About the Mechanics of Shoulder Stability

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Shoulder stability

One of the many remarkable features of the normal shoulder is that the glenohumeral joint (the ball and socket joint between the upper arm bone and shoulder blade) is very stable in spite of the vast range of motion available to the joint (see figure 1).

The glenohumeral joint does not have a deep socket or ligaments that are always under tension (see figures 2 and 3). Thus the glenohumeral joint must be stabilized by mechanisms quite different than those which stabilize the hip or knee. The mechanisms of shoulder stability are different but effective: the humeral head which is slightly smaller than a billiard ball is held precisely centered on the glenoid which is about the size of a tea spoon. It is amazing that such an arrangement can allow the shoulder to throw pull up lift punch and do gymnastics without coming apart!

Figure 1 - Glenohumeral joint

Figure 2 - Deep socket

Figure 3 - Ligaments under tension

Using Dr. John Sidles' Orthokine Laboratory we have done a lot of research into how the normal shoulder is stabilized.

Sockets capsules ligaments

We have found that glenohumeral stability requires that there be a socket into which the ball can be pressed and that the muscles around the joint work in a balanced way to press the humeral head into the glenoid (see figures 4 and 5).

Glenohumeral stability also requires that the capsule and ligaments check the motion of the joint so that it does not rotate to positions where the forces become unbalanced. You can see more about how the ligaments help control shoulder stability in activities such as the baseball pitch. However it is interesting to note that in many important positions of the joint these ligaments do not stabilize the shoulder. The normal joint is in fact quite loose. In most positions the ball is held in the socket primarily by muscle action as described above.

Figure 4 - Socket

Figure 5 - Humeral head and glenoid

Because glenohumeral stability is so important we discuss it in great detail. To keep it from being too dry we have included some very interesting videos prepared by Dr. Douglas Harryman of the UW Shoulder and Elbow Team.

List of movies

Below is a list of movies about the mechanics of shoulder stability.

Click to play

The Mechanisms of
shoulder stability part 1
Click to play

Anatomic dissection
of the shoulder socket
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Anatomic dissection
of the shoulder muscles
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Anatomic dissection
of the shoulder's capsule
Click to play

Capsular ligaments
of the shoulder
Click to play

Joint laxity
Click to play

Anatomy of
shoulder stability
Click to play
The mechanisms of
shoulder stability part 2
Click to play

Knee stability

Click to play

Hip stability
Click to play

suction cup and
adhesion-cohesion forces
Click to play

Glenohumeral balance
Click to play

Click to play

The capsule

Click to play

Limited joint volume
Click to play

Family of
stabilizing mechanisms
Click to play

The ligaments

Precise definitions are necessary in the discussion of glenohumeral stability.

Glenoid center line

The glenoid center line is the line perpendicular to the surface of the glenoid fossa at its midpoint (see figure 6).

Net humeral joint reaction force

The net humeral joint reaction force is the vector sum of all forces acting on the humeral head relative to the glenoid fossa. It is this force that needs to be stabilized at the glenohumeral joint. This force includes component forces applied to the humerus by muscles capsule and ligaments as well as by external factors such as gravity contact with objects and inertia (see figure 7).

Glenohumeral translation

Glenohumeral translation is movement of the center of the humeral head with respect to the face of the glenoid (see figure 8).

Glenohumeral translational laxity

Glenohumeral translational laxity is the translation observed on examination of the joint. A substantial amount of laxity is characteristic of normal glenohumeral joints.

Glenohumeral stability

Glenohumeral stability is the ability to maintain the humeral head centered in the glenoid fossa.

Glenohumeral instability

Glenohumeral instability is the inability to maintain the humeral head centered in the glenoid fossa.

Glenohumeral apprehension

Glenohumeral apprehension is the sense of impending instability in certain glenohumeral positions.

Traumatic glenohumeral instability

Traumatic instability is instability that arises from an injury of sufficient magnitude to tear the glenohumeral capsule ligaments or rotator cuff or to produce a fracture of the humerus or glenoid.

Atraumatic glenohumeral instability

Atraumatic instability is instability that arises without significant trauma.

Figure 6 - Glenoid center line

Figure 7

Figure 8 - Glenohumeral translation

Shoulder & Elbow Articles

  1. About the Mechanics of Shoulder Stability.
  2. Anterior glenoid reconstruction for unstable dislocating shoulders. Surgery to restore lost anterior glenoid bone and deep the socket with a bone graft can restore shoulder anatomy and lessen pain and improve function.
  3. Arthroplasty in Cuff Tear Arthropathy: Surgery for shoulders with a rotator cuff tear and arthritis can lessen shoulder pain and improve function with joint replacement.
  4. Atraumatic Shoulder Instability.
  5. Bankart repair for unstable dislocating shoulders: Surgery to anatomically and securely repair the torn anterior glenoid labrum and capsule without arthroscopy can lessen pain and improve function for active individuals.
  6. Basics of failed shoulder surgery, complications of shoulder surgery and revision shoulder surgery
  7. Chondrolysis
  8. Clinical Presentation and Evaluation of Glenohumeral Arthritis.
  9. Clinical Presentation of Glenohumeral Instability.
  10. Compartmental Syndromes.
  11. Diagnosis of Capsulorraphy Arthropathy.
  12. Diagnosis of the Frozen Shoulder.
  13. Evaluation of Recurrent Instability.
  14. Evaluation of the Rough Shoulder.
  15. Evaluation of the Stiff Shoulder.
  16. Evaluation of the Weak Shoulder.
  17. Examination Under Anesthesia.
  18. Failed Shoulder Replacement and Revision.
  19. Glenohumeral Arthritis References.
  20. Home Exercises for the Rough Shoulder.
  21. Home Exercises for Stiff Shoulder
  22. Home Exercises for the Unstable Shoulder.
  23. Home Exercises for the Weak Shoulder.
  24. Humeroscapular Positions and Motion.
  25. Humerothoracic Positions and Motion.
  26. Injuries Associated with Anterior Dislocations.
  27. Intermediate Shoulder Instability.
  28. Management of Glenohumeral Arthritis.
  29. Mechanics of Glenohumeral Arthritis.
  30. Mechanics of Glenohumeral Arthroplasty.
  31. Mechanics of Glenohumeral Instability.
  32. Mechanics of Shoulder Strength.
  33. More Information on Rotator Cuff Surgery.
  34. Posterior glenoid osteoplasty for unstable dislocating shoulders. Surgery to build up the back of the glenoid socket using an osteotomy and graft can restore shoulder anatomy and lessen pain and improve function.
  35. Ream and Run for Shoulder Arthritis: Conservative Reconstructive Surgery for Selected Individuals Desiring Higher Levels of Activity than Recommended for Traditional Total Shoulder Joint Replacement
  36. Ream and Run non-prosthetic glenoid arthroplasty for shoulder arthritis: Regenerative cementless surgery designed for individuals desiring higher levels of activity than recommended for traditional total joint replacement.
  37. Rehabilitation after Shoulder Arthroplasty.
  38. Rehabilitation following shoulder joint replacement arthroplasty
  39. Relevant Anatomy of Glenohumeral Instability.
  40. Repair of Rotator Cuff Tears: Surgery for shoulders with torn rotator cuff tendons can lessen shoulder pain and improve function without acromioplasty.
  41. Reverse Shoulder Replacement (Delta joint replacement) for arthritis: Surgery with a reverse prosthesis can lessen shoulder pain and improve function in shoulders with failed surgery or combined arthritis, rotator cuff tears and instability.
  42. Reverse Total Shoulder or Delta Shoulder for Shoulder Arthritis Combined with Massive Rotator Cuff Tear and for Failed Conventional Total Shoulder Replacement
  43. Rotator Cuff Clinical Presentation.
  44. Rotator Cuff Differential Diagnosis.
  45. Rotator Cuff Failure.
  46. Rotator Cuff Historical Review.
  47. Rotator Cuff Imaging Techniques.
  48. Rotator Cuff Relevant Anatomy and Mechanics.
  49. Rotator Cuff Tear: When to Repair and When to Smooth and Move the Shoulder
  50. Rotator Cuff Treatment.
  51. SF 36 and Health Status.
  52. Scapulothoracic Positions and Motion.
  53. Shoulder Arthritis
  54. Shoulder Arthritis Book
  55. Shoulder and Elbow Cases to Consider.
  56. Shoulder arthritis and rotator cuff tears: The combination of arthritis and rotator cuff tears is called rotator cuff tear arthropathy. The management of this condition requires thought and experience.
  57. Shoulder arthritis: Osteoarthritis, Chondrolysis, Rheumatoid Arthritis, Degenerative joint disease, and arthritis after shoulder surgery.
  58. Shoulder joint replacement arthroplasty for shoulder arthritis pain and stiffness: two options: total shoulder and ream and run
  59. Shoulder osteoarthritis, chondrolysis, rheumatoid arthritis, degenerative joint disease, and arthritis after shoulder arthroscopy and open surgery
  60. Simple Shoulder Test.
  61. Subacromial Smoothing.
  62. Surface replacement for shoulder arthritis: Surgery with a CAP, a special type of conservative resurfacing joint replacement that resurfaces the ball of the ball and socket joint, can lessen pain and improve function.
  63. Surgery for Atraumatic Instability of the Shoulder.
  64. Surgical release for stiff frozen shoulders: Surgery to remove scar tissue and release contractures can lessen pain and improve function for stiff shoulders that have not responded to rehabilitation or physical therapy.
  65. Total Shoulder Replacement Arthroplasty for Shoulder Arthritis
  66. Total elbow joint replacement for elbow arthritis: Surgery with a dependable, time-tested prosthesis can lessen pain and improve function in elbows, especially in rheumatoid arthritis of the elbow
  67. Total shoulder joint replacement for shoulder arthritis: Surgery with a dependable, time-tested conservative prosthesis and accelerated rehabilitation can lessen pain and improve function in shoulders with arthritis.
  68. Traumatic Shoulder Instability.
  69. Treating Shoulder Dislocation / Subluxation (Instability) and Associated Pain with Minimally Invasive Arthroscopy
  70. Treatment of Recurrent Instability.
  71. Types of Glenohumeral Instability.