Mechanical properties of myxomatous mitral valves.

OBJECTIVE: We sought to characterize the mechanical properties of normal and myxomatous mitral valve tissues. METHODS: We tested 113 mitral valve sections from patients undergoing mitral valve repair or replacement for myxomatous mitral valve prolapse and sections from 33 normal valves obtained at autopsy. RESULTS: Myxomatous mitral valve leaflets were more extensible than normal leaflets when tested parallel to the free edge (41.2% +/- 18.5% vs 17.3% +/- 6.7% circumferential strain [mean +/- SD]; P <.001), as well as perpendicular to the free edge (43.2% +/- 19.4% vs 17.3% +/- 6.7% radial strain; P <.001). Myxoid leaflets were less stiff circumferentially (4.0 +/- 1.6 vs 6.1 +/- 1.4 kN/m; P <.001) and radially (4.5 +/- 1.1 vs 6.1 +/- 1.4 kN/m; P <.001) than normal leaflets. Leaflet strength, however, was similar in both groups. CONCLUSIONS: Myxomatous mitral valve leaflets are physically and mechanically different from normal mitral valve leaflets. They are more extensible and less stiff. Compared with chordae examined previously, however, they are affected much less. Myxomatous mitral valve disease may therefore affect the collagen in the chordae more severely than that in the leaflets.

Anterior-inferior capsular shift of the shoulder: a biomechanical comparison of glenoid-based versus humeral-based shift strategies.

This study compared the biomechanical effects of an anterior-inferior capsular shift based at the humeral side with one on the glenoid side of the joint on resultant multidirectional glenohumeral translation and rotation. Nine matched pairs of fresh cadaveric shoulders were placed in a testing apparatus that constrained 3 rotations but allowed simultaneous free translation of the humeral head with respect to the glenoid. The right and left shoulders of each of the matched pairs were randomized to undergo either a glenoid-based or humeral-based anterior capsular shift. The shoulders were tested vented and following the capsular shift procedure. Translational testing was performed at 0 degrees, 45 degrees, and 90 degrees of glenohumeral elevation with the humerus in neutral rotation, 30 degrees internal rotation, and 30 degrees external rotation. Sequential loads of 30 N in anterior, posterior, and inferior directions were applied while maintaining a 22-N joint compressive load. The maximum arc of internal and external rotation after application of a 1-newton-meter moment was determined for the vented specimens and then after the capsular shift procedure. Both shift strategies resulted in significant limitation of anterior, posterior, and inferior translation in all of the tested positions. No significant differences were noted between the 2 shift strategies with respect to restriction of translation in the anterior or inferior directions. The glenoid-based shift caused a significantly greater decrease in posterior translation at 45 degrees and 90 degrees of abduction. With respect to rotation, the glenoid-based shift exerted significantly greater restriction on external rotation than the humeral-based shift. This study supports the use of either a humeral-based or glenoid-based shift to control multidirectional glenohumeral instability. Greater reduction in external rotation was demonstrated after the glenoid-based shift. Specific differences demonstrated in translation control for humeral-based versus glenoid-based capsular shift procedures may be useful in tailoring a procedure for specific instability patterns.

Tissue damage and calcification may be independent mechanisms of bioprosthetic heart valve failure.

BACKGROUND AND AIM OF THE STUDY: Porcine bioprosthetic valves have excellent hemodynamics and do not require anticoagulation, but have limited durability. Cusp tearing is a major cause of bioprosthetic valve failure. It has been suggested that the mechanism of bioprosthetic valve failure is stiffening by calcification, which leads to elevated stresses and secondary collagen fiber damage and leaflet tearing. This thesis was tested in explanted porcine bioprostheses. METHODS: A total of 60 explanted porcine bioprosthetic valves was tested mechanically, and 15 explanted valves were examined grossly and histologically. Circumferentially and radially oriented samples of cusp tissue were tested uniaxially in a materials testing machine and compared with five controls. RESULTS: Mean (+/-SD) duration of implantation was 10.9+/-5.6 years. Circumferential specimens from explants were less extensible than controls (11.0+/-5.5% versus 24.5+/-2.8% strain, p <0.001), and failed at lower tensions (973+/-733 versus 3075+/-911 N/m, p = 0.001) and at lower strains (21.2+/-8.1% versus 47.3+/-7.1% strain, p <0.001). Radial specimens from explants were less extensible (28.7+/-6.8% versus 39.2+/-5.9% strain, p = 0.002) and failed at lower strains (60.3+/-17.3% versus 112.2+/-24.9% strain, p <0.001) than the controls. The stiffness of the explants was unchanged from controls in both circumferential and radial samples. There were no differences between explants and controls in radial and circumferential stiffness, and in radial failure strength. Calcification was mild and diffuse in most of the tested samples. Tears were found in areas without calcific deposits, along with breaks in collagen fiber bundles. CONCLUSION: These results do not support the thesis that calcification stiffens glutaraldehyde-fixed porcine bioprostheses, except when the entire cusp is transformed into a solid mass of mineral. Rather, leaflet tears may develop as a result of accumulated mechanical damage that is independent of calcification.

Myxomatous mitral valve chordae. I: Mechanical properties.

BACKGROUND AND AIM OF THE STUDY: Chordal rupture is the most common reason for severe mitral regurgitation requiring surgery. The features that predispose myxomatous chordae to rupture, however, have not been studied. Thus, the physical and mechanical properties of normal and myxomatous mitral valve chordae were measured. METHODS: Chordae from 24 normal and 59 myxomatous mitral valves were cut into 10 mm-long segments and mechanically tested to measure extensibility, modulus, failure stress, failure strain, and failure load. After testing, the specimens were weighed and their cross-sectional area and volume measured. RESULTS: Chordae from myxoid mitral valves were larger (1.9 +/- 0.1 mm2 versus 0.8 +/- 0.1 mm2, p < or = 0.001) and heavier (16.6 +/- 1.0 mg versus 6.5 +/- 0.4 mg, p < or = 0.001) than normal chordae. Myxoid chordae had significantly lower moduli (40.4 +/- 10.2 MPa versus 132 +/- 15 MPa, p < or = 0.001) and failed at significantly lower tensile stress (6.0 +/- 0.6 MPa versus 25.7 +/- 1.8 MPa, p < or = 0.001) and absolute load (728 +/- 50 g versus 1,450 +/- 135 g, p < or = 0.001) than normal chordae. Normal and myxoid chordae had similar measurements of extensibility and failure strain. CONCLUSION: Myxomatous degeneration severely affects the mechanical properties of mitral valve chordae. Most notably, myxoid chordae failed at loads one-half of those of normal chordae. This may explain why chordal rupture is the main indication for repair of myxoid mitral valves. These findings also suggest that chordal preservation should be carried out with caution, as myxoid chordae are clearly abnormal with compromised mechanical strength.

Role of preconditioning and recovery time in repeated testing of aortic valve tissues: validation through quasilinear viscoelastic theory.

We investigated how preconditioning history and specimen recovery time affect the accuracy of measurements of mechanical properties obtained from sequential, repeated materials testing of porcine aortic valve (PAV) cusps. Strain history protocols were modeled by quasilinear viscoelastic theory and the results compared with the experimental data. Assuming that the model was predictive, the accuracy of predicting experimental data was related to the suitability of the materials testing protocol. We found that the preconditioned state of the PAV material was not unique but was a function of the deformation history that had occurred before the preconditioning cycles. Preconditioning without an adequate rest period between tests increased predictive errors, whereas allowing the material to rest without preconditioning reduced errors. Modeling more of the strain history reduced errors for specimens briefly rested between tests but had no impact on specimens with long rest periods. The smallest predictive errors were obtained for a loading protocol with a 24 h specimen recovery period followed by material preconditioning. We recommend the use of this protocol for estimating material properties of PAV tissues.

An in vitro comparison of the effect of number and pattern of positional screws on load resistance.

OBJECTIVE: The purpose of this in vitro investigation was to determine whether the number or pattern of placement of positional screws affected their ability to resist vertical loads resembling mastication. MATERIALS AND METHODS: Standardized bone substitutes were secured with varying numbers of 17.0-mm-long, 2.0-mm outer diameter, self-tapping titanium screws (W. Lorenz Surgical, Jacksonville, FL) in various patterns using a positional screw technique. These patterns included one, two, three, four, and five screws in a linear pattern; two screws in a vertical pattern; three screws in an L-pattern, inverted-L pattern, backward-L pattern, inverted-backward-L pattern, and right and left diagonal pattern; four screws in a box pattern; and five screws in a domino pattern. Five models of each group were fabricated along with a control group. The models were secured in a jig and subjected to vertical loads by an Instron 8511.20 Mechanical Testing Unit (Canton, MA) until failure. Common engineering standards, including yield load, yield displacement, stiffness, maximum load, and displacement at maximum load, were measured, and means and standard deviations were derived and compared for statistical significance with an analysis of variance (ANOVA) and Scheffe multiple comparison test. RESULTS: Screw number and pattern of placement affected the mechanical characteristics in resistance to vertical load. Screw numbers less than three and all linear patterns were the least effective. The three-screw L patterns, as a category, provided greater resistance to vertical loads than either the three-screw linear or three-screw diagonal patterns. The addition of more than three screws in geometric patterns offered no greater benefit than the three-screw L patterns. CONCLUSIONS: In this in vitro study, the three-screw L patterns were the most effective and efficient when using the positional screw technique.

An in vitro evaluation of distractors used for osteogenesis.

OBJECTIVE: The objective of this investigation was to evaluate and compare the mechanical behaviors of distractors used for osteogenesis under various conditions by means of common engineering standards. STUDY DESIGN: Five groups of 5 synthetic mandibles were used in this study (N = 25). The first portion of the investigation compared mandibles without intervention (group A [controls]; n = 5), mandibles that had uniform osteotomies stabilized with an external distractor (group B; n = 5), and mandibles that had simulated sagittal osteotomies rigidly fixated with 3 positional screws (group C; n = 5). The second portion of the investigation compared uniform osteotomies (group B; n = 5) and uniform corticotomies (group D; n = 5) that were stabilized with the same external distractor. The last portion of the investigation compared osteotomies stabilized with an external distractor (group B; n = 5) and osteotomies stabilized with an internal distractor (group E; n = 5). Each construct was subjected to vertical loads on a mechanical testing unit. Common engineering standards, including yield load, yield displacement, maximum load, displacement at maximum load, and stiffness, were measured, recorded, and compared by means of a 1-way analysis of variance and a Scheffé multiple comparison test or independent-samples t test. The means between groups were considered significant for P < .05. A polynomial best-fit curve was calculated for the load/displacement data for each group. RESULTS: During the first portion of the investigation, no significant differences were noted between the control, rigidly fixated sagittal osteotomy, and external distractor with osteotomy groups for displacement at maximum load (P = .19). Significant differences were noted between groups for yield displacement (P = .009), yield load (P < .001), maximum load (P < .001), and stiffness (P < .007). Failures occurred in the control and rigidly fixated groups with fractures of the synthetic mandibles. Failures occurred in the external distractor group with permanent deformation or torsion of the pins. During the second portion of the experiment, no statistically significant differences were noted between the corticotomy and osteotomy groups in stiffness (P = .363), maximum load (P = .207), or yield displacement (P = .940). Statistically significant differences were noted between groups for yield load (P = .036) and displacement at maximum load (P = .010). Failures occurred in both groups with permanent deformation or torsion of the pins. During the last portion of the investigation, statistically significant differences were noted between the external distractor and internal distractor groups in yield load (P < .001), yield displacement (P < .001), maximum load (P = .001), and displacement at maximum load (P = .01); no significant differences were noted in stiffness (P = .71). Failures occurred in the external distractor group with permanent deformation or torsion of the pins. Failures occurred in the internal distractor group with fracture of the model or displacement beyond 30.0 mm. CONCLUSIONS: Different patterns of mechanical behavior were found between the control and rigidly fixated sagittal osteotomy groups and the external distractor group, between the corticotomy and osteotomy groups, and between the internal and external distractor groups.

Nitric oxide relaxes human myometrium by a cGMP-independent mechanism.

The role of intracellular guanosine 3',5'-cyclic monophosphate concentration ([cGMP]i) in nitric oxide (NO)-mediated relaxations in the uterus has become controversial. We found the NO donor S-nitroso-L-cysteine (CysNO) to potently (IC50 = 30 nM) inhibit spontaneous contractions in the nonpregnant human myometrium. CysNO treatment increased [cGMP]i significantly (P < 0.001), and this increase was blocked by the guanylyl cyclase inhibitors methylene blue (10 microM) or LY-83583 (1 microM); however, pretreatment with these guanylyl cyclase inhibitors failed to block CysNO-mediated relaxations. Intracellular cAMP concentrations were not altered by treatment of tissues with 10 microM CysNO. Incubation with the cGMP analogs 8-bromo-cGMP or beta-phenyl-1,N2-etheno-cGMP did not significantly affect spontaneous contractility. Pretreatment of tissues with charybdotoxin [a calcium-dependent potassium channel (BK) blocker] completely reversed CysNO-induced relaxations. We conclude that NO is a potent inhibitor of spontaneous contractile activity in the nonpregnant human uterus and that, although guanylyl cyclase and BK activities are increased by NO, increases in [cGMP]i are not required for NO-induced relaxations in this tissue.

An experimental evaluation of traumatic globe rupture.

PURPOSE: The purpose of this investigation was to evaluate the surgeon's ability to assess various types of globe injury, to determine the force necessary to rupture the globe with these types of injuries, and to determine typical orbital retraction forces used in the clinical setting. MATERIALS AND METHODS: Forty-four enucleated globes from recently killed cows were divided into four equal groups-one uninjured control group, one group with a through-and-through scleral laceration, another group with a subtotal scleral laceration, and the last group with an 18-gauge needle perforation. Twenty-seven boarded or board eligible oral and maxillofacial surgeons were asked to assess one sample from each of the four groups. They were then asked to retract a simulated globe on a custom-fabricated jig to determine clinical retraction forces. Ten globes from each of the four groups were then subjected to forces until rupture on an Instron 8501M mechanical testing unit. Accuracy of the clinical assessment was determined, and means and standard deviations of the retraction forces and globe rupture forces were derived. RESULTS: Through-and-through lacerations were assessed by surgeons with 100% accuracy, subtotal lacerations with 96% accuracy, uninjured globes with 74% accuracy, and perforated globes with 15% accuracy. Globe rupture occurred at 16.72+/-7.87 kg in the control group, 20.36+/-7.87 kg in the perforated group, 15.38+/-6.06 kg in the subtotal laceration group, and 4.94+/-2.56 kg in the through-and-through laceration group. Statistically significant differences (P < .001) were noted between the total laceration group and all other groups. The mean retraction force was 0.35+/-0.47 kg, which was statistically less than the force used in all of the rupture groups (P < .001). CONCLUSIONS: Severe injuries (through-and-through lacerations) were assessed with 100% accuracy by the clinicians, and less severe injuries with less accuracy. Rupture forces for globes with perforations and subtotal lacerations were no different than for the control group, but substantially less than for the total laceration group. The simulated clinical retraction forces were substantially more than the rupture forces in all of the groups, including the through-and-through laceration group.

An in vitro comparison of the mechanical characteristics of three sagittal ramus osteotomy fixation techniques.

PURPOSE: The purpose of this in vitro investigation was to compare the mechanical characteristics of three techniques for the rigid internal fixation of simulated sagittal ramus osteotomies in terms of their ability to resist vertical loads resembling masticatory forces. MATERIALS AND METHODS: Fifteen bovine ribs of similar size and shape were subjected to uniform osteotomies resembling a sagittal ramus osteotomy. They were divided into three groups and fixed with three 2.0-mm bicortical positional screws in an inverted L-pattern, eight monocortical screws and a sagittal strut plate, or a hybrid system using three 2.0-mm bicortical positional screws, five 2.0-mm monocortical screws, and a sagittal strut plate. The repaired ribs were secured in a jig, subjected to vertically deforming forces, and evaluated for yield load, yield point, maximum load, displacement at maximum load, load at 3.0 mm displacement, and stiffness. The various groups were compared statistically with a Scheffe multiple comparison test. RESULTS: No statistically significant differences existed between the positional screw group and the hybrid group (P > .05). No statistically significant differences were noted in yield displacement, yield load, displacement at maximum load, for any of the groups (P > .05). There were differences noted between the monocortical strut group and other groups in maximum load, load at 3.0 mm displacement, and stiffness (P > .05). CONCLUSIONS: The monocortically fixed sagittal strut plate offers less resistance to maximum load and loads at 3.0 mm displacement and is less stiff than either a three-positional screw system or a hybrid system. No differences existed between the three positional screw systems and the hybrid system. However, given the low yield loads and yield displacements, these differences may not be clinically important.

A comparison of mandibular angle fracture plating techniques.

OBJECTIVE: The purpose of this investigation was to compare the conventional technique of mandibular angle fracture plating with two biomechanically dissimilar techniques in their abilities to resist vertical loads similar to masticatory forces. STUDY DESIGN: Three groups of five synthetic hemimandibles with simulated fracture repairs were compared for their capabilities to resist vertical deformation. The conventional group was stabilized with a thin tension band system at the superior border and thick stabilization plate system at the inferior border. The nontraditional group was stabilized with a thick tension band system at the superior border and thin stabilization plate system at the inferior border. The two miniplate group was stabilized with a thin tension band system at the superior border and thin stabilization plate at the inferior border. A cantilever beam design was used. Testing was performed with an instron 8511.20 (Instron Corp., Canton, Mass.) mechanical testing device. The three groups were compared with a two way analysis of variance. RESULTS: The forces resisted by the conventional group (167.6 +/- 18.2 N), the nontraditional group (156.3 +/- 33.9 N), and two miniplate group (154.0 +/- 18.4 N) were not statistically different (F = 0.44, p > 0.66). All failures occurred at the tension bands secured with monocortical screws. CONCLUSIONS: Under the conditions described in this in vitro investigation, plate thickness or pattern made no difference. All failures in this experiment occurred with monocortical screws in the superior border tension band system.