Superior Humeral Head Translation Research Articles

Surgical Management of Massive Irreparable Cuff Tears: Superior Capsule Reconstruction and Rotator Cable Reconstruction.

Massive irreparable rotator cuff tears (MIRCTs) present treatment challenges. Recently, superior capsule reconstruction (SCR) and anterior cable reconstruction have emerged as surgical options, but no single approach is superior. This review provides an overview of SCR and cable reconstruction techniques, including biomechanical studies, clinical outcomes, and surgical considerations. Biomechanical studies show SCR with autografts or allografts improves glenohumeral stability and mechanics. Clinical outcomes of SCR demonstrate improved range of motion, function scores, and pain relief in short-term studies. Anterior cable reconstruction reduces superior humeral head translation and subacromial pressures in biomechanical models. Early clinical studies report improved rotator cuff healing and outcomes for cable reconstruction in specific irreparable tear patterns. SCR and cable reconstruction are viable surgical options for MIRCTs based on early encouraging results. However, higher-level comparative studies with long-term follow-up are still needed. Careful consideration of tear pattern, patient factors, and surgical goals is required to optimize treatment of MIRCTs. Further research is necessary to determine the optimal role for these procedures.

Middle trapezius tendon transfer using Achilles allograft for irreparable isolated supraspinatus tendon tears effectively restores the superior stability of the humeral head without restricting range of motion: a biomechanical study

BackgroundMiddle trapezius tendon (MTT) transfer has been suggested for promising treatment of irreparable isolated supraspinatus tendon tears (IISTTs). However, there have been no attempts to assess the biomechanical efficacy of MTT transfer. This study aims to evaluate the biomechanical efficacy of MTT transfer in the setting of IISTTs. MethodsEight fresh-frozen cadaveric shoulders were tested in three conditions (1) intact rotator cuff, (2) IISTTs, and (3) Middle trapezius tendon transfer (MTT) using Achilles allograft for IISTTs. Total humeral rotational range of motion, superior translation of the humeral head, and subacromial contact characteristics were measured at 0°, 20°, and 40° glenohumeral abduction (representing 0°, 30°, and 60° shoulder abduction). Superior translation and subacromial contact pressures were measured at 0°, 30°, 60° and 90° external rotation (ER). Two different MTT muscle loading conditions were investigated. A linear mixed-effects model and Tukey post hoc test were used for statistical analysis. ResultsTotal range of motion was significantly increased after IISTT at 20° abduction (P =.037). There were no changes in total ROM following MTT transfer compared to the IISTT condition (P>.625 for all comparisons). The IISTT condition significantly increased superior translation compared to the intact rotator cuff condition in 0° and 20° abduction with all ER angles (P<.001), 40° abduction/30° ER (P=.016), and 40° abduction/60° ER (P=.002). MTT transfer significantly decreased superior translation of the humeral head at all abduction angles compared to the IISTTs condition (P<.026). MTT transfer significantly decreased peak contact pressure by 638.7 kPa (normal loading) and 726.8 kPa (double loading) at 0° abduction/30° ER compared to the IISTTs condition (P<.001). Mean contact pressure was decreased by 102.8 kPa (normal loading) and 118.0 kPa (double loading) at 0° abduction/30° ER (P<.001) and 101.0 kPa (normal loading) and 99.2 kPa (double loading) at 0° abduction/60° ER (P<.001). MTT transfer at 20° abduction/30° ER with 24N loading significantly decreased contact pressure by 91.2 kPa (P=.035). ConclusionsThe MTT transfer biomechanically restored the superior humeral head translation and reduced the subacromial contact pressure in a cadaveric model of IISTT, while not restricting total ROM. These findings suggest that MTT transfer may have potential as a surgical treatment for IISTTs.

Superior Capsular Reconstruction Combined With Lower Trapezius Tendon Transfer Improves the Biomechanics in Posterosuperior Massive Rotator Cuff Tears

Background: Surgical treatments for chronic posterosuperior massive rotator cuff tear (MRCT) are still controversial. Superior capsular reconstruction (SCR), which provides a static stabilizer to decrease superior humeral head translation, and lower trapezius tendon transfer (LTTT) with centralization of the humeral head, which prevents superior humeral head migration, are potential surgical options. To date, SCR combined with LTTT has not been fully investigated. Hypothesis: Restoration of static stabilizer and dynamic stabilizer together would effectively improve shoulder kinematics in posterosuperior MRCT. Study Design: Controlled laboratory study. Methods: A custom-made shoulder mechanics testing system was used to test 8 fresh-frozen cadaveric shoulders. The testing conditions were as follows: (1) intact; (2) posterosuperior MRCT (supraspinatus and infraspinatus removed); (3) SCR using the fascia lata; (4) LTTT; and (5) SCR combined with LTTT. The total rotational range of motion (ROM), superior translation, anteroposterior translation, and peak subacromial contact pressure were evaluated at 0°, 30°, and 60° of shoulder abduction. Repeated-measures analysis of variance and Tukey post hoc tests were performed. Results: The total rotational ROM, superior translation, anteroposterior translation, and peak subacromial contact pressure increased in posterosuperior MRCTs (all, P < .05). The rotational ROM, superior translation, anteroposterior translation, and peak subacromial contact pressure at 0° and 30° of shoulder abduction decreased in SCR (all, P < .05). However, there was no significant improvement in rotational ROM, superior translation, and peak subacromial contact pressure at 60° of shoulder abduction (P > .05). LTTT resulted in a significant decrease in the superior translation, anteroposterior translation, and peak subacromial contact pressure at 0°, 30°, and 60° of shoulder abduction (P < .05). SCR combined with LTTT restored the total rotational ROM, superior translation, anteroposterior translation, and peak subacromial contact pressure at 0°, 30°, and 60° of shoulder abduction (all, P < .05). Conclusion: In the cadaveric model, SCR combined with LTTT showed improved shoulder kinematics and contact pressures in the posterosuperior MRCT model compared with SCR or LTTT alone. Clinical Relevance: SCR combined with LTTT may be regarded as an alternative surgical procedure for posterosuperior MRCTs.

Poor Results After Arthroscopic Treatment of Irreparable Rotator Cuff Tears Using a Subacromial Balloon Spacer.

Arthroscopic balloon spacer arthroplasty (BSA) is a relatively recent procedure for patients with irreparable rotator cuff tears, conceived to prevent degenerative joint changes and relieve subacromial pain. This is a retrospective case series of 16 consecutive patients treated with BSA and followed up for a minimum 12 months. Besides BSA, partial tendon repair was done whenever possible. Radiographical parameters such as acromiohumeral distance and Hamada stage were measured preoperatively and postoperatively. Range of motion, functional results (Constant score; Simple Shoulder Test; American Shoulder and Elbow Surgeons score; and Disabilities of the Arm, Shoulder, and Hand score), health-related quality of life results with the EuroQol Five Dimensions tool, complications, and patient satisfaction were also analyzed. The mean age was 64 ± 10.3 years, and the male/female ratio was 6/10. Partial repair was achieved in five patients (31.2%) and did not influence functional or health-related quality of life results ( P = 0.11). The mean acromiohumeral distance and Hamada stage worsened from preoperative measures (3.7 ± 2.0 mm and 2.3 ± 1.1) to final follow-up measures (3.1 ± 2.4 mm and 3.1 ± 1.3). Shoulder abduction increased by 7.8° ± 26.8° and forward flexion decreased by 25.5° ± 32.4° after surgery. The functional results at the final follow-up were Constant 49.5 ± 18.0; American Shoulder and Elbow Surgeons 60.2 ± 27.2; Simple Shoulder Test 8.5 ± 4.6; and Disabilities of the Arm, Shoulder, and Hand 32.7 ± 12.1. The mean index value for the EuroQol Five Dimensions was 0.79, and perceived quality of life was 85.6 ± 15.4 of 100 points. 62.5% of the patients (10) were satisfied with surgery. The complication rate was 13% (2), accounting for one deep infection and one balloon migration. Theoretical benefits of BSA for delaying superior humeral head translation and arthropathic changes could not be demonstrated in our series. Its functional results and satisfaction rates are low, but self-reported quality of life after surgery does not seem impaired. Complications do not occur frequently but require reintervention and endanger joint viability. Level IV (case series).

Superior Capsular Reconstruction for Irreparable Rotator Cuff Tears: Technique and Associated Considerations

Background: Massive rotator cuff tears lead to superior humeral head translation and may prevent successful repair. Superior capsular reconstruction (SCR) can overcome this by improving the compression and depressor effects of the rotator cuff and joint capsule simultaneously. Indications: SCR is indicated in young, active patients with massive rotator cuff tears or a functional cuff deficiency from a failed previous rotator cuff repair without significant arthritis, articulation of the humeral head with the acromion, or dysfunctional deltoid. Technique Description: During the procedure, repair of the subscapularis is performed using a polydioxanone (PDS) suture shuttling device to pass suture through the subscapularis in an inverted mattress knotless configuration. Posterior infraspinatus tendon fibers are reduced and the tuberosity is abraded and decorticated followed by completion of the partial repair of the posterior cuff. Anchors are placed on the glenoid and articular margins followed by preparation of the dermal allograft with luggage tags medially and perforation laterally. The graft is passed and secured in a double row configuration laterally with apposition of the graft to the remnant, repaired rotator cuff posteriorly. Results: Passive range of motion (ROM) is begun at week 4, followed by progressive active ROM at weeks 6 to 12, and advancement of full ROM and advanced strengthening begun at 3 months. With SCR, a patient can expect maximal strength and function to be regained at 1 year postoperatively. Significant clinical improvement following SCR has been seen in visual analog scale (VAS) and American Shoulder and Elbow Surgeons Shoulder Score (ASES) scores for most patients, although there is a varied degree of return to sport following this procedure. Discussion/Conclusion: Clinical evidence suggests that SCR is a beneficial therapeutic intervention for short to medium term outcomes in younger patients with massive, irreparable rotator cuff tears. More studies are needed to identify the optimal thickness of the graft.

Superior Capsule Reconstruction With a 3 mm–Thick Dermal Allograft Partially Restores Glenohumeral Stability in Massive Posterosuperior Rotator Cuff Deficiency: A Dynamic Robotic Shoulder Model

Background: Superior capsule reconstruction (SCR) has been shown to improve shoulder function and reduce pain in patients with isolated irreparable supraspinatus tendon tears. However, the effects of SCR on biomechanics in a shoulder with an extensive posterosuperior rotator cuff tear pattern remain unknown. Purpose/Hypothesis: The purpose was to (1) establish a dynamic robotic shoulder model, (2) assess the influence of rotator cuff tear patterns, and (3) assess the effects of SCR on superior humeral head translation after a posterosuperior rotator cuff tear. It was hypothesized that a posterosuperior rotator cuff tear would increase superior humeral head translation when compared with the intact and supraspinatus tendon–deficient state and that SCR would reduce superior humeral head translation in shoulders with massive rotator cuff tears involving the supraspinatus and infraspinatus tendons. Study Design: Controlled laboratory study. Methods: Twelve fresh-frozen cadaveric shoulders were tested using a robotic arm. Kinematic testing was performed in 4 conditions: (1) intact, (2) simulated irreparable supraspinatus tendon tear, (3) simulated irreparable supra- and infraspinatus tendon tear, and (4) SCR using a 3 mm–thick dermal allograft (DA). Kinematic testing consisted of static 40-N superior force tests at 0°, 30°, 60°, and 90° of abduction and dynamic flexion, abduction, and scaption motions. In each test, the superior translation of the humeral head was reported. Results: In static testing, SCR significantly reduced humeral superior translation compared with rotator cuff tear at all abduction angles. SCR restored the superior stability back to native at 60° and 90° of abduction, but the humeral head remained significantly and superiorly translated at neutral position and at 30° of abduction. The results of dynamic testing showed a significantly increased superior translation in the injured state at lower elevation angles, which diminished at higher elevation, becoming nonsignificant at elevation >75°. SCR reduced the magnitude of superior translation across all elevation angles, but translation remained significantly different from the intact state up to 60° of elevation. Conclusion: Massive posterosuperior rotator cuff tears increased superior glenohumeral translation when compared with the intact and supraspinatus tendon–insufficient rotator cuff states. SCR using a 3-mm DA partially restored the superior stability of the glenohumeral joint even in the presence of a simulated massive posterosuperior rotator cuff tear in a static and dynamic robotic shoulder model. Clinical Relevance: The biomechanical performance concerning glenohumeral stability after SCR in shoulders with large posterosuperior rotator cuff tears is unclear and may affect clinical outcomes in daily practice.

A new approach to superior capsular reconstruction with hamstring allograft for irreparable posterosuperior rotator cuff tears: a dynamic biomechanical evaluation.

Superior capsular reconstruction (SCR) treatment of massive, symptomatic, irreparable rotator cuff tears (RCTs) has become a more recently used procedure. However, there is a lack of consensus surrounding optimal graft choice for the SCR technique, and current dermal grafts have increased cost and are technically challenging because of a need for multiple implants. The purpose of this study was to biomechanically investigate a biological lower-cost alternative as a support for the superior capsule reconstruction concept: an isolated semitendinosus tendon (STT) allograft and a combination graft with the long head of the biceps tendon (LHBT) in an established massive posterosuperior RCT cadaver model. Ten fresh-frozen cadaveric shoulders (53.3 ± 12.4 years: range: 26-65) were tested on an established dynamic shoulder simulator using dynamic muscle loading. Cumulative deltoid forces, maximum abduction angle, and superior humeral head translation were compared across 4 testing conditions: (1) intact state, (2) massively retracted (Patte III), irreparable posterosuperior RCT, (3) SCR repair using an STT allograft, and (4) SCR repair using a combined STT-LHBT repair. Intact shoulders required a mean deltoid force of 154.2 ± 20.41 N to achieve maximum glenohumeral abduction (55.3° ± 2.3°). Compared with native shoulders, the maximum abduction angle decreased following a massively retracted posterosuperior RCT by 52% (28.3° ± 8.4°; P < .001), whereas the cumulative deltoid forces increased by 48% (205.3 ± 40.9 N; P = .001). The STT repair and the STT-LHBT repair improved shoulder function compared with the tear state, with a mean maximum abduction angle of 30.6° ± 9.0° and 31.8° ± 7.7° and a mean deltoid force of 205.3 ± 40.9 N and 201.0 ± 34.0 N, respectively, but this was not statistically significant (P > .05). The STT-LHBT repair significantly improved the range of motion with respect to the tear state (P = .04). In a dynamic shoulder simulator model, both the STT and the STT-LHBT repair techniques improved glenohumeral joint kinematics in an amount similar to previously reported "traditional" SCR techniques for treatment of an irreparable posterosuperior RCT.

Arthroscopic Superior Capsule Reconstruction With Doubled Autologous Semitendinosus Tendon Graft

Massive and irreparable rotator cuff tears remain a difficult condition to treat. Fatty infiltration of the muscles and excessive retraction of the tendons predispose to high failure rates of arthroscopic repair techniques. In recent years, studies on the superior capsule have shown that it plays a key role in reducing superior humeral head translation and restoring balance to the force couples required for dynamic shoulder function. Superior capsule reconstruction has become common in clinical practice. Several techniques with different types of grafts have been described, such as fascia lata autograft, dermal allograft patch, and long head of the biceps tendon autograft. More recently, an open technique with semitendinosus tendon autograft has been proposed. Our aim is to describe an all-arthroscopic technique for superior capsule reconstruction using a doubled semitendinosus tendon autograft in a box-shaped configuration. We believe that the technique can combine the advantages of other techniques, such as graft availability, low harvest-site morbidity, limited cost, and mechanical strength.

The biomechanics of the supraspinatus-deficient shoulder treated with superior capsular reconstruction vs. reverse total shoulder arthroplasty-experimental study.

Our objective was to compare biomechanical effects of superior capsular reconstruction (SCR) and reverse total shoulder arthroplasty (rTSA) on shoulder motion, in the setting of an irreparable supraspinatus (SS) tear. We hypothesized that rTSA would produce greater improvement in abduction force and shift the humerus inferiorly, while SCR would produce greater range of motion (ROM) and prevent superior migration of the humerus during abduction. Six cadaveric shoulders were evaluated using a custom biomechanical apparatus. Each shoulder underwent four experimental conditions: (1) intact/control, (2) irreparable SS tear, (3) SCR using dermal allograft, and (4) rTSA without SCR. Deltoid abduction force, superior humeral head translation, and passive range of motion were measured in static tendon loading condition at 0, 30, and 60° of glenohumeral abduction. Both rTSA and SCR restored abduction force to intact levels at all abduction angles. rTSA significantly increased abduction force compared with the SS-deficient shoulder at 0, 30, 60° (p = 0.04), while SCR produced a significant increase at 0° (p = 0.05) abduction. rTSA inferiorly shifted the humeral head by 27 mm (p = 0.002). SCR restored superior humeral head translation to intact SS levels. Compared with SCR, rTSA resulted in 25° less passive abduction (p = 0.001) without significant differences in forward flexion/extension. Compared with SCR, rTSA achieved 10° less passive internal rotation at 0° abduction (p = 0.03) and 26° and 17° greater external rotation at 30° and 60° abduction, respectively (p = 0.03, p = 0.04). Our investigation found that abduction force was restored to intact cuff levels by both procedures, without significant differences between the two techniques. SCR restored superior humeral head migration and rTSA translated the humerus inferiorly. rTSA resulted in decreased passive abduction ROM and increased external rotation, compared with SCR. Both SCR and rTSA restore key biomechanical parameters following an irreparable SS tear, although SCR offers superior passive abduction ROM.

Superior Capsule Reconstruction With Subacromial Allograft Spacer: Biomechanical Cadaveric Study of Subacromial Contact Pressure and Superior Humeral Head Translation

To investigate the biomechanical effects of superior capsule reconstruction with subacromial allograft spacer on superior humeral head translation and subacromial contact pressure. Eight cadaveric shoulder specimens were tested in 4 conditions: (1) intact rotator cuff, (2) supraspinatus tear and superior capsule excision, (3) superior capsule reconstruction with human dermal allograft, and (4) superior capsule reconstruction with subacromial resurfacing using human dermal allograft. In each condition, specimens were tested at 0, 30, 60, and 90° of shoulder abduction in balanced and unbalanced loaded states for subacromial contact pressure and superior humeral head translation. Statistical comparisons were made using a repeated-measures analysis of variance test, followed by a Tukey post hoc test for pairwise comparisons. A P value <.05 was set as statistically significant. Superior humeral head translation and subacromial contact pressure were increased after irreparable rotator cuff tear (P= .001). There was no significant difference between superior capsule reconstruction and intact cuff in regard to superior humeral head translation and subacromial contact pressure at all abduction angles. Superior capsule reconstruction with subacromial resurfacing decreased superior humeral head translation relative to intact (0°, P= .004; 30°, P= .02; 60°, P= .08; 90°, P= .01), superior capsule reconstruction (0°, P= .001; 30°, P= .003; 60°, P= .019; 90°, P= .001), and cuff-deficient states (P= .001). Superior capsule reconstruction with subacromial resurfacing resulted in nonsignificant increases in subacromial contact pressure relative to intact cuff at 0 to 90° abduction angles. Superior capsule reconstruction with subacromial resurfacing using human dermal allograft results in decreased superior humeral head translation relative to superior capsule reconstruction with human dermal allograft only, while increasing subacromial contact pressure. Superior capsule reconstruction with subacromial resurfacing using human dermal allograft reduces superior humeral head translation while increasing subacromial contact pressure in a cadaveric model.

Nonarthroplasty Surgical Treatment Options for Massive, Irreparable Rotator Cuff Tears

Massive, irreparable rotator cuff tears (MIRCTs) provide a significant dilemma for orthopaedic surgeons. One treatment option for MIRCTs is reverse total shoulder arthroplasty. However, other methods of treating these massive tears have been developed. A search of the current literature on nonoperative management, arthroscopic debridement, partial repair, superior capsular reconstruction (SCR), graft interposition, balloon spacer arthroplasty, trapezius transfer, and latissimus dorsi transfer for MIRCTs was performed. Studies that described each surgical technique and reported on clinical outcomes were included in this review. Arthroscopic debridement may provide pain relief by removing damaged rotator cuff tissue, but no functional repair is performed. Partial repair has been suggested as a technique to restore shoulder functionality by repairing as much of the rotator cuff tendon as possible. This technique has demonstrated improved clinical outcomes but also fails at a significantly high rate. SCR has recently gained interest as a method to prohibit superior humeral head translation and has been met with encouraging early clinical outcomes. Graft interposition bridges the gap between the retracted tendon and humerus. Balloon spacer arthroplasty has also been recently proposed and acts to prohibit humeral head migration by placing a biodegradable saline-filled spacer between the humeral head and acromion; it has been shown to provide good clinical outcomes. Both trapezius and latissimus dorsi transfer techniques involve transferring the tendon of these respective muscles to the greater tuberosity of the humerus; these 2 techniques have shown promising restoration in shoulder function, especially in a younger, active population. Arthroscopic debridement, partial repair, SCR, graft interposition, balloon spacer arthroplasty, trapezius transfer, and latissimus dorsi transfer have all been shown to improve clinical outcomes for patients presenting with MIRCTs. Randomized controlled trials are necessary for confirming the efficacy of these procedures and to determine when each is indicated based on specific patient and anatomic factors.

A probabilistic orthopaedic population model to predict fatigue-related subacromial geometric variability

Fatigue-related glenohumeral and scapulothoracic kinematic relationships, in addition to morphological characteristics of the scapula and humerus, affect the dimensions of the subacromial space. Each exhibits considerable interpersonal variability, which if only considering the mean, can lead to misleading population estimations of subacromial impingement risk, particularly for outliers. Additionally, the relative influence of each parameter on subacromial space variability is unclear. Applying empirically-derived morphological and kinematic distributions (n=31), this research used Advanced Mean Value and Monte Carlo probabilistic modeling approaches to predict the distribution of the minimum subacromial space width (SAS) and establish which parameters contributed more to modulating the SAS. The predicted SAS differed by 8mm between 1% and 99% confidence intervals. While the SAS was not influenced by muscle fatigue, the space reduced with arm elevation to magnitudes between 4.5 and 5mm. This reduction resulted in an estimated 65–75% of the population at risk for tissue compression at elevation angles≥90° when considering the interposed tissue thickness. Morphological parameters, notably glenoid inclination, showed higher relative importance for modulating the predicted SAS across conditions, while kinematic parameters (humeral head translation, scapular orientation), which differed by elevation angle and fatigue state, demonstrated less consistent importance levels across experimental conditions. Overall, the findings reinforce the shoulder health risks related to overhead activities, as they pose an increased likelihood of mechanical rotator cuff tendon compression. Further, probabilistic methods are highly innovative, in that they are capable of determining relative parameter importance and subsequently identifying key injury risk factors. As glenoid inclination is difficult to diagnose and treat, and is associated with superior humeral head translation, interventions to improve rotator cuff strength and glenohumeral stability are recommended, particularly in populations exposed to overhead postures.

Fatigue-induced glenohumeral and scapulothoracic kinematic variability: Implications for subacromial space reduction

Superior humeral head translation and scapula reorientation can reduce the subacromial space. While these kinematic abnormalities exist in injured populations, the effect of muscle fatigue is unclear. Additionally, these mechanisms were typically studied independently, thereby neglecting potential covariance. This research evaluated the influence of upper extremity muscle fatigue on glenohumeral and scapulothoracic kinematics and defined their relationship. Radiography and motion tracking systems captured these kinematic relationships, during scapula plane elevation, both before and after fatigue. Fatigue-induced changes in humeral head position, scapular orientation and the minimum subacromial space width were measured. High inter-subject variability existed for each measure which precluded identification of mean differences at the population level. However, significant scapular upward rotation occurred following fatigue (p=0.0002). Despite similar population mean results, between 39% and 57% of participants exhibited fatigue-related changes in disadvantageous orientations. Additionally, correlations between measures were generally fair (0.21–0.40) and highly dependent on elevation, likely attributed to the variable fatigue responses. Overall, the data confirms that fatigue-induced changes in kinematics poses highly variable risk of subacromial impingement syndrome across individuals. Thus, solely considering the “average” or mean population response likely underestimates potentially injurious fatigue consequences.

The potential role of upper extremity muscle fatigue in the generation of extrinsic subacromial impingement syndrome: a kinematic perspective

Background:Subacromial impingement syndrome (SAIS) of the shoulder has a multi-factorial aetiology which includes both intrinsic and extrinsic mechanisms. Upper extremity muscle fatigue has been proposed as a potential causative factor in extrinsic SAIS development.Objectives:This review summarises the current state of knowledge regarding fatigue-induced glenohumeral (GH) and scapulothoracic (ST) kinematic changes as they relate to subacromial space reduction and subsequent SAIS. It specifically addresses evidence for mechanisms including superior humeral head translation and scapular reorientation.Major findings:Global upper extremity muscle fatigue elicits GH kinematic changes that increase SAIS risk. Concurrent compensatory ST kinematic alterations, primarily scapular upward rotation (UR), lessen this risk. Additionally, fatiguing protocols aimed to selectively exhaust certain muscles or muscle groups without fatiguing the rotator cuff were unsuccessful, suggesting that rotator cuff fatigue is likely for a wide range of fatiguing upper extremity tasks.Conclusions:Despite the compensatory response of scapular reorientation in fatigued conditions, the dimensions of the subacromial space still pose increased risk for SAIS development, and the variability in kinematic responses produces highly differential risk, particularly for those exposed to tasks requiring increased rotator cuff muscle demand.

Multivariate analyses of rotator cuff pathologies in shoulder disability.

BackgroundDisability of the shoulder joint is often caused by a tear in the rotator cuff (RC) muscles. Four RC muscles coordinate shoulder movement and stability, among them the supraspinatus and infraspinatus muscle which are predominantly torn. The contribution of each RC muscle to tear pathology is not fully understood. We hypothesized that muscle atrophy and fatty infiltration, features of RC muscle degeneration, are predictive of superior humeral head translation and shoulder functional disability.MethodsShoulder features, including RC muscle surface area and fatty infiltration, superior humeral translation and RC tear size were obtained from a consecutive series of Magnetic Resonance Imaging with arthrography (MRA). We investigated patients with superior (supraspinatus, n = 39) and posterosuperior (supraspinatus and infraspinatus, n = 30) RC tears, and patients with an intact RC (n = 52) as controls. The individual or combinatorial contribution of RC measures to superior humeral translation, as a sign of RC dysfunction, was investigated with univariate or multivariate models, respectively.ResultsUsing the univariate model the infraspinatus surface area and fatty infiltration in both the supraspinatus and infraspinatus had a significant contribution to RC dysfunction. With the multivariate model, however, the infraspinatus surface area only affected superior humeral translation (p<0.001) and discriminated between superior and posterosuperior tears. In contrast neither tear size nor fatty infiltration of the supraspinatus or infraspinatus contributed to superior humeral translation.ConclusionOur study reveals that infraspinatus atrophy has the strongest contribution to RC tear pathologies. This suggests a pivotal role for the infraspinatus in preventing shoulder disability.

Effects of biceps tension and superior humeral head translation on the glenoid labrum.

We sought to understand the effects of superior humeral head translation and load of the long head of biceps on the pathomechanics of the superior glenoid labrum by predicting labral strain. Using micro-CT cadaver images, a finite element model of the glenohumeral joint was generated, consisting of humerus, glenoid bone, cartilages, labrum, and biceps tendon. A glenohumeral compression of 50 N and biceps tensions of 0, 22, 55, and 88 N were applied. The humeral head was superiorly translated from 0 to 5 mm in 1-mm increments. The highest labral strain occurred at the interface with the glenoid cartilage and bone beneath the origin of the biceps tendon. The maximum strain was lower than the reported failure strain. The humeral head motion had relatively greater effect than biceps tension on the increasing labral strain. This supports the mechanistic hypothesis that superior labral lesions result mainly from superior migration of the humeral head, but also from biceps tension

Shoulder labral pathomechanics with rotator cuff tears

Rotator cuff tears (RCTs), the most common injury of the shoulder, are often accompanied by tears in the superior glenoid labrum. We evaluated whether superior humeral head (HH) motion secondary to RCTs and loading of the long head of the biceps tendon (LHBT) are implicated in the development of this associated superior labral pathology. Additionally, we determined the efficacy of a finite element model (FEM) for predicting the mechanics of the labrum. The HH was oriented at 30° of glenohumeral abduction and neutral rotation with 50N compressive force. Loads of 0N or 22N were applied to the LHBT. The HH was translated superiorly by 5mm to simulate superior instability caused by RCTs. Superior displacement of the labrum was affected by translation of the HH (P<0.0001), position along the labrum (P<0.0001), and interaction between the location on the labrum and LHBT tension (P<0.05). The displacements predicted by the FEM were compared with mechanical tests from 6 cadaveric specimens and all were within 1 SD of the mean. A hyperelastic constitutive law for the labrum was a better predictor of labral behavior than the elastic law and insensitive to ±1 SD variations in material properties. Peak strains were observed at the glenoid–labrum interface below the LHBT attachment consistent with the common location of labral pathology. These results suggest that pathomechanics of the shoulder secondary to RCTs (e.g., superior HH translation) and LHBT loading play significant roles in the pathologic changes seen in the superior labrum.

Humeral Head Translation After a Suprascapular Nerve Block

Subacromial impingement syndrome is the most common shoulder disorder. Abnormal superior translation of the humeral head is believed to be a major cause of this pathology. The first purpose of the study was to examine the effects of suprascapular nerve block on superior translation of the humeral head and scapular upward rotation during dynamic shoulder elevation. The secondary purpose was to assess muscle activation patterns during these motions. Twenty healthy subjects participated in the study. Using fluoroscopy and electromyography, humeral head translation and muscle activation were measured before and after a suprascapular nerve block. The humeral head was superiorly located at 60 degrees of humeral elevation, and the scapula was more upwardly rotated from 30 to 90 degrees of humeral elevation after the block. The differences were observed during midrange of motion. In addition, the deltoid muscle group demonstrated increased muscle activation after the nerve block. The study's results showed a compensatory increase in humeral head translation, scapular upward rotation, and deltoid muscle activation due to the nerve block. These outcomes suggest that increasing muscular strength and endurance of the supraspinatus and infraspinatus muscles could prevent any increased superior humeral head translation. This may be beneficial in reducing shoulder impingement or rotator cuff tears over time.

The specificity of fatiguing protocols affects scapular orientation: Implications for subacromial impingement

Background Shoulder impairments are often associated with altered scapular kinematics. As muscles control scapular movement, functionally altering muscle performance through fatigue may produce scapular kinematics that mimic those of injured patients. The aim of this study was to examine if changes in scapular tilt, rotation and pro/retraction following two different upper extremity fatiguing protocols have any implications with respect to subacromial impingement. Methods Scapular orientation was monitored during posturally constrained static holds (at 0°, 45° and 90° of humeral elevation) before and after two fatiguing protocols, one global and one local. Both protocols are associated with producing changes in shoulder skeletal arrangement. Findings Following the global fatiguing protocol, there was significantly more scapular posterior tilt ( P < 0.01) and upward rotation ( P < 0.02), particularly at 90° humeral elevation. No changes in scapular orientation occurred following the local fatiguing protocol. Interpretation Scapular orientation changes following muscle fatigue acted to increase the subacromial space. Thus, the rotator cuff muscles, not the scapular stabilizers, have more influence on actively preventing mechanical subacromial impingement. The lack of evidence of reduction of the subacromial space thus implicates superior humeral head translation as a more likely primary mechanism of the initiation of subacromial impingement.

Biomechanics of Massive Rotator Cuff Tears

Some individuals with massive rotator cuff tears maintain active shoulder abduction, and some maintain good postoperative active range of motion despite high rates of repeat tears after repair. We devised a biomechanical rationale for these observations and measured the increases in residual muscle forces necessary to maintain active shoulder motion with rotator cuff tears of various sizes. A custom cadaver shoulder controller utilizing position and orientation closed-loop feedback control was used. Six cadaver glenohumeral joint specimens were tested in open-chain scapular plane abduction with equivalent upper extremity weight. The shoulder controller limited superior translation of the humeral head to 3.0 mm while maintaining neutral axial rotation by automatically controlling individual rotator cuff forces. Three-dimensional position and orientation and rotator cuff and deltoid force vectors were recorded. Specimens were tested with an intact rotator cuff and with 6, 7, and 8-cm tears. All six specimens achieved full abduction with <or=3.0 mm of superior translation of the humeral head for all rotator cuff tear sizes. The effect of rotator cuff tear was significant for all tear sizes (p < 0.01). Compared with the intact condition, the subscapularis force requirements for the 6, 7, and 8-cm tears were increased by 30%, 44%, and 85%, respectively. For the combined infraspinatus and teres minor, the forces were increased by 32%, 45%, and 86%, respectively. The maximum deltoid force for the simulated tear condition never exceeded the deltoid force required at maximum abduction for the intact condition. However, between 10 degrees and 45 degrees of abduction, the average deltoid force requirement increased 22%, 28%, and 45% for the three tear sizes. In the presence of a massive rotator cuff tear, stable glenohumeral abduction without excessive superior humeral head translation requires significantly higher forces in the remaining intact portion of the rotator cuff. These force increases are within the physiologic range of rotator cuff muscles for 6-cm tears and most 7-cm tears. Increases in deltoid force requirements occur in early abduction; however, greater relative increases are required of the rotator cuff, especially in the presence of larger rotator cuff tears.