Relative contributions of the supraspinatus cord and strap tendons to shoulder abduction and translation.

BACKGROUND: The supraspinatus (SS) is formed by a larger anterior bipennate muscle with a cord-like tendon and a posterior unipennate muscle with a strap-like tendon. There is a tendinous connection between the 2 SS subunits. Yet, the relative mechanical contribution of the SS cord and SS strap musculotendinous units to load transmission and subsequent shoulder abduction force is unknown. We hypothesized that a simulated SS cord vs. an SS strap tear would generate less shoulder abduction force and, further, an intact SS cord would offset the expected abduction loss from an SS strap tear, but the inverse would not be true. MATERIALS AND METHODS: Twenty fresh-frozen cadaveric specimens were tested in a shoulder simulator with physiological load vectors applied to the upper and lower subscapularis, SS cord, SS strap, infraspinatus, and teres minor. The roles of the SS cord and SS strap muscles were delineated by varying their loads, while keeping constant loads on other muscles. The randomized testing trials included a native condition and 4 test cases that simulated tears by dropping the load and force transfer via the SS cord-to-SS strap connection by adding the load. Testing was completed at both 0° and 30° of abduction. During each test, shoulder abduction force, rotator cuff strains, and humeral translation were measured. RESULTS: Simulated isolated SS cord and SS strap tears led to a significantly lower shoulder abduction force (P < .001). A simulated cord tear at 0° and 30° reduced the abduction force by 53% and 38%, respectively. A simulated strap tear at 0° and 30° dropped the abduction force by 27% and 23%, respectively. The decline in the abduction force was larger for the SS cord tear vs. SS strap tear (P ≤ .001). An SS cord tear with full-load transfer to the strap was able to recover to native values at both 0° and 30° (P ≥ .288). Likewise, an SS strap tear with full-load transfer to the SS cord showed a similar recovery to native values at both 0° and 30° (P ≥ .155). During full-load transfer, the tendon strain followed the loading pattern. An SS cord tear or SS strap tear did not cause a change in humeral translation (P ≥ .303). DISCUSSION: The mechanical findings support the efficacy of nonoperative treatment of small (<10 mm) SS tears,11 because an intact SS strap tendon can effectively offset the abduction loss of a torn SS cord tear and vice versa.

Trends and epidemiology of lower trunk fractures in the super elderly population in the United States from 2011 to 2020.

INTRODUCTION: Fractures of the lower trunk are among the most common fractures occurring in the elderly. Super elderly individuals (i.e., those 80 years of age and older) represent a growing segment of the population and are especially prone to these fractures. The contemporary epidemiology of lower trunk fractures in the super elderly population is incompletely described in the literature. MATERIALS AND METHODS: This descriptive epidemiology study used the National Electronic Injury Surveillance System (NEISS) to examine the incidence and recent trends of lower trunk fractures (i.e., fractures of the hip, pelvis, and lumbar spine) occurring among super elderly individuals in the United States (US) from 2011 to 2020. Annual, overall, and age-/sex-specific incidence rates (IRs) were analyzed. Average annual percent change (AAPC) estimates were calculated to indicate the magnitude/direction of trends in annual injury rates. RESULTS: An estimated N=1,226,160 super elderly patients sustained lower trunk fractures over the 10-year study period for an overall IR of 100.2 per 10,000 person-years at-risk (PYR). Hip fractures accounted for the largest percentage of cases (IR=71.7 PYR), followed by lumbar spine fractures (IR=14.7), and pelvic fractures (IR=14.3). The incidence of lower trunk fractures among super elderly females (IR=121.5 PYR) was significantly greater than that of males (IR=65.7 PYR). The incidence of lower trunk fractures among nonagenarians and centenarians was significantly higher than that of octogenarians. Accounting for population growth yielded a significantly increasing annual incidence of lower trunk fractures in super elderly patients over the study period from 86.7 PYR in 2011 to 107.2 PYR in 2020 (AAPC=2.7, p<0.001). The annual incidence of both pelvic (AAPC=5.8) and lumbar spine (AAPC=6.9) fractures increased at a significantly higher rate than that of hip fractures (AAPC=1.4). CONCLUSIONS: This study suggests that the annual incidence of lower trunk fractures in the oldest cohort of patients in the US (80+ years of age) increased significantly during the recent decade from 2011 to 2020, with pelvic and lumbar fractures in particular becoming increasingly common. Increased incidence rates highlight the need for future research aimed at optimizing outcomes and quality of life in this frail and ever-growing segment of the population.