Finite element analysis of mandibular molar protraction mechanics using miniscrews.

BACKGROUND/OBJECTIVES: The aim of this study was to determine the most desirable force system to achieve molar protraction from an interdental miniscrew minimizing side-effects. Several iterations of force delivery were simulated through variations in the height of a miniscrew, length of a molar extension arm, and incorporation of a lingual force. MATERIALS/METHODS: A three-dimensional mesh model of the right posterior segment of the mandible was developed from cone beam computed tomography data from a patient missing a first molar. Protraction appliances were constructed using computer-aided design software and integrated with finite element software. After mesh generation, a total of 80 loading conditions were simulated by altering the extension arm length (2-10mm), miniscrew height (0-8mm), and magnitude of protraction force from the lingual side (0-1.5 N). A constant labial force of 1 N was used in all models. RESULTS: As the length of the extension arm increased, mesial tipping decreased, rotation decreased, and buccolingual inclination remained the same without lingual traction force. Lingual traction reduced rotation but increased tipping. Similar trends were observed in all situations despite of the height of the miniscrew. CONCLUSIONS: The height of the miniscrew is not as critical in affecting tooth movement during mandibular second molar protraction as the length of the extension arm. The most ideal force system in the model appeared to be the longest extension arm (10mm) with the addition of a lingual force of half or equal magnitude of the labial force.

Effect of impeded medial longitudinal arch drop on vertical ground reaction force and center of pressure during static loading.

BACKGROUND: Arch supports commonly used to alleviate foot pain can impede the normal drop of medial longitudinal arch (MLA) thereby altering its function. The purpose of the study was to examine the effect of using arch supports on vertical ground reaction force (GRF) and center of pressure (COP) during simulated midstance while the foot was statically loaded. MATERIALS AND METHOD: Ten healthy young subjects were recruited. Two dimensional (2D) analysis of the MLA was captured for both barefoot (BF) and arch support conditions before and after loading via a custom made weight loading apparatus. The foot was loaded and positioned to simulate the midstance phase of walking. Two-dimensional reflective markers demarcated the MLA and captured with the loaded foot on a force platform. The impeded MLA drop was compared between the unloaded BF, loaded BF and loaded arch support conditions. The vertical GRF, the anterior-posterior and the medial-lateral COP displacements were also measured in response to the impeded MLA by the arch supports. RESULTS: The arch supports impeded the MLA drop (p<0.05) and shifted the COP toward the medial side (p<0.05), specifically for the rearfoot (calcaneal segment region), but no changes were determined for the vertical GRF (p>0.05). CONCLUSION: The impedance of MLA drop by the arch support altered the pattern of the ML COP shift in the rearfoot region. CLINICAL RELEVANCE: The use of arch supports may not relieve painful foot conditions that are associated with excessive calcaneal eversion indicated by altering COP shifts in localized foot regions.

Effects of simulated occupational task parameters on balance.

The effects of single-handed load holding, length of the base of support, and standing surface condition (narrow and wide construction beams) on balance were investigated in twenty-three healthy men between the ages of 18 and 55 years old. Balance during quiet standing was evaluated from postural sway measurements derived from center of pressure (COP) displacement. These measurements included the range or maximal displacement of the COP in the anteroposterior (AP) and mediolateral (ML) directions, the elliptical area, and mean sway velocity. Holding a load in the hand did not significantly affect postural sway measures (p > 0.05), although the effect of surface condition was significant on all COP measures (p < 0.001). Lengthening the base of support did not affect the ranges or elliptical area, but increased the mean velocity of sway (p = 0.001). Changes in the dimensional characteristics of the surface condition and length of base of support affected postural sway, possibly by requiring adjustments to balance and motor control strategies. Further research is required to determine if these changes are detrimental to maintaining balance and increase the risk of falls for workers in similar environments.

Nonlinear behavior of the center of pressure in simulated standing on elevated construction beams.

This study investigated the effects of width of construction beams and single-hand load holding task conditions on nonlinear behavior of the foot center of pressure (COP) exerted on the beam. The foot COP, defined as the point of application of the result of vertical forces acting on the surface of foot support, was measured in the lateral direction under simulated standing task conditions. Twelve healthy male subjects were asked to hold a load of 6.8 kg and 11.3 kg while standing on the elevated construction beams with widths of 10 and 22.5 cm (4 and 9 inches, respectively) under low and high foot separation (foot step). The results showed that both beam width and single-hand load carrying conditions had significant effects on the observed nonlinearity of the foot center of pressure exerted on the beam. Standing on the narrow beam resulted in higher level of chaotic behavior of COP compared to the wide beam condition. The nonlinearity of the COP exerted by the forward (left) foot was higher for the narrow beam condition. For both beams, the nonlinearity of the COP exerted by the forward (left) foot was consistently higher than the COP exerted by the backward (right) foot. Furthermore, for both beams, single-handed holding of the 11.3 kg load resulted in higher levels of COP nonlinearity than carrying 6.8 kg or no load at all. The study results indicate that nonlinear dynamics behavior of the forward foot under single-handed high load holding condition may be critical to preserving lateral stability during standing at the construction beams.

Arm abduction strength and its relationship to shoulder geometry.

This study was conducted to test whether glenohumeral geometry, as measured through MRI scans, is correlated with upper arm strength. The isometric shoulder strength of 12 subjects during one-handed arm abduction in the coronal plane, in a range from 5 degrees to 30 degrees , was correlated with the geometries of their glenoid fossas. Seven parameters describing the glenohumeral joint geometry in the coronal plane were identified as having expected influence on shoulder strength. In addition to these, a new geometric parameter, named the area of glenoid asymmetry (AGA), was considered to reflect the concavity-compression mechanism as well as the inclination of the glenoid surface. As a result of the high correlation between the AGA and mean force and mean moment (0.80, p0.01 and 0.69, p

Ecological models of human performance based on affordance, emotion and intuition.

This paper proposes a complementary approach to Rasmussen's taxonomy of the human skill-, rule-, and knowledge-based performance models by combining the ecological concept of affordances with the neural concepts of human emotion and intuition. The classical cognitive engineering framework is extended through the neuro-ecological approach, including personal human attributes important in exercising control over the work environment. The proposed affordance-, emotion-, and intuition-based models correspond to the three types of human performance, namely: learning, adaptive and tuning control, respectively. The new framework is not a predictive model of the operator behaviour, but rather it describes the processes of neuro-ecological control of the human environment.