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

Specimen-specific finite element representations of implanted hip capsules

The hip capsule is a ligamentous structure that contributes to hip stability. This article developed specimen-specific finite element models that replicated internal-external (I-E) laxity for ten implanted hip capsules. Capsule properties were calibrated to minimize root mean square error (RMSE) between model and experimental torques. RMSE across specimens was 1.02 ± 0.21 Nm for I-E laxity and 0.78 ± 0.33 Nm and 1.10 ± 0.48 Nm during anterior and posterior dislocation, respectively. RMSE for the same models with average capsule properties was 2.39 ± 0.68 Nm. Specimen-specific models demonstrated the importance of capsule tensioning in hip stability and have relevance for surgical planning and evaluation of implant designs.

Image-Based Numerical Analysis for Isolated Type II SLAP Lesions in Shoulder Abduction and External Rotation

The glenohumeral joint (GHJ) is one of the most critical structures in the shoulder complex. Lesions of the superior labral anterior to posterior (SLAP) cause instability at the joint. Isolated Type II of this lesion is the most common, and its treatment is still under debate. Therefore, this study aimed to determine the biomechanical behavior of soft tissues on the anterior bands of the glenohumeral joint with an Isolated Type II SLAP lesion. Segmentation tools were used to build a 3D model of the shoulder joint from CT-scan and MRI images. The healthy model was studied using finite element analysis. Validation was conducted with a numerical model using ANOVA, and no significant differences were shown (p = 0.47). Then, an Isolated Type II SLAP lesion was produced in the model, and the joint was subjected to 30 degrees of external rotation. A comparison was made for maximum principal strains in the healthy and the injured models. Results revealed that the strain distribution of the anterior bands of the synovial capsule is similar between a healthy and an injured shoulder (p = 0.17). These results demonstrated that GHJ does not significantly deform for an Isolated Type II SLAP lesion subjected to 30-degree external rotation in abduction.

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.

Biomechanical effects of laminectomies in the human lumbar spine: a finite element study

BACKGROUND CONTEXT

Previous studies have analyzed the effect of laminectomy on intervertebral disc (IVD), facet-joint-forces (FJF), and range of motion (ROM), while only two have specifically reported stresses at the pars interarticularis (PI) with posterior element resection. These studies have been performed utilizing a single subject, questioning their applications to a broader population.

PURPOSE

We investigate the effect of graded PI resection in a three-dimensional manner on PI stress to provide surgical guidelines for avoidance of iatrogenic instability following lumbar laminectomy. Additionally, quantified FJF and IVD stresses can provide further insight into the development of adjacent segment disease.

STUDY DESIGN

Biomechanical finite element (FE) method investigation of the lumbar spine.

METHODS

FE models of the lumbar spine of three subjects were created using the open-source finite element software, FEBio. Single-level laminectomy, two-level laminectomy, and ventral-to-dorsal PI resection simulations were performed with varying degrees of PI resection from 0% to 75% of the native PI. These models were taken through cardinal ROM under standard loading conditions and PI stresses, FJF, IVD stresses, and ROM were analyzed.

RESULTS

The three types of laminectomy simulated in this study showed a nonlinear increase in PI stress with increased bone resection. Axial rotation generated the most stress at the PI followed by flexion, extension and lateral bending. At 75% bone resection all three types of laminectomy produced PI stresses that were near the ultimate strength of human cortical bone during axial rotation. FJF decreased with increased bone resection for the three laminectomies simulated. This was most notable in axial rotation followed by extension and lateral bending. IVD stresses varied greatly between the nonsurgical models and likewise the effect of laminectomy on IVD stresses varied between subjects. ROM was mostly unaffected by the laminectomies performed in this study.

CONCLUSIONS

Regarding the risk of iatrogenic spondylolisthesis, the combined results are sufficient evidence to suggest surgeons should be particularly cautious when PI resection exceeds 50% bone resection for all laminectomies included in this study. Lastly, the effects seen in FJF and IVD stresses indicate the degree to which the remainder of the spine must experience compensatory biomechanical changes as a result of the surgical intervention.

[Prognostic evaluation of hip joint function following capsule repair based on a threedimensional finite element analysis model]

OBJECTIVE

To construct a three-dimensional (3D) finite element mechanical model of total hip arthroplasty for comparison of biomechanical differences of the hip joint following capsule repair and postoperative rehabilitation.

METHODS

Six frozen specimens of hip joint posterior capsule ligament complex were collected in a bone-capsule-bone manner, and the load-strain curve and other mechanical properties of the specimens were tested using a universal material testing machine. Thin-section CT data of the pelvis and lower limbs obtained from a volunteer were imported into Mimics software to construct a 3D model of the hip joint. Digital models of the cup, femoral prosthesis and joint capsule were created in CATIA software and imported into Mimics to simulate total hip arthroplasty; the assembled data were imported into ABAQUS software. The properties of the capsule were set according to results of the mechanical test, anatomical studies, and constitutive equations, and the biomechanics of the anatomically repaired and conventionally repaired capsules were compared during hip flexion.

RESULTS

The results of testing on the 6 capsule specimens showed a mean ultimate tensile strain of (39.21±5.23)% and a mean of ultimate tensile strength of 1.65±0.38 MPa. The stress-strain curve of the finite element model was consistent with the results of mechanical test on the specimens and the biochemical characteristics of the capsule. The stress was distributed evenly in the anatomically repaired capsule during hip flexion but not in the capsule repaired through the conventional approach; the tensile stress in the lower part of the conventionally repaired capsule reached the ultimate tensile stress measured on the capsule specimens at a 90° flexion.

CONCLUSIONS

The finite element model allows dynamic, quantitative and visual assessment of stress distribution in the hip joint capsule, and compared with the conventional approach, anatomical repair can achieve better biomechanical properties of the capsule.

Altered shoulder kinematics using a new model for multiple dislocations-induced Bankart lesions

BACKGROUND

Many active individuals undergo multiple dislocations during the course of a season before surgical treatment without considering the implications of each successive injury. Therefore, the purpose of this study was to develop a multiple dislocation model for the glenohumeral joint and evaluate the resulting changes in joint function.

METHODS

Eight cadaveric shoulders were evaluated using a robotic testing system. A simulated clinical exam was performed by applying a 50 N anterior load to the humerus at 60° of glenohumeral abduction and external rotation. Each joint was then dislocated. The same loads were applied again and the resulting kinematics were recorded following each of 10 dislocations. The force required to achieve dislocation was recorded and capsulolabral status was assessed.

FINDINGS

A reproducible Bankart lesion was repeatedly created following the dislocation protocol. The force required for all dislocations significantly decreased following the 1st dislocation. In addition, even lower forces were required to achieve the 5th and subsequent dislocations (p < 0.05). Anterior translation in response to an anterior load during the simulated clinical exam increased between the intact and injured joints (p < 0.05). However, anterior translation reached a plateau following the 3rd to 10th dislocations and was increased compared with the 1st dislocation (p < 0.05).

INTERPRETATION

A repeatable Bankart lesion was not surgically made, but created by our new dislocation model. Joint function appeared to reach a constant level after the 3rd to 5th dislocations. Thus, multiple dislocations result in a deleterious dose dependent effect suggesting additional damage is not sustained after the fifth dislocation.

LEVEL OF EVIDENCE

Controlled laboratory study.

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.

Computer simulation of lumbar flexion shows shear of the facet capsular ligament

BACKGROUND CONTEXT

The lumbar facet capsular ligament (FCL) is a posterior spinal ligament with a complex structure and kinematic profile. The FCL has a curved geometry, multiple attachment sites, and preferentially aligned collagen fiber bundles on the posterior surface that are innervated with mechanoreceptive nerve endings. Spinal flexion induces three-dimensional (3D) deformations, requiring the FCL to maintain significant tensile and shear loads. Previous works aimed to study 3D facet joint kinematics during flexion, but to our knowledge none have reported localized FCL surface deformations likely created by this complex structure.

PURPOSE

The purpose of this study was to elucidate local deformations of both the posterior and anterior surfaces of the lumbar FCL to understand the distribution and magnitude of in-plane and through-plane deformations, including the prevalence of shear.

STUDY DESIGN/SETTING

The FCL anterior and posterior surface deformations were quantified through creation of a finite element model simulating facet joint flexion using a realistic geometry, physiological kinematics, and fitted constitutive material.

METHODS

Geometry was obtained from the micro-CT data of a healthy L3-L4 facet joint capsule (n=1); kinematics were extracted from sagittal plane fluoroscopic data of healthy volunteers (n=10) performing flexion; and average material properties were determined from planar biaxial extension tests of L4-L5 FCLs (n=6). All analyses were performed with the non-linear finite element solver, FEBio. A grid of equally spaced 3×3 nodes on the posterior surface identified regional differences within the strain fields and was used to create comparisons against previously published experimental data. This study was funded by the National Institutes of Health and the authors have no disclosures.

RESULTS

Inhomogeneous in-plane and through-plane shear deformations were prominent through the middle body of the FCL on both surfaces. Anterior surface deformations were more pronounced because of the small width of the joint space, whereas posterior surface deformations were more diffuse because the larger area increased deformability. We speculate these areas of large deformation may provide this proprioceptive system with an excellent measure of spinal motion.

CONCLUSIONS

We found that in-plane and through-plane shear deformations are widely present in finite element simulations of a lumbar FCL during flexion. Importantly, we conclude that future studies of the FCL must consider the effects of both shear and tensile deformations.

Does Repair of a Hill-Sachs Defect Increase Stability at the Glenohumeral Joint?

Background:

The effect of osteoallograft repair of a Hill-Sachs lesion and the effect of allograft fit on glenohumeral translations in response to applied force are poorly understood.

Purpose:

To compare the impact of a 25% Hill-Sachs lesion, a perfect osteoallograft repair (PAR) of a 25% Hill-Sachs lesion, and an “imperfect” osteoallograft repair (IAR) of a 25% Hill-Sachs lesion on glenohumeral translations in response to a compressive load and either an anterior or posterior load in 3 clinically relevant arm positions.

Study Design:

Controlled laboratory study.

Methods:

A robotic/universal force-moment sensor testing system was used to apply joint compression (22 N) and an anterior or posterior load (44 N) to cadaveric shoulders (n = 9) with the skin and deltoid removed (intact) at 3 glenohumeral joint positions (abduction/external rotation): 0°/0°, 30°/30°, and 60°/60°. The 25% bony defect state, PAR state, and IAR state were created and the loading protocol was performed. Translational motion was measured in each position for each shoulder state. A nonparametric repeated-measures Friedman test with a Wilcoxon signed-rank post hoc test was performed to compare the biomechanical parameters (P < .05).

Results:

Compared with the defect shoulder, the PAR shoulder had significantly less anterior translation with an anterior load in the 0°/0° (15.3 ± 8.2 vs 16.6 ± 9.0 mm, P = .008) and 30°/30° (13.6 ± 7.1 vs 14.2 ± 7.0 mm, P = .021) positions. Compared with IAR, the PAR shoulder had significantly less anterior translation with an anterior load in the 0°/0° (15.3 ± 8.2 vs 16.6 ± 9.0 mm, P = .008) and 30°/30° (13.6 ± 7.1 vs 14.4 ± 7.1 mm, P = .011) positions, and the defect shoulder had significantly less anterior translation with an anterior load in the 30°/30° (14.2 ± 7.0 vs 14.4 ± 7.0 mm, P = .038) position.

Conclusion:

PAR resulted in the least translational motion at the glenohumeral joint. The defect shoulder had significantly less translational motion at the joint compared with the IAR. An IAR resulted in the most translational motion at the glenohumeral joint. This demonstrates the biomechanical importance of performing an osteoallograft repair in which the allograft closely matches the Hill-Sachs defect and fully restores the preinjury state of the humeral head.

Clinical Relevance:

This study demonstrates the importance of performing an osteoallograft repair of a Hill-Sachs defect that closely matches the preinjury state and restores normal humeral head anatomy.

Hyper extension-internal rotation (HERI): A new test for anterior gleno-humeral instability

BACKGROUND

Anterior shoulder dislocation causes injury to the inferior gleno-humeral ligament (IGHL) and capsule. Clinical manoeuvres currently used to evaluate the IGHL test for, and may induce, apprehension. We developed the hyper extension-internal rotation (HERI) test to assess the IGHL and inferior capsule without causing apprehension or inducing a risk of gleno-humeral dislocation.

HYPOTHESIS

The HERI test is easy to perform and reproducible, induces no risk of gleno-humeral dislocation during the manoeuvre, and causes no apprehension in the patients.

MATERIAL AND METHODS

We studied 14 fresh cadaver shoulders. Each specimen was positioned supine with the lateral edge of the scapula on the table and the upper limb hanging down beside the table under the effect of gravity. This position produced hyperextension and internal rotation of the gleno-humeral joint. For each shoulder, the range of extension (°) was measured before and after isolated IGHL section. Then, we performed the HEIR test in 50 patients with chronic unilateral anterior gleno-humeral instability and we compared the range of extension between the normal and abnormal sides.

RESULTS

In the cadaver study, isolated IGHL section increased the angle of extension by a mean of 14.5° (11°-18°) compared to the pre-injury values. In the clinical study, the mean difference in extension angles between the normal and abnormal sides was 14.5°. The patients reported no apprehension during the HERI test.

CONCLUSION

The angle of extension increases after section or injury of the IGHL in cadaver specimens and patients, respectively. When the inferior capsule and IGHL are damaged, the angle of extension increases compared to the normal side. Lesions to these structures can be evaluated clinically by performing the HERI test.

LEVEL OF EVIDENCE

III.

A method for predicting collagen fiber realignment in non-planar tissue surfaces as applied to glenohumeral capsule during clinically relevant deformation

Previously developed experimental methods to characterize micro-structural tissue changes under planar mechanical loading may not be applicable for clinically relevant cases. Such limitation stems from the fact that soft tissues, represented by two-dimensional surfaces, generally do not undergo planar deformations in vivo. To address the problem, a method was developed to directly predict changes in the collagen fiber distribution of nonplanar tissue surfaces following 3D deformation. Assuming that the collagen fiber distribution was known in the un-deformed configuration via experimental methods, changes in the fiber distribution were predicted using 3D deformation. As this method was solely based on kinematics and did not require solving the stress balance equations, the computational efforts were much reduced. In other words, with the assumption of affine deformation, the deformed collagen fiber distribution was calculated using only the deformation gradient tensor (obtained via an in-plane convective curvilinear coordinate system) and the associated un-deformed collagen fiber distribution. The new method was then applied to the glenohumeral capsule during simulated clinical exams. To quantify deformation, positional markers were attached to the capsule and their 3D coordinates were recorded in the reference position and three clinically relevant joint positions. Our results showed that at 60deg of external rotation, the glenoid side of the posterior axillary pouch had significant changes in fiber distribution in comparison to the other sub-regions. The larger degree of collagen fiber alignment on the glenoid side suggests that this region is more prone to injury. It also compares well with previous experimental and clinical studies indicating maximum principle strains to be greater on the glenoid compared to the humeral side. An advantage of the new method is that it can also be easily applied to map experimentally measured collagen fiber distribution (obtained via methods that require flattening of tissue) to their in vivo nonplanar configuration. Thus, the new method could be applied to many other nonplanar fibrous tissues such as the ocular shell, heart valves, and blood vessels.

Injury to the anteroinferior glenohumeral capsule during anterior dislocation

BACKGROUND

Glenohumeral dislocation commonly results in permanent deformation of the glenohumeral capsule. Knowing the location and extent of tissue damage may aid in improving diagnostic and repair procedures for shoulder dislocations. Therefore, the objectives of this study were to determine: (1) the strain in the anteroinferior capsule at dislocation and (2) the location and extent of injury to the anteroinferior capsule due to dislocation by quantifying the resulting non-recoverable strain.

METHODS

A robotic/universal force-moment sensor testing system was used to anteriorly dislocate six cadaveric shoulders. The magnitude of the maximum principle strain at dislocation and the resulting non-recoverable strain due to dislocation in the anteroinferior capsule were measured by tracking the change in the location of a grid of strain markers from a reference position.

FINDINGS

The glenoid side of the capsule experienced higher strains at dislocation than the humeral side. The greatest strains at dislocation were found on the glenoid side of the anterior band (strain ratio of 0.60), but the greatest non-recoverable strains were found in the posterior axillary pouch (strain ratio of 0.34 on the glenoid side and 0.31 on the humeral side).

INTERPRETATION

These findings suggest that even though the glenoid side of the anterior band undergoes more deformation during anterior dislocation, the most permanent deformation occurs in the posterior axillary pouch, and surgeons should consider also plicating the posterior axillary pouch when performing repair procedures following anterior dislocation. In the future, the mechanical properties of the normal and injured glenohumeral capsules will be compared.

FEBio: finite elements for biomechanics

In the field of computational biomechanics, investigators have primarily used commercial software that is neither geared toward biological applications nor sufficiently flexible to follow the latest developments in the field. This lack of a tailored software environment has hampered research progress, as well as dissemination of models and results. To address these issues, we developed the FEBio software suite (http://mrl.sci.utah.edu/software/febio), a nonlinear implicit finite element (FE) framework, designed specifically for analysis in computational solid biomechanics. This paper provides an overview of the theoretical basis of FEBio and its main features. FEBio offers modeling scenarios, constitutive models, and boundary conditions, which are relevant to numerous applications in biomechanics. The open-source FEBio software is written in C++, with particular attention to scalar and parallel performance on modern computer architectures. Software verification is a large part of the development and maintenance of FEBio, and to demonstrate the general approach, the description and results of several problems from the FEBio Verification Suite are presented and compared to analytical solutions or results from other established and verified FE codes. An additional simulation is described that illustrates the application of FEBio to a research problem in biomechanics. Together with the pre- and postprocessing software PREVIEW and POSTVIEW, FEBio provides a tailored solution for research and development in computational biomechanics.

The effect of a Hill-Sachs defect on glenohumeral translations, in situ capsular forces, and bony contact forces

BACKGROUND

Hill-Sachs defects have been associated with failed repairs for anterior shoulder instability. However, the biomechanical consequences of these defects are not well understood because of the complicated interaction between the passive soft tissue and bony stabilizers.

HYPOTHESIS

The creation of a 25% Hill-Sachs defect would not significantly alter the glenohumeral translations but would increase the in situ forces in the glenohumeral capsule as well as the glenohumeral bony contact forces.

STUDY DESIGN

Controlled laboratory study.

METHODS

A robotic/universal force-moment sensor (UFS) testing system was used to apply joint compression (22 N) and an anterior or posterior load (44 N) to cadaveric shoulders (n = 9) with the skin and deltoid removed (intact) at 3 glenohumeral joint positions (abduction/external rotation): 0°/0°, 30°/30°, and 60°/60° (corresponds to 90°/90° of shoulder abduction/external rotation). A 25% bony defect on the posterolateral humeral head (defect) was then created in the most common position of anterior shoulder dislocation (90°/90°), and the loading protocol was repeated. A nonparametric repeated-measures Friedman test with a Wilcoxon signed-rank post hoc test was performed to compare translations, in situ forces in the capsule, and bony contact forces between each state (P < .05).

RESULTS

At 0°/0°, anterior translation significantly increased from 15.3 ± 8.2 mm to 16.6 ± 9.0 mm (P < .05) in response to an anterior load. At 30°/30°, anterior and posterior translations, respectively, significantly increased in response to both anterior (intact: 13.6 ± 7.1 mm vs defect: 14.2 ± 7 mm; P < .05) and posterior loads (intact: 15.7 ± 5.8 mm vs defect: 17.7 ± 5.1 mm; P < .05). In situ force in the capsule during anterior loading was increased in the defect state at both 60°/60° (intact: 38.9 ± 14.4 N vs defect: 43.2 ± 15.9 N; P < .05) and 30°/30° (intact: 39.6 ± 13.8 N vs defect: 45.6 ± 9.3 N; P < .05). The medial bony contact forces were also increased in the defect state at 30°/30° (intact: 25.0 ± 13.8 N vs defect: 28.9 ± 13.2 N; P < .05) during anterior loading.

CONCLUSION

We believe that the stabilizing function of the intact capsule was the primary contributor to the finding of only small increases of anterior translation, capsule forces, and bony contact forces observed with a 25% Hill-Sachs defect in response to an anterior load.

CLINICAL RELEVANCE

These findings imply that a 25% Hill-Sachs defect in isolation may not be responsible for recurrent instability if the function of the capsule is restored to the intact state and that the presence of the Hill-Sachs defect may be a marker for significant concomitant injury to the anterior glenoid rim. However, the small changes in these parameters may have long-term implications for the development of osteoarthritis.