1. Lecture 11 Orthopaedic Implant: Internal Fixation
BIOMATERIALS ENT 311/4
Lecture 11
Orthopaedic Implant: Internal Fixation
Prepared by: Nur Farahiyah Binti Mohammad
Date: 15th September 2008

2. Teaching Plan COURSE CONTENT DELIVERY MODE LEVEL OF COMPLEXITY
Define various types of internal fixation.
Identify failure modes of internal fixation.
Describe and compare three types of fixation methods
Describe and discuss types of joint replacements.
Describe and recommend biomaterial use to make components of joint replacement.
DELIVERY
MODE
Lecture
Supplement reading
LEVEL OF COMPLEXITY
Knowledge
Repetition
Analysis
Evaluation
COURSE OUTCOME
COVERED
Ability to select biomaterials that can be used for different medical applications and explain the criteria that will lead to a successful implants

3. 1.0 Internal Fixation
Purpose: To stabilize fractured bone until natural healing processes have restored sufficient strength so that implant can be removed.
Material requirement:
Biocompatible
Sufficient strength
Corrosion resistance

4. 1.0 Internal Fixation Material used: Stainless steel Co-Cr alloys
Titanium alloys
Biodegradable polymer
To treat minimally loaded fractures
Eliminate the need for second surgery

5. 1.0 Internal Fixation Types: Wires Pins Screw Plates
Intramedullary nails

6. 1.0 Internal Fixation WIRES Used to reattach large fragments of bone
Useful especially for spiral breaks and reattaching greater trochanter of hip
Wires suffer from twisting and knotting
The deformed region of the wire are more prone to corrosion

7. 1.0 Internal Fixation PINS
Used to hold fragments of bone together temporarily or permanently and to guide large screw insertion.
Have different tip design
Trocar tip – most efficient in cutting
- often used for cortical bone
Diamond tip

8. 1.0 Internal Fixation SCREWS Two types of bone screws:
Cortical (Compact) bone screw-small treads
Cancellous screws –large tread to get more thread-to bone contact

9. 1.0 Internal Fixation
It can be used alone or along with other devices.
The general principle is that bone heals better if the fracture fragments are aligned and pressed closely together.
The idea is to stabilize the fracture and keep the bone in anatomic alignment.

10. 1.0 Internal Fixation

11. 1.0 Internal Fixation Principle application of bone screw:
As interfragmentary fixation devices to lag or fasten bone fragment together.
To attach a metallic plate to bone

12. 1.0 Internal Fixation PLATES
Plates come in several types, and are named for their function.
In general, there are:
Dynamic Compression plates
Neutralization plates
Buttress (support) plates.

13. 1.0 Internal Fixation
The dynamic compression plate is one of the most common types of plates, and can be recognized by its special oval screw holes.
These holes have a special beveled floor to them with an inclined surface.
If desired, this inclined surface can be used to pull the ends of the bone together as the screws are tightened.

14. 1.0 Internal Fixation
Compression plates are used for fractures that are stable in compression.
They may be used in combination with lag screws, and they may provide dynamic compression when used on the tension side of bone.

15. 1.0 Internal Fixation
Neutralization plates are designed to protect fracture surfaces from normal bending, rotation and axial loading forces.
They are often used in combination with lag screws.

16. 1.0 Internal Fixation Resorbable bone plate
As the strength of the fracture site increase due to normal healing processes, the resorption of the implant begins to take place.

17. 1.0 Internal Fixation
Buttress plates are used to support bone that is unstable in compression or axial loading.
These plates are often used in the distal radius and tibial plateau to hold impacted and depressed fragments in position once they have been elevated.

18. Buttress plate bridging a humeral neck fracture -

19. 1.0 Internal Fixation

20. 1.0 Internal Fixation
5. INTRAMEDULLARY NAILS

21. 1.0 Internal Fixation
Used as internal struts to stabilize long bone fracture.
Inserted into medullary cavity
Should have some spring to provide elastic force and prevent rotation.
IM nails are better than plates at resisting multi-directional bending
However, torsional resistance is less than plate

22. 1.0 Internal Fixation When designing IM nails: Problems:
A high polar moment of inertia is desirable to improve torsional rigidity and strength
Problems:
Destroy intramedullary blood supply.

23. 1.0 Internal Fixation Biomaterial applications in Internal Fixation
Properties
Applications
Stainless steel
Low cost, easy fabrication
Surgical wire, Pin, plate, screw, IM nails
Titanium alloy
High cost
Low density and modulus
Excellent bony contact
Surgical wire
Plate, screw, IM nails
Co-Cr alloys
High density and modulus
Difficult fabrication
IM nails
Polylactic acid
Resorbable
Pin, screw

24. Failure modes of internal fixation devices
Failure location
Reason for Failure
Overload
Bone fracture site
Implant screw hole
Screw thread
Small size implant
Unstable reduction
Fatigue
Fracture
Corrosion
Screw head-plate hole
Bent area
Different alloy implant
Over tightening screw
Misalignment of screw
Over bent
Loosening
Screw
Motion
Wrong choice of screw type

25. Lecture 11 (cont) Orthopaedic Implant: Joint replacement
BIOMATERIALS ENT 311/4
Lecture 11 (cont)
Orthopaedic Implant: Joint replacement
Prepared by: Nur Farahiyah Binti Mohammad
Date: 18th September 2008

26. 1.0 Introduction Total joint replacement
An arthritic or damaged joint is removed and replaced with an artificial joint called prosthesis.
Goal - to relieve the pain in the joint caused by the damage done to the cartilage.

27. 1.0 Introduction

28. Joint degradation Also called Osteoarthritis.
Is the end stage of a process of destruction of the articular cartilage.
Results in:
Severe pain
Loss of motion
An angular deformity of the extremity
Cartilage unable to regenerate
So, when exposed to a severe mechanical, chemical or metabolic injury, the damage is permanent.

29. Joint degradation

30. Types of Total Joint Replacement (TJR)
Hip
Ball and socket
Knee
Hinged, semiconstrained, surface replacement, unicompartment or bicompartment
Shoulder
Ankle
Surface replacement
Elbow
Hinged, semiconstrained, surface replacement
Wrist
Ball and socket, space filler
Finger
Hinged, space filler

31. Implant fixation method
Three type of methods of fixation:
Mechanical interlock
This fixation achieved by press-fitting the implant
by using PMMA bone cement
Bone cement
Bone cement is a substance commonly used to hold implants in bone and filling the space between the skeleton and the total joint device.
Often cement is used for hip replacement and knee replacement surgery.
Cement implies that the material sticks the implant into the bone
Fixation=pemasangan

32. Implant fixation method
In reality, bone cement should really be called bone grout.
The reason is that this material actually acts as a space-filler, to create a tight space for the implants to be held against the bone.
Bone cement does not stick substances together, rather it fills the void between the implant and the surrounding bone.
Grout = mortar (simen) cair

33. Implant fixation method
On the X-ray pictures one sees the bone cement as a white layer around the   shadows of the total hip device.

34. Implant fixation method
The microscopic structure of bone cement is made by two substances glued together.
One substance are the small particles of pre-polymerized PMMA (PolyMethylMetaAcrylate), so called "pearls.
These pearls are supplied as a white powder.
The other substance is a liquid monomer of MMA(MethylMetacrylate). Both substances are mixed together at the operation table with added catalyst that starts the polymerization of the monomer fluid.

35. Honeycomb structure gives the bone cement the ability to absorb downward (compression) loads.
An important characteristics in an otherwise brittle material.
the bone cement act mechanically as an shock absorber
(1) The unloaded phase: the bone cement net is regular, not deformed.
( 2) Load applied by body weight impact on the total hip (its femoral component):  the bone cement net within marrow cavity deforms elastically, but does not brake and stays in contact with both total hip and skeleton.
( 3 ) When the load ceases  the bone cement's structure returns to its original regular form.

36. Implant fixation method
Biological fixation
Which is achieved by using textured or porous surface, that allow bone to grow into the interstices.
Porous in growth fixation
Pore size range should be 100 to 350μm
Pores should be interconnected with each other with similar size of opening.

37. Implant fixation method
Direct chemical bonding between implant and bone
for example by coating the implant with calcium hydroxyapatite, which has mineral composition similar to bone.
Material used as coating such as Bioglass and glass ceramic.

38. Biomaterials for TJR Material Application Properties Co-Cr alloy
Stem, head (ball), Cup, porous coating, metal backing
Heavy, hard, stiff
High wear resistance
Titanium alloy
Stem, porous coating
Metal backing
Low stiffness
Low wear resistance
Light
Pure titanium
Porous coating
Excellent osseointegration
Tantalum
Porous structure
Good mechanical strength
Alumina
Ball, cup
Hard, Brittle,
Zirconia
Ball
Heavy and high toughness
UHMWPE
Cup
Low friction, wear debris, low creep resistance
PMMA
Bone cement fixation
Brittle, weak in tension
Low fatigue strength

39. Comparison between Specific Orthopedic Implant Prosthetic Materials

40. Comparison between Specific Orthopedic Implant Prosthetic Materials

41. Total hip replacement Hip joint is a ball and socket joint
Can be divided into 2 types:
Monolithic: Consist of one part, less expensive, less prone to corrosion
Modular: Consist of 2 or more parts , allow customizing of the implant during future revision surgery.

42. Total hip replacement Design aspect
Prosthetic hip component are optimized to provide wide range of motion to prevent dislocation.
Must enable implants to support loads
Proper femoral neck length will decrease bending stress

43. Total hip replacement
If femoral stem is designed with sharp corner: bone in contact with sharp corner may necrose and resorb.
Replaceability: possible to remove one part without removing the other.

44. Hip joint replacement Component or Total Hip joint: Femoral component
Stem –Neck
Ball/head
Acetabular component
Cup
Backing
Insert
Acetabular component
neck
Femoral component
stem

45. Schematic representation
Cortical bone
Trabecular bone
Bone cement
Femoral stem
4 (a). Backing of acetabular cup
Insert of acetabular cup

46. Materials used for each component of THR
Femoral component
Acetabular component
Stem
Ball/head
Cup
Backing
Co-Cr alloy
Titanium alloy
Alumina
Zirconia
UHMWPE
Metal

47. Possible combination of THR
Femoral component
Acetabular component
Fixation
Stem
Head
Cup
Backing
PMMA
Bone ingrowth
Press fitting
Co-Cr alloy
Titanium alloy
Alumina
Zirconia
UHMWPE
Metal
None
Screw or press fitting

48. Total hip replacement STEM Titanium alloy Advantages: Disadvantages:
Excellent corrosion resistance
Highly reactive material
Lowest rate dissolution
Disadvantages:
Wear
Generation of fine wear particles: inflammation and implant loosening

49. Total hip replacement HEAD
Alumina: elicit minimal response from host tissue
Advantages:
High Wear resistance
Reasonable fracture toughness
Extremely stable (undergo little physical/chemical deformation)

50. Total hip replacement Disadvantages: Degradation in-vivo:WEAR
Weakens the material
Change shape that may effect fuction
Produce biology active particles
Low creep resistance: can influence the behaviour of joint

51. Total hip replacement CUP UHMWPE Advantages: Disadvantages:
Tough inert
Disadvantages:
Wear debris cause inflammatory reaction

52. Most frequent problems
Infection
Wear
Migration and failure of implants
Loosening

53. WEAR Degradation in vivo
Wear debris produced by load bearing and motion of the prosthesis
particles generated each step
Cells from the immune system of the host, such as macrophages, will identify the wear particles as foreign matter and initiate a complex inflammatory response

54. WEAR Problem that will be occur as a result of wear are:
Rapid focal bone lose (osteolysis)
Bone resorption
Loosening
Fracture of bone

55. Shoulder Joint Replacement
Humerus fractures -caused by a direct blow or by a fall or osteoporosis (thinning of the bones).
the ball is removed from the top of the humerus and replaced with a cobalt chrome or titanium implant. This is shaped like a half-moon and attached to a stem inserted down the center of the arm bone.
The socket portion of the joint is shaved clean and replaced with a plastic socket that is cemented into the scapula.

56. Elbow Joint replacement
Humeral component
replaces the lower end of the humerus in the upper arm.
has a long stem that anchors it into the hollow center of the humerus.
Ulnar component
replaces the upper end of the ulna in the lower arm.
has a shorter metal stem that anchors it into the hollow center of the ulna.
The hinge between the two components is made of metal and plastic.
The plastic part of the hinge is tough and slick.
allows the two pieces of the new joint to glide easily against each other as you move your elbow.

57. Knee Joint Replacement
Femoral component
upper part of a knee system
made of a strong polished metal.
covers the end of the femur
Patellar component
replaces the kneecap in the center of the knee. 
Tibial component
covers the top end of tibia
covered with a metal tray
topped with a disk-shaped polyethylene insert (sits on a highly polished surface and rotates around a conical post)

58. Knee Joint Replacement

59. Ankle Joint Replacement
Tibial component
replaces the socket portion of the ankle (the top section)
metal tray is attached directly to the tibia bone
plastic cup fits onto the metal piece, forming a socket for the artificial ankle joint.
Talus component
replaces the top of the talus.
made of Titanium
fits into the socket of the tibial component