The biomechanical model of the long finger extensor mechanism and its parametric identification

Cosimulation of the index finger extensor apparatus with finite element and musculoskeletal models

Musculoskeletal modeling has been effective for simulating dexterity and exploring the consequences of disability. While previous approaches have examined motor function using multibody dynamics, existing musculoskeletal models of the hand and fingers have difficulty simulating soft tissue such as the extensor mechanism of the fingers, which remains underexplored. To investigate the extensor mechanism and its impact on finger motor function, we developed a finite element model of the index finger extensor mechanism and a cosimulation method that combines the finite element model with a multibody dynamic model. The finite element model and cosimulation were validated through comparison with experimentally derived tissue strains and fingertip endpoint forces respectively. Tissue strains predicted by the finite element model were consistent with the experimentally observed strains of the 9 postures tested in cadaver specimens. Fingertip endpoint forces predicted using the cosimulation were well aligned in both force (difference within 0.60 N) and direction (difference within 30°with experimental results. Sensitivity of the extensor mechanism to changes in modulus and adhesion configuration were evaluated for ± 50% of experimental moduli, presence of the radial and ulnar adhesions, and joint capsule. Simulated strains and endpoint forces were found to be minimally sensitive to alterations in moduli and adhesions. These results are promising and demonstrate the ability of the cosimulation to predict global behavior of the extensor mechanism, while enabling measurement of stresses and strains within the structure itself. This model could be used in the future to predict the outcomes for different surgical repairs of the extensor mechanism.

An in-depth look at zone III and IV anatomy of the finger extensor mechanism and some clinical implications for use of the relative motion flexion orthosis

BACKGROUND

For hand therapists and hand surgeons acute and chronic injuries of the extensor mechanism (EM) in zones III-IV are challenging to treat with satisfying results.

INTRODUCTION

Early active motion combined with relative motion flexion (RMF) orthoses to manage EM zone III injuries and boutonnière deformity has renewed interest in the complex anatomy and biomechanics of the EM.

PURPOSE

To provide an in-depth discussion of EM zones III-IV anatomy with emphasis on inter-tendinous structures, often omitted in simplified, model-wise illustrations which focus mostly on the tendinous structures.

METHOD

In collaboration the authors combined on the one hand extensive clinical experience and knowledge of the EM literature and on the other hand decades of anatomical, biomechanical and kinesiology research of the EM with special interest for the spiral fibers, through gross anatomy and microdissection anatomy laboratory work, MRI and ultrasonography studies.

RESULTS

The inter-tendinous tissues (i.e., spiral fibers) in zone III are of imminent importance for proper functioning of the EM and to prevent boutonnière deformity to develop after EM surgery or injury.

DISCUSSION

Inter-tendinous links between the tendinous structures of the EM are necessary for balanced finger motion. The spiral fibers are described in more detail because of their role in controlling volar migration of the conjoined lateral bands and because their disruption makes development of boutonnière deformity more likely. Understanding the anatomy and biomechanics of the EM may assist in progress toward 'proof of concept' for use of RMF orthoses and controlled early active motion after EM injury or surgery.

CONCLUSION

Hand surgery and hand therapy practice interventions, including use of RMF orthoses for management of non-surgical and surgical EM injuries may benefit from an in-depth look at the EM zone III and IV anatomy and biomechanics.

Custom-made finger splint versus prefabricated finger splint: finger flexion stabilization

OBJECTIVE:

Finger splints are used as a treatment option in tendon, bone, and soft tissue injuries. For immobilization, custom-made splints and prefabricated finger splints are used. In splints made for immobilization, it is aimed to limit joint movement. The aim of our study is to reveal how much custom-made splints and prefabricated finger splints limit joint motion (flexion angle in proximal interphalangeal and distal interphalangeal joints).

METHODS:

Custom-made splints and prefabricated finger splints were applied to the second fingers of the dominant side in a total of 40 individuals, 20 women and 20 men, not having any health problems. Individuals were asked to flex and joint motion was measured with the iPhone compass application.

RESULTS:

The mean distal interphalangeal joint angle values of the participants measured by prefabricated finger splints were found to be 24.27±8.29, and the mean distal interphalangeal joint angle values measured by custom-made splints was 0.52±1.50. There was a difference between the participants’ distal interphalangeal joint angle values measured by prefabricated finger splints and custom-made splints (p<0.001). distal interphalangeal joint angle values measured with custom-made splints were significantly smaller than those measured with prefabricated finger splint. The mean of the participants’ proximal interphalangeal joint angle values measured by prefabricated finger splints was 16.55±7.90, and the proximal interphalangeal joint angle values measured by custom-made splints was “0” for all participants. There was a difference between the participants’ proximal interphalangeal joint angle values measured by prefabricated finger splints and custom-made splints (p<0.001). Distal interphalangeal joint angle values measured with custom-made splints were significantly smaller than those measured with prefabricated finger splints.

CONCLUSION:

According to our study, custom-made splints can significantly reduce the flexion of the finger interphalangeal joints compared to prefabricated finger splints.

Analysis on synergistic cocontraction of extrinsic finger flexors and extensors during flexion movements: A finite element digital human hand model

Fine hand movements require the synergistic contraction of intrinsic and extrinsic muscles to achieve them. In this paper, a Finite Element Digital Human Hand Model (FE-DHHM) containing solid tendons and ligaments and driven by the Muscle-Tendon Junction (MTJ) displacements of FDS, FDP and ED measured by ultrasound imaging was developed. The synergistic contraction of these muscles during the finger flexion movements was analyzed by simulating five sets of finger flexion movements. The results showed that the FDS and FDP contracted together to provide power during the flexion movements, while the ED acted as an antagonist. The peak stresses of the FDS, FDP and ED were all at the joints. In the flexion without resistance, the FDS provided the main driving force, and the FDS and FDP alternated in a "plateau" of muscle force. In the flexion with resistance, the muscle forces of FDS, FDP, and ED were all positively correlated with fingertip forces. The FDS still provided the main driving force, but the stress maxima occurred in the FDP at the DIP joint.