Pankaj

Techniques to improve the shear strength of impacted bone graft: the effect of particle size and washing of the graft.

Background: When fresh morselized graft is compacted, as in impaction bone-grafting for revision hip surgery, fat and marrow fluid is either exuded or trapped in the voids between particles. We hypothesized that the presence of incompressible fluid damps and resists compressive forces during impaction and prevents the graft particles from moving into a closer formation, thus reducing the graft strength. In addition, viscous fluid such as fat may act as an interparticle lubricant, thus reducing the interlocking of the particles.Methods: We performed mechanical shear testing in the laboratory with use of fresh-frozen human femoral-head allografts that had been passed through different orthopaedic bone mills to produce graft of differing particle-size distributions (grading).Results: After compaction of fresh graft, fat and marrow fluid continued to escape on application of normal loads. Washed graft, however, had little lubricating fluid and better contact between the particles, increasing the shear resistance. On mechanical testing, washed graft was significantly (p < 0.001) more resistant to shearing forces than fresh graft was. This feature was consistent for different bone mills that produced graft of different particle-size distributions and shear strengths.Conclusions: Removal of fat and marrow fluid from milled human allograft by washing the graft allows the production of stronger compacted graft that is more resistant to shear, which is the usual mode of failure. Further research into the optimum grading of particle sizes from bone mills is required.Clinical Relevance: Understanding the mechanical properties of milled human allograft is important when impaction grafting is used for mechanical support. A simple means of improving the mechanical strength of graft produced by currently available bone mills, including an intraoperative washing technique, is described.

The influence of stem length and fixation on initial femoral component stability in revision total knee replacement

Objectives Orthopaedic surgeons use stems in revision knee surgery to obtain stability when metaphyseal bone is missing. No consensus exists regarding stem size or method of fixation. This in vitro study investigated the influence of stem length and method of fixation on the pattern and level of relative motion at the bone–implant interface at a range of functional flexion angles. Methods A custom test rig using differential variable reluctance transducers (DVRTs) was developed to record all translational and rotational motions at the bone–implant interface. Composite femurs were used. These were secured to permit variation in flexion angle from 0° to 90°. Cyclic loads were applied through a tibial component based on three peaks corresponding to 0°, 10° and 20° flexion from a normal walking cycle. Three different femoral components were investigated in this study for cementless and cemented interface conditions. Results Relative motions were found to increase with flexion angle. Stemmed implants reduced relative motions in comparison to stemless implants for uncemented constructs. Relative motions for cemented implants were reduced to one-third of their equivalent uncemented constructs. Conclusions Stems are not necessary for cemented implants when the metaphyseal bone is intact. Short cemented femoral stems confer as much stability as long uncemented stems.

Does bone compaction around the helical blade of a proximal femoral nail anti-rotation (PFNA) decrease the risk of cut-out?: A subject-specific computational study

Objectives Because of the contradictory body of evidence related to the potential benefits of helical blades in trochanteric fracture fixation, we studied the effect of bone compaction resulting from the insertion of a proximal femoral nail anti-rotation (PFNA). Methods We developed a subject-specific computational model of a trochanteric fracture (31-A2 in the AO classification) with lack of medial support and varied the bone density to account for variability in bone properties among hip fracture patients. Results We show that for a bone density corresponding to 100% of the bone density of the cadaveric femur, there does not seem to be any advantage in using a PFNA with respect to the risk of blade cut-out. On the other hand, in a more osteoporotic femoral head characterised by a density corresponding to 75% of the initial bone density, local bone compaction around the helical blade provides additional bone purchase, thereby decreasing the risk of cut-out, as quantified by the volume of bone susceptible to yielding. Conclusions Our findings indicate benefits of using a PFNA over an intramedullary nail with a conventional lag screw and suggest that any clinical trial reporting surgical outcomes regarding the use of helical blades should include a measure of the femoral head bone density as a covariable.

Age‐related optimisation of screw placement for reduced loosening risk in locked plating

When using locked plating for bone fracture fixation, screw loosening is reported as one of the most frequent complications and is commonly attributed to an incorrect choice of screw configuration. Choosing a patient‐optimized screw configuration is not straightforward as there are many interdependent variables that affect device performance. The aim of the study was to evaluate the influence that locking screw configuration has on loosening risk and how this is influenced by bone quality. This study uses finite element models that incorporate cortical bone heterogeneity, orthotropy, and geometrical nonlinearity to examine the effect of screw configuration on variables associated with loosening and interfragmentary motion. Strain levels within the bone were used as indicators of regions that may undergo loosening. The study found that, in healthy bone under axial loading, the most important variables influencing strain levels within the bone were the size of the bridging span (working length) and the plate rigidity. Unlike healthy bone, osteoporotic bone was found to be particularly sensitive to the spacing of the screws within the plate. Using two empty screw holes between the screws closest to the fracture was found to reduce the strain levels at the first screw by 49% in osteoporotic bone (compared to only 2.4% in healthy bone). The study also found that under torsional loading the total number of screws used was the most important variable with a 59% reduction in the strain around the screws closest to the fracture when using six rather than four screws in osteoporotic bone.

Major source of error when calculating bone mechanical properties.

Mechanical testing of bone and fracture callus is performed to assess the functional properties of the tissue. In particular, 3‐ point bending is a commonly performed technique when experimentally evaluating mechanical properties. The Euler‐ Bernoulli equation used to calculate the bending stiffness assumes that the beam is long and slender. In practice, beams that have a span to depth ratio (aspect ratio) of greater than 20:1 are considered to be “slender.” (1) If this method is used on non‐ slender bones an error results as the contribution of the bending due to shear deformation is not taken into account. A review of articles published in the last 2 years in this journal indicated that in the 14 papers that used 3‐point bending to determine elastic modulus none accounted for the effect on deformation due to shear. The error from ignoring the contribution of shear deformation on bones with these properties can be as high as 38%. If the analysis does not take shear deformation into account, large errors in the evaluated material properties will result in an estimated 95% of bones. Three‐point bending is often carried out using long bones from animals such as the mouse, rat, or rabbit that are too small to allow sectioning into smaller samples and must therefore be tested whole. Tests performed on these bones will therefore always be subject to inaccuracies if they assumed to be slender beams. The problem is compounded by the fact that the test length of the specimen is not the length of the bone, but the distance between the supports. In order to provide a suitable location, the supports are generally placed at the metaphyses of the bone. Therefore, the span is less than the total length of the bone, further reducing the aspect ratio, often by 25%, resulting in an increase to the potential error. The problem of not accounting for deflection due to shear is further exacerbated as mechanical testing is often used to determine the influence of medical treatments or physical diseases. In order to study these, animals (usuallymice or rats) are bred with genetic deficiencies. In some cases, such as is found in the FGFR‐3 deficient mouse, this results in musculoskeletal changes. These changes can result in significantly different aspect ratio of limbs in comparison to the wild and the modified specimen as a limb may be longer or thicker without a corresponding change in the complimentary dimension. Assumptions made about the cross‐sectional shape, (ie, whether it is approximately circular as with the shaft of the femur or approximately triangular as for the shaft of the tibia) can also result in errors. However, for whole bones, themajor determinant of this error is the aspect ratio rather than the cross‐sectional geometry. It is recognized that small errors may arise due to non‐ prismatic geometry of the bone, ie, changes in cross‐section from proximal to distal, and inhomogeneous material properties. These are best considered through the use of numerical simulation (eg, finite element analysis), which require a full 3D geometry construction via a CT scan and assignment of variable material properties from CT attenuation data. As a consequence it requires considerably more resources: scanning; conversion of images to numerical models; and analysis of models and interpretation of results. As a result, this type of analysis is rarely performed on small animal studies such as those conducted using rat or mouse limbs. Numerical analysis, due to the additional work and resources required, is usually restricted limited to human bones, as the limited supply of these bones can often warrant the use of additional resources. The deflection due to bending and that attributed to shear can be derived following the methods set out in Wang.

Nonlinear multiscale regularisation in MR elastography: Towards fine feature mapping

&NA; Fine‐featured elastograms may provide additional information of radiological interest in the context of in vivo elastography. Here a new image processing pipeline called ESP (Elastography Software Pipeline) is developed to create Magnetic Resonance Elastography (MRE) maps of viscoelastic parameters (complex modulus magnitude |G*| and loss angle ø) that preserve fine‐scale information through nonlinear, multi‐scale extensions of typical MRE post‐processing techniques. Methods: A new MRE image processing pipeline was developed that incorporates wavelet‐domain denoising, image‐driven noise estimation, and feature detection. ESP was first validated using simulated data, including viscoelastic Finite Element Method (FEM) simulations, at multiple noise levels. ESP images were compared with MDEV pipeline images, both in the FEM models and in three ten‐subject cohorts of brain, thigh, and liver acquisitions. ESP and MDEV mean values were compared to 2D local frequency estimation (LFE) mean values for the same cohorts as a benchmark. Finally, the proportion of spectral energy at fine frequencies was quantified using the Reduced Energy Ratio (RER) for both ESP and MDEV. Results: Blind estimates of added noise (&sgr;) were within 5.3% ± 2.6% of prescribed, and the same technique estimated &sgr; in the in vivo cohorts at 1.7 ± 0.8%. A 5 × 5 × 5 truncated Gabor filter bank effectively detects local spatial frequencies at wavelengths &lgr; ≤ 10px. For FEM inversions, mean |G*| of hard target, soft target, and background remained within 8% of prescribed up to Symbol and mean ø results were within 10%, excepting hard target ø, which required redrawing around a ring artefact to achieve similar accuracy. Inspection of FEM |G*| images showed some spatial distortion around hard target boundaries and inspection of ø images showed ring artefacts around the same target. For the in vivo cohorts, ESP results showed mean correlation of Symbol with MDEV and liver stiffness estimates within 7% of 2D‐LFE results. Finally, ESP showed statistically significant increase in fine feature spectral energy as measured with RER for both |G*| (Symbol) and ø (Symbol). Conclusion: Information at finer frequencies can be recovered in ESP elastograms in typical experimental conditions, however scatter‐ and boundary‐related artefacts may cause the fine features to have inaccurate values. In in vivo cohorts, ESP delivers an increase in fine feature spectral energy, and better performance with longer wavelengths, than MDEV while showing similar stability and robustness. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. Symbol. No caption available. HighlightsNew Magnetic Resonance Elastography (MRE) software pipeline incorporating wavelet‐based denoising and feature‐detection techniques.Systematic noise testing with new Finite Element Method (FEM)–based simulations.Results robust to noise and show new levels of detail for MRE elastograms. Graphical abstract Figure. No caption available.

3D non-linear analysis of the acetabular construct following impaction grafting

The study investigates the short-term behaviour of the acetabular construct following revision hip arthroplasty, carried out using the Slooff–Ling impaction grafting technique; using 3D finite element analyses. An elasto-plastic material model is used to describe the constitutive behaviour of morsellised cortico-cancellous bone (MCB) graft, since it has been shown that MCB undergoes significant plastic deformation under normal physiological loads. Based on previous experimental studies carried out by the authors and others, MCB is modelled using non-linear elasticity and Drucker Prager Cap (DPC) plasticity. Loading associated with walking, sitting down, and standing up is applied to the acetabular cup through a femoral head using smooth sliding surfaces. The analyses yield distinctive patterns of migration and rotation due to different activities. These are found to be similar to those observed in the clinical setting.

Metal-backed versus all-polyethylene unicompartmental knee arthroplasty

Objectives Up to 40% of unicompartmental knee arthroplasty (UKA) revisions are performed for unexplained pain which may be caused by elevated proximal tibial bone strain. This study investigates the effect of tibial component metal backing and polyethylene thickness on bone strain in a cemented fixed-bearing medial UKA using a finite element model (FEM) validated experimentally by digital image correlation (DIC) and acoustic emission (AE). Materials and Methods A total of ten composite tibias implanted with all-polyethylene (AP) and metal-backed (MB) tibial components were loaded to 2500 N. Cortical strain was measured using DIC and cancellous microdamage using AE. FEMs were created and validated and polyethylene thickness varied from 6 mm to 10 mm. The volume of cancellous bone exposed to < -3000 µε (pathological loading) and < -7000 µε (yield point) minimum principal (compressive) microstrain and > 3000 µε and > 7000 µε maximum principal (tensile) microstrain was computed. Results Experimental AE data and the FEM volume of cancellous bone with compressive strain < -3000 µε correlated strongly: R = 0.947, R2 = 0.847, percentage error 12.5% (p < 0.001). DIC and FEM data correlated: R = 0.838, R2 = 0.702, percentage error 4.5% (p < 0.001). FEM strain patterns included MB lateral edge concentrations; AP concentrations at keel, peg and at the region of load application. Cancellous strains were higher in AP implants at all loads: 2.2- (10 mm) to 3.2-times (6 mm) the volume of cancellous bone compressively strained < -7000 µε. Conclusion AP tibial components display greater volumes of pathologically overstrained cancellous bone than MB implants of the same geometry. Increasing AP thickness does not overcome these pathological forces and comes at the cost of greater bone resection. Cite this article: C. E. H. Scott, M. J. Eaton, R. W. Nutton, F. A. Wade, S. L. Evans, P. Pankaj. Metal-backed versus all-polyethylene unicompartmental knee arthroplasty: Proximal tibial strain in an experimentally validated finite element model. Bone Joint Res 2017;6:22–30. DOI:10.1302/2046-3758.61.BJR-2016-0142.R1

Metal backed versus all-polyethylene unicompartmental knee arthroplasty: the effect of implant thickness on proximal tibial strain in an experimentally validated finite element model

Objectives Up to 40% of unicompartmental knee arthroplasty (UKA) revisions are performed for unexplained pain which may be caused by elevated proximal tibial bone strain. This study investigates the effect of tibial component metal backing and polyethylene thickness on bone strain in a cemented fixed-bearing medial UKA using a finite element model (FEM) validated experimentally by digital image correlation (DIC) and acoustic emission (AE). Materials and Methods A total of ten composite tibias implanted with all-polyethylene (AP) and metal-backed (MB) tibial components were loaded to 2500 N. Cortical strain was measured using DIC and cancellous microdamage using AE. FEMs were created and validated and polyethylene thickness varied from 6 mm to 10 mm. The volume of cancellous bone exposed to < -3000 µε (pathological loading) and < -7000 µε (yield point) minimum principal (compressive) microstrain and > 3000 µε and > 7000 µε maximum principal (tensile) microstrain was computed. Results Experimental AE data and the FEM volume of cancellous bone with compressive strain < -3000 µε correlated strongly: R = 0.947, R2 = 0.847, percentage error 12.5% (p < 0.001). DIC and FEM data correlated: R = 0.838, R2 = 0.702, percentage error 4.5% (p < 0.001). FEM strain patterns included MB lateral edge concentrations; AP concentrations at keel, peg and at the region of load application. Cancellous strains were higher in AP implants at all loads: 2.2- (10 mm) to 3.2-times (6 mm) the volume of cancellous bone compressively strained < -7000 µε. Conclusion AP tibial components display greater volumes of pathologically overstrained cancellous bone than MB implants of the same geometry. Increasing AP thickness does not overcome these pathological forces and comes at the cost of greater bone resection. Cite this article: C. E. H. Scott, M. J. Eaton, R. W. Nutton, F. A. Wade, S. L. Evans, P. Pankaj. Metal-backed versus all-polyethylene unicompartmental knee arthroplasty: Proximal tibial strain in an experimentally validated finite element model. Bone Joint Res 2017;6:22–30. DOI:10.1302/2046-3758.61.BJR-2016-0142.R1

Investigation of Modelling Parameters for Finite Element Analysis of MR Elastography

Introduction Magnetic resonance elastography (MRE) utilizes mechanically induced shear waves to attain material property measurements of in vivo tissue. Finite element analysis (FEA) can be used to replicate the technique in silico to aid in the testing and development of the MRE post-processing software. This study aimed to investigate the influence of modelling parameters upon FEA of MRE.