Analysis of finger extensor mechanism strains

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.

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

The extensor mechanism of the finger is a structure transmitting the forces from several muscles to the finger joints. Force transmission in the extensor mechanism is usually modeled by equations with constant coefficients which are determined experimentally only for finger extension posture. However, the coefficient values change with finger flexion because of the extensor mechanism deformation. This induces inaccurate results for any other finger postures. We proposed a biomechanical model of the extensor mechanism represented as elastic strings. The model includes the main tendons and ligaments. The parametric identification of the model in extension posture was performed to match the distribution of the forces among the tendons to experimental data. The parametrized model was used to simulate three degrees of flexion. Furthermore, the ability of the model to reproduce how the force distribution in simulated extensor mechanism changes according to the muscle forces was also demonstrated. The proposed model could be used to simulate the extensor mechanism for any physiological finger posture for which the coefficients involved in the equations are unknown.

The effect of the extensor mechanism on maximum isometric fingertip forces: A numerical study on the index finger

The extensor mechanism is a tendinous network connecting intrinsic and extrinsic muscles of the finger and its function has not yet been fully understood. The goal of this study was to assess the effect of the extensor mechanism on the maximum isometric fingertip forces - a parameter which is essential for grasping. For this purpose, maximum fingertip forces in all directions (i.e. feasible force sets) of two musculoskeletal models of the index finger were compared: the wEM model included a full representation of the extensor mechanism, whereas in the noEM model the extensor mechanism was replaced by a single extensor tendon without connectivity to intrinsic muscles. The feasible force sets were computed in the flexion-extension plane for nine postures. Forces in four predefined directions (palmar, proximal, dorsal, and distal), and the peak resultant forces were evaluated. Averaged forces in all four predefined directions were considerably larger in the wEM model (+187.6%). However, peak resultant forces were slightly lower in the wEM model (-4.3% on average). The general advantage of the wEM model could be explained by co-contraction of intrinsic and extrinsic extensor muscles which allowed reaching larger activation levels of the extrinsic flexors. Only within a narrow range of force directions the co-contraction of intrinsic muscles limited the fingertip forces and lead to lower peak resultant forces in the wEM model. Rather than maximizing peak resultant forces, it appears that the extensor mechanism is a sophisticated tool for increasing maximum fingertip forces over a broad range of postures and force directions - making the finger more versatile during grasping.

Mechanical properties vary for different regions of the finger extensor apparatus

The extensor apparatus, an aponeurosis that covers the dorsal side of each finger, transmits force from a number of musculotendons to the phalanges. Multiple tendons integrate directly into the structure at different sites and the extensor apparatus attaches to the phalanges at multiple points. Thus, prediction of the force distribution within the extensor apparatus, or hood, and the transmission to the phalanges is challenging, especially as knowledge of the underlying mechanical properties of the tissue is limited. We undertook quantification of some of these properties through material testing of cadaver specimens. We punched samples at specified locations from 19 extensor hood specimens. Material testing was performed to failure for each sample with a custom material testing device. Testing revealed significant differences in ultimate load, ultimate strain, thickness, and tangent modulus along the length of the extensor hood. Specifically, thickness, ultimate load, and ultimate strain were greater in the more proximal sections of the extensor hood, while the tangent modulus was greater in the more distal sections. The variations in mechanical properties within the hood may impact prediction of force transmission and, thus, should be considered when modeling the action of the extensor apparatus. Across the extensor hood, tangent modulus values were substantially smaller than values reported for other soft tissues, such as the Achilles tendon and knee ligaments, while ultimate strains were much greater. Thus, the tissue in the extensor apparatus seems to have greater elasticity, which should be modeled accordingly.

Effect of finger posture on the tendon force distribution within the finger extensor mechanism

Understanding the transformation of tendon forces into joint torques would greatly aid in the investigation of the complex temporal and spatial coordination of multiple muscles in finger movements. In this study, the effects of the finger posture on the tendon force transmission within the finger extensor apparatus were investigated. In five cadaver specimens, a constant force was applied sequentially to the two extrinsic extensor tendons in the index finger, extensor digitorum communis and extensor indicis proprius. The responses to this loading, i.e., fingertip force/moment and regional strains of the extensor apparatus, were measured and analyzed to estimate the tendon force transmission into the terminal and central slips of the extensor hood. Repeated measures analysis of variance revealed that the amount of tendon force transmitted to each tendon slip was significantly affected by finger posture, specifically by the interphalangeal (IP) joint angles (p<0.01). Tendon force transmitted to each of the tendon slips was found to decrease with the IP flexion. The main effect of the metacarpophalangeal (MCP) joint angle was not as consistent as the IP angle, but there was a strong interaction effect for which MCP flexion led to large decreases in the slip forces (>30%) when the IP joints were extended. The ratio of terminal slip force:central slip force remained relatively constant across postures at approximately 1.7:1. Force dissipation into surrounding structures was found to be largely responsible for the observed force-posture relationship. Due to the significance of posture in the force transmission to the tendon slips, the impact of finger posture should be carefully considered when studying finger motor control or examining injury mechanisms in the extensor apparatus.

Releasing the A3 pulley and leaving flexor superficialis intact increases pinch force following the Zancolli lasso procedures to prevent claw deformity in the intrinsic palsied finger

Objective estimates of fingertip force magnitude following surgery to prevent digital metacarpophalangeal (MCP) hyperextension (clawing) in cases of paralysis of the hand's intrinsic muscles will assist clinicians in setting realistic expectations for post-operative pinch strength. We used a cadaveric/optimization approach to predict and confirm the maximal biomechanically possible fingertip force in the intrinsic palsied hand before and after two popular tendon transfer methods to the volar plate of the MCP joint. Both surgeries were also evaluated after release of the A3 pulley-a modification predicted by our published computer model of the forefinger to increase fingertip force magnitude. We predicted maximal static fingertip force by mounting eight fresh cadaveric hands on a frame, placing their forefinger in a functional posture (neutral abduction, 45 degrees of flexion at the MCP and proximal interphalangeal joints, and 10 degrees at the distal interphalangeal joint) and pinning the distal phalanx to a 3D dynamometer. We pulled on individual tendons with tensions up to 25% of maximal isometric force of their associated muscle and measured fingertip force and torque output. Using these measurements, we predicted the optimal combination of tendon tensions that maximized palmar force (analogous to pinch force, directed perpendicularly from the midpoint of the distal phalanx, and in the plane of finger flexion-extension) for four cases: (i) the non-paretic case (all muscles available), (ii) intrinsic palsied hand (no intrinsic muscles functioning), (iii) transfer of flexor superficialis tendon to the volar plate of the MCP (Zancolli lasso) in the intrinsic palsied hand, and (iv) leaving flexor superficialis intact and transferring a tendon of comparable strength to the volar plate of the MCP in the intrinsic palsied hand. Lastly, we applied these optimal combinations of tension to the cadaveric tendons and measured fingertip output. With the A3 pulley intact, the maximal palmar force in cases (ii)-(iv) averaged 48 +/- 23% SD (non-paretic = 100%; case (iv) (61 +/- 25%) > cases (ii) and (iii) (43 +/- 23% and 39 +/- 19%, respectively), p < 0.05). Releasing the A3 pulley significantly increased the average palmar force in cases (ii)-(iv) (73 +/- 42%, p < 0.05), with no significant differences among them. Thus, releasing the A3 pulley may improve palmar force magnitude when it is necessary to transfer the digit's own flexor superficialis tendon to the volar plate of the MCP to prevent clawing in the intrinsic palsied hand.

Quantification of fingertip force reduction in the forefinger following simulated paralysis of extensor and intrinsic muscles

Objective estimates of fingertip force reduction following peripheral nerve injuries would assist clinicians in setting realistic expectations for rehabilitating strength of grasp. We quantified the reduction in fingertip force that can be biomechanically attributed to paralysis of the groups of muscles associated with low radial and ulnar palsies. We mounted 11 fresh cadaveric hands (5 right, 6 left) on a frame, placed their forefingers in a functional posture (neutral abduction, 45 degrees of flexion at the metacarpophalangeal and proximal interphalangeal joints, and 10 degrees at the distal interphalangeal joint) and pinned the distal phalanx to a six-axis dynamometer. We pulled on individual tendons with tensions up to 25% of maximal isometric force of their associated muscle and measured fingertip force and torque output. Based on these measurements, we predicted the optimal combination of tendon tensions that maximized palmar force (analogous to tip pinch force, directed perpendicularly from the midpoint of the distal phalanx, in the plane of finger flexion-extension) for three cases: non-paretic (all muscles of forefinger available), low radial palsy (extrinsic extensor muscles unavailable) and low ulnar palsy (intrinsic muscles unavailable). We then applied these combinations of tension to the cadaveric tendons and measured fingertip output. Measured palmar forces were within 2% and 5 degrees of the predicted magnitude and direction, respectively, suggesting tendon tensions superimpose linearly in spite of the complexity of the extensor mechanism. Maximal palmar forces for ulnar and radial palsies were 43 and 85% of non-paretic magnitude, respectively (p<0.05). Thus, the reduction in tip pinch strength seen clinically in low radial palsy may be partly due to loss of the biomechanical contribution of forefinger extrinsic extensor muscles to palmar force. Fingertip forces in low ulnar palsy were 9 degrees further from the desired palmar direction than the non-paretic or low radial palsy cases (p<0.05).

Staged extensor tendon reconstruction in the finger

Staged extensor tendon reconstruction using a silicone implant followed by tendon grafting was done to restore proximal interphalangeal (PIP) joint extension in 6 fingers with severe injuries to the dorsal skin and extensor mechanism. Abrasions to the joint capsule and cortical surfaces were also present. To avoid finger stiffness, the reconstruction was delayed and range of motion exercises were initiated early. The skin injury was managed by split-thickness skin grafting or allowed to heal by secondary intention to avoid prolonged immobilization. During surgery, the peritendinous fascia of the extensor tendon is used to guide insertion of the implant, and it serves as a premade tunnel that appears to aid the gliding and stability of the implant and subsequent tendon graft. Active extension of the PIP joint was restored in all fingers; there was an average extension lag of 15 degrees. PIP joint flexion averaged 95 degrees. On the basis of this experience, the author believes the technique to be a reliable treatment alternative for severely injured fingers with extensor mechanism loss.