The extensor digitorum arises from the lateral epicondyle of the humerus via the common extensor tendon, the adjacent intermuscular septa, and the antebrachial fascia. It divides distally into four tendons for the medial four fingers. These tendons pass in a common synovial sheath with the tendon of extensor indicis, through a tunnel under the extensor retinaculum. The tendons diverge on the dorsum of the hand, one to each finger. The tendon to the index finger is accompanied by extensor indicis, which lies medial to it. On the dorsum of the hand, adjacent tendons are linked by three variable intertendinous connections (juncturae tendinae), which are inclined distally and radially.¹ The digital attachments enter a fibrous expansion on the dorsum of the proximal phalanges to which lumbrical, interosseous, and digital extensor tendons all contribute. The tendons of the extensor digitorum may be variably deficient. More commonly, they are doubled or even tripled in one or more digits, most often the index finger or the middle finger. Occasionally a slip of tendon passes to the thumb. The arrangement of the intertendinous connections on the dorsum of the hand is highly variable. The medial connection is strong and pulls the tendon of the little finger towards that of the ring finger, whereas the connection between the middle two tendons is weak and may be absent. The extensor tendon to the index finger lies lateral to the tendon of the extensor indicis. The extensor tendon to the little finger lies lateral to the tendon of the extensor digiti minimi. The extensor digitorum tendons emerge through the fourth dorsal compartment onto the dorsum of the hand, where they are joined together distally by a

varying pattern of oblique interconnections, the juncturae tendinae. These pass in a distal direction from the middle finger to the index finger and from the ring finger to the middle and little fingers. In case of proximal lacerations to the middle finger extensor tendon, there will be only partial loss of extension because of these tendinous interconnections.¹ There are many skillful movements that are performed by the human hand. Therefore, the anatomy of extensor tendons is very important to know.² The movement of the fingers will be affected by any variations in the tendons. Thus, this study attempted to find out the variations in the anatomy of the tendons of two muscles, extensor digitorum and extensor digiti minimi.

This study was conducted in the Department of Anatomy, Netaji Subhas Medical College & Hospital, Bihta, Patna, Bihar (India) on 25 cadavers i.e. 50 embalmed upper limb specimens which were of unknown age and sex. First, the skin and the superficial fascia were removed on the back of the forearm. Then, the extensor retinaculum was divided longitudinally to expose the tendons of the extensor digitorum and extensor digiti minimi. The tendons of both muscles were traced proximal and distal to the extensor retinaculum. The variations in their number, site of insertion, and intertendinous connections present between the tendons were studied. Photographs were taken and the data was tabulated.

On examining the attachments of tendons of ED over the dorsum of the hand, it was observed in four cadavers i.e. eight hands, that there was no separate tendon of ED to the little finger, i.e. the 4^th tendon of ED for the little finger was absent.

Figure 1: Absence of 4^th tendon of ED for little finger.

The attachment of the usual two slips of EDM to the little finger was accompanied by an extra slip of EDM going to the ring finger. So, altogether EDM had three slips: two for the little finger and one for the ring finger.

Figure 2: Extra slip of EDM for ring finger

Table 1: Distribution of tendons of ED and EDM

Thus, this variation was found in 16% of the specimens.

There have been several studies on the variations of the extensor tendons. There are many reports regarding the deviation in the usual number of tendons present on the dorsum of the hand. This study was attempted to find out the variations in the tendons of two muscles, ED and EDM. However, there are many studies in which other tendons like extensor pollicis longus and brevis, extensor indicis, and extensor carpi ulnaris have been found to show variations in their attachments. The tendon of ED to the digits shows many variations in their number which has been reported by different authors. It may be doubled or tripled and more commonly seen in index or middle finger. Zilber and Oberlin³ stated that the extensor digitorum provided one tendon to the index finger, one to the middle finger, two to the ring finger, and none to the little finger. El-Badawi et al⁴ reported 2 to 6 ED tendons and 3 to 8 tendons proximal & distal to the extensor retinaculum respectively. Celik S et al⁵ reported a single tendon of ED to the index finger in 100% of specimens, a single tendon to the middle finger in 92.6%, a single tendon to the ring finger in 75.9%, and a single tendon to little finger in 24.1% and absent in 68.5%.

Palatty⁶ reported that the ED to index finger was absent in 2%, single in 90%, and double and triple together in 8%. ED to middle finger studied by Palatty⁶ found that it was single in 72%, double and triple together in 26%, and multiple tendons in 2%. They observed ED to ring finger had a single tendon in 44%, double and triple together in 44%, and multiple tendons in 12%. According to Palatty⁶, the tendon to the little finger was single in 22%, double in 20%, and absent in 58%. Gonzalez MH et al⁷ conducted a study on fifty hands in which, three hands lacked both extensor digitorum communis and juncturae. Mehta V et al⁸ reported a case with Extensor digitorum contributing tendons only to the middle and ring fingers, with juncturae tendinum between the extensor digitorum for the ring finger and extensor digiti minimi. According to a case report by Melo C⁹, one tendon of EDM joined the extensor pollicis longus. Palatty⁶ reported that EDM was absent in 2%, single in 18%, double in 70%, and triple in 10%. Zilber & Oberlin³ reported that a tendon slip from the extensor digiti minimi to the ring finger was found in one hand. Mehta V et al⁸ reported that a case of the accessory tendon of EDM was merging proximally with extensor carpi ulnaris. Celik S et al⁵ reported double EDM

tendon in 88.9%. Govsa et al¹⁰ stated that the most frequent pattern of extensor tendons in the 4^th inter-metacarpal space (IMCS) was two tendons from EDM (68.5%). Seradge H et al¹¹ reported that EDM had 3 tendinous slips, two slips to the little finger and one to the ring finger. Swathi Tiwari et al¹² and P.Dass¹³ reported an accessory muscle extensor medius propius. Juncturae tendinum (JT) may coordinate the extension of hand, stabilize and redistributes weight to the metacarpophalangeal joint. Many authors have reported different types of JT. Celik S et al⁵ reported the thickest type of JT between the ring and little finger in 90% of specimens. Govsa et al¹⁰ stated that the thickest type of intertendinous connection in 4^th IMCS was seen in 90% of specimens and also the 4^th IMCS tendons have the greatest tendon length and therefore it is a suitable donor graft tissue for local tendon repair. Hirai Y et al¹⁴ reported that the most common pattern of JT in 2^nd, 3^rd, and 4^th IMCS was type I, type 3r, and type 3y respectively. Von Schroeder HP et al¹⁵ stated that JT was absent in all specimens in 1^st IMCS. Accessory slips can also arise from the extensor carpi ulnaris (ECU) and end on the extensor apparatus of the little finger^8,16,17. According to a study by Nakashima T et al¹⁷ on 240 upper limbs, the accessory slip was observed in 82 limbs. The extension moved towards the proximal phalanx of the little finger and was sometimes connected with the medial slip of EDM at the head of the 5^th metacarpal. It was referred to as an accessory EDM arising from ECU by Kaplan and Spinner¹⁸. Clinical cases of an anomalous tendinous slip were reported by Barfred and Adamsen¹⁹, which connected with ECU and the extensor of the little finger. EDM with three tendinous slips were found from 2% to 10%^{3,4,14,15,20-25}, AND FOUR SLIPS OF EDM WERE REPORTED RARELY^15,20,21,24.

The knowledge of variations in the extensor tendons of the hand at various levels is very important. This knowledge is useful for surgeons undertaking any tendon transfer and reconstructive surgeries. Accessory tendons when present can be easily utilized by the surgeons in cases of trauma or injuries. Detailed knowledge of such variations will also help in better diagnosis and treatment. It will also help in better identification and repair of these structures. It is important to do a routine assessment of a number of extensor tendons and their variations by various radiological methods before proceeding with reconstructive procedures.  This study attempted to provide a data for the prevalence of variations in two important muscles and their tendons which will definitely help the clinicians in this field.

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