Method and Apparatus for the Measurement of Bone Quality in Vivo

A presently preferred embodiment of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of the ultrasound pathways through the cortex and through the cortex and medulla;

FIG. 1A is a schematic diagram showing the ultrasound and measuring apparatus;

FIG. 2 is a diagrammatic representation of the assumption of ovality for the cortex and medulla;

FIG. 3 is a diagrammatic representation of typical ultrasound traces used to determine the time measurements;

FIG. 4 is a diagrammatic representation of the ten measurement sites and sections for the ten selected left metacarpal bones;

FIG. 5 is a graphical comparison of preliminary empirical pathways and experimental values for development of pathway lengths;

FIG. 6 is a graphical comparison of the best empirical formula for the ultrasound pathway length and experimental values;

FIG. 7 is graphical comparison of experimental and measured cross-sectional areas of the bone under consideration, and

FIG. 8 is a graphical comparison of the experimental and measured cross-sectional areas for the experimental bone sections.

The estimate of ultrasound velocity is gained from knowing the distance between two transducers and measuring the time of flight of the transmitted beam. However, as the velocity of sound in the medulla is of the order 1500 ms.sup.-1 and that in the cortical bone of the order of 3000 m s.sup.-1 it is readily seen that the shortest time of flight must be for the sound to travel around the cortex (FIG. 1).

The mathematical relationship of this pathway should be able to be specified as a factor (f) times the total diameter (X) and also as a mathematical function of the ratio (R) of the diameter of the medulla (Y) to the diameter of the cortex (X):

Another pathway, the corticomedullary pathway, (FIG. 1) can be determined for the sound travelling through the cortical walls and the center of the medullas in a straight line. For this second pathway a mean velocity (V) can be determined from x/Tm (where Tm is the transmission time for the second pathway) and this is related to the bone velocity (C.sub.b), the medullary velocity (C.sub.m) and the ratio of medulla to cortex (R) by the formula:

From the apparent velocity (C.sub.a) which is the velocity determined from the shortest time of flight (T.sub.s) and the corlical diameter (X), an estimate of the transverse cortical bone velocity (C.sub.bE) can be made if the path length (f.X) is taken into account:

C.sub.a is the first estimate of the transverse bone velocity and as the bone velocity must be greater than the apparent velocity, by iteration (appendix B), it is possible to calculate both C.sub.bE and R. An empirical formula by means of which f may be estimated will be found in equations (10) and (11). Seeing that the diameter of the bone X is known (transducer spacing minus the average skin and flesh thickness for the particular bone and position) and the ratio R estimated, the cross-sectional area of the cortex can also be estimated using the formula:

Where K is an ovality factor for the external cortical bone cross section and L is an ovality factor for the medulla. These ovality factors are derived from the bone cross sections on the assumption that the sections approximate ellipses as shown in FIG. 2. M is a factor to include the area of the splint bones (MC2 and MC4) as these bones were included in the determination of the BMC. Using this cross sectional area, BMC can be corrected to give an estimate of the bone mineral density (BMD.sub.E) by:

Another estimate of the bone mineral density, BMD.sub.M, can be determined from direct measurements by:

The compact bone density, CBD.sub.E, is estimated using equation (1). For this series the constant for .rho..sub.m was 3120 kg m.sup.-3 and that for .rho..sub.c was 1310 kg m.sup.-3 as published for bovine bone by Lees et al (1979). The CBD.sub.M can also be determined from direct measurement by:

The modulus of elasticity, J.sub.M, is determined using equation (2), CBD.sub.M and C.sub.bM. The modulus of elasticity, J.sub.E, is determined with the same equation but using CBD.sub.E and C.sub.bE.

Referring to FIG. 1A, the ultrasound time of flight measurements were obtained using a pulse module 1 (Panametrics Model 5055), a preamplifier 2 (Panametrics Model 5660B), connected to two 2.25 MHz, 13 mm diameter ultrasound transducers 3 (FIG. 1A) mounted on an electronic digital caliper 4 (Max-cal Model 950-101) connected to a computer (6) via an interface 5 (Mac-cal Model 960-101). A digital storage oscilloscope with signal averaging capacity 7 (Tektronix Model 468) is connected between the pulse module 1 and the preamplifier 2 to store and average the time of flight measurements. An Apple II Plus microcomputer was used to apply the various iterative and calculative algorithms described herein.

The BMC values were determined using a Norland Digital Bone Densitometer (Prototype Model 2781, Large Animal Scanning System, LASS) with an americuim source (.sup.241 Am).

In summary, the method embodying the invention comprises the steps of

(a) measuring the flight times for the ultrasound beam a long the two paths shown in FIG. 1, T.sub.s, the flight time a long the cortical pathway using the oscilloscope 7 and Tm, the flight time a long the cortico-medullary pathway;

(b) calculating the diameter of the bone X from the distance between the transducers less a constant determined experimentally to allow for skin and flesh (in the case of the third metacarpal bone of a horse, 3 mm);

(c) calculating by means of the computer 6 the apparent velocity Ca from x/Ts

(d) calculating by means the computer 6 the "mean" velocity V from x/Tm

(e) calculating by means the computer 6 preferably by iteration, the values of C.sub.bE and R from equations (5), (10) and (4) respectively, Cm being treated as a constant determined experimentally to be approximately 1470 m s.sup.-1 ;

(f) determining the cross-sectional area of the bone using formula (6) M, K and L being experimentally predetermined constants as described above;

(g) determining the bone mineral content BMC using a bone densitometer or by any other suitable method;

(h) determining the estimated compact bone density CBD.sub.E using equation (1);

(i) determining the modulus of elasticity using equation (2), CBD.sub.E and C.sub.bE.

To test the theory outlined above, a series of 15 regular polymethyl methacrylate cylinders of diameter 50 mm and with central holes ranging from 15 to 35 mm in 2 mm steps were machined to simulate bone cross sections. Polymethyl methacrylate was chosen as its ultrasound velocity of about 2800 m s.sup.-1 is close to that of bone at about 3000 m s.sup.-1. The circular central hole was initially left empty to ensure only transmission through the polymethyl methacrylate and later filled with water which with a velocity of about 1500 m s.sup.-1 closely approximated medullary tissue. Time of flight measurements were made on this group of cylinders to establish the best empirical relationship between pathway and ratio of medulla to cortex. Measurements taken with air in the central cavity confirmed that the pathway for the minimum time of flight is through the polymethyl methacrylate, whereas when filled with water both pathways were verified.

Ten metacarpal bones of approximately the same size were selected from the left forelimbs of thoroughbred horses aged from 2 to 5 years. After removal of all soft tissues, the bones were stored at -15.degree. C. until examined. The lateral bone diameter, the times of flight through both the cortex a lone and the contico-medullary pathways (FIG. 3) and the BMC were determined at 10 sites a long the shaft of the bone as shown in FIG. 4. From these measurements C.sub.a, R, C.sub.bE and CSA.sub.E (using factors determined later) were estimated. The bones were then cut to provide 10 transverse cross sections (FIG. 4). The transverse sections 1, 3, 5, 7 and 9 were cut using a band saw to provide approximately 1 cm thick sections and radiographed to record the cross-sectional profile. From these radiographic cross-sectional profiles an estimate of the ovality ratio of the cortical exterior (K) and the ovality ratio of the medulla (L) was determined to allow corrections to be made to estimate the cross-sectional area from the ultrasound measurements. The area of the splint bones (MC2 and MC4) relative to the area of the main shaft (MC3) was also estimated from the radiographs. For these sections the thickness (longitudinal length) was measured by digital calipers, the volume measured by a water displacement method and the cross section area (CSA.sub.M) calculated. On the same sections CBD.sub.M (kg m.sup.-3), ash mass (kg) and ash weight (kg/100 kg dry defatted bone) were measured. Out of the sections 2, 4, 6, 8 and 10, dorsal segments, of as large a size as practical, were cut to allow direct transverse cortical bone velocity measurements (C.sub.bM) to be made. The bone marrow was removed as a plug from all bone sections and the ultrasound velocity of the bone marrow was determined in the dorsopalmar, lateral and longitudinal directions using the same ultrasound equipment.

The modulus of elasticity, J, was determined using equation (2) from two sets of measurements. The first estimate, J.sub.M, is from the directly measured C.sub.bM and the directly measured CBD.sub.M. The other estimate, J.sub.E was made from C.sub.bE and the CBD.sub.E from equation (1). Both determinations are made.

The pathway length determined on the polymethyl methacrylate blocks was compared with several mathematically derived theoretical pathways two of which are shown in FIG. 5. A suitable theoretical pathway based on physical principles has not yet been achieved. Various mathematical correction factors were then tested to find an empirical formula to allow estimation of the path length.

The best empirical formula so far determined is: ##EQU1## where:

This formula was used for all calculations on the selected bones. Equation (10) is shown with the experimental values in FIG. 6. A more accurate value for Rc determined by further experiments may be: Rc=0.79R (0.6+0.15R).

The measurements from the 100 bone marrow samples to determine the average medullary velocity (C.sub.m) are summarized in Table 1. The average value of 1444.+-.14 m s.sup.-1 was used form subsequent calculations.

A comparison of C.sub.bM and C.sub.bE is summarised in Table 2. From the radiographs of the individual bone segments an estimate of ovality of the medulla and the ovality of the external cortical boundary was made. The average ovality factor for the medulla (L) of each bone (n=10) ranged over the 10 bones from 0.65 to 0.90 with a mean of 0.76. The average ovality factor for the cortex (K) ranged from 0.75 to 0.90 with a mean of 0.81. From the same radiographs the areas of the MC2, 3 and 4 were estimated by counting squares and average correction factors were determined for the sections a long the bones. Using these correction factors the cross-sectional areas were estimated from the ultrasound measurements and FIG. 7 shows CSA.sub.E versus CSA.sub.M for positions a long the shaft. FIG. 8 shows the result of a comparison for the 50 bone sites for sections 1, 3, 5, 7 and 9. For the same sections (n=5), Tables 3 and 4 summarize for each bone the average BMC (5 readings at each site), BMC.sub.M, BMD.sub.E, CBD.sub.M, CBD.sub.E, J.sub.M and J.sub.E.

The above values for K, L and M are most valid for the middle region of the bone. The following table A lists corrections factors for K and L and new values for M according to the position of the testing site (the numbers correspond to the numbers in FIG. 4).

Estimates of the precision were obtained for C.sub.a, C.sub.bE, CSA.sub.E and BMC from multiple independent measurements taken at the same site of a bone. Using equations 1, 2, 7, 8 and 9 and estimates of the precision of the measurement of mass and length, further estimates using the method of propagation of errors were made for CSA.sub.M, BMD.sub.E, BMD.sub.M CBD.sub.E, J.sub.E and J.sub.M. These estimates are shown in Table 5.

The accuracy and precision of the methods used here for determining both transverse cortical bone velocity and bone mineral content are encouraging and correlate well with those of Greenfield et al (1981). Greenfield et al utilized the human radius and employed the same single beam photon absorptiometry, but used a pulse echo technique for cortical velocity measurement instead of the transmission method. Both of these ultrasound methods give similar results for cortical bone velocity. In man the cortical velocity of the radius as 3335.+-.300 m s.sup.-1 whereas for the equine metacarpus the velocity was measured at 3109.+-.56 m s.sup.-1. The advantage of the transmission method is that a lateral radiograph to measure cortical thickness is not required and an estimate of the cross-sectional area is obtained at the same time. Greenfield's estimate of uncertainty for cross-sectional area was 2-10% whereas by the transmission method the uncertainty for the equine metacarpus is about 5%. The variation of cross-sectional area with site and area of splint bones correlated well with the results published by Piotrowski et al (1983).

The estimation of cross-sectional area by the ultrasound method was considered good (correlation coefficient r=0.92) and proved to be better than just using an estimate of cross-sectional area based on diameter squared (r=0.82).

The values for bone mineral content in the horse differed from those in man because of the size of the bone being measured. An .sup.241 Am source was used instead of .sup.125 I. However, when the readings for bone mineral density (1060.+-.20 kg m.sup.-3) were compared with Greenfield et al (1120.+-.150 kg m.sup.-3) they were similar. The measurement of bone mineral density to this precision helps to overcome the volumetric problems identified by Ruff and Hayes (1984) found in using just bone mineral content.

The calculated estimates for transverse cortical bone velocity, bone mineral density, compact bone density and modulus of elasticity compared favorably with the measured values for the same parameters. Schryver (1978) measured the bending properties of the metacarpals of ponies and found the elastic modulus to be 18.4.+-.0.14 GN m.sup.-2 for the cranial cortex of the mid-shaft. He obtained values of 16.2 to 20.2 GN M.sup.-2 for different parts of the cortex of the radius and femur and was able to show that diet, exercise and sex of the pony affected the elastic modulus, ultimate strength and energy absorption of the bone.

In this study the transverse cortical bone ultrasound velocity has been determined rather than the apparent transverse ultrasound velocity because it is more uniform along the shaft, is less subject to geometrical effects, allow an estimation of a modulus of elasticity and should provide better correlation between individuals.

These methods, having been developed on post mortem specimens, are now being applied to live animals. Ultrasound measurements (5 sites) take about 10 minutes per limb and absorptiometry (5 readings) takes about 20 minutes per limb. Using these combined techniques it will be possible to establish better parameters for skeletal maturity and to measure the effects of exercise on bone. Whilst these methods are being implemented in equine medicine there appears no reason why the same principles cannot be applied in humans. It will be appreciated that the use of the method to estimate the quality of various human bone would require the experimental determination of different constants (for example the ovality factors K and L would be replaced by different factors and the M factor would be omitted), but the method would otherwise be similarly applied.

(a) Equation 4 ##EQU2##

(b) Equation for curve (2) in FIG. 5 and leading to equation (10)

Assume geometrical symmetry, a straight line path across the cortex and an arc around the medulla.

Let .theta. equal angle between diameter (center to center of transducers) and straight pathway through cortex. .theta. is also half the angle of the arc.

Thus sin .theta.=Y/X=R ##EQU3##

(c) Equation for curve (1) in FIG. 5

Assume symmetry and that the sound path is along an arc of a circle which goes from center to center of the transducers and just touches the edge of the medulla as shown in the sketch. If S is the radius of curvature of the arc and 2 .phi. is the subtended angle of the arc then:

and

The iterative procedure applied in a simple computer program involves equations (4), (5) and (10). C.sub.a, V and C.sub.m are known leaving f, C.sub.b and R to be determined. The only restriction to the iterative process is imposed by equation (4) where:

R=.infin. when C.sub.b =C.sub.m

R<0 when V>C.sub.b >C.sub.m

R=0 when C.sub.b =V

R>0 when C.sub.b >V

Provided the starting point of the iteration is with C.sub.b >V then the correct value is determined. C.sub.a is always greater than V and is used as the first estimate of C.sub.b. The first estimate of R is then determined, equation (4), followed by the first estimate of f, equation (10). Equation (5) then gives a revised estimate of C.sub.b. This process is continued until the change in C.sub.b per cycle is less than some chosen value e.g. 1 m s.sup.-1. It takes only five cycles on average to achieve a suitable result.

f- factor to relate cortical pathway length to diameter of cortex.

f.X- path length of ultrasound beam through cortex of MC3.

K- ovality factor for external perimeter of cortex of MC3.

L- ovality factor for medulla of MC3.

M- factor to compensate for area of splint bones MC2 and MC4.

R- ratio of medulla to cortex of MC3=Y/X.

R.sub.c - corrected ratio of medulla to cortex (R).

X- external diameter of cortex of MC3 (lateral).

Y- diameter of medulla of MC3 (lateral).

BMC- bone mineral content (kg m.sup.-1).

BMD.sub.M - measured bone mineral density (kg m.sup.-3).

BMD.sub.E - estimated bone mineral density (kg m.sup.-3).

C- velocity of ultrasound (m s.sup.-1).

C.sub.a - apparent transverse cortical bone ultrasound velocity (m s.sup.-1)-X/T.sub.s.

C.sub.b - transverse cortical bone ultrasound velocity (m s.sup.-1).

C.sub.bM - measured transverse cortical bone ultrasound velocity (m s.sup.-1).

C.sub.bE - estimated transverse cortical bone ultrasound velocity (m s.sup.-1).

C.sub.m - ultrasound velocity through medulla (m s.sup.-1).

CBD.sub.M - measured compact bone density or specific gravity (kg m.sup.-3).

CBD.sub.E - estimated compact bone density or specific gravity (kg m.sup.-3).

CSA.sub.M - measured cross-sectional area (m.sup.2) (volumetric).

CSA.sub.E - estimated cross-sectional area (m.sup.2) (ultrasound).

J- modulus of elasticity.

J.sub.M - estimated modulus of elasticity using CBD.sub.M and C.sub.bM (GN m.sup.-2).

J.sub.E - estimated modulus of elasticity using CBD.sub.E and C.sub.bE (GN m.sup.-2).

p.sub.m - microscopic bone mineral density (kg m.sup.-3).

p.sub.c - collagen density (kg m.sup.-3).

MC 2, MC 3 and MC 4- second, third and fourth metacarpal bones of the horse.

T.sub.m - time of flight for cortico-medullary path (s).

T.sub.s - shortest time of flight of ultrasound transmission (s).

V- mean velocity of ultrasound through cortex and medulla (m s.sup.-1). =X/T.sub.m.