- Email: [email protected]
ASIF cortex screw in cortical bone in relation to bone mineral
656
Injury (1993) 24, (lo), 656-659
Prinfed in Great Britain
Holding power of the 4.5 mm AO/ASIF cortex screw in cortical bone in relation to bone mineral K. Stwmsoe, W. L. Kok, A. Hsiseth and A. AIho Ulleva Hospital, University of Oslo, Oslo, Norway
To o&fainbasic data about fk holding power of a 4.5 mm AOIASlF corfex screw in corficalbone in relationto bone mineral as eapressedby dpnsifmetic mefhoak untial pull-orrffeds were petformedon 14 human cadaverfemurs. Themechanicalparameterswerecorrelatedwith bone mineral which was assessedby quanfifative computedtomography (QCT) and dual energy X-ray absorptiomefry(DXA). High correlafionswerefound befweenfk QCT mass,fk DXA aixsify and contentvaluesand fk hoMingpower of fk screw. The QCT density values, expressing fk physical den&y of fk bone, did not correlate similarlywell.
Introduction The usual function of a bone screw is to appose bone fragments, or to secure a plate to bone. This is achieved by tensile stress along the length of the screw related to the torsional moment of screwing (Koranyi et al., 1970; Hughes and Jordan, 1972; Cordey et al., 1980: Matter et al., 1986). The holding power of a screw in bone is a property of the screw itself and of the shearing strength of the bone. An osteosynthesis with screws or with plate and screws secures stability and allows functional postoperative care. However, screws may loosen, impairing the result. This problem is met especially in osteoporotic elderly bone. We scheduled the present study to relate the holding power of a cortical screw to the bone mineral as expressed by densitometric methods.
Material and methods Fourteen human cadaver femurs, acquired at autopsy from 10 men and 4 women, were used. The medians and ranges of age, weight and height of the donors were 74 years (range 59-92 years), 66 kg (range 54-76 kg) and 170 cm (range 150-178 cm), respectively. The bone and adjacent joints did not show any macroscopic pathology, such as lower limb fracture, or generalized bone disease which might interfere with the mechanical properties of the bone. Between the studies, the specimens were stored moist in plastic bags at - 20°C (Sedlin and Hirsch, 1966). Before tests they were thawed in the bags and replaced in the bags immediately after testing. The densitometric studies were performed at four sites of 0 1993 Butterworth-HeinemannLtd OOZO-1383/93/100656-04
the femur; namely at the femoral head, the neck, the shaft and the condyle. The single energy quantitative computed tomography (QCT) studies were performed with a Philips Tomoscan 350 as previously described using a window of 100-3000 CT units (Hoiseth et al., 1991). The measurements were made in 9-mm thick slices. Standardized abdominal scan parameters were used. No simultaneous calibrations were performed as the short-term scanner instability was negligible and the periodic scanner maintenance ensured a good long-term stability. Scanner stability was better than 4 CT units. The bone specimens were scanned without water bath or other soft-tissue equivalents. To describe bone geometry, its mass was given by mean density x slice area (Hoiseth et al., in press). The dual energy X-ray absorptiometry (DXA) was performed using a Hologic QDR-1000 with the programme for lumbar spine measurements and using 10 scan lines. No soft-tissue equivalents were used. This may give systematic differences from ordinary scanning, but no alteration of the relative difference between the specimens. An AO/ASIF 4.5 mm cortex screw was inserted 15 cm above the vertex of the lateral condyle using the standard screw insertion technique with bicortical pretapping (Miiller et al., 1991). The pulling device consisted of a bent 7-hole A0 DC tibial plate with a screw in the middle hole. The bone was then placed under two fixed bars, 15 cm apart. The screw was pulled with a velocity of 1 mm/s in a Schenk Trebel RM 100 Universal Material Testing Machine, RM 100, DIN 51220, Class 1, Schenck Trebel (Ratingen, Germany) (FigureI). The pull-out force necessary to loosen the screw was registered using a Watanabe XY-WX 451 recorder (Figure2). The energy absorbed in pulling out the screw was determined as the area under the load deformation curve. The associations were expressed as Pearson correlation coefficients.
Results Table I presents the demographic data, bone mineral values and the results of the pull-out tests. Screw loosening occurred at very different pull-out forces, median 4180N (range 600-6440N). The correlations between the dens-
Stremsee et al.: Holding power of AO/ASIF
cortex screw
657
Table I. Demographic, densitometric and mechanical data in 14 specimens Spec.
A
B
C
D
E
F
G
H
1
64 75 77 74 66 82 61 71 59 84 89 76 66 92
M F F M F F M M M M M M M M
68 56 61 62 56 68 73 73 74 67 76 66 63 54
169 162 165 172 162 158 173 171 162 172 178 172 164 173
496 427 724 529 509 606 575 680 630 533 439 638 476 561
4791 4842 4561 5718
0.71 0.82 0.80 0.80 0.61 0.50 0.88 0.85 0.91 0.91 0.75 0.87 0.53 0.81
3.37 2.89 2.64 2.98 1.99 1.47 3.43 3.33 3.36 3.96 3.07 3.41 2.01 2.86
2 3 4 5 6 7 8 9 10 11 12 13 14
A Age 8 Sex C Weight (kg)
D Height (cm) E Density QCT F Mass QCT
G Density DXA H Content DXA I Pull (Newton)
2557 6544 6548 6287 7803 5589 6622 3919 7977
I 1520 4200 6440 4160 3200 600 5520 4690 5200 5480 2400 4720 2600 2800
J 1.9 9.2 27.4 11.2 0.5 16.0 5.0 6.2 5.8 5.8 5.3 2.1 2.6
J Energy (Joule)
Referred bone mineral values were measured in the distal femoral shaft.
Table II. Pearson correlation coefficients of bone densitometric values and pull-out energies of 4.5 nun cortex screws assessed by pull-out test in cadaver femurs DXA
OCT Measuring Distal shaft
site
Density 0.40 (P=O.O9)
Mass 0.10 (P=O.38)
Density
Content
0.32 (P=O.15)
0.13 (P=O.35)
mm Figure 1. Setup for the uniaxial pull-out test of 4.5 mm AOIASIF cortex screw in human cadaver femur.
Figure 2. Graphics of uniaxial pull-out test of 4.5 mm AOIASIF
itometric values, measured at the site of the screw, and the holding power, with the corresponding plots, are shown in Fipre3, while the relation between load at failure and energy values are given in TableE Loosening of the screw occurred with fractures of the bone at the threads of the screw in a very regular manner. Standard radiographic examination and CT slices at the region of interest were carried out after the test in each specimen. The correlations of maximal pull-out force and QCTdensities were; 0.51 for the femoral head, 0.61 for the
femoral neck, 0.49 for the femoral shaft and 0.54 for the condylar area. The correlation of pull-out force and QCTmass values were not better; 0.52 for the head, 0.53 for the neck, 0.66 for the shaft and 0.53 for the condyle. A significant relationship (r = 0.81) was found between the maximum pull-out force and the maximum bending load in three-point bending which was performed in a separate study of the distal contralateral femoral shaft (Stromsoe et al. unpublished data). No relationships were found between the pull-out energies and densitometric parameters (Table II).
cortex screw. Velocity 1 mm/s.
Injury: International Journal of the Care of the Injured (1993) Vol. 24/No.
658
7000
10
0
t
0
0
0
0
0
8
0” 0
0
0
0000
E
coo 0
:
0
0
0
I
0
i
iti
i? ._ u
3500 -
R = 0.66
0
I
i
F Lg
0
R = 0.49
I
m
3500 8
P
00 0
0 0
oJ 0.30
I
0.65 DXA
0
R = 0.78
I
1.00 density
1.0
R = 0.68
I
I
o’
2.5 DXA
4.0
content
Figure 3. Plots with corresponding correlation factors between holding power of 4.5 mm cortex screws in cadaver femora (one missing value in QCT and bone mineral assessed by QCT and DXA).
Discussion In previous studies, pull-out strength has shown significant correlations with bone mineral density of trabecular and cortical bone (Koranyi et al., 1970; Cordey et al., 1980; Matter et al., 1986). The ratio of holding power of orthopaedic screws to cortical thickness of dog femurs has been shown to be approximately linear (Koranyi et al., 1970). We chose the pull-out test in an effort to study the predictive value of densitometric bone mineral measurements for the bone mechanical strength. QCT and DXA have proven to be precise techniques in the determination of bone mineral (Mazess et a!., 1988; Husby et al., 1989; Benterud et al., 1991; Hoiseth et al., 1991; Markel et al., 1991). The correlation between mechanical strength of the femoral neck and bone mineral expressed as density has been shown in previous works (Mazess et al., 1988; Esses et a!., 1989; Hoiseth et al., 1991; Markel et al., 1991). In our CT studies the pull-out force correlated more closely with the bone mass than the density. Theoretically, bone material properties should depend on density more than mass as defined in Material and methods (mean density x slice area). It seems difficult to design a purely material test. Snyder and Schneider (1991), using cortical tibia1 slices, obtained a correlation coefficient of only 0.50 between bending strength and CT density. Our results (Fixttre.3) are exactly the same. Our correlation was better than the one reported by Snyder and Schneider (1991). Also the correlation between strength and mass was better than the correlation between strength and density. The holding power correlated poorly with density assessed by QCT, while the correlation with density values obtained by DXA
were high. This seems to be explained by the fact that while the QCT density values represent true physical density of the bone, the DXA density reflects bone mineral per projected area and thus takes the geometry of the bone substance into account as does the QCT-mass value (Hsiseth et al., in press). The QCT density is closely related to the calcium concentration in the bone (Benterud et al., 1991). A good correlation with mechanical properties of bone cannot be expected where geometry is an important factor, as in the predominantly cortical bone of the femoral shaft, and bone mineral density (g/cm3), as expressed by QCT. Except for the study of Soshi et al. (x991), high correlations have not been obtained between bone mineral of the cortex and the mechanical properties. Even in a study where bending of cortical slices was used, the correlations were low (Snyder and Schneider, 1991). The poor correlations in our study between the pull-out energies and bone mineral have a methodical explanation as the velocity of the force applied in the tests made the area calculations inaccurate. The pull-out forces varied ten times in different individuals without any explanation in the demographic data. Such conditions make a prediction necessary in clinical fracture treatment. Loosening of bone screws result in unexpected problems in fracture fixation in elderly patients. Thus, the observations made in this study, using non-invasive densitometric methods, become clinically interesting. This study shows that this prediction is possible with the QCT-mass, DXA-density and content values; not, however, with true bone density of cortical bone as expressed by QCT.
Strsmsee
et al.: Holding power of AO/AStF
cortex screw
Acknowledgement This study was supported by the AO/ASIF Foundation, Beme, Switzerland. The DXA studies were performed at Sentrum Rantgeninstitutt,
Oslo, Norway.
References Benterud J. G., Alho A., Stromme J. H. et al. (1991) Calcium equivalence of bone mineral measured by single energy computed tomography -an autopsy study of femur. Trans. Eur. Orthop. Res. Sot. I, 66. Cordey J., SchlHpfer F., Cordey P. et al. (1980) The relation of torque and angular displacement in predicting admissible torque values. Acfa Orfhop. B&. 46,816. Esses S. I., Lotz J. C. and Hayes W. C. (1989) Biomechanical properties of the proximal femur determined in vihv by single energy quantitative computed tomography. 1. BoneMin. Res. 4, 715. Hughes A. N. and Jordan B. A. (1972) The mechanical properties of surgical bone screws and some aspects of insertion practice. Injury 4, 25. Husby T., Hoiseth A., Haffner F. et al. (1989) Quantification of bone mineral by single energy computed tomography. Acta Orthop. SC&. 60,435. Hiseth A., Alho A. and Husby T. (1991) Assessment of bone mineral content in the internal bone volume. Ada h&d. 32,69. Hoiseth A., Stremsoe K. and Alho A. Single energy quantitative computed tomography (QCT) and dual energy X-ray den-
ADVERTISE
659 sitometry (DXA) in femoral bone mineral assessment. Ada Radial. (in press). Koranyi E., Bowman E., Knecht C. D. et al. (1970) Holding power of orthopaedic screws in bone. Clin. Orthop. 72,283. Matter P., Enzler M. and tily A. (1986) Das Verhalten von Osteosyntheseschrauben im Zeitraum zwischen Implantation und !&plantation. Z. Lkfallchir. 79, 161. Markel M. D., Willenheisser M. A., Morin R. L. et al. (1991) The determination of bone fradure properties by dual energy X-ray absorptiometry. A comparative study. Calcif. Tissue Id. 48,392. Mazess R. B., Barden H., Ettinger M. et al. (1988) Bone density of the radius, spine and proximal femur in osteoporosis. 1. Bone Min. Res. 3, 13. Miiller M., Allgijwer M., Willenegger H. et al. (1991) Man& of h&rnal Fixdion. 3rd Ed. Berlin: Springer-Verlag, 179. Sedlin E. D. and Hirsch C. (1966) Factors affecting the determination of physical properties of femoral cortical bone. Ada Orthop. Scud 37,29. Snyder S. M. and Schneider E. (1991) Estimation of cortical bone by computed tomography. j. Orfhop. Res. 9,422. Soshi S., Shiba R., Kondo H. et al. (1991) An experimental study on transpedicular screw fixation in relation to osteoporosis of the lumbar spine. Spine 16, 1335.
Paper accepted 21 June1993.
Requests for reprints shod be addressed lo: K. Strsmsoe MD, Orthopaedic Department, Diakonhjemmets sykehus, Postboks 23, Vindem, N-0319 Oslo, Norway.
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Long ‘shelf life’- advertising goes on working, reinforcing your sales message l High pass-on readership Full details
of advertisement rates, mechanical data, information on circulation and copy dates can be obtained from: MTB Advertising, 1 1 Harts Gardens, Guildford, Surrey GU2 6QA UK Telephone:
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572678
Injury (1993) 24, (lo), 656-659
Prinfed in Great Britain
Holding power of the 4.5 mm AO/ASIF cortex screw in cortical bone in relation to bone mineral K. Stwmsoe, W. L. Kok, A. Hsiseth and A. AIho Ulleva Hospital, University of Oslo, Oslo, Norway
To o&fainbasic data about fk holding power of a 4.5 mm AOIASlF corfex screw in corficalbone in relationto bone mineral as eapressedby dpnsifmetic mefhoak untial pull-orrffeds were petformedon 14 human cadaverfemurs. Themechanicalparameterswerecorrelatedwith bone mineral which was assessedby quanfifative computedtomography (QCT) and dual energy X-ray absorptiomefry(DXA). High correlafionswerefound befweenfk QCT mass,fk DXA aixsify and contentvaluesand fk hoMingpower of fk screw. The QCT density values, expressing fk physical den&y of fk bone, did not correlate similarlywell.
Introduction The usual function of a bone screw is to appose bone fragments, or to secure a plate to bone. This is achieved by tensile stress along the length of the screw related to the torsional moment of screwing (Koranyi et al., 1970; Hughes and Jordan, 1972; Cordey et al., 1980: Matter et al., 1986). The holding power of a screw in bone is a property of the screw itself and of the shearing strength of the bone. An osteosynthesis with screws or with plate and screws secures stability and allows functional postoperative care. However, screws may loosen, impairing the result. This problem is met especially in osteoporotic elderly bone. We scheduled the present study to relate the holding power of a cortical screw to the bone mineral as expressed by densitometric methods.
Material and methods Fourteen human cadaver femurs, acquired at autopsy from 10 men and 4 women, were used. The medians and ranges of age, weight and height of the donors were 74 years (range 59-92 years), 66 kg (range 54-76 kg) and 170 cm (range 150-178 cm), respectively. The bone and adjacent joints did not show any macroscopic pathology, such as lower limb fracture, or generalized bone disease which might interfere with the mechanical properties of the bone. Between the studies, the specimens were stored moist in plastic bags at - 20°C (Sedlin and Hirsch, 1966). Before tests they were thawed in the bags and replaced in the bags immediately after testing. The densitometric studies were performed at four sites of 0 1993 Butterworth-HeinemannLtd OOZO-1383/93/100656-04
the femur; namely at the femoral head, the neck, the shaft and the condyle. The single energy quantitative computed tomography (QCT) studies were performed with a Philips Tomoscan 350 as previously described using a window of 100-3000 CT units (Hoiseth et al., 1991). The measurements were made in 9-mm thick slices. Standardized abdominal scan parameters were used. No simultaneous calibrations were performed as the short-term scanner instability was negligible and the periodic scanner maintenance ensured a good long-term stability. Scanner stability was better than 4 CT units. The bone specimens were scanned without water bath or other soft-tissue equivalents. To describe bone geometry, its mass was given by mean density x slice area (Hoiseth et al., in press). The dual energy X-ray absorptiometry (DXA) was performed using a Hologic QDR-1000 with the programme for lumbar spine measurements and using 10 scan lines. No soft-tissue equivalents were used. This may give systematic differences from ordinary scanning, but no alteration of the relative difference between the specimens. An AO/ASIF 4.5 mm cortex screw was inserted 15 cm above the vertex of the lateral condyle using the standard screw insertion technique with bicortical pretapping (Miiller et al., 1991). The pulling device consisted of a bent 7-hole A0 DC tibial plate with a screw in the middle hole. The bone was then placed under two fixed bars, 15 cm apart. The screw was pulled with a velocity of 1 mm/s in a Schenk Trebel RM 100 Universal Material Testing Machine, RM 100, DIN 51220, Class 1, Schenck Trebel (Ratingen, Germany) (FigureI). The pull-out force necessary to loosen the screw was registered using a Watanabe XY-WX 451 recorder (Figure2). The energy absorbed in pulling out the screw was determined as the area under the load deformation curve. The associations were expressed as Pearson correlation coefficients.
Results Table I presents the demographic data, bone mineral values and the results of the pull-out tests. Screw loosening occurred at very different pull-out forces, median 4180N (range 600-6440N). The correlations between the dens-
Stremsee et al.: Holding power of AO/ASIF
cortex screw
657
Table I. Demographic, densitometric and mechanical data in 14 specimens Spec.
A
B
C
D
E
F
G
H
1
64 75 77 74 66 82 61 71 59 84 89 76 66 92
M F F M F F M M M M M M M M
68 56 61 62 56 68 73 73 74 67 76 66 63 54
169 162 165 172 162 158 173 171 162 172 178 172 164 173
496 427 724 529 509 606 575 680 630 533 439 638 476 561
4791 4842 4561 5718
0.71 0.82 0.80 0.80 0.61 0.50 0.88 0.85 0.91 0.91 0.75 0.87 0.53 0.81
3.37 2.89 2.64 2.98 1.99 1.47 3.43 3.33 3.36 3.96 3.07 3.41 2.01 2.86
2 3 4 5 6 7 8 9 10 11 12 13 14
A Age 8 Sex C Weight (kg)
D Height (cm) E Density QCT F Mass QCT
G Density DXA H Content DXA I Pull (Newton)
2557 6544 6548 6287 7803 5589 6622 3919 7977
I 1520 4200 6440 4160 3200 600 5520 4690 5200 5480 2400 4720 2600 2800
J 1.9 9.2 27.4 11.2 0.5 16.0 5.0 6.2 5.8 5.8 5.3 2.1 2.6
J Energy (Joule)
Referred bone mineral values were measured in the distal femoral shaft.
Table II. Pearson correlation coefficients of bone densitometric values and pull-out energies of 4.5 nun cortex screws assessed by pull-out test in cadaver femurs DXA
OCT Measuring Distal shaft
site
Density 0.40 (P=O.O9)
Mass 0.10 (P=O.38)
Density
Content
0.32 (P=O.15)
0.13 (P=O.35)
mm Figure 1. Setup for the uniaxial pull-out test of 4.5 mm AOIASIF cortex screw in human cadaver femur.
Figure 2. Graphics of uniaxial pull-out test of 4.5 mm AOIASIF
itometric values, measured at the site of the screw, and the holding power, with the corresponding plots, are shown in Fipre3, while the relation between load at failure and energy values are given in TableE Loosening of the screw occurred with fractures of the bone at the threads of the screw in a very regular manner. Standard radiographic examination and CT slices at the region of interest were carried out after the test in each specimen. The correlations of maximal pull-out force and QCTdensities were; 0.51 for the femoral head, 0.61 for the
femoral neck, 0.49 for the femoral shaft and 0.54 for the condylar area. The correlation of pull-out force and QCTmass values were not better; 0.52 for the head, 0.53 for the neck, 0.66 for the shaft and 0.53 for the condyle. A significant relationship (r = 0.81) was found between the maximum pull-out force and the maximum bending load in three-point bending which was performed in a separate study of the distal contralateral femoral shaft (Stromsoe et al. unpublished data). No relationships were found between the pull-out energies and densitometric parameters (Table II).
cortex screw. Velocity 1 mm/s.
Injury: International Journal of the Care of the Injured (1993) Vol. 24/No.
658
7000
10
0
t
0
0
0
0
0
8
0” 0
0
0
0000
E
coo 0
:
0
0
0
I
0
i
iti
i? ._ u
3500 -
R = 0.66
0
I
i
F Lg
0
R = 0.49
I
m
3500 8
P
00 0
0 0
oJ 0.30
I
0.65 DXA
0
R = 0.78
I
1.00 density
1.0
R = 0.68
I
I
o’
2.5 DXA
4.0
content
Figure 3. Plots with corresponding correlation factors between holding power of 4.5 mm cortex screws in cadaver femora (one missing value in QCT and bone mineral assessed by QCT and DXA).
Discussion In previous studies, pull-out strength has shown significant correlations with bone mineral density of trabecular and cortical bone (Koranyi et al., 1970; Cordey et al., 1980; Matter et al., 1986). The ratio of holding power of orthopaedic screws to cortical thickness of dog femurs has been shown to be approximately linear (Koranyi et al., 1970). We chose the pull-out test in an effort to study the predictive value of densitometric bone mineral measurements for the bone mechanical strength. QCT and DXA have proven to be precise techniques in the determination of bone mineral (Mazess et a!., 1988; Husby et al., 1989; Benterud et al., 1991; Hoiseth et al., 1991; Markel et al., 1991). The correlation between mechanical strength of the femoral neck and bone mineral expressed as density has been shown in previous works (Mazess et al., 1988; Esses et a!., 1989; Hoiseth et al., 1991; Markel et al., 1991). In our CT studies the pull-out force correlated more closely with the bone mass than the density. Theoretically, bone material properties should depend on density more than mass as defined in Material and methods (mean density x slice area). It seems difficult to design a purely material test. Snyder and Schneider (1991), using cortical tibia1 slices, obtained a correlation coefficient of only 0.50 between bending strength and CT density. Our results (Fixttre.3) are exactly the same. Our correlation was better than the one reported by Snyder and Schneider (1991). Also the correlation between strength and mass was better than the correlation between strength and density. The holding power correlated poorly with density assessed by QCT, while the correlation with density values obtained by DXA
were high. This seems to be explained by the fact that while the QCT density values represent true physical density of the bone, the DXA density reflects bone mineral per projected area and thus takes the geometry of the bone substance into account as does the QCT-mass value (Hsiseth et al., in press). The QCT density is closely related to the calcium concentration in the bone (Benterud et al., 1991). A good correlation with mechanical properties of bone cannot be expected where geometry is an important factor, as in the predominantly cortical bone of the femoral shaft, and bone mineral density (g/cm3), as expressed by QCT. Except for the study of Soshi et al. (x991), high correlations have not been obtained between bone mineral of the cortex and the mechanical properties. Even in a study where bending of cortical slices was used, the correlations were low (Snyder and Schneider, 1991). The poor correlations in our study between the pull-out energies and bone mineral have a methodical explanation as the velocity of the force applied in the tests made the area calculations inaccurate. The pull-out forces varied ten times in different individuals without any explanation in the demographic data. Such conditions make a prediction necessary in clinical fracture treatment. Loosening of bone screws result in unexpected problems in fracture fixation in elderly patients. Thus, the observations made in this study, using non-invasive densitometric methods, become clinically interesting. This study shows that this prediction is possible with the QCT-mass, DXA-density and content values; not, however, with true bone density of cortical bone as expressed by QCT.
Strsmsee
et al.: Holding power of AO/AStF
cortex screw
Acknowledgement This study was supported by the AO/ASIF Foundation, Beme, Switzerland. The DXA studies were performed at Sentrum Rantgeninstitutt,
Oslo, Norway.
References Benterud J. G., Alho A., Stromme J. H. et al. (1991) Calcium equivalence of bone mineral measured by single energy computed tomography -an autopsy study of femur. Trans. Eur. Orthop. Res. Sot. I, 66. Cordey J., SchlHpfer F., Cordey P. et al. (1980) The relation of torque and angular displacement in predicting admissible torque values. Acfa Orfhop. B&. 46,816. Esses S. I., Lotz J. C. and Hayes W. C. (1989) Biomechanical properties of the proximal femur determined in vihv by single energy quantitative computed tomography. 1. BoneMin. Res. 4, 715. Hughes A. N. and Jordan B. A. (1972) The mechanical properties of surgical bone screws and some aspects of insertion practice. Injury 4, 25. Husby T., Hoiseth A., Haffner F. et al. (1989) Quantification of bone mineral by single energy computed tomography. Acta Orthop. SC&. 60,435. Hiseth A., Alho A. and Husby T. (1991) Assessment of bone mineral content in the internal bone volume. Ada h&d. 32,69. Hoiseth A., Stremsoe K. and Alho A. Single energy quantitative computed tomography (QCT) and dual energy X-ray den-
ADVERTISE
659 sitometry (DXA) in femoral bone mineral assessment. Ada Radial. (in press). Koranyi E., Bowman E., Knecht C. D. et al. (1970) Holding power of orthopaedic screws in bone. Clin. Orthop. 72,283. Matter P., Enzler M. and tily A. (1986) Das Verhalten von Osteosyntheseschrauben im Zeitraum zwischen Implantation und !&plantation. Z. Lkfallchir. 79, 161. Markel M. D., Willenheisser M. A., Morin R. L. et al. (1991) The determination of bone fradure properties by dual energy X-ray absorptiometry. A comparative study. Calcif. Tissue Id. 48,392. Mazess R. B., Barden H., Ettinger M. et al. (1988) Bone density of the radius, spine and proximal femur in osteoporosis. 1. Bone Min. Res. 3, 13. Miiller M., Allgijwer M., Willenegger H. et al. (1991) Man& of h&rnal Fixdion. 3rd Ed. Berlin: Springer-Verlag, 179. Sedlin E. D. and Hirsch C. (1966) Factors affecting the determination of physical properties of femoral cortical bone. Ada Orthop. Scud 37,29. Snyder S. M. and Schneider E. (1991) Estimation of cortical bone by computed tomography. j. Orfhop. Res. 9,422. Soshi S., Shiba R., Kondo H. et al. (1991) An experimental study on transpedicular screw fixation in relation to osteoporosis of the lumbar spine. Spine 16, 1335.
Paper accepted 21 June1993.
Requests for reprints shod be addressed lo: K. Strsmsoe MD, Orthopaedic Department, Diakonhjemmets sykehus, Postboks 23, Vindem, N-0319 Oslo, Norway.
IN THIS JOURNAL
MAKE IT WORK
FOR YOU
journal is highly specific and enables companies to advertise their product, service or event to a well defined and attentive audience within a perfectly-tailored editorial environment. This
Butterworth-Heinemann l
l
journals provide:
Precision targeting
Long ‘shelf life’- advertising goes on working, reinforcing your sales message l High pass-on readership Full details
of advertisement rates, mechanical data, information on circulation and copy dates can be obtained from: MTB Advertising, 1 1 Harts Gardens, Guildford, Surrey GU2 6QA UK Telephone:
$44
(0) 483 578507
Fax: +44
(01483
572678