JBJS | Influence of Metacarpophalangeal Joint Position on Basal Joint-Loading in the Thumb

The Journal of Bone & Joint Surgery, Volume 83, Issue 5

Articles | May 01, 2001

Influence of Metacarpophalangeal Joint Position on Basal Joint-Loading in the Thumb

Mark J.R. Moulton, MD; Michael A. Parentis, MD; Matthew J. Kelly, MD; Christopher Jacobs, PhD; Sanjiv H. Naidu, MD, PhD; Vincent D. PellegriniJr., MD

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Investigation performed at the Department of Orthopaedics and Rehabilitation, The Pennsylvania State University College of Medicine, Milton S. Hershey Medical Center, Hershey, Pennsylvania
Mark J.R. Moulton, MD Michael A. Parentis, MD Matthew J. Kelly, MD Christopher Jacobs, PhD Sanjiv H. Naidu, MD, PhD Vincent D. Pellegrini Jr., MD Department of Orthopaedics and Rehabilitation, The Pennsylvania State University College of Medicine, Milton S. Hershey Medical Center, 500 University Drive, Hershey, PA 17033. E-mail address for V.D. Pellegrini: vpellegrini@psu.edu
No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

The Journal of Bone & Joint Surgery. 2001; 83:709-716

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Abstract

Background: Conventional wisdom holds that hyperextension of the metacarpophalangeal joint of the thumb is secondary to degenerative subluxation of the trapeziometacarpal joint as occurs in osteoarthritis. We propose that a hypermobile metacarpophalangeal joint may have a causative role in the development of primary osteoarthritis at the base of the thumb by concentrating forces on the palmar aspect of the trapeziometacarpal joint.

Methods: Twenty fresh-frozen cadaveric forearm specimens were obtained post mortem from donors with no history of connective-tissue disease. Each specimen was categorized by its passive range of metacarpophalangeal joint motion. Testing was conducted with Fuji ultra-low-pressure-sensitive film while the hand was in the lateral-pinch mode with the metacarpophalangeal joint in each of the following positions: unrestrained, pinned in neutral, pinned in 30° of flexion, and pinned in maximal hyperextension. Quantitative analysis of the trapezial contact surface at each of the metacarpophalangeal joint positions was performed, and the center of pressure was determined. Each specimen was then classified according to the extent of arthritic disease (nonarthritic, moderately arthritic, or affected by end-stage arthritis).

Results: In specimens affected by end-stage osteoarthritis, the center of pressure on the trapeziometacarpal joint moved dorsally by 56.8% of the length of the trapezial surface with metacarpophalangeal joint flexions of 30° (p < 0.01), whereas the corresponding values were 28.2% and 40.9% in the hyperextended and neutral metacarpophalangeal joint positions, respectively. In specimens with moderate osteoarthritis, 30° of metacarpophalangeal joint flexion also produced the most dorsal trapeziometacarpal center of pressure (44.8%); however, this center of pressure was not significantly different from the centers of pressure at the other metacarpophalangeal joint positions. In nonarthritic specimens, the center of pressure was again significantly more dorsal with metacarpophalangeal joint flexion of 30° than it was at the other positions (p < 0.01).

Conclusion: Metacarpophalangeal joint flexion effectively unloaded the most palmar surfaces of the trapeziometacarpal joint regardless of the presence or severity of arthritic disease in this joint.

Clinical Relevance: The presence of hyperextension laxity of the metacarpophalangeal joint may identify individuals who are predisposed to the development of arthritis of the trapeziometacarpal joint; such individuals might benefit from early intervention to stabilize the metacarpophalangeal joint and thus to retard the natural progression of osteoarthritic disease at the base of the thumb. Likewise, in symptomatic patients with a hypermobile metacarpophalangeal joint, fixation of the metacarpophalangeal joint in flexion by either splinting or surgical stabilization may alleviate basal joint symptoms by redirecting trapeziometacarpal joint forces away from the palmar compartment and onto the healthier dorsal aspect of the joint.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

The articulation between the first metacarpal base and the trapezium is functionally unique and a distinguishing feature of the human thumb. The mobility of the trapeziometacarpal joint is derived from the differing radii of curvature of the concavoconvex articular surfaces of its saddle-joint configuration. This incongruous osseous architecture has little inherent constraint, and the trapeziometacarpal joint is largely dependent upon soft-tissue structures for stability.

Generalized ligamentous laxity has been implicated as an important predisposing factor for idiopathic osteoarthritis of the trapeziometacarpal joint¹. Moreover, the stabilizing role of the superficial ligaments has been greatly overestimated^2-8. Anatomic studies have demonstrated that the intracapsular palmar beak ligament has a critical role in providing functional stability to the trapeziometacarpal joint^9-12, and specimens affected by end-stage osteoarthritis have a nonfunctional beak ligament¹³. Analysis of both surgically harvested and cadaveric osteoarthritic specimens has demonstrated the metacarpal base and trapezial surfaces to have a distinctive geographic pattern of eburnation^10,11,14-17. Disease first appears in the palmar compartment, directly adjacent to the beak ligament attachment, where joint surface contact primarily occurs during functional activities. Involvement of the articular surface then typically proceeds in a dorsoradial direction. Importantly, the degeneration of the metacarpal insertion of the beak ligament in cadaveric specimens has been closely correlated with the presence of advanced osteoarthritis¹⁸ and has been implicated in the preferential degeneration of the adjacent palmar articular surface of the trapeziometacarpal joint.

Conventional teaching holds that instability of the metacarpophalangeal joint is secondarily acquired as a compensatory response to progressive subluxation of the trapeziometacarpal joint and adduction of the first ray resulting from osteoarthritic disease^14,19-23. We suggest that, in some patients, hypermobility of the metacarpophalangeal joint may play an important causative role in the expression of osteoarthritis at the base of the thumb. By encouraging reciprocal flexion of the metacarpal shaft and dorsal subluxation of its base, hyperextension of the metacarpophalangeal joint concentrates load on the palmar trapeziometacarpal joint surfaces, which are already predisposed to the development of osteoarthritis. The purpose of this investigation was to test the hypothesis that increased ligamentous laxity resulting in hyperextension of the metacarpophalangeal joint may accelerate the development of trapeziometacarpal osteoarthritis by adversely influencing loading patterns at the base of the thumb.

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Fig. 1:: Schematic drawing depicting the five motor-tendon units loaded in the lateral-pinch-simulation model. ADD = adductor, FPB = flexor pollicis brevis, APB = abductor pollicis brevis, EPL = extensor pollicis longus, and FPL = flexor pollicis longus.

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Fig. 2:: Clean digitized contact imprint used for determination of the center of pressure (+) from the weighted pixel gray-scale image of a right hand specimen tested in lateral pinch with the metacarpophalangeal joint unrestrained. The joint-surface loading in this specimen is concentrated in the volar compartment of the joint.

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Fig. 3:: Relationship of metacarpophalangeal joint position and surface contact area in the trapeziometacarpal joint for an aggregate of all specimens.

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Fig. 4:: Relationship of metacarpophalangeal joint position and surface contact area in the trapeziometacarpal joint for specimens with advanced osteoarthritic disease.

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Fig. 5:: Relationship of metacarpophalangeal joint position and surface contact area in the trapeziometacarpal joint for specimens with moderate osteoarthritic disease.

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Fig. 6:: Relationship of metacarpophalangeal joint position and surface contact area in the trapeziometacarpal joint for specimens with no or minimal osteoarthritic disease.

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Twenty fresh-frozen cadaveric forearm specimens were obtained post mortem from donors with no history of connective-tissue disease; thirteen specimens were obtained from female donors and seven, from male donors (average age at the time of death, 72.4 years). Prior to dissection and biomechanical testing, each specimen was categorized by its passive range of motion at the metacarpophalangeal joint.

A previously described cadaveric lateral-pinch model for dynamic testing of the trapeziometacarpal joint was utilized^12,13. Tendons of the extensor pollicis longus, flexor pollicis longus, and abductor pollicis longus were isolated in the midpart of the forearm, proximal to their natural retinacular pulleys at the wrist. The origins of the abductor pollicis brevis and the flexor pollicis brevis radial to the flexor pollicis longus tendon were dissected together, detached from the trapezium and the scaphoid tubercle, and elevated to expose the trapeziometacarpal joint capsule. The adductor pollicis and the ulnar portion of the flexor pollicis brevis were jointly dissected, in a similar fashion, from the palmar aspect of the third metacarpal. Each specimen was then mounted in a vice clamp on a perforated grid Plexiglas base. The five dissected tendon groups were then extended with monofilament wire and attached to monofilament polymer cord routed through holes in the Plexiglas base in order to recreate their physiologic direction of muscle force application (Fig. 1). The extrinsic tendons were passed close to their site of harvest in the forearm. The abductor pollicis brevis was routed tangential to the plane of the palm in the direction of the pisiform, and the adductor pollicis was extended with a wire through the third intermetacarpal space to the dorsum of the hand. In order to simplify the model and to eliminate the tendency for collapse deformity, Kirschner wires were inserted to transfix the wrist in 30° of extension.

The index and long-finger metacarpophalangeal and proximal interphalangeal joints were similarly fixed in 90° and 30° of flexion, respectively, to provide a stable platform for the thumb in lateral pinch. The five motor-tendon units were then loaded with lead weights in a predetermined ratio to provide reproducible lateral pinch of the thumb with contact on the radial side of the distal interphalangeal joint of the index finger. Simultaneous loading of the extensor pollicis longus and abductor pollicis longus tendons produced a net extension moment, while loading of the abductor pollicis brevis, adductor pollicis, and flexor pollicis longus tendons created a flexion moment. Concurrent loading of the extensor pollicis longus (0.83 N), adductor pollicis (4.17 N), abductor pollicis brevis (5.0 N), flexor pollicis longus (8.34 N), and abductor pollicis longus (13.34 N) tendons in a 1:5:6:10:16 ratio, a modification of the method of Cooney et al.^13,24-26, was found to reproducibly provide appropriate thumb projection and simulation of lateral pinch. This resulted in predictable contact between the thumb pulp and the radial side of the index finger, generating a physiologic pulp pinch pressure of 0.05 to 0.1 MPa as determined by ultra-low-pressure-sensitive Fuji film (Fuji Medical Systems U.S.A., Stamford, Connecticut).

Fuji ultra-low-pressure-sensitive film was cut from a template of each joint into a paddle-shaped configuration to facilitate handling and to avoid smudging of its intra-articular portion. The round portion of each paddle was then inserted into the joint through an incision in the membranous thenar capsule between the abductor pollicis longus and the palmar beak ligament, avoiding disruption of these critical structures. Sequential testing in extension, flexion, and loaded lateral pinch was conducted three times for every specimen in each of four positions of the metacarpophalangeal joint. In this way a total of thirty-six contact imprints was generated for each trapeziometacarpal specimen. The metacarpophalangeal joint was sequentially tested in four predetermined positions: unrestrained, pinned in neutral, pinned in 30° of flexion, and pinned in maximal hyperextension allowed by the joint. Following testing in the unrestrained mode, the metacarpophalangeal joint of each specimen was sequentially secured in the three fixed positions by percutaneous placement of 1.14-mm Kirschner wires.

Prior to analysis of the Fuji film imprints, the handles were cut from each paddle to allow specific study of the joint contact patterns. Light staining at the periphery of each film paddle caused by scissor artifact was easily identified and excluded from the interpretation of the contact area. Pressure intensities were determined at various locations throughout each contact pattern with a Fuji densitometer calibrated for mode of pressure application and humidity conditions. Contact-pressure patterns were scanned and entered into a Power Macintosh 7200 computer (Apple Computer, Cupertino, California). The Fuji film densitometric pressure-intensity scale was calibrated with the pixel density of the computerized gray-scale image, and quantitative analysis of the trapezial joint surface imprint was performed with use of NIH Image 1.60 (National Institutes of Health, Bethesda, Maryland). The center of pressure was found by determining the center of the resulting image weighted by pixel gray-scale density. The dorsal/volar position of the center of pressure was expressed as a percentage of the dorsal-to-volar dimension of the joint surface, with higher values corresponding to a more dorsal area. Accordingly, the most volar aspect of the trapeziometacarpal joint was arbitrarily designated as 0%, and the most dorsal aspect of the joint was designated as 100% (Fig. 2). Contact-pressure patterns and corresponding centers of pressure were determined for each metacarpophalangeal joint position.

Statistical comparison between groups was conducted with multivariate repeated-measures analysis of variance with use of a paired t test. P values of less than 0.05 were considered significant.

At the conclusion of the contact-pressure testing, each joint specimen was opened and was examined directly to determine the status of the articular surfaces and the remaining capsular structures. Radiographic staging was not utilized in this study. The extent of arthritic disease was described as nonarthritic, moderately arthritic, or end-stage arthritis on the basis of the proportion of the joint surface area containing eburnated bone devoid of articular cartilage. These surface assessments were performed on every specimen by the same two independent observers using a predetermined grading scale for the extent of arthritic disease; agreement between observers was 100%. Nonarthritic specimens had either no eburnation or chondromalacia or only very limited volar chondromalacia; moderately arthritic specimens had a thin volar rim of eburnation and/or more extensive chondromalacia in the entire volar compartment; specimens affected by end-stage arthritis had eburnation over more than 50% of the joint surface.

Results

Introduction | Materials and Methods | Results | Discussion | References

On gross examination of the twenty joints, nine (seven from female donors and two from male donors) were affected by end-stage osteoarthritis, five (four from female donors and one from a male donor) were moderately arthritic, and the remaining six (two from female donors and four from male donors) demonstrated very limited disease and were classified as nonarthritic. Maximal passive hyperextension of the metacarpophalangeal joint was £15° in fifteen specimens (Group I) and >15° in the remaining five (Group II).

Overall, 720 specimen contact patterns (three functional test modes ¥ four metacarpophalangeal positions ¥ three repetitions per specimen ¥ twenty specimens), including 240 data points in lateral-pinch testing in one of four metacarpophalangeal positions, were analyzed for this investigation. Aggregate analysis of the center of contact pressure for all specimens, regardless of the position of the metacarpophalangeal joint or the degree of preexisting osteoarthritis of the trapeziometacarpal joint, demonstrated a significant association between dorsal translocation of trapeziometacarpal joint contact and increasing metacarpophalangeal joint flexion. The center of contact appeared at a point 32.9% ± 9.3% (average and standard deviation) along the dorsovolar axis when the metacarpophalangeal joint was pinned in hyperextension, 38.9% ± 6.7% when it was pinned in neutral, 45.4% ± 8.4% when it was not pinned, and 51.6% ± 10.9% when it was pinned in 30° of flexion (p = 0.047) (Fig. 3). With the metacarpophalangeal joint unrestrained, no meaningful difference in contact-pressure centers was observed among the three groups segregated by the extent of arthritic disease (advanced arthritis, 45.4%; moderate osteoarthritis, 42.9%; and no arthritis, 47.5%).

In specimens affected by advanced osteoarthritis (cartilage absent in the volar compartment), metacarpophalangeal joint flexion to 30° shifted the center of joint contact pressure to a position (56.8% ± 13.7% along the dorsovolar axis) that was more dorsal (p < 0.01) than the positions with the joint hyperextended (28.2% ± 10.1%) and in neutral (40.9% ± 6.7%). The center of pressure with the joint in 30° of flexion was also more dorsal than the center with the joint unrestrained (45.4%); however, the difference was not significant (p = 0.075; beta = 0.6) (Fig. 4). Similarly, in specimens with moderate osteoarthritis, a position of 30° of metacarpophalangeal joint flexion resulted in a trend toward dorsal migration of the center of pressure (44.8% ± 3.7% compared with 36.2% 8.5% with the joint in hyperextension, 35.5% ± 8.6% with it in neutral, and 42.9% ± 4.5% with it unrestrained; p = 0.10) (Fig. 5). In nonarthritic specimens, metacarpophalangeal joint flexion of 30° shifted the center of pressure more dorsally than did the other fixed metacarpophalangeal joint positions (49.3% ± 6.5% compared with 37.1% ± 6.0% with the joint in hyperextension and 38.5% ± 4.5% with it in neutral; p < 0.01). However, the center with the joint flexed was not significantly more dorsal than that with the joint unrestrained (47.5% ± 2.6%; p = 0.62) (Fig. 6). Moreover, in each of the three specimen groups categorized by the extent of arthritic disease, the unrestrained metacarpophalangeal joint position produced a center of contact pressure that most closely resembled that obtained with the metacarpophalangeal joint in fixed flexion.

Overall, the center-of-pressure measurements did not significantly differ when they were analyzed according to the degree of mobility of the metacarpophalangeal joint (Groups I and II). However, with the metacarpophalangeal joint fixed in 30° of flexion, specimens in Group I demonstrated a trend toward greater dorsal translation of the center of pressure compared with Group-II specimens (55.1% compared with 50.3%; analysis of variance, p = 0.07).

Discussion

Introduction | Materials and Methods | Results | Discussion | References

The palmar beak ligament has been described as the major static stabilizing structure of the trapeziometacarpal joint, and attrition of this ligament has been implicated as a critical event in the preferential degeneration of the palmar articular surface of the trapeziometacarpal joint in primary osteoarthritis^{6,7,10,14,24,27,28}. In the normal joint, the beak ligament serves to limit dorsal translation of the metacarpal base upon the trapezium during functional activities requiring lateral or key pinch. Attritional changes in the structural integrity of the beak ligament have been observed in parallel with progression of degeneration of articular cartilage^{7,14,24,26-28}. Histologic study of the palmar beak ligament has identified selective degeneration of its metacarpal attachment as an early pathologic event^{7,12,13,18,24,26}. Articular surface disease first appears as cartilage erosion and progresses to eburnation on the palmar-ulnar aspect of the metacarpal base, adjacent to the insertion of the palmar beak ligament, as well as on the central palmar slope of the trapezial surface¹⁰. The relative areas of eburnation of the facing trapezial and metacarpal surfaces occur in a ratio as large as 3:1, suggesting that the metacarpal base translates across the trapezium, shearing the articular surface^11,13. While our experiments with the use of Fuji film measured only static compressive loads and could not directly detect shear, expanding contact patterns in the absence of a change in the actual geometry of the articular surfaces has been taken to indicate translation, or shear, of joint surfaces resulting in superimposition of adjacent contact imprints. This topographical configuration of arthritic wear closely corresponds to joint surface contact patterns that were obtained after division of the beak ligament in a dynamic cadaver model simulating lateral pinch (flexion-adduction) of the thumb^13,29. The beak ligament has been shown to function as the pivot point or fulcrum to which the base of the thumb metacarpal is secured and around which it moves during functional activities^4,9,30.

Distal thumb alignment has been shown to influence loading patterns at the base of the thumb. Basal extension osteotomy of the thumb metacarpal has been proposed as a treatment for osteoarthritis of the trapeziometacarpal joint, but clinical results have been mixed and difficult to interpret³¹. In a dynamic-lateral-pinch model, a 30° extension osteotomy was found to effectively transfer articular pressures onto the normal surfaces of the dorsal compartment in trapeziometacarpal joints affected by mild or moderate osteoarthritis¹². However, no salutary shift in contact area or pressure was seen following osteotomy in specimens with end-stage osteoarthritis. These contact-pressure changes in the trapeziometacarpal joint after extension osteotomy of the metacarpal, which indirectly influences the position of the metacarpophalangeal joint, parallel the observations in our study with variations in the position of that joint.

While the conventional wisdom is that hyperextension of the metacarpophalangeal joint is a late compensatory phenomenon secondary to osteoarthritis of the trapeziometacarpal joint^14,22, we postulated that primary laxity of the metacarpophalangeal joint acts as a contributing and causative factor in the clinical expression of trapeziometacarpal joint disease. Indeed, general hyperextensibility of the hand joints, as indicated by hyperextension of the metacarpophalangeal joint of the little finger, has been correlated with expression of basal joint arthritis in the osteoarthritic hand^1,21. We explored this hypothesis by evaluating the contact pressures in the trapeziometacarpal joint with variable degrees of metacarpophalangeal joint laxity. Indeed, the position of the thumb metacarpophalangeal joint was found to exert an even stronger influence on contact-pressure patterns in the trapeziometacarpal joint than extension metacarpal osteotomy. Even in the face of end-stage arthritis of the trapeziometacarpal joint, securing the metacarpophalangeal joint in a flexed position effectively unloaded the palmar (most severely involved) compartment of the trapeziometacarpal joint. While specimens that had normal or only moderately degenerated trapeziometacarpal joint surfaces demonstrated centers of pressure that translocated in response to metacarpophalangeal joint position, the range of shift of contact centers was narrower than that in specimens with more advanced trapeziometacarpal joint disease. Indeed, the greatest range of translocation of the centers of contact pressure in the trapeziometacarpal joint was observed in specimens affected by end-stage arthritis, with a maximum shift (28.6%) that was more than three times that observed in the moderately arthritic specimens (9.3%). Therefore, even in grossly arthritic specimens, flexion of the metacarpophalangeal joint strongly influenced the center of contact pressure in the trapeziometacarpal joint by effecting a significant dorsal shift. In this group of severely arthritic specimens, the only remaining articular cartilage was found on the most dorsal trapeziometacarpal surfaces. In all specimen groups, hyperextension of the metacarpophalangeal joint produced the most palmar trapeziometacarpal joint contact patterns, whereas flexion resulted in the most dorsal contact areas.

Metacarpophalangeal joint flexion effectively unloaded the palmar compartment of the trapeziometacarpal joint regardless of the presence or severity of arthritic disease at the base of the thumb. Clinical application of this observation may have practical utility in patient care. In a young patient with symptomatic synovitis of the trapeziometacarpal joint and a hyperextensile metacarpophalangeal joint, imposition of metacarpophalangeal joint flexion might alleviate symptoms at the base of the thumb by unloading the palmar trapeziometacarpal compartment. Moreover, increased metacarpophalangeal joint laxity may help to identify individuals who are predisposed to the development of arthritis of the trapeziometacarpal joint and direct them to early intervention that may retard the natural progression of the disease. For patients with symptoms at the base of the thumb and hypermobile metacarpophalangeal joints, metacarpophalangeal joint stabilization by splinting or volar capsulodesis might prove to be a useful therapeutic intervention. Our laboratory investigations showed that the position of the metacarpophalangeal joint has a much more profound influence on regional contact pressures in the trapeziometacarpal joint than does extension metacarpal osteotomy, which has been widely used in the treatment of advanced basal joint osteoarthritis for many years^12,31. Such a strategy may minimize both symptoms and progression of trapeziometacarpal joint osteoarthritis and provide the scientific rationale for a prospective clinical investigation of the efficacy of imposed metacarpophalangeal joint flexion in the management of early trapeziometacarpal joint osteoarthritis.

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Introduction | Materials and Methods | Results | Discussion | References

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