Biomechanics of Hand and Wrist Deformities in Rheumatoid Arthritis
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Overview[edit | edit source]
The structure of the hand provides a great deal of insight into its function. Its multi-joint structure allows the hand to convey forces while simultaneously allowing for intricate movements at the fingers. The carpal tunnel in the wrist allows a large range of motion to finely position the hand in space, without crushing the tendons that are transmitting forces from the large forearm muscles to the hand. As these structures are damaged by rheumatoid arthritis, their capacity to perform their function decreases, severely limiting a person’s ability to adequately complete activities of daily living.
Rheumatoid Arthritis is a systemic, inflammatory disease that can severely compromise hand and wrist function. It is caused by the inflammation of the synovium, which not only lines the many synovial joints in the hand, leading to stiffness and deformity, but also surrounds the tendons that transmit force from the forearm to the fingers. This can cause tendon rupture and inflammation, leading to further weakness and deformity in the hand.
This article examines the biomechanics of the hand in patients with rheumatoid arthritis. It explores the pathogenesis of a variety of hand deformities caused by rheumatoid arthritis, before briefly outlining how this changes hand function.
Radial Deviation and Ulnar Drift[edit | edit source]
Radial deviation of the wrist and ulnar drift at the carpometacarpal joint tend to happen simultaneously. The more that the wrist is deviated radially, the more the fingers deviate in the ulnar direction. This is thought to be due to the biomechanics of the hand as an intercalated segment. The muscles pull the distal bones proximally, causing a collapse into a zig-zag deformity. Typically this deviation gets more extreme as the disease worsens. Other prominent theories of the mechanism of ulnar drift include collateral ligament laxity allowing the drift, increased intra-articular pressure causing deformity, and muscle imbalance. Many activities of daily living also pull the fingers in the ulnar direction, such as holding a cup of coffee. These activities may increase ulnar drift, especially if rheumatoid arthritis is already causing joint laxity.
The main casualty of this deformity is the loss of a range of motion at the wrist. Although patients can still carry out activities of daily living, they may have to compensate for the lack of wrist motion in other ways.
Z Deformity of the Thumb[edit | edit source]
Synovitis of the joint capsule at the metacarpophalangeal (MCP) joint of the thumb causes weakening of the capsule and its ligament. The resulting stretching of Extensor Pollicis Brevis (EPB) allows the Extensor Pollicis Longus (EPL) tendon to sublux. It has also been suggested that the insertion point of EPB may contribute to this disorder. The new volar placement of the tendon forces the MCP joint into flexion, secondarily causing the interphalangeal (IP) joint to hyperextend.
Z deformity of the thumb prevents pinching because the force of the tendons causes the thumb to collapse into MCP flexion and IP hyperextension. Thumb function is important for opposition, which is essential for pinches and grips. Therefore, this deformity can severely interrupt activities of daily living requiring opposition.
Swan Neck Deformity[edit | edit source]
Swan neck deformity can either be caused by flexion at the Distal Interphalangeal (DIP) joint, or by hyperextension at the Proximal Interphalangeal (PIP) joint. In the former, arthritic weakening of the extensor tendon at the DIP causes either stretching or rupture, and the tendon is unable to extend the distal segment. In the latter, rheumatoid arthritis causes inflammation at the PIP joint. This leads to laxity and weakness of the ligaments, joint capsule and structures designed to provide stability to the joints, allowing the PIP to be hyperextended. An alternate cause of hyperextension is that the flexor tendon becomes ruptured. As the PIP joint becomes hyperextended, the extensor tendon becomes shortened and is less able to extend other joints. The resulting tendon laxity leads to an inability to extend the DIP joint. The hyperextension of the PIP also stretches the flexors on the opposite side of the finger, causing them to pull more strongly on the distal segment. This also leads to DIP flexion.
The hyperextension of the PIP joint prevents the formation of a fist, impairing the person’s ability to grasp objects.
Boutonniere Deformity[edit | edit source]
In Boutonniere Deformity, synovitis of the PIP joint causes joint flexion. This may be due in part to damage to the central slip of the extensor tendon, which prevents the joint from extending. Further destruction to the joint tissues by rheumatoid arthritis causes subluxation of the lateral bands. The changed position of the lateral bands means that they pull the joint into flexion, rather than extension. This causes secondary changes in the nearby joints, pulling the DIP joint into extension, due to tightness of the extensor tendon and the shortened flexor tendon. At first the finger may be passively straightened out, but eventually it gets stuck in this position.
A person with a boutonniere deformity might also extend their MCP joints to compensate for PIP joint flexion. As rheumatoid arthritis progresses, and the deformity becomes severe, hand function can become severely limited as they cannot extend their fingers. However, the MCP joint arc of motion is not affected.
Flexor Tenosynovitis and Trigger Finger[edit | edit source]
Flexor tenosynovitis occurs when the synovial tendon sheath is inflamed by rheumatoid arthritis. It occurs in 55% of patients with rheumatoid arthritis. The inflammation and thickening of the tendon sheath can lead to secondary pathologies such as carpal tunnel syndrome. Thickening of the tendon pullies, where the flexor tendons pass through in their hands, can lead to a condition known as trigger finger. Patients may feel a “grinding” sense, as the flexor tendons pass through the pullies. In earlier stages of the disease, the stronger flexors will still be able to pull the hand into flexion, but the weaker extensors will not be able to extend the fingers, as the flexor tendons are stuck in flexion.
In trigger finger, the condition will cause difficulty in finger flexion and extension, and the hand may eventually become stuck in flexion. Flexor tenosynovitis can lead to hand weakness, especially in grasps requiring strong flexion. Bone spurs may cause tendon rupture, leading to further hand weakness and an inability to flex the fingers impacted.
Extensor Tendon Rupture[edit | edit source]
Extension tendon rupture can occur when rheumatoid arthritis causes synovial inflammation of the tendon sheaths. This weakens the tendons, and they eventually rupture. It can also be caused by attrition of the tendons on the ulnar head. Typically the Extensor Digiti Minimi (EDM) tendon ruptures first (excluding the thumb), so by testing for EDM function, clinicians can prevent the ruptures of the remaining tendons.
Extensor tendon rupture can lead to malpositioning of the fingers and painful movement, eventually causing stiffness as motion is limited. However, patients learn to adapt as the function degenerates, and it may not be as obvious as an acute rupture.
Opera Glass Hand[edit | edit source]
In extreme cases, rheumatoid arthritis can lead to bone reabsorption and shortening. This starts at the articular surfaces, and as it progresses, bones become shorter. In a healthy hand, the flexor and extensor tendons pull the distal segments into the proximal segments, creating stability, along with the shear force that they produce that causes rotation around the joint. In the opera glass hand, this compressive force pulls the shortened bones into the hand, leading to collapse of the fingers in a manner similar to how an opera glass might be collapsed into itself. The combination of the bone reabsorption, and external and internal forces can also lead to dislocation.
This deformity causes significant disability. The collapse shortens the span of the fingers, creating a smaller arc of grasp, but unequal shortening between the fingers can also make it difficult to use them. The bone reabsorption at the joint causes profound instability, and also angular deformity. All of these factors make it difficult to complete activities of daily living.
Conclusion and Further Resources[edit | edit source]
Without the appropriate intervention, deformities caused by rheumatoid arthritis will continue to worsen and cause functional issues for the person suffering from the disease. Fortunately there continues to be a great deal of research and development of interventions for both rheumatoid arthritis and each deformity individually. Furthermore, physical therapy can improve the hand function that each person requires to complete their activities of daily living, and also introduce behavioral modifications to decrease joint loading. For more information on hand rheumatoid arthritis and its rehabilitation, consider the following resources:
- The American College of Rheumatology
- Kelley and Firestein’s Textbook of Rheumatology
- Essentials of Physical Medicine and Rehabilitation
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