Discussion:
Current literature review demonstrates no case reports describing documenting distal deltoid tendon rupture from the deltoid tuberosity. This case vignette is the first to describe such injury of humeral site deltoid tendon rupture and describes successful surgical repair. In contrast to our distal deltoid tear, there have been multiple reports on proximal deltoid origin site detachment from the acromion and distal clavicle [4-6]. The mechanism of injury described by prior research for this rare injury includes seatbelt-induced injury after a motor vehicle accident, cricket bowling maneuver, and pull-up exercises [4-6].
Our patient was performing barbell shoulder shrugs with 450 pounds when he experienced acute posterior left shoulder pain during the eccentric phase of the exercise. Andersen and colleagues evaluated the shoulder girdle muscle activity using surface electromyography in a cohort of middle-aged females [7]. The level of muscle activation was measured as a percentage of maximal voluntary static contraction. Activation of the posterior portion of the deltoid muscle was found to be significantly higher during exercises that involved a forward incline of the body including reverse flys (102 +/- 9%), One hand rows (83 +/- 6%) and shoulder shrugs (71 +/- 5%). Comparing this to our study participant, he described a slight forward flexion at his waist in order to bring the barbell out from rubbing on his thighs during the shrug exercise. This suggests an increase force vector directed through the posterior deltoid fibers, which may have contributed to the tendon rupture. There are some limitations of direct comparison between our patient and the reported data. The shrugs exercise performed by Andersen’s female patients utilized dumbbells to the side rather than a barbell.
A recent study analyzing the anatomy of the deltoid muscle tendons examined eight cadaver specimens with an average age of 76 years of age identifying the deltoid origins and end tendons insertion sites. The final model from the study consisted of an end tendon consisting of a continuous succession of bipennate end tendon blades centrally interspaced by unipennate tendon parts, creating a natural segmentation. They identified an average of 2.9 +/- 0.8 end tendon blades inserting posteriorly at the lateral humerus [8]. This is consistent with our intraoperative findings and successful fixation of two bipennate posterior end tendons. There is currently no research on the clinical significance of individual end tendon ruptures.
Treatment of isolated proximal deltoid tears has generated controversy due to limited data on the subject and the diversity of patients currently reported on. Support for surgical fixation in healthy active individuals in based on case report data that demonstrated satisfactory results following operative treatment [5-7]. Following operative fixation patients regained functional deltoid strength and experienced painless range of motion after completing post-operative physical therapy. Support for nonoperative therapy with a supervised rehabilitation program stems from a case report on a partial deltoid detachment from the posterolateral acromion in a healthy 31 year old cricket player. Nonoperative treatment including Indomethacin treatment and physical therapy resulted in a return of function in 8 weeks. However, the patient’s MRI was complicated by areas of myositis ossificans and heterotropic calcification present at the posterolateral acromion [4].
This case is unique due to the posterior deltoid tendon rupture, with confirmed tendon retraction on magnetic resonance imaging; additionally the patient had physical deformity with associated weakness and pain affecting his occupation. Operative management was successful and allowed him to return to work with similar pre-injury strength, range of motion and functional capacity within four months of surgery