Logan Steele
The proximal interphalangeal (PIP) joint is a hinge joint that allows for flexion and extension movements of the fingers. Previous literature has suggested that PIP joint extension is primarily a function of the interosseous and lumbrical muscles. However, recent studies have indicated that the extrinsic extensor muscle, particularly the extensor digitorum communis (EDC), may play a more significant role in PIP joint extension. This is due to the EDC's contribution to the two lateral bands, which have a greater role in PIP joint extension than the central slip alone. The index finger also has additional extrinsic extensor muscles, such as the extensor indicis and extensor digiti minimi. Furthermore, successful restoration and maintenance of active PIP joint extension require a concomitant analysis of forces at the metacarpophalangeal (MCP) joint, as certain conditions, such as MP hyperextension, can impact the PIP joint's function.
| Characteristics | Values |
|---|---|
| Muscles that create PIP extension | Extrinsic extensor (EDC), interosseous, and lumbricals |
| Joint extension | Done by the central slip and the two lateral bands |
| Central slip | Solely from the EDC tendon |
| Lateral band | Composed of extrinsic and intrinsic tendons |
| Intrinsic extensors | Only one pathway, the two lateral bands |
| PIP joint angle | Measured while an extension load is applied on the extensor tendons |
| Specimens | Classified as extrinsic or intrinsic groups |
| Regression analyses | Linear regression analyses were performed to obtain a regression equation and the extension load-PIP joint angle curve |
| Flexor muscles | Extrinsic flexor muscles generate concurrent flexion of all three finger joints |
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What You'll Learn
The extrinsic extensor (EDC) and its role
The extrinsic extensor, also known as the EDC, plays a significant role in PIP joint extension. The EDC tendon trifurcates distal to the MP joint, with its two side branches forming the lateral bands along with the interosseous and lumbrical muscles. This unique anatomy allows the extrinsic extensor to deliver its extension load to the PIP joint through two pathways: the central slip and the two lateral bands. The central slip is solely contributed by the EDC tendon, while the lateral bands are composed of both extrinsic and intrinsic tendons.
Studies have suggested that the EDC may contribute more to PIP joint extension than previously thought, challenging the notion that this movement is primarily driven by the interosseous and lumbrical muscles. This highlights the importance of understanding the intricate anatomy of the dorsal apparatus on the proximal phalanx level.
The EDC also crosses the wrist, influencing the mechanics of MP joint extension. When the wrist is in a neutral position, the EDC excursion required to achieve MP joint extension is reduced compared to when the wrist is in extension. To test the maximum proximal excursion of the EDC, patients are instructed to actively hold both the wrist and MP joints in extension without extending the IP joint.
The EDC's role in PIP joint extension is particularly evident in the index and little fingers, which possess additional extrinsic extensor muscles: the extensor indicis and extensor digiti minimi, respectively. These muscles contribute to the overall extension capabilities of the PIP joint.
Furthermore, the EDC is crucial in the surgical reconstruction of boutonniere deformity. Surgeons must repair the lateral bands to the middle phalanx to ensure proper transfer of the extension load from the extrinsic and intrinsic extensors. This highlights the clinical significance of understanding the role of the EDC in PIP joint extension.
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The central slip and the two lateral bands
PIP joint extension is done by the central slip and the two lateral bands. The central slip is solely from the EDC tendon, while the lateral bands are composed of both the extrinsic and intrinsic tendons. The EDC tendon trifurcates distal to the MP joint, and its two side branches build the lateral bands with the interosseous and lumbricals.
The extrinsic extensor delivers its extension load to the PIP joint through two pathways: the central slip and the two lateral bands. The intrinsic extensor has only one pathway: the two lateral bands. The two lateral bands have a greater role in PIP joint extension than the central slip alone. This is because the two lateral bands are composed of the EDC, interosseous, and lumbricals all together.
For successful surgical reconstruction of boutonniere deformity, surgeons should repair the lateral bands to the middle phalanx to ensure that the extension load is conveyed from the extrinsic and intrinsic extensors. In addition, in a clinical situation with extensor tendon disruption at the finger level, surgeons should thoroughly inspect for injury at different points of the trifurcation of the EDC and lateral bands, not just the central slip.
The index and little fingers have additional extrinsic extensor muscles: the extensor indicis and extensor digiti minimi. The role of the forearm (extrinsic) finger flexor muscles in initiating rotation of the MCP joint and in coordinating flexion at the MCP, PIP, and DIP joints remains a matter of some debate.
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The role of the forearm (extrinsic) finger flexor muscles
The forearm (extrinsic) finger flexor muscles play a crucial role in the movement of the human hand and fingers. These muscles originate outside the hand and cross multiple joints, influencing the movement of all the joints they pass through. One such muscle is the flexor digitorum profundus (FDP), which is a deep muscle in the anterior compartment of the forearm. The FDP is a powerful muscle that provides the main gripping power of the hand. It arises at or below the wrist joint and acts as the sole flexor of the distal interphalangeal (DIP) joint of the 2nd to 5th digits. Additionally, the FDP acts as a flexor of the proximal interphalangeal (PIP) joint, along with the flexor digitorum superficialis (FDS).
The FDP is a hybrid muscle, receiving innervation from two different nerves. The medial half, associated with the ring and little fingers, is supplied by the ulnar nerve. In contrast, the lateral half, associated with the middle and index fingers, is innervated by the anterior interosseous nerve, a branch of the median nerve. The anterior interosseous nerve also supplies other forearm muscles, including the flexor pollicis longus and pronator quadratus.
The FDP tendon perforates the tendon of the flexor digitorum superficialis opposite the proximal phalanx of the corresponding digit. Then, it inserts into the palmar surface of the base of the distal phalanx of the 2nd to 5th digits. This insertion allows the FDP to flex the DIP joint and assist in finger extension at the PIP joint. However, an injury to the FDP tendon distal to the lumbrical origin can result in a paradoxical extension of the interphalangeal joints when attempting to flex the finger, known as "Jersey finger."
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The interosseous and lumbrical muscles
The lumbrical muscles of the hand originate from the flexor digitorum profundus tendons and insert onto the lateral band of the extensor tendons. They have the smallest physiological cross-sectional area in the upper extremity, and their function is difficult to visualize. They have highly specialized architectural properties, with the muscle fibres extending 85% to 90% of their muscle length, indicating that they are designed for long excursions and high contraction velocity. The first and second lumbricals (the most radial two) are innervated by the median nerve, while the third and fourth lumbricals (the most ulnar two) are innervated by the deep branch of the ulnar nerve.
The interosseous muscles, on the other hand, are built for high force generation and low excursion. They have muscle fibres that span only 41% to 52% of muscle length. The combined interossei have about 15 times the PCSA of the lumbricals, indicating that the lumbricals are weak extensors of the PIP joint compared to the interosseous muscles.
While previous literature suggested that PIP joint extension is primarily the function of the interosseous and lumbrical muscles, more recent studies have shown that the extrinsic extensor (EDC) may contribute more to PIP joint extension. The EDC delivers its extension load to the PIP joint through two pathways: the central slip and the two lateral bands. The lateral bands are composed of the EDC, interosseous, and lumbrical muscles all together.
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The impact of previous injuries or surgeries
The proximal interphalangeal (PIP) joint is a complex structure that is influenced by various muscles and tendons. The extension of the PIP joint is primarily facilitated by the extrinsic extensor (EDC) and its two lateral bands, which are composed of the EDC, interosseous, and lumbrical muscles. The central slip, on the other hand, is formed solely by the EDC tendon.
When it comes to the impact of previous injuries or surgeries on PIP extension, there are several important considerations:
Firstly, scar tissue formation from prior injuries or surgeries involving the lateral bands or Landsmeer's ligamentous attachments can significantly hinder the restoration of active PIP extension. This scar tissue can create mechanical imbalances that affect the forces acting on the PIP joint, making it challenging to restore full extension.
Secondly, certain injuries or conditions, such as a proximal phalanx fracture, can lead to complex periarticular contractures if the finger is immobilized incorrectly for an extended period. This can result in adhesions of the extensor tendons, further impacting PIP extension. To prevent this, immobilization should be done in the "safe position," with the metacarpophalangeal (MCP) joints flexed and the PIP joints extended.
Thirdly, PIP joint stiffness, a common issue, can be influenced by previous injuries or surgeries. For example, poor consolidation after a finger fracture or complications during hand surgery can lead to stiffness and a deformed position in flexion, as seen in the case of footballer Karim Benzema.
Additionally, swelling from an injury can cause the normally lax dorsal soft tissues and skin to tighten, pushing the IFP joints into flexion. This alteration in tissue tension can trigger various adverse processes, including the loss of collagen fibre lubricity, connective tissue proliferation, and capsulo-cartilaginous adhesions, ultimately leading to contractures.
In summary, previous injuries or surgeries can impact PIP extension by causing scar tissue formation, contractures, stiffness, and alterations in tissue tension. These factors can create mechanical imbalances and restrict the range of motion at the PIP joint, requiring careful management and rehabilitation to restore full extension.
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Frequently asked questions
PIP extension is primarily a function of the extrinsic extensor (EDC) and its tendons.
Extrinsic extensor muscles are forearm finger flexor muscles that initiate rotation of the MCP joint and coordinate flexion at the MCP, PIP, and DIP joints.
The EDC delivers its extension load to the PIP joint through two pathways: the central slip and the two lateral bands.
The central slip is solely from the EDC tendon. The lateral bands are composed of both the extrinsic and intrinsic tendons, including the interosseous and lumbrical muscles.
Surgeons should repair the lateral bands connected to the middle phalanx to ensure the extension load is conveyed from both extrinsic and intrinsic extensors.
