The 2014 ABJS Nicolas Andry Award: The Puzzle of the Thumb: Mobility, Stability, and Demands in Opposition
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The paradoxical demands of stability and mobility reflect the purpose and function of the human thumb. Its functional importance is underscored when a thumb is congenitally absent, injured, or afflicted with degenerative arthritis. Prevailing literature and teaching implicate the unique shape of the thumb carpometacarpal (CMC) joint, as well as its ligament support, applied forces, and repetitive motion, as culprits causing osteoarthritis (OA). Sex, ethnicity, and occupation may predispose individuals to OA.
What evidence links ligament structure, forces, and motion to progressive CMC disease? Specifically: (1) Do unique attributes of the bony and ligamentous anatomy contribute to OA? (2) Can discrete joint load patterns be established that contribute to OA? And (3) can thumb motion that characterizes OA be measured at the fine and gross level?
We addressed the morphology, load, and movement of the human thumb, emphasizing the CMC joint in normal and arthritic states. We present comparative anatomy, gross dissections, microscopic analysis, multimodal imaging, and live-subject kinematic studies to support or challenge the current understanding of the thumb CMC joint and its predisposition to disease.
The current evidence suggests structural differences and loading characteristics predispose the thumb CMC to joint degeneration, especially related to volar or central wear. The patterns of degeneration, however, are not consistently identified, suggesting influences beyond inherent anatomy, repetitive load, and abnormal motion.
Additional studies to define patterns of normal use and wear will provide data to better characterize CMC OA and opportunities for tailored treatment, including prevention, delay of progression, and joint arthroplasty.
KeywordsUlnar Collateral Ligament Golf Swing Thenar Muscle Thumb Motion Dense Subchondral Bone
The opposable thumb, unique to our species, is one of our essential defining traits. In concert with the human brain, it orchestrates masterful tasks with refined flexibility, creating mobility and stability in paradox. The thumb works in precise coordination with the fingers, whether poised to grasp a fistful of arrows or pen the perfect opening line. The absence, injury, and loss of this critical digit are, appropriately, important topics to surgeons.
Classical treatises and qualitative investigations have been devoted to its purpose and function [9, 21, 34, 35, 105]. The current literature suggests a myriad of etiologies as to why thumb carpometacarpal (CMC) osteoarthritis (OA) is so common, including prevalent theories of anterior oblique ligament degeneration, ligamentous laxity, hormonal changes with menopause, genetic predisposition, repetitive use, and abnormal load transmission [4, 34, 35, 36, 53, 85, 86, 87, 88, 89, 107, 108]. But to date, the absence of quantitative biomechanical, live-subject, and large population studies has impaired formation of a consensus on this topic.
Image modalities related to morphology, load, and motion of the thumb CMC joint
Subject of image
Three-dimensional volumetric computed tomography (3-D CT)
Gorilla and human
Can arthritis be inferred from functional demand?
2nd CMC joint of the gorilla is weightbearing and susceptible to arthritis.
Photographs and video
Child with congenital difference and index finger pollicization
Can a ball-and-socket neotrapezium function like the saddle trapezium?
A simplified “thumb” saddle joint approximates precision grasp and pinch.
Gross inspection, photographs
Cadaveric trapezial specimens
Does bone shape influence motion and laxity?
Concave shape of both trapezial and glenoid surfaces correspond to least muscular support, and most common site of dislocation.
Gross dissections, immunohistochemistry, radiographic markings, arthroscopic evaluation
Cadaveric and surgical thumb CMC specimens
Does ligament integrity correspond to location and presence of proprioceptive mechanoreceptors?
Dorsal ligaments are strong, cellular structures in nonarthritic specimens, in contrast to volar ligaments. Arthritic ligaments have different mechanoreceptors than found in nonarthritic subjects.
Surgical explanted trapezia
Do different wear patterns exist in CMC arthritis?
Three distinct wear patterns occur in surgical patients, inferring differential load.
Flat panel CT, micro-CT, enhanced MRI
Surgical explanted and normal trapezia
How does trabecular bone differ in CMC arthritis?
Increase bone density occurs in the volar ulnar quadrant in CMC arthritis.
Human subject without arthritis grasping object
Can CMC motion be detected in functional loading?
Compression and rotation can be visualized but not quantified given superimposing structures.
Markerless bone registration (MBR) with 3-D CT
Human subjects with and without arthritis grasping, pinching, opening an object
Can CMC motion be quantified in functional loading?
Functional motion coupling occurs with each task. In pinch, the metacarpal translates, internally rotates, and flexes on the trapezium.
Interrelated upper limb motion
Motion analysis with reflective markers
Human subjects with and without arthritis grasping, pinching, opening an object
Can CMC motion be quantified and compared to adjacent joints in functional loading?
The sequence, timing, and compensatory motion of upper limb joints can be measured when CMC joint motion is restricted and altered.
Accessible interactive data
Various technology formats and platforms
Synthesis of various materials and studies
Can complex data information be compiled and repurposed for wide audiences?
An interdisciplinary team can create cross-platform educational modules compiled from complex datasets.
The dorsal deltoid complex emanates from the dorsal tubercle and distally fans across the dorsum of the metacarpal. The stout collagen is organized and cellular and primarily has the proprioceptive mechanoreceptors known as Ruffini endings present close to ligamentous attachments, demonstrated consistently in 10 normal specimens [45, 62, 65]. This supports earlier reports of the principal role of dorsal stabilizing ligaments [13, 36, 49, 50]. Conversely, the volar ligaments are thin, capsular structures, variable in location, with thenar muscles intimal to their presence. Whereas many investigators emphasize importance of the anterior oblique “beak” ligament in the presence and creation of arthritis [32, 33, 34, 35, 85, 86, 87, 88, 89], our studies have focused on normal anatomy analysis, which may in part address this discrepancy. The volar ligaments were lacking in normal ligamentous structure and also mechanoreceptor innervation. Since the volar ligaments in our studies were deemed insufficient to provide static stability, we believe the thenar muscles, intimal to the volar capsule, are of primary importance in volar CMC joint stability. A followup study we performed examining the ligaments of 11 arthritic subjects at time of surgery (10 women, one man) confirmed the predominance of mechanoreceptors in the dorsal radial ligament and paucity in the anterior oblique ligament . These subjects underwent surgery for symptomatic arthritis and thus may have altered neural and ligament structure compared to asymptomatic individuals; the causal relationship of symptoms, ligament pathology, and mechanoreceptors remains unclear.
Furthermore, the role of mechanoreceptors in supporting ligaments versus supporting muscles in the thumb CMC joint has not been determined. Muscle is rich in proprioceptive muscle spindles, and their presence in thenar muscles has been demonstrated with conditioning experiments in both central and peripheral evoked potential testing [95, 97]. These findings support thenar and complementary first dorsal interosseous strengthening in functional CMC rehabilitation with favorable MP flexed positioning, preventing hyperextension [78, 81, 90].
The ulnar complex creates a checkrein effect. The ulnar collateral ligament, or more appropriately, the volar trapeziometacarpal ligament, and the dorsal trapeziometacarpal ligament span from their more central locations proximally to a conjoined attachment directly ulnarly [12, 16]. The stout ulnar complex may be critical to volar concentration of forces.
The height, width, and articular facet interdependence of the CMC joint and other trapezial articulations require further investigation, as well as examining different populations and stages of disease [42, 70]. The role of proprioception, first described by hand philosopher Sir Charles Bell as “muscle sense” with a feedback loop for voluntary control [7, 8], constitutes a future investigation—potentially analyzing the muscle characteristics, sarcomeres, and possibly proprioceptive endings in the thenar muscles of asymptomatic and symptomatic subjects.
Articular and trabecular wear patterns provide an inference of biomechanical loading . The literature varies on reported load patterns in trapezial arthritis [3, 4, 32, 53, 54, 86, 89], with volar wear predominating [86, 87, 89, 107].
Areas of compressive loading have densely packed trabecular bone in volume and width. The vertebral bodies, hip, and knee are among the most widely studied with micro-CT, demonstrating dense subchondral bone and concentrated trabeculae [6, 17, 18, 29]. The trapezium’s middle 1/3 studied with micro-CT in normal and arthritic specimens demonstrated higher trabecular presence in the ulnar column of subchondral bone, with increased volume in the arthritic radial column .
We performed a complementary micro-CT study  evaluating quadrant [26, 108] trabecular characteristics in the trabecular midregion bone of 13 normal and 16 arthritic trapeziae from patients with mean ages of 69 and 61 years, respectively and male-to-female ratios of 4:9 and 10:6, respectively . ANOVA with post hoc Bonferroni/Dunn correction revealed no difference in bone volume fraction between the OA and normal specimens (p = 0.31) although OA trapeziae demonstrated higher trabecular number (p = 0.025) and connectivity (p = 0.018) than non-OA trapeziae. The volar ulnar quadrant of both populations consistently revealed higher bone volume fraction, trabecular number, and connectivity than the dorsoradial and volar radial quadrants. This demonstrates additional evidence of preferential loading to support the thumb’s anatomic juxtaposition to the fingers for grasp and pinch and suggests a stress concentration above the trapezoidal prominence.
Thumb Motion Relative to the Upper Limb
Discrete CMC joint biomechanical studies eliminate the role of fingers, especially as targets and loading platforms for the thumb, and their positional change with different tasks. When either the elbow [82, 96] or wrist  is constrained, other joints compensate; by inference, joint contribution is determined for functional tasks. Electrogoniometric studies with a gimbal coordinate system provide estimates on elbow  and wrist [19, 84, 94] ROM for functional activities. The functional CMC joint range is harder to assess even with constraining experiments [43, 55, 74], given the discrete but exacting motion of this joint and the functional restriction splints might impart .
Several of our experiments focus on analysis of thumb kinematics in relation to the upper limb. First, we simplified the coordinate system and image capture by modeling the phasic and relatively uniplanar orthograde nature of lower limb gait. We reported the upper limb kinematics of reach and grasp in 25 children with normal function and 12 children with spastic hemiplegia [22, 23]. The hemiplegic subjects demonstrated a contracted workspace. Compensatory trunk flexion, shoulder abduction, and forearm supination accompanied the contracted elbow and flexed wrist, and overall movements took longer, had more erratic trajectories, and lacked clear delineation of acceleration at midrange and deceleration at contact. An additional study of phasic golf swing  analyzed the interrelationship of the trunk, shoulder, upper limb, and wrist throughout the swing, with timing, wrist position, and peak torque reproducible among elite golfers. Amateur golfers had inconsistencies in timing, sequence, and reproducibility similar to the deviations seen with hemiplegic subjects. These studies provide the basis for examining the discrete, multiplanar tasks coordinating large and small joints of the upper limb.
Prediction of presence and progression of arthritis, as well as improved means to treat it, may be addressed with computational modeling that incorporates kinetic and kinematic data. Traditional cadaveric biomechanical studies approximate the force on a joint via applied extrinsic and intrinsic tendon loads and employing mechanical models with simplified joint constructs [26, 49, 51]. From such models, we understand the progressive force increase from the tip to the CMC joint, with the force across the CMC joint in grasp exceeding that of lateral pinch approximately 10-fold (120 kg versus 12 kg) . New methods incorporate historical information and contributions from our work using CT, MR, and live kinematic data of divergent geometric bone patterns, joint contact, and movement to predict contact pressures between general 3-D surfaces or to predict functional movement and wear. We are pursuing these with collaborations at Brown University and University of Auckland, including the potential use of high-resolution registration with dual fluoroscopy [73, 104]. The computational methods employ either statistical shape modeling [5, 41, 66] or rigid body dynamics , integrating 3-D data to develop subject-specific models to predict changes over time, such as patterns of joint degeneration, response to fracture, or ligament disruption.
For motion analysis, the complexity of quantifying nonphasic functional activity in a sphere of motion with task measured in a discrete area poses many challenges. We strive for incorporating real-time analysis of complex motion that is readily reproduced and reproducible to quantify motion in congenital anomalies, neurologic and musculoskeletal injuries, and arthritis. Our arthritic subject in the kinematic study responded to visual coaching by reviewing the animation, and subsequent trials produced no compensation. We have pursued pilot studies to create biofeedback tools, including auditory feedback with sonification  for the golf swing, cerebral palsy, and the visual and performing arts . Visual and auditory feedback suggests possible aids for rehabilitation or retraining to correct or restore mechanics associated with imbalance, disease, or injury. We have explored using accelerometers as joint sensors, EMG coupling with motion analysis, and a data glove to either supplement or replace the cumbersome motion analysis setup . Technical considerations of size, ease of use, and quantifying fine coordinated motion required in a discrete area remain a challenge.
At Our Fingertips: Accessible Information
Recent technological developments have permitted improved acquisition and interpretation of the morphology, load, and motion of the thumb, especially in live subjects, which provide clues to the complexity of the thumb as an organ system. The ultimate goal is to decode the action of the musculoskeletal system in both the normal and pathologic states across a spectrum of human development, ethnic diversity, and sex differences. Our investigations in this article emphasizing thumb CMC OA and its pathophysiology have been possible with recent advances in computational power, imaging capabilities, and software enhancements in the realm of “disruptive innovation”  that create new opportunities for acquisition and dissemination of information.
Additional studies in the realm of multimodal technology that define patterns of normal use and wear will provide data to better characterize CMC OA and opportunities for tailored treatment, including prevention, delay of progression, and joint arthroplasty. Such innovation allows us to expand on current methodologies and refurbish others so that deciphering the puzzle of the thumb—in all its beauty and weakness—is well within our grasp.
The breadth of collaborative colleagues includes medical students, graduate students, residents; colleagues at the Stanford University Chase Hand Center, Department of Orthopaedic Surgery, Division of Anatomy, Department of Radiology, Department of Bioengineering; Stanford University Anatomy of Movement class; Brown University; and the Karolinska Institutet. Dr. Robert Chase’s lifelong pursuit of teaching human anatomy, with special emphasis on the thumb, represents the primary inspiration for this work.
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