[Premature wear of total knee arthroplasty in sports shown in a golfer]. / Vorzeitiger Verschleiß von Knieendoprothesen im Sport am Beispiel eines Golfers.

Sport with an endoprosthesis is controversially discussed, whereas golf with a knee endoprosthesis is usually allowed. This case shows that playing golf can lead to severe wear of the prosthesis. The wear pattern of the components of the prosthesis suggests increased rotational loads. A change to a constrained prosthesis was made because of metal-to-metal contact. It is important to inform the patient before surgery about sports with endoprosthesis. Athletic loads are not part of prosthesis testing according to ISO.

[Handling of retrieved implants in orthopedic surgery : Results of a survey within the framework of the EndoCert initiative]. / Bestandsaufnahme zum Umgang mit ausgebauten Implantaten in der orthopädischen Chirurgie : Ergebnisse einer Umfrage im Rahmen der EndoCert-Initiative.

BACKGROUND: Although investigations of retrieved medical implants can provide valuable information about the cause of the revision, there is a lack of information, which could be avoided by consequent failure analyses. In the framework of the EndoCert certification system it is obligatory to record and report incidents. OBJECTIVES: The present work examines how the willingness to report has developed in certified arthroplasty centers and which method of handling retrievals is preferred and actually used. MATERIALS AND METHODS: On the basis of a questionnaire for handling retrievals, all 508 arthroplasty centers that were certified till June 1, 2016, were included (return rate = 97.2%). RESULTS: A total of 93.3% of the centers have established an algorithm for handling of retrievals and 83.0% of the centers prefer to hand out the retrieval to the patient, while only 25.7% wish to store it in the center for research purposes. In the case of a potential incident as the cause of revision, centers prefer to forward the retrieval to damage analysis, whereby the centers act in different ways, depending on the case. An implant fracture is, e.g., considered a reportable event in most cases without temporal limitation. On the other hand, breakage or failure of surgical instruments is considered not to be reported in the case of more than half of the centers. In 2014 and 2015, approximately 71% of EPZs reported no incidents. CONCLUSIONS: According to our survey, many certified arthroplasty centers are sensitized to careful handling of retrievals. The treatment of the explanted components is conducted in different ways. The assessment of whether an incident is to be reported shows large differences. In view of the relatively high number of revision surgeries, the number of reports to the authorities appears to be low.

[Reportable incidents with surgical instruments in orthopedic surgery]. / Meldepflichtige Vorkommnisse mit Operationsinstrumenten in der Orthopädischen Chirurgie.

BACKGROUND: Breakage of instruments in orthopedic surgery is rarely reported but the consequences can be serious for both patients and surgeons. The medical device directive classifies instruments, such as drills and saws into risk class 1 with low approval requirements. Also the number of applications of reusable instruments is not currently limited. OBJECTIVES: The aim of this study was determine to what extent instrument failure can lead to reportable incidents and how these incidents should be processed. METHODS: The study involved an evaluation of clinical cases from our institution with a selective literature review and discussion of the medical device directive. RESULTS: The experience in our clinic showed that especially breakage of rasps in total hip and knee replacement surgery is associated with a major time extension of the operational procedure, a wider surgical access opening as well as complicated procedures to recover the fragments from the incident site. In individual cases a fenestration of the bone had to be conducted in order to collect the broken piece of the rasp. In one case a revision hip stem had to be used instead of the planned primary stem in order to bridge the fenestration site. CONCLUSION: Such consequences of instrument failure were considered to be a reportable incident. A thorough documentation as well as incident reporting to the manufacturer and the Federal authorities are required for a sufficient processing and risk assessment of the incident.

[Influence of surgeon experience in total hip arthroplasty. Dependence on operating time and complication risk]. / Einfluss der Erfahrung des Operateurs in der Hüftendoprothetik. Abhängigkeit von Operationsdauer und Komplikationsrisiko.

BACKGROUND: As a consequence of limited personnel and financial resources, the increase in total hip arthroplasties places higher demands on orthopedic surgeons. OBJECTIVES: In order to maintain high quality treatment, the correlation between surgical experience, duration of surgery and risk of complications was examined. MATERIAL AND METHODS: The surgery time and, if applicable, complications (until discharge from hospital) of 1129 total hip arthroplasties over a period of 4 years were evaluated retrospectively. RESULTS: The group of most experienced surgeons needed an average time of 53.2 ± 17.6 min for each implantation, followed by moderately experienced surgeons (74.5 ± 25.5 min) and less experienced surgeons (80.8 ± 21.9 min). Of all included cases, a total of 41 complications until discharge from hospital occurred. The number of complications increased with duration of surgery, whereby the risk of complications was significantly lower for shorter surgery times conducted by the most experienced surgeons as well as moderately experienced surgeons. The complication risk of less experienced surgeons remained constant independent of surgery duration. CONCLUSION: These results underline the recommendations of the German Endocert system, which determine a minimum number of total joint arthroplasties as a quality indicator not only for hospitals but also for individual surgeons.

[Applications of numerical simulation in musculoskeletal research and its impact on orthopedic surgery]. / Einsatzgebiete der numerischen Simulation in der muskuloskelettalen Forschung und ihre Bedeutung für die Orthopädische Chirurgie.

Finite element analyses (FEA) as well as multibody system dynamics (MSD) are the main tools used for numerical simulation in the field of musculoskeletal research. While FEA is utilized for field problems, such as calculation of stress and strain distribution, MSD is applied for solving kinematic analyses, such as calculation of muscle and joint forces. Depending on the focus of investigation, modelling of biological tissue may vary from simple homogeneous behavior to modelling biochemical processes on the microscale and nanoscale. An important milestone in biomechanical research was the analysis of stress shielding, which led to further research on bone remodelling. Various models of implant-bone fixation used for the prediction of micromotion have been published. New possibilities for biomechanical analyses are achieved by consideration of complex muscle forces which are generated by MSD simulation and imported into FEA models as limiting conditions. A numerical model always requires experimental validation. If the results are confirmed experimentally, various advantages of numerical simulation apply and problems can be analysed isolated from many influencing factors. Therefore, straightforward parameter variation is possible, enabling studies which would be impossible in an experimental or clinical setup.

Influence of stem design parameters on periprosthetic femoral fractures examined by subject specific finite element analyses.

Due to the increasing number of periprosthetic femoral fractures (PFF), the optimisation of implant design gains importance. For the presented research a validated, subject specific finite element model of a human femur with an inlying total hip stem was used to compare the influence of different geometrical implant parameters on the development of PFF. The heterogeneous bone tissue was modelled on the basis of computed tomography scans. A ductile damage model with element deletion was applied to simulate bone fracture in a load case re-enacting a stumbling scenario. The results were compared in terms of fracture load, subsidence and fracture pattern to analyse the influence of friction at the implant-bone interface, implant size and stem length. The results showed that higher friction coefficients lead to an increase of fracture load. Also, the usage of an oversized implant has a negligible effect while an undersized implant reduces the fracture load by 48.9% for the investigated femur. Lastly, a higher fracture load was reached with an elongated stem, but the bending and change in fracture path indicate a more distal force transmission and subsequent stress shielding in the proximal femur.

Subject specific finite element modelling of periprosthetic femoral fractures in different load cases.

Periprosthetic femoral fractures (PFF) around total hip replacements are one of the biggest challenges for orthopaedic surgeons. To understand the risk factors and formation of these fractures, the development of a reliable finite element (FE) model incorporating bone failure is essential. Due to the anisotropic and complex hierarchical structure of bone, the mechanical behaviour under large strains is difficult to predict. In this study, a state-of-the-art subject specific FE modelling technique for bone is utilised to generate and investigate PFF. A bilinear constitutive law is applied to bone tissue in subject specific FE models of five human femurs which are virtually implanted with a straight hip stem to numerically analyse PFF. The material parameters of the models are expressed as a function of bone ash density and mapped node wise to the FE mesh. In this way the subject specific, heterogeneous structure of bone is mimicked. For material mapping of the parameters, computed tomography (CT) images of the original fresh-frozen femurs are used. Periprosthetic fractures are generated by deleting elements on the basis of a critical plastic strain failure criterion. The models are analysed under physiological and clinically relevant conditions in two different load cases re-enacting stumbling and a sideways fall on the hip. The results of the analyses are quantified with experimental data from previous work. With regard to fracture pattern, stiffness and failure load the simulations of the load case stumbling delivered the most stable and accurate results. In general, mapping of material properties was found to be an appropriate way to reproduce PFF with finite element models.

[Influence of acetabular cup design on the primary implant stability : an experimental and numerical analysis]. / Einfluss der Formgebung von künstlichen Hüftpfannen auf die primäre Verankerungsfestigkeit : Eine experimentelle und numerische Analyse.

BACKGROUND: An adequate primary stability and a subsequent stable osseous fixation (secondary stability) of artificial hip cups are required for long-term implant survival. The aim of this study was to analyse the design of cementless press-fit cups as an influencing factor of primary stability. MATERIAL AND METHODS: Different hemispherical and conical cup designs were analysed. The fixation stability of the cups was detected experimentally using a spongiosa and a cortical model based on artificial bone as well as a numerical simulation using a spongiosa model by pull-out and lever-out tests. In addition, the stress on the osseous cup cavity was determined in the finite-element analysis. RESULTS: All tested cup designs revealed higher fixation stability in the cortical bone model compared to the spongiosa model. The experimental tests did not show an increase of fixation stability with the conical cup profile in comparison to hemispherical cup profiles. CONCLUSION: Therefore, cementless press-fit cups with conical cup profile do not provide a higher primary stability in comparison to hemispherical cups. Moreover, the stress on the bone cavity was lower inserting the hemispherical cup profiles in contrast to the conical profiles.

Effect of surgical parameters on the biomechanical behaviour of bicondylar total knee endoprostheses - A robot-assisted test method based on a musculoskeletal model.

The complicated interplay of total knee replacement (TKR) positioning and patient-specific soft tissue conditions still causes a considerable number of unsatisfactory outcomes. Therefore, we deployed a robot-assisted test method, in which a six-axis robot moved and loaded a bicondylar cruciate-retaining (CR)-TKR in a virtual lower extremity emulated by a musculoskeletal multibody model. This enabled us to systematically analyse the impact of the posterior cruciate ligament (PCL), tibial slope, and tibial component rotation on TKR function while considering the physical implant components and physiological-like conditions during dynamic motions. The PCL resection yielded a decrease of femoral rollback by 4.5 mm and a reduction of tibiofemoral contact force by 50 N. A reduced tibial slope led to an increase of tibiofemoral contact force by about 170 N and a decrease of femoral rollback up to 1.7 mm. Although a higher tibial slope reduced the contact force, excessive tibial slopes should be avoided to prevent joint instability. Contrary to an external rotation of the tibial component, an internal rotation clearly increased the contact force and lateral femoral rollback. Our data contribute to improved understanding the biomechanics of TKRs and show the capabilities of the robot-assisted test method based on a musculoskeletal multibody model as a preoperative planning tool.

Investigation of the inverse piezoelectric effect of trabecular bone on a micrometer length scale using synchrotron radiation.

In the present paper we have investigated the impact of electro stimulation on microstructural parameters of the major constituents of bone, hydroxyapatite and collagen. Therapeutic approaches exhibit an improved healing rate under electric fields. However, the underlying mechanism is not fully understood so far. In this context one possible effect which could be responsible is the inverse piezo electric effect at bone structures. Therefore, we have carried out scanning X-ray microdiffraction experiments, i.e. we recorded X-ray diffraction data with micrometer resolution using synchrotron radiation from trabecular bone samples in order to investigate how the bone matrix reacts to an applied electric field. Different samples were investigated, where the orientation of the collagen matrix differed with respect to the applied electric field. Our experiments aimed to determine whether the inverse piezo electric effect could have a significant impact on the improved bone regeneration owing to electrostimulative therapy. Our data suggest that strain is in fact induced in bone by the collagen matrix via the inverse piezo electric effect which occurs in the presence of an adequately oriented electric field. The magnitude of the underlying strain is in a range where bone cells are able to detect it. STATEMENT OF SIGNIFICANCE: In our study we report on the piezoelectric effect in bone which was already discovered and explored on a macro scale in the 1950. Clinical approaches utilize successfully electro stimulation to enhance bone healing but the exact mechanisms taking place are still a matter of debate. We have measured the stress distribution with micron resolution in trabecular bone to determine the piezo electric induced stress. Our results show that the magnitude of the induced stress is big enough to be sensed by cells and therefore, could be a trigger for bone remodeling and growth.

[Recommendations for documentation of incidents with medical devices in orthopaedic surgery]. / Empfehlungen zur Dokumentation von Vorkommnissen mit Medizinprodukten in der Orthopädischen Chirurgie.

The growing number of revisions in orthopaedic surgery as well as the multifactorial reasons for implant failure give cause for taking a closer look at the clinical documentation of adverse events. Based on our long-term experience it is our goal to present recommendations for an adequate documentation and filing. In the framework of the introduction of a quality management system (ISO 9001:2008) in our hospital, a process was developed for reportable incidents with medical devices regulating adequate documentation. Therefore, specific forms were developed. The retrievals are stored for subsequent damage analyses and are available for possible legal claims and tracking. A file should be opened for each reportable incident containing information about the event, a copy of the obligatory BfArM report, surgery report, medical device labels, radiographs and photographs. Declarations of agreement as well as handover certificates should be maintained in order to keep record of the retrievals. In order to assure consistent documentation, we recommend use of specific forms as presented in this paper. Identification of risk factors for implant failure and a long-term reduction of damage cases will only be possible under consequent incident reporting and responsible documentation of adverse events. Processing of cases of damage is accelerated and simplified by the presented recommendations and forms. Together with the newly established joint replacement registry, a higher quality of patient treatment and implant safety should be obtained.

Advanced material modelling in numerical simulation of primary acetabular press-fit cup stability.

Primary stability of artificial acetabular cups, used for total hip arthroplasty, is required for the subsequent osteointegration and good long-term clinical results of the implant. Although closed-cell polymer foams represent an adequate bone substitute in experimental studies investigating primary stability, correct numerical modelling of this material depends on the parameter selection. Material parameters necessary for crushable foam plasticity behaviour were originated from numerical simulations matched with experimental tests of the polymethacrylimide raw material. Experimental primary stability tests of acetabular press-fit cups consisting of static shell assembly with consecutively pull-out and lever-out testing were subsequently simulated using finite element analysis. Identified and optimised parameters allowed the accurate numerical reproduction of the raw material tests. Correlation between experimental tests and the numerical simulation of primary implant stability depended on the value of interference fit. However, the validated material model provides the opportunity for subsequent parametric numerical studies.

[The importance of wear couples for younger endoprosthesis patients]. / Die Bedeutung der Gleitpaarung beim jüngeren Endoprothesenpatienten.

The success and long-term survival rates of modern joint arthroplasty leads to a high patient satisfaction and, together with its technical improvements, has broadened the indications to an increasingly younger population. Limitations to the established systems are the long-term survival rates, which are mainly influenced by wear of the articulating parts and the resulting problems. Beside "classic" long-stemmed cemented shafts articulating with metal against polyethylene, short-stemmed or cup designs with a hard-hard self pairing are increasingly used in total hip arthroplasty. This paper reflects the current state of the art in joint arthroplasty for younger patients with the focus on wear couples and discusses future perspectives. Special interest is focused on the advantages and disadvantages of ceramic bearings, problems with allergies to implant components and the design of endoprostheses with regard to avoidance of impingement.