Clinical Biomechanics of Rotator Cuff Tears
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Rotator cuff tears[edit | edit source]
Rotator cuff tears are one of the leading causes of shoulder pain and disability. A rotator cuff tear involves the tearing of one or more tendons of the rotator cuff muscles, namely supraspinatus, infraspinatus, teres minor, and subscapularis.  Rotator cuff tears may be classified by the specific muscle(s) affected and the size of the tear (partial or full).  Rotator cuff tears may be caused by repetitive microtraumas, degenerative changes with age, or as a result of a traumatic event. 
Biomechanics of the Healthy Rotator Cuff[edit | edit source]
Rotator cuff and glenohumeral stability[edit | edit source]
The glenohumeral joint is formed by the articulation of the round humeral head within the shallow glenoid fossa.  The “socket” of this ball-and-socket joint is deepened by the presence of the glenoid labrum. Together, the glenohumeral ligaments and joint capsule provide passive stability to the joint in the end ranges of motion.  The rotator cuff muscles, in their role as dynamic stabilisers of the shoulder complex, are important for maintaining the stability of the glenohumeral joint, which is highly mobile but inherently unstable.
Rotator cuff biomechanics[edit | edit source]
The rotator cuff provides active stability through a full range of shoulder motion, including flexion/extension, abduction/adduction, and internal/external rotation. Generally, the action of rotator cuff muscles is for supraspinatus to initiate abduction, infraspinatus and teres minor for external rotation, and subscapularis for internal rotation. 
Concavity compression[edit | edit source]
Concavity compression is the main mechanism by which the rotator cuff provides stability.  This involves the compression of the convex humeral head into the concave glenoid fossa.  The function of this mechanism is enhanced by the glenoid labrum, which provides approximately 20% of this stabilization effect.  Concavity compression is particularly important in the mid-ranges of shoulder motion when the joint capsule and ligaments are lax. 
Force couples[edit | edit source]
Force couples are another important mechanism by which the rotator cuff muscles provide stability to the glenohumeral joint. Force couples occur when two opposing muscles create a moment of force about a stable fulcrum or axis of rotation.  The rotator cuff muscles create a force couple around the glenohumeral joint in the transverse and frontal planes. This stabilizes the humeral head within the glenoid fossa through various motions at the shoulder by opposing the forces created by the surrounding musculature. The transverse plane force couple consists of the subscapularis anteriorly and the infraspinatus/teres minor posteriorly. In the frontal plane, the force couple is formed by the inferior rotator cuff muscles which counteract the deltoid acting superiorly.  Balanced force couples are important for glenohumeral stability and require coordinated activation of agonist and antagonist muscles. 
Biomechanical Factors Influencing Rotator Cuff Tears[edit | edit source]
There are a number of factors that may contribute to rotator cuff tears, these include repetitive stress & microtrauma accumulation, age & degenerative changes, and acute or traumatic events.
Repetitive stress & microtrauma accumulation[edit | edit source]
Rotator cuff tears may occur due to repetitive stress and microtrauma accumulation. Repetitive loading can cause microtrauma of the rotator cuff tendon fibres.  This microtrauma or very small tearing of the fibres can lead to partial tearing of the tendon which may gradually progress to a full tear.  This may occur as a result of repetitive overhead motions, as well as when the tendon and musculature are given insufficient rest to repair damage that has accumulated.  Hand dominance has also been identified as a risk factor for rotator cuff tears.  The dominant hand tends to be used more, therefore tends to undergo more repetitive stress, which may contribute to tearing.  Occupational demands from repetitive and/or heavy labour also influence the prevalence of rotator cuff tears.  The rotator cuff is also placed under considerable stress in athletes that participate in overhead or throwing activities.  With repetitive throwing motions, the arm undergoes large amounts of internal and external rotation in which the rotator cuff tendon fibres are subject to twisting, which can lead to torsional overload and shear failure. 
Age & degenerative changes[edit | edit source]
Aging is an important risk factor for the incidence and severity of rotator cuff tears.  It has been observed that the prevalence of rotator cuff tears increases with increasing age.  This may be a result of tendon biomechanics being affected with age, where evidence appears to show a decrease in modulus and strength of tendons with aging.  With increasing age, there are degenerative changes including thinning and disorientation of collagen fibres in the rotator cuff which may be a contributing factor to tearing.  In general, age-related changes may cause tendons to become weaker and more susceptible to injury, such as tearing. 
Acute & traumatic events[edit | edit source]
Rotator cuff tears may also occur as a result of a traumatic event, such as shoulder dislocation. When these types of injuries occur, the torn rotator cuff may contribute to recurrent instability.  Traumatic rotator cuff tears are more likely to occur in young, males who fall on an outstretched arm, most commonly in an abducted and externally rotated position. 
Biomechanical Consequences of Rotator Cuff Tears[edit | edit source]
Rotator cuff tearing affects the biomechanics and stability of the glenohumeral joint.  A tear of the anterior rotator cuff, for example subscapularis, can cause weakness in active internal rotation and increased passive external rotation. A tear in the posterior rotator cuff, for example infraspinatus/teres minor, can cause weakness in active external rotation and increased passive internal rotation.  Full supraspinatus tearing results in the inability to achieve full shoulder abduction, as supraspinatus has an important role in initiating this motion.  When this type of rotator cuff tear progresses and affects the transverse force couple, the force of the deltoid is unable to achieve full abduction, due to the loss of the stable fulcrum for the glenohumeral joint to move.  Furthermore, tear progression involving infraspinatus affects humeral head kinematics.  This is observed as superior and lateral migration of the humeral head at maximum internal rotation and posterior migration in the mid-range of motion.  Again, this is a consequence of an imbalance in the glenohumeral force couple, which effectively decreases joint compression force.  This can be explained by previous biomechanical analysis that rotator cuff tearing leads to a significant decrease in the magnitude of joint reaction forces at the glenohumeral joint.  The direction of the joint reaction forces is also affected, this causes off-centering of the compressive force, which further contributes to instability and migration of the humeral head.  Rotator cuff tears may lead to superior migration of the humeral head and even degeneration of the joint itself.  Overall, a rotator cuff tear leads to an imbalance in force couples, which compromises concavity compression and glenohumeral stability. 
In summary, rotator cuff tears can lead to pain and dysfunction in the shoulder joint. Shoulder rehabilitation should focus on restoring balanced glenohumeral force couples and concavity compression to maintain proper stability and function. 
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