Intact Femur Research Articles

Distal Locking of Femoral Nails

The purpose of this study was to assess the effectiveness of a novel radiation-independent aiming device for distal locking of intramedullary nails in a human cadaver model. A new targeting system was used in 25 intact human cadaver femora for the distal locking procedure after insertion of an intramedullary nail. The number of successful screw placements and the time needed for this locking procedure were recorded. The accuracy of the aiming process was evaluated by computed tomography. The duration of the distal locking process was 8.0 ± 1.8 minutes (mean ± SD; range, 4-11 minutes). None of the screw placements required fluoroscopic guidance. Computed tomography revealed high accuracy of the locking process. The incidence angle (α) of the locking screws through the distal locking holes of the nail was 86.8° ± 5.0° (mean ± SD; range, 80°-96°). Targeting failed in 1 static locking screw because of a material defect in the drilling sleeve. This cadaver study indicated that an aiming arm-based targeting device is highly reliable and accurate. The promising results suggest that it will help to decrease radiation exposure compared with the traditional "free-hand technique."

Intraoperative measurement of femoral antetorsion using the anterior cortical angle method: a novel use for smartphones

Malrotation after femoral nailing is a common problem, yet estimation of the correct rotation during nailing remains a technical challenge. In the current study, a novel technique was developed for determining femoral antetorsion, the anterior cortical angle (ACA) method. The ACA is the angle between a line along the anterior aspect of the femoral neck and the posterior condylar line of the distal femur. The principal advantage of this method is that it facilitates intra-operative assessment of femoral antetorsion by utilizing the positional technology integrated in smartphones. This measurement is directly comparable to measurements made using computed tomography (CT) scans. The objective of the current study was to investigate the possibility and to validate the feasibility and accuracy of the new method and compare the results obtained with the traditional methods of antetorsion estimation via CT and surgical navigation technology. Twelve cadaveric femora were used. Femoral antetorsion was measured with the ACA method, using a smartphone with integrated gyroscope (Apple IPhone, Cupertino, CA, USA) and by a conventional navigated technique (Brainlab, Feldkirchen, Germany). Subsequently, all femora underwent CT scanning to measure their respective antetorsion via the ACA and the method of Jend (1986). Next, a mid-diaphyseal osteotomy was performed and the distal fragment was rotated and were adjusted to 10-15° using ACA by smartphone. All measurements were repeated with this new position of the femoral fragments. Both radiological measurements according ACA and Jend (1986) demonstrated a statistically significant correlation (intact femur, r = 0.773, p = 0.003; after fixation, r = 0.898, p < 0.001). Comparing the measurements derived from the ACA, as analysed on CT images, and that gleaned from the experimental use of the same method with the smartphone, a statistically significant correlation was also demonstrated (intact femur, r = 0.826, p = 0.001; after fixation, r = 0.932, p < 0.001). Comparing the navigation system and the ACA measured by smartphone there was, on intact femora, a fair correlation without statistical significance and after fixation a good correlation with statistical significance (intact femur, r = 0.467, p = 0.126; after fixation, r = 0.869, p = 0.001). The ACA method generated acceptable results and could contribute to improving the results of femoral nailing. The use of this device in a real clinical setting is necessary to truly elucidate its utility.

Toward verified and validated FE simulations of a femur with a cemented hip prosthesis

BackgroundVerified and validated CT-based high-order finite element (FE) methods were developed that predict accurately the mechanical response of patient-specific intact femurs. Here we extend these capabilities to human femurs undergoing a total hip replacement using cemented prostheses. MethodsA fresh-frozen human femur was CT-scanned and thereafter in vitro loaded in a stance position until fracture at the neck. The head and neck were removed and the femur was implanted with a cemented prosthesis. The fixed femur was CT-scanned and loaded through the prosthesis so that strains and displacements were measured. High-order FE models based on the CT scans, mimicking the experiments, were constructed to check the simulations prediction capabilities. ResultsThe FE models were verified and results were compared to the experimental observations. The correlation between the experimental and FE strains and displacements were (R2=0.97, EXP=0.96FE+0.02) for the intact femur and (R2=0.90, EXP=0.946FE+0.0012) for the implanted femur. This is considered a good agreement considering the uncertainties encountered by the heavy distortion embedded in the CT scan of the metallic prosthesis. DiscussionThe patient-specific FE model of the fresh-frozen femur with the cemented metallic prosthesis showed a good correlation to experimental observations, both when considering surface strains, displacements and strains on the prosthesis. The relatively short timescale to generate and analyze such femurs (about 6h) make these analyses a very attractive tool to be used in clinical practice for optimization prostheses (dimensions, location and configuration), and allow to quantify the stress shielding.

Biomechanical properties of an intact, injured, repaired, and healed femur: An experimental and computational study

There is no “gold standard” treatment for femoral mid-shaft fractures near the tip of a hip implant. Moreover, no study has quantified the changes in a femur’s mechanical properties from injury through to healing. The present aim was to predict overall stiffness and peak bone stress in the same femur after injury, repair, and healing with respect to its intact condition. Stage 1 was an intact femur. Stage 2 mimicked a femur with a hip stem. Stage 3 had a 5-mm fracture gap repaired with a plate and screws. Stage 4 represented complete fracture union. Experiments were done on a synthetic femur with strain gages and subjected to 1500N of axial force. Finite element (FE) models were validated against experiments and then re-analyzed using a clinical-level force of 3000N. At 1500N, FE vs. experimental strains had excellent linear agreement (R=0.94; slope=0.97). At 3000N, FE stiffnesses were 2167N/mm (Stage 1), 2359N/mm (Stage 2), 973N/mm (Stage 3), and 3348N/mm (Stage 4), showing that Stage 3 was the least stable compared to Stage 1. At 3000N, FE bone stresses yielded peaks of 75.7MPa at the load application point (Stage 1), 29.0MPa near the hip implant tip (Stage 2), 126.3MPa at the distal portion of the plate (Stage 3), and 69.3MPa at the proximal portion of the plate (Stage 4), showing that Stage 3 was most vulnerable to re-injury compared to Stage 1. Stress shielding and high stresses were present not only after hip implantation and plating, but also after healing.

Anatomical evaluation and stress distribution of intact canine femur

In the biomedical field, three-dimensional (3D) modeling and analysis of bones and tissues has steadily gained in importance. The aim of this study was to produce more accurate 3D models of the canine femur derived from computed tomography (CT) data by using several modeling software programs and two different methods. The accuracy of the analysis depends on the modeling process and the right boundary conditions. Solidworks, Rapidform, Inventor, and 3DsMax software programs were used to create 3D models. Data derived from CT were converted into 3D models using two different methods: in the first, 3D models were generated using boundary lines, while in the second, 3D models were generated using point clouds. Stress analyses in the models were made by ANSYS v12, also considering any muscle forces acting on the canine femur. When stress values and statistical values were taken into consideration, more accurate models were obtained with the point cloud method. It was found that the maximum von Mises stress on the canine femur shaft was 34.8 MPa. Stress and accuracy values were obtained from the model formed using the Rapidform software. The values obtained were similar to those in other studies in the literature.

Sclerostin Antibody Increases Bone Volume and Enhances Implant Fixation in a Rat Model

Previous studies have demonstrated that sclerostin blockade is anabolic for bone. This study examined whether systemic administration of sclerostin antibody would increase implant fixation and peri-implant bone volume in a rat model. Titanium cylinders were placed in the femoral medullary canal of ninety male Sprague-Dawley rats. One-half of the rats (n=45) received murine sclerostin antibody (Scl-Ab, 25 mg/kg, twice weekly) and the other one-half (n=45) received saline solution. Equal numbers of rats from both groups were sacrificed at two, four, or eight weeks after the implant surgery and the femora were examined by microcomputed tomography, mechanical pull-out testing, and histology. Fixation strength in the two groups was similar at two weeks but was 1.9-fold greater at four weeks (p=0.024) and 2.2-fold greater at eight weeks (p<0.001) in the rats treated with sclerostin antibody. At two weeks, antibody treatment led to increased cortical area, with later increases in cortical thickness and total cross-sectional area. Significant differences in peri-implant trabecular bone were not evident until eight weeks but included increased bone volume per total volume, bone structure that was more plate-like, and increased trabecular thickness and number. Changes in bone architecture in the intact contralateral femur tended to precede the peri-implant changes. The peri-implant bone properties accounted for 61% of the variance in implant fixation strength, 32% of the variance in stiffness, and 63% of the variance in energy to failure. The implant fixation strength at four weeks was approximately equivalent to the strength in the control group at eight weeks. Sclerostin antibody treatment accelerated and enhanced mechanical fixation of medullary implants in a rat model by increasing both cortical and trabecular bone volume.

A Preliminary Biomechanical Study of Cyclic Preconditioning Effects on Canine Cadaveric Whole Femurs

Biomechanical preconditioning of biological specimens by cyclic loading is routinely done presumably to stabilize properties prior to the main phase of a study. However, no prior studies have actually measured these effects for whole bone of any kind. The aim of this study, therefore, was to quantify these effects for whole bones. Fourteen matched pairs of fresh-frozen intact cadaveric canine femurs were sinusoidally loaded in 4-point bending from 50 N to 300 N at 1 Hz for 25 cycles. All femurs were tested in both anteroposterior (AP) and mediolateral (ML) bending planes. Bending stiffness (i.e., slope of the force-vs-displacement curve) and linearity R(2) (i.e., coefficient of determination) of each loading cycle were measured and compared statistically to determine the effect of limb side, cycle number, and bending plane. Stiffnesses rose from 809.7 to 867.7 N/mm (AP, left), 847.3 to 915.6 N/mm (AP, right), 829.2 to 892.5 N/mm (AP, combined), 538.7 to 580.4 N/mm (ML, left), 568.9 to 613.8 N/mm (ML, right), and 553.8 to 597.1 N/mm (ML, combined). Linearity R(2) rose from 0.96 to 0.99 (AP, left), 0.97 to 0.99 (AP, right), 0.96 to 0.99 (AP, combined), 0.95 to 0.98 (ML, left), 0.94 to 0.98 (ML, right), and 0.95 to 0.98 (ML, combined). Stiffness and linearity R(2) versus cycle number were well-described by exponential curves whose values leveled off, respectively, starting at 12 and 5 cycles. For stiffness, there were no statistical differences for left versus right femurs (p = 0.166), but there were effects due to cycle number (p < 0.0001) and AP versus ML bending plane (p < 0.0001). Similarly, for linearity, no statistical differences were noted due to limb side (p = 0.533), but there were effects due to cycle number (p < 0.0001) and AP versus ML bending plane (p = 0.006). A minimum of 12 preconditioning cycles was needed to fully stabilize both the stiffness and linearity of the canine femurs. This is the first study to measure the effects of mechanical preconditioning on whole bones, having some practical implications on research practices.

In-vitro comparison of LC-DCP- and LCP-constructs in the femur of newborn calves – a pilot study

BackgroundTo compare the biomechanical in-vitro characteristics of limited-contact dynamic compression plate (LC-DCP) and locking compression plate (LCP) constructs in an osteotomy gap model of femoral fracture in neonatal calves. Pairs of intact femurs from 10 calves that had died for reasons unrelated to the study were tested. A 7-hole LC-DCP with six 4.5 mm cortical screws was used in one femur and a 7-hole LCP with four 5.0 mm locking and two 4.5 mm cortical screws was used in the corresponding femur. The constructs were tested to failure by cyclic compression at a speed of 2 mm/s within six increasing force levels.ResultsThe bone-thread interface was stripped in 21 of 80 cortical screws (26.3%) before a pre-set insertion torque of 3 Nm was achieved. Only 3 corresponding intact pairs of constructs could be statistically compared for relative structural stiffness, actuator excursion and width of the osteotomy gap. Relative structural stiffness was significantly greater, actuator excursion and width of the osteotomy gap were significantly smaller in the LCP constructs. While failure occurred by loosening of the screws in the LC-DCP constructs, locking constructs failed by cutting large holes in the soft distal metaphyseal bone.ConclusionsAn insertion torque sufficient to provide adequate stability in femurs of newborn calves could not be achieved reliably with 4.5 mm cortical screws. Another limiting factor for both constructs was the weak cancellous bone of the distal fracture fragment. LCP constructs were significantly more resistant to compression than LC-DCP constructs.

The assessment of the risk of fracture in femora with metastatic lesions

Previously, we showed that case-specific non-linear finite element (FE) models are better at predicting the load to failure of metastatic femora than experienced clinicians. In this study we improved our FE modelling and increased the number of femora and characteristics of the lesions. We retested the robustness of the FE predictions and assessed why clinicians have difficulty in estimating the load to failure of metastatic femora. A total of 20 femora with and without artificial metastases were mechanically loaded until failure. These experiments were simulated using case-specific FE models. Six clinicians ranked the femora on load to failure and reported their ranking strategies. The experimental load to failure for intact and metastatic femora was well predicted by the FE models (R(2) = 0.90 and R(2) = 0.93, respectively). Ranking metastatic femora on load to failure was well performed by the FE models (τ = 0.87), but not by the clinicians (0.11 < τ < 0.42). Both the FE models and the clinicians allowed for the characteristics of the lesions, but only the FE models incorporated the initial bone strength, which is essential for accurately predicting the risk of fracture. Accurate prediction of the risk of fracture should be made possible for clinicians by further developing FE models.

Numerical analysis of an osseointegrated prosthesis fixation with reduced bone failure risk and periprosthetic bone loss

Currently available implants for direct attachment of prosthesis to the skeletal system after transfemoral amputation (OPRA system, Integrum AB, Sweden and ISP Endo/Exo prosthesis, ESKA Implants AG, Germany) show many advantages over the conventional socket fixation. However, restraining biomechanical issues such as considerable bone loss around the stem and peri-prosthetic bone fractures are present. To overcome these limiting issues a new concept of the direct intramedullary fixation was developed. We hypothesize that the new design will reduce the peri-prosthetic bone failure risk and adverse bone remodeling by restoring the natural load transfer in the femur. Generic CT-based finite element models of an intact femur and amputated bones implanted with 3 analyzed implants were created and loaded with a normal walking and a forward fall load. The strain adaptive bone remodeling theory was used to predict long-term bone changes around the implants and the periprosthetic bone failure risk was evaluated by the von Mises stress criterion. The results show that the new design provides close to physiological distribution of stresses in the bone and lower bone failure risk for the normal walking as compared to the OPRA and the ISP implants. The bone remodeling simulations did not reveal any overall bone loss around the new design, as opposed to the OPRA and the ISP implants, which induce considerable bone loss in the distal end of the femur. This positive outcome shows that the presented concept has a potential to considerably improve safety of the rehabilitation with the direct fixation implants.

How stiffness and distal interlocking of revision hip stems influence the femoral cortical strain pattern

BackgroundStress shielding and nonphysiological load transfer after primary or revision total hip replacement (THR) prepare the ground for resorptive bone remodeling. The quality of the bone stock influences the risk of periprosthetic fractures and the severity of future revision surgeries. The question of whether or not bending stiffness and distal screw interlocking influence load transfer of a modular revision hip stem with a solid, hollow, and hollow-slotted stem extension led to the conception of this experimental study. The results were compared with a standard hip stem for primary THR. MethodsRevision stems were implanted in photoelastically coated composite femora. Cortical strain mapping was conducted before and after insertion of the implants under standardized loading conditions, considering the relevant muscle forces. Statistical analysis was based on a 95% confidence interval and a variance analysis for repeated measurements. ResultsSignificant stress shielding was observed after insertion of all types of hip stems compared with the intact femora. There was also a marked difference between strain alterations induced by standard and revision hip stems. With revision stems, the most distinct stress shielding effects were registered with the solid stem extension, particularly in the femoral diaphysis. Distal interlocking screws only had a local action on strain pattern and tended to enhance stress shielding at the midstem area when using the more flexible components. ConclusionMore flexible revision stems provide a cortical strain pattern of the femur closer to the preoperative status. This may reduce resorptive bone remodeling in the long term. However, any type of revision stem tested in this study caused higher stress shielding than the hip stem for primary THR, especially in the diaphyseal region medially and laterally. With sufficient proximal anchorage, the influence of distal interlocking screws on the femoral strain pattern was localized.

Reconstruction of Femur Length From Its Fragments in South Indian Females ..............................................IJMTFM (2011) 1(2): 45-53

Background: Stature reconstruction from skeletal remains forms part of the forensic anthropological analysis for the purpose of identification of an individual. Material and methods: It is well documented from the previous studies that intact femur has the highest correlation with stature and as such widely used in deriving the regression equations. However, the femur is not always recovered intact in forensic cases. The aim of the present study was to estimate the length of femur from measurements of its fragments. This information is important in archaeological studies and forensic investigations, particularly when fragmented material is examined. Results: Sixty femora of adults’ female individuals were selected for analysis. Maximum length and measures of seven fragments of the femur were obtained by means an osteometrical board and a digital caliper. Simples and multiple linear regressions were made to correlate each fragment with total length of the femur. Regressions formulae were obtained to define these estimates. Conclusion: The derived formulae are population specific and are designed for use in forensic skeletal analyses of South Indian females. Key Words: Forensic Anthropology Population Data, South Indian Females, Fragmentary Femur, Morphometry, Regression equation.

Evaluation of femoral strains with cementless proximal-fill femoral implants of varied stem length

BackgroundThe design intent of proximally-filling lateral flare femoral stems is to load the endosteal surface of the proximal femur both laterally and medially, to achieve normal bone strains. However, the long stem can contact the femoral cortex and may offload the proximal region to some extent. Therefore, in this study, we sought to determine if reducing the stem length, would result in physiologic strain patterns. MethodsUsing the PhotoStress® method we analyzed 13 femurs intact and with three different stem length implants: stemless, ultra-short and short. The test rig loaded the femoral head by simulating the mid-stance single leg support phase of gait with the ilio-tibial band and the hip abductor forces. The strain distribution with each stem length implant was then compared to the intact strain distribution to determine which was most similar. FindingsAs the stem length increased the femurs exhibited a typical pattern of reduced proximal strain and increased distal strain. However, there was some variation in this pattern indicating that the exact stem position and the location of its interaction with the endosteal surface of bone was not the same in each femur. InterpretationThe stemless design provided the best match compared to the native femur and therefore has the greatest potential to address the shortcomings of a stemmed femoral implant. However, the ultra-short implant also exhibited a strain distribution that closely emulated the intact femur, and may represent the best option as there are still several questions pertaining to stability and alignment of a stemless implant.

Experimental evaluation of new concepts in hip arthroplasty

In this thesis we evaluated two different hip arthroplasty concepts trough in vitro studies and numerical analyses. The cortical strains in the femoral neck area were increased by 10 to 15 % after insertion of a resurfacing femoral component compared to values of the intact femur, shown in an in vitro study on human cadaver femurs. There is an increased risk of femoral neck fracture after hip resurfacing arthroplasty. An increase of 10 to 15 % in femoral neck strains is limited, and cannot alone explain these fractures. Together with patient specific and surgical factors, however, increased strain can contribute to increased risk of fracture. An in vitro study showed that increasing the neck length in combination with retroversion or reduced neck shaft angle on a standard cementless femoral stem does not compromise the stability of the stem. The strain pattern in the proximal femur increased significantly at several measuring sites when the version and length of neck were altered. However, the changes were probably too small to have clinical relevance. In a validation study we have shown that a subject specific finite element analysis is able to perform reasonable predictions of strains and stress shielding after insertion of a femoral stem in human cadaver femurs. The usage of finite element models can be a valuable supplement to in vitro tests of femoral strain pattern around hip arthroplasty. Finally, a patient case shows that bone resorption around an implant caused by stress shielding can in extreme cases lead to periprosthetic fracture.

A biomechanical comparison of static versus dynamic lag screw modes for cephalomedullary nails used to fix unstable peritrochanteric fractures

The gamma nail has an option to statically lock its lag screw (static mode) or to allow its lag screw to move within the nail to compress the intertrochanteric fracture (dynamic mode). The purpose of this study was to compare the biomechanical stiffness of static and dynamic lag screw modes for a cephalomedullary nail used to fix an unstable peritrochanteric fracture. Unstable four-part peritrochanteric fractures were created in 30 synthetic femurs and fixed with Long Gamma 3 Nails. Mechanical tests were conducted for axial, lateral, and torsional stiffness with intact femurs, femur-nail constructs with static lag screw mode,and femur-nail constructs with dynamic lag screw mode. A paired Student's t test was used for all statistical comparisons between test groups. Axial and torsional stiffness of intact femurs was significantly greater than femur-nail constructs (p < 0.01 all comparisons),whereas lateral stiffness was significantly less (p < 0.01 all comparisons). Axial stiffness of the femur-nail construct was significantly greater (p < 0.01) in static mode (484.3 N/mm 80.2 N/mm) than in dynamic mode (424.1 N/mm 78.0 N/mm).Lateral stiffness was significantly greater (p < 0.01) in static mode (113.9 N/mm 8.4 N/mm) than in dynamic mode (109.5N/mm 8.8 N/mm). Torsional stiffness was significantly greater (p = 0.02) in dynamic mode (114.5 N/mm 28.2 N/mm) than in static mode (111.7 N/mm 27.0 N/mm). There is a 60 N/mm (12.4%) reduction in axial stiffness when the lag screw is in dynamic mode. Given the statistically significant reduction in axial and lateral stiffness with use of the dynamic mode, static lag screw mode should be further explored clinically for treatment of unstable peritrochanteric fractures.

Reconstruction of femur length from its fragments in South Indian males

Stature is one of the essential parameters in the establishment of identity of an individual. It is well documented from the previous studies that intact femur has the highest correlation with stature and as such widely used in the regression equation derivations. However, the femur is not always recovered intact in forensic cases, thereby rendering the equations derived from the whole bone inappropriate for analysis. This has necessitated the derivation of regression equations for estimating the length of femur, from the fragments of femur. The calculated maximum femoral length can then be used to estimate the stature of the individual by the regression equations, tables or the multiplication factors already established by the various studies.Seven variables were measured in a sample of 60 male femora. All the measurements showed positive degree of correlation. Regression equations for the estimation of maximum length of femur were derived. It is therefore stated that in the absence of intact femur, regression equations derived from the present study can provide a reliable estimate of maximum length of femur in South Indian males and thereby the adult stature.

A PHYSIOLOGICALLY ACCURATE MECHANICAL REPRESENTATION OF THE BONE-IMPLANT CONSTRUCT UNDER GAIT LOADS

Intramedullary nailing is a widely accepted technique utilized in the treatment of femoral fractures. Design of such devices should be accomplished based on physiological constraints, and loading of the femur, simulating in vivo conditions to prevent bone refracture and implant failure after surgical operation. It has been shown in literature that, walking is the most frequent dynamic activity of a patient, which necessitates testing implants mainly under walking loading conditions. In the present study, the response of the implanted femur having femoral mid-fracture as well as the intact femur were investigated at the instance of maximum hip contact force of the gait cycle in a finite element environment. Displacement and strain distribution on both boneimplant construct and intact bone were presented. The results may lead to an accurate estimation of the implants mechanical behavior in design stage, and be used in fatigue based analyses.

In vitro biomechanical comparison of three methods for internal fixation of femoral neck fractures in dogs

The in vitro biomechanical properties of three methods for internal fixation of femoral neck fractures were evaluated. Fifty cadaveric femura from Beagle dogs were used. Ten intact femora served as controls. In 40 femura, an osteotomy of the femoral neck was performed to simulate a transverse fracture. With the remaining 30 femura, three repair methods (two medium Orthofix pins, a 2.7 mm cortical bone screw placed in lag fashion and an anti- rotational Kirschner wire, or three divergent 1.1 mm Kirschner wires) were used to stabilize the osteotomies, and 10 osteotomies were stabilised per repair method. These 30 femura where then subject to monotonic loading to failure. Construct stiffness and load to failure were measured. In the remaining 10 femura, pressure sensitive film was placed at the osteotomy site prior to stabilization with either two Orthofix pins (n = 5) or a screw placed in lag fashion (n = 5) to determine the compressive pressure (MPa), compressive force (KN) and area of compression (cm²). There was no significant difference in the stiffness or load to failure for the three repair methods evaluated. There was no significant difference in the compressive pressure, compressive force or area of compression in osteotomies stabilized with Orthofix pins and 2.7 mm bone screws.

A silk hydrogel-based delivery system of bone morphogenetic protein for the treatment of large bone defects

The use of tissue grafting for the repair of large bone defects has numerous limitations including donor site morbidity and the risk of disease transmission. These limitations have prompted research efforts to investigate the effects of combining biomaterial scaffolds with biochemical cues to augment bone repair. The goal of this study was to use a critically-sized rat femoral segmental defect model to investigate the efficacy of a delivery system consisting of an electrospun polycaprolactone (PCL) nanofiber mesh tube with a silk fibroin hydrogel for local recombinant bone morphogenetic protein 2 (BMP-2) delivery. Bilateral 8 mm segmental femoral defects were formed in 13-week-old Sprague Dawley rats. Perforated electrospun PCL nanofiber mesh tubes were fitted into the adjacent native bone such that the lumen of the tubes contained the defect (Kolambkar et al., 2011b). Silk hydrogels with or without BMP-2 were injected into the defect. Bone regeneration was longitudinally assessed using 2D X-ray radiography and 3D microcomputed topography (μCT). Following sacrifice at 12 weeks after surgery, the extracted femurs were either subjected to biomechanical testing or assigned for histology. The results demonstrated that silk was an effective carrier for BMP-2. Compared to the delivery system without BMP-2, the delivery system that contained BMP-2 resulted in more bone formation (p<0.05) at 4, 8, 12 weeks after surgery. Biomechanical properties were also significantly improved in the presence of BMP-2 (p<0.05) and were comparable to age-matched intact femurs. Histological evaluation of the defect region indicated that the silk hydrogel has been completely degraded by the end of the study. Based on these results, we conclude that a BMP-2 delivery system consisting of an electrospun PCL nanofiber mesh tube with a silk hydrogel presents an effective strategy for functional repair of large bone defects.

Differences in External and Internal Cortical Strain with Prosthesis in the Femur

The contact between a femoral stem prosthesis and the internal surface of the cortical bone with the stress in the interface is of crucial importance with respect to loosening. However, there are no reports of strain patterns at this site, and the main aim of the current study was to investigate differences of internal and external cortical strain in the proximal femur after insertion of a stem prosthesis. The external cortical strain of a human cadaveric femur was measured with strain gauges before and after implantation of a stem prosthesis. By use of optical fibres embedded longitudinally in the endosteal cortex, deformations at the implant–internal cortex interface could also be measured. The main external deformation during loading of the intact femur occurred as compression of the medial cortex; both at the proximal and distal levels. The direction of the principal strain on the medial and lateral aspects was close to the longitudinal axis of the bone. After resection of the femoral neck and insertion of a stem prosthesis, the changes in external strain values were greatest medially at the proximal level, where the magnitude of deformation in compression was reduced to about half the values measured on the intact specimen. Otherwise, there were rather small changes in external principal strain. However, by comparing vertical strain in the external and internal cortex of the proximal femur, there were great differences in values and patterns at all positions. The transcortical differences in strain varied from compression on one side to distraction on the other and vice versa in some of the positions with a correlation coefficient of 0.07. Our results show that differences exist between the external and internal cortical strain when loading a stem prosthesis. Hence, strain at the internal cortex does not correspond and can not be deducted from measured strain at the external cortex.