1. Field of the Invention
The present invention relates to a method and an apparatus for estimating a Bone Mineral Density (hereinafter referred to as xe2x80x9cBMDxe2x80x9d), and in particular, to a method and an apparatus for estimating a Bone Mineral Density utilizing an ultrasound and a bioelectrical impedance.
2. Prior Art
With age, human bones tend to become more brittle, the ratio of fractures caused by falls etc. tends to increase, and it tends to take much more time to recover after such a fracture. For example, if an elderly person fractures a leg, the time period during which it is difficult to walk tends to become longer, and such a person may thus be requested to stay in bed. As a result, this person""s everyday life is significantly effected, and further asthenia tends to progressively advance since it is impossible for this person to move and for other reasons.
Thus, as one of the means for judging the brittleness of human bones, a method and an apparatus for measuring BMD, which is one of the indexes of the degree of bone health, have been developed and are used at hospitals or the like. The term xe2x80x9cBMDxe2x80x9d herein refers to Bone Mineral Calcium (hereinafter referred to as xe2x80x9cBMCxe2x80x9d) contained per unit area of the bone. Although a bone includes a bone radical component (fiber component) and a component of calcification, bone mass is referred to as the Bone Mineral Calcium, since, at the present time, when measuring the bone mass, calcification mass thereof is measured.
In consequence of the measurements made at hospitals etc. utilizing the above measuring method, we have learned that, among elderly people, there are a great many whose bones are in condition in which they easily fracture, that is, osteoporosis, and moreover, half of females more than 65 years old and half of males more than 80 years old are diagnosed as having osteoporosis, and thus this matter is becoming a very significant problem. Also, in recent years, not only among elderly people but also among youths, people who are diagnosed as having osteoporosis have been increasing due to the effects of a lack of exercise, an unbalanced diet, excessive dieting or the like. Also, invalids tend to get osteoporosis for reasons similar to the above given ones, and also expectant mothers are often said to easily get osteoporosis since a large amount of calcium in their bodies is consumed because of giving birth and breast feeding. Thus, it is possible to say that the risk of getting osteoporosis has become very high.
Consequently, various methods and apparatuses for determining the degree of bone health have been developed so as to prevent osteoporosis, and they can be categorized as follows:
(1) Simple X-ray Image
The determination of the degree of bone health will be made, based on an X-ray picture of a bone, by judging the degree of reduction of a calcium fiber, that is, calcification mass in a vertebra or a transcervical.
(2) Second Metacarpus Atrophy Degree Measurement Method (MD Method, DIP Method)
This is a method in which, by utilizing a computer, an X-ray picture of a bone is analyzed and then an atrophy degree of a bone is indicated by a numeral. In the MD method (Micro-Densitometry Method), a picture of a second metacarpus is taken using an X-ray and analyzed, while, in the DIP method (Digital Image Processing Method), an X-ray picture is transmitted to a high resolution processing equipment via a television camera for instrumentation, so that the bone mass can be estimated.
(3) Dual Energy X-ray Absorptiometry (DEXA Method)
This is a method in which, by utilizing two X-rays of different wavelengths, the bones of the umbar spine front, back and side, transcervical or the like are measured and analyzed. In this method, both a cortical bone and a cancellous bone are measured.
(4) QTC Method
This is a method in which an X-ray absorbed dose is estimated without a picture and the bone mass is estimated by a computer process. In this method, only a petrous cancellous bone of a spine can be measured in ring sections thereof without the effects of a bone deformation and a pool of calcium.
(5) Ultrasonography
This is a method in which an ultrasound signal is transmitted to a heel or the like and a stiffness of a bone is estimated based on a traveling speed of the ultrasound etc.
Now, a principle of the ultrasonography will be described below. FIG. 6 is a diagram illustrating a principle of the ultrasonography. This figure shows a sectional view of a heel portion when the ultrasound has passed through a calcaneus. xe2x80x9c30xe2x80x9d indicates a heel, xe2x80x9c31xe2x80x9d indicates the calcaneus, xe2x80x9c33xe2x80x9d indicates a cortical bone which is a peripheral part of a bone and xe2x80x9c32xe2x80x9d indicates respective cancellous bones which are an inner part of a bone. xe2x80x9c34xe2x80x9d indicates a cutis and xe2x80x9c35xe2x80x9d indicates an ankle. xe2x80x9c27axe2x80x9d indicates an ultrasound transmitter for transmitting the ultrasound and xe2x80x9c27bxe2x80x9d indicates an ultrasound receiver for receiving the ultrasound which has passed through the calcaneus.
Even if a bone is thin, if the cortical bone 33 thereof is thick and the cancellous bones 32 thereof are densified, such a bone is resistant to fracture. On the contrary, even if a bone is thick, if the cortical bone 33 thereof is thin and the cancellous bones 32 thereof each have a space therein (loose condition), such a bone is brittle and thus easy to fracture. Thus, it is necessary to comprehensively judge both the thickness of the cortical bone 33 and the condition of the cancellous bones.
A cell such as a cutis has almost the same acoustic character as that of water, and a sound can be transmitted therethrough at a speed of 1500 m/s, an attenuation thereof caused by such transmission being small. Compared to an ordinary cell, a bone is very hard, a speed of sound transmitted therethrough is fast and an attenuation thereof caused by such transmission becomes great. When osteoporosis occurs, the cortical bone 33 is decreased in width, a medullary cavity is increased, and in each of the cancellous bones 32, a trabeculae thereof is decreased and rarified. Since most parts of the rarified medullary cavity are filled with bone marrow liquid, the acoustic character of such a rarified bone becomes more similar to that of water in comparison with that of a healthy bone. That is, as compared to the healthy bone, the speed of sound transmitted through the rarified bone is slow and the attenuation thereof becomes small. Thus, an ultrasound is inputted into the bone from one side thereof and the time it takes for the ultrasound to reach the other side thereof is calculated, so that the calculated speed of sound should reflect the bone mineral density thereof.
As the calcaneus is relatively large and the side shape thereof is almost planar, it is easy to measure the length thereof and such a calcaneus can resist the effect of the sound wave diffracted from the cortical bone. Also, as it is clear that ninety percent of the content in the calcaneus are the cancellous bones, the calcaneus can be adequately measured by the ultrasound measuring method and is often measured by this method. Further, since the flesh portion covering the calcaneus is thin, if this measurement is made from the top of the cutis, any possible error in this measurement is not great.
Now, a method of obtaining a sound of speed SOS in the heel will be described. The time xe2x80x9cTxe2x80x9d that elapsed before the ultrasound, which is transmitted from the ultrasound transmitter 27a, has passed through the calcaneus 31 and reached the ultrasound receiver 27b, is measured. Assuming the distance between the transmitter and the receiver is xe2x80x9cLtxe2x80x9d, the equation:
SOS=Lt÷Txe2x80x83xe2x80x83(equation 1) 
is satisfied.
Now, the method of obtaining the bone mineral density from the SOS will be described. For a number of subjects, their bone mineral densities BMDs are measured by utilizing the highly accurate DEXA method, and their SOSs are measured by means of the ultrasonography to obtain a regression equation between the BMDs and the SOSs. By utilizing such a correlation equation, the bone densities can be obtained from the SOSs.
In addition to the above, in the ultrasonographical methods, there is another type of method which indicates, by means of a frequency analysis, the conditions of the bones accurately by utilizing the fact that the attenuation rates obtained when the ultrasound has passed through the bones are different from one another.
However, since all of the methods described in the above items (1) to (4) utilize an X-ray, the assistance of a radiological technician is indispensable, which creates an obstacle to the widespread use thereof. Also, these methods are not preferable in that the people to be measured are exposed to an X-ray radiation and also that these methods become expensive. On the other hand, the method described in the above item (5) is preferable because the cost thereof is relatively small in comparison with that of the methods described in the above items (1) to (4), and does not utilize an X-ray.
However, a bone is considerably non-uniform in a structure of the trabeculae thereof. Further, although the bone mass of the portion of the bone near the cortical bone 33 is large, the bone mass decreases gradually toward the center of the bone. Thus, there is a problem that the measured values can vary depending on the portion of the bone through which the ultrasound has passed. Further, since the cancellous bone is a sparse bone including some portions having no bone and other portions having bone, the situations at the same portions of the bone and of the portions of the bone having the same bone mineral density can vary from person to person, and whether the trabeculae is present or not can change depending on the portions of the bone through which the ultrasounds have passed, and thus the resultant acoustic characters of bones such as SOSs can differ from bone to bone.
Also, although the portions of a bone where the measurements can be made are limited to specific portions such as the calcaneus etc., the portions where fractures actually easily occur are portions such as the umbar spine, thigh bone or the like. As the bone mineral densities vary from person to person at each of the portions, it is not possible to accurately determine the bone mineral density based on only the specific locations. Thus, the ultrasonography has a problem in that many errors occur when it is used.
The object of the present invention is, in view of the above described present circumstances, to provide a method of estimating bone mineral density which enables anyone to estimate their bone mineral density safely, at a low cost, and with proper accuracy, without worrying about exposure to X-rays.
According to an aspect of the present invention, there is provided a method of estimating a bone mineral density wherein the bone mineral density of a person to be measured is estimated based on an arithmetic expression using a sound of speed in a bone, a weight, and fat free mass of the person to be measured as parameters.
According to one embodiment of the present invention, said arithmetic expression is expressed by BMD=C1xc3x97SOS+C2xc3x97Wt+C3xc3x97FFM+C4, where the sound of speed in the bone is SOS, the weight is Wt, the fat free mass is FFM, the bone mineral density is BMD, and the constants are C1, C2, C3, and C4.
According to another embodiment of the present invention, in said arithmetic expression, a correction is made thereto based on personal parameters of height, sex, age, presence or absence of menstruation, age at menopause, and years which elapsed since the beginning of menopause in the person to be measured.
According to another aspect of the present invention, there is provided a method for estimating a bone mineral density wherein the bone mineral density of a person to be measured is estimated based on an arithmetic expression using at least one of a weight, fat free mass, or cell mass and a sound of speed in a bone of the person to be measured as parameters.
According to further aspect of the present invention, there is provided an apparatus for estimating a bone mineral density comprising: first input device which enters a sound of speed in a bone of a person to be measured; second input device which enters a weight of the person to be measured; third input device which enters fat free mass of the person to be measured; arithmetic device which computes the bone mineral density based on data from said first input device, said second input device and said third input device; and display which displays a bone mineral density value computed by said arithmetic device.
According to one embodiment of the present invention, said display displays a percentage of the bone mineral density computed by said arithmetic device in comparison with a Young Adult Mean of bone mineral densities and a result determined on the basis of the percentage.
According to another embodiment of the present invention, the determined result being that, if the percentage is within 70% to 80%, a decrease in the bone mass is suspected and that, if the percentage is less than 70%, osteoporosis is suspected.
According to one embodiment of the present invention, said display displays a graphical representation in which the bone mineral density is shown in X-axis and at least one of height, weight, age, percent fat and fat free mass is shown in Y-axis, and it further displays a graphical representation of the Young Adult Mean of bone mineral densities.
According to another embodiment of the present invention, said apparatus further comprising fourth input device which enters a percent fat of the person to be measured and health balance computing device which computes a health balance index on the basis of the bone mineral density value computed by said arithmetic device and the percent fat entered from said fourth input device.
According to one embodiment of the present invention, a value of the health balance index is at least any one of excellent, good, normal and bad values.
According to another embodiment of the present invention, said first input device is an ultrasound transducer.
According to one embodiment of the present invention, said first input device is key device for enabling said sound of speed to be manually entered.
According to another embodiment of the present invention, said second input device is a weight sensor.
According to one embodiment of the present invention, said second input device is key device for enabling the weight of the person to be measured to be manually entered.
According to another embodiment of the present invention, said third input device is a part for measuring the fat free mass.
According to one embodiment of the present invention, said second input device is key device for enabling the fat free mass to be manually entered.
According to another embodiment of the present invention, said second input device and said third input device are each a scale with a body fat measuring apparatus.
According to one embodiment of the present invention, said arithmetic device is computed based on an arithmetic expression expressed by BMD=C1xc3x97SOS+C2xc3x97Wt+C3xc3x97FFM+C4, where the sound of speed in the bone is SOS, the weight is Wt, the fat free mass is FFM, the bone mineral density is BMD, and the constants are C1, C2, C3, and C4.
According to another embodiment of the present invention, in said arithmetic expression, a correction is made thereto based on personal parameters of height, sex, age, presence or absence of menstruation, age at menopause, and years which elapsed since the beginning of menopause in the person to be measured.