Numerical and experimental modal analysis applied to an optical test system designed for the form measurements of metre-scale optics

The work focuses on the structural design and performances of a unique optical test system (OTS) used for measuring metre-scale optical surfaces. The investigation was carried out through a modal analysis. Two sets of results are presented. Both modal analysis of the entire OTS and transmissibility function related to its use as an optical system are carried out and analysed. The OTS is used for the measurements of the form accuracy at nanometre level of metre-scale concave surfaces. The OTS is a four and half-metre-tall mechanical structure made of bolted aluminium profiles, two structural platens, two dedicated precision positioning supports, a test piece, and a state-of-the-art laser interferometer. The OTS was numerically modelled and fully instrumented with triaxial accelerometers. The results of the modal analysis highlight the natural modes of the entire OTS. Both numerical and experimental methods are designed. The investigation methods are iterative. Indeed, a preliminary numerical model is created using finite element analysis (FEA). FEA results enable the determination of the dynamic range and suitable locations of accelerometers that are mounted onto the OTS for the experimental validation of the FEA model and further to carry out the transmissibility study. Natural frequencies, damping ratios, and mode shape values are obtained and scrutinized. These results are used for refining the FEA model. In fact, the lack of symmetry and the use of feet are identified as the key design feature that affects the OTS. The correlation between experimental and numerical results is within five percent for the first four modes. The results of the transmissibility study highlight the specific natural modes that influence the OTS measurement capability. Overall, the study enables to guide engineers and researchers towards a robust design using a validated and methodical approach

The course of the superficial peroneal nerve in relation to the ankle position: anatomical study with ankle arthroscopic implications

Despite the fact that the superficial peroneal nerve is the only nerve in the human body that can be made visible; iatrogenic damage to this nerve is the most frequently reported complication in anterior ankle arthroscopy. One of the methods to visualize the nerve is combined ankle plantar flexion and inversion. In the majority of cases, the superficial peroneal nerve can be made visible. The portals for anterior ankle arthroscopy are however created with the ankle in the neutral or slightly dorsiflexed position and not in combined plantar flexion and inversion. The purpose of this study was to undertake an anatomical study to the course of the superficial peroneal nerve in different positions of the foot and ankle. We hypothesize that the anatomical localization of the superficial peroneal nerve changes with different foot and ankle positions. In ten fresh frozen ankle specimens, a window, only affecting the skin, was made at the level of the anterolateral portal for anterior ankle arthroscopy in order to directly visualize the superficial peroneal nerve, or if divided, its terminal branches. Nerve movement was assessed from combined 10° plantar flexion and inversion to 5° dorsiflexion, standardized by the Telos stress device. Also for the 4th toe flexion, flexion of all the toes and for skin tensioning possible nerve movement was determined. The mean superficial peroneal nerve movement was 2.4 mm to the lateral side when the ankle was moved from 10° plantar flexion and inversion to the neutral ankle position and 3.6 mm to the lateral side from 10° plantar flexion and inversion to 5° dorsiflexion. Both displacements were significant (P < 0.01). The nerve consistently moves lateral when the ankle is manoeuvred from combined plantar flexion and inversion to the neutral or dorsiflexed position. If visible, it is therefore advised to create the anterolateral portal medial from the preoperative marking, in order to prevent iatrogenic damage to the superficial peroneal nerve

Anterior ankle arthroscopy, distraction or dorsiflexion?

Anterior ankle arthroscopy can basically be performed by two different methods; the dorsiflexion- or distraction method. The objective of this study was to determine the size of the anterior working area for both the dorsiflexion and distraction method. The anterior working area is anteriorly limited by the overlying anatomy which includes the neurovascular bundle. We hypothesize that in ankle dorsiflexion the anterior neurovascular bundle will move away anteriorly from the ankle joint, whereas in ankle distraction the anterior neurovascular bundle is pulled tight towards the joint, thereby decreasing the safe anterior working area. Six fresh frozen ankle specimens, amputated above the knee, were scanned with computed tomography. Prior to scanning the anterior tibial artery was injected with contrast fluid and subsequently each ankle was scanned both in ankle dorsiflexion and in distraction. A special device was developed to reproducibly obtain ankle dorsiflexion and distraction in the computed tomography scanner. The distance between the anterior border of the inferior tibial articular facet and the posterior border of the anterior tibial artery was measured. The median distance from the anterior border of the inferior tibial articular facet to the posterior border of the anterior tibial artery in ankle dorsiflexion and distraction was 0.9 cm (range 0.7–1.5) and 0.7 cm (range 0.5–0.8), respectively. The distance in ankle dorsiflexion significantly exceeded the distance in ankle distraction (P = 0.03). The current study shows a significantly increased distance between the anterior distal tibia and the overlying anterior neurovascular bundle with the ankle in a slightly dorsiflexed position as compared to the distracted ankle position. We thereby conclude that the distracted ankle position puts the neurovascular structures more at risk for iatrogenic damage when performing anterior ankle arthroscopy

Evaluation and management of injuries of the tibiofibular syndesmosis.

INTRODUCTION: Injury to the tibiofibular syndesmosis often arises from external rotation force acting on the foot leading to eversion of the talus within the ankle mortise and increased dorsiflexion or plantar flexion. Such injuries can present in the absence of a fracture. Therefore, diagnosis of these injuries can be challenging, and often stress radiographs are helpful. Magnetic resonance imaging scans can be a useful adjunct in doubtful cases. The management of syndesmotic injuries remains controversial, and there is no consensus on how to optimally fix syndesmosis. This article reviews the mechanism of injury, clinical features and investigations performed for syndesmotic injuries and brings the reader up-to-date with the current evidence in terms of the controversies surrounding the management of these injuries. SOURCES OF DATA: Embase, Pubmed Medline, Cochrane Library, Elsevier and Google Scholar (January 1950-2014). AREAS OF CONTROVERSY: The management of syndesmotic injuries remains controversial, and there is no consensus on: (i) which ankle fractures require syndesmotic fixation, (ii) the number or the size and the type of screws that should be used for fixation, (iii) how many cortices to engage for fixation, (iv) the level of screw placement above the ankle plafond, (v) the duration for which the screw needs to remain in situ to allow the tibiofibular syndesmosis to heal and (vi) when should patients weight bear. AREAS OF AGREEMENT: (i) A high proportion of syndesmotic fixations demonstrates malreduction of the syndesmosis, (ii) no need to remove screws routinely, (iii) two screws appear to better one alone and (iv) if syndesmosis injury is not detected or not treated long term, it leads to pain and arthritis. GROWING POINTS: (i) How to assess the adequacy of syndesmotic reduction using imaging in the peri-operative period, (ii) the use of bio-absorbable materials and Tightrope and (iii) evidence is emerging not to remove syndesmotic screws unless symptomatic. AREAS OF TIMELY FOR DEVELOPMENT RESEARCH: (i) A bio-absorbable material that can be used to fix the syndesmosis and allow early weight bearing, and (ii) there is a need for developing a surgical technique for adequately reducing the syndesmosis without the exposure to radiation