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Comparison of Resistive and Optical Strain Measurement for Early Fracture Detection

Comparison of Resistive and Optical Strain Measurement for Early Fracture Detection DE GRUYTER Current Directions in Biomedical Engineering 2020;6(3): 20203050 Alina Carabello*, Constanze Neupetsch, Dirk Zajonz, Michael Werner, Christian Rotsch, and Welf-Guntram Drossel Comparison of Resistive and Optical Strain Measurement for Early Fracture Detection https://doi.org/10.1515/cdbme-2020-3050 1 Introduction Abstract: To increase learning success in surgical training, physical simulators are supplemented by measurement tech- The treatment of a hip joint with an endoprosthesis is a main nology to generate and record objective feedback and error de- area of surgical orthopaedics and requires extensive anatomi- tection. An opportunity to detect fractures following hip stem cal knowledge and experience to ensure the treatment of pa- implantation early can be measurement of occurring strains tients without complications. For these purposes physical sim- on bone surface. These strains can be determined while using ulators are used in surgical training. To increase learning suc- strain gauges, digital image correlation (DIC) or photoelastic- cess, integration of appropriate measurement technology can ity. In this research strain gauges and DIC were compared re- generate an objective feedback for the user and enable an early garding their suitability as strain measurement tools for use in error detection. [9] physical simulators. Therefore a testing method was described In literature no methods are described with regard to early to replicate the implantation of a hip stem. Testing devices error detection of intraoperative fractures following hip stem modelled on a realistic prosthesis were pressed into prepared implantation. Thereby strains, that lead to fractures and occur porcine femora in a two-step procedure with a material test- when a prosthesis is impacted, can be measured with strain ing machine. The local strains occurring on bone surface were gauges and optical measurements. [1, 7] determined using an optical measurement system for DIC and Strain gauges are accurate measuring tools for determin- strain gauges. The initial fractures in the tested femora are lo- ing local strains. Due to their size, strain gauges have a high cated medial-anterior in most cases (73,6%). With increasing degree of integration and are cost-effective compared to al- indentation depth of the test device, the strains on bone sur- ternative methods e.g. optical measurements. For determining face increase. Comparing the local strains determined by DIC strain distributions on surfaces, several strain gauges are ap- and strain gauges consistencies in curves are noticeable. Maxi- plied in an array. This results in increasing costs. [4, 7] Opti- mal determined strains before fracturing amount to 0,69% with cal methods can contactless determine strain distributions with strain gauges and 0,75% with DIC. In the range of the fracture high local resolution. Disadvantageous are high costs for pur- gap, strain gradients are determined by using DIC. However chasing the measurement system and necessary visibility of the detected surfaces are of low quality caused by gaps and the specimen. Photoelasticity also requires a complex coating motion artefacts. The results show strains on bone surfaces of the specimen with photoelastic material, that can influence for early fracture detection are measurable with strain gauges the mechanical behavior of the specimen. Also measuring ac- and DIC. DIC is assessed as less suitable compared to strain curacy depends on quality of the coating. [2] DIC, as an alter- gauges. Furthermore strain gauges have greater level of inte- native optical method, requires texturing of the specimen with gration and economic efficiency, so they are preferred the use a speckle pattern instead of a photoelastic coating. Thereby the in surgical training simulators. mechanical behavior of the test specimen is not affected. [4] In this paper, DIC was examined for strain measurement Keywords: strain measurement, bone, strain gauges, digital on bone surface and was compared with strain gauges. The image correlation, fracture main goal was to identify a suitable strain measurement for use in a physical simulator. The determined strains are sup- posed to predict fractures. The requirements for the method- *Corresponding author: Alina Carabello, Chemnitz University of ologies are low costs and accuracy. In order to reduce costs, Technology, Professorship for Adaptronics and Lightweight Design the strain is to be determined locally at one measuring point. in Production, Chemnitz, Germany, e-mail: alina.carabello@mb.tu-chemnitz.de Artificial bone models with biomechanically correct behaviour Constanze Neupetsch, Michael Werner, Christian Rotsch, are not yet available, but are already in development [6]. To be Welf-Guntram Drossel, Fraunhofer Institute for Machine Tools able to investigate a method for recording the strain, biological and Forming Technology IWU, Chemnitz/Dresden, Germany models are being used. The results can then be transferred to Dirk Zajonz, Zeisigwaldkliniken BETHANIEN, Chemnitz, Ger- biomechanically correct bone models. many Open Access. © 2020 Alina Carabello et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License. 2 A. Carabello et al., Strain measurement for early fracture detection 2 Materials and Methods To asses the suitability of DIC and strain gauges, the strains oc- curring when an indention test device is pressed into porcine femur on the basis of the implantation of a hip stem prosthe- sis, were determined. The preparation of specimen and exper- imental setup (s. fig. 1) imitated the process of hip stem im- plantation in laboratory environment under uniform boundary conditions. Therefor, two research objectives were pursued: 1. Localisation of occurring strains and fractures 2. Comparison of DIC and strain gauges Fig. 1: Prepared porcine femora with fracutres medio-anterior determined using DIC and strain gauges. 2.1 Specimen schweig, 5MP, measuring volume 300𝑥230𝑥230 mm). They were positioned opposite each other and recorded the medial Porcine femura show similarities to human femura with regard and lateral bone surfaces. Measurements were made with a to their micro- and marcostructure and are suitable as animal maximum frame rate of 5 Hz. This was limited by the exposure models [8]. 19 porcine femura were tested: Specimen 1 to 14 time. Due to the camera setup, an exposure time of 65 ms was were tested for localisation of occuring strains and fractures necessary. In addition v fi e specimen were tested while only and v fi e additional specimen (A to E) were tested for com- using the ARAMIS system, for the comparison of the method- parison of DIC and strain gauges. To connect the specimen ologies of DIC and strain gauges. This allowed reducing ex- to the test stand, they were embedded in a casting resin. The posure time and increasing frame rate up to 44 Hz. neck of the femur was cut with an oscillating bone saw and The analysis of the image data was done with software an adaptable additive manufactured template. The medullary „ARAMIS Professional 2017 “. The image data were sam- cavity was opened with a rasp. A stochastic, isotropic and pled with facet size of 13 pixels to identify the bone surface homogeneously speckle pattern for DIC was distributed with and generate a digital model. For quantification of occurring graphite dust over the measurement area. strains, the mean values of the axial and radial strains in the re- gion of the fracture gap were calculated. For comparison of the results with the locally determined strains of the strain gauges, 2.2 Experimental Setup a curve was fitted to the digital bone surface in the area of the A hip stem prosthesis from veterinary medicine was scaled strain gauges. The elongation of this curve could be compared and adapted to the dimensions of porcine femura for use as with the measurements of the strain gauges. an indention test device. Two models in different size were In addition to the speckle pattern, two biaxial strain produced for testing and for generating fractures. To connect gauges (Omega , North Ireland, 𝑅 = 350 Ω, 𝑘 = 2, 14) each bone specimen with the material testing machine a positioning were applied to v fi e femora at position 6 after classification ac- table with two degrees of freedom was used. It allowed the cording to Gruen [3]. They were connected to full bridges. Fur- porcine femur to be aligned with the indention test device. thermore, a GSV-1M measurement amplifier and the NI-6009 The insertion of the indention test device into the bone USB box were used. The data acquisition rate was 100 Hz. specimen was realized in a path-controlled manner with a material testing machine (DYNA-MESS Prüfsysteme GmbH, Aachen/Stolberg). For imitating the dynamic load, a crosshead 3 Results −1 speed of 20 mm s was chosen. First, the indention test de- vice was pressed up to 75% into medullary cavity. If no frac- Table 1 shows localisation of occurring fractures and quan- ture has occurred, it was pressed by a further 25%. So the in- tification of largest axial and radial strains of each specimen dention test device was completely pressed into the specimen. before fractures occurred. 73, 6% of specimens (14 out of 19) To determine the localisation with increased incidence were fractured medio-laterally. The limitation of the indention for fractures and occurring strains, 14 specimens were tested test device to two different sizes has thus proved to be in- while using the optical measurement systems ARAMIS sufficient, as 4 bones are not fractured. The applied speckle (GOM GmbH, Braunschweig, 12MP, measuring volume patterns allowed the bone surface to be identified (s. fig 2) 100𝑥80𝑥50 mm) and ATOS Core 300 (GOM GmbH, Braun- A. Carabello et al., Strain measurement for early fracture detection 3 Tab. 1: Fracture localisation and optical quantification of the largest occurring strains for each specimen. Femur Fracture Axial Strain [%] Radial Strain [%] 1 medio-lateral 0, 18 0, 59 2 lateral 0, 01 0, 21 3 medio-lateral 0, 16 0, 79 4 medio-lateral 0, 09 0, 62 5 medio-lateral 0, 05 0, 59 6 medio-lateral 0, 39 0, 98 7 medio-lateral 0, 11 0, 72 Fig. 2: Radial (left) and axial (right) strains determined with DIC 8 none 0, 05 0, 16 on medio-lateral porcine bone surface. 9 none 0, 01 0, 18 10 medio-lateral 0, 08 0, 81 11 none 0, 10 0, 67 with increasing penetration depth. Bone specimen A to C are 12 medio-lateral 0, 07 0, 79 fractured during the first indentation step, while specimen E 13 medio-lateral 0, 04 0, 57 14 medio-lateral n.a. n.a. fractured during the second step and specimen D remained without fracture after two indentation steps. Thereby, the mea- A medio-lateral 0, 09 0, 71 surements were not stopped between the first and the second B medio-lateral 0, 13 0, 67 C medio-lateral 0, 12 0, 65 indention stage, to record possible strain reductions over the D none 0, 09 0, 63 time with constant load. In the graphs a decrease of strains E medio-lateral 0, 10 0, 75 determined with strain gauges can be seen. The optically de- termined strains remains constant. Despite the graphs of bone specimen B and D all graphs show local maximums when frac- except for specimen 14. Because of movement artefacts no ture occurred. surface could be detected. On the lateral side of the bones, After fracturing strain values decrease. The optical mea- neither axial nor radial strains can be determined, too. Maxi- surement shows an irregular course, which is due to partial mum radial strains of 0, 62% ± 0, 23% are determined before and complete losses of the speckle pattern as a result of the the fracture gaps occurred, whereas the axial strains are up to fracture gap. With resistive strain measurement, a plateau is 0, 1% ± 0, 08%. Due to the intervariability of the bone spec- reached after a view seconds. The decreasing strains can be imens, the values of local maxima differ between the bone explained by the reduction of the acting stresses after the frac- specimen. This corresponds to the information given in the lit- ture occurred. Deviations can be also seen in determined strain erature. Jasty et. al [5] have measured radial strains of up to values between DIC and strain gauges. For instance the deter- 0.6% with strain gauges on human preparations, which leads mined maximum strains of specimen A are 0, 71% for optical to fractures and were larger than axially occurring strains. and 0, 69% for strain gauge measurement. In bone specimen Despite the use of two optical measuring systems, the dor- B, an increase in the resistively determined strain up to 0, 36% sal and ventral sides of the bone are not detected. In addition, can be seen. The fracture develops after 2.2 s and was located there is partial and complete loss of the speckle pattern in the below a strain gauge. The strain gauge was detached from the area of the fracture. The image data show movement artefacts bone surface and could no longer provide valid measurement and blurring of the pattern. Furthermore, the exposure condi- results, which is why the measurement is stopped after this tions have a negative effect on the calculation of the surfaces. time. In addition, there is a considerable loss of the speckle Figure 3 shows the curves of the strains over time. The pattern due to a movement artefact. strain values are plotted in separate diagrams for each speci- Due to the low frame rate of 44 Hz in comparison to the men for comparing local strains determined with strain gauges penetration speed, strains can lead to losses of surface infor- and DIC to assess both methodologies. The DIC show an im- mation. In addition, motion artefacts lead to an erroneous iden- proved image quality in comparison with data generated with tification of the facets that are used for the calculation of the frame rate of 5 Hz. Despite the increased frame rate, motion sample surface. This leads to errors in the calculation of the artefacts have led to partial or complete loss of surface infor- strains. This explains strain peaks at the edge of the surface in- mation which resulted in interruptions of the graphs. In each formation and the irregular course of the graphs of the locally diagram an elongation plateau is shown in the first 1.8 s. Dur- determined strains. ing this time, the test device penetrates the bone specimen without acting on it. Then the strain increases over the time 4 A. Carabello et al., Strain measurement for early fracture detection Furthermore, it will be investigated whether the strains during implantation of a hip stem prosthesis can be used to identify critical strains for early fracture detection. Regarding to this, the suitability of strain gauges concerning their measur- ing frequency and local resolution is to investigate, too. Fur- thermore, the results shall be transferred to a realistic bone model [6]. This requires investigations with the appropriate material of the bone model. In further researches this bone models shall be used as surgical simulators with objective feedback. Author Statement Research funding: The authors have no conflict of interest in relation to the present study. The conducted research is not re- lated to either human or animals use. The work presented here will be co-financed with tax funds on the basis of the budget approved by the members of Parliament of Saxony (SAB FKZ 100334004, term: 04/18- 12/20). References [1] Elias JJ, Nagao M, Chu YH, Carbone JJ, Lennox DW, Chao Fig. 3: Local strains determined with DIC and strain gauges. EYS. Medial Cortex Strain Distribution During Noncemented Total Hip Arthroplasty. Clinical orthopaedics and related re- search 2000;250-258 4 Conclusion and Outlook [2] Ellenrieder M, Steinhauser E, Bader R, Mittelmeier W.Influence of cementless hip stems on femoral cortical strain pattern depending on their extent of porous coat- The results show that local strains and strain peaks resulting ing. Biomedizinische Technik. Biomedical engineering from the insertion of a indention test device on bone surfaces 2012;57:121-129 can be determined with strain gauges and DIC. Deviations be- [3] Gruen TA, McNeice GM, Amstutz HC “Modes of failure” of tween strains determined with DIC and strain gauges are pos- cemented stem-type femoral components: a radiographic sible due to the non-linearity of strain gauges on high strains. analysis of loosening. Clinical Orthopaedics and Related Research 1979;141:17–27 Another aspects are the partially losses of surface information [4] Hoult NA, Take A, Andy Take, W, Lee C, Dutton M. Experi- for DIC at the edge of the detected bone surface and in the mental accuracy of two dimensional strain measurements us- fracture gap area. This can be caused by motion artefacts and ing Digital Image Correlation. Engineering Structures 2013; strains between two images which could be too large to iden- 46:718-726 tify common facets and lead to losses of surface information [5] Jasty M, Henshaw RM, O’Connor DO, Harris WH. High As- sembly Strains and Femoral Fractures Produced During In- or breaking up the speckle pattern through the fracture gap. sertion of Uncemented Femoral Components: A Cadaver Both methods allow only localized measurement of the Study. The Journal of Arthroplasty 1993;479-478 strains. Since the fractures occurred at the same location, the [6] Neupetsch C, Hensel E. Werner M, Meißner S, Troge J, methodologies shown are suitable for this application accord- Drossel WG, et al. Development and Validation of Bone Mod- ing to hip stem. If the fracture behaviour is unknown, re- els using Structural Dynamic Measurement Methods. Current newed preliminary examinations are necessary to define mea- Directions in Biomedical Engineering 2019;5(1):343-346 [7] Miles AW, Tanner KE. Strain Measurement in Biomechanics. suring points. Nevertheless the experiments show that strain 1st ed. Springer Netherlands 1992 gauges are a more stable measurement tool for determining lo- [8] Pearce AI, Richards RG, Milz S, Schneider E, Pearce S. cal strains on bone surface before fracture occurs. Furthermore G.Animal models for implant biomaterial research in bone: A they have a greater level of economic efficiency and integra- review. European Cells and Materials 2007; 13:1-10 tion caused by the measurement principle without the need of [9] Sadideen H, Plonczak A, Saadeddin M, Kneebone R. How Educational Theory Can Inform the Training and Practice an optical measurement system and their size. So strain gauges of Plastic Surgeons. Plastic and reconstructive surgery. are assessed more suitable for early fracture detection com- 2018;12:20-42 pared to DIC. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Current Directions in Biomedical Engineering de Gruyter

Comparison of Resistive and Optical Strain Measurement for Early Fracture Detection

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Abstract

DE GRUYTER Current Directions in Biomedical Engineering 2020;6(3): 20203050 Alina Carabello*, Constanze Neupetsch, Dirk Zajonz, Michael Werner, Christian Rotsch, and Welf-Guntram Drossel Comparison of Resistive and Optical Strain Measurement for Early Fracture Detection https://doi.org/10.1515/cdbme-2020-3050 1 Introduction Abstract: To increase learning success in surgical training, physical simulators are supplemented by measurement tech- The treatment of a hip joint with an endoprosthesis is a main nology to generate and record objective feedback and error de- area of surgical orthopaedics and requires extensive anatomi- tection. An opportunity to detect fractures following hip stem cal knowledge and experience to ensure the treatment of pa- implantation early can be measurement of occurring strains tients without complications. For these purposes physical sim- on bone surface. These strains can be determined while using ulators are used in surgical training. To increase learning suc- strain gauges, digital image correlation (DIC) or photoelastic- cess, integration of appropriate measurement technology can ity. In this research strain gauges and DIC were compared re- generate an objective feedback for the user and enable an early garding their suitability as strain measurement tools for use in error detection. [9] physical simulators. Therefore a testing method was described In literature no methods are described with regard to early to replicate the implantation of a hip stem. Testing devices error detection of intraoperative fractures following hip stem modelled on a realistic prosthesis were pressed into prepared implantation. Thereby strains, that lead to fractures and occur porcine femora in a two-step procedure with a material test- when a prosthesis is impacted, can be measured with strain ing machine. The local strains occurring on bone surface were gauges and optical measurements. [1, 7] determined using an optical measurement system for DIC and Strain gauges are accurate measuring tools for determin- strain gauges. The initial fractures in the tested femora are lo- ing local strains. Due to their size, strain gauges have a high cated medial-anterior in most cases (73,6%). With increasing degree of integration and are cost-effective compared to al- indentation depth of the test device, the strains on bone sur- ternative methods e.g. optical measurements. For determining face increase. Comparing the local strains determined by DIC strain distributions on surfaces, several strain gauges are ap- and strain gauges consistencies in curves are noticeable. Maxi- plied in an array. This results in increasing costs. [4, 7] Opti- mal determined strains before fracturing amount to 0,69% with cal methods can contactless determine strain distributions with strain gauges and 0,75% with DIC. In the range of the fracture high local resolution. Disadvantageous are high costs for pur- gap, strain gradients are determined by using DIC. However chasing the measurement system and necessary visibility of the detected surfaces are of low quality caused by gaps and the specimen. Photoelasticity also requires a complex coating motion artefacts. The results show strains on bone surfaces of the specimen with photoelastic material, that can influence for early fracture detection are measurable with strain gauges the mechanical behavior of the specimen. Also measuring ac- and DIC. DIC is assessed as less suitable compared to strain curacy depends on quality of the coating. [2] DIC, as an alter- gauges. Furthermore strain gauges have greater level of inte- native optical method, requires texturing of the specimen with gration and economic efficiency, so they are preferred the use a speckle pattern instead of a photoelastic coating. Thereby the in surgical training simulators. mechanical behavior of the test specimen is not affected. [4] In this paper, DIC was examined for strain measurement Keywords: strain measurement, bone, strain gauges, digital on bone surface and was compared with strain gauges. The image correlation, fracture main goal was to identify a suitable strain measurement for use in a physical simulator. The determined strains are sup- posed to predict fractures. The requirements for the method- *Corresponding author: Alina Carabello, Chemnitz University of ologies are low costs and accuracy. In order to reduce costs, Technology, Professorship for Adaptronics and Lightweight Design the strain is to be determined locally at one measuring point. in Production, Chemnitz, Germany, e-mail: alina.carabello@mb.tu-chemnitz.de Artificial bone models with biomechanically correct behaviour Constanze Neupetsch, Michael Werner, Christian Rotsch, are not yet available, but are already in development [6]. To be Welf-Guntram Drossel, Fraunhofer Institute for Machine Tools able to investigate a method for recording the strain, biological and Forming Technology IWU, Chemnitz/Dresden, Germany models are being used. The results can then be transferred to Dirk Zajonz, Zeisigwaldkliniken BETHANIEN, Chemnitz, Ger- biomechanically correct bone models. many Open Access. © 2020 Alina Carabello et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License. 2 A. Carabello et al., Strain measurement for early fracture detection 2 Materials and Methods To asses the suitability of DIC and strain gauges, the strains oc- curring when an indention test device is pressed into porcine femur on the basis of the implantation of a hip stem prosthe- sis, were determined. The preparation of specimen and exper- imental setup (s. fig. 1) imitated the process of hip stem im- plantation in laboratory environment under uniform boundary conditions. Therefor, two research objectives were pursued: 1. Localisation of occurring strains and fractures 2. Comparison of DIC and strain gauges Fig. 1: Prepared porcine femora with fracutres medio-anterior determined using DIC and strain gauges. 2.1 Specimen schweig, 5MP, measuring volume 300𝑥230𝑥230 mm). They were positioned opposite each other and recorded the medial Porcine femura show similarities to human femura with regard and lateral bone surfaces. Measurements were made with a to their micro- and marcostructure and are suitable as animal maximum frame rate of 5 Hz. This was limited by the exposure models [8]. 19 porcine femura were tested: Specimen 1 to 14 time. Due to the camera setup, an exposure time of 65 ms was were tested for localisation of occuring strains and fractures necessary. In addition v fi e specimen were tested while only and v fi e additional specimen (A to E) were tested for com- using the ARAMIS system, for the comparison of the method- parison of DIC and strain gauges. To connect the specimen ologies of DIC and strain gauges. This allowed reducing ex- to the test stand, they were embedded in a casting resin. The posure time and increasing frame rate up to 44 Hz. neck of the femur was cut with an oscillating bone saw and The analysis of the image data was done with software an adaptable additive manufactured template. The medullary „ARAMIS Professional 2017 “. The image data were sam- cavity was opened with a rasp. A stochastic, isotropic and pled with facet size of 13 pixels to identify the bone surface homogeneously speckle pattern for DIC was distributed with and generate a digital model. For quantification of occurring graphite dust over the measurement area. strains, the mean values of the axial and radial strains in the re- gion of the fracture gap were calculated. For comparison of the results with the locally determined strains of the strain gauges, 2.2 Experimental Setup a curve was fitted to the digital bone surface in the area of the A hip stem prosthesis from veterinary medicine was scaled strain gauges. The elongation of this curve could be compared and adapted to the dimensions of porcine femura for use as with the measurements of the strain gauges. an indention test device. Two models in different size were In addition to the speckle pattern, two biaxial strain produced for testing and for generating fractures. To connect gauges (Omega , North Ireland, 𝑅 = 350 Ω, 𝑘 = 2, 14) each bone specimen with the material testing machine a positioning were applied to v fi e femora at position 6 after classification ac- table with two degrees of freedom was used. It allowed the cording to Gruen [3]. They were connected to full bridges. Fur- porcine femur to be aligned with the indention test device. thermore, a GSV-1M measurement amplifier and the NI-6009 The insertion of the indention test device into the bone USB box were used. The data acquisition rate was 100 Hz. specimen was realized in a path-controlled manner with a material testing machine (DYNA-MESS Prüfsysteme GmbH, Aachen/Stolberg). For imitating the dynamic load, a crosshead 3 Results −1 speed of 20 mm s was chosen. First, the indention test de- vice was pressed up to 75% into medullary cavity. If no frac- Table 1 shows localisation of occurring fractures and quan- ture has occurred, it was pressed by a further 25%. So the in- tification of largest axial and radial strains of each specimen dention test device was completely pressed into the specimen. before fractures occurred. 73, 6% of specimens (14 out of 19) To determine the localisation with increased incidence were fractured medio-laterally. The limitation of the indention for fractures and occurring strains, 14 specimens were tested test device to two different sizes has thus proved to be in- while using the optical measurement systems ARAMIS sufficient, as 4 bones are not fractured. The applied speckle (GOM GmbH, Braunschweig, 12MP, measuring volume patterns allowed the bone surface to be identified (s. fig 2) 100𝑥80𝑥50 mm) and ATOS Core 300 (GOM GmbH, Braun- A. Carabello et al., Strain measurement for early fracture detection 3 Tab. 1: Fracture localisation and optical quantification of the largest occurring strains for each specimen. Femur Fracture Axial Strain [%] Radial Strain [%] 1 medio-lateral 0, 18 0, 59 2 lateral 0, 01 0, 21 3 medio-lateral 0, 16 0, 79 4 medio-lateral 0, 09 0, 62 5 medio-lateral 0, 05 0, 59 6 medio-lateral 0, 39 0, 98 7 medio-lateral 0, 11 0, 72 Fig. 2: Radial (left) and axial (right) strains determined with DIC 8 none 0, 05 0, 16 on medio-lateral porcine bone surface. 9 none 0, 01 0, 18 10 medio-lateral 0, 08 0, 81 11 none 0, 10 0, 67 with increasing penetration depth. Bone specimen A to C are 12 medio-lateral 0, 07 0, 79 fractured during the first indentation step, while specimen E 13 medio-lateral 0, 04 0, 57 14 medio-lateral n.a. n.a. fractured during the second step and specimen D remained without fracture after two indentation steps. Thereby, the mea- A medio-lateral 0, 09 0, 71 surements were not stopped between the first and the second B medio-lateral 0, 13 0, 67 C medio-lateral 0, 12 0, 65 indention stage, to record possible strain reductions over the D none 0, 09 0, 63 time with constant load. In the graphs a decrease of strains E medio-lateral 0, 10 0, 75 determined with strain gauges can be seen. The optically de- termined strains remains constant. Despite the graphs of bone specimen B and D all graphs show local maximums when frac- except for specimen 14. Because of movement artefacts no ture occurred. surface could be detected. On the lateral side of the bones, After fracturing strain values decrease. The optical mea- neither axial nor radial strains can be determined, too. Maxi- surement shows an irregular course, which is due to partial mum radial strains of 0, 62% ± 0, 23% are determined before and complete losses of the speckle pattern as a result of the the fracture gaps occurred, whereas the axial strains are up to fracture gap. With resistive strain measurement, a plateau is 0, 1% ± 0, 08%. Due to the intervariability of the bone spec- reached after a view seconds. The decreasing strains can be imens, the values of local maxima differ between the bone explained by the reduction of the acting stresses after the frac- specimen. This corresponds to the information given in the lit- ture occurred. Deviations can be also seen in determined strain erature. Jasty et. al [5] have measured radial strains of up to values between DIC and strain gauges. For instance the deter- 0.6% with strain gauges on human preparations, which leads mined maximum strains of specimen A are 0, 71% for optical to fractures and were larger than axially occurring strains. and 0, 69% for strain gauge measurement. In bone specimen Despite the use of two optical measuring systems, the dor- B, an increase in the resistively determined strain up to 0, 36% sal and ventral sides of the bone are not detected. In addition, can be seen. The fracture develops after 2.2 s and was located there is partial and complete loss of the speckle pattern in the below a strain gauge. The strain gauge was detached from the area of the fracture. The image data show movement artefacts bone surface and could no longer provide valid measurement and blurring of the pattern. Furthermore, the exposure condi- results, which is why the measurement is stopped after this tions have a negative effect on the calculation of the surfaces. time. In addition, there is a considerable loss of the speckle Figure 3 shows the curves of the strains over time. The pattern due to a movement artefact. strain values are plotted in separate diagrams for each speci- Due to the low frame rate of 44 Hz in comparison to the men for comparing local strains determined with strain gauges penetration speed, strains can lead to losses of surface infor- and DIC to assess both methodologies. The DIC show an im- mation. In addition, motion artefacts lead to an erroneous iden- proved image quality in comparison with data generated with tification of the facets that are used for the calculation of the frame rate of 5 Hz. Despite the increased frame rate, motion sample surface. This leads to errors in the calculation of the artefacts have led to partial or complete loss of surface infor- strains. This explains strain peaks at the edge of the surface in- mation which resulted in interruptions of the graphs. In each formation and the irregular course of the graphs of the locally diagram an elongation plateau is shown in the first 1.8 s. Dur- determined strains. ing this time, the test device penetrates the bone specimen without acting on it. Then the strain increases over the time 4 A. Carabello et al., Strain measurement for early fracture detection Furthermore, it will be investigated whether the strains during implantation of a hip stem prosthesis can be used to identify critical strains for early fracture detection. Regarding to this, the suitability of strain gauges concerning their measur- ing frequency and local resolution is to investigate, too. Fur- thermore, the results shall be transferred to a realistic bone model [6]. This requires investigations with the appropriate material of the bone model. In further researches this bone models shall be used as surgical simulators with objective feedback. Author Statement Research funding: The authors have no conflict of interest in relation to the present study. The conducted research is not re- lated to either human or animals use. The work presented here will be co-financed with tax funds on the basis of the budget approved by the members of Parliament of Saxony (SAB FKZ 100334004, term: 04/18- 12/20). References [1] Elias JJ, Nagao M, Chu YH, Carbone JJ, Lennox DW, Chao Fig. 3: Local strains determined with DIC and strain gauges. EYS. Medial Cortex Strain Distribution During Noncemented Total Hip Arthroplasty. Clinical orthopaedics and related re- search 2000;250-258 4 Conclusion and Outlook [2] Ellenrieder M, Steinhauser E, Bader R, Mittelmeier W.Influence of cementless hip stems on femoral cortical strain pattern depending on their extent of porous coat- The results show that local strains and strain peaks resulting ing. Biomedizinische Technik. Biomedical engineering from the insertion of a indention test device on bone surfaces 2012;57:121-129 can be determined with strain gauges and DIC. Deviations be- [3] Gruen TA, McNeice GM, Amstutz HC “Modes of failure” of tween strains determined with DIC and strain gauges are pos- cemented stem-type femoral components: a radiographic sible due to the non-linearity of strain gauges on high strains. analysis of loosening. Clinical Orthopaedics and Related Research 1979;141:17–27 Another aspects are the partially losses of surface information [4] Hoult NA, Take A, Andy Take, W, Lee C, Dutton M. Experi- for DIC at the edge of the detected bone surface and in the mental accuracy of two dimensional strain measurements us- fracture gap area. This can be caused by motion artefacts and ing Digital Image Correlation. Engineering Structures 2013; strains between two images which could be too large to iden- 46:718-726 tify common facets and lead to losses of surface information [5] Jasty M, Henshaw RM, O’Connor DO, Harris WH. High As- sembly Strains and Femoral Fractures Produced During In- or breaking up the speckle pattern through the fracture gap. sertion of Uncemented Femoral Components: A Cadaver Both methods allow only localized measurement of the Study. The Journal of Arthroplasty 1993;479-478 strains. Since the fractures occurred at the same location, the [6] Neupetsch C, Hensel E. Werner M, Meißner S, Troge J, methodologies shown are suitable for this application accord- Drossel WG, et al. Development and Validation of Bone Mod- ing to hip stem. If the fracture behaviour is unknown, re- els using Structural Dynamic Measurement Methods. Current newed preliminary examinations are necessary to define mea- Directions in Biomedical Engineering 2019;5(1):343-346 [7] Miles AW, Tanner KE. Strain Measurement in Biomechanics. suring points. Nevertheless the experiments show that strain 1st ed. Springer Netherlands 1992 gauges are a more stable measurement tool for determining lo- [8] Pearce AI, Richards RG, Milz S, Schneider E, Pearce S. cal strains on bone surface before fracture occurs. Furthermore G.Animal models for implant biomaterial research in bone: A they have a greater level of economic efficiency and integra- review. European Cells and Materials 2007; 13:1-10 tion caused by the measurement principle without the need of [9] Sadideen H, Plonczak A, Saadeddin M, Kneebone R. How Educational Theory Can Inform the Training and Practice an optical measurement system and their size. So strain gauges of Plastic Surgeons. Plastic and reconstructive surgery. are assessed more suitable for early fracture detection com- 2018;12:20-42 pared to DIC.

Journal

Current Directions in Biomedical Engineering – de Gruyter

Published: Sep 1, 2020

Keywords: strain measurement; bone; strain gauges; digital image correlation; fracture

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