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A Comparative Analysis of Standardised Threads for Use in Implants for Direct Skeletal Attachment of Limb Prosthesis: A Finite Element Analysis

A Comparative Analysis of Standardised Threads for Use in Implants for Direct Skeletal Attachment... Hindawi Applied Bionics and Biomechanics Volume 2019, Article ID 8027064, 10 pages https://doi.org/10.1155/2019/8027064 Research Article A Comparative Analysis of Standardised Threads for Use in Implants for Direct Skeletal Attachment of Limb Prosthesis: A Finite Element Analysis Piotr Prochor and Eugeniusz Sajewicz Department of Biocybernetics and Biomedical Engineering, Faculty of Mechanical Engineering, Bialystok University of Technology, Bialystok 15-351, Poland Correspondence should be addressed to Piotr Prochor; p.prochor@pb.edu.pl Received 1 October 2018; Accepted 25 November 2018; Published 7 February 2019 Academic Editor: Jan Harm Koolstra Copyright © 2019 Piotr Prochor and Eugeniusz Sajewicz. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The aim of the research was to determine the optimal thread’s shape to be used in implants for direct skeletal attachment of limb prosthesis. In addition, by testing appropriate parameters’ modification of the suitable thread, an attempt was made to maximise its effectiveness. The analyses included three thread types described in the ISO standards: shallow, symmetrical, and asymmetrical. The obtained results suggest that shallow thread ensures the lowest equivalent and directional stress peaks generated in the bone as well as favourable stress patterns and profiles during implant loading in relation to symmetrical and asymmetrical threads. Moreover, shallow thread ensured the generation of single equivalent and directional stress peaks, while symmetrical and asymmetrical threads provided additional stress peak for equivalent as well as for each of directional peaks. Subsequently, optimisation of the shallow thread’s shape was conducted by changing two relevant thread’s parameters (flank angle and rounding arc) which influence the generated stress distribution. The highest reduction of stress peaks was obtained while reducing the rounding arc by 0.2 mm. Therefore, it can be stated that the proposed modification of the HA thread can lead to obtaining a higher biomechanical effectiveness of implants for direct skeletal attachment of limb prosthesis. 1. Introduction of Amputees) [5, 8]. It is currently the most common implant solution for direct limb prosthesis in bone fixation, Traditionally, lower limb prostheses are connected to the which includes the use of the thread. Direct skeletal attach- ment of limb prosthesis is currently still developing connect- stump with the use of an individually fitted socket. However, due to a number of disadvantages arising from their use, such ing method being the reason for conducting proper analyses as skin abrasions and poor prosthesis control resulting from with the use of computer or experimental methods [9–16]. the socket loosening, physicians and engineers have started Applying thread as implant anchoring element may to develop new limb-prosthesis connection solutions [1–6]. result in occurring stress peaks thatcan cause local bone One of these solutions is implants for direct skeletal attach- resorption. As a result, it may lead to the loosening of an ment (DSA) of limb prosthesis. These are specially designed implant in a bone and the necessity of its removal. This constructions that are implanted into the marrow cavity of problem has been reported in studies of threaded dental the bone, with a shaft penetrating soft tissues on which the implants, which may suggest possible existence of the same external prosthesis is attached [1–7]. problem in threaded implants for bone-anchored prostheses One way to connect the implant for direct skeletal [17–21]. Stress peaks would explain the reason of the failure attachment to the bone is to use a proper thread connection. in some cases of the currently proposed construction solu- An example of the implant that uses this method is the tions of implants for DSA [2]. The implant’s thread should OPRA (Osseointegrated Prostheses for the Rehabilitation reduce the stress peaks as well as provide the highest 2 Applied Bionics and Biomechanics of the OPRA system [5, 8]. The length of the implanted part implant-bone contact area in order to ensure appropriate secondary stabilisation [18, 22, 23]. However, so far, there was 75.0 mm, while the initial immersion of the implant was are no analyses that optimise a proper thread which could 25.0 mm. The supports were set in the greater trochanter and be used in implants for bone-anchored prostheses. Few femur’s head areas. The exemplary implant-bone model is studies present results that could be used in the process of shown in Figure 2. designing an implant thread for bone-anchored prostheses The analyses included two loading conditions. The first [17, 24]. During the development of implants, engineers was the axial loading of the implant with a force of Fz = can base on numerous research optimising the shape of 1,000 0N. A given load may occur during static load bearing the dental implant’s thread [25, 26]. Nevertheless, due to exercises, that is, during rehabilitation process after implan- the fact that the method of their attachment differs signifi- tation of an implant for bone-anchored prostheses. These cantly from the attaching implant for direct skeletal attach- exercises rely on loading a head of an implant with its user’s ment into the marrow cavity, there is no possibility of own, most of the time, partial mass [29, 30]. The applied applying exactly the same thread design. force occurs in case of extreme implant loading by a man of The aim of this study was to evaluate the efficiency of approx. 100 kg. The second loading method used in the anal- standardised threads, defined in appropriate standards, for yses was taken from the experimental research of the OPRA use in implants for DSA of limb prosthesis. Three types of system and corresponds to the highest forces that are gener- threads, commonly used in medical screws, were considered: ated on the implant’s head during the gait for a patient of HA: shallow thread; HC: symmetrical thread; HD: asymmet- 61 kg [11, 31, 32]. These are obtained during the heel strike rical thread [27, 28]. Additionally, the authors attempted to (F = 100 0N; F = −20 0N; F = 780 0N; M =30 8Nm; x y z x modify the most appropriate standardised thread (which M = −7 2Nm; M = −2 0Nm) and can lead to microcracks y z was determined during the analysis), in order to increase its in bone tissues, which can result in the implant’s loosening. efficiency in implant for DSA. The effectiveness of appropri- The loads, in both analysed cases, were applied onto the ate load transferring by the implants with the analysed head of the implant. In order to reflect the real conditions, threads was determined by comparing the stress patterns a suitable coefficient of friction with a value of 0.4 was and profiles and stress peak values that were generated in applied; as in primary stabilisation, the implant is not yet the bone during different loading methods. The obtained osseointegrated, which can lead to its micromovement results may allow increasing the functionality of currently within the bone tissues [32]. The reciprocal interaction of used threaded implants for bone-anchored prostheses, as the implant and the bone was taken into account by apply- well as newly developed solutions. ing the relevant contact elements. To generate the implant-bone model, 10-node finite 2. Materials and Methods elements were used [33]. The authors focused on analysing the implant-bone interface. For this reason, the mesh was 2.1. Analysed Threads. In the analyses, three types of standar- additionally densified in abovementioned contact to obtain dised threads were considered (Figure 1), which are used the exact location of stress peaks generated in the bone mostly in medical screws. However, after suitable modifica- during implant loading. The discretisation was performed tions, described threads find the use as threads of dental until further densifying was not changing the results more implants [25, 27, 28]. than 3% obtained as Huber-Mises-Hencky stresses. The The highest shapes of threads that have been specified in obtained models contained approx. 350,000 ± 25,000 finite the standards were chosen for analyses (individual dimen- elements with maximal edge length of 3.0 mm for the sions were approximately similar to each other). The threads implant-bone model. were HA 5, HC 4.2, and HD 4.5. HB thread, defined in ISO Orthotropic properties of the cortical tissue were consid- 5835:1991 standard, was omitted in analyses, as it is intended ered in the research: E =12GPa (radial), E =13 4 GPa x y for use as an anchoring element in cancellous bone. Minor (transverse), E = 20 GPa (longitudinal), ν =0 376, ν = z x y diameter of each thread was set as 19.5 mm. Slight radii con- 0 222, ν =0 235, and ρ = 1910 kg/m [3, 14]. Due to the fact sidered in HC and HD threads were equal 0.025 mm. Unde- that the implant is placed primarily in the cortical bone, for scribed threads’ parameters are defined in the appropriate simplification, the isotropic properties of the cancellous tis- standards [27, 28]. sue (E =0 96 GPa, ν =0 3, and ρ = 630 kg/m ) were used [34]. Titanium alloy (Ti6Al4V) was set as implants’ material 2.2. Characteristics of Finite Element Models Created for (E =0 96 GPa, ν =0 3, and ρ = 630 kg/m ) [5, 9]. Analyses. Analyses with the use of finite element method were performed using the Ansys Workbench 16.0 software (Ansys Inc.). Implants with appropriate threads were placed 3. Results in the left femur model of an adult human (male, 44 years old, 85 kg mass, 185 cm height). In order to reflect postampu- 3.1. Comparative Analysis of Standardised Threads. In the tation conditions, the femur was cut in half of its length. first part of the research, a comparative analysis of the biome- Approximate bone shaft diameter was 32 ± 2 mm, while chanical functionality of the included standardised HA, HC, approximate marrow cavity diameter was 16 ± 2 mm and and HD threads was conducted. the length from the head to the amputation level was The first analysed factor was thread-bone contact area 237.5 mm. The implantation method reflects the positioning (Figure 3). According to Hansson and Werke and Lee et al., r5 Applied Bionics and Biomechanics 3 P e P c 𝛼 Slight radius 60° (a) (b) Pe Slight radius (c) Figure 1: Thread types included in analyses. (a) HA: shallow thread [27]. (b) HC: symmetrical thread [28]. (c) HD: asymmetrical thread [28]. Figure 2: The implantation method, considered coordinate system for loads, load location, and the positioning of supports. 7.0E + 03 6.0E + 03 5.0E + 03 4.0E + 03 3.0E + 03 Th 2.0E + 03 1.0E + 03 0.0E + 00 HA HC HD Figure 3: Differences in thread-bone contact area using standardised threads. this parameter can then significantly affect the local concen- secondary stability, by allowing the bone tissue to overgrow trations of bone stresses arising during loading of the through a larger surface. implant [17, 18]. Moreover, wider contact area should Another and at the same time the main analysed factor increase the effectiveness of implant-bone connection in was stress patterns. According to Hansson and Werke, r4 Implant head read-bone contact area (mm ) Implant head d5 d1 Implant head d5 d1 d5 d1 4 Applied Bionics and Biomechanics order to obtain proper implant-bone connection, researchers stresses concentrated in localised area are a direct cause of local bone tissue damage [17]. The obtained results were pre- can base on studies of dental implants that use this anchor- sented as cross sections through the axis of implant-bone ing method [17, 25, 26]. However, these implants use vari- connection and the location of stress peak formed during ous types of threads from symmetrical-, trapezoidal-, to implant loading (Figure 4). Stress patterns created during circular-shaped, which, due to their specific shape, can be the heel strike differed slightly from those resulting from used only in the mandible and maxilla. The aim of the con- axial loading of the implant, while the location of stress peaks ducted study was to determine the optimal shape of the remained the same. thread for the implants for direct skeletal attachment of limb Stress profiles were set in order to increase the precision of prosthesis and modify proper parameters to obtain its max- conducted analyses. For this purpose, suitable paths (Figure 5) imum efficiency. One of the closest studies to the one presented by the were determined, for which the equivalent and directional stresses were designated. authors is the analyses conducted by Hansson and Werke The obtained profiles allow for better understanding of [17]. However, the authors made an attempt to simulate the stress patterns presented in Figure 4. The profiles are implant-bone connection closer to in vivo conditions. presented in Figure 6. Hansson and Werke assumed frictionless contact between The authors additionally analysed the stress peak values the implant and the bone as well as simplified cortical for both loading methods. The obtained results are included bone to isotropic material. Unlike them, the authors con- in Figure 7. sidered the bone as an orthotropic material and used the appropriate coefficient of friction [3, 14, 32]. Due to the 3.2. Comparative Analysis of Modified HA Thread. In the sec- problem with convergence during calculation, Hansson ond part of the research, the influence of changes in param- and Werke applied inflated Young’s modulus of the corti- eters of the most suitable thread defined in the first part cal bone. The authors used appropriate and actual Young’s was examined (HA thread). In both parts, the same mesh modulus value [35, 36]. Because of the adopted simplifica- parameters, loadings, and boundary conditions were used. tions, Hansson and Werke focused on principal stress, while The impact of changes in the two thread’s parameters the authors of the paper decided to examine equivalent was examined (Figure 8): the rounding arc (r1) and the flank Huber-Mises-Hencky as well as directional stress peaks. angle (α). The stress distribution around the thread of the Analysing the location of individual stress peaks may con- implant, presented in Figures 4 and 6, suggests that these tribute to a better understanding of the mechanics of connec- two parameters have the main influence on the load transfer- tion between threaded implants for direct skeletal attachment ring efficiency. Other thread’s parameters should have then of limb prosthesis and bone tissues. Additionally, Hansson primarily impact only on implant’s anchoring effectiveness and Werke in their analyses considered a single implant in the cortical bone. Impacts of the 4 modifications of the member, while the authors of the presented paper analysed rounding arc (0.8 mm, 0.9 mm, 1.1 mm, and 1.2 mm) and 4 a complete model of the implant placed in the femur. This ° ° ° ° modifications of the flank angle (25 ,30 ,40 , and 45 )on method greatly increased the computing time; however, it generated stresses were analysed. Obtained values were led to obtaining more appropriate load transfer from implant compared to values obtained in the first part of the research into bone tissues. The use of the described parameters for HA thread type with standardised parameters (rounding allowed the authors for a better reflection of the actual arc = 1 0mm and flank angle = 35 ). Lastly, a comparative implant-bone connection. As Hansson’s simplification, the analysis of 25 thread geometries was performed. authors took into consideration the full contact of the As in the first part of the results, the thread-bone contact implant with the bone. area was determined in the form of the influence of changes in the flank angle and the rounding arc on analysed factor 4.1. A Comparison of the Effectiveness of HA, HC, and HD (Figure 9). Threads. All stress peaks, equivalent and directional, were The stress peak localisation did not change and remained created in the area of the thread crest, which suggests that it similar as presented in Figure 6 despite the introduced con- is a region of possible bone resorption which occurs due to structional changes of implant-bone connection. The results bone overloading. Obtained results are confirmed in other of stress peak values obtained for axial loading are shown in finite element and experimental thread analyses [17, 25, 26]. Figure 10, while for heel strike in Figure 11. The described feature was observed in all types of analysed threads and can be the reason for the loosening of implants in the bone tissues. It leads to the necessity of its removal, 4. Discussion which was reported in clinical experiments [17–21]. More- The following article presents a comparison of the efficiency over, the results obtained for axial and heel strike loadings (in terms of stress distribution in bone tissues, generated present similarities both in the case of stress peak values as well as stress patterns and profiles. Therefore, it can be while loading of the implant) of standardised threads (shal- low thread HA, symmetrical thread HC, and asymmetrical assumed that axial force is the main factor in the generation of stress peaks. HD) in implants for direct skeletal attachment of limb pros- thesis [27, 28]. Currently, there are no studies that clearly On the basis of the obtained results, it can be concluded determine the optimal shape of the thread for use in this type that the optimal thread type (from the analysed types) for use in implants for DSA of limb prosthesis is the HA thread. of implants. While choosing a suitable type of the thread in Applied Bionics and Biomechanics 5 Axial loading HA thread HC thread HD thread 18 16 14 12 10 8 6 4 2 0 18 16 14 12 10 8 6 4 2 0 18 16 14 12 10 8 6 4 2 0 (MPa) (MPa) (MPa) Heel strike HA thread HC thread HD thread 18 16 14 12 10 8 6 4 2 0 18 16 14 12 10 8 6 4 2 0 18 16 14 12 10 8 6 4 2 0 (MPa) (MPa) (MPa) Figure 4: Equivalent stress patterns (MPa): cross section through stress peak location. HA thread HC thread HD thread Bone Bone Bone Stress path Stress path Stress path Figure 5: Stress paths to define stress profiles at thread-bone interface. It is determined due to the lowest equivalent and directional existence of a second potential bone resorption site due to a stress peak values that are generated in the bone during sudden stress increase. implant loading from all analysed thread-bone connections. The results suggest that the least favourable is the HC Furthermore, high-value stresses generated as a result of thread around which the equivalent stress peak was created stress peak decrease in the HA thread type much faster than with the highest value reaching approx. 16.6 MPa during axial loading and 12.9 MPa during heel strike—HA gener- in other types. Additionally, the use of HA threads signifi- cantly reduces radial (4.0 MPa and 6.5 MPa for HA and ated 13.4 MPa and 10.3 MPa, while HD generated 14.7 MPa HD, respectively, during axial loading and 3.1 MPa and and 11.4 MPa, respectively, for axial and heel strike load- 5.1 MPa during heel strike) stress peaks. This is especially ings. However, the HC thread provided lower radial stress important due to the susceptibility of the bone on shear peak (6.2 MPa for axial loading, 4.7 MPa for heel strike) than the HD thread (6.5 MPa for axial loading, 5.1 MPa stress, which is one of the main reasons for bone resorption around the threaded implant. What is more, in the analysis for heel strike). of stress profiles generated in the case of HC and HD threads, In the analyses, the impact of thread-bone contact area there can be noted a second stress peak (equivalent and on stress peaks was not observed. Nevertheless, it should directional) with about 30% lower value. This indicates the be remembered that higher contact area between an implant 6 Applied Bionics and Biomechanics HA HA (axial loading) (heel strike) 20 16 12 10 4 4 −4 −2 0.75 0.85 0.95 1.05 1.15 0.75 0.85 0.95 1.05 1.15 read-bone contact Th length (mm) read-bone contact Th length (mm) HC HC (axial loading) (heel strike) 20 16 12 10 4 4 −4 −2 0.55 0.65 0.75 0.85 0.95 0.55 0.65 0.75 0.85 0.95 read-bone contact Th length (mm) read-bone contact Th length (mm) HD HD (axial loading) (heel strike) 20 16 12 10 4 4 −4 −2 0.65 0.75 0.85 0.95 1.05 0.65 0.75 0.85 0.95 1.05 read-bone contact Th length (mm) read-bone contact Th length (mm) Equivalent stress Equivalent stress Radial stress Radial stress Transverse stress Transverse stress Longitudinal stress Longitudinal stress Figure 6: Individual stress profiles at the thread-bone interface during different loading types—the profile was limited to the section of stress peak formation (above and below of the presented values of the thread-bone contact length the stress profile changed linearly). and a bone allows for obtaining a more stable secondary 6.5%. This variation modifies the implant-bone interface connection. For this reason, the implant construction affecting the probability of achieving adequate osseointegra- should consider a thread that provides both, small values tion during secondary stabilisation. of stress peaks and a correspondingly large implant-bone In the case of axial loading, the highest values of analysed contact area. stress peaks were generated at a flank angle of 25 . Moreover, the longitudinal stress peak was equally unfavourable at a 45 4.2. A Comparison of the Effectiveness of Modified HA flank angle. Similar dependencies were observed in the case of heel strike, but in this case, equivalent and longitudinal Threads. The modification of the rounding arc by 0.2 mm and the flank angle by 15 from the standardised values stress peaks were the highest with a flank angle of 45 . Addi- (1.0 mm for the rounding arc and 30 for the flank angle) tionally, as in the axial loading, the radial and transverse stress peaks were characterised by the highest value at a flank allows changing the thread-bone contact area by approx. Stress (MPa) Stress (MPa) Stress (MPa) Stress (MPa) Stress (MPa) Stress (MPa) Applied Bionics and Biomechanics 7 Axial loading Heel strike Axial loading Heel strike 20 16 8 6 15 12 6 4.5 10 8 4 3 5 4 2 1.5 0 0 0 0 HA HA HC HC HD HD Axial loading Heel strike Axial loading Heel strike 4 3.2 20 16 3 2.4 15 12 2 1.6 10 8 1 0.8 5 4 0 0 0 0 HA HA HC HC HD HD Figure 7: Stress peak values obtained during the two loading types. (a) (b) Figure 8: Thread geometry modifications used in the study. (a) Rounding arc. (b) Flank angle. angle of 25 . The lowest values of equivalent and longitudinal peak by 6.9%, transverse stress peak by 3.5%, and longitudi- stress peaks during axial loading, as well as heel strike, were nal stress peak by 1.7%. In the case of heel strike, this modi- produced at a standard flank angle of 35 , while the lowest fication reduces the equivalent stress peak by 1.0%, radial values of radial and transverse stress peaks were produced stress peak by 4.9%, transverse stress peak by 2.1%, and lon- at 45 . Decreasing rounding arc caused an increase in the gitudinal stress peak by 0.9%. value of all stress peaks in each of the analysed cases. The most favourable stress peak reduction was obtained 4.3. Limitations of the Study. The assumptions considered by by modifying only the rounding arc increasing it by the authors are characterised by certain simplifications in 0.2 mm, leaving the flank angle at a normalised value of 35 . relation to real conditions. Among them there can be speci- The described modification in the case of axial loading allows fied full contact of the implant with the bone, orthotropic to reduce the equivalent stress peak by 1.7%, radial stress and isotropic properties of the cortical and cancellous bones, 25°-45° R = 0.8-1.2 mm Equivalent stress peak Transverse stress peak (MPa) (MPa) Implant head Equivalent stress peak Transverse stress peak (MPa) (MPa) Longitudinal stress peak Implant head (MPa) Radial stress peak (MPa) Longitudinal stress peak Radial stress peak (MPa) (MPa) Flank angle (%) 8 Applied Bionics and Biomechanics 1.2 1.1 1.0 Standardised HA thread (100%) 0.9 0.8 Figure 9: The influence of flank angle and rounding arc on the thread-bone contact area. −1 −8 −15 −2 25 30 35 40 45 25 30 35 40 45 Flank angle (°) Flank angle (°) R = 0.8 mm R = 1.1 mm R = 0.8 mm R = 1.1 mm R = 0.9 mm R = 1.2 mm R = 0.9 mm R = 1.2 mm R = 1.0 mm R = 1.0 mm 14 6 8 4 2 2 −4 −10 −2 25 30 35 40 45 25 30 35 40 45 Flank angle (°) Flank angle (°) R = 0.8 mm R = 1.1 mm R = 0.8 mm R = 1.1 mm R = 0.9 mm R = 1.2 mm R = 0.9 mm R = 1.2 mm R = 1.0 mm R = 1.0 mm Figure 10: The influence of flank angle and rounding arc on stress peaks during implant axial loading with a force of 1000 N. respectively, or omitting the influence of body fluids as well the HA thread type. It provides the lowest stress peaks’ values as bacteria that often settle in the implant-bone interface. and the most favourable stress pattern and profile. The impact of the stump muscles was also ignored. The modification of the standardised HA thread by reducing the rounding arc by 0.2 mm allows for the reduction of the stress peak values in relation to the stress peaks gener- 5. Conclusions ated in the bone around the standard HA thread. However, it The most appropriate standardised thread for use in should be remembered that this reduction also reduces the implants for direct skeletal attachment of limb prosthesis is thread-bone contact area by 3.7%, which may decrease the Rounding arc (mm) Transverse stress peak Equivalent stress peak change (%) change (%) Radial stress peak change Longitudinal stress peak (%) change (%) Thread-bone contact area in reference to standardised HA thread (%) Applied Bionics and Biomechanics 9 −2 −7 −12 −2 25 30 35 40 45 25 30 35 40 45 Flank angle (°) Flank angle (°) R = 0.8 mm R = 1.1 mm R = 0.8 mm R = 1.1 mm R = 0.9 mm R = 1.2 mm R = 0.9 mm R = 1.2 mm R = 1.0 mm R = 1.0 mm 20 9 12 7 4 3 −4 −1 25 30 35 40 45 25 30 35 40 45 Flank angle (°) Flank angle (°) R = 0.8 mm R = 1.1 mm R = 0.8 mm R = 1.1 mm R = 0.9 mm R = 1.2 mm R = 0.9 mm R = 1.2 mm R = 1.0 mm R = 1.0 mm Figure 11: The influence of flank angle and rounding arc on stress peaks during implant loading with loads generated during heel strike. probability of achieving proper osseointegration in second- [2] R. 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A Comparative Analysis of Standardised Threads for Use in Implants for Direct Skeletal Attachment of Limb Prosthesis: A Finite Element Analysis

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Copyright © 2019 Piotr Prochor and Eugeniusz Sajewicz. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Hindawi Applied Bionics and Biomechanics Volume 2019, Article ID 8027064, 10 pages https://doi.org/10.1155/2019/8027064 Research Article A Comparative Analysis of Standardised Threads for Use in Implants for Direct Skeletal Attachment of Limb Prosthesis: A Finite Element Analysis Piotr Prochor and Eugeniusz Sajewicz Department of Biocybernetics and Biomedical Engineering, Faculty of Mechanical Engineering, Bialystok University of Technology, Bialystok 15-351, Poland Correspondence should be addressed to Piotr Prochor; p.prochor@pb.edu.pl Received 1 October 2018; Accepted 25 November 2018; Published 7 February 2019 Academic Editor: Jan Harm Koolstra Copyright © 2019 Piotr Prochor and Eugeniusz Sajewicz. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The aim of the research was to determine the optimal thread’s shape to be used in implants for direct skeletal attachment of limb prosthesis. In addition, by testing appropriate parameters’ modification of the suitable thread, an attempt was made to maximise its effectiveness. The analyses included three thread types described in the ISO standards: shallow, symmetrical, and asymmetrical. The obtained results suggest that shallow thread ensures the lowest equivalent and directional stress peaks generated in the bone as well as favourable stress patterns and profiles during implant loading in relation to symmetrical and asymmetrical threads. Moreover, shallow thread ensured the generation of single equivalent and directional stress peaks, while symmetrical and asymmetrical threads provided additional stress peak for equivalent as well as for each of directional peaks. Subsequently, optimisation of the shallow thread’s shape was conducted by changing two relevant thread’s parameters (flank angle and rounding arc) which influence the generated stress distribution. The highest reduction of stress peaks was obtained while reducing the rounding arc by 0.2 mm. Therefore, it can be stated that the proposed modification of the HA thread can lead to obtaining a higher biomechanical effectiveness of implants for direct skeletal attachment of limb prosthesis. 1. Introduction of Amputees) [5, 8]. It is currently the most common implant solution for direct limb prosthesis in bone fixation, Traditionally, lower limb prostheses are connected to the which includes the use of the thread. Direct skeletal attach- ment of limb prosthesis is currently still developing connect- stump with the use of an individually fitted socket. However, due to a number of disadvantages arising from their use, such ing method being the reason for conducting proper analyses as skin abrasions and poor prosthesis control resulting from with the use of computer or experimental methods [9–16]. the socket loosening, physicians and engineers have started Applying thread as implant anchoring element may to develop new limb-prosthesis connection solutions [1–6]. result in occurring stress peaks thatcan cause local bone One of these solutions is implants for direct skeletal attach- resorption. As a result, it may lead to the loosening of an ment (DSA) of limb prosthesis. These are specially designed implant in a bone and the necessity of its removal. This constructions that are implanted into the marrow cavity of problem has been reported in studies of threaded dental the bone, with a shaft penetrating soft tissues on which the implants, which may suggest possible existence of the same external prosthesis is attached [1–7]. problem in threaded implants for bone-anchored prostheses One way to connect the implant for direct skeletal [17–21]. Stress peaks would explain the reason of the failure attachment to the bone is to use a proper thread connection. in some cases of the currently proposed construction solu- An example of the implant that uses this method is the tions of implants for DSA [2]. The implant’s thread should OPRA (Osseointegrated Prostheses for the Rehabilitation reduce the stress peaks as well as provide the highest 2 Applied Bionics and Biomechanics of the OPRA system [5, 8]. The length of the implanted part implant-bone contact area in order to ensure appropriate secondary stabilisation [18, 22, 23]. However, so far, there was 75.0 mm, while the initial immersion of the implant was are no analyses that optimise a proper thread which could 25.0 mm. The supports were set in the greater trochanter and be used in implants for bone-anchored prostheses. Few femur’s head areas. The exemplary implant-bone model is studies present results that could be used in the process of shown in Figure 2. designing an implant thread for bone-anchored prostheses The analyses included two loading conditions. The first [17, 24]. During the development of implants, engineers was the axial loading of the implant with a force of Fz = can base on numerous research optimising the shape of 1,000 0N. A given load may occur during static load bearing the dental implant’s thread [25, 26]. Nevertheless, due to exercises, that is, during rehabilitation process after implan- the fact that the method of their attachment differs signifi- tation of an implant for bone-anchored prostheses. These cantly from the attaching implant for direct skeletal attach- exercises rely on loading a head of an implant with its user’s ment into the marrow cavity, there is no possibility of own, most of the time, partial mass [29, 30]. The applied applying exactly the same thread design. force occurs in case of extreme implant loading by a man of The aim of this study was to evaluate the efficiency of approx. 100 kg. The second loading method used in the anal- standardised threads, defined in appropriate standards, for yses was taken from the experimental research of the OPRA use in implants for DSA of limb prosthesis. Three types of system and corresponds to the highest forces that are gener- threads, commonly used in medical screws, were considered: ated on the implant’s head during the gait for a patient of HA: shallow thread; HC: symmetrical thread; HD: asymmet- 61 kg [11, 31, 32]. These are obtained during the heel strike rical thread [27, 28]. Additionally, the authors attempted to (F = 100 0N; F = −20 0N; F = 780 0N; M =30 8Nm; x y z x modify the most appropriate standardised thread (which M = −7 2Nm; M = −2 0Nm) and can lead to microcracks y z was determined during the analysis), in order to increase its in bone tissues, which can result in the implant’s loosening. efficiency in implant for DSA. The effectiveness of appropri- The loads, in both analysed cases, were applied onto the ate load transferring by the implants with the analysed head of the implant. In order to reflect the real conditions, threads was determined by comparing the stress patterns a suitable coefficient of friction with a value of 0.4 was and profiles and stress peak values that were generated in applied; as in primary stabilisation, the implant is not yet the bone during different loading methods. The obtained osseointegrated, which can lead to its micromovement results may allow increasing the functionality of currently within the bone tissues [32]. The reciprocal interaction of used threaded implants for bone-anchored prostheses, as the implant and the bone was taken into account by apply- well as newly developed solutions. ing the relevant contact elements. To generate the implant-bone model, 10-node finite 2. Materials and Methods elements were used [33]. The authors focused on analysing the implant-bone interface. For this reason, the mesh was 2.1. Analysed Threads. In the analyses, three types of standar- additionally densified in abovementioned contact to obtain dised threads were considered (Figure 1), which are used the exact location of stress peaks generated in the bone mostly in medical screws. However, after suitable modifica- during implant loading. The discretisation was performed tions, described threads find the use as threads of dental until further densifying was not changing the results more implants [25, 27, 28]. than 3% obtained as Huber-Mises-Hencky stresses. The The highest shapes of threads that have been specified in obtained models contained approx. 350,000 ± 25,000 finite the standards were chosen for analyses (individual dimen- elements with maximal edge length of 3.0 mm for the sions were approximately similar to each other). The threads implant-bone model. were HA 5, HC 4.2, and HD 4.5. HB thread, defined in ISO Orthotropic properties of the cortical tissue were consid- 5835:1991 standard, was omitted in analyses, as it is intended ered in the research: E =12GPa (radial), E =13 4 GPa x y for use as an anchoring element in cancellous bone. Minor (transverse), E = 20 GPa (longitudinal), ν =0 376, ν = z x y diameter of each thread was set as 19.5 mm. Slight radii con- 0 222, ν =0 235, and ρ = 1910 kg/m [3, 14]. Due to the fact sidered in HC and HD threads were equal 0.025 mm. Unde- that the implant is placed primarily in the cortical bone, for scribed threads’ parameters are defined in the appropriate simplification, the isotropic properties of the cancellous tis- standards [27, 28]. sue (E =0 96 GPa, ν =0 3, and ρ = 630 kg/m ) were used [34]. Titanium alloy (Ti6Al4V) was set as implants’ material 2.2. Characteristics of Finite Element Models Created for (E =0 96 GPa, ν =0 3, and ρ = 630 kg/m ) [5, 9]. Analyses. Analyses with the use of finite element method were performed using the Ansys Workbench 16.0 software (Ansys Inc.). Implants with appropriate threads were placed 3. Results in the left femur model of an adult human (male, 44 years old, 85 kg mass, 185 cm height). In order to reflect postampu- 3.1. Comparative Analysis of Standardised Threads. In the tation conditions, the femur was cut in half of its length. first part of the research, a comparative analysis of the biome- Approximate bone shaft diameter was 32 ± 2 mm, while chanical functionality of the included standardised HA, HC, approximate marrow cavity diameter was 16 ± 2 mm and and HD threads was conducted. the length from the head to the amputation level was The first analysed factor was thread-bone contact area 237.5 mm. The implantation method reflects the positioning (Figure 3). According to Hansson and Werke and Lee et al., r5 Applied Bionics and Biomechanics 3 P e P c 𝛼 Slight radius 60° (a) (b) Pe Slight radius (c) Figure 1: Thread types included in analyses. (a) HA: shallow thread [27]. (b) HC: symmetrical thread [28]. (c) HD: asymmetrical thread [28]. Figure 2: The implantation method, considered coordinate system for loads, load location, and the positioning of supports. 7.0E + 03 6.0E + 03 5.0E + 03 4.0E + 03 3.0E + 03 Th 2.0E + 03 1.0E + 03 0.0E + 00 HA HC HD Figure 3: Differences in thread-bone contact area using standardised threads. this parameter can then significantly affect the local concen- secondary stability, by allowing the bone tissue to overgrow trations of bone stresses arising during loading of the through a larger surface. implant [17, 18]. Moreover, wider contact area should Another and at the same time the main analysed factor increase the effectiveness of implant-bone connection in was stress patterns. According to Hansson and Werke, r4 Implant head read-bone contact area (mm ) Implant head d5 d1 Implant head d5 d1 d5 d1 4 Applied Bionics and Biomechanics order to obtain proper implant-bone connection, researchers stresses concentrated in localised area are a direct cause of local bone tissue damage [17]. The obtained results were pre- can base on studies of dental implants that use this anchor- sented as cross sections through the axis of implant-bone ing method [17, 25, 26]. However, these implants use vari- connection and the location of stress peak formed during ous types of threads from symmetrical-, trapezoidal-, to implant loading (Figure 4). Stress patterns created during circular-shaped, which, due to their specific shape, can be the heel strike differed slightly from those resulting from used only in the mandible and maxilla. The aim of the con- axial loading of the implant, while the location of stress peaks ducted study was to determine the optimal shape of the remained the same. thread for the implants for direct skeletal attachment of limb Stress profiles were set in order to increase the precision of prosthesis and modify proper parameters to obtain its max- conducted analyses. For this purpose, suitable paths (Figure 5) imum efficiency. One of the closest studies to the one presented by the were determined, for which the equivalent and directional stresses were designated. authors is the analyses conducted by Hansson and Werke The obtained profiles allow for better understanding of [17]. However, the authors made an attempt to simulate the stress patterns presented in Figure 4. The profiles are implant-bone connection closer to in vivo conditions. presented in Figure 6. Hansson and Werke assumed frictionless contact between The authors additionally analysed the stress peak values the implant and the bone as well as simplified cortical for both loading methods. The obtained results are included bone to isotropic material. Unlike them, the authors con- in Figure 7. sidered the bone as an orthotropic material and used the appropriate coefficient of friction [3, 14, 32]. Due to the 3.2. Comparative Analysis of Modified HA Thread. In the sec- problem with convergence during calculation, Hansson ond part of the research, the influence of changes in param- and Werke applied inflated Young’s modulus of the corti- eters of the most suitable thread defined in the first part cal bone. The authors used appropriate and actual Young’s was examined (HA thread). In both parts, the same mesh modulus value [35, 36]. Because of the adopted simplifica- parameters, loadings, and boundary conditions were used. tions, Hansson and Werke focused on principal stress, while The impact of changes in the two thread’s parameters the authors of the paper decided to examine equivalent was examined (Figure 8): the rounding arc (r1) and the flank Huber-Mises-Hencky as well as directional stress peaks. angle (α). The stress distribution around the thread of the Analysing the location of individual stress peaks may con- implant, presented in Figures 4 and 6, suggests that these tribute to a better understanding of the mechanics of connec- two parameters have the main influence on the load transfer- tion between threaded implants for direct skeletal attachment ring efficiency. Other thread’s parameters should have then of limb prosthesis and bone tissues. Additionally, Hansson primarily impact only on implant’s anchoring effectiveness and Werke in their analyses considered a single implant in the cortical bone. Impacts of the 4 modifications of the member, while the authors of the presented paper analysed rounding arc (0.8 mm, 0.9 mm, 1.1 mm, and 1.2 mm) and 4 a complete model of the implant placed in the femur. This ° ° ° ° modifications of the flank angle (25 ,30 ,40 , and 45 )on method greatly increased the computing time; however, it generated stresses were analysed. Obtained values were led to obtaining more appropriate load transfer from implant compared to values obtained in the first part of the research into bone tissues. The use of the described parameters for HA thread type with standardised parameters (rounding allowed the authors for a better reflection of the actual arc = 1 0mm and flank angle = 35 ). Lastly, a comparative implant-bone connection. As Hansson’s simplification, the analysis of 25 thread geometries was performed. authors took into consideration the full contact of the As in the first part of the results, the thread-bone contact implant with the bone. area was determined in the form of the influence of changes in the flank angle and the rounding arc on analysed factor 4.1. A Comparison of the Effectiveness of HA, HC, and HD (Figure 9). Threads. All stress peaks, equivalent and directional, were The stress peak localisation did not change and remained created in the area of the thread crest, which suggests that it similar as presented in Figure 6 despite the introduced con- is a region of possible bone resorption which occurs due to structional changes of implant-bone connection. The results bone overloading. Obtained results are confirmed in other of stress peak values obtained for axial loading are shown in finite element and experimental thread analyses [17, 25, 26]. Figure 10, while for heel strike in Figure 11. The described feature was observed in all types of analysed threads and can be the reason for the loosening of implants in the bone tissues. It leads to the necessity of its removal, 4. Discussion which was reported in clinical experiments [17–21]. More- The following article presents a comparison of the efficiency over, the results obtained for axial and heel strike loadings (in terms of stress distribution in bone tissues, generated present similarities both in the case of stress peak values as well as stress patterns and profiles. Therefore, it can be while loading of the implant) of standardised threads (shal- low thread HA, symmetrical thread HC, and asymmetrical assumed that axial force is the main factor in the generation of stress peaks. HD) in implants for direct skeletal attachment of limb pros- thesis [27, 28]. Currently, there are no studies that clearly On the basis of the obtained results, it can be concluded determine the optimal shape of the thread for use in this type that the optimal thread type (from the analysed types) for use in implants for DSA of limb prosthesis is the HA thread. of implants. While choosing a suitable type of the thread in Applied Bionics and Biomechanics 5 Axial loading HA thread HC thread HD thread 18 16 14 12 10 8 6 4 2 0 18 16 14 12 10 8 6 4 2 0 18 16 14 12 10 8 6 4 2 0 (MPa) (MPa) (MPa) Heel strike HA thread HC thread HD thread 18 16 14 12 10 8 6 4 2 0 18 16 14 12 10 8 6 4 2 0 18 16 14 12 10 8 6 4 2 0 (MPa) (MPa) (MPa) Figure 4: Equivalent stress patterns (MPa): cross section through stress peak location. HA thread HC thread HD thread Bone Bone Bone Stress path Stress path Stress path Figure 5: Stress paths to define stress profiles at thread-bone interface. It is determined due to the lowest equivalent and directional existence of a second potential bone resorption site due to a stress peak values that are generated in the bone during sudden stress increase. implant loading from all analysed thread-bone connections. The results suggest that the least favourable is the HC Furthermore, high-value stresses generated as a result of thread around which the equivalent stress peak was created stress peak decrease in the HA thread type much faster than with the highest value reaching approx. 16.6 MPa during axial loading and 12.9 MPa during heel strike—HA gener- in other types. Additionally, the use of HA threads signifi- cantly reduces radial (4.0 MPa and 6.5 MPa for HA and ated 13.4 MPa and 10.3 MPa, while HD generated 14.7 MPa HD, respectively, during axial loading and 3.1 MPa and and 11.4 MPa, respectively, for axial and heel strike load- 5.1 MPa during heel strike) stress peaks. This is especially ings. However, the HC thread provided lower radial stress important due to the susceptibility of the bone on shear peak (6.2 MPa for axial loading, 4.7 MPa for heel strike) than the HD thread (6.5 MPa for axial loading, 5.1 MPa stress, which is one of the main reasons for bone resorption around the threaded implant. What is more, in the analysis for heel strike). of stress profiles generated in the case of HC and HD threads, In the analyses, the impact of thread-bone contact area there can be noted a second stress peak (equivalent and on stress peaks was not observed. Nevertheless, it should directional) with about 30% lower value. This indicates the be remembered that higher contact area between an implant 6 Applied Bionics and Biomechanics HA HA (axial loading) (heel strike) 20 16 12 10 4 4 −4 −2 0.75 0.85 0.95 1.05 1.15 0.75 0.85 0.95 1.05 1.15 read-bone contact Th length (mm) read-bone contact Th length (mm) HC HC (axial loading) (heel strike) 20 16 12 10 4 4 −4 −2 0.55 0.65 0.75 0.85 0.95 0.55 0.65 0.75 0.85 0.95 read-bone contact Th length (mm) read-bone contact Th length (mm) HD HD (axial loading) (heel strike) 20 16 12 10 4 4 −4 −2 0.65 0.75 0.85 0.95 1.05 0.65 0.75 0.85 0.95 1.05 read-bone contact Th length (mm) read-bone contact Th length (mm) Equivalent stress Equivalent stress Radial stress Radial stress Transverse stress Transverse stress Longitudinal stress Longitudinal stress Figure 6: Individual stress profiles at the thread-bone interface during different loading types—the profile was limited to the section of stress peak formation (above and below of the presented values of the thread-bone contact length the stress profile changed linearly). and a bone allows for obtaining a more stable secondary 6.5%. This variation modifies the implant-bone interface connection. For this reason, the implant construction affecting the probability of achieving adequate osseointegra- should consider a thread that provides both, small values tion during secondary stabilisation. of stress peaks and a correspondingly large implant-bone In the case of axial loading, the highest values of analysed contact area. stress peaks were generated at a flank angle of 25 . Moreover, the longitudinal stress peak was equally unfavourable at a 45 4.2. A Comparison of the Effectiveness of Modified HA flank angle. Similar dependencies were observed in the case of heel strike, but in this case, equivalent and longitudinal Threads. The modification of the rounding arc by 0.2 mm and the flank angle by 15 from the standardised values stress peaks were the highest with a flank angle of 45 . Addi- (1.0 mm for the rounding arc and 30 for the flank angle) tionally, as in the axial loading, the radial and transverse stress peaks were characterised by the highest value at a flank allows changing the thread-bone contact area by approx. Stress (MPa) Stress (MPa) Stress (MPa) Stress (MPa) Stress (MPa) Stress (MPa) Applied Bionics and Biomechanics 7 Axial loading Heel strike Axial loading Heel strike 20 16 8 6 15 12 6 4.5 10 8 4 3 5 4 2 1.5 0 0 0 0 HA HA HC HC HD HD Axial loading Heel strike Axial loading Heel strike 4 3.2 20 16 3 2.4 15 12 2 1.6 10 8 1 0.8 5 4 0 0 0 0 HA HA HC HC HD HD Figure 7: Stress peak values obtained during the two loading types. (a) (b) Figure 8: Thread geometry modifications used in the study. (a) Rounding arc. (b) Flank angle. angle of 25 . The lowest values of equivalent and longitudinal peak by 6.9%, transverse stress peak by 3.5%, and longitudi- stress peaks during axial loading, as well as heel strike, were nal stress peak by 1.7%. In the case of heel strike, this modi- produced at a standard flank angle of 35 , while the lowest fication reduces the equivalent stress peak by 1.0%, radial values of radial and transverse stress peaks were produced stress peak by 4.9%, transverse stress peak by 2.1%, and lon- at 45 . Decreasing rounding arc caused an increase in the gitudinal stress peak by 0.9%. value of all stress peaks in each of the analysed cases. The most favourable stress peak reduction was obtained 4.3. Limitations of the Study. The assumptions considered by by modifying only the rounding arc increasing it by the authors are characterised by certain simplifications in 0.2 mm, leaving the flank angle at a normalised value of 35 . relation to real conditions. Among them there can be speci- The described modification in the case of axial loading allows fied full contact of the implant with the bone, orthotropic to reduce the equivalent stress peak by 1.7%, radial stress and isotropic properties of the cortical and cancellous bones, 25°-45° R = 0.8-1.2 mm Equivalent stress peak Transverse stress peak (MPa) (MPa) Implant head Equivalent stress peak Transverse stress peak (MPa) (MPa) Longitudinal stress peak Implant head (MPa) Radial stress peak (MPa) Longitudinal stress peak Radial stress peak (MPa) (MPa) Flank angle (%) 8 Applied Bionics and Biomechanics 1.2 1.1 1.0 Standardised HA thread (100%) 0.9 0.8 Figure 9: The influence of flank angle and rounding arc on the thread-bone contact area. −1 −8 −15 −2 25 30 35 40 45 25 30 35 40 45 Flank angle (°) Flank angle (°) R = 0.8 mm R = 1.1 mm R = 0.8 mm R = 1.1 mm R = 0.9 mm R = 1.2 mm R = 0.9 mm R = 1.2 mm R = 1.0 mm R = 1.0 mm 14 6 8 4 2 2 −4 −10 −2 25 30 35 40 45 25 30 35 40 45 Flank angle (°) Flank angle (°) R = 0.8 mm R = 1.1 mm R = 0.8 mm R = 1.1 mm R = 0.9 mm R = 1.2 mm R = 0.9 mm R = 1.2 mm R = 1.0 mm R = 1.0 mm Figure 10: The influence of flank angle and rounding arc on stress peaks during implant axial loading with a force of 1000 N. respectively, or omitting the influence of body fluids as well the HA thread type. It provides the lowest stress peaks’ values as bacteria that often settle in the implant-bone interface. and the most favourable stress pattern and profile. The impact of the stump muscles was also ignored. The modification of the standardised HA thread by reducing the rounding arc by 0.2 mm allows for the reduction of the stress peak values in relation to the stress peaks gener- 5. Conclusions ated in the bone around the standard HA thread. However, it The most appropriate standardised thread for use in should be remembered that this reduction also reduces the implants for direct skeletal attachment of limb prosthesis is thread-bone contact area by 3.7%, which may decrease the Rounding arc (mm) Transverse stress peak Equivalent stress peak change (%) change (%) Radial stress peak change Longitudinal stress peak (%) change (%) Thread-bone contact area in reference to standardised HA thread (%) Applied Bionics and Biomechanics 9 −2 −7 −12 −2 25 30 35 40 45 25 30 35 40 45 Flank angle (°) Flank angle (°) R = 0.8 mm R = 1.1 mm R = 0.8 mm R = 1.1 mm R = 0.9 mm R = 1.2 mm R = 0.9 mm R = 1.2 mm R = 1.0 mm R = 1.0 mm 20 9 12 7 4 3 −4 −1 25 30 35 40 45 25 30 35 40 45 Flank angle (°) Flank angle (°) R = 0.8 mm R = 1.1 mm R = 0.8 mm R = 1.1 mm R = 0.9 mm R = 1.2 mm R = 0.9 mm R = 1.2 mm R = 1.0 mm R = 1.0 mm Figure 11: The influence of flank angle and rounding arc on stress peaks during implant loading with loads generated during heel strike. probability of achieving proper osseointegration in second- [2] R. 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