Chemical reactions of anions in the gas phase.

Anions of many types, both organic and inorganic, farmiliar and exotic, can be generated in the gas phase by rational chemical synthesis in a flowing afterglow apparatus. Once formed, the rates, products, and mechanisms of their reactions with neutral species of all kinds can be studied, not only at room temperature but at higher energies in a drift field. These completely unsolvated ions undergo a large number of reactions that are analogous to those they undergo in solution, as well as some that are less familiar. New types of ions, for which there are no counterparts in solution, can be produced and their chemical reactions explored.

Superior-inferior stability of the shoulder: role of the coracohumeral ligament and the rotator interval capsule.

OBJECTIVE: To study the superior-inferior stabilizing functions of the coracohumeral ligament (CHL) and the rotator interval capsule (RIC) with use of a material testing machine. MATERIAL AND METHODS: The axial translations of the humerus with the superior-inferior translation force of 30 N applied were recorded under the following joint capsule conditions: (1) intact, (2) vented, (3) the CHL sectioned, and (4) the RIC incised in six cadaver shoulders. The order of sectioning was changed for conditions 3 and 4 in six other cadaver shoulders. RESULTS: With the arm in internal and neutral rotations, venting the capsule significantly increased the superior-inferior translation, which was unaffected by further sectioning of the CHL and the RIC. With the arm in external rotation, only the CHL contributed significantly to inferior stability, whereas both this ligament and the RIC contributed to superior stability to a lesser degree. CONCLUSION: The CHL is a stabilizer in superior inferior directions with the arm in external rotation, and the intra-articular pressure that is maintained by the intact RIC is a stabilizer in superior-inferior directions with the arm in internal and neutral rotations. These findings may provide a scientific background to support closure of the interval space to stabilize the shoulder and may explain part of the superior instability observed in shoulders with rotator cuff tears.

Effects of freeze/thaw conditioning on the tensile properties and failure mode of bone-muscle-bone units: a biomechanical and histological study in dogs.

Eight pairs of canine supraspinatus bone-muscle-bone units were mechanically tested to failure in tension. One side was tested immediately post mortem, and the other side was tested after exposure to a standard freeze/thaw process (-60 degrees C). The failure site was analyzed histologically. Fresh specimens had greater values for ultimate strength (p < 0.001), stiffness (p < 0.001), and energy to failure (p < 0.001). All specimens failed in the muscle close to the musculotendinous junction. The length of muscles subjected to the freezing process was reduced (9.3%). In addition, the load-displacement curves for the fresh and frozen specimens showed marked differences in shape. The loss of tensile strength in muscle tissue is due to damage of the intracellular contractile elements caused by postmortem autolysis; this type of damage is increased as a result of the freeze/thaw process. The freeze/thaw process significantly altered the tensile properties of normal muscle tissue, no matter how carefully it was done. One cannot expect to receive representative data if muscle is frozen and thawed.

Cement viscosity affects the bone-cement interface in total hip arthroplasty.

Quantitative information regarding the interface strength and degree of cement penetration associated with cement viscosity during total hip arthroplasty is limited. The aim of the present study was to determine the effect of the viscosity of bone cement at the time of implantation on the mechanical integrity of total hip arthroplasty. Cement that was injected at an early less viscous stage produced greater failure strength in a push-out test than its more viscous counterpart.

Tensile properties of the supraspinatus tendon.

The tensile properties of the supraspinatus tendon were investigated in 11 shoulders from fresh cadavers. The tendon was divided into three longitudinal strips: anterior, middle, and posterior. Each specimen was mounted on a materials testing machine, with four fluorescent markers placed on both surfaces of the tendon strip. The positions of these markers were recorded during the test by two synchronized video cameras. Load-deformation and strain curves were determined, and the stress-strain curve, strength, and modulus of elasticity were calculated. The posterior strip was thinner in cross section than the others (p = 0.0355). The ultimate load and ultimate stress were significantly greater in the anterior strip (16.5 +/- 7.1 MPa) than in the middle (6.0 +/- 2.6 MPa) and posterior (4.1 +/- 1.3 MPa) strips (p < 0.0001). The modulus of elasticity also was significantly greater in the anterior strip (p < 0.0001), but there was no significant difference between the superficial and deep surfaces. It is concluded that the anterior portion of the supraspinatus tendon is mechanically stronger than the other portions, and it seems to perform the main functional role of the tendon.

Biomechanical evaluation of anterior cervical spine stabilization.

STUDY DESIGN: An in vitro biomechanical study. OBJECTIVES: To simulate a severe compressive flexion injury for determination of the relative stability of different anterior instrumentation systems in a porcine model and to validate this model in human cadaveric specimens. SUMMARY OF BACKGROUND DATA: Anterior plate fixation is useful for high-grade mechanical insufficiency of the cervical spine and may prevent the need for a second procedure. METHODS: The cervical spines of 45 porcine and 12 cadaveric specimens were subjected to nondestructive flexion, lateral bending, and torsional testing on a modified universal testing machine. A corpectomy was performed with release of the posterior ligamentous structures. The specimens were stabilized with one of three anterior plate constructs. The nondestructive testing was repeated to evaluate structural stability (stiffness and neutral zone). Finally, destructive testing examined failure moment, energy to failure, and mechanism of failure. RESULTS: The instrumented specimens had flexural and lateral bending and torsional stiffness values that were similar to or greater than those of their paired intact specimens. The cervical spine locking plate had a significantly higher flexural stiffness ratio (plated:intact), torsional stiffness ratio, lower flexural neutral zone ratio, higher failure moment, and higher energy to failure than did the Caspar plate. CONCLUSIONS: The cervical spine locking plate is theoretically safer than the Caspar system because the posterior vertebral body cortex is not breached by the fixation screws, and the screws are less likely to back out anteriorly and irritate the esophagus. According to these results, the cervical spine locking plate system is biomechanically equivalent to and in some cases more stable than the Caspar system for fixation of a severe compressive flexion injury.

The efficiency of malachite green, free and protein bound, as a photon-to-heat converter.

Dye assisted laser inactivation of proteins has been found to be a methodology that can achieve high selectivity. Despite the fact that the methodology is successful, knowledge of the detailed inactivation mechanism would allow full optimization of this technique. Here, pulsed-laser photoacoustic calorimetry is used to study the photophysical properties, principally the heat release behavior, of protein bound malachite green. We found that when bound to bovine serum albumin the dye is a good photon-to-heat converter, but approximately 2.6% of the absorbed photon energy (lambda(exc) = 624 nm) is not released as heat in less than 10 mus. This observation suggests that a mechanism other than simple heat-induced inactivation may be the principle process; a long lived excited triplet state of malachite green (or species derived from it) is postulated to play a major role.

Capsular properties of the shoulder.

The purpose of this study was to determine the structural properties of the capsule of the glenohumeral joint. Twelve fresh frozen cadaveric shoulders were studied. Capsular strips were prepared from four different sites (anterior, posterior, superior, and inferior) of the capsule. One end of the capsular sections was left attached to the humerus, and the other excised was fixed in a clamp of an Instron universal testing machine. Maximum load, strength (maximum stress), and modulus of elasticity of these four capsular portions were measured. The most common mode of failure was tear at the midsubstance (68%), followed by tear at the clamp-capsule junction (23%), and detachment from the humerus (9%). The posterior capsule (1.0 +/- 0.4 mm) was thinner than the anterior (1.8 +/- 0.3 mm), superior (1.6 +/- 0.4 mm), and inferior capsule (1.5 +/- 0.3 mm). Among the four portions of the capsule, the posterior capsule showed the greatest strength (216.6 +/- 58.2 kg/cm2) and modulus of elasticity (683.1 +/- 228.8 kg/cm2), whereas the superior capsule showed the least strength (82.4 +/- 33.5 kg/cm2). There were no significant differences in maximum load. The greater strength of the posterior capsule may be one explanation for the low incidence of posterior shoulder dislocation.

The anionic oxy-Cope rearrangement: using chemical reactivity to reveal the facile isomerization of the parent substrates in the gas phase.

The rearrangements of 1,5-hexadiene-3-oxide and 3-methyl-1,5-hexadiene-3-oxide have been studied in the gas phase, using both Fourier transform mass spectrometry (FTMS) and the flowing afterglow (FA) technique. Gas-phase studies of ionic rearrangements can be limited by analysis techniques such as collision-induced dissociation, which have the potential of driving the rearrangement prior to fragmentation. In the studies reported here, we have utilized methanol-O-d, methyl nitrite, and dimethyl disulfide as chemical reactivity probes to discern whether rearrangement of either of the alkoxides to their corresponding enolates occurs. Of the three structural probe reagents, dimethyl disulfide has been found to be most ideal, since it reacts efficiently with both alkoxides and enolates to produce a unique product from each. On the basis of the reactions observed between dimethyl disulfide and anions generated from 1,5-hexadien-3-ol and 3-methyl-1,5-hexadien-3-ol, we have found that the gas-phase Cope rearrangement of both tertiary and secondary alkoxides occurs under both FTMS and FA conditions. Use of dimethyl disulfide in the FTMS and evaluation of ion residence time in the FA lead to the establishment of an upper limit on the Delta H(*) of the rearrangement of both the parent secondary and tertiary substrates as approximately 11 kcal mol(-1) at 298 K. This value is consistent with our B3LYP/6-31+G* prediction. The rearrangement is also faster in the gas phase than in solution, in accord with theoretical predictions.

Fluorescence probes in biochemistry: an examination of the non-fluorescent behavior of dansylamide by photoacoustic calorimetry.

Photoacoustic calorimetry is shown to be a simple, precise, and accurate method for the quantification of the photophysics of a fluorescence probe, e.g., dansylamide, in a variety of solvents. This technique, which is described in detail, provides a direct measurement of the energy that is released nonradiatively following photostimulation, and can therefore be used to indirectly determine the amount of energy released via luminescent pathways. Photoacoustic calorimetry combined with established absorption and fluorescence methodologies provides a complete arsenal for characterizing the photophysical properties of many systems. Comparison of the photoacoustic signal for dansylamide versus standard compounds (ferrocene, tetraphenylethylene, 8-anilinonaphthalene-1-sulfonate, and/or 5,5'-dithiobis(2-nitrobenzoic acid) in 12 different solvents gave fh values (fraction of each absorbed 337.1-nm photon returned as heat) from a low of 0.530 in 1,4-dioxane to a high of 0.973 in water. The trend noted with solvent polarity is different and more revealing than that determined by the more classical approach of examining either the wavelength of the emission maximum or the fluorescence quantum yield.

Mechanical environment associated with rotator cuff tears.

A simplified 2-dimensional finite element model was used to investigate the stress environment in the supraspinatus tendon. The extrafibrillar matrix and collagen fiber were modeled with fiber-reinforced composite elements. The stress was evaluated at humeroscapular elevation angles of 0 degree, 30 degrees, and 60 degrees. Two acromion conditions were simulated. In the first set of conditions there was no subacromial impingement. In the second set there was subacromial impingement of the bursal side. Impingement was simulated by producing a 1-mm indentation on the bursal surface, an indentation similar to the type of impingement associated with deltoid contraction. The results demonstrated that subacromial impingement generates high stress concentrations in and around the critical zone. Such high stress could initiate a tear; tears that result from stress point to an extrinsic mechanism. However, we found that high stress and potential tears caused by impingement may occur on the bursal side, the articular side, or within the tendon. This result is unaccounted for by traditional mechanical models in which only bursal-sided partial tears are initiated by subacromial impingement.

Stress analyses of glenoid components in total shoulder arthroplasty.

Finite element analysis was used to characterize the local stresses at the bone-implant interface of 2 different types of glenoid components presently used in unconstrained total shoulder arthroplasty. A series of 2-dimensional finite-element meshes was developed to model the glenoid in 2 mutually perpendicular planes with and without implanted components. One of the implants modeled was a cemented all-polyethylene component, and the second was an uncemented metal-backed component. A variety of parameters were studied including the resultant loading direction (concentric versus eccentric), keel geometry, subchondral bone integrity, and cement mantle size. Results of the analyses show that the cemented all-polyethylene design demonstrated an overall stress pattern that was closer to that of the intact glenoid. When the effects of concentric and eccentric loading conditions were compared, the overall stress magnitudes in the subchondral bone were found to be much lower with the uncemented metal-backed component than with its cemented all-polyethylene counterpart. This finding suggests that some degree of stress shielding may be associated with the metal-backed component. In addition, under both the concentric and eccentric loading conditions, extremely high stress regions were found within the polyethylene near the polyethylene-metal interface of the uncemented metal-backed component.

Biomechanical analysis of prophylactic fixation for middle third humeral impending pathologic fractures.

For determination of the most biomechanically desirable construction for prophylactic fixation of impending central 1/3 humeral fractures, 24 matched pairs of fresh frozen skeletonized human cadaveric humeri were divided randomly into four groups. Group 1 compared intact humeri with matched humeri that had a 50% hemicylindrical cortical central 1/3 defect to show reproducible failure at the defect with significant reduction in strength. Groups 2 through 4 compared prophylactic fixation of the defect combined with cementation and dynamic compression plating, Rush rodding, or locked intramedullary nailing. Each specimen was tested in external rotation torsion to failure by fracture. In Group 1, test specimens with defects failed with significantly lower rotation to failure, peak torque, stiffness, and total energy absorbed to failure. In Groups 2 through 4, intramedullary nailing provided statistically significantly better total energy absorbed to failure and stiffness than did dynamic compression plating. The proximally and distally locked intramedullary nail seems to have biomechanical advantages in the prophylactic stabilization of an impending pathologic fracture of the central 1/3 of the humerus. These biomechanical findings must be considered in light of the clinical context when a means of fixation is selected.

Biomechanical evaluation of a proximal tibial opening-wedge osteotomy plate.

This biomechanical study evaluated the static response of a new opening-wedge osteotomy plate to compression and torsion loads in a human cadaver model. This plate incorporates a metal block that distracts the medial tibial cortices to ensure precise correction and prevent bone collapse. The 15-mm plate was inserted into 23 fresh cadaver specimens using a standard surgical technique. Axial loading of 13 specimens (compression) and external rotation loading of 10 specimens (torsion) was performed using a servohydraulic-testing machine. Compression loading resulted in failure at a mean of 1810 N due to bone collapse, fracture, or translation. Torsional loading resulted in failure at a mean of 10 Nm due to fracture of the lateral tibial cortex in all specimens. The ratio of the experimental failure load to the calculated estimate of the knee joint forces during gait were 1.07 in axial compression and 0.925 in torsion. This opening-wedge osteotomy plate construct appears marginally strong enough to withstand the estimated axial load on the proximal tibia during gait. Estimated torsional load on the knee during level walking slightly exceeds the failure load prior to osteotomy healing. This information can be used to guide further experimental protocols for static and dynamic testing of this device to determine the appropriate rehabilitation guidelines following opening-wedge proximal tibial osteotomy.

Biomechanical evaluation of occipital fixation.

Many studies have shown a positive correlation among screw pullout strength, screw insertional torque, bone thickness, and areal bone mineral density (BMD) measured by dual-energy X-ray absorptiometry. Variations are significant in the anatomy of the occipital bone. But no studies have correlated these variables with respect to the two locations commonly used for plate fixation to the occiput. The purpose of this study was to determine the thickness and quality of the occipital bone and to correlate these variables with the insertional torque of screws and the pullout strength of plates secured into two different locations on the occiput. The occiputs of 12 adult human fresh frozen cadaveric specimens were used. The specimens were analyzed by dual-energy X-ray absorptiometry. Direct thickness measurements of the occiput were performed. Areal and volumetric BMD were measured. A simple pelvic reconstruction plate (3.2 mm) was fixed to the occiput either laterally or at the midline with bicortical 4-mm cancellous screws. Torque was recorded at the time of insertion of each screw. Axial pullout tests were performed on all specimens. The peak load, failure load, stiffness, and energy to failure were recorded for each construct. Statistical analysis showed that the average thickness of occipital bone is greater in the midline than laterally. Occipital bone is thicker and screw torque is greater close to the inion. There is a positive correlation between bone thickness, areal BMD as measured by dual-energy X-ray absorptiometry, screw insertional torque, and strength of fixation. A plate fixed in the midline region of the occiput provides more rigid fixation than a plate fixed laterally. Areal BMD correlates better than volumetric BMD with bone thickness and is a reliable predictor of the strength of occipital fixation.