Skeletal Radiol (2006) 35: 202–211 DOI 10.1007/s00256-005-0065-1
Salil H. Patel Kieran P. Murphy
Received: 1 January 2005 Revised: 2 October 2005 Accepted: 17 October 2005 Published online: 10 February 2006 # ISS 2006
S. H. Patel (*) . K. P. Murphy Radiology, Johns Hopkins School of Medicine, 600 N. Wolfe St Radiology B-100, Baltimore, Maryland 21287, USA e-mail:
[email protected] Tel.: +1410-502-0736 Fax: +1410-614-8238
SCIENTI FIC A RTICLE
Fractures of the proximal femur: correlates of radiological evidence of osteoporosis
Abstract Objective: Fractures of the proximal femur are common sequelae of osteoporosis, and are responsible for significant morbidity and mortality in elderly patients worldwide. Radiographic assessment methods to assess for fracture risk may be of particular value. Design and patients: The authors present the results of biomechanical testing, radiographic imaging, and histologic exam of 20 embalmed human bone specimens, with implications for clinical correlation of radiologic findings. Authors assessed bone architecture using the Singh Index, using a blinded 3-rater system to reduce bias and measure intra-observer reliability. After loading to failure with ultimate tensile strength (UTS), bone specimens were assessed by fracture location type and by trabecular bone volume (TBV). Results: Singh scoring was performed with
Introduction Fracture of the hip affects populations in proportion to age, with an annualized incidence of 0.02 per 1000 women less than 35 years of age, rising to over 30 per 1000 for women older than 85 years; for men, more than 19 per 1000 are affected each year above age 85 [1]. This exponentiallyincreasing incidence of fracture with age is posited to result from postural and gait instability, which causes falls, and from osteoporosis, which leads to bone fragility [2]. Osteoporosis may be defined as an excessive but proportional reduction in the amount of both the mineral and matrix phases, unaccompanied by any abnormality in the
Inter-Class Correlation of 0.80 (F=0.24, by ICC Portney Model 2). A statistically-significant difference among the UTS distributions was noted for UTS by Fracture Site (F=4.49, p=0.026, by ANOVA). No significant association of Singh Index with TBV, or TBV with UTS, was observed, although a trend toward greater UTS with higher Singh grade was observed. Conclusions: The authors propose that the Singh Index is a valuable and reliable indicator which may reflect structural integrity in trabecular bone. Fracture site along the femur is associated with tensile strength. The authors, in the light of these findings, address the promise and potential impact of prophylactic hip augmentation in populations at risk for femoral neck pathology. Keywords Osteoporosis . Fracture . Vertebroplasty
structure of the residual bone [3]. It may occur as a primary process, or as a result of corticosteroid administration or systemic disease. The WHO has proposed a diagnostic criterion of osteoporosis, for screening purposes: bone mineral density greater than 2.5 standard deviations below the mean of young adult females [4]. The condition affects 50 of 1000 women and 24 of 1000 men at 50 years. By age 85, fully half of all females and one-fifth of all males meet an operational definition of osteoporosis [5]. Fractures of the femoral neck and compression fractures of the vertebrae and are the two most common consequences of osteoporosis, the predisposition to fracture being proportional to the degree of bone loss [6]. In fact, 9
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of 10 hip and vertebral fractures in elderly women are due to underlying osteoporosis [7]. The lifetime risk of any fracture in 50-year-olds with osteoporosis is 53% for women and 21% for men [8]. Femoral morphology The loss of bone is generally more marked in the trabecular than the cortical bone. There is thinning and sometimes complete resorption of the trabeculae; this resorption process occurs in a particular sequence. Singh and colleagues described this progression as it appeared radiologically [9]. The Singh Index therefore is an attempt to define levels of osteoporosis on the basis of the alteration of architectural appearance. The data presented by Singh, et al. was obtained by comparing radiologic findings to iliac crest biopsies. Six distinct grades were delineated, designating Grades 6–4 as within the normal range and Grades 3–1 as associated with significant osteoporosis (and an increased likelihood of hip fracture). The architecture of the proximal femur is complex, and underlies its functional utility. Cancellous bone of the femur is composed of two distinct systems of trabeculae: compressive and tensile. There structures are disposed along the lines of maximum forces during load-bearing activity (Figs. 1, 2 and 3). Five groups of trabeculae have been identified: –
–
–
–
–
Primary compressive group: extends from the medial cortex of the shaft to the upper portion of the head of the femur, in curved radial lines. They comprise some of the closely packed trabeculae in the upper end of the femur. Secondary compressive group: arise from the medial cortex of the shaft, and curve upwards and laterally towards the greater trochanter and -upper portion of neck. Typically thin and widely spaced. Greater trochanter group: arise from the lateral cortex just below the greater trochanter, and sweep up to end near its superior surface. Typically slender and of minor functional significance. Primary tensile group: produce the calcar femorale, along a supporting arch; arise from the lateral cortex below the greater trochanter, and arch upward and medially to end in the femoral head. (N.B. Ward’s Triangle is bounded above by the primary tensile group, medially by the primary compressive, and laterally by the secondary compressive group.) Osteoporotic change iniitally occurs at the primary tensile and secondary compressive intersection [9]. Secondary tensile group: arise laterally below the primary tensile groups; they arch upwards and medially, and end by irregularly interdigitating with the secondary compressive group.
Fig. 1 Gross femoral head section illustrating primary compressive trabeculae (upper right), secondary compressive group (mid-to-left region), and Ward’s triangle (center)
Pathological definitions of osteoporosis are couched in descriptors of bone morphometry and related measurements of trabecular bone volume (TBV). The TBV as a proportion of total tissue (bone and bone marrow) is low in osteoporosis. The concept of “critical bone mass” was developed by Meunier, proposing that patients with a trabecular bone volume of 11% or less have osteoporosis, as they are more likely to suffer vertebral fractures at this threshold [10].
Current research in radiographic-functional correlation Numerous investigators have attempted to apply novel analytical methods in order to correlate functional properties of bone with its underlying structural architecture. Notable alternatives to simple roentgenography include the following. –
Fractal dimensional analysis. Pioneered by Majumdar and Benhamou, using direct histologic imaging or secondary analysis of radiographic data [11, 12]. The method usually employed to develop a fractal model is known as Richardson plotting, viz., the log of the perimeter of an outline is related to the log of the measuring unit length, thus allowing determination of the fractal dimension D (N.B., 1-D is the slope of the plot). D is then correlated with other mechanical or imaging
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Fig. 2 Schematic representation of continuum of trabecular patterns corresponding to Singh Indices 6, 5, 3, and 1 (left-to-right)
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properties. One example of such work combines ultrasound imaging and fractal analysis of calcaneus radiographic images, correlating fractal parameters with Body Mass Index and Bone Mineral Content from dual-energy X-ray absorptiometry (DEXA) scanning [13]. Finite element modeling (FEM) and compression analysis [14]. FEM often involves iterative mathematical transformations which divide a target into an array of discrete elements. Structural analysis may be conducted strictly by computational manipulation, for instance using partial differential equations over general domains, or by correlation with experimentallyderived physiologic properties. Realtime manipulation of models may be possible, depending upon parameter complexity. 3D magnetic resonance microimaging and microcomputed tomography combined with FEM [15] and stiffness testing [16]. The strength of these approaches lies in their ability to characterize tissue microstructure in situ, although at present they remain computation-
ally-demanding and are generally reserved for selected research contexts. Aging bone has long been postulated to incorporate effects and mechanical insults in a manner affecting elasticity, stiffness, and strength [17]. As early as in 1896, it was suggested that a fall or a blow delivered over the greater trochanter is a root cause of femoral neck fracture [18]. In this article, the authors present the results of experimentation to evaluate whether the degree of osteoporosis as evidenced on simple plain film radiography can be correlated with fractures occurring in the proximal femur. After stressing cadaveric femurs with a universal load delivery device, the samples were analyzed to correlate histologic findings with apparent radiological character.
Methods Specimens Cadavers stored in 10% buffered formalin were obtained with appropriate review and permission, following the Declaration of Helsinki, from the Department of Anatomy at the University College of Dublin. Proximal femora were subsequently removed. Radiologic examination All bones were studied by radiography (1-sec exposure, 60 Kv, 50 Ma). Films were independently evaluated by three designated physicians, while blinded to findings from contralateral hip specimens. The observer assigned a Singh Index score to each specimen, using the following criteria: –
– – Fig. 3 Femoral head plain radiograph illustrating arch formed by the primary tensile trabeculae and its perpendicular intersection with the primary compressive group
Grade 6. All trabecular groups visible. The upper end of the femur is completely filled with cancellous bone. Ward’s triangle contains fine strands of trabecular bone. Grade 5. Ward’s Triangle, and primary compressive and tensile trabeculae clearly visible. The secondary groups are less clearly demonstrated. Grade 4. The secondary compressive trabeculae are lost; Ward’s Triangle opens laterally. There is a reduc-
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–
– –
tion in 1-inch tensile trabeculae. Resorption extends from the center in a radial fashion. Grade 3. The primary compression trabeculae very evident, with marked reduction of the 1 tensile trabeculae, which can no longer be traced to the greater trochanter and can only be identified in the upper part of the neck. Grade 2. Loss of 1-inch tensile trabeculae; only the compressive group remains. Grade 1. Minimal compressive trabeculae.
Loading The bones were stripped of all connective tissue and muscle. The ultimate tensile stress (UTS) of the remaining specimen was then determined, employing the following method: specimens were mounted in a steel tube of variable diameter, and an Instrom device compressed the femoral head with a force applied at a constant rate of 10 m/min. The peak of each graph was taken to be the UTS.
approximately 2–3 mm in thickness were cut from the lateral end of each specimen, in the area of intersection of the primary tensile and secondary compressive trabeculae. The sections taken for histology were from the most lateral part of that core, because this is a location where it was hoped that early changes of osteoporosis may be found. Decalcification Each pair of specimens was treated in a solution of 10% formic acid and 10% formalin for 48 hours, at room temperature [20]. The specimens were deemed to be adequately decalcified when they became Xray opaque, and when they became pliable between the tips of a forceps. If samples were not adequately decalcified, they were placed in an 18% formic acid solution at 25°C. These specimens were then re-embedded in wax blocks before cutting. Wax embedding and fixation The specimens were dehydrated by an automated process, including formalin fixation and mounting via paraffin embedding, with paraplast treatment. 8-μm sections were stained with hematoxylin & eosin.
Fracturing of bones Microscopic morphometry Fractures were graded by their anatomical location as suggested by Garden [19]. The fractures were divided into three categories: A. Intracapsular fractures. These may be further subdivided into Subcapital and Transcervical. Subcapital refers to fractures which occur immediately beneath the articular surface of the femoral head along the old epiphyseal plate. Transcervical refers to a fracture passing across the femoral neck between the head and the greater trochanter. B. Extracapsular fractures. Several types are possible. A basal fracture occurs at the junction of the base of the femoral neck and the greater and lesser trochanters. Intertrochanteric refers to a fracture along the intertrochanteric line or crest. Subtrochanteric fractures occur within 1 inch below the lesser trochanter. C. Shaft fractures: Occur at levels greater than 1 inch below the lesser trochanter.
Histologic specimen preparation Incision The femoral head and neck were cut using a 20 mm Wadkin bandsaw. Two parallel vertical and two parallel horizontal incisions were made, leaving a central rectangular section. This central section was located just beneath the point of attachment of the ligamentum teres. The specimen was composed of articular surface of head, cortical bone and an inner core of cancellous bone extending as far laterally as the base of the neck. Two pieces
Slides were analyzed using a Nikon Optiphot microscope with a 2.5× objective linked to a computer with a graphical tablet. At least five areas on each section were analyzed for the area covered by trabecular bone. With two sections on each slide, and two slides of each bone examined, 20 areas were analyzed for automated TBV calculation, i.e. TBV= (Volume of Trabeculae/Combined Volume of Trabeculae and Marrow)×100.
Results Of the 20 specimens tested, two pairs of bones were used to develop fracturing technique and were excluded from subsequent analysis. Thus, eighteen cadavers were studied, six males and 13 females. The range of male ages was 64– 80 years (mean 75.6 years), and female ages ranged from 71–93 years (mean 83.7 years). Four females and one male had previously undergone hip surgery: two females and one male with total hip replacements, and two females with dynamic hip screws. One female hip was found to be fractured when removed from the body. Therefore, of the available 40 hips, 30 were available to be studied further. Radiology The radiologists considered all the radiographs to be accurately centered and of good quality. Twenty-nine films were suitable for study; one was not as it was under-
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developed. Figure 4 demonstrates Singh grades. The results are displayed as part of Tables 1 and 2: they are presented in the format “x/y/z”, as the score from the primary author (x), the score from an Orthopedic Registrar (y), and finally the score from a Board-certified radiologist (z). The index was subjectively assessed by all three raters as uncomplicated to apply, and demonstrated relative statistical consistency: inter-rater reliability revealed an Inter-Class Correlation (ICC, Portney Model 2) of 0.80 (F=0.24). Table 1 illustrates the results, classified by the location of the fractures, also showing the Singh Index for comparison and the age and sex of the cadaver and tensile strength. Twenty-seven bones were loaded to failure; Fig. 5 shows two representative specimens.
Table 1 Specimen baseline summary, with radiographer scores and tensile strength Bone
Age (years)
Intercapsular 6R 93 6L 93 9L 75 11L 87 1805L 86 1806R 92 1806L 92 1835L 88 Extracapsular Basal 1800R 89 1814L 82 Intertrochanteric 1801L 86 1816L 88 Subtrochanteric 1L 83 7R 79 Shaft 3 79 11R 87 12L 75 L3L 71 14R 64 1815L 76 1822 79 1817 76 Shaft Fracture (Technical Reasons) 1R 83 9R 75 12R 75 1805R 86 1815R 76
Sex
Singh Index
UTS (N/m2)
F F F F F F F F
4/3/3 4/3/3 2/3/2 4/3/2 4/3/3 5/5/4 5/4/3 4/5/4
220 265 – 510 87 150 150 155
F F
2/2/2 2/2/2
70 130
F F
5/5/4 5/5/5
250 125
F F
4/3/3 4/3/2
210 120
M F F F M M M M
5/5/4 5/6/5 6/5/5 5/5/5 6/5/5 6/5/5 5/5/4 6/6/5
475 320 690 450 640 230 320 320
F F F F M
4/3/3 3/2/2 5/6/5 5/5/4 6/6/6
120 230 160 90 100
(UTS=ultimate tensile strength)
Fractures by site
Fig. 4 Plain radiographs of representative specimens, illustrating Singh Grade 6 (upper left) Grade 5 (upper right), Grade 4 (lower left; note loss of secondary compressive group), Grade 3 (lower right; note that only primary compressive and tensile groups are visible)
A statistically-significant difference among the UTS distributions was noted for UTS by fracture site (F=4.49, p=0.026, by ANOVA), but not for TBV by fracture site (F=0.947, p=0.409, by ANOVA). Shaft fractures sustained the highest UTS loads (Table 3), as may be expected due to the difference in architecture. Fig. 6 reveals generally homogeneous TBV by Singh Index, with higher Singh scores associated with wider ranges in UTS.
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Table 2 Histologic TBV and average Singh Index across raters for each specimen Specimen
Mean Singh Index
1R 1L 3 6R 6L 7R 9R 9L 11R 11L 12R 12L 13L 14R 14L 1800R 1801R 1801L 1805L 1805R 1806L 1806R 1814L 1814R 1815L 1815R 1816L 1817 1822 1835L
3 3 5 3 3 3 2 2 3 5 5 5 5 5 5 2 5 5 3 4 5 4 2 – 6 6 5 6 5 4
–
TBV (%) l5.5 13.1 – 13.2 11.5 9.9 17.2 10.9 19.5 4.7 10.9 14.1 15.2 – 16.8 9.19 12.5 7.72 12.5 10.7 29.6 7.15 16.4 15.0 11.6 12.2 16.4 15.7 22.3 12.2
(TBV=trabecular bone volume)
Fig. 5 Gross specimens illustrating intracapsular (left) and extracapsular (right) fractures
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Shaft. In this study, all the male bones produced shaft fractures, at tensile strength two to three times those of the intra and extracapsular fracture groups. They were younger by an average of 10 years (13 years younger than the intracapsular group). They had Singh Index values in the normal range of 5–6. This suggests that the shaft is weaker than the head in this group. Intrascapular. Intracapsular fractures occurred in women, mean age 88, whose Singh Index shows borderline osteoporosis. These were sheer fractures; this fits with the clinical fracture. There were a wide range of Singh indices for these bones, and it was not possible to correlate either positively or negatively fracture site and indices. Basal. The basal fractures occurred in two bones of two different cadavers. They had the same Singh indices of 2, which means loss of all trabeculae except the primary compressive group. This would theoretically render the base of the femoral neck very vulnerable, as it has no support from secondary compressive or primary tensile trabeculae in this area. Therefore, the Singh Index did correlate with fracture site in the case of the shaft fractures and the basal fractures.
Histologic findings The core of bone taken from samples extended from the articular cartilage to the base of the neck. Four specimens were inadequately decalcified and re-treated before analysis. Normal ranges for TBV stratified by age have been reported previously (see Table 4); these values were obtained using human iliac crest biopsy samples. A gradual decrease in trabecular bone volume with age is welldocumented. Normal females have a slightly higher TBV than normal males in the younger age groups, but have levels the same as or lower than males in groups over 60 years of age [21]. There are, however, no normal ranges available for the trabecular bone volume of the proximal femur. The normal TBV range is wide; in the data presented results from the same individual differ on either side. Most pairs of femurs, however, match reasonably, (e.g.: 6R and 6L=13.2% and 11.5% respectively, 1R and 1L=15.4% and 13.1%, 1805L and 1805 R=12.5% and 10.7%). Further, most of the trabeculae bone volumes do correlate well with Singh Index (R2=0.0062 by linear regression), i.e., those bones with low Singh indices of 4 or 3 have correspondingly low trabecular bone volumes in the 12–9% range. Generally, however, bones with Singh indices of 5–6 had trabecular bone volumes of 14% or greater. UTS did not significantly correlate with TBV (R2=0.0903 by linear regression).
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Table 3 Summary findings and demographics by fracture location Fracture Type
Age (Years)
Sex
Singh Index
Intracapsular Extracapsular Shaft
75–93 (mean 88.3) 79–89 (mean 84.5) 64–87 (mean 75.9)
F only F only F&M
2–5 (mean 4) 2–5 (mean 3) 4–6 (mean 5)
UTS (N/m2) As 95% CI and Mean ± SD 125–317 (221±130) 38.8–281 (160±71.4) 269–460 (431±152)
(CI=confidence interval; SD=standard deviation)
Discussion Bone fracture process Bones are not rigid structures, but instead possess a degree of elasticity whereby stress may produce a transient deformation followed by the recovery of the original form. A fracture is created when stresses exceed the normal limits of elasticity. Adult bone gives away suddenly upon reaching the breaking point (i.e., the UTS), and will not bend into a new shape, as would a steel rod. The force necessary to fracture a bone depends partly on the thickness of the bone and partly on the direction of the force in relation to structure. In addition, the speed at which the force is applied affects the breaking point. Primary stresses may be classified as compression, tension, and shear; combinations of these may give rise secondarily to bending and twisting. The fractures that were produced in this study were shear fractures. A shear fracture is encountered in a long bone when the force is exerted obliquely to the long axis. Using a static load model, it has been estimated that the femoral head encounters up to 2.9 times the body weight acting at 15 degrees to the vertical, as the subject stands on one leg [22]. While these measurements of position are anatomically valid, they are not necessarily consistent physiologically, since the forces acting on the head of the femur at any time may vary during
normal gait [23]. These forces are thus a combination of body weight (i.e., mass × gravity), muscle action, acceleration and deceleration; the resolved vector varies with the position and angulation of the head of the femur. When directed at an angle of 45 degrees, the shear stress equals the compression stress. Bending stresses produce compression on one side and tension on the other of the bone. In the interior, there is a zone were no stress falls. The cortical bone is especially thick in the middle of the body where the greatest bending stresses usually occur. To say that a force must be applied to the head of the femur in the anatomical position to produce a clinically-relevant fracture is therefore to underestimate the design. The trabecular pattern develops in an organized fashion to assist cortical bone resist forces applied to the femoral head from all vectors to which the femur is exposed in life. Provided the femur is loaded in the same plane as these vectors, the fracture will be clinically relevant. It should be noted that the trabecular pattern only becomes organized after we begin weight-bearing as infants [24]. Further, the trabecular pattern of the proximal femur of an arboreal sloth—an animal that lives in an inverted position—shows no definite organization [25]. This correlation with fracture site contradicts prior findings in the literature, wherein the traditional notion is that is that the Singh Index correlates well with tensile stress but not with location of fracture [26]. However, our findings do agree that the tensile strength of the bone is proportional to the Singh index. The literature contends that the fracture site is dependent on the point of application of the force. The femoral head may be conceptualized as comprised of specific areas, which, when loaded, will produce a specific fracture. It is perhaps overly simplistic to consider the trabeculae as the only determinant of
Table 4 Normal TBV ranges (cf. Coupron et al., l978)
Fig. 6 Distribution of UTS in relation to Singh Index and TBV. Datapoint diameter is proportional to TBV
Age (Years)
TBV Male (%)
60–69 70–79 80–89 90–99
17.4±5.6 17.8±4.3
(TBV=Trabecular Bone Volume)
TBV Female (%) 15.3±4.6 14.8±3.3 12.5±2.1 14.0±4.0
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fracture location. The cortical bone thickness varies with, and is proportional to, the local loads experienced in the femoral head and neck. One possible criticism of the methodology employed in this study is that the cadaveric bones studied by load testing were previously embalmed using formalin solution; it may be suggested that such a preservation process may alter the mechanical properties of the specimens. In fact, work from Finlay and co-workers has demonstrated that, during pulsed ultrasonographic and biomechanical testing of bovine femora, formalin treatment does not significantly alter the elastic or failure properties of bone samples [27, 28]. Clinical relevance The femoral head is a structure of elegant design, combining weight-bearing capacity with stability and mobility. In sum, this body of work verifies that the Singh Index is a simple and reproducible metric, with a low degree of interobserver error. Ultimate tensile strength is proportional to Singh Index grade. In applying this index to formal femur samples, shaft and basal fracture groups did correlate with the Singh Index grade. The intracapsular, extracapsular and shaft fractures occurred in very separate distinct groupings, and all the male bones studied produced shaft fractures. Trabecular bone volume estimates did not correlate with the Singh Index, fracture site or tensile strength, which may be explained by the localized nature of the sections taken. Biopsies being taken from Ward’s Triangle itself or from one of the residual bands of trabeculae may have artificially lowered or elevated TBV. It is also possible that the TBV may not clearly correlate with tensile stress if there is variation in cortical bone thickness. Finally, sample size was limited and may restrict generalizability to the population at large. Follow-up experimentation with more widespread histology, under image guidance, with corresponding regional BMD correlation, should be expected to produce results correlating more closely with both the Singh Index and the contralateral bone. Identification of at-risk populations in a primary care setting is critical; strategies currently employed for the detection of osteoporosis include dietary and lifestyle questionnaires, medical history review, and screening for prior bone trauma. A cross-sectional study of women referred for densitometry revealed that a loss in height, controlling for age and weight, of at least 2 inches is predictive of a greater than fourfold increase in risk of hip osteoporosis [29]. Strategies of non-radiologic assessment for osteoporosis may prove insensitive, however. In a meta-analysis by Xu and colleagues, of women older than 35 years of age, dietary calcium levels did not appear to not affect overall risk for hip fracture, although chronic extreme deficiency or intoxication may increase risk [30]. The authors suggest, therefore, that a strategy of periodic screening and cor-
relation of radiographic imaging with functional properties (e.g., Singh Index), may prove to be an important strategy for osteoporotic fracture prevention, particularly in groups at risk for low bone mineral density. Indeed, models suggest that the Singh grade, subject weight, and thickness of the femoral sub-trochanteric region are all useful metrics for BMD [31]. It has been suggested by that computer texture analysis may serve as a surrogate for radiologist assessment in assigning a Singh grade [32], thus facilitating rapid automated screening. In patients with hip Singh grades of 4 or below, modification to daily activity regimens may be indicated to prevent stress fracture [33]. Currently, ultrasonography and DEXA scanning are the standard imaging modalities from clinical practice standpoints. DEXA, which requires testing times in the 10–30 minute range, may used to measure bone density centrally (e.g., hip or spine), or peripherally (e.g., carpals or phalanges) with lower accuracy. Hip fracture presents a significant challenge for physicians, due to the associated high morbidity, economic costs, and psychological hardship conferred by this condition. An estimated 25–35% of all patients die within 1 year of hip fracture [34], and only 30% of patients achieve recovery to functional status [35]. Further, fracture incidence is rising in North America, Africa, Europe, and Asia [36]. Current strategies for treatment of osteoporosis include anti-resorptive therapy with bisphosphonates and calcium supplementation to prevent further bone loss [37]. The use of 3-hydroxy-3-methylglutaryl coenzyme-A reductase inhibitors (statins) has been noted in observational studies of animals and humans to exert a protective effect against fracture development, but a recent meta analysis of human trials has not borne any consistently clear benefit [38]. The most significant implications of this study emerge in the realm of preventative interventional radiology. In the light of the current findings, and inspired by the success of percutaneous veterbroplasty for treatment [39–41] of osteoporotic crush fracture [42, 43], and for fracture prophylaxis [44], the authors are currently pursuing formal investigation of strategies for percutaneous hip augmentation. While the current study does not provide conclusive support for such a strategy, it certainly does suggest value in further study of this approach. Notably, the introduction of polymethylmethacrylate or a newer-generation bioactive bone cements into areas of weakness can alter biomechanical properties of bone [45]. Experimentally, the injection of PMMA into vertebral bodies results in increased fracture load under uniaxial compression [46]. Heini et al., in work that complements the findings presented here, recently demonstrated increased loading thresholds in PMMA- stabilized cadaveric femoral specimens [47]. Tests of plated and cement-fixed cadaveric humeri have shown promising biomechanical stability compared to control specimens [48]. It remains to be seen, however, whether the increased local stress-bearing dynamics produced in hip augmentation, in the setting of
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diffuse osteoporosis, may iatrogenically increase the risk of fracture elsewhere. While the evolution of vertebroplasty has a 17-year history, beginning in Europe [49], the wide range of presentday methods and materials encourages a rapid pace of development in the field of percutaneous hip augmentation. Judicious patient selection, facilitated by systematic imaging and evaluation, in tandem with continued procedural
optimization, thus offers the tantalizing potential for a reduction of morbidity or mortality in at-risk populations. Acknowledgements The authors would like to thank Professor J. B. Coakley and the Staff of University College Dublin for their advice and direction, and Professors W. S. Monkhouse, P. Kelleghan and E. Clarke for their time and assistance. Acknowledgement is also due to Margaret Fenwick for assistance in manuscript production.
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