Glenohumeral mechanics: a study of articular geometry, contact, and kinematics

Benefits of Training the Iron Cross With Herdos Devices and External Load Added to Body Weight for Young Nonachiever Gymnasts

PURPOSE

To verify whether training the iron cross (IC) with assistive devices (herdos; HIC) and added external load (LHIC) to equate the moments of force developed on the rings could be considered an intermediate step between the nonoverloaded herdos situation (HIC) and the IC performed on the rings.

METHODS

Relative levels of surface electromyography (sEMG) activity were normalized with respect to a standing IC before comparing gymnasts who can perform the IC on the rings (achievers) and gymnast who cannot (nonachievers) in the 2 herdos conditions (HIC and LHIC). Seven muscles were chosen for sEMG analysis, namely, pectoralis major (PM), latissimus dorsi, teres major, lower trapezius, serratus anterior, biceps brachii (BB), and triceps brachii. Additionally, 3 indices were calculated to measure levels of coactivation: Elbowidx, Scapulaidx, and Shoulderidx.

RESULTS

The bigger magnitude of differences in sEMG activity among situations was found for the PM and BB (F ≥ 30.7; P < .001). When comparing the global and the PM, teres major, BB, and triceps brachii activity across groups, nonachievers activated their musculature to a greater extent than the achievers independently of the herdos situation (P ≤ .046). Achievers' Elbowidx was the only index that was significantly higher (P ≤ .005) in the IC in comparison to LHIC and HIC.

CONCLUSION

sEMG activity of PM and BB was particularly sensitive between situations, independently of the level of achievement. We recommend training the IC by adding external load in the herdos situation to increase muscle activity to levels closer to the rings situation but avoiding the potential factor of injuries.

Contact Mechanics of Elliptical and Spherical Head Implants during Axial Rotation in Anatomic Total Shoulder Arthroplasty: A Biomechanical Comparison

BACKGROUND

Elliptical humeral head implants have been proposed to result in more anatomic kinematics following total shoulder arthroplasty (aTSA). The purpose of this study was to compare glenohumeral contact mechanics during axial rotation using spherical and elliptical humeral head implants in the setting of aTSA.

METHODS

Seven fresh-frozen cadaveric shoulders were utilized for biomechanical testing in neutral (NR), internal (IR), and external (ER) rotation at various levels of abduction (0°, 15°, 30°, 45°, 60°) with lines of pull along each of the rotator cuff muscles. Each specimen underwent the following three conditions: (1) native, and TSA using (2) an elliptical and (3) spherical humeral head implant. Glenohumeral contact mechanics, including contact pressure (CP; kPa), peak contact pressure (PCP; kPa), and contact area (CA; mm2), were measured in neutral rotation as well as external and internal rotation using a pressure mapping sensor.

RESULTS

Elliptical head implants showed a significantly lower PCP in ER compared to spherical implants at 0° (Δ-712.0 kPa; p = 0.034), 15° (Δ-894.9 kPa; p = 0.004), 30° (Δ-897.7 kPa; p = 0.004), and 45° (Δ-796.9 kPa; p = 0.010) of abduction, while no significant difference was observed in ER at 60° of abduction or at all angles in NR and IR. Both implant designs had similar CA in NR, ER, and IR at all tested angles of abduction (p > 0.05, respectively).

CONCLUSIONS

In the setting of aTSA, elliptical heads showed significantly lower PCP during ER at 0° to 45° of abduction, when compared to spherical head implants. However, in NR and IR, PCP was similar between implant designs. Both designs showed similar CA during NR, ER, and IR at all abduction angles.

LEVEL OF EVIDENCE

basic science; controlled laboratory study.

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.

Empirical joint contact mechanics: A comprehensive review

Empirical joint contact mechanics measurement (EJCM; e.g. contact area or force, surface velocities) enables critical investigations of the relationship between changing joint mechanics and the impact on surface-to-surface interactions. In orthopedic biomechanics, understanding the changes to cartilage contact mechanics following joint pathology or aging is critical due to its suggested role in the increased risk of osteoarthritis (OA), which might be due to changed kinematics and kinetics that alter the contact patterns within a joint. This article reviews and discusses EJCM approaches that have been applied to articulating joints such that readers across different disciplines will be informed of the various measurement and analysis techniques used in this field. The approaches reviewed include classical measurement approaches (radiographic and sectioning, dye staining, casting, surface proximity, and pressure measurement), stereophotogrammetry/motion analysis, computed tomography (CT), magnetic resonance imaging (MRI), and high-speed videoradiography. Perspectives on approaches to advance this field of EJCM are provided, including the value of considering relative velocity in joints, tractional stress, quantification of joint contact area shape, consideration of normalization techniques, net response (superposition) of multiple input variables, and establishing linkages to regional cartilage health status. EJCM measures continue to provide insights to advance our understanding of cartilage health and degeneration and provide avenues to assess the efficacy and guide future directions of developing interventions (e.g. surgical, biological, rehabilitative) to optimize joint's health and function long term.

Physical Therapy for the Treatment of Shoulder Instability

Shoulder instability is the separation of the humeral head from the glenoid. Injury to the static and dynamic stabilizers can result in instability. Anterior shoulder instability is the predominant form of instability. It is usually a result of trauma. Posterior shoulder instability often presents with an insidious onset of pain. Multidirectional instability of the shoulder is symptomatic laxity in more than one plane of motion. The primary goal of rehabilitation is to restore pain-free mobility, strength, and functioning. Rehabilitation implements range of motion and strengthening exercises to restore proprioceptive control and scapular kinematics.

Rotational range of motion of elliptical and spherical heads in shoulder arthroplasty: a dynamic biomechanical evaluation

INTRODUCTION

Elliptical shape humeral head prostheses have been proposed to reflect a more anatomic shoulder replacement. Its effect on the rotational range of motion (ROM) compared to a standard spherical head is still not understood. The purpose was to investigate if there would be a difference in rotational ROM when comparing elliptical and spherical prosthetic heads in a dynamic shoulder model. The authors hypothesized that the use of elliptical heads would result in significantly more rotational ROM compared to the spherical head design.

MATERIALS AND METHODS

Six fresh-frozen, cadaveric shoulders were evaluated using a dynamic shoulder model. After being tested in the native condition, each specimen underwent 6 conditions in the hemiarthroplasty state: (1) matched-fit spherical head, (2) oversized spherical head, (3) undersized spherical head, (4) matched-fit elliptical head, (5) oversized elliptical head, and (6) undersized elliptical head. Following conversion to total shoulder arthroplasty (TSA), the 6 prior conditions were rerun. Each condition was tested at 0°, 30° and 60° of glenohumeral abduction. Rotational ROM was quantified using 3-dimensional tracking, while dynamically applying alternating forces for internal and external rotation via the rotator cuff tendons.

RESULTS

Elliptical and spherical prosthetic heads showed no significant difference in the degree of the total, internal, and external rotational ROM for both the hemiarthroplasty and TSA state. Conversion from hemiarthroplasty to TSA resulted in less degree of total rotational ROM for both head designs in all abduction positions, without reaching statistical significance. There was a significant decrease in total, internal, and external rotational ROM for both elliptical and spherical heads in every replacement condition, when comparing 0° to 30° and 60° of abduction (P < 0.05, respectively).

CONCLUSION

In a dynamic shoulder model, elliptical and spherical prosthetic head designs showed no significant difference in the degree of the total, internal, and external rotational ROM in both the hemiarthroplasty and TSA state.

LEVEL OF EVIDENCE

Controlled laboratory study.

Shoulder Positioning during Superior Capsular Reconstruction: Computational Analysis of Graft Integrity and Shoulder Stability

Simple Summary

In arthroscopic superior capsular reconstruction (ASCR) in irreparable rotator cuff tears (IRCTs), a graft is positioned and fixed between the superior rim of the glenoid and the humeral supraspinatus footprint. The fixation of the graft aims to restore the stability and improve the kinematics of the shoulder. The shoulder position during fixation of the graft may be a key factor impacting the outcome of ASCR; however, biomechanical evidence is lacking, as most studies addressing ASCR have been conducted in cadavers. In this study, graft strain and glenohumeral joint reaction force, estimated using a 3-D musculoskeletal model of the upper limb, were used to evaluate graft integrity and shoulder stability, respectively. The results suggest that ASCR significantly improved shoulder stability compared to the preoperative condition; however, the shoulder positions of fixation associated with the greatest improvements were also associated with the highest risk of compromising the integrity of the graft due to high strains. This study provides new and important information regarding the role of shoulder positioning during fixation of the graft.

Abstract

The shoulder position during fixation of the graft may be a key factor impacting the outcome of arthroscopic superior capsular reconstruction (ASCR) in irreparable rotator cuff tears (IRCTs). However, biomechanical evidence regarding this effect is lacking. The aim of this study was to evaluate the influence of the shoulder position during fixation of the graft on shoulder stability and graft tear risk in ASCR. A 3-D musculoskeletal model of the upper limb was modified to account for the fixation of the graft in ASCR, assuming a full-thickness tear of the supraspinatus tendon. The concomitant tenotomy of the long head of the biceps (LHB) tendon was also studied. The biomechanical parameters evaluated included the strain of the graft and the glenohumeral joint reaction force (GH JRF), which were used to evaluate graft integrity and shoulder stability, respectively. Fixation of the graft considering abduction angles greater than 15° resulted in a high risk for graft tearing when the arm was adducted to the side of the trunk. For abduction angles below 15°, the mean shoulder stability improved significantly, ranging between 6% and 20% (p < 0.001), compared with that in the preoperative condition. The concomitant tenotomy of the LHB tendon resulted in loss of stability when compared to ASCR with an intact LHB tendon. The position of the shoulder during fixation of the graft has a significant effect on shoulder stability and graft tear risk after ASCR in IRCTs. This study provides new and important information regarding the role of shoulder positioning during fixation of the graft.

Comparison of Different Fixation Techniques of the Long Head of the Biceps Tendon in Superior Capsule Reconstruction for Irreparable Posterosuperior Rotator Cuff Tears: A Dynamic Biomechanical Evaluation

BACKGROUND

In the past decade, superior capsular reconstruction has emerged as a potential surgical approach in young patients with irreparable posterosuperior rotator cuff tears (RCT) and absence of severe degenerative changes. Recently, the use of locally available and biological viable autografts, such as the long head of the biceps tendon (LHBT) for SCR has emerged, with promising early results.

PURPOSE/HYPOTHESIS

The purpose of this study was to investigate the effect of using the LHBT for reconstruction of the superior capsule on shoulder kinematics, along with different fixation constructs in a dynamic biomechanical model. The authors hypothesized that each of the 3 proposed fixation techniques would restore native joint kinematics, including glenohumeral superior translation (ghST), maximum abduction angle (MAA), maximum cumulative deltoid force (cDF), and subacromial peak contact pressure (sCP).

STUDY DESIGN

Controlled laboratory study.

METHODS

Eight fresh-frozen cadaveric shoulders (mean age, 53.4 ± 14.2 years) were tested using a dynamic shoulder simulator. Each specimen underwent the following 5 conditions: (1) intact, (2) irreparable posterosuperior rotator cuff tear (psRCT), (3) V-shaped LHBT reconstruction, (4) box-shaped LHBT reconstruction, and (5) single-stranded LHBT reconstruction. MAA, ghST, cDF and sCP were assessed in each tested condition.

RESULTS

Each of the 3 LHBT techniques for reconstruction of the superior capsule significantly increased MAA while significantly decreasing ghST and cDF compared with the psRCT (P < .001 and P < .001, respectively). Additionally, the V-shaped and box-shaped techniques significantly decreased sCP (P = .009 and P = .016, respectively) compared with the psRCT. The V-shaped technique further showed a significantly increased MAA (P < .001, respectively) and decreased cDF (P = .042 and P = .039, respectively) when compared with the box-shaped and single-stranded techniques, as well as a significantly decreased ghST (P = .027) when compared with the box-shaped technique.

CONCLUSION

In a dynamic biomechanical cadaveric model, using the LHBT for reconstruction of the superior capsule improved shoulder function by preventing superior humeral migration, decreasing deltoid forces and sCP. As such, the development of rotator cuff tear arthropathy in patients with irreparable psRCTs could potentially be delayed.

CLINICAL RELEVANCE

Using a biologically viable and locally available LHBT autograft is a cost-effective, potentially time-saving, and technically feasible alternative for reconstruction of the superior capsule, which may result in favorable outcomes in irreparable psRCTs. Moreover, each of the 3 techniques restored native shoulder biomechanics, which may help improve shoulder function by preventing superior humeral head migration and the development of rotator cuff tear arthropathy in young patients with irreparable rotator cuff tears.

Anterior glenohumeral instability: Current review with technical pearls and pitfalls of arthroscopic soft-tissue stabilization

The glenohumeral joint (GHJ) allows for a wide range of motion, but is also particularly vulnerable to episodes of instability. Anterior GHJ instability is especially frequent among young, athletic populations during contact sporting events. Many first time dislocators can be managed non-operatively with a period of immobilization and rehabilitation, however certain patient populations are at higher risk for recurrent instability and may require surgical intervention for adequate stabilization. Determination of the optimal treatment strategy should be made on a case-by-case basis while weighing both patient specific factors and injury patterns (i.e., bone loss). The purpose of this review is to describe the relevant anatomical stabilizers of the GHJ, risk factors for recurrent instability including bony lesions, indications for arthroscopic vs open surgical management, clinical history and physical examination techniques, imaging modalities, and pearls/pitfalls of arthroscopic soft-tissue stabilization for anterior glenohumeral instability.

The effects of a humeral head posterior gliding strategy on changes in muscle activities of the infraspinatus during external rotation of the shoulder

Background/aims

It is important that shoulder rehabilitation programmes incorporate exercises that selectively activate the infraspinatus. In this study, changes in infraspinatus and posterior deltoid electromyography activity during a sitting external rotation exercise with and without the use of a humeral head posterior gliding strategy were investigated.

Methods

A total of 12 healthy men participated in this study. The activity of the infraspinatus and posterior deltoid was measured during the sitting external rotation exercise, performed with and without humeral head posterior gliding. Differences in the electromyography activity of the infraspinatus and posterior deltoid, with and without humeral head posterior gliding, were analysed using paired t-tests.

Results

Infraspinatus activity was significantly higher with humeral head posterior gliding than without it (P<0.001). Conversely, posterior deltoid activity was significantly lower with humeral head posterior gliding compared to without it (P<0.001).

Conclusions

Based on these results, humeral head posterior gliding is recommended for the selective activation of the infraspinatus when performing shoulder external rotation exercises, with the shoulder abducted at 90°.

Glenoid concavity has a higher impact on shoulder stability than the size of a bony defect

PURPOSE

Surgical treatment of shoulder instability caused by anterior glenoid bone loss is based on a critical threshold of the defect size. Recent studies indicate that the glenoid concavity is essential for glenohumeral stability. However, biomechanical proof of this principle is lacking. The aim of this study was to evaluate whether glenoid concavity allows a more precise assessment of glenohumeral stability than the defect size alone.

METHODS

The stability ratio (SR) is a biomechanical estimate of glenohumeral stability. It is defined as the maximum dislocating force the joint can resist related to a medial compression force. This ratio was determined for 17 human cadaveric glenoids in a robotic test setup depending on osteochondral concavity and anterior defect size. Bony defects were created gradually, and a 3D measuring arm was used for morphometric measurements. The influence of defect size and concavity on the SR was examined using linear models. In addition, the morphometrical-based bony shoulder stability ratio (BSSR) was evaluated to prove its suitability for estimation of glenohumeral stability independent of defect size.

RESULTS

Glenoid concavity is a significant predictor for the SR, while the defect size provides minor informative value. The linear model featured a high goodness of fit with a determination coefficient of R² = 0.98, indicating that 98% of the SR is predictable by concavity and defect size. The low mean squared error (MSE) of 4.2% proved a precise estimation of the SR. Defect size as an exclusive predictor in the linear model reduced R² to 0.9 and increased the MSE to 25.7%. Furthermore, the loss of SR with increasing defect size was shown to be significantly dependent on the initial concavity. The BSSR as a single predictor for glenohumeral stability led to highest precision with MSE = 3.4%.

CONCLUSION

Glenoid concavity is a crucial factor for the SR. Independent of the defect size, the computable BSSR is a precise biomechanical estimate of the measured SR. The inclusion of glenoid concavity has the potential to influence clinical decision-making for an improved and personalised treatment of glenohumeral instability with anterior glenoid bone loss.

Concomitant rotator cuff repair and instability surgery provide good patient-reported functional outcomes in patients aged 40 years or older with shoulder dislocation

Background

Recurrent anterior shoulder dislocation in patients aged ≥ 40 years is not as rare as once thought. The mechanism of instability in this patient population is different—more likely to be attributed to rotator cuff pathology—compared with that in younger individuals. With an increasingly aging active population, surgical management has a rising role in preventing morbidity associated with recurrent instability. Our purpose was to evaluate outcomes of anterior shoulder instability repair (ie, Bankart or bony Bankart repair) with and without rotator cuff repair (RCR) in patients aged ≥ 40 years.

Methods

We conducted a retrospective chart review of all patients aged ≥ 40 years who underwent surgical repair for anterior shoulder instability from 2008-2016. Patients were categorized into 4 cohorts: Bankart repair only, bony Bankart repair only, Bankart repair with concomitant RCR, and bony Bankart repair with concomitant RCR. Demographic and history-of-instability data were collected. Clinical and functional outcomes assessed included the Single Assessment Numeric Evaluation score, American Shoulder and Elbow Surgeons score, Penn Shoulder Score, visual analog scale score for pain, Western Ontario Shoulder Instability Index score, and patient satisfaction score.

Results

A total of 146 patients were included in this study, with 103 patients (71%) having ≥2-year outcome scores. Outcome scores were not significantly different among groups. For patients who underwent Bankart repair only, bony Bankart repair only, Bankart repair with RCR, and bony Bankart repair with RCR, the Single Assessment Numeric Evaluation scores were 80.8 ± 19.7, 90.0 ± 10.7, 79.3 ± 29.4, and 87.2 ± 10.6, respectively (P = .284); American Shoulder and Elbow Surgeons scores, 83.8 ± 19.7, 92.4 ± 17.4, 82.5 ± 25.6, and 85.6 ± 12.7, respectively (P = .114); Penn Shoulder Scores for function, 84.5 ± 17.9, 90.9 ± 15.3, 83.6 ± 25.1, and 95.7 ± 13.0, respectively (P = .286); and Western Ontario Shoulder Instability Index scores, 481.0 ± 519.5, 292.1 ± 414.3, 548.9 ± 690.5, and 320.6 ± 258.7, respectively (P = .713). Age at the time of surgery significantly differed between cohorts (P < .001). No patients had recurrence of instability during the study period.

Conclusion

Similar functional outcomes can be achieved in the surgical management of anterior instability in patients aged ≥ 40 years. Rotator cuff tears should be suspected and repaired in patients with anterior instability, especially those aged ≥ 50 years.

Management of Recurrent Anterior Shoulder Instability With Bipolar Bone Loss: A Systematic Review to Assess Critical Bone Loss Amounts

BACKGROUND

There is increasing evidence to suggest that the amount of glenoid bone loss to indicate bone block procedures may be lower than previously thought, particularly in the presence of a Hill-Sachs defect.

PURPOSE

To better establish treatment recommendations for anterior shoulder instability among patients with bipolar bone lesions.

STUDY DESIGN

Systematic review and meta-analysis; Level of evidence, 4.

METHODS

A systematic review of the literature was performed with PubMed, EMBASE, Cochrane Library, and Scopus databases according to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-analyses) guidelines. Studies evaluating outcomes of operative management in anterior shoulder instability that also reported glenoid bone loss in the presence of Hill-Sachs defects were included. Recurrence rates, glenoid bone loss, and humeral bone loss were pooled and analyzed with forest plots stratified by surgical procedure. Methods of quantification were analyzed for each article qualitatively.

RESULTS

Thirteen articles were included in the final analysis, with a total of 778 patients. The mean ± SD age was 24.9 ± 8.6 years. The mean follow-up was 30.1 months (range, 11-240 months). Only 13 of 408 (3.2%) reviewed bipolar bone loss articles quantified humeral and/or glenoid bone loss. Latarjet procedures had the greatest glenoid bone loss (21.7%; 95% CI, 14.8%-28.6%), followed by Bankart repairs (13.1%; 95% CI, 9.0%-17.2%), and remplissage (11.7%; 95% CI, 5.5%-18.0%). Humeral bone loss was primarily reported as percentage bone loss (22.2%; 95% CI, 13.1%-31.3% in Bankart repairs and 31.7%; 95% CI, 21.6%-41.1% in Latarjet) or as volumetric defects (439.1 mm³; 95% CI, 336.3-541.9 mm³ in Bankart repairs and 366.0 mm³; 95% CI, 258.4-475.4 mm³ in remplissage). Recurrence rates were as follows: Bankart repairs, 19.5% (95% CI, 14.5%-25.8%); remplissage, 4.4% (95% CI, 1.3%-14.0%); and Latarjet, 8.7% (95% CI, 5.0%-14.7%). Bankart repairs were associated with significantly greater recurrence of instability in included articles (P = .013).

CONCLUSION

There exists a need for universal and consistent preoperative measurement of humeral-sided bone loss. The presence of concomitant Hill-Sachs defects with glenoid pathology should warrant more aggressive operative management through use of bone block procedures. Previously established values of critical glenoid bone loss are not equally relevant in the presence of bipolar bone loss.

Electromyographic analysis in elite swimmers with shoulder pain during a functional task

The purpose of study was to determine and compare electromyographic activity of selected shoulder girdle muscles in elite swimmers with and without shoulder pain. Twelve professional swimmers with shoulder pain (mean age: 18.55 ± 3.16 years, body mass: 74.33 ± 2.91 kg and height: 179.00 ± 5.29 cm) and 12 swimmers without pain (mean age: 18.11 ± 1.61 years, body weight: 73.33 ± 6.06 kg, height: 178.33 ± 5.07 cm) were recruited. Surface electromyography signals were collected from seven upper limb muscles during a task: participants were instructed to mark points with a pen within each of the three circles counterclockwise. The normalised root-mean-square value was used to determine the muscular activation. Swimmers with shoulder pain demonstrated greater activation of the upper trapezius (pain group mean: 28.04 ± 10.37, control group mean: 13.40 ± 06.04; p = 0.002, ηp2: 0.455), serratus anterior (pain group mean: 30.78 ± 20.09, control group mean: 13.30 ± 5.52; p = 0.023, ηp2: 0.283) and latissimus dorsi (pain group mean: 27.05 ± 17.87, control group mean: 4.99 ± 3.90; p = 0.002, ηp2 : 0.450) muscles. There was no difference (p > 0.05) in the activation of the middle and lower trapezius, middle deltoid and sternocleidomastoid. The altered muscle activation patterns may contribute to the painful shoulder in elite swimmers and need to be considered within the rehabilitation interventions.

Patterns of muscle coordination during dynamic glenohumeral joint elevation: An EMG study

The shoulder relies heavily on coordinated muscle activity for normal function owing to its limited osseous constraint. However, previous studies have failed to examine the sophisticated interrelationship between all muscles. It is essential for these normal relationships to be defined as a basis for understanding pathology. Therefore, the primary aim of the study was to investigate shoulder inter-muscular coordination during different planes of shoulder elevation. Twenty healthy subjects were included. Electromyography was recorded from 14 shoulder girdle muscles as subjects performed shoulder flexion, scapula plane elevation, abduction and extension. Cross-correlation was used to examine the coordination between different muscles and muscle groups. Significantly higher coordination existed between the rotator cuff and deltoid muscle groups during the initial (Pearson Correlation Coefficient (PCC) = 0.79) and final (PCC = 0.74) stages of shoulder elevation compared to the mid-range (PCC = 0.34) (p = 0.020–0.035). Coordination between the deltoid and a functional adducting group comprising the latissimus dorsi and teres major was particularly high (PCC = 0.89) during early shoulder elevation. The destabilising force of the deltoid, during the initial stage of shoulder elevation, is balanced by the coordinated activity of the rotator cuff, latissimus dorsi and teres major. Stability requirements are lower during the mid-range of elevation. At the end-range of movement the demand for muscular stability again increases and higher coordination is seen between the deltoid and rotator cuff muscle groups. It is proposed that by appreciating the sophistication of normal shoulder function targeted evidence-based rehabilitation strategies for conditions such as subacromial impingement syndrome or shoulder instability can be developed.

The role of bone in glenohumeral stability

Shoulder stability depends on several factors, either anatomical or functional. Anatomical factors can be further subclassified under soft tissue (shoulder capsule, glenoid rim, glenohumeral ligaments etc) and bony structures (glenoid cavity and humeral head).

Normal glenohumeral stability is maintained through factors mostly pertaining to the scapular side: glenoid version, depth and inclination, along with scapular dynamic positioning, can potentially cause decreased stability depending on the direction of said variables in the different planes. No significant factors in normal humeral anatomy seem to play a tangible role in affecting glenohumeral stability.

When the glenohumeral joint suffers an episode of acute dislocation, either anterior (more frequent) or posterior, bony lesions often develop on both sides: a compression fracture of the humeral head (or Hill–Sachs lesion) and a bone loss of the glenoid rim. Interaction of such lesions can determine ‘re-engagement’ and recurrence.

The concept of ‘glenoid track’ can help quantify an increased risk of recurrence: when the Hill–Sachs lesion engages the anterior glenoid rim, it is defined as ‘off-track’; if it does not, it is an ‘on-track’ lesion. The position of the Hill–Sachs lesion and the percentage of glenoid bone loss are critical factors in determining the likelihood of recurrent instability and in managing treatment.

In terms of posterior glenohumeral instability, the ‘gamma angle concept’ can help ascertain which lesions are prone to recurrence based on the sum of specific angles and millimetres of posterior glenoid bone loss, in a similar fashion to what happens in anterior shoulder instability.

Cite this article: EFORT Open Rev 2018;3:632-640. DOI: 10.1302/2058-5241.3.180028

Biomechanical comparison of glenoid implants with adaptable and fixed backside curvatures in anatomic total shoulder arthroplasty

BACKGROUND

We evaluated the biomechanical effects and potential advantages of glenoid implants with adaptable backside curvature radii and compared them with standard implants having fixed backside curvatures in anatomic total shoulder arthroplasty (aTSA) for primary glenohumeral osteoarthritis with uniconcave glenoids.

METHODS

A glenoid implant with adaptable backside curvatures (Aequalis PerFORM, Tornier SAS, Montbonnot, France) was compared with its previous model having a fixed curvature radius. Virtual aTSAs were performed in 24 patients from preoperative shoulder computed tomography data sets, using both implants in each patient. For all 48 simulated aTSAs, we first measured the glenoid bone reaming depth, subchondral bone quality after reaming, and implant backside surface and then the predicted cement stress, bone-cement interfacial stress, and bone strain at 60° of arm abduction. These biomechanical quantities were tested for differences between adaptable and fixed implants and for correlations between preoperative measurements and postoperative predictions.

RESULTS

Adaptable glenoid implants induced a significant decrease in cement stress (P = .008), bone-cement interfacial stress (P = .045), and bone strain (P = .039), particularly for glenoids with curvature radii larger than 40 mm. However, these biomechanical effects were not significantly correlated with an increase in subchondral glenoid bone quality.

CONCLUSIONS

Our study confirms the presumed biomechanical advantages of adaptable glenoid implants, even though the effects were not directly due to the adaptation of the backside curvature radius. Benefits were more pronounced for glenoids with large curvature radii. Our initial biomechanical findings should now be corroborated with large-scale clinical studies.

New concepts of radiologic preoperative evaluation of anterior shoulder instability: on-track and off-track lesions

The shoulder is the most frequently dislocated joint in the body due to a larger range of motion and a small area of articulation between the humeral and glenoid surfaces. Traumatic shoulder dislocations, especially those associated with injury to the labroligamentous or bony stabilizers of the joint, lead to further reduction of articular surface contact with resultant glenohumeral instability and recurrent shoulder dislocations. Imaging plays an increasingly important role in the preoperative evaluation of patients with traumatic shoulder instability by evaluating glenohumeral bone loss (uni- or bipolar), assessing soft tissue injuries and identifying patients at risk of postoperative recurrence. Quantification of bone loss is key to differentiate engaging vs. non-engaging Hill-Sachs lesions, while newer concepts of "on-track" vs. "off-track" lesions are being discussed that can determine the required surgical approaches. In this article, we review the preoperative imaging approaches, traditional treatments, outline the bone loss measurement strategies and review these new tracking concepts with relevant case examples.

Evaluation of the Translation Distance of the Glenohumeral Joint and the Function of the Rotator Cuff on Its Translation: A Cadaveric Study

PURPOSE

To evaluate the distance and position of humeral head translation during glenohumeral motion and to investigate the function of the rotator cuff in glenohumeral translation.

METHODS

Using 9 cadavers, glenohumeral translation during passive pendulum motion was tracked by an optical motion capture system. Tension was applied to 5 compartments of the rotator cuff muscles, and 7 different conditions of rotator cuff dysfunction were sequentially simulated. Three-dimensional glenohumeral structure was reconstructed from the computed tomography images of the specimens, and the distance and position of glenohumeral translation were compared among the conditions.

RESULTS

The average radius of glenohumeral translation was 10.6 ± 4.3 mm when static loading was applied to all rotator cuff muscles. The radius increased significantly in the models without traction force on the supraspinatus and total subscapularis tendons (P = .030). The position of the translation center did not change in the mediolateral direction (P = .587) and in the anteroposterior direction (P = .138), but it moved significantly superiorly in the models without supraspinatus and infraspinatus loading (P = .011) and in those without supraspinatus, infraspinatus, and teres minor loading (P < .001).

CONCLUSIONS

The distance and position of humeral head translation during glenohumeral motion changed with rotator cuff deficiency. The present study indicated that the subscapularis plays an important role in maintaining the central position of the humeral head, and that the infraspinatus acts as a major depressor of the humeral head during shoulder motion.

CLINICAL RELEVANCE

The results of this study suggest that extension of a tear into the subscapularis should be avoided to maintain the centering function of the glenohumeral joint in cases with rotator cuff tear.

Controversies In The Surgical Management Of Shoulder Instability: Associated Soft Tissue Procedures

Background:

The glenohumeral joint is a ball-and-socket joint that is inherently unstable and thus, susceptible to dislocation. The traditional and most common anatomic finding is the Bankart lesion (anterior-inferior capsule labral complex avulsion), but there is a wide variety of anatomic alterations that can cause shoulder instability or may be present as a concomitant injury or in combination, including bone loss (glenoid or humeral head), complex capsule-labral tears, rotator cuff tears, Kim´s lesions (injuries to the posterior-inferior labrum) and rotator interval pathology.

Methods:

A review of articles related to shoulder anatomy and soft tissue procedures that are performed during shoulder instability arthroscopic management was conducted by querying the Pubmed database and conclusions and controversies regarding this injury were exposed.

Results:

Due to the complex anatomy of the shoulder and the large range of movement of this joint, a wide variety of anatomic injuries and conditions can lead to shoulder instability, specially present in young population. Recognizing and treating all of them including Bankart repair, capsule-labral plicatures, SLAP repair, circumferential approach to pan-labral lesions, rotator interval closure, rotator cuff injuries and HAGL lesion repair is crucial to achieve the goal of a stable, full range of movement and not painful joint.

Conclusion:

Physicians must be familiarized with all the lesions involved in shoulder instability, and should be able to recognize and subsequently treat them to achieve the goal of a stable non-painful shoulder. Unrecognized or not treated lesions may result in recurrence of instability episodes and pain while overuse of some of the techniques previously described can lead to stiffness, thus the importance of an accurate diagnosis and treatment when facing a shoulder instability.

Hypertrophy of the extra-articular tendon of the long head of biceps correlates with the location and size of a rotator cuff tear

Aims

The aim of this study was to assess hypertrophy of the extra-articular tendon of the long head of biceps (LHB) in patients with a rotator cuff tear.

Patients and Methods

The study involved 638 shoulders in 334 patients (175 men, 159 women, mean age 62.6 years; 25 to 81) with unilateral symptomatic rotator cuff tears. The cross-sectional area (CSA) of the LHB tendon in the bicipital groove was measured pre-operatively in both shoulders using ultrasound. There were 154 asymptomatic rotator cuff tears in the contralateral shoulder. Comparisons were made between those with a symptomatic tear, an asymptomatic tear and those with no rotator cuff tear. In the affected shoulders, the CSAs were compared in relation to the location and size of the rotator cuff tear.

Results

The mean CSA was 21.0 mm2 (4 to 71) in those with a symptomatic rotator cuff tear, 19.9 mm2 (4 to 75) in those with an asymptomatic rotator cuff tear and 14.1 mm2 (5 to 43) in those with no rotator cuff tear. The mean CSA in patients with both symptomatic and asymptomatic rotator cuff tears was significantly larger than in those with no rotator cuff tear (p < 0.001). In the affected shoulders, there were significant differences between patients with more than a medium sized posterosuperior cuff tear and those with an antero-superior cuff tear.

Conclusion

Regardless of the symptoms, there was significant hypertrophy of the extra-articular LHB tendon in patients with a rotator cuff tear. The values were significantly related to the size of the tear. Cite this article: Bone Joint J 2017;99-B:806–11.

Humeral head osteotomy in shoulder arthroplasty: a comparison between anterosuperior and inferoanterior resection techniques

BACKGROUND

The best chance that a shoulder arthroplasty will restore motion and muscle balance across the glenohumeral joint is by closely replicating natural articular morphology. Defining the humeral osteotomy plane along clear landmarks at the anatomic neck is critical. We hypothesized that a new osteotomy, based on alternative landmarks on the anatomic neck, would restore 3-dimensional humeral head morphology more reliably than the traditional osteotomy.

METHODS

The anatomic neck was digitized in 30 human cadaver shoulders and compared with its 3-dimensional computed tomography reconstruction. Two different osteotomy techniques were virtually performed: the traditional, following the anterosuperior anatomic neck; and a new technique, defined by the inferoanterior anatomic neck. The length-width difference and orientation (retroversion, inclination) of the resection area were compared between the techniques and with native anatomy.

RESULTS

Length-width difference of the anterosuperior resection area was higher than in the inferoanterior osteotomy (6 ± 2 mm vs. 3 ± 1 mm; P < .001). Retroversion of the anterosuperior resection plane was higher than the native head (50° ± 12° vs. 37° ± 11°; P < .001), whereas retroversion after the inferoanterior osteotomy (32° ± 12°) did not differ from native (P = .057). Inclination differed after the anterosuperior osteotomy (129° ± 5°) and the inferoanterior osteotomy (127° ± 4°) compared with the native head (134° ± 4°; P ≤ .001).

CONCLUSION

The inferoanterior referenced osteotomy generated a more circular resection area, matching the native humeral head retroversion more closely than in the anterosuperior technique. This study suggests that in shoulder arthroplasty, the humeral resection level should be referenced at the inferoanterior rather than the anterosuperior anatomic neck. Further studies should investigate the biomechanical effects of this alternative resection plane.

CT scan evaluation of glenoid bone and pectoralis major tendon: interest in shoulder prosthesis

Introduction: The shoulder arthroplasty brings satisfaction to patients in terms of quality of life and indolence. However whether anatomic implant or reverse, it does not escape from the loosening of the glenoid component. Moreover, optimal implantation is required to ensure the functional outcome without shortening of the arm. The purpose of this study is obtain CT scan evaluation of the glenoid bone stock in order to optimize glenoid component implantation and obtain a reference to determine optimal humeral component placement in case of humeral proximal fracture.

Materials and methods: Between 2010 and 2011 we have analyzed 200 intact shoulder’s CT. We measured maximal and minimal width in the transverse plane of the glenoid, the distance from the pectoralis major (PM) tendon to the humeral head, the greater tubercle, change of curvature and the anatomical neck.

Results: Mean maximum width was 27.4 ± 3.4 mm and mean minimum width was 15.5 ± 2.8 mm. Distances between upper edge of PM tendon to: humeral head, greater tubercle, change of curvature and anatomical neck were respectively: 67.6 ± 9.98 mm, 57.8 ± 10.3 mm, 28.7 ± 9 mm, and 34.2 ± 9.7 mm.

Conclusion: Our study has produced an assessment of glenoid bone stock for optimal positioning of the glenoid implant but also to obtain a reference to determine the ideal location of the humeral component in the case of proximal humerus fracture.

Distal tibia allograft for glenohumeral instability: does radius of curvature match?

BACKGROUND

A distal tibia osteochondral allograft is a potential graft option for glenoid reconstruction because the distal tibia may have a similar radius of curvature (ROC) as the glenoid. This study evaluated ROC mismatch as measured on computed tomography (CT) scans between the glenoid, distal tibia, and humeral head.

METHODS

Bilateral CT images were formatted for 10 decedents from the Office of the Medical Investigator database, giving 20 specimens per anatomic location. The ROCs of the glenoid, distal tibia, and humeral head were measured. A statistical model was generated to assess ROC mismatch of randomly paired distal tibias and glenoids.

RESULTS

The mean ± standard deviation ROC was 2.9 ± 0.25 cm for the glenoid, 2.3 ± 0.21 cm for the distal tibia, and 2.5 ± 0.12 cm for the humeral head. No differences were found in laterality, intraobserver, or interobserver measurements. The least-squares difference in the ROC between the glenoid and tibia was 0.57 cm, glenoid and humerus was 0.40 cm, and humerus and tibia was 0.17 cm. Only 22% of randomly paired distal tibias and glenoids had a difference in ROC of 0.3 cm or less.

CONCLUSION

CT measurement of the ROC of the glenoid, distal tibia, and humeral head is reliable and reproducible. The probability of obtaining a random distal tibia allograft with a similar ROC to the glenoid is low. Obtaining ROC measurements of the injured glenoid and the distal tibia allograft specimen before use for glenoid reconstruction may be useful.

Stability of the Glenohumeral Joint With Combined Humeral Head and Glenoid Defects: A Cadaveric Study

BACKGROUND

Shoulders with recurrent anterior instability often have combined bony defects of the humeral head and glenoid. Previous studies have looked at only isolated humeral head or glenoid defects.

PURPOSE/HYPOTHESIS

The aim of this study was to define the relationship of combined humeral head and glenoid defects on anterior shoulder instability. Combined bony defects will lead to increased instability compared with an isolated defect, and the "critical" size of humeral head and glenoid defects that need to be addressed to restore stability will be smaller when combined rather than isolated.

STUDY DESIGN

Controlled laboratory study.

METHODS

Eighteen shoulder specimens were tested at 60° of glenohumeral abduction and 80° of glenohumeral external rotation. Humeral head defect sizes included 6%, 19%, 31%, and 44% of the humeral head diameter. Glenoid defect sizes included 10%, 20%, and 30% of the glenoid width. Outcome measures included percentage of intact stability ratio (%ISR; the stability ratio for a given trial divided by the stability ratio in the intact state for that specimen) and percentage of intact translation (%IT; the distance to dislocation for a given trial divided by the distance to dislocation in the intact state for that specimen).

RESULTS

The decrease in %ISR reached statistical significance for humeral head defects of 44%, for glenoid defects of 30%, and for a combined 19% humeral head defect with a 20% glenoid defect (65% mean %ISR). The decrease in %IT reached statistical significance for humeral head defects ≥31%, for glenoid defects ≥20%, and for a combined 19% humeral head defect with a 10% glenoid defect (69% mean %IT).

CONCLUSION

In shoulders with combined humeral head and glenoid defects, bony reconstruction may be indicated for humeral head defects as small as 19% of the humeral head diameter and glenoid defects as small as 10% to 20% of the glenoid width, especially if the glenoid defect produces a significant loss of glenoid concavity depth.

CLINICAL RELEVANCE

In shoulders with combined humeral head and glenoid defects, bony reconstruction may be indicated for defect sizes smaller than would be indicated for either defect found in isolation.

Inclination-dependent changes of the critical shoulder angle significantly influence superior glenohumeral joint stability

BACKGROUND

The critical shoulder angle combines the acromion index and glenoid inclination and has potential to discriminate between shoulders at risk for rotator cuff tear or osteoarthritis and those that are asymptomatic. However, its biomechanics, and particularly the role of the glenoid inclination, are not yet fully understood.

METHODS

A shoulder simulator was used to analyze the independent influence of glenoid inclination during abduction from 0 to 60°. Spindle motors transferred tension forces by a cable-pulley on human cadaveric humeri. A six-degree-of-freedom force transducer was mounted directly behind the polyethylene glenoid to measure shear and compressive joint reaction force and calculate the instability ratio (ratio of shear and compressive joint reaction force) with the different force ratios of the deltoid and supraspinatus muscles (2:1 and 1:1). A stepwise change in the inclination by 5° increments allowed simulation of a critical shoulder angle range of 20° to 45°.

FINDINGS

Tilting the glenoid to cranial (increasing the critical shoulder angle) increases the shear joint reaction force and therefore the instability ratio. A balanced force ratio (1:1) between the deltoid and the supraspinatus allowed larger critical shoulder angles before cranial subluxation occurred than did the deltoid-dominant ratio (2:1).

INTERPRETATION

Glenoid inclination-dependent changes of the critical shoulder angle have a significant impact on superior glenohumeral joint stability. The increased compensatory activity of the rotator cuff to keep the humeral head centered may lead to mechanical overload and could explain the clinically observed association between large angles and degenerative rotator cuff tears.

Bipolar bone defect in the shoulder anterior dislocation

In the anterior shoulder instability with glenoid bone loss among 25 % or more of the inferior glenoid diameter (inverted-pear glenoid), the consensus of recent authors is that glenoid bone grafting (Latarjet procedure) should be performed. The engaging Hill-Sachs lesion has been recognized as a risk factor for recurrent anterior shoulder instability. We have developed a method using radiographic and arthroscopic studies and the concept of the glenoid track to determine whether a Hill-Sachs lesion will engage the anterior glenoid rim, whether or not there is concomitant anterior glenoid bone loss. If the Hill-Sachs lesion engages, it is called an "off-track" Hill-Sachs lesion; if it does not engage, it is an "on-track" lesion. On the basis of our quantitative method, we have developed a treatment paradigm with specific surgical criteria for all patients with anterior shoulder instability (first dislocation or recurrent dislocation), both with and without bipolar bone loss.

Hill-Sachs lesion location: does it play a role in engagement?

OBJECTIVE

To see if there is an association between engagement on physical examination (PE) and the location of the Hill-Sachs lesion (HSL) as assessed by the modified biceps angle.

MATERIALS AND METHODS

Sixty-two patients with a history of anterior shoulder dislocation, who underwent preoperative MRI and arthroscopy at our institution and were tested for engagement on PE, were collected. Two musculoskeletal radiologists reviewed the MR studies, noting the presence of an HSL and documenting the location of the HSL with the modified biceps angle. Statistical analysis included the Mann-Whitney (MW) test and ROC (receiver-operating characteristic) curve.

RESULTS

Of 62 patients, there were 58 males and 4 females with a mean age of 30 (range 18-59 years). Twenty patients demonstrated engagement on PE, while 42 did not. All patients had evidence of an HSL on MRI and arthroscopy. The mean biceps angle for the engaging group was 151.5 ± 13.9°, and 142.4 ± 17.3° for the non-engaging group. The biceps angle was statistically significantly higher among patients who had engagement compared to those who did not (p = 0.027). Overall, diagnostic accuracy was highest for a biceps angle >149°, which resulted in a sensitivity of 70% and specificity of 67%.

CONCLUSION

The modified biceps angle, as measured on MRI, was significantly higher in patients who demonstrated engagement on physical examination than in those who did not. This supports the theory that the location of the Hill-Sachs lesion may play a role in engagement and may be its most important characteristic when determining its significance.

Mid-range shoulder instability modeled as a cam-follower mechanism

In this paper, we model a simplified glenohumeral joint as a cam-follower mechanism during experimental simulated dislocation. Thus, humeral head trajectory and translational forces are predicted using only contact surface geometry and compressive forces as function inputs. We demonstrate this new interpretation of glenohumeral stability and verify the accuracy of the method by physically testing a custom-molded, idealized shoulder model and comparing data to the output of the 2D mathematical model. Comparison of translational forces between experimental and mathematical approaches resulted in r(2) of 0.88 and 0.90 for the small and large humeral head sizes, respectively. Comparison of the lateral displacement resulted in r(2) of 0.99 and 0.98 for the small and larger humeral head sizes, respectively. Comparing translational forces between experiments and the mathematical model when varying the compressive force to 30 N, 60 N, and 90 N resulted in r(2) of 0.90, 0.82, and 0.89, respectively. The preliminary success of this study is motivation to introduce the effects of soft tissue such as cartilage and validation with a cadaver model. The use of simple mathematical models such as this aid in the set-up and understanding of experiments in stability research and avoid unnecessary depletion of cadaveric resources.

A novel osteotomy in shoulder joint replacement based on analysis of the cartilage/metaphyseal interface

BACKGROUND

The accuracy of reconstruction is thought to impact on functional outcome following glenohumeral joint arthroplasty. The objective of this study was to define an area of minimal anatomic variation at the cartilage/metaphyseal interface of the proximal humerus to optimize the osteotomy of the humeral head, enabling accurate reconstruction with a prosthetic component.

METHODS

Hand held digitization and 3D surface laser scanning techniques were used to digitize 24 cadaveric arms and determine the normal geometry. Each humeral head was then examined to identify the most consistent anatomical landmarks for the ideal osteotomy plane to optimize humeral component positioning.

FINDINGS

The novel, posterior referencing, osteotomy resulted in a mean increase in retroversion of only 0.4° when compared to the original geometry. A traditional anterior referencing osteotomy, by comparison, produced a mean increase in retroversion of 11°. In addition, the novel osteotomy only increased axial diameter by 0.71mm and head height by 0.02mm compared to an anterior referencing osteotomy (3.0mm and 2.7mm respectively).

INTERPRETATION

The traditional osteotomy, referencing the anterior border of the cartilage/metaphyseal interface potentially resulted in an increase in prosthetic head size and retroversion. The novel osteotomy, referencing from the posterior cartilage/metaphyseal interface enabled a more accurate recovery of head geometry. Importantly, the increase in retroversion created by the traditional osteotomy was not replicated with the novel technique. Referencing from the posterior cartilage/metaphyseal interface produced a more reliable osteotomy, more closely matching the original humeral geometry.

LEVEL OF EVIDENCE

Basic Science, Anatomic study, Computer model.

Rotator cuff tear and glenohumeral instability : a systematic review

BACKGROUND

The rotator cuff plays a significant role in the static and dynamic stability of the glenohumeral joint. Rotator cuff tears may occur after shoulder dislocations, whether in younger athletes or older patients with age-related tendon degeneration. Untreated tears may cause persistent pain, dysfunction, instability, and degenerative changes. A thorough understanding of when to look for rotator cuff tears after shoulder dislocations and how best to manage them may decrease patients' pain and improve function.

QUESTIONS/PURPOSES

We systematically reviewed the available literature to better understand (1) when a rotator cuff tear should be suspected after a dislocation, (2) whether surgical or nonsurgical approaches result in better scores for pain and satisfaction in patients with rotator cuff tears resulting from shoulder dislocations, and (3) whether intraarticular lesions, rotator cuff tears, or both should be addressed when surgery is performed.

METHODS

We systematically searched MEDLINE(®), CINAHL, and EMBASE for studies published from 1950 to 2012. We included studies reporting outcomes after treatment in patients with rotator cuff tears and shoulder dislocations. We excluded case reports, studies without any treatment, and studies about patients treated with arthroplasty. Five Level III and six Level IV studies were ultimately selected for review.

RESULTS

Patients with persistent pain or dysfunction after a shoulder dislocation often had a concomitant rotator cuff tear. Surgical repair resulted in improved pain relief and patient satisfaction compared to nonoperative management. Repair of the rotator cuff, along with concomitant capsulolabral lesions, helped restore shoulder stability. While these findings are based on Level III and IV evidence, better long-term studies with larger cohorts are needed to strengthen evidence-based recommendations.

CONCLUSIONS

Persistent pain and dysfunction after a shoulder dislocation should prompt evaluation of the rotator cuff, especially in contact or overhead athletes, patients older than 40 years, or those with nerve injury. Surgery should be considered in the appropriately active patient with a rotator cuff tear after dislocation. While the current literature suggests improved stability and function after surgical repair of the rotator cuff, higher-quality prospective studies are necessary to make definitive conclusions.

Shoulder instability in the setting of bipolar (glenoid and humeral head) bone loss: the glenoid track concept

BACKGROUND

An assortment of variables has been used in predicting anterior shoulder instability resulting from pathologic engagement of Hill-Sachs lesions on the glenoid. The glenoid track is a unique biomechanical model that relates both Hill-Sachs and bony Bankart lesions to predict shoulder engagement. We examined the glenoid track concept to determine if it provides a model that unifies glenoid rim and humeral head bone loss in predicting engagement.

QUESTIONS/PURPOSES

In this review we addressed two questions: (1) How are humeral head and glenoid rim bony defects and their interactions quantified? (2) Why is the concept of the glenoid track important?

METHODS

We performed a systematic review of the literature using PubMed (MEDLINE) and OVID for biomechanical studies and peer-reviewed articles published until March 2013. Twenty-four studies fit the inclusion criteria. These were subdivided into four anatomic studies, four studies quantifying glenohumeral bone loss, nine studies biomechanically defining shoulder engagement, six studies analyzing current treatment models, and one clinical study to be included in the final review.

RESULTS

Data demonstrate pathologic engagement is dependent on the medial margin of the Hill-Sachs lesion traveling outside the glenoid track. The width of the glenoid track decreases accordingly if there is a glenoid defect, making engagement more likely. Most treatment models focus on widening the glenoid track before addressing Hill-Sachs lesions.

CONCLUSIONS

The glenoid track uses both glenoid and humeral head bone loss to predict subsequent risk of humeral head engagement and possible dislocation. The glenoid track shows us that restoring the track to its natural width should be among the surgeon's first priority in restoring shoulder stability. Humeral head lesions, also known as Hill-Sachs lesions, are surgically addressed when they cause clinical symptoms. Symptoms arise when the medial margin of the defect engages the glenoid track.

Clavicular hook plate may induce subacromial shoulder impingement and rotator cuff lesion--dynamic sonographic evaluation

Background

Clavicular hook plates are effective fixation devices for distal clavicle fractures and severe acromioclavicular joint dislocations. However, increasing number of studies has revealed that subacromial portion of the hook may induce acromial bony erosion, shoulder impingement, or even rotator cuff damage. By sonographic evaluation, we thus intended to determine whether the presence of hook plate may induce subacromial shoulder impingement and its relationship relative to surrounding subacromial structures.

Methods

We prospectively followed 40 patients with either distal clavicle fracture or acromioclavicular joint dislocation that had surgery using the Arbeitsgemeinschaft für Osteosynthesefragen (AO) clavicular hook plate. All patients were evaluated by monthly clinical and radiographic examinations. Static and dynamic musculoskeletal sonography examinations were performed at final follow-up before implant removal. Clinical results for pain, shoulder function, and range of motion were evaluated using Constant-Murley and Disability of Arm, Shoulder, and Hand (DASH) scores.

Results

Clinically, 15 out of 40 patients (37.5%) presented with subacromial impingement syndrome and their functional scores were poorer than the non-impinged patients. Among them, six patients were noted to have rotator cuff lesion. Acromial erosion caused by hook pressure developed in 20 patients (50%).

Conclusions

We demonstrated by musculoskeletal sonography that clavicular hook plate caused subacromial shoulder impingement and rotator cuff lesion. The data also suggest an association between hardware-induced impingement and poorer functional scores. To our knowledge, the only solution is removal of the implant after bony consolidation/ligamentous healing has taken place. Thus, we advocate the removal of the implant as soon as bony union and/or ligamentous healing is achieved.

Evolving concept of bipolar bone loss and the Hill-Sachs lesion: from "engaging/non-engaging" lesion to "on-track/off-track" lesion

For anterior instability with glenoid bone loss comprising 25% or more of the inferior glenoid diameter (inverted-pear glenoid), the consensus of recent authors is that glenoid bone grafting should be performed. Although the engaging Hill-Sachs lesion has been recognized as a risk factor for recurrent anterior instability, there has been no generally accepted method for quantifying the Hill-Sachs lesion and then integrating that quantification into treatment recommendations, taking into account the geometric interplay of various sizes and various orientations of bipolar (humeral-sided plus glenoid-sided) bone loss. We have developed a method (both radiographic and arthroscopic) that uses the concept of the glenoid track to determine whether a Hill-Sachs lesion will engage the anterior glenoid rim, whether or not there is concomitant anterior glenoid bone loss. If the Hill-Sachs lesion engages, it is called an "off-track" Hill-Sachs lesion; if it does not engage, it is an "on-track" lesion. On the basis of our quantitative method, we have developed a treatment paradigm with specific surgical criteria for all patients with anterior instability, both with and without bipolar bone loss.

The effects of prosthetic humeral head shape on glenohumeral joint kinematics: a comparison of non-spherical and spherical prosthetic heads to the native humeral head

BACKGROUND

The purpose of this study was to quantitatively evaluate the effect of the prosthetic humeral head shape on rotational range of motion and glenohumeral joint kinematics.

METHODS

Six fresh-frozen cadaveric shoulders were tested in multiple positions under anatomic muscle loading. Specimens were tested for the native head, and then the spherical and non-spherical prosthetic heads were randomly implanted in the same stem to preserve the center of rotation. Rotational range of motion was measured with 3.3 Nm of torque. Glenohumeral joint kinematics was quantified by the position vectors of the humeral head apex (HHA) and geometric center of the humeral head (GCHH) to calculate translation of HHA and GCHH per degree of humeral rotation.

RESULTS

The non-spherical prosthetic head replicated the native head shape more accurately than the spherical prosthetic head. Between the non-spherical and native heads, there was no statistical difference in rotational range of motion (P > .05), but a statistical difference in HHA and GCHH translation was found at 60° of scapular plane abduction in the interval from 30° of internal rotation to neutral rotation and at 30° of forward flexion plane abduction in the interval from 30° of external rotation to maximum external rotation, respectively (P < .05). The spherical head significantly decreased rotational range of motion (P < .05), increased HHA translation per degree (P < .05), and decreased GCHH translation per degree (P < .05) in multiple positions compared with the native humeral head.

CONCLUSION

The custom, non-spherical prosthetic head more accurately replicated the head shape, rotational range of motion, and glenohumeral joint kinematics than the commercially available, spherical prosthetic head compared with the native humeral head.

In vivo ultrasound measurement of posterior femoral glide during hip joint mobilization in healthy college students

STUDY DESIGN

Descriptive study.

OBJECTIVES

To measure femoral translational glide in vivo during posterior hip joint mobilization and to interpret data in the context of the normal arthrokinematic glide necessary for hip flexion.

BACKGROUND

Joint play, or translational glide available at a joint, is largely influenced by the geometry of the articular surfaces. The high degree of congruence between the articular surfaces at the hip and the substantial arc of concavity of the acetabulum suggests that the amount of translational glide available at the hip should be very small.

METHODS

Twenty subjects received manual posteriorly directed hip mobilization at a force equal to 50% of their body weight, while concurrent ultrasound imaging of the joint was performed. Images were captured before and during application of the target mobilization force. Femoroacetabular distance for each image was measured, and the average of the 3 greatest differences between corresponding before/during mobilization values represented the translational glide produced. The amplitude of arthrokinematic glide (tangential glide) was calculated using the measured femoral head radius and hip flexion passive range of motion for each subject.

RESULTS

The average posterior femoral translational glide was 2.0 mm (range, 0.8-4.2 mm). The average calculated tangential glide required for hip flexion passive range of motion was 53.8 mm (range, 43.2-64.8 mm). The average translational glide as a percentage of tangential glide was 3.8% (range, 1.5%-7.9%).

CONCLUSION

Translational glides at the hip are small, particularly in the context of the amplitude of glide necessary for normal hip flexion range of motion.

Effect of plane of arm elevation on glenohumeral kinematics: a normative biplane fluoroscopy study

BACKGROUND

Understanding glenohumeral motion in normal and pathologic states requires the precise measurement of shoulder kinematics. The effect of the plane of arm elevation on glenohumeral translations and rotations remains largely unknown. The purpose of this study was to measure the three-dimensional glenohumeral translations and rotations during arm elevation in healthy subjects.

METHODS

Eight male subjects performed scaption and forward flexion, and five subjects (three men and two women) performed abduction, inside a dynamic biplane fluoroscopy system. Bone geometries were extracted from computed tomography images and used to determine the three-dimensional position and orientation of the humerus and scapula in individual frames. Descriptive statistics were determined for glenohumeral joint rotations and translations, and linear regressions were performed to calculate the scapulohumeral rhythm ratio.

RESULTS

The scapulohumeral rhythm ratio was 2.0 ± 0.4:1 for abduction, 1.6 ± 0.5:1 for scaption, and 1.1 ± 0.3:1 for forward flexion, with the ratio for forward flexion being significantly lower than that for abduction (p = 0.002). Humeral head excursion was largest in abduction (5.1 ± 1.1 mm) and smallest in scaption (2.4 ± 0.6 mm) (p < 0.001). The direction of translation, as determined by the linear regression slope, was more inferior during abduction (-2.1 ± 1.8 mm/90°) compared with forward flexion (0.1 ± 10.9 mm/90°) (p = 0.024).

CONCLUSIONS

Scapulohumeral rhythm significantly decreased as the plane of arm elevation moved in an anterior arc from abduction to forward flexion. The amount of physiologic glenohumeral excursion varied significantly with the plane of elevation, was smallest for scaption, and showed inconsistent patterns across subjects with the exception of consistent inferior translation during abduction.

Importance of the subscapularis muscle after total shoulder arthroplasty

BACKGROUND

The rotator cuff muscles are the main stabilizer of the glenohumeral joint. After total shoulder arthroplasty using anterior approaches, a dysfunction of the subscapularis muscle has been reported. In the present paper we tested the hypothesis that a deficient subscapularis following total shoulder arthroplasty can induce joint instability.

METHODS

To test this hypothesis we have developed an EMG-driven musculoskeletal model of the glenohumeral joint. The model was based on an algorithm that minimizes the difference between measured and predicted muscular activities, while satisfying the mechanical equilibrium of the glenohumeral joint. A movement of abduction in the scapular plane was simulated. We compared a normal and deficient subscapularis. Muscle forces, joint force, contact pattern and humeral head translation were evaluated.

FINDINGS

To satisfy the mechanical equilibrium, a deficient subscapularis induced a decrease of the force of the infraspinatus muscle. This force decrease was balanced by an increase of the supraspinatus and middle deltoid. As a consequence, the deficient subscapularis induced an upward migration of the humeral head, an eccentric contact pattern and higher stress within the cement.

INTERPRETATION

These results confirm the importance of the suscapularis for the long-term stability of total shoulder arthroplasty.

Humeral head arthroplasty and its ability to restore original humeral head geometry

BACKGROUND

Modern prosthetic components are designed to enable restoration of proximal humeral morphology, provided that a precise osteotomy of the humeral head at the level of the anatomic neck is performed. To determine whether a simulated osteotomy and replacement arthroplasty with an idealized implant were able to restore original head geometry.

MATERIALS AND METHODS

A handheld digitizer and surface laser scanner were used to digitize 24 humeri. Computer models were used to simulate an osteotomy, performed at the anterior cartilage-metaphyseal interface, and reconstruct the head with a spherical prosthetic head. The head diameter, radius of curvature, and inclination and retroversion angles were calculated for each specimen and compared with the original humeral head.

RESULTS

The simulated osteotomy resulted in a 4.8° decrease in inclination (P < .01) and 11.3° increase in retroversion (P < .001). The radius of curvature in the coronal plane was not significantly different (P = .284). However, in the axial plane, the prosthesis was significantly larger than the original head for both head diameter (P < .001) and radius of curvature (P < .05).

DISCUSSION

The study suggests that the humeral head is not a perfect segment of a sphere and an osteotomy along the anterior cartilage-metaphyseal interface does not remove only the proximal humeral articular surface. Even with a fully adaptable prosthetic implant, replacement arthroplasty is not able to restore original head geometry.

CONCLUSIONS

Alterations to head geometry with the osteotomy described may alter the line of force through the prosthetic joint, producing eccentric loading at the glenoid, and contribute to early failure.

Morphology of the normal and arthritic glenoid

The normal glenoid has a pear-shape aspect and is slightly retroverted. It has a variable orientation in the sagittal plane. The cartilage surface area corresponds to 28 % of the area of the humeral head with a radius of curvature greater than the humeral head. Mechanical properties are significantly higher at the center and posterior edge of the glenoid. With osteoarthritis, the glenoid becomes larger with a greater width and an increasing of the retroversion angle. The wear can be centric or excentric. Mechanical properties are significantly higher at the center and posterior edge of the glenoid.

Importance of polyethylene thickness in total shoulder arthroplasty: a finite element analysis

BACKGROUND

Articular surfaces reconstruction is essential in total shoulder arthroplasty. Because of the limited glenoid bone support, thin glenoid component could improve anatomical reconstruction, but adverse mechanical effects might appear.

METHODS

With a numerical musculoskeletal shoulder model, we analysed and compared three values of thickness of a typical all-polyethylene glenoid component: 2, 4 (reference) and 6mm. A loaded movement of abduction in the scapular plane was simulated. We evaluated the humeral head translation, the muscle moment arms, the joint force, the articular contact pattern, and the polyethylene and cement stress. Findings Decreasing polyethylene thickness from 6 to 2mm slightly increased humeral head translation and muscle moment arms. This induced a small decreased of the joint reaction force, but important increase of stress within the polyethylene and the cement mantel. Interpretation The reference thickness of 4mm seems a good compromise to avoid stress concentration and joint stuffing.

Finite element analysis and physiologic testing of a novel, inset glenoid fixation technique

HYPOTHESIS

The success of shoulder arthroplasty surgery has been limited by a common complication: glenoid implant loosening. Eccentric loading of the glenoid due to migration of the humeral head is considered to be the major cause of glenoid loosening and is referred to as the rocking-horse phenomenon. Glenoid implant loosening may cause pain, limitation of function, and the need for complicated revision surgery. Our hypothesis was that an inset fixation technique could offer increased fixation strength and minimize the effects of the rocking-horse phenomenon on glenoid loosening.

MATERIALS AND METHODS

Fixation strength and stress distribution were analyzed using two methods. First, mechanical simulation of physiologic in vivo cyclic loading was performed on 1 inset glenoid implant design and 2 standard onlay glenoid implant designs currently on the market. Second, 3-dimensional finite element analysis was performed to compare an inset glenoid implant and a standard onlay glenoid implant with a keel and a standard onlay pegged implant.

RESULTS

After cyclic loading to 100,000 cycles, no glenoid implants demonstrated signs of loosening. Mechanical testing after cyclic loading demonstrated less distraction of the glenoid rim using an inset technique compared with an onlay technique. Finite element analysis results indicated that the inset technique achieved up to an 87% reduction in displacement.

CONCLUSIONS

Mechanical tests and finite element analysis support the concept of inset glenoid fixation in minimizing the risk of glenoid loosening.

Glenohumeral instability and rotator cuff tear

The prevalence of rotator cuff tears after traumatic dislocation increases with advancing age, a likely consequence of the age-associated deterioration of the structure and mechanical properties of the tendons of the rotator cuff. These are the effective stabilizers of the glenohumeral joint, compressing the humeral head in the 3-dimensional concavity of the glenohumeral joint. It is impossible to establish whether a lesion of the capsular-labrum complex or of the rotator cuff causes or follows a dislocation, regardless of whether it is anterior or posterior. A peripheral nerve or a brachial plexus injury can be associated with tendon lesion and instability, developing the "terrible triad" of the shoulder. Both conservative and surgical management are possible, and surgeons must choose the most appropriate management modality according to the biologic age, functional demands, and type of lesion.