Forces in the normal and abnormal hand

Comparative Biomechanical Study of Fragment-Specific Plates and Headless Screws for Radial Styloid and Lunate Facet Fixation in Distal Radius Fracture Evaluated in a Synthetic Composite

PURPOSE

To evaluate the compressive stiffness (ability to resist compression under an applied load) of fragment-specific plate and headless screw fixation for radial styloid and volar lunate facet fractures in a synthetic composite distal radius.

METHODS

A simulated radial styloid fracture (AO type B1.1) and simulated volar lunate facet fracture (AO type B3.3) were created in synthetic composite distal radii and fixed with a fragment-specific plate (FSP) using a radial styloid or lunate facet plate or with two- or three-headless screws (2HS, 3HS), creating 6 fixation models: B1.1/FSP, B1.1/2HS, B1.1/3HS, B3.3/FSP, B3.3/2HS, and B3.3/3HS. Compressive stiffness of fixation constructs under initial static load, cyclic load, and final static load was investigated. Nonaxial loadings, including shearing and rotation, were not evaluated.

RESULTS

Regarding AO type B1.1, the mean stiffness of the B1.1/FSP construct was not significantly different from the intact radius, and the mean stiffness was greatest in the B1.1/3HS and lowest in the B1.1/2HS construct. For AO type B3.3, the mean stiffness of the B3.3/3HS construct was not significantly different from the intact radius, and the mean stiffness of the B3.3/FSP and B3.3/2HS construct was greatest and lowest, respectively. Minimal differences in stiffness between initial and final static loads confirmed that there was no evidence of failure implant under cyclic compressive loads.

CONCLUSIONS

Fragment-specific plates and two- or three-headless screw fixation maintained mechanical stability through compressive cyclic loading for radial styloid and volar lunate facet fractures. The FSPs and three-headless screws fixations provided superior stiffness over the two-headless screws fixation. There was no articular fracture failure in all fixation constructs with initial static compression, cyclic loading, and final compression.

CLINICAL RELEVANCE

Fragment-specific plates and headless screws can both be considered as adequate fixation for radial styloid and volar lunate facet fractures.

Intramedullary Nail Fixation for Metacarpal Fracture: A Case Report and Review of the Clinical and Biomechanical Evidence

We present two cases of complex metacarpal fractures treated with an intramedullary locking nail. This is an emerging fixation method that minimizes tissue insult, provides sufficient stability, and allows early mobilization. Locking nails accommodate the capture of fractured fragments in complex unstable patterns and provide longitudinal and rotational stability. The described intrafocal technique is intended to improve coaxial placement of the wire into the medullary canal.

Scapholunate and lunotriquetral joint dynamic stabilizers and their role in wrist neuromuscular control and proprioception

BACKGROUND

Recent research interest has grown in exploring the role of muscles, isometric contraction, proprioception, and neuromuscular control in addressing dynamic scapholunate and lunotriquetral joint instability, marking a shift in the understanding of wrist stability.

PURPOSE

To present a comprehensive review of the carpal ligaments anatomy and wrist biomechanics, with a particular focus on the role of proprioception in dynamic carpal stability and their role in managing scapholunate (SL) and lunotriquetral (LTq) dynamic instabilities.

STUDY DESIGN

We conducted a systematic search of the literature and review of the most relevant papers published and indexed in pubmed, related to wrist biomechanics, proprioception and its contribution to carpal dynamic stability.

METHODS

The study involved a comprehensive review of neuromuscular mechanisms in dynamic stabilization of the carpus, based on cadaver studies. The 3D position of the scaphoid, triquetrum, and capitate was monitored before and after tendon loading.

RESULTS

The extensor carpi ulnaris (ECU) and the flexor carpi radialis (FCR) are identified as the primary pronators of the midcarpal joint. The ECU's pronation effect can potentially strain the scapholunate ligament, while the supinator muscles, the abductor pollicis longus (APL), the extensor carpi radialis longus (ECRL), and the flexor carpi ulnaris (FCU), have a protective role, particularly in cases of scapholunate ligament dysfunctions. The FCR, despite being a pronator of the distal row, has a beneficial effect as it provokes supination of the scaphoid.

CONCLUSIONS

Comprehending carpal dysfunctions and instabilities hinges on understanding carpal anatomy and normal biomechanics. Proprioception, encompassing joint position sensation and neuromuscular control, is pivotal for stability. Biomechanical research informs tailored muscle strengthening for specific carpal issues. Supinator muscles should be strengthened for SL injuries, and ECU-focused strengthening and proprioceptive training are key for dynamic LTq instabilities. Ongoing research should delve into the intricate relationship between carpal ligaments, muscles, and proprioception to enhance wrist stability.

The silicone metacarpophalangeal joint arthroplasty: An in-vitro analysis

BACKGROUND

Silicone is still the gold standard implant in metacarpophalangeal arthroplasty. Whereas the clinical results are acceptable, in follow-ups with >10 years, high rates of implant fracture are common, and 5 to 7% of implants required revision. This work's purpose is to analyse the hypothesis that the joint flexion amplitude has a relevant effect on bone strain level, implant stress and bone-implant micromotion, which can reflect an increase in the risk of bone resorption/fatigue failure, implant fracture and osteolysis.

METHODS

To experimentally predict the cortical loading behaviour, composite metacarpals and proximal phalanges were used in intact and implanted states. A finite element model was developed to evaluate the structural behaviour of cancellous bone and implant. This model was validated by comparing cortical strain and load-displacement curve with experimental measurements.

FINDINGS

Bone strain changes between the intact and the implanted states showed a load transfer effect from the cortical to the cancellous bone that increases significantly with the flexion's amplitude rise. The peak implant stress occurred in the flexion amplitudes further away from the implant neutral angle. The highest implant pistoning motion and the highest phalanx cancellous-bone strain occurred simultaneously at the maximum flexion amplitude.

INTERPRETATION

Limiting joint flexion range will be helpful to reduce the strain-shielding effect on cortical bone, minimizing the overload effect on cancellous bone and decreasing the stress levels and the pistoning motion on the implant, ultimately contributing to the longevity of silicone arthroplasty.

Development and Usability Testing of a Finger Grip Enhancer for the Elderly

As people age, their finger function deteriorates due to muscle, nerve, and brain degeneration. While exercises might delay this deterioration, an invention that enhances elderly people’s pinching abilities is essential. This study aims to design and develop a finger grip enhancer that facilitates the day-to-day pinching activities of elderly people. This research is an extension of a previous study that conceptualised a finger grip enhancer. The device facilitates finger flexion on the thumb and index finger, and weighs 520 g, allowing for improved portability and sufficient force exertion (13.9 N) for day-to-day pinching. To test for usability, eleven subjects aged 65 years and above performed a pinch-lift-hold test on various household objects. The pinch force before and after utilising the device was measured. Using Minitab 18, the statistical significance of using this device was analysed with a paired-samples t-test. With this device, the elderly people’s pinching abilities significantly improved in both pinch force and pinch force steadiness (p < 0.05). The proposed device has the potential to enhance elderly people’s quality of life by supporting a firm pinch in the handling of everyday objects. This research has applicational value in developing exoskeleton devices for patients who require rehabilitation.

The neural mechanisms of manual dexterity

The hand endows us with unparalleled precision and versatility in our interactions with objects, from mundane activities such as grasping to extraordinary ones such as virtuoso pianism. The complex anatomy of the human hand combined with expansive and specialized neuronal control circuits allows a wide range of precise manual behaviours. To support these behaviours, an exquisite sensory apparatus, spanning the modalities of touch and proprioception, conveys detailed and timely information about our interactions with objects and about the objects themselves. The study of manual dexterity provides a unique lens into the sensorimotor mechanisms that endow the nervous system with the ability to flexibly generate complex behaviour.

The Nature of Finger Enslaving: New Results and Their Implications

We present a review on the phenomenon of unintentional finger action seen when other fingers of the hand act intentionally. This phenomenon (enslaving) has been viewed as a consequence of both peripheral (e.g., connective tissue links and multifinger muscles) and neural (e.g., projections of corticospinal pathways) factors. Recent studies have shown relatively large and fast drifts in enslaving toward higher magnitudes, which are not perceived by subjects. These and other results emphasize the defining role of neural factors in enslaving. We analyze enslaving within the framework of the theory of motor control with spatial referent coordinates. This analysis suggests that unintentional finger force changes result from drifts of referent coordinates, possibly reflecting the spread of cortical excitation.

Simulating finger-tip force using two common contact models: Hunt-Crossley and elastic foundation

Musculoskeletal models of the hand rarely include fingerpad contact mechanics, thereby limiting our ability to simulate and examine hand-object interactions. The objective of this study was to evaluate whether two common contact models (Hunt-Crossley and Elastic Foundation) can accurately represent the fingerpad. Two musculoskeletal models of the index finger were created by adding fingerpad contact geometry using either the Hunt-Crossley or Elastic Foundation contact models. Key contact parameters (target force, contact area, and stiffness) were then systematically varied through 432 forward dynamic simulations to examine how these parameters influenced estimation of finger-tip forces. Across all simulations, variation in target force, contact area, and stiffness parameters impacted the computation time required to complete the simulations and the accuracy of the predicted finger-tip force. Computation time was over three times longer in simulations with high versus low values of contact area and stiffness in both contact models. For both contact models, larger contact area and stiffness values resulted in simulations that more closely predicted target force. However, across all simulations, the Hunt-Crossley model produced a greater proportion of accurate finger-tip force simulations than the Elastic Foundation model, suggesting that the Hunt-Crossley contact model may be preferable for modeling the fingerpad. Overall, our study demonstrates how the Hunt-Crossley and Elastic Foundation contact models behave in low-force biomechanical scenarios, such as those experienced during hand-object manipulation, and provides a foundation for incorporating contact mechanics into musculoskeletal models of the hand.

Perturbation-induced fast drifts in finger enslaving

We explored changes in finger forces and in an index of unintentional finger force production (enslaving) under a variety of visual feedback conditions and positional finger perturbations. In particular, we tested a hypothesis that enslaving would show a consistent increase with time at characteristic times of about 1-2 s. Young healthy subjects performed accurate force production tasks under visual feedback on the total force of the instructed fingers (index and ring) or enslaved fingers (middle and little). Finger feedback was covertly alternated between master and enslaved fingers in a random fashion. The feedback could be presented over the first 5 s of the trial only or over the whole trial duration (21 s). After 5 s, the fingers were lifted by 1 cm, and after 15 s, the fingers were lowered to the initial position. The force of the instructed fingers drifted toward lower magnitudes in all conditions except the one with continuous feedback on that force. The force of enslaved fingers showed variable behavior across conditions. In all conditions, the index of enslaving showed a consistent increase with the time constant varying between 1 and 3 s. We interpret the results as pointing at the spread of excitation to enslaved fingers (possibly, in the cortical M1 areas). The relatively fast changes in enslaving under positional finger perturbations suggest that quick changes of the input into M1 from pre-M1 areas can accelerate the hypothesized spread of cortical excitation.

Cricket related hand injury is associated with increased odds of hand pain and osteoarthritis

Radiographic osteoarthritis (OA) is most prevalent in the hand. The association of hand injury with pain or OA is unclear. The objective was to describe the relationship between hand injury and ipsilateral pain and OA in cricketers. Data from former and current cricketers aged ≥ 30 years was used. Data included history of cricket-related hand/finger injury leading to > 4 weeks of reduced exercise, hand/finger joint pain on most days of the last month, self-reported history of physician-diagnosed hand/finger OA. Logistic regression assessed the relationship between injury with hand pain (in former cricketers) and with OA (in all cricketers), adjusted for age, seasons played, playing standard. Of 1893 participants (844 former cricketers), 16.9% reported hand pain, 4.3% reported OA. A history of hand injury increased the odds of hand pain (OR (95% CI) 2.2, 1.4 to 3.6). A history of hand injury also had increased odds of hand OA (3.1, 2.1 to 4.7). Cricket-related hand injury was related to an increased odds of hand pain and OA. This highlights the importance of hand injury prevention strategies within cricket. The high prevalence of hand pain is concerning, and further research is needed to determine the impacts of hand pain.

Modelling force-length-activation relationships of wrist and finger extensor muscles

The wrist and finger extensors play a crucial role in the muscle coordination during grasping tasks. Nevertheless, few data are available regarding their force-generating capacities. The objective of this study was to provide a model of the force-length-activation relationships of the hand extensors using non-invasive methods. The extensor carpi radialis (ECR) and the extensor digitorum communis (EDC) were studied as representative of wrist and finger extensors. Ten participants performed isometric extension force-varying contractions in different postures on an ergometer recording resultant moment. The joint angle, the myotendinous junction displacement and activation were synchronously tracked using motion capture, ultrasound and electromyography. Muscle force was estimated via a musculoskeletal model using the measured joint angle and moment. The force-length-activation relationship was then obtained by fitting a force-length model at different activation levels to the measured data. The obtained relationships agreed with previously reported data regarding muscle architecture, sarcomere length and activation-dependent shift of optimal length. Muscle forces estimated from kinematics and electromyography using the force-length-activation relationships were comparable, below 15% differences, to those estimated from moment via the musculoskeletal model. The obtained quantitative data provides a new insight into the different muscle mechanics of finger and wrist extensors. Graphical abstract By combining in vivo data (kinematics, dynamometry, electromyography, ultrasonography) during isometric force-varying contractions with musculoskeletal modelling, the force-length-activation relationships of both finger and wrist extensors were obtained. The results provided a new insight into the role of hand extensors in the generation and control of hand movements.

Modulation of finger muscle activation patterns across postures is coordinated across all muscle groups

Successful grasp requires that grip forces be properly directed between the fingertips and the held object. Changes in digit posture significantly affect the mapping between muscle force and fingertip force. Joint torques must subsequently be altered to maintain the desired force direction at the fingertips. Our current understanding of the roles of hand muscles in force production remains incomplete, as past studies focused on a limited set of postures or force directions. To thoroughly examine how hand muscles adapt to changing external (force direction) and internal (posture) conditions, activation patterns of six index finger muscles were examined with intramuscular electrodes in 10 healthy subjects. Participants produced submaximal isometric forces in each of six orthogonal directions at nine different finger postures. Across force directions, participants significantly altered activation patterns to accommodate postural changes in the interphalangeal joint angles but not changes in the metacarpophalangeal joint angles. Modulation of activation levels of the extrinsic hand muscles, particularly the extensors, were as great as those of intrinsic muscles, suggesting that both extrinsic and intrinsic muscles were involved in creating the desired forces. Despite considerable between-subject variation in the absolute activation patterns, principal component analysis revealed that participants used similar strategies to accommodate the postural changes. The changes in muscle coordination also helped increase joint impedance in order to stabilize the end-point force direction. This effect counteracts the increased signal-dependent motor noise that arises with greater magnitude of muscle activation as interphalangeal joint flexion is increased. These results highlight the role of the extrinsic muscles in controlling fingertip force direction across finger postures.NEW & NOTEWORTHY We examined how hand muscles adapt to changing external (force direction) and internal (posture) conditions. Muscle activations, particularly of the extrinsic extensors, were significantly affected by postural changes of the interphalangeal, but not metacarpophalangeal, joints. Joint impedance was modulated so that the effects of the signal-dependent motor noise on the force output were reduced. Comparisons with theoretical solutions showed that the chosen activation patterns occupied a small portion of the possible solution space, minimizing the maximum activation of any one muscle.

Mapping Together Kinetic and Kinematic Abilities of the Hand

Millions of people have reduced hand function; this loss of function can be due to injury, disease, or aging. Loss of hand function is identified as reduced motion abilities in the fingers or a decrease in the ability of the fingers to generate force. Unfortunately, there are limited data available regarding each finger's ability to produce force and how those force characteristics vary with changes in finger posture. To relate motion and force abilities of the fingers, first, an approach to measure and map them together is needed. The goal of this work was to develop and demonstrate a method to quantify the force abilities of the fingers and map these forces to the kinematic space associated with each finger. Using motion capture and multiaxis load cells, finger forces were quantified at different positions over their ranges of motion. These two sets of data were then converted to the same coordinate space and mapped together. Further, the data were normalized for the index finger and mapped as a population space model. The ability to quantify motion and force data for each finger and map them together will provide an improved understanding of the effects of treatments and rehabilitation, identifying functional loss due to injury or disease, and device design.

Effects of task dynamics on coordination of the hand muscles and their adaptation to targeted muscle assistance

Dynamic characteristics of a manual task can affect the control of hand muscles due to the difference in biomechanical/physiological characteristics of the muscles and sensory afferents in the hand. We aimed to examine the effects of task dynamics on the coordination of hand muscles, and on the motor adaptation to external assistance. Twenty-four healthy subjects performed one of the two types of a finger extension task, isometric dorsal fingertip force production (static) or isokinetic finger extension (dynamic). Subjects performed the tasks voluntarily without assistance, or with a biomimetic exotendon providing targeted assistance to their extrinsic muscles. In unassisted conditions, significant between-task differences were found in the coordination of the extrinsic and intrinsic hand muscles, while the extrinsic muscle activities were similar between the tasks. Under assistance, while the muscle coordination remained relatively unaffected during the dynamic task, significant changes in the coordination between the extrinsic and intrinsic muscles were observed during the static task. Intermuscular coherence values generally decreased during the static task under assistance, but increased during the dynamic task (all p-values < 0.01). Additionally, a significant change in the task dynamics was induced by assistance only during static task. Our study showed that task type significantly affect coordination between the extrinsic and intrinsic hand muscles. During the static task, a lack of sensory information from musculotendons and joint receptors (more sensitive to changes in length/force) is postulated to have resulted in a neural decoupling between muscles and a consequent isolated modulation of the intrinsic muscle activity.

Biomechanical Comparison of Titanium Locking Fragment-Specific and Volar Locking Plates for AO B1 and B2 Fractures of the Distal Radius

PURPOSE

This biomechanical study compared the stability of volar locking plates (VLPs) and locking fragment-specific (LFS) dorsal and radial styloid plates for the fixation of dorsal (AO 23-B2) and radial styloid (AO 23-B1) shear fractures of the distal radius, respectively.

METHODS

Two groups of 6 composite radii were fixed with a VLP or an LFS dorsal plate over a simulated dorsal shear fracture. Two additional groups of 6 radii received the same VLP or an LFS radial plate to fix a radial styloid fracture. Each plated radius was tested under cyclic axial compression by a servohydraulic testing machine that recorded axial displacement per cycle. Construct stiffness was calculated from the slope of the force-displacement curve.

RESULTS

In the dorsal shear fracture model, the dorsal LFS plate exhibited less displacement than the VLP (0.32 ± 0.04 vs 0.43 ± 0.07 mm, respectively) and showed greater average stiffness (645 ± 64 vs 433 ± 88 N/mm, respectively). Plate type was responsible for 53.1% of the variation in displacement and 68.6% of the variation in stiffness. In the radial styloid fracture model, variations due to number of cycles elapsed and plate type were similar for displacement and stiffness in both groups. The average stiffness during cyclical nondestructive testing was 566 ± 45 and 573 ± 60 N/mm for VLP and LFS radial plating groups, respectively.

CONCLUSIONS

For AO 23-B2 (dorsal rim) fractures, the dorsal LFS plates exhibited significantly less displacement and greater stiffness in axial loading than VLPs. For AO 23-B1 (radial styloid) fractures, the VLP displayed similar displacement and stiffness to the radial LFS plates.

CLINICAL RELEVANCE

All constructs tested could be expected to withstand axial compressive forces typical of early postoperative rehabilitation.

Comparing biomechanical strength of unicortical locking plate versus bicortical compression plate for transverse midshaft metacarpal fracture

PURPOSE

Metacarpal bone fractures constitute 10% of all fractures. Unstable metacarpal fractures require surgical intervention, which poses danger to flexor tendon either due to bicortical drilling or construct of the implant. Unicortical locking plate fixation may be the solution to preventing flexor tendon injury. Studies have compared locking and compression plates. However, in these studies, the biomechanical properties were tested using the static loading method. This study looks into cyclical loading that is more representative of in vivo conditions, particularly for early rehabilitation. We compared the biomechanical strength of the unicortical locking plate and bicortical compression plate system in a transverse metacarpal fracture, tested with cyclical loading and torsion.

METHOD

Twenty pieces of fourth-generation, biomechanical testing grade, left third metacarpal composite bones were used. Resin was used to create the holding block at both ends of the bone. An oscillating saw with 0.8 mm thick saw blade was used to osteotomize the metacarpal sawbones to create a midshaft transverse metacarpal fracture model. Ten pieces were fixed with a 2.0 mm titanium locking plate via unicortical screw purchase and 10 were fixed with a 2.0 mm, four holes, titanium dynamic compression plate, bicortical purchase of screws. They were subjected to cyclic load to failure testing three-point bending and torsion.

RESULTS

There were no significant difference in stiffness and cyclic three-point bending to failure between the unicortical locking plate group and the bicortical compression plate group. The bicortical compression plate group is stiffer and has a higher cyclic bending load to failure as compared to the unicortical locking plate group.

CONCLUSION

Unicortical locking plate fixation of metacarpal fracture can be reliably applied clinically to produce a strong and stable construct that allows early mobilization of the joints. This will not only reduce the complication rate of metacarpal plating, but also improve the functional outcome of the hand.

Wrist kinetics after scaphoidectomy

BACKGROUND

The scaphoid cannot be excised without generating substantial carpal dysfunction. The extent and nature of such a destabilizing procedure, however, has never been properly studied in the laboratory.

METHODS

We used a six-degrees-of-freedom motion tracking device to quantify the changes in carpal alignment produced by isometric simultaneous loading of five wrist motor tendons in 12 fresh normal cadaver arms, before and after excising the entire scaphoid.

FINDINGS

In the intact wrist, tendon loading consistently extended and supinated the capitate while flexing the triquetrum. After scaphoidectomy, the opposite rotations were always found: the capitate collapsed into flexion and pronation, whereas the triquetrum migrated proximally, while extending and radial deviating. All these changes were statistically significant.

INTERPRETATION

Unless it is supplemented by some sort of midcarpal stabilization, scaphoidectomy alone is much too aggressive as a procedure to be considered a treatment option for wrist osteoarthritis.

LEVEL OF EVIDENCE

Laboratory study. Not applicable.

The paraspinal muscle-tendon system: Its paradoxical anatomy

Anatomy of the muscle-tendon system is an important component to musculoskeletal models. In particular, the cross-sectional area of belly (mCSA) and tendon (tCSA) provides information about the maximum force that a muscle may exert. The ratio of mCSA to tCSA (rCSA) demonstrates how muscle force is related to the ability to resist/transmit the force to bone. Previous anatomical studies of the lumbar paraspinal muscles (LPM) showed that their bellies have large mCSA suggesting that they are powerful muscles. Surprisingly, surgical experience shows that the tendons of the LPM are among the thinnest tendons of the body. We therefore hypothesized that traditional biomechanics of the LPM and the rCSA do not correspond for LPM. In 10 fresh-frozen old cadavers, we measured the mCSA, tCSA and rCSA of the LPM (multifidus and the erector spinae, i.e. the longissimus and the iliocostalis); then, we compared these data with those of one of the weakest muscles in the body, i.e. the extensor digitorum communis (EDC) chosen because it shares some common anatomical features with the LPM, in particular with the erector spinae. For instance, the EDC has a polyarticular course and presents long and thin effector tendons. Among the LPM, the longissimus has the greatest mean ACSA with 10.42 cm² compared with 9.16 cm² for the iliocostalis and 0.24 cm² for the multifidus. Mean ACSA of the EDC was almost ten times smaller than those of erector spinae. Regarding the mean tCSA, the EDC was the largest one with 11.48 mm² compared with 2.69 mm² and 1.43 mm² for the longissimus, 5.74 mm² and 2.38 mm²for the iliocostalis and 5.28 mm² and 4.96 mm² for the multifidus. Mean rCSAs of the erector spinae were extremely small, ranged from 1/156 for the spinal attachment of the iliocostalis to 1/739 for the rib attachment of the longissimus that suggests that tendons are an unsuitable size to transmit the force to bone. Mean rCSA of the multifidus and the EDC were in the same range with rCSA = 1/5 and rCSA = 1/9 respectively. The rCSA of the multifidus was substantial, but its ACSA (1cm²) corresponds to low-power muscles. This paradoxical anatomy compels us to consider the biomechanics of the LPM in a different way from that of the classical “chord-like model”, i.e. the muscle belly creates a force that is applied to a bone piece through a tendon. The LPM have large contractile mass in a semi-rigid compartment inside which the pressure may increase. This result strengthens the hypothesis that high pressure and intrinsic stiffness of the LPM create two stiff bodies, closely attached to the spine thus ensuring its stabilization.

Biomechanical analysis of metacarpophalangeal joint arthroplasty with metal-polyethylene implant: An in-vitro study

BACKGROUND

The most common implant options for the metacarpophalangeal joint arthroplasty include silicone, pyrocarbon and metal-polyethylene. A systematic review of outcomes of silicone and pyrocarbon implants was conducted; however, a similar exercise for metal-polyethylene implants revealed a scarcity of published results and lack of long-term follow-up studies. The aim of the present work is to test the hypothesis that the magnitude of metacarpophalangeal joint cyclic loads generates stress and strain behaviour, which leads to long-term reduced risk of metal-polyethylene component loosening.

METHODS

This study was performed using synthetic metacarpals and proximal phalanges to experimentally predict the cortex strain behaviour for both intact and implanted states. Finite element models were developed to assess the structural behaviour of cancellous-bone and metal-polyethylene components; these models were validated by comparing cortex strains predictions against the measurements.

FINDINGS

Cortex strains in the implanted metacarpophalangeal joint presented a significant reduction in relation to the intact joint; the exception was the dorsal side of the phalanx, which presents a significant strain increase. Cancellous-bone at proximal dorsal region of phalanx reveals a three to fourfold strain increase as compared to the intact condition. Interpretation The use of metal-polyethylene implant changes the strain behaviour of the metacarpophalangeal joint yielding the risk of cancellous-bone fatigue failure due to overload in proximal phalanx; this risk is more important than the risk of bone-resorption due to the strain-shielding effect. By limiting the loads magnitude over the joint after arthroplasty, it may contribute to the prevention of implant loosening.

The impact of hand proportions on tool grip abilities in humans, great apes and fossil hominins: A biomechanical analysis using musculoskeletal simulation

Differences in grip techniques used across primates are usually attributed to variation in thumb-finger proportions and muscular anatomy of the hand. However, this cause-effect relationship is not fully understood because little is known about the biomechanical functioning and mechanical loads (e.g., muscle or joint forces) of the non-human primate hand compared to that of humans during object manipulation. This study aims to understand the importance of hand proportions on the use of different grip strategies used by humans, extant great apes (bonobos, gorillas and orangutans) and, potentially, fossil hominins (Homo naledi and Australopithecus sediba) using a musculoskeletal model of the hand. Results show that certain grips are more challenging for some species, particularly orangutans, than others, such that they require stronger muscle forces for a given range of motion. Assuming a human-like range of motion at each hand joint, simulation results show that H. naledi and A. sediba had the biomechanical potential to use the grip techniques considered important for stone tool-related behaviors in humans. These musculoskeletal simulation results shed light on the functional consequences of the different hand proportions among extant and extinct hominids and the different manipulative abilities found in humans and great apes.

Biomechanical Study of the Development of Long Bones: Finite Element Structure Synthesis of the Human Second Proximal Phalanx Under Growth Conditions

Torsional loads are a possible mechanical explanation for the architecture of long bone. Finite element structure synthesis (FESS) has previously successfully been used as a deductive technique using Wolff's Law by applying expected loads to an unspecific homogeneous solid and eliminating stress free parts to verify muscle forces. The extended approach presented in this article includes further mechanobiological rules to model the development from a cartilage model to a finger bone. In contrast to former computational models, simulation of processes leading to both external growth and internal differentiation are included. Combined axial and torsional loads synthesize a complete human secondary proximal phalanx model comparable to form and internal structure to that observed in vivo. While the computational model is very sensitive to initial alterations of loads, changes after growth have a minor effect as observed in animal models. Predictions of cartilage growth and ossification during FESS showed significant similarities to ontogeny indicating the importance of mechanical factors for the morphogenesis of bone during growth. Anat Rec, 302:1389-1398, 2019. © 2018 Wiley Periodicals, Inc.

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

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.

Trabecular bone patterning across the human hand

Hand bone morphology is regularly used to link particular hominin species with behaviors relevant to cognitive/technological progress. Debates about the functional significance of differing hominin hand bone morphologies tend to rely on establishing phylogenetic relationships and/or inferring behavior from epigenetic variation arising from mechanical loading and adaptive bone modeling. Most research focuses on variation in cortical bone structure, but additional information about hand function may be provided through the analysis of internal trabecular structure. While primate hand bone trabecular structure is known to vary in ways that are consistent with expected joint loading differences during manipulation and locomotion, no study exists that has documented this variation across the numerous bones of the hand. We quantify the trabecular structure in 22 bones of the human hand (early/extant modern Homo sapiens) and compare structural variation between two groups associated with post-agricultural/industrial (post-Neolithic) and foraging/hunter-gatherer (forager) subsistence strategies. We (1) establish trabecular bone volume fraction (BV/TV), modulus (E), degree of anisotropy (DA), mean trabecular thickness (Tb.Th) and spacing (Tb.Sp); (2) visualize the average distribution of site-specific BV/TV for each bone; and (3) examine if the variation in trabecular structure is consistent with expected joint loading differences among the regions of the hand and between the groups. Results indicate similar distributions of trabecular bone in both groups, with those of the forager sample presenting higher BV/TV, E, and lower DA, suggesting greater and more variable loading during manipulation. We find indications of higher loading along the ulnar side of the forager sample hand, with high site-specific BV/TV distributions among the carpals that are suggestive of high loading while the wrist moves through the 'dart-thrower's' motion. These results support the use of trabecular structure to infer behavior and have direct implications for refining our understanding of human hand evolution and fossil hominin hand use.

Multi-finger synergies and the muscular apparatus of the hand

We explored whether the synergic control of the hand during multi-finger force production tasks depends on the hand muscles involved. Healthy subjects performed accurate force production tasks and targeted force pulses while pressing against loops positioned at the level of fingertips, middle phalanges, and proximal phalanges. This varied the involvement of the extrinsic and intrinsic finger flexors. The framework of the uncontrolled manifold (UCM) hypothesis was used to analyze the structure of inter-trial variance, motor equivalence, and anticipatory synergy adjustments prior to the force pulse in the spaces of finger forces and finger modes (hypothetical finger-specific control signals). Subjects showed larger maximal force magnitudes at the proximal site of force production. There were synergies stabilizing total force during steady-state phases across all three sites of force production; no differences were seen across the sites in indices of structure of variance, motor equivalence, or anticipatory synergy adjustments. Indices of variance, which did not affect the task (within the UCM), correlated with motor equivalent motion between the steady states prior to and after the force pulse; in contrast, variance affecting task performance did not correlate with non-motor equivalent motion. The observations are discussed within the framework of hierarchical control with referent coordinates for salient effectors at each level. The findings suggest that multi-finger synergies are defined at the level of abundant transformation between the low-dimensional hand level and higher dimensional finger level while being relatively immune to transformations between the finger level and muscle level. The results also support the scheme of control with two classes of neural variables that define referent coordinates and gains in back-coupling loops between hierarchical control levels.

Biomechanical evaluation of pyrocarbon proximal interphalangeal joint arthroplasty: An in-vitro analysis

BACKGROUND

Pyrocarbon proximal interphalangeal joint arthroplasty provided patients with excellent pain relief and joint motion, however, overall implant complications have been very variable, with some good outcomes at short-medium-term follow-up and some bad outcomes at longer-term follow-up. Implant loosening with migration, dislocation and implant fracture were the main reported clinical complications. The aim of the present work was to test the hypothesis that the magnitude proximal interphalangeal joint cyclic loads in daily hand functions generates stress-strain behaviour which may be associated with a risk of pyrocarbon component loosening in the long-term.

METHODS

This study was performed using synthetic proximal and middle phalanges to experimentally predict the cortex strain behaviour and implant stability considering different load conditions for both intact and implanted states. Finite element models were developed to assess the structural behaviour of cancellous-bone and pyrocarbon components, these models were validated against experimentally measured cortex strains.

FINDINGS

Cortex strains showed a significant increase at dorsal side and reduction at palmar side between intact and implanted states. Cancellous-bone adjacent to the condylar implant base components suffers a two to threefold strain increase, comparing with the intact condition.

INTERPRETATION

The use of pyrocarbon implant changes the biomechanical behaviour of the joint phalanges and is associated with a potential risk of support cancellous-bone suffer fatigue failure in mid to long term due to the strain increase for cyclic loads in the range of daily hand activities, this risk is more prominent than the risk of bone resorption due to strain-shielding effect.

El papel de la propiocepción y el control neuromuscular en las inestabilidades del carpo

La congruencia articular, la integridad ligamentaria y la compresión de las superficies articulares ocasionada por la contracción muscular se han considerado históricamente los tres pilares básicos para la estabilidad del carpo. En los últimos años se ha propuesto un nuevo factor para explicar los mecanismos de estabilización carpiana, la propiocepción y el control neuromuscular. La propiocepción en la muñeca se origina en órganos sensoriales localizados en los ligamentos y cápsulas articulares (los mecanorreceptores). La estimulación de los mismos inicia un reflejo involuntario que provoca una respuesta muscular selectiva dirigida a proteger la zona de la muñeca donde se ha originado la señal aferente. En este trabajo de revisión se pretende dar a conocer el concepto de propiocepción y control neuromuscular, el papel que tienen en la estabilidad del carpo y las posibles aplicaciones en la práctica clínica.

The influence of wrist posture, grip type, and grip force on median nerve shape and cross-sectional area

During grasping, the median nerve undergoes mechanical stress in the carpal tunnel which may contribute to carpal tunnel syndrome. This study investigated the effects of wrist posture, grip type, and grip force on the shape and cross-sectional area of the median nerve. Ultrasound examination was used to obtain cross-sectional images of the dominant wrist of 16 healthy subjects (8 male) at the proximal carpal tunnel during grasping. The cross-sectional area, circularity, and axis lengths of the median nerve were assessed in 27 different conditions (3 postures × 3 grip types × 3 force levels). There were no significant changes in median nerve cross-sectional area (P > 0.05). There were significant interactions across posture, grip type, and grip force affecting nerve circularity and axis lengths. When the wrist was flexed, increasing grip force caused the median nerve to shorten in the mediolateral direction and lengthen in the anteroposterior direction (P < 0.04), becoming more circular. These effects were significant during four finger pinch grip and chuck grip (P < 0.05) but not key grip (P > 0.07). With the wrist extended, the nerve became more flattened (less circular) as grip force increased during four finger pinch grip and chuck grip (P < 0.04) but not key grip (P > 0.3). Circularity was lower during the four finger pinch compared to chuck or key grip (P < 0.03). The findings suggest that grip type and wrist posture significantly alter the shape of the median nerve. Clin. Anat. 30:470-478, 2017. © 2017 Wiley Periodicals, Inc.

The Biomechanical Impact of Digital Loss and Fusion Following Trauma: Setting the Patient up for Success

Surgeons managing mutilating hand injures are faced with difficult decisions between attempting to salvage remaining or injured digits or proceeding to amputation and fusion. Through application of a basic understanding of hand biomechanics, the surgeon may more accurately predict what motion and function can best be salvaged. This article provides an explanation of how amputation, fusion, and tendon loss can affect postoperative hand motion. The surgeon can use these concepts in planning the reconstruction or preparing the foundation for secondary reconstructive procedures to achieve the highest functional outcome for the patient.

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.

Carpal Kinematics and Kinetics

The complex interaction of the carpal bones, their intrinsic and extrinsic ligaments, and the forces in the normal wrist continue to be studied. Factors that influence kinematics, such as carpal bone morphology and clinical laxity, continue to be identified. As imaging technology improves, so does our ability to better understand and identify these factors. In this review, we describe advances in our understanding of carpal kinematics and kinetics. We use scapholunate ligament tears as an example of the disconnect that exists between our knowledge of carpal instability and limitations in current reconstruction techniques.

Using Force Sensitive Resistors to Evaluate Hand Tool Grip Design

The pressure distribution of the hand is an important element in evaluating hand tool grip. For measuring the pressure distributions of the hand, a portable sensor glove was developed using force sensitive resistors of the conductive polymer type. Pressure distributions of three hand tools for three male subjects were measured. The pressure distribution pattern varied significantly between type of grip, location, and subjective exertion level. Factor analysis was used to identify the relative importance of each area of the hand during grip action. The results showed that there is an ‘active' area involved in gripping the hand tool and a ‘control’ area which supports the hand motion for the given task. Studies about hand tool pressure distributions will provide an important guideline in designing a new tool or improving an existing one. A microcomputer based system is being developed that records data on grip pressure distribution throughout a tool grip task. The digitized data then will be analyzed both graphically and statistically and will be used to evaluate various hand tools for industrial settings.

Locking Plates for Corrective Osteotomy of Malunited Dorsally Tilted Distal Radial Fractures: A Biomechanical Study

The purpose of the study was to compare the biomechanical properties of five different palmar fixation plate designs in a distal radius osteotomy cadaver model. A 1 cm metaphyseal osteotomy gap was made to simulate a corrective osteotomy and the osteotomy plated. Axial load was applied to the distal end of each construct by a material testing machine under control of a motion analysis video system. The specimens were arranged into five implant groups of eight specimens each. No test group developed deformity and movement of the fracture gap greater than 2 mm with a load of 100 N. Increasing the load to 250 N revealed statistically significant differences in stiffness and failure load between the different plates. Axial failure strength and stiffness were greater for the radial correction plates than for the other implants. The former may provide enough stability for corrective osteotomy of dorsally angulated distal radial malunions, even when the osteotomy gap is only filled with cancellous bone graft instead of cortical bone graft.

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.

Computational models and motor learning paradigms: Could they provide insights for neuroplasticity after stroke? An overview

Computational approaches for modelling the central nervous system (CNS) aim to develop theories on processes occurring in the brain that allow the transformation of all information needed for the execution of motor acts. Computational models have been proposed in several fields, to interpret not only the CNS functioning, but also its efferent behaviour. Computational model theories can provide insights into neuromuscular and brain function allowing us to reach a deeper understanding of neuroplasticity. Neuroplasticity is the process occurring in the CNS that is able to permanently change both structure and function due to interaction with the external environment. To understand such a complex process several paradigms related to motor learning and computational modeling have been put forward. These paradigms have been explained through several internal model concepts, and supported by neurophysiological and neuroimaging studies. Therefore, it has been possible to make theories about the basis of different learning paradigms according to known computational models. Here we review the computational models and motor learning paradigms used to describe the CNS and neuromuscular functions, as well as their role in the recovery process. These theories have the potential to provide a way to rigorously explain all the potential of CNS learning, providing a basis for future clinical studies.

Load distribution on the radio-carpal joint for carpal arthrodesis

BACKGROUND AND OBJECTIVE

Carpal fusions are useful for treating specific carpal disorders, maximizing postoperative wrist motion, hand strength, reducing pain and instability of the joint. The surgeon selects the appropriate treatment by considering the degree of stability, the chronicity of the injury, functional demands of the patient and former patient's outcomes as well. However there are not many studies regarding the load distribution provided by the treatment. So, the purpose of this study is to analyze the load distribution through the wrist joint with an arthrodesis treatment and compare the results with a normal wrist.

METHOD

To this end the rigid body spring model (RBSM) method was used on a three-dimensional model of the wrist joint. The cartilage and ligaments were simulated as springs acting under compression and tension, respectively, while the bones were considered as rigid bodies. To simulate the arthrodesis, the fused bones were considered as a single rigid body.

RESULTS

The changes on the load distribution for each arthrodesis agree with the treatment objective, reducing load transmission through a specific articular surface. For example, for SLAC/SNAC II most of the treatments reduced the load transmitted through the radioscaphoid fossae, almost by 8%. However, the capitolunate (CL) arthrodesis was the treatment that managed to keep the load transmitted through the radiolunate joint closer to normal conditions. Also, in treatments where the scaphoid was excised (3-corner, 4-corner and capitolunate arthrodesis), the joint surface between the lunate surface compensates by doubling the transmitted force to the radius.

CONCLUSIONS

The common arthrodesis for treating SLAC/SNAC II-III, reduces, in fact, the load on the radioscaphoid joint. Alternative treatments that reduce load distribution on the radiocarpal joint should be three corner and capitolunate arthrodesis for treating SLAC/SNAC-II; and for SLAC/SNAC-III four corners with scaphoid excision. On Kienbock's disease. Scaphocapitate (SC) arthrodesis is more effective on reducing the load transmission through the radiolunate and ulnolunate joints. All arthrodesis treatment should consider changes on the load transmission, and also bones' fusion rates and pain reduction on patient's outcomes.

Angular stable lateral plating is a valid alternative to conventional plate fixation in the proximal phalanx. A biomechanical study

BACKGROUND

Dorsal plating is commonly used in proximal phalanx fractures but it bears the risk of interfering with the extensor apparatus. In this study, dorsal and lateral plating fixation methods are compared to assess biomechanical differences using conventional 1.5mm non-locking plates and novel 1.3mm lateral locking plates.

METHODS

Twenty-four fresh frozen human cadaveric proximal phalanges were equally divided into four groups. An osteotomy was set at the proximal metaphyseal-diaphyseal junction and fixed with either dorsal (group A) or lateral (group B) plating using a 1.5mm non-locking plate, or lateral plating with a novel 1.3mm locking plate with bicortical (group C) or unicortical (group D) screws. The specimens were loaded in axial, dorsovolar and mediolateral direction to assess fixation stiffness followed by a cyclic destructive test in dorsovolar loading direction.

FINDINGS

Axial stiffness was highest in group D (mean 321.02, SEM 21.47N/mm) with a significant difference between groups D and B (P=0.033). Locking plates (groups C and D) were stiffer than non-locking plates under mediolateral loading (P=0.007), no significant differences were noted under dorsovolar loading. Furthermore, no significant differences were observed under cyclic loading to failure between any of the study groups.

INTERPRETATION

No considerable biomechanical advantage of using a conventional 1.5mm dorsal non-locking plate was identified over the novel 1.3mm lateral locking plate in the treatment of proximal phalanx fractures. Since the novel low-profile plate is less disruptive to the extensor mechanism, it should be considered as a valid alternative.

Ergonomic evaluation of biomechanical hand function

The human hand is a complex structure that performs various functions for activities of daily living and occupations. This paper presents a literature review on the methodologies used to evaluate hand functions from a biomechanics standpoint, including anthropometry, kinematics, kinetics, and electromyography (EMG). Anthropometry describes the dimensions and measurements of the hand. Kinematics includes hand movements and the range of motion of finger joints. Kinetics includes hand models for tendon and joint force analysis. EMG is used on hand muscles associated with hand functions and with signal-processing technology.

Biomechanical analysis of force distribution in human finger extensor mechanisms

The complexities of the function and structure of human fingers have long been recognised. The in vivo forces in the human finger tendon network during different activities are critical information for clinical diagnosis, surgical treatment, prosthetic finger design, and biomimetic hand development. In this study, we propose a novel method for in vivo force estimation for the finger tendon network by combining a three-dimensional motion analysis technique and a novel biomechanical tendon network model. The extensor mechanism of a human index finger is represented by an interconnected tendinous network moving around the phalanx's dorsum. A novel analytical approach based on the "Principle of Minimum Total Potential Energy" is used to calculate the forces and deformations throughout the tendon network of the extensor mechanism when subjected to an external load and with the finger posture defined by measurement data. The predicted deformations and forces in the tendon network are in broad agreement with the results obtained by previous experimental in vitro studies. The proposed methodology provides a promising tool for investigating the biomechanical function of complex interconnected tendon networks in vivo.

Biomechanical analysis of the human finger extensor mechanism during isometric pressing

This study investigated the effects of the finger extensor mechanism on the bone-to-bone contact forces at the interphalangeal and metacarpal joints and also on the forces in the intrinsic and extrinsic muscles during finger pressing. This was done with finger postures ranging from very flexed to fully extended. The role of the finger extensor mechanism was investigated by using two alternative finger models, one which omitted the extensor mechanism and another which included it. A six-camera three-dimensional motion analysis system was used to capture the finger posture during maximum voluntary isometric pressing. The fingertip loads were recorded simultaneously using a force plate system. Two three-dimensional biomechanical finger models, a minimal model without extensor mechanism and a full model with extensor mechanism (tendon network), were used to calculate the joint bone-to-bone contact forces and the extrinsic and intrinsic muscle forces. If the full model is assumed to be realistic, then the results suggest some useful biomechanical advantages provided by the tendon network of the extensor mechanism. It was found that the forces in the intrinsic muscles (interosseus group and lumbrical) are significantly reduced by 22% to 61% due to the action of the extensor mechanism, with the greatest reductions in more flexed postures. The bone-to-bone contact force at the MCP joint is reduced by 10% to 41%. This suggests that the extensor mechanism may help to reduce the risk of injury at the finger joints and also to moderate the forces in intrinsic muscles. These apparent biomechanical advantages may be a result of the extensor mechanism's distinctive interconnected fibrous structure, through which the contraction of the intrinsic muscles as flexors of the MCP joint can generate extensions at the DIP and PIP joints.

Quantification of finger joint loadings using musculoskeletal modelling clarifies mechanical risk factors of hand osteoarthritis

Owing to limited quantitative data related to the loadings (forces and pressures) acting upon finger joints, several clinical observations regarding mechanical risk factors of hand osteoarthritis remain misunderstood. To improve the knowledge of this pathology, the present study used musculoskeletal modelling to quantify the forces and pressures acting upon hand joints during two grasping tasks. Kinematic and grip force data were recorded during both a pinch and a power grip tasks. Three-dimensional magnetic resonance imaging measurements were conducted to quantify joint contact areas. Using these datasets as input, a musculoskeletal model of the hand and wrist, including twenty-three degrees of freedom and forty-two muscles, has been developed to estimate joint forces and joint pressures. When compared with the power grip task, the pinch grip task resulted in two to eight times higher joint loadings whereas the grip forces exerted on each finger were twice lower. For both tasks, joint forces and pressures increased along a disto-proximal direction for each finger. The quantitative dataset provided by the present hand model clarified two clinical observations about osteoarthritis development which were not fully understood, i.e., the strong risk associated to pinch grip tasks and the high frequency of thumb-base osteoarthritis.

The biomechanics of fixation techniques for hand fractures

Multiple options are available for the fixation of metacarpal and phalangeal fractures; treatment options depend largely on the fracture characteristics, concomitant injuries, and surgeon preference. With an understanding of the options available and the biomechanics involved, appropriate treatment options may be chosen. Although plate and screw constructs provide the strongest fixation for these fractures, many other techniques provide adequate fixation and may be favored for reasons other than biomechanics.

Robot Modeling for Physical Rehabilitation

The science of rehabilitation shows that repeated movements of human limbs can help the patient regain function in the injured limb. There are three types of mechanical systems used for movement rehabilitation: robots, cable-based manipulators, and exoskeletons. Industrial robots can be used because they provide a three-dimensional workspace with a wide range of flexibility to execute different trajectories, which are useful for motion rehabilitation. The cable-based manipulators consist of a movable platform and a base, which are connected by multiple cables that can extend or retract. The exoskeleton is fixed around the patient's limb to provide the physiotherapy movements. This chapter presents a summary of the principal human limb movements, a review of several mechanical systems used for rehabilitation, as well as common mathematical models of such systems.

Impairment in task-specific modulation of muscle coordination correlates with the severity of hand impairment following stroke

Significant functional impairment of the hand is commonly observed in stroke survivors. Our previous studies suggested that the inability to modulate muscle coordination patterns according to task requirements may be substantial after stroke, but these limitations have not been examined directly. In this study, we aimed to characterize post-stroke impairment in the ability to modulate muscle coordination patterns across tasks and its correlation with hand impairment. Fourteen stroke survivors, divided into a group with severe hand impairment (8 subjects) and a group with moderate hand impairment (6 subjects) according to their clinical functionality score, participated in the experiment. Another four neurologically intact subjects participated in the experiment to serve as a point of comparison. Activation patterns of nine hand and wrist muscles were recorded using surface electromyography while the subjects performed six isometric tasks. Patterns of covariation in muscle activations across tasks, i.e., muscle modules, were extracted from the muscle activation data. Our results showed that the degree of reduction in the inter-task separation of the multi-muscle activation patterns was indicative of the clinical functionality score of the subjects (mean value = 26.2 for severely impaired subjects, 38.1 for moderately impaired subjects). The values for moderately impaired subjects were much closer to those of the impaired subjects (mean value = 46.1). The number of muscle modules extracted from the muscle activation patterns of a subject across six tasks, which represents the degree of motor complexity, was found to be correlated with the clinical functionality score (R = 0.68). Greater impairment was also associated with a change in the muscle module patterns themselves, with greater muscle coactivation. A substantial reduction in the degrees-of-freedom of the multi-muscle coordination post-stroke was apparent, and the extent of the reduction, assessed by the stated metrics, was strongly associated with the level of clinical impairment.

The mechanical stability of extra-articular distal radius fractures with respect to the number of screws securing the distal fragment

PURPOSE

The treatment of distal radius fractures with volar locked plating (VLP) has gained popularity. Many different designs and sizes of plates afford a wide variety of configurations of locking screws that can be placed into the distal fracture fragment. The purpose of this study was to determine whether using half of the distal locking screws decreased stability when compared with using all possible distal locking screws with 4 different VLP systems.

METHODS

Twenty-four identical synthetic distal radius sawbone models were instrumented with 1 of 4 designs of VLP devices over a standardized dorsal wedge osteotomy to simulate a dorsally comminuted, extra-articular distal radius fracture. Distal locking screws were placed in varying configurations. Six radii per plate model with different screw configurations then underwent axial loading, volar bending, and dorsal bending using a servohydraulic machine. Distal fragment displacement was recorded using a differential variable reluctance transducer.

RESULTS

There was no significant difference in fracture fragment displacement when using half of the distal locking screw set compared with using the full screw set. Mean differences in displacement between half and full screws were less than 0.1 mm. All configurations had the greatest magnitude of displacement during axial loading. Mean displacement was less in plates containing 2 rows of distal locking screws (-0.4 mm) compared with plates containing 1 row (-0.6 mm).

CONCLUSIONS

Using half of the distal locking screws in VLP in an extra-articular, nonosteoporotic distal radial fracture model with noncyclical, nondestructive loading does not decrease construct stability compared with using all of the screws. Not filling all holes in VLP is more cost effective and does not sacrifice plate stiffness or construct stability. Plates with 2 rows of distal locking screws create more stable fixation than plates with 1 row of distal locking screws.

Radiographic evaluation of chronic static scapholunate dissociation post soft tissue reconstruction

The purpose of this study was to examine the radiographic outcomes of dorsal intercarpal ligament capsulodesis (DILC), documenting the time to carpal collapse postoperatively. From January 2008 to January 2011, 12 patients were identified with chronic scapholunate (SL) dissociation. The average follow-up period was 15.8 months. Paired t-tests were used preoperatively, one month after pin removal, and at final follow-up to determine significance in radiographic outcomes. The Disabilities of the Arm, Shoulder, and Hand (DASH) survey was administered to patients before and after surgery to assess subjective levels of pain, function, and satisfaction. Intraoperatively all deformities were reduced completely. One month after pin removal, the mean SL gap was 3.3 mm, the SL angle was 74°, the radiolunate (RL) angle was 17°, and the lunatocapitate (LC) angle was 8°. Only the SL angle improved; the other measurements remained unchanged. At final follow-up, the mean SL gap was 3.6 mm, the SL angle was 78°, the RL angle was 20°, and the LC angle was 10°. SL angle worsened, but with no statistically significant difference. The other radiographic measurements remained unchanged at final follow up. Wrist flexion and extension decreased from 76% and 69% of the contralateral side to 62% and 56% of the contralateral side after surgery. Grip strength was 64% of the contralateral side before surgery and 83% after surgery. Visual Analog Scale (VAS) results improved from 6.3 to 1.7, and DASH scores improved from 39 to 8 after the surgery. DILC cannot withstand large and repetitive forces. Carpal collapse recurred within a short time after DILC. However, our small patient numbers and short term follow-up preclude any conclusions with respect to clinical efficacy of this procedure. Limitations of this study include the fact that this is a retrospective study with no control group. In addition, it represents a single-surgeon series, which introduces a source of bias and carries the risk of technical and methodological flaws, which may have contributed to the observed radiographic outcomes.

Design optimisation and experimental evaluation of dorsal double plating fixation for distal radius fracture

This study determines the relative effects of changes in osteoporosis condition, plate/screw design factors (plate angle/length/width/thickness and screw diameter) and fixation methods (screw number and screw length) on the biomechanical response of dorsal double plating (DDP) fixation at a distal radius fracture to determine the optimal design and evaluate its biomechanical strength using the dynamic fatigue test. Eighteen CAD and finite element (FE) models corresponding to a Taguchi L18 array were constructed to perform numerical simulations to simulate the mechanical responses of a DDP fixed in a simply distal radius fracture bone. The Taguchi method was employed to determine the significance of each design factor in controlling bone/plate/screw stress and distal fragment displacement under axial (100 N), bending (1 N m) and torsion (1 N m) loads. Simulation results indicated that the order rank to determine the mechanical response was the plate thickness, plate width, screw diameter, and number of screws. Dorsal intermediate (L) plate with 60 mm length, 1.8 mm thickness, 6.0 mm width and 2.8 mm diameter, 20 mm length dual-thread locking screw can be found for optimisation. The DDP, including an L plate with 0°, 30° and 60° angles and a straight I plate, were made with Ti6Al4V to fix onto the sawbones with three corresponding radius fractures to perform the dynamic testing. The specimens were oscillated with loads between 10 N and 150 N at 5 Hz for 20,000 cycles. The average stiffness in 20,000 test cycles was 425.7 N/mm, 461.1 N/mm and 532.1N/mm for the 0°, 30° and 60° constructs, respectively. No difference in stiffness was found in the same angled constructs throughout the 20,000 cycles of testing (p > 0.05). Lack of gross construct failures during cyclic testing and reasonable stiffness corroborated that our new constructs tested to date seem stable enough to support restricted post-operative loads.