Effect of angular stability and other locking parameters on the mechanical performance of intramedullary nails.

To extend the indications of intramedullary nails for distal or proximal fractures, nails with angle stable locking options have been developed. Studies on the mechanical efficacy of these systems have been inconsistent likely due to confounding variables such as number, geometry, or orientation of the screws, as well as differences in the loading mode. Therefore, the aim of this study was to quantify the effect of angular stability on the mechanical performance of intramedullary nails. The results could then be compared with the effects of various locking screw parameters and loading modes. A generic model was developed consisting of artificial bone material and titanium intramedullary nail that provided the option to systematically modify the locking screw configuration. Using a base configuration, the following parameters were varied: number of screws, distance and orientation between screws, blocking of screws, and simulation of freehand locking. Tension/compression, torsional, and bending loads were applied. Stiffness and clearance around the zero loading point were determined. Angular stability had no effect on stiffness but completely blocked axial clearance (p=0.003). Simulation of freehand locking reduced clearance for all loading modes by at least 70% (p<0.003). The greatest increases in torsional and bending stiffness were obtained by increasing the number of locking screws (up to 80%, p<0.001) and by increasing the distance between them (up to 70%, p<0.001). In conclusion, our results demonstrate that the mechanical performance of IM nailing can be affected by various locking parameters of which angular stability is only one. While angular stability clearly reduces clearance of the screw within the nail, mechanical stiffness depends more on the number of screws and their relative distance. Thus, optimal mechanical performance in IM nailing could potentially be obtained by combining angular stability with optimal arrangement of locking screws.

Association of increased myocardial contractility and elevated end-diastolic wall tension with short-term myocardial ischemia: a pressure-volume analysis.

OBJECTIVE: Critical myocardial oxygen imbalance as indicated by elevated interstitial lactate levels may occur in cases of rapidly elevated end-diastolic myocardial wall tension during elevated myocardial contractility in the intact myocardium. Simultaneous administration of beta-adrenergic receptor agonist and antagonist reliably allows for investigating the myocardial response. DESIGN: Experimental using an in vivo animal model. SETTING: Research institution. PARTICIPANTS: Animal model. INTERVENTIONS: Not applicable. MEASUREMENTS AND RESULTS: Fifteen minipigs were investigated in an open-chest model. After a midline sternotomy, a thin dialysis tube was implanted into the LV midmyocardium. Extracellular lactate in perfusate was analyzed every 5 minutes. End-systolic time-varying elastance and end-diastolic wall tension were measured. After a stable period, dobutamine (10 microg/kg/min) was administered to 8 animals. After 20 minutes, esmolol (0.5-mg/kg bolus, repetitively) was added until heart rate decreased to <100 beats/min. For 20 minutes, esmolol was infused at a rate of 3 mg/kg/h, and then dobutamine alone was continued for 10 minutes. With dobutamine, the lactate level did not change, but wall tension decreased and contractility increased. Simultaneous esmolol initially (in the first 10 minutes) increased lactate, whereas LV end-diastolic wall tension and contractility both increased; but after 10 minutes, lactate and contractility decreased significantly. Lactate again increased within 10 minutes after stopping esmolol. A group of 7 animals received esmolol for 20 minutes and showed no changes in lactate; myocardial wall tension increased and contractility decreased. CONCLUSION: Results suggest that oxygen demand/supply is balanced until both end-diastolic wall tension and myocardial contractility are elevated to critical levels.

Determination of preload-recruitable stroke work and elastance by the relationship of arterial blood flow velocity to left ventricular area.

OBJECTIVE: The authors investigated the feasibility of an online sampling and display of LV flow-area loops for the determination of LV elastance and preload-recruitable stroke work (PRSW). Automated LV area measurements by echocardiography may be combined with flow velocity measurements in the internal carotid artery to construct LV flow-area loops as estimates of the systolic pressure-volume relationship. SETTING: University hospital. DESIGN: Open chest model. PARTICIPANTS: Eight anesthetized minipigs. INTERVENTIONS: Inferior vena cava occlusion was performed to simultaneously obtain parameters of the LV flow-area relationship and the LV pressure-area relationship. MEASUREMENTS AND MAIN RESULTS: Parameters were obtained at baseline and during sequential administration of dobutamine (5 microg/kg/min) and halothane (0.8 vol%). Linear regression analysis and analysis of variance were performed to investigate an underlying linear relationship between the corresponding variables. Highly linear elastance and PRSW curves were derived from the flow-area and pressure-area loops (n = 24, R >/= 0.85). Changes of the curve slopes indicated inotropic changes as well as model independent dP/dt(max). Elastance from the pressure-area relationship was expressed by elastance from the flow-area relationship by the term y = 0.52 + 0.04. x (R(2) = 0.84; p < 0.0001). Linear regression of PRSW as derived from the flow-area relationship with PRSW as derived from the pressure-area relationship was expressed by y = 0.43 + 0.02. x (R(2) = 0.77; p < 0.0001). CONCLUSION: Indices of the LV pressure-area relationship can be derived from real-time loops constructed from arterial flow velocity and LV area.