Failure characteristics of all polyethylene cemented glenoid implants in total shoulder arthroplasty

Total shoulder arthroplasty (TSA) still suffers today from mid-term and long-term complications such as glenoid implant loosening, wear, humeral head subluxation/dislocation and implant fracture. Unlike the hip and knee joint replacements, the artificial shoulder joint has yet to offer a long-term satisfactory solution to shoulder replacement. With loosening being the number one reason for TSA revision, investigating methods of monitoring the glenoid implant loosening and investigate the effects of various design parameters on the loosening behaviour of the glenoid fixation is necessary to explore the problem. Several studies were carried out using in-vitro cyclic testing and FEA to; investigate failure progression and its correlation to quantitative measures in a 2D study (n = 60), investigating key glenoid design features in a 2D (n = 60) and 3D study (n = 20), investigating the validity of using bone substitute foam for studying glenoid fixation in a cadaveric study and investigating any correlation between failure and CT or in-vitro quantitative measures (n = 10). Visible failure was observed, for the first time, correlating to inferior rim displacement and vertical head displacement measures. CT failure was detected in 70% of specimens before visible failure was observed. Out of the design pairs tested; smooth-back/rough-back (range of roughnesses), peg/keel, curved-back/flat-back and conforming/non-conforming, roughening the back-surface to 3.4 μm or more improved fixation performance (p < 0.05). Roughening the back-surface changed the mode of failure from implant/cement failure inferiorly due to tensile/shear stresses, to cement/bone failure superiorly due to compressive/shear loading. Differences in the other design pairs were marked showing peg to perform better than keel, conforming over non-conforming and no difference in curved-back over flat-back, although these differences are marginal. Improvements in the standard testing method have also been suggested

Development of a two-axis cyclic loading device for mechanical testing of glenoid component fixation

The fundamental mechanism of aseptic glenoid component loosening, the rocking horse phenomenon, is a reaction to glenohumeral articular forces that are not centered on the component. While glenoid component loosening remains a problem, the underlying mechanisms that lead to fixation failure at the bone-component contact remain controversial. Several studies employing the ASTM F2028 technique have successfully recreated the rocking horse effect. However, no obvious strategy to decrease component loosening has been presented. This thesis investigates the behavior of forces that lead to component loosening on cyclically loaded components using three different protocols and testing apparatuses—a Stewart Platform, a cyclic loading experimental rig, and an ASTM F2028-17-compliant version of the experimental rig. The experimental assessment of response forces acting on the glenoid implant is a desired outcome since it can be used to compare implant designs and allows for controlled testing of alternative materials for prosthesis advancement to lessen the stresses that produce the rocking horse phenomena

Proposta de teste pré-clínico para aferir o desempenho biomecânico de próteses do ombro

Dotoramento em Engenharia MecânicaProsthesis commercialization requires the compliance of regulations that ensure the safe use of prosthesis. However, several prostheses have been withdrawn from the market due to their high failure rates, which is a strong indicator of the lack of suitable pre-clinical tests that allow a more rigorous evaluation of its performance and effectiveness. Thus, the main objective of this doctoral thesis consisted in the development of a pre-clinical test capable of accessing shoulder prosthesis performance. For this purpose, a multi-body model of the intact shoulder containing all muscle groups was used in the first stage in view to identify and characterize those that most contribute to the 90º abduction movement, being them the deltoid, the infraspinatus, the supraspinatus and the subscapularis. Two in vitro models were constructed using composite bone structures of the humerus and of the scapula. In the intact model the cartilage and the inferior glenohumeral ligament were considered and in the implanted model a non-cemented anatomical prosthesis (Comprehensive® Total Shoulder System) and a central post in porous metal for glenoid fixation were used. Strain gage rosettes were used to measure the deformation suffered by the bone structures when positioned at 90º abduction and subjected to loading. Finite element models (FEM) of the intact and implanted shoulder, that replicate the in vitro models, were developed. The FEM were subjected to the same loading scenarios as the in vitro models. The comparison between the strains determined numerically and experimentally allowed FEM validation. Stress and strain distribution inside the bone structures, determined with the FEM of the implanted shoulder, agree with the clinical observations present in literature. This indicates that, in a general way, the developed FEM predicts bone behavior in the presence of a prosthesis and may be considered a pre-clinical test to evaluate shoulder implants performance. To verify that the pre-clinical test developed is sensitive to small differences in implant design and that can be used to predict shoulder prosthesis performance, a new central fixation post in polyethylene was used. Stress and strain distributions determined using the FEM with the new fixation post are (once again) in agreement with clinical observations, confirming that the developed FEM can be used for the pre-clinical evaluation of other shoulder implant designs, allowing to analyze their performance before clinical use.A comercialização de uma prótese requer o cumprimento de regulamentos e normas que garantam a segurança de utilização da mesma. No entanto, diversas próteses têm sido retiradas do mercado devido às elevadas taxas de insucesso que apresentam, sendo este um forte indicador da falta de testes adequados que permitam uma avaliação mais rigorosa do seu desempenho e eficácia. Deste modo, o principal objetivo desta tese de doutoramento consistiu no desenvolvimento de um ensaio pré-clínico capaz de aferir o desempenho biomecânico de próteses do ombro. Para o efeito, numa primeira fase foi utilizado um modelo multi-corpo do ombro intacto contendo todos os grupos musculares com vista a identificar e caracterizar os que mais contribuem para o movimento de abdução de 90°, sendo eles o deltoide, o infraespinhal, o supraespinhal e o subescapular. Foram construídos dois modelos in vitro recorrendo a estruturas ósseas compósitas do úmero e da escápula. No modelo intacto foram consideradas as cartilagens e o ligamento glenohumeral inferior e no modelo implantado foi utilizada uma prótese anatómica não cimentada (Comprehensive® Total Shoulder System) e um pino central de fixação da componente da glenoide revestido com metal poroso. Rosetas de extensometria foram utilizadas para medir as extensões sofridas pelas estruturas ósseas quando posicionadas a 90º de abdução e sob carregamento. Foram desenvolvidos modelos de elementos finitos (EF) do ombro intacto e implantado que replicam os modelos in vitro. Os modelos de EF foram sujeitos aos mesmos cenários de carregamento que os modelos in vitro. A comparação entre as deformações determinadas numericamente e experimentalmente permitiu a validação dos modelos de EF. A distribuição de tensões e deformações no interior das estruturas ósseas, determinadas com o modelo de EF do ombro implantado, estão de acordo com as observações clínicas presentes na literatura. Isto indica que, de uma forma geral, o modelo de EF desenvolvido prevê o comportamento do osso na presença de uma prótese e pode ser considerado um teste pré-clínico para avaliação do desempenho de implantes do ombro. Para verificar que o teste préclínico desenvolvido é sensível a pequenas diferenças no design dos implantes e que pode ser utilizado para prever o desempenho de próteses, foi utilizado um novo pino central de fixação em polietileno. A distribuição de tensões e de deformações determinadas através do modelo de EF usando o novo pino de fixação estão (mais uma vez) de acordo com as observações clínicas, o que confirma que o modelo de EF desenvolvido pode ser utilizado na avaliação préclínica de outros implantes do ombro, permitindo analisar o seu desempenho antes da utilização clínica

Investigating the effect of mechanical loading in a total reversed shoulder implant

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The shoulder joint is a multi-axis synovial ball and socket joint, by having a loose connection it provides a wide degree of freedom; however this means the joint lacks robustness and is prone to damage most commonly from shoulder dislocations. A rotator cuff tear causes major problems in allowing the arm to be lifted beyond a 90˚ abduction position. It is common that this insufficiency aggravates arthritis problems that may have occurred due the rotator cuff tear problem. The study focuses on investigating, describing and quantifying the implant geometric properties to evaluate the joint contact characteristics and use the outcome in redesign the implant. The investigation presents results of finite element analysis on a heavy loading condition on a Verso (reverse) shoulder implant which is validated using experimental data on the same prosthesis. The results are validated within a 5% error margin. A Verso implant is modelled using MIMICS (materialise) and imported into ABAQUS (Simulia, Providence, USA) to analyse the distribution of stress, strain and displacement across the Humerus and Scapula. Details of interaction, boundary conditions, loads and material properties are all obtained from research and applied to the model to portray realistic behaviour. The resulting stress, strain and displacement from this simulation are indicated to show the magnitude and distribution across the entire bone region. This validates the benefits of a Verso implant compared to conventional and long stemmed reverse shoulder implants, as well as provide a basis from which improved designs can be built upon and allow further accurate methods to be developed in analysing shoulder implants effectively

The effect of implant misalignment on shoulder replacement outcomes

Total shoulder arthroplasty is a well-established treatment to relieve pain and restore joint function particularly in arthritis patients. The damaged shoulder joint is replaced with humeral and glenoid components. For success, all replacement components must be aligned properly. However, errors in glenoid component alignment particularly in version is not infrequent due to the complexities such as limited monitoring available during the surgical procedure and glenoid posterior wear, commonly observed in glenohumeral osteoarthritis. Glenoid component version has been found to induce eccentric load and may result in component loosening which is the main indicator for revision surgery. The overall aim of this thesis is to gain the in-depth understanding of how the component version affects the fixation loosening in both cementless and cemented shoulder arthroplasty. Early loosening in cementless arthroplasty is associated with failed biological fixation due to excessive micromotion at the implant-bone interface. To measure interface micromotion, this thesis developed an in-vitro technique based on the application of digital volume correlation (DVC) and micro-computed tomography (μCT). This technique was validated and verified the use of the finite element (FE) method as a tool for investigating the effect of glenoid component version on micromotion. The FE predicted micromotion during a full range of shoulder abduction confirmed that 60° of abduction was the critical position inducting the largest micromotion and large micromotions were shown to be related to increased component retroversion. The condition of the bone was also found to be an important parameter as less stiff bone caused large micromotions.Open Acces

Biomechanical evaluation of Glenoid Component Stability after ATSA under phasic cyclic loading

Background Total shoulder arthroplasty (TSA) is considered a successful curative procedure for many stiff painful shoulder disorders. However, it may be associated with many complications. Glenoid loosening is thought to be the most common complication of anatomic total shoulder arthroplasty (ATSA); its underlying causes could be mechanical (abnormal loading), septic (infections) or aseptic (autoimmune reaction). This study discusses the mechanical glenoid component loosening after ATSA. II. Hypotheses (Hi, Hii & Hiii) (Hi) The recorded mean peak pressure values of the ATSA components are expected to vary greatly according to the motion type; (Hii) the recorded mean peak pressure values are expected to vary between the initial and final phases of each tested specimen; and (Hiii) the occurrence of glenoid component loosening and its degree of extension are expected to be related to the changes of the obtained mean peak pressure values. III. Objectives This study’s aim was to conduct a comprehensive experimental biomechanical evaluation of the stability of ATSA components under phasic cyclic loading, as follows: (i) testing of the degree of artificial glenoid component stability under repetitive phasic cyclic loading; (ii) testing of the relation between the criteria of the applied cyclic loading according to our testing plan and the occurrence of glenoid component loosening; (iii) measurement and assessment of the values, patterns and magnitudes of the contact pressure between the joint components under cyclic loading; (iv) comparison between the obtained mean peak contact pressure values under cyclic loading in the initial and final phases to detect any relations and/or differences; (v) correlation of the measured pressure values during testing with the QCT findings with respect to glenoid component loosening. IV. Materials A series of six fresh-frozen complete cadaveric shoulder joint specimens (bones and soft tissues) was used in this study. The specimens were implanted with ATSA components and tested successively by mounting them on the shoulder simulator. To measure the values mentioned above, we used a TekScan system with a group of two-headed pressure sensor foils, QCT, shoulder pointer and a digitalized 3Dimaging Zebris system with US, in addition to the routinely used surgical and lab instruments in such experiments. V. Methodology The specimens were scanned prior to experimentation to evaluate their articular surfaces morphology. Then the specimens were implanted with ATSA components and a pressure sensor was inserted within the joint cavity of each specimen and situated on the glenoid component surface. The six specimens were successively mounted on the shoulder simulator and each was tested through three phases of cyclic loading in the three directions of motion. The 1st and 3rd short phases took place for each specimen with insertion of a pressure sensor within the joint cavity, while the 2nd long phase took place without sensor insertion. After the completion of all experiments, the specimens were again scanned with QCT to evaluate the position of the implanted glenoids and any presence of radiolucency and/or loosening. VI. Findings (Observations & Examinations) Two specimens were severely unstable during testing, even with the application of lower loads, particularly during abduction/adduction motion cycles. Provisional and/or subsequent controlling physical examinations revealed either a malposition of the glenoid component or a suspected abnormal glenoid morphology. The other four specimens were completely stable during testing in all motion directions with the application of different loading forces and stabilizing weights. Four specimens were radiologically determined to have a massive glenoid component loosening after the completion of testing. VII. Results The recorded mean peak pressure values varied greatly between the testing phases, testing cycles and motion directions. The highest mean peak pressure values were recorded during AA testing episodes, followed by FE testing episodes. The lowest mean peak pressure values were recorded during IE testing episodes. However, high mean peak pressure values were also recorded during IE testing episodes, but with a low frequency. In seven testing episodes, the recorded mean peak pressure values were higher by 16.7 % in all directions of motion in the final testing phase than those recorded in the initial phase of all testing episodes (42 testing episodes). According to the computed t-test values between the initial and final phases per motion direction/per specimen, null hypothesis (Hypothesis (Hii)) was accepted in the whole AA & FE testing cycles with a percentage of 100%, while it was rejected in only one relation of IE testing cycles with a percentage of 5% and accepted in six relations of IE testing cycles with a percentage of 95%. In total, null hypothesis (Hypothesis (Hii)) was rejected in only one relation of the testing cycles, with a percentage of 5%, and accepted in twenty relations of the testing cycles with a percentage of 95%. According to the calculated t-test values between all initial and final phases for each specimen, null hypothesis (Hypothesis (Hii)) was rejected in two experiments with a percentage of 28.6% and accepted in five experiments with a percentage of 71.4%. Four specimens (three keeled and one pegged) were found to be loose, representing 66.7% of all specimens; one of them was unstable during the testing, representing 25% of the loose specimens and 16.7% of all specimens. VIII. Conclusion The recorded mean peak pressure values and load quantities of the tested shoulder joint varied greatly between motion phases, motion cycles and motion types. The resulting contact pressures across the shoulder joint during its action varied greatly according to the acting force, motion type, muscles status and pathologies within the joint and were directly proportional to the motion type, being higher during AA and FE motion cycles than during IE motion cycles. Also, they were directly proportional to the contact surface area and to the degree of compression between joint articulating surfaces during motion. The greatest degree of variability of SD and mean peak pressure values was seen during FE testing cycles. Shoulder joint instability after ATSA could result from component malposition and/or the articular surface morphological abnormalities. Both glenoid loosening and joint instability could incite the occurrence of the other and could worsen its course in a devastating vicious circle. We concluded that glenoid component loosening could be related to joint stability, loads and the mode of load application in relation to the application duration, and to some extent to the component type, which was apparently evident in our study. The first and third hypotheses were approved, while the second hypothesis was statistically rejected (according to the computed t-test values), which may require a further evaluation in future studies

The Application of Digital Volume Correlation Bone Strain Measurements in the Osteoarthritic Glenohumeral Joint

This thesis investigates the accuracy and precision of digital volume correlation measurements derived from micro-computed tomography imagery of interfaces in the upper extremity of clinical relevance, namely, the implant-cement-bone interface of glenoid implants used in total shoulder arthroplasties and the implant-bone interface of shoulder hemiarthroplasties. The works within derive relationships between measurement accuracy and precision and parameters of practical interest such as image quality and measurement spatial resolution. It also analyzes the effects of micro-computed tomography image artifact-inducing materials on the accuracy and precision of digital volume correlation-based measurements and the spatial relationship between distance between the artifacting material and the magnitude of change in accuracy and precision. Finally, it also contains an in-vitro model of the peripheral glenoid peg-cement-bone interface which is subsequently analyzed through digital volume correlation; the relationship between peg/bone region and strain magnitude is elucidated

Design and Development Towards a Novel Prosthesis for Total Shoulder Arthroplasty to Reduce Aseptic Glenoid Loosening

Total shoulder arthroplasty (TSA) is the most common surgical solution, that helps in restoring the structural and functional integrity of a diseased glenohumeral (GH) joint with intact rotator-cuff. A 300% increase in the usage of TSA has been observed since 2007, along with 2.5% increase in revision rate. Aseptic glenoid loosening accounts for 37% of postsurgical failures in TSA. Eccentric loading of the prosthetic glenoid cup, leading to the “rocking horse” effect, is one of the prevalent causes of aseptic glenoid loosening. Current anatomical total shoulder prosthesis (ATSP) geometry does not consider all the GH morphometric features, for example the elliptical shape of the humeral head. Moreover, the morphometric studies leading to the initial ATSP design did not consider the GH morphology of any sub-Saharan population. Hence, there exists a gap in understanding of the implications of certain morphometric features on the functionality of a post-TSA GH joint. This thesis had two primary aims to address this gap in knowledge. Firstly, to study the GH morphometric variations between cohorts representing native European (Swiss) and native sub-Saharan (South African) populations. Secondly, to develop anatomically inspired ATSP design concepts and test them using biomechanical and finite element (FE) models, insilico, under standardised testing protocols. The morphometric analysis suggested that an average Swiss humeral head radius of curvature was larger (P28mm or <19mm. Considering both the populations, the inherent shape of an average humeral head was found to be elliptical. The thickest region of the head was found to lie in the posterior region and not at the geometric center. Hertzian contact theory was applied to calculate the GH stresses produced by symmetric and asymmetric elliptical heads. Higher concentric stresses (P<0.001), within the acceptable limit for polyethylene, were observed to be imparted by the asymmetric heads. Population-specific musculoskeletal models were developed to study the post-TSA kinematic variation. When an identical range of motion (RoM) was performed by these models, population-specific variation in muscle moment arms was observed. The novel glenoid designs were not found to alter the post-surgical kinematics. FE models of the biradial, compartmental and pear-shaped glenoid implant designs were subjected to compressive and shear loading according to the American Society for Testing and Materials (ASTM). Using the bi-radial the glenoid cup, with thickened posterior-superior surface, anatomically relevant force distribution patterns could be replicated. Compartmentalising the glenoid prosthesis into concentric and eccentric regions with the gaps, proved to be highly beneficial. When compared to a commercially available glenoid prosthesis, the compartmental prosthesis was able to contain the GH forces to the concentric region for longer, delaying the eccentric loading and therefore potentially reducing the “rocking horse” effect. In the light of the above observations, two conclusions can be drawn from this thesis. Firstly, it would be beneficial if population-specific ATSP were made available for natives of certain geographic locations. Secondly, glenoid prosthesis designs could be compartmentalised to contain the GH joint forces within the concentric regions of the cup which might aid in the reduction of post-TSA complications

Biomechanical evaluation of Glenoid Component Stability after ATSA under phasic cyclic loading

Background Total shoulder arthroplasty (TSA) is considered a successful curative procedure for many stiff painful shoulder disorders. However, it may be associated with many complications. Glenoid loosening is thought to be the most common complication of anatomic total shoulder arthroplasty (ATSA); its underlying causes could be mechanical (abnormal loading), septic (infections) or aseptic (autoimmune reaction). This study discusses the mechanical glenoid component loosening after ATSA. II. Hypotheses (Hi, Hii & Hiii) (Hi) The recorded mean peak pressure values of the ATSA components are expected to vary greatly according to the motion type; (Hii) the recorded mean peak pressure values are expected to vary between the initial and final phases of each tested specimen; and (Hiii) the occurrence of glenoid component loosening and its degree of extension are expected to be related to the changes of the obtained mean peak pressure values. III. Objectives This study’s aim was to conduct a comprehensive experimental biomechanical evaluation of the stability of ATSA components under phasic cyclic loading, as follows: (i) testing of the degree of artificial glenoid component stability under repetitive phasic cyclic loading; (ii) testing of the relation between the criteria of the applied cyclic loading according to our testing plan and the occurrence of glenoid component loosening; (iii) measurement and assessment of the values, patterns and magnitudes of the contact pressure between the joint components under cyclic loading; (iv) comparison between the obtained mean peak contact pressure values under cyclic loading in the initial and final phases to detect any relations and/or differences; (v) correlation of the measured pressure values during testing with the QCT findings with respect to glenoid component loosening. IV. Materials A series of six fresh-frozen complete cadaveric shoulder joint specimens (bones and soft tissues) was used in this study. The specimens were implanted with ATSA components and tested successively by mounting them on the shoulder simulator. To measure the values mentioned above, we used a TekScan system with a group of two-headed pressure sensor foils, QCT, shoulder pointer and a digitalized 3Dimaging Zebris system with US, in addition to the routinely used surgical and lab instruments in such experiments. V. Methodology The specimens were scanned prior to experimentation to evaluate their articular surfaces morphology. Then the specimens were implanted with ATSA components and a pressure sensor was inserted within the joint cavity of each specimen and situated on the glenoid component surface. The six specimens were successively mounted on the shoulder simulator and each was tested through three phases of cyclic loading in the three directions of motion. The 1st and 3rd short phases took place for each specimen with insertion of a pressure sensor within the joint cavity, while the 2nd long phase took place without sensor insertion. After the completion of all experiments, the specimens were again scanned with QCT to evaluate the position of the implanted glenoids and any presence of radiolucency and/or loosening. VI. Findings (Observations & Examinations) Two specimens were severely unstable during testing, even with the application of lower loads, particularly during abduction/adduction motion cycles. Provisional and/or subsequent controlling physical examinations revealed either a malposition of the glenoid component or a suspected abnormal glenoid morphology. The other four specimens were completely stable during testing in all motion directions with the application of different loading forces and stabilizing weights. Four specimens were radiologically determined to have a massive glenoid component loosening after the completion of testing. VII. Results The recorded mean peak pressure values varied greatly between the testing phases, testing cycles and motion directions. The highest mean peak pressure values were recorded during AA testing episodes, followed by FE testing episodes. The lowest mean peak pressure values were recorded during IE testing episodes. However, high mean peak pressure values were also recorded during IE testing episodes, but with a low frequency. In seven testing episodes, the recorded mean peak pressure values were higher by 16.7 % in all directions of motion in the final testing phase than those recorded in the initial phase of all testing episodes (42 testing episodes). According to the computed t-test values between the initial and final phases per motion direction/per specimen, null hypothesis (Hypothesis (Hii)) was accepted in the whole AA & FE testing cycles with a percentage of 100%, while it was rejected in only one relation of IE testing cycles with a percentage of 5% and accepted in six relations of IE testing cycles with a percentage of 95%. In total, null hypothesis (Hypothesis (Hii)) was rejected in only one relation of the testing cycles, with a percentage of 5%, and accepted in twenty relations of the testing cycles with a percentage of 95%. According to the calculated t-test values between all initial and final phases for each specimen, null hypothesis (Hypothesis (Hii)) was rejected in two experiments with a percentage of 28.6% and accepted in five experiments with a percentage of 71.4%. Four specimens (three keeled and one pegged) were found to be loose, representing 66.7% of all specimens; one of them was unstable during the testing, representing 25% of the loose specimens and 16.7% of all specimens. VIII. Conclusion The recorded mean peak pressure values and load quantities of the tested shoulder joint varied greatly between motion phases, motion cycles and motion types. The resulting contact pressures across the shoulder joint during its action varied greatly according to the acting force, motion type, muscles status and pathologies within the joint and were directly proportional to the motion type, being higher during AA and FE motion cycles than during IE motion cycles. Also, they were directly proportional to the contact surface area and to the degree of compression between joint articulating surfaces during motion. The greatest degree of variability of SD and mean peak pressure values was seen during FE testing cycles. Shoulder joint instability after ATSA could result from component malposition and/or the articular surface morphological abnormalities. Both glenoid loosening and joint instability could incite the occurrence of the other and could worsen its course in a devastating vicious circle. We concluded that glenoid component loosening could be related to joint stability, loads and the mode of load application in relation to the application duration, and to some extent to the component type, which was apparently evident in our study. The first and third hypotheses were approved, while the second hypothesis was statistically rejected (according to the computed t-test values), which may require a further evaluation in future studies