the failure mechanism of cemented glenoid designs: an in
TRANSCRIPT
Poster No. 1885 • 54th Annual Meeting of the Orthopaedic Research Society
The Failure Mechanism of Cemented Glenoid Designs: An In-Vitro and Finite Element Study
Sarah Junaid, Sanjay Gupta, Sanjay Sanghavi, Ulrich HansenMechanical Engineering Department, Imperial College London, London, United Kingdom
Introduction: The problem of glenoid loosening in total shoulder replacement(TSA) is not a new one. Surprisingly, it is still not clear which part of the fixationfails and where this failure initiates. Published works have used FE modelling, radi-ographs and in-vitro tests to assess the superiority of implants and design features[1,2,3,4,5]. All studies thus far have been unable to observe or measure failure. Theauthors propose an in-vitro and FE 2D plane strain study to observe and measurethe failure mechanism of cemented glenoids.
Materials and Methods: Custom-made glenoids were manufactured out ofUHMWPE using design parameters of commercially available implants. Threedesign features were used; keel vs. peg, curve-back vs. flat-back and conforming (25mm) vs. non-conforming (29 mm). Eight designs were made and cemented with a2 mm cement mantle into bone substitute PU foam. All glenoid backs were rough-blasted to 4-6μm. The ASTM standard for glenoid implant cyclic testing [6] wasused to test the specimens at 0.5 Hz using an Instron machine. A 24 mm articu-lating head was used to apply 1800 N compressive and 1100-1300 N vertical loads,corresponding to the subluxation curves of each design. The 29 mm designs wererepeated four times and the 25 mm designs once.
FE models were made of all eight designs using quad elements in Marc/Mentat2005. The in-vitro test was simulated as a linear static problem. Material proper-ties were taken from manufacturers’ data and literature search. The models weretested to convergence and a 3D glenoid model containing 27,000 tetrahedral ele-ments was built in order to compare any changes in compressive, tensile and shearstress distributions in the 2D scenario.
Results: In-vitro, all specimens failed inferiorly at the implant/cement interface(table 1). Failure was progressive and visible. This was also marked by a reductionin vertical load and hence joint stiffness. Differences between the 29 mm and 25mm designs were significant (p=0.012) with 25 mm designs performing better(table 1). For the corresponding conformities, flat-back designs were significantlyweaker to cyclic loading than curve-back designs (p=0.014). Overall there was nosignificant difference between the keel and peg designs. Although the choicebetween keel and peg was not critical in the flat-back designs, the peg was superi-or in the 29 mm curve-back case (p=0.004).
Table 1: Average number of cycles to complete failure. Note: FB=flat-back, CB=curve-back.The subluxation curves of all the FE models were within 10 % of the corre-
sponding in-vitro loads with the exception of the 29 mm flat-back keel, which waswithin 17 %. A comparison between the 2D and 3D models of the 29 mm flat-back keel design showed little difference in compressive, tensile and shear stress
patterns. At the implant/cement interface, inferior tensile stresses reached the ten-sile strength of the interface (2.5-4 MPa [7]) in all designs. The inferior tensile andshear stresses at the implant/cement interface were also analysed and compared toin-vitro failure. Table 2 demonstrates a correlation between shear stress inferiorlyand in-vitro order of failure. The exception to this rule is the 29 mm and 25 mmcurve-back peg, the former being most likely an anomalous result due to relativelyearly failure. On the other hand, inferior tensile stress does not show a correlationto order of failure.
Table 2: 29 mm and 25 mm order of failure and FEA predicted failure using implant/cementshear stress and tensile stress.
Discussion: In-vitro failure in 2D glenoid designs have been observed for thefirst time, showing failure at the implant/cement interface. The 3D analysis indi-cates there is no reason to believe the 2D scenario behaves very differently to the3D case. The curve-back design is mechanically superior and based on this study,is recommended for TSA. The choice of using peg instead of a keel is recom-mended in the curve-back design, however, there are no apparent differences in theflat-back case. Since the bone quality of rheumatic bone was modelled in this study,this may well affect the mechanical difference between keel and peg as shown byLacroix and authors[7]. The conforming designs performed better in this study,however, it must be considered that these designs were tested to lower displace-ments than the non-conforming, based on the testing standard [5]. FE stress plotsindicate inferior shear stresses as a contributor to glenoid failure and as a possiblefailure predictor. A larger sample size is currently being carried out, measuring rimdisplacements and vertical displacement in order to define the correlation withobserved failure progression.
References: 1. Lacroix et al, Proc Inst Mech Engrs Part H, 1997; 2. Anglin etal, J shoulder Elbow Surg, 2000b; 3. Oosterom et al, Proc Inst Mech Engrs Part H,2004; 4. Ianotti et al, J Shoulder Elbow Surg, 2005; 5. ASTM F2028-00; 6.Sanghavi et al, unpublished, 2007; 7. Lacroix et al, J Biomech Eng, 2000.
Acknowledgements: Funded by Arthritis Research Council.