1.2 Micro-traumatic Instability
- As mentioned in MODULE 1, micro-traumatic instability is caused by a gradual or acute
overload of musculature +/- capsuloligamentous structures (ref) This type of instability
is common in patients who regularly go to the gym or participate in repetitive overhead
sports such as swimming and tennis. - With microtraumatic Instability this can be acquired by gradual over use or loading
in a repetitive scenario – such as throwing sports, swimming or gym - Or it can be as a result of an acute overloading incident such as mistiming a throw
or use of the racquet (in tennis) at a particular moment in time (usually with a high load of
force +/- awkward or end range of motion) – Or in a scenario where there is acute
overloading in a situation such in gym where the patient loses control of a motion – usually
under high weight load or in an awkward motion – such as a bench press, incline press or
overhead press. The same stressors could apply in an overuse scenario in a heavy
manual loading job such as a line man or construction worker - Due to the history of micro-trauma, compared to the atraumatic sub-group, this group
have a higher likelihood of acquired structural lesions. Theses structural lesions typically
include small labral tears (anterior, posterior or SLAP lesions) plus or minus stretched
(volumised) and attenuated capsulolabral structures. - Patients often have a previous a history of shoulder pain, especially with high-volume,
activities, such as increasing training loads or high-level tournaments. These episodes
may have been previously conservatively managed (with an ability for the patient to continue
participating in sport) but recently there has been a definite increase pain levels, [pain frequency or
both. - Both non-modifiable and modifiable factors impact on the micro-traumatic shoulder
presentation. - Non-modifiable factors include patient anatomy, age and gender.
- Although shoulder hypermobility is a characteristic feature of atraumatic instability
(ref), the presence of shoulder hypermobility is not uncommon in the micro-
traumatic group. - In the sporting population, up to 13 % of males and up to 42 % of females will have
underlying joint hypermobility(Nathan, Davies, & Swaine, 2018). Hypermobility increases
the prevalence of overuse sporting injuries(Liaghat et al., 2021; Mise et al., 2022)
and increases the risk of macro-traumatic instability in a contact senario
(Cameron et al., 2010; Chahal, Leiter, McKee, & Whelan, 2010) - In the setting of underlying GHJ hypermobility, patients can often present with combined
instability, usually anterior inferior or posterior. Swimming(Pink & Tibone, 2000) (McMaster,
Roberts, & Stoddard, 1998; Zemek & Magee, 1996) and tennis (L. Watson et al., 2023)
in particular, are populations that have an increased incidence of posterior-
inferior laxity and micro-instability. - Alterations in bony anatomy have also been shown to have as association with predisposition
towards instability (Bradley, Baker, Kline, Armfield, & Chhabra, 2006; Hohmann & Tetsworth,
2015; Owens et al., 2007; Weishaupt, Zanetti, Nyffeler, Gerber, & Hodler, 2000) and have
a higher incidence in certain sporting populations.(Greenberg, Fernandez-
Fernandez, Lawrence, & McClure, 2015) - For example, the dominant shoulder of baseball pitchers have a higher incidence
of glenoid retroversion when compared to the non-dominant side(Wyland et al., 2012)or
the shoulders of the non-throwing population(Rassi, Subhas, Bullen, Forney, & Polster,
2019) A significant increase in glenoid retroversion (above 10 degrees) is associated
with the development of PSI. (ref) - Unilateral overhead sportspeople may also have bony adaptation to the humerus which
is often normal and not associated with the development of shoulder pathology. One adaption
that has been found in baseballers and tennis players is an increase in humeral retroversion
(or retrotorsion) where the humerus is oriented in a posterior-medial direction)
(Greenberg et al., 2015). This bony adaptation results in an increase in shoulder ER range
of motion
and a concomitant decrease in shoulder IR range of motion compared to the contralateral
side, which is thought to favour the serving or throwing motion. (Hibberd, Oyama, & Myers,
2014; Reagan et al., 2002; Wyland et al., 2012) Clinicians should be cautious when
screening for and implementing interventions for soft tissue motion deficits based on
clinical ROM measures and mistaking humeral retroversion for pathological glenohumeral
internal rotation deficit (GIRD) (See more in Module 3). - To date, it is unknown if a patient’s anatomy creates a biomechanical advantage for enhanced
performance in particular sports (for example, patients with inherent GHJ hypermobility
have a biomechanical advantage for stroke length in swimming (ref)); or if the acquired
forces and loads of a particular sport alters a patient’s anatomy for enhanced performance
over time (for example, the humeral retrotorsion that occurs in baseballers who commence
throwing at a young age before humeral head physis has occurred). (Wyland et al., 2012) - Volume of play, shoulder muscle strength, scapula position, player position, and participation
in prevention programs are examples of modifiable risk factors. - A recent systematic review(Hoppe et al., 2022) found moderate level evidence that the
following was associated with shoulder injuries in overhead sports:- Player position (e.g.: baseball pitcher in baseball= increased risk of injury)
- Gender (females more likely to sustain a shoulder injury)
- Shoulder rotation ROM (ER ROM deficit only= more likely to have shoulder injury and
more likely to have surgery). - Shoulder rotation strength (reduced ER/IR strength = increased risk of injury)
- Scapula dyskinesia (presence of scapula dyskinesia in abduction =increased
risk of injury) - Prevention programs (decreased the risk to sustain shoulder problems by 28%,
except if the injury was moderate and severe). - Playing volume (increased volume = increased risk of injury)
- Adolescents may be at higher risk for shoulder injuries due to an immature musculoskeletal
system; particularly in regards to playing volume(Zaremski, Zeppieri, & Tripp, 2019)
For example, high levels (volume) of early sports specialisation in baseball, independent
of fatigue, age, and workload, place adolescent athletes at increased risk of injury and
serious overuse injury. (Jayanthi, LaBella, Fischer, Pasulka, & Dugas, 2015) - Certainly throwing (overhead loading) more than 3 hours per week before 10 years
of age has been shown to result in a greater likelihood of shoulder injury (ref) - If a patient has developed anterior dominant microtraumatic instability they will report
positional specific pain in positions of abduction / external rotation at 90° of elevation
such as the cocking phase of throwing or serving in tennis, backstroke in swimming &
opening their arms out in pec fly positions in the gym(Kuhn, 2010) - If a patient has developed posterior dominant microtraumatic instability then they will
report pain with deceleration or letting go of the ball in throwing or follow through or backhand
in tennis or push up into a bench press or overhead press position at the gym(Kuhn,
2010; L. Watson et al., 2023; L. A. Watson et al., 2017) - If a component of inferior instability is present the patient will report not liking or avoiding
carrying things by their side (especially if they are heavy) & often lying on their side at night
(Kuhn, 2010; L. Watson, Pizzari, Balster, Lenssen, & Warby, 2022) - If the instability is chronic or if the underlying joint is hypermobile then patients may
complain of anterior & inferior instability symptoms, posterior inferior instability symptoms
or a combination of all 3 (anterior, posterior & inferior) - The clinician should question the patient on any changes in technique (e.g. changing
serving action in a tennis player) or equipment (e.g. increasing or decreasing the weight of
a racquet by more than 5kg). - Again, the patient can usually determine the worst position for their shoulder or in which
position the shoulder was overloaded – indicating their primary direction of instability
(Kuhn, 2010; Kuhn, Helmer, Dunn, & Throckmorton, 2011).