Susannah L. Gilbert

X-ray reconstruction of moving morphology (XROMM): precision, accuracy and applications in comparative biomechanics research

X-Ray Reconstruction of Moving Morphology (XROMM) comprises a set of 3D X-ray motion analysis techniques that merge motion data from in vivo X-ray videos with skeletal morphology data from bone scans into precise and accurate animations of 3D bones moving in 3D space. XROMM methods include: (1) manual alignment (registration) of bone models to video sequences, i.e., Scientific Rotoscoping; (2) computer vision-based autoregistration of bone models to biplanar X-ray videos; and (3) marker-based registration of bone models to biplanar X-ray videos. Here, we describe a novel set of X-ray hardware, software, and workflows for marker-based XROMM. Refurbished C-arm fluoroscopes retrofitted with high-speed video cameras offer a relatively inexpensive X-ray hardware solution for comparative biomechanics research. Precision for our biplanar C-arm hardware and analysis software, measured as the standard deviation of pairwise distances between 1 mm tantalum markers embedded in rigid objects, was found to be +/-0.046 mm under optimal conditions and +/-0.084 mm under actual in vivo recording conditions. Mean error in measurement of a known distance between two beads was within the 0.01 mm fabrication tolerance of the test object, and mean absolute error was 0.037 mm. Animating 3D bone models from sets of marker positions (XROMM animation) makes it possible to study skeletal kinematics in the context of detailed bone morphology. The biplanar fluoroscopy hardware and computational methods described here should make XROMM an accessible and useful addition to the available technologies for studying the form, function, and evolution of vertebrate animals.

Anatomic Ligament Repair Restores Ankle and Syndesmotic Rotational Stability as Much as Syndesmotic Screw Fixation.

Objectives: To compare the external rotation stability of 3 different syndesmotic stabilization techniques in a cadaveric ankle fracture model. Methods: Nondestructive external rotation stresses of 4 N·m were applied to 8 cadaveric limbs using a hydraulic loading frame. Four conditions were tested using a repeated-measures design: intact and 3 repair conditions after a destabilizing ligamentous ankle injury with syndesmotic disruption. The 3 repair conditions were tricortical trans-syndesmotic screw fixation, posterior inferior tibiofibular ligament (PITFL) repair, and combined PITFL and deltoid ligament repair. External rotation of the ankle joint and syndesmosis was measured using a motion capture system and compared for each test condition. Repeated-measures 1-way analyses of variance statistical tests were performed to compare the ankle and syndesmotic rotation findings between the 3 repair conditions and intact condition. Results: Rotational ankle stability was not fully restored by any of the 3 repair constructs. The intact ankle joint externally rotated approximately half as many degrees as the 3 repair conditions (intact: 10.9; trans-syndesmotic screw: 17.0; PITFL: 21.4; and PITFL/deltoid: 15.6). The intact condition also demonstrated significantly fewer degrees of syndesmotic rotation than the repair constructs (intact 2.4; trans-syndesmotic screw 5.2; PITFL 8.5; and PITFL/deltoid 6.9). Each of the repair conditions resulted in an externally rotated fibula when no loads were applied. The ligamentous repairs externally rotated the fibula twice as much as the trans-syndesmotic screw (P < 0.016). Conclusions: We found that combined repair of the PITFL and deltoid ligament restores an equivalent amount of ankle and syndesmotic rotational stability when compared to trans-syndesmotic screw fixation. Based on our findings, ligamentous repair can potentially be a viable treatment alternative in unstable ankle fracture patients with syndesmotic disruption. Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Using a Statistically Calibrated Biphasic Finite Element Model of the Human Knee Joint to Identify Robust Designs for a Meniscal Substitute

This paper describes a methodology for selecting a set of biomechanical engineering design variables to optimize the performance of an engineered meniscal substitute when implanted in a population of subjects whose characteristics can be specified stochastically. For the meniscal design problem where engineering variables include aspects of meniscal geometry and meniscal material properties, this method shows that meniscal designs having simultaneously large radial modulus and large circumferential modulus provide both low mean peak contact stress and small variability in peak contact stress when used in the specified subject population. The method also shows that the mean peak contact stress is relatively insensitive to meniscal permeability, so the permeability used in the manufacture of a meniscal substitute can be selected on the basis of manufacturing ease or cost. This is a multiple objective problem with the mean peak contact stress over the population of subjects and its variability both desired to be small. The problem is solved by using a predictor of the mean peak contact stress across the tibial plateau that was developed from experimentally measured peak contact stresses from two modalities. The first experimental modality provided computed peak contact stresses using a finite element computational simulator of the dynamic tibial contact stress during axial dynamic loading. A small number of meniscal designs with specified subject environmental inputs were selected to make computational runs and to provide training data for the predictor developed below. The second experimental modality consisted of measured peak contact stress from a set of cadaver knees. The cadaver measurements were used to bias-correct and calibrate the simulator output. Because the finite element simulator is expensive to evaluate, a rapidly computable (calibrated) Kriging predictor was used to explore extensively the contact stresses for a wide range of meniscal engineering inputs and subject variables. The predicted values were used to determine the Pareto optimal set of engineering inputs to minimize peak contact stresses in the targeted population of subjects.

Image based weighted center of proximity versus directly measured knee contact location during simulated gait

To understand the mechanical consequences of knee injury requires a detailed analysis of the effect of that injury on joint contact mechanics during activities of daily living. Three-dimensional (3D) knee joint geometric models have been combined with knee joint kinematics to dynamically estimate the location of joint contact during physiological activities-using a weighted center of proximity (WCoP) method. However, the relationship between the estimated WCoP and the actual location of contact has not been defined. The objective of this study was to assess the relationship between knee joint contact location as estimated using the image-based WCoP method, and a directly measured weighted center of contact (WCoC) method during simulated walking. To achieve this goal, we created knee specific models of six human cadaveric knees from magnetic resonance imaging. All knees were then subjected to physiological loads on a knee simulator intended to mimic gait. Knee joint motion was captured using a motion capture system. Knee joint contact stresses were synchronously recorded using a thin electronic sensor throughout gait, and used to compute WCoC for the medial and lateral plateaus of each knee. WCoP was calculated by combining knee kinematics with the MRI-based knee specific model. Both metrics were compared throughout gait using linear regression. The anteroposterior (AP) location of WCoP was significantly correlated with that of WCoC on both tibial plateaus in all specimens (p<0.01, 95% confidence interval of Pearson׳s coefficient r>0), but the correlation was not significant in the mediolateral (ML) direction for 4/6 knees (p>0.05). Our study demonstrates that while the location of joint contact obtained from 3D knee joint contact model, using the WCoP method, is significantly correlated with the location of actual contact stresses in the AP direction, that relationship is less certain in the ML direction.

Limited-Open Achilles Tendon Repair Using Locking Sutures Versus Nonlocking Sutures An In Vitro Model

Background: Several limited-open Achilles tendon repair techniques that use locking or nonlocking sutures have been developed, but direct comparisons of in vitro mechanical properties have not yet been reported in the literature. It was our hypothesis that loads applied to the repaired Achilles tendon would be better resisted by limited-open techniques that use locking stitches compared with limited-open repairs that use nonlocking stitches. Methods: The Achilles tendons of 31 fresh-frozen cadaver lower limbs were incised 4 cm proximal to the calcaneal insertion. Tendons were then repaired using 1 of 2 limited-open Achilles tendon repair tools, one using 3 nonlocking sutures and the other using a combination of locking and nonlocking sutures. Repaired specimens were cycled to 1000 cycles from 20 to 100 N and from 20 to 190 N followed by a single load to failure test. Nonparametric analyses were performed to compare the number of cycles to gapping and total load to failure between the 2 repair techniques. Results: During cyclic loading, more cycles occurred prior to detection of 2-mm and 9.5-mm gaps in the locking suture construct compared with the nonlocking suture construct (P = .012 and P = .005, respectively). There was no difference in the number of cycles to a gap of 5 mm (P = .053). The locking suture construct also resisted a significantly greater load to failure compared with the nonlocking suture construct (P < .001; median 385.0 and 299.6 N, respectively). Conclusion: Limited-open repair techniques using locking sutures provided greater construct strength under both cyclic and ultimate loads compared with a repair technique that used only nonlocking sutures. Clinical Relevance: Limited-open Achilles tendon repairs using locking sutures are better able to resist forces simulating early accelerated rehabilitation than repairs using nonlocking sutures.

Fluoroquinolones Impair Tendon Healing in a Rat Rotator Cuff Repair Model: A Preliminary Study

Background: Recent studies suggest that fluoroquinolone antibiotics predispose tendons to tendinopathy and/or rupture. However, no investigations on the reparative capacity of tendons exposed to fluoroquinolones have been conducted. Hypothesis: Fluoroquinolone-treated animals will have inferior biochemical, histological, and biomechanical properties at the healing tendon-bone enthesis compared with controls. Study Design: Controlled laboratory study. Methods: Ninety-two rats underwent rotator cuff repair and were randomly assigned to 1 of 4 groups: (1) preoperative (Preop), whereby animals received fleroxacin for 1 week preoperatively; (2) pre- and postoperative (Pre/Postop), whereby animals received fleroxacin for 1 week preoperatively and for 2 weeks postoperatively; (3) postoperative (Postop), whereby animals received fleroxacin for 2 weeks postoperatively; and (4) control, whereby animals received vehicle for 1 week preoperatively and for 2 weeks postoperatively. Rats were euthanized at 2 weeks postoperatively for biochemical, histological, and biomechanical analysis. All data were expressed as mean ± standard error of the mean (SEM). Statistical comparisons were performed using either 1-way or 2-way ANOVA, with P < .05 considered significant. Results: Reverse transcriptase quantitative polymerase chain reaction (RTqPCR) analysis revealed a 30-fold increase in expression of matrix metalloproteinase (MMP)-3, a 7-fold increase in MMP-13, and a 4-fold increase in tissue inhibitor of metalloproteinases (TIMP)-1 in the Pre/Postop group compared with the other groups. The appearance of the healing enthesis in all treated animals was qualitatively different than that in controls. The tendons were friable and atrophic. All 3 treated groups showed significantly less fibrocartilage and poorly organized collagen at the healing enthesis compared with control animals. There was a significant difference in the mode of failure, with treated animals demonstrating an intrasubstance failure of the supraspinatus tendon during testing. In contrast, only 1 of 10 control samples failed within the tendon substance. The healing enthesis of the Pre/Postop group displayed significantly reduced ultimate load to failure compared with the Preop, Postop, and control groups. There was no significant difference in load to failure in the Preop group compared with the Postop group. Pre/Postop animals demonstrated significantly reduced cross-sectional area compared with the Postop and control groups. There was also a significant reduction in area between the Preop and control groups. Conclusion: In this preliminary study, fluoroquinolone treatment negatively influenced tendon healing. Clinical Relevance: These findings indicate that there was an active but inadequate repair response that has potential clinical implications for patients who are exposed to fluoroquinolones before tendon repair surgery.

OARSI osteoarthritis cartilage histopathology assessment system: A biomechanical evaluation in the human knee.

The study compared the OARSI osteoarthritis cartilage histopathology assessment system with the biomechanical quality of human in vivo cartilage samples. In a prospective cohort study, 84 patients (100 knees) with varus deformity of the knee were included between May, 2010 and January, 2012. Osteochondral samples underwent biomechanical and histologic analysis. The dynamic modulus significantly (p < 0.001) decreased with each advancing grade of degeneration from OARSI Grade 0 (surface intact) to OARSI Grade 4 (erosion). For the aggregate modulus, there were significant (p < 0.001) differences between OARSI Grade 0 and OARSI Grade 1 as well as between OARSI Grade 1 and OARSI Grade 2. From OARSI Grade 2 to OARSI Grade 5, no differences in aggregate modulus occurred. The new OARSI grading system provides useful information about the functional properties of cartilage. There is a significant difference in cartilage stiffness between samples with intact surface and no signs of degeneration (OARSI Grade 0) and samples with intact surface and early signs of arthritis (OARSI Grade 1). Surgeons performing joint preserving procedures have to be aware that in knees with an intact cartilage surface (OARSI Grade 0/1), significant differences in the biomechanical properties may exist.

Three-Dimensional Analysis of Fibular Motion After Fixation of Syndesmotic Injuries With a Screw or Suture-Button Construct

Background: Suture-button constructs are an alternative to screw fixation for syndesmotic injuries, and proponents advocate that suture-button constructs may allow physiological motion of the syndesmosis. Recent biomechanical data suggest that fibular instability with syndesmotic injuries is greatest in the sagittal plane, but the design of a suture-button construct, being a rope and 2 retention washers, is most effective along the axis of the rope (in the coronal plane). Some studies report that suture-button constructs are able to constrain fibular motion in the coronal plane, but the ability of a tightrope to constrain sagittal fibular motion is unknown. The purpose of this study was to assess fibular motion in response to an external rotation stress test in a syndesmotic injury model after fixation with a screw or suture-button constructs. Methods: Eleven fresh-frozen cadaver whole legs with intact tibia-fibula articulations were secured to a custom fixture. Fibular motion (coronal, sagittal, and rotational planes) in response to a 6.5-Nm external rotation moment applied to the foot was recorded with fluoroscopy and a high-resolution motion capture system. Measures were taken for the following syndesmotic conditions: intact, complete lateral injury, complete lateral and deltoid injury, repair with a tetracortical 4.0-mm screw, and repair with a suture button construct (Tightrope; Arthrex, Naples, FL) aimed from the lateral fibula to the anterior medial malleolus. Results: The suture-button construct allowed significantly more sagittal plane motion than the syndesmotic screw. Measurements acquired with mortise imaging did not detect differences between the intact, lateral injury, and 2 repair conditions. External rotation of the fibula was significantly increased in both injury conditions and was not restored to intact levels with the screw or the suture-button construct. Conclusion: A single suture-button placed from the lateral fibula to the anterior medial malleolus was unable to replicate the motion observed in the intact specimen when subjected to an external rotation stress test and allowed significantly more posterior motion of the fibula than when fixed with a screw in simulated highly unstable injuries. Clinical Relevance: Fixation of a syndesmotic injury with a single suture-button construct did not restore physiological fibular motion, which may have implications for postoperative care and clinical outcomes.

A statistically-augmented computational platform for evaluating meniscal function

Meniscal implants have been developed in an attempt to provide pain relief and prevent pathological degeneration of articular cartilage. However, as yet there has been no systematic and comprehensive analysis of the effects of the meniscal design variables on meniscal function across a wide patient population, and there are no clear design criteria to ensure the functional performance of candidate meniscal implants. Our aim was to develop a statistically-augmented, experimentally-validated, computational platform to assess the effect of meniscal properties and patient variables on knee joint contact mechanics during the activity of walking. Our analysis used Finite Element Models (FEMs) that represented the geometry, kinematics as based on simulated gait and contact mechanics of three laboratory tested human cadaveric knees. The FEMs were subsequently programmed to represent prescribed meniscal variables (circumferential and radial/axial moduli-Ecm, Erm, stiffness of the meniscal attachments-Slpma, Slamp) and patient variables (varus/valgus alignment-VVA, and articular cartilage modulus-Ec). The contact mechanics data generated from the FEM runs were used as training data to a statistical interpolator which estimated joint contact data for untested configurations of input variables. Our data suggested that while Ecm and Erm of a meniscus are critical in determining knee joint mechanics in early and late stance (peak 1 and peak 3 of the gait cycle), for some knees that have greater laxity in the mid-stance phase of gait, the stiffness of the articular cartilage, Ec, can influence force distribution across the tibial plateau. We found that the medial meniscus plays a dominant load-carrying role in the early stance phase and less so in late stance, while the lateral meniscus distributes load throughout gait. Joint contact mechanics in the medial compartment are more sensitive to Ecm than those in the lateral compartment. Finally, throughout stance, varus-valgus alignment can overwhelm these relationships while the stiffness of meniscal attachments in the range studied have minimal effects on the knee joint mechanics. In summary, our statistically-augmented, computational platform allowed us to study how meniscal implant design variables (which can be controlled at the time of manufacture or implantation) interact with patient variables (which can be set in FEMs but cannot be controlled in patient studies) to affect joint contact mechanics during the activity of simulated walking.

Reconstruction of the medial talonavicular joint in simulated flatfoot deformity.

Background: Reconstructing the ligamentous constraints of the medial arch associated with adult acquired flatfoot deformity remains a challenge. The purpose of this study was to test the efficacy of several reconstruction techniques of the medial arch. We hypothesized that an anatomic reconstruction of the spring ligament complex would correct the deformity better than other techniques tested. Methods: Three reconstructions of the medial support structures were performed on each specimen to recreate the different lines of action and insertions of the medial ligamentous complex in 12 specimens with a simulated flatfoot deformity. Talonavicular and tibiocalcaneal (hindfoot) orientations were measured in the axial, sagittal, and coronal planes in the intact, flatfoot, and reconstructed conditions. Results: While each reconstruction technique corrected the deformity (P < .05), proximal fixation of the graft corrected the greatest amount of talonavicular deformity while also correcting hindfoot valgus (P < .05). Conclusion: The fixation points and lines of action of a medial arch reconstruction have important implications on deformity correction in a flatfoot model. Despite its fidelity to the native structure, the anatomic spring ligament reconstruction provided the least amount of correction. These findings suggest that other ligamentous structures of the medial arch are critical in supporting the midfoot. Clinical Relevance: Reconstruction of the ligamentous supports of the medial arch might be able to correct substantial amounts of deformity without osseous procedures like calcaneal osteotomies or midfoot fusions.