The extensor retinacular system at the metacarpophalangeal joint

THE EXTENSOR RETINACULAR SYSTEM AT THE METACARPOPHALANGEAL JOINT Anatomical and histological study G. M. RAYAN, D. MURRAY, K. W. CHUNG and M. ROHRER From the University of Oklahoma Health Sciences and Baptist Medical Centers, Oklahoma City, USA The anatomy of the sagittal bands was studied in 56 cadaver digits. The sagittal band is part of an extensor retinacular system which is integrated with the extrinsic and intrinsic musculotendinous structures. The extensor retinacular system is a single unit with radial and ulnar components and has transverse, sagittal and oblique fibres. The transverse-sagittal fibres, along with the palmar plate, form a closed cylindrical tube which surrounds the metacarpal head. The oblique fibres form the triangular lamina distal to the sagittal band. The radial component of the sagittal band is often thinner and longer than the ulnar component. The sagittal band envelops the extensor digitorum tendon and the superficial fibres are thinner than deep fibres, especially in the central digits. The central digits have palmar soft tissue confluence on each side consisting of the sagittal band, palmar plate, annular pulley and deep transverse metacarpal ligament. The sagittal band also appears to envelop the superficial interosseous tendons on both sides. Our findings explain the propensity for radial sagittal band injuries and suggest that the sagittal band is the primary stabilizer of the extensor digitorum at the metacarpophalangeal joint. Journal of Hand Surgery (British and European Volume, 1997) 22B: 5." 585-590 The extensor mechanism or extensor retinacular system (ERS) is probably the most complicated and least understood of all anatomical structures of the hand. The ERS of the hand and digits is a composite of tendons and ligaments. The tendinous portion of the ERS constitutes the extensor digitorum (ED), interosseous and lumbrical tendons. The digital retinacular system components were referred to by Milford (1968) as the retaining ligaments of the extensor mechanism. This system includes the sagittal band (SB) at the metacarpophalangeal (MP) joint level, the transverse and oblique retinacular ligaments at the proximal interphalangeal (PIP) joint level and the triangular ligament between the lateral bands. Loss of the integrity of this retinacular system, in particular the SB, by injury or other pathology, can lead to painful extensor tendon instability or digital deformity. The purpose of this investigation was to study the ERS at the M P joint by gross dissections and special histological preparations.

peripheral digits were observed. The descriptions were made with the fingers extended. In one hand the ERS of four digits including the SBs and annular pulleys were examined with transillumination. The annular pulleys were excised and a midline incision was made in the palmar plate. Components of the SB were preserved along with the ED, interosseous and lumbrical tendons. The entire ERS was placed flat against a strong light source and examined.

Histological examination Two fresh frozen hands were examined histologically. A tissue band saw with a diamond tip blade was used to make 4 mm serial transverse sections beginning at the metacarpal neck and progressing distally in a sequential fashion towards the PIP joint level. The tissue blocks were soaked in 10% buffered formalin solution. The specimens were then decalcified, processed and embedded in paraffin. Transverse sections, 5 to 20 microns thick, were made and stained with hematoxylin and eosin (H and E). A light microscope was used for tissue examination.

MATERIALS AND METHODS Gross anatomical dissection Fourteen adult cadaver hands were used, six fresh frozen and eight formalin preserved. Forty digits of ten hands (eight formalin preserved and two fresh frozen) and their extensor systems were examined using 3.5x loupe magnification. Preliminary dissections included removal of skin and subcutaneous tissue from the mid-metacarpal shaft to the PIP joint level. The ERS was isolated, all extraneous tissue was sharply excised and the SB was identified on both sides. The dimensions, extent, attachments, relationship to other structures, fibre orientation and special features of the retinacular system of the

Hard tissue preparation and special staining Two fresh frozen hands were used for hard tissue preparation using a cutting-grinding technique and special staining. Three centimetre blocks were isolated from the frozen hands including the MP joints. They were immediately placed in 10% neutral buffered formalin. After thawing and fixation for 1 hour, the specimens were cut at the MP joint level of the index, middle, ring and small fingers. These were sliced into 5 mm sections using a water-cooled diamond impregnated band saw with a 0.1 mm thick band. 585

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The sections were fixed in 10% neutral buffered formalin for 10 days followed by dehydration with a graded series of alcohols. After dehydration, the specimens were infiltrated with a light-curing embedding resin (Technovit 7200 VLC, Kulzer, Germany). After 96 days of infiltration with constant shaking at normal atmospheric pressure, the specimens were embedded and polymerized with 450 nm light at a temperature not exceeding 40°C. Sections of 150gm were made using a cutting-grinding system (EXAKT Apparatebau, Norderstedt, Germany). The specimens were then prepared to a thickness of 30 gm using a cutting-grinding method. The sections were treated with Masson Trichrom-Goldner stain. The sections were examined by light microscopy and photographs were obtained.

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Dimensions

In all specimens the length of the ERS along the ED tendon border was greater on the radial than the ulnar side (Table 1). The difference in length between radial and ulnar components was 3 mm for the the index, 4 mm for the middle, 2.5 mm for the ring and 0.5 mm for the small finger. Extent

Proximally the ERS has a free border formed by the SB at the metacarpal head and neck level. Its dorsal portion extends farther distally than its palmar portion, reaching the mid-proximal phalanx and converging towards the PIP joint. Distally the superficial interosseous tendon forms the free palmar border of the ERS.

RESULTS Attachments

Gross anatomical dissection The ERS was found to be a complex structure that is shaped as a hollow tunnel and consists of a SB and two triangular laminae. Along with the palmar plate the SB forms a closed cylindrical tube which surrounds the metacarpal head and M P j o i n t (Fig 1). The ED tendon divides this system into radial and ulnar components. Each component contains a triangular lamina and has dorsal and palmar portions in relation to the interosseous tendon. The palmar portion is less defined than the dorsal portion, and becomes thicker as it blends with other structures. The dorsal portion is larger and more substantial than the palmar portion. The radial component of the SB was often thinner than the ulnar component especially in the middle finger.

Fig 1 (a &b) The ERS is shapedas a hollowtunneland incorporates the ED (large arrow) and interosseous tendons. Along with the palmar plate (small arrow) the SB forms a tube which surrounds the metacarpalhead.

The SB has a palmar origin and a dorsal tendinous insertion which glides with the extensor system as the digits move. The origin of the SB blends with the palmar plate, flexor tendon sheath and proximal annular pulley. These fibres also blend with, but do not perforate, the deep transverse metacarpal ligaments (DTML). The SBs on the radial side of the index finger and ulnar side of the small finger, differ from those of the central digits (Fig. 2). In the central digits the soft tissues coalesce palmarly

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Table l ~ M e a n length of extensor retinacular system (ERS) (ram)

Index

Fig 2

Middle

Ring

Small

Radial

Ulnar

Radial

Ulnar

Radial

Ulnar

Radial

Ulnar

31

28

32

28

28

25.5

22

21.5

Cross section of the left index, long, ring and small fingers stained with Masso~Trichrom Goldner. The SB fibres (small arrow) are stained reddish-brown and shown surrounding the metacarpal heads of the index and long fingers. The soft tissue confluence on the ulnar aspect of the index is marked by a large arrow.

forming a confluence on each side consisting of the palmar plate, annular pulley, DTML and the SB (Figs 3 and 4). In the peripheral digits the DTML is not a part of the confluence on the radial side of the index and ulnar side of the small finger. Relationship to other structures

The SB fibres envelop the ED by approaching the tendon from both sides. The fibres superficial to the tendon are usually thinner than the deep fibres. This was often the case in the central digits, especially the long finger. The SB extends palmarly towards the interosseous tendon on the ulnar side and the interosseous and lumbfical tendons on the radial side. Laterally the SB appears to bifurcate into two layers, the fibres of which envelop the superficial interosseous tendons. The deep fibres of the SB are in proximity to the collateral ligaments and capsule of the MP joint (Fig 5). The bony insertion of the deep interosseous tendon into the lateral tubercle of the proximal phalangeal base separates the distal half of the collateral ligament (deep) from the SB (superficial). Some fibres from the dorsal portion of the superficial interosseous tendon extend in a curved manner and blend with the triangular lamina of the ERS.

Fig 3

Magnified section of the long finger from Fig 2 showing the SB in proximity to the collateral ligaments. The radial SB (small arrow) is extending from the ED tendon to the annular pulley and palmar plate. Some fibres from the distal portion of the deep transverse metacarpal ligament can be seen joining

the SB (largearrow).

Fibre orientation

Proximally the ERS fibres are transversely directed in relation to the digital axis and become sagittal as they approach the palmar plate. At the MP joint the fibres run dorsally from proximal to distal. Further distally the fibres are very oblique. The oblique fibres of the ERS also appear to intersect with group of fibres running palmarwards from the lateral slip of the ED, forming a

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Fig 4

In this cross-section the deep transverse metacarpal ligaments (small arrows) of the index-long and long-ring intervals can be seen joining the soft tissue confluence along with the SB, palmar plate and annular pulley.

criss-cross pattern. The deep fibres of the ERS along with a slip of the ED tendon insert into a ridge at the dorsal base of the proximal phalanx.

Special Jeatures The SBs in the middle and ring fingers were more defined than those of the peripheral digits. Dorsally, the SB of the index finger envelops both the ED and extensor indicis tendons. The radial component of the SB envelops the first dorsal interosseous muscle and tendon. Palmarly, the SB is closely related to the radial digital nerve. The superficial tendinous portion of the first dorsal interosseous inserts into the extensor apparatus and blends with the ERS. In the small finger the SB envelops the ED and extensor digiti minimi tendons and does not have a welldefined free border palmarly and distally as in the index finger. The superficial tendinous portion of the abductor digiti minimi is not as well developed as the first dorsal interosseous tendon of the index finger. The SB fibres seem to envelop the abductor digiti minimi and blends with the confluence. The deep fibres of the SB are well developed and have attachment into the proximal phalanx ulnar tubercle.

Fig 5

The SB of the index finger envelops the extensor tendons dorsally. The radial SB also envelops the first dorsal interosseous tendon and joins the palmar plate and annular pulley. An artifact is present in that area.

The divided ERS appeared to have two lateral wings formed by the interosseous tendons. The fibre orientation could be seen readily against the light. The fibres of the triangular laminae between the ED tendon and both interosseous tendons were very thin and sparse (Fig 6). Both components of the SB appeared thick with abundance of transversely oriented fibres. Some of the transverse fibres could be followed and were seen crossing perpendicular to the interosseous muscles and tendons on both sides.

Masson-Trichrom~Goldner preparation showed various tissues to take different staining. The fibres of the ERS stained red or reddish brown. Adipose tissue did not stain and therefore appeared clear. Cartilage and tendons took minimal staining and therefore appeared grey. Muscle and connective tissue including peritenon and retinacular structures had a reddish-orange or brown colour. The condensed muscle fibres, however, stained more deeply than other loose connective tissue structures. Bones appeared very dark green or almost black. With this technique the SBs of all digits could be seen on gross and microscopic examination of sections. The SB could be seen surrounding the ED dorsally, laterally surrounding the interosseous muscles and tendons and palmarly blending with the palmar plate and annular pulley (Fig 7).

Histological evaluation and special staining

DISCUSSION

The H and E stained sections showed the ERS to have dense collagen fibres with no elastic tissue. The

Kaplan (1959) described the dorsal fibrous expansion as an inseparable unit that is divided into elements for

Transillurnination

EXTENSOR RETINACULAR SYSTEM

Fig 6

Transillumination of the unfolded ERS shows the ED (large arrow) and interosseous tendons (small arrow) against a strong light. The triangular laminae between these tendons and the ED tendon are sparse whereas the SB fibres are thick and transversely oriented.

descriptive purposes. In his anatomical study of the dorsal aponeurosis Landsmeer (1949) referred to the retinacular tissue between the ED and the interosseous tendons as the superficial intertendinous lamina. He listed numerous components of the aponeurosis and provided morphological basis for the integrated coordinated function of the extensor and interosseous muscles. Zancolli (1979) described two components of the extensor hood, the interdsseous hood which connects the ED and interosseous tendons and the SB (MP fibrous girdle). The interosseous hood joined the MP fibrous girdle which was deeper to the hood forming the MP extensor hood. He described the superficial insertion of the interosseous muscle as having transverse proximal, oblique middle and very oblique distal fibres. Our dissections showed these two layers (MP fibrous girdle and the whole aponeurotic sleeve) appeared grossly as one sheath which we call the ERS. We also observed three different fibre orientations, but they appeared as one unit and formed a cylindrical tube around the metacarpal head.

589

Fig 7

The ring finger from the cross-section of Fig 4 shows damaged radial SB dorsally (arrow) and ulnar subluxation of the ED tendon. The soft tissue confluence of four structures, the radial SB, palmar plate, annular pulley and deep transverse metacarpal ligament, is also seen.

Traumatic or pathological instability of the ED at the MP joint level is due to compromise of the integrity of the SB (Fig 7) and function. Frequent involvement of the middle finger has been documented (Kettlekamp et al, 1971; Rayan and Murray, 1994) with injury to the radial SB predominantly. The propensity for injury to the SB radial component and for middle finger involvement is probably because the radial components of the SB are thinner and longer than the ulnar SB, with the longest being that of the middle finger. The prominence of the long finger metacarpal head may render its SB more susceptible to trauma. Landsmeer (1949) described two layers of connective tissue, a superficial one which passed over the ED tendon and a deep layer firmly adherent to the lateral border of the ED tendon. We found the extensor tendon to be enveloped within two layers of the SB. The layer superficial to the ED was thinner than the deep, especially in the central digits. Instability of the extensor tendon is probably due to injury of the SB adjacent to the tendon

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and attenuation of the dorsal fibres. The intricate attachment of the SB to the extensor tendon probably explains why dislocation and subluxation are not common (Landsmeer, 1949). Releasing the annular pulley for trigger finger may predispose to ED tendon instability by creating slack in the SB. Kaplan (1959) found the extensor apparatus to be intimately connected with the terminal tendons of the interossei and lumbrical muscles. Tubiana and Valentin (1964) described the sagittal bands as occasionally doubled to form a tunnel for the interosseous tendons. Our dissections showed the SB to have two layers not only dorsally but also laterally that incorporate the interosseous and lumbrical tendons (Fig 5). Milford (1968) referred to the SB as the shroud ligament and described its distal part as fusing with the aponeurotic expansion of the interosseous and lumbrical muscles. In the index finger the SB originates from the proximal phalanx, metacarpal head, tendon sheath and palmar plate. We identified a soft tissue confluence in the second, third and fourth web spaces which was described previously as the assemblage nucleus (Zancolli, 1979). Injuries of the radial component of the SB in the small finger are more common than those of the ulnar component and may cause abduction deformity (Rayan and 'Murray, 1994). This can be explained by our findings that the radial component appeared thinner and weaker than the ulnar component. The incorporation of the strong abductor digiti minimi muscle within the SB may also contribute to this trend. Garcia-Elias et al (1991) observed changes in the length of the extensor mechanism and substantial geometric variations in spatial orientation of the SB fibres with various positions of the finger. This is in agreement with our findings and those of Schultz et al (1981) who described two sets of superficial and deep crossed fibres. We found the ERS to have no elastic properties, a finding which is also in agreement with Schultz et al. Anatomical studies (Kaplan, 1959; Zancolli, 1979; Tubiana and Valentin, 1964) have confirmed the presence of an insertion of the ED tendon into the MP joint capsule and/or the dorsal base of the proximal phalanx. We observed a loose thin flap attached from the deep surface of the ED tendon to the MP joint capsule and proximal phalanx. Harris and Rutledge (1972) found that this fibrous insertion became tight in extreme

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hyperextension of the MP joint. They concluded that the primary extensor of the MP joint is not this slip, but rather the encircling fibres (SB) which connect the extensor mechanism to the flexor sheath. Littler (1967) concluded that the degree of MP joint hyperextension is determined by the special shroud fibres (SB) yoking the extensor tendon to the palmar plate rather than the slip connecting ED tendon to the proximal phalanx. Posner and Kaplan (1984) suggested that this slip may have a stabilizing function for the extensor tendon. We suggest that this stabilizing function by the extensor slip is secondary and the SB constitutes the primary stabilizer of the ED over the MP joint. Acknowledgement Supported by Bennet Research Foundation, Baptist Medical Center.

References Garcia-Elias M, An K, Berglund L, Linscheid R, Cooney W, Chao E (1991). Extensor mechanism of the fingers I. A quantitative geometric study. Journal of H a n d Surgery, 16A: 113~1136. Harris C, Rutledge G (1972). The functional anatomy of the extensor mechanism of the finger. Journal of Bone and Joint Surgery, 54A: 713-726. Kaplan E B (1959). Anatomy, injuries and treatment of the extensor apparatus of the hand and the digits. Clinical Orthopaedics and Related Research, 13: 24~, 1. Kettlekamp D B, Flatt A E, Moulds R (1971). Traumatic dislocation of the long finger extensor tendon: A clinical, anatomical and biomechanical study. Journal of Bone and Joint Surgery, 53A: 229 240. Landsmeer J M (i949). The anatomy of the dorsal aponeurosis of the human finger and its functional significance. Anatomical Record, 1 0 4 : 3 1 4 3 . Littler J W (1967). The finger extensor mechanism. Surgical Clinics of North America, 4 7 : 4 1 5 4 3 2 . Milford L W. Retaining ligaments of the digit of the hand. Philadelphia, WB Saunders Company, 1968. Posner M, Kaplan E. The fingers: osseous and ligamentous structures. In: Spinner M (Ed.) Kaplan'sJimctional andsurgical anatomy of the hand, 3rd edn. Philadelphia, JB Lippincott, 1984: 49. Rayan G, Murray D (1994). Classification and treatment of closed sagittal band injuries. Journal of H a n d Surgery, 19A: 590 594. Schultz R J, Furlong J, Storace A (1981). Detailed anatomy of the extensor mechanism at the proximal aspect of the finger. Journal of H a n d Surgery, 6: 493498. Tubiana R, Valentin P (I964). The anatomy of the extensor apparatus of the fingers. Surgical Clinics of North America, 44: 897-906. Zancolli E A. Structural and dynamic" bases of hand surgery, 2rid edn. Philadelphia, J B Lippincott, 1979:3 36.

Received: 5 August 1996 Accepted after revision: 24 February 1997 G. M. Rayan MD, Physicians Building D, 3366 NW Expressway,#700 Oklahoma City, OK 73112, USA © 1997The British Society for Surgery of the Hand