The thumb carpometacarpal (CMC) joint is one of the most important joints of the thumb and hand, displaying a wide arc of motion with stabilization provided by a complex interaction of internal and external forces.
Over time the thumb CMC joint is subjected to large and repeated forces during use, leading to degenerative changes in susceptible individuals.
Depending on the severity and progression of the thumb CMC joint condition, treatment may include palliative nonsurgical and surgical approaches. Therapists have a large role in both approaches.
Nonsurgical approaches include pain-relieving modalities, orthosis support, targeted exercise, and joint protection.
Postsurgical management includes postoperative orthotic positioning, range of motion (ROM) exercises, edema control, strengthening, scar management, and functional retraining with specific time frames dependent upon the surgical procedure and the clinical setting.
The CMC joint of the thumb is located at the base of the first ray, between the first metacarpal and the trapezium. This joint is one of the most important joints of the thumb, and arguably the entire hand. Aptly referred to as the basal joint of the thumb, the CMC joint permits a wide arc of motion, which substantially amplifies the effectiveness of manual prehension. The CMC joint is stabilized during movement through a complex interaction of internal and external forces. Internal forces are produced by muscles and, to a lesser degree, passive forces created by the structural integrity and natural stiffness of the capsular ligaments. External forces are generated primarily by contact with the environment. Regardless of origin, the majority of forces that act on the active thumb pass directly through the joint surfaces. These “joint forces” can be surprisingly large and, in the healthy thumb, are usually safely absorbed by articular cartilage, periarticular connective tissues, and subchondral bone.
Over the years, an active person’s thumb CMC joint is subjected to a large amount of force. At some point the cumulative impact of the forces may exceed the load tolerance of the tissues. As a consequence the joint may begin to degenerate or, in some cases, develop osteoarthritis (OA), especially if the person is genetically predisposed to this pathology. Additional predisposing factors to CMC joint OA are listed in Box 106-1 . Interestingly, the last two factors may partially account for the greater frequency of OA of the CMC joints in females, most notably in postmenopausal women.
Subtle anatomic anomalies of the joint that create higher than normal contact stress
Acute trauma to the periarticular soft tissues
Fracture of proximal base of the thumb
Thinner layer of articular cartilage combined with relative joint incongruity
Hormonally linked excessive laxity of the capsular ligaments
If present, chronic inflammation (synovitis or capsulitis) of the CMC joint may erode and weaken the ligaments and cartilage, thereby reducing their ability to effectively absorb joint forces. Regardless of the contributory pathology, factors such as pain, muscle weakness and contracture, and instability of the thumb’s CMC joint can significantly reduce the function of the entire hand, as well as the upper extremity.
One goal of this chapter is to describe the primary pathomechanics associated with degenerative instability of the CMC joint of the thumb. As used throughout this chapter, the term instability is defined as a joint that exhibits gross abnormal alignment, often combined with excessive and aberrant mobility. An unstable joint is often painful and, over time, can become rigid and deformed.
Surgeons and hand therapists frequently work as a team to treat the underlying causes of an unstable CMC joint. Optimal treatment requires that the clinician appreciate the anatomy and biomechanical relationships at this joint and, in particular, how these relationships affect articular stability. The first part of this chapter reviews the basic structure and the function of the CMC joint of the thumb, with an emphasis on the structures that stabilize the articulation. This is followed by the description of a classic pattern of instability of the CMC joint, frequently caused by overuse or due to chronic OA. This example will highlight how the weakening of the periarticular connective tissues can initiate a destabilizing process that ultimately affects the mechanics of the entire digit. Understanding the pathomechanics associated with this clinical example establishes much of the rationale for the common forms of therapeutic management of the unstable CMC joint, the primary focus of this chapter.
Structural and Functional Considerations
The wide arc of motion permitted at the thumb’s CMC joint is facilitated by its relatively loose capsular ligaments, and also the joint’s interlocking saddle-shaped morphology. The only other saddle joint in the body is the sternoclavicular joint, which connects the entire upper extremity to the axial skeleton. Apparently, saddle joints are ideal basal joints that support a series of more distal articulations, affording both stability and mobility at the base of the extremity.
A saddle joint is so named because each articular surface is generally concave in one dimension and convex in the other. As indicated for the thumb’s CMC joint in Figure 106-1 , the transverse diameter on the articular surface of the trapezium is generally convex along a medial-to-lateral direction, which is functionally analogous to the side-to-side contour of a horse’s saddle. In contrast, the longitudinal diameter of the articular surface of the trapezium is generally concave from a palmar-to-dorsal direction, functionally analogous to the front-to-rear contour of a horse’s saddle. The articular surface at the base of the thumb metacarpal reciprocally matches the contours of the articular surface of the trapezium.
The primary motions at the CMC joint are palmar abduction and adduction , which occur in the near sagittal plane, and flexion and extension , which occur in the near frontal plane. Opposition and reposition are complex motions derived from these primary biplanar motions.
The arthrokinematics of abduction and adduction are based on the convex articular surface of the thumb metacarpal moving on the fixed concave (longitudinal) diameter of the trapezium. During abduction , for example, the convex articular surface of the metacarpal rolls palmarly almost 45 degrees, and slides dorsally on the concave surface of the trapezium ( Fig. 106-2, online ). Full abduction at the CMC joint elongates the adductor pollicis muscle and most ligaments at the CMC joint, especially those embedded within the posterior aspect of the joint capsule. Full abduction opens the webspace of the thumb, forming a wide curvature useful for grasping large and cylindrical objects.
The arthrokinematics of flexion and extension at the CMC joint are based on the concave articular surface of the metacarpal moving across the convex (transverse) diameter on the trapezium. From a position of full extension, flexion , for example, occurs as the concave surface of metacarpal rolls about 45 to 50 degrees and simultaneously slides in an ulnar (medial) direction ( Fig. 106-3, online ). A shallow groove in the transverse diameter of the trapezium helps to guide the concurrent 45 to 60 degrees medial rotation of the metacarpal.
Opposition involves a wide arc of motion that combines (palmar) abduction and flexion. The flexion component of opposition is mechanically linked to the medial rotation (pronation) of the thumb metacarpal. By inserting along the radial border of the first metacarpal, the opponens pollicis is in optimal alignment to flex and medially rotate the thumb metacarpal, an action that helps to guide its articular surface through a slight groove on the transverse diameter of the articular surface of the trapezium ( Fig. 106-4 ). Full opposition is considered the CMC joint’s close-packed position. This position is stabilized not only by a twisting of several capsular ligaments but also by activation of muscle. Although maximum in full opposition, only about half of the surface area of the joint is load-bearing. Considering the large and frequent loads (forces) that cross this joint, the relatively small contact area may naturally predispose the joint to large and potentially damaging stress. Recall that the stress (or pressure) produced across a joint is inversely proportional to the surface area in contact within the joint.
In addition to the axial rotation of the metacarpal during opposition, fluoroscopic observation reveals a modest twisting of the trapezium relative to the scaphoid and to the trapezoid. These accessory motions in the adjacent carpal bones amplify the full extent of opposition of the thumb. Full opposition allows the tip of the thumb to make pulp-to-pulp contact with the four fingers. This digital interaction allows the palmar surfaces of the thumb and the hand to fit around an extraordinary number of objects of varied shapes and sizes, greatly enhancing the security and adroitness of prehension.
Reposition is a return motion from full opposition that sweeps the thumb back to the anatomic position. This motion combines adduction with a combined extension and lateral rotation (or supination) of the metacarpal.
The capsular ligaments of the CMC joint of the thumb are relatively loose-fitting, a necessity considering the joint’s large ROM ( Fig. 106-5 ). Many names have been used to describe the capsular ligaments at the CMC joint of the thumb. The number of named, distinct ligaments reported to cross the base of the thumb ranges from three to as many as seven. This chapter focuses on five capsular ligaments, which when pulled taut add an important element of stability to the joint ( Table 106-1 ). As a set, the ligaments help control the extent and direction of joint motion, maintain joint alignment, and dissipate forces produced by activated muscle. Failure of the capsular ligaments to perform these functions, whether from pathology associated with arthritis, consequences of aging, or cumulative trauma, can render the joint vulnerable to excessive stress and subsequent degeneration.
|Name||Proximal Attachment||Distal Attachment||Motions That Increase Ligament Tension|
|Anterior oblique †||Palmar tubercle on trapezium||Palmar base of thumb metacarpal||Abduction, extension, and opposition|
|Ulnar collateral ‡||Transverse carpal ligament||Palmar-ulnar base of thumb metacarpal||Abduction, extension, and opposition|
|Intermetacarpal||Dorsal side of base of second metacarpal||Palmar-ulnar base of thumb metacarpal (with ulnar collateral)||Abduction and opposition|
|Posterior oblique||Posterior surface of trapezium||Palmar-ulnar base of thumb metacarpal||Abduction and opposition|
|Radial collateral ‖||Radial (lateral) surface of trapezium||Dorsal surface of thumb metacarpal||All movements to varying degrees, except extension|
In general, extension, abduction, and opposition elongate most of the ligaments of the CMC joint of the thumb. The anterior oblique, radial collateral, and ulnar collateral ligaments have been described as primary dynamic stabilizers of the thumb, especially during pinching and opposition of the thumb. The anterior oblique ligament plays a particularly important role in two functions: (1) passively guiding the medial rotation of the metacarpal during full flexion and (2) restraining the extent of radial (lateral) translation of the metacarpal relative to the trapezium. Radial translation of the metacarpal is driven, in part, by the resultant line of force of the intrinsic flexor muscles of the thumb when performing lateral pinch. Evidence suggests that the large and prolonged forces produced by lateral pinch may, in some persons, be a precursor to degeneration of the CMC joint.
Muscle Actions Across the Base of the Thumb
Table 106-2 (online) lists the primary actions performed by the muscles that cross the CMC joint of the thumb. By necessity, these actions are associated with relatively large joint forces, potentially 10 to 12 times greater than the resistive (external) force applied to the distal end of the digit. The large joint forces ultimately reflect the fact that the leverage available to the muscles (based on moment arm length) is much smaller than the leverage available to the external forces. Even such a seemingly innocuous act as combing the hair, brushing the teeth, or grasping a hammer can, in theory, generate significant muscular-based loads across the CMC joint. A maximal-effort , lateral pinch can concentrate very large joint forces across a relatively small part of the joint’s articular cartilage, just adjacent to the proximal attachment of the anterior oblique ligament. The relatively limited area of this force can create large and potentially damaging intra-articular stress within this region of the joint.
|Flexion||Adductor pollicis, flexor pollicis brevis, flexor pollicis longus, opponens pollicis|
|Extension||Extensor pollicis brevis, extensor pollicis longus, abductor pollicis longus|
|Abduction||Abductor pollicis brevis, abductor pollicis longus|
|Adduction||Adductor pollicis, extensor pollicis longus|
|Opposition||Opponens pollicis, flexor pollicis brevis, abductor pollicis brevis, flexor pollicis longus, abductor pollicis longus|
|Reposition||Extensor pollicis longus, extensor pollicis brevis|
The naturally strongest motions of the thumb typically involve a combination of flexion and adduction of the CMC joint, usually associated with a strong lateral pinch. Figure 106-6 (online) shows the relative torque potential and actions of seven thumb muscles that cross the CMC joint, based on data from cadaver specimens. The location of the dot associated with each muscle indicates the actions of each muscle. For example, the adductor pollicis (oblique head) is shown producing a combined adduction and flexion torque across the CMC joint. Note that the maximum flexion torque is about 50% greater than the maximum adduction torque. As another example, the extensor pollicis longus is capable of producing a combined adduction and extension torque across the CMC joint, with the magnitude of each torque action being nearly equal (at about 0.5 Nm). Note that the oblique head of the adductor pollicis is theoretically capable of producing the greatest torque of all the thumb muscles that cross the CMC joint.
Instability and Deformity
In most persons, the CMC joint functions well throughout a lifetime of relatively large imposed stress. Forces across the joint are partially absorbed and resisted by healthy, strong capsular ligaments. In addition, intact and healthy articular cartilage can help dissipate joint forces. Unfortunately, however, in the case of trauma or disease, capsular ligaments may weaken and lose their ability to stabilize the base of the thumb. As will be described, loss of stability at the CMC joint of the thumb can lead to arthritic changes within the joint, as well as deformity in the more distal joints.
In 1968, Nalebuff outlined a system for classifying deformities of the thumb. (This system of classification is further described in Chapter 123 .) Although the deformities were originally described in the context of rheumatoid arthritis, they can result from any disease or injury that weakens the surrounding ligaments and capsule. The CMC joint is significantly involved in the pathomechanics of most of Nalebuff’s classic thumb deformities (types II–IV). Chronic synovitis associated with rheumatoid arthritis (RA) weakens the capsule and the supporting ligaments of the CMC joint. Subsequent loss in congruity between the joint surfaces typically increases stress on the articular cartilage due to reduced surface area available to disperse the forces. In cases of OA, the CMC joint is frequently affected along with degeneration of other adjacent articulations, especially between the trapezium and second metacarpal.
In either RA or OA, eventual subluxation of the CMC joint often initiates a kinetic imbalance in the more distal joints of the thumb. This phenomenon is nicely exemplified in the pathomechanics associated with the Nalebuff’s type III deformity of the thumb. As depicted in Figure 106-7 , the classic type III deformity is characterized by fixed CMC joint flexion and adduction, metacarpophalangeal (MCP) joint hyperextension, and interphalangeal (IP) joint flexion. This deformity is relatively common in the thumb with RA and typically worsens if left untreated. The following sequence of events is one likely scenario that can lead to a type III deformity.
Ligaments that normally reinforce the medial (ulnar) side of the CMC joint (such as the anterior oblique, ulnar collateral, and intermetacarpal ligaments) can become weak and/or partially ruptured due to chronic synovitis and overuse. The ligaments are no longer able to resist the radial-directed forces that naturally accompany the lateral pinch of the thumb. Subsequently, the base of the first metacarpal slides or migrates radially, or radial-dorsally, relative to the trapezium. Potentially damaging shear forces may concentrate near the palmar aspect of the trapezium, adjacent to the attachments of the anterior oblique ligament. Intrinsic muscles such as the adductor pollicis may become fibrotic and permanently contracted, thereby maintaining the deformity at the CMC joint and a narrowing of the first webspace. In addition, a partially subluxed or dislocated CMC joint affects the moment arms of the muscles that cross the region, which may bias further radial migration of the metacarpal.
Instability of the basal joint of the thumb favors a “zigzag” deformity of the more distal joints. The term zigzag describes a collapse of multiple interconnected joints in alternating directions. In the example of the type III deformity shown in Figure 106-7 , active attempts of extending the thumb away from the palm overstretch the palmar (volar) plate at the MCP joint, especially if synovitis is involved. Overstretching the palmar plate and associated capsule typically leads to a hyperextension deformity at the joint. Subsequent “bowstringing” of the tendons of the extensor pollicis brevis and longus across the hyperextended MCP joint increases the muscles’ leverage to extend the joint, thereby further accentuating the hyperextension deformity. The IP joint may remain flexed due to the passive tension in the stretched and excessively taut flexor pollicis longus.
The pathomechanics associated with the deformity displayed in Figure 106-7 demonstrate the importance of stability or “balance” at the CMC joint. The principles behind most common forms of nonsurgical and surgical therapeutic management of an unstable CMC joint are described in the remaining sections of this chapter.
Nonsurgical and Postsurgical Therapeutic Management of the Carpometacarpal Joint of the Thumb
The first section of the chapter described how weakened periarticular tissue along with excessive forces may cause degeneration, OA, instability, and deformity of the CMC joint of the thumb. Medical treatment typically attempts to reduce the destabilizing forces and accompanying pain, improve joint alignment and integrity, and restore function. Depending on the severity and progression of the condition, treatment may include relatively conservative and palliative nonsurgical therapeutic management, or more progressive surgical management. As will be described, therapists have a large role in both approaches.
Nonsurgical Therapeutic Management
Nonsurgical therapeutic management of the unstable and painful CMC joint typically includes a thorough evaluation of the pain and function of the hand, with careful attention paid to the underlying pathology or cause of the impairments. Common methods of conservative (nonsurgical) treatment include pain control, orthotic positioning, teaching principles of joint protection, exercise, thermal modalities, nonsteroidal anti-inflammatory drugs (NSAIDs), and corticosteroid injections. Table 106-3 (online) lists typical goals for nonsurgical management of the unstable and often painful CMC joint, along with common therapeutic interventions.