Biomechanics of the Steindler flexorplasty surgery: a computer simulation study

Individuals with rotator cuff tears unsuccessfully treated with exercise therapy have less inferiorly oriented net muscle forces during scapular plane abduction

Exercise therapy for individuals with rotator cuff tears fails in approximately 25.0 % of cases. One reason for failure of exercise therapy may be the inability to strengthen and balance the muscle forces crossing the glenohumeral joint that act to center the humeral head on the glenoid. The objective of the current study was to compare the magnitude and orientation of the net muscle force pre- and post-exercise therapy between subjects successfully and unsuccessfully (e.g. eventually underwent surgery) treated with a 12-week individualized exercise therapy program. Twelve computational musculoskeletal models (n = 6 successful, n = 6 unsuccessful) were developed in OpenSim (v4.0) that incorporated subject specific tear characteristics, muscle peak isometric force, in-vivo kinematics and bony morphology. The models were driven with experimental kinematics and the magnitude and orientation of the net muscle force was determined during scapular plane abduction at pre- and post-exercise therapy timepoints. Subjects unsuccessfully treated had less inferiorly oriented net muscle forces pre- and post-exercise therapy compared to subjects successfully treated (p = 0.039 & 0.045, respectively). No differences were observed in the magnitude of the net muscle force (p > 0.05). The current study developed novel computational musculoskeletal models with subject specific inputs capable of distinguishing between subjects successfully and unsuccessfully treated with exercise therapy. A less inferiorly oriented net muscle force in subjects unsuccessfully treated may increase the risk of superior migration leading to impingement. Adjustments to exercise therapy programs may be warranted to avoid surgery in subjects at risk of unsuccessful treatment.

Latarjet's muscular alterations increase glenohumeral joint stability: A theoretical study

The surgical Latarjet procedure aims to stabilise the glenohumeral joint following anterior dislocations. Despite restoring joint stability, the procedure introduces alterations of muscle paths which likely modify the shoulder dynamics. Currently, these altered muscular functions and their implications are unclear. Hence, this work aims to predict changes in muscle lever arms, muscle and joint forces following a Latarjet procedure by using a computational approach. Planar shoulder movements of ten participants were experimentally assessed. A validated upper-limb musculoskeletal model was utilised in two configurations, i.e., a baseline model, simulating normal joint, and a Latarjet model simulating its related muscular alterations. Muscle lever arms and differences in muscle and joint forces between models were derived from the experimental marker data and static optimisation technique. Lever arms of most altered muscles, hence their role, were substantially changed after Latarjet. Altered muscle forces varied by up to 15% of the body weight. Total glenohumeral joint force increased by up to 14% of the body weight after Latarjet, mostly due to increase in compression force. Our simulation indicated that the Latarjet muscular alterations lead to changes in the muscular recruitment and contribute to the stability of the glenohumeral joint by increasing compression force during planar motions.

Redefining the Inclusion Criteria for Successful Steindler Flexorplasty Based on the Outcomes of a Case Series in Eight Patients

Background (rationale)  Steindler flexorplasty (SF) is aimed at restoring independent elbow flexion in the late stages of dysfunction of the primary elbow flexors. Selection criteria for successful SF have been defined. Objectives  The purpose of this study was to redefine the inclusion criteria for successful SF based on functional outcomes. Methods  Eight patients received SF after an average of 50.8 months after injury or dysfunction. Three patients (37.5%) met all five Al-Qattan inclusion criteria (AQIC), and another five patients (62.5%) met four or less AQIC. Patients were followed up for at least 9 months, and the maximum range of active elbow flexion (REF) was measured. Functional results of SF were assessed using the Al-Qattan scale (in accordance with Al-Qattan's scale). Results  The mean maximum REF was 100 degrees (70 to 140 degrees). Five patients reached REF greater than 100 degrees. One patient had a poor outcome, two patients (25%) had a fair outcome, three patients (37.5%) had a good outcome, and two patients (25%) had an excellent outcome of SF on the Al-Qattan scale. The impact of each AQIC on functional outcome has been critically reviewed from a biomechanical point of view. Conclusions  The sufficient number of inclusion criteria required for successful SF can be reduced from five (according to AQIC) to two; Normal or near-normal function (M4 or greater on the MRC scale) of the muscles of the flexor-pronator mass should be considered an obligatory inclusion criterion, while primary wrist extensors may be considered an optional inclusion criterion.

The impact of intrinsic muscle properties on simulated reaching performance

Musculoskeletal modelling is used widely for studying limb motion and its control, but simulation outcomes may depend heavily on the underlying muscle model used. The aim of this study was to investigate how intrinsic muscle properties affect reaching movements in a simple upper limb model. The simulations suggest that more realistic, higher-order activation dynamics requires longer prediction from a forward model and gives rise to a higher level of unplanned co-contraction than simple activation models. Consistent with prior work, muscle force-length-velocity properties stabilised and smoothed limb movements and furthermore helped promote accurate reaching performance with the high-order activation model.

The Role of the Muscle Brachioradialis in Elbow Flexion: An Electromyographic Study

BACKGROUND

In the classical conception, the brachioradialis is a forearm supinator. The hypothesis of this study was that, at least in certain positions of elbow flexion and forearm rotation, the brachioradialis is, along with the biceps and brachialis, one of the main elbow flexors.

METHODS

Fifteen young healthy male volunteers participated in this research. The activities of the biceps, brachialis and brachioradialis muscles were studied using surface electromyography, while the subjects were performing elbow flexions/extensions with as much strength as possible, forearm in neutral position, then in full pronation, then in full supination. The elbow flexion torques were isokinetically measured at 60°/sec for an arc of 120°.

RESULTS

The biceps, brachialis and brachioradialis muscles were electromyographically very active throughout resisted elbow flexion, in all three investigated positions of forearm rotation. At certain positions, the electromyographic activities were much higher than the maximal voluntary contraction signal. For what concerns specifically the brachioradialis, in all three forearm rotation investigated positions, the activity curve demonstrated a slow increase during the first part of elbow flexion, reaching in 73.3% of subjects its peak at the end of flexion; in the remaining 26.7%, the brachioradialis had a flat activity without significant peak. The activity was slightly higher in supination.

CONCLUSIONS

This study indirectly supports the idea that the brachioradialis is one of the main elbow flexors, especially when the elbow flexion is done with the forearm in supination. This observation could be important in clinical elbow and wrist surgical practice.

Sonography of the Pronator Teres: Normal and Pathologic Appearances

The pronator teres muscle is rarely examined during a routine sonographic examination of the elbow joint. Nevertheless, it can be affected by a variety of conditions, including trauma and tumors, and can be implicated in compression of the median nerve. This pictorial essay first illustrates the anatomy and biomechanics of the pronator teres. Then we present the sonographic technique for examination, normal sonographic appearance, and anatomic variations of the pronator teres and adjacent structures as well as sonography of their main disorders. Normal and pathologic sonographic appearances are correlated with magnetic resonance imaging and radiographic results.

Lumped-parameter electromyogram-driven musculoskeletal hand model: A potential platform for real-time prosthesis control

Simple, lumped-parameter musculoskeletal models may be more adaptable and practical for clinical real-time control applications, such as prosthesis control. In this study, we determined whether a lumped-parameter, EMG-driven musculoskeletal model with four muscles could predict wrist and metacarpophalangeal (MCP) joint flexion/extension. Forearm EMG signals and joint kinematics were collected simultaneously from 5 able-bodied (AB) subjects. For one subject with unilateral transradial amputation (TRA), joint kinematics were collected from the sound arm during bilateral mirrored motion. Twenty-two model parameters were optimized such that joint kinematics predicted by EMG-driven forward dynamic simulation closely matched measured kinematics. Cross validation was employed to evaluate the model kinematic predictions using Pearson׳s correlation coefficient (r). Model predictions of joint angles were highly to very highly positively correlated with measured values at the wrist (AB mean r=0.94, TRA r=0.92) and MCP (AB mean r=0.88, TRA r=0.93) joints during single-joint wrist and MCP movements, respectively. In simultaneous multi-joint movement, the prediction accuracy for TRA at the MCP joint decreased (r=0.56), while r-values derived from AB subjects and TRA wrist motion were still above 0.75. Though parameters were optimized to match experimental sub-maximal kinematics, passive and maximum isometric joint moments predicted by the model were comparable to reported experimental measures. Our results showed the promise of a lumped-parameter musculoskeletal model for hand/wrist kinematic estimation. Therefore, the model might be useful for EMG control of powered upper limb prostheses, but more work is needed to demonstrate its online performance.

Prediction of In Vivo Knee Joint Loads Using a Global Probabilistic Analysis

Musculoskeletal models are powerful tools that allow biomechanical investigations and predictions of muscle forces not accessible with experiments. A core challenge modelers must confront is validation. Measurements of muscle activity and joint loading are used for qualitative and indirect validation of muscle force predictions. Subject-specific models have reached high levels of complexity and can predict contact loads with surprising accuracy. However, every deterministic musculoskeletal model contains an intrinsic uncertainty due to the high number of parameters not identifiable in vivo. The objective of this work is to test the impact of intrinsic uncertainty in a scaled-generic model on estimates of muscle and joint loads. Uncertainties in marker placement, limb coronal alignment, body segment parameters, Hill-type muscle parameters, and muscle geometry were modeled with a global probabilistic approach (multiple uncertainties included in a single analysis). 5-95% confidence bounds and input/output sensitivities of predicted knee compressive loads and varus/valgus contact moments were estimated for a gait activity of three subjects with telemetric knee implants from the "Grand Challenge Competition." Compressive load predicted for the three subjects showed confidence bounds of 333 ± 248 N, 408 ± 333 N, and 379 ± 244 N when all the sources of uncertainty were included. The measured loads lay inside the predicted 5-95% confidence bounds for 77%, 83%, and 76% of the stance phase. Muscle maximum isometric force, muscle geometry, and marker placement uncertainty most impacted the joint load results. This study demonstrated that identification of these parameters is crucial when subject-specific models are developed.

Computational sensitivity analysis to identify muscles that can mechanically contribute to shoulder deformity following brachial plexus birth palsy

PURPOSE

Two mechanisms, strength imbalance or impaired longitudinal muscle growth, potentially cause osseous and postural shoulder deformity in children with brachial plexus birth palsy. Our objective was to determine which muscles, via either deformity mechanism, were mechanically capable of producing forces that could promote shoulder deformity.

METHODS

In an upper limb computational musculoskeletal model, we simulated strength imbalance by allowing each muscle crossing the shoulder to produce 30% of its maximum force. To simulate impaired longitudinal muscle growth, the functional length of each muscle crossing the shoulder was reduced by 30%. We performed a sensitivity analysis to identify muscles that, through either simulated deformity mechanism, increased the posteriorly directed, compressive glenohumeral joint force consistent with osseous deformity or reduced the shoulder external rotation or abduction range of motion consistent with postural deformity.

RESULTS

Most of the increase in the posterior glenohumeral joint force by the strength imbalance mechanism was caused by the subscapularis, latissimus dorsi, and infraspinatus. Posterior glenohumeral joint force increased the most owing to impaired growth of the infraspinatus, subscapularis, and long head of biceps. Through the strength imbalance mechanism, the subscapularis, anterior deltoid, and pectoralis major muscles reduced external shoulder rotation by 28°, 17°, and 10°, respectively. Shoulder motion was reduced by 40° to 56° owing to impaired growth of the anterior deltoid, subscapularis, and long head of triceps.

CONCLUSIONS

The infraspinatus, subscapularis, latissimus dorsi, long head of biceps, anterior deltoid, pectoralis major, and long head of triceps were identified in this computational study as being the most capable of producing shoulder forces that may contribute to shoulder deformity following brachial plexus birth palsy.

CLINICAL RELEVANCE

The muscles mechanically capable of producing deforming shoulder forces should be the focus of experimental studies investigating the musculoskeletal consequences of brachial plexus birth palsy and are potentially critical targets for treating shoulder deformity.

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.

Biomechanical contributions of posterior deltoid and teres minor in the context of axillary nerve injury: a computational study

PURPOSE

To determine whether transfer to only the anterior branch of the axillary nerve will restore useful function after axillary nerve injury with persistent posterior deltoid and teres minor paralysis.

METHODS

We used a computational musculoskeletal model of the upper limb to determine the relative contributions of posterior deltoid and teres minor to maximum joint moment generated during a simulated static strength assessment and to joint moments during 3 submaximal shoulder movements. Movement simulations were performed with and without simulated posterior deltoid and teres minor paralysis to identify muscles that may compensate for their paralysis.

RESULTS

In the unimpaired limb model, teres minor and posterior deltoid accounted for 16% and 14% of the total isometric shoulder extension and external rotation joint moments, respectively. During the 3 movement simulations, posterior deltoid produced as much as 20% of the mean shoulder extension moment, whereas teres minor accounted for less than 5% of the mean joint moment in all directions of movement. When we paralyzed posterior deltoid and teres minor, the mean extension moments generated by the supraspinatus, long head of triceps, latissimus dorsi, and middle deltoid increased to compensate. Compensatory muscles were not fully activated during movement simulations when posterior deltoid and teres minor were paralyzed.

CONCLUSIONS

Reconstruction of the anterior branch of the axillary nerve only is an appropriate technique for restoring shoulder abduction strength after isolated axillary nerve injury. When shoulder extension strength is compromised by extensive neuromuscular shoulder injury, reconstruction of both the anterior and posterior branches of the axillary nerve should be considered.

CLINICAL RELEVANCE

By quantifying the biomechanical role of muscles during submaximal movement, in addition to quantifying muscle contributions to maximal shoulder strength, we can inform preoperative planning and permit more accurate predictions of functional outcomes.

On validation of multibody musculoskeletal models

We review the opportunities to validate multibody musculoskeletal models in view of the current transition of musculoskeletal modelling from a research topic to a practical simulation tool in product design, healthcare and other important applications. This transition creates a new need for justification that the models are adequate representations of the systems they simulate. The need for a consistent terminology and established standards is identified and knowledge from fields with a more progressed state-of-the-art in verification and validation is introduced. A number of practical steps for improvement of the validation of multibody musculoskeletal models are pointed out and directions for future research in the field are proposed. It is hoped that a more structured approach to model validation can help to improve the credibility of musculoskeletal models.

Postural dependence of passive tension in the supraspinatus following rotator cuff repair: a simulation analysis

BACKGROUND

Despite surgical advances, repair of rotator cuff tears is associated with 20-70% incidence of recurrent tearing. The tension required to repair the torn tendon influences surgical outcomes and may be dependent on the gap length from torn tendon that must be spanned by the repair. Detailed understanding of forces throughout the range of motion (ROM) may allow surgeons to make evidence-based recommendations for post-operative care.

METHODS

We used a computational shoulder model to assess passive tension and total moment-generating capacity in supraspinatus for repairs of gaps up to 3 cm throughout the shoulder (ROM).

FINDINGS

In 60° abduction, increased gap length from 0.5 cm to 3 cm caused increases in passive force from 3N to 58 N, consistent with those seen during clinical repair. For reduced abduction, passive forces increased substantially. For a 0.5 cm gap, tension throughout the ROM (elevation, plane of elevation, and rotation) is within reasonable limits, but larger gaps are associated with tensions that markedly exceed reported pull-out strength of sutures and anchors. Peak moment for a large 3 cm gap length was 5.09 Nm, a 53% reduction in moment-generating capacity compared to uninjured supraspinatus.

INTERPRETATION

We conclude that shoulder posture is an important determinant of passive forces during rotator cuff repair surgery. Choosing postures that reduce forces intraoperatively to permit repair of larger gaps may lead to failure postoperatively when the shoulder is mobilized. For larger defects, loss of strength in supraspinatus may be substantial following repair even if retear is prevented.

A simulation analysis of the combined effects of muscle strength and surgical tensioning on lateral pinch force following brachioradialis to flexor pollicis longus transfer

Biomechanical simulations of tendon transfers performed following tetraplegia suggest that surgical tensioning influences clinical outcomes. However, previous studies have focused on the biomechanical properties of only the transferred muscle. We developed simulations of the tetraplegic upper limb following transfer of the brachioradialis (BR) to the flexor pollicis longus (FPL) to examine the influence of residual upper limb strength on predictions of post-operative transferred muscle function. Our simulations included the transfer, ECRB, ECRL, the three heads of the triceps, brachialis, and both heads of the biceps. Simulations were integrated with experimental data, including EMG and joint posture data collected from five individuals with tetraplegia and BR-FPL tendon transfers during maximal lateral pinch force exertions. Given a measured co-activation pattern for the non-paralyzed muscles in the tetraplegic upper limb, we computed the highest activation for the transferred BR for which neither the elbow nor the wrist flexor moment was larger than the respective joint extensor moment. In this context, the effects of surgical tensioning were evaluated by comparing the resulting pinch force produced at different muscle strength levels, including patient-specific scaling. Our simulations suggest that extensor muscle weakness in the tetraplegic limb limits the potential to augment total pinch force through surgical tensioning. Incorporating patient-specific muscle volume, EMG activity, joint posture, and strength measurements generated simulation results that were comparable to experimental results. Our study suggests that scaling models to the population of interest facilitates accurate simulation of post-operative outcomes, and carries utility for guiding and developing rehabilitation training protocols.

Biomechanics of latissimus dorsi transfer for irreparable posterosuperior rotator cuff tears

BACKGROUND

Latissimus dorsi transfer is the treatment most frequently used for restoring function in shoulders with irreparable posterosuperior rotator cuff tears. Yet, functional outcomes of the transfers are unpredictable and vary among patients.

METHODS

A three-dimensional upper-extremity computational model was used to simulate and analyze the biomechanical consequences of transferring the latissimus dorsi to four attachment sites: the infraspinatus, supraspinatus, subscapularis and teres minor insertions. Functions of a normal shoulder were simulated, as well as those and of a shoulder with a posterosuperior rotator cuff tear before and after muscle transfers were simulated. Parameters such as active and passive moment-generating capacity, and the moment arm and fiber excursion ratio of the transferred muscle were analyzed.

FINDINGS

All muscle transfers resulted in a large increase in shoulder external rotation strength. The latissimus dorsi was an external rotator after the transfer, but the fiber excursion ratio decreased accordingly. When the latissimus dorsi was transferred to the infraspinatus, supraspinatus or subscapularis insertion, it changed from extensor to flexor at the beginning of flexion. The flexion moment arm of the latissimus dorsi after the transfers was generally decreased. Shoulder abduction strength did not improve. Decrease in fiber excursion ratio during abduction and flexion was observed after the transfer. Side effects of the muscle transfers, such as the reduction of active adduction, extension and internal rotation of the shoulder, were explored.

INTERPRETATION

A transfer to teres minor insertion was not recommended. Infraspinatus insertion was found to be a preferred attachment site in latissimus dorsi transfer, provided that the patient had a strong deltoid.

TRAUMATIC LESIONS OF THE BRACHIAL PLEXUS

OBJECTIVE

To analyze retrospectively the outcomes of primary as well as secondary functional reconstructions in 49 patients with traumatic brachial plexus lesions from a single service. Guidelines for treatment might be extracted from this analysis.

METHODS

Among 152 cases of traumatic lesion of the brachial plexus presented to our clinic, 58 underwent primary brachial plexus reconstructive surgery. On exploration, all patients showed stretching and scarring of plexus elements; root avulsions were found in 28 patients (48%). Outcome evaluation was carried out in 49 of these patients with a follow-up period of 1 year or longer (mean follow-up, 27.9 mo; range, 12–72 mo). A total of 43 secondary reconstructive procedures to improve functionality of the involved arm were performed at a later stage in 25 of 58 patients. Outcomes of the secondary functional restorative procedures were evaluated (mean follow-up, 11.5 mo; range, 3–60 mo in 43 procedures).

RESULTS

Patients with neurolysis as a stand-alone procedure (11 patients) showed an outcome grade of 4 or 5. The average outcome of the 19 patients with C5, C6, and C7 grafting was Grade 3, the same as in patients with nerve transfers to the upper plexus elements (C5–C6 root avulsions, 13 patients). Patients with multiple root avulsions (five cases) showed an overall poor outcome (Grades 0–2). Secondary functional restorative surgery was performed in 43% of the patients and helped improve individual outcomes, providing a favorable effect on the general functionality of the arm. Among the restorative operations performed, the Steindler procedure, wrist extension restoration, claw hand correction, and free functional muscle flap transfer to the arm and forearm were the most rewarding.

CONCLUSION

A combination of primary brachial plexus reconstruction and carefully evaluated, selected, and planned function-restorative secondary procedures might offer favorable outcomes in patients with partial or total brachial plexus lesions.

OpenSim: open-source software to create and analyze dynamic simulations of movement

Dynamic simulations of movement allow one to study neuromuscular coordination, analyze athletic performance, and estimate internal loading of the musculoskeletal system. Simulations can also be used to identify the sources of pathological movement and establish a scientific basis for treatment planning. We have developed a freely available, open-source software system (OpenSim) that lets users develop models of musculoskeletal structures and create dynamic simulations of a wide variety of movements. We are using this system to simulate the dynamics of individuals with pathological gait and to explore the biomechanical effects of treatments. OpenSim provides a platform on which the biomechanics community can build a library of simulations that can be exchanged, tested, analyzed, and improved through a multi-institutional collaboration. Developing software that enables a concerted effort from many investigators poses technical and sociological challenges. Meeting those challenges will accelerate the discovery of principles that govern movement control and improve treatments for individuals with movement pathologies.

Steindler flexorplasty to restore elbow flexion in C5-C6-C7 brachial plexus palsy type

Background

Loss of elbow flexion due to traumatic palsy of the brachial plexus represents a major functional handicap.

Then, the first goal in the treatment of the flail arm is to restore the elbow flexion by primary direct nerve surgery or secondary reconstructive surgery.

There are various methods to restore elbow flexion which are well documented in the medical literature but the most known and used is Steindler flexorplasty.

This review is intended to detail the author's experience with Steindler flexorplasty to restore elbow flexion in patients with brachial plexus palsy C5-C6-C7 where wrist extensors are paralyzed or weakened.

Methods

We conducted a retrospective follow-up study of 12 patients with absent or extremely weak elbow flexion (motor grade 2 or less), wrist/finger extensor and triceps palsy associated; who had undergone surgical reconstruction of the flail upper limb by tendon transfer (Steindler flexorplasty) and wrist arthrodesis to restore elbow flexion. The aetiology of elbow weakness was in all patients brachial plexus palsy (C5-C6-C7 deficit). Data were collected from medical records and from the information obtained during follow-up visits.

Age, sex, preoperative strength (rated on a 0 to 5 scale for the flexors of the elbow, wrist flexors, pronator and triceps), previous surgery, length of follow-up, other associated operative procedures, results and complications were recorded.

Results

The results are the follows: Eleven patients were found to have very good or good function of the transferred muscles. One patient had mild active flexion of the elbow despite the reconstructive procedure.

There were no major intraoperative complications. Two patients experienced transient, intermittent nocturnal ulnar paresthesias postoperatively. In both patients these symptoms subsided without further surgery.

Conclusion

Our study suggests that in patients with C5-C6-C7 palsy where the wrist and finger extensors are paralyzed or weaked, the flexor-pronators muscles of the forearm are strong but the triceps is not available for transfer; Steindler flexorplasty to restore elbow flexion should be complemented with wrist arthrodesis.

Modified Steindler procedure for the treatment of brachial plexus injuries

A retrospective follow-up study was completed on ten patients who suffered from a brachial plexus injury that was treated with a modified Steindler procedure. The mean postoperative period was 6.8 years. The postoperative elbow joint range of motion was -42 degrees of extension (range -5 degrees to -65 degrees ) and 107 degrees of flexion (range 90 degrees -130 degrees ). Manual muscle testing showed grade 4 or 5 in eight patients and grade 3 in two patients. In the subjective assessment, the patients scored 20 out of 30 points and were able to perform almost all activities with the exception of shoulder elevation. Innervation of the musculocutaneous nerve was evaluated by electromyography and no correlation was seen between preoperative and postoperative amplitude of the biceps brachii by electromyogram. Based on these results, we concluded that a modified Steindler procedure is useful for reconstruction of upper brachial plexus injuries, and recovery of the biceps brachii was difficult to predict by an electromyogram.

A model of the upper extremity for simulating musculoskeletal surgery and analyzing neuromuscular control

Biomechanical models of the musculoskeletal system are frequently used to study neuromuscular control and simulate surgical procedures. To be broadly applicable, a model must be accessible to users, provide accurate representations of muscles and joints, and capture important interactions between joints. We have developed a model of the upper extremity that includes 15 degrees of freedom representing the shoulder, elbow, forearm, wrist, thumb, and index finger, and 50 muscle compartments crossing these joints. The kinematics of each joint and the force-generating parameters for each muscle were derived from experimental data. The model estimates the muscle-tendon lengths and moment arms for each of the muscles over a wide range of postures. Given a pattern of muscle activations, the model also estimates muscle forces and joint moments. The moment arms and maximum moment-generating capacity of each muscle group (e.g., elbow flexors) were compared to experimental data to assess the accuracy of the model. These comparisons showed that moment arms and joint moments estimated using the model captured important features of upper extremity geometry and mechanics. The model also revealed coupling between joints, such as increased passive finger flexion moment with wrist extension. The computer model is available to researchers at http://nmbl.stanford.edu.