This article discusses the ligamentous structures of the metacarpophalangeal (MP) joints (see the first image below), proximal interphalangeal (PIP) joints, and distal interphalangeal (DIP) joints. Additionally, the thumb has 2 joints that will be discussed here, the MP joint and the interphalangeal (IP) joint (see the second image below).
The fingers (index through small fingers) are composed of 3 bones each and are all associated with a single metacarpal. Thus, 3 joints per finger exist, all of which have significant motion and require stabilization to prevent subluxation and dislocation. The thumb has 2 bones and a metacarpal for an additional 2 joints. Stabilization is done with a combination of bony restraints, ligaments, and other static soft-tissue stabilizers, and the dynamic action of muscles.
Generally, ligaments are stabilizers of joints. Without them, many of our joints would dislocate or have far too much abnormal motion, leading to degeneration and pain. Providing stability to joints also allows for pressure to be exerted by limbs, fingers, etc, in planes other than the axial plane. Ligaments can be perfectly isometric, meaning they are equally tight throughout the entire arc of motion of a joint, or they can be functionally involved in only a portion of the arc of motion.
The ligaments of the metacarpophalangeal (MP) joints, proximal interphalangeal (PIP) joints, distal interphalangeal (DIP) joints, and the interphalangeal (IP) joint of the thumb are all very similar in configuration; however, their pathology can be different. Specialized ligaments also exist at the MP joints, including the natatory ligament and the deep transverse metacarpal ligament (DTML), which are discussed in this article. Other ligaments are used to stabilize tendons as opposed to joints.
The joint capsule of the thumb MP joint is a stout structure that attaches circumferentially around the joint and seals the joint. The joint capsule blends with the palmar plate and the collateral ligaments. 
The palmar plate is a specialized thickening or continuation of the joint capsule. It is directly palmar to the joint and spans from the metacarpal to the proximal phalanx. Its insertion onto the metacarpal is through 2 elongations called the check rein ligaments (as seen in the image below).
These check rein ligaments fold into the space under the metacarpal neck during flexion and become taut in extension. Distally, the palmar plate attaches directly to the periarticular surface of the proximal phalanx. The palmar plate serves to prevent hyperextension of the joint (see the following video). The amount of hyperextension is very variable amongst individuals. [1, 2, 3, 4] The palmar plate is also the location of the sesamoid bones and attachment for specialized ligaments, where they exist (as in the thumb MP). 
The MP joints have both radial and ulnar collateral ligaments. Within these, each is made up of 2 parts, the accessory ligament and the proper ligament, as seen in the images below. Therefore, each MP joint essentially has 4 collateral ligaments.
The accessory ligament is fan shaped, runs from the metacarpal head near the center of rotation palmarly, and attaches to the palmar plate and (where applicable) the deep transverse metacarpal ligament (see the first image below).  Because these structures are volar, they become tight in extension and loose in flexion (see the second and third images as well as the video, below). Therefore, the accessory ligament is tested clinically in extension . 
The proper ligament is a robust cord and attaches to the metacarpal head at the posterior tubercle (slightly dorsal to the mid axis of the phalanx) and to the proximal phalanx at the base (see the following image). [2, 3, 6] This ligament is most tight in flexion and therefore resists radial and ulnar deviation in flexion. 
The proper ligament can be isolated and therefore examined clinically when the joint is in 30° of flexion. [5, 7] This is achieved due to the "cam" nature of the joint, meaning the arc of motion is not constant; thus, as you flex the MP joint, the phalanx moves further away from the metacarpal and tightens all the supporting structures (see the video below).  This, combined with the diarthrodial structure of the bony surface of the joint, explains why you can abduct and adduct at the MP joint more in extension compared with flexion. 
The radial collaterals are more horizontal than the ulnars.  Certain collaterals are more crucial than others, such as the ulnar collateral of the thumb MP, which is absolutely necessary for the thumb to function as a post for pinch, etc. It is the primary restraint to valgus stress in flexion and also prevents palmar subluxation. 
Deep transverse metacarpal ligament
The deep transverse metacarpal ligament is a thick, wide band connecting the heads of metacarpals 2-5 together through the volar plate of the MP joint. These ligaments hold the metacarpal heads from splaying,  and they allow for some dorsal and volar translation but limit abduction.  See the following images.
The natatory ligament is the most superficial of the MP joint ligaments (see the image below). It attaches just distal to the MP joint and connects the proximal phalanx of all 5 fingers together. The ligament runs in the web space between the fingers and resists excess abduction of the fingers. 
The retaining ligaments are ligaments about the MP, PIP, and DIP joints that do not directly serve to stabilize the joints but that play an indirect role in their function.
The fibers of the sagittal bands originate at the palmar plate and cross around the MP joint toward the volar aspect of the hand. They attach onto the extensor mechanism both radially and ulnarly and keep the extensor tracking midline during flexion of the MP joint.  See the following images.
Landsmeer retinacular ligament (oblique retinacular ligament)
The Landsmeer retinacular ligament is small and has 2 bands, the transverse and oblique. The oblique originates on the lateral volar aspect of the proximal phalanx, transverses over the collaterals, and attaches dorsally to the common extensor (see the image below). The transverse has a shorter course and originates and attaches closer to the joint line and inserts on the lateral border of the proximal phalanx.
These bands are slender and strong ligaments that serve to retain and position the common extensor mechanism during PIP and DIP flexion, much like sagittal bands. [1, 9] Landsmeer originally described them as extending the DIP during active PIP extension; however, other biomechanical research suggests their importance is in stabilizing the distal phalanx under axial load when the PIP is fully extended and the DIP is partially flexed. 
As the name suggests, the triangular ligament is triangular in shape. This ligament is on the dorsal side of each common extensor mechanism and can be found just distal to the PIP joint (see the following images). This ligament counteracts the pull of the oblique retinacular ligament and prevents excessive lateral subluxation of the bands of the common extensor mechanism. 
This article is concerned with the soft-tissue structures stabilizing the joints of the fingers, which include the joint capsule and ligaments at the metacarpophalangeal (MP) and interphalangeal (IP) joints. The tendons are not discussed here.
The finger joints, similar to most joints, are made up of many layers. The joint is filled with fluid called synovial fluid, which helps lubricate the articular cartilage at the end of the bony structures. The synovial fluid is made by synovial membrane found on the inside of the joint capsule.
The capsule and volar plate, which is a specialized segment of the capsule, has many functions. These include proprioception of the joint, sealing the joint, and stability of the joint. The capsule and volar plate is composed of dense, fibrous connective tissue lined by synovial tissue; thus, they are made of bundles of dense collagen fibers. The joint capsule is pierced by both blood vessels and nerve endings; the nerve endings are derived mostly from the muscles surrounding the joint, and they serve proprioceptive and pain functions.
Variable levels of collagens and extracellular matrix in the capsule exist, depending upon the function of the capsule in different locations. These consist of collagen types I, II, and III, and the following glycosaminoglycans (GAGs): chondroitin 4 sulfate, chondroitin 6 sulfate, keratan sulfate, and dermatan sulfate. The capsule and volar plate consist mostly of types I and III collagen. The volar plate stains weakly for all 4 GAGs. 
The ligaments of the finger joints are also made of dense fibrocartilage. The fibrocartilage is made of types I and type III collagen. The ligaments stain for both keratin sulfate and dermatan sulfate in their extracellular matrix. 
The attachment zone is a specialized area of both the capsule and ligaments, and it has special fibrocartilaginous histologic attachment to bone. This zone has 4 specific zones: (1) pure fibrous, (2) uncalcified fibrocartilaginous, (3) calcified fibrocartilaginous, and (4) bone. The attachment zone is the only area of both the ligaments and capsule that stains strongly for type II collagen, as well as stains for types I and III collagen. It also stains strongly for all 4 extracellular matrix GAGs. This area anchors the capsule and ligaments to bone through the specialized fibrocartilaginous zones described above. 
The capsule is blended with the palmar plate and collateral ligaments. Injuries to these structures result in tears. When the structures are torn, the joint space is violated, and the synovial space is no longer isolated.
The palmar plate prevents hyperextension. When it is fibrosed primarily or secondarily to other processes, it contributes to flexion contractures.  The palmar plate is often involved with traumatic finger joint injuries. Because joints are like a box with ligaments on 2 sides, the palmar plate below and the extensor and capsule above, dislocation necessarily requires injuries to 2 or more of these structures. The palmar plate is most commonly associated with dorsal dislocations and associated with lateral or palmar dislocations along with one or more collaterals. Dorsal dislocations are often reducible and stable once reduced.
Volar dislocations, however, are not stable and may require surgical reduction and repair. They are most often treated with immobilization. Also, a straight avulsion injury of the palmar plate due to axial load or hyperextension can occur. This is treated with immobilization in flexion but can lead to chronic deformity or dislocations if not recognized. 
The collateral ligaments are crucial in opposing pinch for hand function, especially at the metacarpophalangeal (MP) level.  Only 1 in 1000 hand injuries involve the collateral ligaments, and of these, 61% are of the thumb. Injury to the finger MP collaterals usually occurs in men in the fourth decade of life and is due to a lateral direct force with the finger in some flexion. Proper ligament tears are most often associated with lateral opening with stress, whereas accessory ligament and plate tears can also have rotatory instability and joint subluxation.  Most collateral tears of the fingers occur at the insertion of the ligament.
High-speed injuries have been shown more likely to be distal, and low-speed injuries are more midsubstance or proximal.  These can be diagnosed by pain at the insertion site, excess deviation with stress, and/or stress radiographs. Complete tears lead to joint instability, a decrease in functional strength, and eventually arthritis. Acute complete tears are more likely to heal when repaired surgically, most often with anchors and direct repair of the ligaments. However, nonoperative management can be attempted and is the first-line treatment for partial or chronic tears. Nonoperative management consists of splinting in 30° of flexion for 6 weeks. Failures of nonoperative treatment require reconstruction with graft material. 
Thumb collateral ligament injuries
Whereas collateral ligament injuries to the fingers and MP joints are relatively uncommon, injuries to the ulnar collateral ligaments of the thumb MP joint are a special circumstance and are relatively common. The injuries result from excessive radial deviation of the thumb about the MP joint and are often known as " gamekeeper's thumb " or " skier’s thumb." Gamekeeper's thumb is derived from chronic attenuation of the ligaments from gamekeepers breaking the necks of game between the thumb and hand; skier's thumb occurs from the ski pole forcibly deviating the thumb radially during a fall while skiing.
The thumb joint can be difficult to examine, because the normal arc of motion is extremely variable (5-115°), and some natural valgus laxity of up to 12° in flexion exists. Similar to other collateral injuries, the thumb ulnar collateral ligament most often tears at the distal insertion site. [6, 7] Less commonly, they can tear midsubstance or proximally. These ligaments can also be associated with a " Stener lesion " or interposition of the adductor aponeurosis between the torn end and its insertion. Stener lesions decrease the rate of healing by preventing direct contact.
These injuries can also occur with a bony avulsion. Tears can be diagnosed by pain at the site, valgus laxity of greater than 30° or 15° more than the unaffected side, and/or magnetic resonance imaging (MRI). Complete tears can have significant functional limitations, greatly reducing the ability of the thumb to act as a post for meaningful activity. Recommended treatment for complete tears is often surgical repair with suture anchors. Free tendon grafts can be used to repair chronic tears. 
The radial collateral ligaments of the thumb MP are also injured at a higher rate than finger MP or IP joints but are less common than the ulnar collateral ligament. The radial collateral ligament is injured in forced adduction and leads to instability and eventually degeneration. If the ulnar collateral ligament is still intact, it also leads to pronation of the thumb about the MP joint. The radial collateral ligament functions in pinch and actions such as button pushing. Examination and treatment follow the same algorithm as the ulnar collateral ligament. 
Deep transverse metacarpal ligament
Isolated injury to the deep transverse metacarpal ligament is unusual. [1, 8] Most such injuries occur with crush injuries and are ruptures between the fourth and fifth metacarpal, allowing for ulnar deviation of the fifth metacarpal. This can be confused with a fifth MP radial collateral ligament tear. Surgical correction is indicated if diagnosed.  Loss of central rays and therefore insufficiency of the deep transverse metacarpal ligament can also lead to scissoring of the remaining rays and decrease in intrinsic function. 
The natatory ligaments limit abduction of the fingers and support the web. When these ligaments are torn or absent, excessive abduction can occur. The natatory ligaments can also become involved in Dupuytren contracture. 
Sagittal band injury can occasionally occur. Pain, swelling, and redness localize the injury. The extensor tendon can sublux from its normal position without a competent sagittal band. The sagittal bands become lax in rheumatoid arthritis, allowing a change in the vector of the tendons, which is a part of the cause of the subluxation of the joints. Sagittal band injuries are repaired surgically. 
Landsmeer retinacular ligament (oblique retinacular ligament)
Tears or insufficiency of the Landsmeer retinacular ligament may lead to distal phalanx instability when the proximal interphalangeal (PIP) joint is extended and the distal interphalangeal (DIP) joint is flexed. The Landsmeer retinacular ligament (oblique retinacular ligament) often becomes contracted in boutonniere deformity (PIP flexion, DIP hyperextension). 
Much like the sagittal bands, the triangular ligament stabilizes the distal extensor mechanism. Without the triangular ligament, excessive volar subluxation of the extensor mechanism occurs during flexion.