osteoporosis is defined by the world health organization ( who ) as having a bone mineral density ( bmd ) of more than 2.5 standard deviations below the norm for healthy young adults ( 1 ) .
bmd , measured by dual - energy x - ray absorptiometry , is used as a bone mass index to predict the risk of fracture ( 1 ) .
osteoporotic bones are porous and fragile , thus making them more susceptible to fracture than healthy bones . in order to maintain a healthy skeleton ,
a steady - state of bone remodeling occurs in adults in which new bone is generated at the same rate that old bone is removed .
the remodeling process is important for maintaining bone strength because it adapts the structure of bone to mechanical loads ( 2 ) . in osteoporosis
, the rate of bone resorption exceeds that of the formation , resulting in a progressive deterioration of bone mass and microarchitecture ( 2 ) .
there are the two kinds of bone that comprise the skeleton are compact and cancellous ( i.e. trabecular ) .
compact bone predominates in the appendicular skeleton and is designed to resist bending forces . on the other hand , cancellous bone is concentrated in the axial skeleton and is structured to resist compressive forces , including gravity . since it is lighter and more porous than compact bone , cancellous bone provides more surface area for bone remodeling and is more metabolically active ( 2 ) .
bisphosphonates , including alendronate and risedronate , are antiresorptive agents which are stable analogues of inorganic pyrophosphate and have a high affinity for hydroxyapatite crystals .
they bind selectively to mineralized surfaces of bone to disrupt degradative osteoclast activity and promote apoptosis of mature osteoclasts ( 4 ) .
bisphosphonates are also thought to prolong the life of osteoblasts by inhibiting apoptosis ( 5 ) .
alendronate and risedronate are highly effective in decreasing the risk of vertebral and hip fractures ( 68 ) .
but , it was also demonstrated that the observed increase in bmd in bisphosphonate - treated patients does not entirely account for the reduction in fracture risk ( 6 ) .
in fact , it is hypothesized that bisphosphonates may act by increasing bone mass , improving trabecular microarchitecture , and/or increasing secondary mineralization of the calcified matrix ( 9 ) .
recently , more emphasis has been placed on the concept of  bone quality  which encompasses the microarchitectural and biomechanical properties of bone ( 10 , 11 ) .
it is estimated that osteoporosis is responsible for about 300 000 hip fractures per year in the united states ( 12 ) .
recent studies have shown that hip - fractured patients rapidly lose bone density within 6 months after fracture , possibly because of the absence of weight bearing for 6 weeks
post - operatively and the fact that peri - prosthetic bone loss rapidly occurs following hip arthroplasty ( 13 ) .
since there is no standard of care for the prevention and the treatment of osteoporosis in these patients , our interest is to determine the best approach to prevent bone loss in this group .
as the effect of bisphosphonates on the femoral neck and surrounding areas has not yet been assessed , we will analyze the bone quality of proximal femurs from treated patients that are discarded after hip replacement surgery . prior to undertake these clinical trials , however , protocols must be established in an untreated population in order to properly assess the quality of bone .
this project aims to determine whether imaging and compression testing of cancellous bone from the femoral neck can accurately assess the bone s microarchitectural and biomechanical properties , respectively , and if together they will serve as a good index of bone quality .
twenty - two bovine ( calf ) femurs were obtained from a butcher ( boucherie charcuterie rivire des prairies , montreal , qc ) .
william fisher ( montreal general hospital , montreal , qc ) from total hip replacement orthopedic surgeries .
a rochester bone biopsy trephine ( item # 87400 - 000 , gauthier medical inc . ,
rochester , mn ) 7.5 mm in diameter was used to extract cylindrical cores of trabecular bone from the centre of the femoral necks .
both ends of the cores were cut at right angles to the long axis in order to minimize shearing during biomechanical compression testing ( 15 ) .
the dimensions of the trabecular bone cores were 7.5 mm in diameter by 14 mm in length , thereby maintaining a valid aspect ratio between 1.5 and 2 ( 16 ) .
the trabecular bone cores were scanned using a lunar piximus densitometer ( lunar corp , madison , wi ) at the bone centre ( mcgill university , montreal , qc ) to measure the bone mineral density ( bmd ) for a region of interest inside the specimen ( 11.3 mm x 5 mm ) .
they were subsequently scanned with a microcomputed tomography ( ct ) instrument ( model 1072 , skyscan , aartselaar , belgium ) at the bone centre ( mcgill university , montreal , qc ) .
the cross - sections along the specimen axis were reconstructed using cone - beam reconstruction software ( skyscan ) , with a distance between each cross - section of 18.25m .
the ct - analyser software ( skyscan ) was used to analyze the trabecular bone of the sample using segmentation method .
specimens were mounted on the testing machine ( instron 5569 , buckinghamshire , uk ) so that the long axes of the cylinders were perpendicular to the lower anvil .
a compressive force was then applied to the upper surface of the samples using a 50kn load cell at a constant crosshead displacement rate of 1 mm / min until failure occurred . the compressive load and the sample length
were recorded at 0.1 second intervals during compression . using this data , the stress ( force / area ) and strain ( length / original length ) were calculated and plotted against one another .
the elastic modulus , an indication of the extent of compressive deformation in a specimen for a given load , was obtained from the linear portion of the stress versus strain curve ( 15 ) .
initially , it was imperative to demonstrate that the protocol used to retrieve cylindrical cores of cancellous bone would provide valid samples for biomechanical testing .
twenty two cores of cancellous bone were extracted from the femoral neck of calf femurs and subjected to compression testing .
the compressive strength and the elastic modulus were calculated for each sample and plotted against each other .
it should be noted that the compressive strength is a measure of the stress within a sample immediately before the yielding point , while the elastic modulus is a measure of the rigidity of a sample . in concordance with previous findings
, the compressive strength correlates strongly to the modulus ( r = 0.98 ) in the calf femur samples ( see figure 1 ) ( 18 ) .
furthermore , these parameters fall within the range of published bovine values ( 19 ) . upon validating our technique ,
the same procedure was applied to the human femoral neck samples obtained from hip replacement surgeries .
comparable to the bovine samples , the compressive strength and the modulus of the human samples are strongly correlated ( r = 0.99 ) , as demonstrated in figure 2 .
moreover , the values for these parameters fall within the range of published human values for the femoral neck ( 20 ) .
prior to subjecting the human cancellous samples to biomechanical testing , they were scanned using three dimensional ct and densitometry .
as demonstrated in figure 3 , the osteoporotic bone appears much more porous than the unaffected bone .
the compressive strength was plotted against the trabecular thickness , number and separation , as shown in figure 4 .
the latter parameters failed to show any statistically significant relationship to the biomechanics of bone .
the trabecular thickness and separation remained fairly constant despite the strength of the bone , whereas the number of trabeculae appeared to increase with the compressive strength although not well .
needless to say , the trabecular data proved to be poor indicators of bone quality .
further investigation of the imaging data revealed that the relative porosity , calculated as the complement of the percent bone volume ( i.e. 1 - % bone volume ) , and the bmd were the best predictors of bone quality .
in fact , the relative porosity and the bmd correlated strongly to one another ( see figure 5 , r = 0.97 ) . upon analyzing the data , the compressive strength and the modulus were shown to vary exponentially with relative porosity and the bmd .
that is to say , the strength and the modulus of the samples increased at a faster rate as the relative porosity decreased or the bmd increased .
thus , the latter were plotted against the natural logarithms of the compressive strength and the modulus for each bone sample ( see figures 69 ) .
the relative porosity and the bmd correlated equally well to the compressive strength and the modulus ( ln(compressive strength ) vs. relative porosity , r = 0.90 ; ln(modulus ) vs. relative porosity , r = 0.88 ; ln(compressive strength ) vs. bmd , r = 0.94 ; ln(modulus ) vs. bmd , r = 0.92 ) .
the assessment of bone quality is of the utmost importance in osteoporotic patients as it correlates well to the risk of fracture ( 1 ) and indicates when it is appropriate to initiate the proper preventative measures ( 68 ) .
bisphosphonates have proven to be quite successful in deterring the progression of osteoporosis ( 6 ) .
recently , there have been trials that have studied the effect of risedronate in osteoporotic patients using ct ( 11 ) .
these studies have indirectly assessed the drug s effect on bone quality using a remote site ( i.e. the iliac crest ) rather than the site of interest ( e.g. vertebrae , femoral neck ) . understandably , the analysis of the iliac crest is convenient because it is easily accessible from the surface and poses minimal risk to the patient .
there has been , however , no evidence to indicate whether any correlation exists between the microarchitecture at the iliac crest and the site of interest . to our knowledge ,
the effects of bisphosphonates on bone quality have not yet been studied in humans at sites than at the iliac crest . prior to embarking on a hip fracture study
, we wanted to establish reliable and reproducible procedures for biomechanical testing to ensure that the data were valid and representative of bone quality . intuitively , bone quality should diminish as the bone becomes more porous and/or less dense with mineral .
indeed , we found the relative porosity and the bmd to be equally strong indicators of bone quality as both of these parameters correlate strongly to the biomechanical properties of bone , though in opposite fashion ( see figures 69 ) .
a potential limitation to this experiment is the inherent design flaw that results from the extraction of trabecular bone . as the cylindrical core of bone
is removed from the sample , the cross struts from the trabeculae along the periphery are severed .
this unavoidably compromises structural integrity , especially since the compact bone significantly contributes to the overall strength of the femur ( 21 ) .
hence , the biomechanical studies will not be entirely representative of the properties of trabeculae within the bone . however , taking into consideration that all the samples are prepared identically before testing , the data analysis remains valid .
we have successfully established a protocol that will be used in a trial setting to analyze the bone quality in human femoral necks using ct and biomechanical compression testing . using cores of cancellous bone from human femoral necks , the relative porosity and the bmd
were shown to be excellent predictive markers of fracture resistance . as this trial evolves in its early stages ,
additional samples collected from hip replacements will be tested and the data compiled to construct a reference database for comparison to bisphosphonate treatment cohorts .
 it is estimated that osteoporosis is responsible for about 300 000 hip fractures per year in the united states . 
 effective prevention of these fractures has been demonstrated using bisphosphonates . 
 however , their mechanism of action has not been elucidated . 
 furthermore , the precise effect of bisphosphonates on the femoral neck and surrounding areas has never been studied . 
 we are interested in establishing a protocol to analyze the bone quality of proximal femurs from patients treated with bisphosphonates . 
 following hip replacement surgery , the aim is to determine whether imaging and compression testing of cancellous bone from the discarded femoral necks can accurately assess the bone s microarchitectural and biomechanical properties , respectively . to validate the technique , it was first tested on an untreated population . 
 a bone biopsy trephine was used to extract cylindrical cores of trabecular bone from the centre of femoral necks . 
 densitometry , microcomputed tomography , and compression testing were used to assess the quality of bone in these samples . 
 the compressive strength was found to be directly proportional to the modulus ( i.e. stiffness ) of the samples , thus reproducing previous findings . the relative porosity and , to a lesser extent , the bone mineral density were capable of predicting the quality of cancellous bone . in conclusion , a protocol to analyze the bone quality in human femoral necks using ct and biomechanical compression testing was successfully established . 
 it will be applied in a clinical setting to analyze bones from bisphosphonate - treated patients following total hip replacement . 