Kevin B. Shelburne

Model Prediction of Anterior Cruciate Ligament Force during Drop-Landings

PURPOSE The aim of this study was to calculate and explain the pattern of force transmitted to the anterior cruciate ligament during soft-style drop-landings. We hypothesized that peak ACL loading is due to the anterior pull of the quadriceps on the tibia, as these muscles develop large eccentric forces upon impact. METHODS A three-dimensional model of the body was used to simulate drop-landing. The simulation was performed by entering into the model muscle excitation patterns based on experimental EMG. The input excitation patterns were modified to create a performance response of the model that matched experimental data. Joint angles, ground reaction forces, and muscle forces obtained from the landing simulation were then applied to a model of the lower limb that incorporated a three-dimensional model of the knee. RESULTS The model ACL was loaded only in the first 25% of the landing phase. Peak ACL force (approximately 0.4 BW) resulted from a complex interaction between the patellar tendon force, the compressive force acting at the tibiofemoral joint, and the force applied by the ground to the lower leg. The patellar tendon force and tibiofemoral contact force both applied significant anterior shear forces to the shank throughout the landing phase. These effects were modulated by another significant posterior shear force applied by the ground reaction, which served to limit the maximum force transmitted to the ACL. CONCLUSION The pattern of ACL force in drop-landing cannot be explained by the anterior pull of the quadriceps force alone.

Effect of posterior tibial slope on knee biomechanics during functional activity.

Treatment of medial compartment knee osteoarthritis with high tibial osteotomy can produce an unintended change in the slope of the tibial plateau in the sagittal plane. The effect of changing posterior tibial slope (PTS) on cruciate ligament forces has not been quantified for knee loading in activities of daily living. The purpose of this study was to determine how changes in PTS affect tibial shear force, anterior tibial translation (ATT), and knee‐ligament loading during daily physical activity. We hypothesized that tibial shear force, ATT, and ACL force all increase as PTS increases. A previously validated computer model was used to calculate ATT, tibial shear force, and cruciate‐ligament forces for the normal knee during three common load‐bearing tasks: standing, squatting, and walking. The model calculations were repeated with PTS altered in 1° increments up to a maximum change in tibial slope of 10°. Tibial shear force and ATT increased as PTS was increased. For standing and walking, ACL force increased as tibial slope was increased; for squatting, PCL force decreased as tibial slope was increased. The effect of changing PTS on ACL force was greatest for walking. The true effect of changing tibial slope on knee‐joint biomechanics may only be evident under physiologic loading conditions which include muscle forces.

Contributions of the Individual Muscles of the Shoulder to Glenohumeral Joint Stability During Abduction

The aim of this study was to determine the relative contributions of the deltoid and rotator cuff muscles to glenohumeral joint stability during arm abduction. A three-dimensional model of the upper limb was used to calculate the muscle and joint-contact forces at the shoulder for abduction in the scapular plane. The joints of the shoulder girdle-sternoclavicular joint, acromioclavicular joint, and glenohumeral joint-were each represented as an ideal three degree-of-freedom ball-and-socket joint. The articulation between the scapula and thorax was modeled using two kinematic constraints. Eighteen muscle bundles were used to represent the lines of action of 11 muscle groups spanning the glenohumeral joint. The three-dimensional positions of the clavicle, scapula, and humerus during abduction were measured using intracortical bone pins implanted into one subject. The measured bone positions were inputted into the model, and an optimization problem was solved to calculate the forces developed by the shoulder muscles for abduction in the scapular plane. The model calculations showed that the rotator cuff muscles (specifically, supraspinatus, subscapularis, and infraspinatus) by virtue of their lines of action are perfectly positioned to apply compressive load across the glenohumeral joint, and that these muscles contribute most significantly to shoulder joint stability during abduction. The middle deltoid provides most of the compressive force acting between the humeral head and the glenoid, but this muscle also creates most of the shear, and so its contribution to joint stability is less than that of any of the rotator cuff muscles.

Effects of foot orthoses and valgus bracing on the knee adduction moment and medial joint load during gait.

BACKGROUND Lateral shoe wedges and valgus knee braces are designed to decrease the force acting in the medial knee compartment by reducing the external adduction moment applied at the knee. The biomechanical changes introduced by these orthoses can be relatively small. Computer modeling and simulation offers an alternative approach for assessing the biomechanical performance of these devices. METHODS A three-dimensional model of the lower-limb was used to calculate muscle, ligament, and joint loading at the knee during gait. A lateral shoe wedge was simulated by moving the center of pressure of the ground reaction force up to 5mm laterally. A valgus knee brace was simulated by applying abduction moments of up to 12 Nm at the knee. FINDINGS Knee adduction moment and medial compartment load decreased linearly with lateral displacement of the center of pressure of the ground reaction force. A 1 mm displacement of the center of pressure decreased the peak knee adduction moment by 2%, while the peak medial compartment load was reduced by 1%. Knee adduction moment and medial compartment force also decreased linearly with valgus moments applied about the knee. A 1 Nm increase in brace moment decreased the peak knee adduction moment by 3%, while the peak medial compartment load was reduced by 1%. INTERPRETATION Changes in knee joint loading due to lateral shoe wedges and valgus bracing are small and may be difficult to measure by conventional gait analysis methods. The relationships between lateral shift in the center of pressure of the ground force, valgus brace moment, knee adduction moment, and medial joint load can be quantified and explained using computer modeling and simulation. These relationships may serve as a useful guide for evaluating the biomechanical efficacy of a generic wedge insole or knee brace.

Effect of muscle compensation on knee instability during ACL-deficient gait

PURPOSE The purpose of this investigation was to determine whether an isolated change in either quadriceps or hamstrings muscle force (quadriceps avoidance and hamstrings facilitation, respectively) is sufficient to stabilize the ACL-deficient (ACLd) knee during gait. METHODS A three-dimensional model of the lower limb was used to calculate anterior tibial translation in the intact and ACLd knee during gait. The model was then used to predict the amount of quadriceps and hamstrings force needed to restore anterior tibial translation (ATT) in the ACLd knee to an intact or maximum allowable level. RESULTS It was possible to reduce ATT in the ACLd knee to the level calculated for the intact knee by increasing the magnitude of hamstrings force (a hamstrings facilitation pattern). Although this strategy decreased the knee extensor moment calculated for walking, the effect was much less than that obtained when quadriceps force was reduced. Reducing quadriceps force to restore normal ATT resulted in complete elimination of the knee extensor moment (a quadriceps avoidance pattern); however, this strategy was insufficient to restore ATT to the level calculated for the intact knee over portions of the gait cycle. CONCLUSION The model simulations showed that increased hamstrings force was sufficient to stabilize the ACLd knee during gait. Reduced quadriceps force was insufficient to restore normal ATT for portions of the gait cycle.

Knee Kinematic Profiles during Drop Landings: A Biplane Fluoroscopy Study

INTRODUCTION The six degrees of freedom knee motion during dynamic activities is not well understood. PURPOSE Biplane fluoroscopy was used to measure the three-dimensional rotations and translations of healthy knees during stiff drop landings and to determine the relationships between three-dimensional rotations and anterior (ATT) and lateral tibial translations (LTT). METHODS Six males performed stiff drop landings from 40 cm while being filmed using a high-speed, biplane fluoroscopy system. Initial, peak, and excursions for rotations and translations were calculated, and relationships and changes in these variables were assessed (α = 0.05). RESULTS Knee flexion at contact was 13.9° ± 9.2° (mean ± SD) and increased to a peak of 44.0° ± 17.2° with an excursion of 31.5° ± 14.1°. Knee varus/valgus angle at contact was -0.3° ± 1.8° varus; subjects progressed into a mean peak valgus position of 1.5° ± 0.9° with total excursion of 2.5° ± 0.9°. Four of six subjects landed externally rotated (2.5° ± 3.0°); two landed internally rotated (-4.9° ± 1.5°), yielding a contact angle of -2.4° ± 3.0° of internal rotation, a peak internal rotation of -5.5° ± 6.0°, and excursion of 3.1° ± 5.5°. Peak ATT were 4.3 ± 0.7 mm (excursion = 2.1 ± 0.9 mm), occurring within 50 ms after contact. Peak LTT were 1.5 ± 1.4 mm (excursion = 2.6 ± 1.6 mm). Significant regressions were found between ATT and knee valgus angle (r2 = 0.39, P = 0.006), between LTT and internal rotation (r2 = 0.96, P < 0.0001), and between LTT and knee valgus angle (quadratic, r2 = 0.90, P < 0.0001). CONCLUSION This study provides a direct correlation between knee valgus angle with knee ATT and LTT during drop landings.

A Dynamic Model of the Knee and Lower Limb for Simulating Rising Movements

A two-dimensional dynamical model of the human body was developed and used to simulate muscle and knee-ligament loading during a fast rising movement. The hip, ankle, and toes were each modeled as a simple hinge joint. Relative movements of the femur, tibia, and patella in the sagittal plane were described using a more detailed representation of the knee. The geometry of the model bones was adapted from cadaver data. Eleven elastic elements described the geometric and mechanical properties of the knee ligaments and joint capsule. The patella was assumed to be massless. Smooth hypersurfaces were constructed and used to calculate the position and orientation of the patella during a forward integration of the model. Each hypersurface was formed by applying the principle of static equilibrium to approximate patellofemoral mechanics during the simulation. The model was actuated by 22 musculotendinous units, each unit represented as a three-element muscle in series with tendon. A first-order process was assumed to model muscle excitation-contraction dynamics. Dynamic optimization theory was used to calculate the pattern of muscle excitations that produces a coordinated rising movement from an initial squatting position in minimum time. The calculations support the contention that squatting is a relatively safe exercise for rehabilitation following ACL reconstruction. ACL forces remain less than 20 N for the duration of the task.

Measurements of Tibiofemoral Kinematics During Soft and Stiff Drop Landings Using Biplane Fluoroscopy

Background Previous laboratory studies of landing have defined landing techniques in terms of soft or stiff landings according to the degree of maximal knee flexion angle attained during the landing phase and the relative magnitude of the ground-reaction force. Current anterior cruciate ligament injury prevention programs are instructing athletes to land softly to avoid excessive strain on the anterior cruciate ligament. Purpose This study was undertaken to measure, describe, and compare tibiofemoral rotations and translations of soft and stiff landings in healthy individuals using biplane fluoroscopy. Study Design Controlled laboratory study. Methods The in vivo, lower extremity, 3-dimensional knee kinematics of 16 healthy adults (6 male and 10 female) instructed to land softly and stiffly in different trials were collected in biplane fluoroscopy as they performed the landing from a height of 40 cm. Results Average and maximum relative anterior tibial translation (average, 2.8 ± 1.2 mm vs 3.0 ± 1.4 mm; maximum, 4.7 ± 1.6 mm vs 4.4 ± 0.8 mm), internal/external rotation (average, 3.7° ± 5.1° vs 2.7° ± 4.3°; maximum, 5.6° ± 5.5° vs 4.9° ± 4.7°), and varus/valgus (average, 0.2° ± 1.2° vs 0.2° ± 1.0°; maximum, 1.7° ± 1.2° vs 1.6° ± 0.9°) were all similar between soft and stiff landings, respectively. The peak vertical ground-reaction force was significantly larger for stiff landings than for soft landings (2.60 ± 1.32 body weight vs 1.63 ± 0.73; P < .001). The knee flexion angle total range of motion from the minimum angle at contact to the maximum angle at peak knee flexion was significantly greater for soft landings than for stiff (55.4° ± 8.8° vs 36.8° ± 11.1°; P < .01). Conclusion Stiff landings, as defined by significantly lower knee flexion angles and significantly greater peak ground-reaction forces, do not result in larger amounts of anterior tibial translation or knee rotation in either varus/valgus or internal/external rotation in healthy individuals. Clinical Relevance In healthy knees, the musculature and soft tissues of the knee are able to maintain translations and rotations within a small, safe range during controlled landing tasks of differing demand. The knee kinematics of this healthy population will serve as a comparison for injured knees in future studies. It should be stressed that because the authors did not compare how the loads were distributed over the soft tissues of the knee between the 2 landing styles, the larger ground-reaction forces and more extended knee position observed during stiff landings should still be considered dangerous to the anterior cruciate ligament and other structures of the lower extremities, particularly in competitive settings where movements are often unanticipated.

Effect of hamstrings muscle action on stability of the ACL-deficient knee in isokinetic extension exercise.

OBJECTIVE To quantify the effect of hamstrings muscle action on stability of the anterior cruciate ligament deficient knee during isokinetic exercise at various speeds. DESIGN Mathematical modeling and forward-dynamics computer simulation were used to study the interactions between knee-extension speed, hamstrings co-contraction activity, and anterior tibial translation in the intact and anterior cruciate deficient knee. BACKGROUND There is much experimental evidence available to believe that hamstrings co-contraction can reduce anterior tibial translation in the anterior cruciate deficient knee. Little is known, however, about the level of hamstrings activation needed to keep anterior tibial translation within normal limits during functional activity. METHODS Isokinetic knee-extension was simulated with a sagittal-plane model used previously to study load sharing between the muscles, ligaments, and bones during isometric knee-extension exercise, isokinetic exercise, and squatting exercise. CONCLUSIONS Some amount of hamstrings activation is needed to stabilize an anterior cruciate deficient knee irrespective of how fast the knee extends. The level of hamstrings co-contraction needed to stabilize an anterior cruciate deficient knee is inversely related to extension speed. Hamstrings co-contraction is more effective in reducing anterior tibial translation than low-resistance extension exercise. RELEVANCE Excessive anterior tibial translation during knee-extension exercise may lead to damage of the meniscus and other passive structures inside the knee. If anterior cruciate deficient patients can be trained to co-contract their hamstrings during isokinetic knee-extension, then this exercise is appropriate for maintaining strength of the thigh muscles without compromising the anterior stability of the knee.

In Vivo Tibiofemoral Kinematics During 4 Functional Tasks of Increasing Demand Using Biplane Fluoroscopy

Background: The anterior cruciate ligament (ACL) has been well defined as the main passive restraint to anterior tibial translation (ATT) in the knee and plays an important role in rotational stability. However, it is unknown how closely the ACL and other passive and active structures of the knee constrain translations and rotations across a set of functional activities of increasing demand on the quadriceps. Hypothesis: Anterior tibial translation and internal rotation of the tibia relative to the femur would increase as the demand on the quadriceps increased. Study Design: Controlled laboratory study. Methods: The in vivo 3-dimensional knee kinematics of 10 adult female patients (height, 167.8 ± 7.1 cm; body mass, 57 ± 4 kg; body mass index [BMI], 24.8 ± 1.7 kg/m2; age, 29.7 ± 7.9 years) was measured using biplane fluoroscopy while patients completed 4 functional tasks. The tasks included an unloaded knee extension in which the patient slowly extended the knee from 90° to 0° of flexion in 2 seconds; walking at a constant pace of 90 steps per minute; a maximum effort isometric knee extension with the knee at 70° of flexion; and landing from a height of 40 cm in which the patient stepped off a box, landed, and immediately performed a maximum effort vertical jump. Results: Landing (5.6 ± 1.9 mm) produced significantly greater peak ATT than walking (3.1 ± 2.2 mm) and unweighted full extension (2.6 ± 2.1 mm) (P < .01), but there was no difference between landing and a maximum isometric contraction (5.0 ± 1.9 mm). While there was no significant difference in peak internal rotation between landing (19.4° ± 5.7°), maximum isometric contraction (15.9° ± 6.7°), and unweighted full knee extension (14.5° ± 7.7°), each produced significantly greater internal rotation than walking (3.9° ± 4.2°) (P < .001). Knee extension torque significantly increased for each task (P < .01): unweighted knee extension (4.7 ± 1.2 N·m), walking (36.5 ± 7.9 N·m), maximum isometric knee extension (105.1 ± 8.2 N·m), and landing (140.2 ± 26.2 N·m). Conclusion: Anterior tibial translations significantly increased as demand on the quadriceps and external loading increased. Internal rotation was not significantly different between landing, isometric contraction, and unweighted knee extension. Additionally, ATT and internal rotation from each motion were within the normal range, and no excessive amounts of translation or rotation were observed. Clinical Relevance: This study demonstrated that while ATT will increase as demand on the quadriceps and external loading increases, the knee is able to effectively constrain ATT and internal rotation. This suggests that the healthy knee has a safe envelope of function that is tightly controlled even though task demand is elevated.