Relationship of anterior knee laxity to knee translations during drop landings: a bi-plane fluoroscopy study.

PURPOSE: Passive anterior knee laxity has been linked to non-contact ACL injury risk. High deceleration movements have been implicated in the non-contact ACL injury mechanism, and evidence suggests that greater anterior tibial translations (ATT) may occur in healthy knees that are lax compared to a tight knee. The purpose of this study was to determine the relationship between anterior knee laxity scores and ATT during drop landings using biplane fluoroscopy. METHODS: Sixteen healthy adults (10 women; 6 men) performed stiff drop landings (40 cm) while being filmed using a high-speed, biplane fluoroscopy system. Initial, peak and excursions for rotations and translations were calculated and regression analysis used to determine the 6DoF kinematic relationships with KT1000 scores with peak ATT occurring during the landing. RESULTS: KT1000 values were (+) correlated with peak ATT values for group (r = 0.89; P < 0.0001) and both genders (males, r = 0.97; P = 0.0003; females, r = 0.93; P = < 0.0001). Regression analysis yielded a significant linear fit for the group (r (2) = 0.80; Y (ATT-group) = - 0.516 + 1.2 × X (KT1000-group)) and for each gender (females: r(2) = 0.86; Y (ATT-females) = 0.074 + 1.2 × X (KT1000-females) and males: r (2) = 0.94; Y (ATT-males) = - 0.79 + 1.2 × X (KT1000-males)). CONCLUSION: A strong relationship was observed between passive anterior knee laxity measured via KT1000 and peak ATT experienced during dynamic activity in otherwise healthy persons performing a stiff drop-landing motion.

Determinants of cruciate-ligament loading during rehabilitation exercise.

OBJECTIVES: To predict and explain the pattern of cruciate-ligament loading during squatting exercises; to determine the effect of hamstrings co-contraction on anterior cruciate ligament (ACL) load during squatting; and to determine the effect of the weightbearing force on ACL load during squatting. DESIGN: Mathematical modeling of the human musculoskeletal system. BACKGROUND: Squatting is a commonly prescribed exercise for strengthening the muscles of the thigh following ACL reconstruction. Although the forces induced in the ACL are purported to be low, no experimental data are available to corroborate this claim. The reason is that measurements of knee-ligament forces are difficult to obtain in vivo. METHODS: The human body was modeled as a four-segment, six-degrees of freedom, planar linkage. The hip, ankle and toes were each modeled as a hinge joint. The relative displacements of the femur, tibia and patella were calculated using a three-degrees of freedom, sagittal-plane model of the knee. Eleven elastic were used to describe the geometric and mechanical properties of the knee ligaments. The model was actuated by 22 musculotendinous units. Optimization theory was used to calculate the forces developed in the muscles and the forces transmitted to the knee ligaments during squatting. RESULTS: The model ACL was loaded from full extension to 10 degrees of knee flexion during squatting; the model PCL was loaded at knee-flexion angles greater than 10 degrees. The pattern of cruciate-ligament loading is determined by the shapes of the articulating surfaces of the bones and by the changing orientation of the hamstrings muscles at the knee. Hamstrings co-contraction is the major determinant of ACL loading during squatting exercises; the weightbearing force has a relatively small effect on the force induced in the ACL. CONCLUSION: The calculations support the contention that squatting is a relatively safe exercise for strengthening the muscles of the thigh following reconstruction of the ACL. RELEVANCE: Knowledge of the forces borne by the knee ligaments is important for designing exercise regimens subsequent to ligament injury and repair. The quadriceps and hamstrings muscles may be strengthened without loading a newly reconstructed ACL by performing squats with the knee flexed to 10 degrees and greater.

Dependence of cruciate-ligament loading on muscle forces and external load.

A sagittal-plane model of the knee is used to predict and explain the relationships between the forces developed by the muscles, the external loads applied to the leg, and the forces induced in the cruciate ligaments during isometric exercises. The geometry of the model bones is adapted from cadaver data. Eleven elastic elements describe the geometric and mechanical properties of the cruciate ligaments, the collateral ligaments, and the posterior capsule. The model is actuated by 11 musculotendinous units, each unit represented as a three-element muscle in series with tendon. For isolated contractions of the quadriceps, ACL force increases as quadriceps force increases for all flexion angles between 0 and 80 degrees; the ACL is unloaded at flexion angles greater than 80 degrees. When quadriceps force is held constant, ACL force decreases monotonically as knee-flexion angle increases. The relationship between ACL force, quadriceps force, and knee-flexion angle is explained by the geometry of the knee-extensor mechanism and by the changing orientation of the ACL in the sagittal plane. For isolated contractions of the hamstrings, PCL force increases as hamstrings force increases for all flexion angles greater than 10 degrees; the PCL is unloaded at flexion angles less than 10 degrees. When hamstrings force is held constant, PCL force increases monotonically with increasing knee flexion. The relationship between PCL force, hamstrings force, and knee-flexion angle is explained by the geometry of the hamstrings and by the changing orientation of the PCL in the sagittal plane. At nearly all knee-flexion angles, hamstrings co-contraction is an effective means of reducing ACL force. Hamstrings co-contraction cannot protect the ACL near full extension of the knee because these muscles meet the tibia at small angles near full extension, and so cannot apply a sufficiently large posterior shear force to the leg. Moving the restraining force closer to the knee-flexion axis decreases ACL force; varying the orientation of the restraining force has only a small effect on cruciate-ligament loading.

A musculoskeletal model of the knee for evaluating ligament forces during isometric contractions.

A model of the knee in the sagittal plane was developed to study the forces in the ligaments induced by isometric contractions of the extensor and flexor muscles. The geometry of the distal femur was obtained from cadaver data. The tibial plateau and patellar facet were modeled as flat surfaces. Eleven elastic elements were used to describe the mechanical behavior of the anterior and posterior cruciate ligaments (ACL and PCL), the medial and lateral collateral ligaments (MCL and LCL), and the posterior capsule. The model knee was actuated by 11 musculotendinous units, each muscle represented by a Hill-type contractile element, a series-elastic element, and a parallel-elastic element. Tendon was assumed to be elastic. The response of the model to anterior-posterior drawer suggests that the geometrical and mechanical properties of the model ligaments approximate the behavior of real ligaments in the intact knee. Calculations for a simulated quadriceps leg raise indicate further that the two-dimensional model reproduces the response of the three-dimensional knee under similar conditions of loading and constraint. During maximum isometric contractions of the quadriceps, the model ACL is loaded from full extension to 80 degrees C of flexion; the model PCL is loaded at 70 degrees of flexion and greater. For maximum isometric extension, ACL forces in the range 0-20 degrees of flexion depend most heavily upon the force-length properties of the quadriceps. At flexion angles greater than 20 degrees, cruciate ligament forces are determined by the geometry of the articulating surfaces of the bones. During isolated contractions of the hamstrings and gastrocnemius muscles, the model ACL is loaded from full extension to 10 degrees of flexion; the model PCL is loaded at all flexion angles greater than 10 degrees. Isolated contractions of the flexor muscles cannot unload the ACL near full extension, as the behavior of the ACL in this region is governed by the shapes of the bones. At 10 degrees of flexion or greater, the overall pattern of PCL force is explained by the force length properties of the hamstrings and by the geometrical arrangement of the flexor muscles about the knee.