understanding the biomechanics of the native pcl provides a framework for reconstruction by replicating the anatomy.15 ) early biomechanical studies characterized the rehabilitationindividual main bundles of the pcl as anterolateral ( al ) and posteromedial ( pm ) bundles.16,17 ) the al bundle is more taut in flexion and more lax in extension ; the reverse is true for the pm bundle , which is more taut in extension and more lax in flexion.17,18 ) in this setting , the al and pm bundles mainly function individually at the flexed and extended positions , respectively . however , more recent biomechanical studies have suggested that , based on length and spatial orientation , the two bundles of the pcl may have a co - dominant relationship rather than a reciprocal one.15,19 - 21 ) this concept means that both bundles function through the range of motion ( rom ) in a synergistic fashion rather than a reciprocal one . mauro et al.19 ) reported no difference in the in situ forces between the al and pm bundles at any of the flexion angles , using a robotic testing system . ahmad et al.21 ) reported that the pm bundle becomes more horizontal with increasing knee flexion and this orientation increased the ability of the pm bundle to resist posterior tibial translation . using magnetic resonance imaging ( mri ) and a dual - orthogonal fluoroscopic system , papannagari et al.20 ) reported that both bundles showed elongation and change of orientation of up to 120 of knee flexion . race and amis22 ) conducted the first biomechanical comparison between isometric single and double bundle reconstruction ; their results showed over - constraint of the isometric single bundle reconstruction in extension with underconstraint at higher degrees of flexion . the double bundle reconstruction resulted in restoration of the posterior laxity from 0 to 120 to within 1 mm of the intact specimens . harner et al.8 ) also reported that double bundle reconstruction resulted in better restoration of posterior stability , compared with single bundle reconstruction in cadaveric knees . however , in some studies,4 - 6,23,24 ) in terms of posterior stability , few differences were observed between single and double bundle pcl reconstruction , even though some different results were reported with different experimental settings . the influence of the femoral attachment site , as well as the number of bundles , was further evaluated.1,2,7 ) in a study using variable femoral attachment sites , mannor et al.7 ) reported that a shallow femoral insertion allows for better control of posterior translation . however , they could not prove the possibility of graft elongation resulting from high graft tension . shearn et al.1 ) reported that the placement of a second bundle in the middle or distal position resulted in a significant reduction in al bundle tension and in cooperative load - sharing ( with the bundles functioning together ) . however , placement of the second bundle in a proximal position resulted in reciprocal loading ( with one bundle functioning in flexion and one in extension ) . the majority of studies have reported improved outcomes from preoperative level of function ; however , when compared with the preinjury activity status , the results are less successful.15 ) the objective knee scores seem to lag behind those of subjective self - reported scoring after surgical reconstruction ; one possible explanation is residual laxity , which has been demonstrated using many reconstructive techniques.15 ) most studies have reported residual laxity ranging from 2 to 6 mm indicating that the surgical result will depend upon the surgical technique.25 - 33 ) few clinical studies comparing the outcomes between single and double bundle pcl reconstruction have been reported . wang et al.34 ) reported no significant difference in the functional score or radiologic evaluation . three studies ( houe and jorgensen,35 ) fanelli and larson,36 ) and kim et al.37 ) ) also reported no difference in subjective and objective outcomes . only one recent study , by yoon et al.,38 ) reported better stability and international knee documentation committee ( ikdc ) distribution ; however , they also stated that it is unclear whether double bundle is definitely superior clinically and functionally because there was no difference in the subjective scores . pcl injuries have potential for intrinsic healing ; several mri studies have reported that the pcl healed with continuity but also with residual laxity.39 - 41 ) in most pcl injuries , some portion of the pcl , or at least the meniscofemoral ligament , is preserved , therefore , in an acute or subacute stage , the pcl has a higher likelihood of spontaneous healing than the anterior cruciate ligament ( acl ) does.25 ) many mri studies have reported that because the ligament is surrounded by a thick synovial sheath that is hardly torn completely and the meniscofemoral ligament remains attached to the lateral meniscus , an injured pcl can heal itself.39 - 42 ) treatment of the isolated pcl injury should depend on the injury status , which is determined by the amount of posterior laxity , the patient 's age and level of activity . in young patients , we can perform cylinder cast immobilization in order to prevent posterior sagging if the instability is less than 8 mm side to the side difference ( which means that there is a stepping between the medial tibial and femoral condyles at 90 of flexion of the knee joint ) and there is some continuity remaining according to the mri.9,43 ) however , during cast immobilization , in order to prevent posterior sagging of the proximal tibia , the cast should be changed if the patient feels that his knee , especially the proximal tibia , is moving anteriorly and posteriorly in the cylinder cast . cylinder cast immobilization is usually maintained for six weeks and then the brace is used with the attachment of two springs with a tibial supporter in order to prevent posterior translation of the tibia for another six weeks.9 ) another option for conservative treatment of the isolated pcl injury is an immediate rehabilitation and quadriceps strengthening exercise program , especially for elderly patients . we recommend pcl reconstruction in patients with more than grade ii pcl injury , even for isolated pcl injury in young patients.25 ) the remnant pcl fibers would be helpful for the improvement of vascularization , and therefore , will promote healing of the graft , and the mechanoreceptors will provide mechanical stability . the center of the femoral tunnel was chosen so that the distal edge of the graft was 2 mm apart from the articular cartilage margin , by placing the guide pin 5 - 6 mm proximal to the articular cartilage at the 11 or 11:30 o'clock position ( left knee ) , depending upon the graft diameter . a tibial tunnel could be created by placement of a guide pin just distal to the center of the tibial insertion or just lateral and distal to the center area in the remnant pcl . the graft can then pass along the medial border of the remnant pcl towards the femoral tunnel , which was located anteromedial to the pcl ( fig . in the case of healed pcl with residual laxity , tensioning with an al bundle reconstruction using a modified inlay technique could be used , and to get very good stability if the remnant pcl is thick and there is a normal signal in the mri studies when the injury is chronic ( more than 12 months ) . however , this technique is a technically demanding procedure and a bigger surgical scar may be produced.29,30,45,46 ) for al bundle reconstruction for the single bundle reconstruction , the femoral tunnel should be made at a distal ( shallow ) and anterior portion . this means that the femoral tunnel should be placed distally ( shallow ) , usually 5 - 6 mm , from the articular margin , and vertically . remnant pcl fibers may provide a soft tissue cushion effect between the graft and the bone at the entrance to the tunnel , which is helpful to prevent the killer turn effect at the femoral and tibial tunnel orifice . if there is no remnant pcl or a very thin pcl remnant , we should do double bundle reconstruction ( fig . 2).47 ) pcl injuries are frequently combined with posterolateral rotatory instability ( plri ) , which occurs in about 43%-80% of cases.47,48 ) although the causes of failure of pcl reconstruction are multifactorial , one of the most common causes is a neglected plri.49,50 ) therefore , identification of concomitant injuries is important in order to obtain a good result . currently , plri can only be evaluated through a physical examination . in particular , 1 to 2 plri ( grade 1 , external rotation [ er ] using a dial test < 10 without varus instability ; grade 2 , er  10 or posterolateral tibial subluxation with grade 0 - 2 varus instability ; and grade 3 , er  20 or posterolateral tibial subluxation + grade 3 varus instability)48 ) often goes unnoticed , especially when the tests are performed with the muscle tensed in acute stage patients with pain . therefore , plri assessment should be performed several times and should become a routine procedure before surgery for the patient under anesthesia.48,51 ) why is the plri misdiagnosed , especially in grade ii plri ? in our opinion , the reason is that in the pcl and posterolateral corner injured patient , the lateral tibial plateau is posterolatearally subluxed at 90 of knee flexion . if a dial test or posterolateral drawer test is performed in this situation , it is difficult to find more er of the leg or subluxation of the posterolateral tibial plateau . therefore , in pcl and posterolateral corner injured patients , reduction of the knee to the normal position using the dial test and posterolateral drawer test is important for making a diagnosis of plri.51 ) we determined that a reduction of the knee in the anteroposterior direction would increase the degree of tibial er in combined pcl - posterolateral corner injuries.51 ) when we performed the dial test in the prone position , this position was also helpful for the same reason . in the prone position , posterior sagging of the proximal tibia would be reduced , which was better than the supine position.51,52 ) for treatment of plri , grade ii injury could be managed with a posterolateral corner sling ( plcs ) through the fibular head.48 ) however , in grade iii plri , anatomical reconstruction would be preferable , as described by laprade and wentorf.53 ) in contrast to acl rehabilitation , accelerated pcl postoperative rehabilitation is generally undesirable and more conservative methods are recommended than acl reconstruction.54,55 ) however , the rehabilitation protocol of a pcl reconstruction is not well established and only a slow and conservative rehabilitation is proposed.14 ) for example , early weight bearing is believed to be hazardous to the pcl because pcl reconstruction is often associated with either a medial or lateral collateral ligament repair or reconstruction and it can cause over - stressing these structures.54,56 ) anatomically , the tibial plateau is inclined posteriorly and an axial load placed on the tibia by weight bearing at relatively extended positions produces an elemental force in the anterior direction . therefore , the joint is stabilized somewhat by weight bearing.57,58 ) in addition , weight bearing can have several benefits . firstly , the patient would have better static stability when standing on both legs , thereby minimizing the risk of falls . fourthly , weight bearing itself can be a co - strengthening exercise and proprioceptive training.54,56,59 ) finally , most patients have a tendency to flex their operated knee to prevent weight bearing.14 ) this means that a posteriorly directed force can be prevented if weight bearing is performed in the fully extended position . accelerated rehabilitation does not mean rapid range of motion exercise.14 ) within 0 to 30 of flexion , the hamstring can not produce a posterior shear force and the anterior angle of the patellar tendon is always larger than that of the hamstring tendons.60,61 ) therefore , within this range of motion , co - strengthening could be performed using calf raising and mini - squatting exercise . quadriceps strengthening extension exercise at angles less than the quadriceps neutral angle produces anterior tibial translation , which is antagonistic to the acl but synergistic to the pcl . therefore , after a pcl reconstruction , quadriceps strengthening knee extension should be restricted to between 60 of flexion and full extension of the knee.62,63 ) in current pcl studies , there has been a shift in biomechanics from reciprocal functioning to co - dominance . surgical devices and reconstructive techniques of the pcl have been developing and a more active approach is used than the past . it is still uncertain whether single or double bundle reconstruction is superior , because of conflicting biomechanical studies and notable limitations of the clinical studies . the remnants of pcl fibers , placement of the femoral tunnel , and combined plri are other hot issues in reconstruction . after the reconstruction of the pcl , a more active and systemic exercise program and early weight - bearing training are increasingly being recognized as important .
there is little consensus on how to optimally reconstruct the posterior cruciate ligament ( pcl ) and the natural history of injured pcl is also unclear . the graft material ( autograft vs. allograft ) , the type of tibial fixation ( tibial inlay vs. transtibial tunnel ) , the femoral tunnel position within the femoral footprint ( isometric , central , or eccentric ) , and the number of bundles in the reconstruction ( 1 bundle vs. 2 bundles ) are among the many decisions that a surgeon must make in a pcl reconstruction . 
 in addition , there is a paucity of information on rehabilitation after reconstruction of the pcl and posterolateral structures . 
 this article focused on the conflicting issues regarding the pcl , and the scientific rationales behind some critical points are discussed .