Sensitivity of the knee joint response, muscle forces and stability to variations in gait kinematics-kinetics.

Sensitivity analysis of the knee joint response to variations in gait kinematics-kinetics as reported in the literature is crucial for improved understanding and more effective prevention-treatment strategies. Using our validated finite element-musculoskeletal (FE-MS) model of lower extremity, we investigate the sensitivity of knee anterior cruciate ligament (ACL), muscle, and contact forces plus stability to the reported scatter in asymptomatic gait data. Three highly loaded stance instants (25, 50 and 75%) with five levels (mean, ±0.5SD and ±SD) for each of six knee joint angles-moments are used employing Taguchi approach (25 experiments) and regression equations. ACL force drops significantly at larger flexion angles (all periods) and smaller internal moment (at 75% only) but increases with the flexion moment. Tibiofemoral (TF) medial-lateral contact force partitioning is found, contrary to the common claim, most sensitive to changes in the adduction angle and not in the adduction moment. Total TF contact force increases significantly at greater moments (but not angles), especially in the sagittal plane. Forces in lateral hamstrings are significantly influenced by changes in adduction angles-moments. Larger flexion moments (at 25 and 50%) significantly increase forces in quadriceps and on patellofemoral (PF) contact. Sagittal moment, adduction moment (at 75%) and flexion angle (at 25%) contribute most to the joint stability. A strong inverse correlation exists between the joint stability and the total TF compression force. These findings can be exploited to adapt and modify intact, injured and reconstructed knee joint responses during gait.

Sensitivity of medial-lateral load sharing to changes in adduction moments or angles in an asymptomatic knee joint model during gait.

BACKGROUND: Osteoarthritis (OA) of the knee joint is a common disease accompanied by pain and impaired mobility. Despite some recent concerns on the lack of correlation between the medial load and the knee adduction moment (KAM), KAM is routinely considered as a surrogate measure of medial load and hence a marker where its reduction is the main focus of preventive and treatment interventions. RESEARCH QUESTION: Determine the relative sensitivity of the tibiofemoral medial-lateral contact load partitioning to changes in the knee adduction angle (KAA) versus KAM. METHODS: Using a lower extremity hybrid musculoskeletal (MS) model driven by gait kinematics and kinetics, we compute here in asymptomatic subjects the sensitivity of the knee joint biomechanical response (muscle and ligament forces) in general and medial/lateral load partitioning in particular to the relative changes in the reported KAA versus changes in reported KAM (both by one standard deviation). RESULTS: As KAA increased (at constant KAM), so did the passive moment resistance of the knee joint which as a result and at all stance periods substantially reduced forces in lateral hamstrings while increasing those in medial hamstrings. At 25% and 75% stance as two highly loaded periods of gait, the drop in KAA (from + SD to -SD while at constant KAM) drastically reduced the medial contact force by 44% and 30% and the medial over lateral contact load and area ratios by 92% and 79% as well as 64% and 51%, respectively. In contrast, the equivalent alterations in KAM (by ± SD at constant KAA) had lower and less consistent effects (<7%) showing much smaller sensitivity to changes in KAM alone. Ligament forces altered at various stance periods with inconsistent trends; peak values of 418 N in the anterior cruciate ligament (90% carried by the posterolateral bundle) and 1056 N in the patellar tendon were computed both at 25% stance and minimum KAA. SIGNIFICANCE: These findings indicate a poor correlation between KAM and tibiofemoral load distribution suggesting instead that KAA and knee alignment should be in focus as the primary marker of knee joint load partitioning and associated prevention and treatment interventions.

Computation of the role of kinetics, kinematics, posterior tibial slope and muscle cocontraction on the stability of ACL-deficient knee joint at heel strike - Towards identification of copers from non-copers.

Rupture of anterior cruciate ligament (ACL) undermines normal activity and function of the knee joint and places it at higher risk of re-injury and degeneration. ACL reconstruction surgery neither necessarily ensures return to pre-injury activities nor alleviates risk of long-term degeneration. Here in this computational investigation of a lower-extremity hybrid model at heel strike (HS) of gait, we search for factors that influence the stability of the joint and hence the distinct performances between post-ACL injury copers and non-copers. Due to the very unstable state of the joint under the mean gait input data, joint rotations-moments, posterior tibial slope (PTS), and cocontraction were altered within the reported data in the literature and the effects on the joint stability (anterior tibial translation (ATT) and critical muscle stiffness coefficient (qcr)) were investigated. Results indicate that, in presence of both a small extension moment (0.1 or 0.2 Nm/kg) and a flexion rotation (∼5-8°), ACL-deficient (ACL-D) knee joint stability substantially improves to levels computed in the pre-injury intact joint. In addition, low cocontraction levels of 1-3% (in hamstrings and quads only and not in gastrocnemii) and reduced PTS (by 5°) further improve ACL-D joint stability. Therefore for a stable joint with ATT < 3 mm and qcr < 25 similar to those in the intact knee at HS, higher flexion angles (>5°) and a small extension moment (∼0.1-0.2 Nm/kg) (i.e., higher activity in hamstrings than quads) are required. A lower posterior tibial slope (by 5°) and a small minimum cocontraction level (1-3%) in hamstrings and quads (but not in gastrocnemii) are also beneficial. These results identify mechanisms likely in play at HS in gait of copers when compared to non-copers.

Computational stability of human knee joint at early stance in Gait: Effects of muscle coactivity and anterior cruciate ligament deficiency.

As one of the most complex and vulnerable structures of body, the human knee joint should maintain dynamic equilibrium and stability in occupational and recreational activities. The evaluation of its stability and factors affecting it is vital in performance evaluation/enhancement, injury prevention and treatment managements. Knee stability often manifests itself by pain, hypermobility and giving-way sensations and is usually assessed by the passive joint laxity tests. Mechanical stability of both the human knee joint and the lower extremity at early stance periods of gait (0% and 5%) were quantified here for the first time using a hybrid musculoskeletal model of the lower extremity. The roles of muscle coactivity, simulated by setting minimum muscle activation at 0-10% levels and ACL deficiency, simulated by reducing ACL resistance by up to 85%, on the stability margin as well as joint biomechanics (contact/muscle/ligament forces) were investigated. Dynamic stability was analyzed using both linear buckling and perturbation approaches at the final deformed configurations in gait. The knee joint was much more stable at 0% stance than at 5% due to smaller ground reaction and contact forces. Muscle coactivity, when at lower intensities (<3% of its maximum active force), increased dynamic stability margin. Greater minimum activation levels, however, acted asan ineffective strategy to enhance stability. Coactivation also substantially increased muscle forces, joint loads and ACL force and hence the risk of further injury and degeneration. A deficiency in ACL decreases total ACL force (by 31% at 85% reduced stiffness) and the stability margin of the knee joint at the heel strike. It also markedly diminishes forces in lateral hamstrings (by up to 39%) and contact forces on the lateral plateau (by up to 17%). Current work emphasizes the need for quantification of the lower extremity stability margin in gait.

3D active-passive response of human knee joint in gait is markedly altered when simulated as a planar 2D joint.

Musculoskeletal models of the lower extremity make a number of important assumptions when attempting to estimate muscle forces and tibiofemoral compartmental loads in activities such as gait. The knee is commonly idealized as a planar 2D joint in the sagittal plane with no consideration of motions and equilibrium in remaining planes. With muscle forces predicted, the static equilibrium in the frontal plane is then used to estimate compartmental loads neglecting also joint passive resistance and assuming condylar contact centers. We aimed here to comprehensively investigate the effects of such assumptions on predicted results. While simulating gait and using a hybrid lower extremity model that incorporates a detailed validated 3D finite element model of the knee joint, analyses are repeated with out-of-sagittal plane rotations and moment equilibrium equations neglected (2D model) and tibial compartmental forces estimated using equilibrium in the frontal plane while disregarding passive resistance and assuming fixed contact centers (1D model). Large unbalanced out-of-sagittal plane moments reaching peaks of 30 Nm abduction moment and 12 Nm internal moment at 25 % stance period are computed that are overlooked in the 2D model. Consideration of the knee as a planar 2D joint substantially diminishes muscle forces, anterior cruciate ligament force and tibiofemoral contact forces/stresses when compared to the 3D reference model. Total tibiofemoral contact force peaks at 25 % stance at 4.2 BW in the 3D model that drops to 3.0 BW in the 2D model. The location of contact centers on each plateau also noticeably alters (by as much as 5 mm). Tibiofemoral contact forces further change when the location of contact centers on each plateau is fixed. Results highlight the importance of accurate simulation of 3D motions and equilibrium equations as well as passive joint properties and contact centers.

Role of gastrocnemius activation in knee joint biomechanics: gastrocnemius acts as an ACL antagonist.

Gastrocnemius is a premier muscle crossing the knee, but its role in knee biomechanics and on the anterior cruciate ligament (ACL) remains less clear when compared to hamstrings and quadriceps. The effect of changes in gastrocnemius force at late stance when it peaks on the knee joint response and ACL force was initially investigated using a lower extremity musculoskeletal model driven by gait kinematics-kinetics. The tibiofemoral joint under isolated isometric contraction of gastrocnemius was subsequently analyzed at different force levels and flexion angles (0°-90°). Changes in gastrocnemius force at late stance markedly influenced hamstrings forces. Gastrocnemius acted as ACL antagonist by substantially increasing its force. Simulations under isolated contraction of gastrocnemius confirmed this role at all flexion angles, in particular, at extreme knee flexion angles (0° and 90°). Constraint on varus/valgus rotations substantially decreased this effect. Although hamstrings and gastrocnemius are both knee joint flexors, they play opposite roles in respectively protecting or loading ACL. Although the quadriceps is also recognized as antagonist of ACL, at larger joint flexion and in contrast to quadriceps, activity in gastrocnemius substantially increased ACL forces (anteromedial bundle). The fact that gastrocnemius is an antagonist of ACL should help in effective prevention and management of ACL injuries.

Alterations in knee contact forces and centers in stance phase of gait: A detailed lower extremity musculoskeletal model.

Evaluation of contact forces-centers of the tibiofemoral joint in gait has crucial biomechanical and pathological consequences. It involves however difficulties and limitations in in vitro cadaver and in vivo imaging studies. The goal is to estimate total contact forces (CF) and location of contact centers (CC) on the medial and lateral plateaus using results computed by a validated finite element model simulating the stance phase of gait for normal as well as osteoarthritis, varus-valgus and posterior tibial slope altered subjects. Using foregoing contact results, six methods commonly used in the literature are also applied to estimate and compare locations of CC at 6 periods of stance phase (0%, 5%, 25%, 50%, 75% and 100%). TF joint contact forces are greater on the lateral plateau very early in stance and on the medial plateau thereafter during 25-100% stance periods. Large excursions in the location of CC (>17mm), especially on the medial plateau in the mediolateral direction, are computed. Various reported models estimate quite different CCs with much greater variations (~15mm) in the mediolateral direction on both plateaus. Compared to our accurately computed CCs taken as the gold standard, the centroid of contact area algorithm yielded least differences (except in the mediolateral direction on the medial plateau at ~5mm) whereas the contact point and weighted center of proximity algorithms resulted overall in greatest differences. Large movements in the location of CC should be considered when attempting to estimate TF compartmental contact forces in gait.

Quantification of the role of tibial posterior slope in knee joint mechanics and ACL force in simulated gait.

The anterior cruciate ligament (ACL) rupture is a common knee joint injury with higher prevalence in female athletes. In search of contributing mechanisms, clinical imaging studies of ACL-injured individuals versus controls have found greater medial-lateral posterior tibial slope (PTS) in injured population irrespective of the sex and in females compared to males, with stronger evidence on the lateral plateau slope. To quantify these effects, we use a lower extremity musculoskeletal model including a detailed finite element (FE) model of the knee joint to compute the role of changes in medial and/or lateral PTS by ±5° and ±10° on knee joint biomechanics, in general, and ACL force, in particular, throughout the stance phase of gait. The model is driven by reported kinematics/kinetics of gait in asymptomatic subjects. Our predictions showed, at all stance periods, a substantial increase in the anterior tibial translation (ATT) and ACL force as PTS increased with reverse trends as PTS decreased. At mid-stance, for example, ACL force increased from 181 N to 317 N and 460 N as PTS increased by 5° and 10°, respectively, while dropped to 102 N and 0 N as PTS changed by -5° and -10°, respectively. These effects are caused primarily by change in PTS at the tibial plateau that carries a larger portion of joint contact force. Steeper PTS is a major risk factor, especially under activities with large compression, in markedly increasing ACL force and its vulnerability to injury. Rehabilitation and ACL injury prevention programs could benefit from these findings.

Knee joint passive stiffness and moment in sagittal and frontal planes markedly increase with compression.

Knee joints are subject to large compression forces in daily activities. Due to artefact moments and instability under large compression loads, biomechanical studies impose additional constraints to circumvent the compression position-dependency in response. To quantify the effect of compression on passive knee moment resistance and stiffness, two validated finite element models of the tibiofemoral (TF) joint, one refined with depth-dependent fibril-reinforced cartilage and the other less refined with homogeneous isotropic cartilage, are used. The unconstrained TF joint response in sagittal and frontal planes is investigated at different flexion angles (0°, 15°, 30° and 45°) up to 1800 N compression preloads. The compression is applied at a novel joint mechanical balance point (MBP) identified as a point at which the compression does not cause any coupled rotations in sagittal and frontal planes. The MBP of the unconstrained joint is located at the lateral plateau in small compressions and shifts medially towards the inter-compartmental area at larger compression forces. The compression force substantially increases the joint moment-bearing capacities and instantaneous angular rigidities in both frontal and sagittal planes. The varus-valgus laxities diminish with compression preloads despite concomitant substantial reductions in collateral ligament forces. While the angular rigidity would enhance the joint stability, the augmented passive moment resistance under compression preloads plays a role in supporting external moments and should as such be considered in the knee joint musculoskeletal models.

Steeper posterior tibial slope markedly increases ACL force in both active gait and passive knee joint under compression.

The role of the posterior tibial slope (PTS) in anterior cruciate ligament (ACL) risk of injury has been supported by many imaging studies but refuted by some in vitro works. The current investigation was carried out to compute the effect of ±5(o) change in PTS on knee joint biomechanics in general and ACL force/strain in particular. Two validated finite element (FE) models of the knee joint were employed; one active lower extremity musculoskeletal model including a complex FE model of the knee joint driven by in vivo kinematics/kinetics collected in gait of asymptomatic subjects, and the other its isolated unconstrained passive tibiofemoral (TF) joint considered under 1400 N compression at four different knee flexion angles (0°-45°). In the TF model, the compression force was applied at the joint mechanical balance point causing no rotations in sagittal and frontal planes. Changes in PTS moderately affected muscle forces and joint contact forces at mid-stance period. Both active (at mid-stance) and passive (at all flexion angles) models showed a substantial increase in the anterior tibial translation and ACL force as PTS increased with reverse trends as PTS decreased. In the active model of gait at mid-stance, ACL force increased by 75% (from 181 N to 317 N) in steeper PTS but decreased by 44% (to 102 N) in flatter PTS. The posterolateral bundle of ACL carried the load at smaller flexion angles with a shift to its anteromedial bundle as flexion increased. In accordance with earlier imaging studies, greater PTS is a major risk factor for ACL rupture especially in activities involving large compression forces.