Download Correlation Between In Vivo Oxidation and Mechanical Properties In

Correlation Between In Vivo Oxidation and Mechanical Properties In Contemporary Polyethylene Bearings
+1Medel, F J; 1Kurtz, S M; 2MacDonald, D W; Shah P, Patel H, Cohen A, 3Rimnac, CM
+1Drexel University, Philadelphia, PA, 2Exponent Inc., Philadelphia, PA, 3Case Western University, Cleveland, OH
[email protected]
Introduction: Ultra-high molecular weight polyethylene oxidation
remains an important concern in total joint arthroplasty, since it
eventually results in a significant drop in mechanical properties that may
compromise implant longevity, especially for TKA [1-3]. In vivo
oxidation of polyethylene can occur in the presence of unstabilized
macroradicals within the polymer and molecular oxygen uptake from
body fluids and tissues [1-3]. The oxidation index (OI) is widely used to
quantify oxidation in polyethylene (ASTM F2102), and the ultimate load
from the small punch test has been used to quantify its ultimate strength
(ASTM 2183). Previously, researchers have suggested that an OI of
greater than 3 was associated with "critical" loss in mechanical
properties and a propensity for fatigue damage in vivo [3]. The
interpretation of OI levels less than 3 remains unclear. The objective of
this study was to employ ASTM techniques to examine the correlation
between in vivo oxidation levels and mechanical properties at the
articular surface of retrieved contemporary acetabular liners and tibial
inserts. We hypothesized that an ASTM OI below 1 can be interpreted as
"low" oxidation and would not significantly affect the ultimate strength
of polyethylene. Finally, the influence of oxidation and mechanical
properties on wear performance and surface damage was also explored.
Methods: A total of 157 polyethylene components (80 acetabular
liners and 77 tibial inserts) were collected after revision surgery. Of the
80 acetabular liners, 63 were annealed highly crosslinked (Crossfire,
Stryker) and 17 conventional gamma inert sterilized, whereas all the
tibial inserts were gamma inert sterilized. Clinical and polyethylene
information, including implantation times, patient characteristics, shelf
lives and polyethylene resin type, were also available (Table 1). Femoral
head penetration was measured in intact acetabular liners implanted for
periods longer than 1 year. In addition, surface damage scores per Hood
[4] were obtained for tibial inserts. FTIR analysis following ASTM
F2102 was conducted on polyethylene sections microtomed from
retrievals. To avoid the interference of absorbed lipids, polyethylene
sections underwent heptane extraction (6 hours) prior to the oxidation
analysis. Maximum oxidation indexes were calculated from FTIR
spectra at the articulating surface of the acetabular liners, and tibial
inserts. Finally, mechanical properties were assessed by means of the
small punch technique (ASTM F2183). Near surface and subsurface
small punch specimens were prepared from superior (worn) and inferior
(unworn) regions of the bearing for retrieved liners. Similarly, small
punch test specimens were machined from the lateral condyle of tibial
inserts near the surface and at the subsurface of anterior, posterior,
medial and lateral locations. General linear models were developed to
explore correlations and examine differences in oxidative, mechanical
and wear performance (p<0.05 level of significance)
Results: Patient cohorts corresponding to acetabular liners were
statistically comparable with respect to age, weight and activity level
(p 0.4). All the gamma inert sterilized polyethylene retrievals oxidized
regardless of polyethylene formulation. Among retrieved acetabular
liners, the highest oxidation (1.6) was found in the annealed highly
crosslinked group. In contrast, ultimate loads were significantly higher
regardless of location for annealed compared to conventional acetabular
liners (p 0.002). Retrieved acetabular liners with oxidation levels less
than 1 demonstrated no significant correlation between oxidation and
ultimate load (p 0.09). General linear models confirmed a significant
correlation between both oxidation and ultimate loads at the superior
subsurface (p=0.02) for conventional liners. Similar correlations were
found for oxidation and ultimate loads near the superior surface
(p<0.0001) and subsurface (p=0.02) of annealed liners. Ultimate loads
dropped up to 40% for oxidation levels near 1 in conventional liners. In
contrast, annealed liners registered a 31% drop when oxidation reached
levels close to 2 (Figure 1).
With regard to tibial inserts, 1900H retrievals had significantly higher
oxidation than retrievals produced from GUR 1050 (p=0.048; Student’s
t-test) and GUR 1020 (p=0.0002). Again, ultimate loads significantly
decreased with oxidation (p<0.0001; linear models), regardless of
polyethylene resin (Figure 2). 1900H retrievals had, however, slightly
higher ultimate loads at zero oxidation levels with respect to the other
resins. Ultimate load and oxidation were not correlated in tibial inserts
with oxidation equal or less than 1 (p 0.2; linear models). With respect
to wear performance, there were no significant correlations between
femoral penetration rate of acetabular liners or bearing surface damage
scores of tibial inserts and oxidation (p=0.2 and p=0.75, respectively) or
near surface mechanical properties (p 0.34 and p 0.28).
Discussion: The current study provides a quantitative basis for
interpreting ASTM OI values based on its effect on mechanical
properties. Our findings supported the hypothesis that ASTM oxidation
levels below 1 do not have a significant effect on the mechanical
performance of polyethylene. Above that oxidation level, polyethylene
mechanical properties start to decrease, confirming a transition in
mechanical behavior when oxidation increases from low (OI<1) to
moderate (OI 1). The lack of correlation of oxidation with clinical
performance in the current study is consistent with previous research,
suggesting that a critical level of oxidation (OI>3) may be necessary to
compromise the mechanical properties of polyethylene.
Acknowledgements: Supported by research grants from NIH
(NIAMS) R01 AR47904, Stryker Orthopedics, Zimmer, Inc., and Sulzer
Settlement.
References: [1] Kurtz et al., CORR, 2006; [2] Medel et al., J Biomed
Mat Res, 2008; [3] Currier et al. JOA 2007; [4] Hood et al. JBMR 1983.
Table 1. Shelf lives and in vivo times for polyethylene retrievals
Shelf
Dose
Life
RETRIEVALS
Resin
(kGy)
(y)
Conventional
GUR 1050
30
0.8±0.7
(n=17)
HIPS
Annealed HXLPE
GUR 1050
100
0.5±0.7
(n=63)
Conventional 1
1900H
30
1.2±1.5
(n=24)
Conventional 2
GUR 1050
30
1.4±1.6
KNEES
(n=25)
Conventional 3
GUR 1020
30
0.7±0.8
(n=28)
In Vivo
Time
(y)
4.0±2.5
2.7±2.0
3.5±2.3
3.1±2.3
1.7±1.4
Figure 1. Correlation between oxidation and ultimate load (superior subsurface) in
annealed liners.
Figure 2. Correlation between oxidation and ultimate load in tibial inserts
Poster No. 2327 • 55th Annual Meeting of the Orthopaedic Research Society