Abstract. Objectives. Ultrasound speckle tracking is a safe and non-invasive diagnostic tool to measure soft tissue deformation and strain. In orthopaedics, it could have broad application to measure how injury or surgery affects muscle, tendon or ligament biomechanics. However, its application requires custom tuning of the speckle-tracking algorithm then validation against gold-standard reference data. Implementing an experiment to acquire these data takes months and is expensive, and therefore prohibits use for new applications. Here, we present an alternative optimisation approach that automatically finds suitable machine and algorithmic settings without requiring gold-standard reference data. Methods. The optimisation routine consisted of two steps. First, convergence of the displacement field was tested to exclude the settings that would not track the underlying tissue motion (e.g. frame rates that were too low). Second, repeatability was maximised through a surrogate optimisation scheme. All settings that could influence the strain calculation were included, ranging from acquisition settings to post-processing smoothing and filtering settings, totalling >1,000,000 combinations of settings. The optimisation criterion minimised the normalised standard deviation between strain maps of repeat measures. The optimisation approach was validated for the medial collateral ligament (MCL) with quasi-static testing on porcine joints (n=3), and dynamic testing on a cadaveric human knee (n=1, female, aged 49). Porcine joints were fully dissected except for the MCL and loaded in a material-testing machine (0 to 3% strain at 0.2 Hz), which was captured using both ultrasound (>14 repeats per specimen) and optical digital image correlation (DIC). For the human cadaveric knee (undissected), 3 repeat ultrasound acquisitions were taken at 18 different anterior/posterior positions over the MCL while the knee was extended/flexed between 0° and 90° in a knee extension rig. Simultaneous optical tracking recorded the position of the ultrasound transducer, knee kinematics and the MCL attachments (which were digitised under direct visualisation post testing). Half of the data collected was used for optimisation of the speckle tracking algorithms for the porcine and human MCLs separately, with the remaining unseen data used as a validation test set. Results. For the porcine MCLs, ultrasound strains closely matched DIC strains (R. 2. > 0.98, RMSE < 0.59%) (Figure 1A). For the human MCL (Figure 1B), ultrasound strains matched the strains estimated from the optically tracked displacements of the MCL attachments. Furthermore, strains developed during flexion were highly correlated with AP position (R = 0.94) with strains decreasing the further posterior the transducer was on the ligament. This is in line with previously reported length change values for the posterior, intermediate and anterior bundles of the MCL. Conclusions. Ultrasound speckle tracking algorithms can be adapted for new applications without ground-truth data by using an optimisation approach that verifies displacement field convergence then minimises variance between repeat measurements. This optimisation routine was insensitive to anatomical variation and loading conditions, working for both porcine and human MCLs, and for quasi-static and dynamic loading. This will facilitate research into changes in musculoskeletal tissue motion due to abnormalities or pathologies. Declaration of Interest. (a) fully declare any financial or other potential conflict of interest

Aims

The aim of the study was to investigate whether the primary stability of press-fit acetabular components can be improved by altering the impaction procedure.

Aims

A conventional arthroscopic capsuloligamentous repair is a reliable surgical solution in most patients with scapholunate instability. However, this repair does not seem to be sufficient for more advanced injuries. The aim of this study was to evaluate the functional results of a wide arthroscopic dorsal capsuloligamentous repair (WADCLR) in the management of severe scapholunate instability.

Aims

Arthroplasty surgery of the knee and hip is performed in two to three million patients annually. Periprosthetic joint infections occur in 4% of these patients. Debridement, antibiotics, and implant retention (DAIR) surgery aimed at cleaning the infected prosthesis often fails, subsequently requiring invasive revision of the complete prosthetic reconstruction. Infection-specific imaging may help to guide DAIR. In this study, we evaluated a bacteria-specific hybrid tracer (^99mTc-UBI_29-41-Cy5) and its ability to visualize the bacterial load on femoral implants using clinical-grade image guidance methods.

Introduction and Objective. Scapholunate instability is the most common cause of carpal instability. When this instability is left untreated, the mechanical relationship between the carpal bones is permanently disrupted, resulting in progressive degenerative changes in the radiocarpal and midcarpal joints. Different tenodesis methods are used in the treatment of acute or early chronic reducible scapholunate instability, where arthritis has not developed yet and the scapholunate ligament cannot be repaired. Although it has been reported that pain is reduced in the early follow up in clinical studies with these methods, radiological results differ between studies. The deterioration of these radiological parameters is associated with wrist osteoarthritis as previously stated. Therefore, more studies are needed to determine the tenodesis method that will improve the wrist biomechanics better and will last longer. In our study, two new tenodesis methods, spiral antipronation tenodesis, and anatomic front and back reconstruction (ANAFAB) were radiologically compared with triple ligament tenodesis (TLT), in the cadaver wrists. Materials and Methods. The study was carried out on a total of 16 fresh frozen cadaver wrists. Samples were randomly allocated to the groups treated with 3 different scapholunate instability treatment methods. These are TLT (n: 6), spiral antipronation tenodesis (n: 5) and ANAFAB tenodesis (n: 5) groups. In all samples SLIL, DCSS, STT, DIC, RSC and LRL ligaments were cut in the same way to create scapholunate instability. Wrist CT scans were taken on the samples in 4 different states, in intact, after the ligaments were cut, after the reconstruction and after the movement cycle. In all of these 4 states, wrist CTs were taken in 6 different wrist positions. For every state and every position through tomography images; Scapholunate (SL) distance, Scapholunate (SL) angle, Radioscaphoid (RS) angle, Radiolunate (RL) angle, Capitolunate (CL) angle, Dorsal scaphoid translation (Dt) measurements were made. Results. Scapholunate distances means were different between intact and cut states only in neutral and clenched fist positions for all groups (p values <0.001). Mean differences were similar between the groups (p > 0.100). In neutral position, for SL center distance, mean difference between cut and reconstruction states were not different between the groups (p=0.497) but it was noted that only TLT group could not restore to the intact state. In neutral position, for SL angle, compared with the cut state, TLT and ANAFAB significantly reduced the angle (TLT: 20° (p=0.005), ANAFAB: 28° (p<0.001)) whereas antipronation tenodesis could not (13°, p=0.080). In clenched fist position, for SL angle, compared with the intact state, only ANAFAB group restored the angle, TLT and antipronation groups were significantly worse than the intact state (TLT: p<0.001, antipronation: p=0.001). In clenched fist position, for RL angle, compared with the intact state, ANAFAB and TLT groups restored the angle but antipronation group was significantly worse than the intact state (p<0.001). In neutral position, for RS angle, compared with the cut state, only ANAFAB significantly reduced the angle (11°, p<0.001) whereas TLT and antipronation groups could not (TLT: 6° (p=0.567), antipronasyon: 4° (p=0.128). Conclusions. In the presence of severe scapholunate instability in which a several number of secondary stabilizers are injured, the ANAFAB tenodesis method may be preferred to the classical method, TLT tenodesis. The results of spiral antipronation tenodesis were not better than the TLT

Abstract. Objectives. A fibril reinforced multiphasic cartilage model was developed to improve the understanding of the depth-dependent cartilage internal structure and its through thickness biomechanical response. The heterogeneous model of cartilage was validated against full-field strain measurement obtained via Digital Image Correlation (DIC) during free swelling experiments. Methods. Hemi-cylindrical cartilage cores of 5mm diameter were obtained from porcine femoral condyles and humeral heads. The full field behaviour of these samples was monitored using DIC during an osmotic free swelling experiment performed following a standardised protocol [1]. Computational models were created in FEBio (version 2.8, . febio.org. ). The cartilage, submerged in saline solution was represented by a 1×1mm cube [2] with geometry and constrains set up to mimic the experimental conditions. Cartilage was modelled as a multiphasic material represented by one inhomogeneous layer with depth-dependent Young's modulus [3], zonally varied water content and zonally oriented collagen fibrils [4]. Experimental and predicted strain maps were compared to each other both qualitatively and quantitatively. Results. The numerical strain map showed high strain localisation close to the cartilage surface, with strain in this region reaching 40% and 12% for femoral and humeral samples respectively, this finding was confirmed in our experimental results. Strain magnitude gradually decreased with depth, reaching near-zero at around 200μm. This behaviour also matched experimental observations. Conclusions. Both sets of computational strain results exhibited very good agreement with experimental data, both in terms of cartilage through-thickness swelling behaviour and strain magnitude. Our results show the importance of including cartilage structural inhomogeneities and inclusions of collagen fibrils when simulating through-thickness cartilage swelling. These findings highlight the crucial role of collagen fibrils on both tissue solute transport properties as well as the overall biomechanical response of cartilage. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Chronic rotator cuff tears are a major problem especially in the elderly population. Refixation is associated with high re-rupture rates. Therefore new implants or healing methods are needed. For a control of success biomechanical characteristics of native as well as treated tendons are of particular importance. Currently, tensile tests with static material testing machines are the most common technique for the biomechanical characterization of tendons. Resulting values are the maximum force (Fmax), stiffness and the Young´s modulus. However, no information is given about the allocation of strains over the tendon area. In addition, the determination of Fmax results in tissue destruction thus foreclosing further evaluation like histology. The Digital Image Correlation (DIC) is a contact-free non-destructive optical measuring method which gives information about distribution of strains by tracking the areal shift of an applied speckle pattern. The needed speckle pattern has to have a high contrast, a homogeneous distribution and a good adhesion to the surface. The method is established for the characterization of construction materials [1] to detect e.g. weak points. The present study examined if DIC is applicable for the complementary biomechanical evaluation of the sheep infraspinatus tendon. Fine ground powder extracted from a printer cartridge was chosen as a starting point. Preliminary to the in vitro experiments, the powder was applied on sheets with different methods: brushing, blowing, sieving and stamping. Stamping showed best results and was used for further in vitro tests on cadaveric native tendons (n=5). First, the toner powder was transferred to coarse-grained abrasive paper using a brush and stamped on the tendon surface. Afterwards DIC analysis was performed. For the in vivo tests, the left infraspinatus tendon of two German black-headed Mutton Sheep was detached and then refixed with bone anchors, the right tendon was used as native control (authorization: AZ 33.19-42502-04-17/2739). 12 weeks after surgery the animals were euthanized, the shoulders were explanted and DIC measurement performed. The speckle pattern could be applied adequately on the smooth tendon surfaces of native tendons. All specimens could be analyzed by DIC with sufficient correlation coefficients. The highest displacements were measured in the peripheral areas, whereas the central part of the tendon showed a low displacement. Repaired left tendons showed obvious differences already macroscopically. The tendons were thicker and showed inhomogeneous surfaces. Application of the toner powder by stamping was distinctly more complicated, DIC analysis could not produce sufficient correlation coefficients. In summary, transfer of DIC to native infraspinatus tendons of sheep was successful and can be further transferred to other animal and human tendons. However, irregular surfaces in tendon scar tissues affect the application of an adequate speckle pattern with a stamp technique. Therefore, further modifications are necessary. This research project has been supported by the German Research Foundation “Graded Implants FOR 2180 – tendon- and bone junctions” WE 4262/6-1

INTRODUCTION. In theory, Finite Element Analysis (FEA) is an attractive method for elucidating the mechanics of modular implant junctions, including variations in materials, designs, and modes of loading. However, the credence of any computational model can only be established through validation using experimental data. In this study we examine the validity of such a simulation validated by comparing values of interface motion predicted using FEA with values measured during experimental simulation of stair-climbing. MATERIALS and METHODS. Two finite element models (FEM) of a modular implant assembly were created for use in this study, consisting of a 36mm CoCr femoral head attached to a TiAlV rod with a 14/12 trunnion. Two head materials were modelled: CoCr alloy (118,706 10-noded tetrahedral elements), and alumina ceramic (124,710 10-noded tetrahedral elements). The quasi-static coefficients of friction (µ. s. ) of the CoCr-TiAlV and Ceramic-TiAlV interfaces were calculated from uniaxial assembly (2000N) and dis-assembly experiments performed in a mechanical testing machine (Bionix, MTS). Interface displacements during taper assembly and disassembly were measured using digital image correlation (DIC; Dantec Dynamics). The assembly process was also simulated using the computational model with the friction coefficient set to µ. s. and solved using the Siemens Nastran NX 11.0 Solver. The frictional conditions were then varied iteratively to find the value of µ providing the closest estimate to the experimental value of head displacement during assembly. To validate the FEA model, the relative motion between the head and the trunnion was measured during dynamic loading simulating stair-climbing. Each modular junction was assembled in a drop tower apparatus and then cyclically loaded from 230–4300N at 1 Hz for a total of 2,000 cycles. The applied load was oriented at 25° to the trunnion axis in the frontal plane and 10° in the sagittal plane. The displacement of the head relative to the trunnion during cyclic loading was measured by a three-camera digital image correlation (DIC) system. The same loading conditions were simulated using the FEA model using the optimal value of µ derived from the initial head assembly trials. RESULTS. For both head materials, the predicted values of axial displacement of the head on the trunnion closely approximated the measured values derived from DIC measurements, with differences of −0.17% to +6.5%, respectively. Larger differences were calculated for individual components of motion for the stair climbing activity. However, the predicted magnitude of interface motion was still within 10% of the observed values, ranging from −7% to −5%. CONCLUSIONS. Our simulations closely approximated physical testing using complex loading, coming within 7% of the target values. By generating a validated computational model of a modular junctions with varying head materials, we will be able to simulate additional activities of daily living to determine micromotion and areas of peak pressure and contact stresses generated. For any figures or tables, please contact authors directly

Introduction. Polyetheretherketone (PEEK) has been proposed as an implant material for femoral total knee arthroplasty (TKA) components. Potential clinical advantages of PEEK over standard cobalt chrome alloys include modulus of elasticity and subsequently reduced stress shielding potentially eliminating osteolysis, thermal conduction properties allowing for a more natural soft tissue environment, and reduced weight enabling quicker quadriceps recovery. Manufacturing advantages include reduced manufacturing and sterilization time, lower cost, and improved quality control. Currently, no PEEK TKA implants exist on the market. Therefore, evaluation of mechanical properties in a pre-clinical phase is required to minimize patient risk. The objectives of this study include evaluation of implant fixation and determination of the potential for reduced stress shielding using the PEEK femoral TKA component. Methods and Materials. Experimental and computational analysis was performed to evaluate the biomechanical response of the femoral component (Freedom Knee, Maxx Orthopedics Inc., Plymouth Meeting, PA; Figure 1). Fixation strength of CoCr and PEEK components was evaluated in pull-off tests of cemented femoral components on cellular polyurethane foam blocks (Sawbones, Vashon Island, WA). Subsequent testing investigated the cemented fixation using cadaveric distal femurs. The reconstructions were subjected to 500,000 cycles of the peak load occurring during a standardized gait cycle (ISO 14243-1). The change from CoCr to PEEK on implant fixation was studied through computational analysis of stress distributions in the cement, implant, and the cement-implant interface. Reconstructions were analyzed when subjected to standardized gait and demanding squat loads. To investigate potentially reduced stress shielding when using a PEEK component, paired cadaveric femurs were used to measure local bone strains using digital image correlation (DIC). First, standardized gait load was applied, then the left and right femurs were implanted with CoCr and PEEK components, respectively, and subjected to the same load. To verify the validity of the computational methodology, the intact and reconstructed femurs were replicated in FEA models, based on CT scans. Results. The cyclic load phase of the pull-off experiments revealed minimal migration for both CoCr and PEEK components, although after construct sectioning, debonding at the implant-cement interface was observed for the PEEK implants. During pull-off from Sawbones the ultimate failure load of the PEEK and CoCr components averaged 2552N and 3814N respectively. FEA simulations indicated that under more physiological loading, such as walking or squatting, the PEEK component had no increased risk of loss of fixation when compared to the CoCr component. Finally, the DIC experiments and FEA simulations confirmed closer resemblance of pre-operative strain distribution using the PEEK component. Discussion. The biomechanical consequences of changing implant material from CoCr to PEEK on implant fixation was studied using experimental and computational testing of cemented reconstructions. The results indicate that, although changes occur in implant fixation, the PEEK component had a fixation strength comparable to CoCr. The advantage of long term bone preservation, as the more compliant PEEK implant is able to better replicate the physiological loads occurring in the intact femur, may reduce stress shielding around the distal femur, a common clinical cause of TKA failure. For any figures or tables, please contact the authors directly

Introduction. Aseptic loosening of total knee replacements is a leading cause for revision. It is known that micromotion has an influence on the loosening of cemented implants though it is not yet well understood what the effect of repeated physiological loading has on the micromotion between implants and cement mantle. This study aims to investigate effect of physiological loading on the stability of tibial implants previously subjected to simulated intra-operative lipid/marrow infiltration. Methods. Three commercially available fixed bearing tibial implant designs were investigated in this study: ATTUNE. ®. , PFC SIGMA. ®. CoCr, ATTUNE. ®. S+. The implant designs were first prepared using a LMI implantation process. Following the method described by Maag et al tibial implants were cemented in a bone analog with 2 mL of bone marrow in the distal cavity and an additional reservoir of lipid adjacent to the posterior edge of the implant. The samples were subjected to intra- operative range of motion (ROM)/stability evaluation using an AMTI VIVO simulator, then a hyperextension activity until 15 minutes of cement cure time, and finally 3 additional ROM/stability evaluations were performed. Implant specific physiological loading was determined using telemetric tibial implant data from Orthoload and applying it to a validated FE lower limb model developed by the University of Denver. Two high demand activities were selected for the loading section of this study: step down (SD) and deep knee bend (DKB). Using the above model, 6 degree of freedom kinetics and kinematics for each activity was determined for each posterior stabilized implant design. Prior to loading, the 3-D motion between tibial implant and bone analog (micromotion) was measured using an ARAMIS Digital Image Correlation (DIC) system. Measurement was taken during the simulated DKB at 0.25Hz using an AMTI VIVO simulator while the DIC system captured images at a frame rate of 10Hz. The GOM software calculated the distance between reference point markers applied to the posterior implant and foam bone. A Matlab program calculated maximum micromotion within each DKB cycle and averaged that value across five cycles. The implant specific loading parameters were then applied to the three tibial implant designs. Using an AMTI VIVO simulator each sample was subjected to 50,000 DKB and 120,000 SD cycles at 0.8Hz in series; equating to approximately 2 years of physiological activity. Following loading, micromotion was measured using the same method as above. Results. Initial micomotion measurements during DKB activity for ATTUNE. ®. , PFC SIGMA. ®. CoCr, ATTUNE. ®. S+ were 155µm, 246µm, and 104µm, respectively, and following physiological loading were 159µm, 264µm, and 112µm, respectively. While there was statistical significance between the micromotion of implant designs (p<0.05), there was no significance between before and after loading. Conclusion. This study shows there is no significant change in micromotion after approximately 2 years of physiological loading. However, there is a significant difference in micromotion between implant designs

Introduction. Although cementless press-fit femoral total knee arthroplasty (TKA) components are routinely used in clinical practice, the effect of the interference fit on primary stability is still not well understood. Intuitively, one would expect that a thicker coating and a higher surface roughness lead to a superior fixation. However, during implant insertion, a thicker coating can introduce more damage to the underlying bone, which could adversely influence the primary fixation. Therefore, in the current study, the effect of coating thickness and roughness on primary stability was investigated by measuring the micromotions at the bone-implant interface with experimental testing. Methods. A previous experimental set-up was used to test 6 pairs of human cadaveric femurs (47–60 years, 5 females) implanted with two femoral component designs with either the standard e.motion (Total Knee System, B. Braun, Germany) interference fit of 350 µm (right femurs) or a novel, thicker interference fit of 700 µm (left femurs). The specimens were placed in a MTS machine (Figure 1) and subjected to the peak loads of normal gait (1960N) and squat (1935N), based on the Orthoload dataset for Average 75. Varus/valgus moments were incorporated by applying the loads at an offset relative to the center of the implants, leading to a physiological mediolateral load distribution. Under these loads, micromotions at the implant-bone interface were measured using Digital Image Correlation (DIC) at different regions of interest (ROIs – Figure 1). In addition, DIC was used to measure opening and closing of the implant-bone interface in the same ROIs. Results. After comparing the micromotions and opening of the two implant designs, we found no significant differences between the standard and novel coating. Loading was a significant factor for both opening (P<0.0001) and micromotions (P=0.019), where the squat produced higher micromotions than gait. Opening was seen anteriorly (MA, LA), and was higher during squat. Closing was noticed distally (MD, LD), particularly during gait (Figure 2). During gait (Figure 3), the highest micromotions were found in the posterior condyles (CM, MP), followed by the medial anterior region (MA). For squat, the largest micromotions were in the anterior flange (ANT), followed by the distal regions (LD, MD). Discussion. In the current study, the primary stability of the same implant with two different coating thicknesses was evaluated. The results demonstrate that increasing the coating thickness does not automatically influence the primary stability of a femoral TKA component. This is likely due to abrasion and damage of the underlying trabecular during implant insertion, which also was observed in previous experiments. The exact relation between coating thickness or interference fit and primary implant stability still remains subject to debate. Obviously, the primary implant stability is compromised when the interference fit is too low. However, the current results suggest that there is a threshold beyond which further improvement of the fixation is not possible. The exact magnitude of this threshold is unknown, and may depend on coating characteristics and bone quality, and requires further evaluation, possibly utilizing a hybrid approach of experimental and computational techniques

Several specimen specific vertebral (VB) models have been proposed in the literature; these replicate the typical set-up of a vertebral body mounted in bone cement and subject to a compressive ramp. VB and cement geometries are obtained from micro-CT images, the cement is typically assigned properties obtained from the literature while VB properties are inferred from the Hounsfield units- where the conversion factor between grayscale data and Young's modulus is optimised using experimental load-displacement data. Typically this calibration is performed on VBs dissected from the same spines as the study group. This, alongside the use of non-specific cement properties, casts some doubts on the predictivity of the models thus obtained. The predictivity of specimen specific FE models was evaluated in this study. VBs obtained from three porcine cervical segments (C2-C6) were stripped of all soft tissues, potted in bone cement and subject to a compressive loading ramp. A speckle pattern was applied to the anterior part of the specimen for DIC imaging. Specimen specific FE models were constructed from these specimens and a conversion factor between grayscale and material properties was optimised. Cement properties were assigned based on literature data. VBs from a further cervical spine (C2-C7) were subject to the same experimental protocol. In this case, the models generated from microCT images the material properties of bone were assigned based on the average conversion factor obtained previously. The predicted load-displacement behaviour thus obtained was compared to experimental data. Generally, poor agreement was found between overall load-displacement. The use of generic cement properties in the models was found to be partly responsible for this. When the load displacement behaviour of the VB was studied in isolation, good agreement within one standard deviation was found with 4 out of 6 models showing good correlation between simulation and DIC data

Digital image correlation (DIC) is rapidly increasing in popularity in biomechanical studies of the musculoskeletal system. DIC allows the re-construction of full field displacement and/or strain maps of the surface of an object. DIC systems typically consist of two cameras focussing on the same region of interest. This constrains the angle between the cameras to be relatively narrow when studying specimens characterised by complex geometrical features, giving rise to concerns on the accuracy of the out of plane estimates of movement. The aim of this research was to compare the movement profiles of bony segments measured by DIC and by an optoelectronic motion capture system. Five porcine cervical spine segments (C2-C6) were obtained from the local butcher. These were stripped of all anterior soft tissues while the posterior structures were left intact. A speckle pattern was applied to the anterior aspect of the specimens, while custom made infrared clusters were rigidly attached to the 3 middle vertebral bodies (C3-C5). The specimens were mounted in a custom made impact rig which fully constrained C6 but allowed C2 to translate in the axial direction of the segment. Images were acquired at 4kHz, both for the DIC (Photron Europe Ltd, UK) and motion capture cameras (Qualisys Oqus 400, Sweden). The in-plane and out of plane displacements of each of the VBs were plotted as a function of time and the similarity between the curves thus obtained was analysed using the SPM1D technique which allowed a comparison to be made in terms of t-statistics. No statistical differences were found between the two techniques in all axis of movement, however the out of plane movements were characterised by higher variance which is attributed to the uncertainty arising from the near parallel positioning of the cameras in the experimental set-up

Objectives

The ability to determine human bone stiffness is of clinical relevance in many fields, including bone quality assessment and orthopaedic prosthesis design. Stiffness can be measured using compression testing, an experimental technique commonly used to test bone specimens in vitro. This systematic review aims to determine how best to perform compression testing of human bone.

INTRODUCTION. Mechanical overloading of the knee can occur during activities of daily living such as stair climbing, jogging, etc. In this finite element study we aim to investigate which parameters could detrimentally influence peri-implant bone in the tibial reconstructed knee. Bone quality and patient variables are potential factors influencing knee overloading (Zimmerman 2016). METHODS. Finite element (FE) models of post-mortem retrieved tibial specimens (n=7) from a previous study (Zimmerman 2016) were created using image segmentation (Mimics Materialise v14) of CT scan data (0.6 mm voxel resolution). Tibial tray and polyethylene inserts were recreated from CT data and measurements of the specimens (Solidworks 2015). Specimens with varying implant geometry (keel/pegged) were chosen for this study. A cohesive layer between bone and cement was included to simulate the behavior of the bone–cement interface using experimentally obtained values. The FE models predict plasticity of bone according to Keyak (2005). Models were loaded to 10 body weight (BW) and then reduced to 1 BW to mimic experimental measurements. Axial FE bone strains at 1 BW were compared with experimental Digital Image Correlation (DIC) bone strains on cut sections of the specimens. After validation of the FE models using strain data, models were rotated and translated to the coordinate system defined in Bergmann (2014). Four loading cases were chosen – walking, descending stairs, sitting down and jogging. Element strains were written to file for post-processing. The bone in all FE models was divided into regions of equal thickness (10 mm) for comparison of strains. RESULTS. Results are shown for two specimens at present. Strain-maps of the specimen cut section compare reasonably well with FE cutting-plane strains. The FE models however show some regions of high strain in certain locations which do not correspond with the experimental results (Figure 1). Plasticity predicted by the models at 10 BW is shown in Figure 2. Median bone strains for two loading cases are shown as a function of distance below the tibial tray in Figure 3. This figure shows that specimen 1 is less likely to be overloaded during jogging when compared with specimen 2. Both specimens remain below the 7300 με threshold for compressive yield. DISCUSSION. Using functioning knee replacement tibial specimens, we study which factors influence bone overloading. Validation using DIC strain measurements is challenging due to the large plasticity regions predicted by the material model used here. The present results were obtained using plasticity relationships from Keyak (2005) for the proximal femur. To further improve on our results, plasticity-bone density relationships for the proximal tibia (Keyak 1996) will be included. Proximal tibial bone has been shown to be stiffer than femoral bone (Morgan 2003). Despite these limitations, FE models provide valuable information on the risk of overloading during daily living activities. The study will be expanded to include an analysis of implant geometry, bone quality and other loading cases. For any figures or tables, please contact the authors directly

Objectives

Unicompartmental knee arthroplasty (UKA) is a demanding procedure, with tibial component subsidence or pain from high tibial strain being potential causes of revision. The optimal position in terms of load transfer has not been documented for lateral UKA. Our aim was to determine the effect of tibial component position on proximal tibial strain.

INTRODUCTION. Stable fixation of cementless tibial trays remains a challenge due bone density variability within the proximal tibia and the spectrum of loads imposed by different activities. This study presents a novel approach to measuring the interface motion of cementless tibial components during functional loading and tests whether interface motion of cementless tibial trays varies around the implant periphery. METHODS. We developed a method to measure relative displacement of a tibial tray relative to the underlying bone using 3D digital image correlation (DIC) and multi-camera stereo photogrammetry. A clinically successful design of cementless total knee prosthesis (Zimmer Inc, Warsaw, IN) was implanted in 6 fresh cadaveric knees. A black-on-white stochastic pattern was applied to the outer surface of the tibia and the cementless prosthesis. High resolution digital images were prepared of the interface region and divided into 25 × 25 pixel regions of interest (ROI). Stereo images of the same ROI were generated using two cameras angled at 60 degrees using image correlation techniques. All specimens were mounted in a custom-built functional activity simulator and loaded with the forces and moments recorded during three common functional activities (standing from a seated position, walking, and stair descent), as reported in the Orthoload database, scaled by 50% for application to cadaveric bone. Prior to functional testing, each implant-tibia construct was preconditioned with 500 cycles of flexion from 5–100 degrees under a vertical tibial load of 1050 N at a frequency of 0.2 Hz. During loading, image data was acquired simultaneously (±20 μs) from the entire circumference of the tibial interface forming 4 stereo images using 8 cameras spaced at 90 degree intervals (Allied Vision Technologies, Exton, PA) using custom image acquisition software (Mathworks, Natick, MA) (Figure 1). The multiple stereo images were registered using the surface topography of each specimen as measured by laser scanning (FARO Inc., Montreal) (Figure 2). During post-processing, the circumferential tray/tibia interface was divided into 10 zones for subsequent analysis (Figure 3). Interface displacements were measured on a point-to-point basis at approximately 700 sites on each specimen using commercial DIC software (Dantec Dynamics, Skovlunde, Denmark) (Figure 4). RESULTS. The average 3D displacement over 10 circumferential zones of the tray was 83.6±41.5 μm (range: 30.8 to 214.9 μm). The anatomic components of tray migration were 0.4±40.8 μm medially (range: 172 μm lateral to 112 μm medial) and 3.1± 40.6 μm posteriorly (range: 86 μm posteriorly to 61 μm anteriorly). The largest tray displacement was observed in the inferior direction with an average inferior displacement of 37.6±63.8 μm (range: 206 μm inferiorly to 81μm superiorly). The largest displacements were observed posteriorly, with the posteromedial aspect subsiding more the posterolateral aspect. DISCUSSION. The stability of tibial trays cannot be accurately assessed by measuring interface motion at a few fixed peripheral sites. If discrete displacement transducers are used for pre-clinical testing, a set of 4–6 transducers should be placed at sites that vary with the pattern of interface motion of each design and the combination of loads and moments applied during testing

The causes of spine disease are often biomechanical ones (e.g. disc degeneration, vertebral fracture). Currently, a deep investigation of the spine biomechanics is missing, due to the high complexity of the spine system (Fung 1980, Brandolini, Cristofolini et al. 2014): vertebrae and intervertebral discs. Recently, the Digital Image Correlation allowed measuring in vitrothe displacement and strain on the surface of soft and hard tissues, upon a specific non-invasive preparation of their surface with a speckle pattern (Palanca, Tozzi et al. 2016). The aim of this explorative work was to evaluate the deformation on spine segments, being able to distinguish between hard and soft tissue in the elastic regime and up to fracture. Segment of four vertebrae were extracted from porcine spines. All ligaments and muscles were removed, without damaging the spine segment (vertebrae and intervertebral discs). A suitable non-conventional white-on-black speckle pattern was prepared on the surface with airbrush airgun to track the movements of the specimen with DIC (Lionello, Sirieix et al. 2014). The endplates of the extreme vertebrae were potted in poly-methyl-methacrylate. The spine segments were tested in pure axial loading with cycles of increasing magnitude, up to fialure. A commercial 3D-DIC (Dantec Dynamics, Denmark) was used. In the present configuration, it allowed a resolution of 30 micrometers. It was used to measure the displacements and strains in a full-field and contactless way on the frontal surface of the spine segments. DIC allowed measuring with success the displacement and strain during the entire test, in the elastic regime and up to failure. The displacements and strains could be measured on the entire specimen, both in the vertebrae (hard tissue) and in the intervertebral discs (soft tissue). The axial strain evaluated prior to failure was close to 10’000 microstrain on the vertebral body surface and exceed 70’000 microstrain on the intervertebral discs, where failure was localized. The pattern, prepared in a dedicated way showed its suitability for both the bone and the disc. The evaluated failure strains were in agreement with the literature (Bayraktar, Morgan et al. 2004) (Spera, Genovese et al. 2011). To the authors' best knowledge, this kind of measurement including strain on soft and hard tissue simultaneously has never been performed before. This work showed the capability of DIC in providing full-field measures on the surface with complex geometry, such as the spine. The assertion of these potentialities could open the way to further application of DIC to study the behaviour of human spines, including improvement of spinal fixation devices

Objectives. Up to 40% of unicompartmental knee arthroplasty (UKA) revisions are performed for unexplained pain which may be caused by elevated proximal tibial bone strain. This study investigates the effect of tibial component metal backing and polyethylene thickness on bone strain in a cemented fixed-bearing medial UKA using a finite element model (FEM) validated experimentally by digital image correlation (DIC) and acoustic emission (AE). Materials and Methods. A total of ten composite tibias implanted with all-polyethylene (AP) and metal-backed (MB) tibial components were loaded to 2500 N. Cortical strain was measured using DIC and cancellous microdamage using AE. FEMs were created and validated and polyethylene thickness varied from 6 mm to 10 mm. The volume of cancellous bone exposed to < -3000 µε (pathological loading) and < -7000 µε (yield point) minimum principal (compressive) microstrain and > 3000 µε and > 7000 µε maximum principal (tensile) microstrain was computed. Results. Experimental AE data and the FEM volume of cancellous bone with compressive strain < -3000 µε correlated strongly: R = 0.947, R. 2. = 0.847, percentage error 12.5% (p < 0.001). DIC and FEM data correlated: R = 0.838, R. 2. = 0.702, percentage error 4.5% (p < 0.001). FEM strain patterns included MB lateral edge concentrations; AP concentrations at keel, peg and at the region of load application. Cancellous strains were higher in AP implants at all loads: 2.2- (10 mm) to 3.2-times (6 mm) the volume of cancellous bone compressively strained < -7000 µε. Conclusion. AP tibial components display greater volumes of pathologically overstrained cancellous bone than MB implants of the same geometry. Increasing AP thickness does not overcome these pathological forces and comes at the cost of greater bone resection. Cite this article: C. E. H. Scott, M. J. Eaton, R. W. Nutton, F. A. Wade, S. L. Evans, P. Pankaj. Metal-backed versus all-polyethylene unicompartmental knee arthroplasty: Proximal tibial strain in an experimentally validated finite element model. Bone Joint Res 2017;6:22–30. DOI:10.1302/2046-3758.61.BJR-2016-0142.R1

Osteoarthritis is one of the most common musculoskeletal diseases. It involves degeneration and loss of articular cartilage, leading to a painful bone on bone articulation during movement. Numerical FEA models exist to predict the mechanical behaviour of degenerated cartilage. One of the limitations of these models arises from the poor validation that can be attained with traditional experimental data. This typically relies on comparison with global mechanical quantities such as total tissue strain, which mask the individual contributions originating from the different layers. In order to improve on this, an experimental method was developed to visualise the through-thickness behaviour of articular cartilage. Four experiments were performed on hemi-cylindrical cartilage plugs, harvested from a porcine femoral head, and immersed in a fluid solution. An Indian ink speckle pattern was applied to the flat surface of each hemi-cylinder. The specimens were equilibrated in 2.5M NaCl solution, transferred to a custom designed testing rig, and a reference image of the tissue cross-section was taken. The solution concentration was then decreased to 0.15M and, predictably, the tissue thickness changed. Images of the tissue cross section were taken every 60s for the duration of the experiment (3600s). All images were analysed using a DIC algorithm (Ncorr open-source 2D digital image correlation matlab program), and documented the strain changes through the tissue thickness as a function of time. The measured total strain in the tissue was consistent with that reported by Lai et al. (1991). However the present technique allows to quantify the strain contribution from any of the tissue layers or sublayer. This poses a significant advantage over traditional methods as resulting information can further the understanding of the factors contributing to the mechanical behaviour of the tissue and provides an ideal platform for validating more and more refined models of tissue behaviour