Last updated on January 17, 2021
A joint is dislocated(1) when the two joint surfaces are no longer in contact with each other. A dislocation or subluxation(2) may be suspected by a visible deformity of a digit, and confirmed by x-ray.
There are five metacarpophalangeal joints in each hand, each of which is a multiaxial diarthrodial- condyloid joint. The joint is formed between the heads of the metacarpal bones and proximal phalanges. All of the five metacarpophalangeal joints are strengthened by a fibrous capsule.
Ligaments of metacarpophalangeal joints:
There are two collateral and one palmar ligament associated with each metacarpophalangeal joint.
Volar Plate: They are thick, dense and fibrocartilaginous. They are placed between the collateral ligaments and are connected to them. The volar plates are attached firmly to the base of proximal phalanx, but the attachment to the head of metacarpal is loose. The loose attachment allows hyperextension of the MP joint.
Collateral Ligaments: The collateral ligament is separable into two layers, with the precise origin arising from the metacarpal head and inserting on the proximal phalanx. The distance between the origin and insertion of the collateral ligament changes in different portions of the ligament when the joint is moved from a position of hyperextension to that of flexion. The dorsal and volar portions of the collateral ligament provide MCP joint constraint at the flexed and extended positions, respectively. (3) (Figure 1)
The radial collateral ligaments (RCL) are more horizontal than ulnar collateral ligaments (UCL)
RCL and UCL have 2 parts each: proper and accessory ligaments. The accessory ligaments are fan shaped, more volar, and become tight in extension They attach from the metacarpal head at the center of rotation to the polar plate and deep transverse metacarpal ligament. The proper collateral ligaments are more cord-like, are more dorsal, and are tight in 30 degrees of flexion. They attach from the posterior tubercle of the metacarpal head (dorsal to the mid-axis) to the proximal phalanx base. The pUCL of the index MCP joint originates at the dorsoulnar MCP head (one-third of the way down) and inserts on the proximal volar aspect of the proximal phalanx (one-quarter of the distance from volar to dorsal).
To isolate the proper collateral ligaments, perform an adduction/abduction stress maneuver in 30 degrees flexion
Blood supply of metacarpophalangeal joints:
These joints receive their blood supply form the dorsal and palmar metacarpal arteries, princeps pollicis artery and radialis indicis artery.
Nerve supply of metacarpophalangeal joints:
These joints recieve their nerve supply for the palmar digital branches of median nerve, the deep terminal branch of ulnar nerve and the posterior interosseous nerve.
Movements of metacarpophalangeal joints:
These joints are highly mobile showing varying degrees of flexion, extension, adduction, abduction and circumduction. Rotation is very limited and cannot take place alone. However, it may accompany flexion and extension.
Muscles producing movements:
- Flexion: Flexor digitorum superficialis, Flexor digitorum profundus, Lumbricals, and Interossei. In the little finger, the flexor digiti minimi, and in the thumb, the flexor pollicis longus and flexor pollicis brevis also produce flexion.
- Extension: Extensor digitorum, extensor indicis (in index finger) and extensor digiti minimi (in little finger). In thumb the extensor pollicis longus and brevis produce extension.
- Abduction: Dorsal interossei, Long extensors, and abductor digiti minimi (in little finger). In thumb, the abduction is produced by abductor pollicis brevis and longus.
- Adduction: Palmar interossei and long flexors. In the thumb, adduction is produced by adductor pollicis.
Mechanism of Injury
Most MP joint dislocations occur as the result of a hyperextension injury – often a fall on an outstretched hand.
Patients will complain of pain and swelling in the area of the MP joint. They will not be able to flex or extend the digit.
The findings include swelling, tenderness, and deformity. The digit may deviate laterally at the joint, and it will be in extension or hyper extension. The patient will not be able to flex or extend the digit, partially because of pain, but also because of tethering or trapping of the tendons around the head of the metacarpal, or the volar plate interposed in the joint. (Figure 2)
Three views of the hand (PA, lateral, oblique) should be obtained. If the joint is dislocated, the PA view will demonstrate loss of the normal joint space, and overlap and loss of congruity of the articular surfaces. (Image needed) The lateral view will demonstrate dorsal displacement and overlap of the base of the proximal phalanx compared to the metacarpal head. (Image needed) The articular surfaces of the joint are not parallel in both subluxation and dislocation. If the articular surfaces are parallel, but there seems to be less contact than normal, then this is not subluxation or dislocation – it is flexion or (hyper-) extension. (Image needed)
Joint dislocations in the hand should be reduced urgently. Chronic dislocation is associated with stiffness, scarring, deformity, and pain. Closed reduction should be attempted in the emergency room. Adequate anesthesia should be obtained through the use of nerve blocks and field blocks. Pain may cause the patient to tense their muscles, and may prevent reduction. The reduction maneuver does not involve traction on the finger. Traction may cause tightening of structures (tendons and ligaments) around the metacarpal neck, thus preventing the proximal phalanx from sliding distally over the metacarpal head. Rather, the MP joint should be hyperextended (to relax the extensor tendon), and then, with the thumbs pressing on the dorsal lip of the base of the proximal phalanx, the phalanx should be pushed over the end of the metacarpal head. (video needed) Closed reduction may be unsuccessful, and a maximum of two attempts should be performed, to minimize the likelihood of additional damage to the joint. Structures preventing closed reduction include the flexor tendons, volar plate, collateral ligaments, and extensor tendons, which all may become interposed in the joint. Repeat x-rays should always be performed after attempted reduction, even if reduction was not achieved. The reasons for getting new x-rays include determination if reduction was successful, identification of fracture or other new injury, and confirmation of parallel joint surfaces. If the joint surfaces are not parallel, then imposition of a soft tissue structure within the joint should be suspected.
If the dislocation is irreducible closed in the ED, then reduction should be performed in the operating room. The patient should not be discharged for follow-up and treatment as an outpatient. Surgery is not emergent, but it should be performed as soon as an anesthesia is safe (NPO, medical clearance, etc). Either peripheral nerve block of the upper extremity or general anesthesia should be performed. Once anesthetized, another attempt at closed reduction can be performed, and intraoperative imaging with a mini c-arm may demonstrate successful reduction because of superior pain control and muscle relaxation. If the dislocation is still irreducible, then open reduction should be performed. A dorsal incision is preferred (longitudinal, centered at the MP joint). The anatomy can be confusing because of abnormal positioning of the extensor tendons and sagittal bands. The volar plate may be interposed in the joint, and it’s surface may look like cartilage, further confusing the picture. Sagittal bands may need to be divided in order to access the collateral ligaments. Once the volar plate is pulled out of the joint, and while the collateral ligaments are levered with a freer elevator, the phalanx can be manipulated back into place. If reduction is still unsuccessful, then a volar incision may be added in order to move the flexor tendons and volar plate out of the joint. A Bruner incision centered at the MP flexion crease can help access these structures, but two incisions should be avoided if possible to prevent excessive swelling and increased stiffness. (Citation needed)
Stability of the joint should be assessed after successful reduction. Limits to radial and ulnar deviation should be present. If there is no hard stop during a deviation maneuver, the there may be a collateral ligament injury, and this may need to be repaired. X-rays should be repeated to confirm reduction and paralleling of the joint surfaces.
MP joint dislocation should be suspected based on a traumatic event with sufficient force to tear joint ligaments. Examination usually confirms loss of function of the MP joint, and x-rays should demonstrate loss of joint surface contact. Reduction should be attempted under adequate anesthesia, and a specific maneuver should be preformed to prevent tightening of the structures around the metacarpal neck that may prevent reduction (see text and video). X-rays should always be performed after any reduction attempt, even if unsuccessful. Assessment of the stability of the joint should be preformed, and ligament repair should be considered if there is instability.
- Minami A, An KN, Cooney WP 3rd, Linscheid RL, Chao EY. J Orthop Res. 1984;1(4):361-8. Ligamentous structures of the metacarpophalangeal joint: a quantitative anatomic study.
- Rozmaryn LM. The Collateral Ligament of the Digits of the Hand: Anatomy, Physiology, Biomechanics, Injury, and Treatment. [Review] Journal of Hand Surgery – American Volume. 42(11):904-915, 2017 Nov.