Muscle force and power following tendon repair at altered tendon length

BACKGROUND: While a great deal is known regarding the performance of muscle with intact tendon, little is known about muscle performance when tendon is surgically lengthened or shortened. This knowledge may allow surgeons to more accurately predict functional outcome following tendon repair when correcting a simple tendon laceration or performing a more complex vascularized neuromuscular transfer. MATERIALS AND METHODS: We studied muscle performance 12 wk following extensor tendon repairs producing altered tendon lengths. Forty male Fischer 344 rats underwent division of the proximal and distal tendons of the extensor digitorum longus muscle. Tendons were immediately repaired producing tendons with increased length, decreased length, or presurgical length (control). Observation confirmed that altered tendon length produced inverse changes in initial resting muscle tension. RESULTS: Muscle in the decreased tendon length group demonstrated a 15.2% greater muscle mass, 4.9% greater muscle length, 9.6% greater physiological cross-sectional area, 12.6% greater maximum isometric force, and 31.9% greater maximum power relative to the control tendon length group (P < 0.05). The increased tendon length group did not differ significantly from the control tendon length group for any measurement. Histologically, muscles set with a decreased tendon length demonstrated normal appearing hypertrophied fibers, without evidence of detrimental histological effects such as fibrosis, denervation, necrosis, inflammation, fiber type changes, or fiber splitting. CONCLUSION: These data support the clinical practice of setting muscles with increased passive tension when performing tendon transfer surgeries. Conversely, setting muscles with decreased tension does not necessarily result in a force or power deficit.     

The determination of moments for extension of the wrist generated by muscles of the forearm

In deciding on suitable tendon transfers to replace denervated muscle-tendon units, important considerations are the strength and effectiveness of possible substitutes. A method is presented by which the strength of the wrist extensor muscles and their moment arms can be determined. The method can be applied to other muscles at other joints. It involves the use of a force transducer which measures the combined forces of the three wrist extensors in an isometric contraction. This moment for wrist extension, measured in the living intact arm, is the same as the sum of the moments of the three wrist extensor muscles. The contribution of each muscle to the total moment is calculated from ratios that have been developed from a quantitative study of moment arms and muscle masses in sixteen cadaver limbs. It is suggested that the ratio of one moment arm to another is fairly constant from subject to subject, and that muscle masses also have sufficiently similar ratios to each other to serve as the basis for practical estimations by the surgeon. Thus the surgeon needs only one or two direct measurements of moments externally and only one or two skeletal measurements on any living subject to be able to estimate the effectiveness of a number of muscles on the basis of cadaver studies such as this, and to project the behavior of a muscle after it has been transferred to a position where it will have new moment arms.     

The relationship between passive ankle plantar flexion joint torque and gastrocnemius muscle and achilles tendon stiffness: implications for flexibility

STUDY DESIGN: Experimental laboratory study. OBJECTIVES: We tested the hypothesis that the muscle fibers and the connective tendinous structures, combined in series, provide the resistance to passive joint movement at the ankle. We also determined the relative association between passive joint torque and each of these 2 elements. BACKGROUND: The reason for individual variation in joint flexibility or tightness is not clearly understood, but the influence of musculotendinous stiffness has been inferred. METHODS AND MEASURES: Each of the subjects (6 women and 6 men) was seated with the right knee extended and right ankle positioned at a 30 degrees , 20 degrees , 10 degrees , 0 degrees , -10 degrees , -20 degrees , and -30 degrees (0, neutral position, positive values reflecting plantar flexion) angle while passive plantar flexion torque was measured. The distal muscle-tendon junction of the medial gastrocnemius was visualized by ultrasonography, and its positional change was defined as muscle belly length change. The whole muscle-tendon unit length change was estimated from joint angle changes, from which Achilles tendon length change was estimated. RESULTS: Both the muscle belly and tendon were significantly elongated as the ankle was dorsiflexed (at 0 degrees the mean +/- SD muscle belly elongation was 10.3% +/- 1.8 %, and the tendon elongation was 2.8% +/- 1.2 %, of the initial length at 30 degrees of ankle plantar flexion), from which stiffness indices were determined both for muscle belly and tendon. The passive torque at 0 degrees , -10 degrees , -20 degrees , and -30 degrees was significantly correlated with the stiffness indices of the Achilles tendon (at 0 degrees , r2 = 0.70 and 0.62 for overall and specific stiffness, respectively; P<.05). A tendon stiffness index, separately obtained from tendon lengthening during maximal isometric contraction, was also correlated with passive ankle plantar flexion torque at 0 degrees , -10 degrees , -20 degrees , and -30 degrees (at 0 degrees , r2 = 0.76; P<.05). The specific stiffness index of the muscle belly was correlated (r2 = 0.47, P<.05) with the passive ankle plantar flexion torque at 0 degrees , but its overall stiffness index was not (r2 = 0.32, P>.05). CONCLUSION: Results suggest that extensibility of the muscle-tendon unit of the Achilles tendon for the most part is related to passive ankle plantar flexion joint torque.     

Analysis of intrinsic-extrinsic muscle function through interactive 3-dimensional kinematic simulation and cadaver studies

PURPOSE: To revisit the analysis of intrinsic muscle function and its relationship to the overall flexion moment at the index metacarpophalangeal (MCP) joint. Prior studies reported inconsistent levels of intrinsic contribution varying from 12.5% to 73% of the total flexion moment at the index MCP joint. This study hypothesized that 26% is a more realistic figure using computer simulation moment arms combined with known muscle-tension fractions. This study tested the hypothesis using a 2-part fresh cadaver study, the objectives of which were to measure actual muscle moment arms throughout index MCP range of motion and then combine these measurements with tension fractions to calculate normal intrinsic minus and low ulnar nerve palsy moments and then using the same specimens to apply loads to the muscles and measure resultant moments directly. METHODS: An interactive computer simulation was constructed to replicate the position and loading orientation of an earlier experiment reported in the literature and known muscle-tension fractions were combined with the resultant moment arm structure to calculate the moment at the index MCP joint for maximum isometric contraction of the normal, intrinsic minus, and low ulnar nerve palsy finger. Eleven fresh cadaver specimens were used. For experiment A the excursion and angle data were used to determine moment arms of each muscle and moments were calculated. For experiment B the muscles were connected to weights based on muscle-tension fractions and the resultant moment was measured at the fingertip. RESULTS: Experiment A determined the muscle-tendon moment arms at the index MCP joint throughout the flexion-extension range of motion. Combining the flexor moment arms with known tension fractions determined that the intrinsics contributed 24% to the total flexion moment, the dorsal and palmar interossei contributed 22%, and the lumbrical contributed 2%. The results of experiment B confirmed the results of experiment A. CONCLUSIONS: Intrinsic muscle contribution to flexion moment at the index MCP joint is not as high or as low as previously reported. The relative moment production is governed by both the moment arms and tension produced by the muscle; thus a more precise knowledge of moment arms is essential to understand muscle balance at a joint. Clinically the hand surgeon should expect a loss of 24% of index MCP flexion moment-generating capacity in the intrinsic-minus index finger and a loss of 22% in the index finger on a hand with low ulnar nerve palsy. Furthermore the flexion moment from the lumbrical of only 2% appears to be adequate to prevent clinical clawing of the digit.     

Implications of muscle design on surgical reconstruction of upper extremities

The basic anatomic and physiologic properties of human upper extremity muscles have been elucidated using a novel intraoperative sarcomere length measuring device in conjunction with quantitative anatomic and biomechanic models of these same muscles. We reviewed the genesis of these studies which began with development of the optical tools, validation using animal models, and application to human upper extremity surgery. Human muscles have a remarkable degree of specialization, even among synergists. Mechanical properties of human tendons provide another source of specialization such that the muscle-tendon unit does not simply retain the properties of a muscle plus a tendon. The operating range of different muscles is also a method whereby the balance of the joint can be maintained in the face of altered moment arms and muscle forces. The derivation of these principles is explained along with their importance in surgical tendon transfers where one muscle substitutes function for another.     

Superior displacement of the hip in total joint replacement: Effects of prosthetic neck length,neck-stem angle,and anteversion angle on the moment-generating capacity of the muscles

The purpose of this study was to determine the effects of superior displacement of the hip center and changes in three prosthetic parameters (neck length, neck-stem angle, and anteversion angle) on the capacity of muscles to generate force and moment about the hip. A three-dimensional model that calculates the maximum isometric forces and moments generated by 25 muscles crossing the hip over a wide range of body positions was used to evaluate the effects of a 2 cm elevation of the hip center and changes in the prosthetic parameters. After superior displacement of the hip center, the neck length was increased from 0 to 3 cm, the neck-stem angle was varied between 110 and 150°, and the anteversion angle was varied between 0 and 40°. Our analysis showed that a 2 cm superior displacement of the hip center would decrease the moment-generating capacity of the four muscle groups studied (abductors, adductors, flexors, and extensors) if neck length were not increased to compensate for decreased muscle length. In the computer model of an adult man that we used, a 2 cm increase in neck length restored the moment-generating capacity of the muscles by increasing muscle length and force-generating capacity. However, a 3 cm increase in neck length increased passive muscle forces substantially, which potentially could limit joint motion. An increased neck-stem angle (i.e., a valgus neck) decreased the abduction moment arm but increased the moment-generating capacity of the other muscle groups. A change in the anteversion angle from 0 to 40° had a relatively small effect on the isometric moment-generating capacity of the muscles studied.     

Effect of radial shortening on muscle length and moment arms of the wrist flexors and extensors

The purpose of this study was to determine the effect of distal radial shortening on muscle length and moment arm of the wrist flexors and extensors. In eight cadaveric upper extremities, distal radius fractures were simulated by an ostectomy. The distal radius was progressively shortened by 2.5, 5.0, 7.5, and 10 mm. Changes in the resting length of the flexor carpi radialis and ulnaris, extensor carpi radialis longus and brevis, and extensor carpi ulnaris muscles were measured with rotary potentiometers at neutral position, flexion, extension, and radial and ulnar deviation of the wrists. The wrists were passively moved through flexion-extension and radioulnar deviation, and tendon excursions and wrist joint angulation were recorded simultaneously. Tendon moment arms were derived from tendon excursions and joint motion. The results showed that either muscle, length or moment arm of the principal wrist flexors and extensors was significantly affected by the radial shortening. Muscle length decreased significantly after radial shortening in all the wrist flexors and extensors except for the extensor carpi ulnaris. The moment arm of the extensor carpi ulnaris tendon decreased significantly during either wrist flexion-extension or radioulnar deviation. The extensor carpi radialis brevis and flexor carpi ulnaris tendons also showed a significant decrease in their moment arms during radioulnar deviation of the wrist. Radial shortening of only 2.5 mm caused statistically significant changes in muscle length and moment arm of the wrist flexors and extensors. Increasing the extent of radial shortening exaggerated the biomechanical changes in the wrist motors. These results validate the importance of normal radial length for wrist kinetics and, from a biomechanical perspective, support complete correction of radial shortening after distal radius fractures.     

The mechanism of the intrinsic-minus finger: a biomechanical study

To demonstrate in the cadaver model that the sequence of extension of the flexed metacarpophalangeal and proximal interphalangeal joints of the intrinsic-minus finger can be explained on the basis of moment ratios about these joints, the engineering concept of free body analysis was applied. Intrinsic-minus fingers, i.e., fingers of a cadaver in which all forces exerted by the intrinsic muscles were removed, were observed to hyperextend maximally at the metacarpophalangeal joint before extension of the proximal interphalangeal joint began. Mathematical calculation of moments provides an explanation of this sequence. This study confirmed that, when an equilibrium of forces at the hyperextended metacarpophalangeal joint is reached, the proximal interphalangeal joint is incompletely extended. These forces include: (1) the proximal pull of the extensor tendon; (2) the distal loading of the extensor tendon by the flexor tendons; (3) the force preventing spanning of the laminae.     

Peroneus brevis is a more effective evertor than peroneus longus

A primary function of the peroneus longus and peroneus brevis is to provide the eversion moment necessary to balance the opposing inversion moments. Surgeons often deal with the loss of or need to sacrifice one of these tendons. This study compares the evertor mechanisms of the peroneus brevis and peroneus longus muscle. This is accomplished in a cadaver model in which the performance of each of the muscle tendons during early heel rise of gait is assessed utilizing the same tendon loads in each so that force is not a variable. Six fresh-frozen cadaver foot-ankle specimens were studied during a simulated early heel rise phase of the gait cycle. The study compared the effect of the peroneus brevis and peroneus longus by separately applying the same load to the each of the tendons. At the talonavicular joint, the peroneus brevis loaded condition externally rotated the navicular 2.1 degrees more than when the peroneus longus was loaded. At the subtalar joint, the peroneus brevis loaded condition resulted in 0.9 degrees more calcaneus valgus relative to the talus than was present during the peroneus longus loaded condition. The experimental data support the hypothesis that the peroneus brevis tendon mechanism is more effective than is the peroneus longus mechanism in rotating the navicular externally and the calcaneus into valgus. This has clinical implications for assisting surgeons in trying to preserve evertor function.     

The effect of position of immobilisation on resting length,resting stiffness,and weight of the soleus muscle of the rabbit

The extent of immobilisation-induced adaptations of muscle length and muscle weight is known to be profoundly influenced by the position (or length) at which the muscle is immobilised. However, the effect of the full range of positions of immobilisation on subsequent adaptations of muscle length and weight has not yet been investigated. To examine further the effect of position of immobilisation, we used cast-immobilisation of the hind limbs of 23 rabbits, in various positions between full plantar flexion and full dorsi-flexion. Six muscles from non-immobilised rabbits were used as the controls. After 10 days of immobilisation, the wet weight of the soleus muscle and the resting length and resting stiffness of the soleus muscle-tendon unit were determined. Immobilisation in a shortened position was associated with a significantly greater decrease in length and weight than was immobilisation in a lengthened position. In addition, immobilisation produced significant increases in the resting stiffness of muscle-tendon units, although there was no evidence of a position-dependent increase in stiffness. Muscle weight was influenced by the position of immobilisation in a nonlinear way. The data support the views that the pre-immobilisation resting length of the muscle represents a threshold length and that immobilisation at lengths longer than this retards immobilisation-induced atrophy.     

The effect of achilles tendon lengthening on ankle dorsiflexion: a cadaver study

BACKGROUND: Tendon lengthening is an important cause of morbidity after Achilles tendon rupture. However, direct measurement of the tendon length is difficult. Ankle dorsiflexion has, therefore, been used as a surrogate measure on the assumption that it is the Achilles tendon that limits this movement. The aim of this investigation was to assess the relationship between Achilles tendon length and ankle dorsiflexion. The primary question was whether or not the Achilles tendon is the structure that limits ankle dorsiflexion. The secondary purpose was to quantify the relationship between Achilles tendon lengthening and dorsiflexion at the ankle joint. METHODS: Five cadaver specimens were dissected to expose the tendons and capsular tissue of the leg and hindfoot. Fixed bony reference points were used as markers for the measurements. In the first specimen, the Achilles tendon was intact and the other structures that may limit ankle dorsiflexion were sequentially divided. In the other specimens the Achilles tendon was lengthened by 1 cm intervals and the effect upon ankle dorsiflexion movement was recorded. RESULTS: Division of the other tendons and the capsular tissue around the ankle joint did not affect the range of ankle dorsiflexion. When the Achilles was divided the foot could be dorsiflexed until the talar neck impinged upon the anterior aspect of the distal tibia. There was a mean increase of 12 degrees of dorsiflexion for each centimeter increase in tendon length. CONCLUSION: The Achilles tendon is the anatomical structure that limits ankle dorsiflexion, even when the tendon is lengthened. There was a linear relationship between the length of the Achilles tendon and the range of ankle dorsiflexion in this cadaver model. Ankle dorsiflexion would appear to be a clinically useful indicator of tendon length.     

How musculotendon architecture and joint geometry affect the capacity of muscles to move and exert force on objects: A review with application to arm and forearm tendon transfer design

This commentary reviews musculotendon architecture and the relation between architectural parameters and the force, speed, and excursion capacity of musculotendon units. It is hoped that this review will help provide the framework within which to appreciate the importance of the data presented by Lieber et al. Muscle fiber pennation hardly affects musculotendon output of forearm and hand muscles. Instead, physiologic cross-sectional area and muscle fiber length affect force capacity and speed and excursion capacity, respectively. How muscles with equal mass can have different force, speed, and excursion capacities is explained. Since the moment arm of a muscle (the shortest distance from the musculotendon unit to the joint center of rotation) transforms muscle output into musculotendon output, it is shown why the capacity for a muscle to exert force on an object, as during grasping, is directly proportional to its moment arm and why the range of joint movement and speed over which muscles exert force is inversely proportional to the moment arm. Finally, tendon, being not stiff in forearm and hand musculotendon units, also affects their output. Criteria are given for designing tendon transfer reconstructions from architectural data and moment arm data to best replicate the biomechanical function of the replaced muscle. To have the same capacity for imparting movement to objects and exerting force on them, the donor muscle should have the same moment arm/physiologic cross-sectional area product, the same fiber length/moment arm ratio, and the same tendon length/muscle fiber length ratio as the replaced muscle.     

Muskelphysiologie und Auswirkung der Operationsmethoden bei infantiler Zerebralparese

The muscle reacts physiologically in neuromuscular disorders which do not affect muscle tissue directly (such as muscular dystrophies), and adapts to altered demands. For this reason, alterations in the muscle-tendon apparatus are grossly secondary. Changing load stimulates an increase in muscle fibres, whereas continuous pull results in an increase in collagen fibres (fibrosis).The length range in which a muscle produces force must be considered with respect to the range of motion of the joint in order to plan soft tissue corrections and to understand the postoperative result. An optimal correction of muscle length can increase muscle force whereas overlengthening will reduce it. The latter is desired in cases where spastic muscle force needs to be reduced. In contrast shortening the muscle-tendon apparatus may be indicated in order to correct overlength and to increase muscle force.The effect of intramuscular lengthening procedures, performed as an aponeurotomy or an intramuscular tenotomy, depends on the stretching program after surgery. Extramuscular tendon lengthening, on the other hand, results in an immediate length gain. The danger of overlengthening and consecutive muscle weakening, however, is greater with the tendinous procedures.     

Effects of tensioning errors in split transfers of tibialis anterior and posterior tendons

BACKGROUND: Split transfers of the tibialis anterior and posterior tendons are commonly performed to address hindfoot varus deformities in patients with cerebral palsy, stroke, or brain injury. Poor outcomes from these procedures are often attributed to a failure to tension the transferred tendon properly, but the mechanical effects of this aspect of the procedure have not been quantified, to our knowledge. The purpose of the present study was to use a cadaver model to examine changes in the actions of these muscles that occur when the tensions in the halves of the split tendon are intentionally balanced or unbalanced to varying degrees. METHODS: Tendon excursion was measured in seven cadaveric specimens before and after split tendon transfer with experimentally controlled tensions in the halves of the split tendon. The muscle moment arm, a quantitative indicator of the action of a muscle about a joint axis, was calculated as the derivative of tendon excursion with respect to the subtalar joint angle. RESULTS: The tibialis anterior had an eversion moment arm with the subtalar joint in a neutral position following surgery, but the tibialis posterior had virtually no action in the neutral position. Following the split transfers with ideally balanced tension, subtalar joint rotations of >5 degrees strongly influenced the moment arm of the tibialis posterior (p < 0.0002), indicating that its action depends on the position of the hindfoot. The moment arm of the tibialis anterior, however, was influenced only by rotations of >/=20 degrees (p > 0.1741 for each angle pair comparison of <20 degrees ). Moment arms were generally insensitive to imbalances in tension between the medial and lateral tendon halves; significant differences in the moment arm (p < 0.05), compared with that in the balanced condition, were seen only when one half was slack or nearly so. CONCLUSIONS: These results suggest that it is possible for a split tendon transfer to be successful over a large range of tensionings. Split transfer of the tibialis posterior tendon produced the desired mechanical outcome in that the tibialis posterior had an eversion moment arm when the foot was inverted and an inversion moment arm when the foot was everted. Split transfer of the tibialis anterior to the cuboid, however, produced a muscle that consistently functioned as an everter regardless of the position of the hindfoot.     

Progressive surgical dissection for tendon transposition affects length-force characteristics of rat flexor carpi ulnaris muscle.

Extramuscular connective tissue and muscular fascia have been suggested to form a myo-fascial pathway for transmission of forces over a joint that is additional to the generally accepted myo-tendinous pathway. The consequences of myo-fascial force transmission for the outcome of conventional muscle tendon transfer surgery has not been studied as yet. To test the hypothesis that surgical dissection of a muscle will affect its length-force characteristics, a study was undertaken in adult male Wistar rats. During progressive dissection of the flexor carpi ulnaris muscle, isometric length-force characteristics were measured using maximal electrical stimulation of the ulnar nerve. After fasciotomy, muscle active force decreased by approximately 20%. Further dissection resulted in additional decline of muscle active force by another 40% at maximal dissection. The muscle length at which the muscle produced maximum active force increased by approximately 0.7 mm (i.e. 14% of the measured length range) after dissection. It is concluded that, in rats, the fascia surrounding the flexor carpi ulnaris muscle is a major determinant of muscle length-force characteristics.     

Response of the tendon during limb lengthening

Little is known about the increase in length of tendons in postnatal life or of their response to limb lengthening procedures. A study was carried out in ten young and nine adult rabbits in which the tibia was lengthened by 20% at two rates 0.8 mm/day and 1.6 mm/day.The tendon of the flexor digitorum longus (FDL) muscle showed a significant increase in length in response to lengthening of the tibia. The young rabbits exhibited a significantly higher increase in length in the FDL tendon compared with the adults. There was no difference in the amount of lengthening of the FDL tendon at the different rates. Of the increase in length which occurred, 77% was in the proximal half of the tendon.This investigation demonstrated that tendons have the ability to lengthen during limb distraction. This occurred to a greater extent in the young who showed a higher proliferative response, suggesting that there may be less need for formal tendon lengthening in young children.     

Abductor tendon tears of the hip: evaluation and management

The gluteus medius and minimus muscle-tendon complex is crucial for gait and stability in the hip joint. There are three clinical presentations of abductor tendon tears. Degenerative or traumatic tears of the hip abductor tendons, so-called rotator cuff tears of the hip, are seen in older patients with intractable lateral hip pain and weakness but without arthritis of the hip joint. The second type of tear may be relatively asymptomatic. It is often seen in patients undergoing arthroplasty for femoral neck fracture or elective total hip arthroplasty (THA) for osteoarthritis. The third type of abductor tendon dysfunction occurs with avulsion or failure of repair following THA performed through the anterolateral approach. Abductor tendon tear should be confirmed on MRI. When nonsurgical management is unsuccessful, open repair of the tendons with transosseous sutures is recommended. Good pain relief has been reported following endoscopic repair. Abductor tendon repair has had inconsistent results in persons with avulsion following THA. Reconstruction with a gluteus maximus muscle flap or Achilles tendon allograft has provided promising short-term results in small series.     

Trapeziometacarpal joint instability affects the moment arms of thumb motor tendons

This study measured the changes in moment arm length of thumb motor tendons after simulated ligamentous instability and subsequent reconstruction of the trapeziometacarpal joint. Excursions of thumb motor tendons were measured simultaneously with the trapeziometacarpal joint angulation during flexion to extension and abduction to adduction motion. Tendon moment arms were calculated based on joint and tendon displacement techniques in the intact joint, after sequential sectionings of the capsuloligamentous restraints, and after the reconstruction procedure of Eaton and Littler. The results showed that moment arms of the abductor pollicis longus and extensor pollicis brevis tendons increased significantly as compared with those for normal joints during flexion to extension motion after sectioning the palmar capsuloligamentous components. After the ulnopalmar structures were cut, the moment arm of the extensor pollicis longus tendon had a statistically significant increase during abduction to adduction motion, and those of the extensor and flexor pollicis longus tendons decreased significantly during flexion to extension motion. Changed moment arms were restored to a normal level after the ligamentous reconstruction. These results indicate that ligamentous disruptions alter the mechanical balance of thumb motor tendons, which may contribute to joint deformities observed in trapeziometacarpal joint arthritis. Restoring joint stability is important to correct mechanical imbalance of the tendons.