Methodology for Robotic In Vitro Testing of the Knee.

The knee joint plays a pivotal role in mobility and stability during ambulatory and standing activities of daily living (ADL). Increased incidence of knee joint pathologies and resulting surgeries has led to a growing need to understand the kinematics and kinetics of the knee. In vivo, in silico, and in vitro testing domains provide researchers different avenues to explore the effects of surgical interactions on the knee. Recent hardware and software advancements have increased the flexibility of in vitro testing, opening further opportunities to answer clinical questions. This paper describes best practices for conducting in vitro knee biomechanical testing by providing guidelines for future research. Prior to beginning an in vitro knee study, the clinical question must be identified by the research and clinical teams to determine if in vitro testing is necessary to answer the question and serve as the gold standard for problem resolution. After determining the clinical question, a series of questions (What surgical or experimental conditions should be varied to answer the clinical question, what measurements are needed for each surgical or experimental condition, what loading conditions will generate the desired measurements, and do the loading conditions require muscle actuation?) must be discussed to help dictate the type of hardware and software necessary to adequately answer the clinical question. Hardware (type of robot, load cell, actuators, fixtures, motion capture, ancillary sensors) and software (type of coordinate systems used for kinematics and kinetics, type of control) can then be acquired to create a testing system tailored to the desired testing conditions. Study design and verification steps should be decided upon prior to testing to maintain the accuracy of the collected data. Collected data should be reported with any supplementary metrics (RMS error, dynamic statistics) that help illuminate the reported results. An example study comparing two different anterior cruciate ligament reconstruction techniques is provided to demonstrate the application of these guidelines. Adoption of these guidelines may allow for better interlaboratory result comparison to improve clinical outcomes.

Age is superior to aortopathy phenotype as a predictor of aortic mechanics in patients with bicuspid valve.

OBJECTIVES: Bicuspid aortic valve (BAV) aortopathy is defined by 3 phenotypes-root, ascending, and diffuse-based on region of maximal aortic dilation. We sought to determine the association between aortic mechanical behavior and aortopathy phenotype versus other clinical variables. METHODS: From August 1, 2016, to March 1, 2023, 375 aortic specimens were collected from 105 patients undergoing elective ascending aortic aneurysm repair for BAV aortopathy. Planar biaxial data (191 specimens) informed constitutive descriptors of the arterial wall that were combined with in vivo geometry and hemodynamics to predict stiffness, stress, and energy density under physiologic loads. Uniaxial testing (184 specimens) evaluated failure stretch and failure Cauchy stress. Boosting regression was implemented to model the association between clinical variables and mechanical metrics. RESULTS: There were no significant differences in mechanical metrics between the root phenotype (N = 33, 31%) and ascending/diffuse phenotypes (N = 72, 69%). Biaxial testing demonstrated older age was associated with increased circumferential stiffness, decreased stress, and decreased energy density. On uniaxial testing, longitudinally versus circumferentially oriented specimens failed at significantly lower Cauchy stress (50th [15th, 85th percentiles]: 1.0 [0.7, 1.6] MPa vs 1.9 [1.3, 3.1] MPa; P < .001). Age was associated with decreased failure stretch and stress. Elongated ascending aortas were also associated with decreased failure stress. CONCLUSIONS: Aortic mechanical function under physiologic and failure conditions in BAV aortopathy is robustly associated with age and poorly associated with aortopathy phenotype. Data suggesting that the root phenotype of BAV aortopathy portends worse outcomes are unlikely to be related to aberrant, phenotype-specific tissue mechanics.

Longitudinal versus circumferential biomechanical behavior of the aneurysmal ascending aorta.

OBJECTIVES: We evaluate the independent effects of patient and aortic tissue characteristics on biaxial physiologic mechanical metrics in aneurysmal and nonaneurysmal tissues, and uniaxial failure metrics in aneurysmal tissue, comparing longitudinal and circumferential behavior. METHODS: From February 2017 to October 2022, 382 aortic specimens were collected from 134 patients; 268 specimens underwent biaxial testing, and 114 specimens underwent uniaxial testing. Biaxial testing evaluated Green-Lagrange transition strain and low and high tangent moduli. Uniaxial testing evaluated failure stretch, Cauchy stress, and low and high tangent moduli. Longitudinal gradient boosting models were implemented to estimate mechanical metrics and covariates of importance. RESULTS: On biaxial testing, nonaneurysmal tissue was less deformable and exhibited a lower transition strain than aneurysmal tissue in the longitudinal (0.18 vs 0.30, P < .001) and circumferential (0.25 vs 0.30, P = .01) directions. Older age and increasing ascending aortic length contributed most to predicting transition strain. On uniaxial testing, longitudinal specimens failed at lower stretch (1.4 vs 1.5, P = .003) and Cauchy stress (1.0 vs 1.9 kPa, P < .001) than circumferential specimens. Failure stretch and Cauchy stress were most strongly associated with tissue orientation and decreased sharply with older age. Age, ascending aortic length, and tissue thickness were the most frequent covariates predicting mechanical metrics across 10 prediction models. CONCLUSIONS: Age was the strongest predictor of mechanical behavior. After adjusting for age, nonaneurysmal tissue was less deformable than aneurysmal tissue. Differences in longitudinal and circumferential mechanics contribute to tissue dysfunction and failure in ascending aneurysms. This highlights the need to better understand the effects of age, ascending aortic length, and thickness on clinical aortic behavior.

Lengthwise regional mechanics of the human aneurysmal ascending thoracic aorta.

The prognosis of patients undergoing emergency endovascular repair of ascending thoracic aortic aneurysm (ATAA) depends on defect location, with root disease bearing worse outcomes than proximal or distal aortopathy. We speculate that a spatial gradient in aneurysmal tissue mechanics through the length of the ascending thoracic aorta may fuel noted survival discrepancies. To this end, we performed planar biaxial testing on 153 root, proximal, and distal segments of ATAA samples collected from 80 patients receiving elective open surgical repair. Following data averaging via surface fitting-based interpolation of strain-controlled protocols, we combined in-vitro and in-vivo measurements of loads and geometry to resolve inflation-extension kinematics and evaluate mechanical metrics of stress, stiffness, and energy at consistent deformation levels. Representative (averaged) experimental data and simulated in-vivo conditions revealed significantly larger biaxial stiffness at the root compared to either proximal or distal tissues, which persisted as the entire aorta stiffened during aging. Advancing age further reduced biaxial stretch and energy storage, a measure of aortic function, across all ATAA segments. Importantly, age emerged as a stronger predictor of tissue mechanics in ATAA disease than either bicuspid aortic valve or connective tissue disorders. Besides strengthening the general understanding of aneurysmal disease, our findings provide specifications to customize the design of stent-grafts for the treatment of ATAA disease. Optimization of deployment and interaction of novel endovascular devices with the local native environment is expected to carry significant potential for improving clinical outcomes. STATEMENT OF SIGNIFICANCE: Elucidating the lengthwise regional mechanics of ascending thoracic aortic aneurysms (ATAAs) is critical for the design of endovascular devices tailored to the ascending aorta. Stent-grafts provide a less invasive alternative to support the long-term survival of ATAA patients ineligible for open surgical repair. In this study, we developed a numerical framework that combines semi-inverse constitutive and forward modeling with in-vitro and in-vivo data to extract mechanical descriptors of ATAA tissue behavior at physiologically meaningful deformation. Moving distally from the aortic root to the first ascending aortic branch, we observed a progressive decline in biaxial stiffness. Furthermore, we showed that aging leads to reduced aortic function and is a stronger predictor of mechanics than either valve morphology or underlying syndromic disorder.

Ex vivo biaxial load testing analysis of aortic biomechanics demonstrates variation in elastic energy distribution across the aortic zone zero.

OBJECTIVES: We hypothesized that tissue characteristics vary significantly along zone zero, which may be reflected by regional differences in stored elastic energy. Our objectives were to (1) characterize the regional variation in stored elastic energy within tissues of the aortic zone zero and (2) identify the association between this variation and patient characteristics. METHODS: From February 2018 to January 2021, 123 aortic tissue samples were obtained from the aortic root and proximal and distal ascending aortas of 65 adults undergoing elective ascending aorta replacement. Biaxial biomechanics testing was performed to obtain tissue elastic energy at the inflection point and compared with patient demographics and preoperative computed tomography imaging. Coefficient models were fit using B-spline to interrogate the relationship among elastic energy, region, and patient characteristics. RESULTS: Mean elastic energy at inflection point was 24.3 ± 15.6 kJ/m3. Elastic energy increased significantly between the root and proximal, and root and distal ascending aorta and decreased with increasing age. Differences due to history of connective tissue disorder and bicuspid aortic valve were significant but diminished when controlled for other patient characteristics. Among covariates, age and region were found to be the most important predictors for elastic energy. CONCLUSIONS: Aortic tissue biomechanical metrics varied across regions and with patient characteristics within the aortic zone zero. Assessment of endovascular outcomes in the ascending aorta must closely consider the region of deployment and variable tissue qualities along the length of the landing zone. Regional variation in tissue characteristics should be incorporated into existing patient-specific models of aortic mechanics.

Aortic diameter is a poor predictor of aortic tissue failure metrics in patients with ascending aneurysms.

OBJECTIVES: There is growing consensus that aortic diameter is a flawed predictor of aortic dissection risk. We hypothesized that aortic tissue metrics would be better predicted by clinical metrics other than aortic diameter. Our objectives were to (1) characterize circumferential aortic failure stress and stretch as a result of aortic size and patient demographics, and (2) identify the influence of bicuspid aortic valve on failure metrics. METHODS: From February 2018 to January 2021, 136 aortic tissue samples were obtained from 86 adults undergoing elective ascending aorta repair. Uniaxial biomechanical testing to failure, defined as a full-thickness central tear, was performed to obtain tissue failure stress and failure stretch and compared with clinical data and preoperative computed tomography imaging. The relationships among aortic diameter, patient demographics, and failure metrics were assessed using random forest regression models. RESULTS: Median failure stress was 1.46 (1.02-1.94) megapascals, and failure stretch was 1.36 (1.27-1.54). Regression models correlated moderately with failure stress (R2 = 0.557) and highly with failure stretch (R2 = 0.806). Failure stress decreased with increasing age, lower body mass index, thicker tissue, and tricuspid aortic valves, whereas failure stretch was most highly correlated with age. Aortic area-to-height index outperformed aortic diameter in all models. CONCLUSIONS: Aneurysmal ascending aortic tissue failure metrics correlated with available clinical metrics. Greater tissue thickness, older age, and tricuspid aortic valve morphology outperformed aortic diameter, warranting further investigation into the role of a patient-specific multifactorial dissection risk assessment over aortic diameter as a sole marker of aortic tissue integrity.

A preliminary evaluation of in vivo response to a filament-wound macroporous collagen midurethral sling in an ovine model.

Stress urinary incontinence (SUI) impacts ~1/3 of women over age 50. Negative publicity around PP meshes used in pelvic prolapse repair drives the need for identifying alternative biomaterials for SUI repair. Our study evaluated in vivo response to collagen sling implanted in an ovine model. Electrocompacted collagen threads were filament wound as slings and crosslinked in genipin. Collagen slings were implanted suburethrally mimicking the transvaginal tape technique. Main study groups were: Collagen sling (n = 3, 6 months) and PP sling (n = 3, 6 months). Collagen sling was also tested at 3-weeks (n = 1) to observe early-stage tissue response and 1-year (n = 2) to assess biomaterial longevity in a preliminary capacity. Collagen slings healed to a fibrous ligament texture at 6 months and maintained such texture to 1 year. Histological scoring indicated biocompatible responses to collagen slings with no adverse events. All study groups exhibited complete tissue ingrowth and interstitial de novo collagen deposition at all time points. Collagen threads induced orderly de novo collagen deposition that was aligned along long axes of threads. Tissue infiltrated collagen slings that were explanted at 6 and 12 months presented similar structural strength with native tissues such as vagina and fascia, and PP (Lynx) slings (p > .05). With the limitation of low number of animals per time point in hindsight, this preliminary study justifies evaluation of collagen slings in a larger sample size of animals, particularly to assess persistence of ligamentous tissue response over longer durations than 1-year.

A Novel and Inexpensive Technique for High-Tension Tendon Clamping With Expandable Mesh Sleeving for In Vitro Foot Biomechanics Testing.

In vitro biomechanical testing is common in the field of orthopedics when novel devices are investigated prior to human trials. It is typically necessary to apply loads through tendons to simulate normal activities, such as walking during a foot and ankle study. However, attachment of tendons to linear actuators has proven challenging because of the tendency of clamps to either slip off or rupture the tendon. Various techniques have been utilized. Freeze clamping is generally accepted as the gold standard for very high load testing in excess of 3000 N, but is expensive, time-consuming, and requires significant ancillary equipment. Purely mechanical solutions such as metal jaw clamps, wire meshes, and others have been explored, but these techniques are either costly, have low load capacities, or have not proven to be reproducible. We have developed a novel tendon clamping technique that utilizes a slip-resistant polyester mesh sleeving that encases the tendon and is fixated at the bottom of the tendon/sleeve interaction with a giftbox suture. The loose end of the sleeving can then be tied in to the linear actuator or load cell apparatus using a timber hitch knot. The sleeving technique allows for loads of 2000-2500 N on the Achilles tendon, and is inexpensive, reproducible, and can be modified to apply loads to smaller tendons as well, though a length of tendon/sleeve overlap of at least 16 cm is required to reach maximum loads. This technique should assist researchers in integrating muscle forces into future biomechanical study designs.

Assessment of L5-S1 anterior lumbar interbody fusion stability in the setting of lengthening posterior instrumentation constructs: a cadaveric biomechanical study.

OBJECTIVE: Excessive stress and motion at the L5-S1 level can lead to degenerative changes, especially in patients with posterior instrumentation suprajacent to L5. Attention has turned to utilization of L5-S1 anterior lumbar interbody fusion (ALIF) to stabilize the lumbosacral junction. However, questions remain regarding the effectiveness of stand-alone ALIF in the setting of prior posterior instrumented fusions terminating at L5. The purpose of this study was to assess the biomechanical stability of an L5-S1 ALIF with increasing lengths of posterior thoracolumbar constructs. METHODS: Seven human cadaveric spines (T9-sacrum) were instrumented with pedicle screws from T10 to L5 and mounted to a 6 degrees-of-freedom robot. Posterior fusion construct lengths (T10-L5, T12-L5, L2-5, and L4-5) were instrumented to each specimen, and torque-fusion level relationships were determined for each construct in flexion-extension, axial rotation, and lateral bending. A stand-alone L5-S1 ALIF was then instrumented, and L5-S1 motion was measured as increasing pure moments (2 to 12 Nm) were applied. Motion reduction was calculated by comparing L5-S1 motion across the ALIF and non-ALIF states. RESULTS: The average motion at L5-S1 in axial rotation, flexion-extension, and lateral bending was assessed for each fusion construct with and without ALIF. After adding ALIF to a posterior fusion, L5-S1 motion was significantly reduced relative to the non-ALIF state in all but one fused surgical condition (p < 0.05). Longer fusions with ALIF produced larger L5-S1 motions, and in some cases resulted in motions higher than native state motion. CONCLUSIONS: Posterior fusion constructs up to L4-5 could be appropriately stabilized by a stand-alone L5-S1 ALIF when using a nominal threshold of 80% reduction in native motion as a potential positive indicator of fusion. The results of this study allow conclusions to be drawn from a biomechanical standpoint; however, the clinical implications of these data are not well defined. These findings, when taken in appropriate clinical context, can be used to better guide clinicians seeking to treat L5-S1 pathology in patients with prior posterior thoracolumbar constructs.

Evaluating stability of the craniovertebral junction after unilateral C1 lateral mass resection: implications for the direct lateral approach.

OBJECTIVE: The direct lateral approach is an alternative to the transoral or endonasal approaches to ventral epidural lesions at the lower craniocervical junction. In this study, the authors performed, to their knowledge, the first in vitro biomechanical evaluation of the craniovertebral junction after sequential unilateral C1 lateral mass resection. The authors hypothesized that partial resection of the lateral mass would not result in a significant increase in range of motion (ROM) and may not require internal stabilization. METHODS: The authors performed multidirectional in vitro ROM testing using a robotic spine testing system on 8 fresh cadaveric specimens. We evaluated ROM in 3 primary movements (axial rotation [AR], flexion/extension [FE], and lateral bending [LB]) and 4 coupled movements (AR+E, AR+F, LB + left AR, and LB + right AR). Testing was performed in the intact state, after C1 hemilaminectomy, and after sequential 25%, 50%, 75%, and 100% C1 lateral mass resection. RESULTS: There were no significant increases in occipital bone (Oc)-C1, C1-2, or Oc-C2 ROM after C1 hemilaminectomy and 25% lateral mass resection. After 50% resection, Oc-C1 AR ROM increased by 54.4% (p = 0.002), Oc LB ROM increased by 47.8% (p = 0.010), and Oc-C1 AR+E ROM increased by 65.8% (p < 0.001). Oc-C2 FE ROM increased by 7.2% (p = 0.016) after 50% resection; 75% and 100% lateral mass resection resulted in further increases in ROM. CONCLUSIONS: In this cadaveric biomechanical study, the authors found that unilateral C1 hemilaminectomy and 25% resection of the C1 lateral mass did not result in significant biomechanical instability at the occipitocervical junction, and 50% resection led to significant increases in Oc-C2 ROM. This is the first biomechanical study of lateral mass resection, and future studies can serve to validate these findings.

Development of a novel in vitro cadaveric model for analysis of biomechanics and surgical treatment of Bertolotti syndrome.

BACKGROUND CONTEXT: Bertolotti syndrome (BS) is caused by pseudoarticulation between an aberrant L5 transverse process and the sacral ala, termed a lumbosacral transitional vertebra (LSTV). BS is thought to cause low back pain and is treated with resection or fusion, both of which have shown success. Acquiring cadavers with BS is challenging. Thus, we combined 3D printing, based on BS patient CT scans, with normal cadaveric spines to create a BS model. We then performed biomechanical testing to determine altered kinematics from LSTV with surgical interventions. Force sensing within the pseudojoint modeled nociception for different trajectories of motion and surgical conditions. PURPOSE: This study examines alterations in spinal biomechanics with LSTVs and with various surgical treatments for BS in order to learn more about pain and degeneration in this condition, in order to help optimize surgical decision-making. In addition, this study evaluates BS histology in order to better understand the pathology and to help define pain generators-if, indeed, they actually exist. STUDY DESIGN/SETTING: Model Development: A retrospective patient review of 25 patients was performed to determine the imaging criteria that defines the classical BS patient. Surgical tissue was extracted from four BS patients for 3D-printing material selection. Biomechanical Analysis. This was a prospective cadaveric biomechanical study of seven spines evaluating spinal motions, and loads, over various surgical conditions (intact, LSTV, and LSTV with various fusions). Additionally, forces at the LSTV joint were measured for the LSTV and LSTV with fusion condition. Histological Analysis: Histologic analysis was performed prospectively on the four surgical specimens from patients undergoing pseudoarthrectomy for BS at our institution to learn more about potential pain generators. PATIENT SAMPLE: The cadaveric portion of the study involved seven cadaveric spines. Four patients were prospectively recruited to have their surgical specimens assessed histologically and biomechanically for this study. Patients under the age of 18 were excluded. OUTCOME MEASURES: Physiological measures recorded in this study were broken down into histologic analysis, tissue biomechanical analysis, and joint biomechanical analysis. Histologic analysis included pathologist interpretation of Hematoxylin and Eosin staining, as well as S-100 staining. Tissue biomechanical analysis included stiffness measurements. Joint biomechanical analysis included range of motion, resultant torques, relative axis angles, and LSTV joint forces. METHODS: This study received funding from the American Academy of Neurology Medical Student Research Scholarship. Three authors hold intellectual property rights in the simVITRO robotic testing system. No other authors had relevant conflicts of interest for this study. CT images were segmented for a representative BS patient and cadaver spines. Customized cutting and drilling guides for LSTV attachment were created for individual cadavers. 3D-printed bone and cartilage structural properties were based on surgical specimen stiffness, and specimens underwent histologic analysis via Hematoxylin and Eosin, as well as S-100 staining. Joint biomechanical testing was performed on the robotic testing system for seven specimens. Force sensors detected forces in the LSTV joint. Kruskal-Wallis tests and Dunnett's tests were used for statistical analysis with significance bounded to p<.05. RESULTS: LSTV significantly reduces motion at the L5-S1 level, particularly in lateral bending and axial rotation. Meanwhile, the LSTV increases adjacent segment motion significantly at the L2-L3 level, whereas other levels have nonsignificant trends toward increased motion with LSTV alone. Fusion involving L4-S1 (L4-L5 and L5-S1) to treat adjacent level degeneration associated with an LSTV is associated with a significant increase in adjacent segment motion at all levels other than L5-S1 compared to LSTV alone. Fusion of L5-S1 alone with LSTV significantly increases L3-L4 adjacent segment motion compared to LSTV alone. Last, ipsilateral lateral bending with or without ipsilateral axial rotation produces the greatest force on the LSTV, and these forces are significantly reduced with L5-S1 fusion. CONCLUSIONS: BS significantly decreases L5-S1 mobility, and increases some adjacent segment motion, potentially causing patient activity restriction and discomfort. Ipsilateral lateral bending with or without ipsilateral axial rotation may cause the greatest discomfort overall in these patients, and fusion of the L5-S1 or L4-S1 levels may reduce pain associated with these motions. However, due to increased adjacent segment motion with fusions compared to LSTV alone, resection of the joint may be the better treatment option if the superior levels are not unstable preoperatively. CLINICAL SIGNIFICANCE: This study's results indicate that patients with BS have significantly altered spinal biomechanics and may develop pain due to increased loading forces at the LSTV joint with ipsilateral lateral bending and axial rotation. In addition, increased motion at superior levels when an LSTV is present may lead to degeneration over time. Based upon results of LSTV joint force testing, these patients' pain may be effectively treated surgically with LSTV resection or fusion involving the LSTV level if conservative management fails. Further studies are being pursued to evaluate the relationship between in vivo motion of BS patients, spinal and LSTV positioning, and pain generation to gain a better understanding of the exact source of pain in these patients. The methodologies utilized in this study can be extrapolated to recreate other spinal conditions that are poorly understood, and for which few native cadaveric specimens exist.