A dynamic analysis of glenohumeral motion after simulated capsulolabral injury. A cadaver model

Muscle and joint function in the rotator cuff deficient shoulder

Full-thickness rotator cuff tears can lead to poor coaptation of the humeral head to the glenoid, disrupting muscle forces required for glenohumeral joint stability, ultimately leading to joint subluxation. The aim of this study was to evaluate muscle forces and glenohumeral joint translations during elevation in the presence of isolated and combined full-thickness rotator cuff tears. Eight fresh-frozen upper limbs were mounted to a computer-controlled testing apparatus that simulated joint motion by simulated muscle force application. Scapular-plane abduction was performed, and glenohumeral joint translations were measured using an optoelectronic system. Testing was performed in the native shoulder, a following an isolated tear to the supraspinatus, as well as combined tears involving the supraspinatus and subscapularis, as well as supraspinatus, infraspinatus, and teres minor. Rotator cuff tears significantly increased middle deltoid force at 30°, 60°, and 90° of abduction relative to that in the native shoulder (p < 0.05). Significantly greater superior translations were observed relative to the intact shoulder due to combined tears to the supraspinatus and infraspinatus at 30° of abduction (mean increase: 1.6 mm, p = 0.020) and 60° of abduction (mean increase: 4.8 mm, p = 0.040). This study illustrates the infraspinatus-teres minor complex as a major humeral head depressor and contributor to glenohumeral joint stability. An increase in deltoid force during abduction occurs in the presence of rotator cuff tears, which exacerbates superior migration of the humeral head. The findings may help in the development of clinical tests in rotator cuff tear diagnostics, in surgical planning of rotator cuff repair, and in planning of targeted rehabilitation.

Development of a Cadaveric Shoulder Motion Simulator with Open-Loop Iterative Learning for Dynamic, Multiplanar Motion: A Preliminary Study

Ex vivo shoulder motion simulators are commonly used to study shoulder biomechanics but are often limited to performing simple planar motions at quasi-static speeds using control architectures that do not allow muscles to be deactivated. The purpose of this study was to develop an open-loop tendon excursion controller with iterative learning and independent muscle control to simulate complex multiplanar motion at functional speeds and allow for muscle deactivation. The simulator performed abduction/adduction, faceted circumduction, and abduction/adduction (subscapularis deactivation) using a cadaveric shoulder with an implanted reverse total shoulder prosthesis. Kinematic tracking accuracy and repeatability were assessed using maximum absolute error (MAE), root mean square error (RMSE), and average standard deviation (ASD). During abduction/adduction and faceted circumduction, the RMSE did not exceed 0.3, 0.7, and 0.8 degrees for elevation, plane of elevation, and axial rotation, respectively. During abduction/adduction, the ASD did not exceed 0.2 degrees. Abduction/adduction (subscapularis deactivation) resulted in a loss of internal rotation, which could not be restored at low elevation angles. This study presents a novel control architecture, which can accurately simulate complex glenohumeral motion. This simulator will be used as a testing platform to examine the effect of shoulder pathology, treatment, and rehabilitation on joint biomechanics during functional shoulder movements.

Ex vivo experimental strategies for assessing unconstrained shoulder biomechanics: A scoping review

BACKGROUND

Biomechanical studies of the shoulder often choose an ex vivo approach, especially when investigating the active and passive contribution of individual muscles. Although various simulators of the glenohumeral joint and its muscles have been developed, to date a testing standard has not been established. The objective of this scoping review was to present an overview of methodological and experimental studies describing ex vivo simulators that assess unconstrained, muscular driven shoulder biomechanics.

METHODS

All studies with ex vivo or mechanical simulation experiments using an unconstrained glenohumeral joint simulator and active components mimicking the muscles were included in this scoping review. Static experiments and humeral motion imposed through an external guide, e.g., a robotic device, were excluded.

RESULTS

Nine different glenohumeral simulators were identified in 51 studies after the screening process. We identified four control strategies characterized by: (a) using a primary loader to determine the secondary loaders with constant force ratios; (b) using variable muscle force ratios according to electromyography; (c) calibrating the muscle path profile and control each motor according to this profile; or (d) using muscle optimization.

CONCLUSION

The simulators with the control strategy (b) (n = 1) or (d) (n = 2) appear most promising due to its capability to mimic physiological muscle loads.

Effect of 8-week Shoulder External Rotation Exercise with Low Intensity and Slow Movement on Infraspinatus

OBJECTIVE

Generally, low-intensity training is recommended as selective training of the infraspinatus muscle. This study aimed to investigate whether an 8-week intervention of low-intensity, slow-movement, external rotation exercise of the shoulder led to an increase in muscle strength with shoulder external rotation and cross-sectional area (CSA) infraspinatus muscle.

METHODS

Sixteen healthy male volunteers were randomly assigned to the low-intensity and slow-movement (LS) group (N = 8) or the normal-intensity and normal-speed (NN) group (N = 8). The LS and NN groups performed shoulder external rotation exercises with low intensity and slow movement, and normal intensity and normal speed, respectively. The exercise session consisted of three sets of 10 repetitions, which were performed three times per week for 8 weeks. We measured the CSA of the infraspinatus and muscle strength of the shoulder external rotation before and after the 8-week intervention.

RESULTS

A significant increase in infraspinatus CSA from baseline to 8 weeks was found in the LS group (7.3% of baseline) but not in the NN group. No significant differences were found in the muscle strength of shoulder external rotation.

CONCLUSION

Our results suggest that low-intensity exercise of the infraspinatus is effective for muscle hypertrophy when performed with slow movement. This finding may help patients who should avoid excessive stress in the early phase of rehabilitation.

The development and evaluation of an in-vitro shoulder simulator with active muscle simulation

The purpose of the present study was to develop a novel active in-vitro shoulder simulator to emulate all forms of planar and non-planar glenohumeral motions with active muscle simulation on cadaver specimens or shoulder models and to critically evaluate its performance. A physiologic shoulder simulator, driven using simulated muscle force, was developed to dynamically realize accurate kinematic control in all three rotational degrees of freedom (DOF) under physiological kinetic boundaries. The control algorithm of the simulator was implemented using three parallel running independent control loops, which regulate the forces of individual muscles in the respect DOF and work asynchronously in disparate sequences adapted to specific motions (abduction, flexion/extension and rotation). Three cadaveric specimens were used to evaluate the kinematic and kinetic performance of the simulator during simulated motions. High kinematic accuracy (maximum mean deviation ≤ 2.35° and RMSE 1.13°) and repeatability (maximum and average SD of ≤ 1.21° and 0.67°) were observed in all three rotational DOF investigated. The reliabilities of all individual muscle forces actuated in the simulator during planar and non-planar motions were generally excellent, with the 95% CIs of ICC estimates of > 0.90 for most instances (30/36). A novel shoulder simulator with active muscle simulation was developed and evaluated. Its capability to reproduce kinematics and kinetics in a physiological range for all DOF was systematically evaluated for multiple kinetic and kinematic outcome variables. The presented simulator is a powerful tool for investigating the biomechanics of physiological and pathological shoulder joints and to evaluate various surgical interventions. Acquisition of reliable data in joint kinetics and translational kinematics during active motions is critical to assess shoulder pathologies and appropriate treatments. We provide a unique muscle activated physiologic shoulder simulator, which allows the comprehensive acquisition of joint kinematic and kinetic data during repeated realistic planar and non-planar motions.

Humeral and Glenoid Version in Reverse Total Shoulder Arthroplasty: A Systematic Review

There is increasing interest in reverse total shoulder arthroplasty (RTSA) as a reliable treatment for arthritic, rotator cuff deficient shoulders. Humeral and glenoid version are controversial parameters that can influence internal and external rotation, muscular forces, and implant stability as outcomes of RTSA. The aim of this study was to obtain an overview of the current knowledge on the effect of both humeral component version and glenoid component version and give recommendations on their most optimal degree for RTSA. A comprehensive quantitative review of the published literature on the effect of humeral version and glenoid version in RTSA was performed, to identify its influence on the range of movement, muscle forces, and intrinsic stability of the reverse prosthesis. Eleven studies were included: nine were biomechanical studies, one was a clinical-radiographic study, and one was an implant design consideration. Anterior stability can be improved by implanting the humeral component in neutral or with some anteversion. Glenoid component retroversion has been proven to reduce the likelihood of subluxation, while increasing ER and ROM at the same time. The study was conducted considering 5° anteversion; neutral; and 5°, 10°, and 20° retroversion of the glenoid component. Although a final opinion has not been yet expressed on the matter, the general consensus tends to agree on restoring 0° to 20° of retroversion of the humeral and glenoid component to yield the best outcomes.

Verification of Shoulder External Rotators Strength Measurement Using a Suspension Scale

Background

Of the shoulder external rotators, the infraspinatus and teres minor are the key muscles that contribute to the dynamic stability of the shoulder. It is crucial to properly measure the strength values to evaluate muscle function and training load for shoulder external rotators. A suspension scale (SPS) can measure the mass of the suspended object, and it may be possible to apply it to measure strength. However, the utility of strength measurements using an SPS has not been clarified in previous studies. In this study, we aimed to investigate the intra-rater reliability of measuring the strength of shoulder external rotators using an SPS and the relationship between strength measurement using an SPS and a handheld dynamometer (HHD).

Methodology

The participants were 10 healthy males with 20 shoulders (24.5 ± 2.5 years old; height = 172.8 ± 5.4 cm; weight = 69.6 ± 8.1 kg). Upper extremity strength was measured at 90° shoulder abduction, 90° external rotation, 0° horizontal adduction/abduction, 90° elbow flexion, and 0° forearm pronation/supination in the prone position. The isometric strength of shoulder external rotation was measured with the SPS and HHD, and one examiner measured the maximum strength value. The intra-rater reliability of the two methods using SPS and HHD was evaluated using the intraclass correlation coefficient (ICC_1,2), standard error of measurement (SEM), minimum detectable change (MDC), and Bland-Altman analysis. The relationship between the SPS and HHD was calculated as the correlation coefficient between the strength values of SPS and HHD.

Results

The intra-rater reliability of the strength measurement of shoulder external rotators using SPS was ICC_1,2 0.98 (95% confidence interval = 0.95-0.99), and SEM and MDC were 0.3 and 0.9, respectively. The measurements using SPS had no fixed and proportional biases. A significant positive correlation was observed between SPS and HHD (r = 0.94, p < 0.01).

Conclusions

The SPS is an alternative to the HHD for measuring the strength of shoulder external rotators. Thus, measuring the strength of shoulder external rotators using an SPS may be applied as a cost-effective and portable assessment method for shoulder function.

Effects of a Novel Rotator Cuff Rehabilitation Device on Shoulder Strength and Function

ABSTRACT

Savitzky, JA, Abrams, LR, Galluzzo, NA, Ostrow, SP, Protosow, TJ, Liu, SA, Handrakis, JP, and Friel, K. Effects of a novel rotator cuff rehabilitation device on shoulder strength and function. J Strength Cond Res 35(12): 3355-3363, 2021-The glenohumeral joint, a multiaxial ball and socket joint, has inherent instability counterbalanced by the muscular stability of the rotator cuff (RC) and connective tissue. Exercise has been shown to alleviate pain and disability arising from degenerative changes of the RC due to overuse, trauma, or poor posture. This study compared the training effects of ShoulderSphere (SS), an innovative device that uses resistance to centrifugal force, to TheraBand (TB), a traditional device that uses resistance to elasticity. Thirty-five healthy male and female adults (24.2 ± 2.4 years) were randomized into 3 groups: SS, TB, and control. Five outcomes were assessed before and after the twice-weekly, 6-week intervention phase: strength (shoulder flexion [Fx], extension [Ext], external rotation [ER], and internal rotation [IR]), proprioception (6 positions), posterior shoulder endurance (ShEnd), stability (Upper Quarter Y-Balance Test [YBal] (superolateral [YBalSup], medial [YBalMed], and inferolateral [YBalInf]), and power (seated shot put [ShtPt]). Data were analyzed using a 3 (group: SS, TB, and control) × 2 (time: pre and post) generalized estimating equation. Analyses demonstrated a main effect of time for all strength motions (p < 0.01): YBalInf (p < 0.0001), ShtPt (p < 0.05), and ShEnd (p < 0.0001) but no interaction effects of group × time. There were no main or interaction effects for proprioception. Both SS and TB groups had significant within-group increases in Ext, IR, YBalInf, and ShEnd. Only the SS group had significant increases in ER, Fx, and ShtPt. ShoulderSphere demonstrated comparable conditioning effects with TB and may afford additional strength gains in Fx and ER, and power. ShoulderSphere should be considered a viable alternative in RC conditioning.

Graft Tensioning in Superior Capsular Reconstruction Improves Glenohumeral Joint Kinematics in Massive Irreparable Rotator Cuff Tears: A Biomechanical Study of the Influence of Superior Capsular Reconstruction on Dynamic Shoulder Abduction

Background:

Superior capsular reconstruction (SCR) for massive, irreparable rotator cuff tears has become more widely used recently; however, ideal tensioning of the graft and the influence on joint kinematics remain unknown.

Purpose/Hypothesis:

The purpose of this study was to assess the effects of graft tensioning on glenohumeral joint kinematics after SCR using a dermal allograft. The hypothesis was that a graft fixed under tension would result in increased glenohumeral abduction motion and decreased cumulative deltoid forces compared with a nontensioned graft.

Study Design:

Controlled laboratory study.

Methods:

A total of 10 fresh-frozen cadaveric shoulders were tested using a dynamic shoulder simulator. Each shoulder underwent the following 4 conditions: (1) native, (2) simulated irreparable supraspinatus (SSP) tear, (3) SCR using a nontensioned acellular dermal allograft, and (4) SCR using a graft tensioned with 30 to 35 N. Mean values for maximum glenohumeral abduction and cumulative deltoid forces were recorded. The critical shoulder angle (CSA) was also assessed.

Results:

Native shoulders required a mean (±SE) deltoid force of 193.2 ± 45.1 N to achieve maximum glenohumeral abduction (79.8° ± 5.8°). Compared with native shoulders, abduction decreased after SSP tears by 32% (54.3° ± 13.7°; P = .04), whereas cumulative deltoid forces increased by 23% (252.1 ± 68.3 N; P = .04). The nontensioned SCR showed no significant difference in shoulder abduction (54.1° ± 16.1°) and required deltoid forces (277.8 ± 39.8 N) when compared with the SSP tear state. In contrast, a tensioned graft led to significantly improved shoulder abduction compared with the SSP tear state (P = .04) although abduction and deltoid forces could not be restored to the native state (P = .01). A positive correlation between CSA and maximum abduction was found for the tensioned-graft SCR state (r = 0.685; P = .02).

Conclusion:

SCR using a graft fixed under tension demonstrated a significant increase in maximum shoulder abduction compared with a nontensioned graft; however, abduction remained significantly less than the intact state. The nontensioned SCR showed no significant improvement in glenohumeral kinematics compared with the SSP tear state.

Clinical Relevance:

Because significant improvement in shoulder function after SCR may be expected only when the graft is adequately tensioned, accurate graft measurement and adequate tension of at least 30 N should be considered during the surgical procedure. SCR with a tensioned graft may help maintain sufficient acromiohumeral distance, improve clinical outcomes, and reduce postoperative complications.

The Superior Capsular Reconstruction: Lessons Learned and Future Directions

The ideal solution for the irreparable rotator cuff tear remains in question. A recent innovative technique, superior capsular reconstruction (SCR), has demonstrated promising results with some early clinical outcomes demonstrating statistically notable improvements in active forward flexion and American Shoulder and Elbow Surgeons scores. Multiple biomechanical studies have also demonstrated its ability to reduce superior translation of the humerus after massive rotator cuff tear. Even so, these results are still early and durability of the reconstruction over time needs to be determined. Although more than 15,000 SCRs have been done worldwide, there remains a paucity of outcome data and one must be vigilant to not allow enthusiasm to overtake critical evaluation. The potential mechanisms of action, including tenodesis effect, force coupler, or subacromial spacer, need further elucidation and the ideal indications for this procedure, as well as its technical optimization, and limitations have yet to be fully determined. The purpose of this review is to critically evaluate the biomechanical and clinical literature that has assessed SCR, along with the controversies and considerations encountered with this procedure.

Effect of rotator cuff muscle activation on glenohumeral kinematics: A cadaveric study

Healthy shoulder function requires the coordination of the rotator cuff muscles to maintain the humeral head's position in the glenoid. While glenohumeral stability has been studied in various settings, few studies have characterized the effect of dynamic rotator cuff muscle loading on glenohumeral translation during shoulder motion. We hypothesize that dynamic rotator cuff muscle activation decreases joint translation during continuous passive abduction of the humerus in a cadaveric model of scapular plane glenohumeral abduction. The effect of different rotator cuff muscle activity on glenohumeral translation was assessed using a validated shoulder testing system. The Dynamic Load profile is a novel approach, based on musculoskeletal modeling of human subject motion. Passive humeral elevation in the scapular plane was applied via the testing system arm, while the rotator cuff muscles were activated according to the specified force profiles using stepper motors and a proportional control feedback loop. Glenohumeral translation was defined according to the International Society of Biomechanics. The Dynamic load profile minimized superior translation of the humeral head relative to the conventional loading profiles. The total magnitude of translation was not significantly different (0.805) among the loading profiles suggesting that the compressive forces from the rotator cuff primarily alter the direction of humeral head translation, not the magnitude. Rotator cuff muscle loading is an important element of cadaveric shoulder studies that must be considered to accurately simulate glenohumeral motion. A rotator cuff muscle activity profile based on human subject muscle activity reduces superior glenohumeral translation when compared to previous RC loading profiles.

Electromyographic analysis of selected shoulder muscles during rehabilitation exercises

BACKGROUND

Overhead work-activities and repetitive use of arm in occupational tasks are known risk factors to cause shoulder injury that often develops to functional disability. Rehabilitation exercises (REs) are capable of slowing this development.

OBJECTIVE

The main objective of the present work is to experiment with common REs and to identify those which produce significant integrated electromyography (iEMG) activation of selected shoulder muscles namely, posterior deltoid, infraspinatus, teres minor, upper trapezius, middle trapezius and lower trapezius.

METHODS

  Ten healthy male occupational workers, exposed to the risk of shoulder injury, performed six REs while iEMG activities of concerning muscles are recorded using Biopac MP 150 system. REs are cross-examined statistically to select those which yield higher iEMG activations.

RESULTS

Results indicate that upper trapezius presents the highest iEMG activity in exercise 6 (standing ER at 90∘ abduction and 90∘ elbow flexion). Middle trapezius and lower trapezius show highest activation in exercise 1 (prone horizontal abduction at 90∘ with full external rotation with thumb right up). Also, posterior deltoid, infraspinatus, and teres minor have the highest iEMG activity in this exercise.

CONCLUSIONS

Results provide basic information to select appropriate rehabilitation programs for occupational workers exposed to the risk of shoulder injury.

Biomechanical Evaluation of Glenoid Version and Dislocation Direction on the Influence of Anterior Shoulder Instability and Development of Hill-Sachs Lesions

BACKGROUND

Abnormal glenoid version is a risk factor for shoulder instability. However, the degree to which the variance in version (both anteversion and retroversion) affects one's predisposition for instability is not well understood.

PURPOSE

To determine the influence of glenoid version on anterior shoulder joint stability and to determine if the direction of the humeral head dislocation is a stimulus for the development of Hill-Sachs lesions.

STUDY DESIGN

Controlled laboratory study.

METHODS

Ten human cadaveric shoulders (mean age, 59.4 ± 4.3 years) were tested using a custom shoulder dislocation device placed in a position of apprehension (90° of abduction with 90° of external rotation). Glenoid version was adjusted in 5° increments for a total of 6 version angles tested: +10°, +5°, 0°, -5°, -10°, and -15° (anteversion angles are positive, and retroversion angles are negative). Two humeral dislocation directions were tested. The first direction was true anterior through the anterior-posterior glenoid axis. The second dislocation direction was 35° inferior from the anterior-posterior glenoid axis based on the deforming force role of the pectoralis major. The force and energy to dislocate were recorded.

RESULTS

Changes in glenoid version manifested a linear effect on the dislocation force. The energy to dislocate increased as a second-order polynomial as a function of increasing glenoid retroversion. Glenoid version of +10° anteversion and -15° retroversion was highly unstable, resulting in spontaneous dislocation in one-quarter (10/40) and one-half (25/40) of the specimens anteriorly and posteriorly, respectively, in the absence of an applied dislocation force. The greater tuberosity was observed to engage with the anterior glenoid rim, consistent with Hill-Sachs lesions, 40% more frequently when the dislocation direction was true anterior compared with 35° inferior from the anterior-posterior glenoid axis. The engagement of the greater tuberosity caused an increase in the energy required to dislocate.

CONCLUSION

Glenoid version has a direct effect on the force required for a dislocation. An anterior-inferior dislocation direction requires less energy for a dislocation and results in a lower risk of the development of a Hill-Sachs lesion than a direct anterior dislocation direction.

CLINICAL RELEVANCE

Consideration should be given to glenoid version when choosing a surgical treatment option for anterior shoulder instability.

Biomechanical Effect of Thickness and Tension of Fascia Lata Graft on Glenohumeral Stability for Superior Capsule Reconstruction in Irreparable Supraspinatus Tears

PURPOSE

To investigate the effects of graft length and thickness on shoulder biomechanics after superior capsule reconstruction.

METHODS

Subacromial peak contact pressure and glenohumeral superior translation were measured at 0°, 30°, and 60° of glenohumeral abduction in 8 fresh-frozen cadaveric shoulders under 5 conditions: (1) intact shoulder; (2) irreparable supraspinatus tendon tear, (3) superior capsule reconstruction with a fascia lata allograft 4-mm thick and 15 mm longer than the distance from the superior glenoid to the lateral edge of the greater tuberosity, as determined during placement at 30° of glenohumeral abduction; (4) superior capsule reconstruction with a fascia lata allograft 8-mm thick and with the same 15 mm relative length determined at 10° of glenohumeral abduction, and (5) superior capsule reconstruction with a fascia lata allograft 8-mm thick and with the 15-mm relative length determined at 30° of glenohumeral abduction. To investigate the effect of graft thickness, we compared the data from conditions 1, 2, 3, and 5. To assess the effect of graft length, we compared conditions 1, 2, 4, and 5.

RESULTS

With superior capsule reconstruction using a 4-mm graft, subacromial peak contact pressure (but not superior translation) was significantly lower than with irreparable supraspinatus tears (at 0° abduction: 259% decrease; P = .0002; at 30° abduction: 113% decrease; P = .01). The superior capsule reconstruction using an 8-mm graft significantly decreased both subacromial peak contact pressure (at 0° abduction: 246% decrease, P = .0002; at 30° abduction: 158% decrease; P = .0008; at 60° abduction: 57% decrease; P = .04) and superior translation (at 0° abduction: 135% decrease; P = .02; at 30° abduction; 130% decrease; P = .004). Graft length with placement at 10° glenohumeral abduction was 5 mm greater than that at 30° abduction. The 8-mm superior capsule reconstruction performed at 10° or 30° of glenohumeral abduction significantly decreased subacromial peak contact pressure (placement at 10° and 30°: 0° abduction, P = .0002 and .0002, respectively; 30° abduction, P = .0004 and .0005, respectively; 60° abduction, P = .04 and .04, respectively) and superior translation (placement at 10° and 30°; 0° abduction, P =.04 and .02, respectively; 30° abduction, P = .02 and .004, respectively) compared with irreparable supraspinatus tears.

CONCLUSIONS

Superior capsule reconstruction normalized the superior stability of the shoulder joint when the graft was attached at 10° or 30° of glenohumeral abduction. An 8-mm-thick graft of fascia lata had greater stability than did a 4-mm-thick graft.

CLINICAL RELEVANCE

Grafts 8-mm thick and attached at 15° to 45° of shoulder abduction (equal to 10° to 30° of glenohumeral abduction) biomechanically restore shoulder stability during superior capsule reconstruction using fascia lata.

Electromyographic Analysis of the Shoulder Girdle Musculature During External Rotation Exercises

Background:

Implementation of overhead activity, a key component of many professional sports, requires an effective and balanced activation of the shoulder girdle muscles, particularly during forceful external rotation (ER) motions.

Purpose:

To identify activation strategies of 16 shoulder girdle muscles/muscle segments during common shoulder ER exercises.

Study Design:

Descriptive laboratory study.

Method:

Thirty healthy subjects were included in this study, and 16 shoulder girdle muscles/muscle segments were investigated (surface electrode: anterior, middle, and posterior deltoid; upper, middle, and lower trapezius; serratus anterior; teres major; upper and lower latissimus dorsi; and upper and lower pectoralis major; fine wire electrodes: supraspinatus, infraspinatus, subscapularis, and rhomboid major) using a telemetric electromyography (EMG) system. Five ER exercises (standing ER at 0° and 90° of abduction, with underarm towel roll, prone ER at 90° of abduction, side-lying ER with underarm towel) were studied. Exercise EMG amplitudes were normalized to EMG at maximum ER force in a standard position. Univariate analysis of variance and post hoc analysis applied on EMG activity of each muscle were used to assess the main effect of the exercise condition.

Results:

Muscular activity differed significantly among the ER exercises (P < .05 to P < .001). The greatest activation for anterior and middle deltoid, supraspinatus, upper trapezius, and serratus anterior occurred during standing ER at 90° of abduction; for posterior deltoid, middle trapezius, and rhomboid during side-lying ER with underarm towel; for lower trapezius, upper and lower latissimus dorsi, subscapularis, and teres major during prone ER at 90° of abduction; and for the clavicular and sternal part of the pectoralis major during standing ER with underarm towel.

Conclusion:

Key glenohumeral and scapular muscles can be optimally activated during specific ER exercises, particularly in positions that stimulate athletic overhead motions.

Clinical Relevance:

These results enable sports medicine professionals to target specific muscles during shoulder rehabilitation protocols while minimizing the effect of others, providing a foundation for optimal evidence-based exercise prescription. They also provide information for tailored muscle training and injury prevention in overhead sports.

The effects of finger extension on shoulder muscle activity

[Purpose] This study aims to examine the effects of the extension of the fingers (distal upper limb) on the activity of the shoulder muscles (proximal upper limb). [Subjects and Methods] This study involved 14 healthy male adults with no musculoskeletal disorder or pain related to the shoulders and hands. The subjects in a sitting posture abducted the angle of the shoulder joints at 60° and had their palms in the front direction. Electromyography (EMG) was comparatively analyzed to look at the activities of the infraspinatus (IS) and rhomboid major (RM) when the fingers were extended and relaxed. [Results] The activity of the IS was statistically significantly higher when the fingers were extended than when they were relaxed. [Conclusion] According to the result of this study, finger extension is considered to affect the muscles for connected shoulder joint stability.

Development and performance evaluation of a multi-PID muscle loading driven in vitro active-motion shoulder simulator and application to assessing reverse total shoulder arthroplasty

In vitro active shoulder motion simulation can provide improved understanding of shoulder biomechanics; however, accurate simulators using advanced control theory have not been developed. Therefore, our objective was to develop and evaluate a simulator which uses real-time kinematic feedback and closed loop proportional integral differential (PID) control to produce motion. The simulator's ability to investigate a clinically relevant variable-namely muscle loading changes resulting from reverse total shoulder arthroplasty (RTSA)-was evaluated and compared to previous findings to further demonstrate its efficacy. Motion control of cadaveric shoulders was achieved by applying continuously variable forces to seven muscle groups. Muscle forces controlling each of the three glenohumeral rotational degrees of freedom (DOF) were modulated using three independent PID controllers running in parallel, each using measured Euler angles as their process variable. Each PID controller was configured and tuned to control the loading of a set of muscles which, from previous in vivo investigations, were found to be primarily responsible for movement in the PID's DOF. The simulator's ability to follow setpoint profiles for abduction, axial rotation, and horizontal extension was assessed using root mean squared error (RMSE) and average standard deviation (ASD) for multiple levels of arm mass replacement. A specimen was then implanted with an RTSA, and the effect of joint lateralization (0, 5, 10 mm) on the total deltoid force required to produce motion was assessed. Maximum profiling error was <2.1 deg for abduction and 2.2 deg for horizontal extension with RMSE of <1 deg. The nonprofiled DOF were maintained to within 5.0 deg with RMSE <1.0 deg. Repeatability was high, with ASDs of <0.31 deg. RMSE and ASD were similar for all levels of arm mass replacement (0.73-1.04 and 0.14-0.22 deg). Lateralizing the joint's center of rotation (CoR) increased total deltoid force by up to 8.5% body weight with the maximum early in abduction. This simulator, which is the first to use closed loop control, accurately controls the shoulder's three rotational DOF with high repeatability, and produces results that are in agreement with previous investigations. This simulator's improved performance, in comparison to others, increases the statistical power of its findings and thus its ability to provide new biomechanical insights.

Biomechanical analysis of articular-sided partial-thickness rotator cuff tear and repair

BACKGROUND

Articular-sided partial-thickness rotator cuff tears are common injuries in throwing athletes. The superior shoulder capsule beneath the supraspinatus and infraspinatus tendons works as a stabilizer of the glenohumeral joint.

PURPOSE

To assess the effect of articular-sided partial-thickness rotator cuff tear and repair on shoulder biomechanics. The hypothesis was that shoulder laxity might be changed because of superior capsular plication in transtendon repair of articular-sided partial-thickness rotator cuff tears.

STUDY DESIGN

Controlled laboratory study.

METHODS

Nine fresh-frozen cadaveric shoulders were tested by using a custom shoulder-testing system at the simulated late-cocking phase and acceleration phase of throwing motion. Maximum glenohumeral external rotation angle, anterior translation, position of the humeral head apex with respect to the glenoid, internal impingement area, and glenohumeral and subacromial contact pressures were measured. Each specimen underwent 3 stages of testing: stage 1, with the intact shoulder; stage 2, after creation of articular-sided partial-thickness tears of the supraspinatus and infraspinatus tendons; and stage 3, after transtendon repair of the torn tendons by using 2 suture anchors.

RESULTS

Articular-sided partial-thickness tears did not significantly change any of the shoulder biomechanical measurements. In the simulated late-cocking phase, transtendon rotator cuff repair resulted in decreased maximum external rotation angle by 4.2° (P = .03), posterior shift of the humeral head (1.1-mm shift; P = .02), decreased glenohumeral contact pressure by 1.7 MPa (56%; P = .004), and decreased internal impingement area by 26.4 mm(2) (65%; P < .001) compared with values in the torn shoulder. In the acceleration phase, the humeral head shifted inferiorly (1.2-mm shift; P = .03 vs torn shoulder), and glenohumeral anterior translation (1.5-mm decrease; P = .03 vs torn shoulder) and subacromial contact pressure (32% decrease; P = .004 vs intact shoulder) decreased significantly after transtendon repair.

CONCLUSION

Transtendon repair of articular-sided partial-thickness supraspinatus and infraspinatus tears decreased glenohumeral and subacromial contact pressures at time zero; these changes might lead to reduced secondary subacromial and internal impingements and consequently progression to full-thickness rotator cuff tear. However, repair of the tendons decreased anterior translation and external rotation and changed the positional relationship between the humeral head and the glenoid.

CLINICAL RELEVANCE

Careful attention should be paid to shoulder laxity and range of motion when transtendon repair is chosen to treat articular-sided partial-thickness rotator cuff tears, specifically in throwing athletes.

Role of the superior shoulder capsule in passive stability of the glenohumeral joint

BACKGROUND

The shoulder capsule is the main static stabilizer of the glenohumeral joint. However, few studies specifically address the function of the superior shoulder capsule, which is usually damaged in patients with complete rotator cuff tears. Therefore, the purpose of this study was to determine the biomechanical contribution of the superior shoulder capsule to passive stability of the glenohumeral joint.

METHODS

Seven cadaveric shoulders were tested with a custom testing system. Glenohumeral translations, subacromial contact pressure, and glenohumeral external and internal rotations were quantified at 5°, 30°, and 60° of glenohumeral abduction. Data were compared among 3 conditions: (1) intact superior capsule, (2) after detaching the superior capsule from the greater tuberosity (tear model), and (3) after complete removal of the superior capsule from the greater tuberosity to the superior glenoid (defect model).

RESULTS

A tear of the superior capsule significantly (P < .05) increased anterior and inferior translations compared with those in the intact capsule. Creation of a superior capsular defect significantly (P < .05) increased glenohumeral translation in all directions, subacromial contact pressure at 30° of glenohumeral abduction, and external and internal rotations compared with those of the intact capsule.

CONCLUSION

The superior shoulder capsule plays an important role in passive stability of the glenohumeral joint. A tear in the superior capsule at the greater tuberosity, which may be seen with partial rotator cuff tears, increased anterior and inferior translations. A defect in the superior capsule, seen in massive cuff tears, increased glenohumeral translations in all directions.

Changes in Shoulder External Rotator Muscle Activity during Shoulder External Rotation in Various Arm Positions in the Sagittal Plane

[Purpose] The aim of this study was to investigate changes in electromyographic (EMG) activity of the infraspinatus and posterior deltoid muscles during shoulder external rotation under different shoulder flexion angles. [Subjects] Thirteen participants were included in this study. [Methods] The participants performed isometric shoulder external rotation at 45°, 90°, and 135° of shoulder flexion. A surface EMG system recorded the EMG activity of the infraspinatus and posterior deltoid muscles during shoulder external rotation. The changes in the muscle activity of infraspinatus and posterior deltoid and ratio of infraspinatus to posterior deltoid muscle activity were analyzed using one-way repeated-measures analysis of variance with Bonferroni’s correction. [Results] The posterior deltoid activity was significantly decreased, while the ratio of the infraspinatus to posterior deltoid activity was significantly increased at 45° of shoulder flexion compared with 90° and 135° of shoulder flexion (p < 0.05). There were no significant differences in the EMG activity of the infraspinatus among the three conditions (p > 0.05). [Conclusion] These findings indicate that shoulder external rotation at 45° of shoulder flexion effectively reduced the contribution of the posterior deltoid activation to shoulder external rotation.

Control strategies to re-establish glenohumeral stability after shoulder injury

BACKGROUND

Muscles are important "sensors of the joint instability". The aim of this study was to identify the neuro-motor control strategies adopted by patients with anterior shoulder instability during overhead shoulder elevation in two planes.

METHODS

The onset, time of peak activation, and peak magnitude of seven shoulder muscles (posterior deltoid, bilateral upper trapezius, biceps brachii, infraspinatus, supraspinatus and teres major) were identified using electromyography as 19 pre-operative patients with anterior shoulder instability (mean 27.95 years, SD = 7.796) and 25 age-matched asymptomatic control subjects (mean 23.07 years, SD = 2.952) elevated their arm above 90 degrees in the sagittal and coronal planes.

RESULTS

Temporal characteristics of time of muscle onsets were significantly different between groups expect for teres major in the coronal plane (t = 1.1220, p = 0.2646) Patients recruited the rotator cuff muscles earlier and delayed the onset of ipsilateral upper trapezius compared with subjects (p<0.001) that control subjects. Furthermore, significant alliances existed between the onsets of infraspinatus and supraspinatus (sagittal: r = 0.720; coronal: r = 0.756 at p<0.001) and ipsilateral upper trapezius and infraspinatus (sagittal: r = -0.760, coronal: r = -0.818 at p<0.001). The peak activation of all seven muscles occurred in the mid-range of elevation among patients with anterior shoulder instability whereas subjects spread peak activation of all 7 muscles throughout range. Peak magnitude of patients' infraspinatus muscle was six times higher (sagittal: t = -8.6428, coronal: t = -54.1578 at p<0.001) but magnitude of their supraspinatus was lower (sagittal: t = 36.2507, coronal: t = 35.9350 at p<0.001) that subjects.

CONCLUSIONS

Patients with anterior shoulder instability adopted a "stability before mobility" neuro-motor control strategy to initiate elevation and a "stability at all cost" strategy to ensure concavity compression in the mid-to-150 degrees of elevation in both sagittal and coronal planes.

Anterior shoulder dislocation increases the propensity for recurrence: a cadaveric study of the number of dislocations and type of capsulolabral lesion

BACKGROUND

The number of anterior shoulder dislocations that predispose to recurrence is unknown; some clinicians recommend surgical repair after the initial episode and others after multiple recurrences. The purpose of this study was to quantify the forces during successive anterior dislocations of cadaveric shoulders and to inspect the capsule and labrum afterwards, in order to assess the propensity for recurrence.

MATERIALS AND METHODS

Twenty-two human cadaveric shoulders were tested using a custom cadaveric shoulder dislocation device with simulated muscle loading. Each was positioned in the apprehension position and the humerus was moved in horizontal abduction until the shoulder dislocated. The joint reaction force was measured, as was the force that developed passively in the pectoralis major muscle. Following 3 successive dislocations, each was inspected for anterior capsulolabral lesions.

RESULTS

There was a significant decrease in force after the second dislocation. In 11, there was no labral avulsion and a significant decrease in force after the first dislocation. In the other 11, there was a labral avulsion and a significant decrease in force after the second dislocation.

CONCLUSION

Two successive anterior shoulder dislocations may increase propensity for recurrence; but this is influenced by the type of capsulolabral lesion that occurs. No labral avulsion, likely a result of capsular stretching, may be a worse prognostic finding than labral avulsion after the initial episode.

Preliminary evaluation of a robotic apparatus for the analysis of passive glenohumeral joint kinematics

Background

The shoulder has the greatest range of motion of any joint in the human body. This is due, in part, to the complex interplay between the glenohumeral (GH) joint and the scapulothoracic (ST) articulation. Currently, our ability to study shoulder kinematics is limited, because existing models isolate the GH joint and rely on manual manipulation to create motion, and have low reproducibility. Similarly, most established techniques track shoulder motion discontinuously with limited accuracy.

Methods

To overcome these problems, we have designed a novel system in which the shoulder girdle is studied intact, incorporating both GH and ST motions. In this system, highly reproducible trajectories are created using a robotic actuator to control the intact shoulder girdle. High-speed cameras are employed to track retroreflective bone markers continuously.

Results

We evaluated this automated system’s capacity to reproducibly capture GH translation in intact and pathologic shoulder conditions. A pair of shoulders (left and right) were tested during forward elevation at baseline, with a winged scapula, and after creation of a full thickness supraspinatus tear.

Discussion

The system detected differences in GH translations as small as 0.5 mm between different conditions. For each, three consecutive trials were performed and demonstrated high reproducibility and high precision.

The measurement of joint mechanics and their role in osteoarthritis genesis and progression

Mechanics play a role in the initiation and progression of osteoarthritis. However, our understanding of which mechanical parameters are most important, and what their impact is on the disease, is limited by the challenge of measuring the most important mechanical quantities in living subjects. Consequently, comprehensive statements cannot be made about how mechanics should be modified to prevent, slow or arrest osteoarthritis. Our current understanding is based largely on studies of deviations from normal mechanics caused by malalignment, injury, and deformity. Some treatments for osteoarthritis focus on correcting mechanics, but there appears to be scope for more mechanically based interventions.

Effect of lateral offset center of rotation in reverse total shoulder arthroplasty: a biomechanical study

BACKGROUND

Lateral offset center of rotation (COR) reduces the incidence of scapular notching and potentially increases external rotation range of motion (ROM) after reverse total shoulder arthroplasty (rTSA). The purpose of this study was to determine the biomechanical effects of changing COR on abduction and external rotation ROM, deltoid abduction force, and joint stability.

MATERIALS AND METHODS

A biomechanical shoulder simulator tested cadaveric shoulders before and after rTSA. Spacers shifted the COR laterally from baseline rTSA by 5, 10, and 15 mm. Outcome measures of resting abduction and external rotation ROM, and abduction and dislocation (lateral and anterior) forces were recorded.

RESULTS

Resting abduction increased 20° vs native shoulders and was unaffected by COR lateralization. External rotation decreased after rTSA and was unaffected by COR lateralization. The deltoid force required for abduction significantly decreased 25% from native to baseline rTSA. COR lateralization progressively eliminated this mechanical advantage. Lateral dislocation required significantly less force than anterior dislocation after rTSA, and both dislocation forces increased with lateralization of the COR.

CONCLUSION

COR lateralization had no influence on ROM (adduction or external rotation) but significantly increased abduction and dislocation forces. This suggests the lower incidence of scapular notching may not be related to the amount of adduction deficit after lateral offset rTSA but may arise from limited impingement of the humeral component on the lateral scapula due to a change in joint geometry. Lateralization provides the benefit of increased joint stability, but at the cost of increasing deltoid abduction forces.

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.

Effect of deltoid tension and humeral version in reverse total shoulder arthroplasty: a biomechanical study

BACKGROUND

No clear recommendations exist regarding optimal humeral component version and deltoid tension in reverse total shoulder arthroplasty (TSA).

MATERIALS AND METHODS

A biomechanical shoulder simulator tested humeral versions (0°, 10°, 20° retroversion) and implant thicknesses (-3, 0, +3 mm from baseline) after reverse TSA in human cadavers. Abduction and external rotation ranges of motion as well as abduction and dislocation forces were quantified for native arms and arms implanted with 9 combinations of humeral version and implant thickness.

RESULTS

Resting abduction angles increased significantly (up to 30°) after reverse TSA compared with native shoulders. With constant posterior cuff loads, native arms externally rotated 20°, whereas no external rotation occurred in implanted arms (20° net internal rotation). Humeral version did not affect rotational range of motion but did alter resting abduction. Abduction forces decreased 30% vs native shoulders but did not change when version or implant thickness was altered. Humeral center of rotation was shifted 17 mm medially and 12 mm inferiorly after implantation. The force required for lateral dislocation was 60% less than anterior and was not affected by implant thickness or version.

CONCLUSION

Reverse TSA reduced abduction forces compared with native shoulders and resulted in limited external rotation and abduction ranges of motion. Because abduction force was reduced for all implants, the choice of humeral version and implant thickness should focus on range of motion. Lateral dislocation forces were less than anterior forces; thus, levering and inferior/posterior impingement may be a more probable basis for dislocation (laterally) than anteriorly directed forces.

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.

Imaging the role of biomechanics in osteoarthritis

Osteoarthritis is widely believed to result from local mechanical factors acting within the context of systemic susceptibility. This narrative review delineates current understanding of the etiopathogenesis of osteoarthritis and more specifically examines the critical role of biomechanics in disease pathogenesis. There are several ways the mechanical forces across the joint can be measured, including some that rely heavily on imaging methods. These are described and methods to advance the field are proposed.

The measurement of joint mechanics and their role in osteoarthritis genesis and progression

Mechanics play a role in the initiation, progression, and successful treatment of osteoarthritis. However, we don't yet know enough about which specific mechanical parameters are most important and what their impact is on the disease process to make comprehensive statements about how mechanics should be modified to prevent, slow, or arrest the disease process. The objectives of this review are (1) to summarize methods for assessing joint mechanics and their relative merits and limitations, (2) to describe current evidence for the role of mechanics in osteoarthritis initiation and progression, and (3) to describe some current treatment approaches that focus on modifying joint mechanics.

Biomechanics of open Bankart and coracoid abutment procedures in a human cadaveric shoulder model

The specific aims of this experiment were (1) to develop a clinically relevant model of anteroinferior shoulder dislocation in the apprehension position to compare the biomechanics of the intact anterior capsuloligamentous structures, and (2) to evaluate the initial strength of an open Bankart and of a coracoid abutment procedure. Fifteen shoulders from deceased donors were used. For the intact shoulders, mean peak load was 486 N, and stiffness was 26,7 N/mm. For the Bankart repair, the mean peak load was 264 N, and mean stiffness was 14.1 N/mm. Transosseous repairs failed by suture pullout through soft tissues. For the coracoid abutment repair, the mean peak load was 607 N and stiffness was 25.57 N/mm. This study reveals that the biomechanical performance of the Bankart and coracoid abutment repairs fails to reproduce the properties of the natural intact state.

Moment arms of the muscles crossing the anatomical shoulder

The objective of the present study was to determine the instantaneous moment arms of 18 major muscle sub-regions crossing the glenohumeral joint during coronal-plane abduction and sagittal-plane flexion. Muscle moment-arm data for sub-regions of the shoulder musculature during humeral elevation are currently not available. The tendon-excursion method was used to measure instantaneous muscle moment arms in eight entire upper-extremity cadaver specimens. Significant differences in moment arms were reported across sub-regions of the deltoid, pectoralis major, latissimus dorsi, subscapularis, infraspinatus and supraspinatus (P < 0.01). The most effective abductors were the middle and anterior deltoid, whereas the most effective adductors were the teres major, middle and inferior latissimus dorsi (lumbar vertebrae and iliac crest fibers, respectively), and middle and inferior pectoralis major (sternal and lower-costal fibers, respectively). In flexion, the superior pectoralis major (clavicular fibers), anterior and posterior supraspinatus, and anterior deltoid were the most effective flexors, whereas the teres major and posterior deltoid had the largest extensor moment arms. Division of multi-pennate shoulder muscles of broad origins into sub-regions highlighted distinct functional differences across those sub-regions. Most significantly, we found that the superior sub-region of the pectoralis major had the capacity to exert substantial torque in flexion, whereas the middle and inferior sub-regions tended to behave as a stabilizer and extensor, respectively. Knowledge of moment arm differences between muscle sub-regions may assist in identifying the functional effects of muscle sub-region tears, assist surgeons in planning tendon reconstructive surgery, and aid in the development and validation of biomechanical computer models used in implant design.

Consequences of a Perthes-Bankart lesion in twenty cadaver shoulders

This study investigated whether an anteroinferior capsulolabral lesion is sufficient to allow the humeral head to dislocate and whether a limited inferior approach for creating the lesions influenced the results compared with an all-arthroscopic approach. Four ligamentous zones of the glenohumeral capsule were sequentially detached from the glenoid neck and labrum in 20 cadaver shoulders through an inferior approach. Before and after each resection step, inferior stability was tested using a sulcus test and anterior stability using a drawer test and an apprehension maneuver. Dislocation was only possible when at least 3 zones were cut. This study confirmed that superior and posterior extension of the classic anteroinferior Perthes-Bankart lesion is necessary before the capsular restraint in external rotation and abduction is overcome and dislocation occurs. Lesions other than the Perthes-Bankart need to be investigated when recurrent dislocation is treated, because this anteroinferior injury is most probably not the sole factor responsible for the instability.

In vivo articular cartilage contact at the glenohumeral joint: preliminary report

BACKGROUND

Little is known about normal in vivo mechanics of the glenohumeral joint. Such an understanding would have significant implications for treating disease conditions that disrupt shoulder function. The objective of this study was to determine articular contact locations between the glenoid and humeral articular surfaces in normal subjects during shoulder abduction with neutral, internal, and external rotations. We hypothesized that glenohumeral articular contact is not perfectly centered and is variable in normal subjects tested under physiological loading conditions.

METHODS

Orthogonal fluoroscopic images and magnetic resonance image-based computer models were used to characterize the centroids of articular cartilage contact of the glenohumeral joint at various static, actively stabilized abduction and rotation positions in five healthy shoulders. The shoulder was investigated at 0 degrees , 45 degrees , and 90 degrees abduction with neutral rotation and then at 90 degrees abduction combined with active maximal external rotation and active maximal internal rotation.

RESULTS

For all the investigated positions, the centroid of contact on the glenoid surface for each individual, on average, was more than 5 mm away from the geometric center of the glenoid articular surface. Intersubject variation of the centroid of articular contact on the glenoid surface was observed with each investigated position, and 90 degrees abduction with maximal internal rotation showed the least variability. On the humeral head surface, the centroids of contact were located at the superomedial quarter for all investigated positions, except in two subjects' positions at 0 degrees abduction, neutral rotation.

CONCLUSIONS

The data showed that the in vivo glenohumeral contact locations were variable among subjects, but in all individuals they were not at the center of the glenoid and humeral head surfaces. This confirms that "ball-in-socket" kinematics do not govern normal shoulder function. These insights into glenohumeral articular contact may be relevant to an appreciation of the consequences of pathology such as rotator cuff disease and instability.

Initial fixation strength of massive rotator cuff tears: in vitro comparison of single-row suture anchor and transosseous tunnel constructs

PURPOSE

The purpose of this study was to compare the in vitro repair integrity of massive rotator cuff tears fixed with transosseous tunnel and single-lateral row suture anchor techniques.

METHODS

A 5 x 2-cm crescent-shaped rotator cuff tear was created in 6 matched pairs of cadaveric shoulders. Paired shoulders were repaired with 3 transosseous tunnels and 6 Mason-Allen sutures or with 3 screw-in suture anchors and 6 simple sutures. The repairs were cyclically loaded at physiologic forces along the respective directions of pull when the arm was in 90 degrees of scapular plane elevation. Gap formation and repair displacements were monitored with digital video imaging at 3 sites for each repair.

RESULTS

There was no significant difference between the maximal gapping of the repair constructs. After 4,000 cycles, the mean maximal gapping at any position along the repair was 6.2 +/- 2.99 mm in the transosseous tunnel construct and 4.9 +/- 1.27 mm in the suture anchor repair construct (P = .40). Gapping was significantly less in the anterior region when compared with the posterior region of the repair (P = .015).

CONCLUSIONS

There is no difference in cyclic loading of transosseous and single-row suture anchor repair techniques. Significantly greater gap formation occurs at the posterior aspect of repairs of massive rotator cuff tears in this in vitro model.

CLINICAL RELEVANCE

Initial fixation strength of single-row suture anchor repairs is equivalent to that of transosseous repairs. Further research is required to determine the unknown clinical significance of increased posterior repair gap formation.

The effect of muscle loading on the kinematics of in vitro glenohumeral abduction

This in vitro study evaluated the effects of four different muscle-loading ratios on active glenohumeral joint abduction. Eight cadaveric shoulders were tested using a shoulder simulator designed to reproduce unconstrained abduction of the humerus via computer-controlled pneumatic actuation. Forces were applied to cables that were sutured to tendons or fixed to bone, to simulate loading of the supraspinatus, subscapularis, infraspinatus/teres minor, and anterior, middle, and posterior deltoid muscles. Four sets of muscle-loading ratios were employed, based on: (1) equal loads, (2) average physiological cross-sectional areas (pCSAs), (3) constant values of the product of electromyographic (EMG) data and pCSAs, and (4) variable ratios of the EMG and pCSA data which changed as a function of abduction angle. The investigator generated passive motions with no muscle loads simulated. Repeatability was quantified by five successive trials of the passive and simulated active motions. There was improved repeatability in the simulated active motions versus passive motions, significant for abduction angles less than 40 degrees (p=0.02). No difference was found in the repeatability of the four different muscle-loading ratios for simulated active motions (p0.067 for all angles). The improved repeatability of active over passive motion suggests simulated active motion should be employed for in vitro simulations of shoulder motion.

Simulated capsulolabral lesion in cadavers: dislocation does not result from a bankart lesion only

PURPOSE

Although an anteroinferior capsulolabral detachment (typical Bankart lesion) has been evaluated in other experimental studies, it has not yet been tested with an apprehension test in an intact shoulder model.

METHODS

Adjacent combinations of 4 zones of the capsuloligamentous complex were sequentially detached from the glenoid neck in 50 cadaveric shoulders. Stability was tested before and after each resection step: inferior stability with a sulcus test and anterior stability with an anterior drawer test and with a load-and-shift test in the apprehension position.

RESULTS

A metastable anteroinferior dislocation occurred in 18 specimens after section of 3 zones and in 14 only after section of 4 zones. A locked dislocation occurred after section of all 4 zones in 33 specimens and in the other 17 shoulders only after the posterior capsule was also cut.

CONCLUSIONS

The humeral head cannot dislocate anteroinferiorly when there only is a Bankart lesion. In our study superior and posterior extension was necessary before the tensioning mechanism in external rotation and abduction failed enough for dislocation to occur.

CLINICAL RELEVANCE

Because the Bankart lesion is most likely not the only lesion present in patients with recurrent dislocation, a careful search for other lesions needs to be done when one is attempting surgical treatment. These lesions would need to be treated as well if one wants to avoid the risk of residual instability.

Anatomical glenoid reconstruction for recurrent anterior glenohumeral instability with glenoid deficiency using an autogenous tricortical iliac crest bone graft

BACKGROUND

Anterior shoulder instability associated with severe glenoid bone loss is rare, and little has been reported on this problem. Recent biomechanical and anatomical studies have suggested guidelines for bony reconstruction of the glenoid.

HYPOTHESIS

Anatomical glenoid reconstruction will restore stability in shoulders with recurrent anterior instability owing to glenoid bone loss.

STUDY DESIGN

Case series; Level of evidence, 4.

METHODS

Eleven cases of traumatic recurrent anterior instability that required bony reconstruction for severe anterior glenoid bone loss were reviewed. In all cases, the length of the anterior glenoid defect exceeded the maximum anteroposterior radius of the glenoid based on preoperative assessment by 3-dimensional CT scan. Surgical reconstruction was performed using an intra-articular tricortical iliac crest bone graft contoured to reestablish the concavity and width of the glenoid. The graft was fixed with cannulated screws in combination with an anterior-inferior capsular repair.

RESULTS

At mean follow-up of 33 months, the mean American Shoulder and Elbow Surgeons score was 94, compared with a preoperative score of 65. The University of California, Los Angeles score improved to 33 from 18. The Rowe score improved to 94 from a preoperative score of 28. The mean motion loss compared with the contralateral, normal shoulder was 7 degrees of flexion, 14 degrees of external rotation in abduction, and one spinous process level for internal rotation. All patients returned to preinjury levels of sport, and only 2 complained of mild pain with overhead sports activities. No patients reported any recurrent instability (dislocation or subluxation). The CT scans with 3-dimensional reconstructions obtained 4 to 6 months postoperatively demonstrated union of the bone graft with incorporation along the anterior glenoid rim and preservation of joint space.

CONCLUSION

Anatomical reconstruction of the glenoid with autogenous iliac crest bone graft for recurrent glenohumeral instability in the setting of bone deficiency is an effective form of treatment for this problem.

Unreduced chronic dislocation of the humeral head with ipsilateral humeral shaft fracture: a case report

The purpose of reporting this case is to illustrate a treatment plan for a chronically anteriorly dislocated shoulder associated with an ipsilateral humerus fracture, a condition heretofore not addressed in the literature to our knowledge. An 18-year-old female, left hand dominant, injured her left upper extremity and liver in a motor vehicle accident. X-rays at time of injury revealed a diaphyseal facture of her left humerus. No x-rays of the shoulder were taken at time of injury. Treatment consisted of a plaster cast application and discharge at 1 week. The patient was seen again 4 weeks postinjury, at which time only humerus films were taken and the immobilization was continued. At 45 days postinjury, the patient complained of left shoulder pain, and shoulder x-rays at that time revealed an anterior subcoracoid dislocation of the left humeral head. At surgery 52 days postinjury, the humeral shaft fracture was found to be unstable and external fixation of both the fracture (2 pins above and below the fracture) and the reduced but still unstable humeral head was performed (a pin through the humeral head into the glenoid). The external fixator was removed at 3 weeks, and at a 3-year follow-up, the patient had acquired nearly full range of motion of her shoulder without pain and no significant limitations of her arm movements or activities. In conclusion, given a patient with a chronic anteriorly dislocated shoulder and a healing ipsilateral shaft fracture, an external fixation stabilization of both the fracture and the relocated repaired dislocation is a viable treatment option.

Reliability of palpation of humeral head position in asymptomatic shoulders

The purpose of this study was to determine if, within a normal population: (1) palpation of the humeral head, relative to the acromion, in three static positions, was a reliable technique (2) there was a difference in humeral head position between the dominant and non-dominant shoulders in the three positions (3) there was a difference in humeral head position relative to the acromion between the arm at side (AS), the 90 degrees abduction/external rotation (AER) and 90 degrees abduction/internal rotation (AIR) positions. This test-retest study recorded palpation landmarks using a standardized protocol. Intra-tester reliability was above 0.8 for both AS and AER and all other ICCs were below 0.6. There was no systematic difference between dominant and non-dominant sides in any of the three positions (AS P=0.408, AER P=0.448, AIR P=0.233). There was a significant difference in measurements between each position (P<0.001). It can be concluded that, palpation of humeral head position in relation to the acromion is a reliable technique in the AS position. These normative data provide a baseline that can be used for future comparison if differences are found to exist in subgroups with pathological shoulder conditions where larger glenohumeral translations are thought to exist.

Quantitation of ligament laxity in anterior shoulder instability: an experimental cadaver model

Three groups of cadaver specimens were studied. In group 1 (20 shoulders) glenohumeral ligaments were detached from the humerus until a permanent dislocation of the humeral head occurred in abduction plus external rotation. On the dislocated joint the ligament was reconstructed using a fascia lata lengthening plasty. After the plasty had been completed, the shoulder was reduced and instability checked in the same position. Then the capsule (including the plasty) was harvested and measured. In group 2 (20 shoulders), after the plasty had been completed in the same conditions as above, the capsule was progressively reduced by 2 mm steps until the instability disappeared. Then the capsule (including the plasty) was harvested and measured. In group 3 (12 shoulders), measurements of the head and of the capsule were done. To dislocate the shoulder the section of the three glenohumeral ligaments was required. Lengthening of the capsule in group 1 was 240-250%. In all cases shortening of the capsule led to the stabilization of the shoulder. After stabilization of the shoulder was reached a residual lengthening of 175-185% was recorded. In 3 out of 4 shoulders the amount of capsule shortening required to return to a stable shoulder was between 16 and 18 mm. This experiment did not reproduce the Bankart lesion; therefore it only concerns atraumatic instability. The main limitation of this model is the low lever force that may be used to dislocate the shoulder; consequently the elasticity of the glenohumeral ligament was not taken in account. The experimental values were likely over-estimated. Nevertheless the present results provide useful information for building an experimental model of atraumatic instability of the shoulder.