Artificial Limbs 3

what are joints

joints are the areas where 2 or more bones meet (most joints are mobile, allowing the bones to move)

what are the 3 types of joints

1. synarthrosis (immovable) or fibrous

2. amphiarthrosis (slightly moveable) or cartilaginous

3. diarthrosis (freely moveable) or synovial

hip joint

a ball-and-socket joint that allows motion and gives stability needed to bear body weight (one of the most stable joints in the body)

what is the acetabulum and femur

The socket area (acetabulum) is inside the pelvis. The ball part of this joint is the top of the thighbone (femur)

what does the cartilage act as in the hip joint

the cartilage acts as a slippery coating between the ball and the socket that allows the ball to glide and rotate smoothly when the leg moves

what allows the rotational motion without any detectable translational motion

Congruity of the femoral head with the acetabulum allows the rotational motion without any detectable translational motion

what defines motion limits

The osseous anatomy of the joint & stabilizing forces of the fibrous capsule and neuromuscular anatomy defines motion limits

what are the degrees of flexion, extension, abduction, adduction, internal rotation, external rotation

• Flexion– 120 degrees

• Extension– 10 degrees

• Abduction– 45 degrees

• Adduction– 25 degrees

• Internal Rotation– 15 degrees

• External rotation– 35 degrees In

what are the 4 types of hip joint pathologies

osteoarthritis, rheumatoid arthritis, hip fracture, bone dysplasia

osteoarthritis

• wear and tear arthritis

• cartilage in the hip joint gradually wears away

• causes: increasing age, family history, previous injury, obesity, developmental hip dysplasia

rheumatoid arthritis

• autoimmune disease

• causes inflammation of the synovial membrane

• causes: genetics, environmental factors, hormones

hip fracture

• injury

• causes: older age, lifestyle, osteoporosis

bone dysplasia

• when the acetabulum is too shallow to support the femoral head

• congenital condition

total hip replacement surgery

• The orthopaedic surgeon, upon accessing the hip joint, proceeds to extract the femoral head. They prepare the thigh bone's surface to accommodate the corresponding part of the prosthesis.

• Once the articular cavity is stabilized, the surgeon readies the interior of the thigh bone to receive the prosthetic shaft

• The prosthetic shaft can be affixed with or without the use of bone cement. The femoral head is affixed to the upper end of the stem

• This head is positioned within acetabular component and connects directly to the liner

components of hip prosthesis

stem, head, liner, metal back (acetabular cup)

what can the stem be made from

stainless steel, cobaltum-chromium alloys, titanium alloys

why is stainless steel used

resistance to oxidation coupled with relative ease of machining, forming, and hardening makes stainless steel a strong candidate for material choice (however does have lower biocompatibility)

why is cobaltum-chromium alloys used

high strength, low corrosion, low wear, high Youngs modulus(however more suitable for cemented prostheses)

why are titanium alloys used

low density, high mechanical strength, excellent corrosion resistance, biocompatible (however poor wear resistance and cementless)

metal on plastic articulation (MOP)

METAL ON PLASTIC ARTICULATION (MOP)

• Head in CoCr and Insert(liner) in UHMWPE

• Traditional coupling

• Shock absorption

• Prone to wear and to create debris ->periprosthetic osteolysis

• Cross-linked UHMWPE decreasees debris by 10 time

CERAMIC ON CERAMIC ARTICULATION (COC)

Head and insert in CoCr

• Low wear- Inhert debris

• 10 year survival rate is up to 89% (age<60)

• Expensive- Complex manufacturing

• No shock absorption

• Complex revision

METAL ON METAL (MOM)

Head and insert in Metal

• Low wear Requires precise positioning

• Reactive debris, worse reaction than MOP

• Systemic issues (granuloma in liver)

• No shock absorption

main advantages and disadvantages of bearing surfaces

see image

what is the difference of cemented vs cementless stem insertion

cemented: bone cement is used for fixing the implant to bone interface

cementless: press fit (short term), osteointegration (long term)

Polymethyl methacrylate (PMMA)

Polymethyl methacrylate (PMMA), is commonly known as bone cement, and is widely used for implant fixation in various Orthopaedic and trauma surgery, not really cement just substance that bonds two things together, acts as a space filler kinda like grout, not adhesive instead relies on close mechanical interlock between the irregular bone surface and the prosthesis

what are the pros of cemented stell insertion

• Immediate stability

• Better load distribution (if PMMA is uniformly distributed

what are the cons of cemented stem insertion

• prone to long term cement fracture (due to high stress)

• cement polymerisation is prone to bubbes

• PMMA polymerisation is exothermal (up to 60C) which can cause bone necrosis in the first layer

• large bore

what are the pros of cementless stem insertion

• small bore

• more stable (as per clinical evidence) if osteointegration is successful

what are the cons of cementless stem insertion

more complex procedure, very sensitive to surgical technique

what does polished surface finish do

Polished is preferred in case of cemented prostheses, since porous coating can cause fracture in the cement layer and lead to aseptic loosening

what does surface finishing enhance

Surface finishing to enhance rugosity, which helps with osteointegration in cementless implantes.

• Porous coating to achieve macro porosity (discontinued)

• Hydroxyapatite coating (HA) by means of plasma-spray coating, to help osteointegration

• Bioglass

• PMMA precoat

short term complications of hip joint replacements

• septic mobilisation

• dislocation

• allergy

medium term complications of hip joint replacements

• structural failure

• aseptic mobilisation

how can the implant be mobilised through stress shielding/necrosis

The load on the operated femur is shared between the implant and the native bone. If the load on the bone is too low, it tends to remodel negatively and reabsorb If the load on the bone is too high, bone cells go into necrosis and tissue dies. In both cases, the implant gets mobilised

long term complications of hip joint replacements

• fatigue

• osteolysis

Osteolysis

Osteolysis is the process of progressive destruction of periprosthetic bony tissue, visible on radiographs. Wear particles (worn off the contact surface of the head-linear interface) migrate along the prosthesis. As the body attempts to clean up these wear particles, it triggers an autoimmune reaction which causes resorption of living bone tissue and implant loosening.

fatigue

The average individual performs roughly 2.5M steps a year. Hip implants experience a complex stress pattern (combination of compression, torsion, bending) which repeats cyclically. Due to cyclic loading, hip implants can undergo fatigue fracture in specific areas (neck, stem).

what is W,D,G,B,F,C,M.S

W=wear

D=deformation

G=Galvanic corrosion

B=bending fatigue

C=crevice corrosion

M=micromotion and wear

S=shock absorber

what are the design requirements for a hip joint replacement

• Allow physiological DoF (rotation) (biomechanical)

• Sustain physiological loads (biomechanical)

• Strength (mechanical)

• Fatigue Resistance (mechanical)

• Wear Resistance (mechanical)

• Biocompatibility (biomechanical)

• Surface Stability (biomechanical)

• Implantability (surgical)

• Replaceability (surgical)

• Biomechanical compatibility (biomechanical)

what are the components are in a knee plant

kneecap component, femoral component, tibial component, plastic spacer

what is the material of the femoral component

CoCr alloy, Ti alloy, Oxinium, ceramic

what is the material of the tibial insert

UHMWPE

what is the material of the tibial component

CoCrMo alloy, Ti alloty UHMWPE

what is the material of the patellar compoent

UHMWPE