Rotator Cuff Force Research Articles

Three-Point Contact Repair Technique for Partial Subscapularis Tears Using U-Stitch and Cow Hitch Knot Loaded on a Single Knotless Anchor: Simple, Reliable and Quick Method

The importance of repairing the subscapularis tendon to facilitate restoration of the rotator cuff force couple is obvious. However, identifying and repairing subscapularis tears can be challenging for surgeons. In this technical note, we present our preferred surgical method for Lafosse type 1, 2, and 3 and Yoo and Rhee type 1-2A and 2B subscapularis tears, providing a 3-point suture contact achieved with FiberTape (Arthrex) and knotless anchors. In this technique, fiber suture is placed into the superolateral edge of the subscapularis tendon as a traction stitch (cow hitch knot). Second, the FiberTape is passed through the medial upper and lower parts of the tendon, just perpendicular to the fibers, using a suture passer. The FiberTape’s free ends remain on the tendon’s anterior side, shaping a U form. The U-shaped FiberTape and traction suture are loaded to a knotless anchor and placed on the footprint area. This creates a 3-point contact between the FiberTape U-suture and the cow hitch knot on the area. We believe that this technique provides fast and reliable fixation due to its 3-point contact application and simplicity.

Inlay vs. onlay humeral components in reverse total shoulder arthroplasty: a biorobotic shoulder simulator study

BackgroundBoth inlay and onlay humeral implants are available for reverse total shoulder arthroplasty (rTSA), but biomechanical data comparing these components remains limited. This study investigated the effects of inlay and onlay rTSA humeral components on shoulder biomechanics using a biorobotic shoulder simulator. MethodsTwenty fresh-frozen cadaver shoulders were tested before and after rTSA with either an inlay or onlay humeral implant. Comparisons were performed between the most commonly implanted configurations for each implant (baseline), and a modification to provide equivalent neck shaft angle (NSA) for the inlay and onlay configurations. Specimens underwent passive range of motion (ROM) assessment with the scapula held static, and performed scapular plane abduction driven by previously collected human-subject scapulothoracic and glenohumeral kinematics. Passive ROM glenohumeral joint angles were compared using t-tests while muscle force and excursion data during scapular plane elevation were evaluated with statistical parametric mapping and t-tests. ResultsMaximum passive elevation was reduced for the inlay versus onlay humeral component, while both implants caused reduced passive elevation versus the native joint. Inlay rTSA also demonstrated reduced passive internal rotation at rest and increased external rotation at 90° humerothoracic elevation versus the native joint. All preoperative plan ROM estimates differed from experiments. Rotator cuff forces were elevated with an onlay versus inlay humeral implant, but simulated muscle excursions did not differ between systems. Compared to the native joint, rotator cuff forces were increased for both inlay and onlay implants and deltoid forces were reduced for inlay. The rTSA dramatically altered muscle excursions versus the native joint. Comparisons of inlay and onlay humeral implants with equivalent NSA were consistent with the baseline comparisons. ConclusionsRotator cuff forces required to perform scapular plane abduction increase following rTSA using both inlay and onlay implants. Rotator cuff forces are lower with inlay implants compared to onlay, although inlay implants also result in reduced passive elevation ROM. Deltoid forces are lower with inlay implants in comparison to native, but not with onlay implants. The differences between inlay and onlay components are largely unaffected by NSA, indicating that these differences are inherent to the inlay and onlay designs. In those patients with an intact rotator cuff, decreased rotator cuff forces to perform abduction with an inlay humeral implant compared to an onlay implant may promote improved long-term outcomes due to reduced deltoid muscle fatigue when using an inlay implant.

Reverse total shoulder glenoid component inclination affects glenohumeral kinetics during abduction: a cadaveric study.

Optimal implant placement in reverse total shoulder arthroplasty (rTSA) remains controversial. Specifically, the optimal glenoid inclination is unknown. Therefore, a cadaveric shoulder simulator with 3-dimentional human motion specific to rTSA was used to study joint contact and muscle forces as a function of glenoid component inclination. Eight human cadaver shoulders were tested before and after rTSA implantation. Scapular plane abduction kinematics from control subjects and those with rTSA drove a cadaveric shoulder simulator with 3-dimentional scapulothoracic and glenohumeral motion. Glenoid inclination varied from -20° to +20°. Outputs included compression, superior-inferior (S/I) shear, and anterior-posterior shear forces from a 6° of freedom load cell in the joint, and deltoid and rotator cuff muscle forces. Data were evaluated with statistical parametric mapping and t-tests. Inferior glenoid inclination (-) reduced S/I shear by up to 125% relative to superior inclination, with similar compression to the neutral condition (0°). Superior inclinations (+) increased the S/I shear force by approximately the same magnitude, yet decreased compression by 25% in the most superior inclination (+20°). There were few differences in deltoid or rotator cuff forces due to inclination. Only the middle deltoid decreased by approximately 7% for the most inferior inclination (-20°). Compared with native shoulders, the neutral (0°) rTSA inclination showed reduced forces of 30%-75% in the anterior deltoid and a trend toward decreased forces in the middle deltoid. Force demands on the rotator cuff varied as a function of elevation, with a trend toward increased forces in rTSA at peak glenohumeral elevation. Inferior inclination reduces superior shear forces, without influencing compression. Superior inclination increased S/I shear, while decreasing compression, which may be a source of component loosening and joint instability after rTSA. Inferior inclination of the rTSA glenoid may reduce the likelihood of glenoid loosening by reducing the magnitude of cyclic shear and compressive loading during arm elevation activities, although this may be altered by specific-subject body habitus and motion. These factors are especially important in revision rTSA or glenoid bone grafting where there is already a 3-fold increase in glenoid baseplate loosening vs. primary rTSA.

Anatomic total shoulder glenoid component inclination affects glenohumeral kinetics during abduction: a cadaveric study.

Although typically favorable in outcome, anatomic total shoulder arthroplasty (aTSA) can require long-term revision. The most common cause for revision is glenoid loosening, which may result from eccentric cyclic forces and joint translations. "Rocking" of the glenoid component may be exacerbated by the joint geometry, such as glenoid inclination and version. Restoration of premorbid glenoid inclination may be preferable, although laboratory and computational models indicate that both superior inclination and inferior inclination have benefits. This discrepancy may arise because previous studies were limited by a lack of physiological conditions to test inclination. Therefore, a cadaveric shoulder simulator with 3-dimensional human motion was used to study joint contact and muscle forces with isolated changes in glenoid inclination. Eight human cadaveric shoulders were tested before and after aTSA. Scapular-plane abduction kinematics from human subjects were used to drive a cadaveric shoulder simulator with 3-dimensional scapulothoracic and glenohumeral motion. Glenoid inclination was varied from -10° to +20°, whereas compressive, superior-inferior shear, and anterior-posterior shear forces were collected with a 6-df load cell during motion. Outputs also included muscle forces of the deltoid and rotator cuff. Data were evaluated with statistical parametric mapping repeated-measures analysis of variance and t tests. Inferior glenoid inclination (-10°) reduced both compressive and superior-inferior shear forces vs. neutral 0° inclination by up to 40%, and even more when compared with superior inclination (P<.001). Superior inclinations (+10° and +20°) tended to increase deltoid and rotator cuff forces vs. neutral 0° inclination or inferior inclination, on the order of 20%-40% (P≤.045). All force metrics except anterior-posterior shear were lowest for inferior inclination. Most aTSA muscle forces for neutral 0° inclination were not significantly different from native shoulders and decreased 45% and 15% in the posterior deltoid and supraspinatus, respectively (P≤.003). Joint translations were similar to prior reports in aTSA patients and did not differ between any inclinations or compared with native shoulders. Joint reaction forces were similar to those observed in human subjects with instrumented aTSA implants, providing confidence in the relative magnitude of our results. Inferior inclination reduces overall forces in the shoulder. Superior inclinations increase the muscle effort required for the shoulder to achieve similar motion, thus increasing the forces exerted on the glenoid component. These results suggest that a preference toward aTSA glenoid components in inferior inclination may reduce the likelihood of glenoid loosening by reducing excessive muscle and joint contact forces.

No Strength Differences Despite Greater Posterior Rotator Cuff Intramuscular Fat in Patients With Eccentric Glenohumeral Osteoarthritis.

BackgroundWhen nonoperative measures do not alleviate the symptoms of glenohumeral osteoarthritis (OA), patients with advanced OA primarily are treated with anatomic total shoulder arthroplasty (TSA). It is unknown why TSAs performed in patients with eccentric (asymmetric glenoid wear) compared with concentric (symmetric glenoid wear) deformities exhibit higher failure rates, despite surgical advances. Persistent disruption of the posterior-to-anterior rotator cuff (RC) force couple resulting from posterior RC intramuscular degeneration in patients with eccentric deformities could impair external rotation strength and may contribute to eventual TSA failure. Pain and intramuscular fat within the RC muscles may impact external rotation strength measures and are important to consider.Questions/purposes(1) Is there relative shoulder external rotation weakness in patients with eccentric compared with concentric deformities? (2) Is there higher resting or torque-dependent pain in patients with eccentric compared with concentric deformities? (3) Do patients with eccentric deformities have higher posterior-to-anterior RC intramuscular fat percent ratios than patients with concentric deformities?MethodsFrom February 2020 to November 2021, 65% (52 of 80) of patients with OA met study eligibility criteria. Of these, 63% (33 of 52) of patients enrolled and provided informed consent. From a convenience sample of 21 older adults with no history of shoulder pain, 20 met eligibility criteria as control participants. Of the convenience sample, 18 patients enrolled and provided informed consent. In total for this prospective, cross-sectional study, across patients with OA and control participants, 50% (51 of 101) of participants were enrolled and allocated into the eccentric (n = 16), concentric (n = 17), and control groups (n = 18). A 3-degree-of-freedom load cell was used to sensitively quantify strength in all three dimensions surrounding the shoulder. Participants performed maximal isometric contractions in 26 1-, 2-, and 3-degree-of-freedom direction combinations involving adduction/abduction, internal/external rotation, and/or flexion/extension. To test for relative external rotation weakness, we quantified relative strength in opposing directions (three-dimensional [3D] strength balance) along the X (+adduction/-abduction), Y (+internal/-external rotation), and Z (+flexion/-extension) axes and compared across the three groups. Patients with OA rated their shoulder pain (numerical rating 0-10) before testing at rest (resting pain; response to “How bad is your pain today?”) and with each maximal contraction (torque-dependent pain; numerical rating 0-10). Resting and torque-dependent pain were compared between patients with eccentric and concentric deformities to determine if pain was higher in the eccentric group. The RC cross-sectional areas and intramuscular fat percentages were quantified on Dixon-sequence MRIs by a single observer who performed manual segmentation using previously validated methods. Ratios of posterior-to-anterior RC fat percent (infraspinatus + teres minor fat percent/subscapularis fat percent) were computed and compared between the OA groups.ResultsThere was no relative external rotation weakness in patients with eccentric deformities (Y component of 3D strength balance, mean ± SD: -4.7% ± 5.1%) compared with patients with concentric deformities (-0.05% ± 4.5%, mean difference -4.7% [95% CI -7.5% to -1.9%]; p = 0.05). However, there was more variability in 3D strength balance in the eccentric group (95% CI volume, %3: 893) compared with the concentric group (95% CI volume, %3: 579). In patients with eccentric compared with concentric deformities, there was no difference in median (IQR) resting pain (1.0 [3.0] versus 2.0 [2.3], mean rank difference 4.5 [95% CI -6.6 to 16]; p = 0.61) or torque-dependent pain (0.70 [3.0] versus 0.58 [1.5], mean rank difference 2.6 [95% CI -8.8 to 14]; p = 0.86). In the subset of 18 of 33 patients with OA who underwent MRI, seven patients with eccentric deformities demonstrated a higher posterior-to-anterior RC fat percent ratio than the 11 patients with concentric deformities (1.2 [0.8] versus 0.70 [0.3], mean rank difference 6.4 [95% CI 1.4 to 11.5]; p = 0.01).ConclusionPatients with eccentric deformities demonstrated higher variability in strength compared with patients with concentric deformities. This increased variability suggests patients with potential subtypes of eccentric wear patterns (posterior-superior, posterior-central, and posterior-inferior) may compensate differently for underlying anatomic changes by adopting unique kinematic or muscle activation patterns.Clinical RelevanceOur findings highlight the importance of careful clinical evaluation of patients presenting with eccentric deformities because some may exhibit potentially detrimental strength deficits. Recognition of such strength deficits may allow for targeted rehabilitation. Future work should explore the relationship between strength in patients with specific subtypes of eccentric wear patterns and potential forms of kinematic or muscular compensation to determine whether these factors play a role in TSA failures in patients with eccentric deformities.

Incomplete Rotator Cable Did Not Cause Rotator Cuff Dysfunction in Case of Rotator Cuff Tear: A Biomechanical Study of the Relationship Between Rotator Cable Integrity and Rotator Cuff Function

This study seeks to evaluate the biomechanical relationship between the severity of rotator cable tears and the function of the rotator cuff. Twelve cadaveric shoulders with intact rotator cuff, existing rotator cable, and a critical shoulder angle below 35° were included. For each shoulder, a posterosuperior rotator cuff tear (PSRCT) (model 2) in the crescent area was formed. Then anterior insertion detached (model 3), anterior insertion detached together with the middle cable tear (model 4), and the whole rotator cable tear (model 5) were subsequently created. The rotator cuff that lay above the humeral head rotation center was detached as a global tear control (model 6), along with the primitive status as the intact control (model 1). Glenohumeral abduction was initiated by simulating deltoid and remaining rotator cuff force. Functioning of the remaining rotator cuff was evaluated using the middle deltoid force (MDF), as required for abduction. No statistically significant differences in peak MDF values were seen among the 4 PSRCT statuses (44.10 ± 7.30 N [model 2], P= .96; 45.50 ± 9.55 N [model 3], P= .86; 45.90 ± 3.53 N [model 4], P= 0.30; 44.20 ± 8.19 N [model 5], P= .80) and intact control status (39.79 ± 7.65 N [model 1]). However, significant differences in peak MDF values were found among the 4 PSRCT statuses and the global tear control status (54.53 ± 7.46 N [model 6], P < .01). The PSRCT, regardless of the severity of the rotator cable tear, does not induce functionally significant biomechanical impairment. Tear extension involving all rotator cuff tissue above the geometric rotation center of the humeral head results in obvious functional impairment. For PSRCT, the remaining rotator cuff tissue above the geometric rotation center may contribute to the preservation of shoulder function in RCT patients.

Arthroscopic superior capsular reconstruction of the shoulder: a narrative review.

Irreparable rotator cuff tears (IRCTs) in young and considerably active patients are difficult to treat because it is mostly associated with poor outcome which may lead to a painful and dysfunctional shoulder. Most of the IRCTs are encountered in massive size rotator cuff tears which associated with high failure rate following surgical repair. Thus, the IRCTs was considered challenging for its poor healing rate following repair which may induce the arthritic changes. Since the advent of arthroscopic superior capsular reconstruction (ASCR) of the shoulder in 2013, it has gained its popularity. The procedure has become the most popular option for joint-preserving shoulder surgery for patients with IRCTs. It works by providing a static restraint to the superior humeral head migration to optimize the rotator cuff force couples, hence improving joint kinematics. The acceptance of superior capsular reconstruction has made it rapidly evolving in terms of a wider variety of procedures and broader surgical indications. Despite the enthusiasm and widely acceptance towards the procedure, there are still many queries that exist regarding the best indications, surgical technique particularly graft of choice, the long-term outcome, and the complication and risk of the superior capsular reconstruction (SCR). This narrative review provide the current evidence of SCR in an attempt to provide a state-of-the-art knowledge.

Strain measurement of the deep layer of the supraspinatus tendon using fresh frozen cadaver: The influence of shoulder elevation

The shoulder is a uniquely complex large joint. Effective and safe physical therapy efforts towards rehabilitating injured and repaired shoulders requires a thorough understanding of shoulder mechanics from both generalized and very specific perspectives. Numerous biomechanical studies have been published on the shoulder. None of the studies, to our knowledge, considered the strain of the deep layers of the supraspinatus tendon for scapular plane elevation. Ten unilateral fresh-frozen human cadaveric specimens were used for the study. Scapular plane angles ranging from -10 to 30 degrees were evaluated for tensile loads, ranging from 0 to 120Newtons, exerted on the supraspinatus tendon. Strain measurements that specifically targeted the deep layer of the supraspinatus tendon during tensile loading were recorded. Strains recorded in the supraspinatus deep layer while increasing tendon force of the supraspinatus were significant for isometric gleno-humeral elevation of 30° and 20°. The response of strain to tendon force was less pronounced for 10° or less of gleno-humeral elevation. When performing isometric shoulder exercise regimens, rotator cuff forces and both surface and deep tendon strain, relative to scapular positioning, are relevant and should be considered.

Superior Capsular Reconstruction: Indications, Techniques, and Clinical Outcomes.

* The rotator cuff itself acts as a dynamic stabilizer to the glenohumeral joint, resisting superior migration of the humeral head when the deltoid muscle contracts. In the coronal plane, the deltoid muscle and the supraspinatus tendon function as a coordinated couple, and in the transverse plane, the infraspinatus and teres minor (posterior) and subscapularis (anterior) tendons function to maintain a balanced force couple.* Superior capsular reconstruction works by providing the glenohumeral joint with a passive constraint to superior humeral-head translation, optimizing the otherwise dysfunctional rotator cuff force couples.* Superior capsular reconstruction is indicated in patients with intolerable pain and/or unacceptable dysfunction who have attempted and undergone failed nonoperative treatment, with massive, irreparable rotator cuff tears (typically involving supraspinatus and infraspinatus tendons), with minimal to no rotator cuff arthropathy (Hamada Grade 1 or 2), with an intact or reparable subscapularis tendon, and with a functional deltoid muscle.* Superior capsular reconstruction is contraindicated in patients with rotator cuff tears associated with moderate to severe rotator cuff arthropathy (Hamada Grade ≥3), patients with glenohumeral osteoarthritis, and patients with a torn or irreparable subscapularis.* Passing the acellular dermal allograft into the joint can be perhaps the most challenging aspect of the procedure, particularly in the setting of grafts of &gt;35 mm in the anterior-posterior dimension.* Despite its rapid growth in popularity and several biomechanical studies evaluating the technique, clinical outcome literature for superior capsular reconstruction remains limited to date.

Isolated loss of active external rotation: a distinct entity and results of L'Episcopo tendon transfer

The purpose of this study was to characterize a subgroup of cuff-deficient patients with isolated loss of active external rotation (ILER) but preserved active elevation and to evaluate the outcomes of the L'Episcopo procedure to restore horizontal muscle balance. During a 10-year period, 26 patients (14 men, 12 women) were identified with ILER in the setting of massive irreparable posterosuperior cuff tears. A modified L'Episcopo tendon transfer was performed to restore active external rotation and to improve shoulder function. The mean age at surgery was 64.5 years (29-83 years). Patients were evaluated with a mean follow-up of 52 months (range, 24-104 months). Preoperatively, despite maintained active elevation (average of 161°), ILER patients complained about loss of spatial control of the arm and difficulties with activities of daily living. On computed tomography scan or magnetic resonance imaging, there was severe fatty infiltration of infraspinatus and absent or atrophic teres minor. After L'Episcopo transfer, 84% of patients were satisfied. The gain in active external rotation was +26° in arm at the side and +18.5° in 90° abduction. Adjusted Constant score and Simple Shoulder Value increased from 63.6% to 86.9% and from 36.9% to 70.8%, respectively (P < .001). The ADLER score increased from 10 to 24.5 points (P < .002). Two patients with advanced cuff tear arthropathy (Hamada stage 3 and 4) required conversion to a reverse shoulder arthroplasty (RSA) 7 and 9 years after the index surgery. ILER is a distinct entity that is a cause of severe handicap because of loss of spatial control of the upper limb. This symptom is related to absent or atrophied infraspinatus and teres minor. In properly selected cases (Hamada stage 1 or 2), the modified L'Episcopo transfer is effective at restoring anterior-posterior rotator cuff force balance. In more advanced cuff tear arthropathy (Hamada stage ≥3), the tendon transfer should be performed with an RSA because of possible secondary degeneration of the glenohumeral joint.

Superior Capsular Reconstruction.

Superior capsular reconstruction (SCR) of the shoulder has recently gained popularity as an option for joint-preserving shoulder surgery for patients with an irreparable rotator cuff tear. In the absence of glenohumeral arthritis, rotator cuff tear irreparability should only be diagnosed for most patients after a careful diagnostic arthroscopy. Superior capsular reconstruction adds biological, passive, superior constraint to the glenohumeral joint, thereby optimizing the rotator cuff force couples and improving joint kinematics. At short-term follow-up, SCR has been shown to be effective for pain relief and restoration of active shoulder motion, even in the worst cases of shoulder dysfunction (true shoulder pseudoparalysis). The rapid early adoption and expansion of SCR is justified by its excellent anatomical, biomechanical, and short-term clinical results. The techniques for arthroscopic SCR using dermal allograft continue to improve; however, the operation remains technically demanding. Patients with risk factors for irreparability and who might benefit from reconstruction of the superior capsule should be counseled about the operation as an additional, joint-preserving procedure that can be done in conjunction with arthroscopic, partial rotator cuff repair. [Orthopedics. 2017; 40(5):271-280.].

Volumetric evaluation of the rotator cuff musculature in massive rotator cuff tears with pseudoparalysis

If the balance of the rotator cuff force couple is disrupted, pseudoparalysis may occur, but the exact mechanism remains unknown. This study investigated the effect of rotator cuff force couple disruption on active range of motion in massive rotator cuff tear (mRCT) by rotator cuff muscle volume analysis. The study included 53 patients with irreparable mRCT: 22 in the nonpseudoparalysis group and 31 in the pseudoparalysis group. The volumes of the subscapularis (SBS), infraspinatus (ISP), and teres minor (TM) muscles were measured using magnetic resonance imaging (MRI), and the ratios of each muscle volume to the anatomic external rotator (aER) volume were calculated. A control group of 25 individuals with normal rotator cuffs was included. Anterior-to-posterior cuff muscle volume ratio (SBS/ISP + TM) was imbalanced in both mRCT groups (1.383 nonpseudoparalysis and 1.302 pseudoparalysis). Between the 2 groups, the ISP/aER ratio (0.277 vs. 0.249) and the inferior SBS/aER ratio (0.426 vs. 0.390) were significantly decreased in the pseudoparalysis group (P= .022 and P= .040, respectively). However, neither the TM/aER ratio (0.357 vs. 0.376) nor the superior SBS/aER ratio (0.452 vs. 0.424) showed a significant difference between the two groups (P= .749 and P= .068, respectively). If the inferior SBS was torn, a high frequency of pseudoparalysis was noted (81.0%, P= .010). The disruption of transverse force couple was noted in both irreparable mRCT groups, although no significant difference was found between the nonpseudoparalysis and pseudoparalysis groups. ISP and inferior SBS muscle volumes showed a significant decrease in pseudoparalysis group and, therefore, were considered to greatly influence the loss of active motion in mRCT. The TM did not exert significant effect on the incidence of pseudoparalysis.

Biomechanical Investigation of Locked Plate Fixation with Suture Augmentation in a Comminuted Three-Part Proximal Humerus Fracture Model

Background: Locked plating of displaced proximal humerus fractures is a reliable fixation method. Greater tuberosity (GT) failure is a known complication that may occur in the early post-operative period. Despite postoperative immobilization, the rotator cuff continues to exert significant forces on fracture fragments. Our hypothesis is that suture augmentation will provide greater stability of the GT than locked plating alone. To prove this we developed a three-part proximal humerus fracture model to test fracture fixation. Methods: A biomechanical study was performed on nine fresh frozen cadaveric humeri, simulating a three-part proximal humerus fracture (Neer Classification). Rotator cuff tendon insertions were preserved to physiologically load the proximal humerus. The fracture was reduced and fixed with a locked plate alone or a locked plate with suture augmentation of the GT to the rotator cuff tendons. Biomechanical testing utilized a materials testing machine and a three-dimensional motion capture system to quantify interfragmentary motion under torsional loading as a function of fixation type. Results: Greater torsional stability was observed in the suture-augmented group compared to the plate only group (p = 0.0012). There were two catastrophic failures in the plate only group while none of the suture reinforced constructs failed. Conclusions: In our model, suture-augmentation of the GT to the rotator cuff provided greater stability than locked plating alone. The current study provides a biome-chanical basis for reinforcing locked plate constructs with sutures. The added stability afforded by suture-augmentation may mitigate rotator cuff forces in the clinical setting, avoiding fracture displacement in the early postoperative rehabilitation period.

The effect of coracoacromial ligament excision and acromioplasty on the amount of rotator cuff force production necessary to restore intact glenohumeral biomechanics

Coracoacromial ligament (CAL) excision and acromioplasty increase superior and anterosuperior glenohumeral translation. It is unknown how much of an increase in rotator cuff force production is required to re-establish intact glenohumeral biomechanics after these surgical procedures. We hypothesized that, after CAL excision and acromioplasty, an increase in rotator cuff force production would not be necessary to reproduce the anterosuperior and superior translations of the intact specimens. Nine cadaveric shoulders were subjected to loading in the superior and anterosuperior directions in the intact state after CAL excision, acromioplasty, and recording of the translations. The rotator cuff force was then increased to normalize glenohumeral biomechanics. After CAL excision at 150 and 200 N of loading, an increase in the rotator cuff force by 25% decreased anterosuperior translation to the point where there was no significant difference from the intact specimen's translation. After acromioplasty (and CAL excision) at 150 and 200 N, an increase in the rotator cuff force of 25% and 30%, respectively, decreased superior translation to the point where there was no significant difference from the intact specimen's translation. At 150 to 200 N of loading, CAL excision and acromioplasty increase the rotator cuff force required to maintain normal glenohumeral biomechanics by 25% to 30%. After a subacromial decompression, the rotator cuff has an increased force production requirement to maintain baseline glenohumeral mechanics. Under many circumstances, in vivo force requirements may be even greater after surgical attenuation of the coracoacromial arch. Basic Science Study; Biomechanics.

A larger critical shoulder angle requires more rotator cuff activity to preserve joint stability.

Shoulders with rotator cuff tears (RCT) tears are associated with significantly larger critical shoulder angles (CSA) (RCT CSA = 38.2°) than shoulders without RCT (CSA = 32.9°). We hypothesized that larger CSAs increase the ratio of glenohumeral joint shear to joint compression forces, requiring substantially increased compensatory supraspinatus loads to stabilize the arm in abduction. A previously established three dimensional (3D) finite element (FE) model was used. Two acromion shapes mimicked the mean CSA of 38.2° found in patients with RCT and that of a normal CSA (32.9°). In a first step, the moment arms for each muscle segment were obtained for 21 different thoracohumeral abduction angles to simulate a quasi-static abduction in the scapular plane. In a second step, the muscle forces were calculated by minimizing the range of muscle stresses able to compensate an external joint moment caused by the arm weight. If the joint became unstable, additional force was applied by the rotator cuff muscles to restore joint stability. The model showed a higher joint shear to joint compressive force for the RCT CSA (38.2°) for thoracohumeral abduction angles between 40° and 90° with a peak difference of 23% at 50° of abduction. To achieve stability in this case additional rotator cuff forces exceeding physiological values were required. Our results document that a higher CSA tends to destabilize the glenohumeral joint such that higher than normal supraspinatus forces are required to maintain modeled stability during active abduction. This lends strong support to the concept that a high CSA can induce supraspinatus (SSP) overload. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:961-968, 2016.

Disruption of the anterior–posterior rotator cuff force balance alters joint function and leads to joint damage in a rat model

The rotator cuff assists in shoulder movement and provides dynamic stability to the glenohumeral joint. Specifically, the anterior-posterior (AP) force balance, provided by the subscapularis anteriorly and the infraspinatus and teres minor posteriorly, is critical for joint stability and concentric rotation of the humeral head on the glenoid. However, limited understanding exists of the consequences associated with disruption of the AP force balance (due to tears of both the supraspinatus and infraspinatus tendons) on joint function and joint damage. We investigated the effect of disrupting the APforce balance on joint function and joint damage in an overuse rat model. Twenty-eight rats underwent 4 weeks of overuse to produce a tendinopathic condition and were then randomized into two surgical groups: Detachment of the supraspinatus only or detachment of the supraspinatus and infraspinatus tendons. Rats were then gradually returned to their overuse protocol. Quantitative ambulatory measures including medial/lateral, propulsion, braking, and vertical forces were significantly different between groups. Additionally, cartilage and adjacent tendon properties were significantly altered. These results identify joint imbalance as a mechanical mechanism for joint damage and demonstrate the importance of preserving rotator cuff balance when treating active cuff tear patients.

Evaluation of the muscle volumes of the transverse rotator cuff force couple in nonpathologic shoulders

The balance between the subscapularis muscle and the infraspinatus/teres minor muscles, often referred to as the rotator cuff transverse force couple (TFC), has been proposed to be a critical component for normal shoulder function. The relationship between the muscle volume and the power means that TFC can be evaluated with the measurement of the muscle volume of the subscapularis muscle and the infraspinatus/teres minor. The aim of this study is to evaluate an innovative computed tomography (CT)-based technique to measure the muscle volume and to evaluate if there is a significant difference between muscle volumes of both the subscapularis muscle and the infraspinatus/teres minor in nonpathologic shoulders. CT images of 27 shoulders (21 patients) with a full scapula and a proximal humeral head were evaluated. Two volume masks (subscapularis and infraspinatus/teres minor) were calculated on the basis of the assigned muscle contours on the transverse slices. The intraobserver and interobserver correlation coefficient was calculated. The intraobserver and interobserver correlation coefficient was excellent. The correlation between the anterior and posterior part of the TFC was strong. There was no significant difference between the volume masks. Muscle volume of the TCF can be quantified using CT images. In nonpathologic shoulders, there is no significant difference between the muscle volume of the anterior (subscapularis) and posterior part (teres minor/infraspinatus) of the TFC.

Human evolution and tears of the rotator cuff

Humans differ from other great ape species in their propensity to develop tears of the rotator cuff. The aim of this study was to compare the anatomical risk factors for subacromial impingement and rotator cuff tears amongst the great apes and to determine which features may be accentuated in humans and therefore play a more significant role in disease aetiology. Orthogonal digital photographs of 22 human, 17 gorilla, 13 chimpanzee and 12 orangutan dry bone scapula specimens oriented in the glenoid plane were taken. Anatomical measurements were preformed using a calibrated digital image technique and the results scaled according to scapula vertebral border length. Of the ten anatomical features associated with subacromial impingement and rotator cuff tears in humans, none were shown to be accentuated and significantly different to the other species studied. However the human supraspinatus fossa was shown to be significantly smaller. These results indicate that an alternative primary aetiological factor for rotator cuff tears must exist. A reduction in the size of the supraspinatus fossa in human scapulae suggests that structural insufficiency of the supraspinatus or a change in rotator cuff force vectors could play a role.

Soft-tissue Management in Revision Total Shoulder Arthroplasty

Revision total shoulder arthroplasty can be technically challenging. Results are inferior to those of primary arthroplasty, particularly when the indications for revision are related to soft-tissue problems. Patient selection is important. In the stiff arthroplasty, the surgeon must address rotator cuff and capsular contractures as well as exuberant soft-tissue adhesions. Unstable arthroplasty can be the result of asymmetric soft-tissue balancing or deficiencies (eg, subscapularis deficiency) that can lead to loss of the rotator cuff force couples and, subsequently, to instability on attempted glenohumeral motion. Infection must be considered in the workup of the failed total shoulder. In this era of ever-increasing use of shoulder arthroplasty, surgeons will be presented with growing numbers of patients who require revision surgery. An organized approach is needed to diagnose and manage the stiff or unstable total shoulder arthroplasty.

The effect of crank position and backrest inclination on shoulder load and mechanical efficiency during handcycling

Handbikes come in different models and setups, but only limited knowledge is available on the handbike-user interface. The aim of this study was to identify optimal handbike setups, assuming that in such a setup mechanical efficiency is high, while shoulder load is low. Thirteen subjects with a spinal cord injury (paraplegia) performed handcycling with different handbike setups at constant power output: four crank positions (two distances, two heights) and four backrest inclinations. The O2-consumption, kinetics, and kinematics were measured to calculate mechanical efficiency and shoulder load (glenohumeral contact force, net shoulder moments, and rotator cuff force). The analysis showed that more upright backrest positions resulted in lower shoulder load compared with the most reclined position [glenohumeral contact force (260 vs 335 N), supraspinatus (14.4% vs 18.2%), and infraspinatus force (5.4% vs 9.8%)], while there was no difference in efficiency. Except for a reduction in subscapularis force at the distant position, no differences in shoulder load or efficiency were found between crank positions. Recreational handbike users, who want to improve their physical capacity in a shoulder-friendly way, should set up their handbike with a more upright backrest position and a distant crank placement.