Implant Technology Unit 1 Flashcards
what are ortho implant devices and their function
devices made from non-biological materials to improve structure and/or function
- either provide structural support after an injury (i.e. bone fixators)
=- replace or modify injured, diseased and painful joints
what qualities must an implant have
- be tolerated by human body with no short or little long term risk (e.g. carcinogenesis)
- relieve pain and enable sufficient mobility for ADL
- adequate strength
- function w/out failure for as long as it is required. Ideally should last expected life space.
- practicability of insertion; predictable outcome reasonably guaranteed by competent surgeon
- cost effective
what are most implants made of
metal e.g. stainless steel, titanium alloy
[in joints, bearing surface is made of a plastic material as metal-to-metal causes unsatisfactory result]
what is the main problems associated with implants
infection
- bacteria attracted to metal/cement surfaces
- commonly the normal bacteria found on the skin the culprit
- implant has to be removed
from a structural standpoint, what is the most important factors in the design of an implant
strength
stability
- its fixation to body tissue should be free from movement
- should function in harmony with the natural structures of the body, especially bone
what is important to remember about the cost of the implant
same implants have different prices in different countries
despite low cost of hip replacement it still remains outside the reach of the majority of countries
what are the 3 categories of performance of an implant technology
structural factors
kinematic factors
biocompatibility
what are the factors under ‘structural factors’
strength
- Components must withstand loads acting on them w/out deforming permanently or breaking
stiffness
- components must be rigid enough to bear load without excessive deflection, while not being so stiff that they adversely affect the loading on adjacent tissues
lubrication
- Moving parts must be adequately lubricated or require no lubrication
wear
- The rate of wear of bearing surfaces must not cause failure or generate wear particles which damage body tissues
fatigue
- fatigue life should be greater than the intended life of the implant
what are the factors under ‘kinematics factors’
Motion
- ROM must be sufficient to enable daily living functions, even if it is less than normal joint ROM
- The directions and patterns of motion must be controlled to ensure stability
what are factors under ‘biocompatibility’
biological integration
- Harmful reactions of implant materials w/ body tissues shouldn’t exceed accepted safe levels
- corrosion of materials by the body should not cause the implant to fail
functional integration
- implant should perform such that it does not adversely affect the function of other parts of the body
what are the 2 types of bones
compact bone
- a.k.a cortical bone
cancellous bone
how are bones designed to bear load
- 5 things
- end regions of the bones are shaped so as to accommodate the joint i.e. wider at the ends
- end regions of the bones contain cancellous bone which is more porous and less stiff (more flexible) than cortical bone, giving shock absorbing properties
- in cancellous bone, trabecular are aligned along the directions of greatest stress
- region below articular surface is more dense than the cancellous bone below it, provides a rigid underlying surface for the joint to bear on w/out causing excessive deformation
- shafts of bones contain dense compact bone, more rigid than cancellous bone, provides the necessary resistance to deformation under bending and torsional loads
how is stiffness measure
Young’s modulus (E)
- ratio of stress to strain
how does Young’s modulus change in most material when loaded
remains approximately constant irrespective of the load applied or the rate of loading
What does is mean when a material is said to be isotropic
that their mechanical properties are the same no matter which direction they are loaded
what does anisotropic mean and what materials exhibit this behaviour
it’s young’s modulus depends on the direction in which it is being loaded
bone
what direction is cortical bone stiffest and strongest
when loaded longitudinally
- [main direction in which it is loaded naturally]
what is meant when a bone is described as viscoelastic
the stiffness of bone changes according to the rate at which it is loaded
- faster it is loaded the stiffer it becomes
when is cortical bone strongest and weakest
strongest - under compressive loading
weakest - under shear loading
how are implants designed in terms of loading the bone
try to load the bone in compression
avoid shear stress especially but also try avoid tensile
avoid excessive stress shielding
what is stress shielding/stress protection and what is an example in orthopaedics
when bone is reabsorbed because of reduced loading
i.e. Wolff’s Law
caused in orthopaedics by bone plates; bone around plate gets reabsorbed; can lead to loosening of fixation screws meaning it is no longer effective in supporting the bone
what is the main difference in structure between bone in diaphysis and bone in the region of a joint
Diaphyseal bone
- made from compact bone
- rigid and provides resistance to deformation under loading.
Bone in the region of a joint
- cancellous
- the trabeculae are aligned along directions of greatest stress
- much less rigid than cortical bone and has good shock absorbing properties
- Bones are generally wider at the joints than at the diaphyses.
what is the major role of orthopaedic implants
provide structural support
in an orthopaedic implant:
BONE FIXATOR
- what are the 3 regions of the implant, assuming the bones touch at the fracture site
Region 1 = LOAD TRANSFER
- where the screws fix the plate to the bone
- here, part of the applied load in the bone is transferred to the plate
Region 2 = LOAD SHARING
- the fracture site, where the broken bones are supported by the plate
- here, part of the load is taken by the plate and part by the bone
Region 3 = LOAD TRANSFER
- where the screws fix the plate to the bone at the other side of the fracture
in an orthopaedic implant:
INTRAMEDULLARY STEM OF A CEMENTED JOINT REPLACEMENT
- what are the 3 regions of the implant, assuming the bones touch at the fracture site
Region 1 = LOAD TRANSFER
- where part of applied load is transferred from the stem to the bone
Region 2 = LOAD SHARING
- load sharing between the bone and the stem
Region 3 = LOAD TRANSFER
- remaining part of the load is transferred from the stem to the bone
- the bone below this region, takes all the load
where is the load transferred for bone plate and for the intra-medullary stem
bone plate
- load transferred at bone screw region
intra-medullary stem
- load transferred at end regions of the stem only
what is meant by load sharing and load transfer
Load sharing region
- For an implant attached to bone, the regions where the load is partly taken by the bone and partly taken by the implant
Load transfer region
- where load is transferred from an implant to a bone (or from a bone to an implant)
- load passes across interfaces between them
what does the load generate at the interface
and what serious complication can occur due to this
stresses or relative movement at the interface
stresses = occur when the two materials are bonded together
relative movement = occurs either if they are not bonded or if a bone comes loose
Loosening - serious complication in joint replacement
if there was two materials, one on top of another, and the bottom material was more flexible than the top
what would happen under loading
the bottom half would compressed more under loading and expand laterally more than the top
if there was two materials, one on top of another, and the bottom material was more flexible than the top
what would happen under loading if the two materials were bonded together
any lateral strain at the interface is the same for both materials, so a shear stress is generated at the interface, because one material is trying to expand more than the other one
if there was two materials, one on top of another, and the bottom material was more flexible than the top
what would happen under loading if the two materials were not bonded together
if it is also lubricate
sliding can occur freely so there are no shear stresses
what is the general rule about the difference in the young’s modulus and shear stress
greater the difference in young’s modulus then the greater the shear stress generated
if there was two materials, one on top of another, and the top material is less stiff than the bottom
what is stress at the interface like
stresses at the interface under the region of an applied load from above will be much greater
if there was two materials, one on top of another, and the top material is less stiff than the bottom
what is the advantage of using a stiff tibial component for a knee prosthesis
distributes loads more evenly over the underlying bone than a material with a lower stiffness, such as polyethylene
why does shear stress occur at a bone-implant interface
occurs at a bone-implant interface because the bone and implant each have a different material stiffness i.e. young’s modulus
so they try to deform by different amounts under a load
If joined together they cannot deform separately so a shear stress develops between them along line of interface
what is important to note about shear stress at the interface
shear stress is not constant across the whole length of the interface
[there is no shear stress in the central portion, which is a region of load sharing]
what is the equation for shear stress
shear stress = applied force / area being sheared
what causes stress concentration
sharp corners, notches, holes
what is a consequence of stress shielding
osteopenia due to bone reabsorption
structural stiffness is determined by what 2 factors
material stiffness
- basic property of the material
- i.e. its Youngs Modulus
geometrical stiffness
- shape of the cross section of the structural component
how does metal’s properties compare to bone
10 times stiffer than cortical bone
many more times stiffer than cancellous bone
are isotropic unlike bone
how do you calculate shear modulus
G = shear stress/shear strain
[calculated by applying a twisting load to a material]
what do you measure when you are measuring how stiff something is
measure of how much it deflects under load
defined as = force require to produce a unit deflection
[stiffness = S or k ]
what is the equation for stiffness
S = force / displacement
S = EA/L
E = youngs modulus A = area L = length
