Stress and strain in the anterior band of the inferior glenohumeral ligament during a simulated clinical examination

A Validated Open-Source Shoulder Finite Element Model and Investigation of the Effect of Analysis Precision

Understanding the loads and stresses on different tissues within the shoulder complex is crucial for preventing joint injury and developing shoulder implants. Finite element (FE) models of the shoulder joint can be helpful in describing these forces and the biomechanics of the joint. Currently, there are no validated FE models of the intact shoulder available in the public domain. This study aimed to develop and validate a shoulder FE model, then make the model available to the orthopaedic research community. Publicly available medical images of the Visible Human Project male subject's right shoulder were used to generate the model geometry. Material properties from the literature were applied to the different tissues. The model simulated abduction in the scapular plane. Simulated glenohumeral (GH) contact force was compared to in vivo data from the literature, then further compared to other in vitro experimental studies. Output variable results were within one standard deviation of the mean in vivo experimental values of the GH contact force in 0°, 10°, 20°, 30°, and 45° of abduction. Furthermore, a comparison among different analysis precision in the Abaqus/Explicit platform was made. The complete shoulder model is available for download at github.com/OSEL-DAM/ShoulderFiniteElementModel.

Effect of the medial collateral ligament and the lateral ulnar collateral ligament injury on elbow stability: a finite element analysis

Ligaments are the most important stabilizer of elbow. However, the stress of ligaments is hard to measure because of the complex biomechanical environment in the elbow. Our objective was to develop a human elbow finite element model and to validate it by a comparison with previous experimental data. Then several different ligaments injury conditions and elbow flexion were simulated to analyse the elbow instability and to stress the biomechanical consequences. The computational investigation of different effects of ligament constraints of elbow was studied by means of finite element analysis. The stress of the anterior bundle was almost greater than other ligaments in all conditions, which played the most important role during the elbow flexion. The posterior bundle was the secondary stabilizer during flexion after the anterior bundle. The lateral ulnar collateral ligament (LUCL) injury could result in an increase of the ulnar cartilage stress. The anterior bundle and the LUCL were recommended to be repaired in elbow joint dislocations and fractures. This study could help understand the dynamic effects of ligaments on the joint over the entire extension by investigating the tissue stress.

Finite element analysis of the rotator cuff: A systematic review

BACKGROUND

Finite element modeling serves as a promising tool for investigating underlying rotator cuff biomechanics and pathology. However, there are currently no concrete guidelines for reporting in finite element model studies. This has compromised the reliability, validity, and reproducibility of literature due to omission of pertinent items within publications. Recently a Finite Element Model Grading Procedure has been proposed as a reporting guideline for model developers. The aim of this study was to conduct a systematic review of rotator cuff focused finite element models and characterize the reporting quality of those articles.

METHODS

A comprehensive literature search was performed in PubMed, Web of Science, and Embase to find relevant articles. Each article was graded and given a reporting quality ranking based on a score generated from the Finite Element Model Grading Procedure.

FINDINGS

We found that only 5/22 articles had scores of 75% or higher and fell within the "exceptional" reporting quality range. Most of the articles (16/22) fell within the "good" reporting quality range with scores between 50% and 75%. However, 9/16 articles within the "good" reporting quality range had scores below 60%.

INTERPRETATION

This study indicates that improved guidelines and standards for good reporting practices must be made in the field of finite element modeling. Furthermore, it supports the use of the Finite Element Model Grading Procedure as an objective method for evaluating the quality of finite element model reporting in the literature.

A Validated, Subject-Specific Finite Element Model for Predictions of Rotator Cuff Tear Propagation

Rotator cuff tears are a significant clinical problem previously investigated by unvalidated computational models that either use simplified geometry or isotropic elastic material properties to represent the tendon. The objective of this study was to develop an experimentally validated, finite element model of supraspinatus tendon using specimen-specific geometry and inhomogeneous material properties to predict strains in intact supraspinatus tendon. Three-dimensional tendon surface strains were determined at 60°, 70°, and 90° of glenohumeral abduction for articular and bursal surfaces of supraspinatus tendon during cyclic loading to serve as validation data. A finite element model was developed using the tendon geometry and inhomogeneous material properties to predict surface strains for loading conditions mimicking experimental loading conditions. Experimental strains were directly compared with computational model predictions to validate the model. Overall, the model successfully predicted magnitudes of strains that were within the experimental repeatability of 3% strain of experimental measures on both surfaces of the tendon. Model predictions and experiments showed the largest strains to be located on the articular surface (~8% strain) between the middle and anterior edge of the tendon. Importantly, the reference configuration chosen to calculate strains had a significant effect on strain calculations, and therefore must be defined with an innovative optimization algorithm. This study establishes a rigorously validated, specimen-specific computational model using novel surface strain measurements for use in investigating the function of the supraspinatus tendon and to ultimately predict the propagation of supraspinatus tendon tears based on the tendon's mechanical environment.

Finite element models of the human shoulder complex: a review of their clinical implications and modelling techniques

The human shoulder is a complicated musculoskeletal structure and is a perfect compromise between mobility and stability. The objective of this paper is to provide a thorough review of previous finite element (FE) studies in biomechanics of the human shoulder complex. Those FE studies to investigate shoulder biomechanics have been reviewed according to the physiological and clinical problems addressed: glenohumeral joint stability, rotator cuff tears, joint capsular and labral defects and shoulder arthroplasty. The major findings, limitations, potential clinical applications and modelling techniques of those FE studies are critically discussed. The main challenges faced in order to accurately represent the realistic physiological functions of the shoulder mechanism in FE simulations involve (1) subject-specific representation of the anisotropic nonhomogeneous material properties of the shoulder tissues in both healthy and pathological conditions; (2) definition of boundary and loading conditions based on individualised physiological data; (3) more comprehensive modelling describing the whole shoulder complex including appropriate three-dimensional (3D) representation of all major shoulder hard tissues and soft tissues and their delicate interactions; (4) rigorous in vivo experimental validation of FE simulation results. Fully validated shoulder FE models would greatly enhance our understanding of the aetiology of shoulder disorders, and hence facilitate the development of more efficient clinical diagnoses, non-surgical and surgical treatments, as well as shoulder orthotics and prosthetics. © 2016 The Authors. International Journal for Numerical Methods in Biomedical Engineering published by John Wiley & Sons Ltd.

Biomechanical Role of Capsular Continuity in Superior Capsule Reconstruction for Irreparable Tears of the Supraspinatus Tendon

BACKGROUND

Patients with irreparable rotator cuff tears have a defect of the superior capsule, which creates discontinuity of the shoulder capsule in the transverse direction (anterior-posterior direction). This effect is one of the causes underlying shoulder instability after rotator cuff tears.

PURPOSE/HYPOTHESIS

The purpose of this study was to assess the effects of anterior and posterior continuity on shoulder biomechanics after superior capsule reconstruction (SCR). The hypothesis was that capsular continuity in the transverse direction would improve glenohumeral stability after SCR.

STUDY DESIGN

Controlled laboratory study.

METHODS

Seven fresh-frozen cadaveric shoulders were tested by using a custom shoulder testing system. Subacromial peak contact pressure, glenohumeral superior translation, glenohumeral compression force, and glenohumeral range of motion (ROM) were compared among 5 conditions: (1) intact shoulder, (2) simulated irreparable supraspinatus tendon tear, (3) SCR without side-to-side suturing, (4) SCR with posterior side-to-side suturing, and (5) SCR with both anterior and posterior side-to-side suturing.

RESULTS

The creation of an irreparable supraspinatus tear significantly increased glenohumeral superior translation (0° of abduction: 254% of intact [P = .04]; 30° of abduction: 200% of intact [P = .04]) and subacromial peak contact pressure (0° of abduction: 302% of intact [P = .0001]; 30° of abduction: 239% of intact [P = .0006]), decreased glenohumeral compression force (0° of abduction: 85% of intact [P = .004]; 30° of abduction: 87% of intact [P = .0002]; 60° of abduction: 88% of intact [P = .0001]), and increased total ROM (0° of abduction: 16° increase [P = .008]). SCR without side-to-side suturing significantly decreased subacromial peak contact pressure (0° of abduction: 79% of intact [P = .0001]; 30° of abduction: 91% of intact [P = .001]; 60° of abduction: 55% of intact [P = .04]) but did not inhibit glenohumeral superior translation. By adding posterior side-to-side sutures, both glenohumeral superior translation (0° of abduction: 93% of intact [P = .02]; 30° of abduction: 110% of intact [P = .04]) and subacromial peak contact pressure decreased significantly (0° of abduction: 56% of intact [P = .0001]; 30° of abduction: 83% of intact [P = .0003]; 60° of abduction: 46% of intact [P = .04]). Neither SCR with nor SCR without side-to-side suturing ameliorated the tear-associated decrease in glenohumeral compression force and increase in total ROM. Adding anterior side-to-side sutures did not change any measurements compared with SCR with posterior side-to-side suturing.

CONCLUSION

SCR with side-to-side suturing completely restored the superior stability of the shoulder joint by establishing posterior continuity between the graft, residual infraspinatus tendon, and underlying shoulder capsule.

CLINICAL RELEVANCE

Side-to-side suturing between the graft, residual infraspinatus tendon, and underlying shoulder capsule is recommended for SCR in patients with irreparable supraspinatus tendon tears to restore superior stability after surgery.

A Normative Anatomic Study of the Glenohumeral Joint and Rotator Cuff Tendons

Introduction:

The purpose of this study is to determine normative values relating to the glenohumeral joint and rotator cuff tendons namely length, width, thickness and footprint of supraspinatus (SS), infraspinatus (IS), teres minor ™ and subscapularis (SC) from 19 specimens.

Methods:

Rotator cuff tendons and humerus dimensions were measured directly using a digital vernier calliper. In addition, moduli values of rotator cuff tendons were obtained from tensile tests on twelve cadaveric shoulders. The tendon footprints were delineated onto a tracing paper and their dimensions were calculated using digitiser software. The glenoid dimensions were obtained from marking its outline onto a tracing paper and the cavity dimensions from wax moulds. The results were tabulated and statistical analyses, including Student's t-test and ANOVA, were performed.

Conclusion:

The normative anatomic values obtained can be used in finite element modeling, computer simulation, design of implants and surgery.

Shoulder labral pathomechanics with rotator cuff tears

Rotator cuff tears (RCTs), the most common injury of the shoulder, are often accompanied by tears in the superior glenoid labrum. We evaluated whether superior humeral head (HH) motion secondary to RCTs and loading of the long head of the biceps tendon (LHBT) are implicated in the development of this associated superior labral pathology. Additionally, we determined the efficacy of a finite element model (FEM) for predicting the mechanics of the labrum. The HH was oriented at 30° of glenohumeral abduction and neutral rotation with 50N compressive force. Loads of 0N or 22N were applied to the LHBT. The HH was translated superiorly by 5mm to simulate superior instability caused by RCTs. Superior displacement of the labrum was affected by translation of the HH (P<0.0001), position along the labrum (P<0.0001), and interaction between the location on the labrum and LHBT tension (P<0.05). The displacements predicted by the FEM were compared with mechanical tests from 6 cadaveric specimens and all were within 1 SD of the mean. A hyperelastic constitutive law for the labrum was a better predictor of labral behavior than the elastic law and insensitive to ±1 SD variations in material properties. Peak strains were observed at the glenoid-labrum interface below the LHBT attachment consistent with the common location of labral pathology. These results suggest that pathomechanics of the shoulder secondary to RCTs (e.g., superior HH translation) and LHBT loading play significant roles in the pathologic changes seen in the superior labrum.

In-vivo glenohumeral translation and ligament elongation during abduction and abduction with internal and external rotation

Study Design

Basic Science. To investigate humeral head translations and glenohumeral ligament elongation with a dual fluoroscopic imaging system.

Background

The glenohumeral ligaments are partially responsible for restraining the humeral head during the extremes of shoulder motion. However, in-vivo glenohumeral ligaments elongation patterns have yet to be determined. Therefore, the objectives of this study were to 1) quantify the in-vivo humeral head translations and glenohumeral ligament elongations during functional shoulder positions, 2) compare the inferred glenohumeral ligament functions with previous literature and 3) create a baseline data of healthy adult shoulder glenohumeral ligament lengths as controls for future studies.

Methods

Five healthy adult shoulders were studied with a validated dual fluoroscopic imaging system (DFIS) and MR imaging technique. Humeral head translations and the superior, middle and inferior glenohumeral ligaments (SGHL, MGHL, IGHL) elongations were determined.

Results

The humeral head center on average translated in a range of 6.0mm in the anterior-posterior direction and 2.5mm in the superior-inferior direction. The MGHL showed greater elongation over a broader range of shoulder motion than the SGHL. The anterior-band (AB)-IGHL showed maximum elongation at 90° abduction with maximum external rotation. The posterior-band (PB)-IGHL showed maximum elongation at 90° abduction with maximum internal rotation.

Discussion

The results demonstrated that the humeral head translated statistically more in the anterior-posterior direction than the superior-inferior direction (p = 0.01), which supports the concept that glenohumeral kinematics are not ball-in-socket mechanics. The AB-IGHL elongation pattern makes it an important static structure to restrain anterior subluxation of the humeral head during the externally rotated cocking phase of throwing motion. These data suggest that in healthy adult shoulders the ligamentous structures of the glenohumeral joint are not fully elongated in many shoulder positions, but function as restraints at the extremes of glenohumeral motion. Clinically, these results may be helpful in restoring ligament anatomy during the treatment of anterior instability of the shoulder.

Finite element modelling of the glenohumeral capsule can help assess the tested region during a clinical exam

The objective of this research was to examine the efficacy of evaluating the region of the glenohumeral capsule being tested by clinical exams for shoulder instability using finite element (FE) models of the glenohumeral joint. Specifically, the regions of high capsule strain produced by glenohumeral joint positions commonly used during a clinical exam were identified. Kinematics that simulated a simple translation test with an anterior load at three external rotation angles were applied to a validated, subject-specific FE model of the glenohumeral joint at 60° of abduction. Maximum principal strains on the glenoid side of the inferior glenohumeral ligament (IGHL) were significantly higher than the maximum principal strains on the humeral side, for all three regions of the IGHL at 30° and 60° of external rotation. These regions of localised strain indicate that these joint positions might be used to test the glenoid side of the IGHL during this clinical exam, but are not useful for assessing the humeral side of the IGHL. The use of FE models will facilitate the search for additional joint positions that isolate high strains to other IGHL regions, including the humeral side of the IGHL.

Finding consistent strain distributions in the glenohumeral capsule between two subjects: implications for development of physical examinations

The anterior-inferior glenohumeral capsule is the primary passive stabilizer to the glenohumeral joint during anterior dislocation. Physical examinations following dislocation are crucial for proper diagnosis of capsule pathology; however, they are not standardized for joint position which may lead to misdiagnoses and poor outcomes. To suggest joint positions for physical examinations where the stability provided by the capsule may be consistent among patients, the objective of this study was to evaluate the distribution of maximum principal strain on the anterior-inferior capsule using two validated subject-specific finite element models of the glenohumeral joint at clinically relevant joint positions. The joint positions with 25 N anterior load applied at 60° of glenohumeral abduction and 10°, 20°, 30° and 40° of external rotation resulted in distributions of strain that were similar between shoulders (r² ≥ 0.7). Furthermore, those positions with 20-40° of external rotation resulted in capsule strains on the glenoid side of the anterior band of the inferior glenohumeral ligament that were significantly greater than in all other capsule regions. These findings suggest that anterior stability provided by the anterior-inferior capsule may be consistent among subjects at joint positions with 60° of glenohumeral abduction and a mid-range (20-40°) of external rotation, and that the glenoid side has the greatest contribution to stability at these joint positions. Therefore, it may be possible to establish standard joint positions for physical examinations that clinicians can use to effectively diagnose pathology in the anterior-inferior capsule following dislocation and lead to improved outcomes.

The Impact of Glenoid Labrum Thickness and Modulus on Labrum and Glenohumeral Capsule Function

The glenoid labrum is an integral component of the glenohumeral capsule’s insertion into the glenoid, and changes in labrum geometry and mechanical properties may lead to the development of glenohumeral joint pathology. The objective of this research was to determine the effect that changes in labrum thickness and modulus have on strains in the labrum and glenohumeral capsule during a simulated physical examination for anterior instability. A labrum was incorporated into a validated, subject-specific finite element model of the glenohumeral joint, and experimental kinematics were applied simulating application of an anterior load at 0 deg, 30 deg, and 60 deg of external rotation and 60 deg of glenohumeral abduction. The radial thickness of the labrum was varied to simulate thinning tissue, and the tensile modulus of the labrum was varied to simulate degenerating tissue. At 60 deg of external rotation, a thinning labrum increased the average and peak strains in the labrum, particularly in the labrum regions of the axillary pouch (increased 10.5% average strain) and anterior band (increased 7.5% average strain). These results suggest a cause-and-effect relationship between age-related decreases in labrum thickness and increases in labrum pathology. A degenerating labrum also increased the average and peak strains in the labrum, particularly in the labrum regions of the axillary pouch (increased 15.5% strain) and anterior band (increased 10.4% strain). This supports the concept that age-related labrum pathology may result from tissue degeneration. This work suggests that a shift in capsule reparative techniques may be needed in order to include the labrum, especially as activity levels in the aging population continue to increase. In the future validated, finite element models of the glenohumeral joint can be used to explore the efficacy of new repair techniques for glenoid labrum pathology.

Effects of region and sex on the mechanical properties of the glenohumeral capsule during uniaxial extension

Surgical repair of the glenohumeral capsule after dislocation ignores regional boundaries of the capsule and is not sex specific. However, each region of the capsule functions to stabilize the joint in different positions, and differences in joint laxity between men and women have been found. The objectives of this research were to determine the effects of region (axillary pouch and posterior capsule) and sex on the material properties of the glenohumeral capsule. Boundary conditions derived from experiments were used to create finite-element models that applied tensile deformations to tissue samples from the capsule. The material coefficients of a hyperelastic constitutive model were determined via inverse finite-element optimization, which minimized the difference between the experimental and finite-element model-predicted load-elongation curve. These coefficients were then used to create stress-stretch curves representing the material properties of the capsule regions for each sex in response to uniaxial extension. For the axillary pouch, the C1 (men: 0.28+/-0.39 MPa and women: 0.23+/-0.12 MPa) and C2 (men: 8.2+/-4.1 and women: 7.7+/-3.0) material coefficients differed between men and women by only 0.05 MPa and 0.5, respectively. Similarly, the posterior capsule coefficients differed by 0.15 MPa (male: 0.49+/-0.26 MPa and female: 0.34+/-0.20 MPa) and 0.6 (male: 7.8+/-2.9 and female: 7.2+/-3.0), respectively. No differences could be detected in the material coefficients between regions or sexes. As a result, surgeons may not need to consider region- and sex-specific surgical repair techniques. Furthermore, finite-element models of the glenohumeral joint may not need region- or sex-specific material coefficients when using this constitutive model.

In vivo three-dimensional evaluation of the functional length of glenohumeral ligaments

BACKGROUND

Glenohumeral ligaments play an important role in stabilizing the shoulder. However, it is impossible to know how they function in vivo during shoulder motion. To help elucidate this stabilizing role, we studied the in vivo three-dimensional kinematics of the normal shoulder joint using a markerless bone-registration technique.

METHODS

Our technique utilized image registration to determine corresponding relations between several image volumes represented at different coordinates. Magnetic resonance images of 14 shoulder joints of seven healthy volunteers were acquired for seven isometric abduction orientations between 0 degrees and 180 degrees . We then calculated three-dimensional shortest path between the origin and insertion of each ligament based on anatomical study in each abduction orientation.

FINDINGS

At 0 degrees of abduction, the posterior band of the coracohumeral ligament displayed the maximum length. At 30 degrees of abduction, the superior glenohumeral ligament displayed the maximum length. At 60 degrees of abduction, the anterior band of the coracohumeral ligament and the middle glenohumeral ligament displayed the maximum length. At 120 degrees of abduction, the anterior band of the inferior glenohumeral ligament displayed the maximum length.

INTERPRETATION

Based on progressive abduction of the arm, each ligament had different pattern in change of length. At different arm orientation of abduction, each ligament displayed the maximum length. We think that each ligament might play an important role in stabilizing the shoulder at different arm orientation.

The glenohumeral capsule should be evaluated as a sheet of fibrous tissue: a validated finite element model

The function of the glenohumeral capsule has typically been evaluated by isolating several discrete, ligamentous regions during experimental and computational investigations. However, recent data suggests that the regions of the glenohumeral capsule have significant interactions and function multiaxially. Therefore, examining the function of the inferior glenohumeral ligament as a discrete structure may not be appropriate. The objective of this work was to validate the predicted strain distribution and deformed shape of the inferior glenohumeral ligament using experimental data for two subject-specific finite element models: (1) a continuous model including all capsular regions, and (2) a discrete model including only the inferior glenohumeral ligament. The distribution of maximum principal strain and deformed shape of the glenohumeral capsule was determined for a cadaveric shoulder in a joint position frequently associated with dislocation (60 degrees of glenohumeral abduction, 52 degrees of external rotation, and a 25 N anterior load applied to the humerus). The experimental kinematics were then applied to the two finite element models constructed from the geometry and material properties from the same cadaveric shoulder and the predicted strain distributions and deformed shapes were determined. For the continuous model, the average difference between predicted strains and experimental strains was less than 5%. The predicted deformed shape was also similar to experimental data, with the anterior band of the inferior glenohumeral ligament clearly wrapped around the humeral head. In contrast, large differences existed between the strains predicted by the discrete model when compared to the experimental strains for this joint position (average difference from experimental data was 20%). In addition, the predicted deformed shape of the inferior glenohumeral ligament did not wrap around the humeral head. These differences may be attributed to neglecting the complex interactions between the anterior band of the inferior glenohumeral ligament with the neighboring capsular regions. Thus, the glenohumeral capsule should not be evaluated as several discrete structures. Rather, it should be evaluated as a single sheet of fibrous tissue.

Relationships between total and non-recoverable strain fields in glenohumeral capsule during shoulder subluxation

Non-recoverable strain in the glenohumeral capsule is of prime clinical significance, but the factors that contribute to non-recoverable strain are largely unknown. This study examined the relationship between total and non-recoverable strain in the antero-inferior glenohumeral capsule using an experimental model. Maximum principal total strain alone explained up to 35% of the variance in non-recoverable strain. A multiple regression model, including variables for lateral position and specimen, explained 50% of the variance in non-recoverable strain. Both linear and quadratic terms for maximum principal total strain were significant predictors of non-recoverable strain. The correlation of total and non-recoverable strain directions exhibited a slope of nearly 1:1. The regression model showed that non-recoverable strain is likely to be low for small levels of total strain, and increase non-linearly with total strain. Non-recoverable strain tended to be higher closer to the glenoid, even when controlling for total strain. Minimum principal total strain was not a significant predictor of non-recoverable strain for the cases examined, indicating that the glenohumeral capsule may demonstrate uniaxial failure behavior even when loaded biaxially. These results are important toward prediction of non-recoverable strain in computational models of glenohumeral subluxation, as well as for theoretical models of ligament failure.

Twenty-five years of tendon and ligament research

Twenty-five years ago, the Journal of Orthopaedic Research published its first volume, which included five articles covering topics in tendon and ligament research. Since then, the body of tendon and ligament research has continued to increase exponentially. This review summarizes major advancements in tendon and ligament research since the initial publication of this journal. The purpose of this article is not to provide an in-depth review of all of tendon and ligament research, but instead to provide a concise literature review of some of the major and recurring areas of research. The general topics covered over the last 25 years include tissue properties, tendinopathy, healing, and engineered scaffolds.

A subject-specific finite element model of the anterior cruciate ligament

The anterior cruciate ligament (ACL) is commonly injured. The stress distribution in the ACL is the key for understanding its function and injury mechanism, as well as for developing optimal surgical reconstruction protocols. In this study, a three-dimensional subject-specific finite element model of human ACL was developed. Bony geometries were reconstructed from CT scan images, while the geometry of the ACL and the orientation of its fiber bundles were measured via a mechanical digitizer. A transversely isotropic, hyperelastic, and nearly incompressible constitutive model was implemented to describe the mechanical properties of the ACL. A 134N anterior tibial load were applied to a cadaveric knee specimen at full extension, 30 degrees , and 60 degrees of flexion by a 6-DOF Robotic/Universal Force-moment Sensor (UFS) system, which was also used to measure the ACL resultant force. Knee kinematics was collected by digitizing two registration blocks attached to the femur and the tibia, respectively, and was input into the FE model as boundary conditions. The resultant force of the ACL calculated by the FE model was comparable to the experimental data, with the error within 10%, thus validated the model. The FE results showed that the average stress in the ACL was between the range 4.7-5.0MPa, with a peak stress between the range 9.8-10.9MPa, which shifted from the posterior lateral (PL) bundle to the anterior medial (AM) bundle as the knee flexed.

Effects of rotator cuff tears on muscle moment arms: a computational study

Rotator cuff tears cause morphologic changes to cuff tendons and muscles, which can alter muscle architecture and moment arm. The effects of these alterations on shoulder mechanical performance in terms of muscle force and joint strength are not well understood. The purpose of this study was to develop a three-dimensional explicit finite element model for investigating morphological changes to rotator cuff tendons following cuff tear. The subsequent objectives were to validate the model by comparing model-predicted moment arms to empirical data, and to use the model to investigate the hypothesis that rotator cuff muscle moment arms are reduced when tendons are divided along the force-bearing direction of the tendon. The model was constructed by extracting tendon, cartilage, and bone geometry from the male Visible Human data set. Infraspinatus and teres minor muscle and tendon paths were identified relative to the humerus and scapula. Kinetic and kinematic boundary conditions in the model replicated experimental protocols, which rotated the humerus from 45 degrees internal to 45 degrees external rotation with constant loads on the tendons. External rotation moment arms were calculated for two conditions of the cuff tendons: intact normal and divided tendon. Predicted moment arms were within the 1-99% confidence intervals of experimental data for nearly all joint angles and tendon sub-regions. In agreement with the experimental findings, when compared to the intact condition, predicted moment arms were reduced for the divided tendon condition. The results of this study provide evidence that one potential mechanism for the reduction in strength observed with cuff tear is reduction of muscle moment arms. The model provides a platform for future studies addressing mechanisms responsible for reduced muscle force and joint strength including changes to muscle length-tension operating range due to altered muscle and tendon excursions, and the effects of cuff tear size and location on moment arms and muscle forces.

Methodology and sensitivity studies for finite element modeling of the inferior glenohumeral ligament complex

The objectives of this research were to develop a methodology for three-dimensional finite element (FE) modeling of the inferior glenohumeral ligament complex (IGHL complex) as a continuous structure, to determine optimal mesh density for FE simulations, to examine strains and forces in the IGHL complex in clinically relevant joint positions, and to perform sensitivity studies to assess the effects of assumed material properties. A simple translation test in the anterior direction was performed on a cadaveric shoulder, with the humerus oriented at 60 degrees of glenohumeral abduction and 0 degrees of flexion/extension, at 0 degrees , 30 degrees and 60 degrees of humeral external rotation. The geometries of the relevant structures were extracted from volumetric CT data to create a FE model. Experimentally measured kinematics were applied to the FE model to simulate the simple translation test. First principal strains, insertion site forces and contact forces were analyzed. At maximum anterior humeral translation, strains in the IGHL complex were highly inhomogeneous for all external rotation angles. The motion of the humerus with respect to the glenoid during the simple translation test produced a tangential load at the proximal and distal edges of the IGHL complex. This loading was primarily in the plane of the inferior glenohumeral ligament complex, producing an in-plane shear-loading pattern. There was a significant increase in strain with increasing angle of external rotation. The largest insertion site forces occurred at the axillary pouch insertion to the humerus (36.7N at 60 degrees of external rotation) and the highest contact forces were between the anterior band of the IGHL complex and the humeral cartilage (7.3N at 60 degrees of external rotation). Strain predictions were highly sensitive to changes in the ratio of bulk to shear modulus of the IGHL complex, while predictions were moderately sensitive to changes in elastic modulus of the IGHL complex. Changes to the material properties of the humeral cartilage had little effect on predicted strains. The methodologies developed in this research and the results of the mesh convergence and sensitivity studies provide a basis for the subject-specific modeling of the mechanics of the IGHL complex.

Three-dimensional finite element modeling of ligaments: technical aspects

The objective of this paper is to describe strategies for addressing technical aspects of the computational modeling of ligaments with the finite element (FE) method. Strategies for FE modeling of ligament mechanics are described, differentiating between whole-joint models and models of individual ligaments. Common approaches to obtain three-dimensional ligament geometry are reviewed, with an emphasis on techniques that rely on volumetric medical image data. Considerations for the three-dimensional constitutive modeling of ligaments are reviewed in the context of ligament composition and structure. A novel approach to apply in situ strain to FE models of ligaments is described, and test problems are presented that demonstrate the efficacy of the approach. Approaches for the verification and validation of ligament FE models are outlined. The paper concludes with a discussion of future research directions.