- Email: [email protected]
Accuracy of radiographic versus cast measurement of canine proximal femoral canal dimensions
Ciin/cai Mater/a/s 1987; 2: 231-235
f radiographic versus cas measure roximal femoral canal dimensions y&t, AU Qaniels, Russell Van der Wilde and Harold K Dunn, DIVISIONof Orthopaedlc Surgery, Urwersity
of Utah
Medical Cenrer
Radiographrc measurements of the femur are commonly used in the design of femoral prostheses and in preoperative planning and follow-up of hip arthroplasty patients. In this study, a canine model was used to test the accuracy of radiographic measurements of the proximal femur. The femoral canal width of seven purebred greyhounds was measured on anteriorposterior and lateral radiographs. These dimensions were compared to direct measurements of metal casts of the proximal medullary canal. The mean difference between the two methods of measurement was .07 cm in the AP view and .05 cm in the lateral view, with an error of 4.5% and 3.9% respective!\/. Identifying potential sources of measurement error should lead to improved standardization of X-ray technique, thus allowing a more meaningful evaluation of research data and indrvidual patient radiographs.
introduction Age-related increases in the transverse human femoral shaft diameter and medullary canal diameter have been documented by several authors.‘-” With increasing age, endosteal surface resorption surpasses periosteal accretion, with subsequent cortical thinning and femoral canal expansion.‘J These changes correlate with decreases in femoral cortical bone density and osteoporosis,6 placing the elderly at increased risk of fracture. Recent studies have implicated femoral endosteal expansion as a cause of component loosening in total hip arthroplasty.7 Address for correspondence: Ronald W B Wyatt, MD, Division of Orthopedic Surgery, University of Utah Medical Center, Salt Lake City, Utah 84132, USA.
While direct measurements of cortical width and femoral canal diameters are possible with cadavers, quantitating femoral expansion in living subjects has usually been accomplished with radiographic measurements. However) few studies have investigated the accuracy of radiographic measurements of femoral canai dimensions. Trotter and Peterson* compared femoral cortical diameters obtained by direct measurement to those obtained radiographically and consistently found the radiographic diameters to be greater, even when corrected for magnification. surements were highly dependent on the femur’s rotational alignment. Smith et dY compared CT and conventional radiography in measuring cadaver femoral cortical thickness and found both to overestimate the actual values. The purpose
232
RWB Wyatt et al.
of this study was to determine the accuracy of femoral medullary canal dimensions obtained by radiographic measurement in a canine model. Methods
and material
The femurs of seven male purebred greyhounds weighing between 30 kg and 36 kg were obtained and dissected free of soft tissue. The femurs were placed on a work table, resting on the femoral head and femoral condyles. A 3/16” smooth reference pin was then drilled across both cortices parallel to the table at the middiaphyseal level. A radiograph was then obtained with the pin remaining horizontal, which, approximated the normal canine anteriorposterior orientation. The femur was then turned 90 degrees, with the pin vertical, and a lateral radiograph was obtained. The femoral head was resected and the femoral
Figure 1
Femoral casts with reference
pin removed
shaft transected at the middiaphyseal level. All trabecular bone was then curetted from the medullary canal. Removal of all trabecular bone was confirmed by direct observation and transillumination. The distal femur was then plugged, and the femoral canal and neck were filled with a low melting temperature (70°C) lead alloy (Fusible Alloy 1.58, Affiliated Metals, Salt Lake City, Utah). The femur was cooled and the bone removed from the metal cast with an oscillating bone saw (Figure 1). On removing the bone, it was noted that the metal cast was firmly adhered to the femoral endosteal surface, with no evidence of shrinkage or space between bone and metal. Therefore, the dimensions of the metal cast were an accurate representation of the femoral endosteal dimensions. Using the most prominent point of the lesser trochanter as a reference point, measurements of the cast width in the AP projection were taken
Accuracy of radiographic versus cast measuremerzt
at 5 mm increments using a digital caliper reading to the nearest 0.01 mm. The reference pin, which remained incorporated in the metal cast, was kept horizontal initially to maintain a consistent rotational position of the femur. The cast was then rotated 90 degrees and canal widths in the lateral projection were measured with the caliper. Femoral canal diameters were next measured from radiographs in both AP and lateral projections. A computer graphics system, consisting of a microcomputer,, backlit digitizing pad, and related software (Orthographies Inc., SLC, Utah) were used. The system has a precision of 0.13 mm and an accuracy of 0.25 mm. Again, the lesser trochanter was used as a landmark and consecutive measurements were taken at 5 mm intervals.
A comparison of the femoral canal dimensions obtained by radiographic measurements to those obtained by direct measurements of the casts is shown in Tables 1 and 2. In the anteriorposterior view, the mean discrepancy between direct and radiographic measurements ranged from 1.9 mm in the proximal femur to .1 mm in the middiaphyseal region. This represented an 8% error in the proximal femur, and a 1% error distally. Overall, the mean error was 5%. In the lateral view, the mean discrepancy between radiographic and direct measurements ranged from 1.1 mm proximally to .2 mm at the middiaphysis. The mean error was 4%.
nification ranged from a calculated 2% in the proximal femur to 4% in the middia~h~seal region. However, the discrepancy between cast and radiographic measurements was smallest in the diaphyseal region, indicating that other factors were involved. Table 1
Mean medial-laterai femoral canal width IAP view)
Centimetresfrom reference point 0.5 1.0 7.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0
X-ray km) 2.17 1.96 1.67 1.57 1.44 1.32 1.24 1.19 1.13 1.10 1.07 1.06 1.05 1.05 1.07 1.09
Cast km) 2.36 2.09 1.90 1.68 1.48 1.39 1.30 1.19 1.16 1.11 1.06 1.03 1.03 1.02 1.04 1.06 AVG
Mean abs. diff. * 0.19 0.12 0.16 0.12 0.11 0.11 0.08 0.08 0.04 0.02 0.02 0.02 0.01 0.02 0.02 0.04 0.07
% of Mean castwidth + 8.0 5.6 8.5 7.3 7.0 7.5 5.9 6.4 3.1 2.0 2.1 1.6 0.9 1.5 2.0 3.6 4.5
* Mean difference between x-ray and cast measurementsfor all seven femurs + Mean absolute difference divided by mean cast width
Table 2 Mean anterior-posterior (lateral view)
femoral canal width
Centimetresfrom reference point 0.5
Mean abs. diff. * 0.06 0.11 0.10 0.08 0.05 0.02 0.03 0.03 0.05 0.04 0.03 0.04 0.04 0.05 0.04 0.03 0.05
1.o
Determining the accuracy of femoral canal measurements obtained from radiographs is important in interpreting studies reporting these dimensions. The age-related change in femoral canal width has been reported as less than 1 mm per year,“’ which is probably less than the potential measurement error. Possible sources of discrepancy between actual femoral canal dimensions and radiographic measurements are discussed below. I) Magnification: although in this study the femurs were laid directly on the X-ray cassette, there was some magnification of the bony image due to the anterior bow of the femur. This mag-
233
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0
X-ray (cm) 1.59 1.54 1.44 1.38 1.32 1.26 1.22 1.20 1.17 1.15 1.10 1.07 1.09 1.11 1.13 1.14
Cast (cm) 1.62 1.52 1.41 1.34 1.28 1.24 1.21 1.18 1.13 1.10 1.07 1.07 1.08 1.09 1.12 1.15 AVG
% of Mean cast width * 3.6 6.8 6.8 5.6 4.3
I.3 2.2 2.8 4.0 3.8 3.1 3.7 3.4 5.0 3.2 2.7 3.9
* Mean difference between x-ray and cast measurements all seven femurs + Mean absolute difference divided by mean cast width
for
234
RWB Wyatt et al.
In living subjects, magnification would be expected to be greater due to interposition of soft tissue between the bone and cassette. This magnification error can be minimized if a radiopaque object of known dimensions is placed in the same plane as the object to be measured when the radiograph is obtained. All measurements can then be scaled to the known dimensions. 2) Femoral rotation: the femoral canal is not perfectly circular in cross section and thus small amounts of rotation will change the canal width on radiographs. A recent study of human femurs demonstrated an 8% change in femoral diameter with a 10 degree change in rotation, and a 11% change in diameter with 20 degree change in rotation.lO In this study, the amount of rotation was controlled by aligning a threaded pin which was fixed to bone. In clinical studies, the amount of femoral rotation in successive radiographs is difficult to control. 3) Reference point: in this study the most prominent portion of the lesser trochanter was chosen as the reference point for all measurements. Although this point seemed easy to recognize on both radiographs and casts, the two may not have correlated exactly. Measuring from the threaded pin, which was fixed both to the bone and the metal cast, may have improved accuracy. This same principle can be applied to clinical studies, where measuring serial radiographs from a relatively fixed object in the bone, such as the tip of a femoral prosthesis, may improve accuracy. 4) Variability in X-ray radiodensities: radiographs of the femur demonstrate a transition of radiodensity from cortical bone to medullary canal. In the diaphysis there is a narrow transition from cortical bone to marrow contents. However, in the metaphyseal region there are variable amounts of cancellous bone and a subsequent wider zone of transition. Determining exactly where cortical bone ends and femoral canal begins is thus more difficult in the metaphysis than the diaphysis. This is reflected in the results, which show greater accuracy in measurements of the midfemur and less accuracy in the measurements of the proximal femur. The visual phenomenon of contrasting radiodensities, called ‘Mach bands’, can also be a source of error in ra.diographic evaluations. Lanell has shown that differentiating object density in objects less than 1 mm is difficult.
5) Observer variability: reproducibility and accuracy in measurements of roentgenographic dimensions also depend on the experience and ability of the investigator. Brand12 demonstrated that interobserver differences in radiographic measurements can be substantial, especially when small measurements are involved. In summary, many variables affect the accuracy of radiographic measurements. X-ray magnification causes radiographic values to exceed actual values, but other sources of error are less predictable. In our study, the cummulative effect of all potential sources of measurements error were such that radiographic values were sometimes larger and sometimes smaller than actual femoral dimensions. Although the shape of the human proximal femur is different from the canine femur, the same principles in achieving measurement accuracy apply. Controlling the variables which introduce measurement error will permit a more meaningful interpretation of the data on proximal femoral dimensions.
References
Smith RW, Walker RR. Femoral expansion in aging women: implications for osteoporosis and fractures. Science 1964; 145: 156-57. Dewey JR, Murray HB, Armelagos GJ. Rates of femoral cortical bone loss in two Nubian populations. Clin Orthop 1969; 65: 61-66. Van Gerven DP. Thickness and area measurements as parameters of skeletal involution of the humerus, femur, and tibia. J Gerontology 1973; 28: 40-45. Ericksen MF. Aging changes in the medullary cavity of the proximal femur in American blacks and whites. Am J Phys Anthrop 1979; 51: 563-70. Ruff CB, Hayes WC. Subperiosteal expansion and cortical remodelling of the human femur and tibia with aging. Science 1982; 217: 945-48. Atkinson PJ, Weatherell JA, Weidmann SM. Variation in the density of the femoral diaphysis with age. J Bone Joint Surg (Br) 1967; 49: 781-88. Hofmann AA, Bigler GT, France EP et al. Increased endosteal bone loss after hip arthroplasty. Orthop Trans 1986; 10: 236.
Accuracy of radiographic versus cast measurement 8 Trotter M, Peterson RR. Transverse diameter of the femur: on roentgenograms and on bones. Ciin Orthop 1967; 52: 233-39. 9 Smith HW, D&met AA, Levine E. Measurement of cortical thickness in a human cadaver femur. Clin Orthop 1982; 169: 269-74. l( 3 West JD, Mayor MB, Collier JP. Potential errors inherent in quantitative densitometric J Bone analysis of orthopaedic radiographs. Joint Surg (Am) 1987; 69: 58-64.
235
11 Lane EJ, Proto AV, Phillips TW. Mach bands and density perception. Radiology 1976; 121 I 9-17. 12 Brand RA, Yoder SA, Pedersens Radiographic lucencies about total bin reconstructions. Clin Orthop 1985; 1 39. (Received 1 April 1987, revised 14 accepted 22 May 1987.)
f radiographic versus cas measure roximal femoral canal dimensions y&t, AU Qaniels, Russell Van der Wilde and Harold K Dunn, DIVISIONof Orthopaedlc Surgery, Urwersity
of Utah
Medical Cenrer
Radiographrc measurements of the femur are commonly used in the design of femoral prostheses and in preoperative planning and follow-up of hip arthroplasty patients. In this study, a canine model was used to test the accuracy of radiographic measurements of the proximal femur. The femoral canal width of seven purebred greyhounds was measured on anteriorposterior and lateral radiographs. These dimensions were compared to direct measurements of metal casts of the proximal medullary canal. The mean difference between the two methods of measurement was .07 cm in the AP view and .05 cm in the lateral view, with an error of 4.5% and 3.9% respective!\/. Identifying potential sources of measurement error should lead to improved standardization of X-ray technique, thus allowing a more meaningful evaluation of research data and indrvidual patient radiographs.
introduction Age-related increases in the transverse human femoral shaft diameter and medullary canal diameter have been documented by several authors.‘-” With increasing age, endosteal surface resorption surpasses periosteal accretion, with subsequent cortical thinning and femoral canal expansion.‘J These changes correlate with decreases in femoral cortical bone density and osteoporosis,6 placing the elderly at increased risk of fracture. Recent studies have implicated femoral endosteal expansion as a cause of component loosening in total hip arthroplasty.7 Address for correspondence: Ronald W B Wyatt, MD, Division of Orthopedic Surgery, University of Utah Medical Center, Salt Lake City, Utah 84132, USA.
While direct measurements of cortical width and femoral canal diameters are possible with cadavers, quantitating femoral expansion in living subjects has usually been accomplished with radiographic measurements. However) few studies have investigated the accuracy of radiographic measurements of femoral canai dimensions. Trotter and Peterson* compared femoral cortical diameters obtained by direct measurement to those obtained radiographically and consistently found the radiographic diameters to be greater, even when corrected for magnification. surements were highly dependent on the femur’s rotational alignment. Smith et dY compared CT and conventional radiography in measuring cadaver femoral cortical thickness and found both to overestimate the actual values. The purpose
232
RWB Wyatt et al.
of this study was to determine the accuracy of femoral medullary canal dimensions obtained by radiographic measurement in a canine model. Methods
and material
The femurs of seven male purebred greyhounds weighing between 30 kg and 36 kg were obtained and dissected free of soft tissue. The femurs were placed on a work table, resting on the femoral head and femoral condyles. A 3/16” smooth reference pin was then drilled across both cortices parallel to the table at the middiaphyseal level. A radiograph was then obtained with the pin remaining horizontal, which, approximated the normal canine anteriorposterior orientation. The femur was then turned 90 degrees, with the pin vertical, and a lateral radiograph was obtained. The femoral head was resected and the femoral
Figure 1
Femoral casts with reference
pin removed
shaft transected at the middiaphyseal level. All trabecular bone was then curetted from the medullary canal. Removal of all trabecular bone was confirmed by direct observation and transillumination. The distal femur was then plugged, and the femoral canal and neck were filled with a low melting temperature (70°C) lead alloy (Fusible Alloy 1.58, Affiliated Metals, Salt Lake City, Utah). The femur was cooled and the bone removed from the metal cast with an oscillating bone saw (Figure 1). On removing the bone, it was noted that the metal cast was firmly adhered to the femoral endosteal surface, with no evidence of shrinkage or space between bone and metal. Therefore, the dimensions of the metal cast were an accurate representation of the femoral endosteal dimensions. Using the most prominent point of the lesser trochanter as a reference point, measurements of the cast width in the AP projection were taken
Accuracy of radiographic versus cast measuremerzt
at 5 mm increments using a digital caliper reading to the nearest 0.01 mm. The reference pin, which remained incorporated in the metal cast, was kept horizontal initially to maintain a consistent rotational position of the femur. The cast was then rotated 90 degrees and canal widths in the lateral projection were measured with the caliper. Femoral canal diameters were next measured from radiographs in both AP and lateral projections. A computer graphics system, consisting of a microcomputer,, backlit digitizing pad, and related software (Orthographies Inc., SLC, Utah) were used. The system has a precision of 0.13 mm and an accuracy of 0.25 mm. Again, the lesser trochanter was used as a landmark and consecutive measurements were taken at 5 mm intervals.
A comparison of the femoral canal dimensions obtained by radiographic measurements to those obtained by direct measurements of the casts is shown in Tables 1 and 2. In the anteriorposterior view, the mean discrepancy between direct and radiographic measurements ranged from 1.9 mm in the proximal femur to .1 mm in the middiaphyseal region. This represented an 8% error in the proximal femur, and a 1% error distally. Overall, the mean error was 5%. In the lateral view, the mean discrepancy between radiographic and direct measurements ranged from 1.1 mm proximally to .2 mm at the middiaphysis. The mean error was 4%.
nification ranged from a calculated 2% in the proximal femur to 4% in the middia~h~seal region. However, the discrepancy between cast and radiographic measurements was smallest in the diaphyseal region, indicating that other factors were involved. Table 1
Mean medial-laterai femoral canal width IAP view)
Centimetresfrom reference point 0.5 1.0 7.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0
X-ray km) 2.17 1.96 1.67 1.57 1.44 1.32 1.24 1.19 1.13 1.10 1.07 1.06 1.05 1.05 1.07 1.09
Cast km) 2.36 2.09 1.90 1.68 1.48 1.39 1.30 1.19 1.16 1.11 1.06 1.03 1.03 1.02 1.04 1.06 AVG
Mean abs. diff. * 0.19 0.12 0.16 0.12 0.11 0.11 0.08 0.08 0.04 0.02 0.02 0.02 0.01 0.02 0.02 0.04 0.07
% of Mean castwidth + 8.0 5.6 8.5 7.3 7.0 7.5 5.9 6.4 3.1 2.0 2.1 1.6 0.9 1.5 2.0 3.6 4.5
* Mean difference between x-ray and cast measurementsfor all seven femurs + Mean absolute difference divided by mean cast width
Table 2 Mean anterior-posterior (lateral view)
femoral canal width
Centimetresfrom reference point 0.5
Mean abs. diff. * 0.06 0.11 0.10 0.08 0.05 0.02 0.03 0.03 0.05 0.04 0.03 0.04 0.04 0.05 0.04 0.03 0.05
1.o
Determining the accuracy of femoral canal measurements obtained from radiographs is important in interpreting studies reporting these dimensions. The age-related change in femoral canal width has been reported as less than 1 mm per year,“’ which is probably less than the potential measurement error. Possible sources of discrepancy between actual femoral canal dimensions and radiographic measurements are discussed below. I) Magnification: although in this study the femurs were laid directly on the X-ray cassette, there was some magnification of the bony image due to the anterior bow of the femur. This mag-
233
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0
X-ray (cm) 1.59 1.54 1.44 1.38 1.32 1.26 1.22 1.20 1.17 1.15 1.10 1.07 1.09 1.11 1.13 1.14
Cast (cm) 1.62 1.52 1.41 1.34 1.28 1.24 1.21 1.18 1.13 1.10 1.07 1.07 1.08 1.09 1.12 1.15 AVG
% of Mean cast width * 3.6 6.8 6.8 5.6 4.3
I.3 2.2 2.8 4.0 3.8 3.1 3.7 3.4 5.0 3.2 2.7 3.9
* Mean difference between x-ray and cast measurements all seven femurs + Mean absolute difference divided by mean cast width
for
234
RWB Wyatt et al.
In living subjects, magnification would be expected to be greater due to interposition of soft tissue between the bone and cassette. This magnification error can be minimized if a radiopaque object of known dimensions is placed in the same plane as the object to be measured when the radiograph is obtained. All measurements can then be scaled to the known dimensions. 2) Femoral rotation: the femoral canal is not perfectly circular in cross section and thus small amounts of rotation will change the canal width on radiographs. A recent study of human femurs demonstrated an 8% change in femoral diameter with a 10 degree change in rotation, and a 11% change in diameter with 20 degree change in rotation.lO In this study, the amount of rotation was controlled by aligning a threaded pin which was fixed to bone. In clinical studies, the amount of femoral rotation in successive radiographs is difficult to control. 3) Reference point: in this study the most prominent portion of the lesser trochanter was chosen as the reference point for all measurements. Although this point seemed easy to recognize on both radiographs and casts, the two may not have correlated exactly. Measuring from the threaded pin, which was fixed both to the bone and the metal cast, may have improved accuracy. This same principle can be applied to clinical studies, where measuring serial radiographs from a relatively fixed object in the bone, such as the tip of a femoral prosthesis, may improve accuracy. 4) Variability in X-ray radiodensities: radiographs of the femur demonstrate a transition of radiodensity from cortical bone to medullary canal. In the diaphysis there is a narrow transition from cortical bone to marrow contents. However, in the metaphyseal region there are variable amounts of cancellous bone and a subsequent wider zone of transition. Determining exactly where cortical bone ends and femoral canal begins is thus more difficult in the metaphysis than the diaphysis. This is reflected in the results, which show greater accuracy in measurements of the midfemur and less accuracy in the measurements of the proximal femur. The visual phenomenon of contrasting radiodensities, called ‘Mach bands’, can also be a source of error in ra.diographic evaluations. Lanell has shown that differentiating object density in objects less than 1 mm is difficult.
5) Observer variability: reproducibility and accuracy in measurements of roentgenographic dimensions also depend on the experience and ability of the investigator. Brand12 demonstrated that interobserver differences in radiographic measurements can be substantial, especially when small measurements are involved. In summary, many variables affect the accuracy of radiographic measurements. X-ray magnification causes radiographic values to exceed actual values, but other sources of error are less predictable. In our study, the cummulative effect of all potential sources of measurements error were such that radiographic values were sometimes larger and sometimes smaller than actual femoral dimensions. Although the shape of the human proximal femur is different from the canine femur, the same principles in achieving measurement accuracy apply. Controlling the variables which introduce measurement error will permit a more meaningful interpretation of the data on proximal femoral dimensions.
References
Smith RW, Walker RR. Femoral expansion in aging women: implications for osteoporosis and fractures. Science 1964; 145: 156-57. Dewey JR, Murray HB, Armelagos GJ. Rates of femoral cortical bone loss in two Nubian populations. Clin Orthop 1969; 65: 61-66. Van Gerven DP. Thickness and area measurements as parameters of skeletal involution of the humerus, femur, and tibia. J Gerontology 1973; 28: 40-45. Ericksen MF. Aging changes in the medullary cavity of the proximal femur in American blacks and whites. Am J Phys Anthrop 1979; 51: 563-70. Ruff CB, Hayes WC. Subperiosteal expansion and cortical remodelling of the human femur and tibia with aging. Science 1982; 217: 945-48. Atkinson PJ, Weatherell JA, Weidmann SM. Variation in the density of the femoral diaphysis with age. J Bone Joint Surg (Br) 1967; 49: 781-88. Hofmann AA, Bigler GT, France EP et al. Increased endosteal bone loss after hip arthroplasty. Orthop Trans 1986; 10: 236.
Accuracy of radiographic versus cast measurement 8 Trotter M, Peterson RR. Transverse diameter of the femur: on roentgenograms and on bones. Ciin Orthop 1967; 52: 233-39. 9 Smith HW, D&met AA, Levine E. Measurement of cortical thickness in a human cadaver femur. Clin Orthop 1982; 169: 269-74. l( 3 West JD, Mayor MB, Collier JP. Potential errors inherent in quantitative densitometric J Bone analysis of orthopaedic radiographs. Joint Surg (Am) 1987; 69: 58-64.
235
11 Lane EJ, Proto AV, Phillips TW. Mach bands and density perception. Radiology 1976; 121 I 9-17. 12 Brand RA, Yoder SA, Pedersens Radiographic lucencies about total bin reconstructions. Clin Orthop 1985; 1 39. (Received 1 April 1987, revised 14 accepted 22 May 1987.)