Kinematic evaluation of the finger’s interphalangeal joints coupling mechanism—variability, flexion–extension differences, triggers, locking swanneck deformities, anthropometric correlations
Introduction
Study aims: The human finger contains tendinous mechanisms that are essential for proper control (Leijnse et al., 2005). One such mechanism couples the distal interphalangeal (DIP) and proximal interphalangeal (PIP) joint angles in the (unloaded) finger when flexed with active deep flexor. This study’s aim was to quantify, in a large sample, coupled DIP versus PIP trajectories (DvPT) more accurately than previously, for clinical reference and model validation. The scope is descriptive analysis. The biomechanics underlying the measured DvPT properties and correlations will be considered in future work.
Previous versus present DvPT measurement methods: The present measurements of coupled DvPT were obtained with substantially greater accuracy than previous (Hahn et al., 1995, Holguin et al., 1999), by amplifying finger segment motions via marker frames. The present study also involved the largest and anthropometrically most varied sample to our knowledge to date.
New data: Increased measurement accuracy allowed for detailed quantification of DvPT properties, variability, previously undescribed systematic differences between flexion and extension DvPT and identification of three types of finger motion triggers that are distinct from flexor tendon triggers. A trigger is a consistent trajectory deceleration (trigger activation) followed by an acceleration (trigger release), indicating elastic energy buildup and release in the finger’s tendon/muscle/ligaments.
Applications: The present data may advance insight into normal and pathological finger biomechanics (e.g., swanneck deformities), and may also be relevant to humanoid robotic hand design, e.g., (Miller et al., 2005). The marker frame measurement method may be useful for surgical/rehabilitative outcome studies and differential diagnostics of trigger fingers.
Section snippets
Data acquisition
Subjects: DvPTs were obtained from 68 normal fingers in eight subjects bilaterally and a ninth left hand. Subjects were 7 male, 2 female, 6 Caucasian, 3 Asian, with mean age of 33±11 years (Table 1). Prior to data collection, subjects signed an informed consent document approved by the University of Louisville Institutional Review Board.
Anthropometrics: Hand/finger lengths/widths were measured with calipers. Metacarpophalangeal (MCP), PIP and DIP maximum passive extension (MPE) and flexion
Passive and active joint ranges, gravitational marker frame effects, influence of MCP flexion angle, reference position accuracy
The mean and individual passive joint ranges obtained by goniometer are presented in Table 1, Table 2. Recorded IPJT, which included possible reference angle measurement errors, exceeded a passive joint range in at most 13 of the 68 fingers by a mean difference of less than 5°. These IPJT were corrected by translating them within the passive joint range by the exceeded joint range difference (Table 3). No adjustments were needed for both extension and flexion ranges of a joint. Gravitational
Discussion
Summary: In 68 fingers of nine subjects with highly variable joint laxity, DvPTs reproduced highly consistently, demonstrating the underlying IPJ coupling mechanism. DvPT shapes ranged from linear (mostly in D5) to full sigmoid (most pronounced in D3). The mean DvPT DIP range was smallest in D2 and greatest in D3.
Inconsistent DvPT caused by superficial flexor dominance: Some fingers initially exhibited inconsistent f-DvPTs, with DIP angles remaining smaller than in e-DvPTs (Fig. 3A). This
Conflict of interest statement
There are no conflicts of interest in this study.
Acknowledgements
The authors thank G. Prater Jr., Ph.D., W.P. Hnat, Ph.D., N.H. Kyureghyan, Ph.D., J.C. Jones, supporting engineer, and the volunteer subjects of the University of Louisville, USA, and F. Schuind, MD, Ph.D., of the Université Libre de Bruxelles, Belgium, for their support.
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