Shoulder Dislocation & Instability: Time, Mechanism & Direction

Time

• Acute

  • Acute dislocation or subluxation is referring to a first time fully dislocated or subluxed shoulder.

• Recurrent

  • Recurrent dislocation implies having repeated dislocations of the shoulder. Patients with a tendency for recurrent (repeated) dislocation or subluxation are said to have an unstable shoulder or shoulder instability (unstable shoulder).

• Chronic

  • Although most dislocations are diagnosed and managed promptly, there are rare cases that are missed or neglected, leading to a chronically dislocated state of the joint. They are usually irreducible and cause considerable pain and functional disability in most affected patients, prompting the need to find a surgical method to reverse the worsening conditions caused by the dislocated joint.

     

    

Mechanism

• Subluxation

  • Glenohumeral subluxation is defined as a partial or incomplete dislocation that usually stems from changes in the mechanical integrity of the joint.

• Dislocation

  • The shoulder dislocation (more accurately termed a glenohumeral joint dislocation) involves a complete  separation of the humerus from the glenoid of the scapula at the glenohumeral joint.

•  Atraumatic Involuntary Subluxation

  • Atraumatic involuntary subluxation is a form of instability in which the shoulder starts to slip part way out of joint without having had a significant injury.

    

Direction

• Anterior

  • Accounts for 45 - 50% of all joint dislocations (Chalidis et al., 2007)
  • 90-98% are anterior dislocations (Smith, 2006)
  • Bimodal distribution: Individuals in 20s and 60s (Cutts et al., 2009)
  • Male to female ratio of 3:1 

• Posterior

• Represents 2-5% of all shoulder dislocations (need citation)
• Seizures associated with 39% of cases (Rouleau & Hebert-Davies, 2012)
• Posterior dislocations can also come about from electrocution 
• Diagnosis is missed or delayed in 24 - 79% of cases (McLaughlin, 1952)
• Standard AP radiographs appear normal in 50% of cases (humeral head appears to be normally aligned with the glenoid) (Gor, 2002)
• Strongest shoulder muscles (latissimus dorsi, pectoralis major, subscapularis) overpower others and pull shoulder internally, posteriorly
• Other mechanisms of a posterior dislocation have to do with an anterior-directed shoulder trauma (such as a grabbing the dashboard in a motor vehicle collision or falling on an outstretched hand or an american football lineman contacting an opponent at the line of scrimmage. Posterior shoulder dislocations come about from a large force at anteriorshoulder directed posteriorly against internally rotated arm, flexed Shoulder and adducted arm
• The most common associated injuries with a posterior shoulder dislocation includes a reverse Hill-Sachs defect (compression fracture in the anterior aspect of the humeral head), reverse Bankart lesion (posterior labral tear) and proximal humeral fractures - in particular fracture of the anatomical neck and/or lesser tuberosity.

• Inferior

  • This type of dislocation is commonly referred to as luxatio erecta which means "erect dislocation" in Latin. This name derives from the typical way in which the arm is usually fully abducted and held above the head on presentation.

  •  This is an unusual injury. The majority are traumatic. The classic example is when a motorcycle rider falls off their bike. The humerus neck is pushed against the acromion and this can cause an inferior capsule tear. Another common mechanism of injury for inferior dislocations is when a patient  falls and grasps for an object overhead, hyperabducting the humeral neck against the acromion.  This forces the humeral head out of the socket, tearing the inferior capsule. Soft tissue injury or fractures are common with this type of shoulder dislocation. In addition, neurovascular injury can occur, particularly axillary nerve damage, with these dislocations.

  •  Although it accounts for less that one percent of all shoulder dislocations, when luxatio erecta occurs, it is much more frequent in men than in women with a reported ratio of approximately 10:1.

• Multidirectional

  • Multidirectional instability is defined as symptomatic involuntary instability in two or more directions which differs from hyperlaxity that is characterized by increased length and elasticity of normal joint restraints, resulting in a greater degree of translation of the articular surfaces, but still, physiological and asymptomatic. (Saccomanno et al., 2014)

  •  MDI is associated with hyperlaxity, which can be either congenital or acquired. Congenital can be related to connective tissue disorders such as Ehlers-Danlos syndrome, Marfan syndrome, benign hypermobility syndrome and osteogenesis imperfecta (Zweers et al., 2004). Acquired is more typically related to sports in which athletes are exposed to repetitive microtrauma and overuse, as in gymnastics, swimming and throwing.

  •  The Beighton´s criteria is the most widely used scoring system. (Folci & Capsoni, 2016) A positive Beighton score for adults is 5 out of the 9 possible points; for children, a positive score is at least 6 out of 9 points.