Results The peak compression occurs at an insertion depth of −3.1 mm, −2.8 mm, 0.9 mm, and 1.5 mm for the Acutrak Mini, Acutrak Standard, Herbert-Whipple, and Synthes screws respectively (insertion depth is positive when the screw is proud above the bone and negative when buried). The compression and insertion torque at a depth of −2 mm were found to be 113 ± 18 N and 0.348 ± 0.052 Nm for the Acutrak Standard, 104 ± 15 N and 0.175 ± 0.008 Nm for the Acutrak Mini, 78 ± 9 N and 0.245 ± 0.006 Nm for the Herbert-Whipple, and 67 ± 2N, 0.233 ± 0.010 Nm for the Synthes headless compression screws. Conclusions All 4 screws generated a sizable amount of compression (> 60 N) over a wide range of insertion depths. The compression at the commonly recommended insertion depth of −2 mm was not significantly different between screws; thus, implant selection should not be based on compression profile alone. Conically shaped screws (Acutrak) generated their peak compression when they were fully buried in the foam whereas the shanked screws (Synthes and Herbert-Whipple) reached peak compression before they were fully inserted.

Because insertion torque correlated poorly with compression, surgeons should avoid using tactile judgment of torque as a proxy for compression. The scaphoid is the most commonly fractured carpal bone, accounting for approximately 60% of all carpal fractures and an estimated incidence of 30 fractures per 100,000 person-years. This injury occurs predominantly in young healthy adults and is associated with a high incidence of delayed union, nonunion, and osteonecrosis owing to the tenuous blood supply to the bone., Management of these injuries was transformed in the 1980s with the introduction of the scaphoid specific Herbert screw and subsequent screw variations.

A headless screw generates interfragmentary compression through differential pitches between the leading and the trailing threads. The compression thereby provides rigid internal fixation without the intra-articular prominence of standard headed screws. A headless compression screw (HCS) has become the implant of choice for the internal fixation of displaced and nondisplaced scaphoid fractures., Given the popularity of the HCS, sundry commercial designs have emerged, each with its own variation in thread pitch, shaft diameter, and shape. Despite an abundance of recent papers describing biomechanical testing of these screws in both human cadaver – and polyurethane foam – scaphoid models, the results are discordant, and there is little consensus on optimal screw design. Moreover, the vast majority of studies report peak compression force; however, it is unlikely this force is achieved consistently in clinical practice. In the absence of a load cell measuring compression, the surgeon must rely on screw position (insertion depth) and tactile feedback from the screwdriver (insertion torque), which may or may not correlate with compression.

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Rather than using peak compression as the end point to compare screws, the compression in relation to insertion depth and torque (the insertion profile) is of greater clinical interest. Knowledge of such a profile would improve our understanding of the implant, provide a better basis for comparing HCSs, and enable the surgeon to optimize compression. The purpose of this study was to determine the insertion profiles of 4 popular, commercially available HCSs.

Using a customized setup, both inter-fragmentary compression and insertion torque were measured simultaneously as the test screws were driven into a polyurethane foam scaphoid bone model. The goals of our study were to identify the relationship between the compression force, the insertion torque, and the insertion depth; to determine the insertion depth that yields peak compression for each screw; and to measure compression and torque at an insertion depth of −2 mm below the cortex. This depth maximizes screw length while ensuring a buried depth of −2 mm below the articular cartilage and is frequently recommended in the literature. Implants Four commercially available HCSs were tested (). All screws were chosen to have similar length (24–25 mm) in order to control for bone purchase. The Acutrak Standard (Acumed, Hillsboro, OR) is a highly polished titanium, conically shaped, self-tapping, fully threaded, cannulated screw with a variable thread pitch spanning the entire screw. It has a distal outer diameter (DOD) of 3.3 mm, and proximal outer diameter (POD) of 4.4 mm.