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Extensor digitorum brevis manus
Extensor Digitorum Brevis M a n u s - - J . A. L. Hart
EXTENSOR D,IGITORUM BREVIS M A N U S J. A. L. H A R T , Oxford Extension of the forelimb digits in amphibia is achieved solely by an intrinsic, short, extensor nmscle (Bunnell, 1944). Remnants of this muscle, extensor digitorum brevis manus, occur in man (Wood, 1864). TWO patients are described in whom an extensor digitorum brevis manus muscle was found at surgical exploration and where its presence was regarded as contributing to each patient's complaint.
Fig. 1 The muscle bellies of extensor digitorum brevis manus are seen lying deep to the extensor tendons. The exostosis which was excised is shown at E. Fig. 2 Two muscle bellies are seen lying on a plane deep to the extensor tendon. Case 1
A twenty-nine year old housewife complained of a swelling on the dorsum of her right hand present for eighteen months. For the last six weeks the swelling had increased in size and she had recently noted discomfort, associated with a clicking sensation over the swelling, on flexing her wrist. There was a bony swelling at the base of the second metacarpal with a diffuse, overlying, soft tissue swelling. A distinct click was palpable as the extensor tendons passed over the prominence during flexion and extension of the wrist. X-rays showed an exostosis on the dorsum of the base of the second metacarpal and the adjacent capitate. The operative fi~adings are shown in Fig. 1. Two muscle bellies were seen on the dorsum of the hand, lying in lhe plane between the long extensors to the index and middle fingers and the underlying metacarpals with their attached dorsal interossei. These muscle bellies extended proximally, deep ~to the extensor retinaculum. Their distal attachments were not determined. The more lateral muscle belly impinged on the exostosis during flexion of the wrist. The exostosis was excised and the muscle not disturbed. At final follow up ten months later, the patient stated that, although a slight swelling had persisted since the operation, the uncomfortable clicking and pain had not recurred. Case 2
A secretary, a girl of seventeen, complained of a painful swelling on the dorsum of her left wrist present for fourteen months. The swelling initially appeared after lifting a heavy object at work. She stated that the swelling had fluctuated in The H a n d - - V o l . 4
No. 3
1972
265
Extensor Digitorum Brevis Manus--J. A. L. Hart
size. Examination revealed a diffuse, slightly tender, soft tissue swelling at the base of the third metacarpal, which was more prominent on flexion of the wrist. Although the swelling did not appear to be typical, a diagnosis of ganglion was thought most likely. X-rays of the left wrist and hand showed no abnormality. At exploration through a dorsal transverse incision centred on the swelling, two muscle bellies were found lying medial and lateral to the extensor to the index finger in the plane between the long extensors and the underlying metacarpals (Fig. 2). No evidence of a ganglion or any other lesion which could have caused the swelling was found. In this patient the lateral muscle belly was excised. At follow up ten months later, the patient's symptoms had completely resolved. DISCUSSION
A n a t o m i c a l Features of Extensor Digitorurn Brevis M a n u s
According to Macalister (1875), an intrinsic muscle on the dorsum of the hand in man was first noted by Albinus who named it extensor brevis digiti indicts. Wood (1864) introduced the term extensor digitorum brevis manus which is accepted as a term signifying the presence of an intrinsic muscle on the dorsum of the hand. This muscle, although rarely seen in its complete form, is equivalent to the extensor digitorum brevis muscle of the foot. Many variations have been described, with slips passing from one to four fingers (Macalister, 1875; Cauldwell, 1943). Slips to the index finger are most common, where they may replace the extensor indicts proprius muscle (Macalister, 1875; Cauldwell, 1943; Jones, 1959), or form a short second head of the muscle (Wood, 1864; Macalister, 1875; Bunnell, 1942; Cau~dwell, 1943; Bhadkamkar and Mysorekar, 1960; Glasgow, 1967). The latter occurs frequently in combination with slips to other fingers. The attachments of the extensor digitorum brevis manus muscle vary, but in general the muscle arises from the dorsum of the carpus and is inserted into the dorsal expansion or the corresponding extensor tendon. The intrinsic muscle lies in the plane between the extensor tendons and the underlying metacarpals. Wood (1864) suggested that the extensor digitorum brevis manus muEcle was derived from the dorsal interossei, but the commonly accepted view is that it forms part of the deep extensors since it is innervated by the posterior interosseous nerve (Bhadkamkar and Mysorekar, 1960; Glasgow, 1967). The incidence of this muscle in man has been quoted as high as 10% (Glasgow, 1967), but Cauldwell and his associates (1943) found an incidence of approximately 2%. Most of these findings are from dissections of human cadavers, whereas instances of cases presenting clinically are uncommon and have been recognised only at surgery. McGregor (1926) described a case in a Bantu patient, who had sustained a laceration on the dorsum of the hand, where an excessive number of tendons were present in the distal wound. Further exploration revealed a muscle belly of extensor digitorum brevis manus. Bunnell (1942) mentioned that he had encountered two patients with an extensor digitorum brevis manus Muscle. Jones (1959) described a case diagnosed pre-operatively as a ganglion, where the extensor indicts proprius was replaced by an intrinsic extensor muscle to the index finger. Clinical Presentation
The muscle may present clinically as a swelling on the dorsum of the hand. The diagnosis is suggested by the diffuse nature of the swelling and its elongated form. It is not translucent. Jones (1959) described a sign whereby extension of the finger increased the size of the swelling. In the former case described here this sign was not present after operation, but post operative scarring may have negated this sign. 266
The Hand--Vol. 4
No. 3
1972
Extensor Digitorum Brevis M a n u s - - J . A. L. Hart
The most important factor in pre-operative diagnosis is awareness of the presence of the muscle, particularly when a soft tissue swelling on the dorsum of ~he wrist does not appear to be a typical ganglion. Electromyography may be useful where the presence of the muscle is suspected.
Recommended Treatment Jones (1959) divided the extensor retinaculum in his case with only partial relief. In Case 1 described here the exostosis was removed, which relieved the patient's symptoms, but she still complained of a residual swelling. In Case 2 the muscle belly was excised, which produced relief of symptoms with no loss of power of extension of the digit. This would appear to be the ideal form of treatment, as no case has been described where the intrinsic muscle has been the sole extensor for the finger. This lesion may be more common than is usually recognised, particularly in female adolescents who complain of pain on the dorsum of the wrist, without well defined clinical signs. Careful clinical examination in such cases may reveal an extensor digitorum brevis manus muscle. SUMMARY
Two patients with symptoms caused by an extensor digitorum brevis manus muscle are described. The literature is discussed, Excision of the abnormal muscle belly would appear to be the most certain method of obtaining complete relief. ACKNOWLEDGEMENTS
I a m grateful to the Consultants at the Nuffield O r t h o p a e d i c Centre under whose care the patients were admitted, for permission to publish the case reports; to Mr. R. E m a n u e l of the Nuffield O r t h o p a e d i c Centre P h o t o g r a p h i c D e p a r t m e n t for the operative p h o t o g r a p h s ; and to Miss A n i t a L y z b a for typing the manuscript.
REFERENCES
BHADKAMKAR, A. R. and MYSOREKAR, V. R. (1960) Bilateral Extensor Digitorum Brevis Muscle in the Hand. Journal of the Anatomical Society of India 9, 104-105. BUNNELL, S (1942) Surgery of the Intrinsic Muscles of the Hand Other Than Those Producing Opposition of the Tlmmb. Journal of Bone and Joint Surgery 24, 1-31. BUNNELL, S. (1944) Surgery of the Hand. First Edition. Philadelphia. J. B. Lippincott Company. CAULDWELL, E. W., ANSON, B. J. and WRIGHT, R. R. ,(1943) The Extensor Indicis Proprius Muscle. A Study of 263 Consecutive Specimens. Quarterly Bulletin of Northwestern University Medical School 17, 267 279. GLASGOW, E. F. (1967) Bilateral Extensor Digitorum Brevis Manus. The Medical Journal of Australia 2, 24-25. JONES, B. V. (1959) An Anomalous Extensor Indieis Muscle. Journal of Bone and Joint Surgery 41B, 763-765. MACALISTER, A. (1875) On Muscular Anomalies in Human Anatomy. Transactions of the Royal Irish Academy 25, Series S, 1-t34 McGREGOR, A. L. (1926) A Contribution to the Morphology of the Thumb. The Jomnal of Anatomy 60, 259-273. WOOD, J. (1864) On Some Varieties in Human Myology. Proceedings of the Royal Society London 13, 299-303. l'he Hcmd--Vol. 4
No. 3
1972
267
EXTENSOR D,IGITORUM BREVIS M A N U S J. A. L. H A R T , Oxford Extension of the forelimb digits in amphibia is achieved solely by an intrinsic, short, extensor nmscle (Bunnell, 1944). Remnants of this muscle, extensor digitorum brevis manus, occur in man (Wood, 1864). TWO patients are described in whom an extensor digitorum brevis manus muscle was found at surgical exploration and where its presence was regarded as contributing to each patient's complaint.
Fig. 1 The muscle bellies of extensor digitorum brevis manus are seen lying deep to the extensor tendons. The exostosis which was excised is shown at E. Fig. 2 Two muscle bellies are seen lying on a plane deep to the extensor tendon. Case 1
A twenty-nine year old housewife complained of a swelling on the dorsum of her right hand present for eighteen months. For the last six weeks the swelling had increased in size and she had recently noted discomfort, associated with a clicking sensation over the swelling, on flexing her wrist. There was a bony swelling at the base of the second metacarpal with a diffuse, overlying, soft tissue swelling. A distinct click was palpable as the extensor tendons passed over the prominence during flexion and extension of the wrist. X-rays showed an exostosis on the dorsum of the base of the second metacarpal and the adjacent capitate. The operative fi~adings are shown in Fig. 1. Two muscle bellies were seen on the dorsum of the hand, lying in lhe plane between the long extensors to the index and middle fingers and the underlying metacarpals with their attached dorsal interossei. These muscle bellies extended proximally, deep ~to the extensor retinaculum. Their distal attachments were not determined. The more lateral muscle belly impinged on the exostosis during flexion of the wrist. The exostosis was excised and the muscle not disturbed. At final follow up ten months later, the patient stated that, although a slight swelling had persisted since the operation, the uncomfortable clicking and pain had not recurred. Case 2
A secretary, a girl of seventeen, complained of a painful swelling on the dorsum of her left wrist present for fourteen months. The swelling initially appeared after lifting a heavy object at work. She stated that the swelling had fluctuated in The H a n d - - V o l . 4
No. 3
1972
265
Extensor Digitorum Brevis Manus--J. A. L. Hart
size. Examination revealed a diffuse, slightly tender, soft tissue swelling at the base of the third metacarpal, which was more prominent on flexion of the wrist. Although the swelling did not appear to be typical, a diagnosis of ganglion was thought most likely. X-rays of the left wrist and hand showed no abnormality. At exploration through a dorsal transverse incision centred on the swelling, two muscle bellies were found lying medial and lateral to the extensor to the index finger in the plane between the long extensors and the underlying metacarpals (Fig. 2). No evidence of a ganglion or any other lesion which could have caused the swelling was found. In this patient the lateral muscle belly was excised. At follow up ten months later, the patient's symptoms had completely resolved. DISCUSSION
A n a t o m i c a l Features of Extensor Digitorurn Brevis M a n u s
According to Macalister (1875), an intrinsic muscle on the dorsum of the hand in man was first noted by Albinus who named it extensor brevis digiti indicts. Wood (1864) introduced the term extensor digitorum brevis manus which is accepted as a term signifying the presence of an intrinsic muscle on the dorsum of the hand. This muscle, although rarely seen in its complete form, is equivalent to the extensor digitorum brevis muscle of the foot. Many variations have been described, with slips passing from one to four fingers (Macalister, 1875; Cauldwell, 1943). Slips to the index finger are most common, where they may replace the extensor indicts proprius muscle (Macalister, 1875; Cauldwell, 1943; Jones, 1959), or form a short second head of the muscle (Wood, 1864; Macalister, 1875; Bunnell, 1942; Cau~dwell, 1943; Bhadkamkar and Mysorekar, 1960; Glasgow, 1967). The latter occurs frequently in combination with slips to other fingers. The attachments of the extensor digitorum brevis manus muscle vary, but in general the muscle arises from the dorsum of the carpus and is inserted into the dorsal expansion or the corresponding extensor tendon. The intrinsic muscle lies in the plane between the extensor tendons and the underlying metacarpals. Wood (1864) suggested that the extensor digitorum brevis manus muEcle was derived from the dorsal interossei, but the commonly accepted view is that it forms part of the deep extensors since it is innervated by the posterior interosseous nerve (Bhadkamkar and Mysorekar, 1960; Glasgow, 1967). The incidence of this muscle in man has been quoted as high as 10% (Glasgow, 1967), but Cauldwell and his associates (1943) found an incidence of approximately 2%. Most of these findings are from dissections of human cadavers, whereas instances of cases presenting clinically are uncommon and have been recognised only at surgery. McGregor (1926) described a case in a Bantu patient, who had sustained a laceration on the dorsum of the hand, where an excessive number of tendons were present in the distal wound. Further exploration revealed a muscle belly of extensor digitorum brevis manus. Bunnell (1942) mentioned that he had encountered two patients with an extensor digitorum brevis manus Muscle. Jones (1959) described a case diagnosed pre-operatively as a ganglion, where the extensor indicts proprius was replaced by an intrinsic extensor muscle to the index finger. Clinical Presentation
The muscle may present clinically as a swelling on the dorsum of the hand. The diagnosis is suggested by the diffuse nature of the swelling and its elongated form. It is not translucent. Jones (1959) described a sign whereby extension of the finger increased the size of the swelling. In the former case described here this sign was not present after operation, but post operative scarring may have negated this sign. 266
The Hand--Vol. 4
No. 3
1972
Extensor Digitorum Brevis M a n u s - - J . A. L. Hart
The most important factor in pre-operative diagnosis is awareness of the presence of the muscle, particularly when a soft tissue swelling on the dorsum of ~he wrist does not appear to be a typical ganglion. Electromyography may be useful where the presence of the muscle is suspected.
Recommended Treatment Jones (1959) divided the extensor retinaculum in his case with only partial relief. In Case 1 described here the exostosis was removed, which relieved the patient's symptoms, but she still complained of a residual swelling. In Case 2 the muscle belly was excised, which produced relief of symptoms with no loss of power of extension of the digit. This would appear to be the ideal form of treatment, as no case has been described where the intrinsic muscle has been the sole extensor for the finger. This lesion may be more common than is usually recognised, particularly in female adolescents who complain of pain on the dorsum of the wrist, without well defined clinical signs. Careful clinical examination in such cases may reveal an extensor digitorum brevis manus muscle. SUMMARY
Two patients with symptoms caused by an extensor digitorum brevis manus muscle are described. The literature is discussed, Excision of the abnormal muscle belly would appear to be the most certain method of obtaining complete relief. ACKNOWLEDGEMENTS
I a m grateful to the Consultants at the Nuffield O r t h o p a e d i c Centre under whose care the patients were admitted, for permission to publish the case reports; to Mr. R. E m a n u e l of the Nuffield O r t h o p a e d i c Centre P h o t o g r a p h i c D e p a r t m e n t for the operative p h o t o g r a p h s ; and to Miss A n i t a L y z b a for typing the manuscript.
REFERENCES
BHADKAMKAR, A. R. and MYSOREKAR, V. R. (1960) Bilateral Extensor Digitorum Brevis Muscle in the Hand. Journal of the Anatomical Society of India 9, 104-105. BUNNELL, S (1942) Surgery of the Intrinsic Muscles of the Hand Other Than Those Producing Opposition of the Tlmmb. Journal of Bone and Joint Surgery 24, 1-31. BUNNELL, S. (1944) Surgery of the Hand. First Edition. Philadelphia. J. B. Lippincott Company. CAULDWELL, E. W., ANSON, B. J. and WRIGHT, R. R. ,(1943) The Extensor Indicis Proprius Muscle. A Study of 263 Consecutive Specimens. Quarterly Bulletin of Northwestern University Medical School 17, 267 279. GLASGOW, E. F. (1967) Bilateral Extensor Digitorum Brevis Manus. The Medical Journal of Australia 2, 24-25. JONES, B. V. (1959) An Anomalous Extensor Indieis Muscle. Journal of Bone and Joint Surgery 41B, 763-765. MACALISTER, A. (1875) On Muscular Anomalies in Human Anatomy. Transactions of the Royal Irish Academy 25, Series S, 1-t34 McGREGOR, A. L. (1926) A Contribution to the Morphology of the Thumb. The Jomnal of Anatomy 60, 259-273. WOOD, J. (1864) On Some Varieties in Human Myology. Proceedings of the Royal Society London 13, 299-303. l'he Hcmd--Vol. 4
No. 3
1972
267