2.3 Hip Prosthesis Design
The origins of THA
The origins of THA
34. What is Youngs Modulus of elasticity?
How is it defined?
The ability of material to resist deformation.
E= Stress / Strain
35. What are the most commonly used materials from which hip prosthesis stems are manufactured?
- Stainless steel
- Cobalt Chrome Alloy
- Titanium and its alloys
36. In terms of elasticity, how do Cobalt Chrome and Titanium Alloy compare?
How do they compare to that of bone?
Cobalt Chrom E= 230
Titanium E=106
Bone E= 7.25
43. What is the difference between Hydrostatic:
- Hydrostatic fluid film lubrication
- Hydrodynamic fluid film lubrication?
- Hydrostatic:high pressure into bearing
- Hadrodynamic: fluid film generated by the motion of bearing
44. Where Hydrodynamic fluid film lubrication exists what factors can influence the esstablishment and maintenance of the film?
- Squeeze pressure
- viscosity of th fluid
45. What is mixed lubrication?
A mixture of fluid film and boundary lubracation.
Happen when low viscosities or on high load.
48. Describe the five possible bearing couples used in total hip arthroplasty.
How do volumetric wear rates compare for a given head size?
- Metal head on polyethylene
- Ceramic head on polyethylene
- Ceramic on ceramic
- Metal on metal
- Ceramic head on metal
46. What type of lubrication is observed in synovitic joints?
Mixed lubrication
28. How can offset be increased in prosthetic hip stems?
head offset increases proportionally with stem sizes
additional set of high-offset stems with medially moved neck segment
- +4 offset liner cup
- Face-changing liner cup
- reduce neck angle and increase neck lenght
42. What are
- friction
- boundary lubrication
- fluid film lubrication?
2.3S69
- resistance to relativ sliging movement of contacting surfaces
- no fluid in the contact area of the surfaces
- entrained layer of fluid film between contacting surfaces
25. What is the purpose of augmented polyethylene cups and liners (e.g. the "Chanley LPW" cup, or the "lipped liner")?
2.3-S65
increasing stability by increasing coverage of the head
To stabilise partially the joint against dislocate but not in the whole angle
Charnley: impingement points reduced posterior by 110°
Liner: snaped in, adjust the centre of rotation and restore appropriate soft tissue tension
37. Ceramic femoral heads are connected to the stems by tape coupling.
What are the reasons for this?
Metal Sleeves Taper Delta TS, wird für Revision verwendet. Wenn der Stem weiterverwendet wir, kann der Konus leicht beschädigt werden, und die Keramik köpfe dadurch kapput gehen
24. How is range of motion between a hip stem and a cup influenced by the head : neck ratio?
2.3S89
The loss of motion is greatest in designs with small diameter heads an large diameter tapers
ROM: ends at point of impingement of prosthetic femoral neck against
the month of the bearing
41. Why are Titanium alloys inferior bearing metals?
zu weiches Metal für Gelenke, harte Metalle werden benötigt
20. What is the minimum range of motion which modern hip replacement prosthesis are designed to achieve?
132°
arc of motion: was das Implant kann
range of motion: was der Pation kann
29.Distinguish (Unterscheide) between "composite beam" and "taper slip" loading in cemented hip prostheses.
2.3S91,92
Composite beam
Thick cement mantle, stem absorbs bending forces & increases stiffness
taper-slip: stem designed to accomoate small platic changes in the cement mantle ==> self locking by moving deepter into the canal
==> der soll nachrutschen
sehe keinen Unterschied!!!
39. Incongurent bearing surfaces are associated with high contact stresses.
Which wear process is associated with this?
abrasive third body wear
40. In metal/Polyethylene hip bearing couples, how does surface finisch (and scratches influence wear rates?
scratches increas the of the softer side of the bearing
size of grain structure influences wear on soft side
47. What is the relationship between femoral head diameter and polyethylene wear in a metal head/polyethylene liner bearing couples?
Grösserer Durchmesser mehr Fläche mehr Abrieb
38. Why is it important to avoid pairing ceramic components (i.e. head and liner) from different manufacturers?
Konus muss sehr exakt sein, da der Keramik sehr sensible sind und ist sehr anfällig auf schlechte Passform
01. What features of the external form of the cemented acetabular component interact with bone cement to facilitate fixation?
Grooves are machined into the outer surface.
On implantation, cement flows into these grooves providing a mechanical interllock often referred to as inteterigitation.
02. What does interdigitation mean?
What is necessary to ensure adequate interdigitation?
the cement must also interdigitate with the bone and can only be adequately achieved by sustained pressurisation of the cement at the time of cup insertion.
03. What is the potential consequence of failure to adequately pressurize bone cement in the presence of bleeding bone?
If the pressure is released too early in the polymersiation cycle, blood pressure within the bone will displace the cement and little or no interdigitation will be achieved.
04. What is considered the minimum acceptable cement mantle thickness over the entire implant surface?
minimum of 2mm of cement over the entire surface.
05. What feature of the Charnley PIJ and Ogee cups enhance cemented fixation?
to solve the problem about cement extrusion.
06. Polyethlene is invisble on x-ray. What feature of polyethylene cups enable the surgeon to visualise their position?
Equatorial and or polar x-ray marker wires are imbedded in the outer surface of the cup.
07. What types of fixation method are used in cementless acetabular components?
is achieved by bone ingrowth into a microporous structure applied to the outer surface of the shell - called Porocoat
08. The DePuy proprietary porous coating is called Porocoat.
What are the characteristics of this coating?
pore size is 100-400 microns (avg. 250 microns.
Volume porosity 40-50%
porosity increases from 20% to 87% at the surface
09. What form of biological fixation is achieved with the Hydroxyapatite coated implants?
The HA forms a biocompatible surface which encourages bone formation.
Gaps of up to 2mm between the implant and the host bone can be bridged as new bone is formed on the surface of the HA.
10. ??How does the thickness of Hydroxyaptite coating influence it's use in cemented hip arthroplasty?
??
11. What is DuoFix?
combines HA and Porocoat to provide an osteo-inductive environment creating more extensive and rapid bone ingrowth.
12. Why might bone screws be used to fix a cementless acetabular compenent?
Where press-fit fixation cannot be achieved, some form of initial mechanical fixation must be used.
Bone screws are inserted into the pelvis through holes in the roof of the shell.
13. What is the potential disadvantage of screw holes in modular cementless acetabular cups?
How is this minimised?
Screw holes in the shell allow the ingress of polyethylene wear debris. This debris has been proven to cause osteolysis.
This hole can be closed by inserton using a titanium screw
14. Why is the internal surface of a hemispherical modular cementless shell highly polished?
to reduce the potential for wear debris generation from micro-motion between the liner and the shell.
15. How do hard bearing liner inserts transfer loads to the acetabular component?
Give one example.
Loads in the liner are transferred to the shell via the VIP taper, which also prevents liner rotation.
The liners are flat-topped and have smaller external radii than the internal shell radius; this prevents polar loading and maintains the load at the VIP taper interface.
16. What is the size range of the diameter of femoral heads.
Which is most commonly used?
cementless / cemented polyethylene: 22-26mm
comonly used = 28mm
17. What is the minimum recommended polyethylene bearing wall thickness below which it is likely to suffer fatigue wear?
6mm wall thickness
18. How does polyethylene wear debris production relate to head diameters, in metal/polythylene bearing couples.
(i.e. Volumetric wear)?
The larger sliding area (higher diameter), while reducing contact pressure, doees produce a greater volume of wear debris.
19. How does head diameter influence penetration rate in metal/polyethylene bearing couples (i.e. Linear wear)
small head, beuse of their higher contact presures tend to"burrow" into the cup.
Large heads: will help to reduce plastic deformation and fatigue wear of the polyethylene.
21. What might a limited range of motion cause?
- thick neck diameter
- lare head diameter
- no Chamfer
-
- 1 / 48
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Lernkarten
24. How is range of motion between a hip stem and a cup influenced by the head : neck ratio?
2.3S89
The loss of motion is greatest in designs with small diameter heads an large diameter tapers
ROM: ends at point of impingement of prosthetic femoral neck against
the month of the bearing
41. Why are Titanium alloys inferior bearing metals?
zu weiches Metal für Gelenke, harte Metalle werden benötigt
20. What is the minimum range of motion which modern hip replacement prosthesis are designed to achieve?
132°
arc of motion: was das Implant kann
range of motion: was der Pation kann
29.Distinguish (Unterscheide) between "composite beam" and "taper slip" loading in cemented hip prostheses.
2.3S91,92
Composite beam
Thick cement mantle, stem absorbs bending forces & increases stiffness
taper-slip: stem designed to accomoate small platic changes in the cement mantle ==> self locking by moving deepter into the canal
==> der soll nachrutschen
sehe keinen Unterschied!!!
39. Incongurent bearing surfaces are associated with high contact stresses.
Which wear process is associated with this?
abrasive third body wear
40. In metal/Polyethylene hip bearing couples, how does surface finisch (and scratches influence wear rates?
scratches increas the of the softer side of the bearing
size of grain structure influences wear on soft side
47. What is the relationship between femoral head diameter and polyethylene wear in a metal head/polyethylene liner bearing couples?
Grösserer Durchmesser mehr Fläche mehr Abrieb
38. Why is it important to avoid pairing ceramic components (i.e. head and liner) from different manufacturers?
Konus muss sehr exakt sein, da der Keramik sehr sensible sind und ist sehr anfällig auf schlechte Passform
