25–40% of unicompartmental knee replacement (UKR) revisions are performed for unexplained pain possibly secondary to elevated proximal tibial bone strain. This study investigates the effect of tibial component metal backing and polyethylene thickness on cancellous bone strain in a finite element model (FEM) of a cemented fixed bearing medial UKR, validated using previously published acoustic emission data (AE). FEMs of composite tibiae implanted with an all-polyethylene tibial component (AP) and a metal backed one (MB) were created. Polyethylene of thickness 6–10mm in 2mm increments was loaded to a medial load of 2500N. The volume of cancellous bone exposed to <−3000 (pathological overloading) and <−7000 (failure limit) minimum principal (compressive) microstrain (µ∊) and >3000 and >7000 maximum principal (tensile) microstrain was measured. Linear regression analysis showed good correlation between measured AE hits and volume of cancellous bone elements with compressive strain <−3000µ∊: correlation coefficients (R= 0.947, R2 = 0.847), standard error of the estimate (12.6 AE hits) and percentage error (12.5%) (p<0.001). AP implants displayed greater cancellous bone strains than MB implants for all strain variables at all loads. Patterns of strain differed between implants: MB concentrations at the lateral edge; AP concentrations at the keel, peg and at the region of load application. AP implants had 2.2 (10mm) to 3.2 (6mm) times the volume of cancellous bone compressively strained <−7000µ∊ than the MB implants. Altering MB polyethylene insert thickness had no effect. We advocate using caution with all-polyethylene UKR implants especially in large or active patients where loads are higher

We investigated the static and cyclical strength of parallel and angulated locking plate screws using rigid polyurethane foam (0.32 g/cm. 3. ) and bovine cancellous bone blocks. Custom-made stainless steel plates with two conically threaded screw holes with different angulations (parallel, 10° and 20° divergent) and 5 mm self-tapping locking screws underwent pull-out and cyclical pull and bending tests. The bovine cancellous blocks were only subjected to static pull-out testing. We also performed finite element analysis for the static pull-out test of the parallel and 20° configurations. In both the foam model and the bovine cancellous bone we found the significantly highest pull-out force for the parallel constructs. In the finite element analysis there was a 47% more damage in the 20° divergent constructs than in the parallel configuration. Under cyclical loading, the mean number of cycles to failure was significantly higher for the parallel group, followed by the 10° and 20° divergent configurations. In our laboratory setting we clearly showed the biomechanical disadvantage of a diverging locking screw angle under static and cyclical loading

Introduction. Atypical femoral fracture focused on relation of bisphosphonate use, frequently. However, the mechanism of atypical femoral fracture was not yet clarified. Atypical femoral fractures have been kept femoral shaft cortical thickness and BMD, practically. We hypothesized that atypical femoral fractures were associated with impaired bone quality and curvature of femoral shaft. Materials & Methods. We experienced four atypical femoral fractures. One was subtrochanteric and three were shaft fracture. Two cases received bisphosphonate therapy for 3–5 years. BMD, bone metabolic markers, and bone quality markers were evaluated. Histomorphometry and collagen cross-link analysis were performed. Curvature of femoral shaft and 3-D finite element analysis in one incomplete fracture case were assessed. Results. BMD values were either maintained or not severely decreased. Deterioration of bone quality were verified by the results of histomorphometry, collagen cross-link analysis, and bone quality maker. Especially, homocystine values, such as one of bone quality markers, were increased in all cases. All atypical femoral shaft fractures showed outward curvature of femoral bone. In one case of incomplete atypical femoral shaft fracture, stress was concentrated at the fracture region according 3-D finite element analysis. Conclusions. The results of this study suggest that atypical femoral fractures were estimated associated with deterioration of bone quality and curvature of femoral shaft

Introduction. Short-segment posterior instrumentation for spine fractures is threatened by unacceptable failure rates. Two important design objectives of pedicle screws, bending and pullout strength, may conflict with each other. Hypothesis. Multiobjective optimization study with artificial neural network (ANN) algorithm and genetic algorithm (GA). Materials & Methods. Three-dimensional finite element (FE) methods were applied to investigate the optimal designs of pedicle screws with an outer diameter of 7 mm using a multiobjective approach for these two objectives. Based on the FE results on an L25 orthogonal array, two objective functions were developed by an ANN algorithm. Then, the trade-off solutions known as Pareto optima were explored by a GA. The optimal design was validated by mechanical tests. Results. The knee solutions of the Pareto fronts had simultaneous high bending and pullout strength ranging from 92 to 94 percent of their maxima. The corresponding range of the design parameters was 3.8 to 4.06 mm for inner diameter and 3.21 to 3.3 mm for pitch; 0 mm for beginning position of conical angle, 0.4 mm for proximal root radius, 5 degrees for proximal half angle, and 0.1 mm for thread width. The optimal design was well validated by mechanical tests, comparing with commercially available pedicle screws. Discussion & Conclusions. The optimal design of pedicle screws obtained could achieve an ideal with high mechanical performance in both bending and pullout tests

Unicompartmental knee replacements (UKRs) have inconsistent and variable survivorships reported in the literature. It has been suggested that many are revised for ongoing pain with no other mode of failure identified. Using a medial UKR with an all-polyethylene non-congruent tibial component from 2004–7, we noted a revision rate of 9/98 cases (9.2%) at a mean of 39 months. Subchondral sclerosis was noted under the tibial component in 3/9 revisions with well fixed implants, and the aim of this study was to investigate this as a mode of failure. 89 UKRs in 77 patients were investigated radiographically (at mean 50 months) and with SF-12 and Oxford Knee scores at mean follow up 55 months. Subjectively 23/89 cases (25%) had sclerosis under the tibial component. We describe a method of quantifying this sclerosis as a greyscale ratio (GSR), which was significantly correlated with presence/absence of sclerosis (p<0.001). Significant predictors of elevated GSR (increasing sclerosis) were female sex (p<0.001) and elevated BMI (P=0.010) on both univariate and multivariate analysis. In turn, elevated GSR was significantly associated with poorer improvement in OKS (p<0.05) at the time of final follow up. We hypothesise that this sclerosis results from repetitive microfracture and adaptive remodelling in the proximal tibia due to increased strain. Finite element analysis is required to investigate this further, but we suggest caution should be employed when considering all polyethylene UKR implants in older women and in those with BMI >35

Background. Locking internal fixation through a relatively small surgical dissection presents an innovative technique for managing distal tibial extra-articular fractures. The aim of this study is to evaluate the biomechanical properties of one locking internal fixation plate used to treat these injuries. Method. An AO/OTA43-A3 fracture was created in synthetic composite tibiae. Locking internal fixation was achieved with an anatomically pre-contoured medial distal tibial locking plate. Comparisons were made between different screw configurations in holes proximal to the fracture and monocortical versus bicortical fixation. Axial stiffness was measured using a universal materials testing machine. Finite element analysis (FEA) was used to model the elastic deformation of the constructs. Outcome measures were axial stiffness under physiological loading conditions and compression load to failure. Results. A trend towards reduced mean axial stiffness from the bicortical to the monocortical fixation constructs was observed. The physical model demonstrated no difference in measured mean axial stiffness between constructs with all screw holes filled and constructs with 2 screws in the holes closest and furthest from the fracture site. There was a 19% reduction in mean measured axial stiffness between constructs with all holes filled and in constructs with 2 screws in adjacent holes furthest from the fracture site (p<0.05). FEA predicted increased plate deflection and reduced construct axial stiffness with increasing distance of screw placement from the osteotomy site. Conclusion. Axial stiffness of distal tibial extra-articular metaphyseal fractures stabilized by locking internal fixation is dependent upon the configuration of the screw in the plate

Introduction. The MITCH PCR is an anatomic, flexible, horse-shoe shaped acetabular component, with 2 polar fins. The rationale of the PCR cup design is to reproduce a near-physiological stress distribution in the bone adjacent to the prosthesis. The thin composite cup is designed to fuse and flex in harmony with the surrounding bony structure. Only the pathological acetabular cartilage and underlying subchondral bone of the horseshoe-shaped, load-bearing portion of the acetabular socket is replaced, thus preserving viable bone stock. The PCR is manufactured from injection moulded carbon fibre reinforced polyetheretherketone (PEEK), with a two layer outer surface comprising hydroxyapatite and plasma sprayed commercially pure titanium. It is implanted in conjunction with a large diameter low wear femoral head, producing a bearing that will generate minimal wear debris with relatively inert particles. Pre-clinical mechanical testing, finite element analysis and biocompatibility studies have been undertaken. FEA evaluation predicts preservation of host bone density in the load bearing segments. A pilot clinical study was completed on a proto-type version of the PCR cup (the “Cambridge” cup), achieving excellent 5 and 10 year results. Subjects and Methods. We report the three-year results from a two-centre, prospective clinical evaluation study of the MITCH PCR cup. Patient outcome has been assessed using standardised clinical and radiological examinations and validated questionnaires. The change in physical level of activity and quality of life has been assessed using the Oxford Hip Score, Harris Hip score and the EuroQol-5D score, at scheduled time-points. Serial radiographs have been analysed to monitor the fixation and stability of the components. Results and Conclusions. In total 25 PCR cups were implanted by 3 surgeons. There were 12 men and 13 women. The mean patient age at time of surgery was 67 years (range 57–74). An Accolade TMZF stem was used as the femoral component in 19 patients and an Exeter stem in 6. The mean Oxford Hip score improved from 19.8 pre-operatively to 45 at the latest follow-up. The mean Euroqol-5D score improved from 62.6 to 83.6 and the Harris Hip score improved from 49.9 to 90.6. Three adverse events were noted in 2 patients (2 chest infections and 1 deep vein thrombosis). One revision of the acetabular component was performed at 21 months for squeaking. This has been investigated and modification of the articular geometry has resolved the problem on in-vitro testing

Introduction. Spinal aBMD only explains 50–80% of vertebral strength, and the application of aBMD measurements in isolation cannot accurately identify individuals who are likely to eventually experience bone fracture, due to the low sensitivity of the test. For appropriate treatment intervention, a more sensitive test of bone strength is needed. Such a test should include not only bone mineral density, but also bone quality. Quantitative computed tomography-based finite element methods (QCT/FEM) may allow structural analyses taking these factors into consideration to accurately predict bone strength (PBS). To date, however, basic data have not been reported regarding the prediction of bone strength by QCT/FEM with reference to age in a normal population. The purpose of this study was thus to create a database on PBS in a normal population as a preliminary trial. With these data, parameters that affect PBS were also analyzed. Methods. Participants in this study comprised individuals who participated in a health checkup program with CT at our hospital in 2009. Participants included 217 men and 120 women (age range, 40–89 years). Exclusion criteria were provided. Scan data of the second lumber vertebra (L2) were isolated and taken from overall CT data for each participant obtained with simultaneous scans of a calibration phantom containing hydroxyapatite rods. A FE model was constructed from the isolated data using Mechanical Finder software. For each of the FE models, A uniaxial compressive load with a uniform distribution and uniform load increment was applied. For each participant, height and weight were measured, BMI was calculated. Simple linear regression analysis was used to estimate correlations between age and PBS as analyzed by QCT/FEM. Changes in PBS with age were also evaluated by grouping participants into 5-year age brackets. One-way analysis of variance was used to compare average PBS for participants in each age range. Mean PBS in the 40–44 year age range was taken as the young adult mean (YAM). The ratio of mean PBS in each age group to YAM was calculated as a percentage. A multivariate statistical technique was used to determine how PBS was affected by age, height, weight, and BMI. Result/Discussion. Mean PBS was lower in women than in men for all age ranges. PBS in men and women significantly decreased with age. Simple linear regression between age and PBS showed the annual rate of decline in PBS was 55 N/year in men and 164 N/year in women. Mean PBS in the 75–79 year age range was 77% of YAM in men and in women, that in the 70–79 year age range was 47% of YAM. PBS was strongly dependent on age, while physical status had less effect

Introduction. In knee arthroplasty a ceramic component has several advantages: first, there is no ion release implying a risk for potential allergies. Second, the hardness of the material leads to a scratch resistance which ultimately reduces PE wear over time. In the past, ceramic components in knee applications were limited in the variety of design possibilities due to necessary thickness of the component resulting from the associated fracture risk of ceramics. By the development of an alumina matrix composite material with increased mechanical properties it is possible to develop ceramic knee components which have nearly the same design as a metal component and use the same implantation technique as well as the same instruments. This offers the surgeon the opportunity to choose intraoperatively between metal or ceramic knee components. Extensive in-vitro testing shows that ceramic knee components achieve superior mechanical test results. The reliability of the components is proven by two different burst tests and a fatigue test for both a femoral and a tibial ceramic knee component. Material and method. The mechanical proof-test was developed by subsequent steps of numerical load/stress analysis and design of an adequate mechanical test equipment. The procedure was organized as follows:. Oncologic: Analysis of relevant maximum in-vivo loading conditions. Analysis of the “boundary conditions”. Finite Element analysis: Identifying regions of highest stress concentration. Design analysis and accommodation if necessary. Development of an adequate mechanical test equipment which produces stresses comparable to the in-vivo conditions. Performing mechanical tests with ceramic femoral components. Validation of the test concept: comparison of test results and stress analysis. Assign “safety margin”,. Establish “proof test”. Results. Two independent load scenarios have been determined for each type of components as being in-vivo relevant. Hence, the developed proof-test consists of two subsequent load tests, the so-called regular test and the tension test for the femoral components, and the upper side test and the lower side test for the tibial components. In the regular test, the mechanical strength of the polished outer condyles is tested using a force which is equivalent to an in-vivo loading of 16 times bodyweight. In the tension test, the interior sides of the condyles are stressed in the sagittal plane ensuring a mechanically reliable implantation. This test is performed with a force equivalent to 10 times bodyweight. Discussion. The procedure to determine the proof loads using the maximum in-vivo loads together with a safety factor ensures the mechanical safety of the ceramic knee component. Together with the well-known excellent wear and biological behaviour of ceramics, this application provides an alternative to common metallic knee components. Clinical observations in the framework of a multi-centre study in different European countries have been started and show very promising results

Aims

Postoperative malalignment of the femur is one of the main complications in distal femur fractures. Few papers have investigated the impact of intraoperative malalignment on postoperative function and bone healing outcomes. The aim of this study was to investigate how intraoperative fracture malalignment affects postoperative bone healing and functional outcomes.

Aims

The best method of treating unstable pelvic fractures that involve the obturator ring is still a matter for debate. This study compared three methods of treatment: nonoperative, isolated posterior fixation and combined anteroposterior stabilization.

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).

It is becoming increasingly common for a patient to have ipsilateral hip and knee replacements. The inter-prosthetic (IP) distance, the distance between the tips of hip and knee prostheses, has been thought to be associated with an increased risk of IP fracture. Small gap distances are generally assumed to act as stress risers, although there is no real biomechanical evidence to support this.

The purpose of this study was to evaluate the influence of IP distance, cortical thickness and bone mineral density on the likelihood of an IP femoral fracture.

A total of 18 human femur specimens were randomised into three groups by bone density and cortical thickness. For each group, a defined IP distance of 35 mm, 80 mm or 160 mm was created by choosing the appropriate lengths of component. The maximum fracture strength was determined using a four-point bending test.

The fracture force of all three groups was similar (p = 0.498). There was a highly significant correlation between the cortical area and the fracture strength (r = 0.804, p <  0.001), whereas bone density showed no influence.

This study suggests that the IP distance has little influence on fracture strength in IP femoral fractures: the thickness of the cortex seems to be the decisive factor.

Cite this article: Bone Joint J 2014;96-B:1378–84.

This article presents a unified clinical theory that links established facts about the physiology of bone and homeostasis, with those involved in the healing of fractures and the development of nonunion. The key to this theory is the concept that the tissue that forms in and around a fracture should be considered a specific functional entity. This ‘bone-healing unit’ produces a physiological response to its biological and mechanical environment, which leads to the normal healing of bone. This tissue responds to mechanical forces and functions according to Wolff’s law, Perren’s strain theory and Frost’s concept of the “mechanostat”. In response to the local mechanical environment, the bone-healing unit normally changes with time, producing different tissues that can tolerate various levels of strain. The normal result is the formation of bone that bridges the fracture – healing by callus. Nonunion occurs when the bone-healing unit fails either due to mechanical or biological problems or a combination of both. In clinical practice, the majority of nonunions are due to mechanical problems with instability, resulting in too much strain at the fracture site. In most nonunions, there is an intact bone-healing unit. We suggest that this maintains its biological potential to heal, but fails to function due to the mechanical conditions. The theory predicts the healing pattern of multifragmentary fractures and the observed morphological characteristics of different nonunions. It suggests that the majority of nonunions will heal if the correct mechanical environment is produced by surgery, without the need for biological adjuncts such as autologous bone graft.

Cite this article: Bone Joint J 2016;98-B:884–91.