36 The Interphalangeal Joint

Hilton P. Gottschalk and Randy S. Bindra

36 The Interphalangeal Joint

36.1 Introduction

The proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints are critical hinges that lend dexterity to the fingers to perform fine manipulative tasks while affording stability to the hand to allow a powerful grip. While the bony anatomy of the joint is the major determinant of the joint stability and mobility, all structures enveloping the bony scaffold, including joint capsule, ligaments, surrounding tendons, and skin, also contribute to joint integrity and function.

36.2 Proximal Interphalangeal Joint

Of the interphalangeal joints, the PIP joint is most critical to function due to its large arc of motion and its long lever arm. Like elsewhere in the hand, form and function are closely related. ¹ Although it has been described as a simple hinge joint, ² ^, ³ the PIP joint is considerably more complex. ¹ The PIP joint is responsible for 85% of total encompassment as we grasp an object, which allows us to adapt to surface irregularities. This adaptability allows us to perform an amazing array of functions.

36.2.1 Bony Anatomy

The PIP joint is located midway between the metacarpophalangeal (MCP) joint and the tip of the finger, forming the anatomical and functional center of the digit. ¹ ^, ⁴ The central location is not a coincidence but rather is designed to allow congruous finger movement. Like many things in nature, the relative lengths of the phalanges and metacarpals in the hand follow the Fibonacci series (▶Fig. 36.1). This is a mathematical series in which each number is the sum of the previous two numbers and was first described by Fibonacci. ¹ ^, ⁴ Thus, the length of the proximal phalanx equals the sum of the lengths of the middle and distal phalanges. Similarly, the length of the metacarpal is equal to the sum of the lengths of the middle and proximal phalanges (See also Chapter 2).

**Fig. 36.1** (a) Line drawing of a Fibonacci spiral approximating a golden spiral. Square sizes are 1,1,2,3,5,8,13,21, and 34. (b) The Fibonacci spiral and tiles are superimposed on a clenched fist. Illustrating the length of proximal phalanx is the sum of the lengths of the middle and distal phalanges.

The PIP joint can be best described as a “sloppy hinge” that allows motion in three dimensions. The sagittal plane motion (flexion/extension) is the main movement. ¹ ^, ² Not only do the fingers have to flex into the palm, but they also have to stay closely approximated despite their individual lengths.

Proximal Phalangeal Head

The proximal phalangeal head is bicondylar, separated by an intercondylar notch that articulates with a projection, or trochlea, from the middle phalanx (▶Fig. 36.2a, b). This articulation provides some inherent stability in medial, lateral, and rotational directions when the joint is flexed or extended. ³ ^, ⁵ ^, ⁶ The three-dimensional shape of the proximal phalangeal head/neck allows for a rotational component to motion at the PIP joint such that the tips of the fingers converge toward each other with flexion of the joint. Following three components contribute to this phenomenon:

**Fig. 36.2** (**a, b**) Volar view of the proximal phalangeal joint.

The proximal phalangeal heads are roughly trapezoidal in shape, with the volar margin being greater than the dorsal margin (▶Fig. 36.3). In addition, the dorsal and volar margins are tilted such that the index, middle, and small fingers tilt slightly toward the ring finger. ¹ ^, ² ^, ⁷
The radius of curvature of each condyle is slightly different.
In the coronal plane, the distal margin of the head of the proximal phalanx tilts away from the second web space. ¹

**Fig. 36.3** End–on view of the head of the proximal phalanx showing the wider volar portion and the narrower dorsal part.

Essentially, the index finger supinates at the PIP joint in flexion while the ring and small fingers slightly pronate.

Additional features of the proximal phalangeal head include (▶Fig. 36.4a, b, and c):

**Fig. 36.4** (a) The lateral aspect of the proximal phalanx (b). Cadaver specimen (c) Dried bone specimen with (*) marking flat area.

A dorsally placed pit on the lateral side where the “proper” collateral ligament originates.
A flattened area just volar to the pit which serves as an extension of the attachment site for the proper collateral ligament and allows for the ligament complex to glide during flexion and extension.
Extension of the intercondylar groove to the volar aspect of the phalanx to accommodate the volar lip of the middle phalanx and allow a 110° arc of motion.

Base of Middle Phalanx

The base of the middle phalanx has two concavities separated by an intercondylar ridge (▶Fig. 36.5). The fit is not perfectly congruous to the head of the proximal phalanx, to allow for a small amount of rotation. Additional features of the base of the middle phalanx include the following (▶Fig. 36.6):

**Fig. 36.5** End on view of the base of the middle phalanx showing the bicondylar shape, the volar intercondylar ridge, and the insertion area of the central slip dorsally.

**Fig. 36.6** Base of the middle phalanx. (a) Volar aspect. (b) Lateral aspect.

A base wider than the dorsal margin, with a lateral tubercle on each side where portions of the collateral ligament attach. ¹ ^, ²
An additional thickening in the dorsal lip for insertion of the central slip of the extensor tendon.
The palmar aspect of the middle phalanx contains a roughened area close to the joint surface for attachment of the palmar plate, as well as two small tubercles for capsular attachment.

36.2.2 Stabilizing Restraints

There are three main restraints to the PIP joint: the proper collateral ligament, accessory collateral ligament, and the volar plate. Interestingly, the dorsal aspect of the joint is devoid of major stabilizing structures and only has a thin-walled capsule that allows for some “give” in finger extension with a synovial pouch lying deep to the extensor mechanism.

Proper Collateral Ligament

The proper collateral ligament is the major restraint to varus and valgus angulation of the PIP joint. ⁶ The proper collateral ligament (PCL) possesses the following properties:

It originates from the pit of the proximal phalangeal head and fans out to attach to the thickened lateral margin of the middle phalanx (▶Fig. 36.7).
It is thicker than the accessory collateral ligament and can cover up to one-third of the lateral width of the PIP joint.
At its insertion onto the lateral tubercle of the middle phalanx, the fibers blend with the periosteum into a portion of the metaphysis (▶Fig. 36.8).
The PCL is tight in flexion secondary to the trapezoid cross-sectional shape of the proximal phalanx. The PCL stretches across the condyles and is the major lateral stabilizer at 60° flexion. ¹ ^, ⁶
The volar edge blends with the thinner accessory collateral ligament.