Metacarpal Joint Anatomy: Bones, Ligaments, and Movement

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-19

The metacarpal joint is the articulation between a metacarpal bone of the hand or forepaw and the first bone of the digit, the proximal phalanx. In humans and most other mammals this articulation is called the metacarpophalangeal joint, abbreviated MCP joint, and in horses and other hoofed animals the same joint in the forelimb is the fetlock.

This joint matters because it is the hinge that converts forearm muscle action into a working hand, paw, or hoof. It is also a common site of injury. In people, thumb MCP injuries and long-finger dislocations are frequent athletic and household problems [1][2]. In horses, the fetlock is one of the most heavily loaded joints in the body and a major site of racehorse fracture and joint disease [3][4]. In dogs and cats, the metacarpophalangeal joints are small but functionally important for grip, landing, and normal gait. Understanding the bones, ligaments, and allowed movements of this joint is the foundation for reading imaging, recognizing injury patterns, and reasoning about treatment.

What Exactly Counts as a Metacarpal Joint

A metacarpal joint, in the strict anatomical sense used here, is a metacarpophalangeal joint. It sits between the distal end (head) of a metacarpal bone and the proximal end (base) of the first phalanx of the corresponding digit.

Two nearby joints are often confused with it.

The carpal joints sit between the forearm bones and the metacarpal bones, one level higher in the limb. The carpometacarpal joint of the thumb is a separate, saddle-shaped joint at the base of the thumb, not the MCP joint at the thumb's knuckle.

The metatarsophalangeal joints are the matching joints of the hind limb, between the metatarsal bones and the first phalanges. They are built on the same plan as the metacarpophalangeal joints but belong to the pelvic limb. In horses, the fetlock can be either metacarpophalangeal (forelimb) or metatarsophalangeal (hind limb), and the two are studied together because their anatomy is nearly identical [5][6].

Summary Table: Metacarpal Joint Versus Its Neighbors

Bones of the Metacarpal Joint

The bony partners of a metacarpophalangeal joint are the metacarpal head and the base of the proximal phalanx. Their shapes set the limits of motion.

The Metacarpal Head

The metacarpal head is rounded and wider than the shaft behind it. In the human thumb, the head is flatter dorsally and has a slight ridge that helps resist shear. In the long fingers, the head is a broad, nearly spherical condyle that allows flexion, extension, and a small amount of side-to-side and rotational motion [2].

In horses, the distal end of the third metacarpal bone carries two rounded condyles separated by a sagittal ridge. That ridge runs into a matching groove on the proximal phalanx and acts as a built-in guide rail, keeping the joint from sliding sideways during heavy loading [7][4].

The Proximal Phalanx Base

The base of the proximal phalanx is concave where it meets the metacarpal head. In the human thumb, the base is broader and flatter, which trades some mobility for stability [8]. In horses, the base of the proximal phalanx has a central sagittal groove that receives the metacarpal ridge, plus flat articular facets on the palmar side where the sesamoid bones articulate [5].

Sesamoid Bones

Sesamoid bones are small bones embedded within tendons or ligaments near a joint. They change the angle of pull of a tendon and protect the joint surface.

In horses, two proximal sesamoid bones sit on the palmar surface of the fetlock, one on each side, and articulate with the palmar facets of the distal metacarpus and the proximal phalanx [5][4]. In dogs, palmar sesamoid bones are also present at the metacarpophalangeal joints, embedded in the digital flexor tendons. In humans, two small sesamoids sit at the palmar aspect of the thumb MCP joint, one radial and one ulnar. These thumb sesamoids can become trapped inside the joint after a hyperextension injury and lock it, a documented cause of a locked MCP joint in a teenager after a basketball injury [9].

Comparative Table: Metacarpophalangeal Articulations Across Species

Ligaments of the Metacarpal Joint

Ligaments are tough bands of connective tissue that connect bone to bone and limit unwanted motion. At the metacarpophalangeal joint, three groups matter most: the collateral ligaments, the palmar or plantar ligaments, and, in horses, the suspensory apparatus.

Collateral Ligaments

Every metacarpophalangeal joint has a medial collateral ligament and a lateral collateral ligament. They run from the side of the metacarpal head to the side of the proximal phalanx base. They resist side-to-side bending and, when the joint is flexed, they tighten and help control rotation.

The thumb MCP joint is a special case. Its stability depends heavily on the ulnar collateral ligament, which resists forced abduction of the thumb, and on the radial collateral ligament, which resists forced adduction [8][10]. Injury to the ulnar collateral ligament is the classic "skier's thumb" pattern. When the torn ligament end is displaced above the adductor aponeurosis, it cannot heal on its own, a situation called a Stener lesion. A mirror-image problem can occur on the radial side, where the ligament end becomes trapped under the abductor pollicis brevis aponeurosis, a Stener-like lesion that also blocks healing and requires surgery [11].

In the long fingers, the collateral ligaments are the primary stabilizers of the MCP joint. They keep the joint surfaces in constant tension through the arc of motion, while the muscles and tendons provide dynamic stability during active movement [2]. Isolated injury to the joint capsule without collateral ligament damage can still cause rotational instability, and surgeons should consider repair when a finger rotates abnormally and does not improve with conservative care [12].

Palmar (Volar) Ligaments and the Volar Plate

On the palm side of each MCP joint, a thick fibrocartilaginous structure called the volar plate reinforces the joint capsule. It prevents the joint from hyperextending too far and forms part of the floor of the joint. In the thumb, the volar plate is one of the main reasons the MCP joint is so stable, and it helps explain why volar dislocation of the thumb MCP joint is rare [13].

In dogs and cats, an equivalent palmar fibrocartilage pad sits under each metacarpophalangeal joint and works with the palmar ligaments to stop hyperextension during landing and push-off.

The Suspensory Apparatus in Horses

The horse fetlock has a specialized support system that lets it bear enormous loads while still allowing the joint to extend during the stride. This system is the suspensory apparatus, and it has several parts.

The suspensory ligament (third interosseous muscle) runs down the back of the cannon bone and splits into two branches that attach to the proximal sesamoid bones [5].

The sesamoid bones themselves are joined to each other by the intersesamoidean ligament, which forms a groove that the deep digital flexor tendon glides through [5].

The sesamoids are anchored to the proximal phalanx by the straight sesamoidean ligament and to the middle phalanx region by the oblique sesamoidean ligaments [5]. Together these structures form a sling that supports the fetlock from behind, limits overextension, and stores elastic energy that is returned during the stride.

How Movement Works at the Metacarpal Joint

The metacarpophalangeal joint is a condylar synovial joint. Condylar means the joint surfaces are rounded, with one convex surface (the metacarpal head) fitting into one concave surface (the phalanx base). Synovial means the joint has a fluid-filled cavity lined by synovium, which lubricates the cartilage and nourishes it.

Primary Motion: Flexion and Extension

The main movement at every metacarpophalangeal joint is flexion (bending the digit toward the palm or sole) and extension (straightening it away). In humans, the long-finger MCP joints flex to about 90 degrees. The thumb MCP joint has a smaller arc and prioritizes stability over mobility, which is why it tolerates less laxity before function suffers [8].

In horses, the fetlock's dominant motion is dorsiflexion, also called extension or overextension of the fetlock, in which the joint angle opens as the hoof lands and the limb loads. The joint is capable of a large dorsiflexion arc during fast work, and the suspensory apparatus limits how far it can go. The sagittal ridge of the distal metacarpus and the matching groove of the proximal phalanx keep this motion in a single plane [7].

Secondary Motions: Side-to-Side and Rotation

In humans, the long-finger MCP joints allow a small amount of abduction and adduction (side-to-side spread) and axial rotation (twisting) when the joint is extended. These motions disappear as the joint flexes because the collateral ligaments tighten. This is the classic "locking" behavior of the MCP joints that lets you make a firm fist. The thumb MCP joint allows flexion, extension, abduction, adduction, pronation, and supination, but with a much smaller range than the long fingers [8].

The Role of the Sesamoids in Movement

Sesamoid bones change the direction of tendon pull. In horses, the proximal sesamoids act as a fulcrum for the suspensory apparatus, increasing the leverage of the supporting structures and protecting the flexor tendons as they pass over the back of the fetlock [5]. In dogs and cats, palmar sesamoids do the same job for the digital flexor tendons. In the human thumb, the sesamoids sit within the flexor pollicis brevis and adductor pollicis tendons and help distribute load across the palmar joint surface.

How the Metacarpal Joint Is Examined and Imaged

Physical Examination

In people, the examiner tests MCP stability by holding the metacarpal and stressing the proximal phalanx in different directions. Laxity compared with the opposite hand suggests collateral ligament injury. Pain with axial loading, a block to motion, or a finger that sits in an abnormal rotation suggests a more complex injury such as a dislocation or an entrapped structure [1][12].

In small animals, veterinarians assess the metacarpophalangeal joints by palpating for swelling, testing collateral stability, and checking for pain on flexion and extension. In horses, fetlock examination includes palpation of the joint capsule, the suspensory ligament branches, and the sesamoids, plus flexion tests and observation at the walk and trot.

Imaging

Radiographs remain the first imaging step for most metacarpophalangeal problems. In the thumb, standard static and dynamic views under fluoroscopy help detect instability patterns [10]. Computed tomography and cone beam computed tomography give detailed views of bone, including the subchondral bone under the joint surface [10].

In horses, cone beam CT is a practical way to see the bony parts of the fetlock. It can also show the common digital extensor tendon, the superficial and deep digital flexor tendons, the suspensory ligament, and the straight and oblique sesamoidean ligaments. Some structures, including the collateral sesamoidean ligaments and the intersesamoidean ligament, are not clearly seen on cone beam CT, and soft tissue detail is better on conventional multidetector CT [5][14]. Articular cartilage at the fetlock can only be seen after contrast medium is injected into the joint, on either modality [5][14].

Ultrasound is the preferred tool for soft tissue in the thumb MCP joint, and high-field MRI gives the most complete view of ligaments, tendons, and the volar plate [8][10]. In camels, advanced imaging and arthroscopy have mapped the MCP joint in detail, showing a prominent dorsal synovial plica, dorsally flattened metacarpal condyles, and complete separation between adjacent MCP joints [7].

Arthroscopy

Arthroscopy lets a surgeon look directly at the joint surfaces and remove loose fragments. In horses, a direct arthroscopic approach to the distal pouch of the palmar recess of the fetlock has been described for removing free-floating fragments, and cartilage damage was found in 18 of 25 operated fetlocks in one case series [6]. In camels, dorsal arthroscopy allows examination of the articular cartilage of both the distal metacarpus and the proximal phalanx, while palmar access is limited by the shape of the sesamoid and the sagittal ridge [7].

Comparative Range of Motion and Function

Humans

The long-finger MCP joints are mobile hinges with a small amount of side-to-side and rotational play. The thumb MCP joint is much stiffer, with stability as its main job [8][2]. This difference explains why thumb MCP injuries are so disabling: the joint has little reserve to compensate for laxity.

Dogs and Cats

Dogs and cats have several metacarpophalangeal joints per paw. Their motion is more varied than in humans because the digits can spread, flex independently, and, in cats, work with the retractile claw mechanism. The joints are small and are supported by collateral ligaments, palmar ligaments, and a palmar fibrocartilage pad. Cats use these joints for gripping and for controlled landing, while dogs use them for propulsion and for gripping uneven ground.

Horses

The horse fetlock has the largest dorsiflexion range of the joints discussed here. During gallop, the fetlock drops toward the ground as the suspensory apparatus stretches and then recoils, returning energy to the stride. This extreme range is possible because of the sagittal ridge and groove that keep the joint in one plane, and because the sesamoids and suspensory apparatus share the load [5][7].

Cattle

Cattle have a fused third and fourth metacarpal (the cannon bone) and two weight-bearing digits, each with its own metacarpophalangeal joint and proximal sesamoid bones. Motion is dominated by flexion and extension during walking, with limited side-to-side movement because the digits are bound together by the axial and abaxial sesamoidean ligaments.

Clinical Relevance: Why This Anatomy Matters

Thumb MCP Injuries

The thumb MCP joint is injured more often than most people realize. Ulnar collateral ligament tears are common in skiing and other sports where the thumb is forced into abduction. Radial collateral ligament tears are rarer and often occur with forced adduction [15]. Both can become chronic problems if not treated, and chronic radial collateral ligament injuries may need reconstruction using a slip of the abductor pollicis brevis tendon [15]. Locked thumb MCP joints can result from an incarcerated sesamoid, a metacarpal osteophyte, or a torn ligament that blocks motion [9][16].

Long-Finger MCP Dislocations

Dislocations of the long-finger MCP joints are usually dorsal and often involve the index finger [1]. Some are irreducible because soft tissue, such as the volar plate or a sesamoid, becomes trapped in the joint. Surgical treatment through a dorsal approach has been associated with greater postoperative range of motion, less extension lag, and lower disability scores, while a volar approach has been associated with better grip strength in one series [1].

Rheumatoid Arthritis

In rheumatoid arthritis, the MCP joints of the long fingers are frequently destroyed, leading to pain, ulnar drift, and volar subluxation. Arthroplasty, usually with a flexible silicone implant, remains the main reconstructive option and gives reproducible correction of deformity and improved hand alignment and function. Outcomes depend heavily on soft tissue balancing and tendon realignment, and alternative implants such as surface-replacement and pyrocarbon prostheses have shown less predictable results in this setting [17].

Equine Fetlock Disease

The horse fetlock is under extreme load, and racehorses are prone to palmar osteochondral disease, parasagittal groove lysis, and condylar fracture. In a study of 86 elite Thoroughbred racehorses, palmar osteochondral disease was present in 74 of 86 horses (86.0%) and in 231 of 342 limbs (67.5%) on first scan [3]. Parasagittal groove lysis, a feature linked to condylar fracture, was seen in 16 of 86 horses (18.6%) and 20 of 342 limbs (5.8%) on first scan [3]. A separate post-mortem study found that lysis in the subchondral bone under the parasagittal groove was strongly associated with condylar fracture, with an odds ratio of 9.6 [4]. These findings show why the bony anatomy of the fetlock, especially the parasagittal groove and the sesamoids, is central to equine lameness work.

Common Mistakes and Limitations

Students and clinicians frequently mix up the metacarpal joint with the carpal joints or with the metatarsophalangeal joints. The metacarpal joint is at the knuckle or fetlock level, not the wrist. The carpal joints are between the forearm and the metacarpus. The metatarsophalangeal joints are the hind-limb equivalents.

Another common error is assuming that all metacarpophalangeal joints behave the same way. The thumb MCP joint is much stiffer than the long-finger MCP joints, and the horse fetlock has a far larger dorsiflexion range than any human MCP joint. Range of motion values from one species or one joint should not be applied to another.

A third mistake is treating sesamoid bones as unimportant. In horses, the proximal sesamoids are central to the suspensory apparatus and to fracture risk [4]. In the thumb, an incarcerated sesamoid can lock the joint [9]. In dogs and cats, palmar sesamoids affect flexor tendon mechanics.

Imaging has limits. Cone beam CT is excellent for bone and for cartilage when contrast is used, but it is limited for soft tissue structures compared with conventional CT [5][14]. Some ligaments, such as the collateral sesamoidean and intersesamoidean ligaments in horses, are not clearly seen on cone beam CT [5]. Ultrasound and MRI are needed when ligament detail matters.

Finally, individual cases vary. A veterinarian or physician who can examine the patient, review imaging, and consider the whole limb is the right person to make diagnostic and treatment decisions.

Quick Review

1. A metacarpal joint is a metacarpophalangeal joint between a metacarpal head and a proximal phalanx base. In horses it is the fetlock.
2. It is a condylar synovial joint with a convex metacarpal head and a concave phalanx base.
3. Collateral ligaments resist side-to-side motion. The palmar ligaments and volar plate resist hyperextension.
4. Horses have two proximal sesamoid bones and a suspensory apparatus that supports the fetlock and stores energy.
5. The thumb MCP joint prioritizes stability. The long-finger MCP joints prioritize mobility.
6. Horses have the largest dorsiflexion range. Dogs and cats have more varied, digit-specific motion.
7. Do not confuse the metacarpal joint with the carpal joints or the metatarsophalangeal joints.

Frequently Asked Questions

What is the metacarpal joint?

The metacarpal joint is the metacarpophalangeal joint, the articulation between the head of a metacarpal bone and the base of the proximal phalanx of the same digit. In horses and other hoofed animals, the forelimb version is called the fetlock.

Is the fetlock the same as the metacarpal joint?

Yes, in the forelimb. The fetlock is the metacarpophalangeal joint of the horse. The hind-limb fetlock is the metatarsophalangeal joint, which has nearly identical anatomy.

What type of joint is the metacarpophalangeal joint?

It is a condylar synovial joint. The metacarpal head is convex and the base of the proximal phalanx is concave, and the joint has a synovial cavity that lubricates the cartilage.

Do horses have sesamoid bones at the fetlock?

Yes. Horses have two proximal sesamoid bones on the palmar aspect of each fetlock, one on each side. They articulate with the distal metacarpus and proximal phalanx and are part of the suspensory apparatus [5][4].

Which ligaments stabilize the metacarpophalangeal joint?

The medial and lateral collateral ligaments resist side-to-side motion, and the palmar ligaments plus the volar plate resist hyperextension. In horses, the suspensory ligament and the sesamoidean ligaments form an additional supporting sling [5][2].

How does range of motion differ between species?

Horses have the largest dorsiflexion range at the fetlock, supported by the suspensory apparatus. Dogs and cats have more varied motion across multiple digits. Humans have mobile long-finger MCP joints and a much stiffer thumb MCP joint [8][2].

Related Articles

• Equine Hindlimb Anatomy: Bones, Joints, and Ligaments
• Canine Stifle Joint Anatomy and Cranial Cruciate Ligament Rupture
• Equine Forelimb Tendon and Ligament Anatomy: Clinical Relevance
• Equine Bone and Joint Infections: Diagnosis and Treatment
• Cranial Cruciate Ligament CCL Injury in Cats: Feline Joint Stifle Stabilization
• Bovine Musculoskeletal Anatomy: Axial Skeleton and Joints
• Condyloid Joint: Structure and Movement

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