Biomechanical Factors That Contribute To Rotator Cuff Function And Injury
By Richard J. Vahl, D.C.,
and James B. Vahl, CPT
 |
Chronic shoulder problems are the most common upper extremity problem in the general population and in sports. These injuries can occur at any age but are especially more common in the over 40 age group and among men. However, gender is making less of a difference. Therefore, women are no longer exempt. Rotator cuff impingement, tendonitis/tendonosis, capsular tears and structural injuries are all common in athletes male or female. Especially, overhead motion and contact sport athletes are venerable to these injuries. Injuries causing damage to the small delicate rotator cuff muscles are common, often debilitating and very often can easily become chronic.
Synchronous and unimpeded joint motion are both necessary for proper shoulder function. Raising the arm overhead requires a fine combination of shoulder mobility and dynamic stability. Because of its structural mechanics, the shoulder joint relies heavily on support from the group of relatively small muscles collectively known as the “rotator cuff complex.” The rotator cuff complex enables the shoulder joint to produce its numerous characteristic movements while still maintaining a balance between shoulder mobility and stability. This structural arrangement of the glenohumeral joint contributes to its highly mobile but somewhat unstable status. This synchronous and unimpeded motion is a byproduct of:
- proper scapular motion along the thorax
- balanced muscle strength and function
- efficient neurological timing of synergistic muscle contractions
- sensorimotor integration.
Biomechanical Structure
The primary role of the shoulder joint complex is to place the upper extremities in positions that allow the hand and arm to function. The upper extremities can assume an infinite number of positions in a three-dimensional perspective in space. Therefore, the shoulder joint complex is a semi or modified ball and socket type of joint which is capable of movement in all three cardinal and oblique planes of motion. The shoulder joint complex does sacrifice some of its stability as compared to the hip joint for the sake of extra mobility. Therefore, it lacks a deep, fuller socket and broader socket surface area as compared to the hip joint and socket. So, instead of a fairly closed ball and socket joint like the hip joint, it is more of a “golf ball on a tee” joint. The humeral head articulates with a smaller open and shallow saucer- type of articulation, the glenoid fossa, which is located on the anterolateral surface of the scapula. Therefore, the glenohumeral joint stability is classified as being more dynamically stable than statically stable. The major function of the four rotator cuff muscles is to work in concert with each other to allow the arm to move relatively free in numerous positions. They do all this while pulling the humeral head downward and inward within the glenoid fossa.
Biomechanical Function
Since the humeral head is three to four times larger than the glenoid fossa, only approximately 25 percent of the humeral head is in contact with the glenoid fossa at any point in time. One of the main functions of the rotator cuff muscles is to compress and depress the humeral head within the glenoid fossa, to prevent it from spinning, sliding and rolling off the top of the glenoid fossa and striking up against the undersurface of the acromion process. Under normal circumstances, the rotator cuff muscles enable the humeral head to be constrained within a couple millimeters of the center of the glenoid fossa throughout most of the arc of shoulder motion.
The four rotator cuff muscles include:
- the supraspinatus
- infraspinatus
- teres minor
- subscapularis
These muscles all work synergistically. However, they each have an individual function as well:
The supraspinatus arises from the supraspinous fossa of the scapula and attaches to the greater tuberosity on the humeral head. Its function is to work closely with the deltoid muscle to raise the arm in flexion and abduction. The supraspinatus fibers maintain a horizontal line of pull, much like guide wires, which resolves or modifies the deltoid’s vertical line of pull. Weakness or extensive damage to the supraspinatus allows the vertical pull of the deltoid to drive the humeral head directly against the undersurface of the acromion process. When this happens it is called a sub-acromial impingement.
The subscapularis is an internal rotator. It has a diagonal arrangement of its fibers and therefore, a diagonal line of pull as well. It occupies the subscapular fossa on the anterior surface of the scapula, and inserts on the lesser tubercle of the humeral head.
The infraspinatus occupies the infraspinatus fossa on the posterior surface of the scapula, below the spine of the scapula. The infraspanatus also has diagonal fibers. The diagonal orientation of the infraspinatus fibers and their line of pull create external rotation of the arm and shoulder. The teres minor attaches to the greater tubercle of the humeral head.
The teres minor occupies the upper two-thirds of the axillary border of the scapula. The infraspinatus like the teres minor attaches to the greater tubercle of the humeral head and create external rotation of the arm and shoulder. The humeral head rolls up the glenoid during abduction and flexion of the arm and shoulder.
Force couples of the subscapularis, infraspinatus, teres minor and long head of the biceps brachi muscles create a concavity compression mechanism which pulls the humeral head into the glenoid fossa.
Balanced Strength And Function:
Balanced rotator cuff strength and function are necessary to prevent upward migration of the humeral head and subacromial impingement of the rotator cuff tendons. The supraspinatus and teres minor are considered the most efficient abductors and humeral head depressors in the rotator cuff group respectively. As a group, the rotator cuff directs and stabilizes the humeral head within the glenoid while the larger extrinsic muscles like the ltissimus dorsi, pectoralis major and deltoid produce the forces necessary for gross arm and shoulder movements.
Injury Mechanism:
The most common injuries to the rotator cuff include:
- primary impingement
- secondary impingement
- degenerative anatomical changes
- tendonitis
- rotator cuff tears
Rotator cuff tears are generally classified by the extent and depth of the fiber damage as either partial or full thickness tears. Encroachment of the humeral head beneath the acromion process can reduce the shoulder space. This space, between the top of the humeral head and the inferior surface of the acromion process is normally only five to ten millimeters deep. It enables the rotator cuff tendons and their lubricating bursa to glide unscathed beneath the acromion. A reduction in this space can cause impingement or abrasion of the rotator cuff and can cause impingement or abrasion of the rotator cuff and long head of the biceps tendons. On the average, up to 40 percent of rotator cuff tears are considered full thickness tears and the majority of them are considered asymptomatic. Scapular instability is found in two-thirds of individuals with rotator cuff problems. Poor scapular kinetics can lead to “reverse scapulohumeral rhythm” or hiking motion during humeral elevation can lead to impingement as the humeral head is driven upward into the acromion. During shoulder flexion and abduction, both upward rotation and posterior scapular tilt move the acromiom process away from the greater tubercle on the humeral head, allowing the humeral head and rotor cuff muscles and tendons. Factors leading to sub-acromial impingements and rotator cuff tears include the following:
- acute sudden trauma
- repetitive or cumulative trauma
- anatomical variants in acromion process shape
- capsuloligamentous lavity
- excessive tension or tightness of posterior and inferior joint capsule
- impaired sensorimotor function
- improper exercise selection and technique.
Other causes of injury and rotator cuff dysfunction include kyphotic posture with rounded shoulders and abducted scapulae, and periscapular and rotator cuff muscle fatigue and weakness.
Injury Prevention:
Exercises to isolate and strengthen the rotator cuff muscles are important components of an injury prevention and/or rehabilitation program. Exercise, strengthening and conditioning, sensorimotor training, and postural awareness are all essential components of either a rehabilitation or injury prevention program.
The periscapular muscles are:
- seratus anterior
- pectoralis minor
- levator scapula
- rhomboids minor and major
- trapezius
These should be the initial focus of a proximal shoulder stability program. The rotator cuff muscles are generally included in conjunction with the periscapular muscles. Gentle stretching of the posterior joint capsule is indicated for tight shoulders. Excessive capsular tension has long been associated with upward migration of the humeral head during shoulder abduction, i.e. frozen shoulder or causing adhesive capsulitis. Proximal stability from periscapular balanced rotator cuff muscle strength and adequate flexibility help to maintain normal, healthy shoulder function. The intrinsic rotator cuff muscles are important and offset some of the potentially destabilizing forces created by the larger extrinsic muscles.
The rotator cuff muscles oblique line of pull elevation creates downward, inward compression forces. The rotator cuff and periscapular muscles forc7e maintains the humeral head center of rotation. The glenoid labrum, a fibrocartilagenous ring, attaches around the rim of the glenoid fossa and increases the depth of the articular surface area. It augments the rotator cuff’s stabilizing effects. Even experimental removal of the glenoid labrum has led to a 20 percent reduction in shoulder stability in cadavers. Specific scapular motions against the thorax provide the stable, yet mobile supporting base from which the rotator cuff muscles work. The scapula must move with the humeral head in order to maintain a supportive surface. This in turn, enhances glenohumeral stability and maintains optimal height and length tension of the rotator cuff and deltoid muscles. Failure of the periscapular muscles to stabilize and guide the scapula during glenohumeral motion can lead to scapular “dyskinesia.”
Conclusion
Rotator cuff injuries to the shoulder are common, painful and debilitating. Many arm and shoulder movements especially including overhead activities are made possible by the collective actions of four small muscles in the rotator cuff group and structure of the shoulder joint. Understanding the biomechanics, anatomic structure and function of the shoulder and rotator cuff group of muscles can only assist healthcare professionals in establishing a better rationale and understanding for selecting specific technique and exercises for the patient and clients in both rehabilitation and injury prevention.
Dr. Richard J. Vahl is a former Professor and Department Chairman at the Palmer College of Chiropractic in Davenport, Iowa. He is an Adjunct Emeritus Professor on the ICA: Council of Fitness and Sports Health Science. In addition to his D.C. Degree he has a Ph.D. in health, physical education and sports science. He is a diplomat of the American Academy of Pain Management and a fellow in Applied Spinal Biomechanical Engineering. Vahl is a Certified Master of Fitness Science and Sports Science by the International Sports Science Association (ISSA). He has published and lectured both nationally and internationally and is a member of the National Association of Sport and Physical Education (NASPE) and the National Academy of Sports Medicine (NASM). He has been a team doctor for many state, national and international sporting and martial events and has been the team doctor and medical advisor for the USA Kendo Team and Team Miletich Fighting Arts.
James B. Vahl is a graduate of San Diego State University (SDSU) in pre-professional healthcare, molecular biology and Japanese, and is currently a student at Palmer College of Chiropractic. He is a Certified Physical Trainer by the American Council on Exercise (ACE) and a Health/Fitness Instructor, Certified by the American College of Sports Medicine (ACSM). James is also a member of the National Academy of Sports Medicine (NASM) and is the director of gobodyfitness.com.
