A novel experimental design for the measurement of metacarpal bone loading and deformation and fingertip force

Abstract

Background: Musculoskeletal and finite element modelling are often used to predict joint loading and bone strength within the human hand, but there is a lack of in vitro evidence of the force and strain experienced by hand bones.

Methods: This study presents a novel experimental setup that allows the positioning of a cadaveric digit in a variety of postures with the measurement of force and strain experienced by the third metacarpal. The setup allows for the measurement of fingertip force as well. We tested this experimental setup using three cadaveric human third digits in which the flexor tendons were loaded in two tendon pathways: (1) parallel to the metacarpal bone shaft, with bowstringing; (2) a semi-physiological condition in which the tendons were positioned closer to the bone shaft.

Results: There is substantial variation in metacarpal net force, metacarpal strain and fingertip force between the two tendon pathways. The net force acting on the metacarpal bone is oriented palmarly in the parallel tendon condition, causing tension along the dorsum of the metacarpal shaft, while the force increases and is oriented dorsally in the semi-physiological condition, causing compression of the dorsal metacarpal shaft. Fingertip force is also greater in the semi-physiological condition, implying a more efficient grip function. Inter-individual variation is observed in the radioulnar orientation of the force experienced by the metacarpal bone, the fingertip force, and the strain patterns on the metacarpal shaft.

Conclusion: This study demonstrates a new method for measuring force and strain experienced by the metacarpal, and fingertip force in cadaveric digits that can, in turn, inform computation models. Inter-individual variation in loads experienced by the third digit suggest that there are differences in joint contact and/or internal bone structure across individuals that are important to consider in clinical and evolutionary contexts.

Keywords: Biomechanics; Force; Hand; Strain.

Figure 1. Experimental setup.

A customized experimental setup was designed to fix the fingertip and the base of metacarpal bone using the distal and proximal clamps, respectively. A six-axis load cell was fixed onto the proximal clamp and the proximal stand to measure the force experienced by the metacarpal bone, and a set of pulleys and weights were used to load the tendons to simulate the muscle contraction during hand function. Photo credit: Dr. Szu-Ching Lu.

Figure 2. Metacarpal loading and deformation measurement and fingertip force evaluation.

(A) The load cell was attached to the proximal clamp to measure the force experienced by the metacarpal bone, and the strain gauges were attached to the metacarpal shaft to measure the bone deformation. The third digit was in a flexed posture with the flexor tendons loaded and guided parallel to the bone shaft, and then a low-friction metal bar was applied to place the tendons in a semi-physiological pathway (the dash line). (B) The load cell was fixed to the distal clamp for the fingertip force measurement. DIP, distal interphalangeal joint; PIP, proximal interphalangeal joint; MCP, metacarpophalangeal joint. Photo credit: Dr. Szu-Ching Lu.

Figure 3. Stain gauge attachment.

(A) Three strain gauges were applied to the radial-palmar, dorsal and ulnar-palmar sides of the metacarpal bone at its midshaft to quantify the bone deformation. (B) A computed tomography image shows the coronal cross-sectional view of the metacarpal bone with three strain gauges attached. Photo credit: Dr. Szu-Ching Lu.

Figure 4. Net force acting on the metacarpal bone.

The force is depicted in the flexion-extension plane (A) and the radial-ulnar deviation plane (B) with respect to the third metacarpal bone. The arrow shows the mean value of the three specimens and the box shows the range of one standard deviation. Data of each specimen are also presented.

Figure 5. Metacarpal bone strain.

The strain experienced by the three sides of the metacarpal bone is presented as the mean value (box) with one standard deviation (whiskers). Data of individual specimen are also presented. Positive value represents tension while negative value means compression.

Figure 6. Fingertip force.

The force is presented in the flexion-extension plane (A) and the radial-ulnar deviation plane (B) with respect to the distal phalanx. The arrow shows the mean value of the three specimens and the box shows the range of one standard deviation, and the data of individual specimen are also presented.