M. James Rudert

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.

Indentation assessment of biphasic mechanical property deficits in size-dependent osteochondral defect repair

The apparent biphasic material properties of 10-month osteochondral defect repair tissue were determined for a series of full thickness defects of 1, 3, or 5 mm diameter, created in weight-bearing regions of 48 canine femoral condyles. Load cell recordings from indentation tests were compared with resultant contact forces computed using a corresponding linear biphasic finite element model. The spread of cartilage engagement by a spherical ended indentor was modeled by successively imposing an impenetrability kinematic boundary condition at cartilage surface nodes for which incipient indentor surface penetration was detected. For each indentation test, a least-squares-error curve fitting procedure was used to identify a set of biphasic coefficients (aggregate modulus, permeability, Poisson ratio) that closely modeled experimental behavior. In the near neighborhood of best-fit, the finite element solutions were found to be much more sensitive to aggregate modulus perturbations than to permeability permutations, suggesting that perceived permeability increase may be of lesser value as a discriminant of repair tissue inadequacy. Compared to surrounding cartilage, the repair tissue for all defect sizes had statistically significant decreases in aggregate modulus and in Poisson ratio (much more so for 3 and 5 mm defects than for 1 mm defects). The two larger defect diameters had significant increases in permeability, whereas the 1 mm defects did not. While the material property deficits were consistent, substantial and comparable to those in other recent animal models of osteochondral defect repair, the size-dependence per se of the observed constitutive differences was only modest.

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 capsules 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. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 29:1642–1648, 2011

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.

THE ROLE OF THE INTEROSSEOUS TALOCALCANEAL LIGAMENT IN SUBTALAR JOINT STABILITY

Background: Injury of the interosseous talocalcaneal ligament (ITCL) has been recognized as a cause of subtalar instability, though lack of an accepted clinical test has limited the ability of clinicians to reliably make the diagnosis. Clinical effects of ITCL failure remain unclear because of insufficient understanding of the role of the ligament. Methods: Load-displacement characteristics of the subtalar joint were studied in six cadaver specimens using an axial distraction test and a transverse multi-direction drawer test. In all tests, cyclic loading (+/−60 N) was applied, and load-displacement responses were collected before and after sectioning of the ITCL. Two parameters were used to analyze the data: neutral-zone laxity as a measure of joint play, and flexibility as a measure of resistance to applied force. Results: In the axial distraction test, sectioning increased both neutral-zone laxity and flexibility (p = .01 and .02, respectively). In the transverse test, sectioning caused increase of both neutral-zone laxity and flexibility (p <.001, for each). Neutral-zone laxity increased most greatly along an axis defined roughly by the posterior aspect of the fibula and the central region of the medial malleolus. Flexibility increased most in the medial direction (p <.05, for each). Conclusions: Results confirmed the role of the ITCL in maintaining apposition of the subtalar joint, as well as suggested its role in stabilizing the subtalar joint against drawer forces applied to the calcaneus from lateral to medial. The dominant direction of increased neutral-zone laxity described above suggests the optimal direction for detecting subtalar instability involved with ITCL injury. Clinical Relevance: ITCL failure may result in subtalar instability and should be examined with a drawer force along the preferential axis roughly from the posterior aspect of the fibula to the central region of the medial malleolus. Further clinical evaluation is required to determine whether ITCL failure is reliably detectable.

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.

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.

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

Summary: 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.

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.

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.