Biomechanical evaluation of the pedicle screw insertion depth effect on screw stability under cyclic loading and subsequent pullout.

STUDY DESIGN: A biomechanical ex vivo study of the human lumbar spine. OBJECTIVE: To evaluate the effects of transpedicular screw insertion depth on overall screw stability and pullout strength following cyclic loading in the osteoporotic lumbar spine. SUMMARY OF BACKGROUND DATA: Although much is known about the clinical outcomes of spinal fusion, questions remain in our understanding of the biomechanical strength of lumbar pedicle screw fixation as it relates to screw sizing and placement. Biomechanical analyses examining ideal pedicle screw depth with current pedicle screw technology are limited. In the osteoporotic spine, optimized pedicle screw insertion depth may improve construct strength, decreasing the risk of loosening or pullout. METHODS: A total of 100 pedicles from 10 osteoporotic lumbar spines were randomly instrumented with pedicle screws in mid-body, pericortical, and bicortical depths. Instrumented specimens underwent cyclic loading (5000 cycles of ±2 N m pure flexion moment) and subsequent pullout. Screw loosening, failure loads, and energy absorption were calculated. RESULTS: Cyclic loading significantly (P<0.001) reduced screw-bone angular stiffness between prefatigue and postfatigue conditions by 25.6%±17.9% (mid-body), 20.8%±14.4% (pericortical), and 14.0%±13.0% (bicortical). Increased insertion depth resulted in lower levels of reduction in angular stiffness, which was only significant between mid-body and bicortical screws (P=0.009). Pullout force and energy of 583±306 N and 1.75±1.98 N m (mid-body), 713±321 N and 2.40±1.79 N m (pericortical), and 797±285 N and 2.97±2.33 N m (bicortical) were observed, respectively. Increased insertion depth resulted in higher magnitudes of both pullout force and energy, which was significant only for pullout force between mid-body and bicortical screws (P=0.005). CONCLUSION: Although increased screw depth led to increased fixation and decreased loosening, additional purchase of the stiff anterior cortex is essential to reach superior screw-bone construct stability and stiffness.

Finite element model of the knee for investigation of injury mechanisms: development and validation.

Multiple computational models have been developed to study knee biomechanics. However, the majority of these models are mainly validated against a limited range of loading conditions and/or do not include sufficient details of the critical anatomical structures within the joint. Due to the multifactorial dynamic nature of knee injuries, anatomic finite element (FE) models validated against multiple factors under a broad range of loading conditions are necessary. This study presents a validated FE model of the lower extremity with an anatomically accurate representation of the knee joint. The model was validated against tibiofemoral kinematics, ligaments strain/force, and articular cartilage pressure data measured directly from static, quasi-static, and dynamic cadaveric experiments. Strong correlations were observed between model predictions and experimental data (r > 0.8 and p < 0.0005 for all comparisons). FE predictions showed low deviations (root-mean-square (RMS) error) from average experimental data under all modes of static and quasi-static loading, falling within 2.5 deg of tibiofemoral rotation, 1% of anterior cruciate ligament (ACL) and medial collateral ligament (MCL) strains, 17 N of ACL load, and 1 mm of tibiofemoral center of pressure. Similarly, the FE model was able to accurately predict tibiofemoral kinematics and ACL and MCL strains during simulated bipedal landings (dynamic loading). In addition to minimal deviation from direct cadaveric measurements, all model predictions fell within 95% confidence intervals of the average experimental data. Agreement between model predictions and experimental data demonstrates the ability of the developed model to predict the kinematics of the human knee joint as well as the complex, nonuniform stress and strain fields that occur in biological soft tissue. Such a model will facilitate the in-depth understanding of a multitude of potential knee injury mechanisms with special emphasis on ACL injury.

The dynamic response of human lungs due to underwater shock wave exposure.

Since the 19th century, underwater explosions have posed a significant threat to service members. While there have been attempts to establish injury criteria for the most vulnerable organs, namely the lungs, existing criteria are highly variable due to insufficient human data and the corresponding inability to understand the underlying injury mechanisms. This study presents an experimental characterization of isolated human lung dynamics during simulated exposure to underwater shock waves. We found that the large acoustic impedance at the surface of the lung severely attenuated transmission of the shock wave into the lungs. However, the shock wave initiated large bulk pressure-volume cycles that are distinct from the response of the solid organs under similar loading. These pressure-volume cycles are due to compression of the contained gas, which we modeled with the Rayleigh-Plesset equation. The extent of these lung dynamics was dependent on physical confinement, which in real underwater blast conditions is influenced by factors such as rib cage properties and donned equipment. Findings demonstrate a potential causal mechanism for implosion injuries, which has significant implications for the understanding of primary blast lung injury due to underwater blast exposures.

Suture capsulorrhaphy versus capsulolabral advancement for shoulder instability.

PURPOSE: The purpose of this study was to test the strength of a suture capsulorrhaphy repair versus a capsulolabral repair with knotless suture anchors in a cadaveric model with anteroinferior shoulder instability. METHODS: Fourteen cadaveric shoulders were tested with either a suture capsulorrhaphy to the intact labrum or a capsulolabral advancement using a knotless suture anchor into the glenoid. Specimens were translated with the shoulder in an abducted, externally rotated position to failure. RESULTS: The capsulolabral advancement showed a significantly higher load to failure than did the suture capsulorrhaphy group (P = .030). CONCLUSIONS: Capsulolabral advancement with suture anchors may offer greater initial strength when compared with a suture capsulorrhaphy. In the setting of shoulder instability without evidence of a labral tear, the capsulolabral advancement technique may be considered biomechanically superior. CLINICAL RELEVANCE: In the setting of shoulder instability due to capsular insufficiency, the capsulolabral advancement may be considered biomechanically superior to a traditional suture capsulorrhaphy.

Dynamic Response of the Thoracolumbar and Sacral Spine to Simulated Underbody Blast Loading in Whole Body Post Mortem Human Subject Tests.

Fourteen simulated underbody blast impact sled tests were performed using a horizontal deceleration sled with the aim of evaluating the dynamic response of the spine in under various conditions. Conditions were characterized by input (peak velocity and time-to-peak velocity for the seat and floor), seat type (rigid or padded) and the presence of personnel protective equipment (PPE). A 50% (T12) and 30% (T8) reduction in the thoracic spine response for the specimens outfitted with PPE was observed. Longer duration seat pulses (55 ms) resulted in a 68-78% reduction in the magnitude of spine responses and a reduction in the injuries at the pelvis, thoracic and lumbar regions when compared to shorter seat pulses (10 ms). The trend analysis for the peak Z (caudal to cranial) acceleration measured along the spine showed a quadratic fit (p < 0.05), rejecting the hypothesis that the magnitude of the acceleration would decrease linearly as the load traveled caudal to cranial through the spine during an Underbody Blast (UBB) event. A UBB event occurs when an explosion beneath a vehicle propels the vehicle and its occupants vertically. Further analysis revealed a relationship (p < 0.01) between peak sacrum acceleration and peak spine accelerations measured at all levels. This study provides an initial analysis of the relationship between input conditions and spine response in a simulated underbody blast environment.

Mechanisms and timing of injury to the thoracic, lumbar and sacral spine in simulated underbody blast PMHS impact tests.

During an underbody blast (UBB) event, mounted occupants are exposed to high rate loading of the spine via the pelvis. The objective of this study was to simulate UBB loading conditions and examine mechanisms of injury in the thoracic, lumbar and sacral spine. Fourteen instrumented, whole-body, postmortem human subject (PMHS) experiments were performed using the WSU-decelerative horizontal sled system. The specimens were positioned supine on a decelerative sled, which then impacted an energy absorbing system mounted to a concrete barrier. Variables included the peak velocity and time-to-peak velocity for seat and floor, and the presence or absence of personal protective equipment (PPE) and seat padding. Post-test CT scans and autopsies were performed to identify the presence and severity of injuries. Acceleration and angular rate data collected at vertebra T1, T5, T8, T12, and S1 were used to assess injury timing and mechanisms. Additionally, joint time-frequency analysis (JTFA) of the spinal Z acceleration of the sacrum and vertebrae was developed with the aim of verifying spinal fracture timing. Injuries observed in the spine were attributed to axial compression applied through the pelvis, together with flexion moment due to the offset in the center of gravity of the torso, and are consistent with UBB-induced combat injuries reported in the literature. The injury timing estimation techniques discussed in this study provide a time interval when the fractures are predicted to have occurred. Furthermore, this approach serves as an alternative to the estimation methods using acoustic sensors, force and acceleration traces, and strain gauges.

Evaluation of the Whole Body Spine Response to Sub-Injurious Vertical Loading.

It is critical to understand the relationship between under-body blast (UBB) loading and occupant response to provide optimal protection to the warfighter from serious injuries, many of which affect the spine. Previous studies have examined component and whole body response to accelerative based UBB loading. While these studies both informed injury prediction efforts and examined the shortcomings of traditional anthropomorphic test devices in the evaluation of human injury, few studies provide response data against which future models could be compared and evaluated. The current study examines four different loading conditions on a seated occupant that demonstrate the effects of changes in the floor, seat, personal protective equipment (PPE), and reclined posture on whole body post-mortem human surrogate (PMHS) spinal response in a sub-injurious loading range. Twelve PMHS were tested across floor velocities and time-to-peak (TTP) that ranged from 4.0 to 8.0 m/s and 2 to 5 ms, respectively. To focus on sub-injurious response, seat velocities were kept at 4.0 m/s and TTP ranged from 5 to 35 ms. Results demonstrated that spine response is sensitive to changes in TTP and the presence of PPE. However, spine response is largely insensitive to changes in floor loading. Data from these experiments have also served to develop response corridors that can be used to assess the performance and predictive capability of new test models used as human surrogates in high-rate vertical loading experiments.

Whole Body PMHS Response in Injurious Experimental Accelerative Loading Events.

Previous studies involving whole-body post-mortem human surrogates (PHMS) have generated biomechanical response specifications for physically simulated accelerative loading intended to reproduce seat and floor velocity histories occurring in under-body blast (UBB) events (e.g.,. References 10, 11, 21 These previous studies employed loading conditions that only rarely produced injuries to the foot/ankle and pelvis, which are body regions of interest for injury assessment in staged UBB testing using anthropomorphic test devices. To investigate more injurious whole-body conditions, three series of tests were conducted with PMHS that were equipped with military personal protective equipment and seated in an upright posture. These tests used higher velocity and shorter duration floor and seat inputs than were previously used with the goal of producing pelvis and foot/ankle fractures. A total of nine PMHS that were approximately midsize in stature and mass were equally allocated across three loading conditions, including a 15.5 m/s, 2.5 ms time-to-peak (TTP) floor velocity pulse with a 10 m/s, 7.5 ms TTP seat pulse; a 13 m/s, 2.5 ms TTP floor pulse with a 9.0 m/s, 5 ms TTP seat pulse; and a 10 m/s, 2.5 ms TTP floor pulse with a 6.5 m/s, 7.5 ms TTP seat pulse. In the first two conditions, the seat was padded with a ~ 120-mm-thick foam cushion to elongate the pulse experienced by the PMHS. Of the nine PMHS tests, five resulted in pelvic ring fractures, five resulted in a total of eight foot/ankle fractures (i.e., two unilateral and three bilateral fractures), and one produced a femur fracture. Test results were used to develop corridors describing the variability in kinematics and in forces applied to the feet, forces applied to the pelvis and buttocks in rigid seat tests, and in forces applied to the seat foam in padded seat tests. These corridors and the body-region specific injury/no-injury response data can be used to assess the performance and predictive capability of anthropomorphic test devices and computational models used as human surrogates in simulated UBB testing.

Failure of coracoclavicular artificial graft reconstructions from repetitive rotation.

PURPOSE: To assess how suture type and suture construct in an augmented Weaver-Dunn reconstruction affect coracoclavicular sling failure and rotary stability. METHODS: Fifteen cadaveric shoulders were tested in rotation about the long axis of the clavicle with 10 lb of simulated arm weight. The clavicle was rotated 50 degrees about its long axis, and the applied torque was recorded. Next, modified Weaver-Dunn reconstruction was conducted. Two types of coracoclavicular sling (opposed drill holes through the clavicle and complete loop around the clavicle) were tested by use of 3 different sutures (FiberWire [Arthrex, Naples, FL], Mersilene tape [Ethicon, Somerville, NJ], and braided polydioxanone [PDS] [Ethicon]). For each sling-suture combination, the joint was retested over 50 degrees of rotation and then cycled over 40 degrees of rotation for 15,000 cycles or until failure. RESULTS: After modified Weaver-Dunn reconstruction with either sling construct, mean torque over 50 degrees of acromioclavicular rotation was significantly reduced in posterior (P < .0001) and anterior (P < .0001) rotation, with any suture material tested. When the coracoclavicular sling was placed through opposed drill holes, no wear to the bone or suture was observed. When the sling material was looped around the clavicle, FiberWire and PDS resulted in abrasion of soft tissue and periosteum. In all cases sawing motion between bone and suture was observed at the coracoid. The FiberWire itself failed at a mean of 8,213 cycles. Some wear was noted in the Mersilene tape. PDS suture showed no wear. CONCLUSIONS: In a cadaveric model of modified Weaver-Dunn reconstruction, a coracoclavicular suture loop was used to augment coracoacromial ligament transfer. Suture loops secured around the entire clavicle were shown to contribute to increased abrasive wear. Securing suture loops through opposed drill holes in the clavicle resulted in decreased abrasive wear. CLINICAL RELEVANCE: Proper selection of suture type and suture construct may affect the failure rate of augmented Weaver-Dunn reconstructions.

Effects of hindfoot constraint on syndesmotic displacement.

BACKGROUND: The effects of altered hindfoot kinematics on the syndesmosis have not been previously studied. Our purpose was to test how the magnitude of displacement across the syndesmosis changes under simulated subtalar (ST) and/or talonavicular (TN) fusion and with altered hindfoot position in a cadaveric model. MATERIALS AND METHODS: Six cadaveric specimens (three matched pairs) age 33 to 43 years were disarticulated at the knee and mounted into a custom six-degree-of-freedom testing frame with a simulated ground reaction force of 700 N and a tensile Achilles load of 500 N. Specimens were then tested through four cycles of internal and external rotation under four conditions: simulated combined ST + TN fusion, ST fusion alone, TN fusion alone and no fusion. Each condition was tested in the neutral coronal position and 9 degrees of inversion and eversion. Infrared light emitting diode (irLED) marker arrays were used to track displacement across the anterior tibiofibular ligament (ATiFL) in order to assess displacement across the syndesmosis. RESULTS: Without fusion, displacement across the ATiFL in inversion is greater than that in neutral (p = 0.015). With ST, the measured ATiFL displacement in inversion is greater than that in neutral (p = 0.042). In neutral, the combined ST + TN significantly increased ATiFL displacement when compared to no fusion (p = 0.0043). Increased displacement was seen in inversion compared to eversion in all testing conditions. CONCLUSION: Simulated ST and TN fusion increases displacement across the ATiFL during simulated physiologic loading. Hindfoot inversion also increases displacement of the ATiFL. CLINICAL RELEVANCE: These observations may have clinical implications with respect to syndesmotic injury and total ankle arthroplasty.

Anatomically-based skeletal coordinate systems for use with impact biomechanics data intended for anthropomorphic test device development.

Post-mortem human subjects (PMHS) are frequently used to characterize biomechanical response and injury tolerance of humans to various types of loading by means of instrumentation installed directly on the skeleton. Data extracted from such tests are often used to develop and validate anthropomorphic test devices (ATDs), which function as human surrogates in tests for injury assessment. Given that the location and orientation of installed instrumentation differs between subjects, nominally similar measurements made on different PMHS must be transformed to standardized, skeletal-based local coordinate systems (LCS) before appropriate data comparisons can be made. Standardized PMHS LCS that correspond to ATD instrumentation locations and orientations have not previously been published. This paper introduces anatomically-defined PMHS LCS for body regions in which kinematic measurements are made using ATDs. These LCS include the head, sternum, single vertebrae, pelvis, femurs (distal and proximal), and tibiae (distal and proximal) based upon skeletal landmarks extracted from whole body CT scans. The proposed LCS provide a means to standardize the reporting of PMHS data, and facilitate both the comparison of PMHS impact data across institutions and the application of PMHS data to the development and validation of ATDs.

Kinematic and Biomechanical Response of Post-Mortem Human Subjects Under Various Pre-Impact Postures to High-Rate Vertical Loading Conditions.

Limited data exist on the injury tolerance and biomechanical response of humans to high-rate, under-body blast (UBB) loading conditions that are commonly seen in current military operations, and there are no data examining the influence of occupant posture on response. Additionally, no anthropomorphic test device (ATD) currently exists that can properly assess the response of humans to high-rate UBB loading. Therefore, the purpose of this research was to examine the response of post-mortem human surrogates (PMHS) in various seated postures to high-rate, vertical loading representative of those conditions seen in theater. In total, six PMHS tests were conducted using loading pulses applied directly to the pelvis and feet of the PMHS: three in an acute posture (foot, knee, and pelvis angles of 75°, 75°, and 36°, respectively), and three in an obtuse posture (15° reclined torso, and foot, knee, and pelvis angles of 105°, 105°, and 49.5°, respectively). Tests were conducted with a seat velocity pulse that peaked at ~4 m/s with a 30-40 ms time to peak velocity (TTP) and a floor velocity that peaked at 6.9-8.0 m/s (2-2.75 ms TTP). Posture condition had no influence on skeletal injuries sustained, but did result in altered leg kinematics, with leg entrapment under the seat occurring in the acute posture, and significant forward leg rotations occurring in the obtuse posture. These data will be used to validate a prototype ATD meant for use in high-rate UBB loading scenarios.

Strain Response of the Anterior Cruciate Ligament to Uniplanar and Multiplanar Loads During Simulated Landings: Implications for Injury Mechanism.

BACKGROUND: Despite basic characterization of the loading factors that strain the anterior cruciate ligament (ACL), the interrelationship(s) and additive nature of these loads that occur during noncontact ACL injuries remain incompletely characterized. HYPOTHESIS: In the presence of an impulsive axial compression, simulating vertical ground-reaction force during landing (1) both knee abduction and internal tibial rotation moments would result in increased peak ACL strain, and (2) a combined multiplanar loading condition, including both knee abduction and internal tibial rotation moments, would increase the peak ACL strain to levels greater than those under uniplanar loading modes alone. STUDY DESIGN: Controlled laboratory study. METHODS: A cadaveric model of landing was used to simulate dynamic landings during a jump in 17 cadaveric lower extremities (age, 45 ± 7 years; 9 female and 8 male). Peak ACL strain was measured in situ and characterized under impulsive axial compression and simulated muscle forces (baseline) followed by addition of anterior tibial shear, knee abduction, and internal tibial rotation loads in both uni- and multiplanar modes, simulating a broad range of landing conditions. The associations between knee rotational kinematics and peak ACL strain levels were further investigated to determine the potential noncontact injury mechanism. RESULTS: Externally applied loads, under both uni- and multiplanar conditions, resulted in consistent increases in peak ACL strain compared with the baseline during simulated landings (by up to 3.5-fold; P ≤ .032). Combined multiplanar loading resulted in the greatest increases in peak ACL strain (P < .001). Degrees of knee abduction rotation (R(2) = 0.45; ß = 0.42) and internal tibial rotation (R(2) = 0.32; ß = 0.23) were both significantly correlated with peak ACL strain (P < .001). However, changes in knee abduction rotation had a significantly greater effect size on peak ACL strain levels than did internal tibial rotation (by ~2-fold; P < .001). CONCLUSION: In the presence of impulsive axial compression, the combination of anterior tibial shear force, knee abduction, and internal tibial rotation moments significantly increases ACL strain, which could result in ACL failure. These findings support multiplanar knee valgus collapse as one the primary mechanisms of noncontact ACL injuries during landing. CLINICAL RELEVANCE: Intervention programs that address multiple planes of loading may decrease the risk of ACL injury and the devastating consequences of posttraumatic knee osteoarthritis.

Evaluation of WIAMan Technology Demonstrator Biofidelity Relative to Sub-Injurious PMHS Response in Simulated Under-body Blast Events.

Three laboratory simulated sub-injurious under-body blast (UBB) test conditions were conducted with whole-body Post Mortem Human Surrogates (PMHS) and the Warrior Assessment Injury Manikin (WIAMan) Technology Demonstrator (TD) to establish and assess UBB biofidelity of the WIAMan TD. Test conditions included a rigid floor and rigid seat with independently varied pulses. On the floor, peak velocities of 4 m/s and 6 m/s were applied with a 5 ms time to peak (TTP). The seat peak velocity was 4 m/s with varied TTP of 5 and 10 ms. Tests were conducted with and without personal protective equipment (PPE). PMHS response data was compiled into preliminary biofidelity response corridors (BRCs), which served as evaluation metrics for the WIAMan TD. Each WIAMan TD response was evaluated against the PMHS preliminary BRC for the loading and unloading phase of the signal time history using Correlation Analysis (CORA) software to assign a numerical score between 0 and 1. A weighted average of all responses was calculated to determine body region and whole body biofidelity scores for each test condition. The WIAMan TD received UBB biofidelity scores of 0.62 in Condition A, 0.59 in Condition B, and 0.63 in Condition C, putting it in the fair category (0.44-0.65). Body region responses with scores below a rating of good (0.65-0.84) indicate potential focus areas for the next generation of the WIAMan design.

Dynamic evaluation of contact pressure and the effects of graft harvest with subsequent lateral release at osteochondral donor sites in the knee.

PURPOSE: To dynamically evaluate contact pressure about the periphery of the lateral femoral condyle in intact knees, to qualify the effects of osteochondral donor graft harvest on this contact pressure, and to quantify the effects of lateral release on contact pressure after graft harvest. TYPE OF STUDY: Cadaveric analysis. METHODS: Digital electronic pressure-sensing cells were used to measure contact pressure over the periphery of the lateral femoral condyle in 10 fresh-frozen knee specimens. Nonweightbearing resistive extension was simulated as the knees were placed through a functional range of motion. Dynamic pressure readings were evaluated over intact cartilage, around the rims of four 5-mm osteochondral defects, and after lateral release. RESULTS: The pressure cells were all subjected to contact pressures as the knees were placed through a functional range of motion. Average maximal contact pressure progressed distally as the knees were flexed. The creation of 5-mm osteochondral defects did not lead to a significant increase in rim stress concentration over the surrounding cartilage. Lateral release resulted in small decreases in contact pressure over the osteochondral defects. CONCLUSIONS: The creation of 5-mm donor defects about the lateral aspect of the lateral femoral condyle does not lead to significant alterations in local contact pressure. CLINICAL RELEVANCE: Our biomechanical findings may have important implications relating to cartilage restoration using osteochondral autografting procedures. Donor-site morbidity may be minimized if donor-site defects are limited to 5 mm and smaller.

Uni-directional coupling between tibiofemoral frontal and axial plane rotation supports valgus collapse mechanism of ACL injury.

Despite general agreement on the effects of knee valgus and internal tibial rotation on anterior cruciate ligament (ACL) loading, compelling debate persists on the interrelationship between these rotations and how they contribute to the multi-planar ACL injury mechanism. This study investigates coupling between knee valgus and internal tibial rotation and their effects on ACL strain as a quantifiable measure of injury risk. Nineteen instrumented cadaveric legs were imaged and tested under a range of knee valgus and internal tibial torques. Posterior tibial slope and the medial tibial depth, along with changes in tibiofemoral kinematics and ACL strain, were quantified. Valgus torque significantly increased knee valgus rotation and ACL strain (p<0.020), yet generated minimal coupled internal tibial rotation (p=0.537). Applied internal tibial torque significantly increased internal tibial rotation and ACL strain and generated significant coupled knee valgus rotation (p<0.001 for all comparisons). Similar knee valgus rotations (7.3° vs 7.4°) and ACL strain levels (4.4% vs 4.9%) were observed under 50 Nm of valgus and 20 Nm of internal tibial torques, respectively. Coupled knee valgus rotation under 20 Nm of internal tibial torque was significantly correlated with internal tibial rotation, lateral and medial tibial slopes, and medial tibial depth (R(2)>0.30; p<0.020). These findings demonstrate uni-directional coupling between knee valgus and internal tibial rotation in a cadaveric model. Although both knee valgus and internal tibial torques contribute to increased ACL strain, knee valgus rotation has the ultimate impact on ACL strain regardless of loading mode.

Osteochondral defects in the human knee: influence of defect size on cartilage rim stress and load redistribution to surrounding cartilage.

PURPOSE: To determine the influence of osteochondral defect size on defect rim stress concentration, peak rim stress, and load redistribution to adjacent cartilage over the weightbearing area of the medial and lateral femoral condyles in the human knee. METHODS: Eight fresh-frozen cadaveric knees were mounted at 30 degrees of flexion in a materials testing machine. Digital electronic pressure sensors were placed in the medial and lateral compartments of the knee. Each intact knee was first loaded to 700 N and held for 5 seconds. Dynamic pressure readings were recorded throughout the loading and holding phases. Loading was repeated over circular osteochondral defects (5, 8, 10, 12, 14, 16, 18, and 20 mm) in the 30 degrees weightbearing area of the medial and lateral femoral condyles. RESULTS: Stress concentration around the rims of defects 8 mm and smaller was not demonstrated, and pressure distribution in this size range was dominated by the menisci. For defects 10 mm and greater, distribution of peak pressures followed the rim of the defect with a mean distance from the rim of 2.2 mm on the medial condyle and 3.2 mm on the lateral condyle. An analysis of variance with Bonferroni correction revealed a statistically significant trend of increasing radius of peak pressure as defect size increased for defects from 10 to 20 mm (P = .0011). Peak rim pressure values did not increase significantly as defects were enlarged from 10 to 20 mm. Load redistribution during the holding phase was also observed. CONCLUSIONS: Rim stress concentration was demonstrated for osteochondral defects 10 mm and greater in size. This altered load distribution has important implications relating to the long-term integrity of cartilage adjacent to osteochondral defects in the human knee. Although the decision to treat osteochondral lesions is certainly multifactorial, a size threshold of 10 mm, based on biomechanical data, may be a useful adjunct to guide clinical decision making.

The Effect of Ligament Modeling Technique on Knee Joint Kinematics: A Finite Element Study.

Finite element (FE) analysis has become an increasingly popular technique in the study of human joint biomechanics, as it allows for detailed analysis of the joint/tissue behavior under complex, clinically relevant loading conditions. A wide variety of modeling techniques have been utilized to model knee joint ligaments. However, the effect of a selected constitutive model to simulate the ligaments on knee kinematics remains unclear. The purpose of the current study was to determine the effect of two most common techniques utilized to model knee ligaments on joint kinematics under functional loading conditions. We hypothesized that anatomic representations of the knee ligaments with anisotropic hyperelastic properties will result in more realistic kinematics. A previously developed, extensively validated anatomic FE model of the knee developed from a healthy, young female athlete was used. FE models with 3D anatomic and simplified uniaxial representations of main knee ligaments were used to simulate four functional loading conditions. Model predictions of tibiofemoral joint kinematics were compared to experimental measures. Results demonstrated the ability of the anatomic representation of the knee ligaments (3D geometry along with anisotropic hyperelastic material) in more physiologic prediction of the human knee motion with strong correlation (r ≥ 0.9 for all comparisons) and minimum deviation (0.9º ≤ RMSE ≤ 2.29°) from experimental findings. In contrast, non-physiologic uniaxial elastic representation of the ligaments resulted in lower correlations (r ≤ 0.6 for all comparisons) and substantially higher deviation (2.6° ≤ RMSE ≤ 4.2°) from experimental results. Findings of the current study support our hypothesis and highlight the critical role of soft tissue modeling technique on the resultant FE predicted joint kinematics.

Diagnostic value of knee arthrometry in the prediction of anterior cruciate ligament strain during landing.

BACKGROUND: Previous studies have indicated that higher knee joint laxity may be indicative of an increased risk of anterior cruciate ligament (ACL) injuries. Despite the frequent clinical use of knee arthrometry in the evaluation of knee laxity, little data exist to correlate instrumented laxity measures and ACL strain during dynamic high-risk activities. Purpose/ HYPOTHESES: The purpose of this study was to evaluate the relationships between ACL strain and anterior knee laxity measurements using arthrometry during both a drawer test and simulated bipedal landing (as an identified high-risk injurious task). We hypothesized that a high correlation exists between dynamic ACL strain and passive arthrometry displacement. The secondary hypothesis was that anterior knee laxity quantified by knee arthrometry is a valid predictor of injury risk such that specimens with greater anterior knee laxity would demonstrate increased levels of peak ACL strain during landing. STUDY DESIGN: Controlled laboratory study. METHODS: Twenty cadaveric lower limbs (mean age, 46 ± 6 years; 10 female and 10 male) were tested using a CompuKT knee arthrometer to measure knee joint laxity. Each specimen was tested under 4 continuous cycles of anterior-posterior shear force (±134 N) applied to the tibial tubercle. To quantify ACL strain, a differential variable reluctance transducer (DVRT) was arthroscopically placed on the ACL (anteromedial bundle), and specimens were retested. Subsequently, bipedal landing from 30 cm was simulated in a subset of 14 specimens (mean age, 45 ± 6 years; 6 female and 8 male) using a novel custom-designed drop stand. Changes in joint laxity and ACL strain under applied anterior shear force were statistically analyzed using paired sample t tests and analysis of variance. Multiple linear regression analyses were conducted to determine the relationship between anterior shear force, anterior tibial translation, and ACL strain. RESULTS: During simulated drawer tests, 134 N of applied anterior shear load produced a mean peak anterior tibial translation of 3.1 ± 1.1 mm and a mean peak ACL strain of 4.9% ± 4.3%. Anterior shear load was a significant determinant of anterior tibial translation (P < .0005) and peak ACL strain (P = .04). A significant correlation (r = 0.52, P < .0005) was observed between anterior tibial translation and ACL strain. Cadaveric simulations of landing produced a mean axial impact load of 4070 ± 732 N. Simulated landing significantly increased the mean peak anterior tibial translation to 10.4 ± 3.5 mm and the mean peak ACL strain to 6.8% ± 2.8% (P < .0005) compared with the prelanding condition. Significant correlations were observed between peak ACL strain during simulated landing and anterior tibial translation quantified by knee arthrometry. CONCLUSION: Our first hypothesis is supported by a significant correlation between arthrometry displacement collected during laxity tests and concurrent ACL strain calculated from DVRT measurements. Experimental findings also support our second hypothesis that instrumented measures of anterior knee laxity predict peak ACL strain during landing, while specimens with greater knee laxity demonstrated higher levels of peak ACL strain during landing. CLINICAL RELEVANCE: The current findings highlight the importance of instrumented anterior knee laxity assessments as a potential indicator of the risk of ACL injuries in addition to its clinical utility in the evaluation of ACL integrity.