Flexor Tendon Anatomy:

Each finger has two flexor tendons:

• Flexor digitorum profundus (FDP): attaches to the distal phalanx and flexes the distal interphalangeal (DIP) joint.
• Flexor digitorum superficialis (FDS): splits into two slips and attaches to the middle phalanx, flexing the proximal interphalangeal (PIP) joint​ ncbi.nlm.nih.gov. (Only the FDP crosses the DIP joint.) The thumb’s long flexor is the flexor pollicis longus (FPL), which flexes the thumb IP joint ​ncbi.nlm.nih.gov.

All these tendons run through fibro-osseous tunnels in the hand, stabilized by a system of pulleys. There are five annular pulleys (A1–A5) in each finger; the A2 and A4 pulleys are especially important to prevent tendon bowstringing. The thumb has a similar pulley system (two annular pulleys and an oblique pulley) ncbi.nlm.nih.gov.

Vascular Supply:

Flexor tendons have limited blood supply. Within the sheath, small vessels from the digital arteries reach the tendons via the vincula (one long and one short vinculum for each tendon) wheelessonline.com. These structures provide nutrition and also act as tethers to limit proximal retraction of a cut tendon​ wheelessonline.com. However, a significant portion of tendon nourishment comes from diffusion of synovial fluid in the sheath. Tendon healing involves both intrinsic (internal tendon cell activity) and extrinsic (influx of cells from the sheath) processes. Excessive extrinsic healing leads to adhesions (scar tissue binding the tendon to the sheath) ​ncbi.nlm.nih.gov, which can impair gliding. This is why managing healing to minimize adhesions is critical.

Zones of Injury:

Flexor tendon injuries are classified into five zones (Verdan’s classification) based on location​ pmc.ncbi.nlm.nih.gov:

• Zone I: distal to the FDS insertion (fingertip to mid-middle phalanx) – contains only FDP tendon. (Injuries here often involve FDP avulsion, a “jersey finger.”)
• Zone II: from the FDS insertion to the proximal edge of the A1 pulley (distal palmar crease) – contains both FDS and FDP within the fibrous sheath (historically called “no man’s land” due to past difficulty in repairing these).
• Zone III: palm region (A1 pulley to distal edge of the transverse carpal ligament) – often associated with neurovascular injuries because tendons are near nerves/arteries here.
• Zone IV: within the carpal tunnel – multiple tendons in a tight space (injuries here often also involve the median nerve).
• Zone V: forearm (proximal to the carpal tunnel to the musculotendinous junctions) – may involve injuries to flexor tendons at their origin and often accompanied by major nerve or vessel injuries.

The thumb is divided into zones T1–T5 (T1 = thumb tip, T5 = forearm). Notably, Zone II in the fingers (and Zone T2 in the thumb) has both FDP and FDS in one sheath and is the most common area for flexor tendon lacerations​ ncbi.nlm.nih.gov. It also historically had the worst outcomes due to adhesions – hence the “no man’s land” label – although modern techniques have improved results.

Mechanisms of Injury:

Most flexor tendon injuries are caused by open lacerations to the volar (palmar) hand or fingers (e.g., from knives, glass)​ncbi.nlm.nih.gov. These sharp injuries can partially or completely sever the tendons. Avulsion injuries are another mechanism: a sudden forceful extension of a flexed finger (as in grabbing a jersey in sports) can tear the FDP tendon from its insertion – this is the classic jersey finger injury, most often involving the ring finger​ ncbi.nlm.nih.gov. Less commonly, crush injuries can rupture or injure tendons (often with fractures and soft tissue damage), and degenerative/attritional ruptures can occur in conditions like rheumatoid arthritis (where chronic inflammation weakens the tendons).

When a flexor tendon is completely lacerated or avulsed, the patient loses the ability to actively flex the affected joint. Partially cut tendons may still flex but usually with pain, and they are at risk of rupturing fully if untreated. Additionally, a partially cut tendon may catch on a pulley, causing a locking or triggering sensation.

Clinical Presentation

Patients with a flexor tendon injury typically present with inability to bend the finger at the affected joint. Key features include:

Loss of flexion: If the FDP is completely cut or avulsed, the patient cannot flex at the DIP joint; if the FDS is cut (and FDP intact), loss of PIP flexion may be noted. With both FDS and FDP lacerated (as in a Zone II injury), the finger rests in an extended position at PIP and DIP and cannot be flexed actively ncbi.nlm.nih.gov. The normal slight flexed posture of the finger (“flexor cascade”) is disrupted. The fingertip may drop into extension if the FDP is detached (inability to keep the DIP flexed).

• Pain: Patients often have pain in the injured finger, especially when attempting to flex it. A partial tendon tear may present with pain and limited bending rather than complete loss of motion ncbi.nlm.nih.gov.
• Visible injury: In open injuries, there is usually a cut on the palmar side of the hand or finger. The wound may be deep, and you might even see a tendon end in the wound. There can be bleeding and swelling. In a closed avulsion (jersey finger), there is no open wound, but there might be bruising, and the finger shows the loss of DIP flexion.
• Retracted tendon “lump”: In an FDP avulsion, the torn tendon may retract toward the palm. Patients with a jersey finger often have a tender lump in the palm or at the base of the finger, which is the retracted tendon end or a bony fragment ncbi.nlm.nih.gov.
• Associated injuries: Because volar finger cuts can also injure the digital nerves and arteries that run alongside the tendons, there may be numbness in the fingertip (if a nerve is cut) or impaired capillary refill (if an artery is cut). Always assess sensation on both sides of the finger and check fingertip perfusion.

Physical Exam Tests:

A careful hand exam will confirm the diagnosis:

• Tenodesis effect: Observe the finger posture with wrist motion. Normally, when the wrist is extended, the fingers passively flex (and when the wrist is flexed, the fingers passively extend) due to tension in the tendons. If a finger’s flexor tendon is ruptured, that finger may remain extended even when the wrist is extended orthobullets.com ncbi.nlm.nih.gov. For example, with an FDP tear, as the wrist is extended, the DIP of that finger will not flex as it normally would. This absence of the normal tenodesis flexion indicates tendon discontinuity.
• Isolated FDP test: Stabilize the PIP joint in extension and ask the patient to flex the fingertip. This isolates the FDP (since the FDS cannot act with the PIP held extended). Inability to flex at the DIP joint indicates an FDP tendon injury to that finger ncbi.nlm.nih.gov.
• Isolated FDS test: Hold the other fingers fully extended (this prevents their FDP tendons from contributing, due to the common muscle belly of FDS). Then ask the patient to flex the injured finger at the PIP. Failure to flex the PIP joint suggests the FDS is cut ncbi.nlm.nih.gov. (Keep in mind some people lack an independent FDS to the small finger.)
• Neurovascular exam: Test sensation in the finger (two-point discrimination on the pulp of the finger) to check for digital nerve injury ncbi.nlm.nih.gov. Also check capillary refill in the pulp. If there is numbness or tingling in the fingertip, suspect a nerve laceration until proven otherwise. An Allen test can evaluate arterial supply if needed.

In summary, the combination of mechanism, loss of function, and these exam findings usually makes the diagnosis of a flexor tendon injury clear on clinical grounds.

Flexor tendon injuries can be managed either conservatively or with surgery, but the majority of significant injuries (complete tendon ruptures) require surgical repair for any meaningful return of function. The goals of treatment are to re-establish a continuous tendon (if cut) and maximize the glide of the tendon through the sheath (minimizing scar adhesions).

Conservative Management:

Non-surgical treatment is reserved for select cases:

• Partial lacerations: If the tendon is less than ~50% cut and the finger can still flex against resistance, one can treat with splinting and careful observation surgeryreference.aofoundation.org. The finger is typically immobilized in a slightly flexed position (to relax the tendon) for around 3-4 weeks to allow the injured fibers to heal. During this period and afterwards, therapy is used to gently restore motion. Small partial tears can heal, but there is a risk of the tendon rupturing if the injury is underestimated.
• Patient factors: If a patient has a contraindication to surgery (such as severe medical comorbidities or a local infection that makes immediate repair unsafe), one might temporize with splint immobilization and plan for a delayed repair or tendon graft later. In some very low-demand patients (elderly or those willing to accept loss of motion), a partial tendon injury might be managed without surgery, but this is uncommon.

Conservative treatment has limited indications because an untreated complete tendon rupture will result in permanent loss of flexion. The downsides of non-operative management include possible tendon scarring to the surrounding tissue (causing a stiff finger) and risk of late rupture. Thus, complete lacerations or large partial tears (>50%) are generally managed surgically.

Surgical Treatment:

The standard of care for a fully lacerated flexor tendon is surgical repair. Timing of surgery can be:

• Primary repair: performed within the first 24–48 hours (up to a few days) after injury. This is ideal for clean cuts. If the wound is clean, primary repair should not be delayed – early repair is associated with easier tendon mobilization and better outcomes ncbi.nlm.nih.gov.
• Delayed primary: if the injury is dirty or the patient has other issues, repair can be done up to ~10-14 days after injury (once the wound is clean and the patient stable). Repairs done within the first 2 weeks after injury can still achieve results comparable to immediate repair ncbi.nlm.nih.gov.
• Secondary repair/reconstruction: for injuries older than ~2 weeks, the tendon ends will have retracted and scarred in place. A direct end-to-end repair at this stage is often not possible or has poor outcomes ncbi.nlm.nih.gov. In these cases, a two-stage tendon reconstruction is often done: first a silicone rod is implanted to preserve the sheath (Stage 1), and a few months later a tendon graft (often from the palmaris longus) is threaded through that sheath and sewn in to replace the tendon (Stage 2). Alternatively, if the distal tendon stump is long enough, a one-stage graft or tendon transfer may be performed. These secondary procedures are complex, so whenever possible, primary repair is preferable.

Surgical Repair Technique: In surgery, the two cut ends of the tendon are identified (often they retract into the palm or arm, especially if vincula are ruptured ncbi.nlm.nih.gov). The surgeon brings the ends together and uses strong sutures to stitch them end-to-end. Key points of modern flexor tendon repair include:

• Using a multistrand core suture technique: Traditional repairs used two-strand sutures (e.g., the classic modified Kessler stitch), but these have relatively low strength. Current best practice is to use at least a 4-strand core suture (often 4 or 6 strands crossing the repair) for improved strength ncbi.nlm.nih.gov. Common techniques include modified Lim-Tsai, Adelaide, or cruciate 4-strand, and various 6-strand configurations. A stronger repair allows for earlier motion in rehab. (Notably, a recent trial showed that both 4-strand and 6-strand repairs in Zone II had similarly high success rates pmc.ncbi.nlm.nih.gov, so what matters is a well-done technique with sufficient strands.)
• Adding an epitendinous suture: This is a fine running suture around the outside of the tendon repair. It smooths the juncture and significantly increases the repair strength (by about 50%) ncbi.nlm.nih.gov. Typically a 5-0 or 6-0 Prolene is used for this running stitch after the core sutures are tied.
• Minimizing bulk and friction: Especially in the tight Zone II sheath, too much suture material or repairing both slips of FDS can create a bulky repair that won’t glide. Surgeons often repair only one slip of the FDS (or even leave FDS unrepaired) if both were cut, focusing on the FDP repair ncbi.nlm.nih.gov. This maintains PIP flexion strength but reduces crowding in the sheath. Additionally, tight pulleys (usually A2 or A4) may be partially released (vented) to allow the repair to glide more easily if needed ncbi.nlm.nih.gov. Care is taken to balance a strong repair with a smooth, gliding repair site.
• Tendon-to-bone repair: In Zone I avulsion injuries (jersey finger), the FDP tendon must be reattached to the distal phalanx. This is often done using a small suture anchor in the bone (which secures the tendon end to the bone) ncbi.nlm.nih.gov. Older methods use a pull-out wire or button. If a piece of bone avulsed with the tendon, sometimes a screw or anchor can fix that bony fragment back to the distal phalanx. These repairs should be done as early as possible (Type I avulsions within a week) to prevent tendon degeneration ncbi.nlm.nih.gov.

After the repair, the surgeon will test the finger through gentle range of motion to ensure the tendon glides and the repair holds. The wound is then closed, often leaving the skin open or loosely closed if there was contamination (to avoid infection).

Postoperative Rehabilitation

After repair, the hand is placed in a protective dorsal blocking splint (wrist and fingers flexed) to prevent the repaired tendons from being stretched. Early motion is initiated to prevent adhesions – often passive flexion exercises (e.g. the Duran protocol) starting in the first few days, or even gentle active “place and hold” flexion if the repair is strong asht.org. Motion is carefully guided by a hand therapist. Over the first 4–6 weeks, the patient performs controlled range of motion within the splint’s protection. By around 6 weeks post-op, the tendon has healed enough to begin more active use and weaning off the splint. The exact rehab plan can vary (early passive mobilization vs. early active mobilization protocols), but the principle is the same: allow tendon glide to minimize adhesions, while protecting the repair from excessive force. Early passive motion (Kleinert or Duran protocol) has slightly lower risk of repair rupture, whereas early active motion (if the repair is robust) can yield better final range of motion asht.org asht.org. Adherence to therapy is crucial; the patient must not use the hand for any strong grip until the tendon is sufficiently healed (usually around 10-12 weeks post-op for full strength.

Complications

Flexor tendon repairs carry several potential complications. Adhesions are very common – scar tissue can bind the tendon to the surrounding sheath, leading to limited flexion despite the tendon being intact. If significant, a secondary tenolysis (surgery to cut adhesions) may be needed once healing is mature. Rerupture of the tendon repair can occur (reported in ~4-10% of cases, depending on protocol) asht.org, especially if the rehab is overly aggressive or the repair was weak; a ruptured repair usually requires re-operation. Joint stiffness (especially PIP joint contractures) can result from prolonged immobilization or adhesions. Bowstringing can occur if important pulleys (A2, A4) were lost and not reconstructed – the tendon will bow forward when the finger flexes, causing loss of flexion power; this may require later pulley reconstruction. Attention to surgical technique (preserving or reconstructing pulleys, strong multi-strand repair) and a balanced therapy protocol (early motion but protected) helps minimize these complications orthobullets.com orthobullets.com.

Key Points

• Flexor tendon injuries often result from lacerations (e.g. knives, glass) or avulsions (e.g. jersey finger) and lead to loss of active finger flexion.
• Anatomy: FDP inserts on the distal phalanx (flexes DIP); FDS on the middle phalanx (flexes PIP). Both tendons run in a sheath with pulleys (A2 and A4 are crucial to prevent bowstringing). Blood supply is via small vincula wheelessonline.com.
• Clinical exam: Check the tenodesis effect – when the wrist is extended, fingers should passively flex; a finger that stays extended may have a cut tendon ncbi.nlm.nih.gov. Test FDP by isolating DIP flexion (hold PIP straight) and FDS by isolating PIP flexion (hold other fingers straight).
• Zones: Flexor tendon injuries are categorized by zones I–V pmc.ncbi.nlm.nih.gov. Zone II (FDS/FDP within the finger) is the hardest to treat (historically “no man’s land”) due to the high risk of adhesions.
• Diagnosis: Usually clinical. X-ray can detect avulsion fragments or foreign bodies; ultrasound can show tendon continuity ncbi.nlm.nih.gov. Always assess for concomitant nerve or artery injury.
• Treatment: Complete tendon ruptures almost always need surgical repair. Partial tears <50% may be treated with splinting surgeryreference.aofoundation.org, but larger tears should be repaired to restore function. Aim for early repair (within 1–2 weeks) for best results.
• Surgery: Modern flexor tendon repair uses a multi-strand core suture technique (at least 4 strands) plus a running epitendinous suture ncbi.nlm.nih.gov ncbi.nlm.nih.gov. In Zone II, often only one slip of FDS is repaired (to reduce bulk) ncbi.nlm.nih.gov. If the FDP avulsed from bone, reattach it (often with an anchor) ncbi.nlm.nih.gov.
• Rehabilitation: Early controlled motion is key to preventing adhesions. Typically use a dorsal blocking splint and begin passive flexion or gentle active flexion in the first week or two asht.org. Gradually increase motion; avoid full force use until ~8–12 weeks.
• Complications: Adhesions (scar binding tendon) can limit motion – may require tenolysis. Rupture of the repair (4–6% risk asht.org) causes loss of flexion again – needs re-operation. Joint stiffness is common, and bowstringing can occur if pulleys are lost. Diligent therapy helps mitigate these.

 

Practice Questions

Question 1: A 25-year-old rugby player grabs an opponent’s jersey and feels a pop in his ring finger. He cannot flex the fingertip of that finger. On exam, there is no wound, but the ring finger’s DIP joint stays extended and there is a tender lump in the palm. PIP motion is intact and other fingers flex normally.

Which of the following statements regarding this injury are True or False?

1. This injury is an avulsion of the flexor digitorum profundus (FDP) tendon, commonly called a “jersey finger.” (T/F)
2. The ring finger is the most frequently involved digit in this type of injury. (T/F)
3. In this injury, the FDP tendon is likely retracted into the palm and the blood supply through the vincula is disrupted. (T/F)
4. Treatment usually involves surgical reattachment of the tendon to the distal phalanx (often using a suture anchor). (T/F)
5. If the flexor digitorum superficialis (FDS) tendon is intact, the patient will still be able to flex the DIP joint of the injured finger. (T/F)

Answers and Explanations:

1. True. This mechanism (forceful finger extension during a tackle) classically causes an FDP avulsion – a jersey finger. The patient loses active DIP flexion in that finger (the FDP is torn from its insertion) and a retracted tendon end may form a mass in the palm ncbi.nlm.nih.gov.
2. True. The ring finger is the most commonly affected digit in jersey finger injuries ncbi.nlm.nih.gov. It is more exposed during grip (being the weakest finger) and tends to absorb the force, making it prone to this avulsion.
3. True. In a Type I FDP avulsion (no bone fragment), the tendon retracts into the palm and the vincula are usually ruptured, depriving the tendon of its blood supply ncbi.nlm.nih.gov. This is why such injuries require urgent surgical repair (within ~1 week) – without blood supply, the tendon begins to degenerate. The tender lump in the palm in this case is the retracted tendon.
4. True. Surgical reattachment of the tendon to the distal phalanx is required to regain DIP flexion. A small suture anchor drilled into the base of the distal phalanx is a common method ncbi.nlm.nih.gov. Without surgical repair, the finger would permanently lack active flexion at the DIP joint.
5. False. The FDS can flex the PIP joint but cannot flex the DIP joint (only the FDP inserts on the distal phalanx). In an FDP avulsion, the fingertip cannot be flexed at all. The intact FDS does not restore DIP flexion, so the patient will still be unable to bend the fingertip. Therefore, surgery is needed to restore DIP flexion.

Question 2: A 30-year-old chef sustains a deep volar laceration to his left index finger at the proximal phalanx level. He cannot flex the index finger at the PIP or DIP joints, though other fingers flex normally. The radial side of that index finger is numb (indicating a digital nerve injury). He is taken for surgical repair.

Regarding this scenario (Zone II flexor tendon injury of the index finger), determine whether the following statements are True or False:

1. Zone II extends from the FDS insertion in the middle phalanx to the A1 pulley in the palm; it contains both FDP and FDS tendons in the sheath. (T/F)
2. Both the FDP and FDS tendons are likely lacerated in this injury. (T/F)
3. Primary surgical repair in this zone typically involves a multi-strand core suture technique, and often only one slip of the FDS is repaired to minimize bulk. (T/F)
4. After flexor tendon repair, the finger is immobilized in full extension for 6 weeks to protect the repair. (T/F)
5. Adhesion formation is a major concern in Zone II, which is why early controlled motion is recommended after repair. (T/F)

Answers and Explanations:

1. True. Zone II is defined as the region from the FDS insertion (mid-middle phalanx) to the distal palmar crease (entry of the flexor sheath at the A1 pulley) pmc.ncbi.nlm.nih.gov. In this zone, the FDP and FDS tendons lie together within a tight fibro-osseous sheath. This patient’s laceration at the proximal phalanx is indeed in Zone II – the classic “no man’s land” with both tendons in one sheath.
2. True. Inability to flex at both the PIP and DIP joints implies that both the FDS and FDP tendons to the index finger have been completely cut. In a Zone II injury like this, a deep laceration often transects both tendons. (The numbness on the radial side of the finger also indicates a likely laceration of the radial digital nerve, given their close proximity in this area.)
3. True. In Zone II repairs, surgeons use a strong multi-strand core suture (usually 4 or 6 strands) to repair each tendon ncbi.nlm.nih.gov, allowing for early mobilization. Repairing both FDS slips in this tight zone can create excess bulk, so often only one slip of the FDS is repaired (or the FDS is left partially unrepaired) to reduce crowding in the sheath ncbi.nlm.nih.gov. The priority is a robust FDP repair to restore DIP flexion; FDS repair is secondary and done in a way that won’t impede tendon gliding.
4. False. After flexor tendon repair, the finger is splinted in flexion, not extension. A dorsal blocking splint (with the wrist and fingers flexed) is used to relax the repaired tendon. Moreover, the hand is not kept immobile for a full 6 weeks – early controlled motion begins in the first week or two after surgery asht.org. Prolonged complete immobilization would lead to severe adhesions and joint stiffness.
5. True. Adhesions (scar tissue sticking the tendon to surrounding tissue) are a notorious problem in Zone II injuries because of the confined sheath space and dual tendons. Early controlled mobilization (either passive flexion exercises or carefully supervised active flexion) is therefore utilized to promote tendon gliding and reduce the likelihood of disabling adhesions ncbi.nlm.nih.gov.

 

References

1. Stevens KA, et al. “Flexor Tendon Lacerations.” StatPearls [Internet], StatPearls Publishing, 2023​ ncbi.nlm.nih.gov​ ncbi.nlm.nih.gov. (Comprehensive overview of flexor tendon injuries and repair techniques)
2. AO Surgery Reference – AO Foundation. “Hand – Flexor Tendon Injuries.” (Online reference detailing flexor tendon injury zones, repair methods, and rehabilitation)​ surgeryreference.aofoundation.org​ surgeryreference.aofoundation.org
3. Heydari MB, et al. (2023). “Comparison of Six-strand and Four-strand Techniques on Zone II Flexor Tendon Repair: A RCT.” World J Plast Surg 12(2):34-40​ pmc.ncbi.nlm.nih.gov. (Study showing 4-strand vs 6-strand repairs have similar outcomes)
4. Starr HM, et al. (2013). “Flexor Tendon Repair Rehabilitation Protocols: A Systematic Review.” J Hand Surg Am 38(9):1712-1717​ asht.org​ asht.org. (Comparison of early passive vs early active motion rehab after tendon repair)
5. Wheeless CR. “Tendon Vascular Supply and Nutrition.” Wheeless’ Textbook of Orthopaedics​ wheelessonline.com​ wheelessonline.com. (Discussion of tendon blood supply, vincula function, and the role of synovial fluid in tendon healing)

Disclaimer

This content is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for medical concerns.