Pathology & Treatment

Posterior shoulder instability is one of the most commonly misdiagnosed or not-recognized shoulder pathologies due to its variety of clinical presentations. According to the ABC Classification different types of posterior shoulder instability can be distinguished based on the underlying pathomechanical principle.[1] There is structural posterior shoulder instability (Type B2) which is caused by a structural defect e.g. due to trauma and there is functional posterior shoulder instability caused by inappropriate muscle activation without any trauma (Type B1). [2] 

In patients suffering from functional posterior shoulder instability the humeral head translates excessively due to lack of contraction of stabilizing muscles, provoking a posterior subluxation or dislocation every time the shoulder passes a particular phase of movement. Common symptoms reported include pain during movement of the arm, a loss of range of motion due to weakness or blockage that inhibits any further movement, as well as a strong feeling of instability that extensively limits shoulder function.

While structural posterior shoulder instability can be addressed surgically, in functional posterior shoulder instability operations should be avoided as there are typically no structural defects that can be addressed and therefore surgery often results in additional pain and further limitation of shoulder function.[3-7] Regular physiotherapy is the current gold- standard of treatment for patients suffering from functional posterior shoulder instability including tactile biofeedback, coordination and strengthening exercises.[5, 7] Unfortunately, conventional conservative treatment can often be unsuccessful in achieving a satisfying outcome in affected patients.


Recent studies showed that patients suffering from functional posterior shoulder instability could benefit from electric muscle stimulation which re-activates the previously hypoactive muscles.[8, 9] The Shoulder Pacemaker is a motion activated electrical muscle stimulator developed to retrain the patients’ muscle pattern in order to treat posterior shoulder instability. In a cohort of patients with previously failed conventional physiotherapy the Shoulder Pacemaker therapy concept led to a highly significant improvement of stability, function, as well as pain and several patients were able to return to physically demanding and even athletic activities.

Future assessment includes the beneficial effect of the Shoulder Pacemaker treatment in patients suffering from structural shoulder instability during the postoperative rehabilitation phase as well as in patients with combined structural and functional deficiencies.


1. Moroder, P. and M. Scheibel, ABC classification of posterior shoulder instability. Obere Extrem, 2017. 12(2): p. 66-74.

2. Moroder, P., et al., Characteristics of functional shoulder instability. J Shoulder Elbow Surg, 2020. 29(1): p. 68-78.

3. Hawkins, R.J., G. Koppert, and G. Johnston, Recurrent posterior instability (subluxation) of the shoulder. J Bone Joint Surg Am, 1984. 66(2): p. 169-74.

4. Huber, H. and C. Gerber, Voluntary subluxation of the shoulder in children. A long-term follow-up study of 36 shoulders. J Bone Joint Surg Br, 1994. 76(1): p. 118-22.

5. Jaggi, A. and S. Lambert, Rehabilitation for shoulder instability. Br J Sports Med, 2010. 44(5): p. 333-40.

6. Kuroda, S., et al., The natural course of atraumatic shoulder instability. J Shoulder Elbow Surg, 2001. 10(2): p. 100-4.

7. Takwale, V.J., P. Calvert, and H. Rattue, Involuntary positional instability of the shoulder in adolescents and young adults. Is there any benefit from treatment? J Bone Joint Surg Br, 2000. 82(5): p. 719-23.

8. Moroder, P., et al., Use of shoulder pacemaker for treatment of functional shoulder instability: Proof of concept. Obere Extrem, 2017. 12(2): p. 103-108.

9. Moroder, P., et al., The Shoulder Pacemaker treatment concept for posterior positional functional shoulder instability: prospective clinical trial. Am J Sports Med, 2020.

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