Wrist movements induce torque and lever force in the scaphoid: an ex vivo study

PURPOSE We hypothesised that intercarpal K-wire fixation of adjacent carpal bones would reduce torque and lever force within a fractured scaphoid bone. METHODS In eight cadaver wrists, a scaphoid osteotomy was stabilised using a locking nail, which also functioned as a sensor to measure isometric torque and lever forces between the fragments. The wrist was moved through 80% of full range of motion (ROM) to generate torque and force within the scaphoid. Testing was performed with and without loading of the wrist and K-wire stabilisation of the adjacent carpal bones. RESULTS Average torque and lever force values were 49.6 ± 25.1 Nmm and 3.5 ± 0.9 N during extension and 41 ± 26.7 Nmm and 8.1 ± 2.8 N during flexion. Torque and lever force did not depend on scaphoid size, individual wrist ROM, or deviations of the sensor versus the anatomic axis. K-wire fixation did not produce significant changes in average torque and lever force values except with wrist radial abduction (P = 0.0485). Other than wrist extension, torque direction was not predictable. CONCLUSION In unstable scaphoid fractures, we suggest securing rotational stability with selected implants for functional postoperative care. Wrist ROM within 20% extension and radial abduction to 50% flexion limit torque and lever force exacerbation between scaphoid fragments

Rotational Stability of Scaphoid Osteosyntheses: An In Vitro Comparison of Small Fragment Cannulated Screws to Novel Bone Screw Sets

<div><p>Background</p><p>The current standard of care for operative repair of scaphoid fractures involves reduction and internal fixation with a single headless compression screw. However, a compression screw in isolation does not necessarily control rotational stability at a fracture or nonunion site. The single screw provides rotational control through friction and bone interdigitation from compression at the fracture site. We hypothesize that osteosyntheses with novel bone screw sets (BSS) equipped with anti-rotational elements provide improved rotational stability.</p><p>Methods</p><p>Stability of osteosynthesis under increasing cyclic torsional loading was investigated on osteotomized cadaveric scaphoids. Two novel prototype BSS, oblique type (BSS-obl.) and longitudinal type (BSS-long.) were compared to three conventional screws: Acutrak2^®mini, HCS^®3.0 and Twinfix^®. Biomechanical tests were performed on scaphoids from single donors in paired comparison and analyzed by balanced incomplete random block design. Loading was increased by 50 mNm increments with 1,000 cycles per torque level and repeated until a rotational clearance of 10°. Primary outcome measure was the number of cycles to 10° clearance, secondary outcome measure was the maximum rotational clearance for each torque level.</p><p>Findings</p><p>BSS-obl. performed significantly better than Acutrak2^®mini and HCS^® (p = 0.015, p<0.0001). BSS-long. performed significantly better than HCS^® (p = 0.010). No significant difference in performance between BSS-obl. and BSS-long. (p = 0.361), between BSS obl. and Twinfix^® (p = 0.50) and BSS long. and Twinfix^® (p = 0.667) was detected. Within the torque range up to 200 mNm, four of 21 (19%) BSS-long. and four of 21 (19%) BSS-obl. preparations showed early failure. The same loading led to early failure in four (29%) Twinfix^®, seven (50%) Acutrak2^®mini and 10 (71%) HCS^® of 14 screw samples, respectively.</p><p>Conclusions</p><p>For both BSS and to a lesser extent for Twinfix^® (as dual-component screw), higher rotational stabilities were identified in comparison to single component headless compression screws.</p></div

Statistics for secondary outcome by implant.

<p>Adjustment for Multiplicity: Holm- Simulated.</p

a-d. Sample of a force/angle diagram.

<p>Force/angle diagrams of the cyclic load of a scaphoid bone with a torque load from 150 to 300 mNm and resulting loss of stability. In Fig 4d loosening of the screw fastening is clearly visible by the curves which run out to the right and the line up to 10°.</p

a-e. Screw types.

<p>The shown screws were used for the test series: Acutrak^®2mini (Acumed) (Fig 1a), Twinfix^® (Stryker) (Fig 1b), HCS^® 3.0 (Synthes) (Fig 1c), and prototypes of the two new BSS sets with an additional crosswise drill hole with an inner thread in oblique type (BSS-obl.) (Fig 1d) and with a longitudinal groove with an inner thread and longitudinal screw as a longitudinal type (BSS-long.) (Fig 1e).</p

primary outcome (Least squares means estimates for screw comparisons): dataset on cycles until failure with reference to torque loads (restricted to load range, where majority of failures occurred); N stands for number of screws which didn’t fail at a previous torque level and a further tested in the indicated load range.

<p>primary outcome (Least squares means estimates for screw comparisons): dataset on cycles until failure with reference to torque loads (restricted to load range, where majority of failures occurred); N stands for number of screws which didn’t fail at a previous torque level and a further tested in the indicated load range.</p

Test array.

<p>Draft of the test array for cyclic load to the scaphoid bone osteosyntheses, which were clamped into the holder.</p

primary outcome (Statistics for primary outcome by implant): dataset on total cycles until failure for all tested screws.

<p>primary outcome (Statistics for primary outcome by implant): dataset on total cycles until failure for all tested screws.</p

Primary outcome.

<p>Observed Cycles until failure by Implant. Median of maximum attained cycles of the respective screw models with average parameter and standard deviation. Box plot of maximum cycles per screw type. Median, 25% and 75% percentiles, the points correspond to the individual results per screw.</p