Effect of interfragmentary gap on compression force in a headless compression screw used for scaphoid fixation.

We investigated the effect of an interfragmentary gap on the final compression force using the Acutrak 2 Mini headless compression screw (length 26 mm) (Acumed, Hillsboro, OR, USA). Two blocks of solid rigid polyurethane foam in a custom jig were separated by spacers of varying thickness (1.0, 1.5, 2.0 and 2.5 mm) to simulate an interfragmentary gap. The spacers were removed before full insertion of the screw and the compression force was measured when the screw was buried 2 mm below the surface of the upper block. Gaps of 1.5 mm and 2.0 mm resulted in significantly decreased compression forces, whereas there was no significant decrease in compression force with a gap of 1 mm. An interfragmentary gap of 2.5 mm did not result in any contact between blocks. We conclude that an increased interfragmentary gap leads to decreased compression force with this screw, which may have implications on fracture healing.

Kinematic analysis of the scaphoid using gated four-dimensional CT.

AIM: To investigate the kinematics of the scaphoid while the wrist was in radioulnar motion using gated four-dimensional computed tomography (4DCT) imaging. MATERIALS AND METHODS: Six cadaveric wrist specimens were scanned in the following order of capsulotomy and ligament sectioning: (1) ligaments intact, (2) capsulotomy, (3) scapholunate ligament division, and lastly, (4) lunotriquetral ligament division. A three-dimensional model was then reconstructed to analyse the translation and angular displacements of the scaphoid. RESULTS: The magnitude of displacement was found to increase with each consecutive ligament sectioning. Translation along the y-axis was statistically significant, with the scaphoid shifting up to 1.39 mm from its original position after complete sectioning of the ligaments while the wrist was deviating radially. Angular displacement about all three axes was statistically significant, with the highest occurring in the flexion-extension plane when the scaphoid flexed by 9.1° from its original position after total sectioning of the ligaments. CONCLUSION: The present study showed that changes in the kinematics of the scaphoid can be detected using 4DCT, thus demonstrating its feasibility in the diagnosis of dynamic carpal instability, which only presents during motion.

Biomechanical investigation of 'figure of 8' flexor tendon repair techniques.

This biomechanical study compared the original Al-Qattan repair with other modifications postulated to reduce bulk and friction, thereby potentially improving outcome. A total of 32 cadaveric digits with intact flexor apparatus were used. In each digit, the flexor digitorum profundus and flexor digitorum superficialis tendons were cut cleanly in Zone 2. We tested Al-Qattan's technique along with three modifications using stronger suture material and varying the number of strands across the repair site. Of the four repair techniques, the modified Al-Qattan's technique using two 'figure of 8' 4-0 Fiberwire core sutures (Group 4) had the best balance of ultimate tensile strength (50.9 N), 2 mm gapping force (38 N) and friction. The modified technique provided a stronger repair for early active mobilization and has less friction than the originally described repair.