1. A tendon is a strong, somewhat elastic, modified connective tissue which arises from a muscle and attaches to a bone by way of a fibrous or fibrocartilaginous tissue. When the muscle contracts, the tendon pulls on the bone causing a movement in a joint proximal to its attachment.
2. The tendon has a layer of epitenon on the outside which encloses multiple fasciculi composed of fibres, each fibre is made up of cross-linked fibrils which is in turn made up of collagen. The cross linking provides the tendons with their tensile strength. The tendon also has its blood vessels, nerves and lymphatics. Its internal lubrication is provided by a ground substance composed of large molecular carbohydrates combined with amino acids (glycosamine glycans).
3. Because a tendon must glide smoothly and also not bow string, it is covered with a variety of layers; the smooth epitenon, loose areolar tissue called paratenon which contains mucopolysaccharides to allow smooth gliding and it also passes through a variety of tunnels called tendon sheaths. These sheaths or tunnels prevent bow stringing. They are not continuous but are interrupted and occasionally give an appearance of rings which are called pulleys. The flexion of fingers which may begin at the distal phalanx can therefore be smoothly completed via all the proximal joints to make a fist without bow stringing.
4. In the hand, two bursae, each for the long flexor of the thumb and the flexors of the little finger, a palmar bursa for the flexors of the central three fingers and separate bursae beyond this central bursa for the long flexors of the central fingers along their length are an additional lubricating environment. The reasons for this peculiar arrangement lie buried in the evolutionary process.
5. The epitenon is a cellular structure and its invagination between the fascicles is named endotenon (also cellular). In addition, the tendon has distinct cells called tenocytes. The exact function of each of these cells is not known. The tendon also has a mesotenon which is a condensation of the soft tissue of the paratenon and carries blood vessels. Similar blood vessels also supply the tendon via viniculii located in cruciate structures near the pulleys. The tendon is also usually supplied by arterioles near its origin from the muscle and this supply can be called intrinsic. The tendon therefore has a dual blood supply. The tendon does not get any blood supply from its distal, bony attachment. The tendon is enveloped by the paratenon or the bursae by a process of invagination (as in the abdomen or thorax, parietal or visceral peritoneum or pleura) and the space created by invagination is filled with a fluid which has a role to play in both nourishing the tendons and in its gliding. Tendons which glide very near the bone and/or skin are also protected by bursae (e.g. the pre-patellar or the supra patellar bursa). The nerve supply to the tendon is almost completely afferent, relaying its impulses through muscular or peripheral nerves.
6. When a tendon snaps as in a closed injury, or is cut and retracts because of both its elasticity and the proximal muscular pull, up to a point allowed by its anatomy, it must suffer a loss of some or almost all of its extrinsic blood supply. Yet tendons heal when repaired (probably by virtue of their intrinsic supply). But grafts of tendons used as a bridge also heal at the repaired ends with a tensile function approaching that of a normal tendon system. These facts suggest that the tendon, with a very low rate of metabolism, survives with a diminished or an almost non-existent blood supply, and can heal when joined, either through its own cells or by way of a repair process which starts in its external environment.
7. If a tendon is sutured, after it has been divided as a part of an open wound, its repair at least partly, will be through an extrinsic process. As it is when the ends of a cut tendon are joined, a scar will form during the process of repair within its fascicles as well as its surroundings leading to adhesions and loss of gliding function which is not uncommon when tendons are repaired. Yet this surprisingly does not occur in all cases leading to two possible deductions.
a. That the external reparative process withdraws once the repair is achieved or
b. The tendons repair intrinsically on their own when joined, notwithstanding their reduced blood supply, by virtue of nourishment from the fluid surrounding them without taking recourse to the use of extracellular matrix deposited around them (for ECM or extracellular matrix, see previous chapter on wound healing).
8. This matter is not settled, certainly not through verifiable human trials but the weight of evidence may now be veering to the view that the intrinsic repair of tendons is very valuable to the final outcome and is indeed possible without any external contribution as confirmed by experiments on animals.
9. That a long flexor of a finger avulsed from its bony attachment or a mallet finger caused by avulsion of its osseous attachment of the extensor have by far the best results when their bony attachments are firmly secured is a common experience. The bone in these cases unites with vigour and the tendon in fact has not been repaired at all.
10. What flows from the above is the need to repair tendons in a manner which will approximate the epitenon of the cut tendons most meticulously to avoid as much as possible if not eliminate all together the extrinsic reparative process. Unfortunately the other ‘tension overcoming’ sutures, made of whatever substance used in repair of tendons, go through the epitenon at more than one point and also leave knots on the outer surface of the tendon, inviting a natural but a possibly complicating reaction from the surrounding tissue. This situation can improve only if a glue like substance is discovered which will hold the tendon together while overcoming its natural pull and must also not provoke any reaction in the surrounding tissue.
11. The technique or method of repairing a tendon has seen many variations over the last fifty years without significantly improving the outcomes of these repairs. What has changed the results dramatically towards the better is a paradigm shift towards a primary repair both for closed ruptures and open lacerations at all levels. This has been necessitated because of the earlier experience that to avoid primary repairs is to invite a collapse of the sheaths and the tunnels. Any reconstruction of these sheaths and tunnels invited more scars. This has particular reference to the hand because unlike in other parts of the body, particularly the foot, the movements in the hand need to be far more dexterous.
12. In the event, the division of the hand into zones to indicate if primary or secondary repairs should be undertaken has somewhat lapsed. There is no denying though that the area lying over the two proximal phalanges of the fingers presents difficulties. The sheath and the pulleys, the attachment of the split sublimis over the middle phalanx and the profundus traversing through this bifurcation is a crowded situation and any healing in this area with any extrinsic scar could be a recipe for poor results. On the other hand as mentioned earlier, a delayed repair invariably leads to a collapse of the system of tunnels and their sheaths which are crucial to the gliding motion of the tendon and also to prevent bow stringing. Because retunnelling the sheaths was invariably found to be difficult and any neo-reconstruction of the collapsed tunnels lead to adhesions, a meticulous primary repair of the profundus tendons has in the long run proved to be a much better option when both the profundus and the sublimis have been cut. In this scenario, the attachment of the sublimis to the bone is not removed but the rest of the tendon is excised to allow more space and play for the profundus tendon in the proximal part of the sheath. It helps to reduce scarring in this area by keeping the exposure in the finger to the minimum and to find the tunnels blindly by way of a probe or a rigid catheter and then thread the retracted tendon rather than doing a wide exposure. An ultrasound examination, particularly of the palm, can be very valuable to locate the proximal retracted end of the lacerated tendon and minimize the palmar exposure. In the primary repair, it is easier to get the tendons to their place of repair which might become very difficult after several weeks by which time they may get quite rigid, another reason for undertaking a primary repair.
13. In order that adhesions are minimized the total length of the tunnelled area can be judiciously reduced after ensuring that some gliding zones are kept intact and it is confirmed on the table that bow stringing is not occurring.
14. The results of primary repair of tendons are very good when the wounds are clean, sharp and small.
15. When the injury includes fractures, they must be rigidly stabilized so that primary healing of bones occurs and minimizes scars.
16. In larger contaminated wounds with avulsed and / or contused skin, which requires replacement with a flap, where the tendon sheaths are intact, yet the tendons are cut and have retracted, a tendon repair with a flap cover might be attempted if post operatively the hand can be maintained in a position necessary to take away the tension from the repaired ends of a tendon. This might be possible with a free flap or a distally based radial artery flap but is almost impossible with a distant two stage flap except perhaps when a cross finger flap will suffice. This particularly pertains to repairs of flexor tendons in the hand.
17. When amputated digits or the hand at the level of the wrist or below are reimplantable because the nature of the injury is sharp, tendon repairs are best done together with repair of nerves and vessels as well as rigid fixation of fractures.
18. When wounds are in the nature of avulsion crush injuries, and are contaminated, involve multiple planes and are ragged, whether they are accompanied by fractures or not, it is best to achieve flap cover and postpone reconstruction of tendons till after healing is complete.
19. When tendons are repaired secondarily, ideally a retracted tendon is located, pulled through the tunnels and sutured to its distal cut end.
20. When a retracted tendon has shrunk and cannot be pulled through, a tendon graft becomes necessary.
When wounds are more complicated (as in para 16 ) silicon rods are the best option.
21. Whether such rods can be placed at the time that flap cover is achieved will remain a matter of judgment and choice. A silicon rod achieves two important things. It creates a tunnel around it and its placement means that a tendon graft can be threaded through the tunnel with minimum exposure, temporarily attached to the rod as it is withdrawn. In effect, it entails two small exposures at the proximal and distal sites of repair.
Photographs of patient with use of silicon rod courtesy: Sudhir Warrier, Mumbai
22. A tendon graft is devascularised tissue and the nature of its survival is a matter of debate. While a necrosis of this graft (at its ends or along its length) is uncommon and it certainly has a fairly high survival rate with or without adhesions. Its unpredictable survival also means that its final functional length might shorten, allowing only restricted movement across joints across which it had been placed, and will produce a tenodesing effect. The functional nature and extent of this contracture in each individual case needs to be determined when one is considering whether re-exploration can be undertaken to reduce adhesions if any, or placing a new graft or a silicon rod.
23. In the surgery of tendon transfers, if the transferred tendon is not lying adjacent to the tendon to which it is transferred, and it requires extensive mobilisation (for e.g. extensor to a flexor or vice-versa), the transferred tendon is relatively devascularised and this might affect the results.
24. When adhesolysis in the hand needs to be undertaken it will need patience, must be done through properly planned incisions, preferably aided by magnification to avoid injury to nerves or vessels.
25. Incisions in the hand should as far as possible not violate natural creases in the wrist or the palm. In the fingers a mid neutral incision along the radial or ulnar side or a zig zag incision over the palmar surface of fingers give adequate exposure and result in good scars without any resultant contracture.
26. One of the changes that has accompanied primary repair of tendons is in the manner in which the fingers are mobilized post operatively. The idea behind this current method of mobilization is to keep the inter-phalangeal joints mobile without putting any strain on the repaired tendons. The technique involves immobilization of the hand by a dorsal plaster slab or a splint with the wrist at about 20° flexion and the meta carpo phalangeal joints at 90° flexion but the IP joints are kept free. The patient is advised to press on the fingers, effect passive flexion and then allow the fingers to extend by voluntary effort. This manoeuvre avoids stiffness of IP joints and also allows some tension free movement of the repaired tendons. This position is maintained for 3 weeks followed by active voluntary flexion while keeping the wrist flexed but with the metacarpo-phalyngeal joint free from the splints.
27. The plantaris or the Palmaris longus are two tendons frequently employed as tendon grafts because they are relatively dispensible, can be easily accessed and are of a dimension that can be threaded through the tendon sheaths of the hand. Of the two Palmaris longus, if present, is preferred particularly if it is on the same side on which surgery is done so that two operative fields are avoided. When more than one tendon graft is needed, up to four tendons, two each of the Palmaris and Plantaris can be harvested, though occasionally, a thick plantaris can be split vertically in a precise manner and can be used as two grafts.
28. Post operative adhesions in the hand, particularly after any form of reconstruction of a tendon, remain unpredictable. There is no verifiable animal evidence available to suggest that a pharmacological intervention will prevent formation of adhesions. The same applies to any form of radiation treatment or treatment with ultrasound waves.
29. This chapter as has been the practice in the past focuses more on the hand than other parts of the body because the hand is a vital, multi-dimensional organ as compared to the leg or the foot and also because injuries and repairs of tendons of other parts is also the province of orthopedic surgery. The hand is also prone to more intricate injuries.