The effect of percutaneous pin fixation of the interphalangeal joint on the thumb-tip force produced by the flexor pollicis longus: a cadaver study

The Sensitivity of the Endpoint Forces of Thumb Extrinsic and Intrinsic Muscles to Changes in Joint Angles, Muscle Moment Arms and Bone Lengths in the Flexed Thumb

A previous study showed in situ measurements of thumb-tip forces produced by muscles vary substantially among cadaveric specimens. Potential sources of variability include inter-specimen anatomic differences and postural deviations from the nominal posture in which the specimens were tested. This study aimed to theoretically determine the variation in thumb-tip force caused by inter-specimen differences in thumb anatomy and posture. We developed a two-dimensional mathematical model of force production at the thumb tip based on published estimates of muscle moment arms, bone length, and joint angle measurements from nine cadaveric specimens. The model was placed in a flexed posture. Using the model, we calculated variations in magnitude and direction of each muscle's thumb-tip force induced by a ±1 standard deviation (or equivalent) variation in each bone length, the moment arm of the muscle (i.e., anatomic factors), and each joint angle (i.e., postural factor). For most muscles, inter-specimen differences in the metacarpophalangeal (MP) joint angle produced at least a 75% larger variation in thumb-tip force magnitude than that produced by other factors. For all muscles, differences in the interphalangeal joint angle among specimens produced the largest variation in thumb-tip force direction. For some muscles, inter-specimen differences in bone lengths, moment arms, and MP joint angles also produced large variations in thumb-tip force direction. This study suggests deviation from the nominal flexed thumb posture and large measurement variability in muscle moment arms are primary and secondary sources, respectively, of variability in thumb-tip forces produced by the majority of thumb muscles. Further, this study suggests a more careful approach to standardizing the thumb posture would likely improve current measurements of thumb-tip forces.Clinical Relevance- This work describes the influence of anatomic and postural factors on thumb-tip forces that thumb muscles produce. The results of this work have implications for musculoskeletal modeling and surgical reconstruction of grasp.

Measurement of the Three-Dimensional Muscle Endpoint Forces in the Extended Thumb and Its Application to Determining Muscle Combinations that Enable Lateral Pinch Force Production Throughout the Plane of Flexion-Extension

Functional outcomes of tendon transfer surgeries, designed to restore lateral pinch grasp to persons following cervical spinal cord injury, have been mixed. That is, pinch force magnitudes have varied by 10-fold and have been reported to be as low as low as tenths of a pound. We believe a novel tendon transfer approach in which the donor muscle actuates a small group of paralyzed thumb muscles, instead of just the flexor pollicis longus (FPL) muscle (the current approach), will enable endpoint forces that are better directed and therefore a consistently stronger pinch force following surgery. We further believe that such surgeries can be better designed to account for grasp force production throughout the entire plane of flexion-extension if muscle endpoint forces in the extended thumb are known. Consequently, we measured muscle endpoint forces in the extended thumb in 6 cadaveric specimens after a force of 10 N was applied to each muscle. Further, we simulated a tendon transfer surgery in which the donor muscle applied equal force to each muscle in 246 small groups of muscles, calculated the direction of the resulting endpoint force throughout the flexion-extension plane, and determined if those groups of muscles produced a better directed force than FPL's. While we found that 3 individual muscles and 52 muscle groups could produce desirably directed endpoint forces in parts of the flexion-extension plane, no muscle or muscle group could produce well-directed endpoint forces throughout the flexion-extension plane. We concluded that a group of muscles could likely be found if the donor muscle provided different levels of force to each of the muscles in a muscle group. This would be possible through intentional geometric manipulation of the donor-to-recipient muscle attachment to allow for unequal splitting of donor muscle force.Clinical Relevance-This work aims to determine whether the same combination of thumb muscles can produce well-directed endpoint forces throughout the flexion-extension plane. If so, then this work informs surgeons which muscle groups could be involved in a tendon transfer to restore lateral pinch grasp ability throughout the plane of flexion-extension in person with cervical spinal cord injury.

Benefits of Additional Procedures for Metacarpophalangeal Hyperextension on Simple Trapeziectomy in Thumb Basal Osteoarthritis: A Biomechanical Cadaver Study

Background: Patients with advanced osteoarthritis of the first carpometacarpal joint (CMC-1) may develop hyperextension of the first metacarpophalangeal joint (MCP-1). No clear clinical benefice has been reported consecutively to the surgical treatment of the MCP-hyperextension combined to a trapeziectomy. The reason of the missing benefit may be due to changes in the thumb position impairing the thumb stability secondary to the surgical procedures. We assessed changes in the transmission of forces at the thumb's end phalanx following a trapeziectomy combined with the surgical adjustment of the hyperextension of the MCP-1-joint in a biomechanical investigation using cadavers. Methods: The thumb muscles were loaded with nylon cables connected to a tension meter in 8 forearm cadavers. A 6-axis force sensor assessed the termino-lateral key-pinch orthogonal strength vectors at the level of the thumb distal phalanx prior to any surgery, and following a simple trapeziectomy, a trapeziectomy combined to a MCP-1-capsulodesis and the transfer of the extensor pollicis brevis over the metacarpal-1 head, or to an MCP-1-arthrodesis. Results: Combination of the trapeziectomy with the MCP-1- joint palmar capsulodesis and EPB-transfer or with a MCP-arthrodesis in neutral pronation-supination resulted in a significant shift of the thumb in pronation-abduction with respect to the preoperative assessment. The lowest shift was achieved when performing the arthrodesis in 20° supination or by overloading of the adductor pollicis. Conclusions: Combining the trapeziectomy with surgeries addressing the MCP-1-joint hyperextension induced a shift of the thumb in pronation-abduction that could impair the key-pinch stability. When considering additional procedures for MCP-1-joint hyperextension deformities, it should be recommended to fix the EPB-tendon on the radial aspect of the metacarpal head if a tendon transfer is considered, otherwise the MCP joint arthrodesis should be performed in supinated position, in order to achieve lateral key-pinch stability.

Assessing Finger Joint Biomechanics by Applying Equal Force to Flexor Tendons In Vitro Using a Novel Simultaneous Approach

Background

The flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) are critical for finger flexion. Although research has recently focused on these tendons’ coactivity, their contributions in different tasks remain unclear. This study created a novel simultaneous approach to investigate the coactivity between the tendons and to clarify their contributions in different tasks.

Methods

Ten human cadaveric hands were mounted on our custom frame with the FDS and FDP of the third finger looped through a mechanical pulley connected to a force transducer. Joint range of motion, tendon excursion and loading force were recorded during individual joint motion and free joint movement from rest to maximal flexion. Each flexor tendon’s moment arm was then calculated.

Results

In individual motions, we found that the FDP contributed more than the FDS in proximal interphalangeal (PIP) joint motion, with an overall slope of 1.34 and all FDP-to-FDS excursion (P/S) ratios greater than 1.0 with force increase. However, the FDP contributed less than the FDS in metacarpophalangeal (MCP) joint motion, with an overall slope of 0.95 and P/S ratios smaller than 1.0 throughout the whole motion except between 1.9% and 13.1% force. In free joint movement, the FDP played a greater role than the FDS, with an overall ratio of 1.37 and all P/S ratios greater than 1.0.

Conclusions

The new findings include differences in finger performance and excursion amounts between the FDS and FDP throughout flexion. Such findings may provide the basis for new hand models and treatments.

Transducer and base compliance alter the in situ 6 dof force measured from muscle during an isometric contraction in a multi-joint limb

Although musculoskeletal models are commonly used, validating the muscle actions predicted by such models is often difficult. In situ isometric measurements are a possible solution. The base of the skeleton is immobilized and the endpoint of the limb is rigidly attached to a 6-axis force transducer. Individual muscles are stimulated and the resulting forces and moments recorded. Such analyses generally assume idealized conditions. In this study we have developed an analysis taking into account the compliances due to imperfect fixation of the skeleton, imperfect attachment of the force transducer, and extra degrees of freedom (dof) in the joints that sometimes become necessary in fixed end contractions. We use simulations of the rat hindlimb to illustrate the consequences of such compliances. We show that when the limb is overconstrained, i.e., when there are fewer dof within the limb than are restrained by the skeletal fixation, the compliances of the skeletal fixation and of the transducer attachment can significantly affect measured forces and moments. When the limb dofs and restrained dofs are matched, however, the measured forces and moments are independent of these compliances. We also show that this framework can be used to model limb dofs, so that rather than simply omitting dofs in which a limb does not move (e.g., abduction at the knee), the limited motion of the limb in these dofs can be more realistically modeled as a very low compliance. Finally, we discuss the practical implications of these results to experimental measurements of muscle actions.

The sensitivity of endpoint forces produced by the extrinsic muscles of the thumb to posture

This study utilizes a biomechanical model of the thumb to estimate the force produced at the thumb-tip by each of the four extrinsic muscles. We used the principle of virtual work to relate joint torques produced by a given muscle force to the resulting endpoint force and compared the results to two separate cadaveric studies. When we calculated thumb-tip forces using the muscle forces and thumb postures described in the experimental studies, we observed large errors. When relatively small deviations from experimentally reported thumb joint angles were allowed, errors in force direction decreased substantially. For example, when thumb posture was constrained to fall within +/-15 degrees of reported joint angles, simulated force directions fell within experimental variability in the proximal-palmar plane for all four muscles. Increasing the solution space from +/-1 degrees to an unbounded space produced a sigmoidal decrease in error in force direction. Changes in thumb posture remained consistent with a lateral pinch posture, and were generally consistent with each muscle's function. Altering thumb posture alters both the components of the Jacobian and muscle moment arms in a nonlinear fashion, yielding a nonlinear change in thumb-tip force relative to muscle force. These results explain experimental data that suggest endpoint force is a nonlinear function of muscle force for the thumb, support the continued use of methods that implement linear transformations between muscle force and thumb-tip force for a specific posture, and suggest the feasibility of accurate prediction of lateral pinch force in situations where joint angles can be measured accurately.

Use of intrinsic thumb muscles may help to improve lateral pinch function restored by tendon transfer

BACKGROUND

For surgical reconstruction of lateral pinch following tetraplegia, the function of the paralyzed flexor pollicis longus is commonly restored. The purpose of this study was to investigate if one of the intrinsic muscles could generate a more suitably directed thumb-tip force during lateral pinch than that of flexor pollicis longus.

METHODS

Endpoint force resulting from 10 N applied to each thumb muscle was measured in eleven upper extremity cadaveric specimens. We utilized the Kruskal-Wallis test (alpha=0.05) to determine whether thumb-tip forces of intrinsic muscles were less directed toward the base of the thumb, i.e., proximally directed, than the thumb-tip force produced by flexor pollicis longus. Additionally, a biomechanical model was used to assess the effect of an increase in tendon force on intrinsic muscle endpoint forces.

FINDINGS

All of the intrinsic muscles produced thumb-tip force vectors, ranging from 127 degrees to 156 degrees , that were significantly (P<0.009) less proximally directed than that of flexor pollicis longus (66 degrees (46 degrees )). A biomechanical model predicted that intrinsic muscle thumb-tip forces would vary non-linearly with tendon force. A 2-fold increase in tendon force produced, on average, a 2.3-fold increase in force magnitude and an 8 degrees shift in force direction across all intrinsic muscles.

INTERPRETATION

This study suggests the possibility of using an intrinsic muscle, e.g., the flexor pollicis brevis (ulnar head), instead of flexor pollicis longus, to produce a more advantageously directed thumb-tip force during lateral pinch in the surgically-reconstructed tetraplegic thumb and thus potentially enhance function.