Key Pinch Force is More Misdirected in Individuals with Tetraplegia Compared to Non-impaired Subjects

M. Elise Johanson, PT MS; Wendy M. Murray, PhD; Niels Smaby, MS; GJ van de Pol, MS; Joseph D. Towles, MS; Felix E. Zajac, PhD; Vincent R. Hentz, MD

Objectives: After spinal cord injury, tendon transfer and joint stabilization procedures are performed to improve the ability of the thumb to produce adequate key pinch forces to accomplish functional tasks. Function is predicted to improve if pinch force magnitude can be increased. However the ability to direct the force between the pad of the thumb and the lateral aspect of the index finger is also critical to securing objects. The purpose of this study is to compare the magnitude and orientation of the pinch force produced during maximal effort in non-impaired subjects and individuals with spinal cord injury (SCI). We hypothesized that the force produced by individuals with SCI would be more misdirected and smaller in magnitude compared to the force produced by non-impaired subjects.

Methods: Five non-impaired subjects, five individuals with C5-7 level spinal cord injury and no prior reconstructive hand surgery, and 4 subjects who had flexor pollicis longus (FPL) function surgically restored were tested. Each subject was instructed to produce a maximal force in the key pinch posture by pressing the thumb against a 6-axis force sensor. We recorded the magnitude of the pinch force and calculated the angle of the pinch force vector relative to the axis of the force sensor that was oriented perpendicular to the thumbtip. Data were analyzed using the Mann-Whitney U non-parametric test for independent samples.

Results: On average, the force produced by individuals with SCI without surgical reconstruction was oriented 12 degrees (2.6 SD) from the axis perpendicular to the thumb pad, which was significantly greater (p < .01) than the non-impaired subjects (5.3 degrees, 1.9 SD). In addition, mean force magnitude was significantly lower (3.8 N vs. 53.6 N) for the individuals with SCI (p < .01). The post-operative group had a mean pinch force of 27.3 N (SD 7.1) and mean force direction angle of 10.5 (SD 5.2) degrees from the reference axis.

Conclusions: Our previous study of non-impaired subjects suggests the abductor pollicis brevis and extensor pollicis longus muscles are important for directing pinch force. The absence of control of these muscles in individuals with SCI at the C5-7 level may be a factor in how the pinch force is oriented. The findings from the postoperative group suggest that tendon transfer surgery improved the thumbtip force magnitude. However, the ability to direct the force more accurately may require surgical modifications to restore the action of the EPL and APB whenever possible. Understanding how pinch forces are directed may influence the choice of muscle-tendon transfers or joint stabilization procedures performed to restore function to the severely paralyzed thumb.

Johanson ME, Valero-Cuevas FJ, and Hentz VR. Activation patterns of the thumb muscles during stable and unstable pinch tasks. J Hand Surg: 26A: 698-705, 2001.

Funding: This research was supported by the Rehabilitation Research and Development Service of the Department of Veterans Affairs project number B1967-R.