Mechanics of Tendon Transfers




The goal of tendon transfer operations is to restore balance to a hand after one or more of its muscles have been paralyzed or destroyed. In so doing, the surgeon must compare the usefulness of the action of the lost muscle with that of the muscle that will have to be transferred, leaving a defect in the place where the muscle was before.


It is not enough to consider the function and usefulness of these muscles in a qualitative sense. One also has to make an attempt to quantify the gains and losses at each joint, so that one does not overbalance a hand in an attempt to restore balance.


In this process of planning tendon transfers it is useful to think about those mechanical qualities that are transferred with a tendon and those that remain in the distal part of the limb as passive structures that have to be moved and that sometimes resist movement.


Factors Transferred with a Muscle


When a muscle-tendon unit is transferred, it carries with it some but not all of the qualities it had in its original situation.


Strength


In this context, strength refers to the ability to generate tension in the tendon. The tension capability of a muscle depends on the number of muscle fibers that it has and on the total cross-sectional area of all its fibers. Its ability to sustain its tension over time and over a number of repeated contractions depends also on the adequacy of its blood supply; however, the act of transferring a tendon should not change the vascular supply or the nerve supply of its muscle. There used to be a widely quoted rule that said when a muscle-tendon unit was transferred, its strength dropped one level in the scale of 0 to 5 by which muscles were graded. This rule was worked out when polio was the most common cause of paralysis demanding tendon transfers and surgeons had to grade muscles carefully, not only in terms of paralyzed versus unparalyzed but also in various grades of paralysis. We were warned not to use a grade 3 muscle without realizing that after transfer it would only work as grade 2. Any truth in this generalization must have resulted from factors other than muscle strength, such as “drag,” which is discussed later. The actual strength of a muscle is unchanged by transfer.


Just within the last two decades surgeons have started to transplant muscles by microvascular and nerve anastomosis. This is quite a different thing from simple transfer; it involves removing a muscle from another limb, with its major artery, vein, and nerve, and placing it in a new situation, using locally available vessels and the motor nerve of the muscle that it is to replace. This is a new challenge, and one for which some of the rules have not yet been worked out. The fascicular patterns of the grafted nerve and the recipient stump may be very different, and even with the highest skill there is no chance that every nerve fiber will get through to a muscle unit. The published reports so far have indicated that the successful cases are those in which the transplanted muscles are bigger and bulkier than the muscles they are to replace, presumably to allow for considerable loss of muscle units and yet allow for adequate survival. How much discrepancy to allow between the strength of the donor and the required strength after transplant has not yet been estimated. I suspect that a ratio of 2|mX1 would be a conservative estimate.


For transfer, however, the task is simpler. Here the muscle keeps its nerve supply. The muscle is only redirected, or “transferred.” We choose one that is about the right size to restore the balance of the hand that has been disturbed by paralysis. This is not the same as choosing a muscle of the same size as the one it is to replace. In most cases of paralysis, the sum of the strengths of all the muscles that remain will be substantially less than the original total. Therefore, to obtain a new balance for the hand, the surgeon should be content to replace only part of the strength of the muscles that have been paralyzed.


Fortunately, there is a good deal of flexibility in this choice because muscles fairly quickly adapt their tension output to the demand. A strong muscle will become weaker if it is not used, and a weak muscle will become stronger, up to a point, as long as it is used to its maximum tension capability. However, it will become stronger only if it is used, and used in phase. A muscle will not become stronger just by being placed in a situation that demands strength. It must be used daily and hourly to its maximum by active contraction. Passive lengthening may result from the activity of other muscles, but active contraction results from the patient’s recognition of the new function of the muscle. Here the therapist may be a great help. As an aid in the selection of a muscle of the right strength, the tables published by Brand, Beach, and Thompson may be used. At operation the relative diameters of exposed tendons serve as a good approximation. If one muscle is twice as strong as another, then the cross-sectional area of all its muscle fibers will be double that of the other, and the cross-sectional area of its tendon also will be about double that of the other tendon. This is true only of the preparalysis strength. The extent to which tendon diameters change after periods of paralysis or of hypertrophy of the muscle is not known.


Excursion


Another feature of a muscle that is transferred with it is its potential range of excursion. However, this statement needs to be qualified by defining terms. We may recognize three kinds of excursion: potential, required, and available.


If a muscle is freed from all its connective tissue attachments and the naked muscle is stimulated from its fully stretched position, it should contract through a distance that is approximately equal to the resting length of its individual muscle fibers. This is a basic quality of muscle fibers, depending on the number of sarcomeres they contain. We have called this the potential excursion of the muscle. However, in the intact limb, very few muscles are able to achieve their full potential range of excursion, either because of restrictions imposed by surrounding connective tissue or because the joints they control do not have the range of motion (ROM) that requires that much excursion.


Required excursion is determined more by joints than by muscles. It is the excursion that is needed to put the joints through their whole ROM. The extensor carpi radialis brevis, for example, has fibers that are about 6 cm long and therefore could potentially contract through 6 cm. However, the full ROM of the average wrist can be accomplished with about 3.5 cm. Thus, in the average hand, this muscle never uses more than 3.5 cm of excursion. Previously published lists of tendon excursions have been mostly estimates of what we now call required excursion.


Perhaps the most significant information needed by the surgeon is the available excursion, as Freehafer calls it. Freehafer measures this at operation after cutting the tendon distally. We think that excursion is permitted by the investing connective tissue. This available excursion varies from case to case and probably largely depends on the extent to which the patient has actually used the joints and muscles during the previous months. Connective tissue is responsive to the pattern of use. For example, people who have not previously done jogging or ballet dancing find that they cannot stretch their calf muscles far enough to run or dance effectively. By persistent stretching exercises, they finally obtain a larger range. They probably have not actually lengthened their muscle fibers; they have merely lengthened the connective tissue of the paratenon and perimysium. After such activity, their available excursion would have increased.


Available excursion may be measured at operation after the tendon to be transferred has been divided. It may be held at its end by a hemostat or stitch and pulled out to its full stretch. At this point, the muscle is stimulated by a tetanizing current while the movement of the tendon is measured. If the patient is awake, he or she may make the contraction voluntarily. The figure should be recorded for reference after surgery. Available excursion is transferred to the new site at operation only if the transfer involves minimal change of position and minimal dissection. Sometimes, a long-fibered (long potential excursion) muscle is transferred from a site where it had a short required excursion to a site where it has a long required excursion. In such a case, it will take time and active use to lengthen the connective tissue in and around the muscle and tendon so that the available excursion increases to match the new required excursion. If a tendon is to be widely rerouted and if the transfer is done through open wounds, the normal compliant connective tissue and paratenon are divided and will be replaced by scar. In many operations for tendon transfer, the final success or failure is determined more by the mechanical qualities of the paratendinous scar than by any other single factor. It is difficult enough for the patient to have to learn to use a muscle for a new action in an unfamiliar situation, but if that action cannot be accomplished until scar has been mobilized and lengthened, it may never be accomplished at all.


There is wide variation in the mechanical qualities of various types of connective tissue. Paratenon typically is easily stretchable. It has a long, low length-tension curve. This means that a great deal of lengthening results from very little tension. This situation permits nearly free tendon movement over a wide range of excursion. The common fatty areolar connective tissue that serves to fill spaces between structures in the body is not quite as compliant, but it also will lengthen with moderate tension and then, with repeated movement, will become modified into a kind of paratenon.


Fascia and retinaculum, fibrous septa, and scar all tend to have steep length-tension curves, lengthening only about 10% even under considerable tension. Thus, if a tendon comes to lie on a ligament that has been cut or scarified at surgery, the tendon may become united to the ligament through a collagen scar whose short fibers become parallel oriented by the pull of the tendon. Such fibers, only 1 or 2 mm long, would allow almost no tendon movement. If, instead, the transferred tendon is passed through a tunnel in loose fat and connective tissue, the scar that forms around the tendon will attach it only to the soft and compliant tissues through which the tendon tunneler has found its way. When traction is applied to that tendon, the new scar may not stretch but the surrounding fat and areolar tissue will stretch and move with the tendon through several millimeters. This early movement will allow the patient to sense the new action and use it. Thus the patient will continue to stimulate further muscle contraction and movement and further stretch and then lengthening and remodeling of the new paratendinous tissue.


Thus the concept of excursion of a tendon is complex and involves many factors. The true potential excursion, dependent only on the number of sarcomeres in each fiber, is not responsive to movement or active use; it is responsive to the tension in the resting fiber. Tarbary et al. have shown in experimental animals that if a muscle fiber is immobilized in a slack position it becomes shorter by loss of sarcomeres, and if it is immobilized in a stretched position, it becomes longer by the addition of sarcomeres, until neutral tension is achieved.


Thus there is no way that a patient or a therapist can make a permanent change in the length of a muscle fiber. If a hand is kept at rest (splinted) in a posture that keeps some muscles slack and others stretched, every muscle fiber will undergo cell activity, with whole sarcomeres being added to the stretched fibers and removed from the slack fibers until all are returned to their normal tension. When the hand is released and allowed movement again, the patient will find that it is difficult to return to the normal position of rest. It will take time until a reverse process of sarcomere adjustment restores fiber lengths to what they were.


If, while transferring a tendon, a surgeon makes the new attachment at a high tension, he or she might assume that the result will be a stronger muscle action. This is not so. As soon as the muscle is at a high resting tension, more sarcomeres will be added in series to each fiber until the resting tension is the same as all other muscles. The fibers will be longer but not stronger.




Mechanical Factors At Distal End Of Transfer


Leverage


All that a muscle can give to a joint is tension and excursion of the tendon. At the joint, this is turned into action according to the leverage. The actual movement of a joint around an axis is accomplished by “torque,” or “moment.” These two terms mean the same thing and are the product of force times leverage, or tension times moment arm. We all know that a lever enables a small force to move heavy objects ( Fig. 46-1 ). We also know that a heavy weight can be moved most easily by a small force if the force is applied far up the lever, away from the axis, or if the load is close to the axis. This increases the mechanical advantage. (Mechanical advantage is equal to the moment arm of the force divided by the moment arm of the load.) What we often forget is that the force has to move farther if its lever arm is long and that the load will move very little if its lever arm is short.





Figure 46-1


Equilibrium in lever system.


Now this concept must be translated into the terminology of tendons and joints. The leverage that a tendon has at a joint is the perpendicular distance between the joint axis and the tendon as it crosses the joint. The lever arm beyond that is the length of the bone or digit distal to the joint axis. Thus, in the body, almost all levers work with the force at the short end of the lever and the load at the long end. We are using a system of muscles that can produce enormous forces over rather short distances. The lever and pulley systems around joints are designed to take big forces with short ranges and make them effective over bigger ranges, with reduced force. There is no way to beat this system. The price of increased power is reduced range. I often hear surgeons say that they have found a way to increase the “strength” of a transfer at a joint. I rarely hear them mention the amount of excursion that has been used up, leaving less “strength” for other joints in the same sequence, or the fact that the transfer will now be effective over a smaller range.


One reason that surgeons usually do not try to work out the moments or leverages of the tendons they transfer is that they know it is so difficult to identify exactly where the axis of a joint is located. Therefore it is not possible at surgery to measure the perpendicular distance between the axis and the tendon.


However, there is a simple and practical method of estimating the moment arms of tendons at joints that will enable surgeons to know exactly how effective each tendon will be at each joint it crosses. The method is based on two rules of geometry ( Fig. 46-2 ). The first is that when a lever (bone) moves around an axis (joint), every point on the lever moves through a distance proportional to its own distance from the axis. The second rule grows out of the first and is an example of it ( Fig. 46-3 ). If a lever moves around an axis through an angle of 57.29 degrees, then every point on the lever moves a distance equal to its own distance from the axis. If a length of a radius is marked on the circumference of a circle and the two ends of that radius are joined to the center, the angle between them is called a radian and measures about 57.29 degrees ( Fig. 46-4 ). As a lever moves around an axis, a number of points on its length may be thought of as marking out a number of arcs of concentric circles. When the lever has moved a radian, every arc that has been described is the same length as the distance (radius) of that point on the lever from the axis. This becomes a way to relate angles to distances; angles of joint movement can be related to excursions of the tendons that cause the movement or are affected by it.





Figure 46-2


Pulley-wheel axis and lever (tendon and bone at joint). Every point on lever (bone) moves through distance proportional to distance from axis.



Figure 46-3


Rope moving off pulley has same moment arm in many directions and will move same distance for same angular movement of lever.



Figure 46-4


Radius along circumference of circle subtends radian at center.


In the stressful, time-dominated atmosphere of the operating room, nobody is going to measure tenths of a degree, or even single degrees. I find it useful to have in the operating room a metal triangle with angles of 30, 60, and 90 degrees ( Fig. 46-5 ). I also have a metal millimeter scale. Keeping a little tension on a tendon, I move a joint through 60 degrees, while my assistant measures exactly how far the tendon has moved. The excursion of the tendon that matches 60 degrees of joint movement is the same as the moment arm or leverage of that tendon at that joint. I often use 90 degrees of joint movement because I find it is easier to guess a right angle. In that case, the tendon excursion will be 1.5 times the moment arm ( Fig. 46-6 ). If I use only 30 degrees of joint motion, the tendon will move half the moment arm.



Apr 21, 2019 | Posted by in PHYSICAL MEDICINE & REHABILITATION | Comments Off on Mechanics of Tendon Transfers
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