Achilles Tension Mitigates Fibular Malalignment Measured in Cadaveric Studies of Syndesmotic Clamping.

BACKGROUND:: Fibular malreduction is becoming a commonly recognized complication of surgical repair of the syndesmosis when a reduction clamp is used. The goal of this work was to determine the interdependent effects of transsyndesmotic reduction clamp position and applied compression force on fibular alignment in a realistic cadaveric preparation of complete syndesmotic injury. METHODS:: Six through-the-knee cadaveric specimens were CT scanned intact, with the distal syndesmosis fully destabilized, and with 53, 102, and 160 N clamping forces each applied along an anteriorly, centrally, and posteriorly directed transsyndesmotic axis. Testing was repeated incorporating 178 N of Achilles tendon tension using all 3 clamping forces applied along the centrally directed axis. Fibular reduction was automatically quantified from CT scan-generated bony surfaces as rotation of the fibula around the tibia, rotation of the fibula within the incisura, medial/lateral fibular displacement, and anterior/posterior fibular displacement. RESULTS:: Transsyndesmotic clamping along the anteriorly directed axis resulted in the best reduction quality by all 4 quantified measures. Along the centrally and posteriorly directed axes, progressively greater forces caused significantly greater sagittal plane fibular malreduction. Addition of Achilles tension reduced the magnitude of fibular malreduction and overcompression. CONCLUSION:: Placing the medial tine of a transsyndesmotic reduction clamp on the anterior medial tibia resulted in the most accurate syndesmotic reduction and provided some protection against overcompression with large reduction clamp forces. Achilles tension appeared to contribute to reduction, decreasing the magnitude of measured malreduction from clamping. CLINICAL RELEVANCE:: Previous studies estimating fibular malpositioning in cadaveric models that lacked passive muscle tension may have overestimated expected magnitudes of malalignment in patients treated with syndesmotic clamping. However, syndesmotic malreduction, particularly in the sagittal plane, was a real complication of syndesmotic clamping that was reduced by using an anterior position of the medial tine on the tibia.

Screw fixation of the syndesmosis alters joint contact characteristics in an axially loaded cadaveric model.

BACKGROUND: The purpose of this study was to quantify the effects of rigid syndesmotic fixation on functional talar position and cartilage contact mechanics. METHODS: Twelve below-knee cadaveric specimens with an intact distal syndesmosis were mechanically loaded in four flexion positions (20° plantar flexion, 10° plantar flexion, neutral, 10° dorsiflexion) with zero, one, or two 3.5-mm syndesmotic screws. Rigid clusters of reflective markers were used to track bony movement and ankle-specific pressure sensors were used to measure talar dome and medial/lateral gutter contact mechanics. RESULTS: Screw fixation caused negligible anterior and inferior shifts of the talus within the mortise. Relative to no fixation, mean peak contact pressure decreased by 6%-32% on the talar dome and increased 2.4- to 6.6-fold in the medial and lateral gutters, respectively, depending on ankle position and number of screws. CONCLUSIONS: Two-way ANOVA indicated syndesmotic screw fixation significantly increased contact pressure in the medial/lateral gutters and decreased talar dome contact pressure while minimally altering talar position.

Biomechanical Comparison of Syndesmotic Repair Techniques During External Rotation Stress.

BACKGROUND: The purpose of this study was to compare mechanical behavior of conventional syndesmosis fixation devices with new anatomic repair techniques incorporating various repair augmentations to determine which approach would return rotational ankle mechanics closer to those of an intact ankle. METHODS: Ten pairs of fresh-frozen through-the-knee cadaveric lower limbs were subjected to 7.5 Nm of external rotation torque while under 750 N of axial compression. After testing specimens intact and with the deltoid and syndesmotic ligament complexes completely destabilized, specimens underwent syndesmotic fixation using a screw, a suture button construct, a prototype structurally augmented flexible trans-syndesmotic fixation device, or the prototype device plus suture repairs of the anterior-inferior tibiofibular ligament and deep deltoid ligament. Syndesmotic repair devices were exchanged between tests so that each specimen was tested with 2 different fixation techniques. Whole-foot rotation angles at 7.5 Nm of applied torque were measured for comparison of the different repair strategies, and reflective markers mounted on the tibia, fibula, and talus were used to track translations and rotations of the talus and the fibula relative to the tibia during testing. RESULTS: Syndesmotic destabilization significantly ( P < .001) increased whole-foot, talus, and fibula rotation in an axial plane and posterior fibula translation under 7.5 Nm of torque. Neither the suture button nor the augmented flexible trans-syndesmotic fixation device reduced those increases. Screw fixation or addition of anatomic ligament repairs to the augmented flexible fixation device successfully reduced axial plane rotations and sagittal plane translations to near intact levels. CONCLUSION: Flexible trans-syndesmotic fixation alone was found to be insufficient for restoring rotational stability to the ankle/talus or preventing sagittal plane displacement of the fibula. CLINICAL RELEVANCE: Repairs to simulate anatomic structures disrupted during a syndesmosis injury were required to restore rotational stability to the foot when using flexible trans-syndesmotic fixation that may have clinical applicability.

Discrete element analysis is a valid method for computing joint contact stress in the hip before and after acetabular fracture.

Evaluation of abnormalities in joint contact stress that develop after inaccurate reduction of an acetabular fracture may provide a potential means for predicting the risk of developing post-traumatic osteoarthritis. Discrete element analysis (DEA) is a computational technique for calculating intra-articular contact stress distributions in a fraction of the time required to obtain the same information using the more commonly employed finite element analysis technique. The goal of this work was to validate the accuracy of DEA-computed contact stress against physical measurements of contact stress made in cadaveric hips using Tekscan sensors. Four static loading tests in a variety of poses from heel-strike to toe-off were performed in two different cadaveric hip specimens with the acetabulum intact and again with an intentionally malreduced posterior wall acetabular fracture. DEA-computed contact stress was compared on a point-by-point basis to stress measured from the physical experiments. There was good agreement between computed and measured contact stress over the entire contact area (correlation coefficients ranged from 0.88 to 0.99). DEA-computed peak contact stress was within an average of 0.5 MPa (range 0.2-0.8 MPa) of the Tekscan peak stress for intact hips, and within an average of 0.6 MPa (range 0-1.6 MPa) for fractured cases. DEA-computed contact areas were within an average of 33% of the Tekscan-measured areas (range: 1.4-60%). These results indicate that the DEA methodology is a valid method for accurately estimating contact stress in both intact and fractured hips.

Cadaveric validation of a finite element modeling approach for studying scapular notching in reverse shoulder arthroplasty.

Cadaveric experiments were undertaken to validate a finite element (FE) modeling approach for studying impingement-related scapular notching in reverse shoulder arthroplasty (RSA). The specific focus of the validation was contact at the site of impingement between the humeral polyethylene component and the inferior aspect of the scapula during an adduction motion. Lateralization of the RSA center of rotation was varied because it has been advocated clinically to reduce impingement and presumably decrease the risk of scapular notching. Tekscan sensors were utilized to directly measure contact stress at the impingement site, and FE was used to compute contact stresses. Favorable agreement was seen between physically measured and FE-computed impingement site location (within one sensing element of the Tekscan sensor) and contact loads (mean absolute difference of 14.9%). Contact stresses and contact areas were difficult to compare directly due to the disparate spatial resolutions of the Tekscan sensor and the FE model. FE-computed contact at the impingement site was highly focal, with a total contact area comparable to the area of an individual Tekscan sensing element. The good agreement between the physically measured and FE-computed contact data (i.e., contact load and location) support the use of FE modeling as a tool for computationally testing the efficacy of changing various surgical variables associated with RSA.

Single-bundle versus double-bundle ACL reconstructions in isolation and in conjunction with extra-articular iliotibial band tenodesis.

BACKGROUND: Intra-articular anterior cruciate ligament (ACL) reconstruction has been the primary treatment option for isolated ACL injuries for many years. An anatomic double-bundle reconstruction has been devised in an effort to improve rotational control. The role of the extra-articular iliotibial band tenodesis in ACL injuries has evolved from primary treatment, to an adjuvant secondary procedure, to being used more selectively in revision ACL reconstructions. HYPOTHESES: 1) Single-bundle and doublebundle intra-articular ACL reconstructions will both restore pre-injury laxity measurements in an isolated ACL injury cadaver model. 2) The deep iliotibial band structures contribute to rotational control and in a dual ACL + ITB injury cadaver model, ACL reconstruction alone cannot restore rotational control. STUDY DESIGN: Controlled Laboratory Design. METHODS: 17 fresh frozen cadavers received intra-articular reconstructions, seven single-bundle and ten double-bundle; laxity was measured with the ACL intact/ITB intact, ACL reconstructed/ITB intact, after cutting the ITB, and after an ITB tenodesis procedure; laxity measurements of anterior tibial translation(ATT) and internal rotation(IR) were measured following applications of an anterior shear force, an internal torque and a coupled anterior shear force-internal torque at 30 and 90 degrees of flexion. RESULTS: Single-bundle and double-bundle ACL reconstructions both restored IR to a native knee state under isolated internal torques and under coupled forces. Both reconstruction techniques also re-established anterior tibial translation to at least the pre-ACL injury level, with over-constraint in the double-bundle subgroup [5.00 (+2.11) to 3.50(+1.18), p-value 0.026] under coupled loads at 30 degrees of flexion. With the individual ACL reconstructions held constant, under coupled forces mean IR increased in the single-bundle subgroup [13.7(+1.1) to 17.6(+1.2), p-value 0.004] and the double-bundle subgroup [9.5(+1.0) to 12.4(+1.0), p-value 0.009] with the cutting of the ITB at 30 degrees. Under internal torque, mean IR increased in the single-bundle subgroup [14.0(+1.0) to 18.4(+1.6), p-value 0.016] with the cutting of the ITB at 30 degrees, while IR increased in the double-bundle subgroup [10.0(+1.3) to 13.4(+1.5), p-value 0.002] under the same internal torque at 90 degrees. With the ACL reconstruction held constant, ATT did not significantly change when the ITB was cut or when it was tenodesed under any specific loading condition. CONCLUSION: Single-bundle and double-bundle intra-articular reconstructions were both able to restore internal rotation and anterior tibial translation to at least native knee laxity levels after an isolated laboratory ACL injury. When the ACL reconstructions were held constant, internal rotation statistically increased with the cutting of the ITB under multiple testing conditions in both the single-bundle and double-bundle subgroups.

The capsule's contribution to total hip construct stability--a finite element analysis.

Instability is a significant concern in total hip arthroplasty (THA), particularly when there is structural compromise of the capsule due to pre-existing pathology or due to necessities of surgical approach. An experimentally grounded fiber-direction-based finite element model of the hip capsule was developed, and was integrated with an established three-dimensional model of impingement/dislocation. Model validity was established by close similarity to results from a cadaveric experiment in a servohydraulic hip simulator. Parametric computational runs explored effects of graded levels of capsule thickness, of regional detachment from the capsule's femoral or acetabular insertions, of surgical incisions of capsule substance, and of capsule defect repairs. Depending strongly upon the specific site, localized capsule defects caused varying degrees of construct stability compromise, with several specific situations involving over 60% decrement in dislocation resistance. Construct stability was returned substantially toward intact-capsule levels following well-conceived repairs, although the suture sites involved were often at substantial risk of failure. These parametric model results underscore the importance of retaining or robustly repairing capsular structures in THA, in order to maximize overall construct stability.

Instability dependency of osteoarthritis development in a rabbit model of graded anterior cruciate ligament transection.

BACKGROUND: Joint instability has long been empirically recognized as a leading risk factor for osteoarthritis. However, formal mechanistic linkage of instability to osteoarthritis development has not been established. This study aimed to support a clinically accepted, but heretofore scientifically unproven, concept that the severity and rapidity of osteoarthritis development in unstable joints is dependent on the degree of instability. In a survival rabbit knee model of graded joint instability, the relationship between the magnitude of instability and the intensity of cartilage degeneration was studied at the organ level in vivo. METHODS: Sixty New Zealand White rabbits received either complete or partial (medial half) transection of the anterior cruciate ligament or sham surgery (control) on the left knee. At the time that the animals were killed at eight or sixteen weeks postoperatively (ten animals for each treatment and/or test-period combination), the experimental knees were subjected to sagittal plane stability measurement, followed by whole-joint cartilage histological evaluation with use of the Mankin score. RESULTS: Sagittal plane instability created in the partial transection group was intermediate between those in the complete transection and sham surgery groups. The partial and complete transection groups exhibited cartilage degeneration on the medial femoral and/or medial tibial surfaces. The average histological score (and standard deviation) for the medial compartment in the partial transection group (2.9 ± 0.9) was again intermediate, significantly higher than for the sham surgery group (1.9 ± 0.8) and significantly lower than for the complete transection group (4.5 ± 2.3). The average histological scores for the medial compartment in the partial transection group correlated significantly with the magnitude of instability, with no threshold effect being evident. The significance level of alpha was set at 0.05 for all tests. CONCLUSIONS: The severity of cartilage degeneration increased continuously with the degree of instability in this survival rabbit knee model of graded instability.

Apparent transverse compressive material properties of the digital flexor tendons and the median nerve in the carpal tunnel.

Carpal tunnel syndrome is a frequently encountered peripheral nerve disorder caused by mechanical insult to the median nerve, which may in part be a result of impingement by the adjacent digital flexor tendons. Realistic finite element (FE) analysis to determine contact stresses between the flexor tendons and median nerve depends upon the use of physiologically accurate material properties. To assess the transverse compressive properties of the digital flexor tendons and median nerve, these tissues from ten cadaveric forearm specimens were compressed transversely while under axial load. The experimental compression data were used in conjunction with an FE-based optimization routine to determine apparent hyperelastic coefficients (µ and α) for a first-order Ogden material property definition. The mean coefficient pairs were µ=35.3 kPa, α=8.5 for the superficial tendons, µ=39.4 kPa, α=9.2 for the deep tendons, µ=24.9 kPa, α=10.9 for the flexor pollicis longus (FPL) tendon, and µ=12.9 kPa, α=6.5 for the median nerve. These mean Ogden coefficients indicate that the FPL tendon was more compliant at low strains than either the deep or superficial flexor tendons, and that there was no significant difference between superficial and deep flexor tendon compressive behavior. The median nerve was significantly more compliant than any of the flexor tendons. The material properties determined in this study can be used to better understand the functional mechanics of the carpal tunnel soft tissues and possible mechanisms of median nerve compressive insult, which may lead to the onset of carpal tunnel syndrome.

Organ-level histological and biomechanical responses from localized osteoarticular injury in the rabbit knee.

The processes of whole-joint osteoarthritis development following localized joint injuries are not well understood. To demonstrate this local-to-global linkage, we hypothesized that a localized osteoarticular injury in the rabbit knee would not only cause biomechanical and histological abnormalities in the involved compartment but also concurrent histological changes in the noninvolved compartment. Twenty rabbits had an acute osteoarticular injury that involved localized joint incongruity (a 2-mm osteochondral defect created in the weight-bearing area of the medial femoral condyle), while another 20 received control sham surgery. At the time of euthanasia at 8 or 16 weeks post-surgery, the experimental knees were subjected to sagittal-plane laxity measurement, followed by cartilage histo-morphological evaluation using the Mankin score. The immediate effects of defect creation on joint stability and contact mechanics were explored in concomitant rabbit cadaver experimentation. The injured animals had cartilage histological scores significantly higher than in the sham surgery group (p < 0.01) on the medial femoral, medial tibial, and lateral femoral surfaces (predominantly on the medial surfaces), accompanied by slight (mean 20%) increase of sagittal-plane laxity. Immediate injury-associated alterations in the medial compartment contact mechanics were also demonstrated. Localized osteoarticular injury in this survival animal model resulted in global joint histological changes.

Traveling-load calibration of grid-array transient contact stress sensors.

Thin, pliant transducers with grid arrays of sensing elements (sensels) have been widely used for transient measurements of intra-articular contact stresses. Conventional calibration procedures for this class of sensors are based upon spatially uniform scaling of sensel output values so as to recover two known fiducial loads, physically applied with the sensor either compressed between platens or mounted in situ. Because of the nonlinearities involved, it is desirable to have the highest of those two calibration loadings be such that all individual sensels are engaged at/near the peak of their expected functional range. However, for many situations of practical interest, impracticably large total calibration forces would be required. We report development of a novel pneumatically actuated wringer-like calibration device, and companion iterative post-processing software, that bypasses this longstanding difficulty. Sensors passed through the rollers of this device experience constant-distribution traveling fiducial loads propagating across their surface, thus allowing efficient calibration of all sensels individually to contact stress levels that would be impracticably high to simultaneously apply to all sensels. Sensel-specific calibration curves are rapidly and easily generated using this new approach and compare favorably to those obtained with less expeditious conventional platen-based protocols.

Effect of implantation accuracy on ankle contact mechanics with a metallic focal resurfacing implant.

BACKGROUND: Talar osteochondral defects can lead to joint degeneration. Focal resurfacing with a metallic implant has shown promise in other joints. We studied the effect of implantation accuracy on ankle contact mechanics after focal resurfacing of a defect in the talar dome. METHODS: Static loading of seven cadaver ankles was performed before and after creation of a 15-mm-diameter osteochondral defect on the talar dome, and joint contact stresses were measured. The defect was then resurfaced with a metallic implant, with use of a custom implant-bone interface fixture that allowed fine control (in 0.25-mm steps) of implantation height. Stress measurements were repeated at heights of -0.5 to +0.5 mm relative to an as-implanted reference. Finite element analysis was used to determine the effect of implant height, post axis rotation, and valgus/varus tilt over a motion duty cycle. RESULTS: With the untreated defect, there was a 20% reduction in contact area and a 40% increase in peak contact stress, as well as a shift in the location of the most highly loaded region, as compared with the values in the intact condition. Resurfacing led to recovery of 90% of the contact area that had been measured in the intact specimen, but the peak contact stresses remained elevated. With the implant 0.25 mm proud, peak contact stress was 220% of that in the intact specimen. The results of the finite element analyses agreed closely with those of the experiments and additionally showed substantial variations in defect influences on contact stresses across the motion arc. Talar internal/external rotations also differed for the unfilled defect. Focal implant resurfacing substantially restored kinematics but did not restore the stresses to the levels in the intact specimens. CONCLUSIONS: Focal resurfacing with a metallic implant appears to have the potential to restore normal joint mechanics in ankles with a large talar osteochondral defect. However, contact stresses were found to be highly sensitive to implant positioning.

Compliance-dependent load allocation between sensing versus non-sensing portions of a sheet-array contact stress sensor.

Piezoresistive array pressure sensors are widely used in orthopaedic research to determine contact stress distributions across articular joint surfaces. Experience with such sensors has shown there can be inaccuracies in how the sensor perceives applied load, depending on the material stiffnesses between which it is compressed experimentally, versus in calibration. A study was undertaken to quantify the relationship between load perception of one such sensor design (Tekscan) and the stiffness of the materials between which it is compressed. A three-dimensional finite element model of a 3x3 sensel portion of the sensing matrix was formulated, along with a layer of compression test material on each side of the sensor. The elastic modulus of the test material was varied across the range representative of cartilage (12 MPa) to hard plastic (10 GPa). Using the computed contact pressure results between contacting surfaces of the sensor layers, the percentage of load passing through the active conductor intersections was determined. The results revealed that with increase of the elastic modulus of the material between which the sensor was compressed, the percentage of load on the active conductor intersections increased monotonically. The highest sensitivity of perceived loading to test material modulus (0.1%/MPa) was seen at the low end of the modulus range. The more compliant the test material, the more the sensor layers conformed around each other's geometric incongruities, the larger the true contact areas, and the higher the fraction of the total load that passed through the intermediate (non-sensing) regions between the conductors.

Correlation of dynamic cartilage contact stress aberrations with severity of instability in ankle incongruity.

Joint instability is presumed to cause abnormality in cartilage contact mechanics, which accumulatively damages the articular surface, leading to osteoarthritis. The purpose of this study was to clarify the effect of instability on dynamic cartilage contact mechanics. Using human ankle cadaver specimens, potentially unstable ankles were modeled by introducing a coronally directed step-off incongruity of the anterior tibial surface and/or by transecting the anterior talofibular ligament. Specimens were subjected to a duty cycle with quasi-physiologic stance-phase motion and loading. AP tibial forces were modulated, causing a controlled, quantifiable ankle subluxation during the duty cycle. Instantaneous changes in local articular contact stresses were continuously measured using a thin, flexible pressure transducer. Tests were repeated while varying the tibial surface condition (anatomic, 1-mm step-off, and 2-mm step-off), both before and after transection of the anterior talofibular ligament, with various AP force magnitudes, so that situations of various degrees of instability were created for each specimen. Instability events occurred when the step-off incongruity was introduced, with the abnormality in joint kinematics being greater after ligament transection. Contact stress data revealed that these instability events involved distinctly abrupt increases/decreases in local articular contact stresses, and that the degree of abruptness was correlated nearly linearly with the abnormality in kinematics. The severity of contact stress aberration appeared to be correlated with the degree of instability. Given this linear relationship, even small instability events presumably involve appreciable abnormality in dynamic joint contact mechanics.

Instability-associated changes in contact stress and contact stress rates near a step-off incongruity.

BACKGROUND: Intra-articular fractures can result in articular surface incongruity and joint instability, both of which can lead to posttraumatic osteoarthritis. The purpose of this study was to quantify changes in contact stresses and contact stress rates in incongruous human cadaveric ankles that were either stable or unstable. It was hypothesized that joint instability, superimposed on articular incongruity, would cause significant increases in contact stresses and contact stress rates. METHODS: Intact human cadaveric ankles were subjected to quasi-physiologic stance-phase motion and loading, and instantaneous contact stresses were captured at 132 Hz. The anterior one-third of the distal part of the tibia was displaced proximally by 2.0 mm, and testing was repeated. Anterior/posterior forces were modulated during loading to cause incongruous ankles to either remain stable or become unstable during loading. Transient contact stresses and contact stress rates were measured for seven ankles under intact, stable-incongruous, and unstable-incongruous conditions. Peak and 95th percentile values of contact stress and contact stress rates for all three conditions were compared to determine the pathomechanical effects of incongruity and instability. RESULTS: The addition of instability caused 95th percentile and peak contact stresses to increase approximately between 20% and 25% in the unstable-incongruous specimens compared with the stable-incongruous specimens. In contrast, the addition of instability increased the magnitude of peak positive and peak negative contact stress rates by 115% and 170% in the unstable-incongruous specimens compared with the stable-incongruous specimens. Similarly, the 95th percentile contact stress rates increased 112% in the unstable-incongruous specimens compared with the stable-incongruous specimens. CONCLUSIONS: In human cadaveric ankles, instability superimposed on an existing articular surface incongruity causes disproportionate increases in contact stress rates compared with the increases in contact stresses.

In vivo measurement of translational stiffness of rabbit knees.

This paper describes the design, evaluation, and preliminary results of a specialized testing device and surgical protocol to determine translational stiffness of a rabbit knee, replicating the clinical anterior drawer test. Coronal-plane transverse pins are inserted through the rabbit leg, two in the tibia and one in the distal femur, to hold and reproducibly position the leg in the device for tests at multiple time points. A linear stepper motor draws the tibia upward then returns to the home position, and a load cell measures the resisting force; force-displacement knee stiffness is then calculated. Initial evaluation of this testing device determined the effects of preconditioning, intra-operator repeatability, rabbit-to-rabbit variability, knee flexion angle (90 degrees vs. 135 degrees ), and anterior cruciate ligament (ACL) sectioning (0%, 25%, 50%, 75%, 100%). Knee stiffness generally decreased as ACL sectioning increased. This testing device and surgical protocol provide an objective and efficient method of determining translational rabbit knee stiffness in vivo, and are being used in an ongoing study to evaluate the effect of knee instability (via partial to complete ACL sectioning) on the development of post-traumatic osteoarthritis.

Contribution of articular surface geometry to ankle stabilization.

BACKGROUND: Passive ankle stability under weight-bearing conditions has been found to depend substantially on the role of the articular surface geometry. In the present study, it was hypothesized that, in the ankle under axial loading, contact-stress changes in response to alterations of external load involve reproducible and specific patterns to maintain ankle stability. METHODS: Six cadaver ankles with the peri-ankle ligaments intact were tested. Each specimen, held at several predetermined ankle positions under a primary one-body-weight axial force, was subjected to an additional secondary load. The secondary load-specifically, anterior/posterior shear force, inversion/eversion torque, or internal/external rotation torque-was applied independently, while motion associated with the two other secondary loading directions was unconstrained. Contact stress in the tibiotalar articulation was monitored by a real-time contact-stress sensor. Site-specific stress changes solely due to secondary loading at each load/position were identified by subtraction of the corresponding axial-force-only baseline distribution. The role of these stress changes in ankle stabilization was studied for each specimen by analyzing the data with a computer model of ankle geometry. RESULTS: In the cadaver experiment, anterior and posterior shear forces caused reproducible positive changes in articular contact stresses on the anterior and posterior regions, respectively. Similar changes with version torques occurred on the medial and lateral regions. Positive changes with internal/external rotation torques occurred at two diagonal locations: anterolateral and posteromedial, or anteromedial and posterolateral. In the model analysis, these stress-change patterns were found to be effective in ankle stabilization, and the levels of contribution by the articular surface were calculated as accounting for approximately 70% of anterior/posterior stability, 50% of version stability, and 30% of internal/external rotation stability. CONCLUSIONS: The documented changes in contact stress illustrate the major role of articular geometry in passive ankle stabilization. The levels of contribution by the articular surface that we calculated are consistent with those reported in the literature. These findings support the conceptual mechanism of ankle stabilization by redistribution of articular contact stress.

Incongruity-dependent changes of contact stress rates in human cadaveric ankles.

Cartilage biosynthetic transduction and injury characteristics have been shown to be particularly sensitive to changes in contact stress rates. This study investigated incongruity-associated changes in contact stress rates that resulted from an articular surface stepoff of the distal tibia in human cadaveric ankles. Ten human cadaveric ankles were subjected to quasi-physiologic stance-phase motion and loading and instantaneous contact stresses were captured at 132 Hz over the entire articular surface using a custom-fabricated stress transducer. An osteoarticular fragment consisting of the anterolateral 25% of the distal tibia was osteotomized. Testing was repeated after displacing the fragment proximally between 0.0 mm to 4.0 mm in 1.0 mm increments. Transient contact stress measurements were used to calculate contact stress rates. Compared to intact ankles, the anatomic configuration had modest increases in global and peak postitive and negative contact stress rates throughout the motion cycle. In contrast, stepoff specimens had significant increases in global and complete motion cycle peak positive and negative contact stress rates, as high as 3.1X intact values in specimens with a 4.0 mm stepoff. Contour plots of contact stress rates also demonstrated an instability event during motion. An anterolateral stepoff of the distal tibia caused significant changes in positive and negative contact stress rates in cadaveric ankles. Incongruity-associated changes in contact stress rates and incongruity-associated instability events may be important pathomechanical determinants of post-traumatic arthritis.

Contact stress transients during functional loading of ankle stepoff incongruities.

Cartilage deformation demonstrates viscoelastic behavior due to its unique structure. However, nearly all contact studies investigating incongruity-associated changes in cartilage surface stresses have been static tests. These tests have consistently measured only modest increases in contact stresses, even with large incongruities. In this study, an experimental approach measuring real-time contact stresses in human cadaveric ankles during quasi-physiologic motion and loading was used to determine how stepoff incongruities of the distal tibia affected contact stresses and contact stress gradients. Peak instantaneous contact stresses, in ankles with stepoffs between 1.0 and 4.0mm of the anterolateral articular surface, increased by between 2.3 x and 3.0 x compared to the corresponding intact ankle values. Peak instantaneous contact stress gradients in stepoff configurations increased by between 1.9 x and 2.6 x the corresponding intact configuration values. Anatomic reduction of the displaced fragment restored intact contact stresses and contact stress gradients. Intact and anatomic configurations demonstrated a heterogeneous population of low-magnitude, randomly oriented contact stress gradient vectors in contrast to high-magnitude, preferentially oriented gradients in stepoff configurations. Peak instantaneous contact stresses may be important pathomechanical determinants of post-traumatic arthritis. Abnormal contact stress gradients could cause regional pathological disturbances in cartilage stress and interstitial fluid distribution. Measuring contact stresses and contact stress gradients during motion allowed potential incongruity-associated pathologic changes in loading that occur over the complete motion cycle to be investigated.

Tensile engagement of the peri-ankle ligaments in stance phase.

BACKGROUND: Development of reconstructive operative procedures to restore normal ankle kinematics after injury requires an understanding of the biomechanics of the ankle during gait. The contribution of the peri-ankle ligaments to ankle motion control is not yet well understood. Knowledge of the tensile engagement of the peri-ankle ligaments during stance phase is necessary to achieve physiologic motion patterns. METHODS: Eleven fresh-frozen cadaver ankles were subjected to a dynamic loading sequence simulating the stance phase of normal level gait. Simultaneously, ligament strain was continuously monitored in the anterior talofibular, calcaneofibular, and posterior talofibular ligaments, as well as in the anterior, middle, and posterior superficial deltoid ligaments. Eight of these specimens underwent further quasi-static range-of-motion testing, where ligament tension recruitment was assessed at 30 degrees plantarflexion and 30 degrees dorsiflexion. RESULTS: In the dynamic loading tests, none of the ligaments monitored showed a reproducible strain pattern indicating a role in ankle stabilization. However, in the extended range-of-motion tests, most ligaments were taut in plantarflexion or dorsiflexion. CONCLUSIONS: A consistent combination of individual ligament strain patterns that principally control ankle motion was not identified; none of the ligaments studied were reproducibly recruited to be a primary stabilizing structure. The peri-ankle ligaments are likely to be secondary restraining structures that serve to resist motion to avoid extreme positions. Stance phase ankle motion appears to be primarily controlled by articular congruity, not by peri-ankle ligament tension.