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.