Numerical modelling of the shoulder for clinical applications

How does computed tomography inform our understanding of shoulder kinematics? A structured review

The objective of this structured review was to review how computed tomography (CT) scanning has been used to measure the kinematics of the shoulder. A literature search was conducted using Evidence-based Medicine Reviews (Embase) and PubMed. In total, 29 articles were included in the data extraction process. Forty percent of the studies evaluated healthy participants' shoulder kinematics. The glenohumeral joint was the most studied, followed by the scapulothoracic, acromioclavicular, and sternoclavicular joints. Three-dimensional computed tomography (3DCT) and 3DCT with biplane fluoroscopy are the two primary imaging techniques that have been used to measure shoulder joints' motion under different conditions. Finally, many discrepancies in the reporting of the examined motions were found. Different authors used different perspectives and planes to report similar motions, which results in confusion and misunderstanding of the actual examined motion. The use of 3DCT has been widely used in the examination of shoulder kinematics in a variety of populations with varying methods employed. Future work is needed to extend these methodologies to include more diverse populations, to examine the shoulder complex as a whole, and to standardize their reporting of motion examined to make study to study comparisons possible.

Joint-line medialization after anatomical total shoulder replacement requires more rotator cuff activity to preserve joint stability

BACKGROUND

The biomechanical effects of joint-line medialization during shoulder surgery are poorly understood. It was therefore the purpose of this study to investigate whether medialization of the joint line especially associated with total shoulder arthroplasty leads to changes in the rotator cuff muscle forces required to stabilize the arm in space.

METHODS

A validated computational 3-D rigid body simulation model was used to calculate generated muscle forces, instability ratios, muscle-tendon lengths and moment arms during scapular plane elevation. Measurements took place with the anatomical and a 2 mm and 6 mm lateralized or medialized joint line.

RESULTS

When the joint line was medialized, increased deltoid muscle activity was recorded throughout glenohumeral joint elevation. The rotator cuff muscle forces increased with medialization of the joint line in the early phases of elevation. Lateralization of the joint line led to higher rotator cuff muscle forces after 52° of glenohumeral elevation and to higher absolute values in muscle activity. A maximum instability ratio of >0.6 was recorded with 6 mm of joint-line medialization.

CONCLUSION

In this biomechanical study, medialization and lateralization of the normal joint line during total shoulder arthroplasty led to substantial load changes on the shoulder muscles used for stabilizing the arm in space. Specifically, medialization does not only lead to muscular shortening but also to increased load on the supraspinatus tendon during early arm elevation, the position which is already most loaded in the native joint.

Chronic Pseudoparalysis Needs to Be Distinguished From Pseudoparesis: A Structural and Biomechanical Analysis

BACKGROUND

Chronic pseudoparalysis is generally defined as the inability to actively elevate the arm above 90° with free passive range of motion and no neurological deficits. It has been suggested that this arbitrary cutoff needs to be refined.

PURPOSE

To analyze whether there are structural and biomechanical differences in patients with chronic pseudoparalysis and those with chronic pseudoparesis.

STUDY DESIGN

Case-control study; Level of evidence, 3.

METHODS

In this retrospective study, 50 patients with chronic massive rotator cuff tears (mRCTs; ≥2 tendons) and free passive and active scapular plane abduction <90° were divided into 2 groups: pseudoparalysis group (n = 24; active scapular plane abduction, <45°) and pseudoparesis group (n = 26; active scapular plane abduction, >45° and <90°). Radiographic measurements included the critical shoulder angle, acromiohumeral distance, posterior acromial tilt, anterior and posterior acromial coverages, and posterior acromial height on outlet views. Measurements on magnetic resonance imaging (MRI) included fatty infiltration of the rotator cuff muscles, anterior (subscapularis) and posterior (infraspinatus/teres minor) tear extensions, and global (anterior + posterior) tear extension in the parasagittal plane. A published musculoskeletal model was used to simulate the effect of different mRCTs on the muscle force required for scapular plane abduction.

RESULTS

Plain radiographs revealed no differences between patients with chronic pseudoparalysis and those with pseudoparesis. MRI assessment showed significant differences between patients with chronic pseudoparalysis and those with pseudoparesis with respect to fatty infiltration of the subscapularis (2.9 vs 1.6; P < .001) and infraspinatus (3.6 vs 3.0; P < .001) muscles, and anterior (-23° vs 4°; P < .001), posterior (-23° vs -14°; P = .034), and global rotator cuff (225° vs 190°; P < .001) tear extensions. The anterior tear extension in patients with chronic pseudoparalysis always involved more than 50% of the subscapularis, which was associated with an odds ratio of 5 for inability to actively abduct more than 45°. The biomechanical model was unable to find a combination of muscles that could balance the arm in space when the tear extended beyond the supraspinatus and the cranial subscapularis.

CONCLUSION

This study confirms that chronic pseudoparalysis and pseudoparesis are associated with different structural lesions. In the setting of a chronic mRCT, involvement of more than 50% of the subscapularis tendon with fatty infiltration of stage 3 is associated with pseudoparalysis of active scapular plane abduction <45°. The key function of the subscapularis was confirmed in the biomechanical model.

Evaluation of musculoskeletal modelling parameters of the shoulder complex during humeral abduction above 90°

Based on electromyographic data and force measurements within the shoulder joint, there is an indication that muscle and resulting joint reaction forces keep increasing over an abduction angle of 90°. In inverse dynamics models, no single parameter could be attributed to simulate this force behaviour accordingly. The aim of this work is to implement kinematic, kinetic and muscle model modifications to an existing model of the shoulder (AnyBody™) and assess their single and combined effects during abduction up to 140° humeral elevation. The kinematics and the EMG activity of 10 test subjects were measured during humeral abduction. Six modifications were implemented in the model: alternative wrapping of the virtual deltoid muscle elements, utilization of a three element Hill model, strength scaling, motion capture driven clavicle elevation/protraction, translation of the GH joint in dependency of the acting forces and an alteration of the scapula/clavicle rhythm. From the six modifications, 16 different combinations were considered. Parameter combinations with the Hill model changed the resultant GH joint reaction force and led to an increase in force during abduction of the humerus above 90°. Under the premise of muscle activities and forces within the GH joint rising after 90° of humeral abduction, we propose that the Hill type muscle model is a crucial parameter for accurately modelling the shoulder. Furthermore, the outcome of this study indicates that the Hill model induces the co-contraction of the muscles of the shoulder without the need of an additional stability criterion for an inverse dynamics approach.

Double-Blinded Randomized Study of the Correlation between Simple Radiography and Magnetic Resonance Imaging in the Evaluation of the Critical Shoulder Angle: Reproducibility and Learning Curve

Objective  To evaluate the feasibility of magnetic resonance imaging (MRI) to obtain the critical shoulder angle (CSA) comparing the results obtained through radiography and MRI, and assess the learning curves. Methods  In total, 15 patients were evaluated in a blinded and randomized way. The CSA was measured and compared among groups and subgroups. Results  The mean angles measured through the radiographic images were of 34.61 ± 0.67 and the mean angles obtained through the MRI scans were of 33.85 ± 0.53 ( p  = 0.29). No significant differences have been found among the groups. The linear regression presented a progressive learning curve among the subgroups, from fellow in shoulder surgery to shoulder specialist and radiologist. Conclusion  There was no statistically significant difference in the X-rays and MRI assessments. The MRI seems to have its efficacy associated with more experienced evaluators. Data dispersion was smaller for the MRI data regardless of the experience of the evaluator.

The lever arm ratio of the rotator cuff to deltoid muscle explains and predicts pseudoparalysis of the shoulder

Aims

In patients with a rotator cuff tear, tear pattern and tendon involvement are known risk factors for the development of pseudoparalysis of the shoulder. It remains unclear, however, why similar tears often have very different functional consequences. The present study hypothesizes that individual shoulder anatomy, specifically the moment arms (MAs) of the rotator cuff (RC) and the deltoid muscle, as well as their relative recruitment during shoulder abduction, plays a central role in pseudoparalysis.

Materials and Methods

Biomechanical and clinical analyses of the pseudoparalytic shoulder were conducted based on the ratio of the RC/deltoid MAs, which were used to define a novel anatomical descriptor called the Shoulder Abduction Moment (SAM) index. The SAM index is the ratio of the radii of two concentric spheres based on the centre of rotation of the joint. One sphere captures the humeral head (numerator) and the other the deltoid origin of the acromion (denominator). A computational rigid body simulation was used to establish the functional link between the SAM index and a potential predisposition for pseudoparalysis. A retrospective radiological validation study based on these measures was also undertaken using two cohorts with and without pseudoparalysis and massive RC tears.

Results

Decreased RC activity and improved glenohumeral stability was predicted by simulations of SAM indices with larger diameters of the humeral head, being consequently beneficial for joint stability. Clinical investigation of the SAM index showed significant risk of pseudoparalysis in patients with massive tears and a SAM < 0.77 (odds ratio (OR) 11).

Conclusion

The SAM index, which represents individual biomechanical characteristics of shoulder morphology, plays a determinant role in the presence or absence of pseudoparalysis in shoulders with massive RC tears.

A survey of human shoulder functional kinematic representations

In this survey, we review the field of human shoulder functional kinematic representations. The central question of this review is to evaluate whether the current approaches in shoulder kinematics can meet the high-reliability computational challenge. This challenge is posed by applications such as robot-assisted rehabilitation. Currently, the role of kinematic representations in such applications has been mostly overlooked. Therefore, we have systematically searched and summarised the existing literature on shoulder kinematics. The shoulder is an important functional joint, and its large range of motion (ROM) poses several mathematical and practical challenges. Frequently, in kinematic analysis, the role of the shoulder articulation is approximated to a ball-and-socket joint. Following the high-reliability computational challenge, our review challenges this inappropriate use of reductionism. Therefore, we propose that this challenge could be met by kinematic representations, that are redundant, that use an active interpretation and that emphasise on functional understanding.

The Stabilizing Function of Superficial Shoulder Muscles Changes Between Single-Plane Elevation and Reaching Tasks

OBJECTIVE

The goal of this study was to determine whether and how much the stabilizing role of the shoulder muscles changes as a function of humeral elevation and the plane of elevation.

METHODS

A musculoskeletal model, comprising a personalized scapulohumeral rhythm, was used to calculate the ratio of shear over compressive force (stability ratio) of three rotator cuff muscles (supraspinatus, infraspinatus and subscapularis) and three superficial shoulder muscles (middle deltoid, clavicular part of pectoralis major and latissimus dorsi) during abduction, flexion and reaching movements in 10 healthy adults.

RESULTS

The range of the stability ratios was [Formula: see text] for the rotator cuff muscles compared to [Formula: see text] for the superficial shoulder muscles. In the superior-inferior direction, the stability ratios of all muscles changed with humeral elevation and for infraspinatus, subscapularis, latissimus dorsi and deltoid also with the plane of elevation. In the anterior-posterior direction, the stability ratios of all muscles changed with humeral elevation, except for the deltoid, and with the plane of elevation, except for the supraspinatus, with interaction effects in all muscles.

CONCLUSION

The rotator cuff muscles provide greater compression than shear forces during all tasks. The stabilizing function of the superficial shoulder muscles examined in this study varies during tasks.

SIGNIFICANCE

The findings can be used to predict in which movements the shoulder joint becomes more unstable and can be applied to understand how shear and compressive forces change in populations with abnormal shoulder motion.

Glenohumeral joint reaction forces increase with critical shoulder angles representative of osteoarthritis-A biomechanical analysis

Osteoarthritis (OA) of the glenohumeral joint constitutes the most frequent indication for nontraumatic shoulder joint replacement. Recently, a small critical shoulder angle (CSA) was found to be associated with a high prevalence of OA. This study aims to verify the hypothesis that a small CSA leads to higher glenohumeral joint reaction forces during activities of daily living than a normal CSA. A shoulder simulator with simulated deltoid (DLT), supraspinatus (SSP), infraspinatus/teres minor (ISP/TM), and subscapularis (SSC) musculotendinous units was constructed. The DLT wrapping on the humerus was simulated using a pulley that could be horizontally adjusted to simulate the 28° CSA found in OA or the 33° CSA found in disease-free shoulders. Over a range of motion between 6° and 82° of thoracohumeral abduction joint forces were measured using a six-axis load cell. An OA-associated CSA yielded higher net joint reaction forces than a normal CSA over the entire range of motion. The maximum difference of 26.4 N (8.5%) was found at 55° of thoracohumeral abduction. Our model thus suggests that a CSA typical for OA predisposes the glenohumeral joint to higher joint reaction forces and could plausibly play a role in joint overloading and development of OA. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1047-1052, 2016.

Upper limb strength estimation of physically impaired persons using a musculoskeletal model: A sensitivity analysis

Sensitivity of upper limb strength calculated from a musculoskeletal model was analyzed, with focus on how the sensitivity is affected when the model is adapted to represent a person with physical impairment. Sensitivity was calculated with respect to four muscle-tendon parameters: muscle peak isometric force, muscle optimal length, muscle pennation, and tendon slack length. Results obtained from a musculoskeletal model of average strength showed highest sensitivity to tendon slack length, followed by muscle optimal length and peak isometric force, which is consistent with existing studies. Muscle pennation angle was relatively insensitive. The analysis was repeated after adapting the musculoskeletal model to represent persons with varying severities of physical impairment. Results showed that utilizing the weakened model significantly increased the sensitivity of the calculated strength at the hand, with parameters previously insensitive becoming highly sensitive. This increased sensitivity presents a significant challenge in applications utilizing musculoskeletal models to represent impaired individuals.

Supraspinatus tendon load during abduction is dependent on the size of the critical shoulder angle: A biomechanical analysis

Shoulders with supraspinatus (SSP) tears are associated with significantly larger critical shoulder angles (CSA) compared to disease-free shoulders. We hypothesized that larger CSAs increase the ratio of joint shear to joint compression forces (defined as "instability ratio"), requiring substantially increased compensatory supraspinatus loads. A shoulder simulator with simulated deltoid, supraspinatus, infraspinatus/teres minor, and subscapularis musculotendinous units was constructed. The model was configured to represent either a normal CSA of 33° or a CSA characteristic of shoulders with rotator cuff tears (38°), and the components of the joint forces were measured. The instability ratio increased for the 38° CSA compared with the control CSA (33°) for a range of motion between 6° to 61° of thoracohumeral abduction with the largest differences in instability observed between 33° and 37° of elevation. In this range, SSP force had to be increased by 13-33% (15-23 N) to stabilize the arm in space. Our results support the concept that a high CSA can induce SSP overload particularly at low degrees of active abduction.

Clinical applications of musculoskeletal modelling for the shoulder and upper limb

Musculoskeletal models have been developed to estimate internal loading on the human skeleton, which cannot directly be measured in vivo, from external measurements like kinematics and external forces. Such models of the shoulder and upper extremity have been used for a variety of purposes, ranging from understanding basic shoulder biomechanics to assisting in preoperative planning. In this review, we provide an overview of the most commonly used large-scale shoulder and upper extremity models and categorise the applications of these models according to the type of questions their users aimed to answer. We found that the most explored feature of a model is the possibility to predict the effect of a structural adaptation on functional outcome, for instance, to simulate a tendon transfer preoperatively. Recent studies have focused on minimising the mismatch in morphology between the model, often derived from cadaver studies, and the subject that is analysed. However, only a subset of the parameters that describe the model's geometry and, perhaps most importantly, the musculotendon properties can be obtained in vivo. Because most parameters are somehow interrelated, the others should be scaled to prevent inconsistencies in the model's structure, but it is not known exactly how. Although considerable effort is put into adding complexity to models, for example, by making them subject-specific, we have found little evidence of their superiority over current models. The current trend in development towards individualised, more complex models needs to be justified by demonstrating their ability to answer questions that cannot already be answered by existing models.

Biomechanics of the normal and arthritic ankle joint

Understanding biomechanics of the normal and arthritic ankle joint can aid in analysis of an underlying clinical problem and provide a strategic basis for a more optimal management. The challenge to the clinician and the biomechanist is that the mechanical complexity of the ankle joint still clouds current understanding. This article provides an overview of current understanding of functional ankle anatomy, how this function can be altered in the degenerated ankle, and how surgical intervention further affects foot and ankle biomechanics. The focus is on how altered loading of neighboring joints in the midfoot and hindfoot may induce postoperative joint remodeling and can manifest in secondary clinical problems.

Soft tissue structures resisting anterior instability in a computational glenohumeral joint model

The glenohumeral joint is the most dislocated joint in the body due to the lack of bony constraints and the dependence on soft tissue for stability. The roles that various structures provide to joint function are important for understanding injury treatment and orthopaedic device design purposes. The goal of this study was to develop a computational model of the glenohumeral joint whereby joint behaviour was dictated by articular contact, ligamentous constraints, muscle loading and external perturbations. The bone structure of the computational model consisted of assembled computer tomographic images of the scapula, humerus and clavicle. The soft tissue elements were composed of forces and tension-only springs that represented muscles and ligaments. Validation of this model was achieved by comparing computational predictions to the results of a cadaveric experiment in which the relative contribution of muscles and ligaments to anterior joint stability was examined. The computational model predicted an anterior subluxation force that was similar to the cadaveric experimental results in humeral external rotation. The individual structure results showed the subscapularis to be critical to stabilisation in both neutral and external rotations, the biceps stabilised the joint in neutral but not in external rotation, and the inferior glenohumeral ligament resisted anterior displacement only in external rotation. The model's predictions were similar to the conclusions of the cadaveric experiment and the literature. Knowledge gained from this type of model could assist in further understanding the contribution of soft tissue stabilisers to joint function, pre-operative planning or the design of orthopaedic implants.

"What if": the use of biomechanical models for understanding and treating upper extremity musculoskeletal disorders

To aid understanding of the working of the upper extremity, several musculoskeletal models of the shoulder and arm have been developed. These models comprise the full shoulder girdle, which implies that the thoracohumeral link is formed by a scapular and clavicular segment. These models are based upon limited anatomical parameter sets and work on the assumption of a general control principle. Upper Extremity models have proven to be useful for different categories of applications, such as quantification of the load on musculoskeletal structures, or the evaluation of changes in the musculoskeletal structure on function and mechanical integrity ("what if" questions). Although these models are increasingly used, validation has long been a difficult issue. With the development of instrumented endoprostheses, a new method for model validation has come within reach. Up till now results have indicated that to obtain 'true' force values, models should be scaled, and should allow for cocontraction. Musculoskeletal models will be finding their way in education and in clinical decision making. On the longer run individualized models might become important for application to individual patients, although scaling will for some time remain a difficult issue.