Mechanical properties of the human achilles tendon

Longitudinal strain estimation in incompressible cylindrical tissues from magnetic resonance imaging

In this paper, we present a simple approach for estimating the average longitudinal strains from models reconstructed from medical images. It can be used for many incompressible generalized cylindrical tissues, such as tendons, ligaments, and fusiform muscles; the major deformation directions of these soft tissues are along the longitudinal axes. The method is especially useful when pre- and post-deformation tissue correspondences are difficult to establish directly from images for various reasons, such as insufficient image resolution, homogenous image intensity, and noise. Incompressibility, which is accepted as a good approximation for soft tissues, is exploited as a constraint on the tissue deformation. Experiments with Magnetic Resonance Imaging (MRI) of tissue phantoms and computer simulations show that the method is accurate and practical even in the presence of noise. Finally, we demonstrate the usefulness of our approach on studying extraocular muscle deformation.

Construction of collagen scaffolds that mimic the three-dimensional architecture of specific tissues

Every tissue and organ has its own 3-dimensional (3D) extracellular matrix (ECM) organization. Cells in a 3D bioscaffold for tissue engineering typically align new ECM components according to the bioscaffold provided. Therefore, scaffolds with a specific 3D structural design resembling the actual ECM of a particular tissue may have great potential in tissue engineering. Here, we show that, using specific freezing regimes, 3D scaffolds that mimic the 3D architecture of specific tissues can be made from collagen. Three examples are given, namely, scaffolds resembling the cup-shaped parenchymal (alveolar) architecture of lung, scaffolds that mimic the parallel collagen organization of tendon, and scaffolds that mimic the 3D organization of skin. For the preparation of these tissue-specific scaffolds, we relied on simple techniques without the need for expensive or customized equipment. Freezing rate, type of suspension medium, and additives (e.g., ethanol) were found to be prime parameters in controlling scaffold morphology.

Clinical outcomes of allograft versus autograft in anterior cruciate ligament reconstruction

Anterior cruciate ligament (ACL) injuries are the most common complete ligamentous injury to the knee. The optimal graft should be able to reproduce the anatomy and biomechanics of the ACL, be incorporated rapidly with strong initial fixation, and cause low graft-site morbidity. This article reviews the literature comparing the clinical outcomes following allograft and autograft ACL reconstruction and examines current issues regarding graft choice.

The effect of therapeutic ultrasound intensity on the ultrastructural morphology of tendon repair

This study evaluated the effects of ultrasound intensity on the ultrastructural morphology of Achilles tendon healing. Twenty Sprague-Dawley rats with surgically hemi-transected Achilles tendons were randomly assigned into four groups of 0, 0.5, 1.2 and 2 W/cm(2) for ultrasound treatment, with five rats in each group. The treatments were administered with 1 MHz continuous ultrasound daily starting from day 5 after injury. On day 30, ultrathin slides of the Achilles tendons were prepared and examined with transmission electron microscopy. Results showed that the mean collagen fibril size of all treatment groups was higher than the control (p < 0.05). There was no significant difference in the collagen fibril size among the treatment groups. These findings suggest that therapeutic ultrasound can enhance the maturation of collagen fibrils of repairing tendons, and this was not dependent on the intensity of ultrasound applied.

Quantitative histology and ultrastructure fail to explain weakness of immobilized rabbit Achilles' tendons

OBJECTIVE

To test the hypothesis that mechanical weakness caused by immobilization is the result of characteristic histologic and ultrastructural changes in rabbit Achilles' tendons.

DESIGN

Single-blind, randomized controlled trial.

SETTING

University animal care facility.

ANIMALS

Twenty-three New Zealand rabbits.

INTERVENTION

Twenty weight-matched rabbits underwent unilateral hind leg immobilization. The Achilles' tendons of immobilized and contralateral legs were harvested after 4 or 8 weeks. For ultrastructural outcomes, 6 normal Achilles' tendons of 3 rabbits served as controls.

MAIN OUTCOME MEASURES

Light microscopic assessments were made on the tendons for cross-sectional area of the tendon, number of tenocytes, adipocytes and blood vessels, roundness of nuclei, area of intense metachromasia, and area of spatially aligned collagen fibers. Transmission electron microscopy (TEM) measured mean collagen fibril diameter and density.

RESULTS

Light microscopic assessment failed to reveal a statistical difference in any of the outcome measures between immobilized and contralateral tendons. TEM did not show a statistical difference in mean fibril diameter between the immobilized groups (4 wk, 113.8+/-1.6 nm; 8 wk, 113.5+/-1.4 nm) compared with their respective contralateral tendons (4 wk, 111.4+/-1.4 nm; 8 wk, 116.2+/-1.8 nm) and normal controls (111.8+/-2.0 nm). Eight-week contralateral fibrils were statistically larger than 4-week contralateral fibrils, which was attributed to a training effect of the leg opposite of the casted leg.

CONCLUSIONS

Our results add to the recent literature about the absence of characteristic histologic or collagen fibril size markers that could explain the mechanical weakness of immobilized tendons. Further research using biochemical, gene expression, and functional imaging markers of tendon is needed to pinpoint the alterations responsible for the mechanical weakness. Such markers would be crucial for the rehabilitation and secondary prevention of tendon injuries.

Diagnosis and management of the painful ankle/foot part 1: clinical anatomy and pathomechanics

Distinctive anatomical features can be witnessed in the ankle/foot complex, affording specific pathological conditions. Disorders of the ankle/foot complex are multifactoral and features in both the clinical anatomy and biomechanics contribute to the development of ankle/foot pain. The superior tibiofibular, distal tibiofibular, talocrural, subtalar, and midtarsal joint systems must all participate in function of the ankle/foot complex, as each biomechanically contributes to functional movements and clinical disorders witnessed in the lower extremity. A clinician's ability to effectively evaluate, diagnose, and treat the distal lower extremity is largely reliant upon a foundational understanding of the clinical anatomy and biomechanics of this complex complex. Thus, clinicians are encouraged to consider these distinctions when examining and diagnosing disorders of the ankle/foot.

In Vitro System for Applying Cyclic Loads to Connective Tissues Under Displacement or Force Control

Overuse is thought to be the primary cause of chronic tendon injuries, in which forceful or repetitive loading results in an accumulation of micro-tears leading to a maladaptive repair response. In vitro organ culture models provide a useful method for examining how specific loading patterns affect the cellular response to load which may explain the early mechanisms of tissue injury associated with tendinopathies and ligament injuries. We designed a novel tissue loading system which employs closed-loop force feedback, capable of loading six tissue samples independently under force or displacement control. The system was capable of applying loads up to 40 N at rates of 100 N s(-1) and frequencies of 2 Hz, well above loads and rates measured in rabbit tendons in vivo. Loading parameters such as amplitude, rate, and frequency can be controlled while biomechanical factors such as creep, force relaxation, tangent modulus and Young's modulus can be assessed. The system can be used to examine the relationship between each loading parameter and biomechanical factors of connective tissues maintained in culture which may provide useful information regarding the etiology of overuse injuries.

Fiber kinematics of small intestinal submucosa under biaxial and uniaxial stretch

Improving our understanding of the design requirements of biologically derived collagenous scaffolds is necessary for their effective use in tissue reconstruction. In the present study, the collagen fiber kinematics of small intestinal submucosa (SIS) was quantified using small angle light scattering (SALS) while the specimen was subjected to prescribed uniaxial or biaxial strain paths. A modified biaxial stretching device based on Billiar and Sacks (J. Biomech., 30, pp. 753-7, 1997) was used, with a real-time analysis of the fiber kinematics made possible due to the natural translucency of SIS. Results indicated that the angular distribution of collagen fibers in specimens subjected to 10% equibiaxial strain was not significantly different from the initial unloaded condition, regardless of the loading path (p=0.31). Both 10% strip biaxial stretch and uniaxial stretches of greater than 5% in the preferred fiber direction led to an increase in the collagen fiber alignment along the same direction, while 10% strip biaxial stretch in the cross preferred fiber direction led to a broadening of the distribution. While an affine deformation model accurately predicted the experimental findings for a biaxial strain state, uniaxial stretch paths were not accurately predicted. Nonaffine structural models will be necessary to fully predict the fiber kinematics under large uniaxial strains in SIS.

Stretching for prevention of Achilles tendon injuries: a review of the literature

Professional and recreational athletes commonly perform pre-exercise stretching to prevent musculoskeletal injuries. Little definitive evidence exists that clearly demonstrates the efficacy of stretching in reducing injury. Achilles tendon injuries are among the most common injuries affecting active individuals in the United States today. Clinicians commonly recommend stretching the Achilles tendon without concrete scientific evidence to support such a claim. Few studies have addressed the effect of stretching in Achilles tendon injuries, and it is unclear if the conclusions made for musculoskeletal injuries can be applied to the Achilles tendon. Biomechanical studies of the Achilles tendon and measurements of the tendon's reflex activity have demonstrated possible mechanisms for the potential benefit of stretching, including load-induced hypertrophy and increased tendon tensile strength. Recent prospective studies have contended that reductions in plantarflexor strength and increases in ankle dorsiflexion range of motion from stretching the Achilles tendon may increase the risk of injury. Studies examining stretching in injury prevention, the biomechanical properties of injuries to the Achilles tendon were compiled and reviewed. Although many theories have been published regarding the potential benefits and limitations of stretching, few studies have been able to definitively demonstrate its utility in injury prevention.

Corticosteroids reduce the tensile strength of isolated collagen fascicles

BACKGROUND

Overuse tendon injuries are frequent. Corticosteroid injections are commonly used as treatment, although their direct effects on the material properties of the tendon are poorly understood.

PURPOSE

To examine the influence of corticosteroids on the tensile strength of isolated collagen fascicles.

STUDY DESIGN

Controlled laboratory study.

METHODS

Single strands (300-500 mum) of rat-tail collagen fascicles were incubated in either high (1 mL of 40 mgmL(-1) mixed with 0.5 mL saline 9%) or low (1 mL of 40 mgmL(-1) mixed with 2 mL saline 9%) concentration of methylprednisolone acetate (Depomedrol) for 3 or 7 days, while the control segment from the same fascicle was kept in saline (N = 64). After the incubation period, the fascicles underwent displacement to failure in a mechanical test rig at 0.13 mm/s, and thereafter hydroxylysyl pyridinoline and lysyl pyridinoline cross-link content was evaluated in a high-performance liquid chromatography system. Data for each group were analyzed with a 2-way analysis of variance (time x incubation) for ultimate stress (mean +/- standard deviation).

RESULTS

In the high-concentration groups, strength was reduced after 3 (16.6 +/- 4.6 MPa) and 7 (8.6 +/- 1.7 MPa) days compared to the controls (30.2 +/- 5.0 MPa and 25.6 +/- 4.6 MPa, respectively; P < .05). In the low-concentration groups, strength was reduced after 3 (12.0 +/- 3.1 MPa) and 7 days (10.9 +/- 2.5 MPa) compared to the controls (31.5 +/- 5.0 MPa and 32.4 +/- 5.6 MPa, respectively; P < .05). The amount of cross-linking was unaffected by the intervention.

CONCLUSION

Data show that the tensile strength of isolated fascicles is markedly reduced after 3- and 7-day incubation in both high and low concentration of corticosteroids, although the observed effect on whole tendon remains unknown.

CLINICAL RELEVANCE

Corticosteroids may weaken specific regions of the injected tendon and leave it more prone to rupture. This weakening effect is manifested in the individual collagen fascicles that constitute the tendon.

A serrated jaw clamp for tendon gripping

Mechanical testing of tendon and application of muscular tendon forces in cadaveric or animal studies requires the use of a clamp to hold the tendon rigidly at high loads without damaging it. Frozen type serrated clamp was able to achieve the objective but is less readily available and manageable because of its complex and massive configuration. In this study, a custom-made, serrated jaw clamp was fabricated. Maximum tension before visual slippage was monitored during tensile testing of a bovine tendon. The non-frozen serrated jaw clamp was able to sustain tension forces of more than 2500 N and strain of about 30% without slippage and cutting of the tendon. The jaw clamp can be easily customised and fabricated for biomechanical studies to hold tendons of different sizes. The commercially available, serrated plastic material enables the jaw clamp to be custom-made with minimal workmanship and manufacturing time.

The Pathomechanics of Plantar Fasciitis

Plantar fasciitis is a musculoskeletal disorder primarily affecting the fascial enthesis. Although poorly understood, the development of plantar fasciitis is thought to have a mechanical origin. In particular, pes planus foot types and lower-limb biomechanics that result in a lowered medial longitudinal arch are thought to create excessive tensile strain within the fascia, producing microscopic tears and chronic inflammation. However, contrary to clinical doctrine, histological evidence does not support this concept, with inflammation rarely observed in chronic plantar fasciitis. Similarly, scientific support for the role of arch mechanics in the development of plantar fasciitis is equivocal, despite an abundance of anecdotal evidence indicating a causal link between arch function and heel pain. This may, in part, reflect the difficulty in measuring arch mechanics in vivo. However, it may also indicate that tensile failure is not a predominant feature in the pathomechanics of plantar fasciitis. Alternative mechanisms including 'stress-shielding', vascular and metabolic disturbances, the formation of free radicals, hyperthermia and genetic factors have also been linked to degenerative change in connective tissues. Further research is needed to ascertain the importance of such factors in the development of plantar fasciitis.

The measurement of the variation in the surface strains of Achilles tendon grafts using imaging techniques

Uniaxial tensile tests are commonly used to characterize the structural and material properties of tendons and ligaments. During these tests, the stress and strain distributions applied to the specimen are assumed to be uniform. However, few studies have investigated the strain distributions throughout the tissue. The purpose of this study was to use imaging techniques to measure the strains around the circumference of 11 mm wide Achilles tendon grafts during a uniaxial tensile test. Pairs of radiopaque beads with a diameter of 2mm were affixed around the mid-substance of the tendon in four different locations. The motion of the beads was recorded using a cine fluoroscope. This system was shown to measure the displacement of the beads with an accuracy of 0.02 mm. During the uniaxial tensile test, large variations in local tissue strains were observed. At 10 MPa of applied stress, the local tissue strain varied from an average of 2.5-8.7%, an increase in strain of more than three times. As a result of these large variations, the modulus calculated from the stress-strain data varied from an average of 217 to 897 MPa, an increase of approximately 4 times. Furthermore, these data suggest that underestimates of the elastic modulus may result if a uniform strain distribution is assumed. These results indicate that during uniaxial tensile tests, the assumption of uniform stress and strain distributions should be carefully considered and small, uniform specimens should be used when measuring the material properties of soft tissues.

A novel microstructural approach in tendon viscoelastic modelling at the fibrillar level

Novel applications in rehabilitation, surgery and tissue engineering require the knowledge of the mechanical behaviour of the tissues at microstructural level. The aim of this work is to investigate the viscoelastic properties of the tendon from the interaction of its biological constituents in the fibrillar network. Traction, relaxation and creep in-vitro tests have been performed on porcine flexor digital tendons. A viscoelastic constitutive equation at finite deformation is presented. The fibrillar deformation modes are described through a network of adaptive links between collagen type I and decorin. The theoretical predictions fit accurately the experimental data. The results of the model demonstrate the mechanical importance of glycosaminoglycan chains of decorin for the differential recruitment and the activation of fibrillar collagen.

Muscle performance during maximal isometric and dynamic contractions is influenced by the stiffness of the tendinous structures

Contractile force is transmitted to the skeleton through tendons and aponeuroses, and, although it is appreciated that the mechanocharacteristics of these tissues play an important role for movement performance with respect to energy storage, the association between tendon mechanical properties and the contractile muscle output during high-force movement tasks remains elusive. The purpose of the study was to investigate the relation between the mechanical properties of the connective tissue and muscle performance in maximal isometric and dynamic muscle actions. Sixteen trained men participated in the study. The mechanical properties of the vastus lateralis tendon-aponeurosis complex were assessed by ultrasonography. Maximal isometric knee extensor force and rate of torque development (RTD) were determined. Dynamic performance was assessed by maximal squat jumps and countermovement jumps on a force plate. From the vertical ground reaction force, maximal jump height, jump power, and force-/velocity-related determinants of jump performance were obtained. RTD was positively related to the stiffness of the tendinous structures ( r = 0.55, P < 0.05), indicating that tendon mechanical properties may account for up to 30% of the variance in RTD. A correlation was observed between stiffness and maximal jump height in squat jumps and countermovement jumps ( r = 0.64, P < 0.05 and r = 0.55, P < 0.05). Power, force, and velocity parameters obtained during the jumps were significantly correlated to tendon stiffness. These data indicate that muscle output in high-force isometric and dynamic muscle actions is positively related to the stiffness of the tendinous structures, possibly by means of a more effective force transmission from the contractile elements to the bone.

Structural Achilles tendon properties in athletes subjected to different exercise modes and in Achilles tendon rupture patients

The prevalence of Achilles tendon (AT) injury is high in various sports, and AT rupture patients have been reported to have a 200-fold risk of sustaining a contralateral rupture. Tendon adaptation to different exercise modes is not fully understood. The present study investigated the structural properties of the AT in male elite athletes that subject their AT to different exercise modes as well as in Achilles rupture patients. Magnetic resonance imaging of the foot and leg, anthropometric measurements, and maximal isometric plantar flexion force were obtained in 6 male AT rupture patients and 25 male elite athletes (kayak/control group n = 9, volleyball n = 8 and endurance running n = 8). AT cross-sectional area (CSA) was normalized to body mass. Runners had a larger normalized AT CSA along the entire length of the tendon compared with the control group (P < 0.05). The volleyball subjects had a larger normalized CSA compared with the control group (P < 0.05) in the area of thinnest tendon CSA. No structural differences of the AT were found in the rupture subjects compared with the control group. Rupture subjects did not subject their AT to greater force or stress during a maximal voluntary isometric plantar flexion than the other groups. The CSA of the triceps surae musculature was the strongest predictor of AT CSA (r(s) = 0.569, P < 0.001). This study is the first to show larger CSA in tendons that are subjected to intermittent high loads. AT rupture patients did not display differences in structural or loading properties of the tendons.

Region-specific mechanical properties of the human patella tendon

The present study investigated the mechanical properties of tendon fascicles from the anterior and posterior human patellar tendon. Collagen fascicles from the anterior and posterior human patellar tendon in healthy young men (mean ± SD, 29.0 ± 4.6 yr, n = 6) were tested in a mechanical rig. A stereoscopic microscope equipped with a digital camera recorded elongation. The fascicles were preconditioned five cycles before the failure test based on pilot data on rat tendon fascicle. Human fascicle length increased with repeated cycles ( P < 0.05); cycle 5 differed from cycle 1 ( P < 0.05), but not cycles 2–4. Peak stress and yield stress were greater for anterior (76.0 ± 9.5 and 56.6 ± 10.4 MPa, respectively) than posterior fascicles (38.5 ± 3.9 and 31.6 ± 2.9 MPa, respectively), P < 0.05, while yield strain was similar (anterior 6.8 ± 1.0%, posterior 8.7 ± 1.4%). Tangent modulus was greater for the anterior (1,231 ± 188 MPa) than the posterior (583 ± 122 MPa) fascicles, P < 0.05. In conclusion, tendon fascicles from the anterior portion of the human patellar tendon in young men displayed considerably greater peak and yield stress and tangent modulus compared with the posterior portion of the tendon, indicating region-specific material properties.

Differential displacement of the human soleus and medial gastrocnemius aponeuroses during isometric plantar flexor contractions in vivo

The human triceps surae muscle-tendon complex is a unique structure with three separate muscle compartments that merge via their aponeuroses into the Achilles tendon. The mechanical function and properties of these structures during muscular contraction are not well understood. The purpose of the study was to investigate the extent to which differential displacement occurs between the aponeuroses of the medial gastrocnemius (MG) and soleus (Sol) muscles during plantar flexion. Eight subjects (mean ± SD; age 30 ± 7 yr, body mass 76.8 ± 5.5 kg, height 1.83 ± 0.06 m) performed maximal isometric ramp contractions with the plantar flexor muscles. The experiment was performed in two positions: position 1, in which the knee joint was maximally extended, and position 2, in which the knee joint was maximally flexed (125°). Plantarflexion moment was assessed with a strain gauge load cell, and the corresponding displacement of the MG and Sol aponeuroses was measured by ultrasonography. Differential shear displacement of the aponeurosis was quantified by subtracting displacement of Sol from that of MG. Maximal plantar flexion moment was 36% greater in position 1 than in position 2 (132 ± 20 vs. 97 ± 11 N·m). In position 1, the displacement of the MG aponeurosis at maximal force exceeded that of the Sol (12.6 ± 1.7 vs. 8.9 ± 1.5 mm), whereas in position 2 displacement of the Sol was greater than displacement of the MG (9.6 ± 1.0 vs. 7.9 ± 1.2 mm). The amount and “direction” of shear between the aponeuroses differed significantly between the two positions across the entire range of contraction, indicating that the Achilles tendon may be exposed to intratendinous shear and stress gradients during human locomotion.

Functional efficacy of tendon repair processes

Despite various attempts to repair and replace injured tendon, an understanding of the repair processes and a systematic approach to achieving functional efficacy remain elusive. In this review the epidemiology of tendon injury and repair is first examined. Using a traditional paradigm for repair assessment, the biology and biomechanics of normal tendon, natural healing, and repair are then explored. New treatment strategies such as functional tissue engineering are discussed, including a functional approach to treatment that involves the development of in vivo functional design parameters to judge the acceptability of a repair outcome. The paper concludes with future directions.

Achilles tendon rupture: surgical versus non-surgical treatment

OBJECTIVE

To ascertain the treatment method of choice for Achilles tendon rupture, which results in the most favourable functional outcome.

METHODS

A comprehensive literature search was performed to retrieve relevant English language articles comparing surgical with non-surgical treatment.

RESULTS

The literature search identified five prospective randomised controlled trials, three of which compare surgical with non-surgical treatment, one which compares functional early mobilisation with cast immobilisation after surgical repair and one which compares functional and cast immobilisation in non-surgical management of Achilles tendon rupture.

CONCLUSION

Surgical treatment of Achilles tendon rupture is associated with a significantly lower incidence of re-rupture and therefore is the treatment method of choice. Non-surgical treatment may be acceptable for patients who refuse surgery or who are unfit for surgery. Functional early mobilisation appears to be associated with an improved functional outcome and should be considered in preference to plaster cast immobilisation where appropriate.

The effect of length on the structural properties of an Achilles tendon graft as used in posterior cruciate ligament reconstruction

BACKGROUND

The clinical outcomes of posterior cruciate ligament reconstruction are varied. No previous studies have investigated the effect of graft length on the structural properties of the graft.

HYPOTHESIS

Graft length significantly affects the structural properties of posterior cruciate ligament grafts.

STUDY DESIGN

Controlled laboratory study.

METHODS

Eight Achilles tendon grafts were tested under tensile loads up to 400 N at 3 different lengths: long (75 mm), medium (48 mm), and short (34 mm). These 3 lengths represent midtunnel fixation, inlay fixation, and fixation near the ligament insertions.

RESULTS

Shortening the graft from both long to medium and from medium to short increased the stiffness by approximately 25%. Long and medium grafts displaced significantly more than medium and short grafts, respectively.

CONCLUSION

The effective length of a graft, which is determined by where it is fixed, should be considered an important variable in posterior cruciate ligament reconstruction.

Methods for quasi-linear viscoelastic modeling of soft tissue: application to incremental stress-relaxation experiments

The quasi-linear viscoelastic (QLV) model was applied to incremental stress-relaxation tests and an expression for the stress was derived for each step. This expression was used to compare two methods for normalizing stress data prior to estimating QLV parameters. The first and commonly used normalization method was shown to be strain-dependent. Thus, a second normalization method was proposed and shown to be strain-independent and more sensitive to QLV time constants. These analytical results agreed with representative tendon data. Therefore, this method for normalizing stress data was proposed for future studies of incremental stress-relaxation, or whenever comparing stress-relaxation at different strains.

Effect of fiber orientation and strain rate on the nonlinear uniaxial tensile material properties of tendon

Tendons are exposed to complex loading scenarios that can only be quantified by mathematical models, requiring a full knowledge of tendon mechanical properties. This study measured the anisotropic, nonlinear, elastic material properties of tendon. Previous studies have primarily used constant strain-rate tensile tests to determine elastic modulus in the fiber direction. Data for Poisson's ratio aligned with the fiber direction and all material properties transverse to the fiber direction are sparse. Additionally, it is not known whether quasi-static constant strain-rate tests represent equilibrium elastic tissue behavior. Incremental stress-relaxation and constant strain-rate tensile tests were performed on sheep flexor tendon samples aligned with the tendon fiber direction or transverse to the fiber direction to determine the anisotropic properties of toe-region modulus (E0), linear-region modulus (E), and Poisson's ratio (v). Among the modulus values calculated, only fiber-aligned linear-region modulus (E1) was found to be strain-rate dependent. The E1 calculated from the constant strain-rate tests were significantly greater than the value calculated from incremental stress-relaxation testing. Fiber-aligned toe-region modulus (E(1)0 = 10.5 +/- 4.7 MPa) and linear-region modulus (E1 = 34.0 +/- 15.5 MPa) were consistently 2 orders of magnitude greater than transverse moduli (E(2)0 = 0.055 +/- 0.044 MPa, E2 = 0.157 +/- 0.154 MPa). Poisson's ratio values were not found to be rate-dependent in either the fiber-aligned (v12 = 2.98 +/- 2.59, n = 24) or transverse (v21 = 0.488 +/- 0.653, n = 22) directions, and average Poisson's ratio values in the fiber-aligned direction were six times greater than in the transverse direction. The lack of strain-rate dependence of transverse properties demonstrates that slow constant strain-rate tests represent elastic properties in the transverse direction. However, the strain-rate dependence demonstrated by the fiber-aligned linear-region modulus suggests that incremental stress-relaxation tests are necessary to determine the equilibrium elastic properties of tendon, and may be more appropriate for determining the properties to be used in elastic mathematical models.

Anterior cruciate ligament reconstruction using cryopreserved allografts

Primary ACL reconstruction historically has been done using autograft tissues whereas allografts have been limited to revision cases and patients who are older or with lower physical demands because some animal studies suggested a slower biologic incorporation rate. The purpose of the current study was to evaluate the effectiveness of the cryopreserved Achilles tendon allograft in primary ACL reconstruction in a consecutive series of athletes. Fifty consecutive patients with a strenuous or moderate preinjury activity level, as defined by the International Knee Documentation Committee (IKDC), had ACL reconstruction using cryopreserved Achilles tendon allografts secured with bioabsorbable interference screws. Five patients were professional athletes. The average age of the patients was 36 years (range, 17-50 years). A 3- to 5-year followup study was done in all of the patients using the IKDC form. Tunnel widening was measured in the lateral radiographs at the widest level. The overall outcome was normal or nearly normal in 94% of the patients. No failures were reported in this series. Forty-six patients (92%) returned to their same preinjury sport activity level. The average KT-1000 side-to-side difference was 2.3 mm. Average tibial tunnel widening was 2.7 mm (range, 0-6 mm); no significant correlation was observed between increased tunnel size and a fair or poor clinical outcome. This experience shows that favorable results can be obtained with cryopreserved Achilles tendon allografts in athletes in whom avoiding donor site morbidity may be an issue in terms of a prompt return to sport.

Comparison of the effects of laser, ultrasound, and combined laser + ultrasound treatments in experimental tendon healing

BACKGROUND AND OBJECTIVE

Therapeutic ultrasound (US) and laser (L) treatments accelerate and facilitate wound healing, and also have beneficial effects on tendon healing. This randomized control study was designed to evaluate the effects of low-intensity US and low-level laser therapy (LLLT) on tendon healing in rats.

STUDY DESIGN/MATERIALS AND METHODS

Eighty-four healthy male Swiss-Albino rats were divided into three groups consisting of 28 rats, the left Achilles tendons were used as treatment and the right Achilles tendons as controls. The right and left Achilles tendons of rats were traumatized longitudinally. The treatment was started on postinjury day one. We applied the treatment protocols including low-intensity US treatment in Group I (US Group), Sham US in Group II (SUS Group), LLLT in Group III (L Group), Sham L in Group IV (SL Group), US and LLLT in Group V (US + L Group), and Sham US and Sham L in Group VI (SUS + SL Group). The US treatment was applied with a power of 0.5 W/cm2, a frequency of 1 MHz, continuously, 5 minutes daily. A low-level Ga-As laser was applied with a 904 nm wavelength, 6 mW average power, 1 J/ cm2 dosage, 16 Hz frequency, for 1 minute duration, continuously. In the control groups, the similar procedures as in the corresponding treatment groups were applied with no current (Sham method). The treatment duration was planned for 9 days (sessions) in all groups, except the rats used for biochemical evaluation on the 4th day of treatment, which were treated for 4 days. We measured the levels of the tissue hydroxyproline for biochemical evaluation on the 4th, 10th, and 21st days following the beginning of treatment and the tendon breaking strength on the 21st day following the beginning of treatment for biomechanical evaluation. Seven rats in each group were killed on the 4th, 10th, and 21st days for biochemical evaluation and on the 21st day for biomechanical evaluation.

RESULTS

The hydroxyproline levels were found to be significantly increased in the treatment groups on the 10th and 21st days compared to their control groups (P < 0.05). In comparison of the treatment groups on the 4th, 10th, and 21st days of the treatment, the levels of tissue hydroxyproline were found to be more increased in combined US+L Group compared with US Group and L Group, but the difference was not significant (P > 0.05). In comparison of the tendon breaking strengths, it was found as significantly increased in the treatment groups compared with their control groups (P < 0.05), although there was no significant difference between the treatment groups.

CONCLUSIONS

Although US, L, and combined US + L treatments increased tendon healing biochemically and biomechanically more than the control groups, no statistically significant difference was found between them. Also we did not find significantly more cumulative positive effects of combined treatment. As a result, both of these physical modalities can be used successfully in the treatment of tendon healing.

Diagnosis and Management of the Painful Ankle/Foot. Part 2: Examination, Interpretation, and Management

Diagnosis, interpretation, and subsequent management of ankle/foot pathology can be challenging to clinicians. A sensitive and specific physical examination is the strategy of choice for diagnosing selected ankle/foot injuries and additional diagnostic procedures, at considerable cost, may not provide additional information for clinical diagnosis and management. Because of a distal location in the sclerotome and the reduced convergence of afferent signals from this region to the dorsal horn of the spinal cord, pain reference patterns are low and the localization of symptoms is trustworthy. Effective management of the painful ankle/foot is closely linked to a tissue-specific clinical examination. The examination of the ankle/foot should include passive and resistive tests that provide information regarding movement limitations and pain provocation. Special tests can augment the findings from the examination, suggesting compromises in the structural and functional integrity of the ankle/foot complex. The weight bearing function of the ankle/foot compounds the clinician's diagnostic picture, as limits and pain provocation are frequently produced only when the patient attempts to function in weight bearing. As a consequence, clinicians should consider this feature by implementing numerous weightbearing components in the diagnosis and management of ankle/foot afflictions. Limits in passive motion can be classified as either capsular or non-capsular patterns. Conversely, patients can present with ankle/foot pain that demonstrates no limitation of motion. Bursitis, tendopathy, compression neuropathy, and instability can produce ankle/foot pain that is challenging to diagnose, especially when they are the consequence of functional weight bearing. Numerous non-surgical measures can be implemented in treating the painful ankle/foot, reserving surgical interventions for those patients who are resistant to conservative care.

Region-specific differences in Achilles tendon cross-sectional area in runners and non-runners

The present study examined the cross-sectional area (CSA) of the Achilles tendon in subjects who repeatedly exposed their tendons to large loads (habitual runners) compared to control subjects (non-runners). Six male habitual runners [36 (7) years, 70.9 (4.4) kg and 1.84 (0.05) m, +/-SD] who had performed distance running (approximately 80 km per week) for the last 5 years were compared to six non-runners [34 (3) years, 81.2 (8.7) kg and 1.81 (0.02) m, +/-SD]. Tendon CSA was obtained from MR images obtained with the ankle in a neutral position (90 degrees ). The most proximal aspect of the tuberosity of calcaneus was used as a landmark to standardize the levels of images: the most distal image (1) was obtained 10 mm above the proximal tuberosity of calcaneus, and the most proximal image (7) was obtained 70 mm above the proximal tuberosity of calcaneus. There was a significant difference in CSA along the length of the tendon both in runners ( P<0.001) and non-runners ( P<0.01). In non-runners and runners the CSA of the most distal part was 51% and 85% greater than the most proximal part of the tendon, respectively. Furthermore, there was a difference in tendon CSA between the groups, such that runners had a greater CSA (36%) than non-runners at the most distal part of the tendon ( P<0.05). The greater CSA in the distal tendon may reflect differences in structural properties along the length of the human Achilles tendon, while the greater CSA in runners compared to non-runners may indicate a region-specific hypertrophy in response to the habitual loading of running.

Therapeutic ultrasound improves strength of achilles tendon repair in rats

The purpose of this study was to evaluate the effects of therapeutic ultrasound on structural properties and functional performance of Achilles tendon healing. Thirty Sprague-Dawley rats with surgical hemitransected Achilles tendon were studied. Ten were treated daily with 1 MHz continuous ultrasound at 1.0 W/cm2 for 4 min, 11 at 2.0 W/cm2 for 4 min and nine served as control without treatment. Achilles functional index (AFI) was recorded preoperatively and on postoperative days 3, 10 and 30. On day 30, the rats were sacrificed and Achilles tendons were tested for load-relaxation, stiffness and ultimate tensile strength (UTS). Results showed that UTS of both low-dose (p=0.023) and high-dose (p=0.002) groups was significantly greater than in controls. No significant differences in AFI (p=0.179), load-relaxation (p=0.205) and stiffness (p=0.842) were found among groups. These findings suggested that both low- and high-dose therapeutic ultrasound accelerate the healing process of ruptured tendon.

Effect of strength training on human patella tendon mechanical properties of older individuals

This study investigated the effect of strength training on the mechanical properties of the human patella tendon of older individuals. Subjects were assigned to training (n = 9; age 74.3 +/- 3.5 years, body mass 69.7 +/- 14.8 kg and height 163.4 +/- 9.1 cm, mean +/- S.D.) and control (n = 9; age 67.1 +/- 2 years, body mass 73.5 +/- 14.9 kg and height 168.3 +/- 11.5 cm) groups. Strength training (two series of 10 repetitions at 80 % of five-repetition maximum) was performed three times per week for 14 weeks using leg extension and leg press exercises. Measurements of tendon elongation during a ramp isometric knee extension were performed before and after training and control periods in vivo using ultrasonography. Training caused a decreased tendon elongation and strain at all levels of force and stress (P < 0.01). Baseline tendon elongation and strain at maximal tendon load were 4.7 +/- 1.1 mm and 9.9 +/- 2.2 %, respectively (maximum force: 3346 +/- 1168 N; maximum stress: 40 +/- 11 MPa). After training, these values decreased to 2.9 +/- 1.2 mm and 5.9 +/- 2.4 % (P < 0.01), respectively (maximum force: 3555 +/- 1257 N; maximum stress: 42 +/- 11 MPa). Tendon stiffness increased by 65 % (2187 +/- 713 to 3609 +/- 1220 N mm-1; P < 0.05) and Young's modulus increased by 69 % (1.3 +/- 0.3 to 2.2 +/- 0.8 GPa; P < 0.01). As a result of these changes, the rate of torque development increased by 27 % (482.8 +/- 302.5 to 612.6 +/- 401 N m s-1; P < 0.01) following training. No significant changes occurred in any measured variables in the control group (P > 0.05). This study shows for the first time that strength training in old age increases the stiffness and Young's modulus of human tendons. This may reduce the risk of tendon injury in old age and has implications for contractile force production and the rapid execution of motor tasks.

Determination of optimal graft lengths for posterior cruciate ligament reconstruction--a theoretical analysis

Various graft materials have been used in posterior cruciate ligament (PCL) reconstruction. However, it is unclear if these grafts can reproduce the structural behavior of the PCL. This paper analyzed the effect of graft length on the structural behavior of the graft using a minimal deformation energy method. An analytical solution was obtained to determine the optimal effective graft length that can best reproduce the structural behavior of the PCL. This optimal graft length was determined as a function of the axial rigidity of the graft. Two typical grafts, bone-patella tendon-bone (BPTB) and Achilles tendon, were analyzed. The data demonstrated that in order to reproduce the PCL behavior, the effective length of a BPTB graft (10 mm width) should be 34 mm, while the Achilles tendon graft (with a cross sectional area of 55 mn2) needs to be 48 mm in length. Longer grafts result in less resistance and shorter graft increased the graft resistance. An initial graft tension cannot help recreate the overall structural behavior of the PCL. These results suggest that graft length is an important surgical variable in PCL reconstruction. An optimal reconstruction of the PCL should reproduce the structural properties of the PCL by using a graft with an optimal length.

Differential strain patterns of the human gastrocnemius aponeurosis and free tendon, in vivo

AIM

The mechanical characteristics of the human free tendon and aponeurosis, in vivo, remains largely unknown. The present study evaluated the longitudinal displacement of the separate free Achilles tendon and distal (deep) aponeurosis of the medial gastrocnemius muscle during voluntary isometric contraction.

METHODS

Ultrasonography-obtained displacement of the free tendon and tendon-aponeurosis complex, electromyography of the gastrocnemius, soleus, and dorsiflexor muscles, and joint angular rotation were recorded during isometric plantarflexion (n = 5). Tendon cross-sectional area, moment arm and segment lengths (L(o)) were measured using magnetic resonance imaging. Tendon force was calculated from joint moments and tendon moment arm, and stress was obtained by dividing force by cross-sectional area. The difference between the free tendon and tendon-aponeurosis complex deformation yielded separate distal aponeurosis deformation. Longitudinal aponeurosis and tendon strain were obtained from the deformations normalized to segment lengths.

RESULTS

At a common tendon force of 2641 +/- 306 N, the respective deformation and Lo were 5.85 +/- 0.85 and 74 +/- 0.8 mm for the free tendon and 2.12 +/- 0.64 and 145 +/- 1.3 mm for the distal aponeurosis, P < 0.05. Longitudinal strain was 8.0 +/- 1.2% for the tendon and 1.4 +/- 0.4% for the aponeurosis, P < 0.01. Stiffness and stored energy was 759 +/- 132 N mm(-1) and 6.14 +/- 1.89 J, respectively, for the free tendon. Cross-sectional area of the Achilles tendon was 73 +/- 4 mm2, yielding a stress of 36.5 +/- 4.6 MPa and Young's modulus of 788 +/- 181 MPa.

CONCLUSION

The free Achilles tendon demonstrates greater strain compared with that of the distal (deep) aponeurosis during voluntary isometric contraction, which suggests that separate functional roles may exist during in vivo force transmission.

The evolution of tendon--morphology and material properties

Phylogenetically, tendinous tissue first appears in the invertebrate chordate Branchiostoma as myosepta. This two-dimensional array of collagen fibers is highly organized, with fibers running along two primary axes. In hagfish the first linear tendons appear and the myosepta have developed specialized regions with unidirectional fiber orientation-a linear tendon within the flat sheet of myoseptum. Tendons react to compressive load by first forming a fibrocartilaginous pad, and under severe stress, sesamoid bones. Evidence for this ability to react to load first arises in the cartilaginous fish, here documented in a tendon from the jaw of a hard-prey crushing stingray. Sesamoid bones are common in bony fish and also in tetrapods. Tendons will also calcify under tensile loads in some groups of birds, and this reaction to load is seen in no other vertebrates. We conclude that the evolutionary history of tendon gives us insight into the use of model systems for investigating tendon biology. Using mammal and fish models may be more appropriate than avian models because of the apparent evolution of a novel reaction to tensile loads in birds.

Achilles rupture in the athlete. Current science and treatment

Achilles tendon ruptures became increasingly common in the latter half of the 20th century. Once the diagnosis is made, the patient's goals and objectives should be clearly stated. The treatment choice should incorporate the patient's needs, desires, objectives, and functional goals to assure an optimal result.