interference screws used for knee ligament reconstructive surgery are a popular application of bioabsorbable materials in orthopedics .
they offer various advantages [ 1 , 2 ] over traditional metallic implants , such as the ability to engineer them to provide an optimum degradation profile , a reduced need for implant removal , and less distortion on magnetic resonance imaging ( mri ) .
polylactide carbonate ( plc ) is a combination of an amorphous bioabsorbable polymer , poly dl - lactide - coglycolide ( pdlg ) , with calcium carbonate , a bone - stimulating material and neutralizing agent .
plc was used to develop the  calaxo  ( smith & nephew , andover , ma , usa ) interference screw for knee ligament reconstruction surgery .
a 26-year - old man underwent arthroscopic reconstruction of his anterior cruciate ligament ( acl ) , posterior cruciate ligament ( pcl ) and posterolateral corner following a multiple - ligament knee injury obtained while playing rugby .
the patient had undergone a twisting knee injury during a tackle and was immediately diagnosed as having a severe knee injury and referred to the senior author ( fsh ) .
an mri scan performed within twenty - four hours of the injury confirmed a tear of the acl , pcl and posterolateral corner . for acl reconstruction ,
two semitendinosus allografts were used to fashion a four - strand 10  mm - thick graft which was fixed with an endoloop ( smith & nephew ) on the femoral side and a 9  mm by 25  mm plc ( calaxo , smith & nephew ) interference screw on the tibial side . for the pcl reconstruction ,
an achilles tendon allograft was fixed to the femoral side by a 9  mm by 25  mm plc ( calaxo , smith & nephew ) screw and to the tibial side by a 9  mm by 35  mm plla ( rci , smith & nephew ) screw . for the reconstruction of the posterolateral corner of the knee ,
two semitendinosus allografts were passed : one into an anterior - to - posterior through - and - through hole in the fibula , and the other through an anteroposterior tunnel in the tibia adjacent to the fibula .
the tibial end of the graft was fixed with an 8  mm by 25  mm plc ( calaxo , smith & nephew ) .
the two free ends of the fibular allograft and one free end of the tibial allograft were then passed subfascially and fixed into a 9  mm tunnel drilled at an isometric point above the lateral epicondyle of the femur .
the allograft was then fixed in the femoral tunnel using a 10  mm by 35  mm plc ( calaxo , smith & nephew ) screw .
meticulous care was taken to ensure that , when bone tunnels were adjacent , they were placed such that no compromise to bone strength would occur ( see postoperative radiographs  fig .  1 ) .
1immediate postoperative plain radiographs with illustration of tunnel positions on the lateral femoral condyle illustrated on the ap view .
the lateral tunnel is placed in a more anterior plane in relation to the acl tunnel immediate postoperative plain radiographs with illustration of tunnel positions on the lateral femoral condyle illustrated on the ap view .
the lateral tunnel is placed in a more anterior plane in relation to the acl tunnel postoperatively the patient was mobilized nonweight - bearing for 2  months in a knee brace , and specialist physiotherapy was initiated for muscle strengthening and range of movements .
the immediate postoperative recovery was unremarkable and the patient made satisfactory progress with regards to rehabilitation .
six months after the procedure , the patient presented with sudden onset of knee pain after minor trauma that involved slipping but not falling in a changing room . there was no preceding trauma . on a computed tomography scan ( ct  fig . 
2 ) , the femoral fixation screw [ 10  mm by 35  mm plc ( calaxo , smith & nephew ) ] of the posterolateral corner allograft was found to be completely degraded , and a minimally displaced fracture was noted through the tunnel ( fig .  3 ) .
was not compromised , the patient was managed conservatively by immobilization in a brace for six  weeks .
the recovery was satisfactory , with a return to premorbid function by approximately 14  months after surgery ( see weight - bearing radiographs  fig . 
, the patient continues to have very good functional results , with bilaterally equal range of movements in the knee .
clinical examination using lachman s test , the drawer test and the collateral stress test have confirmed a stable knee joint.fig . 
4plain radiographs taken at 14  months suggest a healed fracture ct image of the fracture in the femoral tunnel .
please note the  empty tunnels  on the rotation views plain radiographs taken at 14  months suggest a healed fracture
for acl reconstruction , two semitendinosus allografts were used to fashion a four - strand 10  mm - thick graft which was fixed with an endoloop ( smith & nephew ) on the femoral side and a 9  mm by 25  mm plc ( calaxo , smith & nephew ) interference screw on the tibial side . for the pcl reconstruction ,
an achilles tendon allograft was fixed to the femoral side by a 9  mm by 25  mm plc ( calaxo , smith & nephew ) screw and to the tibial side by a 9  mm by 35  mm plla ( rci , smith & nephew ) screw . for the reconstruction of the posterolateral corner of the knee ,
two semitendinosus allografts were passed : one into an anterior - to - posterior through - and - through hole in the fibula , and the other through an anteroposterior tunnel in the tibia adjacent to the fibula .
the tibial end of the graft was fixed with an 8  mm by 25  mm plc ( calaxo , smith & nephew ) .
the two free ends of the fibular allograft and one free end of the tibial allograft were then passed subfascially and fixed into a 9  mm tunnel drilled at an isometric point above the lateral epicondyle of the femur .
the allograft was then fixed in the femoral tunnel using a 10  mm by 35  mm plc ( calaxo , smith & nephew ) screw .
meticulous care was taken to ensure that , when bone tunnels were adjacent , they were placed such that no compromise to bone strength would occur ( see postoperative radiographs  fig .  1 ) .
1immediate postoperative plain radiographs with illustration of tunnel positions on the lateral femoral condyle illustrated on the ap view .
the lateral tunnel is placed in a more anterior plane in relation to the acl tunnel immediate postoperative plain radiographs with illustration of tunnel positions on the lateral femoral condyle illustrated on the ap view .
the lateral tunnel is placed in a more anterior plane in relation to the acl tunnel postoperatively the patient was mobilized nonweight - bearing for 2  months in a knee brace , and specialist physiotherapy was initiated for muscle strengthening and range of movements .
the immediate postoperative recovery was unremarkable and the patient made satisfactory progress with regards to rehabilitation .
six months after the procedure , the patient presented with sudden onset of knee pain after minor trauma that involved slipping but not falling in a changing room . there was no preceding trauma . on a computed tomography scan ( ct  fig . 
2 ) , the femoral fixation screw [ 10  mm by 35  mm plc ( calaxo , smith & nephew ) ] of the posterolateral corner allograft was found to be completely degraded , and a minimally displaced fracture was noted through the tunnel ( fig .  3 ) .
was not compromised , the patient was managed conservatively by immobilization in a brace for six  weeks .
the recovery was satisfactory , with a return to premorbid function by approximately 14  months after surgery ( see weight - bearing radiographs  fig . 
, the patient continues to have very good functional results , with bilaterally equal range of movements in the knee .
clinical examination using lachman s test , the drawer test and the collateral stress test have confirmed a stable knee joint.fig . 
4plain radiographs taken at 14  months suggest a healed fracture ct image of the fracture in the femoral tunnel .
please note the  empty tunnels  on the rotation views plain radiographs taken at 14  months suggest a healed fracture
the main disadvantages associated with the use of bioabsorbable implants are their comparatively low mechanical strength , their relatively high cost , and the undesirable biological response   they may produce .
the persistence of bioabsorbable screws for up to 3  years after their insertion is well documented in mri studies after knee ligament reconstruction surgery [ 46 ] . in ovine models , the plc screws have shown gradual and controlled degradation , stimulating ossification of the graft within the bone tunnel .
new bone thus formed was noted around the margin of the screw as early as 612  weeks .
this was followed by partial integration of the screw with bone by 26  weeks and complete replacement with new bone at 52  weeks . computed tomography data followed this pattern , with extensive integration of the screw to bone by 26  weeks and disappearance of the screw by 52  weeks .
an interference screw that would replenish the bone tunnel would be an ideal choice for knee ligament reconstruction surgery . however , clinical studies looking at the outcome of plc screws are lacking .
there are few reports of femoral fractures at the sites of graft fixation associated with acl reconstruction ( table  1 ) .
noah et al .   in 1992 and radler et al .   in 2000 reported an extracondylar femur fracture at the site of extra - articular augmentation of acl reconstruction . in the former case ,
the fracture occurred 6  months after an iliotibial band augmentation of a patellar tendon graft acl reconstruction , whereas the latter case followed 20  months after the removal of a lateral augmentation device used in an acl reinsertion.table  1femur fractureauthorprocedurefixationtime from procedurenoah et al .
patellar tendon acl reconstruction and itb extra - articular tenodesisinterference screw6  monthsternes et al .
gore - tex graft acl revisionn / a8  weekswiener and siliski patellar tendon acl revision7  monthsmanktelow et al . hamstring tendon acl reconstruction with extra - articular tenodesis24  months fracture from staple of extra - articular augmentation to intraosseous tunnelberg et al .
patellar tendon acl reconstruction7    25 metal interference screw for a 10  mm tunnel5  months .
size9  months , fallsheps et al . quadrupled st - g autograft acl reconstructionendoloop button5  months , head injury , ?
the fracture extended from the staple of the extra - articular augmentation to the intraosseous tunnel , 24  months after a hamstring autograft acl reconstruction with extra - articular tenodesis . in a case reported by ternes et al .   in 1993 ,
the femoral tunnel used for the placement of a gore - tex prosthetic graft was the site of the fracture , which occurred 8  weeks following the procedure .
multiple anterolateral femoral cortex perforations were reported as the etiology for the fracture reported 7  months following a patellar tendon acl reconstruction by weiner et al .   in 1996 , and 5  months following quadrupled hamstring graft acl reconstruction by sheps et al .   in 2006 .
berg et al .   in 1999 reported a fracture through the femoral tunnel 8  weeks after a bone  patellar tendon  bone acl reconstruction . at the time of the operation , the femoral tunnel had been over - drilled and
the femoral tunnel was itself noted to be the stress riser , precipitating a fracture 5  months following patellar tendon acl reconstruction by mithoefer et al .  
in 2005 , and 9  months following a similar procedure by wilson et al .   in the same year
. cases of tibial plateau fracture through the distal fixation site of the acl grafts have also been reported [ 1621 ] ( table  2 ) .
in the case reported by thaunat et al .   in 2006 , a 9  mm bioabsorbable screw ( polyglycolide - co - trimethylenecarbonate , endofix , mansfield , ma , usa ) was used to fix the tibial end of a bone patellar tendon bone acl reconstruction .
four years later the patient presented with tibial plateau fracture following a valgus compression trauma to his knee .
imaging demonstrated screw resorption and tunnel enlargement.table  2tibial plateau fractureauthorprocedurefixationtimeel - hage et al .
achilles tendon acl reconstructionrichards interference screw18  months post - traumamorgan and steensen acl reconstructiondelcogliano et al .
patellar tendon acl reconstruction10  mm tibial tunnel and 9    25 interference screws7  monthsmithofer et al .
acl reconstruction8  mm tunnel fixed with 9    25 rci screws12  months , fallthaunat et al .
acl reconstruction11  mm tunnel , 9  mm pga - co - tmc screw4  years , valgus compression injury after fall tibial plateau fracture biomechanical studies have shown that a bone defect such as a screw hole can concentrate stress and decrease the bone strength to torsional loading [ 2224 ] .
this may explain the fractures noted in the osseous tunnels following knee ligament reconstruction surgery .
both mechanical and biological factors have been recognized to contribute to tunnel enlargement after acl reconstruction .
mechanical factors include motion of the graft within the tunnel , fixation methods / devices , stress shielding of the graft , improper graft placement , and accelerated rehabilitation .
graft swelling , the use of allograft tissue , synovial fluid propagation within bony tunnels , and increased cytokine levels within the knee are all biologic modes of osteolysis contributing to tunnel enlargement . in most cases
unpredictable bioabsorbable screw resorption results in graft in empty tunnel and perhaps synovial reaction and cytokines , all contributing to persistent tunnel enlargement and bone weakening . in our case ,
rapid degradation of the plc screws in the osseous tunnel in the absence of any attempted bone integration may have predisposed to the fracture by a similar mechanism .
ongoing clinical concerns with the unpredictable absorption of the plc screws ( calaxo , smith & nephew ) lead to their withdrawal from the market in 2007 .
it is crucial to be aware of adverse effects where this particular interference screw has been used . in conclusion ,
stress risers may occur following the use of bioabsorbable screws for ligament reconstruction surgery , particularly if screw resorption is rapid and bone integration is not complete .
femoral fracture presenting as a late complication following the use of bioabsorbable interference screws in knee ligament reconstruction surgery is rare .
it is important to be aware of this potential postoperative complication when considering which form of screw fixation to use .
 we report an unusual case of femoral fracture from minimal trauma , due to the rapid disappearance of a bioabsorbable interference screw used for reconstruction of the posterolateral corner of the knee . the literature on bone tunnel fractures following knee ligament reconstruction surgery is also reviewed . 