Beyond flexibility: Unraveling the mechanics and assessment of digital stiffness

INTRODUCTION

Clinical evaluation of digital stiffness has a significant impact on functional recovery, yet traditional assessments remain largely subjective. This article explores the mechanics underlying stiffness, the limitations of traditional evaluation methods, and the necessity of objective assessment tools.

Digital stiffness is defined as the “reduction or loss of the mobility arc of the articular chain that constitutes the finger.”^[1] This condition arises not from the hardening of tissues but from the reduced sliding between various tissue planes, leading to a loss of motion at the macroscopic level. Differentiating stiffness from rigidity – a mechanical property referring to resistance to deformation – is critical for understanding its treatment. To properly frame digital stiffness, it is essential to consider the structural composition of tissues, their mechanical behavior under force, and the impact of post-traumatic edema.^[2,3]

This report is based on a narrative review of the existing literature and clinical experience in the assessment and management of digital stiffness. It aimed to explore the biomechanical principles underlying this condition, differentiate it from rigidity, and discuss the influence of viscoelastic properties and post-traumatic edema. This report highlights the advantages and limitations of current evaluation techniques by critically analyzing subjective and objective assessment methods. The goal was to advocate for evidence-based rehabilitation strategies and encourage the integration of advanced technologies for improved diagnosis and treatment outcomes.

VISCOELASTIC PROPERTIES OF TISSUES

Biological tissues exhibit viscoelastic properties, combining elasticity and viscosity. Elasticity refers to the tissue’s ability to return to its original shape after deformation, whereas viscosity involves internal friction that impedes motion when subjected to force. Viscoelastic behavior is evident when force application leads to an initial rapid elongation followed by reduced elongation despite increasing load (stiffening effect).^[4]

For instance, connective tissues are composed primarily of collagen and elastin, two key structural elements with distinct mechanical properties. Collagen provides significant tensile strength but has limited elongation capability (10%), while elastin is more compliant, allowing reversible elongation up to 200%.^[5] The balance of these components varies by tissue type, with tendons and ligaments being collagen-dense to ensure stability, while skin requires greater elasticity to accommodate motion.

The presence of interstitial fluid introduces viscosity into the tissue matrix, further influencing motion. Increased fluid density, as seen in post-traumatic edema, generates resistance to sliding between layers, resulting in restricted motion.^[6]

IMPACT OF EDEMA ON DIGITAL STIFFNESS

While the intrinsic mechanics of tissues contribute to stiffness, external factors, such as fluid dynamics and inflammation, also play a critical role.

Post-traumatic edema significantly contributes to digital stiffness by altering the biomechanical properties of tissues. The fluid accumulation increases tension between fibers (cross-linking) and prevents normal gliding. For example, dorsal edema at the level of the proximal interphalangeal (PIP) joint increases the radius of curvature, pre-tensioning the skin and restricting flexion. In normal conditions, the skin elongates by 12 mm to allow 90° of PIP flexion. When edema increases the radius by 5 mm, the skin requires 19 mm of elongation, which is physiologically unattainable.^[7]

Furthermore, intra-articular edema can pre-tension the collateral ligaments and capsule, further limiting joint motion. The application of pressure, as described by Pascal’s principle, redistributes fluid within the joint, increasing tension in otherwise lax structures [Figure 1].^[8]

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Figure 1:

(a) Normal dorsal skin elongation (12 mm) allows 90° of PIP flexion. (b) In the presence of dorsal edema, the increased radius of curvature requires 19 mm of elongation, which exceeds physiological capacity and restricts joint motion.

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ASSESSMENT OF DIGITAL STIFFNESS

A reliable assessment of digital stiffness is crucial for guiding effective treatment strategies. The commonly used end-feel method relies on subjective manual evaluation of passive range of motion (PROM), classifying tissues into soft end-feel (responding to force with improved PROM) and hard end-feel (resisting elongation with minimal improvement).^[9]

However, the end-feel method is highly operator-dependent and influenced by factors such as force application, patient positioning, and temperature.

OBJECTIVE MEASURES

Objective assessment tools, such as goniometry and torque range of motion (TROM), address the limitations of manual evaluation.

• Goniometry provides quantitative data on PROM changes, enabling the monitoring of stiffness progression over time. However, it remains subject to intra- and inter-operator variability

TROM standardizes the force application and measures the resulting angular displacement, significantly reducing variability. This method involves incrementally applying predefined forces (e.g., 200–1000 g) and recording the corresponding range of motion. The resulting torque-angle curve (TAC) provides insights into tissue behavior under stress [Figure 2].The slope of the TAC indicates the tissue’s responsiveness to elongation. A gradual slope suggests tissues with good elasticity that respond well to treatment, while a steep slope indicates tissues that rapidly stiffen under load, requiring prolonged force application.^[10] This distinction is critical for predicting treatment outcomes and tailoring rehabilitation strategies.

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Figure 2:

Torque-angle curve illustrating tissue responsiveness under increasing loads.

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CLINICAL IMPLICATIONS

Accurate assessment of digital stiffness has significant implications for clinical practice. It allows therapists to:

1. Identify tissue characteristics (different Young modulus) to select appropriate interventions

2. Monitor treatment progression and detect plateaus, prompting early referral for surgical evaluation if necessary

3. Differentiate between reversible edema-related stiffness and structural contractures requiring more intensive interventions.

The TAC approach provides objective data that can inform decisions regarding splinting strategies (e.g., static, static progressive, or serial casting splinting) and the duration of force application. For instance, tissues with a low Young’s modulus (low stiffness) may respond well to short-duration stretching. In contrast, tissues with a high Young’s modulus (indicating greater stiffness) may benefit from prolonged, low-load stretching.^[11]

CLINICAL EXAMPLE

To better understand the diagnostic capabilities of the TAC assessment system, consider this example: A patient presents with a flexion contracture of the volar plate of a PIP joint. The test reveals a very narrow foot of the curve and a nearly vertical linear zone with a slight shift to the right (indicating tissue elongation under tension). Anamnesis reveals that the trauma occurred 3 months before the evaluation. In this case, choosing a dynamic orthosis such as a Capener splint offers a high likelihood of treatment failure in regaining joint extension.

TREATMENT RECOMMENDATIONS

In the first scenario, a static postural or static progressive orthosis is recommended, while in the second, a serial cast system may be preferable. However, the distinction is often not clear-cut but rather nuanced, depending on various factors such as the type and number of joints involved, patient compliance, functional goals, and the etiology of stiffness.

CONCLUSION

The evaluation of digital stiffness requires a shift from subjective assessments to standardized, objective measures such as TROM and torque-angle analysis. The integration of objective and data-driven evaluations such as TROM and TAC will not only refine our understanding of stiffness but also improve rehabilitation strategies and patient outcomes.

These tools provide precise data on tissue behavior, facilitating evidence-based decision-making in rehabilitation. Recognizing the distinct contributions of viscoelastic and edema-related factors enables clinicians to optimize treatment strategies and improve patient outcomes.