+ * Stolk

R + Huiskes

Verdonschot

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CAN FINITE ELEMENT BASED PRE-CLINICAL TESTS DIFFERENTIATE BETWEEN CEMENTED HIP 
REPLACEMENT STEMS ACCORDING TO CLINICAL SURVIVAL RATES? 
 
+*Stolk, J; *Verdonschot, N; **Huiskes, R 
+*Orthopaedic Research Laboratory, University Medical Center Nijmegen, The Netherlands 
 
Introduction: Conventional cemented total hip arthroplasty (THA) 
implants are highly successful, but some studies still report unacceptably 
high failure rates for new implants that are introduced on the orthopaedic 
market [1]. Proper pre-clinical testing of these implants might prevent 
such disasters. The present study is performed within the frame-work of 
a large European project, aimed at developing validated pre-clinical tests 
to test cemented THA implants against the cement damage accumulation 
failure scenario. According to this scenario damage accumulation in the 
bulk cement and along the interfaces eventually leads to gross loosening 
of the implant. Retrieval studies have shown that this is one of the most 
important failure scenarios for the femoral component [2]. A numerical 
pre-clinical test was developed in the form of a finite element (FE) 
simulation that allows monitoring of cement crack formation and stem 
migration, in cemented THA reconstructions subjected to cyclic loading. 
The simulation was already validated experimentally in an earlier study 
[3]. The current study is concerned with the final clinical validation of 
the simulation. The damage accumulation failure scenario was simulated 
for four cemented hip stems, with distinct differences in clinical survival 
rates as reported by the Swedish Hip Registrer. The question was: does a 
ranking of the stems from superior to inferior, based on the FE 
predictions concur with a ranking based on the clinical survival rates?  
 
Methods 
Four stems were selected from the Swedish Hip Register: the Lubinus 
SPII, the Exeter Polished, the Charnley Roundback and the Mueller 
Curved. At 10 years after surgery the survival rates of these implants 
were 4%, 5%, 8% and 13%, respectively (Fig. 1)[4]. For each stem an 
FE model was created, representing a cemented THA reconstruction in a 
composite femur (Fig. 2). The stem-cement interfaces were unbonded 
around all stems (µ=0.25), as it was assumed that the rate of the 
debonding process was much higher than that of the failure process of 
the cement mantle. A loading history was applied to the models, 
representing 20 million cycles of alternating walking and stair climbing, 
in a ratio of 9:1 cycles [5]. 
     The FE simulation to model the damage accumulation failure 
scenario was based on a 3-D continuum damage mechanics approach. 
Damage (micro-cracks) was thought to accumulate in the cement mantle 
as a function of the number of loading cycles and the local stress levels 
and orientations. As damage accumulated, macro-cracks were formed, 
reducing the load carrying capacity of the cement. The simulation also 
accounted for cement creep. The simulation was able to predict the 
locations and orientations of the macro-cracks, and the amount of stem 
migration attributable to the mechanical failure processes. Fatigue and 
creep properties of the cement were taken from [6,7]. 
 
 
 
 
Fig. 1. Survival curves as 
reported by the Swedish 
Hip Register for the stems 
analyzed [4]. 
Fig 2. FE models of cemented 
reconstructions, with a Lubinus SPII, 
Exeter, Mueller C, and Charnley stem 
(from left to right). 
 
Results: After 20 million cycles, the Mueller C. had produced a con-
siderably higher number of cement cracks than the other three stems 
(Fig. 3). Cracks were formed around the entire stem (Fig. 5). Proximo-
distal damage pathways were formed, and the cracked zones often 
extended over the thickness of the mantle. The Charnley performed 
better, with a lower number of cracks. The rate at which proximo-distal 
damage pathways were formed was lower than around the Mueller C. 
The Exeter performed even better. Full thickness crack zones were 
produced only in the proximo-medial region. The crack zones 
progressed distally at a low rate. The Lubinus performed best, with the 
lowest number of cement cracks. No full thickness cracks occurred 
around that stem, and no proximo-distal damage paths were formed. 
Concerning migration, the Exeter migrated more than the other stems 
(Fig. 4). From the collared implants, the Lubinus SPII showed the lowest 
migration values. For all stems, the highest component of migration was 
torsional. 
 
 
Fig 3. Number of cracks in the 
cement around the stems 
Fig 4. Migration of head center of 
the stems after 20 million cycles 
 
 
Fig. 5. The crack distributions in the cement mantles around the Lubinus 
SPII stem (A), the Exeter stem (B), the Mueller Curved stem (C) and the 
Charnley stem (D). The cement mantles are split open along the mid-
frontal plane, and the inside of the anterior and posterior sides is shown. 
 
Discussion: Based on the clinical survival rates, the ranking of the stems 
would be, from superior to inferior: Lubinus SPII, Exeter, Charnley, 
Mueller Curved. This ranking was reproduced exactly by the FE 
simulations, when taking the number of cement cracks produced as the 
ranking criterion. This ranking was further justified by the finding that 
inferior stems produced proximo-distal damage pathways at a higher 
rate, and more full thickness cracks. Concerning the migration patterns, 
the stems behaved according to their design concepts, with the highest 
migration values for the Exeter stem. For the Exeter stem, high 
migration values did not accord with high damage accumulation rates. 
     In conclusion, the FE simulations were able to differentiate between 
four cemented THA implants, in a clinically valid way. This 
corroborates the use of the FE simulation for pre-clinical testing 
purposes. 
 
Acknowledgments: This work was performed within EU-project 
SMT4-CT96-2076 
References: [1] Sylvain et al, J Arthrop 16:141, 2001; [2] Jasty et al, 
JBJS 73B:551, 1991; [3] Stolk et al, 48th ORS, Dallas, TX; [4] Herberts 
et al, Acta Orthop Scand 71:111, 2000; [5] Morlock et al, J Biomech 
34:873, 2001; [6] Murphy et al, J Biomed Mater Res 59:646-654, 2002; 
[7] Verdonschot et al, J Biomed Mater Res 29: 575, 1995. 
 
Affiliated instituations for co-authors 
** Faculty of Biomedical Engineering, Eindhoven University of 
Technology, The Netherlands. 
49th Annual Meeting of the Orthopaedic Research Society
Paper #0213


CAN FINITE ELEMENT BASED PRE-CLINICAL TESTS DIFFERENTIATE BETWEEN CEMENTED HIP REPLACEMENT STEMS ACCORDING TO CLINICAL SURVIVAL RATES?

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CAN FINITE ELEMENT BASED PRE-CLINICAL TESTS DIFFERENTIATE BETWEEN CEMENTED HIP REPLACEMENT STEMS ACCORDING TO CLINICAL SURVIVAL RATES?

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