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

Abstract

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.

Keywords: Biomechanics; Scaphoid fracture; Torque and lever force; Wrist movement.

Fig. 1

The testing setup with fastening of the forearm, retaining the ability to produce wrist movements. Loading of the wrist was achieved by weights attached to a pulley system, generating tendon loading. The figure shows an enlarged version of the sensor located in the osteotomised scaphoid that measured rotation and torque

Fig. 2

Photo of the sensor and schematic drawing of the sensor that was implanted and interlocked within the osteotomised scaphoid. The position of strain gauges on the sensor and draft of Wheatstone bridge circuits have been published previously [7]

Fig. 3

X-ray of the experimental wrist setup. The fixation locking nail was used as a sensor between the two scaphoid fragments. In addition, the midcarpal joint was partially stabilised with K-wires

Fig. 4

Representation of torque at 20%, 50%, and 80% of ROM in radial and ulnar directions of the wrist. Using MATLAB (MathWorks, Adalperostraße, Ismaning, Germany), digital goniometer-based ROM, torque, and two simultaneously recorded forces were collected over time

Fig. 5

Torque within the scaphoid at 20%, 50%, and 80% of wrist ROM (flexion, extension, radial abduction, ulnar abduction) in the unloaded and loaded states with and without K-wire stabilisation