For cementless femoral stems, there is debate as to whether a collar enhances primary stability and load transfer compared to collarless designs. Finite Element (FE) analysis has the potential to compare stem designs within the same cohort, allowing for subtle performance differences to be identified, if present. Subject-specific FE models of intact and implanted femora were run for a diverse cohort (21 males, 20 females; BMI 16.4-41.2 kg/m , age 50-80 yrs). Collared and collarless versions of Corail (DePuy Synthes, Warsaw, IN) were sized and positioned using an automated algorithm that aligns the femoral/stem axes, preserves the head-center location, and maximizes metaphyseal fit. Joint contact and muscle forces simulating peak forces in level gait and stair climbing and were scaled to the body mass and applied to each subject. Three failure scenarios were assessed: Potential for peri-prosthetic fibrous tissue formation (stem micromotion), potential for peri-prosthetic bone damage (equivalent strains), and calcar bone remodeling (changes in strain-energy density). Comparisons were performed using paired t-tests. Only subtle differences were found (mean 90th percentile micromotion: Collared = 86 µm, collarless = 92.5 µm, mean 90th percentile interface strains: Collared = 733 µϵ, collarless = 767 µϵ, and similar remodeling stimuli were predicted). The slight differences observed were small in comparison with the inter-patient variability. Statement of clinical significance: Our results suggest that the presence/absence of a collar is unlikely to substantially alter the bone-implant biomechanics nor the initial mechanical environment. Hence, a collar is likely to have minimal clinical impact. Analysis using different femoral stem designs is recommended before generalising these findings. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1185-1195, 2018.

The primary stability achieved during total hip arthroplasty determines the long-term success of cementless acetabular cups. Pre-clinical finite element testing of cups typically use a model of a single patient and assume the results can be extrapolated to the general population. This study explored the variability in predicted primary stability of a Pinnacle cementless acetabular cup in 103 patient-specific finite element models of the hemipelvis and examined the association between patient-related factors and the observed variability. Cups were inserted by displacement-control into the FE models and then a loading configuration simulating a complete level gait cycle was applied. The cohort showed a range of polar gap of 284-1112 μm and 95th percentile composite peak micromotion (CPM) of 18-624 μm. Regression analysis was not conclusive on the relationship between patient-related factors and primary stability. No relationship was found between polar gap and micromotion. However, when the patient-related factors were categorised into quartile groups, trends suggested higher polar gaps occurred in subjects with small and shallow acetabular geometries and cup motion during gait was affected most by low elastic modulus and high bodyweight. The variation in primary stability in the cohort for an acetabular cup with a proven clinical track record may provide benchmark data when evaluating new cup designs. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1012-1023, 2018.

Current practices for designing new cushions for seats depend on superficial measurements, such as pressure mapping, which do not provide sufficient information about the condition of sub-dermal tissues. Finite element (FE) modelling offers a unique alternative to integrate assessment of sub-dermal tissue condition into seat/cushion design and development processes. However, the development and validation of such FE models for seated humans requires accurate representation of the anatomy and material properties, which remain challenges that are yet to be addressed. This paper presents the development and validation of a detailed 3D FE model with high anatomical fidelity of the buttock and thigh, for a specific seated subject. The developed model consisted of 28 muscles, the pelvis, sacrum, femur, and one layer of inter-muscular fat, subcutaneous fat and skin. Validation against in vivo measurements from MRI data confirmed that the FE model can simulate the deformation of soft tissues under sitting loads with an accuracy of (mean ± SD) 4.7 ± 4.4 mm. Simulation results showed that the maximum strains (compressive, shear and von-Mises) on muscles (41, 110, 79%) were higher than fat tissues (21, 62, 41%). The muscles that experienced the highest mechanical loads were the gluteus maximus, adductor magnus and muscles in the posterior aspect of the thighs (biceps femoris, semitendinosus and semimembranosus muscles). The developed FE model contributes to the progression towards bio-fidelity in modelling the human body in seated postures by providing insight into the distribution of stresses/strains in individual muscles and inter-muscular fat in the buttock and thigh of seated individuals. Industrial applications for the developed FE model include improving the design of office and household furniture, automotive and airplane seats and wheelchairs as well as customisation and assessment of sporting and medical equipment to meet individual requirements.

Aims The primary aim of this study was to address the hypothesis that fracture morphology might be more important than posterior malleolar fragment size in rotational type posterior malleolar ankle fractures (PMAFs). The secondary aim was to identify clinically important predictors of outcome for each respective PMAF-type, to challenge the current dogma that surgical decision-making should be based on fragment size. Methods This observational prospective cohort study included 70 patients with operatively treated rotational type PMAFs, respectively: 23 Haraguchi Type I (large posterolateral-oblique), 22 Type II (two-part posterolateral and posteromedial), and 25 (avulsion-) Type III. There was no standardized protocol on how to address the PMAFs and CT-imaging was used to classify fracture morphology and quality of postoperative syndesmotic reduction. Quantitative 3D-CT (Q3DCT) was used to assess the quality of fracture reduction, respectively: the proportion of articular involvement; residual intra-articular: gap, step-off, and 3D-displacement; and residual gap and step-off at the fibular notch. These predictors were correlated with the Foot and Ankle Outcome Score (FAOS) at two-years follow-up. Results Bivariate analyses revealed that fracture morphology (p = 0.039) as well as fragment size (p = 0.007) were significantly associated with the FAOS. However, in multivariate analyses, fracture morphology (p = 0.001) (but not fragment size (p = 0.432)) and the residual intra-articular gap(s) (p = 0.009) were significantly associated. Haraguchi Type-II PMAFs had poorer FAOS scores compared with Types I and III. Multivariate analyses identified the following independent predictors: step-off in Type I; none of the Q3DCT-measurements in Type II, and quality of syndesmotic reduction in small-avulsion Type III PMAFs. Conclusion PMAFs are three separate entities based on fracture morphology, with different predictors of outcome for each PMAF type. The current debate on whether or not to fix PMAFs needs to be refined to determine which morphological subtype benefits from fixation. In PMAFs, fracture morphology should guide treatment instead of fragment size. Cite this article: Bone Joint J 2020;102-B(9):1229–1241.

Traditionally, diaphyseal stems have been utilized to augment the stability of revision total knee replacement (rTKR) implants. More recently metaphyseal augments, such as sleeves, have been introduced to further augment component fixation. The effect of augments such as stems and sleeves have on the primary stability of a rTKR implant is poorly understood, however it has important implications on the complexity, costs and survivorship of the procedure. Finite element analysis was used to investigate the primary stability and strain distribution of various size stems and sleeves used in conjunction with a cementless revision tibial tray. The model was built from computer tomography images of a single healthy tibia obtained from an 81-year-old patient to which an Anderson Orthopaedic Research Institute (AORI) IIA defect was virtually added. The influences of varying body mass index (BMI) and bone modulus were also investigated. Stemless sleeves were found to provided adequate primary implant stability (average implant micro-motion <50 μm) for the studied defect. Addition of a stem did not enhance the primary stability. Furthermore, this study found that varying BMI and bone modulus had a considerable effect on strain distribution but negligible effect on micro-motion in the sleeve area. In conclusion, the addition of diaphyseal stem to a metaphyseal sleeve had little benefit in enhancing the primary stability of tibial trays augmented when simulating reconstructions of AORI IIA tibial defects. Additional studies are required to determine the relative benefit of the diaphyseal stem when using metaphyseal sleeves defects with more extensive bone loss. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1876-1886, 2018.