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Review Article

Primary and Secondary Consequences of Rotator Cuff Injury on Joint Stabilizing Tissues in the Shoulder OPEN ACCESS

[+] Author and Article Information
Hafizur Rahman

Department of Mechanical Science
and Engineering,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801
e-mail: mrahman3@illinois.edu

Eric Currier

Department of Mechanical Science
and Engineering,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801
e-mail: ecurrier0@gmail.com

Marshall Johnson

Department of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332
e-mail: mvjohns2@gatech.edu

Rick Goding

Department of Orthopaedic,
Joint Preservation Institute of Iowa,
West Des Moines, IA 50266
e-mail: R.goding@jointpreservationiowa.com

Amy Wagoner Johnson

Department of Mechanical Science
and Engineering,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801
e-mail: ajwj@illinois.edu

Mariana E. Kersh

Department of Mechanical Science
and Engineering,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801
e-mail: mkersh@illinois.edu

1Corresponding author.

Manuscript received May 13, 2017; final manuscript received September 13, 2017; published online September 29, 2017. Assoc. Editor: Kyle Allen.

J Biomech Eng 139(11), 110801 (Sep 29, 2017) (10 pages) Paper No: BIO-17-1211; doi: 10.1115/1.4037917 History: Received May 13, 2017; Revised September 13, 2017

Rotator cuff tears (RCTs) are one of the primary causes of shoulder pain and dysfunction in the upper extremity accounting over 4.5 million physician visits per year with 250,000 rotator cuff repairs being performed annually in the U.S. While the tear is often considered an injury to a specific tendon/tendons and consequently treated as such, there are secondary effects of RCTs that may have significant consequences for shoulder function. Specifically, RCTs have been shown to affect the joint cartilage, bone, the ligaments, as well as the remaining intact tendons of the shoulder joint. Injuries associated with the upper extremities account for the largest percent of workplace injuries. Unfortunately, the variable success rate related to RCTs motivates the need for a better understanding of the biomechanical consequences associated with the shoulder injuries. Understanding the timing of the injury and the secondary anatomic consequences that are likely to have occurred are also of great importance in treatment planning because the approach to the treatment algorithm is influenced by the functional and anatomic state of the rotator cuff and the shoulder complex in general. In this review, we summarized the contribution of RCTs to joint stability in terms of both primary (injured tendon) and secondary (remaining tissues) consequences including anatomic changes in the tissues surrounding the affected tendon/tendons. The mechanical basis of normal shoulder joint function depends on the balance between active muscle forces and passive stabilization from the joint surfaces, capsular ligaments, and labrum. Evaluating the role of all tissues working together as a system for maintaining joint stability during function is important to understand the effects of RCT, specifically in the working population, and may provide insight into root causes of shoulder injury.

FIGURES IN THIS ARTICLE
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The Shoulder Complex.

The shoulder is among the most mobile joints in the body allowing for significant range of motion in multiple planes. The shoulder complex is made of the scapula, clavicle, humerus, and the soft tissues that span the joint including cartilage, capsular ligaments, the labrum, and surrounding muscle-tendon units (Fig. 1(a)) [1,2]. The articulations in the shoulder complex include the glenohumeral joint, scapulothoracic articulation, and the acromioclavicular joint. These tissues work in unison to complete a wide range of kinematic tasks.

Often modeled as a ball-and-socket joint, the three shoulder rotational degrees-of-freedom include flexion and extension, abduction and adduction, and internal and external rotation. Abduction and flexion account for the largest ranges of motion (170 ± 10.8 deg and 164 ± 10.2 deg, respectively) compared to extension (81 ± 11.3 deg). The shoulder joint can rotate more internally (86 ± 4.6 deg) than externally (67 ± 11.3 deg) [3].

Rotator Cuff Tear and Treatment.

Injuries to the upper extremities can occur as a result of a wide range of activities from sports to occupational, and account for more work-related injuries (31%) than any other body region [4]. Among the shoulder injuries, rotator cuff tears (RCTs) warrant specific attention because of the high incidence among workers and the variable success rate of repairs. Rotator cuff problems account for over 4.5 million physician visits per year [5], and rotator cuff repair is one of the most common surgeries performed on the shoulder with 250,000 surgeries performed annually in the U.S. [6,7]. Unfortunately, the success rate of rotator cuff repair is variable with many resulting in retears. Revision surgeries can be as high as 30% for isolated supraspinatus tendon tears [8]. Surprisingly, there is a disproportionately low amount of published research with regard to work-related injuries of the shoulder and rotator cuff tears (Fig. 2). This paucity of data and the current revision rate suggests that the relationship between the injury mechanism, repair, and rehabilitation with respect to rotator cuff tears is not well understood.

A rotator cuff tear is described as a tear of one or more of the rotator cuff tendons (supraspinatus, infraspinatus, teres minor, and subscapularis (Figs. 1(b) and 1(c)) [2]) and is classified by the size of the tear. A full thickness tear indicates a through-thickness tear of the shoulder (Fig. 1(e)) while a partial thickness tear is described as the fraying of the tendon–bone connection (Fig. 1(f)) and can lead to a full tear if not treated properly [911]. RCTs cause pain, depending on the severity of the tear, and can lead to limited function in the affected shoulder, especially during overhead activities [11,12]. Other symptoms include, but are not limited to weakness, tenderness, and snapping sounds coming from the joint [11]. Individuals with RCTs have also reported difficulty sleeping on the effected side [12]. In contrast, RCTs can also be asymptomatic with little to no clinical symptoms [13].

More Than Muscle: Evaluating the Consequences of RCTs on the Shoulder Complex.

Due to the asymptomatic nature of many RCTs, it is not possible to know how many tears go unreported; however, it has been suggested that symptomatic RCTs accounted for 34.7% of all tears and asymptomatic tears for 65.3% [14]. While the tear is often considered to be an injury to the tendons, and is consequently treated as such, there has been evidence in the literature that the RCTs may have significant effects on the remaining surrounding tissues. The mechanical basis of normal shoulder joint function depends on the balance between active muscle forces and passive stabilization from the joint surfaces, capsular ligaments, and labrum. Understanding the effect of rotator cuff tears on the mechanics of both the injured tendon and surrounding tissues is important for connecting and translating the results that arise from studies of in vivo shoulder kinematics, cadaveric studies using simulators, or in vivo muscle volume studies [1518]. We suggest that an improved comprehension of the mechanisms underlying shoulder function, before and after injury, can lead to improved diagnosis and treatment.

Therefore, we aimed to summarize the mechanical consequences of rotator cuff tears on both the injured tendons and the surrounding tissues. Specifically, tears in the supraspinatus and infraspinatus rotator cuff tendons have an immediate primary effect on the balance of muscle forces at the shoulder joint [19] and are the subject of Sec. 2 of this review. In Sec. 3, we compared longitudinal changes in the mechanical properties of the bone–tendon interface as a result of RCT. Next in Sec. 4, we evaluated the secondary consequences of rotator cuff tears on the mechanical properties of uninjured tendons, ligaments, and cartilage. Finally, we identify opportunities for further study that may lead to provide better outcomes of rotator cuff surgeries.

We define the primary effects of rotator cuff tears as changes in the mechanical or structural properties of the torn tendon. Rotator cuff tears predominantly occur in the supraspinatus tendon [12]. Using rodent models, the elastic modulus of the supraspinatus decreased by (72%) after 14 days of detachment, and was likely associated with the increased area (200%) observed at the same time period (Figs. 3(a) and 3(b)) [20]. The thickening of the remaining tendons after injury is the physiological adaptive response of the remaining tendons to the increased load that they bear. However, after 20 days of that detachment, modulus values returned to pre-injury values.

This trend was different when both supraspinatus and infraspinatus were injured [21]. Interestingly, supraspinatus area increased (33%) after 56 days of detachment and remained higher (26%) compared to the control at 112 days (Fig. 3(c)) [21]. However, the modulus of elasticity of the supraspinatus tendon did not change for any time period following for multitendon tears (Fig. 3(d)), in contrast to the results of the single tear. Therefore, the mechanical change in supraspinatus seems to be dependent on whether or not it alone is torn, or whether there are multiple tendon tears present.

While the supraspinatus tends to be the most common tendon torn, the infraspinatus was more sensitive to multitendon tears than supraspinatus. When both infraspinatus and supraspinatus were torn, the modulus of elasticity of the infraspinatus tendon changed: modulus of elasticity decreased at 28 days and increased at 112 days, while stiffness increased only after 112 days (Figs. 3(e) and 3(f)) [21]. The rationale for why infraspinatus is more sensitive to multitendon tears than supraspinatus is not clear and remains a point for further investigation.

Experiments have also been conducted to investigate the effects of tendon repairs on tendon mechanical properties [6,2224]. The supraspinatus tendon in rabbit was repaired immediately after detachment, but both stiffness and peak load decreased after 7 days compared to the uninjured supraspinatus tendon (52% for stiffness and 60% for peak load) [22]. Another study compared different repair time periods (1, 2, 3 months delay repair) with control (uninjured) in rabbits [23]. Results showed that if the injured supraspinatus tendon was repaired after 1 month of disruption, stiffness increased (19%) relative to control. However, if repaired after 2 or 3 months of disruption, stiffness did not change paradoxically suggesting that waiting to repair the tendon restores pre-injury stiffness levels.

Galatz et al. compared the effect of immediate and delayed repair (repaired after 3 weeks of detachment) in rat supraspinatus tendon [6]. Area increased (46%) for delayed repair compared to immediate repair when measured after 28 days of repair. Maximum stress decreased (80%) for the delayed repair group while measured after 10 days of repair. Plate et al. also investigated the influence of age on immediate repair of supraspinatus tendon in rats compared to control (uninjured) conditions [24]. For the older group (24 months of age), failure stress and peak failure load significantly decreased in the immediately repaired tendon at both 14 and 56 days after repair compared to the control. In contrast, for a younger group (8 months of age), failure stress and peak failure load of the immediate repair group only decreased after 14 days of repair. Therefore, properties of the repaired tendon depend not only on the timing of the surgical repair but also on the age. Earlier repair providing better mechanical properties may lower the risk of the tendon retear. Finally, results also showed that aging has a negative impact on the healing of the tendons.

Other species like canine and ovine also showed the change in tendon properties after tears [25,26]. Studies have investigated the change in the properties of the infraspinatus muscle and tendon after disruption [25,26]. The stiffness and modulus of elasticity significantly increased in detached infraspinatus muscle after 84 days of detachment compared to uninjured muscle in canine [25]. Similarly, the modulus of elasticity of the infraspinatus tendon increased after 42 days and 126 days of detachment compared to the uninjured tendon in ovine (60% for 42 days, and 70% for 126 days) [26].

Different animal species including rat, rabbit, canine, and sheep have been used to evaluate the effects of RCTs. However, Chaudhury et al. used human biopsy samples to measure the effects of RCTs [27]. The storage modulus, calculated by dynamic shear analysis, showed that the torn tendons had significantly lower modulus (20%) compared to normal tendons.

In summary, the increment or decrement of properties of the injured tendons and muscles depends not only on the number of tears but also on the time period after injury. Even after repair, the tendons may experience changes in properties compared to control. These changes in tendon properties can further affect the balance between muscle forces and passive stabilization and can lead to shoulder joint instability and abnormal joint kinematics.

The tendon to bone insertion site consists of functionally graded tissue whose function is to transfer load between the hard bone and soft tendon. Without this transitional area, high stress concentrations would form at the interface of these two materials, leading to an increased potential for failure [2831]. The insertion site is divided into four zones: tendon, fibrocartilage, mineralized fibrocartilage, and bone [32]. Each zone contributes to the overall gradient in cell phenotype, tissue organization, tissue composition, and tissue mechanical properties [33].

The studies on mechanical properties of insertion site that are reviewed in this paper compared normal, healthy supraspinatus tendons to injured and repaired supraspinatus tendons. The repaired tendons were tested after several weeks to observe the effect of time on the healing of the insertion site. Studies also considered the effect of different activity levels on the healing process [32,3437]. While the measured properties varied with time and occasionally among activity levels, most properties changed from their corresponding control values (Fig. 4) [32,3437]. It is important to note that the control measurements of Manning et al. [37] differ from the control measurements of Thomopoulos et al. [34] and Gimbel et al. [35] even though test methods were similar. The studies showed that the quality of the healing tissue differs for the normal insertion site even after an extended test period. This is consistent with other studies that show that the normal four-zone insertion site does not reform once damaged [6,28,34,3849]. All insertion studies that are reviewed in this paper used rat shoulders as test specimens. While rat shoulders have anatomy and repair procedures comparable to the human shoulder, the conclusions made in these studies cannot be directly applied to the humans [34,50]. Another limitation of these studies is that the supraspinatus tendons were “detached” or “transected” and not torn by an acute traumatic or chronic degenerative process. These tendons were completely separated from the humeral head while many human patients experience only partial tears.

These studies of the insertion site measured apparent properties and do not account for changes in properties along the tendon to bone insertion site. The insertion site was categorized as bone or tendon compartments, and only viscoelastic measurements of the intact insertion site were considered [28]. While the current studies focus on the properties of the insertion site after repair, no studies have tested the damaged insertion site. Due to insufficient knowledge about the natural healing process of the insertion site, little can be done to regenerate the normal tissue [32]. Therefore, it is necessary to investigate the development of the normal four-zone insertion site.

The stability of the glenohumeral joint depends on the balance between static and dynamic structures including the glenoid articular cartilage, glenoid labrum, ligaments, joint capsule, osseous structures, rotator cuff muscles, and other muscle structures surrounding the shoulder joint. In healthy shoulders, these structures allow for concentric rotation of the humeral head on the glenoid surface. However, the loss of muscle force due to RCTs likely leads to glenohumeral joint instability, and the articular surfaces are exposed to abnormal joint mechanics. Therefore, in addition to the primary injured tendon, RCTs also have secondary effects on the remaining intact tendons, cartilage, and ligaments of the shoulder.

Intact Rotator Cuff Tendons.

Due to the dependence between glenohumeral joint structures, tears in any of rotator cuff tendons will eventually affect the properties of other surrounding intact rotator cuff tendons. Several studies have shown that the mechanical properties of the infraspinatus and subscapularis tendons change due to tears in surrounding rotator cuff tendons. Properties of the intact rotator cuff tendons were measured for both control (uninjured, i.e., no tears in surrounding rotator cuff tendons) and injured tendons (at least one of surrounding rotator cuff tendons is torn). We calculated the percent change of intact tendon properties after tearing in surrounding tendons relative to the control condition and summarized these findings graphically in Fig. 5.

Single Tear of Supraspinatus.

Detachment of the supraspinatus tendon caused changes in both structural and mechanical properties of the infraspinatus and subscapularis tendons (Fig. 5(a)) [51]. The area of infraspinatus and subscapularis tendons increased after 4 weeks and 8 weeks of detachment. However, the elastic modulus decreased and the percent relaxation increased only after 8 weeks of detachment. No changes were observed for peak load and equilibrium load [51].

Experiments have been performed to compare between normal cage activity and overuse activity of rats due to supraspinatus tear. After supraspinatus tendon detachment, area and modulus did not change for the infraspinatus and subscapularis [52]. Although supraspinatus is the most frequent torn tendon among rotator cuffs, there are only two studies that measured the biomechanical effects of supraspinatus torn tendons on surrounding intact rotator cuff tendons [51,52].

Multitendon Tear: Supraspinatus and Infraspinatus.

Multitendon tears in both supraspinatus and infraspinatus also affected the properties of subscapularis (Fig. 5(b)) [51], but to different degrees compared to the single-tendon tear. The initial (4 week) increase in area of the subscapularis was nearly twice as high in the presence of both supraspinatus and infraspinatus tears compared to a supraspinatus tear only. By 8 weeks, the change was less profound (68% in double tear compared to 50% in single tear). The modulus decrease in the double tear was nearly identical to the single tear scenario. Stiffness only decreased after 8 weeks and no changes were seen for percent relaxation, peak load, and equilibrium load [51].

Comparing between normal cage and overuse activity in rats with multitendon tears (supraspinatus and infraspinatus) showed that the elastic modulus decreased in the midsubstance of the lower subscapularis and upper subscapularis, and insertion site of lower subscapularis for overuse activity group [53]. Also, elastic modulus increased in the insertion site of upper subscapularis. However, area did not change for any region of these tendons. In contrast, for a single tear, no changes were observed for both modulus and area as mentioned earlier. Therefore, elastic moduli are affected by overuse activity if the infraspinatus and supraspinatus are both detached.

Comparisons were also performed between single tear (supraspinatus) and multitendon tears (supraspinatus and infraspinatus) to measure the contributions of the additional tear compared to the single tear [54]. No area changes were observed for lower subscapularis and upper subscapularis for multitendon tears compared to single tendon tear. Elastic modulus decreased for the midsubstance region of the lower subscapularis and upper subscapularis. However, the modulus increased for the insertion region only in the upper subscapularis [54].

Multitendon Tear: Supraspinatus and Subscapularis.

The structural and mechanical properties in the infraspinatus changed due to tears in the supraspinatus and the subscapularis (Fig. 5(c)) [51]. For the infraspinatus tendon, the area increased after 4 weeks and 8 weeks in a similar fashion as single tear. (For 4 weeks, 11% in double tear compared to 16% in single tear; for 8 weeks, 35% in double tear compared to 37% in single tear). Modulus decreased and percent relaxation increased after 8 weeks of detachment (for modulus, 22% in double tear relative to 26% in single tear; for percent relaxation, 14% for double tear relative to 13% in single tear). Stiffness, peak load and equilibrium load were not affected by multitendon tears, as was also reported for single tear. It is interesting that the degree to which these properties are altered for multitendon tears are similar to single tear, and suggests that detachment of subscapularis in addition to supraspinatus would not further change the infraspinatus properties. In summary, the area of the intact tendons increased and modulus decreased after tears in surrounding tendons.

Comparison Between Single Tear and Multitendon Tears.

While comparing between single tear and multitears, the area increased and modulus of elasticity decreased for infraspinatus and subscapularis irrespective of the number of the tears. However, the degree to which these properties would change depends on the number of the tears. For example, changes in properties in subscapularis were more for multitears compared to single tear. In contrast, the change in properties was identical for single tear and multitears in infraspinatus. Furthermore, different degrees of loadings have significant contributions on how these properties are changed.

Cartilage.

The effect of supraspinatus and infraspinatus tears on cartilage thickness and elastic modulus has been evaluated using rat models. Glenoid cartilage thickness decreased in the antero-inferior region after the detachment of the supraspinatus and the infraspinatus tendons while the elastic modulus decreased over a larger region of the glenoid (Fig. 6) [55]. Reuther et al. reported that after 8 weeks of supraspinatus tendon detachment, within the overuse activity rat group, the equilibrium modulus increased significantly in antero-inferior and superior regions of glenoid cartilage compared to normal cage activity group [52]. But no change in thickness was observed in any regions. However, for detachment of supraspinatus and infraspinatus tendons together, the cartilage modulus decreased in the center and posterior–superior regions in the overuse activity group [53].

The detachment of the biceps tendon in addition to multitendon tears (supraspinatus and infraspinatus) also reduced glenoid cartilage thickness in the anterior–inferior region, with no change in elastic modulus [56]. However, the elastic modulus decreased in the center of the glenoid for multitendon tears (supraspinatus and infraspinatus) compared to supraspinatus tendon tear [54]. Current studies only show the effect of rotator cuff injuries on glenoid cartilage mechanics for rats, no properties data available for human glenohumeral cartilage.

Previous studies have suggested that there is a correlation between the glenohumeral joint arthrosis and tear/atrophy of the rotator cuff [5761]. However, the exact pathogenesis of how the articular cartilage degeneration occurs is not well understood. Therefore, further investigation is necessary to understand the cartilage degeneration process during the progression of RCTs.

Ligament.

There are four ligaments surrounding the glenohumeral joint: coracohumeral, superior glenohumeral, middle glenohumeral, and inferior glenohumeral (Fig. 1(d)) [2,62]. These ligaments are characterized as a thickening of the glenohumeral capsule. Yet, changes in mechanical and geometric properties due to RCTs are only available for the coracoacromial ligament (CAL), which extends between the coracoid process and acromion of the scapula. No studies have evaluated the effects of RCTs on the mechanical properties of superior, middle, and inferior glenohumeral ligaments. Human cadaveric studies have reported how the CAL properties changed for RCT for different age groups (Table 1) [63,64]. The thickness, width, and area of the medial band of the CAL did not change for any age groups after RCT; however, the length of the medial band decreased 18% in younger subjects.

Whether or not the medial band length differs in older subjects after RCT is less clear: one study reported decreased length [64] while another showed no change in length [63]. The length and area of the lateral band of the CAL have been shown to be sensitive to RCTs. Similar to the medial band, the length of the lateral band consistently decreased after RCT. In contrast to the medial band, area increased after RCT in subjects over 60 years of age. For older groups, the total and ligamentous modulus were significantly lowered after RCT [63]. There were no changes in width, thickness, stiffness, failure load/displacement, total or ligamentous failure strain after RCT.

There are five types of CAL for the human shoulder joint: Y-shaped (two bands), broad-band (one band), quadrangular (one band), V-shaped (two bands), and multiple-banded [65]. Comparing these five types with RCT, no statistical relations were found between types or geometric measurement of these CAL and RCT. However, if these five types were divided into two groups based on bundle numbers like unique bundle (broad-band and quadrangular) and more than one bundle (Y-shaped, V-shaped, and multiple-banded), CAL with more than one bundle showed a significant association with RCT with a longer lateral border and larger coracoid insertion [65].

In addition to mechanical testing, several other methods have been used to measure the elasticity of ligaments. Kijima et al. measured the strain ratio, defined as the ratio of strain of CAL to that of the RCT, as an index of the elasticity of CAL by ultrasound elastography. Here, the higher strain ratio indicates that the CAL is softer. The strain ratio of CAL with RCT (23.75 ± 15.05) was higher than that of the older ligaments without rotator cuff tear (12.62 ± 7.94) suggesting that the CAL softens in the presence of RCT [66]. Scanning acoustic microscopy has also been used to measure the speed of sound through the CAL, which is directly proportional to the square root of the Young's modulus [67]. Using this method, the modulus of the CAL in those with the RCT was higher than without the RCT group.

Risk Factors Associated With RCTs.

While RCTs can occur during traumatic events such as a fall or an accident, most tears develop gradually [68]. The risk of rotator cuff tears increases with age [6971], and more than half of the population over 60 years of age is currently living with some degree of rotator cuff injury [71]. Arm dominance and gender have not been correlated with risk of RCT [13]; however, women have worse rotator cuff repair outcomes over time than men based on postoperative pain and level of abduction [69]. Individuals who have a history of cigarette smoking or have higher body mass index have also been shown to be at increased risk for RCT [72]. Athletes who perform high stress repetitive motions at the shoulder are more prone to developing RCTs [68], but majority of injuries occur in the lay population and often stem from work-related injuries. The distinction between sport and occupational tears becomes blurred when the mechanism behind the tears is considered.

A number of workplace risk factors have been identified for RCTs including large and sudden forces, heavy lifting, extensive overhead activity, repetitive or long duration actions, and vibration. A recent review paper [73] compiled an extensive list of reported odds ratios and confidence intervals associated with rotator cuff tears as a function of activity types. We extracted the odds ratios and grouped them according to the following risk factors: repetition, duration, vibration, force, posture, and heavy physical work (Fig. 7) [73]. A one-way analysis of variance with Tukey posthoc analysis was conducted to determine if there was a statistically significant difference in the risk factors (α = 0.05) (OriginPro 2015, OriginLab Corporation, Northampton, MA). Repetition had the highest odds ratio (4.22) while vibration had the lowest average odds ratio (2.46) (Table 2) [73]. There was a wide range of odds ratios within any given risk factor, and comparisons of the means across risk factors did not reveal significant differences between risk factors. Differences in study methodologies and variations in risk factors (such as high vibration versus low vibration tool use) may partly explain this variability.

Limited studies have aimed to understand rotator cuff tears in the populations that experience tears most: occupational workers in comparison to athletes, the latter of which have a unique anatomy and physiology. Understanding the compensatory effects of injury to a rotator cuff tendon/tendons on the remaining uninjured tissue is important both from a clinical perspective and, in the cases of a Worker's Compensation claim, the medical legal perspective. Localizing the time that the injury occurred can have the obvious benefit of elucidating causality in a Worker's Compensation claim.

Improving the Treatment of RCTs.

Rotator cuff tears are one of the most common injuries affecting the upper extremity with variable success rate in repair surgeries. Animal species (rat, rabbit, sheep, canine) are considered as the most appropriate models and have been used in understanding the consequences of RCTs. However, none of them can exactly replicate the human shoulder. Although studies on the effect of RCTs have made considerable progress in the last few decades providing the biomechanical alterations in animals; still there are some limitations, specifically, how the properties of the soft tissues (intact tendons, ligaments, and cartilages) will be altered for humans.

Many studies focus on RCTs from the point of injury forward (diagnosis, classifications, repair, etc.), and it is widely appreciated that the degree of return to pre-injury functional activities is related to both the rehabilitation protocol as well as the treatment intervention [74]. Ideally, postintervention decisions are made in concert between (1) the surgeon who initially prescribes which activities of daily living the patient can resume and (2) the physical therapist prescription of rehabilitation protocols. However, these recommendations are variable with respect to timing and progression [34,7577], and in many cases can be based on clinical experience rather than quantitative data [74].

The successful repair of RCTs requires understanding how the primary and secondary consequences might affect the shoulder joint stability and function. Furthering the understanding of the tissue biomechanics of the shoulder girdle is essential to improving the care delivered to patients. The natural history of shoulder pathology is very complex as changes in the anatomy and dynamic stability of the shoulder, as a result of specific pathologies, are an area where there is disagreement and at times controversy within the clinical community. For instance, the role of the long head of Biceps in humeral head position is not well defined. Understanding this role would have a direct effect on surgical decision making, such as whether performing a tenotomy during rotator cuff repair is warranted.

Recently, the reconstruction of the superior capsule for complete rotator cuff tear has received more attention. However, the biomechanics of this surgery are not well defined and would assist in understanding the surgery further. Surgical planning for the shoulder is highly dependent on understanding the static and dynamic biomechanical issues in the normal and pathologic shoulder. Finally, in terms of rehabilitation, some patients with massive rotator cuff tears do well with treatment by physical therapy alone while others go on to a pseudoparalysis and have a nearly nonfunctional shoulder. These contrasting outcomes indicate that in some cases, the remaining tissues in the shoulder complex can work toward stabilizing the joint but the mechanisms behind this are not understood. Understanding the biomechanics including adaptive reconfiguration of the remaining soft tissues in this situation would be helpful in identifying those patients who may benefit most from specific postsurgery rehabilitation.

Identifying changes in the mechanics after RCTs with development in tissue engineering will allow researchers to improve the current surgeries available to treat rotator cuff tears. Future research should aim to elucidate the effects of RCTs on human tissues to better the understanding of RCTs on human shoulder function and develop better treatment algorithms.

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Figures

Grahic Jump Location
Fig. 1

(a) Shoulder joint including bones, cartilage, and capsule, (b) posterior, and (c) anterior view of muscles that span the shoulder with rotator cuff muscles highlighted, (d) ligaments of the shoulder joint with coracoacromial, coracohumeral, and glenohumeral ligaments highlighted, (e) a full thickness tear in supraspinatus tendon, and (f) a partial thickness tear in supraspinatus tendon [1,2]

Grahic Jump Location
Fig. 2

Number of articles found during the Pubmed search. For Pubmed search, we used the keywords as “A” and “B”, where “A” indicates either “Shoulder injury” or “RCT.” “B” indicates “Sports” or “Occupational” or “Work.” Graph shows that there was higher number of papers published for “Sports” compared to work-related injuries.

Grahic Jump Location
Fig. 3

Change in supraspinatus tendon (a) area and (b) modulus of elasticity over time following its injury in rat (n = 10 for each data point). Change in supraspinatus tendon (c) area and (d) modulus of elasticity over time following both supraspinatus and infraspinatus injuries (n = 12 for each data point). Change in infraspinatus (e) area and (f) modulus of elasticity over time following both supraspinatus and infraspinatus injury (n = 12 for each data point). The X-axis represents the time after injury. The Y-axis represents the properties. Closed and open symbols represent data for the control and injured tendons, respectively. * indicates statistically significant difference between control (uninjured) and injured tendon [20,21].

Grahic Jump Location
Fig. 4

Change in insertion: (a) stiffness, (b) area, and (c) modulus of elasticity over time after operation. The X-axis represents the time after injury and repair. Y-axis represents the properties. Each unique symbol denotes a different study. All studies have been done using rodents and n indicates the number of the rodents used in different cases. Closed symbols represent the control data for each study. Studies 1, 2, 3, 4, and 5 represent Refs. [35], [32], [37], [34], and [36], respectively.

Grahic Jump Location
Fig. 5

Change in (a) infraspinatus and subscapularis properties due to supraspinatus tendon detachment (b) subscapularis properties after supraspinatus and infraspinatus tendons detachment, and (c) infraspinatus properties and supraspinatus and subscapularis tendons detachment. Properties are expressed as the percent change after the detachment compared to control (uninjured). 4 wks and 8 wks indicate properties measured at 4 weeks and 8 weeks after injury. The squiggly lines near the tendon insertion site represent tendon detachment. Properties above and below the solid lines indicate percent increase and percent decrease from control, respectively. “nsd” indicates no significant differences from control for that property [51].

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Fig. 6

Thickness and elastic modulus change in glenoid cartilage after RCT. nsd indicates no statistically significant difference after multitendon tears (supraspinatus and infraspinatus) compared to the control (uninjured). ↓ indicates statistically significant decrease after multitendon tears compared to the control [55].

Grahic Jump Location
Fig. 7

Distribution of statistically significant odds ratios with RCTs as a function of risk factors [73]

Tables

Table Grahic Jump Location
Table 1 Changes in ligament properties after RCT [63,64]
Table Footer Note−decreased significantly, + increased significantly.
Table Grahic Jump Location
Table 2 Quantitative summary of odds ratios associated with risk factors shown in Fig. 7 (N = number of studies, SD = standard deviation, SE = standard error) [73]

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