Fracture frequency of all-ceramic crowns during dynamic loading in a chewing simulator using different loading and luting protocols

OBJECTIVE: The purpose of this laboratory study was to compare the frequency of failures (complete fractures or partial cracks) of molar crowns made of two different all-ceramic materials during dynamic loading in a chewing simulator, as well as the fracture load when subjected to static loading, in relation to different dynamic loading and luting protocols. METHODS: One hundred and forty-four molar crowns fabricated with IPS Empress or an experimental e.max Press material with high translucency (e.max Press Exp) were luted on CAD/CAM milled PMMA abutments (first lower molar, circular chamfer) either with Variolink or glass-ionomer cement (GIC). All crowns were loaded according to three different loading protocols (n=12 per group) and two force profiles (sinusoidal, rectangular) in a pneumatically driven chewing simulator with a steel stylus (? 8mm) and they underwent simultaneous thermocycling (5 degrees C/55 degrees C). After each phase the crowns were evaluated with regard to fractures or cracks. After dynamic testing, the crowns that did not fail were subjected to compression loading until complete fracture in a universal testing machine (UTM). As control groups, unloaded crowns were also subjected to a UTM. Survival statistics with log-rank tests were applied for the results of the dynamic loading, while ANOVA with post hoc Tukey B was used for the fracture load results and two-way ANOVA was carried out for logarithmically transformed data. Weibull statistics were calculated for pooled fracture load data of the dynamically loaded and control groups. RESULTS: In the 144 IPS Empress crowns, complete fractures were observed in 9 crowns and partial cracks in another 3 crowns. When the data was pooled, a statistically significant increase in fractures occurred when the sinusoidal force profile was applied compared to a rectangular force profile (log-rank, p<0.05). No fractures occurred in the e.max Press Exp crowns. The two-way ANOVA showed that the type of luting protocol used had the most significant effect on the fracture load of both materials. In conjunction with Empress, however, the luting material influenced the variability twice as much as in e.max Press Exp. There was no statistically significant difference in the fracture load of GIC-luted e.max Press Exp crowns and that of the Variolink luted Empress crowns. The force profile had a significant effect on the fracture load only of the Empress crowns but not of the e.max Press Exp crowns. Weibull statistics revealed a higher scattering of the data of dynamically loaded crowns compared to that of the control groups. CONCLUSIONS: For testing all-ceramic materials, dynamic loading is indispensable to draw valid conclusions on clinical performance of all-ceramic molar crowns. A sinusoidal profile is advisable, while a gradual increase of the force amplitude does not significantly affect the results.     

Optimal pole assignment into specified regions and its application to rotating mechanical systems

This paper considers a pole assignment problem to cluster all poles of a closed-loop system into some specified regions by introducing the complex state for systems having an isotropic property and by using the Riccati equation. The algebraic relations for the distribution of the eigenvalues of a complex matrix are used in order to cluster the poles into specified regions, which guarantees the relative stability margin, e.g. uniform damping or uniform damping ratio. The proposed scheme is essentially a combination of the pole assignment approach and linear quadratic design, taking the advantages of both. A block modal control method—an extension of recursive pole assignment—is also developed for the vibration control of rotating systems by clustering the forward and backward modes in order. Vibration control simulations with an isotropic rotor—bearing system, a magnetic bearing system and a rotating circular disc are treated in order to demonstrate the advantages of the proposed method.     

Renal acid excretion in early infancy

In early infancy, complex disorders of acid base metabolism are more frequent than in any other age group, with a predisposition to metabolic acidosis due to an age-related low renal capacity for acid excretion and an unphysiologically high actual renal acid load in nutrition with common formulas. Recently in preterm and small-for-gestational-age infants, persistent maximum renal net acid excretion (NAE) with subnormal or normal blood acid base status, impaired weight gain, and adaptive hormonal reactions have been observed. Incipient late metabolic acidosis is one example of a mixed disorder of acid base metabolism with maximum renal NAE in early infancy. Alkali therapy is highly effective and can be realized both on an individual basis, using urine pH screening as a diagnostic criterium for maximum renal acid stimulation, or on a general preventive level using modified standard formula with a reduced actual renal NAE similar to that seen on alimentation with human milk. From an integrated point of view, the low glomerular filtration rate and renal capacity for acid excretion beyond the developmental age of more than 44 weeks, may well be interpreted as the result of a specific adaptation to breast feeding sparing energy, and thus an evolutionary advantage for the survival of mother and child. Received July 10, 1996; received in revised form and accepted October 7, 1996     

双相气道内正压呼吸机在老年慢性阻塞性肺疾病康复期和急性肺水肿患者中的临床应用

应用双相气道内正压呼吸机（ＢｉＰＡＰ）对９例慢性阻塞性肺疾病（ＣＯＰＤ）康复期和呼吸衰竭稳定期进行临床观察发现，治疗后动脉血氧分压与治疗前相比显著升高（Ｐ＜０．０１）；肺活量及用力肺活量轻度升高；１例３次发生急性左心衰、急性肺水肿致急性呼衰患者治疗后，病情很快改善；１例阻塞性夜间睡眠呼吸暂停综合征者应用效果满意。该机应用简便、安全，对心率血压无明显影响，为ＣＯＰＤ康复期、急性左心衰兼肺水肿的治疗提供了新途径。     

Shoulder strain in keyboard workers and its alleviation by arm supports

Summary Keyboard work consists mostly of dynamic contractions of the small muscles of the forearms and hands. This is accompanied by continuous activity in the arm, shoulder and neck muscles keeping the head and hand in the correct position. Eliminating the weight from the arm by means of support and the position of the arms influences the electrical activity of shoulder muscles when working at a keyboard. We studied the influence of elbow angle, as well as that of different arm supports, on electrical activity of upper trapezius muscle during keyboard work in healthy workers and persons suffering from shoulder pains. The measurements were carried out in the laboratory. EMG activities, which where measured as mean square root (RMS)-values at every 100-millisecond period in trapezius muscle when working, were lower, the greater the elbow angle. Furthermore electrical activity decreased when subjects used arm supports while working. It is evident that the static load to shoulder muscles can be lowered significantly in keyboard work, when the forearms are at an angle of at least 100 degrees and by using arm supports. The most convienient and ergonomic working position can also be found individually be the method used here.     

Does a tensioning device pinned to the tibia improve knee anterior–posterior load‐displacement compared to manual tensioning of the graft following anterior cruciate ligament reconstruction? A cadaveric study of two tibial fixation devices

Devices that are pinned to the tibia to tension an anterior cruciate ligament (ACL) graft produce joint reaction loads that in turn can affect the maintenance of graft initial tension after tibial fixation and hence knee anterior-posterior (AP) load-displacement. However, the effect of these devices on AP load-displacement is unknown. Our objectives were to determine whether tensioning by device versus tensioning by hand causes differences in AP load-displacement and intraarticular graft tension for two commonly used tibial fixation devices: a bioresorbable interference screw and a WasherLoc. AP load-displacement and intraarticular graft tension were measured in 20 cadaveric knees using a custom arthrometer. An initial tension of 110 N was applied to a double-looped tendon graft with the knee at extension using a tensioning device pinned to the tibia and a simulated method of tensioning by hand. After inserting the tibial fixation device, the 134 N anterior limit (i.e., anterior position of the tibia with respect to the femur with a 134 N anterior force applied to the tibia) and 0 N posterior limit (i.e., AP position of the tibia relative to the femur with a 0 N force applied to the tibia) were measured with the knee in 25 degrees flexion. Intraarticular graft tension was measured at extension. These limits and intraarticular graft tension were also measured after cyclically loading the knee 300 times. Compared to a simulated method of tensioning by hand, tensioning with a device pinned to the tibia did not decrease the 134 N anterior limit and did not cause posterior tibial translation. However, intraarticular graft tension was maintained better with a tensioning device pinned to the tibia for the Washerloc, but not the interference screw. For two commonly used tibial fixation devices, a tensioning device pinned to the tibia does not improve AP load-displacement at 25 degrees flexion over tensioning by hand when the graft is tensioned at full extension, but does improve the maintenance of intraarticular graft tension for the Washerloc.     

ADJUSTABLE SUTURES IN STRABISMUS SURGERY: A PERSONAL SERIES OF CASES

A personal series of 77 separate primary adjustable suture procedures performed on 69 patients is analysed. Adjustable suture surgery was performed on patients over ten years of age undergoing rectus muscle surgery, and a few selected younger patients. The ages ranged from 7 to 74 years. A 24-month-old infant who had a two-stage-anaesthesia is included in the series.
Thirty-eight of the 69 patients were reoperations. Of the 77 primary procedures, 48 (62.3%) required postoperative adjustment; 29 (37.7%) were tied down without adjustment.
Sixty-two of the 69 patients (89.8%) achieved a "cure". A further four patients (5.8%) were initially in desired alignment, but deteriorated within the follow-up period. Three of these were successfully reoperated. Of the three primary failures (4.4 %) two were reoperated with one achieving orthophoria.
Apart from alignment failure in the primary procedure, complications were minimal. Current follow-up periods range from one to 24 months.     

Epitope specificity is critical for high and moderate avidity cytotoxic T lymphocytes associated with control of viral load and clinical disease in horses with equine infectious anemia virus

Equine infectious anemia virus (EIAV) is a lentivirus that causes persistent infections in horses. We hypothesized that high-avidity CTL specific for nonvariable epitopes might be associated with low viral load and minimal disease in EIAV-infected horses. To test this hypothesis, memory CTL (CTLm) responses were analyzed in two infected horses with high plasma viral loads and recurrent disease (progressors), and in two infected horses with low-to-undetectable viral loads and mild disease (nonprogressors). High-avidity CTLm in one progressor recognized an envelope gp90 epitope, and the data documented for the first time in EIAV that viral variation led to CTL escape. Each of the nonprogressors had high-to-moderate avidity CTLm directed against epitopes within Rev, including the nuclear export and nuclear localization domains. These results suggested that the epitope specificity of high- and moderate-avidity CTLm was an important determinant for disease outcome in the EIAV-infected horses examined.     

The ventilatory threshold gives maximal lactate steady state

Summary The purpose of this study was to examine whether the ventilatory threshold (Th _v) would give the maximal lactate steady state ([1a]_{ss, max}), which was defined as the highest work rate (W) attained by a subject without a progressive increase in blood lactate concentration [1a]_b at constant intensity exercise. Firstly, 8 healthy men repeated ramp-work tests (20 W·min^–1) on an electrically braked cycle ergometer on different days. During the tests, alveolar gas exchange was measured breath-by-breath, and theW atTh _v (W _{Th v}) was determined. The results of two-way ANOVA showed that the coefficient of variation of a singleW _{Th v} determination was 2.6%. Secondly, 13 men performed 30-min exercise atW _{Th v} (Th _v trial) and at 4.9% aboveW _{Th v} (Th _v + trial), which corresponded to the 95% confidence interval of the single determination. The [1a]_b was measured at 15 and 30 min from the onset of exercise. The [1a]_b at 15 min (3.15 mmol·1^–1, SEM 0.14) and at 30 min (2.95 mmol·1^–1, SEM 0.18) were not significantly different inTh _v trial. However, the [1a]_b ofTh _v+ trial significantly increased (P<0.05) from 15 min (3.62 mmol·1^–1, SEM 0.36) to 30 min (3.91 mmol·1^–1, SEM 0.40). These results indicate thatTh _v gives the [1a]_ss,max, at which one can perform sustained exercise without continuous [1a]_b accumulation.     

External load can alter the energy cost of prolonged exercise

Summary The present study was undertaken to examine the energy cost of prolonged walking while carrying a backpack load. Six trained subjects were tested while walking for 120 min on a treadmill at a speed of 1.25 m · s^–1 and 5% elevation with a well fitted backpack load of 25 and 40 kg alternately. Carrying 40 kg elicited a significantly higher (p<0.01) enery cost than 25 kg. Furthermore, whereas carrying 25 kg resulted in a constant energy cost, 40 kg yielded a highly significant (p<0.05) increase in energy cost over time. The study implies that increase in load causes physical fatigue, once work intensity is higher than 50% maximal work capacity. This is probably due to altered locomotion biomechanics which in turn lead to the increase in energy cost. Finally, the prediction model which estimates energy cost while carrying loads should be used with some caution when applied to heavy loads and long duration of exercise, since it might underestimate the acutal enery cost.