BRINGING SCIENCE TO TREATMENT

Scapular dyskinesis and overhead injury risk in athletes

Scapular dyskinesis is strongly associated with an increased risk of ‘overhead’ injuries in athletes. Andrew Hamilton looks at recent evidence on the evaluation of scapular function in overhead athletes and the implications for rehab.

In overhead sports like tennis or volleyball, the shoulder is at increased risk of injury due to the high loading and impact forces experienced in the joint region during serving and smashing. Many shoulder injuries occur over time, with chronic overload leading to injury. As part of this process, pain may also arise from sportspecific adaptations, alterations in strength, flexibility and posture changes in the shoulder joint. These alterations change biomechanics and movement strategies during serving and striking, aggravating the risk of an overload injury.

Evidence suggests that while the structure and function of the glenohumeral joint i s pivotal in determining the risk of shoulder injury, there actually exists a whole kinetic chain in which deficits anywhere along the chain can increase the risk of shoulder injury1 2 3 4 5. In the shoulder itself, glenohumeral internal-rotation deficit (GIRD) and rotator cuff strength imbalance play an important role in the kinetic chain, but other deficits such as scapular dyskinesis, thoracic spine stiffness and hyperkyphosis, lumbar core instability, and hip range of motion can all contribute to a ‘kinetic chain failure’, increasing injury risk in both young and old overhead athletes6 7. In this article however, we’ll concentrate on scapular dyskinesis (SD).

What is scapular dyskinesis (SD)?

Physiologically, the scapula is the stable base of origin for muscles that contribute to the dynamic glenohumeral stability and produce arm motion, and scapular stability is needed for force production from muscles arising from the scapula. The definition of SD is the alteration of normal scapular kinematics, resulting in the loss of normal control of scapular motion. SD by itself is not an injury or a musculoskeletal diagnosis – rather it has been hypothesised to relate to changes in glenohumeral angulation, acromioclavicular joint strain, subacromial space dimension, shoulder muscle activation and humeral position and motion (see box 1).

Box 1: Classification of scapular dyskinesis
Kibler et al reported the reliability of a visually based classification system for scapular dysfunction that defined 3 different types of motion abnormalities(17): type 1 = inferior angle prominence, type II = medial border prominence, and type III = excessive superior border elevation. Normal, symmetric scapular motion was considered type IV.
Type 1: Inferior Angle Prominence
-At Rest: the inferior medial scapular border may be prominent dorsally
-Abnormal Motion: inferior angle tilts dorsally and the acromion tilts ventrally over the top of the thorax. The axis of the rotation is in the horizontal plane
-Common Associated Findings: Tightness/shortening of pectoralis minor (Borstad & Ludewig, 2005)
Type 2: Medial Border Prominence
-At Rest: medial border is prominent dorsally
-Abnormal Motion: medial border of scapula tilts dorsally of the thorax. Rotation axis is in the frontal plane
-Common Associated Findings: weakness/reduced activation serratus anterior and lower traps.
Type 3: Superior Border Prominence
-At Rest: Superior border generally elevated, scapula often anteriorly displaced
-Abnormal Motion: shoulder-shrug motion evident. Axis of motion in the saggital plane.
-Common Associated Findings: Tightness/overactive upper trapezius, weakness/ reduced activation lower trapezius.
Type 4: Normal and symmetric scapular motion

Various shoulder soft tissue pathologies including impingement (internal and external) anterior capsular laxity, labral injury and rotator cuff weakness have been found in association with SD in overhead athletes complaining of shoulder pain8 9 10 11 12 13. To confuse matters however, scapular asymmetries have been noted in overhead athletes that are asymptomatic as well as those who are injured. This makes it difficult to determine whether SD is simply an effect of shoulder injury in overhead athletes or is implicated as a prime cause.

Some studies have found no causative relationship between SD and shoulder pain14 15, whereas others have clearly identified scapular dyskinesis as a possible risk factor for chronic shoulder pain in overhead athletes 16 17. In particular, obvious SD as defined by Kibler et al has been found to increase the risk for shoulder pain (see box 1).

SD assessment

As discussed above, glenohumeral range of motion, rotator cuff strength or imbalance and scapular position and movement are important factors in the assessment of healthy and previously injured overhead athletes. By performing a clinical assessment, clinicians can define risk/causal factors and then help guide the athlete in a suitable rehab programme with the goal of full return-to-play after injury.

However, unlike the measurement of glenohumeral range of motion and rotator cuff strength, where it’s relatively straightforward for the clinician to measure defined ranges of motion or levels of strength and strength imbalances, assessing SD is rather more challenging. Part of the reason is that evidence supporting ‘recommended values’ for the prevention of injury or return to play after injury with respect to scapular function is scarce.

Studies have investigated the validity of using simple visual observation (performed either by using the yes/no method for SD being present/absent) as a means of assessment. In general, they suggest that simple visual observation is a fairly reliable and valid method on the proviso that the clinician is trained in an appropriate and standardised manner 18 19. Categorising SD into different types, based on the specific position of the scapula (ie Kibbler’s method) is also a reliable means of assessment when used by a clinician during repeated assessments. However, the evidence also suggests that interclinician reliability is not nearly as high 20.

SD and scapular asymmetry

Although difficult to separate cause and effect, the evidence does suggest that an obvious degree of SD is a risk factor for shoulder pain in overhead athletes. In one study for example, researchers rated SD in handball players by dividing them into one of three categories – normal scapular control; slight SD; obvious SD – and found that obvious dyskinesis was a major risk factor for shoulder pain21.

Having said that, the studies carried out to date do not support the notion that scapular behaviour should be symmetrical in overhead athletes. For example, studies on volleyball and handball players have looked at scapular asymmetry in resting scapular postur22 23 and found that a degree of asymmetry was both common and not predictive of shoulder pain. Other researchers have also reported that the prevalence of SD is almost identical in subjects with and without shoulder pain, questioning the clinical value of scapular asymmetry. Although this may sound confusing, what it boils down to is that clinicians treating overhead athletes should be aware that some degree of scapular asymmetry is common and should not be considered automatically as a pathological sign – but rather an adaptation to sports practice and extensive use of upper limb.

Assessment in the clinic

When assessing scapula function in overhead athletes, the measurement of a number of parameters can be useful. These include, scapular upward rotation, and inter and intra-muscular strength. Scapular inclination – a number of studies have measured scapular inclination following upward rotation in healthy overhead athletes24 25. The data from these studies can be used as a reference base, providing cut-off values for correct scapular positioning at several elevation angles. It’s common to observe a large variation in scapular upward inclination through the midrange of motion – most likely due to large natural anatomical variations between individuals. However, in full elevation, most studies suggest that upward inclination should be at least 45-55 degrees26 27. Figure 1 shows the use of a digital inclinometer for measuring inclination. The use of a digital inclinometer has been shown to exhibit high inter – and intra-rater reliability. However, this is on the proviso that there is adequate palpation of the reference points in the different humeral elevation angles and that the clinician is able to control any additional tilting of the inclinometer in planes other than the scapular plane28.

Figure 1: Measuring scapular rotation

Figure 1: Measuring scapular rotation

Scapular muscle strength testing

It’s useful to assess strength ratios and absolute strength of the scapular muscles. In a healthy but non-athletic population the isokinetic ratio protraction/retraction has been shown to be around 1:1 whereas there are slight changes in overhead athletes. In the case of throwing athletes, a slight shift in strength towards the protractors is normal29 30 31. In one-handed overhead sports such as tennis, an increase of 10% in scapular muscle strength is advised on the dominant side. In particular, the strength levels of lower trapezius and serratus anterior should receive special attention, since these muscles are shown to be susceptible to weakness in injured athletes32 33. And while in bilateral sports such as swimming, rowing etc there shouldn’t be significant side-to-side differences in scapular muscle strength, clinicians should expect observe some strength bias towards the handed side.

For the measurement of scapular muscle strength, several protocols have been described in the literature34 35. Clinicians should bear in mind that different testing procedures will produce different outcomes depending on the equipment used, the positioning of the dynamometer and patient positioning. An example of a well-validated set of strength tests for scapular function (lower, middle and upper trapezius, and serratus anterior is shown below in figures 2-5.)

The following four tests for scapular muscle strength tests have demonstrated to provide excellent intra-rater test-retest reliability. Muscle testing is performed by first prepositioning the scapula in the midrange position of scapular motion for the specific muscle test. The midrange position is located by having the subject go through the available scapular range of motion, and then the midrange was estimated as the midpoint of the motion. This midrange position is selected to optimize the length-tension relationship of the tested muscle and, therefore, the generation of a maximum isometric contraction. A ‘make test’ is then performed – subjects are instructed to maintain the midrange position during each muscle test while resistance is gradually applied via the handheld dynamometer (HHD) until the clinician matches the subject’s effort.

Figure 2: Lower trapezius
The resistance force from the HHD is applied to the spine of the scapula midway between the acromial process and the root of the spine. The force on the scapula is applied in the superior and lateral direction parallel to the long axis of the humerus, which is maintained at 140 degrees of elevation. The scapular motion used is scapular adduction and depression.
Figure 3: Middle trapezius
The HHD resistance force is applied to the spine of the scapula midway between the acromial process and the root of the spine. The force is applied in the lateral direction parallel to the long axis of the humerus, which is placed in 90 degrees of abduction. The scapular motion used is scapular retraction.
Figure 4: Upper trapezius
The HHD is placed over the superior scapula. Force is applied directly downward (inferior) through the HHD in the direction of scapular depression. The scapular motion used is scapular elevation.
Figure 5: Serratus anterior
The elbow is placed in 90 degrees of flexion, and resistance is applied to the ulna at the olecranon process along the long axis of the humerus. The triceps muscle should be monitored visually and by palpation to ensure that it does not contribute to force production during the SA muscle test. The scapular motion used in this test is scapular protraction.

Scapular strength and flexibility rehab

Once deficits and imbalances in scapular behaviour have been identified, an intervention program to restore flexibility and muscle performance is the logical step. In a recent paper, Cagnie et al outlined a ‘scapular intervention algorithm’ (see figure 6) to help guide the clinician through the different steps and progressions required36.

In this paper, Cagnie et al describe a detailed 4-stage approach for scapular strength rehab consisting of the following:

First stage – conscious muscle control performed in the early stage of scapular training. The goal of this stage is to improve proprioception and to normalise scapular resting position.

Second stage – muscle control and strength necessary for daily activities. The goal here is to focus more on muscle control and co-contraction (advanced control during basic activities) or muscle strength (in case, for instance, manual muscle testing or isokinetic testing shows isolated strength deficit in one or more scapular muscles).

Third stage – advanced control during sports movements using general scapular exercises to increase muscle strength. The goal of this stage is to exercise advanced scapular muscle control and strength during sport-specific movements. Special attention is given to integration of the kinetic chain into the exercise programme, and implementation of sport-specific demands by performing plyometric exercises and eccentric exercises. In particular, overhead athletes should perform exercises in which the external rotators are eccentrically loaded, for instance, using weight balls and elastic resistance-tubing.

In addition, the authors provide a protocol for the treatment of flexibility problems in the scapular muscles. They point out that these are often characterised by loss of flexibility in the pectoralis minor and the levator scapulae muscles, and stiffness and tightness of the posterior shoulder structures (the capsule as well as the glenohumeral external rotator muscles). Both of these flexibility deficits may lead to scapular malpositioning, in particular, anterior tilting and downward rotation. For a very detailed discussion together with detailed information on how clinicians can use this approach, you can visit: http://bjsm. bmj.com/content/48/8/692.long to see the original paper.

The elbow is placed in 90 degrees of flexion, and resistance is applied to the ulna at the olecranon process along the long axis of the humerus. The triceps muscle should be monitored visually and by palpation to ensure that it does not contribute to force production during the SA muscle test. The scapular motion used in this test is scapular protraction.

Summary

While SD may not be the root cause of shoulder injury in overhead athletes, investigation and rehab (if necessary) of scapular function is an important part of overcoming shoulder injury. In particular , identifying scapular asymmetries, strength imbalances and deficits in flexibility of the scapular muscles should form an important part of any treatment protocol.

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