lateral closed wedge high tibial osteotomy ( hto ) described by jackson and waugh   and coventry   is an established technique to treat osteoarthritis of the medial compartment and varus deformity .
the advantages of a medial opening wedge osteotomy compared with a lateral closing wedge osteotomy include easier technique , achievement of more predictable correction , maintenance of bone stock , correction of the deformity close to its origin , and avoidance of the peroneal nerve , and , proximal tibiofibular joint . however , a suitable material to fill the opened defect in the medial opening procedure has not been identified .
although autogenous bone is the preferred graft material for filling bone defects , autogenous bone grafting has the disadvantages of co - morbidity at the donor site .
allografts have commonly been used as substitutes for autogenous bone grafts ; significant problems associated with allografts include a low bone fusion rate and disease transmission [ 2 , 8 ] .
hydroxyapatite is widely used as a bone substitute because of its excellent biocompatibility and osteoconductive properties [ 9 , 14 ] .
although it biodegrades slowly and there is no progressive bone formation during the bone repair .
.   reported good long - term results after opening hto using hydroxyapatite as a bone filler . if , however , severe varus or valgus knee deformities were to occur , fixing a component in the tibia during total knee arthroplasty might be difficult .
in contrast , most implanted porous -tricalcium phosphate ( tcp ) can be resorbed within a few years . in previous studies , we used -tcp with 75  % porosity , but this has a compressive strength of only 3  mpa , which is inadequate for weight - bearing sites until bone incorporation occurs .
thus , we recently developed a wedge - shaped -tcp block with 60  % porosity for opening hto .
this -tcp has a compressive strength of 22  mpa , which is approximately sevenfold greater than -tcp with 75  % porosity .
we previously reported the clinical results of bone formation and resorption of -tcp using -tcp blocks with 75 and 60  % porosity at 33  months . during opening hto ,
the opened defect was fixed with a puddu plate , after which -tcp with 75  % porosity was used to fill the cancellous bone defect . on the medial cortical bone side , however , a wedge - shaped -tcp block with 60  % porosity was implanted in front of and at the back of the plate .
the use of a -tcp block with 60  % porosity avoided autogenous bone grafting and shortened the surgical time .
the decreased porosity reduced pore interconnection , suggesting a longer period for -tcp resorption . in that study ,
radiographic examination showed that -tcp with 75  % porosity was almost completely resorbed , but some -tcp with 60  % porosity remained .
other reports also evaluated -tcp resorption in opening hto using plain x - rays [ 1 , 4 , 11 , 16 , 19 ] . to our knowledge , no radiological rating system to monitor remodelling of -tcp using ct images has been reported .
the aim of this study was to establish a novel evaluation system to monitor bone formation and -tcp resorption in opening hto .

 the -tcp block used in this study was highly pure and provided by olympus biomaterials co. ltd . ,
briefly , cahpo4/h2o and caco3 at a molar ratio of 2:1 were mixed into a slurry with pure water and particles of zirconium in a pot mill for 24  h and dried at 80  c .
calcium - deficient hydroxyapatite was converted to -tcp by calcination at 750  c for 1  h. after sintering of the -tcp powder at 1,050  c for 1  h , a porous -tcp block with a mean pore size of 200  m and a porosity of 75  % was obtained .
a bimodal pore size distribution in the -tcp block with 75  % porosity was observed , where one peak existed in a region of > 100  m and the other peak existed in a region of < 5  m .
the -tcp block with 60  % porosity was synthesized by the same method as the 75  % material , except that the amount of forming agent was changed .
the mean compressive strength of -tcp blocks with 60 and 75  % porosity was 22 and 3  mpa , respectively . during opening hto ,
the opened defect was fixed with a puddu plate after which the cancellous bone defect was filled with 4 wedge - shaped -tcp blocks of 75  % porosity . on the medial side , however , 3 wedge - shaped -tcp blocks with 60  % porosity were implanted in front of and at the back of the plate .
eighteen patients received an opening of 10  mm and 13 patients had an opening of 12.5  mm .
the volume of each piece of -tcp block with 60  % porosity for a 10  mm opening was 1.06  cm and for a 12.5  mm opening was 1.39  cm .
three pieces of -tcp were implanted in all cases , but 1/3 of one piece was removed in the case of small knees . to fill the cancellous bone defects , 67 and 78  cm of -tcp blocks with 75  % porosity were used for 10 and 12.5  mm openings , respectively . between 2003 and 2005 ,
opening wedge hto was performed on 36 patients using a puddu plate and -tcp with 60 and 75  % porosity .
thirty - one patients ( 22 women and 9 men ) with a mean age of 64  years ( range 5177  years ) at the time of surgery , were evaluated with a mean follow - up point of 75  months ( range 7278  months ) .
exclusion criteria in this study included ( 1 ) patients with standing lateral femorotibial angle of more than 184 ; ( 2 ) patients who had an extension loss of more than 10 ; ( 3 ) patients who had severe osteoarthritis in the patellofemoral joint ; and ( 4 ) patients whose bmi was more than 27 ( kg / m ) .
after surgery , the knee was protected with a hinged brace for 12  weeks .
partial weight - bearing was allowed at 34  weeks , and total weight - bearing was allowed at 56  weeks .
all patients received follow - up evaluations at regular intervals in our outpatient clinic , and underwent radiographs and ct examinations at 2  weeks ( fig . 
ct images parallel to the osteotomy plane were made and images at the centre were used to evaluate -tcp resorption and bone formation .
the ct image data were divided into 3 areas by tracing manually on the screen of the computer ( fig .  1-a2 , b2 ) .
ct attenuation values [ in hounsfield units ( hu ) ] of the area implanted with -tcp of 60  % porosity , the area implanted with -tcp of 75  % porosity , and hinged - cancellous bone were analysed using the imaging software , osirix .
traces of the -tcp blocks were still visible at 6  years post - operatively in some cases , but in most cases , it was hard to define the -tcp implanted area . in those cases , ct
circumferential cortical bone of the tibia was excluded from each value of -tcp and cancellous bone.fig . 
1ct images of a 64-year - old man showing the centre of the osteotomy plane at 2  weeks ( a1 , a2 ) and 6  years
( b1 , b2 ) . the mean ct values ( hu ) of the area implanted with -tcp of 60  % porosity (  ) , the area implanted with -tcp of 75  % porosity (  ) , hinged - cancellous bone (  ) , and the area with insufficient -tcp implantation (  ) were , 1,717.3 , 1,044.8 , 175.1 , and 39.1 , respectively .
six years after surgery , these values were 440.5 , 162.0 , 164.3 , and 21.9 , respectively .
 indicates the area of the metal plate ct images of a 64-year - old man showing the centre of the osteotomy plane at 2  weeks ( a1 , a2 ) and 6  years ( b1 , b2 ) .
the mean ct values ( hu ) of the area implanted with -tcp of 60  % porosity (  ) , the area implanted with -tcp of 75  % porosity (  ) , hinged - cancellous bone (  ) , and the area with insufficient -tcp implantation (  ) were , 1,717.3 , 1,044.8 , 175.1 , and 39.1 , respectively .
six years after surgery , these values were 440.5 , 162.0 , 164.3 , and 21.9 , respectively .
 indicates the area of the metal plate the ct images were evaluated independently by two investigators ( tt and yk ) .
the two investigators re - evaluated the ct data one more time at 11  months after the first analysis .
inter - observer variability as well as intra - observer variability during the first and second rounds were determined using intraclass correlation coefficient , an appropriate summary statistic for determining the reliability of measurement [ 12 , 20 ] .
all patients provided informed consent prior to participation in this study , which was approved by nho utsunomiya national hospital ethical committee ( i d 15 - 1 ) .
we used a paired t test to compare ct values of -tcp implanted areas and cancellous bone .

 the -tcp block used in this study was highly pure and provided by olympus biomaterials co. ltd . ,
briefly , cahpo4/h2o and caco3 at a molar ratio of 2:1 were mixed into a slurry with pure water and particles of zirconium in a pot mill for 24  h and dried at 80  c .
calcium - deficient hydroxyapatite was converted to -tcp by calcination at 750  c for 1  h. after sintering of the -tcp powder at 1,050  c for 1  h , a porous -tcp block with a mean pore size of 200  m and a porosity of 75  % was obtained .
a bimodal pore size distribution in the -tcp block with 75  % porosity was observed , where one peak existed in a region of > 100  m and the other peak existed in a region of < 5  m .
the -tcp block with 60  % porosity was synthesized by the same method as the 75  % material , except that the amount of forming agent was changed .
the mean compressive strength of -tcp blocks with 60 and 75  % porosity was 22 and 3  mpa , respectively . during opening hto ,
the opened defect was fixed with a puddu plate after which the cancellous bone defect was filled with 4 wedge - shaped -tcp blocks of 75  % porosity . on the medial side , however , 3 wedge - shaped -tcp blocks with 60  % porosity were implanted in front of and at the back of the plate .
eighteen patients received an opening of 10  mm and 13 patients had an opening of 12.5  mm .
the volume of each piece of -tcp block with 60  % porosity for a 10  mm opening was 1.06  cm and for a 12.5  mm opening was 1.39  cm .
three pieces of -tcp were implanted in all cases , but 1/3 of one piece was removed in the case of small knees . to fill the cancellous bone defects , 67 and 78  cm of -tcp blocks with 75  % porosity were used for 10 and 12.5  mm openings , respectively . between 2003 and 2005 ,
opening wedge hto was performed on 36 patients using a puddu plate and -tcp with 60 and 75  % porosity .
thirty - one patients ( 22 women and 9 men ) with a mean age of 64  years ( range 5177  years ) at the time of surgery , were evaluated with a mean follow - up point of 75  months ( range 7278  months ) .
exclusion criteria in this study included ( 1 ) patients with standing lateral femorotibial angle of more than 184 ; ( 2 ) patients who had an extension loss of more than 10 ; ( 3 ) patients who had severe osteoarthritis in the patellofemoral joint ; and ( 4 ) patients whose bmi was more than 27 ( kg / m ) .
after surgery , the knee was protected with a hinged brace for 12  weeks .
partial weight - bearing was allowed at 34  weeks , and total weight - bearing was allowed at 56  weeks .
all patients received follow - up evaluations at regular intervals in our outpatient clinic , and underwent radiographs and ct examinations at 2  weeks ( fig . 
ct images parallel to the osteotomy plane were made and images at the centre were used to evaluate -tcp resorption and bone formation .
the ct image data were divided into 3 areas by tracing manually on the screen of the computer ( fig .  1-a2 , b2 ) .
ct attenuation values [ in hounsfield units ( hu ) ] of the area implanted with -tcp of 60  % porosity , the area implanted with -tcp of 75  % porosity , and hinged - cancellous bone were analysed using the imaging software , osirix .
traces of the -tcp blocks were still visible at 6  years post - operatively in some cases , but in most cases , it was hard to define the -tcp implanted area . in those cases , ct
circumferential cortical bone of the tibia was excluded from each value of -tcp and cancellous bone.fig . 
1ct images of a 64-year - old man showing the centre of the osteotomy plane at 2  weeks ( a1 , a2 ) and 6  years
( b1 , b2 ) . the mean ct values ( hu ) of the area implanted with -tcp of 60  % porosity (  ) , the area implanted with -tcp of 75  % porosity (  ) , hinged - cancellous bone (  ) , and the area with insufficient -tcp implantation (  ) were , 1,717.3 , 1,044.8 , 175.1 , and 39.1 , respectively .
six years after surgery , these values were 440.5 , 162.0 , 164.3 , and 21.9 , respectively .
 indicates the area of the metal plate ct images of a 64-year - old man showing the centre of the osteotomy plane at 2  weeks ( a1 , a2 ) and 6  years ( b1 , b2 ) .
the mean ct values ( hu ) of the area implanted with -tcp of 60  % porosity (  ) , the area implanted with -tcp of 75  % porosity (  ) , hinged - cancellous bone (  ) , and the area with insufficient -tcp implantation (  ) were , 1,717.3 , 1,044.8 , 175.1 , and 39.1 , respectively .
six years after surgery , these values were 440.5 , 162.0 , 164.3 , and 21.9 , respectively .
the two investigators re - evaluated the ct data one more time at 11  months after the first analysis .
inter - observer variability as well as intra - observer variability during the first and second rounds were determined using intraclass correlation coefficient , an appropriate summary statistic for determining the reliability of measurement [ 12 , 20 ] .
all patients provided informed consent prior to participation in this study , which was approved by nho utsunomiya national hospital ethical committee ( i d 15 - 1 ) .
we used a paired t test to compare ct values of -tcp implanted areas and cancellous bone .
the mean preoperative standing lateral femorotibial angle was 181 ( range 177184 ) , which was corrected to a mean angle of 169 post - operatively .
ct image analysis containing the centre of the osteotomy plane showed that the mean ct value ( hu ) of the implanted area with -tcp of 60  % porosity at 2  weeks and 6  years were 1,694.0    94.2 , and 574.1    273.5 , respectively .
the mean ct value of the implanted area with -tcp of 75  % porosity at 2  weeks and 6  years were 1,010.9    81.1 , and 168.8    75.1 , respectively .
the mean ct value of cancellous bone at 2  weeks and 6  years were 178.0    45.1 , and 174.9    69.3 , respectively ( fig .  2 ) .
the values with the smallest standard deviation ( sd ) were for hinged - cancellous bone at 2  weeks and values with the largest sd were for areas implanted with -tcp of 60  % porosity at 6  years.fig . 
2ct values ( hu ) obtained from each area of 31 cases at 2  weeks and 6  years .
the ct values of -tcp with 75  % porosity implanted area at 6  years were not statistically different from those of hinged - cancellous bone , indicating that -tcp with 75  % porosity was completely resorbed and replaced by cancellous bone .
the labels  60  ,  75  , and  bone  indicate the area implanted with -tcp of 60  % porosity , the area implanted with -tcp of 75  % porosity , and hinged - cancellous bone , respectively ct values ( hu ) obtained from each area of 31 cases at 2  weeks and 6  years .
the ct values of -tcp with 75  % porosity implanted area at 6  years were not statistically different from those of hinged - cancellous bone , indicating that -tcp with 75  % porosity was completely resorbed and replaced by cancellous bone .
the labels  60  ,  75  , and  bone  indicate the area implanted with -tcp of 60  % porosity , the area implanted with -tcp of 75  % porosity , and hinged - cancellous bone , respectively six years after surgery , there was no statistical difference of the ct values between the area implanted with -tcp of 75  % porosity and cancellous bone , indicating that the -tcp with 75  % porosity had been completely resorbed and replaced by bone .
in contrast , the ct values indicated that approximately 1/3 of the -tcp with 60  % porosity remained even 6  years after surgery ( fig .  2 ) .
the intraclass correlation coefficients of the ct values of 60  % porosity -tcp , 75  % porosity -tcp , and hinged - cancellous bone at 2  weeks and 6  years were determined .
the intraclass correlation coefficients of the inter - observer measurements for the first and second rounds were 0.670.94 and 0.790.96 , respectively .
the intraclass correlation coefficients of the intra - observer measurements for the first and second investigators were 0.890.96 and 0.660.94 , respectively .
the most important finding of this study was the quantitative evaluation of the -tcp resorption and bone formation in opening hto .
there are several reports that evaluate -tcp resorption in opening hto [ 1 , 4 , 11 , 16 , 19 ] .
van hemert et al .   developed a radiological rating system to monitor bone healing in opening wedge osteotomies .
remodelling of bone gaps filled with -tcp granules or blocks with 70  % porosity was graded into five categories .
the results were only analysed by conventional anteroposterior and lateral radiographs , potentially leading to observer variance of radiological measurements .
as yet , no radiological rating system using ct images to monitor bone healing in opening hto is available .
ct values of 60  % and 75  % porosity -tcp blocks at the centre of ct images at 2  weeks and 6  years were originally measured ; however , ct images at 6  years showed that -tcp with 60  % porosity was not uniformly resorbed ( fig .  1 ) .
this strongly suggested that measurements focused on a certain area ( region of interest ) of -tcp were inadequate to evaluate -tcp resorption .
we therefore used the imaging software , osirix , which was developed in 2004 by a team of radiologists from geneva and ucla , working on apple computers , and can be downloaded free of charge .
-tcp blocks were still visible at 6  years in some cases , but in most of the cases , it was hard to define the -tcp implanted area . in those cases , we referred to ct images taken at 2  weeks to define the -tcp - implanted area .
the present study showed that there was no statistical difference between the ct values of the area implanted with -tcp of 75  % porosity at 6  years and cancellous bone , indicating that -tcp with 75  % porosity was completely resorbed and replaced by bone .
in contrast , from the ct values , approximately 1/3 of -tcp with 60  % porosity remained even 6  years after surgery .
figure  1 showed that ct values in the area with insufficient -tcp implantation was lower than cancellous bone , indicating that poor bone formation occurred in this area .
.   reported on smoking as a risk factor of non - union in opening hto , but the present study did not show significant difference of -tcp resorption in the two patients who smoked .
first , the ct values of each circumferential cortical bone of the tibia were excluded from each value of -tcp and cancellous bone , because it was hard to distinguish newly formed bone from residual -tcp .
second , it was hard to obtain completely matched images at 2  weeks and 6  years , even though we tried to measure the centre of the osteotomy plane .
third , the ct images were divided into three areas by tracing manually on the screen of the computer .
although this manual process caused inter- and intra - observer differences , intraclass correlation coefficients analysis showed substantial to almost perfect agreement between the two independent investigators .
the present study showed that -tcp with 75  % porosity was completely resorbed and replaced by bone , but -tcp with 60  % porosity was only partially resorbed .
however , the amount of residual -tcp with 60  % porosity was so small that it would not hinder total knee arthroplasty should it be necessary in the future . to promote complete remodeling of -tcp , additional stimulations such as growth factors may also be necessary .
this system is the first to quantitatively evaluate the residual amount of -tcp and bone formation in opening hto
. it will be a useful technique to evaluate -tcp resorption and bone formation in any -tcp implanted area .
the aim of this study was to establish a novel evaluation system to monitor bone formation and -tcp resorption in opening hto using the image software , osirix .
the results showed that -tcp with 75  % porosity was completely resorbed and replaced by bone , but -tcp with 60  % porosity was only partially resorbed even 6  years after surgery .
the imaging software , osirix , enabled us to evaluate -tcp resorption and bone formation in both opening hto and other -tcp implanted areas .
 purposethe aim of this study was to establish an evaluation system to monitor bone formation and beta - tricalcium phosphate ( tcp ) resorption in opening high tibial osteotomy ( hto).methodsfrom 2003 to 2005 , opening hto was performed in 36 patients using a puddu plate and -tcp blocks with 60 and 75  % porosity . 
 thirty - one patients were used for evaluation . 
 all patients underwent ct examination at 2  weeks and 6  years . 
 the ct image data were divided into three areas , and ct values of each area were analysed using the imaging software , osirix.resultsct image analysis at 2  weeks showed that the mean ct - attenuation values ( in hounsfield units ) of the implanted area with -tcp of 60  % porosity , the implanted area with -tcp of 75  % porosity , and cancellous bone were , 1,694.0    94.2 , 1,010.9    81.1 , and 178.0    45.1 , respectively . 
 six years after surgery , these values were 574.1    273.5 , 168.8    75.1 , and 174.9    69.3 , respectively.conclusion-tcp with 75  % porosity was completely resorbed and replaced by bone . 
 -tcp with 60  % porosity was resorbed , but approximately 1/3 still remained even 6  years after surgery . 
 the imaging software , osirix , enabled scanning of the whole area to measure ct values . 
 this system is the first to quantitatively evaluate -tcp resorption and bone formation in opening hto.level of evidencelaboratory studies . 
Level of evidence
Introduction
Materials and methods
Preparation of -TCP
Reproducibility of the radiological evaluating system
Statistical analysis
Results
Discussion
Conclusions