Study of a constrained finite element elbow prosthesis: the influence of the implant placement

BackgroundThe functional results of total elbow arthroplasty (TEA) are controversial and the medium- to long-term revision rates are relatively high. The aim of the present study was to analyze the stresses of TEA in its classic configuration, identify the areas of greatest stress in the prosthesis-bone-cement interface, and evaluate the most wearing working conditions.Materials and methodsBy means of a reverse engineering process and using a 3D laser scanner, CAD (computer-aided drafting) models of a constrained elbow prosthesis were acquired. These CAD models were developed and their elastic properties, resistance, and stresses were studied through finite element analysis (finite element method-FEM). The obtained 3D elbow-prosthesis model was then evaluated in cyclic flexion-extension movements (> 10 million cycles). We highlighted the configuration of the angle at which the highest stresses and the areas most at risk of implant mobilization develop. Finally, we performed a quantitative study of the stress state after varying the positioning of the stem of the ulnar component in the sagittal plane by +/- 3 degrees.ResultsThe greatest von Mises stress state in the bone component for the 90 degrees working configuration was 3.1635 MPa, which occurred in the most proximal portion of the humeral blade and in the proximal middle third of the shaft. At the ulnar level, peaks of 4.1763 MPa were recorded at the proximal coronoid/metaepiphysis level. The minimum elastic resistance and therefore the greatest stress states were recorded in the bone region at the apex of the ulnar stem (0.001967 MPa). The results of the analysis for the working configurations at 0 degrees and 145 degrees showed significant reductions in the stress states for both prosthetic components; similarly, varying the positioning of the ulnar component at 90 degrees (- 3 degrees in the sagittal plane, 0 degrees in the frontal plane) resulted in better working conditions with a greater resulting developed force and a lower stress peak in the ulnar cement.ConclusionThe areas of greatest stress occur in specific regions of the ulnar and humeral components at the bone-cement-prosthesis interface. The heaviest configuration in terms of stresses was when the elbow was flexed at 90 degrees. Variations in the positioning in the sagittal plane can mechanically affect the movement, possibly resulting in longer survival of the implant.Level of evidence:

Le infezioni tubercolari dell’apparato locomotore

Gli AA dopo aver presentato i dati epidemiologici più importanti della TBC osteoarticolare, si soffermano sulla fisiopatologia, il quadro clinico e radiografico delle localizzazioni più frequenti sottolineando i segni clinici e radiografici tipici di queste lesioni e l’importanza della RM, specie se eseguita con mezzo di contrasto, nella diagnosi precoce della malattia. Vengono inoltre presi in considerazione gli aspetti evolutivi della malattia sia prima che dopo l’introduzione dei farmaci antitubercolari, la terapia medica ed il trattamento incruento e chirurgico delle varie forme. Gli AA concludono che la TBC è una malattia purtroppo sempre di attualità contrariamente a quanto si pensava alla fine degli anni settanta quando si diffuse la convinzione che essa fosse stata debellata. Importante è fare la diagnosi il più precocemente possibile, perché l’inizio precoce della terapia medica associata al trattamento ortopedico può assicurare la guarigione nel 90% de

Indications and limitations of the fixator TGF "Gex-Fix" in proximal end humeral fractures

The incidence of fractures of the humerus has increased exponentially in recent years. The most used classifications for humerus fracture are morphological (Neer), biological (AO/ASIF) and descriptive (Hertel). The types of surgical treatment for humerus fracture include prosthetic replacement and synthesis using different devices, including the Tension Guide Fixator (TGF), Gex-Fix. External fixation for displaced proximal humeral fractures avoids dissection and soft tissue stripping and has been reported by some authors to be associated with higher union rates, a lower incidence of avascular necrosis, less scarring of the scapulohumeral interface, and faster rehabilitation compared with open reduction and internal fixation. Other authors have reported that external fixation does not ensure acceptable reduction and fracture stability, particularly in patients with osteoporosis. The external fixation technique involves the introduction of Steinmann's pin to keep manual reduction, the introduction of two K-wires in the humeral head, the removal of the Steinmann's pin, and the introduction of two fiches on the humeral shaft. Hub connectors are mounted on the wires and on the chips to connect the outer bar and tensioning system. A total of 84 patients aged 42-84 years with proximal end humeral fractures (66% had two-part fractures) were treated with Fixator TGF in this study from December 2007 to June 2012. The postoperative recovery was earlier and the active-assisted motion was less painful than has been reported with other surgical techniques. The TGF was removed without anaesthesia at the outpatient clinic at a mean of 7 weeks (range 5-8 weeks) after surgery, and there was no loss of reduction or secondary displacement after removal. These results, after five years of experience, confirm that the best indication for this fixator is two- or three-part fractures because the device enables early active mobilisation. The limitations of this fixator are evident in fractures in which closed reduction is not possible and in three-part fractures with varus displacement because the TGF has less stability than other systems, such as the plate or cage. The short learning curve, reduced surgical time and risk, and low cost encourage the use of this technique