What Are the Joints in Your Fingers Called?

Each finger has three joints, and each has its own name based on where it sits along the finger. The joint closest to your palm is the metacarpophalangeal joint, usually shortened to MCP. The middle joint is the proximal interphalangeal joint, or PIP. The joint nearest your fingertip is the distal interphalangeal joint, or DIP. The thumb is slightly different, having only two joints instead of three, but those names follow the same logic once you understand the system.

The Three Joints and What Their Names Mean

The naming convention is less intimidating than it looks. “Interphalangeal” just means “between the phalanges,” referring to the small bones that make up your fingers. Each finger has three phalanges: the proximal phalanx (closest to your palm), the middle phalanx, and the distal phalanx (the one under your fingernail). The joint between the proximal and middle phalanx is the PIP, and the joint between the middle and distal phalanx is the DIP. “Proximal” means closer to the body, “distal” means farther away.

The MCP joint sits where the finger meets the hand. Its name reflects the two bones it connects: the metacarpal bone in the palm and the proximal phalanx of the finger. You can see your MCP joints easily when you make a fist; they form the knuckles that protrude most visibly. The collateral ligaments at the MCP joints play a key role in defining how far and in which directions these joints can move.¹

The PIP joint is the middle hinge of each finger and is considered one of the most important joints for normal hand function. Its anatomy involves subtle differences in bone shape that tightly control how the joint bends, keeping motion on a precise track.² When people injure or stiffen their PIP joints, the impact on grip and dexterity is often surprisingly large. The DIP joint, by contrast, has a smaller range of motion and contributes less to overall grip strength, but it matters for fine pinching tasks like picking up a coin or turning a key.

Why the Thumb Only Has Two Joints

The thumb has just two phalanges instead of three, so it only has two joints below its MCP. The MCP joint of the thumb works similarly to those in the other fingers, though it tends to have somewhat less side-to-side mobility. Below it sits a single interphalangeal joint, referred to simply as the IP joint of the thumb, since there is no need to distinguish between “proximal” and “distal” when there is only one.

One anatomical curiosity of the thumb’s IP joint is that it frequently contains a sesamoid bone, a small, seed-shaped bone embedded within a tendon. A radiographic study found sesamoid bones at the thumb’s IP joint in about two-thirds of people examined. In the fingers, sesamoid bones at the PIP or DIP joints are extremely rare, showing up in fewer than one percent of cases.³ Those tiny extra bones are thought to help redirect tendon forces and reduce friction, and their near-absence in the fingers versus their abundance in the thumb hints at how differently the thumb operates mechanically.

The thumb also has a carpometacarpal joint at its base, where the metacarpal meets the wrist bones. This saddle-shaped joint is what gives the thumb its wide range of independent motion, including opposition, the ability to touch the fingertips of the same hand. That joint is not technically a finger joint in the way the MCP, PIP, and DIP are, but it is worth knowing about because it is one of the most common sites for arthritis in the hand.

What Holds Finger Joints Together

Bones alone do not explain how your fingers work. Each joint is wrapped in a capsule lined with synovial membrane, which produces a small amount of lubricating fluid. Surrounding the capsule are collateral ligaments on each side that prevent the joint from wobbling laterally, and a thick plate of fibrocartilage on the palm side called the volar plate that stops the joint from bending backward too far. When the volar plate tears, the PIP joint can hyperextend painfully, a common injury in ball sports.⁴

On the back of the hand, the extensor mechanism is a sheet-like arrangement of tendons that controls finger straightening. It is more complex than most people realize. Beyond the well-known central bands that run along the top of each finger, there are layers of intercrossing fiber bundles connecting different tendon components. Researchers have found that these crossing fibers significantly affect how muscle forces are transmitted through the finger, and the effect changes depending on what position the finger is in.⁵ Earlier biomechanical models had simplified this structure away, which likely introduced errors into predictions about how fingers move and where forces concentrate.

How Much Force Your Finger Joints Handle

Finger joints look small, but they handle remarkably heavy loads during everyday tasks. A biomechanical analysis of common activities found that the MCP joint can experience forces exceeding 100 newtons just from carrying objects like grocery bags or twisting open a jar. The stresses at the MCP joint reached levels comparable to those seen at the hip joint, around 2 megapascals, with the spongy bone inside the joint playing a major role in distributing that load.⁶ If you have ever wondered why arthritis in the hands is so common and so debilitating, the answer partly lies in those numbers. These joints are tiny but under constant mechanical stress, especially the MCP and PIP joints, which bear the brunt of grip forces.

The precision grip, the pinch between thumb and index finger, loads joints differently from a power grip. Research on grip mechanics in primates has shown that both joint mobility and the proportions of the digits are critical for precision manipulation. Having a long thumb or very flexible joints alone does not guarantee good precision grip; both factors need to work in concert.⁷ This helps explain why hand injuries that seem minor, like a stiff PIP joint or a slightly shortened finger after a fracture, can have outsized effects on tasks requiring fine motor control.

Common Conditions at Each Joint

Different diseases tend to target different finger joints, and knowing which joint is affected can be a diagnostic clue. Osteoarthritis, the “wear and tear” type, most commonly hits the DIP joints, where it produces bony enlargements called Heberden’s nodes. In a multi-center cohort study, Heberden’s nodes were present in over 86 percent of hand osteoarthritis patients, most frequently at the middle finger. Bouchard’s nodes, the equivalent enlargements at the PIP joint, were less common, appearing in about 37 percent of patients.⁸ Interestingly, patients with Bouchard’s nodes at the PIP joints reported more functional difficulty than those with only Heberden’s nodes, which makes sense given how important the PIP joint is for grip.

Rheumatoid arthritis, by contrast, tends to attack the MCP joints and the wrist. One of its hallmark signs is ulnar drift, where the fingers gradually angle toward the pinky side of the hand at the MCP joints. A review of the biomechanics behind ulnar drift identified several mechanical factors, including failure of the collateral ligaments, pressure changes inside the joint, and degenerative changes in the bones of the wrist and hand.⁹ Even routine daily activities can worsen the drift once the process has started, which is why occupational therapists often teach people with rheumatoid arthritis to use their hands in ways that reduce sideways stress on the MCP joints.

Rarer conditions can appear at unexpected sites. Synovial chondromatosis, a disorder where cartilage nodules form inside a joint, typically affects large joints like the knee. When it shows up in a finger’s PIP joint, it is unusual enough to be reported as a clinical case study. One such case involved a woman whose PIP joint in the index finger became painful and swollen, with multiple small cartilage nodules found during surgery.¹⁰ The rarity of this presentation underscores how diagnostically important it is to know which joint is involved and what normally happens there.

Swan Neck and Boutonniere Deformities

Two classic finger deformities are defined entirely by what is happening at the PIP and DIP joints, and their names come up constantly in hand therapy. In a swan neck deformity, the PIP joint hyperextends while the DIP joint droops into flexion, giving the finger a profile that vaguely resembles a swan’s neck. In a boutonniere deformity, the pattern reverses: the PIP joint is stuck in a bent position and the DIP joint hyperextends. Both deformities result from disruptions to the extensor mechanism, the tendon system that runs along the back of the finger.¹¹

These are not just cosmetic problems. Because the PIP and DIP joints work as a linked system, when the tendon balance at one joint shifts, it changes the forces at the adjacent joint. A boutonniere deformity that starts with a torn central slip of the extensor tendon at the PIP joint, for example, will progressively worsen as the lateral bands of the extensor mechanism slip to the wrong side of the PIP joint’s axis of rotation. The resulting imbalance pulls the DIP into extension while the PIP flexes further. Treatment depends heavily on whether the deformity is still flexible or has become fixed, and the specific joint involved dictates whether splinting, tendon surgery, or joint reconstruction is the appropriate option.

How Therapists and Doctors Measure Finger Joint Motion

When a hand surgeon or therapist evaluates your finger, they measure the range of motion at each joint individually. A survey of hand therapy practice found that the most common method is measuring each joint separately using a finger goniometer, a small protractor-like device placed on the top of the finger. Over 92 percent of therapists in the survey used a flat-armed finger goniometer as their primary tool, and none reported using smartphone measurement apps.¹²

That may be starting to change. Researchers are now testing AI-based three-dimensional hand models that can measure MCP, PIP, DIP, and thumb IP joint ranges of motion from photographs, comparing their accuracy against traditional manual goniometry.¹³ A pilot study using this approach had participants perform six standardized hand gestures while a surgeon captured photos, which were then analyzed by an AI system.¹⁴ If these tools prove accurate enough, they could make it easier to track recovery after surgery or injury without requiring an in-person visit every time, though the technology is still in early validation.

How Your Fingers Know Where They Are

Your finger joints do more than just bend and straighten; they constantly report their position back to your brain. This sense, called proprioception, relies on multiple types of nerve endings embedded in and around the joints. Research on the thumb’s IP joint identified three distinct types of nerve endings in the joint capsule: one type that senses pressure changes, another that contributes to the sense of movement, and a third that provides proprioceptive information about joint position.¹⁵

But the joints and skin are not the only players. A classic experiment on the DIP joint of the middle finger showed that full proprioceptive accuracy depends on input from muscles, skin, and joint receptors all working together. When researchers blocked the digital nerves to eliminate joint and skin signals, leaving only muscle receptors active, people could still sense finger position, but less accurately. The reverse was also true: removing muscle receptor input while keeping joint and skin signals intact also degraded performance. Neither source alone was enough for full accuracy.¹⁶ This is one reason why conditions that damage peripheral nerves, like diabetes or carpal tunnel syndrome, can make fine finger movements feel clumsy even when the joints themselves are healthy.

Joint Replacement in Fingers

When arthritis destroys a finger joint beyond what medication or therapy can manage, joint replacement becomes an option. The MCP joint has been the most common target for arthroplasty, especially in rheumatoid arthritis patients with severe ulnar drift. Research into implant design has explored using soft layered materials that mimic the way natural cartilage works, with models predicting that such implants produce a thicker lubricating film between surfaces, which should reduce long-term wear.¹⁷

PIP joint replacement has been more challenging because the PIP is a tightly constrained hinge, and even small errors in implant shape can limit motion or cause instability. A recent approach used a logarithmic spiral design, inspired by the natural geometry of the PIP joint’s articular surfaces, to create an implant that better replicates the finger’s normal arc of motion. In both computer simulation and cadaver testing, the spiral-shaped implant maintained consistent contact throughout the bending range and achieved a flexion arc greater than 90 degrees.¹⁸ DIP joint replacement is performed far less often; because the DIP contributes less to overall hand function, surgeons more commonly fuse the DIP in a functional position rather than replacing it, which eliminates pain at the cost of eliminating motion at that joint.

What Happens When You Crack Your Knuckles

The satisfying pop when you pull or bend your fingers comes from the MCP joints most of the time, though PIP joints can crack too. For decades, the leading explanation was that a gas bubble inside the joint fluid collapsed and caused the sound. But real-time MRI imaging of a finger being cracked told a different story. Researchers watched the joint space as it was pulled apart and saw a gas cavity form rapidly at the moment of the crack, not collapse. The observation was consistent with a process called tribonucleation, where two lubricated surfaces resist being separated until they reach a critical point and then pull apart suddenly, creating a sustained gas-filled space.¹⁹ The sound comes from the rapid creation of the cavity, not its destruction. The cavity then slowly reabsorbs over the next several minutes, which is why you cannot crack the same knuckle again right away.

Whether habitual knuckle cracking causes arthritis is one of the most persistent hand-related health myths. The available evidence, including one widely cited self-experiment by a physician who cracked only one hand’s knuckles for over 60 years, does not support a link between cracking and osteoarthritis. That said, no large-scale randomized trial has addressed the question, so the reassurance is based on observational data and case reports rather than definitive proof. What cracking does demonstrate is how much negative pressure can develop inside these small synovial joints when the bones are distracted, a force that the joint capsule and ligaments ordinarily resist throughout normal use.