Scapulohumeral rhythm is the interplay between the scapula and the humerus during motion of the shoulder. There is a definitive pattern of interaction of scapula and humerus when contributing to the motion of the shoulder. Scapulohumeral rhythm is also called glenohumeral rhythm.
Optimal function of the shoulder is reliant on the coordinated movement of the scapula and the humerus.
When there is a change of the normal position of the scapula related to the humerus, the scapulohumeral rhythm is disturbed.
Scapulohumeral rhythm may get disturbed in various conditions and lead to dysfunction of the scapulohumeral rhythm, termed as scapular dyskinesia due to changes in scapular position and control exerted by the scapula stabilizing muscles.
Altered scapulohumeral rhythm affects the function of the glenohumeral joint and contributes shoulder impingement, rotator cuff pathology and shoulder instability. Scapular dyskinesis has been reported in 70-100% people with shoulder injuries including glenohumeral instability, rotator cuff abnormalities, and labral tears.
Study of scapulohumeral rhythm forms an important part of the clinical examination of the shoulder.
Normal Shoulder Motion and Scapulohumeral Rhythm
[Read Shoulder joint Anatomy]
There is a three-dimensional scapular motion pattern during normal arm elevation that includes upward rotation, posterior tilting and varying internal/external rotation dependent [varying on the plane and angle of elevation].
Main contributors to the Shoulder elevation are
- Humeral movement at glenohumeral joint
- Scapular movement at scapulothoracic joint
Joints with minor contribution are sternoclavicular Joint and an acromioclavicular Joint.
These results in a coordinated movement pattern of the arm elevation.
All the joints continuously participate in the movements at different rates at different phases of elevation.
While the limb is elevated overhead the scapula undergoes
- While the limb is elevated overhead the scapula undergoes gliding/rotatory movement over the thorax turning glenoid cavity upward or downward depending on the direction of the motion. This movement occurs in frontal plane around an anteroposterior axis.
- Tilts in lateral-lateral axis [anterior and posterior tilting] External and internal rotation occurs around a cephalo-caudal (longitudinal) axis.
- In addition, there is external and external rotation in longitudinal axis [cephalo-caudal axis] which cause glenoid cavity more into the frontal plane [external rotation] or sagittal plane [internal rotation].
Contribution of the scapula in Shoulder Elevation
The scapula on the thorax contributes to elevation (flexion and abduction) of the humerus by upwardly rotating the glenoid fossa 50° to 60° from its resting position.[if there is no glenohumeral motion, scapula could contribute to about 60° of elevation of the humerus.]
Glenohumeral -joint alone contributes 90-120°. The combination of scapular and humeral movement result in a maximum range of elevation of 150-180°.
The contribution of these joints is as follows
First 30 degrees of shoulder elevation – Setting Phase:
- The movement is largely glenohumeral.
- Scapulothoracic movement is small and inconsistent.
After First 30 degrees of shoulder elevation:
- The glenohumeral and scapulothoracic joints move simultaneously.
- Overall 2:1 ratio of glenohumeral to scapulothoracic movement.
Purpose of Scapulohumeral Rhythm
- Preserves the length-tension relationships of the glenohumeral muscles
- the muscles do not shorten as much as they would without the scapula’s upward rotation, so can maintain the force through a substantial range of motion.
- Prevents impingement between the humerus and the acromion by moving glenoid
The scapulohumeral rhythm can be defined as the ratio of the glenohumeral movement to the scapulothoracic movement during arm elevation.
Scapulohumeral ratio is calculated by dividing the total amount of shoulder elevation (humerothoracic) by the scapular upward rotation (scapulothoracic).
Humeral elevation: scapulothoracic rotation is 2:1 during arm elevation.
Variations of the scapulothoracic ratio
- children showed a higher scapulohumeral
- Frozen or stiff shoulders- the scapular contribution increases
- Differences in BMI may cause different types of rhythms.
Causes of Abnormal Scapulohumeral Rhythm
- Bony Factors
- Fracture Clavicle
- Joint Pathologies
- Acromioclavicular arthritis
- Glenohumeral joint degeneration
- Spinal accessory nerve palsy
- Pectoralis minor
- Biceps short head
- Soft tissue posterior shoulder inflexibility
- Ligamentous stiffness
- Muscular causes
- Serratus anterior weakness
- Abnormal trapezius force couple
- Decreased rotator cuff strength
Measurement of scapulohumeral rhythm
Measures only 2-dimensional kinematics.
This uses tape measures for linear assessment and compares abnormal motion between the pathological and healthy scapula. This examination is based on simple bilateral visual observation of scapular position.
Lateral Scapular Slide Test
This test evaluates the scapular symmetry when loads are placed in three positions of the upper limbs
- Position 1 – Arm is relaxed at the side (0° of humeral elevation)
- Position 2 – Hand on the lateral iliac crest
- Position 3 – Internally rotated and abducted arm to 90°.
Two measurements are performed using a tape in each position (between the inferior angle of the scapula and the closest spinous process) in order to allow calculation of an average value. ]
A 1.5-cm asymmetry in any of the positions is established as a threshold for an abnormal pattern.
Scapular dyskinesis test
In this test, the patient performs weighted shoulder flexion and abduction movements and scapular motion is visually observed. Dyskinesis is defined as
- Presence of either winging (prominence of any portion of the medial border or inferior angle away from the thorax)
The motion can be x-rayed and studied.
Other measures are MRI, goniometry, cinematography and 3-dimensional tracking systems
Management of Altered Glenohumeral Rhythm
Therapy needs to be individualized. The causative factors need to be determined and then addressed accordingly.
The general options for the treatment are
- Stretching exercises
- Active and passive mobilizations
- Coordination between muscles
- Functional training [sports] if required.