Objectives. This investigation sought to advance the work published in our prior biomechanical study (Journal of Orthopaedic Research, 2016). We specifically sought to determine whether there are additional easy-to-measure parameters on plain radiographs of the proximal humerus that correlate more strongly with ultimate fracture load, and whether a parameter resembling the Dorr strength/quality characterisation of proximal femurs can be applied to humeri. Materials and Methods. A total of 33 adult humeri were used from a previous study where we quantified bone mineral density of the proximal humerus using radiographs and dual-energy x-ray absorptiometry (DEXA), and regional mean cortical thickness and cortical index using radiographs. The bones were fractured in a simulated backwards fall with the humeral head loaded at 2 mm/second via a frustum angled at 30° from the long axis of the bone. Correlations were assessed with ultimate fracture load and these new parameters: cortical index expressed in areas (“areal cortical index”) of larger regions of the diaphysis; the canal-to-calcar ratio used analogous to its application in proximal femurs; and the recently described medial cortical ratio. Results. The three new parameters showed the following correlations with ultimate fracture load: areal cortical index (r = 0.56, p < 0.001); canal-to-calcar ratio (r = 0.38, p = 0.03); and medial cortical ratio (r = 0.49, p < 0.005). These correlations were weaker when compared with those that we previously reported: mean cortical thickness of the proximal diaphysis versus ultimate fracture load (r = 0.71; p < 0.001); and mean density in the central humeral head versus ultimate fracture load (r = 0.70; p < 0.001). Conclusion. Simple-to-measure radiographic parameters of the proximal humerus reported previously are more useful in predicting ultimate fracture load than are areal cortical index, canal-to-calcar ratio, and medial cortical ratio. Cite this article: J. G. Skedros, C. S. Mears, W. Z. Burkhead. Ultimate fracture load of cadaver proximal humeri correlates more strongly with mean combined cortical thickness than with areal cortical index, DEXA density, or canal-to-calcar ratio. Bone Joint Res 2017;6:1–7. DOI: 10.1302/2046-3758.61.BJR-2016-0145.R1

Background and Purpose: Periprosthetic tibial plateau fractures are a rare but serious complication of UKA. Since they usually appear perioperatively they can be associated with sawing defects during implantation. The aim of the study was to evaluate fracture loads and fracture patterns under particular consideration whether extended vertical saw cuts reduce the stability of the tibial plateau and increase the risk of periprosthetic tibial plateau fractures. Material and Methods: In 6 matched paired fresh frozen tibiae (donor data: f/m = 2/4, mean age 81.2 years, mean weight 61.7kg) tibial implantation of the cemented Oxford Uni was performed in group A and with an extended vertical saw cut of 10° in group B in a randomized fashion. Before fracturing the tibiae with a maximum load of 10.0kN under standard conditions, DEXA bone density measurement and standard X-Ray were accomplished. After load induction fracture patterns and maximum fracture loads were analyzed and correlated to BMD, BMI, bodyweight (BW), age and surface area of the tibial implant. Results: In group A a maximum load of Fmax = 3.912 (2.346–8.500) kN lead to fractures, whereas in group B all tibiae fractured with a mean load of Fmax = 2.622 (1.085–5.036) kN. The difference was statistically different with p=0.028. The induced fractures were similar to those observed in clinical practice. Between BMI and the maximum fracture loads inducing tibial plateau fractures a significant correlation could be proven for all tibiae (r=0.643). Discussion: The observed fracture pattern showed metaphyseal fractures similar to those observed in clinical practise. Extended vertical saw cuts weaken the bone structure and therefore raise the risk of medial tibial plateau fractures. In our study extended vertical saw cuts of 10° reduce maximum fracture loads about 30%. We recommend special training and modified instruments for inexperienced surgeons to minimize the incidence of extended vertical saw cuts and to reduce the risk of periprosthetic fractures

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.

Introduction:. In an attempt to reduce stress shielding in the proximal femur multiple new shorter stem design have become available. We investigated the load to fracture of a new polished tapered cemented short stem in comparison to the conventional polished tapered Exeter stem. Method:. A total of forty-two stems, twenty-one short stems and twenty-one conventional stems both with three different offsets were cemented in a composite sawbone model and loaded to fracture. Results:. study showed that femurs will break at a significantly lower load to failure with a shorter compared to conventional length Exeter stem. Conclusion:. This Both standard and short stem design are safe to use as the torque to failure is 7–10 times as much as the torques seen in activities of daily living

Abstract. Introduction. In cementless UKR, primary fixation of the tibial component is achieved by press-fitting a keel (i.e. with interference) into a vertical slot cut into the proximal tibia. This may adversely affect the structural integrity of surrounding bone. Early post-operative peri-prosthetic tibial fractures are 7x more common in very small knees, but the aetiology of these fractures is unknown - such sizes are rarely used in the UK but more common in Asian populations. This study explores the effect of keel-related features in fracture risk of these very small tibias. Method. This in vitro study compares the effect of keel and slot depth (standard vs 33% shallower vs nil) and loading position (anterior/posterior gait range limits: mid-tibia vs 8mm posterior) on fracture load and path. 3D-printed titanium components were implanted using surgical instrumentation/technique, in bone-analogue foam machined to a CT-reconstructed very small tibia which subsequently experienced a peri-prosthetic fracture. Results. Introducing a standard slot reduces load-to-fracture by 50% (1421N-vs-710N, p<0.0001). Press-fitting a standard keel further reduces load-to-fracture by 40% (710N-vs-423N, p=0.0001). A shallower slot/keel increases load-to-fracture substantially (slot: 27% increase, 904N-vs-710N p=0.0003, slot+keel: 60% increase, 683N-vs-423N p=0.0004). Deeper keels fractured more vertically (current 8.2° vs shallow 15.5° vs nil 21°, degrees-to-vertical, p<0.0001). There was no difference caused by loading position. Conclusion. In very small tibias, a standard cementless keel significantly weakens the bone and may contribute to fractures. Therefore, decreasing interference or using a shallower keel should decrease the risk of fracture, although it might compromise fixation

Introduction. Up to 60% of total hip arthroplasties (THA) in Asian populations arise from avascular necrosis (AVN), a bone disease that can lead to femoral head collapse. Current diagnostic methods to classify AVN have poor reproducibility and are not reliable in assessing the fracture risk. Femoral heads with an immediate fracture risk should be treated with a THA, conservative treatments are only successful in some cases and cause unnecessary patient suffering if used inappropriately. There is potential to improve the assessment of the fracture risk by using a combination of density-calibrated computed tomographic (QCT) imaging and engineering beam theory. The aim of this study was to validate the novel fracture prediction method against in-vitro compression tests on a series of six human femur specimens. Methods. Six femoral heads from six subjects were tested, a subset (n=3) included a hole drilled into the subchondral area of the femoral head via the femoral neck (University of Leeds, ethical approval MEEC13-002). The simulated lesions provided a method to validate the fracture prediction model with respect of AVN. The femoral heads were then modelled by a beam loaded with a single joint contact load. Material properties were assigned to the beam model from QCT-scans by using a density-modulus relationship. The maximum joint loading at which each bone cross-section was likely to fracture was calculated using a strain based failure criterion. Based on the predicted fracture loads, all six femoral heads (validation set) were classified into two groups, high fracture risk and low fracture risk (Figure 1). Beam theory did not allow for an accurate fracture load to be found because of the geometry of the femoral head. Therefore the predicted fracture loads of each of the six femoral heads was compared to the mean fracture load from twelve previously analysed human femoral heads (reference set) without lesions. The six cemented femurs were compression tested until failure. The subjects with a higher fracture risk were identified using both the experimental and beam tool outputs. Results. The computational tool correctly identified all femoral head samples which fractured at a significantly low load in-vitro (Figure 2). Both samples with a low experimental fracture load had an induced lesion in the subchondral area (Figure 3). Discussion. This study confirmed findings of a previous verification study on a disease models made from porcine femoral heads (Preutenborbeck et al. I-CORS2016). It demonstrated that fracture prediction based on beam theory is a viable tool to predict fracture. The tests confirmed that samples with a lesion in the weight bearing area were more likely to fracture at a low load however not all samples with a lesion fractured with a low load experimentally, indicating that a lesion alone is not a sufficient factor to predict fracture. The developed tool takes both structural and material properties into account when predicting the fracture risk. Therefore it might be superior to current diagnostic methods in this respect and it has the added advantage of being largely automated and therefore removing the majority of user bias. For any figures or tables, please contact the authors directly

Multiple myeloma (MM) is a chronic, malignant B-cell disorder, with a less than 50% 5-year survival rate [1]. This disease is responsible for vertebral compression fractures (VCFs) in 34 to 64% of diagnosed patients [1], and at least 80% of MM patients experience pathological fractures [3]. Even though reduced DXA-derived bone mineral density (BMD) has been observed in MM patients with vertebral fractures [4], the current quantitative standard method is insufficient in MM due to the osteo-destructive bone changes. Finite-element (FE) analysis is a computational and non-destructive modeling and testing approach to determine bone strength using 3D bone models from CT images. Thus, this study aimed to assess the differences in FE-predicted critical fracture load in MM patients with and without VCFs in the thoracic and lumbar segments of the spine. Multi-detector CT (MDCT) images of two radiologically assessed MM patients (1 with VCFs and 1 without VCFs) were used to generate three-dimensional (3D) models of the whole spine. For each subject, the thoracic segments, 1 to 12 (T1-T12) and lumbar segments, 1 to 5 (L1-L5) were segmented and meshed. Heterogeneous, non-linear anisotropic material properties were applied by discretizing each vertebral segment into 10 distinct sets of materials. A compressive load was simulated by constraining the surface nodes on the inferior endplate in all directions, and a displacement load was applied on the surface nods on the superior endplate [2]. This analysis was performed using ABAQUS version 6.10 (Hibbitt, Karlsson, and Sorensen, Inc., Pawtucket, RI, USA). The MM subject with VCFs had originally experienced fractures in the T4, T5, T12, L1, and L5 segments whereas the MM subject without VCFs experienced none. The former displayed large and abrupt differences in fracture loads between adjacent vertebrae segments, unlike the latter, which exhibited progressive differences instead (no abrupt changes between adjacent vertebrae segments observed). Results from this preliminary study suggest that segments at high risk of fracture are collectively involved in an unstable network, which place the vertebral segments with high values of fracture loads (peaks) as well as the adjacent segments at risk of VCF. For instance, the high fracture load at T11 places T10, T11 and T12 at risk of fracture. Accordingly, T12 has already fractured, and T10 and T11 remain at risk. The relative changes between adjacent vertebrae segments that indicate instability (extremely high fracture load values) enables ease of identification of segments at high fracture risk. Clinicians would be able to work with pre-emptive treatment strategies in future as they can focus on more targeted therapy options at the high-risk vertebrae segments [3]

INTRODUCTION. Ceramic hip components are known for their superior material properties and longevity. In comparison to other materials commonly used, ceramics have a very low friction coefficient and a high fracture load. However, even though in-vivo fractures of ceramic ball heads are a relatively rare occurrence compared to other reasons for revision, they are of concern to the surgeon using ceramic components. The goal of this work was to evaluate the most probable causes for fracture and to quantify the influence of the metal taper contamination and shell deformation, respectively. METHODS. An experimental set-up imitating the in-vivo loading situation was used to analyze different scenarios that may lead to the fracture of the ball heads, such as dynamic loading, edge loading and the metal taper contamination. 58 ceramic ball heads made of pure alumina were loaded until fracture under various conditions. Parameters under investigation were the inclination of the insert, the loading velocity, and the contamination of the interface between taper and ball head. RESULTS. The behavior of the ball heads for the different scenarios showed a large variation. If the inclination of the insert equaled 45°, it is not possible to break the ceramic ball head prior to the failure of the metal taper due to high plastic deformation. In case of edge loading, due to the reduction of load transfer area, the load required to fracture dropped significantly. The loading rate had no measurable influence on this value. The largest effect on the fracture load had a contamination with osseous tissue and a damage of the metal taper. The fracture load decreases to approximately 20% compared to the value measured without the contamination. DISCUSSION. Contamination of the interface with osseous tissue or damages on the metal taper lead to a minimum fracture load in the range of the maximum forces ever measured in vivo. According to these findings, diligence is recommended during the implantation of the ceramic hip components in order to avoid disturbances or contamination of this interface. Because the reduction of the friction and the damage or contamination of the ceramic/metal interface results in a reduction of the fracture load, the presence of any material on the component tapers should be avoided

Ceramic hip components are known for their superior material properties concerning the invivo loading situation. In comparison to other commonly used materials, ceramics have a very low friction coefficient and a high fracture load. However, there are a few reported occasions of in-vivo fracture of ceramic ball heads. An experimental set-up imitating the in-vivo loading situation is used to analyze different scenarios that may lead to the fracture of the ball heads, such as dynamic loading, edge loading and the metal taper condition. It will be shown that even the worst-case set-up does not lead to fracture loads if the interface between ceramic ball head and metal taper is clean and dry. In contrast, certain disturbances/impurities of this interface can cause a further reduction of the fracture load. Ceramic ball heads made of pure alumina have been loaded until fracture under various conditions. The angle between the loading direction and the metal taper equals 35°, the ceramic ball is mounted in an alumina insert. Parameters under investigation were the inclination of the insert, the loading rate, and the condition of taper and ball head (contamination of the interface between taper and ball with adipose and osseous tissue; stripe wear on the outside of the ball head). Altogether 58 specimens (all alumina heads mounted on a titanium taper) have been tested, To resemble the position of the human acetabulum during walking and standing up, the inclination of the insert was chosen to differ between 45° (walking) and 80° (standing up). A variation of the loading speed is also tested, with a maximal speed in the range of the in-vivo loading rate (chosen parameters: 0,5 kN/sec and 25 kN/sec). For fabric samples, bovine femur (corticalis) and porcine adipose tissue were used. All fractured ball heads were statistically analyzed regarding the appearance of fracture in general, the fracture origin, and the metal transfer in the cone of the ceramic ball head. The behavior of the ball heads for the different scenarios shows a great variation: If the inclination of the insert equals 45°, it is not pos sible to break the ceramic ball head at all because of the high plastic deformation of the metal taper. In case of edge loading, the fracture load drops to 20 kN for 28-12/14 S ball heads and 36 kN for 28-12/14 L ball heads. The loading rate and the contamination of the interface between ball head and taper with adipose tissue have no measurable influence on this value. The largest effect on the fracture load has a contamination with osseous tissue. The fracture load decreases to 32% compared to the value measured without the contamination. A minimal fracture load of approximately 8 kN (KK 28-12/14 L) was measured. Statistical analysis shows that the fracture load depends linearly on the stiffness of the system (ball heads 28-12/14 S). Because none of the other parts changes during the experiments, the cause of the change in stiffness is most likely due to a change of the friction coefficient between ball head and taper: A reduced stiffness indicates a lower friction coefficient which results in higher normal forces in the ball head and, therefore, leads to lower fracture loads. This theory is supported by numerical calculations. The influence of edge loading and contamination of the interface between taper and ball with osseous tissue on the fracture load can be shown. If the insert has a high inclination angle, high bending forces are applied to the ball head amplifying the effect of edge loading. It should be accentuated, that the minimum fracture load of a ball head without contamination of the interface is still twice as high as the maximum forces measured in-vivo. Contamination with osseous tissue leads to a minimum fracture load of approximately eight times of the body weight, a value being close to the maximum forces ever measured invivo. Therefore, diligence is recommended during the implantation of the ceramic hip components in order to avoid disturbances of this interface. Because the reduction of the stiffness results in a reduction of the fracture load, the lubrication of the taper should be avoided

Purpose: To examine whether neutral or valgus placement results in greater fracture strength ex vivo, when the femoral neck is notched superolaterally as sometimes occurs during hip resurfacing arthroplasty. Methods: We loaded 10 paired fresh-frozen notched proximal cadaveric femora (8F/2M, 66 to 80 years) to failure. In each case, the right femur was implanted, using bone cement, with a machined resurfacing component aligned neutrally with respect to the femoral neck whereas the left femur was implanted at 10° valgus. The superolateral notch was 3 mm wide by 2 mm deep directly beside the component. Potted femurs were loaded to failure using an Instron materials testing machine. All 20 femora were scanned using Dual-Energy X-Ray Absorptiometry. Results: The effect of neutral versus valgus placement was complex. (1) Two pairs slowly crushed; the remaining femurs exhibited a clear fracture. When only the fracture-type failures were analyzed, valgus placement resulted in fracture loads on average 22% higher than for neutral placement (paired t-test, p< 0.05). All femurs failed within the notch, as occurs clinically. (2) Femurs with high bone density (BMD> 0.65 g/cm2) showed a significant increase in fracture load (p< 0.05) whereas femurs with low BMD (< 0.65 g/cm2) were unaffected by component placement. BMD was highly correlated with fracture load (Pearson r=0.87, p=0.0003). (3) The greatest improvements occurred when the neck-shaft angle was relatively low, 128°–132°. (4) Two of ten femurs required larger head sizes at 10° valgus. Conclusions: (1) Fracture load was primarily controlled by bone quality (BMD); (2) nevertheless, varus/valgus placement did affect the fracture load; (3) the magnitude and direction of this effect depended on fracture type, bone mineral density and the original neck-shaft angle; (4) for the level of bone quality typical of patients undergoing hip resurfacing, and for low-to-average neck-shaft angles (up to 132°), the fracture load for 10° valgus placement was significantly higher than for neutral placement. Funding: Other Education Grant

The disadvantage of removing a well-fixed femoral stem are multiple (operating time, risk of fracture, bone and blood loss, recovery time and post-op complications. Ceramic heads with titanium adapter sleeves (e.g. BIOLOX®OPTION, Ceramtec) are a possibility for putting a new ceramic head on slightly damaged used tapers. ‘Intolerable’ taper damages even for this solution are qualitatively specified by the manufacturers. The aim of this study was to determine the fracture strength of ceramic heads with adapter sleeves on stem tapers with such defined damage patterns. Pristine stem tapers (Ti-6Al-4V, 12/14) were damaged to represent the four major stem taper damage patterns specified by the manufacturers: . -. ‘Truncated’: Removal of 12.5% of the circumference along the entire length of the stem taper at a uniform depth of 0.5mm parallel to the taper slope. -. ‘Slanted’: Removal of 33.3% of the proximal diameter perimeter with decreasing damage down to 3.7mm from the proximal taper end. -. ‘Cut’: Removal of the proximal 25% (4mm) of the stem taper. -. ‘Scratched’: Stem tapers from a previous ceramic fracture test study with a variety of scratches and crushing around the upper taper edge from multiple ceramic head fractures. -. The ‘Control’ group consisted of three pristine tapers left undamaged. BIOLOX®OPTION heads (Ø 32mm, length M) with Ti adapter sleeves were assembled to the damaged stem tapers and subjected to ISO7206-10 ultimate compression strength testing. The forces required to fracture the head were high and caused complete destruction of the ceramic heads in all cases. The ‘Truncated’ group showed the lowest values (136kN ± 4.37kN; Fig. 3). Forces were higher and similar for the ‘Cut’ (170kN ± 8.89kN), ‘Control’ (171.8 ± 16.5kN) and ‘Slanted’ (173kN ± 21.9kN) groups, the ‘Scratched’ group showed slightly higher values (193kN ± 11.9kN). The Ti adapter sleeves were plastically deformed but did not fail catastrophically. The present study suggests that manufacturer's recommendations for removal of a well fixed femoral stem could be narrowed down to the ‘Truncated’ condition. Even this might not be necessary since the fracture load is still substantially higher than the ASTM standard requires. Surgeons should consider to keep stems with larger taper damages as previously thought and spare the patient from stem revision. The greatest reservation regarding adapter sleeves is the introduction of the new metal-on-metal interface between stem and sleeve, which could possibly facilitate fretting-corrosion, which is presently one of the major concerns for modular junctions (3). Clinically such problems have not been reported yet. Ongoing FE-simulations are performed to investigate whether micromotions between stem and head taper are altered by the investigated damages

Introduction: The incidence of contralateral, second hip fractures after a first hip fracture is as high as 20% in the elderly. Femoroplasty using an injectable and resorbable bi-phosphonate loaded bone substitute to prevent controlateral hip fracture may represent a promising preventive therapy. We aimed to evaluate the biomechanical consequences of the femoroplasty using this bone substitute. Materials and Methods: Twelve paired human cadaveric femora from donors with a mean age of 86 years (7 women and 6 men) were randomly assigned for femoroplasty and biomechanically tested for fracture load against their native contralateral control. Anterior–posterior and lateral radiographs and DXAscan’s were made before injection. Femoroplasty were performed under fluoroscopic guidance with an injectable and resorbable bi-phosphonate loaded bone substitute. All femurs were fractured by simulating a fall on the greater trochanter by an independent observer. Results: Mean T-score of the tested femur were −3. Bone density was comparable for each pair of femur. All the observed fractures were Kyle II throchanteric fractures. Mean fracture load was 2786 Newton in the femoroplasty group (group F) versus 2116 Newton in the control group (group C) (p< 0.001). Fracture loads were always higher in the group F: mean 41.6% (mini: 1.2%/maxi:102.1%). Effect of femoroplasty was significantly superior for women and also correlated to initial bone density (p< 0.0001). Discussion:According to our results, femoroplasty with an injectable and resorbable bi-phosphonate loaded bone substitute can provide significant biomechanical reinforcement of the proximal femur to prevent controlateral fracture

Introduction. Periprosthetic medial tibial plateau fractures (TPF) are rare but represent a serious complication in unicompartmental knee arthroplasty (UKA). Most common treatment of these fractures is osteosynthesis with canulated screws or plates. Aim. The aim of this study was to evaluate these two different treatment options of periprosthetic fractures. The hypothesis was that osteosynthetic treatment with plates show significantly higher maximum fracture loads than fixation with cannulated screws. Materials and Methods. 12 matched paired fresh frozen tibias with periprosthetic tibial plateau fractures were used for this study. In group A osteosyntheses with angle-stable plates were performed, whereas in group B cannulated screws were utilized to fixate the periprosthetic fractures. DEXA bone density measurement and standard X-rays (ap and lateral) were accomplished before loading the tibias under standardised conditions with a maximum load of up to 10.0kN. Results. In the plate group all tibias fractured with a median load of Fmax=2.64 (0.45–5.68) kN, whereas in the group with cannulated screws fractures occurred at a mean load of Fmax=1.50 (0.27–3.51) kN. The difference was statistically significant with p<0.05. Discussion. Angle-stable plates showed significantly higher fracture load resistance than fixation with cannulated screws. Therefore osteosynthesis with angle-stable plates in periprosthetic tibial plateau fractures should be recommended. MULTIPLE DISCLOSURES

Purpose. Twelve case reports of distal femur fractures as post-operative complications after anterior cruciate ligament (ACL) reconstruction have been described in the literature. The femoral tunnel has been suggested as a potential stress riser for fracture formation. The recent increase in double bundle ACL reconstructions may compound this risk. This is the first biomechanical study to examine the stress riser effect of the femoral tunnel(s) after ACL reconstruction. The hypotheses tested in this study are that the femoral tunnel acts as a stress riser to fracture and that this effect increases with the size of the tunnel (8mm versus 10mm) and with the number of tunnels (one versus two). Method. Femoral tunnels simulating single bundle (SB) hamstring graft (8 mm), bone-patellar tendon-bone graft (10 mm), and double bundle (DB) ACL reconstruction (7mm, 6 mm) were drilled in fourth generation saw bones. These three experimental groups and a control group consisting of native saw bones without tunnels, were loaded to failure. Result. All fractures occurred through the tunnels in the double tunnel group whereas fractures did not consistently occur through the tunnels in the single tunnel groups. The mean fracture load was 6145 N 471 N in the native group, 5691 N 198 N in the 8 mm single tunnel group, 5702 N 282 N in the 10 mm single tunnel group, and 4744 N 418 N in the double tunnel group. The mean fracture load for the double tunnel group was significantly different when compared to native, 8 mm single bundle, and 10 mm single bundle groups independently (p value = 0.0016, 0.0060, and 0.0038 respectively). No other statistically significant differences were identified. Conclusion. An anatomically placed femoral tunnel in single bundle ACL reconstruction in our experimental model was not a stress riser to fracture, whereas the two femoral tunnels in double bundle ACL reconstruction significantly decreased load to failure. The results support the sparcity of reported peri-ACL reconstruction femur fractures in single femoral tunnel techniques. However, the increased fracture risk in double bundle ACL reconstruction is a cause for concern and may impact patient selection

Aims

The distal radius is a major site of osteoporotic bone loss resulting in a high risk of fragility fracture. This study evaluated the capability of a cortical index (CI) at the distal radius to predict the local bone mineral density (BMD).

Although resurfacing hip replacement (RHR) is associated with a more demanding patient cohort, it has achieved survivorship approaching that of total hip replacement. Occasional failures from femoral neck fracture, or migration and loosening of the femoral head prosthesis have been observed, the causes of which are multifactorial, but predominately biomechanical in nature. Current surgical technique recommends valgus implant orientation and reduction of the femoral offset, reducing joint contact force and the femoral neck fracture risk. Radiographic changes including femoral neck narrowing and ‘pedestal lines’ around the implant stem are present in well performing hips, but more common in failing joints indicating that loosening may involve remodelling. The importance of prosthesis positioning on the biomechanics of the resurfaced joint was investigated using finite element analysis (FEA). Seven FE models were generated from a CT scan of a male patient: the femur in its intact state, and the resurfaced femur with either a 50mm or 52mm prosthesis head in. neutral orientation,. 10° of relative varus or. 10° of relative valgus tilt. The fracture risk during trauma was investigated for stumbling and a sideways fall onto the greater trochanter, by calculating the volume of yielding bone. Remodelling was quantified for normal gait, as the percentage volume of head and neck bone with over 75% post-operative change in strain energy density for an older patient, and 50% for a younger patient. Resurfacing with the smaller, 50mm prosthesis reduced the femoral offset by 3.0mm, 4.3mm and 5.1mm in varus, neutral and valgus orientations. When the 52mm head was used, the natural joint centre could be recreated rrespective of orientation, without notching the femoral neck. The 50mm head reduced the volume of yielding femoral neck bone relative to the intact femur in a linear correlation with femoral offset. When the natural femoral offset was recreated with the 52mm prosthesis, the predicted neck fracture load in stumbling was decreased by 9% and 20% in neutral and varus orientations, but remained in line with the intact bone when implanted with valgus orientation. This agrees with clinical experience and justifies currently recommended techniques. In oblique falling, the neck fracture load was again improved slightly when the femoral offset was reduced, and never fell below 97% of the natural case for the larger implant in all orientations. Predicted patterns of remodelling stimulus were consistent with radiographic clinical evidence. Stress shielding increased slightly from varus to valgus orientation, but was restricted to the superior femoral head in the older patient. Bone densification around the stem was predicted, indicating load transfer. Stress shielding only extended into the femoral neck in the young patient and where the femoral offset was reduced with the 50mm prosthesis. The increase in remodelling correlated with valgus orientation, or reduced femoral offset. The trend would become more marked if this were to reduce the joint contact force, but there was no such correlation for the 52mm prosthesis, when the natural femoral offset was recreated. Only in extreme cases would remodelling alone be sufficient to cause visible femoral neck narrowing, i.e. patients with a high metabolism and considerably reduced femoral offset, implying that other factors including damage from surgery or impingement, inflammatory response or retinacular blood supply interruption may also be involved in femoral neck adaptation. The results of this FEA biomechanical study justify current surgical techniques, indicating improved femoral neck fracture strength in stumbling with valgus position. Fracture risk under oblique falling was less sensitive to resurfacing. Furthermore, the results imply that reduced femoral offset could be linked to narrowing of the femoral neck; however the effects of positioning alone on bone remodelling may be insufficient to account for this. The study suggests that surgical technique should attempt to recreate the natural head centre, but still aim primarily for valgus positioning of the prosthesis, to reduce the femoral neck fracture risk

Most of researches related to osteoporosis emphasized on trabecular bone loss. However, cortical bone has a prominent role on bone strength determined by bone quality, such as 2D or 3D geometry and microstructure of bone, not only density.[1] The focal thinning of cortical bone associated with aging in post-menopausal osteoporotic bone in the proximal femur may predispose a hip to fracture.[2, 3] As the trabecular bone is lost with progression of osteoporosis, the remaining cortical bone take more predominant role on bone strength.[4] To date, no effective osteoporotic agent was demonstrated to enhance both cortical geometric change and bone strength. Herein, we investigate the effect of Teriparatide (rhPTH(1–34)) on cortical bone at femoral diaphysis in OVX rat model. Twenty 12-week-old, female Sprague Dawley rats were used in this study. Bilateral ovariectomies were performed in 16 animals and randomly divided to three groups as control (N=6), OVX (N=6) and treatment group after OVX (OVX+F) by teriparatide (N=8). After twelve weeks of intervention, all rats were euthanized and right femurs and L5 vertebrae were extracted for further tests. All bone specimens were subjected to dual-energy X-ray absorptiometer (DXA) to evaluate areal bone mineral density (aBMD) of L5 vertebrae and femurs, micro-computed tomography (micro-CT) to analyze cortical bone parameters of femoral diaphysis, including cortical cross section area (CSA), cortical thickness and cross-sectional moment of inertia (CSMI). A three-point bending test was applied to determine fracture load of each femurs. Compare to OVX group, increase of aBMD by 14.6 % at L5 vertebrae and 13.3% at femoral diahpysis in treatment group. The cortical parameters of femoral diaphysis, CSA and cortical thickness, analyzed by micro-CT were significantly increased but the increasing tendency of CSMI did not have significant changes statistically after teriparatide intervention for 3 months duration. The increase of cortical bone strength (OVX vs OVX+F group, 120.72±2.72 vs 137.93±5.02, p < 0.05) at femoral diaphysis after treatment were also noticed. This study has point out a deeper look at geometric change of cortical bone after teriparatide treatment. This finding imply teirparatide has the ability to change the geometry of cortical bone and increase bone strength at femoral diaphysis

The high risk and the associated high mortality of secondary, contralateral hip fractures [1,2] could justify internal, invasive prophylactic reinforcement of the osteoporotic proximal femur to avoid these injuries in case of a low energy fall. Previous studies have demonstrated high potential of augmentation approaches [3,4,5], but to date there has no ideal solution been found. The development of optimized reinforcement strategies can be aided with validated computer simulation tools that can be used to evaluate new ideas. A validated non-linear finite element (FE) simulation tool was used here to predict the yield and fracture load of twelve osteoporotic or osteopenic proximal femora in sideways fall based on high resolution CT images. Various augmentation strategies using bone cement or novel metal implants were developed, optimized and virtually performed on the bone models. The relative strengthening compared to the non-augmented state was evaluated using case-specific FE analyses. Strengthening effect of the cement-based augmentation was linearly proportional to cement volume and was significantly affected by cement location. With the clinically acceptable 12.6 ± 1.2 ml volume and optimized location of the cement cloud, compared to the non-augmented state, 71 ± 26% (42 – 134%) and 217 ± 166% (83 – 509%) increase in yield force and energy was reached, respectively. These were significantly higher than previously published experimental results using the “central” cement location [5], which could be well predicted by our FE models. The optimized metal implant could provide even higher strengthening effect: 140 ± 39% (76 – 194%) increase in yield force and +357 ± 177% (132 – 691%) increase in yield energy. However, for metal implants, a higher risk of subcapital fractures was indicated. For both cement and metal, the originally weaker bones were strengthened exponentially more compared to the stronger ones. The ideal solution for prophylactic augmentation should provide an appropriate balance between the requirements of being clinically feasible, ethically acceptable and mechanically sufficient. Even with the optimized location, the cement-based approach may not provide enough strengthening effect and adequate reproducibility of the identified optimal cement cloud position may not be achieved clinically. While the metal implant based strategy appears to be able to deliver the required strengthening effect, the ethical acceptance of this more invasive option is questionable. Further development is therefore required to identify the ideal, clinically relevant augmentation strategy. This may involve new cement materials, less invasive metal implants, or a combination of both. The FE simulation approach presented here could help to screen the potential ideas and highlight promising candidates for experimental evaluation

Since artificial joints are expected to operate for more than decades in human body, animal and clinical studies are not suitable for evaluation of their durability. Instead, in-vitro mechanical tests have been employed, but they cannot fully reproduce complex in-vivo mechanical and biochemical environment. For instance, lipids in synovial fluid have been known to be absorbed in ultra-high molecular weight polyethylene (UHMWPE) components of artificial joints in vivo, and recently it was found that absorbed lipids have potential to degrade UHMWPE. In order to assure clinical relevance of the in-vitro mechanical tests, understanding of the effect of the in-vivo environment on mechanical properties is indispensable. However, well-developed mechanical tests cannot be applied to retrieved components, because they require large specimens. In this study, we attempted to develop methods to evaluate mechanical properties of retrieved UHMWPE components. We prepared five kinds of UHMWPE. Those are molded UHMWPE made from GUR 1020 resin without any further treatment, remelted highly crosslinked UHMWPE, annealed highly crosslinked UHMWPE, squalene absorbed UHMWPE which was prepared by immersing in squalene at 80°C for 7 days (SQ) and squalene absorbed and artificially aged UHMWPE which was prepared by artificially aging SQ at 80°C for 21 days in air (SQA). SQ and SQA were employed in this study to mimic lipid absorption and lipid induced degradation. These materials were tested by two well-established mechanical tests, namely, tensile tests and compression tests, and two proposed mechanical tests that can be applied to retrieved components, namely, tensile punch tests and micro indentation tests. It was possible to clearly identify the difference between materials by any of test methods used in this study. Stiffness obtained from tensile punch tests and elastic modulus obtained from micro indentation tests were shown to be highly correlated with elastic modulus obtained from compression tests except for SQA, which was inhomogeneous due to degradation at the surfaces. The results showed that the elastic modulus of the local surface could be evaluated by micro indentation tests, while the average of that of the entire specimen could be evaluated by compression tests. ield load, fracture load and maximum load obtained from tensile punch tests showed little correlation with yield stress, fracture stress and maximum stress obtained from tensile tests, respectively. These differences were considered to be attributed to the differences in a stress condition between these two test methods. It is multi-axial tension in tensile punch tests, while it is uniaxial in tensile tests. Although some of the parameters obtained by tensile punch tests showed no or limited correlation with those obtained by tensile tests, it was possible to clearly identify the difference between materials by these proposed test methods. In particular, micro indentation tests could evaluate the mechanical properties very locally. These proposed test methods have the potential to provide useful information on mechanical properties of retrieved UHMWPE components

Periprosthetic tibial plateau fractures (PTPF) represent a rare but serious complication in unicompartmental knee arthroplasty (UKA). Although excellent long-term results have been reported with cemented UKA, surgeons continue to be interested in cementless fixation. The aim of the study was to compare fracture loads of cementless and cemented UKA. Tibial components of the Oxford UKA were implanted in six paired fresh-frozen tibiae. In one set surgery was performed with cement fixation and in the other cementless components were implanted. Loads were then applied under standardised conditions to fracture the specimens. Mean loads of 3.6 (0.7–6.9) kN led to fractures in the cemented group, whereas the tibiae fractured in the cementless group with a mean load of 1.9 (0.2–4.3) kN (p< 0.05). The loading capacity in tibiae with cementless components is significantly less compared to cemented fixation. Our results suggest that, patients with poor bone quality who are treated with a cementless UKA are at higher risk for periprosthetic fractures