The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 80, Issue 11

Current Concepts Review | November 01, 1998

Current Concepts Review - Bone Densitometry in Orthopaedic Practice*

ERIC C. MIRSKY, M.D.†; THOMAS A. EINHORN, M.D.‡, BOSTON, MASSACHUSETTS

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

J Bone Joint Surg Am, 1998 Nov 01;80(11):1687-98

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During the last twenty-five years, there has been increasing interest within the orthopaedic community in the noninvasive measurement of the bone-mineral content of various regions of the skeleton. This interest has been stimulated, in part, by the recognition and understanding that conventional radiographs are neither sensitive nor accurate for the diagnosis of early bone loss. It has been reported, for example, that a reduction in bone-calcium content must exceed 30 percent to be observed with certainty on conventional radiographs¹. In addition, factors including radiographic technique and positioning of the patient lead to variability in radiodensity and affect the accuracy of conventional radiographs. Bone densitometry originally was developed to aid in the diagnosis and treatment of the so-called bone-loss syndromes, especially osteoporosis⁷¹. Current methods include radiographic absorptiometry, single-energy x-ray absorptiometry, dual-energy x-ray absorptiometry, quantitative computed tomography, and quantitative ultrasound. All of these modalities are relatively safe and allow good accuracy and precision of measurement.

Figures in this Article

Introduction

Introduction | Basic Principles and Technical Considerations | Clinical Indications for the Use of Bone Densitometry | Overview | References

Bone densitometry has far-reaching implications for orthopaedic practice, in terms of both diagnosis and treatment. Diagnostically, one of the major applications is in the evaluation and management of patients who have osteoporosis, as this technology allows an assessment of the risk of fracture as well as the quantity of bone before pharmacological treatment or operative intervention. Bone densitometry also allows an evaluation of periprosthetic bone-remodeling after total hip arthroplasty. This information has been shown to be useful in research protocols for evaluation of the response of the proximal aspect of the femur to the altered strain environment imposed by the implant^21,23,48. Clinically, periprosthetic measurements may allow the detection of bone-remodeling that is not otherwise apparent because of the limited sensitivity of radiographs.

In this article, the basic principles of bone densitometry are reviewed, the different modalities available for the measurement of bone-mineral content are described, and the existing data on how information obtained from these measurements can be used to manage patients who have bone disease or who have had a total hip arthroplasty are summarized.

†120 Summit Avenue, Summit, New Jersey 07901.

‡Doctors Office Building, Suite 808, Boston Medical Center, 720 Harrison Avenue, Boston, Massachusetts 02118-2393. Please address requests for reprints to Dr. Einhorn.

†120 Summit Avenue, Summit, New Jersey 07901.

‡Doctors Office Building, Suite 808, Boston Medical Center, 720 Harrison Avenue, Boston, Massachusetts 02118-2393. Please address requests for reprints to Dr. Einhorn.

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Fig. 1 Standardized radiograph of the hand, used in radiographic absorptiometry. The hand is positioned adjacent to an aluminum reference wedge, and direct-exposure settings are used to make a single radiograph. The mean density of the middle phalanges of the second, third, and fourth fingers is calculated, and the results are reported in aluminum-equivalent values.

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Figs. 2-A and 2-B: Quantitative computed tomography. Fig. 2-A: Lateral computed tomography scan, localizing the mid-plane of three lumbar vertebral bodies.

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Fig. 2-B Axial computed tomography scan through the mid-plane of a vertebral body. The rectangular region of interest within the vertebral body is used to determine cancellous bone-mineral content. The circular images below the vertebral body represent the so-called phantoms containing solutions of varying densities of K2HPO4. Computed tomographic determinations of the bone density of the vertebrae are compared with known density readings of the solutions in the phantoms. The measurements for the vertebrae are then averaged, and a commercially available software package is used to convert Hounsfield units to bone-mineral equivalents.

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Figs. 3-A and 3-B: Dual-energy x-ray absorptiometry bone-mineral report. Fig. 3-A: Graph showing the bone-mineral densities (BMD) for the first through the fourth lumbar vertebra. Bone-mineral density is plotted according to the age of the patient. The dark region represents values that are within one standard deviation of the normal bone-mineral density at a given age, and the mirror-image region below it represents values that are within one standard deviation below the normal values. The line distinguishing these two regions represents the normal bone-density value at any given age. The broken line represents the fracture threshold, a value that is calculated and expressed on bone-densitometry reports but is no longer considered useful for making decisions regarding treatment⁵⁸. The hash mark represents the bone-mineral density measured for the patient. In this example, the patient is approximately sixty-seven years old and has a bone-mineral density of the lumbar spine that is below, but less than one standard deviation below, the normal value for the patient's age. The density is also below the fracture threshold as well as more than two standard deviations below the values for normal young individuals.

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Fig. 3-B Table showing bone-mineral-density (BMD) values for each vertebra as well as the sum for the four lumbar vertebrae. T scores and Z scores are also reported. The percentages show the ratio between the value at each vertebra or vertebrae and the value for normal young individuals of the same race and gender (T score) or the value for individuals of the same age, race, and gender (Z score).

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Fig. 4 Anteroposterior dual-energy x-ray absorptiometry (DEXA) scan of the proximal aspect of the femur of a patient who had a total hip arthroplasty. Regional percentage variations in bone-mineral content (BMC) and density (BMD) over the length of the proximal aspect of the femur are shown in the adjacent table.

Basic Principles and Technical Considerations

Introduction | Basic Principles and Technical Considerations | Clinical Indications for the Use of Bone Densitometry | Overview | References

The unit of measurement for bone densitometry is bone-mineral content, expressed in grams. Although the instrumentation varies with different modalities, all record the attenuation of a beam of energy as it passes through bone and soft tissue. The energy originates from gamma rays from either isotope sources or x-ray tubes. Quantitative computed tomography is the only modality that allows the direct measurement of volumetric density, expressed as grams per cubic centimeter. When other techniques are used, the values for bone-mineral content may be converted into areal bone-mineral measurements (that is, bone-mineral density, expressed as grams per square centimeter) by dividing the bone-mineral content by the area that is scanned¹¹. As a result, comparisons of results are necessarily limited to bones of equal shape, which assumes a constant relationship between the thickness of the bone and the area that is scanned. Moreover, the measurements are strictly skeletal-site-specific; thus, individuals can be compared only when identical locations in the skeleton are studied.

Radiographic Absorptiometry

Radiographic absorptiometry is a technique for measuring radiographic density with use of standardized radiographs of peripheral sites, most commonly the hand or the heel. This method requires that the personnel in a standard radiology facility follow a simple protocol for making two radiographs—for example, of the fingers of the hand—at different radiographic energies and with use of an aluminum reference wedge. A single radiograph is made with use of direct-exposure settings (Fig. 1). The radiograph then is mailed to a central reading facility, where the image is captured electronically with use of a high-resolution video camera and is analyzed to determine the mean density of the middle phalanges of the second, third, and fourth fingers. The results are given in aluminum-equivalent values.

The advantages of radiographic absorptiometry include a relatively low cost and a lack of a need for specialized equipment¹⁵. Radiographic absorptiometry has been used mostly by primary-care physicians as a screening technique for the diagnosis of osteoporosis. It is less expensive and more widely available than other bone-densitometry techniques such as dual-energy x-ray absorptiometry and quantitative computed tomography. In addition, it has been shown to be both precise and accurate for obtaining bone-mineral-content measurements of the phalanges of the hand⁸⁰. Yang et al., in a study of cadavera, reported that the short-term precision error was small, with a coefficient of variation of 1 percent for bone-mineral content, and the correlation between radiographic absorptiometry determinations of bone-mineral content in the hand and dual-energy x-ray absorptiometry measurements of bone-mineral content in the forearm was good (r = 0.887)⁸⁰. A significant correlation between bone-mineral content as determined by radiographic absorptiometry and the ash weight of the bones also was reported (r = 0.983)⁸⁰. The major disadvantage of radiographic absorptiometry is that, because the measurements are sensitive to changes in the overlying soft tissues, the technique is limited to the appendicular skeleton.

Single-Energy X-Ray Absorptiometry

Single-energy x-ray absorptiometry is a technique for measuring the bone-mineral content of the appendicular skeleton (usually the distal aspect of the radius or the calcaneus). A collimated photon beam is directed from an x-ray source through the measurement site. The photon attenuation of the beam by bone then is measured and converted to bone-mineral content with use of a known standard.

Single-energy x-ray absorptiometry is commonly used because it is relatively simple to perform and the total dose of radiation to the body is negligible. Single-energy x-ray absorptiometry has largely replaced single-photon absorptiometry, an earlier version of this technique that used a photon source and emitted much more radioactivity. The major disadvantage of single-energy x-ray absorptiometry is that it is restricted to the appendicular skeleton. The measurements correspond well with the status of the peripheral long bones but poorly with that of the axial skeleton⁶⁸.

Dual-Energy X-Ray Absorptiometry

Dual-energy x-ray absorptiometry, which was introduced in 1987, is currently the most widely used modality for the clinical measurement of bone-mineral content¹². Together with single-energy x-ray absorptiometry and single-photon absorptiometry, this technology has replaced dual-energy photon absorptiometry. Specifically, the x-ray tube used in dual-energy x-ray absorptiometry has replaced the radionuclide source employed in dual-energy photon absorptiometry. Compared with dual-energy photon absorptiometry, dual-energy x-ray absorptiometry requires less time for the examination, is more reproducible, and involves less exposure to radiation.

With this technique, the x-ray tube emits an x-ray beam, the attenuation of which is detected by an energy-discriminating photon-counter. The x-rays are generated either by an energy-switching system (Hologic, Waltham, Massachusetts) or by rare-earth-filtered x-ray sources (Lunar, Madison, Wisconsin, or Norland, Fort Atkinson, Wisconsin). An energy-switching system is produced by rapidly switching the x-ray potential between two energies synchronously with line frequency, resulting in rapid pulses of different frequencies and different energy levels. Filtered x-ray systems use different effective energies that are emitted simultaneously. The output from constant potential x-ray generators is passed through a rare-earth filter with specific absorption characteristics, resulting in energy output at different levels of voltage. Perhaps the major advantage of an x-ray source compared with a radioisotope is the greater intensity, which greatly improves precision and accuracy. The photon flux produced by an x-ray source with a mean tube current of one milliampere is 500 to 1000 times greater than that produced by a one-curie gadolinium-153 source used in standard dual-photon absorptiometry systems⁶⁶.

Dual-energy x-ray absorptiometry provides bone-mineral measurements both axially and peripherally^47,77 as well as total-body scans. Scans of the spine and the femur can be performed in approximately one minute and two minutes, respectively, and total-body scans require approximately four minutes. The dose of radiation is so low (0.5 to 5.0 microsieverts) as to be essentially unimportant⁹; hence, there is no need to shield either the patient or the personnel who operate the equipment². High-resolution images are produced^54,66; consequently, the precision and the accuracy are excellent (0.5 to 2 percent and 3 to 5 percent, respectively^54,73). Thus, dual-energy x-ray absorptiometry can be used to detect small changes in bone-mineral content at multiple anatomical sites, with little exposure to radiation, short examination times, and excellent precision, accuracy, and resolution^5,67,76. A major disadvantage of the technique is that it does not enable the examiner to differentiate between cortical and trabecular bone.

Recently, new software has made it possible to evaluate bone-mineral content in the forearm and the calcaneus with use of standard dual-energy x-ray absorptiometry equipment^30,79. Yamada et al. reported that assessment of the bone-mineral density of the calcaneus with dual-energy x-ray absorptiometry revealed substantially lower measurements in women who had osteoporosis compared with those in a control population of women who did not have osteoporosis⁷⁹. Those authors concluded that assessment of the bone-mineral density of the calcaneus with dual-energy x-ray absorptiometry can be useful for predicting the risk of fracture of the femoral neck, intertrochanteric fracture, or fracture of the spine, particularly when other methods are not available. Compared with quantitative computed tomography, dual-energy x-ray absorptiometry has superior precision, is less expensive, and is associated with lower absorbed doses of radiation.

Quantitative Computed Tomography

Quantitative computed tomography is another modality that can be used to measure bone-mineral content. This technique involves the use of a mineral calibration phantom in conjunction with a computed tomography scanner. The vertebral body is the usual site of measurement. The phantom (a reference source used to calibrate measurements of bone density) usually consists of hydroxyapatite in plastic that is scanned simultaneously with the vertebrae. A lateral computed tomography scan localizes the mid-plane of two, three, or four lumbar vertebral bodies. Quantitative readings are then obtained from a region of trabecular bone in the anterior portion of the vertebra (Figs. 2-A and 2-B). The computed tomographic determinations of vertebral bone density are compared with known density readings of solutions in the phantoms. The measurements of the vertebrae are then averaged, and a commercially available software package is used to convert Hounsfield units (provided by standard computed tomography scanners) to bone-mineral equivalents. A Hounsfield unit is a measure of x-ray attenuation for computed tomography scans in which each pixel is assigned a value on a scale, with air being equivalent to -1000; water, to 0; and compact bone, to +1000.

Quantitative computed tomography has several theoretical and practical advantages compared with other techniques for the evaluation of bone-mineral content. First, it is the only method available that allows separate assessments of trabecular and cortical bone areas; in fact, it may be used to measure cancellous bone, cortical bone, or an integrated sum of both^7,28,36,65. Furthermore, it is the only currently available modality that allows the direct measurement of a volume of bone, which can be expressed directly as density (grams per cubic centimeter). With other instruments, the density of the region of interest is calculated by dividing the bone-mineral content (grams) by the area that is scanned (square centimeters) and then is expressed as grams per square centimeter. Thus, quantitative computed tomography provides the examiner with an accurate measure of the three-dimensional geometry of bone and, specifically, its trabecular component. Finally, quantitative computed tomography can be performed with use of standard computed tomography systems, which are available in most hospitals and radiology offices⁷. Most manufacturers provide a quantitative computed tomography option with their operating systems.

The principal disadvantage of quantitative computed tomography is that it exposes the patient to a higher dose of radiation than do other bone-densitometry techniques. The dose of radiation with modern quantitative computed tomography has been reported to be approximately twenty-nine microsieverts³⁷, whereas the dose with a typical dual-energy x-ray absorptiometry scan of the hip ranges from 0.5 to 5.0 microsieverts and that with radiography of the chest is fifty microsieverts^9,37.

Quantitative computed tomography is available in both single-energy and dual-energy modalities. The single-energy technique offers better reproducibility and is most commonly recommended and widely indicated. However, standard single-energy computed tomographic analysis of the lumbar spine fails to account for increases in bone-marrow fat concentration that occur with increasing age^45,53. As a result, measurements in elderly, osteoporotic individuals may be falsely decreased by 20 to 25 percent. In part to address these concerns, dual-energy quantitative computed tomography was developed. The accuracy error is reportedly decreased with dual-energy computed tomography, but the dose of radiation is higher than that with the single-energy modality⁷.

In general, densitometry techniques can be performed in either the axial or the appendicular skeleton. Peripheral measurements, performed in the appendicular skeleton, help to predict the risk of fracture; however, they are less sensitive for the monitoring of therapy than are measurements in the axial skeleton because changes due to age, therapeutic intervention, and estrogen deficiency occur less rapidly in appendicular bone than they do in the axial skeleton. Peripheral quantitative computed tomography systems originally used a nuclear energy source; however, newer systems use an x-ray source. The technique requires a special computed tomography unit with a small circular gantry^31,64. The principal advantage of this technique is the ability to investigate separately the mineral contents of cortical and trabecular bone with use of a cross-sectional x-ray image that localizes the site to be studied. Again, the measurements are expressed as apparent density in grams per cubic centimeter. The most commonly studied appendicular site is the distal aspect of the radius⁴⁹.

Perhaps the major advantage of quantitative computed tomography scanning of the appendicular skeleton is that it delivers a lower dose of radiation than does conventional computed tomography. To this end, a low-radiation-dose gamma-ray computed tomography scanner was developed for the measurement of the trabecular and cortical bone-mineral content of the distal aspect of the radius^33,72. The dose of radiation associated with this procedure is typically 0.4 microsievert to the skin³⁴. An additional, theoretical advantage is that bone strength at the tissue or organ level can be determined, and this may ultimately prove to be of practical value.

Quantitative Ultrasound

The use of ultrasound for the measurement of bone density recently has received widespread attention because it involves no radiation, is relatively simple to implement and process, is portable, and is inexpensive. Some investigators have suggested that quantitative ultrasound, in contrast to other bone-densitometry methods that measure only bone-mineral content, can measure additional properties of bone such as mechanical integrity^39,75. The most accessible sites for ultrasound measurement are the calcaneus and the patella, and, to a lesser extent, the radius, tibia, and phalanges.

Ultrasound assessment of bone is based on the velocity and attenuation of an ultrasound wave, as determined by a pair of coaxially aligned transducers. An ultrasound signal, generated by one transducer, is sent through the bone. A second (receiver) transducer detects the ultrasound wave as it emerges from the bone. This technology assumes that bones with different biomechanical properties have different ultrasound-determined values for attenuation and velocity^39,75. Specifically, propagation of the ultrasound wave through bone is affected by bone mass, bone architecture, and the directionality of loading.

Quantitative ultrasound measurements as a means for assessing the strength and stiffness of bone are based on the processing of the received ultrasound signals. The speed of sound and the ultrasound velocity both provide measurements on the basis of how rapidly the ultrasound wave propagates through the bone and the soft tissue. Newer ultrasound-imaging devices create a parametric image of broadband ultrasound attenuation at the calcaneus^46,63. This is a measure of the increase in attenuation of the ultrasound wave as a function of increasing frequency. Roux et al. reported that broadband ultrasound attenuation at the calcaneus was highly associated with local bone-mineral content and was also associated with bone-mineral content in the lumbar spine and the femur⁶³. The precision of this technique was 1.4 to 3.3 percent. Roux et al. noted that parametric imaging enhanced the reproducibility of ultrasound measurements of the calcaneus. However, the value of the technique with regard to the prediction of future fracture requires additional investigation.

The values obtained with use of quantitative ultrasound have been shown to correlate with those obtained with use of standard bone-densitometry techniques such as dual-energy x-ray absorptiometry. At the calcaneus, quantitative ultrasound and dual-energy x-ray absorptiometry measurements have been shown²⁹ to have a correlation of approximately 0.80 to 0.85. This high correlation led the United States Food and Drug Administration to recommend that quantitative ultrasound be used clinically.

Clinical Indications for the Use of Bone Densitometry

Introduction | Basic Principles and Technical Considerations | Clinical Indications for the Use of Bone Densitometry | Overview | References

Many factors that can lead to a decrease in bone mass have been identified. As a result, numerous potential indications for bone densitometry have been proposed. However, there are insufficient data to justify routine screening with use of this technique. Recently, the Health Care Financing Administration defined five diagnostic categories that it considers to be indications for the use of bone densitometry³². These categories include estrogen deficiency in women at clinical risk for osteoporosis, evidence of vertebral abnormalities, long-term glucocorticoid therapy, a diagnosis of primary hyperparathyroidism, and the need for monitoring in order to assess the response to or the efficacy of an approved drug therapy for the treatment of osteoporosis. Congress passed legislation requiring Medicare to reimburse for the cost of both performing and interpreting the examination for individuals in these diagnostic categories.

Use of Bone-Mineral Data for the Management of Patients Who Have Osteoporosis

Osteoporosis is a pathological condition of bone that is characterized by decreased bone mass and increased risk of fracture^13,38. It is well accepted that bone-mineral content and bone-mineral density are associated with bone strength^10,24,56,57. In addition, it has been shown that most fractures in elderly individuals are related, at least in part, to low bone mass⁶⁹. Thus, measurements provided by bone densitometry are important for assessing bone strength and the corresponding risk of fracture^8,25,61,62.

Fracture of the proximal aspect of the femur is perhaps the most serious consequence of osteoporosis. Approximately 250,000 such fractures occur in the United States each year, resulting in annual health-care costs of more than 8.7 billion dollars⁶⁰. The risk of fracture of the proximal aspect of the femur is associated with advancing age and is more common in women. One of every six white women in the United States will sustain such a fracture, and as many as 20 percent will die as a result¹⁶. Because of the devastating medical and economic impact of these fractures, the hip is a major site of interest for the information provided by bone densitometry. The ability to predict an individual's risk of sustaining a fracture of the proximal aspect of the femur, and the subsequent initiation of prophylactic measures to avoid this occurrence, is one of the most important applications of this technology.

Bone-densitometry measurements can be used to help to identify individuals who are at risk for fracture and to stratify that risk⁶². The probabilities of fracture of the proximal aspect of the femur, the vertebrae, the radius, and the calcaneus have all been shown to be predictable on the basis of bone densitometry^{3,17,18,27,35,43,55,59}. Some studies have indicated that information regarding bone-mineral content at any anatomical site is equally valuable for predicting the risk of fracture in general^3,55; however, other studies have suggested that measurements obtained at a particular site of interest may provide the most important information for the prediction of fracture at that site¹⁸.

Bone densitometry also has been used to analyze subtle morphological differences in the anatomy of the proximal aspect of the femur between individuals. Such analysis includes measurement of the length of the hip axis (the length along the axis of the femoral neck from a point distal to the lateral aspect of the greater trochanter, along the femoral neck, and to the inner pelvic brim), measurement of the femoral neck-shaft angle, and measurement of the width of the femoral neck at its mid-portion. Faulkner et al. performed dual-energy x-ray absorptiometry scans in white women in an attempt to predict the risk of subsequent fracture of the femoral neck or intertrochanteric fracture²⁶. The precision error of measurement of the hip axis was less than 1 percent. Those investigators reported that the length of the hip axis predicted fracture independently of bone-mineral density, age, weight, or height. A hip-axis length that was one standard deviation greater than the mean was associated with a twofold increase in the risk of subsequent fracture of the femoral neck or intertrochanteric fracture. Moreover, each decrease in the standard deviation for the bone-mineral density increased the risk of fracture by a factor of 2.7. No association was found between the risk of fracture and the diameter of the femoral neck or the neck-shaft angle²⁶. Although we consider these data to be important, we believe that the length of the hip axis is only one of many independent risk factors for fracture of the proximal aspect of the femur.

Finally, bone densitometry may be useful in the preoperative evaluation of osteoporotic patients when the success of an operation may depend on the quality and quantity of the bone. For example, patients who have indications for internal fixation of a fracture or for procedures such as spinal arthrodesis may be candidates for bone densitometry. In these instances, a preoperative assessment of bone-mineral content may help to identify the necessity for enhancement of existing bone mass, augmentation of a fusion mass, internal fixation, or, alternatively, consideration of other procedures. However, such a use for bone densitometry has not yet been studied extensively, and data to help the physician to determine directly which individuals would or would not benefit from operative intervention are not available.

Interpretation of a Bone-Densitometry Report

A standard bone-mineral report consists of measurements expressed as bone-mineral content (the amount of hydroxyapatite, in grams) and converted to areal density (grams per square centimeter) within the region of interest. In addition, normal values are provided according to gender and race and are plotted according to age. Demographic data, including the clinical indications and the patient's age, gender, race, weight, and height, also are listed (Figs. 3-A and 3-B). In order to interpret a standard bone-mineral report, a region of interest must be selected. In order to compare individuals, the sites of measurement should be constant because the bone-mineral content may vary between different bones and between different regions of the same bone. In order to avoid error based on differences in patients' heights, it is important that the region of interest be an anatomical region, such as a percentage of the total length of the bone, and not a region of fixed length. The results are compared with normative values, and standard curves of normative values are provided for individuals of both genders and several races. Comparison of measured values with mean values for normal young or age-matched individuals permits an assessment of the risk of fracture.

Both a Z score and a T score are determined for each record to help in analyzing the results. The Z score is used to compare the patient's bone-mineral density with the mean value for individuals of the same age. A low Z score indicates an etiology other than age-related bone loss. The Z score is calculated by subtracting the patient's result from the mean value for age-matched controls and dividing this value by the standard deviation of the mean. Therefore, by definition, the Z score is zero at the mean value for the population. The Z score is expressed as a standard deviation. The T score is used to compare the patient's bone-mineral density with the mean value for young adults of the same gender and race. Like the Z score, it is expressed as a standard deviation. It is calculated by dividing the difference between the patient's result and the mean value for normal young adults by the standard deviation of the mean. The T score is used for the diagnosis of low bone mass or osteoporosis.

The World Health Organization recently published a document in which it attempted to clarify definitions and to assist clinicians in their interpretation of bone-densitometry results⁷⁸. According to that report, a normal value for bone-mineral content is within one standard deviation of the mean value for young adults of the same age and gender (that is, the T score is more than -1). Osteopenia is considered to be present when the value for bone-mineral content is more than one standard deviation but not more than 2.5 standard deviations below the mean for young adults (that is, the T score is less than -1 and more than -2.5). Osteoporosis is considered to be present when the value is more than 2.5 standard deviations below the mean bone-mineral content for young adults (that is, the T score is less than -2.5). Severe osteoporosis is considered to be present when the value for bone-mineral content is more than 2.5 standard deviations below the mean for young adults and there is at least one so-called fragility fracture (a fracture assumed to be associated with osteoporosis because it occurred as a result of slight trauma).

Physicians should initiate therapy to reduce the risk of fracture in patients on the basis of the presence or absence of risk factors for osteoporosis¹⁹. For white women, those risk factors include a maternal history of fracture of the proximal aspect of the femur, a previous fracture of any type after the age of fifty years, a tall height at the age of twenty-five years, fair or poor health (as rated by the woman), previous hyperthyroidism, treatment with long-acting benzodiazepines or anticonvulsant drugs, excessive intake of caffeine, a duration of less than four hours per day on the feet, an inability to rise from a chair without use of the upper extremities, poor depth perception, poor contrast sensitivity, tachycardia at rest, and low calcaneal bone density¹⁹. Therapy should be initiated to reduce the risk of fracture in women who have a bone-mineral-density T score of less than -2 in the absence of risk factors and in those who have a T score of less than -1.5 if other risk factors are present. Pharmacological treatment should be offered to all individuals—especially women older than the age of seventy years⁵⁹—with risk factors who are seen with a fracture of the proximal aspect of the femur or a vertebral fracture.

The therapeutic options for patients who have osteoporosis include hormone-replacement therapy, bisphosphonates (currently, only alendronate has been approved for marketing by the Food and Drug Administration for use in the treatment of osteoporosis), selective estrogen-receptor modulators (currently, only raloxifene has been approved for marketing by the Food and Drug Administration for use in the treatment of osteoporosis), and calcitonin.

Bone Densitometry for the Evaluation of Periprosthetic Remodeling of Bone After Total Hip Arthroplasty

Total hip arthroplasty alters the strain environment in the proximal aspect of the femur, and the resultant effects on bone-remodeling lead to a redistribution of bone mass adjacent to the prosthesis. This sometimes results in substantial and progressive bone loss that is characterized by extensive resorption in the remodeled femur, with the greatest mean decrease in bone-mineral content occurring adjacent to the proximal one-third of the femur^{22,23,44,48,52}. Although osteolysis associated with wear debris has been implicated as the dominant etiology of periprosthetic bone loss, stress-shielding also has been suggested as a cause of the observed changes^44,52. The evaluation and quantification of periprosthetic bone-remodeling is important clinically, as mechanical loosening of the implant is the most frequently reported complication of total hip arthroplasty⁵¹.

Resorption of bone from the proximal aspect of the femur is an important factor contributing to failure of total hip implants that have been inserted either with or without cement. Prosthetic loosening or subsidence, and fracture of the femur or the prosthesis, are associated with bone loss^6,14,74. Consequently, an accurate assessment of progressive quantifiable changes in periprosthetic bone-mineral content may help the treating surgeon to determine when to intervene in order to preserve bone stock for revision arthroplasty. This information is also useful to manufacturers in their efforts to redesign and improve implants, and it gives physicians a means of determining when an unfavorable situation may be developing in a prosthetic system. In the future, pharmacological agents may be used to inhibit progressive bone loss⁷⁰, and bone densitometry may be useful in determining when and how to use these drugs.

Dual-energy x-ray absorptiometry has been used to assess the bone-mineral content of the proximal aspect of the femur in vivo^{4,22,40,44,48,50,52}. The use of special software supplied by the manufacturer of the device enables the magnitude of the loss (or gain) of periprosthetic bone to be determined. Furthermore, dual-energy x-ray absorptiometry requires only a small volume of bone and thus is appropriate for the evaluation of an osteoporotic femoral shaft adjacent to a prosthesis that has been inserted with or without cement. Dual-energy x-ray absorptiometry software also allows analysis of regional percentage variations in bone-mineral content over the length of the proximal aspect of the femur. The entire femoral component, as well as surrounding bone and soft tissue, may be included in an anteroposterior scan. Local soft-tissue density may be subtracted from the scan with use of a standardized soft-tissue baseline value. Areas of the scan in which the x-ray beams are attenuated by the implant also may be subtracted (Fig. 4).

Dual-energy x-ray absorptiometry provides both the accuracy and the precision that are necessary to detect and to quantify changes in bone that occur after total hip arthroplasty^40-42,50,54. Error attributable to nonuniform distribution of soft tissue or to observer bias is virtually eliminated. Kiratli et al. found that dual-energy x-ray absorptiometry was accurate, with an error of less than 1 percent, for the determination of bone-mineral content in patients who had had a total hip arthroplasty⁴². The precision error in vivo was 2 to 4.5 percent and was attributed mainly to variable positioning of the patient and to the nonhomogeneous distribution of soft tissue⁴². In the clinical setting, positioning of the patient is probably the most important variable.

It is well established that the initial bone stock in the femur has an important influence on the extent of bone-remodeling²². Accordingly, some authors have advocated the use of dual-energy x-ray absorptiometry for the routine preoperative analysis of bone-mineral content in order to predict the change in bone mass after total hip arthroplasty, especially for patients who have poor bone stock and those who are at risk for osteoporosis²². However, the efficacy of dual-energy x-ray absorptiometry as a means for determining whether or not cement should be used in an arthroplasty has not yet been demonstrated.

There is concern about the adverse effects of remodeling and subsequent mechanical loosening of extensively porous-coated implants. Several investigators have used dual-energy x-ray absorptiometry to quantitate the remodeling changes characteristic of periprosthetic bone after total hip arthroplasty without cement^{22,40,42,44,52}. Engh et al. performed dual-energy x-ray absorptiometry analyses of the femora obtained from five cadavera in which an anatomic medullary locking prosthesis had been in situ for at least seventeen months before death²². The contralateral, normal femur in each cadaver was used as a control. None of the contralateral femora had sustained a previous fracture. It is widely accepted that fractures cause permanent changes in bone-mineral content, even remote from the fracture site. The largest decreases in bone-mineral content were noted at the most proximal aspect of the remodeled femora. The percentage decrease in bone-mineral content also was inversely related to the corresponding bone-mineral content of the contralateral, control femur. On the basis of these results, Engh et al. suggested that dual-energy x-ray absorptiometry may be useful preoperatively for predicting the extent of bone-remodeling that will occur after total hip arthroplasty.

Several authors have noted that more extensively porous-coated femoral implants inserted without cement produce greater stress-shielding and more marked bone resorption^4,20,21,40. Kilgus et al. used dual-energy x-ray absorptiometry to compare the bone-mineral content adjacent to a femoral implant that had been inserted without cement with that of the normal, contralateral femur⁴⁰. That study included forty-six patients who had an extensively porous-coated implant and twenty-six who had a proximally coated implant. The greatest decreases in bone-mineral content compared with the content in the controls occurred in the most proximal one centimeter of the medial cortex around the extensively porous-coated implants.

Bone-remodeling after total hip arthroplasty is most pronounced in the first two postoperative years, after which time it continues at a much slower rate. Kiratli et al., using dual-energy x-ray absorptiometry, reported a rapid decrease in bone-mineral density in the proximal aspect of the femur, compared with the immediate postoperative values, during the first two years after total hip arthroplasty⁴². The density of both cortical and cancellous bone adjacent to the proximal portion of extensively porous-coated implants was decreased. Engh et al. noted that, although radiographic decreases in bone density continued for at least five years after implantation, these changes were most marked during the first two years and subsequently proceeded at a slower rate²³.

In summary, dual-energy x-ray absorptiometry provides a precise and accurate means for the evaluation of periprosthetic bone-remodeling after total hip arthroplasty. It can be performed with use of a relatively small volume of bone. In addition, dual-energy x-ray absorptiometry software allows the subtraction of surrounding soft tissue and metal implants. The literature supports the use of this modality for evaluation of the magnitude and rate of changes in bone-mineral content after total hip arthroplasty, particularly in patients in whom a femoral component has been inserted without cement. This information may be useful to manufacturers, who must evaluate the response of the bone to the implant in order to minimize any deleterious effects of bone-remodeling. If current research on the use of antiosteoclastic drug regimens proves useful in the management of patients who have an implant⁷⁰, data on bone-mineral content may be used to help guide treatment. The efficacy of bone densitometry in the evaluation of the proximal aspect of the femur before primary total hip arthroplasty has not been established.

Overview

Introduction | Basic Principles and Technical Considerations | Clinical Indications for the Use of Bone Densitometry | Overview | References

Bone densitometry provides critical information about osseous integrity, the risk of fracture, and periprosthetic bone-remodeling. Consequently, an understanding of this technology is important in current orthopaedic practice. Proposed clinical indications for the measurement of bone-mineral content have been based on both medical need and cost-effectiveness. Universal screening for prophylaxis against osteoporosis and monitoring of bone-mineral content to assess the efficacy of therapeutic intervention are not currently recommended uses for bone densitometry. Perhaps the major value of bone densitometry in current orthopaedic practice is the identification of patients with osteoporosis who are at increased risk for fracture. With the numerous modalities that are available for measuring bone-mineral content, it is important for the clinician to choose the proper technique and to interpret the information in a useful manner.

References

Introduction | Basic Principles and Technical Considerations | Clinical Indications for the Use of Bone Densitometry | Overview | References

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Topics

osteoporosis ; fracture ; femur ; minerals ; hip replacement arthroplasty ; x-ray absorptiometry, dual-energy ; bone densitometry

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ERIC C. MIRSKY, THOMAS A. EINHORN; Current Concepts Review - Bone Densitometry in Orthopaedic Practice*. The Journal of Bone & Joint Surgery. 1998 Nov;80(11):1687-98.

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