Regional distribution of circulating microspheres in the femur of the rabbit

Evidence of an autoregulatory mechanism of regional bone blood flow at hypotension

BACKGROUND

Blood flow in various organs is determined by an autoregulatory mechanism that guarantees constant organ perfusion over a wide range of arterial blood pressure changes. This physiological principle has been proven for the kidney, brain and intestinal tract, but so far not for bone. This study was carried out to determine whether there is an autoregulatory mechanism of bone or not.

METHODS

The fluorescent microsphere reference sample method was used to determine blood flow within the bone and kidneys. Eight anesthetized female New Zealand rabbits received left ventricular injections of fluorescent microspheres over a wide range of arterial pressure levels prior to removal of kidney, femur and tibia. Blood flow values were calculated by measurement of fluorescence intensity in kidney and bone and correlated to fluorescence intensity in the peripheral blood (reference sample).

RESULTS

Despite a reduction of mean arterial pressure from 100 to 80 mmHg bone blood flow remained constant. Further reduction of mean arterial pressure results in a linear decrease in bone blood flow.

CONCLUSION

The correlation between arterial pressure and organ perfusion in the bone is similar to blood flow within the kidney, indicating the presence of an autoregulated blood flow mechanism within the bone tissue.

Fluorescent microspheres are reliable for serial bone blood flow measurements

The fluorescent microsphere method is one of the current techniques to determine regional blood flow in various organs. The purpose of this study was to examine the suitability of fluorescent microspheres for serial measurement of regional bone blood flow. Six anesthetized female New Zealand rabbits received five left ventricular injections of fluorescent microspheres in 20-minute intervals. To test the precision of the measurement two types of fluorescent microspheres were injected simultaneously at the first and last injections. Blood flow was calculated in the kidneys, lungs, brain, femurs, and tibias after measuring the fluorescence intensity in each reference blood and tissue sample. Comparison of blood-flow values obtained by simultaneously injected microspheres showed an excellent correlation and a minimal percentage difference at the first and last injections, indicating valid measurements of regional bone blood flow. No significant differences were observed when comparing blood flow in the corresponding regions of bones on the right side and left side. Mean blood flow in the femur and tibia significantly increased at the fourth injection whereas flow distribution within the femur and tibia essentially remained unchanged throughout the experiment. Comparison of blood flow values obtained by simultaneously injected microspheres showed moderate agreement for the kidneys and lungs at the last injections. Because this finding might be attributable to disturbances of microcirculation caused by accumulation of spheres in high-flow organs, the increase in regional bone blood flow observed in our experiments has to be interpreted carefully. This study showed that bone blood flow can be determined reliably in anesthetized rabbits by as many as three serial injections of fluorescent microspheres.

Microspheres accurately predict regional bone blood flow

Even though the microsphere method frequently is used to determinate bone blood flow, validation of this technique for bone blood flow measurement is incomplete. The method is based on the principle that injected microspheres are distributed with the arterial blood and trapped in the capillaries because of their diameter (15 microm). The number of spheres lodged in an organ is proportional to its blood flow. The number of radioactive or fluorescent microspheres in a specific organ is determined indirectly by measuring radioactivity or fluorescence intensity in the organ. In this study the reliability and precision of the microsphere method for determining bone blood flow was established using radioactive and fluorescent microspheres. Six female, anesthetized New Zealand rabbits received left ventricular injections of pairs of fluorescent and/or radioactive microspheres. The humerus, femur, and tibia were dissected in a standardized manner and blood flow was determined in each sample. Comparison of relative blood flow values showed an excellent correlation between radioactive and fluorescent microspheres. The percentage difference and variation between two simultaneously injected sets of microspheres was minimal for radioactive microspheres (0.8% +/- 9.6%) and for fluorescent microspheres (0.2% +/- 11.4%). Regional bone blood flow in different regions of the femur, tibia, or humerus ranged from 2.2-28.1 mL/minute/100 g, but there was no significant difference between right and left bone samples of the same region after repeated measurement. Radioactive and fluorescent microspheres allow precise determination of regional bone blood flow.

Fluorescent microsphere method is suitable for chronic bone blood flow measurement: a long-term study after meniscectomy in rabbits

The fluorescent microsphere (FM) method is considered a reliable technique to determine regional bone blood flow (RBBF) in acute experiments. In this study, we verified the accuracy and validity of this technique for measurement of RBBF in a long-term experiment and examined RBBF after meniscectomy. Twenty-four anesthetized female New Zealand white rabbits (3 groups, each n = 8) received consecutive left ventricular injections of FM in defined time intervals after meniscectomy: group 1 from preoperation to 3 wk postoperation; group 2 from 3 to 7 wk postoperation; and group 3 from 7 to 11 wk postoperation. To test the precision of the FM method, two FM species were injected simultaneously at the first and last measurement. After the experiment, humeri, femora, tibiae, and reference organs (kidney, lung, brain) were removed and dissected according to standardized protocols. Fluorescence was determined in each reference blood and tissue sample, and blood flow values were calculated. Blood flow in kidney, lung, and brain revealed no significant difference between right and left side and remained unchanged during the observation period, thus excluding errors due to shunting and dislodging of spheres in our experiments. Comparison of relative bone blood flow values obtained by simultaneously injected FM showed an excellent correlation at the first and last injection, indicating valid RBBF measurements in long-term experiments. We found a significant increase in RBBF 3 wk after meniscectomy in the right tibial condyles compared with the nonoperated left side. Similar changes were found in the femoral condyles. RBBF in other regions of tibia, femur, and humerus revealed no significant differences between right- and left-sided bone samples of the same region. Our results demonstrate that the FM method is valid for measuring RBBF in long-term experiments. In addition, we were able to demonstrate that meniscectomy leads to an increase in RBBF in the tibial condyles at a very early stage. This increase might be caused by stress-induced alterations of the subchondral bone.

Validity of fluorescent microspheres method for bone blood flow measurement during intentional arterial hypotension

In this study, we compared bone blood flow values obtained by simultaneously injected fluorescent (FM) and radiolabeled microspheres (RM) at stepwise reduced arterial blood pressure. Ten anesthetized female New Zealand White rabbits received simultaneous left ventricular injections of FM and RM at 90, 70, and 50 mmHg mean arterial blood pressure (MAP). After the experiments, both kidneys and long bones of all four limbs were removed and dissected in a standardized manner. Radioactivity (corrected for decay, background, and spillover) and fluorescence were determined, and blood flow values were calculated. Relative blood flow values estimated for each bone sample by RM and FM were significantly correlated (r = 0.98, slope = 0.99, and intercept = 0.04 for 90 mmHg; r = 0.98, slope = 0.94, and intercept = 0.09 for 70 mmHg; r = 0.98, slope = 0.96, and intercept = 0.07 for 50 mmHg). Blood flow values (ml x min-1 x 100 g-1) of right and left bone samples determined at the different arterial blood pressures were identical. During moderate hypotension (70 mmHg MAP), blood flow in all bone samples remained unchanged compared with 90 mmHg MAP, whereas a significant decrease of bone blood flow was observed at severe hypotension (50 mmHg MAP). Our results demonstrate that the FM technique is valid for measuring bone blood flow. Differences in bone blood flow during altered hemodynamic conditions can be detected reliably. In addition, changes in bone blood flow during hypotension indicate that vasomotor control mechanisms, as well as cardiac output, play a role in setting bone blood flow.

Approaches to study of ischemia in bone

Ischemic osteonecrosis is a complication of certain traumatic and a number of idiopathic conditions. The course of the disease may result in collapse of the convex member of a joint and osteoarthritis, often requiring arthroplasty. Increasing incidence in young adults poses a challenge for development of long-term joint prostheses. Current status of research into the disease is discussed and three new models using intravital microscopy described. The first, an arterial occlusion (AO) model, creates ischemia by occluding the common iliac artery exclusively, avoiding direct trauma on other tissues in the limb. The second, a total occlusion (TO) model utilizes classical tourniquet occlusion of the thigh vessels. The third, a venous occlusion (VO) model, is also a tourniquet procedure but it blocks occlusion of the femoral artery with a protective sheath. Preliminary results from AO and TO studies are reported which show that reperfusion injury is detectible after ischemia doses as short as 4 h. This complication was confirmed by observation of leukocyte adherence, secondary ischemia, and abnormal vessel leakage. Also, a new quantitation of osteonecrosis is introduced whereby fluorescently-tagged dead osteocytes and computer-based image processing provide values for an "osteonecrosis index." Viewing of all vascular events is made possible by intravital microscopy through a bone chamber window implanted in rabbit tibias. It is proposed that such chronic visual techniques allow quantitation of events leading to osteonecrosis as well as the revascularization, resorption and bone apposition of creeping substitution which characterizes postischemia recovery.

Cast immobilization and tibial diaphyseal blood flow: an initial study

The effect of cast immobilization on blood flow to the tibial diaphysis was studied by the microsphere method, both before and after casting of one hindlimb of adult New Zealand White rabbits. Preliminary studies were undertaken to investigate the possibility of the reduction of tibial flow by the microspheres used for the control measurements of blood flow. There were no significant differences in the flows to the tibial diaphysis or skeletal muscle between the immobilized and control limbs after either 1 or 2 weeks. This similarity between control and experimental limbs indicates that immobilization had no major effect on tibial blood flow over and above the systemic effects of the procedure.

Regional blood flow and strontium-85 incorporation rate in the rat hindlimb skeleton

Regional distribution of radioactive microspheres and 85Sr incorporation rate in the os ilium, femur, and tibia were determined in 44 rats. In 15 additional animals, regional blood flow was measured with the reference sample technique. Highest numbers of microspheres were found in the distal femoral and proximal tibial metaphyses. Highest blood flow rates were found in the os ilium, the caput femoris, and distal femoral and proximal tibial metaphyses. The diaphyses and epiphyses of the femur and tibia had a relatively lower perfusion. The regional 85Sr incorporation rate was correlated (r = 0.67) to the regional distribution of the microspheres. The dispersion of the data was large and this was mainly due to biological variation. The results describe the regional distribution of blood flow in the rat hindlimb and demonstrate a close association between blood flow and mineral turnover in normal rat skeletal tissue as shown by the correlation between microsphere and 85Sr incorporation rate distribution.