35.1.1 Skin and Integument

The skin on the dorsal aspect of the metacarpophalangeal joints is mobile, while the skin on the palmar aspect of the metacarpophalangeal joints is firmly fixed to the palmar fascia. The adherence of the palmar skin via the vertical septae that run from the skin to the palmar fascia facilitates gripping. The palmar fascia includes the pretendinous bands, superficial transverse metacarpal ligament (also called the transverse ligament of the palmar aponeurosis), and vertical septa. ^{3 – 5} The pretendinous bands run superficially and longitudinally in line with the flexor tendons and insert into the skin at the level of the metacarpophalangeal joint. The superficial transverse metacarpal ligament is perpendicular and deep to the pretendinous bands. The vertical septa are the deepest structures and are adjacent to the metacarpal necks. The vertical septa, including the septa of Legueu and Juvara, connect the superficial and deep metacarpal ligaments. ³ The vertical septa form seven longitudinal compartments in the palm and divide the flexor tendons and A1 pulleys from the neurovascular bundles and lumbricals. The palmar fascia of the thumb includes the thenar muscle fascia, the poorly defined pretendinous band, the proximal commissural ligament, and the distal commissural ligament. ⁶ The proximal commissural ligament is the radial continuation of the superficial transverse metacarpal ligament. The distal commissural ligament is the radial continuation of the natatory ligament of the digital web spaces.

35.1.2 Osteology

The metacarpophalangeal joint is composed of the metacarpal head and the base of the proximal phalanx (▶Fig. 35.1a–d). The radius of curvature of the metacarpal head is smaller than the radius of curvature of the base of the proximal phalanx. ⁷ The metacarpal head is larger in the sagittal plane than in the transverse plane (▶Fig. 35.2). The opposite is true for the base of the proximal phalanx. The base of the proximal phalanx is larger in the transverse plane than in the sagittal plane.

The metacarpal head is convex, irregular, and asymmetrical (▶Fig. 35.2). It is narrow dorsally and widens volarly (▶Fig. 35.2), allowing progressively more contact with the base of the proximal phalanx with increasing flexion (▶Fig. 35.3). ² The metacarpal head and the arc of its articular surface are offset volarly relative to the metacarpal shaft (▶Fig. 35.1d and ▶Fig. 35.4). The radial condyle is larger than the ulnar condyle especially in the second and third metacarpal heads (▶Fig. 35.4a). ⁸ The asymmetry of the condyles of the metacarpal head may contribute to ulnar drift of the metacarpophalangeal joints in inflammatory arthritis. The base of the proximal phalanx is concave, shallow, and oval (▶Fig. 35.3).

The metacarpal head has dorsal lateral tubercles for the attachment of the collateral ligaments ⁵ (▶Fig. 35.5). It also has grooves on its lateral surface that are oriented from palmar proximal to dorsal distal (▶Fig. 35.5). The tendons of the interosseous muscles lie in these grooves. The base of the proximal phalanx has a dorsal and palmar ridge for the attachment of the metacarpophalangeal joint capsule and, rarely, the direct insertion of the extensor tendon. ⁵ The palmar ridge extends laterally to form tubercles for the attachment of the collateral ligaments. The base of the proximal phalanx also has dorsal lateral abductor and adductor tubercles. In addition, there is a palmar depression located centrally for the passage of the flexor tendons (▶Fig. 35.3).

The metacarpophalangeal joint is an ellipsoid joint that allows biaxial motions, including flexion, extension, abduction, adduction, and rotation. ⁵ The intra-articular space is greater in the sagittal than in the transverse plane, and the synovium has enlarged dorsal and palmar pouches. The metacarpophalangeal joint volume is largest in extension. With flexion, the metacarpophalangeal joint volume decreases and joint constraint increases. The shape of the metacarpal head produces a cam effect: flexion transforms into translation and elongation of collaterals. ¹ The collateral ligaments are loose in extension but tighten in flexion and further increase the constraint of the joint. The primary stabilizers of the metacarpophalangeal joint are the collateral ligaments, volar plate, and capsule. ⁵ The flexor tendons, extensor tendons, and intrinsics add to the dynamic stability of the joint.

35.1.3 Capsule, Collateral Ligaments, and Volar Plate

The capsule surrounds the metacarpophalangeal joint, attaching to the base of the proximal phalanx and the metacarpal neck. It is relatively lax. The metacarpophalangeal joint is supported dorsally by the sagittal bands and the loose insertion of the common extensor tendon (▶Fig. 35.6). Volarly, the joint is supported by the volar plate (▶Fig. 35.7 and ▶Fig. 35.8) that is continuous with the deep transverse metacarpal ligament (or the inter palmar plate ligament) (▶Fig. 35.9) and laterally by the collateral ligaments, sagittal bands, and intrinsic tendons. ²

The collateral ligaments arise from the dorsal lateral tubercles of the metacarpal head (▶Fig. 35.5) and insert on the lateral tubercles on the base of the proximal phalanx ⁵ (▶Fig. 35.10). The accessory collateral ligaments run from the palmar grooves of the dorsal lateral tubercles of the metacarpal head to the lateral edge of the volar plate (▶Fig. 35.11). The accessory collaterals then penetrate the deep transverse metacarpal ligament and joint with the fibrous flexor tendon sheath. ⁵ The collateral ligaments are dorsal to the center of rotation of the metacarpophalangeal joints. ⁹ They are loose in extension but tighten in flexion (▶Fig. 35.12). The accessory collaterals are volar to the center of rotation. ⁹ These ligaments are loose in flexion but tighten in extension.

The collateral ligaments of the metacarpophalangeal joint are asymmetric. ⁸ The ulnar collateral ligaments are more parallel to the long axis of the finger in the sagittal plane. The radial collateral ligaments are more oblique and attach closer to the joint margins. The angulation of the radial collateral ligament is greatest in the index finger and decreases progressively from the radial to the ulnar side of the hand. ¹⁰ The obliquity of the radial collateral ligament of the index finger resists the pronation force on the index finger generated by the first dorsal interosseous muscle. The radial collateral ligament also resists the volar translational forces of the flexors and intrinsic muscles. Biomechanically, the asymmetry of the collateral ligaments leads to supination and ulnar deviation with metacarpophalangeal joint flexion. ^{1 , 8 , 10}

The volar plate of the metacarpophalangeal joint is fibrocartilaginous distally and membranous proximally (▶Fig. 35.7 and ▶Fig. 35.8). The distal portion is thicker laterally and is attached to the lateral tubercles at the base of the proximal phalanx. ⁵ The proximal portion inserts on the metacarpal neck. Laterally, the volar plate attaches to the deep transverse metacarpal ligament. The volar plate is collapsible and rarely forms checkreins, making metacarpophalangeal joint contractures less common than proximal phalanx contractures. ¹