total hip replacement the choice of implant based on the various charecteristics

1. Choice of implant in THR
Dr. Sairamakrishnan S

2. • Total hip femoral and acetabular components
of various materials and a multitude of
designs are currently available.
• Certain design features of a given implant may
provide an advantage in selected situations.

3. • No implant design or system is appropriate for
every patient, and a general knowledge of the
variety of component designs and their
strengths and weaknesses is an asset to the
surgeon.

4. • Total hip femoral and acetabular components
are commonly marketed together as a total
hip system.
• Although these systems are often convenient,
the variety of modular head sizes with most
femoral components allows use with other
types of acetabular components if necessary.

5. FEMORAL COMPONENTS
• The primary function of the femoral
component is the replacement of the femoral
head and neck.

6. • Height
• Offset
• Version
• Head size

7. • Larger head – increased ROM, reduced
displacement (0.05% with 32 head), increased
wear.

8. • The ideal configuration of the prosthetic head
and neck segment includes a trapezoidal neck
and a larger diameter head without a skirt.

9. CEMENTED FEMORAL COMPONENTS
• The stem should be fabricated of highstrength
superalloy. Most designers favor cobalt-
chrome alloy because its higher modulus of
elasticity may reduce stresses within the
proximal cement mantle.
• The cross section of the stem should have a
broad medial border and preferably broader
lateral border to load the proximal cement
mantle in compression.

10. • Sharp edges produce local stress risers that
may initiate fracture of the cement mantle
and should be avoided.
• A collar aids in determining the depth of
insertion and may diminish resorption of bone
in the medial neck.

11. • Failure of cemented stems is initiated at the
prosthesis-cement interface with debonding
and subsequent cement fracture.
• Noncircular shapes, such as a rounded
rectangle or an ellipse, and surface
irregularities, such as grooves or a longitudinal
slot, also improve the rotational stability of
the stem within the cement mantle.

12. • If debonding does occur, a stem with a
roughened or matte surface generates more
debris with motion than a stem with a
smooth, polished surface.

13. • Stem should occupy approximately 80% of the
cross section of the medullary canal with an
optimal cement mantle of approximately 4
mm proximally and 2 mm distally.
• Preformed PMMA centralizers

14. • The lengths of current stem designs range
from 120 to 150 mm.
• In longer stems the tip of the stem may
impinge on the anterior cortex or even
perforate it.

15. CEMENTLESS FEMORAL
COMPONENTS
• The two prerequisites for biologic fixation are
immediate mechanical stability at the time of
surgery and intimate contact between the
implant surface and viable host bone.
• Current cementless stem designs differ in their
materials, surface coating, and shape.

16. • Traditionally
• (1) titanium alloy with one of a variety of
surface enhancements
• (2) cobalt-chromium alloy with a sintered
beaded surface.

17. • A variety of surface modifications including
porous coatings, grit blasting, plasma
spraying, and hydroxyapatite coating have
been used to enhance implant fixation.

19. • Khanuja, Vakil, Goddard, and Mont proposed
a classification system for cementless stems
based on shape.
• Types 1 through 5 are straight stems, and
fixation area increases with type. Type 6 is an
anatomic shape.

20. • Type 1 stems are so
called single-wedge
stems.
• They are flat in the
anteroposterior
plane and tapered in
the mediolateral
plane.

21. • Fixation is by cortical engagement only in the
mediolateral plane and by three-point fixation
along the length of the stem.
• The femoral canal is prepared by broaching
alone with no distal reaming.
• Dorr type A femurs, distal engagement alone
risks fracture or rotational instability.
• These stems have performed well in Dorr type
B and C femurs.

22. • Type 2 stems engage
the proximal femoral
cortex in both
mediolateral and
anteroposterior
planes.

23. • So-called dualwedge designs fill the proximal
femoral metaphysis more completely than
type 1 stems
• Femoral preparation typically requires distal
reaming followed by broaching of the
proximal femur.
• They can be used safely in Dorr type A femurs.

24. • Type 3 represents a more disparate group of
implants.
• These stems are tapered in two planes, but
fixation is achieved more at the metaphyseal-
diaphyseal junction than proximally as with
types 1 and 2.

25. • Type 3A stems are tapered with a round
conical distal geometry. Longitudinal cutting
flutes are added to type 3B stems. These
implants have recently gained popularity in
complex revision cases.
• Type 3C implants are rectangular and thus
provide four-point rotational support

27. • Type 4 are extensively
coated implants with
fixation along the entire
length of the stem.
• Canal preparation
requires distal cylindrical
reaming and proximal
broaching.

28. • Excellent long-term results have been
achieved with these implants.
• Femoral stress shielding and thigh pain have
been reported with various designs.
• Their use in Dorr type C femurs can be
problematic because of the large stem
diameter required.

29. • Type 5 or modular
stems have separate
metaphyseal sleeves
and diaphyseal
segments that are
independently sized
and instrumented

30. • Such implants often are recommended for
patients with altered femoral anatomy,
particularly those with rotational
malalignment.
• Both stem segments are prepared with
reamers, leading to a precise fit with
rotational stability obtained both proximally
and distally.

31. • This feature makes modular stems an
attractive option when femoral osteotomy is
required.
• Modular stems can be used for all Dorr bone
types, but increased cost and potential
problems with modular junctions should be
taken into account.

32. • Type 6 or anatomic
femoral components
incorporate a posterior
bow in the metaphyseal
portion and variably an
anterior bow in the
diaphyseal portion,
corresponding to the
geometry of the femoral
canal.

33. • Right and left stems are required, and
anteversion must be built into the neck segment.
• Anatomic variability in the curvature of the femur
usually requires some degree of overreaming of
the canal; if the tip of the stem is eccentrically
placed, it impinges on the anterior cortex.
• This point loading has been suggested to be a
source of postoperative thigh pain.
• The popularity of anatomic stems has declined
over the past decade in favor of straight designs.

34. • With cementless devices, the requirements for
canal filling often mean the stem must be of
sizable diameter.
• Because stiffness of a stem is proportional to the
fourth power of the diameter, an increased
prevalence of femoral stress shielding can be
seen with larger stems.
• The mismatch in stiffness between implant and
bone also has been cited as a cause of
postoperative thigh pain.

35. • Current stem designs deal with this problem in
several ways.
• The elastic section modulus of the stem can
be changed to allow greater flexibility while
leaving the implant diameter unchanged so
that stability is not compromised.
• The addition of deep, longitudinal grooves
reduces bending and torsional stiffness.

36. • The bending stiffness in the distal third of the
stem also can be reduced substantially by
splitting the stem in the coronal plane, similar
to a clothespin.
• Tapered distal stem geometries are inherently
less stiff than cylindrical ones and have been
associated with minimal thigh pain.

37. Acetabular component

38. CEMENTED ACETABULAR
COMPONENTS
• The original sockets for cemented use were
thick-walled polyethylene cups.
• Vertical and horizontal grooves often were
added to the external surface to increase
stability within the cement mantle.

39. • Wire markers were embedded in the plastic to
allow better assessment of position on
postoperative radiographs.
• PMMA spacers, typically 3 mm in height,
ensure a uniform cement mantle.
• Bottoming out

41. • The simplicity and low cost of all-polyethylene
components make them a satisfactory option
in older, low demand patients.

42. CEMENTLESS ACETABULAR
COMPONENTS
• Most cementless acetabular components are
porous coated over their entire circumference
for bone ingrowth.
• Instrumentation typically provides for
oversizing of the implant 1 to 2 mm larger
than the reamed acetabulum as the primary
method of press-fit fixation.

43. • Fixation with screws increases the rotational
stability.
• Studies have shown that extensive bone
ingrowth occurs at the sites of initial fixation.

44. • Most systems feature a metal shell with an
outside diameter of 40 to 75 mm that is used
with a modular liner.
• The liner must be fastened securely within the
metal shell.
• “Backside wear”

45. • Monoblock acetabular components with
nonmodular polyethylene also have been
produced to alleviate the problem of backside
wear but have not proven to be superior to
modular implants.

46. • High stresses within the polyethylene are
likely when the thickness of the plastic is less
than 5 mm, leaving the component at risk for
premature failure as a result of wear.

47. • Most modern modular acetabular
components are supplied with a variety of
polyethylene liner choices.
• Some designs incorporate an elevation over a
portion of the circumference of the rim,
whereas others completely reorient the
opening face of the socket.

49. • A constrained acetabular component includes
a mechanism to lock the prosthetic femoral
head into the polyethylene liner.
• The tripolar-style mechanism features a small
inner bipolar bearing that articulates with an
outer true liner.

51. • Other designs use a liner with added
polyethylene at the rim that deforms to
capture the femoral head.
• A locking ring is applied to the rim to prevent
escape of the head.

53. • A dual mobility acetabular component is an
unconstrained tripolar design.
• The implant consists of a porous coated metal
shell with a polished interior that accepts a
large polyethylene ball into which a smaller
metal or ceramic head is inserted.

55. • Custom components for acetabular
reconstruction rarely are indicated.
• Most deficient acetabula can be restored to a
hemispherical shape, and a standard, albeit
large, acetabular component can be inserted.

56. • A cementless acetabular component with
modular porous metal augments can be used
instead of a large structural graft or excessively
high placement of a hemispherical component.
• Augments of various sizes are screwed into bony
defects to support the acetabular component.
• The augments are joined to the implant with the
use of bone cement.

58. • Reconstruction rings have been introduced to
allow bone grafting of the deficient acetabulum
behind the ring, rather than relying on cement on
both sides of the device.
• Reconstruction ring provides immediate support
for the acetabular component and protects bone
grafts from excessive early stresses while union
occurs.
• These devices are commonly referred to as
antiprotrusio rings and cages.

60. • These implants do not provide for long-term
biologic fixation and are prone to fracture and
loosening.

61. Wear
• Linear wear
• Volumetric wear
– V = 3.14 x r2 x W

62. Bearing surfaces

63. HIGHLY CROSSLINKED POLYETHYLENE
• Crosslinking is accomplished by either gamma
or electron-beam radiation at a dose between
5 and 10 Mrad.
• Initial testing of this material has shown
remarkable wear resistance.
• But exposure to these high amounts of
radiation increases the free radicals

64. • Free radicals oxidise the implant and make
them britle.
• Free radicals can be reduced by
– Post irradiation heating
– Annealing
– Vitamin E doping
• Terminal sterilization is most commonly done
with either gas plasma or ethylene oxide

65. • Each company have their own proprietary way
of crosslinking.
• 80-90% reduction in wear compared to
traditional polyethylene.
• Muratoglu et al. showed that the wear rate of
this material is not related to the size of the
femoral head within the range of 22 to 46 mm

66. • There are now a sufficient number of studies
with 10-year follow-up to conclude that the
performance of highly crosslinked
polyethylenes surpasses that of conventional
polyethylene.

67. METAL-ON-METAL BEARINGS
• Some implants have survived with a
remarkably low wear rate, others have failed
because of flawed materials selection, poor
manufacturing tolerances, inadequate
clearance between implants, and
impingement.

68. • High-carbon (>0.20% carbon) cobalt-
chromium alloy has been demonstrated to
have lower wear rates than low carbon alloys.
• “Diametral clearance” - 100 to 200 μm.

69. • “self-healing”
• Large-diameter heads (≥54 mm) have been
associated with reduced wear.

70. • The particle size is much smaller than
polyethylene, however, and the number of
particles is larger.
• Elevated levels of cobalt and chromium ions in
serum, erythrocytes and urine
• The long-term exposure to these ions raises
concerns of malignancies, renal complications.

71. • ALVAL - aseptic lymphocytic vasculitis
associated lesion
• Delayed hypersensitivity reaction

72. • Other reactions
– including pain
– periarticular fluid accumulation
– solid mass formation (or so-called pseudotumor),
– extensive tissue necrosis including the hip
abductors.

73. CERAMIC-ON-CERAMIC BEARINGS
• Because of its high density, implants have a
surface finish smoother than metal implants.
• Ceramic is harder than metal and more
resistant to scratching from third-body wear
particles.
• The linear wear rate of alumina-on-alumina
has been shown to be 4000 times less than
cobalt-chrome alloy-on-polyethylene.

74. • Earlier ceramic implants had failure because
– flawed implant designs
– inadequate fixation
– implant fracture
– occasional cases of rapid wear with osteolysis

75. • Impingement between the femoral neck and rim
of the ceramic acetabular component causes
impact loading of the rim can produce chipping
or complete fracture of the acetabular bearing.
• Repetitive contact at extremes of motion also can
lead to notching of the metal femoral neck by the
harder ceramic and initiate failure through this
relatively thin portion of the implant.

76. • Recently there has been a reduction in the use
of CoC because of a squeaking noise produced
by the implants (~10%). Occurs after 1 year of
use.

77. • Oxidized zirconium is a zirconium metal alloy
that is placed through an oxidation process to
yield an implant with a zirconia ceramic
surface.
• So-called ceramicized metals are not
susceptible to chipping, flaking, or fracture as
are other ceramics.

78. • Oxidized zirconium is currently available only
in femoral head components mated with
polyethylene and not as a ceramic-on-ceramic
couple.
• There has been a mixed reports on the wear
rate of these implants compared with
traditional cobalt chromium heads.