak pressures during loading response; functional elongation of theprosthesis during swing phase.

y Caused by poor suspension, a poorly designed socket, or volume fluctuationsof the limb.

Skin healthy Cause reddening of tissues and may lead to blisters, ulcerations, and

ultimately such problems as verrucous hyperplasia.

Poor gaity Cause an unnatural gait pattern, such as limping or an asymmetrical gait.

y Put unnecessary forces on the sound limb as well as the rest of the body, andcan cause overuse problems to these areas.

PROBLEMS IN LOWER LIMB SOCKET FIT

Loading

response

Initial

swing


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ADVANCES IN SOCKETS

Illustration 1 - Example of Anatomically Contoured and

Controlled Interface socket. Photo courtesy ofRandallAlley, CP, FAAOP

Illustration 2 - Comparison of narrow ML socket to

conventional socket shape. Based on information from J.Thomas Andrew, CP, FAAOP. Used by permission.

Illustration 3 - Range of motion possible with a roll-on

suspension prosthesis.

Illustration 4 - Shielded cables and electrodes.


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MECHANICAL TESTING

Prosthetic devices: ISO10328.

Proper materials combination or recipe to fabricatea strong and light patient interface.

Thermoplastic materials reinforced plastics known

as composites. Textile preform materials in the form of braids,

stockinet, and other woven goods are used incombination with thermosetting resins to providestrength, durability and adjustability.

I-beam principle: two thin plates of composite areseparated at a great distance from each other by alight weight material such as urethane foam orhoneycomb

y Increase strength without greatly increasing weight.


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MECHANICAL TESTING

The dimensional stability of the composite

structure is primary to resisting buckling

moments of inertia during the compression cycles

of walking and running.

Tests:

y Axial compression test

y Bending test


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PRESSURE ULCERS Affects people who are confined to bed for

extended period of time, or wheelchair users, or

those with sensory deficiency such as diabetics.

Their development and severity may be affectedby vascular occlusion, ischemia and/or pressure

intensity.


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PRESSURE ULCERS Extended periods of uninterrupted pressure and shear occlude

blood and lymphatic circulation, causing deficient tissuenutrition and a buildup of waste products due to ischemia.

Blood vessels collapse and thrombose if the pressure is notrelieved over time. Once occlusion of the blood flow hasoccurred, ulceration continues the process.

When tissues have been compressed for prolonged timeperiods, tissue damage continues to occur even after thepressure is relieved.y Tissue damage can be produced by low pressures over an extended

period of time or high pressures for shorter periods of time.


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PRESSURE ULCERS Factors that augment the risk of pressure ulcer development in

the perioperative setting:y Shear: The presence of shear may decrease the time that tissue can remain

under pressure before ischemia occurs.

y Pressure: The sustained high pressure from specific patient positions and/or

various devices such as use of supports and straps, pneumatic tourniquets, andunyielding electrode adhesives for an extended time (>2- 3 hrs) may shorten the

time to pressure ulcer development.

y Temperature: Elevated tissue temperatures increase the oxygen consumption

rate of the local cells, thereby shortening the time to death from ischemia.

y Time: The severity of the ulcer is determined by the length of time pressure is

applied to any given location. The probability of developing a pressure ulcer

increases with the duration and intensity of the pressure and shearing force

acting upon the tissue during surgery.


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ULCER STAGING


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PREVENTION/TREATMENT STRATEGIES

Preoperative strategies:y Collect information on devices to be used during

surgery.

y

Identify patients at risk.y Maximize nutritional status.

y Develop a normothermia strategy to be employed.

y Minimize skin exposure to moisture.

y Assess and modify increased pressure situations.

y Reduce or eliminate friction and shear.

y Avoid massaging over bony prominences or areas

that have been damaged by pressure.


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Intraoperative strategies:y Protect and position patient properly.

y Protect pressure-sensitive areas when placing a patient in

a prone, supine, or lateral position.

y Ensure adequate peripheral perfusion in pressure areas.

y Minimize skin exposure to moisture intraoperatively.

y Smooth out all sheets, pads, and other materials beneath

the patient while on the operating room table.

y Consider additional criteria if patient temperature

regulators are used (i.e. hypo/hyperthermia unitoperation), e.g.

Thermostat settings, heat exposure temperature (both direct

and indirect), temperature probe placement, system and

patient temperature monitoring and patient pressure

maintenance (especially on bony prominences).


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Postoperative strategies:y Consider removing adhesive and gel interfaces from the skin immediately

following the surgical procedure.

y Record any observed changes or abnormalities.

y Mobilize early after the surgical procedure.

y

Position patients to prevent them from lying directly on their trochanters.y Reposition patients who are confined to bed at least once every two hours.

y Provide complete and total relief of pressure for the injured area postoperatively.

y Consider placing the patient on a pressure relieving device

y Select pressure relief systems that reduce pressure at the interface between the

underlying supporting surface (e.g., dynamic alternating pressure support

systems).

y

Use positioning devices to prevent contact with bony prominences.y Cleanse skin at the time of soiling and at routine intervals.

y Maintain the head of the bed at the lowest degree of elevation (30 laterally

inclined) consistent with the patients medical conditions.

y Minimize the environmental risk factors such as low humidity (i.e., less than

40%).


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PEDICLE SCREW

Used sometimes in a spinal fusion to add extrasupport and strength to the fusion while it heals.

Pedicle screws are placed above and below thevertebrae that were fused. A rod is used to connectthe screws which prevents movement and allows the

bone graft to heal. After the fusion is completely healed, the screws and

rods can be removed. Removal isn't necessary unlessthey cause the patient discomfort.


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STRENGTH

The strength of the stabilized segment will

depend upon the soundness of the screw fixation

within the pedicles and the design of the fixation

system itself.

Decreasing the rod diameter resulted in a loss of

stiffness and bending strength.

Strength at the bonescrew interface may also be

altered by the screw orientation, its depth of

penetration and other parameters associatedwith the design of a pedicle screw.


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MECHANICAL TESTING OF PEDICLE SCREW

Prior to implantation, spinal implants are

subjected to rigorous testing to ensure safety and

efficacy.

A full battery of tests for the devices may includemany steps ranging from biocompatibility tests to

in vivo animal studies.


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MECHANICAL TESTING OF PEDICLE SCREW

The implant must be able to

y Withstand, without failure, the forces created in the

spine during daily activities

y Maintain the necessary position of the motion

segments during the process of bony fusion.

Test likely failure modes

y Bending compression quasi-static

y Bending compression cyclic test


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SPINAL IMPLANT ASTM F1798: Standard guide for evaluating the

static and fatigue properties of interconnection

mechanisms and subassemblies used in spinal

arthrodesis implants

ASTM F1717: Standard test methods for spinal

implant constructs in a vertebrectomy model


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ASTM F1798 This test evaluates the properties of

interconnection mechanisms of a single screw,

rod and lock nut as shown in the picture.

4 tests to be performed:y Static axial grip (slip test)

y Static torsion grip

y Static flexion-extension (FE) moment

y Dynamic flexion-extension (FE)


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TESTING REQUIREMENTSComponent Size Qty Remarks (Refer Fig. 1 and 2)

Fixed A ngle Screw 6.6 x L40mm 32 pcs F1798

i. Static Axial Grip: Four (4) samples

ii. Static Torsion Grip: Four (4) samples

iii. Static FE Moment: Four (4) samples

iv. Dynamic FE: Four (4) samples

F1717

i. Static Tension: Eight (8) samples making up two (2) constructsii. Static Compression: Eight (8) samples making up two (2)

constructs

Lock Nut 32 pcs Quantity corresponds to the number of screws

Rod 5.5 x L70mm 24 pcs F1798

i. Static Axial Grip: Four (4) samples

ii. Static Torsion Grip: Four (4) samples

iii. Static FE Moment: Four (4) samples

iv. Dynamic FE: Four (4) samplesF1717

i. Static Tension: Four (4) samples making up two (2) constructs

ii. Static Compression: Four (4) samples making up two (2)

constructs

Cross connector 4 sets F1798

Not required.

F1717

i. Static Tension: Two (2) sets making up two (2) constructs

ii. Static Compression: Two (2) sets making up two (2) constructs


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ECRI (1 CASE)

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Mechanical Safety Lecture 6