research interest is increasing in biomaterials with the potential to stimulate bone healing have gained interest to improve treatment outcome for patients with osteoporotic fractures . calcium phosphate cements ( cpc ) has been used in bone surgery for many years . the bio - inspired combination of the inorganic component silica and the organic component collagen has lead to a novel class of innovative biomaterials because silica and collagen turned out to be ideal partners for composite formation , resulting in advantageous synergistic effects in material properties suitable for soft - tissue or hard - tissue substitution applications . the chemical and technological flexibility of the silica / collagen system has generated a remarkably wide range of biomaterials individually adapted for a large number of biomedical applications , including soft hydrogels , macroposous scaffolds , and solid compact xerogels . in addition , metal foams open new opportunities for design of bone implants due to their mechanically stability . in cardiovascular applications , iron has been used successfully in animal implantation studies and in clinical cases . the use of pet scanning as a diagnostic tool for fracture healing is attractive because it can provide a direct quantitative assessment of metabolic activity in a region of interest . we recently showed that the fixation of fluoride in the bone matrix may be enhanced using cpc , especially cpc with strontium carbonate as biomaterial for bone replacement and defect healing in osteoporotic bone . the present study was designed to examine the efficacy of multifunctional bioabsorbable implants , including cpc - based , silica / collagen - based , and iron - based composites for the treatment of experimental osteoporotic fractures . the treatment response was assessed by f-18-sodium fluoride ( naf ) dynamic pet - ct ( dpet - ct ) . all animal procedures were carried out in accordance with the guide for care and use of laboratory animals of the national institutes of health . the animal studies were approved by the corresponding authority ( regierungsprsidium gieen , gi 20/28 nr . the study included 65 3-month - old female sprague - dawley rats ( charles rivers wiga , sulzbach , germany ) with a body weight between 250 and 400 g. after an acclimatization of 4 weeks , an osteoporotic bone status was induced by bilateral ovariectomy and a calcium- , phosphorous- , vitamin d3- , soy- , and phytoestrogen - free diet , as previously described by heiss et al . . after 3 months , the animals underwent femur surgery under general anaesthesia with xylazine ( 4 mg / kg body weight , rompun , bayer ) and ketamine ( 100 mg / kg body weight , hostaket , hoechst ) by intraperitoneal injection . the left femur was desinfected with povidone iodine ( braunol , braun , melsungen , germany ) and draped in a sterile manner . after making a 4-cm skin incision at the lateral aspect of the distal thigh , the lateral vastus muscle and the lateral head of the femoral biceps muscle were dissected in their septum intermusculare and the femur from shaft to the condyle massif was exposed . subsequently , a 7-hole t - shaped miniplate ( leibinger xs - miniplate , stryker , schnkirchen , germany ) was slightly bent and fixed to the lateral femur with 1.2-mm screws . afterwards , a wedge - shaped defect with a lateral height of 4 mm was then created at the metaphyseal area of the femur . the cutting lines were marked with a custom - made triangular sawing guide with a lateral length of 4 mm . the distal transverse osteotomy was made just above the border of the femoropatellar joint , without bruising the articular cartilage . calcium phosphate cement ( cpc ) and cpc enriched with strontium ( srcpc ) were prepared as described previously . in brief , cpc powder was composed of -tricalcium phosphate ( -ca3(po4)2 ) , calcium hydrogen phosphate ( cahpo4 ) , calcium carbonate ( caco3 ) , and hydroxyapatite ( ca10(po4)6(oh)2 ) , and was supplied by innotere gmbh ( radebeul , germany ) . srcpc powder was obtained by substitution of caco3 by srco3 ( alfa aesar ) in the cpc cement precursor powder . a mouldable paste was obtained by mixing the cement powder with na2hpo4 solution using a liquid - to - powder ratio of 500 and 350 l / g for cpc and srcpc , respectively , immediately prior to implantation . srcpc was previously shown to release significant amounts of sr upon immersion in aqueous media ( e.g. , cell culture medium ) and to positively influence both in vitro osteoblast proliferation and differentiation and in vivo bone formation ( 15 ) . composites of silica and fibrillar bovine collagen were used for implantation as monolithic xerogels ( b30 , 70 wt% silica , 30 wt% collagen ) or as porous scaffold ( sc - b30 , xerogel particles b30 , size < 250 m , embedded in a collagen matrix with xerogel / matrix weight ratio of 1.0 ) . for preparation of compact silica / collagen xerogels , bovine tropocollagen type i ( gfn , germany ) was dialysed ( nominal molecular weight cut - off 1214 kda , roth , germany ) against deionized water followed by fibrillation in 30-mm neutral sodium phosphate buffer solution , lyophilisation ( christ alpha 14 laboratory freeze - dryer , osterode , germany ) , and resuspension in 0.1 m trishcl ph 7.4 ( roth ) to obtain homogeneous 30 mg / ml suspensions . the silica component was prepared by hydrolysing tetraethoxysilane ( teos , 99% , sigma , germany ; molar ratio teos / water=1/4 ) under acidic conditions ( 0.01 m hcl ) to obtain silicic acid . vigorous stirring of calculated volumes of silicic acid and collagen suspension to obtain a final composition of 70% silica and 30% collagen ( b30 ) resulted in the formation of 800-l hydrogels . a modification of the scaffolds was prepared by using xerogel particles consisting of 50 wt% silica , 30 wt% fibrillar bovine collagen , and 20 wt% strontium carbonate ( sc - b30sr20 ) . strontium carbonate was introduced as a third phase by being previously added to the collagen suspension . b30 or b30sr20 gels were transferred to molds , stabilized , and dryed for 7 days in an espec sh-221 climate chamber ( japan ) at 37c and 95% relative humidity . the resulting xerogel samples were ground and classified according to the particle size . on the one hand , b30 xerogel particles < 0.250 mm were compacted using a custom - made pressing tool to obtain monolithic b30 xerogel samples exhibiting exactly the above - described bone defect shape . on the other hand , b30 or b30sr20 xerogel particles < 0.120 mm were added to a 30-mg / ml collagen suspension adjusting a xerogel / collagen weight ratio of 1/1 . these xerogel particle / collagen suspensions were transferred to custom - made silicon molds to again obtain the above - described bone defect shape , cooled at 0.5 k / min to 20c ( espec sh-221 climate chamber , osaka , japan ) followed by lyophilisation ( christ alpha 14 laboratory freeze - dryer , osterode , germany ) . the scaffolds were chemically cross - linked by immersing in 1 wt% n-(3-dimethylaminopropyl)-n0-ethylcarbodiimide ( edc)/n - hydroxysuccinimide ( nhs ) ( sigma ) in 40% ethanol for 24 h. finally , the scaffolds were rinsed in deionized water and freeze - dried again . according to the xerogel particle composition used , scaffold sample labels were scb30 or scb30sr20 , respectively . all xerogel or scaffold samples were gamma - sterilized at 25 kgy before used for implantation . moreover , iron foam with interconnected pores ( fe ) was applied . for preparation of strontium coating of iron foams , 153 ml of ultrapure water and 17 ml of phosphoric acid ( 85% h3po4 ) were added in a 250-ml glass bottle . the supernatant was diluted 20-fold using ultrapure water , which yielded the solution of sr(h2po4)2 . ten fe foams eroded and cleaned by ifam ( fraunhofer institute for manufacturing technology and advanced materials , dresden ) with an average weight of 170 mg were coated by suspension in 100 ml of sr(h2po4)2 solution for 4 h under vacuum . sr - coated foams were settled in fresh coating solution for another 3 days at room temperature and rinsed 3 times using absolute ethanol with subsequent drying at 40c in a drying cabinet . sr - coating resulted in an average weight gain of 37 mg or approximately 22% . in addition , the alloy element phosphor is added with fe3p - powder ( atmix , japan ) with a particle size of 3.8 m . to achieve a phosphate ratio of 0.6% the iron foam was coated with zolendronic acid ( fe - bp ) that is a member of the bisphosphonate family . . then the complex was carefully washed , dried , ground , and finally led to a coating of 35-g zolendronic acid , the basic iron foam . these 8 different formulations of bone substitution materials were implanted in the current animal model ( n=78 ) ( table 1 ) . eighteen weeks after induction of osteoporosis and 6 weeks after femoral surgery , dpet - ct with f - sodium fluoride ( naf ) scans were performed . during scanning , rats were anesthetized using a mixture of nitrous oxide ( 1 l / min ) , oxygen ( 0.5 l / min ) and isoflurane ( 1.5 vol.% ) . dpet - ct studies were performed for 60 min after the intravenous application of 20 to 40 mbq f - sodium fluoride , using a 28-frame protocol ( 10 frames of 30 s , 5 frames of 60 s , 5 frames of 120 s , and 8 frames of 300 s ) . a dedicated pet - ct system ( biograph mct , 128 s siemens co , erlangen , germany ) with an axial field of view of 21.6 cm with truev and truepoint , operated in a 3-dimensional mode , was used for all animal studies . the system provides the simultaneous acquisition of 369 transverse pet slices with a slice thickness of 0.6 mm . two rats were examined in parallel and were positioned in the axial plane of the system to maintain the optimum resolution in the center of the system . a high - resolution ct scan was performed prior the pet scanning with 160 ma , 80 kv , pitch of 0.85 cm in addition to the low - dose attenuation ct ( 80 ma , 80 kv ) for attenuation correction of the acquired dynamic emission pet data . all pet images were attenuation - corrected and an image matrix of 400400 pixels was used for iterative image reconstruction ( voxel size 1.5651.5650.6 mm ) based on the syngo mi pet / ct 2009c software version . the reconstructed images were converted to standardized uptake value ( suv ) images based on the formula ( 18 ) : suv = tissue concentration ( bq / g)/[injected dose ( bq)/body weight ( g ) ] . volumes of interest were placed over the created defect area in the femur , and rotated pet images according to the ct images . irregular regions of interest were drawn manually according to the fused pet - ct images , as mentioned before . to precisely select the region to draw the voi of the created defect , the fusion of pet images and ct scans by co - registration preprocessing was performed after rotation of each rat individually . first , we rotated the ct series to determine the optimum angle for the visualisation of the complete defect and the adjacent metal plates in the longitudinal direction . the next step was to fuse ct and pet and to use these fused images for the quantitative pet evaluation . the spatial resolution was 2 mm for pet image and 0.33 mm for ct image . the voi for the defect was placed between the 2 screws located at the left and the right side of the 4-mm created defect . for input , we used 10 contiguous pet slices in the middle and lower third of the aorta . we avoided using the upper part of the thoracic aorta due to spillover from the heart . herein , a 2-tissue compartment model was used to evaluate the dynamic studies and to gain quantitative data about the kinetics . in the current study , the learning - machine 2-tissue compartment model was used for the fitting , and provided 5 parameters : the plasma clearance to the bone extracellular fluid ( ecf ) compartment and the rate constant for return of tracer to plasma ; k1 and k2 ; the rate constants describing movement of tracer into and out of the bound bone compartment ; k3 and k4 ; and the fractional blood volume , also called vessel density ( vb ) , which reflects the amount of blood in the voi . following compartment analysis , we calculated the plasma clearance of tracer to bone mineral from the compartment data using the formula : influx ( ki)=(k1*k3)/(k2+k3 ) . compared to the standard iterative method , the machine learning method has the advantage of a fast convergence and avoidance of over - fitting . the model parameters were accepted when k1k4 was less than 1 and vb exceeded 0 . the fractal dimension is a parameter of heterogeneity and was calculated for the time - activity data of each individual volume of interest . the values for fractal dimension vary from 0 to 2 , showing the deterministic or chaotic distribution of tracer activity . we used a subdivision of 77 and a maximal suv of 20 for the calculation of fractal dimension . statistical evaluation was performed with stata / se 11.1 ( statacorp , college station , tx , usa ) using the descriptive statistics and box plots . linear discriminant analysis was performed for calculating the sensitivity , specificity , positive predictive value ( ppv ) , negative predictive value ( npv ) and highest accuracy . the 2-sided wilcoxon rank sum test was applied for all pet parameters , which also included suv , using a single parameter analysis to assess groups . all animal procedures were carried out in accordance with the guide for care and use of laboratory animals of the national institutes of health . the animal studies were approved by the corresponding authority ( regierungsprsidium gieen , gi 20/28 nr . the study included 65 3-month - old female sprague - dawley rats ( charles rivers wiga , sulzbach , germany ) with a body weight between 250 and 400 g. after an acclimatization of 4 weeks , an osteoporotic bone status was induced by bilateral ovariectomy and a calcium- , phosphorous- , vitamin d3- , soy- , and phytoestrogen - free diet , as previously described by heiss et al . . after 3 months , the animals underwent femur surgery under general anaesthesia with xylazine ( 4 mg / kg body weight , rompun , bayer ) and ketamine ( 100 mg / kg body weight , hostaket , hoechst ) by intraperitoneal injection . the left femur was desinfected with povidone iodine ( braunol , braun , melsungen , germany ) and draped in a sterile manner . after making a 4-cm skin incision at the lateral aspect of the distal thigh , the lateral vastus muscle and the lateral head of the femoral biceps muscle were dissected in their septum intermusculare and the femur from shaft to the condyle massif was exposed . subsequently , a 7-hole t - shaped miniplate ( leibinger xs - miniplate , stryker , schnkirchen , germany ) was slightly bent and fixed to the lateral femur with 1.2-mm screws . afterwards , a wedge - shaped defect with a lateral height of 4 mm was then created at the metaphyseal area of the femur . the cutting lines were marked with a custom - made triangular sawing guide with a lateral length of 4 mm . the distal transverse osteotomy was made just above the border of the femoropatellar joint , without bruising the articular cartilage . calcium phosphate cement ( cpc ) and cpc enriched with strontium ( srcpc ) were prepared as described previously . in brief , cpc powder was composed of -tricalcium phosphate ( -ca3(po4)2 ) , calcium hydrogen phosphate ( cahpo4 ) , calcium carbonate ( caco3 ) , and hydroxyapatite ( ca10(po4)6(oh)2 ) , and was supplied by innotere gmbh ( radebeul , germany ) . srcpc powder was obtained by substitution of caco3 by srco3 ( alfa aesar ) in the cpc cement precursor powder . a mouldable paste was obtained by mixing the cement powder with na2hpo4 solution using a liquid - to - powder ratio of 500 and 350 l / g for cpc and srcpc , respectively , immediately prior to implantation . srcpc was previously shown to release significant amounts of sr upon immersion in aqueous media ( e.g. , cell culture medium ) and to positively influence both in vitro osteoblast proliferation and differentiation and in vivo bone formation ( 15 ) . composites of silica and fibrillar bovine collagen were used for implantation as monolithic xerogels ( b30 , 70 wt% silica , 30 wt% collagen ) or as porous scaffold ( sc - b30 , xerogel particles b30 , size < 250 m , embedded in a collagen matrix with xerogel / matrix weight ratio of 1.0 ) . for preparation of compact silica / collagen xerogels , bovine tropocollagen type i ( gfn , germany ) was dialysed ( nominal molecular weight cut - off 1214 kda , roth , germany ) against deionized water followed by fibrillation in 30-mm neutral sodium phosphate buffer solution , lyophilisation ( christ alpha 14 laboratory freeze - dryer , osterode , germany ) , and resuspension in 0.1 m trishcl ph 7.4 ( roth ) to obtain homogeneous 30 mg / ml suspensions . the silica component was prepared by hydrolysing tetraethoxysilane ( teos , 99% , sigma , germany ; molar ratio teos / water=1/4 ) under acidic conditions ( 0.01 m hcl ) to obtain silicic acid . vigorous stirring of calculated volumes of silicic acid and collagen suspension to obtain a final composition of 70% silica and 30% collagen ( b30 ) resulted in the formation of 800-l hydrogels . a modification of the scaffolds was prepared by using xerogel particles consisting of 50 wt% silica , 30 wt% fibrillar bovine collagen , and 20 wt% strontium carbonate ( sc - b30sr20 ) . strontium carbonate was introduced as a third phase by being previously added to the collagen suspension . b30 or b30sr20 gels were transferred to molds , stabilized , and dryed for 7 days in an espec sh-221 climate chamber ( japan ) at 37c and 95% relative humidity . the resulting xerogel samples were ground and classified according to the particle size . on the one hand , b30 xerogel particles < 0.250 mm were compacted using a custom - made pressing tool to obtain monolithic b30 xerogel samples exhibiting exactly the above - described bone defect shape . on the other hand , b30 or b30sr20 xerogel particles < 0.120 mm were added to a 30-mg / ml collagen suspension adjusting a xerogel / collagen weight ratio of 1/1 . these xerogel particle / collagen suspensions were transferred to custom - made silicon molds to again obtain the above - described bone defect shape , cooled at 0.5 k / min to 20c ( espec sh-221 climate chamber , osaka , japan ) followed by lyophilisation ( christ alpha 14 laboratory freeze - dryer , osterode , germany ) . the scaffolds were chemically cross - linked by immersing in 1 wt% n-(3-dimethylaminopropyl)-n0-ethylcarbodiimide ( edc)/n - hydroxysuccinimide ( nhs ) ( sigma ) in 40% ethanol for 24 h. finally , the scaffolds were rinsed in deionized water and freeze - dried again . according to the xerogel particle composition used , all xerogel or scaffold samples were gamma - sterilized at 25 kgy before used for implantation . moreover , iron foam with interconnected pores ( fe ) was applied . for preparation of strontium coating of iron foams , 153 ml of ultrapure water and 17 ml of phosphoric acid ( 85% h3po4 ) were added in a 250-ml glass bottle . the supernatant was diluted 20-fold using ultrapure water , which yielded the solution of sr(h2po4)2 . ten fe foams eroded and cleaned by ifam ( fraunhofer institute for manufacturing technology and advanced materials , dresden ) with an average weight of 170 mg were coated by suspension in 100 ml of sr(h2po4)2 solution for 4 h under vacuum . sr - coated foams were settled in fresh coating solution for another 3 days at room temperature and rinsed 3 times using absolute ethanol with subsequent drying at 40c in a drying cabinet . sr - coating resulted in an average weight gain of 37 mg or approximately 22% . in addition , the alloy element phosphor is added with fe3p - powder ( atmix , japan ) with a particle size of 3.8 m . to achieve a phosphate ratio of 0.6% the iron foam was coated with zolendronic acid ( fe - bp ) that is a member of the bisphosphonate family . . then the complex was carefully washed , dried , ground , and finally led to a coating of 35-g zolendronic acid , the basic iron foam . these 8 different formulations of bone substitution materials were implanted in the current animal model ( n=78 ) ( table 1 ) . eighteen weeks after induction of osteoporosis and 6 weeks after femoral surgery , dpet - ct with f - sodium fluoride ( naf ) scans were performed . during scanning , rats were anesthetized using a mixture of nitrous oxide ( 1 l / min ) , oxygen ( 0.5 l / min ) and isoflurane ( 1.5 vol.% ) . dpet - ct studies were performed for 60 min after the intravenous application of 20 to 40 mbq f - sodium fluoride , using a 28-frame protocol ( 10 frames of 30 s , 5 frames of 60 s , 5 frames of 120 s , and 8 frames of 300 s ) . a dedicated pet - ct system ( biograph mct , 128 s siemens co , erlangen , germany ) with an axial field of view of 21.6 cm with truev and truepoint , operated in a 3-dimensional mode , was used for all animal studies . the system provides the simultaneous acquisition of 369 transverse pet slices with a slice thickness of 0.6 mm . two rats were examined in parallel and were positioned in the axial plane of the system to maintain the optimum resolution in the center of the system . a high - resolution ct scan was performed prior the pet scanning with 160 ma , 80 kv , pitch of 0.85 cm in addition to the low - dose attenuation ct ( 80 ma , 80 kv ) for attenuation correction of the acquired dynamic emission pet data . all pet images were attenuation - corrected and an image matrix of 400400 pixels was used for iterative image reconstruction ( voxel size 1.5651.5650.6 mm ) based on the syngo mi pet / ct 2009c software version . the reconstructed images were converted to standardized uptake value ( suv ) images based on the formula ( 18 ) : suv = tissue concentration ( bq / g)/[injected dose ( bq)/body weight ( g ) ] . volumes of interest were placed over the created defect area in the femur , and rotated pet images according to the ct images . irregular regions of interest were drawn manually according to the fused pet - ct images , as mentioned before . to precisely select the region to draw the voi of the created defect , the fusion of pet images and ct scans by co - registration preprocessing was performed after rotation of each rat individually . first , we rotated the ct series to determine the optimum angle for the visualisation of the complete defect and the adjacent metal plates in the longitudinal direction . the next step was to fuse ct and pet and to use these fused images for the quantitative pet evaluation . the spatial resolution was 2 mm for pet image and 0.33 mm for ct image . the voi for the defect was placed between the 2 screws located at the left and the right side of the 4-mm created defect . for input , we used 10 contiguous pet slices in the middle and lower third of the aorta . we avoided using the upper part of the thoracic aorta due to spillover from the heart . herein , a 2-tissue compartment model was used to evaluate the dynamic studies and to gain quantitative data about the kinetics . in the current study , the learning - machine 2-tissue compartment model was used for the fitting , and provided 5 parameters : the plasma clearance to the bone extracellular fluid ( ecf ) compartment and the rate constant for return of tracer to plasma ; k1 and k2 ; the rate constants describing movement of tracer into and out of the bound bone compartment ; k3 and k4 ; and the fractional blood volume , also called vessel density ( vb ) , which reflects the amount of blood in the voi . following compartment analysis , we calculated the plasma clearance of tracer to bone mineral from the compartment data using the formula : influx ( ki)=(k1*k3)/(k2+k3 ) . compared to the standard iterative method , the machine learning method has the advantage of a fast convergence and avoidance of over - fitting . the model parameters were accepted when k1k4 was less than 1 and vb exceeded 0 . the unit for the rate constants k1k4 was 1/min . besides the compartmental analysis , a noncompartmental model based on the fractal dimension was used . the fractal dimension is a parameter of heterogeneity and was calculated for the time - activity data of each individual volume of interest . the values for fractal dimension vary from 0 to 2 , showing the deterministic or chaotic distribution of tracer activity . we used a subdivision of 77 and a maximal suv of 20 for the calculation of fractal dimension . statistical evaluation was performed with stata / se 11.1 ( statacorp , college station , tx , usa ) using the descriptive statistics and box plots . linear discriminant analysis was performed for calculating the sensitivity , specificity , positive predictive value ( ppv ) , negative predictive value ( npv ) and highest accuracy . the 2-sided wilcoxon rank sum test was applied for all pet parameters , which also included suv , using a single parameter analysis to assess groups . we have reported that k3 was the most sensitive pet parameter for the characterization of healing process and revealed the best differentiation for the empty defect and the cpc group , as well as the srcpc , for the defect region . however , no pet parameter demonstrated a significant difference between the cpc and srcpc group in the defect region . in this study , the volume of interest ( voi ) of the biomaterial - bone interface was selected for evaluation ( figure 1 ) . k3 showed a statistically significant increase in the bone formation for the srcpc group compared to the cpc group ( p=0.018 ) in the biomaterial - bone interface ( figure 2 ) . three types of the composites were used : monolithic xerogels ( b30 ) and porous scaffolds ( sc - b30 and sc - b30sr20 ) . comparing the pet parameters , we could not detect significant changes between the 3 composites and the empty defect for both rois . only k3 in the sc - b30 group exhibited a significant increase in comparison to the b30 group in the defect region ( figure 3 , p=0.022 ) . three different formulations of the iron foam were used as implants in the different rat groups : a ) plain iron foam ( fe ) , b ) strontium functionalized iron foam ( fe - s ) , c ) iron foam with coating of the bisphosphonate zoledronic acid ( fe - bp ) . no significant differences were found in the pet parameters between the 3 implants and the empty defect for both rois . however , there was a significant increase of ki in the fe - bp group as compared with the fe group for the defect region ( figure 4 , p=0.037 ) . a multiparameter analysis was performed to find differences between the 2 different groups based on the dpet - ct data . the aim was to identify a combination of variables that best discriminate each group of rats with different biomaterials in the defect region . an overall accuracy of > 75% was achieved for the combination of some kinetic parameters between the 2 different groups . furthermore , all sensitivity and specificity values from the multiparameter analysis were high ( > 71% ) ( table 2 ) . the highest overall accuracy of 78.6% was achieved for k3 between the cpc and srcpc group based on the biomaterial - bone interface . the positive predictive value ( npv ) and the negative predictive value ( ppv ) were 85.7% and 71.4% , respectively . therefore , 6/7 rats with cpc in the defect and as 5/7 rats with srcpc in the defect could be correctly classified based on the k3 as measured . between the sc - b30 and sc - b30sr group , the highest overall accuracy of 86.7% was achieved for a combination of k1 , k3 , ki , and suv . we could correctly classify 7/8 rats with sc - b30 in the defect ( npv of 86.3% ) and 6/7 rats with sc - b30sr ( ppv of 85.7% ) . between the sc - b30 and b30 group , the highest overall accuracy of 92.3% was achieved for a combination of k3 and ki . we could correctly classify 5/6 rats with b30 in the defect ( npv of 83.3% ) and 7/7 rats with sc - b30 ( ppv of 100% ) . between the sc - b30sr20 and b30 group , the highest overall accuracy of 78.6% was achieved for a combination of k3 and vb . we could correctly classify 3/6 rats with b30 in the defect ( npv of 50.0% ) and 8/8 rats with sc - b30sr20 ( ppv of 100% ) . between the fe and fe - s group , we could correctly classify 6/8 rats with fe - s in the defect ( npv of 75.0% ) and 5/7 rats with fe ( ppv of 71.4% ) . between the fe and fe - bp groups , the highest overall accuracy of 80.0% we could correctly classify 7/8 rats with fe - bp in the defect ( npv of 86.3% ) and 5/7 rats with fe ( ppv of 71.4% ) . between the fe - s and fe - bp , the highest overall accuracy of 75.0% was achieved for a combination of vb , k1 , k3 , ki , and suv . we could correctly classify 6/8 rats with fe - bp in the defect ( npv of 75.0% ) as well as 6/8 rats with fe ( ppv of 75.0% ) . we have reported that k3 was the most sensitive pet parameter for the characterization of healing process and revealed the best differentiation for the empty defect and the cpc group , as well as the srcpc , for the defect region . however , no pet parameter demonstrated a significant difference between the cpc and srcpc group in the defect region . in this study , the volume of interest ( voi ) of the biomaterial - bone interface was selected for evaluation ( figure 1 ) . k3 showed a statistically significant increase in the bone formation for the srcpc group compared to the cpc group ( p=0.018 ) in the biomaterial - bone interface ( figure 2 ) . three types of the composites were used : monolithic xerogels ( b30 ) and porous scaffolds ( sc - b30 and sc - b30sr20 ) . comparing the pet parameters , we could not detect significant changes between the 3 composites and the empty defect for both rois . only k3 in the sc - b30 group exhibited a significant increase in comparison to the b30 group in the defect region ( figure 3 , p=0.022 ) . three different formulations of the iron foam were used as implants in the different rat groups : a ) plain iron foam ( fe ) , b ) strontium functionalized iron foam ( fe - s ) , c ) iron foam with coating of the bisphosphonate zoledronic acid ( fe - bp ) . no significant differences were found in the pet parameters between the 3 implants and the empty defect for both rois . however , there was a significant increase of ki in the fe - bp group as compared with the fe group for the defect region ( figure 4 , p=0.037 ) . a multiparameter analysis was performed to find differences between the 2 different groups based on the dpet - ct data . the aim was to identify a combination of variables that best discriminate each group of rats with different biomaterials in the defect region . an overall accuracy of > 75% was achieved for the combination of some kinetic parameters between the 2 different groups . furthermore , all sensitivity and specificity values from the multiparameter analysis were high ( > 71% ) ( table 2 ) . the highest overall accuracy of 78.6% was achieved for k3 between the cpc and srcpc group based on the biomaterial - bone interface . the positive predictive value ( npv ) and the negative predictive value ( ppv ) were 85.7% and 71.4% , respectively . therefore , 6/7 rats with cpc in the defect and as 5/7 rats with srcpc in the defect could be correctly classified based on the k3 as measured . between the sc - b30 and sc - b30sr group , the highest overall accuracy of 86.7% was achieved for a combination of k1 , k3 , ki , and suv . we could correctly classify 7/8 rats with sc - b30 in the defect ( npv of 86.3% ) and 6/7 rats with sc - b30sr ( ppv of 85.7% ) . between the sc - b30 and b30 group , the highest overall accuracy of 92.3% was achieved for a combination of k3 and ki . we could correctly classify 5/6 rats with b30 in the defect ( npv of 83.3% ) and 7/7 rats with sc - b30 ( ppv of 100% ) . between the sc - b30sr20 and b30 group , the highest overall accuracy of 78.6% was achieved for a combination of k3 and vb . we could correctly classify 3/6 rats with b30 in the defect ( npv of 50.0% ) and 8/8 rats with sc - b30sr20 ( ppv of 100% ) . between the fe and fe - s group , we could correctly classify 6/8 rats with fe - s in the defect ( npv of 75.0% ) and 5/7 rats with fe ( ppv of 71.4% ) . between the fe and fe - bp groups , the highest overall accuracy of 80.0% we could correctly classify 7/8 rats with fe - bp in the defect ( npv of 86.3% ) and 5/7 rats with fe ( ppv of 71.4% ) . between the fe - s and fe - bp , the highest overall accuracy of 75.0% was achieved for a combination of vb , k1 , k3 , ki , and suv . we could correctly classify 6/8 rats with fe - bp in the defect ( npv of 75.0% ) as well as 6/8 rats with fe ( ppv of 75.0% ) . in this study , 3 different classes of biomaterials , including cpc - based implants , composites of silica , and collagen and iron foam implants , were investigated using dpet - ct after they were implanted in 4-mm created rat femoral defects . cpc is a well - defined , hydroxyapatite - forming cement with a formulation based on -tricalcium phosphate as initially described by driessens et al . . srcoc is a strontium - enriched modification thereof , in which caco3 was substituted with srco3 during cement precursor fabrication . this srcpc was recently shown to enhance osteoblast precursor proliferation and osteogenic differentiation in vitro as well as to support new bone formation in vivo . the 4-mm defect represented a critical - size defect with a fracture gap , which has been proven to be a good model to investigate potential bone enhancement effects of biomaterials . our data demonstrate differences in the tracer kinetics within the region of the 4-mm created defect , which includes new bone formation in the defects filled with biomaterials . for f - sodium fluoride , pet parameter k3 describes the formation of fluoroapatite , which is indicative of new bone formation . both cpc and srcpc revealed statistically significant enhanced new bone formation as compared to the empty defect in the defect region . however , no significant difference between the 2 different biomaterials in the defect region was detected . recently , ventura et al . reported pet - suv only indicated the speed of new bone formation but not the absolute amount by correlation of pet - ct and histology . therefore , it might be possible that pet in our study also describes the speed of new bone formation . bone healing is a dynamic process over time , and the use of a single time point is a limitation of this study . we are considering a series of measurements in further related studies , as part of an ongoing project . in addition , in this study , srcpc - treated animals exhibited a statistically significant higher rate of new bone formation at the biomaterial - bone interface , suggesting that a considerable amount of strontium is released from the srcpc into the interface region and into the surrounding tissue , which are most likely related to enhanced bone formation . recently , andersen et al . revealed that local delivery of strontium from surface - functionalized titanium implants could increase bone - to - implant interaction for implants in the femoral shaft of healthy female wister rats . demonstrated a high sr concentration in the interface region of the srcpc implant by tof - sims , which is in line with our findings . it may be concluded that strontium is released into the local milieu of osseointegrating implants , enhancing bone ingrowth into the implant surface . bone tissue has a highly nano - hierarchical structure , consisting mainly of collagen type i fibres and nano hydroxyapatite crystals as the matrix . scaffolds based on polymeric materials act as temporary templates for bone regeneration and actively stimulate vascularized bone growth by improving material - tissue interaction due to the porous form of the biomaterial structure . our results demonstrate significantly higher k3 values of f - sodium fluoride in the defect of rats filled with porous silicate / collagen ( sc - b30 ) than with silicate / collagen ( b30 ) ( figure 3 ) , which allows us to conclude that the formation of fluoroapatite , and therefore the new bone formation , is enhanced in the porous form of the biomaterial structure rather than in the solid form . however , a non - significant increase of k3 in the sc - b30sr20 group was detected as compared with the b30 group . it is uncertain if this is caused by a lack of statistical power or by a greater degree of tissue heterogeneity than can be explained by a 2-tissue compartmental model . the efficacy of bisphosphonates in reducing bone resorption has been clinically estimated using biochemical indices of bone turnover . reported a highly significant correlation ( r=0.84 ) between ki and bone formation rate assessed by levels of serum alkaline phosphatase , as well as histomorphometry in patients with renal osteodystrophy . our results demonstrate that ki , which reflects bone remodeling , was significantly higher in the defect with fe - bp than in the defect with fe only ( figure 4 ) . the increase of ki is related to the enhanced bone formation , confirming the reduced rate of bone resorption . however , a non - significant increase of ki in the fe - s group was detected as compared with the fe group . although we did not find further significant differences for any pet parameters between the other 2 different material group in the defect or in the biomaterial - tissue interface region , each group could be still well classified by the combination of some parameters as measured in the defect ( table 2 ) . it is unclear whether this is because pet is not sensitive enough for discriminating differences of bone metabolism due to the small changes of bone remodeling using different biomaterials and their enrichments , or is due to lack of statistical power . with f - sodium fluoride , k3 was the most significant parameter for the characterization of healing processes and showed the best differentiation between the 2 different biomaterials . pet scanning using f - sodium fluoride seems to be a sensitive and useful method for evaluation of bone healing after replacement with these biomaterials .