Flexor tendon and synovial gliding during simultaneous and single digit flexion in idiopathic carpal tunnel syndrome

The characteristic pathological finding in carpal tunnel syndrome (CTS) is non-inflammatory fibrosis of the subsynovial connective tissue (SSCT), which lies between the flexor tendons and the visceral synovium (VS). How this fibrosis might affect tendon function is unknown. To better understand the normal function of the SSCT, the relative motion of the middle finger flexor digitorum superficialis (FDS III) tendon and VS was observed during finger flexion in patients with CTS and cadavers with a history of CTS and compared to normal cadavers. A digital camcorder was used to monitor the gliding motion of the FDS III tendon and SSCT in eight patients with idiopathic CTS undergoing carpal tunnel release surgery (CTR), in eight cadavers with an antemortem history of CTS and compared these with eight cadaver controls. There were no significant differences noted in the total movement of the SSCT relative to the FDS III. However, the pattern of SSCT movement relative to the FDS III in the CTS patients and cadavers with an antemortem history of CTS differed from the controls in one of two patterns, reflecting either increased SSCT adherence to FDS III or increased SSCT dissociation from FDS III. In CTS, the gliding characteristics of the SSCT are qualitatively altered. These changes may be the result of increased fibrosis within the SSCT, which in some cases has ruptured, resulting in SSCT-tendon dissociation. Similar changes are also identified postmortem in the CTS patient.     

The mechanical properties of the rabbit carpal tunnel subsynovial connective tissue

The rabbit model is commonly used to study carpal tunnel syndrome (CTS). It has been proposed that the subsynovial connective tissue (SSCT) in the carpal tunnel may play a role in the etiology of CTS, but the material properties of the rabbit SSCT are unknown. The purpose of this study was to develop a method to measure the shear properties of the rabbit SSCT. In six rabbit cadaver forepaws, the excursion of the third digit flexor digitorum superficialis (FDS) and load to failure of the SSCT were measured in a custom device. The mean excursion to full flexion in this model was 7.08 mm (S.D. 0.77). The mean shearing force at full flexion was 317 mN (S.D. 166). At full flexion percentage of maximum shear force in the SSCT was 54.5% (S.D. 19.4). The mean energy absorbed at full flexion was 0.29 mJ (S.D. 0.31). The mean excursion needed to reach 5% of the maximum shear force was 3.04 mm (S.D. 0.99). The testing model presented in this study demonstrates structural parameters to evaluate the shear properties of the SSCT in a rabbit model. The data presented could be used for estimating sample sizes in a more comprehensive study of the effect of CTS on the SSCT properties.     

In vivo forces generated by finger flexor muscles do not depend on the rate of fingertip loading during an isometric task

Risk factors for activity-related tendon disorders of the hand include applied force, duration, and rate of loading. Understanding the relationship between external loading conditions and internal tendon forces can elucidate their role in injury and rehabilitation. The goal of this investigation is to determine whether the rate of force applied at the fingertip affects in vivo forces in the flexor digitorum profundus (FDP) tendon and the flexor digitorum superficialis (FDS) tendon during an isometric task. Tendon forces, recorded with buckle force transducers, and fingertip forces were simultaneously measured during open carpal tunnel surgery as subjects (N=15) increased their fingertip force from 0 to 15N in 1, 3, and 10s. The rates of 1.5, 5, and 15N/s did not significantly affect FDP or FDS tendon to fingertip force ratios. For the same applied fingertip force, the FDP tendon generated more force than the FDS. The mean FDP to fingertip ratio was 2.4+/-0.7 while the FDS to tip ratio averaged 1.5+/-1.0 (p<0.01). The fine motor control needed to generate isometric force ramps at these specific loading rates probably required similar high activation levels of multiple finger muscles in order to stabilize the finger and control joint torques at the force rates studied. Therefore, for this task, no additional increase in muscle force was observed at higher rates. These findings suggest that for high precision, isometric pinch maneuvers under static finger conditions, tendon forces are independent of loading rate.     

Tendon displacements during voluntary and involuntary finger movements

In the human hand, independent movement control of individual fingers is limited. One potential cause for this is mechanical connections between the tendons and muscle bellies corresponding to the different fingers. The aim of this study was to determine the tendon displacement of the flexor digitorum superficialis (FDS) of both the instructed and the neighboring, non-instructed fingers during single finger flexion movements. In nine healthy subjects (age 22–29 years), instructed and non-instructed FDS finger tendon displacement of the index, middle and ring finger was measured using 2D ultrasound analyzed with speckle tracking software in two conditions: active flexion of all finger joints with all fingers free to move and active flexion while the non-instructed fingers were restricted. Our results of the free movement protocol showed an average tendon displacement of 27 mm for index finger flexion, 21 mm for middle finger flexion and 17 mm for ring finger flexion. Displacements of the non-instructed finger tendons (≈12 mm) were higher than expected based of the amount of non-instructed finger movement. In the restricted protocol, we found that, despite minimal joint movements, substantial non-instructed finger tendon displacement (≈9 mm) was still observed, which was interpreted as a result of tendon strain. When this strain component was subtracted from the tendon displacement of the non-instructed fingers during the free movement condition, the relationship between finger movement and tendon displacement of the instructed and non-instructed finger became comparable. Thus, when studying non-instructed finger tendon displacement it is important to take tendon strain into consideration.     

Comparison of two ways of altering carpal tunnel pressure with ultrasound surface wave elastography

Higher carpal tunnel pressure is related to the development of carpal tunnel syndrome. Currently, the measurement of carpal tunnel pressure is invasive and therefore, a noninvasive technique is needed. We previously demonstrated that speed of wave propagation through a tendon in the carpal tunnel measured by ultrasound elastography could be used as an indicator of carpal tunnel pressure in a cadaveric model, in which a balloon had to be inserted into the carpal tunnel to adjust the carpal tunnel pressure. However, the method for adjusting the carpal tunnel pressure in the cadaveric model is not applicable for the in vivo model. The objective of this study was to utilize a different technique to adjust carpal tunnel pressure via pressing the palm and to validate it with ultrasound surface wave elastography in a human cadaveric model. The outcome was also compared with a previous balloon insertion technique. Results showed that wave speed of intra-carpal tunnel tendon and the ratio of wave speed of intra-and outer-carpal tunnel tendons increased linearly with carpal tunnel pressure. Moreover, wave speed of intra carpal tunnel tendon via both ways of altering carpal tunnel pressure showed similar results with high correlation. Therefore, it was concluded that the technique of pressing the palm can be used to adjust carpal tunnel pressure, and pressure changes can be detected via ultrasound surface wave elastography in an ex vivo model. Future studies will utilize this technique in vivo to validate the usefulness of ultrasound surface wave elastography for measuring carpal tunnel pressure.     

Predicting tenocyte expression profiles and average molecular concentrations in Achilles tendon ECM from tissue strain and fiber damage

In this study, we propose a method for quantitative prediction of changes in concentrations of a number of key signaling, structural and effector molecules within the extracellular matrix of tendon. To achieve this, we introduce the notion of elementary cell responses (ECRs). An ECR defines a normal reference secretion profile of a molecule by a tenocyte in response to the tenocyte’s local strain. ECRs are then coupled with a model for mechanical damage of tendon collagen fibers at different straining conditions of tendon and then scaled up to the tendon tissue level for comparison with experimental observations. Specifically, our model predicts relative changes in ECM concentrations of transforming growth factor beta, interleukin 1 beta, collagen type I, glycosaminoglycan, matrix metalloproteinase 1 and a disintegrin and metalloproteinase with thrombospondin motifs 5, with respect to tendon straining conditions that are consistent with the observations in the literature. In good agreement with a number of in vivo and in vitro observations, the model provides a logical and parsimonious explanation for how excessive mechanical loading of tendon can lead to under-stimulation of tenocytes and a degenerative tissue profile, which may well have bearing on a better understanding of tendon homeostasis and the origin of some tendinopathies.     

Stability of reconstructed paralyzed shoulders using a reflected long head biceps technique

A new tendon transfer technique is proposed for the reconstruction of the paralyzed shoulders secondary to Brachial Plexus Injury (BPI). In this tendon transfer, the long head of the biceps tendons is utilized as a bridging tendon graft. It is reflected at the exit of the bicipital groove, passed through the deltoid and directed to the trapezius. The technique is referred to here as the Reflected Long Head Bicepts (RLHB) technique. This study evaluated the effect of this tendon transfer on the anterior, posterior, and inferior stability of the reconstructed should using cadaveric specimens. It was shown that loading of the RLHB contributed significantly to anterior stability of the reconstructed shoulder for 90 deg elevation in the scapula plane. The mean displacement was reduced by 56 percent with RLHB loaded (p<0.01), by 56 percent with the rotator cuff loaded (p <0.005), and by 67 percent with both the RLHB and the rotator cuff loaded (p<0.004). For the post-operation conditions, variation of the directions of RLHB had no significant effect on joint displacement in response to anterior loading. The RLHB tendon also contributed to the posterior and inferior stability for the low and middle elevations in the plane of scapula. Two variations of the RLHB tendon transfer procedures, namely the "Sub-Deltoid" and the "Through-Deltoid" techniques, were introduced and studied. These two techniques did not seem to have significantly different effects on the displacement of the humeral head in response to both posterior and inferior loading. The results of this study seemed to support the clinical feasibility of this tendon transfer approach as far as the biomedical stability of the reconstruction is concerned.     

Histochemistry defines a proteoglycan-rich layer in bovine flexor tendon subjected to bending

Mid-substance fibrocartilage develops in bovine deep flexor tendon at the point where the tendon wraps under sesamoid bones of the foot and receives transverse compressive loading during locomotion. Fibrocartilage extends several millimeters into the tendon at this location and the proteoglycan-rich tissue stains intensely with Alcian blue. Using histochemical techniques we demonstrate the presence of aggrecan, type VI collagen, and hyaluronic acid in the extracellular matrix of this region of tendon. Biglycan staining was localized to the cells, however. Adjacent to the fibrocartilage, at the outer curvature of the tendon as it bends, the tissue resembles typical tensile tendon with dense bundles of linearly arranged collagen. Longitudinal sections revealed discrete layers of Alcian blue-stained material between the collagen bundles. We demonstrate that these layers of loose matrix also contain aggrecan, type VI collagen, and hyaluronic acid. However, the dense collagen bundles of this region are negative for these components. Transverse sections of tendon in the area adjacent to fibrocartilage show a distinct Alcian blue-stained structure surrounding vascular elements at the point where several fiber bundles come together. This is concluded to be the same structure as the Alcian blue-stained layers seen in longitudinal sections. These observations suggest that proteoglycan-rich matrices in tendon subjected to mechanical loading other than pure tension may serve multiple roles. Such matrices can not only provide compressive stiffness and separate and lubricate collagen bundles that move relative to each other, but may also protect the integrity of vasculature in tendon subjected to bending and shear.     

Middle phalanx skeletal morphology in the hand: Can it predict flexor tendon size and attachments?

Specific sites on the palmar diaphysis of the manual middle phalanges provide attachment for the flexor digitorum superficialis (FDS) tendon. It has been assumed in the literature that lateral palmar fossae on these bones reflect locations for these attachments and offer evidence for relative size of the flexor tendon. This assumption has led to predictions about relative FDS muscle force potential from sizes of fossae on fossil hominin middle phalanges. Inferences about locomotor capabilities of fossil hominins in turn have been drawn from the predicted force potential of the flexor muscle. The study reported here provides a critical first step in evaluating hypotheses about behavioral implications of middle phalangeal morphology in fossil hominins, by testing the hypothesis that the lateral fossae reflect the size of the FDS tendon and the location of the terminal FDS tendon attachments on the middle phalanx. The middle phalangeal region was dissected in 43 individuals from 16 primate genera, including humans. Qualitative observations were made of tendon attachment locations relative to the lateral fossae. Length measurements of the fossae were tested as predictors of FDS tendon cross-sectional area and of FDS attachment tendon lengths. Our results lead to the conclusion that the hypothesis must be rejected, and that future attention should focus on functional implications of the palmar median bar associated with the lateral fossae.     

An MRI-based method to align the compressive loading axis for human cadaveric knees

There is a need to align the mechanical axis of the tibia with the axis of loading for studies involving tibiofemoral compression to interpret results and to ensure repeatability of loading within and among specimens. Therefore, the objectives of this study were (1) to develop a magnetic resonance imaging (MRI)-based alignment method for use with apparatuses applying tibiofemoral joint compression, (2) to demonstrate the usefulness of the method by aligning cadaveric knees in an apparatus that could apply tibiofemoral joint compression, and (3) to quantify the error associated with the alignment method. A four degree-of-freedom adjustable device was constructed to allow determination and alignment of the mechanical axis of the tibia of cadaveric knee joints with the axis of loading of an apparatus applying tibiofemoral joint compression. MRI was used to determine the locations of bony landmarks in three dimensions defining the mechanical axis of the tibia relative to an initial orientation of the four degree-of-freedom device. Adjustment values of the device were then computed and applied to the device to align the mechanical axis of the tibia with the axis of a compressive loading apparatus. To demonstrate the usefulness of the method, four cadaveric knees were aligned in the compressive loading apparatus. The vectors describing the mechanical axis of the tibia and the loading axis of the apparatus before and after adjustment of the four degree-of-freedom device were computed for each cadaveric knee. After adjustment of the four degree-of-freedom device, the mechanical axis of the tibia was collinear with the loading axis of the apparatus for each cadaveric knee. The errors in the adjustment values introduced by inaccuracies in the MR images were quantified using the Monte Carlo technique. The precisions in the translational and rotational adjustments were 1.20 mm and 0.90 deg respectively. The MR-based alignment method will allow consistent interpretation of results obtained during tibiofemoral compressive studies conducted using the apparatus described in this paper by providing a well-defined loading axis. The alignment method can also be adapted for use with other apparatuses applying tibiofemoral compression.     

Finite element analysis to characterize how varying patellar loading influences pressure applied to cartilage: model evaluation

A finite element analysis (FEA) modeling technique has been developed to characterize how varying the orientation of the patellar tendon influences the patellofemoral pressure distribution. To evaluate the accuracy of the technique, models were created from MRI images to represent five knees that were previously tested in vitro to determine the influence of hamstrings loading on patellofemoral contact pressures. Hamstrings loading increased the lateral and posterior orientation of the patellar tendon. Each model was loaded at 40°, 60°, and 80° of flexion with quadriceps force vectors representing the experimental loading conditions. The orientation of the patellar tendon was represented for the loaded and unloaded hamstrings conditions based on experimental measures of tibiofemoral alignment. Similar to the experimental data, simulated loading of the hamstrings within the FEA models shifted the center of pressure laterally and increased the maximum lateral pressure. Significant (p < 0.05) differences were identified for the center of pressure and maximum lateral pressure from paired t-tests carried out at the individual flexion angles. The ability to replicate experimental trends indicates that the FEA models can be used for future studies focused on determining how variations in the orientation of the patellar tendon related to anatomical or loading variations or surgical procedures influence the patellofemoral pressure distribution.     

Reliability of ultrasound speckle tracking with singular value decomposition for quantifying displacement in the carpal tunnel

Inhibited movement patterns of carpal tunnel structures have been found in carpal tunnel syndrome (CTS) patients. Motion analysis on ultrasound images allows us to non-invasively study the (relative) movement of carpal tunnel structures and recently a speckle tracking method using singular value decomposition (SVD) has been proposed to optimize this tracking. This study aims to assess the reliability of longitudinal speckle tracking with SVD in both healthy volunteers and patients with CTS.Images from sixteen healthy volunteers and twenty-two CTS patients were used. Ultrasound clips of the third superficial flexor tendon and surrounding subsynovial connective tissue (SSCT) were acquired during finger flexion-extension. A custom made tracking algorithm was used for the analysis. Intra-class correlation coefficients (ICCs) were calculated using a single measure, two-way random model with absolute agreement and Bland-Altman plots were added for graphical representation.ICC values varied between 0.73 and 0.95 in the control group and 0.66–0.98 in the CTS patients, with the majority of the results classified as good to excellent. Tendon tracking showed higher reliability values compared to the SSCT, but values between the control and CTS groups were comparable.Speckle tracking with SVD can reliably be used to analyze longitudinal movement of anatomical structures with different sizes and compositions within the context of the carpal tunnel in both a healthy as well as a pathological state. Based on these results, this technique also holds relevant potential for areas where ultrasound based dynamic imaging requires quantification of motion.     

Finite element analysis of the cranium: Validity,sensitivity and future directions

Finite element analysis (FEA) is increasingly applied in skeletal biomechanical research in general, and in fossil studies in particular. Underlying such studies is the principle that FEA provides results that approximate reality. This paper provides further understanding of the reliability of FEA by presenting a validation study in which the deformations experienced by a real cadaveric human cranium are compared to those of an FE model of that cranium under equivalent simulated loading. Furthermore, model sensitivity to simplifications in segmentation and material properties is also assessed. Our results show that absolute deformations are not accurately predicted, but the distribution of the regions of relatively high and low strains, and so the modes of global deformation, are reasonably approximated.     

Tendon excursion and gliding: clinical impacts from humble concepts

As integral components of the musculoskeletal system, the primary function is transmission of muscle forces to the skeletal system. Proper excursion and gliding of the tendon determine the efficiency of this function. Studies of the tendon excursion and gliding based on two simple mechanical concepts have resulted in several significant clinical implications.     

Rate-independent characteristics of an arthroscopically implantable force probe in the human achilles tendon

The effect of loading rate on specimen calibration was investigated for an implantable force sensor of the two-point loading variety. This variety of sensor incorporates a strain gage to measure the compressive load applied to the sensor due to tensile loading in a soft tissue specimen. The Achilles tendon in each of four human cadaveric lower extremities was instrumented with a force sensor and then loaded in tension using a materials testing machine. Each specimen was tensile tested at three different displacement rates, 0.25, 2.5 and 12.7 cm s(-1), corresponding with mean loading rates of 33.8, 513.2, and 2838.6 N s(-1), respectively. A calibration curve relating the force sensor signal and applied tendon tension was generated for each specimen/ displacement rate combination. For each specimen, calibration curves were compared by calculating an RMS error for the entire data set (eRMS = 1.6% of the full load value) and a coefficient of determination, R2, of a curve fit through all of the data (R2 = 99.6%). Over the range of rates tested, no measurable change in sensor sensitivity due to loading rate was observed. Hysteresis for all displacement rates was on the order of 2.4%.     

Wrist and digital joint motion produce unique flexor tendon force and excursion in the canine forelimb

The force and excursion within the canine digital flexor tendons were measured during passive joint manipulations that simulate those used during rehabilitation after flexor tendon repair and during active muscle contraction, simulating the active rehabilitation protocol. Tendon force was measured using a small buckle placed upon the tendon while excursion was measured using a suture marker and video analysis method. Passive finger motion imposed with the wrist flexed resulted in dramatically lower tendon force (approximately 5 N) compared to passive motion imposed with the wrist extended (approximately 17 N). Lower excursions were seen at the level of the proximal interphalangeal joint with the wrist flexed (approximately 1.5 mm) while high excursion was observed when the wrist was extended or when synergistic finger and wrist motion were imposed (approximately 3.5 mm). Bivariate discriminant analysis of both force and excursion data revealed a natural clustering of the data into three general mechanical paradigms. With the wrist extended and with either one finger or four fingers manipulated, tendons experienced high loads of approximately 1500 g and high excursions of approximately 3.5 mm. In contrast, the same manipulations performed with the wrist flexed resulted in low tendon forces (4-8 N) and low tendon excursions of approximately 1.5 mm. Synergistic wrist and finger manipulation provided the third paradigm where tendon force was relatively low (approximately 4 N) but excursion was as high as those seen in the groups which were manipulated with the wrist extended. Active muscle contraction produced a modest tendon excursion (approximately 1 mm) and high or low tendon force with the wrist extended or flexed, respectively. These data provide the basis for experimentally testable hypotheses with regard to the factors that most significantly affect functional recovery after digital flexor tendon injury and define the normal mechanical operating characteristics of these tendons.     

Modeling of time-dependent force response of fingertip to dynamic loading

An extended exposure to repeated loading on fingertip has been associated to many vascular, sensorineural, and musculoskeletal disorders in the fingers, such as carpal tunnel syndrome, hand-arm vibration syndrome, and flexor tenosynovitis. A better understanding of the pathomechanics of these sensorineural and vascular diseases in fingers requires a formulation of a biomechanical model of the fingertips and analyses to predict the mechanical responses of the soft tissues to dynamic loading. In the present study, a model based on finite element techniques has been developed to simulate the mechanical responses of the fingertips to dynamic loading. The proposed model is two-dimensional and incorporates the essential anatomical structures of a finger: skin, subcutaneous tissue, bone, and nail. The skin tissue is assumed to be hyperelastic and viscoelastic. The subcutaneous tissue was considered to be a nonlinear, biphasic material composed of a hyperelastic solid and an invicid fluid, while its hydraulic permeability was considered to be deformation dependent. Two series of numerical tests were performed using the proposed finger tip model to: (a) simulate the responses of the fingertip to repeated loading, where the contact plate was assumed to be fixed, and the bone within the fingertip was subjected to a prescribed sinusoidal displacement in vertical direction; (b) simulate the force response of the fingertip in a single keystroke, where the keyboard was composed of a hard plastic keycap, a rigid support block, and a nonlinear spring. The time-dependent behavior of the fingertip under dynamic loading was derived. The model predictions of the time-histories of force response of the fingertip and the phenomenon of fingertip separation from the contacting plate during cyclic loading agree well with the reported experimental observations.     

Development and validation of ultrasound speckle tracking to quantify tendon displacement

Ultrasound can be used to study tendon movement. However, measurement of tendon movement is mostly based on manual tracking of anatomical landmarks such as the musculo-tendinous junction, limiting the applicability to a small number of muscle-tendon units. The aim of this study was to quantify tendon displacement without anatomical landmarks using a speckle tracking algorithm optimized for tendons in long B-mode image sequences. A dedicated two-dimensional multi-kernel block-matching scheme with subpixel motion estimation was devised to handle large displacements over long sequences. The accuracy of the tracking on porcine tendons was evaluated during different displacements and velocities. Subsequently, the accuracy of tracking the flexor digitorum superficialis (FDS) of a human cadaver hand was evaluated. Finally, the in-vivo accuracy of the tendon tracking was determined by measuring the movement of the FDS at the wrist level. For the porcine experiment and the human cadaver arm experiment tracking errors were, on average, 0.08 and 0.05 mm, respectively (1.3% and 1.0%). For the in-vivo experiment the tracking error was, on average, 0.3 mm (1.6%). This study demonstrated that our dedicated speckle tracking can quantify tendon displacement at different physiological velocities without anatomical landmarks with high accuracy. The technique allows tracking over large displacements and in a wider range of tendons than by using anatomical landmarks.     

Hyaluronic acid diminishes the resistance to excursion after flexor tendon repair: an in vitro biomechanical study

Adhesion between the tendon and tendon sheath after primary flexor tendon repair is seen frequently, and postoperative finger function is occasionally unsatisfactory. A reduction of the friction may facilitate tendon mobilization, which in turn may reduce the risk of the adhesion and restriction of range of motion. We considered the possibility of utilizing the hyaluronic acid (HA) as a lubricant. To evaluate the effect of HA, the gliding resistance between the canine flexor digitorum profundus tendon repaired by a modified Kessler suture technique with running epitendinous suture and the annular pulley located on the proximal phalanx (corresponding to the A2 pulley in humans) was evaluated and compared before and after administration of HA. The HA solution measurement groups were identified as follows; intact tendon as a control; repaired tendon; tendon soaked in 0.1, 1, and 10 mg/ml HA. The resistance increased after repairing, then it decreased after soaking in 10 mg/ml HA solution. The results of this study revealed that HA diminishes the excursion resistance after flexor tendon repair. We believe that some style of administration of the HA might reduce the excursion resistance and prevent adhesion until the synovial surface is fully developed.     

Muscle moment-arms: a key element in muscle-force estimation

A clear and rigorous definition of muscle moment-arms in the context of musculoskeletal systems modelling is presented, using classical mechanics and screw theory. The definition provides an alternative to the tendon excursion method, which can lead to incorrect moment-arms if used inappropriately due to its dependency on the choice of joint coordinates. The definition of moment-arms, and the presented construction method, apply to musculoskeletal models in which the bones are modelled as rigid bodies, the joints are modelled as ideal mechanical joints and the muscles are modelled as massless, frictionless cables wrapping over the bony protrusions, approximated using geometric surfaces. In this context, the definition is independent of any coordinate choice. It is then used to solve a muscle-force estimation problem for a simple 2D conceptual model and compared with an incorrect application of the tendon excursion method. The relative errors between the two solutions vary between 0% and 100%.