Direct contribution of axial impact compressive load to anterior tibial load during simulated ski landing impact

Anterior tibial loading is a major factor involved in the anterior cruciate ligament (ACL) injury mechanism during ski impact landing. We sought to investigate the direct contribution of axial impact compressive load to anterior tibial load during simulated ski landing impact of intact knee joints without quadriceps activation. Twelve porcine knee specimens were procured. Four specimens were used as non-impact control while the remaining eight were mounted onto a material-testing system at 70° flexion and subjected to simulated landing impact, which was successively repeated with incremental actuator displacement. Four specimens from the impacted group underwent pre-impact MRI for tibial plateau angle measurements while the other four were subjected to histology and microCT for cartilage morphology and volume assessment. The tibial plateau angles ranged from 29.4 to 38.8°. There was a moderate linear relationship (Y=0.16X; R²=0.64; p<0.001) between peak axial impact compressive load (Y) and peak anterior tibial load (X). The anterior and posterior regions in the impacted group sustained surface cartilage fraying, superficial clefts and tidemark disruption, compared to the control group. MicroCT scans displayed visible cartilage deformation for both anterior and posterior regions in the impacted group. Due to the tibial plateau angle, increased axial impact compressive load can directly elevate anterior tibial load and hence contribute to ACL failure during simulated landing impact. Axial impact compressive load resulted in shear cartilage damage along anterior–posterior tibial plateau regions, due to its contribution to anterior tibial loading. This mechanism plays an important role in elevating ACL stress and cartilage deformation during impact landing.     

Extent and distribution of tibial osteochondral disruption during simulated landing impact with axial tibial rotation restraint

Post-traumatic knee osteochondral injuries are often coupled with anterior cruciate ligament (ACL) injury mechanisms during landing. However, it is not well understood whether restraining axial tibial rotation during landing would influence the extent and distribution of osteochondral disruption. Using ski landing as an example, this study subjected knee specimens to simulated landing impact without and with axial tibial rotation restraint, and investigated the extent and distribution of osteochondral disruption at the tibial plateau. Twenty-one porcine knee specimens were randomly divided into three test conditions, namely: (1) control, (2) impact only (I), and 3) impact with restraint (IR). Simulated landing impact was applied to the specimens based on a single 10 Hz haversine. Osteochondral explants were obtained from anterior, middle and posterior regions of medial and lateral tibial compartments. The extent of cartilage and trabecular disruption in these explants was examined based on histology, SEM and microCT. Only specimens in unrestrained condition incurred ACL failure upon impact. Restraining axial tibial rotation during simulated impact generally inflicted cartilage damage and deformation, and further caused trabecular disruption. Axial tibial rotation restraint did not necessarily restrict anterior tibial translation, as indicated by the presence of relative posterior femoral translation and osteochondral disruption at anterior–posterior tibial regions. While the results obtained in the current study may not be completely translatable to human models, there is likelihood that restraining axial tibial rotation during landing may help to prevent ACL failure, but will also induce osteochondral disruption in most tibial regions.     

Mechanisms of anterior-posterior stability of the knee joint under load-bearing

The anterior-posterior (AP) stability of the knee is an important aspect of functional performance. Studies have shown that the stability increases when compressive loads are applied, as indicated by reduced laxity, but the mechanism has not been fully explained. A test rig was designed which applied combinations of AP shear and compressive forces, and measured the AP displacements relative to the neutral position. Five knees were evaluated at compressive loads of 0, 250, 500, and 750 N, with the knee at 15° flexion. At each load, three cycles of shear force at ±100 N were applied. For the intact knee under load, the posterior tibial displacement was close to zero, due to the upward slope of the anterior medial tibial surface. The soft tissues were then resected in sequence to determine their role in AP laxity. After anterior cruciate ligament (ACL) resection, the anterior tibial displacement increased significantly even under load, highlighting its importance in stability. Meniscal resection further increased displacement but also the vertical displacement increased, implying the meniscus was providing a buffering effect. The PCL had no effect on any of the displacements under load. Plowing cartilage deformation and surface friction were negligible. This work highlighted the particular importance of the upward slope of the anterior medial tibial surface and the ACL to AP knee stability under load. The results are relevant to the design of total knees which reproduce anatomic knee stability behavior.     

Anterior laxity,lateral tibial slope,and in situ ACL force differentiate knees exhibiting distinct patterns of motion during a pivoting event: A human cadaveric study

Knee instability following anterior cruciate ligament (ACL) rupture compromises function and increases risk of injury to the cartilage and menisci. To understand the biomechanical function of the ACL, previous studies have primarily reported the net change in tibial position in response to multiplanar torques, which generate knee instability. In contrast, we retrospectively analyzed a cohort of 13 consecutively tested cadaveric knees and found distinct motion patterns, defined as the motion of the tibia as it translates and rotates from its unloaded, initial position to its loaded, final position. Specifically, ACL-sectioned knees either subluxated anteriorly under valgus torque (VL-subluxating) (5 knees) or under a combination of valgus and internal rotational torques (VL/IR-subluxating) (8 knees), which were applied at 15 and 30° flexion using a robotic manipulator. The purpose of this study was to identify differences between these knees that could be driving the two distinct motion patterns. Therefore, we asked whether parameters of bony geometry and tibiofemoral laxity (known risk factors of non-contact ACL injury) as well as in situ ACL force, when it was intact, differentiate knees in these two groups. VL-subluxating knees exhibited greater sagittal slope of the lateral tibia by 3.6 ± 2.4° (p = 0.003); less change in anterior laxity after ACL-sectioning during a simulated Lachman test by 3.2 ± 3.2 mm (p = 0.006); and, at the peak applied valgus torque (no internal rotation torque), higher posteriorly directed, in situ ACL force by 13.4 ± 11.3 N and 12.0 ± 11.6 N at 15° and 30° of flexion, respectively (both p ≤ 0.03). These results may suggest that subgroups of knees depend more on their ACL to control lateral tibial subluxation in response to uniplanar valgus and multiplanar valgus and internal rotation torques as mediated by anterior laxity and bony morphology.     

Excessive compression of the human tibio-femoral joint causes ACL rupture

The knee is one of the most frequently injured joints in the human body. A recent study suggests that axial compressive loads on the knee may play a role in injury to the anterior cruciate ligament (ACL) for the flexed knee, because of an approximate 10 degrees posterior tilt in the tibial plateau (J. Orthop. Res. 16 (1998) 122-127). The hypothesis of the current study was that excessive axial compressive loads in the human tibio-femoral (TF) joint would cause relative displacement and rotation of the tibia with respect to the femur, and result in isolated injury to the ACL when the knee is flexed to 60 degrees , 90 degrees or 120 degrees . Sixteen isolated knees from eleven fresh cadaver donors (74.3+/-10.5 yr) were exposed to repetitive TF compressive loads increasing in intensity until catastrophic injury. ACL rupture was documented in 14/16 cases. The maximum TF joint compressive force for ACL failure was 5.1+/-2.1 kN for all flexion angles combined. For the 90 degrees flexed knee, the injury occurred with a relative anterior displacement of 5.4+/-3.8mm, a lateral displacement of 4.1+/-1.4mm, and a 7.8+/-7.0 degrees internal rotation of the tibia with respect to the femur.     

Combined measurement and modeling of specimen-specific knee mechanics for healthy and ACL-deficient conditions

Quantifying the mechanical environment at the knee is crucial for developing successful rehabilitation and surgical protocols. Computational models have been developed to complement in vitro studies, but are typically created to represent healthy conditions, and may not be useful in modeling pathology and repair. Thus, the objective of this study was to create finite element (FE) models of the natural knee, including specimen-specific tibiofemoral (TF) and patellofemoral (PF) soft tissue structures, and to evaluate joint mechanics in intact and ACL-deficient conditions. Simulated gait in a whole joint knee simulator was performed on two cadaveric specimens in an intact state and subsequently repeated following ACL resection. Simulated gait was performed using motor-actuated quadriceps, and loads at the hip and ankle. Specimen-specific FE models of these experiments were developed in both intact and ACL-deficient states. Model simulations compared kinematics and loading of the experimental TF and PF joints, with average RMS differences [max] of 3.0° [8.2°] and 2.1° [8.4°] in rotations, and 1.7 [3.0] and 2.5 [5.1] mm in translations, for intact and ACL-deficient states, respectively. The timing of peak quadriceps force during stance and swing phase of gait was accurately replicated within 2° of knee flexion and with an average error of 16.7% across specimens and pathology. Ligament recruitment patterns were unique in each specimen; recruitment variability was likely influenced by variations in ligament attachment locations. ACL resections demonstrated contrasting joint mechanics in the two specimens with altered knee motion shown in one specimen (up to 5 mm anterior tibial translation) while increased TF joint loading was shown in the other (up to 400 N).     

A comparison of patellofemoral cartilage morphology and deformation in anterior cruciate ligament deficient versus uninjured knees

Anterior cruciate ligament (ACL) deficient patients have an increased rate of patellofemoral joint (PFJ) osteoarthritis (OA) as compared to the general population. Although the cause of post-injury OA is multi-factorial, alterations in joint biomechanics may predispose patients to cartilage degeneration. This study aimed to compare in vivo PFJ morphology and mechanics between ACL deficient and intact knees in subjects with unilateral ACL ruptures. Eight male subjects underwent baseline MRI scans of both knees. They then performed a series of 60 single-legged hops, followed by a post-exercise MRI scan. This process was repeated for the contralateral knee. The MR images were converted into three-dimensional surface models of cartilage and bone in order to assess cartilage thickness distributions and strain following exercise. Prior to exercise, patellar cartilage was significantly thicker in intact knees as compared to ACL deficient knees by 1.8%. In response to exercise, we observed average patellar cartilage strains of 5.4 ± 1.1% and 2.5 ± 1.4% in the ACL deficient and intact knees, respectively. Importantly, the magnitude of patellar cartilage strain in the ACL deficient knees was significantly higher than in the intact knees. However, while trochlear cartilage experienced a mean strain of 2.4 ± 1.6%, there was no difference in trochlear cartilage strain between the ACL deficient and uninjured knees. In summary, we found that ACL deficiency was associated with decreased patellar cartilage thickness and increased exercise-induced patellar cartilage strain when compared to the uninjured contralateral knees.     

The effects of knee joint kinematics on anterior cruciate ligament injury and articular cartilage damage

This study determined which knee joint motions lead to anterior cruciate ligament (ACL) rupture with the knee at 25° of flexion. The knee was subjected to internal and external rotations, as well as varus and valgus motions. A failure locus representing the relationship between these motions and ACL rupture was established using finite element simulations. This study also considered possible concomitant injuries to the tibial articular cartilage prior to ACL injury. The posterolateral bundle of the ACL demonstrated higher rupture susceptibility than the anteromedial bundle. The average varus angular displacement required for ACL failure was 46.6% lower compared to the average valgus angular displacement. Femoral external rotation decreased the frontal plane angle required for ACL failure by 27.5% compared to internal rotation. Tibial articular cartilage damage initiated prior to ACL failure in all valgus simulations. The results from this investigation agreed well with other experimental and analytical investigations. This study provides a greater understanding of the various knee joint motion combinations leading to ACL injury and articular cartilage damage.     

Estimation of ACL forces by reproducing knee kinematics between sets of knees: A novel non-invasive methodology

In situ force in the anterior cruciate ligament (ACL) has been quantified both in vitro in response to relatively simple loads by means of robotic technology, as well as in vivo in response to more complex loads by means of force transducers and computational models. However, a methodology has been suggested to indirectly estimate the in situ forces in the ACL in a non-invasive, non-contact manner by reproducing six-degree of freedom (six-DOF) in vivo kinematics on cadaveric knees using a robotic/UFS testing system. Therefore, the objective of this study was to determine the feasibility of this approach. Kinematics from eight porcine knees (source knees) were collected at 30 degrees , 60 degrees , and 90 degrees of flexion in response to: (1) an anterior load of 100 N and (2) a valgus load of 5 N m. The average of each kinematic data set was reproduced on a separate set of eight knees (target knees). The in situ forces in the ACL were determined for both sets of knees and compared. Significant differences (rho<0.05) were found between the source knees and the target knees for all flexion angles in response to an anterior load. However, in response to valgus loads, there was no significant difference between the source knees and the target knees at 30 degrees and 90 degrees of flexion. It was noted that there was a correlation between anterior knee laxity (the distance along the displacement axis from the origin to the beginning of the linear region of the load-displacement curve) and internal-external rotation. These data suggest that in order to obtain reproducible results one needs to first match knees to knees with comparable anterior knee laxity. Thus, an estimate of the in situ forces in the ACL during in vivo activities might be obtainable using this novel methodology.     

Iliotibial band tension affects patellofemoral and tibiofemoral kinematics

The iliotibial band (ITB) has an important role in knee mechanics and tightness can cause patellofemoral maltracking. This study investigated the effects of increasing ITB tension on knee kinematics. Nine fresh-frozen cadaveric knees had the components of the quadriceps loaded with 175 N. A Polaris optical tracking system was used to acquire joint kinematics during extension from 100° to 0° flexion. This was repeated after the following ITB loads: 30, 60 and 90 N. There was no change with 30 N load for patellar translation. On average, at 60 and 90 N, the patella translated laterally by 0.8 and 1.4 mm in the mid flexion range compared to the ITB unloaded condition. The patella became more laterally tilted with increasing ITB loads by 0.7°, 1.2° and 1.5° for 30, 60 and 90 N, respectively. There were comparable increases in patellar lateral rotation (distal patella moves laterally) towards the end of the flexion cycle. Increased external rotation of the tibia occurred from early flexion onwards and was maximal between 60° and 75° flexion. The increase was 5.2°, 9.5° and 13° in this range for 30, 60 and 90 N, respectively. Increased tibial abduction with ITB loads was not observed. The combination of increased patellar lateral translation and tilt suggests increased lateral cartilage pressure. Additionally, the increased tibial external rotation would increase the Q angle. The clinical consequences and their relationship to lateral retinacular releases may be examined, now that the effects of a tight ITB are known.     

Differences in tibial rotation during walking in ACL reconstructed and healthy contralateral knees

This study tested the hypotheses that in patients with a successful anterior cruciate ligament (ACL) reconstruction, the internal–external rotation, varus–valgus, and knee flexion position of reconstructed knees would be different from uninjured contralateral knees during walking. Twenty-six subjects with unilateral ACL reconstructions (avg 31 years, 1.7 m, 68 kg, 15 female, 24 months past reconstruction) and no other history of serious lower limb injury walked at a self-selected speed in the gait laboratory, with the uninjured contralateral knee as a matched control. Kinematic measurements of tibiofemoral motion were made using a previously-described point-cluster technique. Repeated-measures ANOVA (α=0.017) was used to compare ACL-reconstructed knees to their contralateral knees at four distinct points during the stance phase of walking. An offset towards external tibial rotation in ACL-reconstructed knees was maintained over all time points (95%CI 2.3±1.3°). Twenty-two out of twenty-six individuals experienced an average external tibial rotation offset throughout stance phase. Varus–valgus rotation and knee flexion were not significantly different between reconstructed and contralateral knees. These findings show that differences in tibial rotation during walking exist in ACL reconstructed knees compared to healthy contralateral knees, providing a potential explanation why these patients are at higher risk of knee osteoarthritis in the long-term.     

Regulation of the patellofemoral contact area: An essential mechanism in patellofemoral joint mechanics?

Although the relationship between contact area and pressure under physiological loading has been described in the feline patellofemoral joint, this interaction has only been examined under simplified loading conditions and/or considerably lower forces than those occurring during demanding activities in humans. We hypothesized that patellofemoral contact area increases non-linearly under an increasing joint reaction force to regulate patellofemoral pressure. Eight human cadaveric knees were ramp loaded with muscle forces representative of the stance phase of stair climbing at 30° knee flexion. Continuous pressure data were acquired with a pressure sensitive film that was positioned within the patellofemoral joint. While pressure was linearly dependent upon the resulting joint reaction force, contact area asymptotically approached a maximum value and reached 95% of this maximum at patellofemoral forces of 349–723 N (95% CI). Our findings indicate that the regulatory influence of increasing contact area to protect against high patellofemoral pressure is exhausted at relatively low loads.     

Fatigue injury risk in anterior cruciate ligament of target side knee during golf swing

A golf-related ACL injury can be linked with excessive golf play or practice because such over-use by repetitive golf swing motions can increase damage accumulation to the ACL bundles. In this study, joint angular rotations, forces, and moments, as well as the forces and strains on the ACL of the target-side knee joint, were investigated for ten professional golfers using the multi-body lower extremity model. The fatigue life of the ACL was also predicted by assuming the estimated ACL force as a cyclic load. The ACL force and strain reached their maximum values within a short time just after ball-impact in the follow-through phase. The smaller knee flexion, higher internal tibial rotation, increase of the joint compressive force and knee abduction moment in the follow-through phase were shown as to lead an increased ACL loading. The number of cycles to fatigue failure (fatigue life) in the ACL might be several thousands. It is suggested that the excessive training or practice of swing motion without enough rest may be one of factors to lead to damage or injury in the ACL by the fatigue failure. The present technology can provide fundamental information to understand and prevent the ACL injury for golf players.     

A knee-specific finite element analysis of the human anterior cruciate ligament impingement against the femoral intercondylar notch

This work presents a finite element analysis of anterior cruciate ligament (ACL) impingement against the intercondylar notch during tibial external rotation and abduction, as a mechanism of noncontact ACL injuries. Experimentally, ACL impingement was measured in a cadaveric knee in terms of impingement contact pressure and six degrees-of-freedom tibiofemoral kinematics. Three-dimensional geometries of the ACL, femur and tibia were incorporated into the finite element model of the individual knee specimen. A fiber-reinforced model was adopted, which accounts for the anisotropy, large deformation, nonlinearity and incompressibility of the ACL. With boundary conditions specified based on the experimental tibiofemoral kinematics, the finite element analysis showed that impingement between the ligament and the lateral wall of intercondylar notch could occur when qthe knee at 45° was externally rotated at 29.1° and abducted at 10.0°. Strong contact pressure and tensile stress occurred at the impinging and nonimpinging sides of the ligament, respectively. The impingement force and contact area estimated from the model matched their counterparts from the corresponding cadaver experiment. The modeling and experimental approach provides a useful tool to characterize potential ACL impingement on a knee-specific basis, which may help elucidate the ACL injury mechanism and develop more effective treatments.     

Static and fatigue strength of a fixation device transducer for measuring anterior cruciate ligament graft tension

To determine which exercises do not overload the graft-fixation complex during intensive rehabilitation from reconstructive surgery of the anterior cruciate ligament (ACL), it would be useful to measure ACL graft loads during rehabilitative activities in vivo in humans. A previous paper by Ventura et al. (1998) reported on the design of an implantable transducer integrated into a femoral fixation device and demonstrated that the transducer could be calibrated to measure graft loads to better than 10 percent full-scale error in cadaveric knees. By measuring both the static and fatigue strengths of the transducer, the purpose of the present study was to determine whether the transducer could be safely implanted in humans without risk of structural failure. Eight devices were loaded to failure statically. Additionally, seven devices were tested using the up-and-down method to estimate the median fatigue strength at a life of 225,000 cycles. The average ultimate strength was 1856 +/- 74 N and the median fatigue strength was 441 N at a life of 225,000 cycles. The maximum graft load during normal daily activities is estimated to be 500 N and the 225,000 cycle life corresponds to that of the average healthy individual during a 12-week period. Considering that patients who have had an ACL reconstruction are less ambulatory than normal immediately following surgery and that biologic incorporation of the graft should be well developed by 12 weeks thus decreasing the load transmitted to the fixation device, the FDT can be safely implanted in humans without undue risk of structural failure.     

Effect of pelvis impact angle on stresses at the femoral neck during falls

Improved understanding is required of how the mechanics of the fall affect hip fracture risk. We used a hip impact simulator to determine how peak stresses at the femoral neck were affected by pelvis impact angle, hip abductor muscle force, and use of a wearable hip protector.We simulated falls from standing (2 m/s impact velocity) involving initial hip abductor muscle forces of 700 or 300 N. Trials were acquired for impact to the lateral aspect of the greater trochanter, and impact to the pelvis rotated 5°, 10° and 15° anteriorly (positive) or posteriorly (negative). Measures were acquired with and without a commercially available hip protector. During trials, we measured three-dimensional forces with a load cell at the femoral neck, and derived peak compressive and tensile stresses.Peak compressive stress increased 37% (5.91 versus 4.31 MPa; p < 0.0005) and peak tensile stress increased 209% (2.31 versus 0.75 MPa; p < 0.0005) when the pelvis impact angle changed from 15° anterior to −15° posterior. For lateral impacts, the peak tensile and compressive stresses averaged 73% and 8% lower, respectively, in the 700 N than 300 N muscle force condition, but the effect was reversed for anteriolateral or posteriolateral impacts. The attenuation in peak compressive stress from the hip protector was greatest for posteriolateral impacts (−15 to −5°; 36–41%), and least for anteriolateral (+15°; 10%).These results clarify the effects on hip fracture risk during a fall of pelvis impact angle and muscle forces, and should inform the design of improved hip protectors.     

Forces in anterior cruciate ligament during simulated weight-bearing flexion with anterior and internal rotational tibial load

This study determined in-vitro anterior cruciate ligament (ACL) force patterns and investigated the effect of external tibial loads on the ACL force patterns during simulated weight-bearing knee flexions. Nine human cadaveric knee specimens were mounted on a dynamic knee simulator, and weight-bearing knee flexions with a 100N of ground reaction force were simulated; while a robotic/universal force sensor (UFS) system was used to provide external tibial loads during the movement. Three external tibial loading conditions were simulated, including no external tibial load (termed BW only), a 50N anterior tibial force (ATF), and a 5Nm internal rotation tibial torque (ITT). The tibial and femoral kinematics was measured with an ultrasonic motion capture system. These movement paths were then accurately reproduced on a robotic testing system, and the in-situ force in the ACL was determined via the principle of superposition. The results showed that the ATF significantly increased the in-situ ACL force by up to 60% during 0-55 degrees of flexion, while the ITT did not. The magnitude of ACL forces decreased with increasing flexion angle for all loading conditions. The tibial anterior translation was not affected by the application of ATF, whereas the tibial internal rotation was significantly increased by the application of ITT. These data indicate that, in a weight-bearing knee flexion, ACL provides substantial resistance to the externally applied ATF but not to the ITT.     

Finite element modeling of the breakage behavior of agricultural biomass pellets under different heights during handling and storage

It is very important to determine the amount of mechanical damage to biomass pellets during handling, transportation, and storage. However, it is difficult to determine the amount of damage to biomass pellets caused by existing external forces. However, a useful method is the finite element methods, which can be used in different engineering fields to simulate the posture of the material under defined boundary conditions. In this research, a drop test simulation of biomass pellet samples was performed by using the finite element method. An experimental study (compressive test) was carried out to measure some mechanical properties of the sample and use the obtained data in the finite element method simulation. The stress–strain curve of different biomass pellets was determined. Yield strength, Poisson’s ratio, ultimate strength and modulus of elasticity, and stress were identified. In the end, the maximum equivalent stress, highest contact force (generated normal force from target surface at impact), and shape of deformation of samples at impact were obtained from simulation results. The drop scenario was created with 25 steps after the impact site, and the FEM simulation was solved. The maximum stress value was 9.486 MPa, and the maximum generated force was 485.31 N. at step 8 of the FEM simulation. When the stress magnitudes were assessed, simulation outputs indicated that simulation stress values are inconsistent with experimental data.     

Effect of Compressive Strain on Cell Viability in Statically Loaded Articular Cartilage

Physiological loading of articulating joints is necessary for normal cartilage function. However, conditions of excessive overloading or trauma can cause cartilage injury resulting in matrix damage and cell death. The objective of this study was to evaluate chondrocyte viability within mechanically compressed articular cartilage removed from immature and mature bovine knees. Twenty-three mature and 68 immature cartilage specimens were subjected to static uniaxial confined-creep compressions of 0–70% and the extent of cell death was measured using fluorescent microscopic imaging. In both age groups, cell death was always initiated at the articular surface and increased linearly in depth with increasing strain magnitude. However, most of the cell death was localized within the superficial zone (SZ) of the cartilage matrix with the depth never greater than ~ 500 μm or 25% of the thickness of the test specimen. The immature cartilage was found to have a significantly greater (> 2 times) amount (depth) of cell death compared to the mature cartilage, especially at the higher strains. This finding was attributed to the lower compressive modulus of the immature cartilage in the SZ compared to that of the mature cartilage, resulting in a greater local matrix strain and concomitant cell surface membrane strain in this zone when the matrix was compressed. These results provide further insight into the capacity of articular cartilage in different age groups to resist the severity of traumatic injury from compressive loads.     

Computational assessment of the influence of vastus medialis obliquus function on patellofemoral pressures: Model evaluation

A study was performed to evaluate a computational model used to characterize the influence of vastus medialis obliquus (VMO) function on the patellofemoral pressure distribution. Ten knees were tested in vitro at 40°, 60° and 80° of knee flexion with quadriceps loads applied to represent a normal VMO, and with the VMO force decreased by approximately 50% to represent a weak VMO. The tests were performed with the cartilage intact and with a full thickness cartilage lesion centered on the lateral facet of the patella. The experimental tests were replicated computationally by applying discrete element analysis to a model of each knee constructed from MRI images. Repeated measures statistical comparisons were used to compare computational to experimental data and identify significant (p<0.05) differences due to the lesion and the applied VMO force. Neither the lateral force percentage nor the maximum lateral pressure varied significantly between the computational and experimental data. Creating a lesion significantly increased the maximum lateral pressure for all comparisons, except for the experimental data at 40°. Both computationally and experimentally, decrease in the VMO force increased the lateral force percentage by approximately 10% for all cases, and each increase was statistically significant. The maximum lateral pressure increase was typically less than 10% but was still significant for the majority of comparisons focused on the VMO strength. The results indicate that computational modeling can be used to characterize how varying quadriceps loading influences the patellofemoral force and pressure distributions while varying the condition of cartilage.