Avoiding artefacts in MicroCT imaging of collagen scaffolds: Effect of phosphotungstic acid (PTA)-staining and crosslink density.

X-ray micro-computed tomography (µ-CT) can be used to provide both qualitative and quantitative information on the structure of three-dimensional (3D) bioactive scaffolds. When performed in a dry state, µ-CT accurately reflects the structure of collagen-based scaffolds, but imaging in a wet state offers challenges with radiolucency. Here we have used phosphotungstic acid (PTA) as a contrast agent to visualise fully hydrated collagen scaffolds in a physiologically relevant environment. A systematic investigation was performed to understand the effects of PTA on the results of µ-CT imaging by varying sample processing variables such as crosslinking density, hydration medium and staining duration. Immersing samples in 0.3% PTA solution overnight completely stained the samples and the treatment provided a successful route for µ-CT analysis of crosslinked samples. However, significant structural artefacts were observed for samples which were either non-crosslinked or had low levels of crosslinking, which had a heterogeneous interior architecture with collapsed pores at the scaffold periphery. This work highlights the importance of optimising the choice of processing and staining conditions to ensure accurate visualisation for hydrated 3D collagen scaffolds in an aqueous medium.

The Mechanics of Brow-Suspension Ptosis Repair: A Comparative Study of Fox Pentagon and Crawford Triangle Techniques.

PURPOSE: To perform quantitative analysis of the most commonly used brow-suspension configurations. METHODS: The inflection positions for Fox pentagon and Crawford triangle configurations were marked on 49 healthy volunteers (male and female) and photographs taken in 3 states: "normal," "closed," and "raised." The skin marks were measured vectorially with respect to the medial canthus, and displacement changes were evaluated for "normal-to-closed" ("blinking") and from "closed-to-raised" ("eye-opening") states. The distance between a pair of inflection marks, representing the approximate path of sling configurations, were also measured and analyzed in relation to the mechanical properties of a variety of synthetic brow-suspension materials. RESULTS: "Blinking" resulted in the greatest displacement in the medial eyelid incision, resulting in the greatest strain on the line connecting the medial eyelid and medial brow inflections. No significant differences in the strains for individual lines were found between the Fox and Crawford techniques, although the former shows a significantly lower overall strain in the whole loop than the latter. The displacements of some inflections and of the strains of a few lines differed significantly in men and women. CONCLUSIONS: Within the scope of this study, the blinking action was shown to result in the maximum strain of ~40%, which lies within the elastic region of stress-strain curves for some commonly used synthetic brow-suspension materials. No one method was statistically superior, although the Fox pentagon gave a significantly lower overall strain when the sling material was assumed to move somewhat around the inflections within a closed loop.

Analysis of blink dynamics in patients with blepharoptosis.

Owing to the rapid movements of the human upper eyelid, a high-speed camera was used to record and characterize voluntary blinking and the blink dynamics of blepharoptosis patients were compared to a control group. Twenty-six blepharoptosis patients prior to surgery and 45 control subjects were studied and the vertical height of the palpebral aperture (PA) was measured manually at 2 ms intervals during each blink cycle. The PA and blinking speed were plotted with respect to time and a predictive model was generated. The blink dynamic was analysed in closing and opening phases, and revealed a reduced speed of the initial opening phase in ptotic patients, suggesting intrinsic muscle function change in ptosis pathogenesis. The PA versus time curve for each subject was reconstructed using custom-built parameters; however, there were significant differences between the two groups. Those parameters used included the rate of closure, the delay between opening and closing, rate of initial opening, rate of slow opening (nonlinear function) and the 'switch point' between those two rates of opening. The model was tested against a new group of subjects and was able to discriminate ptosis patients from controls with 80% accuracy.

Stress-relaxation and fatigue behaviour of synthetic brow-suspension materials.

Ptosis describes a low position of the upper eyelid. When this condition is due to poor function of the levator palpebrae superioris muscle, responsible for raising the lid, "brow-suspension" ptosis correction is usually performed, which involves internally attaching the malpositioned eyelid to the forehead musculature using brow-suspension materials. In service, such materials are exposed to both rapid tensile loading and unloading sequences during blinking, and a more sustained tensile strain during extended periods of closure. In this study, various mechanical tests were conducted to characterise and compare some of commonly-used synthetic brow-suspension materials (Prolene(®), Supramid Extra(®) II, Silicone rods (Visitec(®) Seiff frontalis suspension set) and Mersilene(®) mesh) for their time-dependent response. At a given constant tensile strain or load, all of the brow-suspension materials exhibited stress-relaxation or creep, with Prolene(®) having a statistically different relaxation or creep ratio as compared with those of others. Uniaxial tensile cyclic tests through preconditioning and fatigue tests demonstrated drastically different time-dependent response amongst the various materials. Although the tests generated hysteresis force-strain loops for all materials, the mechanical properties such as the number of cycles required to reach the steady-state, the reduction in the peak force, and the cyclic energy dissipation varied considerably. To reach the steady-state, Prolene(®) and the silicone rod required the greatest and the least number of cycles, respectively. Furthermore, the fatigue tests at physiologically relevant conditions (15% strain controlled at 6.5 Hz) demonstrated that the reduction in the peak force during 100,000 cycles ranged from 15% to 58%, with Prolene(®) and the silicone rod exhibiting the greatest and the least value, respectively. Many factors need to be considered to select the most suitable brow-suspension material for ptosis correction. These novel data on the mechanical time-dependent performance could therefore help to guide clinicians in their decision-making process for optimal surgical outcome.

Microstructure and mechanical properties of synthetic brow-suspension materials.

Levator palpebrae superioris (LPS) is a muscle responsible for lifting the upper eyelid and its malfunction leads to a condition called "ptosis", resulting in disfigurement and visual impairment. Severe ptosis is generally treated with "brow-suspension" surgery, whereby the eyelid is cross-connected to the mobile tissues above the eyebrow using a cord-like material, either natural (e.g. fascia lata harvested from the patient) or a synthetic cord. Synthetic brow-suspension materials are widely used, due to not requiring the harvesting of fascia lata that can be associated with pain and donor-site complications. The mechanical properties of some commonly-used synthetic brow-suspension materials were investigated--namely, monofilament polypropylene (Prolene®), sheathed braided polyamide (Supramid Extra® II), silicone frontalis suspension rod (Visitec® Seiff frontalis suspension set), woven polyester (Mersilene® mesh), and expanded polytetrafluoroethylene (Ptose-Up). Each material underwent a single tensile loading to the failure of the material, at three different displacement rates (1, 750 and 1500 mm/min). All the materials exhibited elastic-plastic tensile stress-strain behaviour with considerable differences in elastic modulus, ultimate tensile strength, elastic limit and work of fracture. The results suggest that, as compared to other materials, the silicone brow-suspension rod (Visitec® SFSS) might be the most suitable, providing relatively long-lasting stability and desirable performance. These findings, together with other factors such as commercial availability, cost and clinical outcomes, will provide clinicians with a more rational basis for selection of brow-suspension materials.

High-speed camera characterization of voluntary eye blinking kinematics.

Blinking is vital to maintain the integrity of the ocular surface and its characteristics such as blink duration and speed can vary significantly, depending on the health of the eyes. The blink is so rapid that special techniques are required to characterize it. In this study, a high-speed camera was used to record and characterize voluntary blinking. The blinking motion of 25 healthy volunteers was recorded at 600 frames per second. Master curves for the palpebral aperture and blinking speed were constructed using palpebral aperture versus time data taken from the high-speed camera recordings, which show that one blink can be divided into four phases; closing, closed, early opening and late opening. Analysis of data from the high-speed camera images was used to calculate the palpebral aperture, peak blinking speed, average blinking speed and duration of voluntary blinking and compare it with data generated by other methods previously used to evaluate voluntary blinking. The advantages of the high-speed camera method over the others are discussed, thereby supporting the high potential usefulness of the method in clinical research.