In 1833, Sir Charles Bell wrote: “On the length, strength, free lateral motion, and perfect mobility of the thumb, depends the power of the human hand.” With such a significant portion of hand function being attributed to the thumb, it is curious that so little is found in the literature regarding the biomechanics of splinting the thumb. Whether thumb motion is affected by an isolated injury or is the result of severe hand trauma or stiffness, an in-depth understanding of its complex biomechanics is required to enable precise splinting.
Motions of the Thumb Joints
Individual thumb joint motions, as well as the complex composite motions, are reviewed in the following sections. Before one is able to comprehend the complex motions of the thumb, a clear understanding of the individual joint biomechanics is necessary.
The Interphalangeal Joint
Flexion and extension describe the motions of the stable hinge joint of the thumb interphalangeal (IP) joint. The inherent lateral stability of the IP joint allows force to be efficiently transmitted to the thumb tip. Flexion of the IP joint is in a plane perpendicular to the fingernail ( Fig. 116-1 ). Extension of the thumb IP joint is in the opposite direction but identical plane. Some describe limited pronation of the IP joint as it flexes due to the greater radial condyle height.
The thumb IP joint may readily hyperextend as much as 15 to 20 degrees or more and flex as much as 80 degrees. Individuals with normal thumb IP hyperextension often incorporate the hyperextension during pinching because this allows the thumb pulp and the index finger pulp to have full contact ( Fig. 116-2 ). Normal hyperextension of the uninjured thumb should be measured, and if present, regaining hyperextension should be a goal of mobilization splinting.
Although slight flexion is considered the ideal position for thumb IP joint fusion, the loss of full IP joint flexion greatly limits the ability of the thumb to participate in fine motor skills. In a patient’s dominant hand, preserving full IP joint motion is vital for activities such as sewing, model building, and other precision work.
The Metacarpophalangeal Joint
The thumb metacarpophalangeal (MP) joint combines characteristics of both a condyloid and ginglymus joint and has been described as having “no exact mechanical equivalent.” Flexion and extension, ulnar deviation (adduction) and radial deviation (abduction), and rotation are all present at this joint. Flexion and extension are primary motions, with radial and ulnar deviation and rotation being accessory motions. The accessory motions of the MP joint are a reflection of the related motion occurring at the carpometacarpal (CMC) joint. For example, during thumb opposition, the thumb MP joint radially deviates approximately 20 degrees and rotates slightly to aid the thumb pulp meeting the index finger pulp. The combined mobility of the thumb MP joint and CMC joint range of motion (ROM) allows a greater functional ROM because abduction occurs simultaneously at both joints. It is important to note that one cannot fully flex the MP and IP joints of the thumb while holding the CMC joint in an abducted or extended position, a relevant point when splinting.
The shape of the head of the first metacarpal determines the ROM for flexion and extension of the MP joint, and the degree of ligament laxity determines the amount of adduction, abduction, and rotation available at the joint. The configuration of the collateral ligaments of the MP joint of the thumb allows stability of the MP joint in both flexion and extension ( Fig. 116-3 ).
The normal ROM at the thumb MP joint is more variable among individuals than either the thumb IP joint or the CMC joint and is said to have the greatest variation of any joint in the body. The MP joint may flex only a few degrees in some individuals and bend to 90 degrees in others. Those with naturally limited MP joint flexion often tend to have a greater range of IP joint flexion, and those with a larger range of MP joint flexion may have more limited IP joint motion.
Whenever splinting or mobilizing the MP joint of the thumb for flexion, one must examine and measure the contralateral thumb to establish realistic ROM goals.
The Carpometacarpal Joint
The large ROM of the thumb CMC joint reflects the lack of bony stability of the joint and the accompanying ligamentous laxity. Because of this laxity, most problems associated with the thumb CMC joint are problems of decreased stability rather than stiffness. Often, one encounters the competing goals of achieving stability and maintaining mobility at this joint.
The thumb CMC joint is a saddle joint with the primary contours allowing motion to occur at right angles. The convexity at the proximal end of the first metacarpal allows flexion and extension, and the convexity of the distal aspect of the trapezium allows abduction and adduction. The ligamentous laxity of the joint allows rotation (described by some as pronation) ( Fig. 116-4 ). Zancolli describes retropulsion, antepulsion, adduction, and abduction as the four directions of movement possible at the CMC saddle joint; others reserve the use of the terms retropulsion and antepulsion for combined joint motions. In any case, there is consensus regarding the description of specific static positions of individual thumb joints, although there may be controversy regarding the terminology to describe combined motions.
Retropulsion describes the position of the first metacarpal rising above the plane of the other metacarpals ( Fig. 116-5 ). This motion is full abduction and extension of the thumb CMC joint in concert with full extension (and abduction) of the MP and IP joints of the thumb. This position may also correctly be described as full thumb extension and abduction. When less than full retropulsion (extension and abduction) of the thumb is attained but full abduction of the first metacarpal is achieved in the same plane as the other metacarpals (as when the hand is flat on a surface), this may be described as radial abduction ( Fig. 116-6 ).
When the thumb moves toward the fingertips and continues to move across the palm to the ulnar border, the terminology may be somewhat confusing. When the thumb rests against the radial aspect of the index finger, it is described as zero position or adduction ( Fig. 116-7 ). Note that this describes only the motion of the CMC joint. Some clinicians refer to palmar adduction when the thumb is held against the hand on the palmar surface of the index finger. These two positions reflect a change in the position of the MP and IP joints, but the position of the CMC joint remains relatively unchanged.
When the IP, MP, and CMC joint motions are combined, the terms opposition and circumduction describe these composite motions. If the plane of the fingernail stays at a 90-degree angle to the plane of the palm and the thumb is pulled directly away from the index finger in a plane 90 degrees to the palm, this is termed palmar abduction ( Fig. 116-8 ). As the thumb proceeds toward the tips of the ulnar fingers, rotation at the CMC joint is accompanied by rotation and extension at the MP and IP joints. When full thumb rotation has occurred, the angle of the thumbnail becomes almost parallel to the plane of the palm, creating a position of opposition ( Fig. 116-9 ). When the thumb is touching the base of the ulnar digits and the fingernail is more parallel to the plane of the palm, the CMC joint is maximally rotated and flexed, describing the position of antepulsion ( Fig. 116-10 ).
The collective motion from the position of maximum radial abduction and extension (retropulsion) to the ulnar border (antepulsion) of the hand is termed circumduction. Circumduction describes the maximum arc of motion of the CMC joint accompanied by the associated MP and IP joint motion, whereas the previously defined terms describe a position of a joint or joints.
Specific Joint Problems
The Interphalangeal Joint
The thumb IP joint, a stable hinge joint, is vulnerable to injury because it is involved in all manipulative tasks. Crush injuries or lacerations are usually the cause of IP joint injury, whereas sprain and dislocation injuries to the thumb IP joint are rare. This may be related to the greater stability of the IP joint because of the shorter lever arm of the distal phalanx. Because the stability of the IP joint is greater than either the MP or CMC joints, force is transferred to the looser proximal joints and they are more frequently injured.
Capsular Tightness
When IP joint flexion is limited following a fracture, dislocation, or crush injury, the thumb MP and CMC joints must be stabilized by a splint so the flexion mobilization force can be directed specifically to the IP joint ( Fig. 116-11 ). As the flexion range increases, the outrigger is shortened to maintain the line of pull at an angle of 90 degrees to the axis of the distal phalanx.
Splinting to regain full IP joint extension is often at the request of the patient to reestablish the normal hyperextended pinch pattern. This may be accomplished with serial splinting, either with a molded palmar thermoplastic shell or a small serial plaster cast. At times it may be better to provide a dynamic extension mobilization force to this joint if the other alternatives are unrealistic in relation to the patient’s lifestyle or if alternating flexion and extension are required.
Instability
Instability of the thumb IP joint is rarely seen after injury but may be present as a result of osteoarthritis or rheumatoid arthritis of this joint (see rheumatoid arthritis discussion later in this chapter). Lateral instability, occasionally seen in osteoarthritis, allows excessive radial deviation at the thumb IP joint during pinch. Although fusion of the joint can eliminate pain and provide stability to increase pinch strength, a small splint can retain some mobility in the flexion/extension plane while stabilizing the joint in the radial and ulnar planes. A temporary splint may be made of thermoplastic material ( Fig. 116-12 ), or the patient may choose a long-term Siris splint, custom made of sterling silver (available from Silver Ring Splint Company, Charlottesville, Virginia).
The Metacarpophalangeal Joint
Although capsular tightness may limit thumb MP joint motion following fracture, dislocation, tendon injury, or crush injury, the most common injury to the thumb MP joint is injury to the ulnar collateral ligament (UCL), followed by injury to the radial collateral ligament.
Collateral Ligament Injury
A fall on the outstretched hand forces the thumb MP joint into radial deviation (abduction), injuring the UCL. The injury may be partial where stability of the joint is retained, or the injury can result in a complete tear causing the joint to no longer be stable. Ligamentous integrity is important at the MP joint to provide stability during pinching and grasping of large objects.
Many use the term gamekeeper’s thumb to describe a thumb MP joint UCL injury. Originally called gamekeeper’s thumb because of the chronic UCL strain seen as a result of the gamekeeper’s twisting the heads of wounded rabbits, it is recommended now that the term gamekeeper’s thumb be reserved only for chronic strains of the UCL. Acute injuries are best described as skier’s thumb because of the frequency of this injury when the skier falls on the open hand.
It is generally agreed that incomplete tears of either the UCL or the radial collateral ligament can be appropriately treated with immobilization splinting to prevent radial and ulnar deviation. Surgical repair is required for complete lesions.
Immobilization can be accomplished with a casting material, or in compliant patients, a removable splint that immobilizes the MP joint is acceptable. Although some authors illustrate a thumb MP and CMC joint immobilization splint that includes the wrist, shorter splints that do not impede the CMC joint or wrist are also recommended. A well-fitting splint need not include the wrist to protect the thumb MP joint ( Fig. 116-13 ).
Even patients with chronic MP joint instability may benefit from a period of immobilization splinting and antiinflammatory medication. If the instability is relatively mild, the patient may manage symptoms by intermittent use of the splint. The splint is ideally used for protection during resistive activities that create symptoms but also may be helpful to provide a period of immobilization to diminish symptoms following aggravating activity.
To completely immobilize the thumb MP joint, the splint must obtain adequate purchase on the thumb metacarpal and the proximal phalanx. A small extension can be added to a CMC joint immobilization splint to encircle the proximal phalanx, preventing motion at the MP joint (see Fig. 116-13 ). The patient retains the ability to pinch while in the splint because the IP joint is free. The advantage to this splint design is that it opens and closes around the proximal phalanx to allow more specific stabilization of the proximal phalanx. Splints designed to slip over the IP joint usually allow some MP joint motion because the IP joint is always larger than the proximal phalanx. An identical design may be used for a period of continued immobilization after surgical repair of a collateral ligament injury.
Capsular Tightness
Ligamentous injuries to the MP joint or fractures at or near the MP joint often limit active and passive motion. If an MP joint flexion mobilization force is attached to a wrist cuff, the looser CMC joint will be more influenced by this force than the stiff MP joint. To mobilize the thumb MP joint into flexion, the CMC joint must be stabilized before adding a flexion mobilization force at a 90-degree angle to the axis of the proximal phalanx of the thumb ( Fig. 116-14 ). As the thumb MP joint motion increases, this outrigger length is shortened to maintain the correct line of pull.
The Carpometacarpal Joint
As discussed previously, the configuration of the trapeziometacarpal ligaments and the saddle shape of the CMC joint allow rotation and the four other motions of flexion/extension and abduction/adduction (see Fig. 116-4 ). The trapezium, on which the first metacarpal articulates, sits palmar of the other carpal bones, allowing the thumb to move in an arc around the fingers. Because of its large ROM and normal capsular laxity, the CMC joint most often requires immobilization splinting rather than mobilization splinting.
Osteoarthritis
The most common problem seen at the thumb CMC joint is increased laxity as a result of osteoarthritis (see Chapter 101 ).
As the laxity of the CMC joint increases with osteoarthritis, pain becomes present with resisted motion (pinch) ( Fig. 116-15 ). Before arthroplasty is considered, corticosteroid injections, oral antiinflammatory drugs, and splinting may be used to control the symptoms.
Most splints that immobilize the thumb CMC joint include one or both adjacent joints. * The author advocates a smaller splint that immobilizes only the thumb CMC joint and can be more comfortably worn during activities ( Fig. 116-16 ). When the thenar muscle bulk increases during pinch, the splint stabilizes the first metacarpal against the molded palmar portion of the splint.
* References .
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