Aim: This study investigated the difference in proximal tibial cortical strain distribution using a fixed or mobile bearing design for TKA. Methods: Eight fresh frozen human cadaver tibias were used. The strain magnitude and distribution on the anterior cortex of the proximal tibia during axial and rotational loading of the knee were measured with a quantitative full-field strain measurement technique (Electronic Speckle Pattern Interferometry). First, strain distributions of the intact knee were acquired. Subsequently, strain distributions after implementation of conventional and mobile bearing PCL retaining total knee implants (Scorpio®) were measured. Results: Under each loading condition, the minimum principal strain was greater in magnitude as compared to the maximum principal strain. Under 1′500 N axial loading, the resulting minimum principal strain magnitude and orientation was nearly identical between the mobile bearing configuration (500 ± 287 με), and the fixed bearing configuration (500 ± 286 μ ε). In response to 10° internal rotation, this strain increased to 782 ± 371 μ ε and 1000 ± 389 μ ε for the mobile and fixed tibial component, respectively. In response to 10° external rotation, minimal principal strain decreased to 421 ± 233 μ ε for the mobile bearing, but increased to 632 ± 293 μ ε for the fixed bearing. These differences between mobile and fixed bearing scenarios were statistically highly significant. Conclusion: For this in-vitro study under exact controlled loading conditions the mobile bearing design induced less strain in the proximal tibia as the fixed bearing tibial component. The difference in strain levels may be of importance to understand bone remodeling and osseointegration

INTRODUCTION:. The purpose of this study was to determine if a short femoral stem (Lima Corporate, Udine, Italy) would result in a strain distribution which mimicked the intact bone better than a traditional length stem, thereby eliminating the potential for stress-shielding. METHODS:. A 2 mm thick moldable plastic (PL-1, Vishay Micromeasurements, Raleigh, NC) was contoured to six fourth-generation composite femoral bones (Pacific Research Laboratories, Vashon, WA). The intact femurs were then loaded (82 kg) in a rig which simulated mid-stance single limb support phase of gait (Figure 1). During testing, the femurs were viewed and video recorded through a model 031 reflection polariscope. Observing the photoelastic coating through the polariscope, a series of fringes could be seen, which represented the difference in principal strain along the femur. The fringes were quantified using Fringe Order, N, as per the manufacturers technical notes. In order to analyze the strain distribution, the femur was separated into 6 zones, 3 lateral and 3 medial, and the maximum fringe order determined. Upon completion of testing of the intact femur, the short length femoral stem was inserted and tested, and finally the traditional length femoral stem was inserted and tested. Anterior and lateral radiographs were obtained of the femur with each femoral stem in order to confirm proper alignment. RESULTS:. Fringes formed in a similar pattern for all femurs, intact and with stems. The fringes first occurred medially and laterally in a proximal-distal direction and radiated outward, decreasing in fringe order, toward the neutral axis of bending (anterior and posterior). The magnitude of the fringe order, N, remained the same or increased in the proximal to distal direction. This became more prominent, particularly on the lateral side, with the traditional length femoral stem, when a distal migration of the fringes was seen compared to the intact femur (Figures 2 and 3). Medially, with the traditional length femoral stem, the fringes remained but were of a lower magnitude than the intact femur. The femoral strain pattern, resulting from implantation of the short length femoral stem, was found to closely match the intact femur. X-rays confirmed proper alignment of all implants. CONCLUSIONS:. The distal migration of strain seen with the traditional length femoral stem was indicative of potential stress shielding. As an alternative, this study suggests that the short length femoral stem most closely replicates the strain distribution of the intact femur and may limit this type of failure

This study investigated the difference in proximal tibial cortical strain distribution using a fixed or mobile bearing design for TKA. Eight fresh frozen human cadaver tibias were used. The strain magnitude and distribution on the anterior cortex of the proximal tibia during axial and rotational loading of the knee were measured with a quantitative full-field strain measurement technique (Electronic Speckle Pattern Interferometry). First, strain distributions of the intact knee were acquired. Subsequently, strain distributions after implantation of conventional and mobile bearing PCL retaining total knee implants (Scorpio®) were measured. Under each loading condition, the minimum principal strain was greater in magnitude as compared to the maximum principal strain. Under 1,500 N axial loading, the resulting minimum principal strain magnitude and orientation was nearly identical between the mobile bearing configuration(500 ± 287m;e;), and the fixed bearing configuration (500 ± 286m;e;). In response to 10° internal rotation, this strain increased to 782 ± 371m;e; and 1000± 389m;e; for the mobile and fixed tibial component, respectively. In 10° external rotation, minimal principal strain decreased to 421 ± 233m;e; for the mobile bearing, but increased to 632 ± 293m;e; for the fixed bearing. These differences between mobile and fixed bearing scenarios were highly statistically significant. For this in-vitro study under exact controlled loading conditions the mobile bearing design induced less strain in the proximal tibia than the fixed bearing tibial component. The difference in strain levels may be of importance for bone remodeling and osseointegration

Total shoulder arthroplasty is a well-tested procedure that offers pain relief and restores the joint function. However, failure rate is still high, and glenoid loosening is pointed as the main reason in orthopedic registers. In order to understand the principles of failure, the principal strain distributions after implantation with Comprehensive® Total Shoulder System of Biomet® were experimental and numerically studied to predict bone behavior. Fourth generation composite left humerus and scapula from Sawbones® were used. These were implanted with Comprehensive® Total Shoulder System (Biomet®) with a modular Hybrid® glenoid base and Regenerex® glenoid and placed in situ by an experienced surgeon. The structures were placed in order to simulate 90º abduction, including principal muscular actions. Muscle forces used were as follows: Deltoideus 300N, Infraspinatus 120N, Supraspinatus 90N, Subscapularis 225N. All bone structures were modeled considering cortical and the trabecular bone of the scapula. The components of prosthesis were placed in the same positions than those in the in vitro models. Geometries were meshed with tetrahedral linear elements, with material properties as follows: Elastic modulus of cortical bone equal to 16 GPa, elastic modulus of trabecular bone equal to 0.155 GPa, polyethylene equal to 1GPa and titanium equal to 110 GPa. The assumed Poisson's ratio was 0.3 in all except for polyethylene where we assumed a value of 0.4. The prosthesis was considered as glued to the adjacent bone. The finite element model was composed of 336 024 elements. At the glenoid cavity, the major influence of the strain distributions was observed at the posterior-superior region, in both cortical and trabecular bone structures. The system presents critical region around holes of fixation in glenoid component. At the trabecular bone, the maximum principal strains at the posterior-superior region ranged from 2250 µε to 3000 µε. While at the cortical bone, the maximum principal strains were 300 µε to 400 µε. The results observed evidence some critical regions of concern and the effect of implant in the bone strains mainly at the posterior-superior region of the glenoid cavity is pronounced. This indicates that this region is more affected by the implant if bone remodeling is a concern and it is due to the strain-shielding effect, which has been connected with loosening of the glenoid component

Summary Statement. An MRI-derived subject-specific finite element model of a knee joint was loaded with subject-specific kinetic data to investigate stress and strain distribution in knee cartilage during the stance phase of gait in-vivo. Introduction. Finite element analysis (FEA) has been widely used to predict the local stress and strain distribution at the tibiofemoral joint to study the effects of ligament injury, meniscus injury and cartilage defects on soft tissue loading under different loading conditions. Previous studies have focused on static FEA of the tibiofemoral joint, with few attempts to conduct subject-specific FEA on the knee during physical activity. In one FEA study utilising subject-specific loading during gait, the knee was simplified by using linear springs to represent ligaments. To address the gap that no studies have performed subject-specific FEA at the tibiofemoral joint with detailed structures, the present study aims to develop a highly detailed subject-specific FE model of knee joint to precisely simulate the stress distribution at knee cartilage during the stance phase of the gait cycle. Method. A detailed three-dimensional model of a healthy human knee was developed from MRI images of a living subject, including the main anatomical structures (bones, all principal ligaments, menisci and articular cartilages). The femur, tibia and fibula were considered as rigid bodies, while the menisci and articular cartilage were modelled as linearly elastic, isotropic and homogeneous while the ligaments were considered to be hyperelastic. Loading and boundary condition assignment was based on the kinematic and kinetic data recorded during gait analysis. Ten time intervals during the stance phase of gait were separately simulated to quantify the time–dependent stress distribution throughout the cycle from heel-strike to toe-off. Loading condition of the tibiofemoral joint varys during the gait cycle since the joint angle changes from extension to flextion, therefore different joint angles at relative time interval were determined to accurately simulate the varing loading condition. Results. The compressive stress and tensile strain distributions in the femoral cartilage, tibia cartilage and menisci of each selected time interval during the stance phase of gait cycle were quantified and corresponded to specific amount of varus/valgus knee moment obtained by inverse dynamics analysis of the kinematic and kinetic data from gait analysis. Therefore a correlation between stress/strain and the frontal movement was established and analysed. For example, at 10% of stance phase, the stress concentration was observed on the lateral compartment due to the valgus moment created at heel strike. At the next interval, the stress concentration shifted to the medial side as the frontal knee moment shifted to a varus orientation. Discussion. The results suggest that the stress distribution of tibiofemoral articular cartilage is qualitatively consistent with the valgus and varus moment observed during the stance phase of gait. The methods described could be applied to investigate the effects of injury and reconstruction on stress distribution within the tibiofemoral joint

Objective: To examine the effect of varying the thickness of the cement mantle on the strain distribution near the bone-cement interface. Background: An insufficient cement mantle is thought to generate cement fractures near the bone-cement interface. Debonding at the bone-cement interface may accompany such fractures, and, mechanical failure of the prosthesis may follow. In this study, we aim to analyse the relationship between the cement mantle thickness and the acetabular strain distribution near the bone-cement interface. Experimental model: Four hemi-pelvic saw bones specimens were implanted with six protected precision strain gauges. All specimens were prepared to receive a 53/28 cemented polyethylene cup (Depuy Charnley Elite). Methods: We simulated hip joint force relative to the cup during normal walking for quasi-static tests on an Instron 1603 testing machine. The magnitude of the maximum and minimum principal strains, and the orientation of the maximum principal strains were calculated based on the readings of strains from a 32 channel digital acquisition system. Results: Statistically significant differences in the total strains per gait cycle (p< 0.001) have been noted at all gauge locations. In the principal load bearing quadrants, the recorded tensile strains are reduced by 50% as a result of the thicker mantle, while the transmission of compressive strain is enhanced. Conclusion: A cement mantle thickness of 5-6mm may preserve the structural integrity of the principal load bearing quadrants of the acetabulum better than a mantle thickness of 2-3mm, by minimising the acetabu-lar strains. This maybe desirable in total hip replacements for conditions such as rheumatoid arthritis and osteoporosis, where the poorer quality bone can be assisted by recruitment of a larger surface area to participate in load bearing. Keywords: Principal strains; Cement mantle; Mantle thickness; Bone-cement interface; Acetabular strains

Objective: To develop in-vitro experiments that measure the strain distributions at the bone-implant and bone-cement interface of the acetabular region under physiological loading conditions for cemented and cementless sockets. Experimental model: Four hemi-pelvic specimens of saw bones were used. Following careful placement of six protected precision strain gauges, two specimens were prepared to receive a cemented polyethylene cup (Depuy Charnley Elite 53/28). Another two specimens were prepared and implanted with un-cemented Duraloc 58/28 cups. Press-fit technique was validated by torque measurements. Background: Symptoms associated with prosthetic migration result from osteoclast induced bone resorption at the interface adjacent to bone. We aim to develop a new and more accurate method of measuring strains at this critical interface. Methods: To simulate quasi-static loading, selected variables of hip joint force relative to the cup during normal walking was used for quasi-static tests on an Instron 1603 testing machine. The magnitude and orientation of the principal strains (maximum and minimum) were calculated based on the readings of strains from a 32 channel digital acquisition system. Results: The magnitude and distribution of acetabular trabecular bone strains are dependent on the type of cup material (un-cemented/cemented) implanted. At the position of maximum load, the maximum principal strain in the un-cemented specimens was 14.4 times higher than that for the cemented specimens (T-value = −96.40, P-value = 0.007). The highest recorded tensile strains in these specimens were localised to the acetabular rim of the posterior-superior quadrant. For the cemented specimens, the maximum principal strains are highest in the dorsal acetabulum, at a location that approximates to the centre of rotation of the replaced hip joint. Shear strains in the posterior-superior quadrant of both cementless and cemented acetabuli surpass the maximum principal strains. Conclusion: In both cemented and un-cemented specimens, the maximum shear and principal strains magnitude show similar spatial and statistical distribution. As indicators of local failure prospect within the acetabulum, these strains suggest that the posterior-superior quadrant is the most likely site for load-induced micro-fractures, in both cemented and cementless acetabuli

Background: It is thought that the forces transmitted across the hip joint produce migration of the prosthesis by failure at either the bone-cement or the prosthesis-cement interface. As symptoms associated with such motions often result from failure at the cement-bone interface, it is this interface and its sub-surfaces that are the critical areas of prosthesis loosening. Our aim is to produce a new and more accurate method of measuring strains at this critical interface. Objective: To develop in-vitro experiments to measure the strain distributions near the bone-cement interface of the acetabulum region under physiological, quasi-static loading conditions. Experimental Model: Two hemi-pelvic specimens of saw bones were used. Following careful placement of six protected precision strain gauges (4.6 x 6.4mm, tri-axial EA-13-031RB-120/E). One specimen was prepared to receive a cemented polyethylene cup (Depuy Charnley Ogee LPW 53/22). An uncemented 58mm Duraloc cup was implanted into a second specimen. Methods: Hip joint force relative to the cup during normal walking (Bergmann, G., 2001. HIP98) was used for quasi-static tests on a Llody LR30K loading machine. The magnitude of the maximum and minimum principal strains, and the orientation of the maximum principal strains were calculated from a 32 channel digital acquisition system. Results: For both specimens, the maximum principal strains at the maximum loading were highest in the medial wall (dome area) of the acetabulum. The tensile strain from the dome of the uncemented specimen at the maximum loading was twice that of the cemented specimen. In the cemented specimen, the compressive strains in the medial wall were almost twice the tensile strains at the maximum load. Within the acetabular quadrants, the highest strains were recorded in the posterio-inferior quadrant. Compressive strains in the posterio-inferior wall of the acetabulum seem to be comparable to those in the anterior-superior wall. Conclusion: The critical areas for load transfer in the acetabulum are the medial wall (dome area), the posterio-inferior and the anterior-superior quadrants. The uncemented cup appears to provide a better load transfer mechanism than the cemented cup

Introduction. A new conservative hip stem has been designed to address the complex problem of total hip arthroplasty in the younger population. Objectives. To assess the stability and strain distribution of a new conservative hip stem. Materials and Methods. The prosthesis is tapered and collared and made from titanium (Ti6Al4V) with a titanium porous plasma spray to encourage bony ingrowth (Figure.1). It is circular-trapezoidal in cross-section to provide optimal ‘fit and fill’ in the femoral neck. (i) Finite Element Analysis (FEA). Computed tomography scans of an intact femur were modelled using MARC software and consisted of 161390 elements and 174881 nodes. The implant was modelled (Unigraphics) as a titanium alloy stem with a cobalt-chrome alloy head and consisted of 93440 hexahedral elements and 101133 nodes. This study compared the strains in the femoral calcar of an intact femur with a stem ‘implanted’ in neck shaft angles of 125°, 135°, and 145°. The head of all models received a load of 2.3KN at 7 degrees medially. (ii) Photoelastic Coating. A photoelastic coating was moulded around the medial cortices of ten third generation femora Sawbones. Strain before and after prosthesis insertion was measured at one-centimetre intervals down the medial cortex of the bones using a polariscope. The bones were positioned in a simplified single leg stance (7° physiological alignment), and loaded at 2.3 KN with strain recorded. (iii) Linear Variable Differential Transducers (LVDT's). Micromotion and migration of the prosthesis was measured using LVDT's. The femoral heads were cyclically loaded with 2.3KN at 1Hz for 2,500 cycles and held in a single leg stance. The bones were then repositioned at 70° of flexion to produce torsional (stair climbing) forces and loaded with 0.5KN for 2,500 cycles. Statistical analysis of non-parametric data was performed using a two-tailed Wilcox signed rank test (p<0.05). Results. The FEA analysis revealed strains in the neutral position most closely resembled that of an intact femur (Figure.2). Photoelastic strain readings for intact bone and following insertion were paired and statistically analysed using the Wilcox signed rank test (two tailed). The composite bones with prostheses inserted at 125° and 145° demonstrated a significant difference to the intact bones, whereas those at 135° showed no significant difference in the surface strain pattern of the femur following prosthetic insertion (Figure.3). Under single leg stance loading all prostheses produced axial micromotion of less than 200 µm and 50 µm in the varus-valgus direction. Implants inserted at 135° and 125° produced the least micromotion, the implants inserted at 145° had the greatest magnitude of motion and may be more susceptible to loosening. Under torsional load the same was true with the 135° and 125° producing the least micromotion while with the angulation of 145° micromotion increased over the test period – again suggesting loosening. Conclusion. This design transfers load in a physiological manner and the prosthesis is most stable in the neutral position. The findings from this study have been translated into clinical practice with the prosthesis implanted into two patients with promising results

Introduction. Traditional applied loading of the knee joint in experimental testing of RTKR components is usually confined to replicating the tibiofemoral joint alone. The second joint in the knee, the patellofemoral joint, can experience forces of up to 9.7 times body weight during normal daily living activities (Schindler and Scott 2011). It follows that with such high forces being transferred, particularly in high flexion situations such as stair climbing, it may be important to also represent the patellofemoral joint in all knee component testing. This research aimed to assess the inclusion of the patellofemoral joint during in vitro testing of RTKR components by comparing tibial strain distribution in two experimental rigs. The first rig included the traditional tibiofemoral joint loading design. The second rig incorporated a combination of both joints to more accurately replicate physiological loading. Five implanted tibia specimens were tested on both rigs following the application of strain gauge rosettes to provide cortical strain data through the bone as an indication of the load transfer pattern. This investigation aimed to highlight the importance of the applied loading technique for pre-clinical testing and research of knee replacement components to guide future design and improve patient outcomes. Methods. Five composite tibias (4th Generation Sawbones) were prepared with strain gauge rosettes (HBM), correctly aligned and potted using guides for repeatability across specimens. The tibias were then implanted with Stryker Triathlon components according to surgical protocol. The first experimental rig was developed to replicate traditional knee loading conditions through the tibiofemoral joint in isolation. The second experimental rig produced an innovative method of replicating a combination of the tibiofemoral and patellofemoral joint loading scenarios. Both rigs were used to assess the load distribution through the tibia using the same tibia specimens and test parameters for comparison integrity (Figure 1). The cortical strains were recorded under an equivalent 500 N cyclical load applied at 10° of flexion by a hydraulic test machine. Results. The average results comparing both experimental rigs at three strain gauge locations are shown in Figure 2. Paired t-tests were performed on all results and a p value of p<0.05 was considered significant. No significant differences were found between the rigs. There was a trend towards a reduction in proximal principal strain with the inclusion of the patellofemoral joint (p=0.058). Discussion. The results of this study indicate that there is no significant difference in tibial load transfer between the traditional and novel applied loading techniques at small flexion angles. There is a trend towards a reduction in proximal strain when including the patellofemoral joint. This reduction may be linked to the patella tendon force counteracting the effect of tibiofemoral loading at this small flexion angle. At high flexion angles the patellofemoral reaction load increases significantly relative to the tibiofemoral load. This will have a significant effect on tibial strains and so it is recommended that testing at higher flexion angles should be performed in a combined loading rig

Aims

Vertebrates have adapted to life on Earth and its constant gravitational field, which exerts load on the body and influences the structure and function of tissues. While the effects of microgravity on muscle and bone homeostasis are well described, with sarcopenia and osteoporosis observed in astronauts returning from space, the effects of shorter exposures to increased gravitational fields are less well characterized. We aimed to test how hypergravity affects early cartilage and skeletal development in a zebrafish model.

Six pairs of human cadaver femora were divided equally into two groups one of which received a non-cemented reference implant and the other a very short non-dependent experimental implant. Thirteen strain-gauge rosettes were attached to the external surface of each specimen and, during application of combined axial and torsional loads to the femoral head, the strains in both groups were measured.

After the insertion of a non-cemented femoral component, the normal pattern of a progressive proximal-to-distal increase in strains was similar to that in the intact femur and the strain was maximum near the tip of the prosthesis. On the medial and lateral aspects of the proximal femur, the strains were greatly reduced after implantation of both types of implant. The pattern and magnitude of the strains, however, were closer to those in the intact femur after insertion of the experimental stem than in the reference stem. On the anterior and posterior aspects of the femur, implantation of both types of stem led to increased principal strains E1, E2 and E3. This was most pronounced for the experimental stem.

Our findings suggest that the experimental stem, which has a more anatomical proximal fit without having a distal stem and cortex contact, can provide immediate postoperative stability. Pure proximal loading by the experimental stem in the metaphysis, reduction of excessive bending stiffness of the stem by tapering and the absence of contact between the stem and the distal cortex may reduce stress shielding, bone resorption and thigh pain.

Hydroxyapatite-coated standard anatomical and customised femoral stems are designed to transmit load to the metaphyseal part of the proximal femur in order to avoid stress shielding and to reduce resorption of bone. In a randomised in vitro study, we compared the changes in the pattern of cortical strain after the insertion of hydroxyapatite-coated standard anatomical and customised stems in 12 pairs of human cadaver femora. A hip simulator reproduced the physiological loads on the proximal femur in single-leg stance and stair-climbing. The cortical strains were measured before and after the insertion of the stems.

Significantly higher strain shielding was seen in Gruen zones 7, 6, 5, 3 and 2 after the insertion of the anatomical stem compared with the customised stem. For the anatomical stem, the hoop strains on the femur also indicated that the load was transferred to the cortical bone at the lower metaphyseal or upper diaphyseal part of the proximal femur.

The customised stem induced a strain pattern more similar to that of the intact femur than the standard, anatomical stem.

We implanted titanium and carbon fibre-reinforced plastic (CFRP) femoral prostheses of the same dimensions into five prosthetic femora. An abductor jig was attached and a 1 kN load applied. This was repeated with five control femora. Digital image correlation was used to give a detailed two-dimensional strain map of the medial cortex of the proximal femur. Both implants caused stress shielding around the calcar. Distally, the titanium implant showed stress shielding, whereas the CFRP prosthesis did not produce a strain pattern which was statistically different from the controls. There was a reduction in strain beyond the tip of both the implants.

This investigation indicates that use of the CFRP stem should avoid stress shielding in total hip replacement.

In vitro loading of the proximal femur has improved our understanding of stress shielding after total hip arthroplasty. However, previous load simulators often use simplified loading regimens that may not produce physiologic baseline strains. The purpose of this study was to compare the femoral strain levels produced when using simplified and more complex loading.

A mechanical load simulator was developed which could simultaneously apply a spinal load and nine of eleven available muscle loads to the proximal femur in heelstrike and stair climbing modes. Computer controlled electromechanical actuators were attached to a strain gauged fresh cadaver femur (donor body weight 39 6kg) with metal cables. A spinal load of 668 N (SPL) was applied alone and in combination with individual muscle loads of 267 N to determine the effect of each muscle on femoral strain. The magnitude and direction of the joint reaction force (JRF) was monitored in real time by a three-dimensional force transducer proximal to a metal acetabulum. Anterior, middle and posterior portions of the gluteus medius (ABD), iliotibial band (ITB), short external rotators (SER), vastus lateralis, adductors, rectus femoris, hamstrings, iliopsoas, and gluteus maximus were simulated.

SPL was applied and ABD and ITB were adjusted to produce a JRF magnitude of 2.0 BW. SPL was applied with two combinations of nine muscle loads adjusted in heelstrike mode to produce a JRF magnitude of 2.0 and 2.5 BW and JRF trajectory aligned within one degree of the radiographically determined compression trabecular stream axis.

Both nine-muscle combinations produced lower medial compression strains and substantially lower lateral tension strains than SPL+ABD+ITB in heelstrike and stair climbing. Simplified loading caused a bending moment in the proximal femur resulting in higher strains. Combined loading at 2.5 BW produced compression at 10 of 12 gauges in heelstrike mode and 9 of 12 gauges in stairclimbing.

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. Vertebral compression fractures are the most common type of osteoporotic fracture. Though 89% of clinical fractures occur anteriorly, it is challenging to replicate these ex vivo with the underlying intervertebral discs (IVDs) present. Furthermore, the role of disc degeneration in this mechanism is poorly understood. Understanding how disc morphology alters vertebral strain distributions may lead to the utilisation of IVD metrics in fracture prediction, or inform surgical decision-making regarding instrumentation type and placement. Aim. To determine the effect of disc degeneration on the vertebral trabecular bone strain distributions in axial compression and flexion loading. Methods. Eight cadaveric thoracolumbar segments (T11-L3) were prepared (N=4 axial compression, N=4 flexion). µCT-based digital volume correlation was used to quantify trabecular strains. A bespoke loading device fixed specimens at the resultant displacement when loaded to 50N and 800N. Flexion was achieved by adding 6° wedges. Disc degeneration was quantified with Pfirrmann grading and T2 relaxation times. Results. Anterior axial strains were 80.9±39% higher than the posterior region in flexion (p<0.01), the ratio of which was correlated with T2 relaxation time (R. 2. =0.80, p<0.05). In flexion, the central-to-peripheral axial strain ratio in the endplate region was significantly higher when the underlying IVDs were non-degenerated relative to degenerated (+38.1±12%, p<0.05). No significant differences were observed in axial compression. Conclusion. Disc degeneration is a stronger determinant of the trabecular strain distribution when flexion is applied. Load transfer through non-degenerate IVDs under flexion appears to be more centralised, suggesting that disc degeneration predisposes flexion-type compression fractures by shifting high strains anteriorly. Conflicts of interest. The authors declare none. Sources of funding. This work was funded by the Engineering & Physical Sciences Research Council (EP/V029452/1), and Back-to-Back

Aims. Periprosthetic hip fractures (PPFs) after total hip arthroplasty are difficult to treat. Therefore, it is important to identify modifiable risk factors such as stem selection to reduce the occurrence of PPFs. This study aimed to clarify differences in fracture torque, surface strain, and fracture type analysis between three different types of cemented stems. Methods. We conducted biomechanical testing of bone analogues using six cemented stems of three different types: collarless polished tapered (CPT) stem, Versys Advocate (Versys) stem, and Charnley-Marcel-Kerboull (CMK) stem. Experienced surgeons implanted each of these types of stems into six bone analogues, and the analogues were compressed and internally rotated until failure. Torque to fracture and fracture type were recorded. We also measured surface strain distribution using triaxial rosettes. Results. There was a significant difference in fracture torque between the three stem types (p = 0.036). Particularly, the median fracture torque for the CPT stem was significantly lower than that for the CMK stem (CPT vs CMK: 164.5 Nm vs 200.5 Nm; p = 0.046). The strain values for the CPT stem were higher than those for the other two stems at the most proximal site. The fracture pattern of the CPT and Versys stems was Vancouver type B, whereas that of the CMK stem was type C. Conclusion. Our study suggested that the cobalt-chromium alloy material, polished surface finish, acute-square proximal form, and the absence of a collar may be associated with lower fracture torque, which may be related to PPF. Cite this article: Bone Joint Res 2022;11(5):270–277

Introduction and Objective. The patients with a total hip arthroplasty is growing in world manly in Europe and USA, and this solution present a high success at 10years in several orthopaedic registers. The application of total press-fit hip fixation presents the most used solution, but presents some failures associated to the acetabular component fixation, associated to the load transfer and bone loss at long term. The aim of this work is to investigate the influence of different acetabular bone loss in the strain distribution in iliac bone. To evaluate implant fixation, an experimental study was performed using acetabular press-fit component simulating different acetabular bone loss and measuring the strain distribution. Materials and Methods. The experimental samples developed was based in an iliac bone model of Sawbones supplier and a acetabular component Titanium (Stryker) in a condition press-fit fixation and was implanted according surgical procedure with 45º inclination angle and 20º in the anteversion angle. Were developed five models with same initial bone, one with intact condition simulating the cartilage between bones and four with different bone loss around the acetabular component. These four models representing the evolution of bone support of acetabular components presented in the literature. The evolution of bone loss was imposed with a CAD CAM process in same iliac bone model. The models were instrumented with 5 rosettes in critical region at the cortical bone to measure the strain evolution along the process. Results. The results of strain gauges present the influence of acetabular component implantation, reducing the bone strains and presented the effect of the strain shielding. The acetabular component works as a shield in the load transfer. The critical region is the posterior region with highest principal strains and the strain effect was observed with different bone loss around acetabular component. The maximum value of principal strain was observed in the intact condition in the anterior region, with 950μ∊. In the posterior superior region, the effect of bone loss is more important presenting a reduction of 500% in the strains. The effect of bone loss is presented in the strains induced with acetabular implantation, in the first step of implantation the maximum strain was 950μ∊ and in the last model the value was 50μ∊, indicating lower press-fit fixation. Conclusions. The models developed allows study the effect of bone loss and acetabular implant fixation in the load transfer at the hip articulation. The results presented a critical region as the anterior-superior and the effect of strain shielding was observed in comparison with intact articulation. The results of press-fit fixation present a reduction of implant stability along bone loss. The process of bone fixation developed present some limitation associated to the bone adhesion in the interface, not considered. Acknowledgement. This work was supported by POCI-01-0145-FEDER-032486,– FCT, by the FEDER, with COMPETE2020 - (POCI), FCT/M