Locking Strengths of Ceramic, Metal and Polyethylene Inserts in a Multi-Bearing Acetabular Shell
*
Popoola, O.; *West C.; and *Fryman C.
Zimmer Inc., Research, Warsaw, USA
[email protected]
*
INTRODUCTION
New multi-bearing acetabular shells are designed with integral locking
features that permit the use of polyethylene, ceramic and metal inserts.
The hard bearing inserts are locked via a Morse taper while the polyethylene inserts engage a snap feature located below the taper and anti
rotation scallops (Figure 1). Clinically the acetabular liners may be
subjected to an axial dissociation force (e.g. hip head suction), a leverout moment transmitted via the head or caused by neck impingement,
and/or a frictional torque due to rotation and translation of the hip
head/neck. Moreover, the polyethylene inserts may undergo sufficient
deformation and backside wear through micro-motion and loading to
potentially compromise the shell’s locking ability during articulation.
The shell/inserts locking mechanism must provide adequate strength to
counter these forces and provide for safe and effective use of the implant.
a
b
c
Figure 1: Multi-bearing shell (a) with polyethylene (b) and ceramic (c)
inserts
The objectives of this study are to (i) determine the shell locking mechanism strength for each bearing material using three test methods which
simulate clinically relevant static loading scenarios: push-out, lever-out
and torque-out; (ii) characterize the locking reliability and backside wear
of the polymer inserts when the shell/insert assembly is subjected to
physiologically-relevant loading and motions.
MATERIALS AND METHODS
The shell is made of Ti6Al4V substrate and Tantalum foam (Trabecular Metal™) fixation surface. Fifty inserts (15 ceramic, 15 metal and
20 polyethylene) were impacted into respective size shells using a drop
tower with impact force magnitudes ranging from 4 to 9kN. The impact
forces were derived from a prior cadaver study in which 37 liner insertions forces by eight different surgeons were measured.
Push-out, torque-out and lever-out tests: Push out, lever out and torque
out tests were performed on five assemblies of each material. The pushout test was performed using the ASTM F1820 [1] method. The leverout and torque-out tests were performed using modified versions of the
methods proposed by Tradonsky et al. and CeramTec AG [2]. The
required minimum forces / moments values for safe and effective use of
the implants were based on predicate performance, external literature
values or calculated. The values are also compared to those obtained
from clinically successful predicates [3]. The torque-out tests were
performed under a 30N vertical load.
Polyethylene Locking Reliability and Backside Wear: Five 28mm
polyethylene inserts were engraved with perpendicular grooves on their
backside (Figure 2), impacted into 44mm shells, and subjected to ISO
14242-1 motion waveforms for 5 million cycles under a peak load of
4000N at 2 Hz. The tested inserts were levered-out of their respective
shells as previously described and the lever-out forces were compared to
those obtained from as-assembled untested inserts. The disassembled
liners were melt-annealed for relaxation and creep recovery as described
by Muratoglu et al. [4]. Pre and post test (after creep recovery) measurements of the grooves depth were performed using a Zygo white light
interferometer. Photographs of the machining marks on the backside of
the liners were taken before and after the test.
RESULTS AND DISCUSSION
Table I shows the push-out, torque-out and lever-out forces needed to
disengage the inserts from the shells. The push-out and lever-out forces
are comparable to those reported by Tradonsky et al. [2] and by Paech et
al. [5]. Moreover, the torque-out moments are significantly higher than
the maximum of theoretically predicted values of 20Nm for hard bearings and 4Nm for polyethylene in a titanium alloy shell (assuming
Figure 2: Machined grooves on the back of a polyethylene insert
extreme friction coefficients of 0.1 and 0.7 for the polyethylene and the
hard bearings respectively) as described by Volz et al [6]. Volz calculations assumed a vertical load of 3KN across the head/insert interface,
while the current tests were performed under 30N load. Significantly
higher torque-out moments would be obtained if the experiment were
performed under 3KN. These torque-out strengths are significantly
higher than predicted in vivo value of 2.4Nm [3]. The push out and lever
out forces as well as the torque out moments are significantly higher
than those obtained from clinically successful predicates measured in the
same manner. Figure 3 shows typical photographs of the machining lines
on the backside of the polyethylene inserts before and after the durability
test. The depths of these lines and the lever out forces needed to disengage the inserts from their shells are shown in Table II. The differences
in groove depths are within experimental error and the backside machining lines are visibly present before and after the durability test. This
indicates that no significant backside wear occurred during the test.
Table I: Disengagement of 28mm and 40mm inserts from 44mm,
54mm and 56mm shells
Approximate
Ceramic/
Polyethylene
Predicate
Ranges
Metal
Push-out (N)
600-2400
1800-1900
400 - 1200
Torque-out (Nm)
17- 49
29-32
33 - 42
Lever-out (N)
460-940
850-1200
170 - 475
Table II: Lever out forces and groove depths
Lever-out force (N)
Groove depth ( m)
Pretest
1045 ± 149
135 ± 45
Post test
1253 ± 61
110 ± 40
After
Before
Figure 3: Backside photographs of inserts before and after the durability test
CONCLUSION
Multi-bearing shell designs offer the same in vitro insert locking reliability as shells designed for single bearing (polymer or hard bearing) use.
These shells offer surgeons maximum insert choice flexibility during
primary or revision surgeries.
REFERENCES [1]ASTM International. 2003. Standard Test Method
for Determining the Axial Disassembly Force of a Modular Acetabular
Device. [2] S. Tradonsky et al., 1993. CORR, 26, 154, [3] Smith and
Nephew, 2008. R3 Acetabular System. [4] A. Muratoglu et al., 2004.
Journal of Arthroplasty, Vol. 19, No. 1. [5] A. Paech et al. 2008, Res.
Journal of Biomedical Science 2(1) 43, [6] RG Volz et al., 1977, JBJS
59:5
Poster No. 2228 • 56th Annual Meeting of the Orthopaedic Research Society