Medical Engineering and Physics

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T A Correa et al Medical Engineering and Physics 62 2018 22 28 23. Primary TKR,Primary TKR,tibial tray,tibial tray,Bone cement Subframe 2 mm. layer 1 2mm,Bone cement,layer 1 2mm,Finned keel,partly filling Finned keel partly. bone defect filling bone defect,Extracortical,Intramedullary plates with. stem extension angle stable,with bone cement,bone screws. Fig 1 Representative CAD images of the implants used a tibial component with cemented intramedullary stem xation and b tibial component with medial and lateral. extracortical plate xation Labels highlight the cementing technique. intramedullary stems when increased stability is required due to distally were 25 mm in length and those used proximally two. bone loss Plates and screws for xation of articulating components on the medial plate and one on the lateral plate were 20 mm in. have been used in a number of tumour prostheses 17 19 For a length so as not to interfere with the stem and keels of the tib. revision knee replacement this type of xation when compared ial component The screw holes in the plates were tapped with a. with an intramedullary stem could offer several bene ts reduced 6 mm diameter M6 thread thereby mimicking angle stable lock. strain shielding 20 elimination of end of stem pain 21 22 or ing screw plate technology as commonly used in modern fracture. fracture 23 no need for the removal of healthy bone adjacent xation products. to the medullary canal and easier removal if required Further the. extracortical assembly could be designed to support a primary TKA 2 2 Simulation of bone defects and implantation technique. component during revision potentially reducing inventory and ex. pense Thus the feasibility of an extracortical revision system war Twelve composite synthetic tibia bone models medium left. rants investigation and advantages and disadvantages with respect fourth generation model 3401 Sawbones AG Sweden were pre. to conventional intramedullary xation must be quantitatively as pared with a proximal tibial cut using a surgical saw The position. sessed Given that a reduction in strain shielding is the primary and orientation of the cut was de ned by a custom made cutting. bene t considered due to its impact on component survivability guide based on the geometry of the composite bone model which. this must be assessed prior to further concept development was digitized from computed tomography CT images to ensure a. The aim of the present study was to quantify strain shielding near identical cut for all bones Similar patient speci c instruments. of proximal tibial bone by RTKA tibial components with additional have achieved repeatable implantation within 2 and 1 mm in syn. xation provided by either a a conventional intramedullary stem thetic knee joints 24 The cut was oriented with a 3 posterior. or b medial and lateral extracortical plates It was hypothesised slope relative to a plane normal to the bone s anatomical axis and. that the extracortical xation would reduce bone strain shielding was located 9 mm distal to the outer rim of the lateral articulat. when compared to conventional intramedullary RTKA components ing surface of the tibial plateau Four intact specimens were left. unimplanted to provide the baseline surface strain results from. 2 Methods which evaluation of the strain shielding behaviour of the two im. plant designs was performed, 2 1 Implant component design A second stage of bone removal was performed for the re.
maining eight synthetic bones to simulate the bone lost during. The cemented intramedullary implant Fig 1 a comprised a the removal of a keeled tibial tray component such as the afore. PFC Sigma Fixed Bearing tibial tray DePuy Orthopaedics Inc mentioned PFC Sigma design including a moderate amount. Warsaw IN USA size 3 47 mm anterior posterior by 71 mm of bone loss adjacent to such a component This was performed. medial lateral with a stainless steel stem extension 14 mm diam by rst drilling a hole through to the medullary canal simulat. eter 115 mm length attached through the provided internal screw ing a previous use of the intramedullary alignment technique for. thread within the tray s short stem Both the diameter and length primary component implantation and second removing all bone. were typical of medium sized femoral and tibial stem extensions within 5 mm of the primary component s keel to a depth of 25 mm. currently provided by major knee replacement manufacturers below the resection plane using a hand drill and a separate custom. The extracortical implant xation system Fig 1 b was com guide block This represented removal of the keeled primary TKA. prised of four major components and eight screws A titanium al component plus the layer of bone cement around it plus several. loy Ti6Al4V sub frame was placed below the same PFC Sigma mm of adherent cancellous bone and or a cavity that had become. Fixed Bearing tibial tray Two Ti6Al4V extracortical plates were also resorbed and replaced by brous tissue after loosening of the xa. attached to this sub frame one located medially and one antero tion. laterally through mating features at the proximal ends one pro Finally four of the specimens with simulated bone loss were. truding boss on each plate and two corresponding sockets in the implanted with the conventional cemented intramedullary tibial. sub frame These plates matched the longitudinal curvature of the revision implant design and the other four were implanted with. bone were 16 mm wide and 3 mm thick and extended approxi a novel tibial component with extracortical plate xation For the. mately 95 mm medial and 80 mm lateral distal to the joint line intramedullary design poly methyl methacrylate PMMA bone. Five steel screws 6 mm diameter M6 thread were used to attach cement was used between the base of the tray and the resected. the medial plate to the adjacent synthetic bone cortex and three bone a 1 2 mm layer covering the entire area of the mating sur. screws were used similarly on the lateral side The screws used faces to ll the areas of simulated bone defect and between the. 24 T A Correa et al Medical Engineering and Physics 62 2018 22 28. Four loading conditions used for experimental measurement of longitudinal. Medial load Medial Lateral,weighting load kN load kN. Gait 3 0 BW 2 03 kN 70 1 42 0 61,50 1 02 1 02,Stair ascent 3 6 BW 2 47 kN 70 1 73 0 74. 50 1 24 1 24, stem and the widened medullary canal of the bone a 1 2 mm. layer adjacent to the entire surface area of the stem 25 26 For. the extracortical design PMMA was applied between the proximal. cut surface of the tibia and the sub frame and between the sub. frame and the tibial tray with both of these cement layers being. 2 mm thick and covering the entire areas of the respective mating. surfaces and to ll the areas of simulated bone defect. Fig 2 The testing set up The mounting frame around the femoral component is. attached to the loading frame above it by bearings that allow freedom to rotate in. 2 3 Loading and strain measurement ab adduction the medial lateral position of this pivot was located to create the. desired load on each of the femoral and tibial condyles Left full setup view. right lateral aspect, Digital image correlation DIC an optical technique was used. to determine bone surface strain patterns under axial compressive. loading to assess the strain shielding behaviour of the implants Three loading cycles with the greatest peak load 2 47 kN were. 27 28 It combines the advantages of a conventional in vitro strain applied prior to strain measurement to allow the implant system. gauge experiment such as realistic contact mechanics between the to bed in to the synthetic bone All loads were applied at a rate. implant components and at the bone implant interface with the of 1 mm min and the gait and stair ascent peak loads were held for. full eld strain measurements akin to the predictions obtainable 5 s before strain measurement Two repetitions per loading condi. from nite element models tion were performed for each condition. High contrast speckle patterns were applied to the most Throughout loading the speckle pattern features were imaged. proximal 200 mm of each synthetic bone using matt paint The by two charge coupled DIC cameras with 50 mm lenses located. distal ends were potted in a 60 mm diameter by 100 mm long 1 3 m from the bone providing a 210 mm 175 mm eld of view. cylinder with PMMA and mounted with the proximal resection and a depth of focus eld of 165 mm Calibration was performed. plane aligned horizontally which corresponded to the mechanical using a 175 140 mm panel The medial lateral and posterior. axis of the tibia having the same orientation as the electromechan surfaces of the bone were imaged separately Displacements and. ical testing machine s loading axis strains were calculated using an ARAMIS 5 M software system. Synthetic tibiae were loaded through a femoral component and GOM mbH Braunschweig Germany and consistent coordinate. the tibial tray s corresponding XLKTM cross linked polyethylene in systems were generated for each surface medial lateral and pos. sert DePuy Orthopaedics Inc Warsaw IN USA with the knee in terior based on common landmarks. extension using a single axis electromechanical materials testing. machine Model 5866 Instron High Wycombe UK with a 10 kN 2 4 Data analysis. load cell Four loading conditions were applied to each tibia gait. with a balanced mediolateral load gait with a medially weighted Compressive strains were reported as positive values Pilot test. load stair ascent with a balanced load and stair ascent with a ing showed that the minimum principal strains on the areas un. medially weighted load Table 1 Peak load magnitudes for gait der greatest compression typically postero medial were gener. 3 0 BW and stair ascent 3 6 BW were based on published values ally below 0 20 20 0 0 microstrain and that transverse strain and. 29 30 and on in vivo loading data from the database Orthoload shear strain were of much lower magnitude than the longitudinal. 31 Medial load weighting was controlled through the medial strain in the direction of compressive loading Therefore similar. lateral position of a pivot axis between the condyles of the custom to Sztefek et al 28 and Zimmermann et al 33 only the lon. femoral component xture this xture allowed for the medial gitudinal component of the strains was used for strain analysis. weighting to be controlled with a maximum error of 2 During as the small values of transverse and shear strain were affected. gait and stair ascent the peak load vector direction is between by noise when the eld of view was enlarged to allow simulta. 1 and 6 anteriorly proximally depending on the patient 31 and neous measurement of strains adjacent to the tibial plateau and. therefore no anterior component of load was applied during test also distal to the tip of the long cemented intramedullary stem. ing Tibial bending stresses will be in uenced by the knee contact Strain elds were analysed in three longitudinal regions a prox. point in the sagittal plane In vivo measurements suggest that the imal region 0 90 mm from the resection plane corresponding to. position and trends vary between patients implant designs and the length where the medial extracortical plate could be placed. the degree of weight bearing 32 The testing set up simulated a middle region 90 150 mm corresponding to the length where. a weight bearing posterior stabilized knee Fig 2 for which the the stem could be present but not the extracortical plate and a. data suggest that there is little change in contact point with ex distal region 150 200 mm where neither stem nor plates would. ion being positioned between 4 and 5 mm posteriorly from the be present. centre of the tibial tray in the sagittal plane 32 The tibial mount Due to the orientation shape and loading of the synthetic bone. ing was adjusted such that when the femoral component was low specimens during the experiment which simulated the effects of. ered into weight bearing engagement with the PE insert it did not the knee adduction moment during gait and stair ascent large ar. cause anterior posterior de ection of the tibia eas of both the medial and posterior surfaces were placed under. T A Correa et al Medical Engineering and Physics 62 2018 22 28 25. axial compression while the lateral surfaces were largely uncom. pressed therefore results are only presented for the medial and. posterior faces where the largest differences in compressive strains. were observed The mean percentage strain shielding caused by. the implants compared to the intact case was quanti ed by nding. the mean strain along two repeatable proximal distal lines parallel. to the anatomical axis the rst on the medial surface was located. 30 mm posterior to the anterior crest and the second on the pos. terior surface was located 5 mm medial to the surface centreline. These lines were chosen based on pilot testing which con rmed. that the data obtained were repeatable with low noise. 2 5 Strain measurement technique comparison, Stereo DIC measured strains can be sensitive to the speckle pat.
tern design and the system set up lighting focal lengths cam. era spacing calibration etc To ensure that the experimental. set up used in this experiment produced meaningful results. one synthetic bone specimen was prepared with six pre wired. 120 strain gauge rosettes 3 mm 0 45 90 grid KFH 3 120 Fig 3 Strain strain plot for cross validation of DIC with strain gauges showing the. a Biomechanics Group Mechanical Engineering Department Imperial College London London SW7 2AZ UK b School ofEngineering University Portsmouth Portsmouth PO1 3DJ UK c Department of Orthopaedics and Traumatology Marche Polytechnic University Ancona Italy d Musculoskeletal SurgeryGroup Department ofand Cancer Imperial College LondonSchool Medicine W6 8RF UK a r t i c l e i n f o

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