FAILURE OF A TOTAL KNEE PROSTHESIS TIBIAL TRAY DUE TO THE FATIGUE WEAR

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1 7 FAILURE OF A TOTAL KNEE PROSTHESIS TIBIAL TRAY DUE TO THE FATIGUE WEAR Lucian CAPITANU, Aron IAROVICI, Justin ONIŞORU Institute of Solid Mechanics, Bucharest, Romanian Academy, Romania luciancapitanu@yahoo.com ABSTRACT Some studies report the failure of a tibial insert of knee prostheses due to the subsurface cracking and delamination wear [1]. The present study starts by analysing retrieved total knee prosthesis revealing the loss of a functionality of the prosthesis due to the fact that a larger part of the polyethylene tibial insert was dislocated. Some qualitatively remarks on the depreciative phenomenon are done based on optical and microscopic analysis of the affected areas of the polyethylene component. The failure mechanism was postulated as a combination of the high level of loading during normal activities and a non-conforming contact mechanism between the metallic femoral condyles and the tibial insert. The question that arises is: Could be this phenomenon evaluated quantitatively a priori, e.g. could be the failure due to delamination wear predicted? In order to do that some finite element simulations were performed to dynamically determinate the area of the contact and the contact pressure for three different activities, stair ascending and that could be considered as defining the level of the activity of the patient. The finite element model uses a modern contact mechanism, including friction, between the metallic femoral condyles (considered rigid) and the tibial polyethylene insert. The results reveal the disadvantages induced by the nonconformity of the contact, it means the fact that the loads are transferred by relatively small areas of higher stresses. A special summation technique was adopted in order to consider the cumulative effect of all activities, based on the frequency of each activity and on the magnitude of the determined damage function. The computed surface of cumulative maximum damage traversing the layers of the polyethylene insert could be identified as the cracking surface in the retrieved prosthesis. The conclusion is that a partial answer to the question enunciated before could be achieved by the method presented here, that is able to determinate the path of the subsequent failure of the polyethylene inserts even that for an accurate and complete answer some extensions of the simulations needed to be done. KEYWORDS: Total knee prosthesis, wear, experimental methods. 1. INTRODUCTION Recent clinical studies (Blunn et al. [1], Wasielevski et al. [2]) identify the massive wear of the polyethylene tibial insert due to the mechanics of non-conforming contact as the main cause to prosthetic failure. In [3] Knight et al. report that from 209 cases of primary TKR, 18 prostheses failed, all tibial inserts showing wear pits and delamination. The average time to failure was around 80 months, close to that reported by Heck et al. in [4] (i.e. 72 months), but other studies (Engh et al. [5], Jones et al. [6], Kilgus et al. [7], Mintz et al. [8]) show that gross delamination could occur even earlier. Examining an early retrieved prosthesis, Ries et al. [9] noticed gross wear pits and delamination of the tibial insert, accompanied by an increase of percentage crystallinity at a plane coincident with the plane of delamination and the presence of subsurface oxidation peak typically observed in UHMWPE inserts, gamma-irradiated in air and aged. Still controversial, the idea that increasing the thickness of tibial inserts will reduce wear was rather false. Indeed, it is the contact non-conformity which conducts to high contact pressure localized on small areas and subsequently to the fatigue wear and delamination of polyethylene part [3, 4].

2 8 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI a) b) c) d) Fig. 1. Pictures of retrieved TKP after 21 years operation: (a) CoCr alloy femoral-component; (b) UHMWPE insert and 16 years operation; (c) CoCr alloy femoral-component; (d) UHMWPE insert [15]. Fig. 2. Optical image of a UHMWPE insert estimated by using a shape tracer. Lines indicate surface profiles. (+) marks indicate the maximum deformation point at each area. [15]. Analysing two retrieved TKP (after 21 and 16 years from implantation), Oonishi et al. [15] noticed the presence of parallel scratches (oriented anteroposterior) on femoral component (fig. 1), and large delamination of polyethylene tibial insert (fig. 2). Our study, based on examination of two retrievals (see fig. 3 and fig. 4) identifies also some characteristic marks of fatigue wear phenomena. The insert in figure 3a is from an early retrieved tibial part (the revision was due to the loosening of the implant) and one could observe the wear pits of the contact surface, that are specific for adhesive wear, and also delamination of the peripheral area of the medial part as effect of fatigue wear. The insert in figure 3b was from a total knee prosthesis retrieved at 67 months from implantation. In this case, the fatigue wear has more catastrophic effects massive loss of polyethylene in the medial part, and severe delamination in the lateral part. From the qualitative evaluation of the two retrieved implants we could conclude that the fatigue wear debuted with some wear pits of the contact surface and delamination (especially in the peripheral medial part of the tibial insert) but, in time, severe damages occur (gross delaminations, massive losses of polyethylene). 2. METHODOLOGY AND FE MODEL The methodology for predicting the fatigue wear phenomena combines FE analyses of active loading cycles of relevant routine activities with a summation technique that is based on computation of a cumulative estimator of damage.

3 9 a) b) Fig. 3. Two retrieved polyethylene inserts showing (a) pitting and (b) severe delamination [6]. Fig. 4. Pictures of fatigue wear of a total hip prostheses components. A FE model of the artificial joint contact was used (fig. 5) for all analyses. The model includes one femoral condyle and one half (the medial one) of the polyethylene insert, and of the metallic tibial tray. For deformable parts (the tibial parts) solid brick elements with 8 nodes and 3 DOF s per node (all three translations) are used. Fig. 5. Finite element model used in analyses [16]. The femoral condyle having a thoroidal shape with a radius of 22 mm in the sagital plane (the flexion plane), and a radius of 30 mm in the transversal plane, was considered rigid. Elastic linear constitutive laws are considered for metallic as for the plastic parts. The materials properties are listed in Table 1. The routine activities considered relevant for this study are the active cycles of the, stair descending, and stair ascending activities. The kinematics of all three movements are illustrated in figure 6 in (a), (b), and (c) the segmental movement of the lower member are sketched (Bergmann et al. [10]), in (d) the compressive force of contact are listed comparative for all activities (Taylor and Walker, [11]), in (e) and (f) two determinants of the cinematic conditions (the flexion angle and the internal-external rotation of tibia) are plotted (Bergmann et al. [10]). The compressive force is applied to the tibial tray (as distributed pressure on the lower basis of it). The two rotations (flexion and internal-external rotation) are dynamically constraining the femoral part. We considered that the actual level of flexion (not higher than 70 degrees) does not induce an antero-posterior translation (usually appearing when the ligaments are overstrained) as for extended flexions.

4 10 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI Joint contact force [%BW] a) d) Flexion angle [deg] b) e) 0 25 Internal-external rotation angle [deg] c) f) -10 Fig. 6. Dynamic conditions during all activities: kinematics of lower member [10] (a, b stairs descending, c ) and loading [11] and kinematics [10] of the joint surfaces (d contact force, e flexion angle, f internal-external rotation angle). Table 1. The mechanical properties of materials used in FE analyses Material Young Modulus E [GPa] Poisson ratio ν Remarks Co-Cr alloy UHMWPE from ISO from Lewis [12]

5 11 The contact mechanism includes rolling and sliding of the two joint surfaces the femoral condyliar thoroidal surface and the planar surface of tibial insert; the friction obeys the Coulomb law with a friction coefficient of 0.12 (Villa et al. [13]). For every activity a damage estimator could be computed from the variation of the shear maximum principal stress in every element (Sathasivam and Walker [14]): n k 1 D f = i 1 i ( i 1 i) 2 τ + τ τ + + τ (1) i= 1 where: k D f - damage function for activity k, τ i - shear maximum stress on elements for time t i. The effect of all activities could be cumulated by evaluating a weighted sum: tot k Df = wkdf (2) where the weights are depending on the frequency of the activity. 3. RESULTS AND DISCUSSION Performing the dynamic analyses of the contact between the femoral metallic condyles and the polyethylene tibial insert (in the above-described conditions) one could determinate the characteristics of the contact mechanism in the artificial knee. For example, the trajectories of the contact spot for all three activities are plotted on the medial part of the tibial insert (fig. 7). Examining the sketch one could see that all activities do not involve extension of the knee it means that only the median and posterior parts of insert are used. For the trajectory is a closed curve (like a hysteretic loop). The stair ascending and stair descending trajectories are quite similar (even opposite as direction), both of them having a large internalexternal rotation (for stair ascending loading at the beginning of the cycle, for stair descending at the end of it). One could see that the loading cycle will affect the median area of the medial part of the insert, the stair ascending and descending activities having their maxima located in the posterior parts of the insert. As expected, due to the geometry of the contact (the joint surfaces are a toroidal surface for femoral condyle and a plane surface for tibial insert) it is clear that the contact spot will have an elliptical form with the major axis oriented transversally to the saggital plane of the femoral condyle. The maximum contact pressure is a good estimate of the intensity of the loading. The variation in time of this contact parameter will follow the variation of loading (see fig. 8). One could notice for all activities an initial step maxima (corresponding to the initiation of the movement the contact between the foot and the ground) followed by even a higher value but distributed on a large domain of time (corresponding to the weight bearing stance). The contact pressure is very important as estimator of loading magnitude, but even more important for estimating the fatigue wear will be the maximum shear stress, as being the parameter used in the damage estimator stated by formula (1). From the contact mechanism (which involves both rolling and frictional sliding) it results that the maximum shear stress will be located under the contact surface at a distance depending of the aspect ratio of the contact elliptic spot. Using the cumulative estimator defined in formula (2) the areas were the damage is likely to occurs could be identified from the cumulative distributions plotted in figure 9 (the L-shaped region of different colour). One could see that the most affected areas are the medial and posterior parts of the tibial insert. One could also notice that the maximum damage will occur in the subsurface of the insert (where the shear stresses from are maximal), which is usual the starting plane of severe delaminations. The magnitude of the damage estimator (~477MPa 2 at the surface and ~535 MPa 2 in the subsurface) is in concordance with the results of Sathasivam and Walker [14] which obtained a damage score of ~230 MPa 2 for a constant loading of 1,000 N (approx. 120% BW) which represent one half of the maximum loading from or from stair ascending or stair descending activities. 4. CONCLUSIONS A method combining the Finite Element Method for evaluating the contact mechanism in an artificial knee joint with a summation technique based on a damage estimator was successfully used in order to predict the areas were the damage is likely to occur. Based on the above-described study, one can formulate several conclusions. First, the fatigue wear was identified as the main phenomenon responsible for the polyethylene tibial insert failure. The clinical studies show that even those tibial parts retrieved for other reasons (implant loosening, misalignment etc.) presented markers of fatigue wear (cracks in the subsurface, wear pits, delaminations, loss of large parts of polyethylene). The cumulative nature of the wear fatigue phenomena requires a qualitative and quantitative evaluation of the transfer of loadings through the joint and a summation technique for encountering the variety of the human activities. The first part was assessed by intermediate of some dynamic finite element analyses of the joint contact mechanism. For the second part, we assumed that, stair ascending and stair descending are the regular activities dominant for the phenomena.

6 12 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI stair ascending Posterior stair descending Medial Lateral normal walking Anterior Fig. 7. The contact paths for all three cases considered Contact pressure [MPa] Fig. 8. The contact pressure variation for all three cases. a) b) Fig. 9. The damage score map (a) at the contact surface, (b) in the subsurface.

7 13 Thus, for all three activities the finite element analyses are performed and a damage score was computed. Finally, a cumulative damage score (that accounts for all three activities) was determined and the areas were the fatigue wear is likely to occur, are identified. From the distribution of the cumulative damage score (plotted in figure 9) one could see that the areas that could be damaged are located medially in the median and posterior regions of the tibial insert. Even the stair ascending and descending are activities that generate higher contact pressure, due to the increased frequency, the is still dominant. A closer look to the distribution of damage score reveals that the maximum damage is likely to occur not at the contact surface but, rather, in the subsurface. The presence of maximum values in an area were other studies (see Ries et al. [9]) identifies an increase of the percentage cristallinity and the presence of subsurface oxidation peak induced by the gamma-irradiation and ageing of the prosthesis could be an explanation of the early delamination of the polyethylene inserts. We intend to extend the study in two directions: firstly, the contact mechanism would be improved by considering the polyethylene viscoelastoplasticity and a better friction mechanism (accounting for the dependency of the friction coefficient to the contact pressure and for the velocity of the relative movement between the joint surfaces); secondary, the summation technique would be extended to a large area of activities and the influence of the frequency of the loadings would be better estimated. REFERENCES 1. Blunn G.W., Joshi A.B., Minns R.J. et all, 1997, Wear in retrieved condylar knee arthroplasties, Journal of Arthroplasty, Vol. 12, pp Wasielewski R.C., Galante J.O., Leighty R.M., Natarajan R.N., Rosenberg A.G., 1994, Wear patterns on retrieved polyethylene tibial inserts and their relationship to technical considerations during total knee arthroplasty, Clinical Orthopaedics, Vol. 299, pp Knight J.L., Gorai P.A., Atwater R.D., Grothaus L., 1995, Tibial Polyethylene Failure After Primary Porous-coated Anatomic Total Knee Arthroplasty, J. of Arthroplasty, Vol. 10, pp Heck D.A., Clingman J.K., Kettelkamp D.G., 1992, Gross polyethylene failure in total knee arthroplasty, Orthopedics, Vol. 15, pp. 23ff. 5. Engh G.A., Dwyer K.A., Hanes C.K., 1992, Polyethlyene wear of metal-backed tibial components in total and unicompartmental knee prostheses, J Bone Joint Surg, Vol. 74B, pp Jones S.M.G., Pinder I.M., Moran C.G., Malcolm A.J., 1992, Polyethylene wear in uncemented knee replacements, J Bone Joint Surg, Vol. 74B, pp Kilgus D.J., Moreland J.R., Finerman G.A. et al, 1991, Catastrophic wear of tibial polyethylene inserts, Clin Orthop, Vol. 273, pp Mintz L., Tsao A.K., McCrae C.R. et al, 1991, The arthroscopic evaluation and characteristics of severe polyethylene wear in total knee arthroplasty, Clin Orthop, Vol. 273, pp Ries M.D., Bellare A., Livingston B.J., Cohen R.E., Spector, M., 1996, Early Delamination of a Hylamer-M Tibial Insert, Journal of Arthroplasty, Vol. 11, pp Bergmann G., Deuretzbacher G., Heller M., Graichen F., Rohlmann A., Strauss J., Duda G.N., 2001, Hip Contact Forces and Gait Patterns from Routine Activities, Journal of Biomechanics, Vol. 34, pp Taylor S.J.G., Walker P.S., 2001, Forces and moments telemetered from two distal femoral replacements during various activities, Journal of Biomechanics, Vol. 34, pp Lewis G., 2001, Properties of cross-linked ultra-highmolecular-weight polyethylene, Biomaterials, Vol. 22, pp Villa T., Migliavacca F., Gastaldi D., Colombo M., Pietrabissa R., 2004, Contact stresses and fatigue life in a knee prosthesis: comparison between in vitro measurements and computational simulation, Journal of Biomechanics, Vol. 37, pp Sathasivam, S., Walker, P.S., 1999, The conflicting requirements of laxity and conformity in total knee replacement, Journal of Biomechanics, Vol. 32, pp Oonishi H., Kim S.C., Kyomoto M., Iwamoto M., Ueno M., 2006, PE wear in ceramic/pe bearing surface in total knee arthroplasty; Clinical experiences of more than 24 years, Bioceramics and Alternative Bearings in Joint Arthroplasty, 11-th BIOLOX Symposium Proceedings, pp Onişoru J., Capitanu L., Iarovici A., 2006, Failure of a tibial insert of a total knee prosthesis due to fatigue wear, Buletinul Institutului Politehnic din Iasi, Tomul LII (LVI), Fasc. 6A, ACME 2006/1, pp