- See:
Osteolysis
- Discussion:
- originally Charnley used High Density Polyethylene and later this was changed to Ultrahigh Molecular Wt Polyethylene
- impact strength of UHMWPE is higher than that for HDPE and the density of UHMWPE is much lower than for HDPE;
- UHMWPE has better abrasion resistance, strength, resistance to deformation, and fatigue strength;
-
resistance to wear:
- conformation of the bearing surfaces is important because it is factor in the determination of contact stress;
- increases in material strength, will usually result in increases in stiffness and increases in contact stresses;
- hence, it is important that gains in gains in strength offset increases in surface contact;
- increases in the modulus of the polymer (or increases in the density) will have the
effect of increasing contact stress and may result in increased wear;
- higher contact stresses between ultra-high molecular wt poly & other biomaterials are thought to result in greater polymeric wear;
- over the lifetime of an implant, polyethylene becomes stiffer near the surface, and therfore, contact forces will increase w/ time;
- incongruent contact:
- occurs from the "round on flat" contact of some early
total knee designs;
- these systems attempted to re-create
normal knee kinematics;
- these knee systems induce high contact stresses in the polyethylene component which leads to wear;
-
thermoplastic properties:
- deformation of polyethylene is dependent on temperature;
- heat pressing alters the physical properties of polyethylene near the surface of the component and makes the device more
susceptible for fracture at the subsurface juncture of heat-press material and the bulk polyethylene;
-
sterilization methods:
- gas plasma
- ethylene oxide
- gamma sterilization:
- sterilization in air may be particularly harmful, because it may initiate a long term oxidative process which has a negative impact
on the implant's mechanical properties, and which can dramatically increase the propensity for volumetric wear;
- shelf aging may cause oxidation and free radical formation of insert;
- direct molded 1900 resin may have less oxidation than other methods;
- tibial inserts may be more prone to oxidation that acetabular inserts;
- radiation does promote polyethylene cross linking;
- w/ low oxygen environment, free radicals created during irradiation can form carbon-carbon cross-links between polyethylene molecules;
- cross-linking improves the wear resistance of the polymer;
- subsequent heating of acetabular polyethylene cups after irradiation may reduce
the level of residual free radicals, which reduces oxidation;
- in the study by H. Mckellop et al, a hip-joint simulator was used to assess whether it is preferable to sterilize ultra-high molecular weight
polyethylene cups without gamma irradiation, to avoid radiation-induced oxidative degradation, or to sterilize with gamma
irradiation while the cups are packaged in a suitable low-oxygen atmosphere to minimize oxidation while retaining the
increased wear resistance conferred by the radiation-induced crSome manufacturers take the additional step of
heating acetabular polyethylene cups after irradiation to reduce the level of residual free radicals, thereby reducing subsequent oxidationss-linking;
- irradiation while the cups are packaged in a suitable low-oxygen atmosphere to minimize oxidation while
retaining the increased wear resistance conferred by the radiation-induced cross-linking;
- results of the present study demonstrated that the cross-linking induced by gamma irradiation improves
the wear resistance of ultra-high molecular weight polyethylene, while oxidation reduces it;
- sterilizing with use of gamma irradiation with the implant packaged in a low-oxygen atmosphere avoids
immediate oxidation and cross-links the polyethylene, thereby increasing its wear resistance,
but long-term oxidation of the residual free radicals may markedly reduce the wear resistance;
- Effect of Sterilization Method and Other Modifications on the Wear Resistance of Acetabular Cups Made of UHMWP. A Hip-Simulator Study*
H. Mckellop PH.D. et al.
J Bone Joint Surg [Am] 82-A: 1708-25, 2000
- Polyethylene Wear in TKR:
- Linear Wear in THR:
- wear is measured as smallest radius from the center of femoral head to outer border of the acetabuluar cup;
- ranges from 0.01 to 0.56 mm per year (avg about 0.29) but this often decreases with time to 0.15 mm
per year after 9 years;
- linear wear increases w/ heavier pts and pts that exercise;
- 22 mm femoral heads have a greater amount of linear wear than do 28 mm heads;
- excessive wear may be associated w/
osteolysis, which may be more common w/
non-cemented components (Devane et al);
- this finding that ostelysis is more common in non-cemented components may be due
to design characteristics of the cup or may be due to the fact that thinner
polyethylene cups were inserted w/ the non-cemented group;
- the cement may also prevent the polyethylene debris from reaching the bone interface;
- as pointed out by Sychterz et al, it is important to measure the initial amount of "apparent
linear wear" since this can measure as much as 1.1 mm on the initial postop films;
- this may be a result of liner design and polyethylene plastic deformation,
and should not be counted in final assessment of volumetric wear;
-
radiographic techniques:
- as noted by PN Smith et al, it is important that films be taken wt bearing when measuring
polyethylene wear;
- supine films will consistently underestimate polyethylene wear;
- in contrast, in the report by KD Moore MD et al (JBJS Jan 2000), the authors found no evidence
that radiographs must be taken w/ the patient wt bearing (vs supine) in order to accurately
measure the position of the femoral head within the polyethylene liner;
- references:
- The Effect of Weight-Bearing on the Radiographic Measurement of the Position of the Femoral Head After THA.
PN Smith et al.
J Bone Joint Surg 82-A: 62-9, 2000.
- Volumetric Wear in THR:
- related in part to size of femoral head;
- lowest rate of volumetric wear can be found w/ 22 mm head, wear w/ 26 mm head is intermediate, and and the
highest rate of wear is found w/ 32 mm heads (and result increase in
osteolysis);
- as pointed out by M. Jasty et al (JBJS 1997), the thickness of the polyethylene component is
especially important in metal backed press fit components;
- w/ smaller acetabular components consider using a 26 mm rather than a 28 mm head;
The Otto Aufranc Award. Impact of gamma sterilization on clinical performance of polyethylene in the hip.
The natural history of ultra high molecular weight polyethylene.
Post-irradiation aging of ultra-high molecular weight polyethylene.
Measurement of Polyethylene Wear in Acetabular Components Inserted with and withou Cement.
A randomized trial.
P.A. Devane, E.J. Robinson MD, R.B. Bourne, MD, C.H. Rorabeck MD, N.N. Nayak MD, J.G. Horne. JBJS Vol 79-A. No. 5, May 1997. p 682.
Use of Polyethylene for Joint Replacement Prostheses
DL Bartel PhD and S Li PhD.
Advances in Operative Orthopaedics, vol 3. 1995.
Radiographic Evaluation of Penetration by the Femoal Head into the Polythylene Liner over Time.
CJ Sychterz et al. JBJS Vol 79-A. No 7. Jul 97. p 1040.
The effect of conformity, thickness, and material on stresses in ultra high molecular weight components for total joint replacement.
DL Bartel et al.
JBJS. Vol 68-A. 1986. p 1041-1051.
The influence of wt bearing on the measurement of polyethylene wear in THA.
PN Smith et al.
JBJS. Vol 81-B.
1999. p 259.
The Basis for a Second-generation Highly Cross-linked UHMWPE.
Nonsurgical Management of Osteolysis: Challenges and Opportunities.
