In Vivo Biomechanics Of The Knee During Weight Bearing In High Flexion

Purpose: The objective of this study is to investigate the6-DOF kinematics,tibiofemoral cartilage contact biomechanics and the changes in length of the majorligaments of the knee during weight bearing flexion from full extension to maximal flexionof living subjects.Methods: Eight healthy subjects (eight knees; age,23～49;6male and2female; BMI,19.9～29.3kg/m2) with no history of knee injuries or chronic knee pain were recruited.Each subject signed a consent form approved by the IRB of Massachusetts GeneralHospital. The knees were MRI scanned to create3D models of the patella, tibia and femur,including the articular cartilage of the patella, tibia and femur and the insertions of theACL, PCL, sMCL, dMCL and LCL. Base on the functional anatomy, the ACL was dividedinto two bundles: the anteromedial bundle (AMB) and posterolateral bundle (PLB).Similarly, the PCL was divided into two bundles: the anterolateral bundle (ALB) andposteromedial bundle (PMB). The sMCL, dMCL and LCL were each divided into threeequal portions: the anterior bundle (AB), the middle bundle (MB) and the posterior bundle(PB). The subjects were then imaged using a dual fluoroscopic image system in twoorthogonal directions while performing a quasi-static single-leg lunge （every15°）fromfull extension to the maximal flexion they could perform. The knee models andfluoroscopic images were used to reproduce the in vivo motion of the knee. The6-DOFkinematics, the motion of articular cartilage contact points and the elongation of eachbundle of the major ligaments were measured along the flexion path of the knee. Thecontact points on the medial and lateral tibial plateau were calculated by finding thecentroid of the intersection of the tibial cartilage layers. A mathematical algorithm wasimplemented to find the shortest3D wrapping path of a ligament bundle around the bones.The length of each bundle at the full extension was set as the initial length. The flexionpath of the knee was divided into three ranges: low flexion range (full extension to30°offlexion), middle flexion range (30°to120°of flexion); high flexion range (120°tomaximal flexion). A repeated measure ANOVA was used to detect statistically significantdifferences in the6-DOF kinematics, the motion of articular cartilage contact points andthe bundles at different flexion angles and ranges. Results: The posterior femoral translation consistently increased from full extension tothe maximum flexion. The lateral femoral translation increased slightly and internal tibialrotation and varus increased at low flexion angles (full extension to30°), the medialfemoral translation and the internal tibial rotation maintained a small variation, and thetibial varus consistently increased in the middle range of flexion (30°to120°), and thenmedial femoral translation and internal tibial rotation sharply increased, and tibia varussharply decreased at high flexion angles (120°to maximal flexion). The contact pointmoved similarly in the medial and lateral compartments before120°of flexion, but less onthe medial compartment at high flexion angles. The ACL was shown to have largerreduction in length at mid-range of flexion and less reduction at maximal flexion. The PCLwas shown to have maximal increase in length around90°of flexion. For collateralligaments, all AB bundles were shown to have increasing length with flexion except that ofthe sMCL showing reduction in length at high flexion. The MB bundles showed minimalchange in lengths except that of the sMCL showing consistent reduction in length withflexion. All PB bundles showed reduction in lengths with flexion.Conclusions: The results indicated that the knee motion couldn’t be described usingone character in the entire range of flexion, especially in high flexion. The kneebiomechanical data in the entire range of flexion of the knee could be instrumental fordesigning new knee prostheses to achieve physical high flexion, designing anddevelopment of cruciate retaining total knee arthroplasty and improving rehabilitationprotocols after knee injuries.