Comparison of numerical results of bone remodelling and clinical observations in short-stem hip-jointprosthesis

Transcription

1 The Journal of Orthopaedcs Trauma Surgery and Related Research Comparson of numercal results of bone remodellng and clncal observatons n short-stem hp-jontprosthess J ORTHOP TRAUMA SURG REL RES 12(3) 2017 Research Artcle ULRICH WIEBKING 1, BASTIAN EBBECKE 2, THOMAS GÖSLING 3, UDO NACKENHORST 4 AND CHRISTIAN KRETTEK 1 1 Department of Trauma Surgery, Medznsche Hochschule Hannover, Germany 2 Volkswagen AG, Wolfsburg, Germany 3 Department of Trauma and Orthopaedc Surgery, Städtsches Klnkum Braunschweg GmbH, Holwedestraße 16, Braunschweg, Germany 4 Insttute of Mechancs and Computatonal Mechancs, Gottfred Wlhelm Lebnz Unverstät Hannover, Welfengarten 1, Hannover, Germany Address for correspondence: Dr. Ulrch Webkng, Department of Trauma Surgery, Medznsche Hochschule Hannover, Carl-Neuberg-Str. 1, Hannover, Germany Tel: 0049/(0)511/ , Fax: 0049/(0)511/ , E-mal: U.Webkng@t-onlne.de Statstcs Fgures 10 Tables 01 References 22 Receved: Accepted: Publshed: Abstract Background: Investgatons of the densty dstrbuton of bone caused by mplants make predctons n bone remodellng possble. For the executon of these analyses, a method s presented wth fnte element calculaton. Methods: A bophyscal model based upon a fnte-element calculaton s examned wth numercal calculatons of short-stemmed (Mayo and Spron), uncemented prostheses. Changes n bone mneral densty -- measured over a perod of 5 years n 100 patents after total hp replacement, -- were used as reference for the quanttatve check calculatons. The developed algorthm underlyng these FE-calculatons was subjected to an applcaton test, and both an approved and new short-stemmed artfcal lmb system regardng ts densty dstrbuton s smultaneously examned. Results: The spron-system was found to have the lowest resorpton behavour. Conclusons: For the short-stem prosthess bone remodellng actvty n the area of the medal proxmal cortcals s obvously. Keywords: Fnte-elements, Bone densty, Stress sheldng, Short-stem prosthess 12 (3) 2017

2 41 INTRODUCTION Hp replacement n young patents often leads to a hgh probablty of revson operaton later n lfe [1]. Therefore, the am already at prmary mplantaton should be to remove few bones, and to obtan a stand tme as long as possble. The varety of artfcal lmb models offered for ths ndcaton underlnes the fact a soluton s yet to be found [2-5]. A commonly used artfcal lmb s the Mayo hp prosthess (Zmmer Inc., Warsaw, USA). A newer alternatve s the Spron prosthess (ARGE, Hannover, Germany), ntroduced n 2000 (Fg. 1a and 1b). The dfferences between these two artfcal lmb types are prmarly n constructon, overcoatng, and mplantaton technque [2,4]. Fg. 1 (a). Mayo prosthess. (b). Spron prosthess. Measurement of the relatve moton between a prosthess and bone n vtro s the frst step n testng the prmary stablty of a new prosthess. We dd ths for the Spron prosthess [6]. She has sgnfcantly less moton n the bone/prosthess nterface compared wth the Zweymuller prosthess. Whle clncal course observaton only allows for successful lmb mplantaton to be determned after years, an mmnent desgn problem can be recognzed sooner wth a numercal calculaton. Numercal methods about the adaptaton of the bone have been establshed n prevous studes, datng back more than 15 years [5,7-9]. Wth the help of numercal methods, statements about the change of the bone remodellng are possble wthout rsks for the patent. In connecton wth ths, the queston about the valdty of the mposed data has been posed [10-12]. In the study presented here, a fnte element method s valdated wth clncal data from bone densty analyss of the proxmal femur [13], usng the method of the calculaton of both artfcal lmb types (Mayo /Spron ) [2,14,15]. The am of the study s to test the prncple effcency and valdty of the algorthm, and to obtan results about bone remodellng n artfcal lmb types. METHODS Theoretcal framework Computatonal methods for the smulaton of stress adaptve bone remodelng have been under development for more than 15 years. After ntal problems wth the stablty of the algorthms, current methods are stable and effcent methods can be provded. The work presented here s a follow-up to the prevous work done by Nackenhorst and colleagues [8,16]. By ths approach, the bone s modeled n a contnuum sense, meanng that the network structure of cancellous bone n the proxmal regon of the femur s smeared by an average mass densty ρ. However, the trabecular structure can be descrbed wth ts ansotropc mechancal behavour [8, 17]. Furthermore, due to lmted knowledge about tme-dependent jont loads and muscle forces, the model s based on statcally equvalent forces. These statcally equvalent forces are the result of an optmzaton procedure, whch targets the best ft of the computed mass densty dstrbuton wth the physologcal bone. Ths optmzaton procedure can be performed for ndvdual anatomy and moblty condtons whenever CT--data are provded. Basc assumpton of the theory s an equaton of evoluton. ρ =k ψ ψ -1 ref that relates the tme-rate of mass densty to the mechancal demand, (1) whch s expressed by the stran-energy densty Ψ n the sotropc case. When the stran-energy densty exceeds a physologcal target value Ψ locally, the bone densty ncreases, otherwse decreasng untl each pont of the bone s stressed physologcally. It should be noted that ths physologcal target value s a result of the optmzaton procedure prevously mentoned. Other groups have suggested a tolerance-regon around the physologcal target value (so called dead zone) [7,9] whereby numercal nstabltes can be expected. However, the values suggested are vague, and specal numercal treatment s requred to prevent the soluton from dependng on the ntal condtons. The parameter k n Eq. (1) s a measure for the tme scale n whch the remodellng processes take place. Ths s observed when Eq. (1) s scaled by a reference mass-densty ρo,.. ρ ψ λ = = k 1 ρ 0 ψ ref k where k = has the dmenson of one over tme; λ s referred to ρ0 as desgn-varable below. Untl recently, the tme scales n whch bone remodellng processes take place were not well nvestgated, but some gudelnes could be derved from fracture healng. Ths value s of essental nterest for remoblzaton therapes, but t does not need to be specfed n ths context because only the long term behavour s of nterest for the current nvestgatons. Hence, the tme-dscretzed verson of Eq. (2), are.e., ψ λ = τ 1 ψ (3) ref wll be treated, where a dmensonless tme ncrement τ = k t has been ntroduced. The computaton of the stran-energy densty dstrbuton s performed by a lnear-elastc fnte element analyss, whch s an approxmate soluton of the mechancal equlbrum for arbtrary geometres under consderaton of the specfc boundary condtons. A standard procedure leads to a system of coupled lnear equatons K u = f (4) where K s the stffness matrx, u collects the unknown nodal dsplacements and f represents the appled external forces actng at the nodes of the fnte element model. From the soluton, the stran energy-densty s derved by a smple post-processng step: 1 Ψ= ε T Cε (5) 2ρ where C s the elastcty-matrx and ε the Vogt-representaton of the lnear stran tensor, whch s derved from the dsplacementfeld computed from Eq. (4). It s essental to recognze that the elastcty-matrx depends on the spatal densty dstrbuton. A consttutve relaton has to be stated between the Youngsmodulus E and mass-densty ρ. Wthn a thermodynamc consstent framework, a relaton: E E ρ = ρ has been proven prevously [8]. Wth the constants E 0 and ρ 0, reference values (e.g. for cortcal bone) have been ntroduced for a dmensonless representaton. Equaton (6) s n accordance wth a statstcal analyss of varous ndependently performed measurements [15]. By combnng these fndngs, Eq. (5) s rewrtten as λ ψ = ε T C0ε (7) 2ρ 0 and therefore, a nonlnear problem consstng of Eqs. (3), (4), (6), and (8) has to be solved. By an explct approach, the scheme (2) (6)

3 42 sketched n Fg. 2a has to be solved. Heren, the marker t denotes the ncremental step, and the suffx represents the fnte element number. The update loop has to be performed untl a convergence crteron s fulflled; for example, the stran energy and the mass densty wthn the whole system do not change. The real course of the tme s not known. It s not clear whether the long-term stuaton s reached after 6 months or 6 years. Fg. 2a. Explct soluton procedure to the soluton of the non-lnear problem. The fnte element model has D 4-knots planar elemtents (Fg. 2b left) and D 4-knots tetrahedron volume elements (Fg. 2a rght). The rgdty of the volume elements s varable accordng to Eq. (6). That means a volume element wth a densty lke zero, has no rgdty and does not exst mathematcal. The nets are lnked drectly to each other, therefore an deal ngrowth behavor between bone and mplant are smulated. The Young s modulus of the prostheses correspondents to the materal propertes of ttan. Fg. 2b. 2-D and 3-D fnte elements net, respectvely wth load vectors. The net buldng was realzed wth the software MSC Patran 2001r2. For the fnte element calculaton, we used a new algorthm developed wth Matlab 6 [18]. We assume for 3D statc aequvalent loads [18], for 2D we assume the one-leg load. Healthy femur The frst step n numercal analyss performed here s the calculaton of the bomechancal balance (Fg. 3). The computed mass densty dstrbuton s compared wth a radograph of the proxmal femur. The hollow bone, as well as the characterstc trabecular structure of the cancellous proxmal femur, s approxmated by the fnte element model. The medullary cavty, trabecular, and Ward s trangle can be recognzed n the comparson. Fg. 3 (a) Calculated densty dstrbuton of the proxmal femur, (b) Comparson wth an x-ray mage. Model valdaton Valdaton of the model was executed wth data whch were nvestgated from a measurement of the bone mneral densty of the proxmal femur after cementless hp arthroplasty, over a perod of 5 years n 100 patents [13]. The BMD of the proxmal femur was measured by DEXA usng a QDR Hologc scanner (Hologc Inc, Waltham, Massachusetts). Analyss of the changes of bone mneral densty n the 7 Gruen zones of the proxmal femur therefore was possble for clncal condtons. As n the cted reference, we used an Alloclassc Zweymüller SL cementless stem (Zmmer, Inc., Warsaw, USA) for the smulaton of change n bone mneral densty, and a comparson of estmatons wth the data [13] was executed. In ths model, computed densty of the fnte elements was added n each Gruen zone. To compare ths data wth the examnatons n [19], the relatve change of BMD (compared wth the base data) of each Gruen zone G k n each fnte element at tme step t was calculated: V = det ( j ) t t λv t n Gk = λ v n 1 1 Therefore, t s possble to compare the data of the examnatons wth the data of the computaton (Table 1). Ths procedure also was used for the quanttatve descrpton of the calculated changes of densty of femoral neck arthroplastes. Prostheses In 1981, B.F. Morrey suggested replacng the mmedate rgd fxaton of the prosthess n the shaft by several selectve contacts at certan regons of the proxmal thgh -- leadng to the development of the Mayo hp conservatve prosthess [4]. The optmal stress dstrbuton pattern for ntramedullary loaded systems s not alone ensured by anatomcal shaft shapes [4]. A desgn was developed to make the mmedate fxng possble n the medullary cavty, ensurng the greatest possble compatblty wthn the medullary cavty wthout a greater manpulaton of the anatomy. Studes of fatgue, proxmal femoral anatomy, fracture characterstcs, and fnte-element stress analyss produce a short-stemmed femoral mplant wth a double-wedged contour and neck shaft angle of 150 (Fg. 1, left). The Spron prosthess, developed n 1999, ams to avod the rsk of a secondary varaton and ncrease stablty [2]. Durng development of ths uncemented, short-stemmed prosthess attenton was pad to the recept of the regon of the femoral neck, snce these ncreases the stablty n the prmary phase and garners a suffcent bone stock n the revson for a conventonal ntramedullary prosthess. Furthermore, use of the femoral neck should receve the mechancs of the natural jont by unchanged muscular stran. The cementless concal basc body and self-tappng thread works lke a screw. Ths shape produces a surface enlargement n the compact volume of the femoral neck. Because of the sharp-edged thread formaton, osteonducton s reached n addton to the ttanum alloy and bont coatng of the prosthess. RESULTS Valdaton Whle the measurements of the patents showed a sgnfcant decrease of the mean bone mneral densty of 4% after 6 months, our estmatons showed a slght ncrease of the mean bone mneral densty of 1.5% after 1 year. After 1 year, the values reganed ther ntal standard, so that a fnal value of 0.5% over the baselne was presented after 5 years. The fnal value of the bone mneral densty of the clncal measurements after 5 years was 1% over the baselne value. Wthn the frst 6 months, the bone mneral densty decreased n our model nto 4 Gruen zones (1, 4, 6 and 7) and ncreased n the other 3 zones (2, 3 and 5). Except for zone 2 and 3, the clncal

4 43 Table 1. Relatve change of the densty dstrbuton at 11-tme ponts for each Gruen zone caused by mplantaton of the Alloclassc Zweymüller SL cementless stem. Reproduced wth permsson and copyrght of the Brtsh Edtoral Socety of Bone and Jont Surgery [13]. Varables 1 week 6 months 1 yr 1,5 yrs 2 yrs 2,5 yrs 3 yrs 3,5 yrs 4 yrs 4,5 yrs 5 yrs 1 1, , , , , , , Mean 1,0000 1,0081 1,0158 1,0148 1,0130 1,0115 1,0094 1,0073 1,0062 1,0059 1,0059 measurements showed smlar courses at the patent: The bone mneral densty nto zone 2 remaned the same, and decreased nto zone 3. Bone mneral densty ncreased n the other zones between the frst 6 months and the 1st year n the model presented here, except for n zones 3, 6, and 7. The bone mneral densty n vvo measurements n zones 3, 6, and 7 showed a decrease for zone 3 and no changes for zones 6 and 7. The bone mneral densty for all other zones was n accordance wth estmatons. Between year 1 and 5 the bone mneral densty ncreased n zone 4 and 5 n the calculated model lke also n the clncal measurement. The bone mneral densty nto zone 2 and 3 removed over the same tme perod n the estmatons. The clncal data showed a sgnfcant ncrease n these zones. Zones 1, 6, and 7 showed none or only weak changes n the model presented here for the bone mneral densty for ths tme perod. Ths s n accordance wth the fact that the clncal data dd not show any sgnfcant changes for these zones. Compared to the baselne n the clncal measurements 5 years after surgcal operaton, less bone mneral densty was seen n zones 1, 6, and 7, more bone mneral densty was seen n zones 2, 4, and 5, and an ncrease was seen n zone 3. Except for zone 3, these data corresponded to our estmatons. Zone 3 contaned a removal of the bone mneral densty n the model after 5 years (Fg. 4). ncrease of 4% after 6 months. Subsequently, t decreased to a fnal value of 9% beyond the baselne after 5 years. After an ncrease of the densty dstrbuton n the greater trochanter (Gruen zone 1) of 16% wthn the frst 6 months, the dstrbuton was found to be 34% under the baselne after 5 years. The tp of the prosthess (Gruen zone 2) contaned an ncrease of the densty dstrbuton of 23% as the only area after 6 months. Ths dstrbuton rebounded up to the end-pont after 5 years only on 18% over the baselne. The calcar (Gruen zone 7) and every zone 3-6 showed reductons of the densty dstrbuton between 7 and 13% (Fg. 6). Fg. 5. Numercal calculaton of the densty change after mplantaton of the Mayo prosthess. Fg. 4 (a) Results of the numercal smulaton of the changes of densty for more than 5 years, dstrbuted on 7 Gruen zones n the proxmal femur due to mplantaton of a Zweymueller Allo classc SL stem. Value after 1 week corresponds to the baselne. Values n percentage of the baselne; (b) Course of the changes of densty because of mplantaton of a Zweymueller Allo classc stem at n= 100 patents. Reproduced wth permsson and copyrght of the Brtsh Edtoral Socety of Bone and Jont Surgery [20]. Mayo-prosthess The bone remodelng charged for the Mayo prosthess are shown n Fg. 5. The mass densty dstrbuton n ths representaton was compared to the bomechancal long-tme balanced state. We found changes n the remanng cancellous bone n whch the orgnal stran trabecular structure was reduced largely n the trochanter regon. In the proxmal area of the medal cortcals was found a decrease n densty. A condensaton at the lateral cortcals appeared at the tp of the prosthess. The mean densty dstrbuton n the complete bone contaned an Fg. 6. Course of calculaton of the change of densty due to mplantaton of Mayo prosthess. Value after 1 week corresponds to the baselne. Value n percentage of the baselne. The rarefcaton of the trabecular structure n the area of the trochanter major and the condensaton at the end of the Mayoprosthess shows Fg. 7. Fg y.o. strongly adpose patent 1.8 years after operatvely suppled acetabulum fracture on the rght. In the course development of a post-

5 44 traumatc arthross and after that mplantaton of a Mayo prothess 8 months after the frst supply. (a) 1 month, (b) 2 months, (c) 4 months, and (d) 9 months after hp replacement. Stll Duchenne-lmpng at nsuffcency of the M. gluteus medus at ths nstant. Otherwse recovered. Spron prosthess Results of the Spron prosthess smulaton are shown n Fg. 8. The stress-sheldng and bone response at the lateral cortcals n the area at the end of the screw was proved n a seres (Fg. 9). Intally, the mean densty dstrbuton n the complete bone showed an ncrease of 1% after 6 months. Subsequently, t decreased to a fnal value from 9% beyond the baselne after 5 years. The densty dstrbuton n the greater trochanter (Gruen zone 1) was 3% over the baselne after 6 months. After ths, t came to a decrease to 21% over the baselne. The calcar (Gruen zone 7) contaned a gradual removal of the densty dstrbuton of 14% under the baselne after 5 years. All further zones (2-6) contaned only decreases of the densty dstrbuton 4% to 6% under the baselne (Fg. 10). Fg. 8. Numercal calculaton of the change of densty after mplantaton of the Spron prosthess. Fg. 9. A-43 year-old patent 3 years after acetabulum fracture wth dslocaton of hp treated conservatvely on the rght. Wth mmunosuppresson after heart transplantaton and a marathon, development of an aseptc hp necross. (a) postoperatve mage, (b) 2 months, (c) 3 months, and (d) 6 months after the hp replacement. Fg. 10. Course of the calculated change of densty due to mplantaton of Spron prosthess. Value after 1 week corresponds to the baselne. Value n percentage of the baselne. DISCUSSION Numerous efforts have been made recently n the feld of hp prostheses to mnmze the weaknesses of conventonal prmary hp arthroplasty by optmzaton of desgn [2,3,5]. The am s to stablze the femoral neck regon and prevent a stress-sheldng of the proxmal femur, as well as to make ths regon a stable mplant bearng. The deal prosthess would be made wth respect to ndvdual bone densty for the respectve patent. A potental tool to acheve ths s the method of smulaton of bone densty dstrbuton under gven load falls, wth the help of the fnte element method [19]. However, t s also possble to make statements about the expected femoral stress [17]. A new algorthm developed to address these challenges was used for the analyss of the reorganzaton behavor of bone densty at dfferent short stemmed prostheses [4,8,15]. The frst objectve was to determne whether the data calculated wth ths algorthm could make accurate predctons about the change n bone densty. Secondly, the objectve was to test the calculaton procedure wth dfferent geometres of short-stemmed prostheses, and to obtan nformaton about the reorganzaton behavor of bone under these prostheses. Bone densty data were measured after total hp replacement n a prevous study [13] n the 7 Gruen zones of the proxmal femur and over a perod of 5 years. The data were ascertaned by dualenergy x-ray absorptometry (DEXA). The DEXA s regarded as an establshed method for the acquston of changes n bone densty around prostheses, snce patents do not have to undergo any further nvasve procedures to obtan data [20]. The study perod was longer and the patent populaton was greater than that of smlar studes [21,22]. The ascertaned data contaned a suffcent valdty for our calculated data as a reference. The calculated changes of the bone densty n the 7 Gruen zones of the model presented here corresponded to the changes n bone densty measured n vvo. However, the concrete proportonal changes to the baselne dd not correspond. The calculated changes of the bone densty n zones 1, 2, 4, 5, 6 and 7 after operaton n the 5th year were n the average 5% hgher than the clncal data (Fg. 4). Zone 2 contaned the lowest change n bone densty (13 vs. 11), and zone 7 presented the hghest dfference (3 vs. 14) compared to clncal measurements. Furthermore, zones 2 and 4 showed a more rapd ncrease n bone mneral densty, and zone 6 showed a slower ncrease n bone mneral densty compared to clncal measurements. However, zone 3 showed the greatest devaton, and the course of modfcatons of densty n ths zone s contrary to the clncal data. Ths means a loss of bone at the lateral mean thrd of the femur therefore after 5 years. However, the clncal measurements showed an ncrease n densty. Thus the model was computed n two-dmensonal, ths devaton s not surprsng: The threedmensonal load behavour has been smulated by a sde plate. Ths plate had a constant thckness. In addton, the smplfed load case of the forces n 2D was used -- one hp jont force dstrbuted on the Caput femors and the Adductor longus muscle on the Trochanter major explanng the devatons n zone 3. Despte these restrctons, the calculated course of the change n bone mneral densty at the greater trochanter, at the tp of the stem and at the medal dstal thrd (zones 1, 4, and 5), corresponded to the changes measured n vvo at the bone over the complete tme perod. A devaton was found at the medal proxmal thrd (zone 6) and calcar (zone 7) only between 6 months and 1 year after the operaton for bone mneral densty compared to clncal data. The bone mneral densty n the lateral proxmal thrd (zone 2) was n agreement wth clncal measurement n ts course only n the 5th year. The lateral dstal thrd (zone 3) was not n agreement wth the clncal values at any tme. The Alloclassc Zweymüller SL cementless stem allows transmsson of mechancal forces to the lateral and dstal proxmal thrd of the proxmal femur (zones 2, 4, and 5) [13]. Removal of the bone mneral densty s seen n the greater trochanter (zone 1) and calcar (zones 6 and 7). Ths data were confrmed by the estmatons from bone densty measurements n vvo. The loss n calcar wth 3% s less than data (14%) measured n vvo. Other clncal studes contan hgher losses of bone densty n ths range [20-22]. As n other clncal studes, the estmatons made here show more loss

6 45 n the greater trochanter (zone 1) and n the medal proxmal thrd (zone 6) (9% and 10%, respectvely) than the n vvo measurements (3%) provded n prevous studes [20-22]. Comparson of calculated and actual reorganzaton behavor of the femoral neck arthroplastes appled here showed agreement qualtatvely n x-ray mages. In addton, the proportonal changes of densty dstrbutons were calculated from fnte element models of the short stem prostheses. The mean bone densty of the 7 Gruen zones showed an ncrease of the BMD n the frst 6 months for the two prostheses after mplantaton. The Spron prosthess produced a hgher dssmlaton of bone densty under the ntal value compared to the Mayoprosthess. Analyss of the ndvdual courses of the changes of densty n the 7 Gruen zones showed an apparent decrease n the greater trochanter and the calcar. For the Mayo-prosthess, the dynamcs of the dssmlaton was greater than that of the Spron prosthess n the greater trochanter, whereas n the calcar the Spron prosthess trggered a far stronger dssmlaton processes than the Mayo prosthess. Due to ts desgn, the Mayo prosthess can wthstand a greater bendng load than the Spron prosthess, and a stronger rarefcaton for the trabecula than the greater trochanter. The changes of densty n the area of the calcar at the Spron prosthess showed that the plate of the prosthess s a guarantor for the prmary fxaton up to osseontegraton [2]. Upon completon of osseontegraton, there was lttle danger of an after-varzaton due to vertcal forces, and therefore the supportng functon of the plate of the prosthess was dspensable. However, these changes n densty were not n all patents provded wth Spron prostheses. In the case descrbed here, ths resorpton can already be recognzed after 9 months. Consderng the challenges of the two-dmensonal representaton, n connecton wth the valdaton of the model and the radologcal changes of mplanted prostheses descrbed above, the Mayo prosthess (lke the Spron prosthess) cannot prevent a stress sheldng n the proxmal femur. The Mayo prosthess caused a rarefcaton of the greater trochanter lke a conventonal hp endoprostheses, but caused an ncrease n densty of the lateral cortcals and a stable bone for later revsons. The changes n densty dstrbuton due to the Spron prosthess are confned to the area of the greater trochanter and calcar, and spare the stem of the bone. A rarefcaton of the greater trochanter took place n a lower measure than n the Mayo prosthess. The consequences of the resorpton of the calcar on means and long-term results reman to be determned. CONCLUSION Based on observed trends, the prostheses used n ths study appear to prevent lower stress sheldng. The Spron -system showed n the fnte element model the lowest resorpton behavor. Wth applcaton of a FEM-algorthm, changes n bone densty were examned after the mplantaton of nnovatve hp endoprostheses. The numercal smulaton method was found to qualtatvely predct prmary effects of the stress-adaptve bone remodellng. Tools are avalable for the development of new prostheses, wth fnte element calculatons servng as a check for the sutablty of prostheses already n the desgn phase. These procedures can be further developed wth relable materal data and load szes, to support ndvdual operaton plannng and quanttatve concluson. COMPETING INTERESTS The authors declare that they have no competng nterests. References: 1. Johnsson R., Franzén H., Nlsson LT.: Combned survvorshp and multvarate analyses of revsons n 799 hp prostheses. A 10- to 20-year revew of mechancal loosenng. J Bone Jont Surg. 1994;76: Brkenhauer B., Kstmacher H., Res J.: Concepton and frst results of the Spron cementless femoral neck screw prosthess. 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