Cementless implant composition and femoral stress. A finite element analysis

Proximal atrophy and thigh pain are recognized problems with some cementless femoral stems in total hip arthroplasty. It is thought that reduced femoral stress from alterations in load transfer caused by an intramedullary stem contributes to proximal femoral atrophy. An increase in flexural rigidity and bone stress near the stem tip is thought to contribute to thigh pain. A three-dimensional finite element analysis study was performed to calculate stresses in the proximal femur and bone near the stem tip before and after implantation of a collared, proximally coated, cementless femoral prosthesis. The influence of prosthetic material was examined by changing implant composition from cobalt chrome to titanium alloy and leaving all other parameters constant. Femoral stress was increased twofold immediately below the collar with the titanium implant compared with the cobalt chrome. However, the proximal femoral stress in the titanium implanted model was still 1/10 that in the corresponding region of the unimplanted femur model. At the stem tip, as much as a 30% reduction in femoral stress was seen with the titanium stem compared with the cobalt chrome. These findings suggest biomechanical evidence of an advantage for titanium as an implant material compared with cobalt chrome for cementless femoral stems.

Multiple regions of ligand discrimination revealed by analysis of chimeric parathyroid hormone 2 (PTH2) and PTH/PTH-related peptide (PTHrP) receptors

PTH and PTH-related peptide (PTHrP) bind to the PTH/PTHrP receptor and stimulate cAMP accumulation with similar efficacy. Only PTH activates the PTH2 receptor. To examine the structural basis for this selectivity, we analyzed receptor chimeras in which the amino terminus and third extracellular domains of the two receptors were interchanged. All chimeric receptors bound radiolabeled PTH with high affinity. Transfer of the PTH2 receptor amino terminus to the PTH/PTHrP receptor eliminated high-affinity PTHrP binding and significantly decreased activation by PTHrP. A PTH/PTHrP receptor N terminus modified by deletion of the nonhomologous E2 domain transferred weak PTHrP interaction to the PTH2 receptor. Introduction of the PTH2 receptor third extracellular loop into the PTH/PTHrP receptor increased the EC50 for PTH and PTHrP, while preserving high-affinity PTH binding and eliminating high-affinity PTHrP binding. Similarly, transfer of the PTH/PTHrP receptor third extracellular loop preserved high-affinity PTH binding by the PTH2 receptor but decreased its activation. Return of Gln440 and Arg394, corresponding residues in the PTH/PTHrP and PTH2 receptor third extracellular loops, to the parent residue restored function of these receptors. Simultaneous interchange of wild-type amino termini and third extracellular loops eliminated agonist activation but not binding for both receptors. Function was restored by elimination of the E2 domain in the receptor with a PTH/PTHrP receptor N terminus and return of Gln440/Arg394 to the parent sequence in both receptors. These data suggest that the amino terminus and third extracellular loop of the PTH2 and PTH/PTHrP receptors interact similarly with PTH, and that both domains contribute to differential interaction with PTHrP.

An unusual over-gravid female of Enterobius vermicularis recovered from a child

An unusual over-gravid female of Enterobius vermicularis was recovered from a 15-month old child by cello-tape anal swab. The patient resided in Inchon and complained of severe anal itching. The worm measured 7.8 mm in length and 0.5 mm in width, and retained typical morphologic features of E. vermicularis such as cephalic alae and a sharply pointed posterior end. In this gravid female, peculiarly, the uterus was tremendously distended, and about 99% of the whole body length was completely packed with a great number of eggs. Other internal organs were difficult to observe. This paper describes a peculiar over-gravid female of E. vermicularis.

Compartment model for measuring myocardial oxygen consumption using [1-11C]acetate

Although [1-11C]acetate has been validated as a PET tracer for myocardial oxygen consumption (MVO2) in animals and humans, mono- and biexponential fitting of the tissue time-activity curve yields only estimates of MVO2. This study attempts to develop and validate a simple tracer kinetic model in vivo for estimation of regional MVO2.

METHODS

Twenty-seven experiments were performed in 12 anesthetized dogs with [1-11C]acetate and serial PET images under different MBF and MVO2 (baseline, ischemia, xylazine, dobutamine and dipyridamole). Estimates of MVO2 were obtained from dynamic [1-11C]acetate PET and model fitting. MBF was measured by radiolabeled microspheres, and MVO2 was calculated by the Fick method using arterial and coronary blood samples.

RESULTS

The proposed model fitted equally well for all study conditions with a multiple correlation coefficient of 0.985 +/- 0.026. Estimated MVO2 correlated linearly with measured MVO2 (y = 0.033 + 0.690x, r = 0.92, s.e. of estimates = 0.020).

CONCLUSION

This study indicates that MVO2 can be assessed with PET and [1-11C]acetate over a wide range with a simple tracer kinetic model.

Reproducibility of measurements of regional resting and hyperemic myocardial blood flow assessed with PET

PET with 13N-ammonia permits the noninvasive quantification of myocardial blood flow (MBF) in humans. The present study was done to assess the reproducibility of quantitative blood flow measurements at rest and during pharmacologically induced hyperemia in healthy individuals.

METHODS

Thirty healthy volunteers (26 men, 4 women) were studied. Paired measurements of MBF at rest (n = 21), during adenosine (n = 15) and during dipyridamole (n = 7) were performed using a two-compartment model for 13N-ammonia PET. The mean difference between baseline and follow-up blood flow (% difference) was calculated to assess reproducibility.

RESULTS

No significant difference was observed between resting blood flow at baseline or follow-up (15.8% +/- 15.8%; p = ns). Baseline and follow-up resting blood flow were linearly correlated (r = 0.63, p < 0.005). Normalization of resting blood flow to the rate pressure product improved the reproducibility significantly (15.8% +/- 15.8% versus 10.1% +/- 10.5%, p < 0.05). Baseline and follow-up hyperemic myocardial blood flow did not differ (11.8% +/- 9.4%; p = ns) and were linearly correlated (r = 0.69, p < 0.0005).

CONCLUSION

MBF at rest can be measured reproducibly with 13N-ammonia PET. The individual response to pharmacologic stress appears to be relatively consistent. Thus, serial blood flow measurements with 13N-ammonia PET can be used to quantify the effect of various interventions on MBF and vasodilatory reserve.

Two human cases of Thelazia callipaeda infection in Korea

Thelazia callipaeda were observed from a 7-month old baby who lived in Uijongbu in 1989 and from a 42-year old man who lived in Anyang in 1994. These are the 23rd and 24th records of human thelaziasis in Korea as the literature are concerned.

Correlation of computed finite element stresses to bone density after remodeling around cementless femoral implants

Fundamental to the development of a durable, uncemented femoral component is an understanding of the remodeling process that goes on after implantation. Predicting the bone remodeling that results from the use of a given hip implant would facilitate the design of a prosthesis that would optimize bone adaptation. This report combines the results of finite element stress analysis of the AML prosthesis implanted in vitro into a proximal femur with quantitative bone mineral density measured in vivo in the medial and lateral aspects of human femora at periods after implantation. Unimplanted femora were also analyzed for comparison purposes. Bone density measurements were obtained using dual energy x-ray absorptiometry. Absolute values of the maximum principal stress and maximum shear stress calculated in the femur at the time of implantation accurately predict bone density resulting from remodeling caused by the prosthesis. The calculated initial strain was not found to correlate with resultant bone density. These findings suggest that the results of stress analyses using three dimensional models of femora implanted in vitro can predict bone remodeling around prostheses and may be used to quantitate appropriate design criteria for total hip replacements.

Femoral prosthesis implantation induces changes in bone stress that depend on the extent of porous coating

The objective of this study was to evaluate the effect of implantation of porous-coated anatomic medullary fitting prostheses on stress in the proximal femur. Three-dimensional finite element models of a cadaveric femur before and after implantation were used to evaluate the resulting changes in stress in the bone. Models of the femur were generated automatically from computed tomographic scan data with use of an innovative mesh-generation technique. The models were analyzed for three levels of porous coating (proximal, 5/8, and full), with the assumption of ideal ingrowth (perfect bonding) over porous areas and a frictionless, tension-free surface on smooth areas. All models were loaded and restrained to represent conditions of normal gait. The stresses predicted in the implanted femur are consistent with clinical observations of proximal cortical atrophy (normal stress reduced to 6-9% of normal at the calcar and 50-55% at mid-prosthesis) and of hypertrophy at the porous coating junctions (normal stress at the 5/8-coating junction, 123% of stress proximal to the junction) and hypertrophy near the distal tip of the prosthesis (anterior and posterior normal stresses 200-800% of normal). The fully coated prosthesis induced stresses in the bone near the tip of the prosthesis that were most like stresses in the normal femur (medial and lateral normal stress 105 and 102% of the stress in the normal femur). Below the collar, the normal stress associated with the proximally coated prosthesis was 6% greater than that produced with the other two levels of coating but still was only 2% of normal. The 5/8-coated prosthesis appeared to combine the worst features of the fully coated and proximally coated prostheses--greater stress-shielding at the calcar and higher stress near the tip of the prosthesis.

Are contraction and relaxation coupled in patients with and without congestive heart failure?

BACKGROUND

Although changes in contractility are often accompanied by changes in relaxation, a mathematical model of ventricular coupling has not been described. A model we examined suggests a hyperbolic relation between measurements of contraction and relaxation. We thus tested the hypothesis that relatively load-independent measurements of contractility (end-systolic elastance [Ees]) and relaxation (the slope of the tau-to-end-systolic pressure relation [R]) were coupled.

METHODS AND RESULTS

To establish the validity of the model, an assessment of Ees and R was made in 30 subjects who underwent sequential digital ventriculography and micromanometer pressure measurements during atrial pacing (93 +/- 10 min-1) before and after graded doses of nitroprusside. To establish if a cyclic AMP (cAMP)-mediated intervention alters coupling, seven of the 30 subjects were studied before and after 3 months of beta-blockade. To determine if a non-cAMP-mediated intervention alters coupling, 12 other patients were studied before and after deslanoside. Nonlinear regression analysis for the initial 30 patients suggested a hyperbolic relation: (Ees) (R) = 1.05 (r = 0.79, p less than 0.001) with an inflection point near Ees = 1.02 mm Hg/ml. Thus, with normal or near-normal contractility, relaxation is normal and not load dependent (R is close to 0). With systolic dysfunction, relaxation becomes very afterload dependent and so must be normalized for load. After long-term beta-blockade in patients with severe left ventricular dysfunction, small improvements in contractility (elastance) occurred with larger changes in relaxation, but the curve describing the relation was not displaced. Acute administration of deslanoside resulted in a large increase in elastance and a smaller change in relaxation but did not alter coupling. However, the magnitude of the change in R was dependent on the predrug R value.

CONCLUSIONS

These data suggest contraction and relaxation may be physiologically coupled with relaxation relatively preserved in early heart failure and more rapid deterioration in relaxation as elastance falls under 1.02 mm Hg/ml. Both beta-blockers (which may act through cAMP) and digitalis (which is cAMP independent) improve contraction and relaxation, but both mechanisms appear to maintain coupling. The hyperbolic relation between contraction and relaxation may have important implications regarding therapeutic response and selection of patients for clinical trials in heart failure.

Alleviation of cocaine-induced coronary vasoconstriction by nitroglycerin

Cocaine induces vasoconstriction of epicardial coronary arteries in patients with and without coronary artery disease, and this vasoconstriction is particularly marked in segments narrowed by atherosclerosis. To assess the effect of nitroglycerin on cocaine-induced coronary vasoconstriction, computer-assisted quantitative analysis was performed on non-diseased and diseased coronary artery segments in 23 patients (18 men, 5 women, aged 43 to 65 years) 1) at baseline, 2) after administration of intranasal saline solution (in 8 patients) or 2 mg/kg of cocaine (in 15 patients), and then 3) after administration of sublingual placebo (in 6 patients) or 0.4 or 0.8 mg of nitroglycerin (in 9 patients) in the 15 patients given cocaine. In response to cocaine administration, coronary artery cross-sectional area decreased 22 +/- 7% (mean +/- SD) in non-diseased segments (p less than 0.05) and 45 +/- 18% in diseased segments (p less than 0.02). The magnitude of vasoconstriction was greater (p = 0.01) in the diseased segments. Sublingual nitroglycerin abolished the vasoconstriction in both non-diseased and diseased segments. Thus, nitroglycerin alleviates cocaine-induced vasoconstriction in patients with coronary artery disease.

Potentiation of cocaine-induced coronary vasoconstriction by beta-adrenergic blockade

STUDY OBJECTIVE

To determine whether beta-adrenergic blockade augments cocaine-induced coronary artery vasoconstriction.

DESIGN

Randomized, double-blind, placebo-controlled trial.

SETTING

A cardiac catheterization laboratory in an urban teaching hospital.

PATIENTS

Thirty clinically stable patient volunteers referred for catheterization for evaluation of chest pain.

INTERVENTIONS

Heart rate, arterial pressure, coronary sinus blood flow (by thermodilution), and epicardial left coronary arterial dimensions were measured before and 15 minutes after intranasal saline or cocaine administration (2 mg/kg body weight) and again after intracoronary propranolol administration (2 mg in 5 minutes).

MEASUREMENTS AND MAIN RESULTS

No variables changed after saline administration. After cocaine administration, arterial pressure and rate-pressure product increased; coronary sinus blood flow fell (139 +/- 28 [mean +/- SE] to 120 +/- 20 mL/min); coronary vascular resistance (mean arterial pressure divided by coronary sinus blood flow) rose (0.87 +/- 0.10 to 1.05 +/- 0.10 mm Hg/mL.min); and coronary arterial diameters decreased by between 6% and 9% (P less than 0.05 for all variables). Subsequently, intracoronary propranolol administration caused no change in arterial pressure or rate-pressure product but further decreased coronary sinus blood flow (to 100 +/- 14 mL/min) and increased coronary vascular resistance (to 1.20 +/- 0.12 mm Hg/mL.min) (P less than 0.05 for both).

CONCLUSIONS

Cocaine-induced coronary vasoconstriction is potentiated by beta-adrenergic blockade. Beta-adrenergic blocking agents probably should be avoided in patients with cocaine-associated myocardial ischemia or infarction.

Cocaine-induced coronary-artery vasoconstriction

Intranasal cocaine is used frequently as a local anesthetic during many rhinolaryngologic procedures. Although its "recreational" use in high doses has been associated with chest pain and myocardial infarction, this association has not been established when cocaine is used in low doses as a topical anesthetic, and its effect on the coronary vasculature of humans is unknown. We studied the effects of intranasal cocaine (10 percent cocaine hydrochloride; 2 mg per kilogram of body weight) on the blood flow in and dimensions of the coronary arteries and on myocardial oxygen demand in 45 patients (34 men and 11 women, 36 to 67 years of age) who were undergoing cardiac catheterization for the evaluation of chest pain. Heart rate, arterial pressure, blood flow in the coronary sinus (measured by thermodilution), and the dimensions of the epicardial left coronary artery (measured by quantitative arteriography) were measured before and 15 minutes after the intranasal administration of saline (in 16 patients) or cocaine (in 29). No variables changed after the administration of saline. After cocaine was administered, the heart rate and arterial pressure rose, the coronary-sinus blood flow fell (from a mean [+/- SD] of 149 +/- 59 ml per minute to 124 +/- 53 ml per minute), and the diameter of the left coronary artery decreased by 8 to 12 percent (P less than 0.01 for all comparisons). No patient had chest pain or electrocardiographic evidence of myocardial ischemia after the administration of cocaine. Subsequently, the administration of the alpha-adrenergic blocking agent phentolamine caused all these values to return to base-line levels. There was no difference in response between the patients found to have disease of the left coronary artery (n = 28) and those without such disease (n = 17). We conclude that the intranasal administration of cocaine near the dose used for topical anesthesia causes vasoconstriction of the coronary arteries, with a decrease in the coronary blood flow, despite an increase in myocardial oxygen demand, and that these effects are mediated by alpha-adrenergic stimulation. It is reasonable to assume that these effects would be more pronounced at the much higher doses associated with the recreational use of cocaine.