| IntroductionAnterior cruciate ligament (ACL) rupture leads to the motion instability of the knee,resulting in changed tibiofemoral kinematics. Tibiofemoral motion is relatively complexand includes components of6degrees of freedom kinematics and femoral condylar motion(anterior-posterior, medial-lateral, and proximal-distal translations). ACL is the keyligament for normal knee motion. After ACL rupture, anterior and medial translations andinternal rotation of the tibia increase abnormally during knee flexion. Many previousstudies on tibiofemoral motion after ACL deficiency regarded femoral motion as onemovement, neglecting the motion of the medial and lateral condyles. Therefore, the effectof ACL deficiency on femoral condylar motion remains poorly understood at present.Inconsistent results still exist about the effect of ACL deficiency on femoral condylaranterior-posterior motion. Furthermore, there is no report in documents about the effect ofACL deficiency on femoral condylar medial-lateral and proximal-distal motions. Athorough knowledge of abnormal tibiofemoral6degrees of freedom kinematics andcondylar motion in ACL-deficient knees might further understand the influence of ACLdeficiency on femoral motion, and contribute to improving ACL reconstructive surgicaltechniques.The changed tibiofemoral kinematics caused by ACL deficiency might lead tosecondary injury of the meniscus and cartilage. With the changed tibiofemoral translationand rotation in ACL deficiency, posterior and lateral translations of tibiofemoral cartilagecontact point increase abnormally during flexion, resulting in a load shift fromweight-bearing area to non-weight-bearing area, which causes increased cartilage contactstress on the non-weight-bearing area. In addition, the stress on the medial meniscus alsoincreases obviously after ACL deficiency. So it is clear that ACL rupture leads to a redistribution of the interarticular stress, which predisposes the meniscus and cartilage tosecondary injury. Few previous studies have discussed the stress on each part of themeniscus (the anterior horn, the body and the posterior horn) and cartilage (tibial cartilageand femoral cartilage) after ACL rupture. Clinical observations have showed that the effectof ACL deficiency on each part of the meniscus and cartilage is not exactly the same.Therefore, a thorough knowledge of the abnormal stress on each part of the meniscus andcartilage in ACL deficiency might contribute to understanding the frequent areas ofsecondary injury in the meniscus and cartilage and the potential injury mechanism.The finite element analysis should be an effective method to obtain the stressdistributions of knee meniscus and cartilage. The work process of this method is as follows:A finite element model is created based on the geometric structure of the object. After themodel is given the right material property, load and boundary conditions, the finite elementanalysis of the force and motion is conducted for the object. The finite element analysis canovercome some disadvantages of in-vitro experiments, such as specimens being not readilyavailable or not used repeatedly. Furthermore, it can obtain the data which are not easy toobtain in in-vitro experiments, such as ligament tension, joint contact force and area, stressand strain on the meniscus and cartilage. The finite element analysis has become a reliablemethod to study the joint biomechanics.In the present article, we used biplane radiography technique to measure the thoroughtibiofemoral translation and rotation during upright weight-bearing flexion. The uprightweight-bearing condition is better than the non-weight-bearing condition to reflect andevaluate the pathological knee motion caused by ACL deficiency. The biplane radiographytechnique was first developed to measure the tibiofemoral motion, and accuracy of thistechnique was evaluated. Next, we used biplane radiography to measure and analyze thechanged tibiofemoral6degrees of freedom kinematics and femoral condylar motion(anterior-posterior, medial-lateral, and proximal-distal translations) in ACL-deficient kneesduring upright weight-bearing flexion. Then, we inputted the changed tibiofemoralkinematics after ACL deficiency into the finite element model, so as to analyze the changedstress distributions of the medial and lateral menisci, including the anterior horn, the bodyand the posterior horn, and of the medial and lateral tibiofemoral cartilages in ACL-deficient knees.Methods1. Biplane radiography technology was developed to measure tibiofemoral kinematicsand was validated in an in-vitro experiment. Two X-ray machines were used tosimultaneously capture the biplane X-ray images of the knee at6flexion positions. Next,the knee at extension was scanned by CT, and the three-dimensional tibiofemoral modelwas reconstructed using CT data. Then, the tibiofemoral model was matched to the biplanetibiofemoral images at6flexion positions, obtaining6tibiofemoral models based onbiplane radiography (BR models). The tibiofemoral kinematics of BR models was measuredthrough a joint coordinate system established on the model. The process of validation is asfollows: The same knee at6flexion positions was directly scanned by CT, and6tibiofemoral models were reconstructed (CT models). The tibiofemoral kinematics of CTmodels was also measured through the same joint coordinate system. Then, the accuracy ofbiplane radiography was evaluated by comparing the tibiofemoral kinematics of BR modelsto CT models (as a reference standard).2. The tibiofemoral motion after ACL rupture was measured during uprightweight-bearing flexion. Biplane radiography was used to measure the tibiofemoral motionin ACL-deficient knees and contralateral normal knees during squatting from extension to120°in patients with unilateral ACL rupture. And the tibiofemoral motion between the twosides was compared to obtain the changed kinematics after ACL rupture.3. To investigate the effect of ACL rupture on the stress distributions of the meniscusand cartilage, the finite element analysis was used to obtain the changed stress on each partof the meniscus and cartilage in ACL-deficient knees. The finite element tibiofemoralmodel was created, and the model‘s validity was verified. Next, the ACL-deficient modelwas created using the changed kinematics after ACL deficiency as boundary conditions.Then, the stress on each part of the meniscus and cartilage was calculated in theACL-deficient model and the normal model and compared between the two models atextension,15°, and30°flexions.Results1. Biplane radiography was able to accurately measure the tibiofemoral kinematics. The accuracy of biplane radiography was0.88mm,0.65mm, and0.61mm inanterior-posterior, medial-lateral, and proximal-distal translation, respectively; and1.03°,1.09°and0.76°in flexion-extension, internal-external and adduction-abduction rotation,respectively.2. At extension and15°of flexion, increased posterior translation of the lateral condylewas found in ACL-deficient knees, which was accompanied by excess posterior translationand external rotation of the femur during squatting. The anterior-posterior translation of themedial condyle, the medial-lateral and proximal-distal translations of femoral condyles andthe femur, and the femoral adduction-abduction were all comparable betweenACL-deficient knees and normal knees.3. On flexion phase from15°to60°during squatting, ACL deficiency led to asignificantly reduced extent of posterior movement of the lateral condyle, which wasaccompanied by reduced extents of posterior movement and external rotation of the femur.The extents of anterior-posterior movement of the medial condyle, of medial-lateral andproximal-distal movements of femoral condyles and the femur, and of femoraladduction-abduction were all comparable between ACL-deficient knees and normal knees.4. Between extension and30°of flexion, the stress increased on the medial and lateralmenisci after ACL deficiency. At extension, ACL rupture led to markedly increased stresson the anterior horn of the medial meniscus and slightly increased stress on the anteriorhorn and body of the lateral meniscus. At15°and30°of flexion, the stress increasedobviously on the posterior horn of the medial meniscus, and increased slightly on each partof the lateral meniscus after ACL rupture.5. ACL rupture led to increased stress on the medial and lateral tibiofemoral cartilagesbetween extension and30°of flexion. The growth rate of the stress increased gradually onthe medial femoral cartilage but decreased gradually on the medial tibial cartilage in theACL-deficient model during flexion. The growth rate of the stress was small ontibiofemoral cartilage in the lateral compartment after ACL rupture.Conclusions1. Biplane radiography technology was developed successfully to measure the thoroughtibiofemoral motion and was valid and feasible after verification. 2. During squatting from extension to120°, ACL deficiency primarily changed theanterior-posterior motion of the lateral condyle, producing not only posterior subluxation atearly flexion positions but also reduced extent of posterior movement on the middle flexionphase.3. Between extension and30°of flexion, ACL deficiency primarily changed the stressdistribution of the medial tibiofemoral compartment. After ACL rupture, the stress increasedobviously on the anterior and posterior horns of the medial meniscus at extension and atflexion, respectively. The growth rate of the stress increased gradually on the medialfemoral cartilage in the ACL-deficient model during flexion. |
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