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Mechanics of Metacarpophalangeal Joint Extension

Published:February 03, 2018DOI:https://doi.org/10.1016/j.jhsa.2017.12.010

      Purpose

      It is a common belief that extension of the metacarpophalangeal (MCP) joint of the finger is achieved via the sagittal bands acting as a sling or lasso to attach the extensor tendon to the base of the proximal phalanx. The aim of this study was to test the hypotheses that (1) division of the sagittal bands reduces extension force or torque of the MCP joint, and (2) division of the extensor tendon distal to the sagittal band will not affect the extension force or torque of the MCP joint.

      Methods

      Ten cadaver limbs were secured to a jig to allow for testing of the extension force of the MCP joints of the index, middle, and ring fingers. A 1-kg load was applied to the forearm extensor digitorum communis tendon and the extension force was measured with the MCP joint positioned at 0° (neutral extension) and again at 45° flexion. These measurements were repeated after the sagittal bands were divided in 15 specimens; in the other 15 specimens, the extensor tendon was divided just distal to the sagittal bands.

      Results

      After sagittal band division, extension force was similar in the 2 groups (0.11 N reduction after division with the MCP joints in neutral and 0.14 N in 45° flexion). There was significantly less extension force after division of the extensor tendon in both joint positions (0.95 N reduction after division in neutral extension and 0.66 N in 45° flexion).

      Conclusions

      The sagittal bands do not primarily extend the MCP as a sling or lasso. The extensor tendon continuation to the extensor hood and middle phalanx is the major extension motor. The MCP joint is extended by the torque generated by the extensor tendon passing the joint carrying a force and possessing an extension moment arm.

      Clinical relevance

      This principle should be correctly understood in the literature to ensure that clinical decisions related to injury and/or repair of the extensor tendon and sagittal bands are based on a sound understanding of their mechanics.

      Key words

      Introduction

      “Metacarpophalangeal joint extension is accomplished via fibers of the sagittal band that extend from the extensor hood to the palmar plate.” This unreferenced statement first appeared in the fifth edition of Green’s Operative Hand Surgery and has been repeated and expanded in the sixth edition and restated again in the seventh edition as “Extension of the MP joint is transmitted by the pull of the EDC through the sagittal bands.”
      • Baratz M.E.
      • Schmidt C.C.
      • Hughes T.B.
      Extensor tendon injuries.
      • Strauch R.J.
      Extensor tendon injury.
      • Strauch R.J.
      Extensor tendon injury.
      No reference is given, much in the way of an accepted wisdom. Searching the literature supplies some clues regarding the origin of this idea.
      • Zancolli E.A.
      • Cozzi E.P.
      Atlas of Surgical Anatomy of the Hand.
      • Winslow J.B.
      Section 3: A Description of the Muscles; Article 1: The general Doctrine of the Muscles; Sub-section 8: Extensor Digitorum Communis.
      • Winslow J.B.
      Section 3: A Description of the Muscles; Article 21: The particular Uses of the Muscles, which are wholly inserted in Bones; Sub-section 6: Uses of the Muscles which move the Fingers.
      • Smith R.J.
      Balance and kinetics of the fingers under normal and pathological conditions.
      • Dy C.J.
      • Rosenblatt L.
      • Lee S.K.
      Current methods and biomechanics of extensor tendon repairs.
      • Wehbe M.A.
      Anatomy of the extensor mechanism of the hand and wrist.
      • Newport M.L.
      Early repair of extensor tendon injuries.
      • Posner M.A.
      • Green S.M.
      Diagnosis and treatment of finger deformities following injuries to the extensor tendon mechanism.
      • Thompson S.T.
      • Wehbe M.A.
      Extensor physiology in the hand and wrist.
      • Harris Jr., C.
      • Rutledge Jr., G.L.
      The functional anatomy of the extensor mechanism of the finger.
      The concept seems to arise to satisfy the question of how the metacarpophalangeal (MCP) joint extends without a direct insertion of extensor digitorum communis (EDC) into the base of the proximal phalanx. The sagittal band (SB) passes from the EDC to the volar plate, and hence indirectly to the volar base of the proximal phalanx.
      One origin of this theory appears in the work of Zancolli and Cozzi.
      • Zancolli E.A.
      • Cozzi E.P.
      Atlas of Surgical Anatomy of the Hand.
      Their concept seems to apply only to the posture of hyperextension, such as in a claw hand, and not physiologic extension through the normal MCP joint arc of motion. Zancolli and Cozzi referenced an 18th-century anatomy text by Winslow,
      • Winslow J.B.
      Section 3: A Description of the Muscles; Article 1: The general Doctrine of the Muscles; Sub-section 8: Extensor Digitorum Communis.
      • Winslow J.B.
      Section 3: A Description of the Muscles; Article 21: The particular Uses of the Muscles, which are wholly inserted in Bones; Sub-section 6: Uses of the Muscles which move the Fingers.
      who stated that “each (extensor) tendon having reached the bases of the first phalanx, is slightly inserted therein by some lateral expansions fixed in each side of the bases,”
      • Winslow J.B.
      Section 3: A Description of the Muscles; Article 1: The general Doctrine of the Muscles; Sub-section 8: Extensor Digitorum Communis.
      and “The lateral Expansions extend the first Phalanx.”
      • Winslow J.B.
      Section 3: A Description of the Muscles; Article 21: The particular Uses of the Muscles, which are wholly inserted in Bones; Sub-section 6: Uses of the Muscles which move the Fingers.
      It is not clear whether Winslow was referring to the SBs or the lateral aponeurotic expansions of the extensor hood distal to the SB.
      Smith
      • Smith R.J.
      Balance and kinetics of the fingers under normal and pathological conditions.
      also conceptualized that the primary extensor of the MCP joint is the encircling fibers connecting the extensor mechanism to the flexor sheath, volar plate, and the periosteum of the proximal phalanx, and that “the principle function of the sagittal bands is to extend the proximal phalanx.” Other authors stated a role for the SB in MCP joint extension,
      • Dy C.J.
      • Rosenblatt L.
      • Lee S.K.
      Current methods and biomechanics of extensor tendon repairs.
      • Wehbe M.A.
      Anatomy of the extensor mechanism of the hand and wrist.
      • Newport M.L.
      Early repair of extensor tendon injuries.
      • Posner M.A.
      • Green S.M.
      Diagnosis and treatment of finger deformities following injuries to the extensor tendon mechanism.
      but many cited no reference. Thompson and Wehbe
      • Thompson S.T.
      • Wehbe M.A.
      Extensor physiology in the hand and wrist.
      called this the lasso effect but provided no reference. Some authors referenced an article by Harris and Rutledge,
      • Harris Jr., C.
      • Rutledge Jr., G.L.
      The functional anatomy of the extensor mechanism of the finger.
      which infers the lateral expansion as the indirect connection rather than the SB. However, the article anecdotally notes full extension of the MCP joint after division of the EDC at the distal margin of the SB.
      The purpose of this study was to test empirically the extension effect in a cadaver model of (1) the SBs and (2) the extensor tendon beyond the SBs. The hypotheses were that (1) division of the SB would weaken MCP extension force or torque, and (2) division of the extensor tendon distal to the SB would not.

      Materials and Methods

      After obtaining approval from the institutional human research ethics committee, a sample of convenience composed of 10 fresh-frozen cadaver forearms amputated distal to the elbow were prepared for testing. All cadavers had a full passive range of motion at the MCP and interphalangeal (IP) joints. The EDC tendons to the index, middle, and ring fingers were dissected proximal to the extensor retinaculum and a whipstitch with a large-caliber, nonabsorbable suture was placed at the musculotendinous junction to apply traction. The extensor apparatus overlying the proximal phalanges of the digits was exposed. The length of each proximal phalanx was measured using calipers and an indelible mark was placed at the three-quarter length position from the MCP joint. The limbs were then allocated into 1 of 2 groups: (1) those in which the SBs were to be divided; and (2) those in which the extensor tendon at the level of the MCP joint, at the distal margin of the SB, was to be divided. Each group contained 15 digits that were assumed to be independent specimens for purposes of the statistical analyses.
      Each forearm was positioned on a wooden frame with the metacarpal heads placed just distal to the edge of a longitudinal wooden block. The forearms were pinned to the block in a position of neutral rotation and with the wrist neutral by one 1.6-mm K-wire passing through the radial and ulnar shafts and another through the midshaft of all metacarpals. A height-adjustable load cell frame was secured in place to the underlying plinth (Fig. 1).
      Figure thumbnail gr1
      Figure 1Positioning of the cadaver and load cell for measurement of extension force.
      The load cell was aligned (perpendicular to it and at the correct height) with the three-quarter mark on the proximal phalanx of each digit. The MCP joint was placed at 0° flexion. Force exerted with no tendon load was measured as 0 for the load cell. A 1-kg weight was then attached to the tendon via the proximal suture and a pulley. After a delay of 1 minute to allow for stress relaxation, the force exerted under load was recorded. These measurements were repeated with the MCP joint in 45° flexion (the limit of flexion before extensor tendon subluxation occurred after division of the SBs) by moving the wooden frame on the underlying plinth.
      After this, the relevant structure (either both SBs at the margins of the extensor tendon or the extensor tendon at the distal extent of the SB) was completely divided. Force exerted on the load cell was remeasured both at rest and with weight attached at 0° and 45° flexion. This process was repeated through the index, middle, and ring digits of the 10 hands.
      The results were recorded by an independent observer and paired t tests were applied for statistical analysis.

      Results

      A total of 30 digits were prepared from 10 cadaveric forearms, with 10 each of index, middle, and ring fingers. These were divided into 2 groups of 15, for division of EDC tendons and SBs, respectively. One specimen in the EDC group failed to extend fully before intervention, which decreased this group to 14 valid results for analysis.
      The MCP extension force decreased by an average of 8% (MCP joint in neutral) and 7% (45° flexion) after SB division, with mean differences of 0.11 and 0.14 N, respectively (Table 1, Fig. 2). However, a post hoc analysis based on 80% power and 5% significance levels concluded that the sample size was not large enough in this group to exclude a type II error.
      Table 1Mean (Range) Force in Newtons Before and After Division of EDC or SB, With MCP Joint in Neutral and Flexed Positions
      PositionEDCSB
      IntactDividedP ValueIntactDividedP Value
      Force (N) with MCP joint in extension1.14 (0.32–1.92)0.19 (0.00–0.47)< .051.00 (0.42–1.48)0.89 (0.14–1.48).13
      Force (N) with MCP joint in 45° flexion1.88 (1.24–2.35)1.22 (0.58–2.02)< .051.88 (1.49–2.15)1.74 (1.08–2.33).16
      Figure thumbnail gr2
      Figure 2Extension force of specimens before and after SB division (n = 15). MP, metacarpophalangeal.
      Significant mean decreases of 85% (0.95 N; P < .05) were noted in extension force after division of the EDC tendon with the MCP joint in extension and 35% (0.66 N; P < .05) with the MCP joint flexed (Table 1, Fig. 3).
      Figure thumbnail gr3
      Figure 3Extension force of specimens before and after EDC division (n = 14). MP, metacarpophalangeal.

      Discussion

      The extensor mechanism of the digits consists of both tendinous and retinacular elements. The tendinous components are the extensor digitorum and interosseous and lumbrical tendons, which combine to form the extensor hood over the proximal phalanx. The retinacular system includes the SB at the MCP joint level, an aponeurotic layer at the base of the proximal phalanx between the lateral band and central tendon, the transverse and oblique retinacular ligaments at the proximal IP joint level, and the triangular ligament between the lateral bands.
      • Rayan G.M.
      • Murray D.
      • Chung K.W.
      • Rohrer M.
      The extensor retinacular system at the metacarpophalangeal joint: anatomical and histological study.
      The mechanics of extension of the MCP joint has been the subject of conjecture. An insertion of the EDC to the base of the proximal phalanx has been suggested and sought.
      • Tubiana R.
      • Valentin P.
      The anatomy of the extensor apparatus of the fingers.
      • Tubiana R.
      • Valentin P.
      The physiology of the extension of the fingers.
      • Van Sint Jan S.
      • Rooze M.
      • Van Audekerke J.
      • Vico L.
      The insertion of the extensor digitorum tendon on the proximal phalanx.
      Thin fibrous expansions have been found extending from the undersurface of the EDC tendon to the dorsal MCP joint capsule and the base of the proximal phalanx, but anatomical and radiological studies have proven these to be inconsistent and lax at all functional MCP joint positions and thus functionally unimportant.
      • Van Sint Jan S.
      • Rooze M.
      • Van Audekerke J.
      • Vico L.
      The insertion of the extensor digitorum tendon on the proximal phalanx.
      No tendinous insertion of the EDC to the base of proximal phalanx has been demonstrated.
      The SB is a collagenous structure with a palmar origin from a confluence with the volar plate, flexor tendon sheath, A1 annular pulley, and transverse metacarpal ligament. It runs superficial to the capsule and collateral ligaments of the MCP joint and splits into a thinner superficial layer and thicker deep layer to envelop the EDC tendon. The proximal margins of the SB remain free whereas the distal border blends with the aponeurotic expansion of the interossei.
      • Rayan G.M.
      • Murray D.
      • Chung K.W.
      • Rohrer M.
      The extensor retinacular system at the metacarpophalangeal joint: anatomical and histological study.
      • Young C.M.
      • Rayan G.M.
      The sagittal band: anatomic and biomechanical study.
      The SB acts as the primary stabilizer of the EDC tendon at the MCP joint.
      • Rayan G.M.
      • Murray D.
      • Chung K.W.
      • Rohrer M.
      The extensor retinacular system at the metacarpophalangeal joint: anatomical and histological study.
      • Young C.M.
      • Rayan G.M.
      The sagittal band: anatomic and biomechanical study.
      The SB tension limits proximal excursion of the EDC tendon during MCP joint hyperextension, resulting in an inability of the long extensor to extend the IP joints in that circumstance.
      Green’s Operative Hand Surgery theorizes the SB to be a means of MCP joint extension, by identifying a lasso or sling, and therefore an insertion, onto the base of the proximal phalanx, and a simple origin and insertion mechanical concept.
      • Baratz M.E.
      • Schmidt C.C.
      • Hughes T.B.
      Extensor tendon injuries.
      • Strauch R.J.
      Extensor tendon injury.
      • Strauch R.J.
      Extensor tendon injury.
      Another function or effect of the SB, along with the loose connections between the EDC and MCP capsule and the base of the proximal phalanx, is to limit excursion of the EDC. This is relevant only at the limits of range of motion or in certain pathologies such as claw hand. This effect does not explain or help conceptualize normal mechanics in normal arcs of motion.
      This study attempted to test the role of the SB empirically in the extension of the proximal phalanx at the MCP joint. It found that the SB’s role in extension is negligible. The decrease in extension force after division of the SB was minimal compared with that after EDC division. Therefore, the theory of a major role for the SB in extension of the MCP is not supported.
      A full discussion of finger biomechanics is not possible here. Readers are directed to Brand and Hollister.
      • Brand P.W.
      • Hollister A.M.
      Clinical Mechanics of the Hand.
      Metacarpophalangeal joint extension can be explained by simple or complex concepts. Simply, the EDC has a moment arm and carries a force. It therefore generates an extensor moment at the MCP joint. Its insertion is further along the composite finger structure of articulated phalanges.
      • Littler J.W.
      The finger extensor mechanism.
      The complex concept is the solving of static and dynamic mechanics that result in the posture, motion, and work done by the composite structure, and its multitude of links, motors, retinacular structures, and external forces.
      This study has some flaws. The instrumentation used was simple, which may have led to measurement error. It was a single pull test with no conditioning or repeated loading. The sample size employed was based on cadaver availability at our institution; after a post hoc analysis, we determined it not to be sufficiently large to allow for statistical analysis in the SB group. However, a more sophisticated test system may increase the precision of data to allow for a statistical analysis in a sample of this size, especially because the preliminary results of this test suggested that the effects observed in the 2 groups were of different magnitudes.
      The EDC extends the MCP joint by carrying force across the joint with a moment arm, and an attachment further along the kinetic chain of the digit. The minimal decrease in extension force after SB division, compared with that after EDC division, suggests that the SB has no or little extension torque on the MCP joint. The main function of the SB is stabilization of the extensor tendon.

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