The Functional Anatomy And Assessment Of The Extensor Mechanism The Functional Anatomy And Assessment Of The Extensor Mechanism Henk Giele MS, FRCS, FRACS Oxford Hand Unit, Radcliffe Infirmary, Oxford, UK Donald Sammut FRCS (Plast) Windsor Hand Clinic, Windsor, UK Injury, paralysis or deterioration of the extensor mechanism in the digits is common and can be profoundly debilitating. Despite its apparent simplicity, the extensor mechanism is, in fact, intricate and finely balanced. It is important to familiarise oneself with the anatomical detail and biomechanics if one is to treat disability with success. INTRODUCTION xtension is produced by a complex combination of extensor and intrinsic muscle action. The extensor mechanism of the fingers can be subdivided into extrinsic extensors, intrinsic extensors, and a retinacular system. These subdivisions do not persist as distinct entities at the level of the digit. The three elements combine and interchange, forming the extensor expansion, which interlinks and synchronises the movement of the metacarpophalangeal (MCP), proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints. EXTRINSIC EXTENSORS Extrinsic extension is produced by the Extensor digitorum communis tendon (EDC) (Figure 1a). The EDC tendon inserts into the dorsal hood (the proximal part of the extensor expansion) (Figure 1b), overlying the MCP joint, and is the principle extensor of this joint. Minor attachments, deep to the tendon, adhere it to articular margins and to the base of the proximal phalanx. The majority of the tendon fibres pass freely over the joint and flatten out into the dorsal hood. Extension is aided by the sagittal bands (Figure 1c), which extend transversely around the axis of the MCP joint and encircle the proximal phalanx. Over the mid-shaft of the proximal phalanx, the fibres of the dorsal hood divide into three groups, one central slip (Figure 1d) and two lateral bands (Figure 1e). The central slip attaches to the base of the middle phalanx and is the prime extensor of the PIP joint. The two lateral bands diverge around the central slip (as it attaches to the base of the middle phalanx), and converge over the distal half of the middle phalanx. At this point they are held together by the triangular ligament (Figure 1l) (part of the retinacular system). More distally, they coalesce and insert into the base of the distal phalanx. Extension of the PIP and DIP joints occurs mainly by direct tendon action on the bone insertion. In an arrangement analogous to that at the MCP joint level, this extension is aided by transverse bands (Figure 1f) emanating from the extensor fibres and encircling both the PIP joint and the base of the middle phalanx. INTRINSIC EXTENSORS Intrinsic finger extension is produced by the interossei (Figure 1g) and lumbricals (Figure1h). These produce interphalangeal extension of the fingers and do not contribute to MCPJ extension – in fact they flex the MCPJ. The interossei originate from the metaphyses of adjacent metacarpals and the lumbrical from the flexor digitorum profundus tendon. The lumbricals are unique in that they have no bony attachment, arising from tendon (the FDP) and inserting into tendon (the extensor fibres). The interossei have two sites of insertion. The deep insertion is into the base of the proximal phalanx. The superficial and predominant bulk of muscle inserts distally into the lateral aspects of the dorsal hood. This part of the interosseous is separated from the EDC tendon by the sagittal bands. The interosseous tendon forms the bulk of the lateral bands (Figure 1i), aided by fibres of the long extensor (EDC) and, on the radial side, the insertion of the lumbrical. There is also a merging of fibres of the lateral bands with the central slip so that these lateral bands contribute to PIPJ extension (Figure 1j). This criss-crossing of extensor tendon and intrinsic tendon fibres has been likened to a Chinese finger trap – when loose, this allows expansion and movement; when tense, it grips and straightens the enclosed structures. Both the interossei and the lumbrical pass palmar to the axis of the MCP joint and thus cause flexion at this level. It is also important to note that the interossei insert into both bone and extensor mechanism. The lumbrical inserts exclusively into the extensor apparatus. This difference, coupled with the fact that the lumbrical is placed more palmar, and therefore further from the joint axis, explains why the lumbricals are relatively more powerful in modulating extensor function. Their origin from the flexor digitorum profundus also enables them to operate constantly on a favourable part of the tension/length curve (the฀Blix฀Curve,฀1891). The curve describes the optimal muscle length at which it generates optimal maximal tension. E PAGE 104 The British Journal of Hand Therapy Winter 2006 Vol 11 No 4 by guest on April 2, 2016 hth.sagepub.com Downloaded from