Flexion of the digits is controlled by the flexor digitorum superficialis (FDS), flexor digitorum profundus (FDP), and flexor pollicis longus (FPL) muscles. The FDP originates from the proximal ulna and interosseous membrane and inserts into the distal phalanges to provide flexion at the distal interphalangeal (DIP) joints and contributes to proximal interphalangeal (PIP) joint flexion. It is also attached to the lumbricals in the palm, and this is important after flexor tendon division, as the proximal end of the FDP tendon will therefore not retract further proximal than the midpalm. The FDS originates from the medial epicondyle of the humerus and coronoid process of the ulna and inserts into the middle phalanges to provide flexion at the PIP joints. The FPL originates at the proximal radius, interosseous membrane, and the accessory head (known as Gantzer’s muscle) and arises from the coronoid process and the medial epicondyle of the humerus. It inserts into the distal phalanx of the thumb to provide flexion of the interphalangeal (IP) joint as well as the metacarpophalangeal (MCP) joint. Camper’s chiasm represents the union of the two slips of FDS over the distal aspect of the proximal phalanx after they have divided into two slips to pass either side of the FDP tendon just distal to the level of the MCP joint. The two slips of FDS insert separately to the middle phalanx. Knowledge of the anatomic position of the FDS tendons at the wrist is useful during spaghetti wrist repair, as this can help identify the tendons accurately. The index and little finger FDS tendons lie deep, not superficial to the ring and middle FDS tendons.1–3
2. Doyle JR, Blythe WF. Anatomy of the flexor tendon sheath and pulleys of the thumb. J Hand Surg Am 1977;2(2):149–151
3. Strickland JW. Development of flexor tendon surgery: twenty-five years of progress. J Hand Surg Am 2000;25(2):214–235 INNERVATION TO THE LONG FLEXORS OF THE HAND 2. Which of the following flexor muscles receives its innervation entirely from the anterior interosseous nerve? The anterior interosseous nerve is a branch of the median nerve in the forearm. It supplies the FPL, the pronator quadratus, and the radial two FDP tendons. The ulnar two FDP tendons are supplied by the ulnar nerve and the FDS are supplied by the median nerve directly. When not due to neuritis, anterior interosseous nerve palsy can occur secondary to compression of the nerve by the tendinous edge of the deep head of the pronator teres, the tendinous origin of FDS, or an abnormal accessory head of FPL (Gantzer’s muscle). Patients with this condition cannot flex the index DIP joint or thumb IP joint normally, resulting in an abnormal tip pinch grip. Furthermore, patients cannot make the “OK” sign where a closed ring shape is made using the index and thumb. There is no sensory deficit present because the anterior interosseous has no cutaneous sensory branches.1–3
Tendon nutrition in the digits depends significantly on diffusion from the synovial sheath. The other sources of nutrition listed each make a variable contribution to tendon nutrition, but the vascular network within the digital flexor tendons is sparse. The bony insertion and musculotendinous junction blood supply is limited to around 1 cm of tendon length at each site, with the remaining blood supply arising from the vincula. In the forearm and proximal digit, there is also a vascular supply from segmental vessels in the paratenon.1 Axial blood supply within the flexors tends to be located in the dorsal portion of the tendon; so, many surgeons prefer to place sutures in the volar portion during repair in the hope of maximizing blood supply. In contrast, there is some evidence that placing the core sutures dorsally within the tendon may be biomechanically advantageous.2,3
The unique feature of the lumbrical muscles is that they each originate and insert into tendon without inserting or originating from bone at either end. There are four lumbricals, with the index and long (middle) finger lumbricals being unipennate and the ring and small (little) finger lumbricals being bipennate. The lumbricals originate from the FDP tendons in the midpalm and insert onto the lateral bands of the extensor mechanism. This means they serve to flex the MCP joints while extending the PIP and DIP joints (intrinsic positive position). The lumbricals do not share a common nerve supply. The ulnar two lumbricals receive innervation from the ulnar nerve and the radial two lumbricals are innervated by the median nerve.1,2
There is a universal nomenclature for flexor tendon injuries first described by Verdan.1,2 This categorizes the zone of injury into five areas: Zone I: Distal to insertion of the FDS tendon Zone II: From proximal edge of A1 pulley to FDS insertion (a.k.a. “no man’s land”) Zone III: From distal end of the carpal tunnel to proximal edge of A1 pulley Zone IV: Within the carpal tunnel Zone V: Proximal to the carpal tunnel This has clinical relevance as the approach to repair and functional outcome will differ according to the zone involved. Zone II has been described as “no man’s land” because historically the results following repair were poorest in this zone. This zone is complicated because there are multiple flexors within the tight space of the flexor sheath, i.e., two FDS slips and one FDP. Therefore, achieving a functional repair without damage to the vincula blood supply, the sheath itself or the pulley system is challenging. Beyond this, achieving satisfactory rehabilitation without adhesions and tendon re-injury are also difficult.3
The thumb has five flexor zones, but these are different to the flexor zones for the other digits: • Zone T I: Distal to IP joint • Zone T II: From proximal edge of A1 pulley to IP joint • Zone T III: Over thenar eminence • Zone T IV: Within the carpal tunnel • Zone T V: Proximal to the carpal tunnel The anatomy of the thumb flexor mechanism differs to that of the digits in a number of ways. First, there is only a single flexor (the FPL); so, even a zone II injury involves only one tendon repair and thereby reduces some of the complexities of injury in this area despite it running within the flexor sheath. Second, there is only a single IP joint and some degree of stiffness or flexion of the thumb IP joint is far better tolerated than it is at PIP joint in the fingers. The pulley system is different too with just two annular pulleys and one intervening oblique pulley. The A1 pulley is over the MCP joint and the A2 pulley is over the IP joint. The pulleys should ideally be preserved during repair to avoid bowstringing, with the oblique pulley being most important.1
The management of partial flexor tendon repairs will depend on a number of factors such as the location, the flexor tendon involved, and the type of injury sustained. It will also be affected by surgeon and patient preferences. While there is some biomechanical evidence that shows larger tendon lacerations can still withstand protected active mobilization without repair, most agree that tendon lacerations involving more than 50% of the tendon diameter should be formally repaired with core and epitendinous sutures. Lesser divisions should be smoothed off or repaired with simple sutures to avoid the edges catching and to ensure the tendon ends are neatly apposed.1–4
Zone I closed avulsion injuries of the FDP were classified by Leddy and Packer in 1977 into three categories based on the presence of a bony fragment on radiograph, the blood supply, and the position of the retracted proximal tendon end.1 Additional fourth and fifth categories were subsequently added.2 The classification is useful to guide management. The patient described has a type II injury based on the bony fragment position and should undergo an open repair using a pull-out suture or bone anchor device. The modified Leddy-Packer classification includes the following: Type I: The FDP tendon retracts into the palm with rupture of both vincula. Type II: The FDP tendon avulses with a small fragment of distal phalanx, the long vinculum remains intact, and the tendon retracts to the level of the PIP joint (A3 pulley). Type III: A large bony fragment is avulsed with the tendon and is prevented from retraction beyond the middle phalanx by the A4 pulley. Type IV: An avulsion fracture of the distal phalanx combines with tendon avulsion from the fragment, along with tendon retraction. Type V: A bony avulsion of the FDP is coupled with a distal phalanx fracture (either intra-articular or extra- articular).
Bone anchor devices may be used successfully in primary tendon repairs. The outcomes using bone anchors are comparable to pull-out suture techniques. The recommended technique is to insert either a mini-Mitek or two micro-Mitek anchors at a 45-degree angle from distal/volar to proximal/dorsal, while taking care not to violate the DIP joint surface or nail bed.1,2 When using a pull-out suture technique where the core suture is passed externally over the nail, a monofilament such as nylon or polypropylene is preferred. Braided sutures glide less well, are difficult to remove, and are also more prone to infection. FDP avulsion may also be repaired by drilling a small hole transversely through the distal phalanx for the core suture to pass through, enabling a fully internal repair. Although the normal recommendation for core suture repair is that there is 10 mm of tendon proximal and distal to the repair, a distal stump of 5 mm is considered sufficient for standard core suture repair of zone I flexor tendon injuries. For most flexor tendon injuries, repair within a week of injury is recommended. However, some injuries that involve avulsion of the tendon insertion from the distal phalanx, with minimal proximal retraction (due to either intact vincula or a bony fragment anchored against a pulley) may still be repaired after several weeks. A delay in repairing a retracted tendon can lead to shortening, a quadriga effect, and constriction of the flexor sheath.
The main benefit of using an epitendinous suture is that it adds between 10 and 50% additional strength to the repair. In addition, it helps smooth the repair and improve gliding within the flexor sheath. A meta-analysis of complications after flexor tendon repair also suggested that the rate of reoperation is reduced by 84% if an epitendinous suture is used.1 There is debate surrounding the optimal positioning of core sutures with volar positioning considered to be best for tendon vascularity and dorsal positioning considered more biomechanically favorable during active flexion. In reality, many tendons are sufficiently thin that reliably placing the suture anywhere other than centrally is challenging. A minimum of four-strand (not two) core suture repair is required in order to commence early active mobilization. This may be undertaken as a two-strand core suture combined with a two-strand mattress, as two separate two-strand core sutures, or as a single four-strand core suture repair. There is no definitive evidence that a four-strand single-knot technique is superior to two adjacent two-strand core sutures. Some surgeons prefer to use a six-strand technique when the tendon diameter is sufficient to allow this. The core suture should ideally have 10 mm (not 5 mm) proximal and distal to the repair. A Pulvertaft weave is not used in primary tendon repair in the digit, as it would entail significant tendon shortening and would create too much bulk to fit within the tendon sheath. It is reserved for tendon transfer cases outside a tendon sheath, such as the extensor indices transfer to extensor pollicis longus.1–4
The most important pulley to preserve in the thumb is the oblique pulley. This is done to avoid bowstringing. Although the use of an epitendinous suture can add a further 10 to 50% strength to a flexor tendon repair, it is not necessary for zone V repairs. A four- or six-strand core repair is advocated for most injuries. Zone II injuries have the poorest functional outcomes. This zone was previously termed “no man’s land” because of the difficulties with achieving satisfactory repair and outcomes. The carpal ligament should be repaired in zone IV injuries to prevent bowstringing during rehabilitation. In most cases, it is best to repair both FDS and FDP. However, many people do not have an FDS in the little finger and the FDS may not always be repaired in zone II if it is very small and likely to interfere with satisfactory movement of the FDP repair. Where there is a problem with bulk, some surgeons opt to repair only one slip of FDS and trim the other in zone II.1–3
This is likely to be a Leddy-Packer type I FDP tendon avulsion injury, with the tendon end lying in the palm. The FDP tendon does not usually retract to the wrist because of the lumbrical attachments. In this scenario, the window for successful direct repair is reduced to 7 to 10 days compared to type II and III injuries, because the tendon has lost both the vincula and synovial nutrient supplies, and the flexor sheath is largely empty and able to shrink down. There is a slim chance that this will actually be a type II injury, with an intact vinculum longum and the tendon end at the A3 pulley, in which case a direct repair might still be possible. However, this is much less likely, and beyond 4 weeks there may have been sufficient musculotendinous contraction to lead to an excessively tight repair, causing a troublesome quadriga effect. A quadriga effect occurs when shortening or tethering of one FDP tendon reduces power and excursion in the remaining three healthy tendons, as described by Verdan.1 If a primary repair is not possible in a delayed presentation, it is not advisable to perform an immediate palmaris graft, because the sheath is usually scarred and contracted. Even if there is adequate space for a primary graft, the risk of adhesions is much higher than if a staged approach is used. A silicone (Hunter) rod spacer is usually placed to generate a pseudosheath for subsequent tendon grafting from 8 to 12 weeks later.2–4
Immediate repair is not recommended in bite wounds because of contamination and an element of crush injury. A thorough debridement and washout in addition to antibiotic therapy is preferable, followed by a delayed primary tendon repair if the wound is clean at 48 to 72 hours. When exploring an open tendon injury, serous fluid in the sheath is not uncommon; however, turbid or frankly purulent fluid is a contraindication to immediate repair. In the other scenarios, it is optimal to repair the divided tendon as soon as possible after presentation.1,2
The patient in this scenario has sustained an injury in flexor tendon zone III, which lies in the palm distal to the carpal tunnel until the distal palmar crease and MCP joint. There are two possible explanations for his pain on flexion. Either there is a partial flexion tendon injury or a hematoma from the injury causing discomfort. The safest management is to proceed with formal exploration to confirm the nature of the injury. Although many partial flexor tendon injuries do not require repair, exploration is warranted, because the tendon may subsequently rupture or cause triggering without repair.1–3
The patient in this scenario is at risk of flexor carpi radialis and radial artery injury given the anatomic site involved, but clinically these structures do not appear to be injured. The chances of her sustaining an isolated FDS little finger injury are slim given the site of injury. It is far more likely that she has a congenital absence of the FDS; this occurs in approximately 20% of the population. It is important therefore to first assess the contralateral limb, since the condition may well be bilateral. It is good to do this as a standard practice when assessing hand function. In this scenario, consideration should still be given to formal wound exploration because glass injuries often penetrate to the bone. Where there is no functional loss and therefore no concern about tendon retraction of the forearm, local anesthesia is a reasonable option for this. Hand therapy is not indicated in this scenario given the current functional state. Ultrasound investigation can be useful to assess damaged tendons or the thumb MCP joint ulnar collateral ligament but would not be indicated as a first-line investigation in this scenario.1–3
The patient in this scenario displays the Linburg-Comstock anomaly, in which there are attachments between the FPL and index FDP tendons in the carpal tunnel. This occurs in up to a third of the population and will have been present before the tendon repair but may not have been recognized or documented. Where this condition is associated with pain, i.e., it becomes a functional problem, it is described as Linburg-Comstock syndrome.1–3 Adhesions occur in approximately 4% of all flexor tendon repairs and can result in limited active range of motion caused by scar tissue forming between tendon and surrounding structures.4 Wartenburg’s sign relates to a situation where the small finger is held in abduction secondary to unopposed action of the extensor digiti minimi and may be due to ulnar nerve injury with intrinsic muscle weakness. Absence of the little finger FDS is demonstrated by an inability to flex the PIP joint when the adjacent digits are held extended to eliminate FDP function.
Precise postoperative care after flexor tendon repair is critical to achieving a good outcome. The patient must be placed in a dorsal blocking splint to prevent excessive extension of the repair. The wrist is commonly placed in neutral or slight flexion, as this weakens the flexor tendons and can reduce the risk of postoperative rupture. However, many surgeons prefer slight wrist extension, as this is a more natural posture for digital flexion and is thought to improve range of glide and reduce excessive strain in attempted active digital flexion.1 The dorsal blocking splint should usually maintain the MCP joints in approximately 60 to 70 degrees of flexion. There is a wide range of preferred MCP joint splint angles reported (20–90 degrees); however, most authors agree that the IP joints should be able to straighten fully, otherwise flexion contractures will develop. Trying to force all four MCP joints into 90 degrees of flexion often results in an MCP joint angle of around 70 degrees and unintended flexion at the IP joints. Most techniques do not require elastic band use in conjunction with the splint. The Kleinert protocol uses elastic band traction to facilitate active extension and passive flexion after repair. Other techniques rely on early active mobilization and splinting alone.2,3
The most commonly re-ruptured flexor tendon is the FPL, but ring and little finger FDP tendons also have high rates of re-rupture. The overall rate of re-rupture after primary flexor tendon repair is around 5% (not 15%). Although re-rupture can be due to poor surgical technique during the repair, the main factors tend to be related to the postoperative care, such as patients removing the splint or not complying with rehabilitation advice. Contractures occur in around 20% of cases, and again this is affected by the rehabilitation and aftercare provided and patient compliance. Few patients require joint release (capsulotomy) for managing this complication. Tenolysis may be required in patients with adhesions that are unresponsive to focused hand therapy. These patients typically display an intact tendon repair with good passive range of motion but poor active range of motion.1