Intra-articular partial-thickness rotator cuff tears: analysis of injured and repaired strain behavior.

BACKGROUND: There are few biomechanical studies regarding partial-thickness rotator cuff tears and subsequent repair. HYPOTHESIS: Partial-thickness intra-articular supraspinatus tendon tears increase articular-sided tendon strain as they increase in size. Repair of these tears will return strain to the intact state. STUDY DESIGN: Controlled laboratory study. METHODS: Twenty fresh-frozen human cadaveric shoulders were prepared by dissecting to the supraspinatus tendon and leaving the native footprint intact. The tendon footprint was measured with digital calipers and divided into thirds (anterior, middle, and posterior). The middle third was the area where a consistent partial tear was created based on the thickness of the specimens' particular footprint. Created were 25%, 50%, and 75% tears. Image analysis software and differential variable reluctance transducers strain gauges were used to measure strain. A 100 N load at 1 Hz for 30 cycles was conducted for glenohumeral angles of 45 degrees, 60 degrees, and 90 degrees. This was completed for the intact tendon, 25%, 50%, and 75% tears. Shoulders were then repaired using the in situ fixation method. The in situ method consisted of either a parachute anchor or metal corkscrew anchor. Eight shoulders were examined for load-to-failure testing with the Materials Testing System. RESULTS: There was a significant difference (P < .05) in rotator cuff strain between the intact rotator cuff tendon and 50% and 75% partial-thickness tears. The cuff strain was returned to the intact state with repair. This was consistent for 3 different glenohumeral abduction angles and for all 3 intra-articular tendon areas. The bursal strain did not have any significant differences between groups. CONCLUSION: Articular-sided tendon strain increases consistently across the supraspinatus tendon with greater partial tears. Repair returned strain close to the intact state. CLINICAL RELEVANCE: This study may add credence to the clinical practice of repairing intra-articular partial-thickness rotator cuff tears greater than 50%.

Gestational mercury vapor exposure and diet contribute to mercury accumulation in neonatal rats.

Exposure of pregnant Long-Evans rats to elemental mercury (Hg0) vapor resulted in a significant accumulation of Hg in tissues of neonates. Because elevated Hg in neonatal tissues may adversely affect growth and development, we were interested in how rapidly Hg was eliminated from neonatal tissues. Pregnant rats were exposed to 1, 2, or 4 mg Hg0 vapor/m3 or air (controls) for 2 hr/day from gestation day 6 (GD6) through GD15. Neonatal brain, liver, and kidney were analyzed for total Hg at various times between birth and postnatal day 90 (PND90). Milk was analyzed for Hg between birth and weaning (PND21). Before weaning, the Hg levels in neonatal tissues were proportional to maternal exposure concentrations and were highest in kidney followed by liver and then brain. There was no elimination of Hg between birth and weaning, indicating that neonates were exposed continuously to elevated levels of Hg during postpartum growth and development. Consumption of milk from exposed dams resulted in a slight increase in kidney Hg concentration during this period. Unexpectedly, neonatal Hg accumulation increased rapidly after weaning. Increased Hg was measured in both control and exposed neonates and was attributed to consumption of NIH-07 diet containing trace levels of Hg. By PND90, tissue Hg levels equilibrated at concentrations similar to those in unexposed adult Long-Evans rats fed the same diet. These data indicate that dietary exposure to trace amounts of Hg can result in a significantly greater accumulation of Hg in neonates than gestational exposure to high concentrations of Hg0 vapor.