Influence of age-related bone density changes on primary stability in stemless shoulder arthroplasty: A multi-implant finite element study.

BACKGROUND: Stemless implants were introduced to prevent some of the stem-related complications associated with the total shoulder arthroplasty. Although general requirements for receiving these implants include good bone quality conditions, little knowledge exists about how bone quality affects implant performance. The goal of this study was to evaluate the influence of age-induced changes in bone density, as a metric of bone quality, in the primary stability of five anatomic stemless shoulder implants using 3D finite element (FE) models. METHODS: The implant designs considered were based on the Global Icon, Sidus, Simpliciti, SMR, and Inhance stemless implants. Shoulder arthroplasties were virtually simulated in Solidworks. The density distributions of 20 subjects from two age groups, 20 to 40 and 60 to 80 years old, were retrieved from medical image data and integrated into three-dimensional FE models of a single humerus geometry, developed in Abaqus, to avoid confounding factors associated with geometric characteristics. For the designs which do not have a solid collar covering the entire bone surface, i.e., the Sidus, Simpliciti, SMR, and Inhance implants, contact and non-contact conditions between the humeral head component and bone were considered. Primary stability was evaluated through the assessment of micromotions at the bone-implant interface considering eight load cases related to rehabilitation activities and demanding tasks. Three research variables, considering 20 µm, 50 µm, and 150 µm as thresholds for osseointegration, were used for a statistical analysis of the results. RESULTS: The decreased bone density registered for the 60-80 age group led to larger micromotions at the bone-implant interface when compared to the 20-40 age group. The Global Icon-based and Inhance-based designs were the least sensitive to bone density, whereas the Sidus-based design was the most sensitive to bone density. The establishment of contact between the humeral head component and bone for the implants that do not have a solid collar led to decreased micromotions. DISCUSSION: Although the age-induced decline in bone density led to increased micromotions in the FE models, some stemless shoulder implants presented good overall performance regardless of the osseointegration threshold considered, suggesting that age alone may not be a contraindication to anatomic total shoulder arthroplasty. If only primary stability is considered, the results suggested superior performance for the Global Icon-based and Inhance-based designs. Moreover, the humeral head component should contact the resected bone surface when feasible. Further investigation is necessary to combine these results with the long-term performance of the implants and allow more precise recommendations.

Progression of partial to complete ruptures of the Achilles tendon during rehabilitation: A study using a finite element model.

Substantial research on complete Achilles tendon ruptures is available, but guidance on partial ruptures is comparatively sparse. Conservative management is considered acceptable in partial tendon ruptures affecting less than 50% of the tendon's width, but supporting experimental evidence is currently lacking. Using a previously validated finite element model of the Achilles tendon, this study aimed to assess whether loading conditions simulating an early functional rehabilitation protocol could elicit progression to a complete rupture in partial ruptures of varying severity. In silico tendon rupture simulations were performed to locate the most likely rupture site for least, moderate, and extreme subtendon twist configurations. These three models were split at the corresponding rupture site and two sets of partial ruptures were created for each, starting from the medial and lateral sides, and ranging from 10% to 50% loss of continuity. Simulations were conducted with material parameters from healthy and tendinopathic tendons. Partial ruptures were considered to progress if the volume of elements showing a maximum principal strain above 10% exceeded 3 mm3. To assess whether the tendinopathic tendons typical geometric characteristics could compensate for the inferior material properties found in tendinopathy, an additional model with increased cross-sectional area in the free tendon region was developed. Progression to complete ruptures occurred even with less than a 50% loss of continuity, regardless of subtendon twisting, and material parameters. The tendinopathic tendon model with increased cross-sectional area showed similar results. These findings suggest the current criteria for surgical treatment of partial ruptures should be reconsidered. Statement of clinical significance: The clinical significance and most appropriate treatment of partial ruptures of the Achilles tendon is unclear. Despite the widespread use of the "50% rule" in treatment decisions of partial tendon ruptures, experimental evidence supporting it is missing. The present study provides new data, from a validated aponeurotic and free Achilles tendon finite element model, showing that partial ruptures may progress to complete ruptures under loading conditions elicited from functional rehabilitation protocols, even for partial ruptures affecting less than 50% of the tendon's width. Under these novel findings, the current criteria for surgical treatment of partial ruptures should be reconsidered.

Influence of the rotator cuff tear pattern in shoulder stability after arthroscopic superior capsule reconstruction: a computational analysis.

OBJECTIVES: To assess the ability of the arthroscopic superior capsule reconstruction (SCR) in restoring glenohumeral stability in the presence of different preoperative patterns of irreparable rotator cuff tears (RCTs). METHODS: A computational musculoskeletal (MSK) model of the upper limb was used to simulate isolated SCR and to estimate the stability of the shoulder. Four patterns of preoperative irreparable RCTs were modeled: Supraspinatus (SSP); SSP â€‹+ â€‹Subscapularis (SSC); SSP â€‹+ â€‹Infraspinatus (ISP); and SSP â€‹+ â€‹SSC â€‹+ â€‹ISP. The muscles involved in the irreparable RCT were removed from the MSK model to simulate an irreparable full-thickness tear. In the MSK model, the muscle and joint forces were estimated for a set of upper limb positions, from four types of motions (abduction in the frontal plane, forward flexion in the sagittal plane, reaching behind the back, and combing the hair) collected in a biomechanics laboratory, through inverse dynamic analysis. The stability of the shoulder was estimated based on the tangential and compressive components of the glenohumeral joint reaction force. The comparison of pre- and post-operative conditions, for the four patterns of irreparable RCTs, with the healthy condition, was performed using ANOVA and Tukey's tests (statistical level of p â€‹< â€‹0.05). RESULTS: In the setting of an isolated irreparable SSP tear, SCR statistically significantly improved stability compared with the preoperative condition (p â€‹< â€‹0.001). For the irreparable SSP â€‹+ â€‹SSC pattern, a statistically significant loss in stability was observed (p â€‹< â€‹0.001) when SCR was applied. For the irreparable SSP â€‹+ â€‹ISP and SSP â€‹+ â€‹SSC â€‹+ â€‹ISP patterns, the postoperative condition increased shoulder stability, compared to the preoperative condition; however, the improvement was not statistically significantly different. CONCLUSION: Isolated SCR for irreparable RCTs extending beyond the SSP does not statistically significantly improve the stability of the glenohumeral joint. LEVEL OF EVIDENCE: Level IV.

Design and validation of a finite element model of the aponeurotic and free Achilles tendon.

The Achilles tendon (AT) is a common injury site. Ruptures are usually located in the free tendon but may cross the myotendinous junction into the aponeurotic region. Considering the possibility of aponeurotic region involvement in AT ruptures, a novel three dimensional (3D) finite element (FE) model that includes both the aponeurotic and free AT regions and features subtendon twisting and sliding was developed. It was hypothesized that the model would be able to predict in vivo data collected from the literature, thus being considered valid, and that model outputs would be most sensitive to subtendon twist configurations. The 3D model was constructed using magnetic resonance images. The model was divided into soleus and gastrocnemius subtendons. In addition to a frictionless contact condition, the interaction between subtendons was modeled using two contact formulations: sliding with anisotropic friction and no sliding. Loads were applied on the tendon's most proximal cross-section and anterior surface, with magnitudes estimated from in vivo studies. Model outputs were compared with experimental data regarding 3D deformation, transverse plane rotation, and nodal displacements in the free tendon. The FE model adequately simulated the free tendon behavior regarding longitudinal strain, cross-section area variation, transverse plane rotation, and sagittal nodal displacements, provided that subtendon sliding was allowed. The frictionless model exhibited noticeable medial transverse sliding of the soleus subtendon, which was present to a much lesser degree in the anisotropic friction model. Model outputs were most sensitive to variations in subtendon twist and dispersion of the collagen fiber orientations. Clinical Significance: This Achilles tendon finite element model, validated using in vivo experimental data, may be used to study its mechanical behavior, injury mechanisms, and rupture risk factors.

Influence of Graft Positioning during the Latarjet Procedure on Shoulder Stability and Articular Contact Pressure: Computational Analysis of the Bone Block Effect.

The Latarjet procedure is the most popular surgical procedure to treat anterior glenohumeral (GH) instability in the presence of large anterior glenoid bone defects. Even though the placement of the bone graft has a considerable influence on its efficacy, no clear indications exist for the best graft position. The aim of this study was to investigate the influence of the medial-lateral positioning of the bone graft on the contact mechanics and GH stability due to the bone block effect. Four finite element (FE) models of a GH joint, with a 20% glenoid bone defect, treated by the Latarjet procedure were developed. The FE models differed in the medial-lateral positioning of the bone graft, ranging from a flush position to a 4.5 mm lateral position with respect to the flush position. All graft placement options were evaluated for two separate shoulder positions. Anterior GH instability was simulated by translating the humeral head in the anterior direction, under a permanent compressive force, until the peak translation force was reached. Joint stability was computed as the ratio between the shear and the compressive components of the force. The lateralization of the bone graft increased GH stability due to the bone block effect after a 3 mm lateralization with respect to the flush position. The increase in GH stability was associated with a concerning increase in peak contact pressure due to the incongruous contact between the articulating surfaces. The sensitivity of the contact pressures to the medial-lateral positioning of the bone graft suggests a trade-off between GH stability due to the bone block effect and the risk of osteoarthritis, especially considering that an accurate and consistent placement of the bone graft is difficult in vivo.

Metaphyseal sleeves in revision total knee arthroplasties: Computational analysis of bone remodeling.

BACKGROUND: Metaphyseal sleeves help maintain long term stability and reduce revision rate for aseptic loosening in total knee arthroplasty (TKA) revision. However, their performance regarding bone remodeling is still poorly known for the long term. This study aimed to investigate the impact of metaphyseal sleeves on the bone remodeling of the tibia. METHODS: Five finite element models of a female tibia with different implant configurations (regarding stem length and metaphyseal sleeve application) were developed. Loading conditions included joint reaction force, muscle, and tibia-fibula loads from 6 instances of the gait cycle. The bone remodeling model applied was adapted to the subject under analysis by selecting the bone remodeling parameters that best replicated the bone density distribution of the tibia estimated from the CT data. Changes in bone density after TKA were evaluated in 8 regions of interest. RESULTS: Global bone loss ranged from -31.16%, in 115 mm stemmed configurations, to -20.93%, in 75 mm stemmed configurations. Apart from the lateral and posterior regions in the proximal tibia, whose bone loss reduced and increased, respectively, due to the incorporation of a metaphyseal sleeve, changes in bone density were similar with and without a metaphyseal sleeve for each stem length. CONCLUSION: The results suggest that bone remodeling of the tibia is not critically affected by the incorporation of metaphyseal sleeves. Considering that sleeves are believed to present a favorable clinical outcome in stability and osseointegration, reducing the revision rate for aseptic loosening, their advantages seem to outweigh their disadvantages regarding bone remodeling.

Shoulder Positioning during Superior Capsular Reconstruction: Computational Analysis of Graft Integrity and Shoulder Stability.

The shoulder position during fixation of the graft may be a key factor impacting the outcome of arthroscopic superior capsular reconstruction (ASCR) in irreparable rotator cuff tears (IRCTs). However, biomechanical evidence regarding this effect is lacking. The aim of this study was to evaluate the influence of the shoulder position during fixation of the graft on shoulder stability and graft tear risk in ASCR. A 3-D musculoskeletal model of the upper limb was modified to account for the fixation of the graft in ASCR, assuming a full-thickness tear of the supraspinatus tendon. The concomitant tenotomy of the long head of the biceps (LHB) tendon was also studied. The biomechanical parameters evaluated included the strain of the graft and the glenohumeral joint reaction force (GH JRF), which were used to evaluate graft integrity and shoulder stability, respectively. Fixation of the graft considering abduction angles greater than 15° resulted in a high risk for graft tearing when the arm was adducted to the side of the trunk. For abduction angles below 15°, the mean shoulder stability improved significantly, ranging between 6% and 20% (p < 0.001), compared with that in the preoperative condition. The concomitant tenotomy of the LHB tendon resulted in loss of stability when compared to ASCR with an intact LHB tendon. The position of the shoulder during fixation of the graft has a significant effect on shoulder stability and graft tear risk after ASCR in IRCTs. This study provides new and important information regarding the role of shoulder positioning during fixation of the graft.

Influence of the PFNA screw position on the risk of cut-out in an unstable intertrochanteric fracture: a computational analysis.

The position of the lag screw in the femoral head is a key factor to cut-out, the most reported complication in the internal fixation of intertrochanteric fractures. Considering that the best position for the lag screw remains controversial, the aim of this study was to evaluate the influence of different lag screw positions on the risk of cut-out of an unstable intertrochanteric fracture fixed with a Proximal Femoral Nail Anti-Rotation (PFNA) implant. The relationship between cut-out and the tip-apex distance (TAD) or the calcar referenced tip-apex distance (CalTAD) was also investigated. Finite element models of one male and one female femur treated with a PFNA implant were developed considering the lag screw positioned centrally and inferiorly on the anteroposterior view, and for each of these, the screw tip at 4 discrete positions along its longitudinal axis. All 8 positions simulated for each femur considered the lag screw in a centre position on the lateral view. The risk of cut-out was evaluated for two loading conditions assuming it is related with high compressive strains. The bone region at the fracture line, near the tip of the missing medial fragment, was always the most concerning regarding high compressive strains. The inferior positioning of the lag screw reduced the volume of bone susceptible to yielding compared to the centre positioning. The deep placement of the screw tip improved the outcome for both centre and inferior positions. The results suggested the inferior and deep placement of the screw to be the best position to reduce the risk of cut-out. The volume of bone susceptible to yielding was found not to be correlated to TAD or CalTAD, suggesting that further investigation is necessary to identify other, more reliable, predictors of cut-out.

Proximal and mid-thigh fascia lata graft constructs used for arthroscopic superior capsule reconstruction show equivalent biomechanical properties: an in vitro human cadaver study.

BACKGROUND: The proximal fascia lata (FL) graft construct used for arthroscopic superior capsule reconstruction (ASCR) is openly harvested, whereas the mid-thigh FL graft construct is minimally invasively harvested. The purpose of the current study was to compare the biomechanical properties of proximal thigh and mid-thigh-harvested FL graft constructs used for ASCR. The hypothesis was that, despite the different morphological characteristics of the proximal thigh and mid-thigh FL graft constructs used for ASCR, their biomechanical properties would not significantly differ. This information may assist orthopedic surgeons in the choice of the harvest location, technique, and type of graft construct for ASCR. METHODS: Forty FL specimens, 20 proximal thigh and 20 mid-thigh, were harvested from the lateral thighs of 10 fresh human cadavers (6 male, 4 female; average age, 58.60 ± 17.20 years). The thickness of each 2-layered proximal thigh and 6-layered mid-thigh FL graft construct was measured. Each construct was mechanically tested in the longitudinal direction, and the stiffness and Young's modulus were computed. Data were compared by Welch's independent t-test and analysis of variance, and statistical significance was set at P < .05. RESULTS: The average thickness of the proximal thigh FL graft construct (7.17 ± 1.97 mm) was significantly higher than that of the mid-thigh (5.54 ± 1.37 mm) [F (1,32) = 7.333, P = .011]. The average Young's modulus of the proximal thigh and mid-thigh graft constructs was 32.85 ± 19.54 MPa (range, 7.94 - 75.14 MPa; 95% confidence interval [CI], 23.71 - 42.99) and 44.02 ± 31.29 MPa (range, 12.53 -120.33 MPa; 95% CI, 29.38 - 58.66), respectively. The average stiffness of the proximal thigh and mid-thigh graft constructs was 488.96 ± 267.80 N/mm (range, 152.96 - 1086.49 N/mm; 95% CI, 363.63 - 614.30) and 562.39 ± 294.76 N/mm (range, 77.46 - 1229.68 N/mm; 95% CI, 424.44 - 700.34), respectively. There was no significant difference in the average Young's modulus or stiffness between the proximal thigh and mid-thigh graft constructs (P = .185 and P = .415, respectively). CONCLUSION: Despite the different morphological characteristics of the proximal thigh and mid-thigh FL graft constructs used for ASCR, their Young's modulus and stiffness did not significantly differ.

Stress analysis in a bone fracture fixed with topology-optimised plates.

The design of commercially available fixation plates and the materials used for their fabrication lead to the plates being stiffer than bone. Consequently, commercial plates are prone to induce bone stress shielding. In this study, three-dimensional fixation plates are designed using topology optimisation aiming to reduce the risk of bone stress shielding. Fixation plate designs were optimised by minimising the strain energy for three levels of volume reduction (i.e. 25%, 45% and 75%). To evaluate stress shielding, changes in bone stress due to the different fixation plate designs were determined on the fracture plane of an idealised shaft of a long bone under a four-point bending load considering the effect of a patient walking with crutches of a transverse fractured tibia. Topology optimisation is a viable approach to design less stiff plates with adequate mechanical strength considering high volume reductions, which consequently increased the stress transferred to the bone fracture plane minimising bone stress shielding.

Bone adaptation impact of stemless shoulder implants: a computational analysis.

BACKGROUND: Despite stemless implants showing promising functional and radiologic clinical outcomes, concerning signs of complications, such as bone resorption, have been reported. The aim of this study was to investigate the influence of 5 stemless designs on the bone adaptation process of the humerus. METHODS: Three-dimensional finite element models of shoulder arthroplasties were developed considering stemless designs based on the Eclipse, Global Icon, SMR, Simpliciti, and Sidus stemless systems. For the designs not possessing a collar that covers the entire resected surface of the humerus, conditions of contact and no contact were simulated between the humeral head components and the bone surface. By use of a bone remodeling model, computational simulations were performed considering 6 load cases of standard shoulder movements. The bone adaptation process was evaluated by comparing differences in bone density between the implanted models and the intact model of the humerus. RESULTS: Overall, the design of the stemless implants had a relevant impact on the bone adaptation process of the humerus. The Eclipse-based design caused the largest bone mass loss, whereas the SMR-based design caused the least. When contact was simulated between the humeral head components of the SMR-, Simpliciti-, and Sidus-based designs and the resected bone surface, bone resorption increased. DISCUSSION: Considering only the bone adaptation process, the results suggest that the SMR-based implant presents the best performance and that contact between the humeral head component and the resected bone surface should be avoided. However, because other factors must be considered, further investigation is necessary to allow definite recommendations.

Computational design and fabrication of a novel bioresorbable cage for tibial tuberosity advancement application.

Tibial tuberosity advancement (TTA) is a promising method for the treatment of cruciate ligament rupture in dogs that usually implies the implantation of a titanium cage as bone implant. This cage is non-biodegradable and fails in providing adequate implant-bone tissue integration. The objective of this work is to propose a new process chain for designing and manufacturing an alternative biodegradable cage that can fulfill specific patient requirements. A three-dimensional finite element model (3D FEM) of the TTA system was first created to evaluate the mechanical environment at cage domain during different stages of the dog walk. The cage microstructure was then optimized using a topology optimization tool, which addresses the accessed local mechanical requirements, and at same time ensures the maximum permeability to allow nutrient and oxygen supply to the implant core. The designed cage was then biofabricated by a 3D powder printing of tricalcium phosphate cement. This work demonstrates that the combination of a 3D FEM with a topology optimization approach enabled the design of a novel cage for TTA application with tailored permeability and mechanical properties, that can be successfully 3D printed in a biodegradable bioceramic material. These results support the potential of the design optimization strategy and fabrication method to the development of customized and bioresorbable implants for bone repair.

Critical analysis of musculoskeletal modelling complexity in multibody biomechanical models of the upper limb.

The inverse dynamics technique applied to musculoskeletal models, and supported by optimisation techniques, is used extensively to estimate muscle and joint reaction forces. However, the solutions of the redundant muscle force sharing problem are sensitive to the detail and modelling assumptions of the models used. This study presents four alternative biomechanical models of the upper limb with different levels of discretisation of muscles by bundles and muscle paths, and their consequences on the estimation of the muscle and joint reaction forces. The muscle force sharing problem is solved for the motions of abduction and anterior flexion, acquired using video imaging, through the minimisation of an objective function describing muscle metabolic energy consumption. While looking for the optimal solution, not only the equations of motion are satisfied but also the stability of the glenohumeral and scapulothoracic joints is preserved. The results show that a lower level of muscle discretisation provides worse estimations regarding the muscle forces. Moreover, the poor discretisation of muscles relevant to the joint in analysis limits the applicability of the biomechanical model. In this study, the biomechanical model of the upper limb describing the infraspinatus by a single bundle could not solve the complete motion of anterior flexion. Despite the small differences in the magnitude of the forces predicted by the biomechanical models with more complex muscular systems, in general, there are no significant variations in the muscular activity of equivalent muscles.

Subject-specific bone remodelling of the scapula.

Finite element analyses, with increasing levels of detail and complexity, are becoming effective tools to evaluate the performance of joint replacement prostheses and to predict the behaviour of bone. As a first step towards the study of the complications of shoulder arthroplasty, the aim of this work was the development and validation of a 3D finite element model of an intact scapula for the prediction of the bone remodelling process based on a previously published model that attempts to follow Wolff's law. The boundary conditions applied include full muscle and joint loads taken from a multibody system of the upper limb based on the same subject whose scapula was here analysed. To validate the bone remodelling simulations, qualitative and quantitative comparisons between the predicted and the specimen's bone density distribution were performed. The results showed that the bone remodelling model was able to successfully reproduce the actual bone density distribution of the analysed scapula.

Bone remodelling analysis of the humerus after a shoulder arthroplasty.

The shoulder arthroplasty has become an efficient treatment for some pathologies. However there are complications that can compromise its success. Among them, the stress shielding effect on the humerus has been reported as a possible cause of failure. The objective of this work was to investigate the bone remodelling in the humerus after a shoulder arthroplasty. For this purpose, computational models were developed to analyse the stress shielding contribution to the humeral component failure of shoulder arthroplasties, with a cemented and an uncemented prosthesis. A computational remodelling model was used to characterize the bone apparent density at each site of the humerus. The density distribution was obtained by the solution of a problem that takes into account both structural stiffness and the metabolic cost of bone maintenance. Bone was subjected to 6 load cases that include the glenohumeral reaction force and the action of 10 muscles. In the implanted models, different interface conditions were tested for the bone-implant and the cement-implant interfaces. Moreover, a pathological case defined by a poorer quality of bone was considered. In the healthy situation, the models that better model in vivo conditions showed no significant changes in bone mass. However, the results for the pathological case showed some bone resorption which supports the importance given to the quality of bone in the success of the joint replacement. Bearing in mind the conditions addressed, the results lead to conclude that the stress shielding is not a key factor for the humeral component failure of shoulder arthroplasties in a healthy situation though several issues, including muscle function and bone quality, may heighten its effect.