Biomechanical comparison of expandable and locked intramedullary femoral nails.

OBJECTIVES: The expandable intramedullary (IM) nail does not require locking and fluoroscopy use is minimized. However, the lack of cross-locking screws may adversely affect the fixation's rotational stability. The purpose of our study was to compare the rotational stability afforded by an expandable nail with that of a standard locked nail. METHODS: In a cadaver model of a diaphyseal femoral fracture (OTA type 32-A3), we compared first-generation expandable IM nails with standard locked IM nails in osteoporotic and nonosteoporotic femora (10 pairs each) and second-generation expandable nails with standard locked IM nails only in nonosteoporotic femora (10 pairs). To simulate torsional loads during walking, we applied an external rotation moment of -1 to 10 Nm at 1 Hz to each construct for 5000 cycles. Failure was defined as 15 degrees of rotation at the fracture site. We used McNemar's test to check for significant (P < 0.05) differences in failure between groups. RESULTS: Of the first-generation expandable nails, 90% failed (9/10 in osteoporotic and 9/10 in nonosteoporotic femora) within the first 1000 cycles. Of the respective locked nails, significantly fewer failed in nonosteoporotic femora than in osteoporotic femora (0/10 and 3/10, respectively). Of the second-generation nails, 8/10 failed within 100 cycles of testing. Of the comparative locked nails, none failed at 5000 cycles. CONCLUSIONS: We concluded that the expandable IM femoral nail, when tested in purely axial rotation, has poor rotational stability compared with the standard locked IM femoral nail.

Tendon and neurovascular bundle displacement in the palm with hand flexion and extension: an MRI and gross anatomy correlative study.

This study evaluated the correlative use of MRI methods and gross anatomy to monitor tendon displacement in the central region of the palm at rest and during flexion and extension of the metacarpophalangeal and interphalangeal joints with respect to the corresponding neurovascular bundles (NVBs). In all of the samples the neutral and extended positions showed the NVB to be palmar with respect to the flexor tendons, while during flexion tendon displacement caused the NVB to be dorsal to both the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) tendons. T1-weighted MR images correlated with gross anatomical slides demonstrated that significant changes occur in the relative positions of the flexor tendons and associated NVBs of the palm upon flexion and extension.

A biomechanical comparison of locking and nonlocking plates for the fixation of calcaneal fractures.

BACKGROUND: Locking plate systems have been developed in an attempt to increase the strength of fracture fixation and, in so doing, allow earlier mobilization and rehabilitation. The purpose of our study was to compare the mechanical integrity of the locking plate and traditional nonlocking plate fixation for calcaneal fractures in a cadaver model. Our hypothesis was that the locking plate construct provides stronger fixation than the nonlocking plate construct. METHODS: We created a Sanders type-IIB fracture in 10 pairs of fresh-frozen cadaver feet (bone mineral density, 0.50 +/- 0.14 g/cm2 age, 69 +/- 16 years). One foot of each pair was fixed with a nonlocking calcaneal plate (Synthes, Paoli, PA), and the contralateral foot was fixed with the Locking Calcaneal Plate (Synthes, Paoli, PA). The specimens then were cyclically loaded through the tibia from 0 to 700 N at 1 Hz on a materials testing machine to simulate weightbearing. Fragment displacement was measured with a three-dimensional kinematic analysis system. Significance was set at p < 0.05. RESULTS: There was no significant difference between the two plating systems with respect to the mean (+/- SD) number of cycles to failure (locking plate, 3261 +/- 2355; nonlocking plate, 2271 +/- 2465). CONCLUSION: In a cadaver model of type-IIB calcaneal fractures, locking plate fixation did not provide a biomechanical advantage over traditional nonlocking plate fixation.