Cemented Acetabular Fixation
Henry W. Hamilton, M.D.,
F.R.C.S.(C)
Orthopaedic Surgeon
Port Arthur Clinic
Thunder Bay, Ontario
In
1995, Murray, from the Nuffield Orthopaedic Centre in Oxford posed the
question, “Which Primary Total Hip
Replacement?” JBJS 77B, 520-527, 1995.
His
review studied the situation in the UK, but the points he makes apply just as
much to North America.
 He found 62 different primary THRs manufactured by
19 companies.
The cost was 250 – 750 Pounds for cemented
implants and 486 – 2000 Pounds for cementless implants.
No clinical tests are required before a new design
is released onto the market.
There were 5 year survival results in peer
reviewed journals for only 8 of the 62 implants.
There
were 10 year survival results for only 3: Charnley, Stanmore & Muller.
15 year results on 2: Charnley & Stanmore.
And 20 year results on 1: Charnley.
The 2 cheapest implants have the longest follow
up.
Malachu
(Acta Orthop Scand 64, 497-506, 1993) has questioned whether 5 year results
will hold up at 10 years.
There
is debate whether laboratory tests can predict the clinical outcome.
Murray
recommended that, “If clinical results
are not available, a new implant
should only be used if it is included in a properly conducted clinical trial”
We
should remember that innovations often
fail, e.g.:
Carbon fibre reinforced PE.
Wright, JBJS 70A, 1312 –1321, 1988.
The change from a polished to a matt stem
Anthony, JBJS 72B, 971 – 979,1990.
Metal backed sockets.
Huk, JBJS 76B, 568 – 574, 1994.
Ranawat, CORR 344, 297 –215, 1997.
Cement porosity reduction.
Hamilton, Orthop. Review XVII, 48-54 1988.
Ling, CORR 355, 249 – 253, 1998.
Hylamer.
Livingston, JBJS 79A, 481 – 492, 1999.
Bonding of cement to Iowa stem.
Sporer, Callaghan, Olejniczak, Goetz, &
Johnston.
JBJS 81A, 481-492, 1999.
Cementless joints!
Dr Rosenberg & Dr McAuley will tell whether
there is any evidence to justify the continued use of cementless
implants.
The
classic publication on bone cement is of course John Charnley’s “Acrylic Cement in Orthopaedic Surgery”,
Churchill Livingstone, Edinburgh & London, 1972.
Our
understanding of pressurisation of
cement owes much to Ling, Oh, Harris, & Bourne.
“Socket design &
cement pressurisation in Charnley LFA.”
Shelley
& Wroblewski, JBJS 70B, 358- 363, 1988.
A party trick.
Hand mix CMW cement for about 3 minutes until doughy and ready for
use. Mold cement into the shape of a cross.
Hurl the cross onto the floor as hard as you can.
The cross maintains its shape. Leave the cross on the floor, and rest a
ballpoint pen on the cross. You will observe that the pen will gradually sink
into the cement.
PMMA is
highly viscoelastic and will resist sudden large forces, but will yield to
gentle sustained forces.
Shelley & Wroblewski designed an experimental
apparatus with an acetabular shaped cavity with a central 5mm orifice leading
to a tube-type rheometer. Above the acetabular cavity a mechanism allowed a
pressuriser, or different designs of sockets, to be pushed into the cement
filled cavity with a given force. The rheometer tube was filled with liquid
paraffin and connected to a calibrated mercury column, and to a piston
arrangement which could maintain a constant back pressure.
With an insertion force of 8kg the peak acetabular
pressure with:
Unflanged cup was 42mmHg
Flanged cup was 170mmHg
Exeter pressuriser was 193mmHg
Intrusion into a 5mm cement keying hole:
An unflanged cup inserted with a force of 8kg
pushes less than
1 cu cm cement into the keying hole in the first
minute. Once the cup “bottoms out”, no further pressurisation is possible.
Back pressure of 25mmHg , representing the blood in the bone, now causes the
acetabular pressure to decline, and by 7 minutes there is net extrusion of
cement from the keying hole.
With a flanged cup 3 cu cm of cement is pushed
into the keying hole and maintained there for 7 minutes.
The Exeter pressuriser is 6% by volume more
effective than the flanged cup.
Pressurisation must
continue throughout polymerisation.
When pressurisation stops
viscoelastic cement is extruded by 25mmHg back pressure from blood in the
bone.
“Relationship of
acetabular wear to osteolysis and loosening in THA”
David
Sochart while at Wrightington Centre for Hip Surgery. CORR 363, 135-150,
1999.
235 LFAs in 163 patients, 48 male & 115
female.
All patients were under 40 years old, average age
31.7 years.
The diagnoses were ankylosing spondylitis 32, CDH
57, OA 59, & RA 87. Septic cases were excluded.
Follow up was from 6-30 years, average 19.5 years.
Wear (penetration) 0-7.2mm
Rate of wear 0-0.55mm/year
Penetration due to creep 0.14mm
Creep occurs in first 18 months, thereafter
penetration due to wear.
Short studies exaggerate wear by including creep.
Definition of cup failure:
Loose (demarcation in 3 zones).
Migration.
Revision.
35% cups failed.
25 year cup
survival to failure with:
Rate of wear
< 0.1mm/year 77%
Rate of wear
> 0.2mm/year 0%
Marked variability in rates of wear.
Highly significant relationship between cup wear
and component failure and revision.
“A tribological study of retrieved hip prostheses”
Graham
Isaac, Michael Wroblewski, John Atkinson, & Duncan Dowson. CORR 276,
115-125, 1992.
100 Charnley cemented acetabular cups, 78 of which
had associated femoral prostheses, retrieved at revision surgery for
mechanical problems. 92% of the failures were associated with cup loosening,
and 54% with stem loosening.
Cup damage:
Socket erosion – 34
(external PE movement against bone when cup
“bottoms out”)
Rim wear – 48
(impingement of femoral neck)
Discoloration – 30
(yellow oxidation)
Cement ingress – 60
(particles of cement embedded in PE)
Cratering – 89
(formation of pits by embedded & subsequently dispersed
cement particles)
Scoring – Every case
(“third bodies*” drawn across articular surface as
seen with EM)
abrasive roentgenographic contrast medium in
cement.
“Comparison of the wear of
aged & non-aged UHMWPE sterilized by gamma irradiation & gas plasma”
Fisher,
Reeves, Isaac, Saum, & Sanfford. Journal of Materials Science: Materials
in Medicine 8, 375-378, 1997.
Prior to 1969, at Wrightington Centre for Hip
Surgery, acetabular cups were chemically sterilised.
By 1969, Thackray the manufacturers of the LFA,
sterilized all the cups by gamma
irradiation.
In laboratory tests GUR415 ram extruded bar PE
pins were tested against highly polished stainless steel plates.
PE sterilized by 2.5Mrads in air and accelerated
aged (storage at 70C for 14 days in pure oxygen) equivalent to 4-5 years
caused marked oxidative degradation (Fourier transform infrared analysis).
The rate of
wear of irradiated & aged PE was 3X that of un-aged and un-sterilized PE,
and of aged gas plasma sterilized PE.
Since 1997, gas plasma sterilization has been the
policy of Dupuy International, formerly Thackray, and now Johnson &
Johnson.
“Charnley Low Friction
Torque Arthroplasty of the Hip, 20-30 year results” Wroblewski, Fleming,
Siney. JBJS 81B, 427-430, 1999.
This study includes a brief outline of the
evolution of the LFA over the last 36 years.
320 LFAs with a mean age of 43 years were followed
for a mean of over 22 years.
82.0% were pain free.
5.3% were revised.
Key observations were:
There is poor correlation between clinical &
radiological results.
Serial radiographs are helpful in recognising
failure.
The loss of bone stock found at revision is always
worse than it appears on the radiographs.
If the bone
loss is progressive, the arthroplasty should be revised even if it is
asymptomatic.
Hamilton 1004 LFAs prior
to 1990.
Attempt to follow every case with an annual
examination and radiograph.
Details of every case recorded on a custom
computer programme (Orthopaedic Review, Vol XVII, Jan 1988)
834 of the 1004 LFAs were primary operations
followed for 10- 27 years.
Primary LFAs 1972-1989 and deaths 1972-1999.
Year of implantations destined to fail & year
of revision.
You will harvest what you sow!
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