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Anatomic study of variations of hypothenar muscles and arborization patterns of the ulnar nerve in the hand1
Anatomic Study of Variations of Hypothenar Muscles and Arborization Patterns of the Ulnar Nerve in the Hand Keiichi Murata, MD, FRCS, Makoto Tamai, MD, Amit Gupta, MD, Louisville, KY
Purpose: To show the variations in the hypothenar muscles and the arborization patterns of the ulnar nerve and to investigate the relationship between the hypothenar muscles and the ulnar nerve. Methods: We performed an anatomic study of 35 hands from embalmed cadavers. After dissecting the ulnar side of the hand we recorded the number of hypothenar muscles and their variations, the site of the hiatus for the deep branch of the ulnar nerve, and the branching patterns of the ulnar nerve in each hand. We then investigated the relationship between the variations in the hypothenar muscles and the arborization patterns of the ulnar nerve. Results: The abductor digiti minimi (ADM) had 1 belly in 6 hands, 2 bellies in 28 hands, and 3 bellies in 1 hand. The flexor digiti minimi brevis was absent in 8 hands, had 1 belly in 24 hands, and had 2 bellies in 3 hands. The opponens digiti minimi had 2 layers of origin and the deep branch of the ulnar nerve passed between these 2 layers in all hands. We classified the structural patterns of the hiatus into 3 types, the arborization patterns of the ulnar nerve into 5 types, and the branching patterns of the motor branch to the ADM into 4 types according to their morphologic characteristics. We found variations of the arborization pattern in which the deep branch originated from the ulnar trunk of the ulnar nerve distal to bifurcation in 3 hands, a communicating branch was present between the 2 sensory branches to the fingers in 3 hands, and the ulnar sensory branch pierced the ADM in 2 hands. Conclusions: We confirmed that the anatomic relationship between the ulnar nerve and the hypothenar muscle is complex and that the formation of the hiatus varies. This knowledge can assist the surgeon in the diagnosis and treatment of conditions associated with the ulnar aspect of the hand. (J Hand Surg 2004;29A:500 –509. Copyright © 2004 by the American Society for Surgery of the Hand.) Key words: Aberrant muscle, Guyon’s canal, hypothenar muscle, ulnar nerve.
From the Christine M. Kleinert Institute for Hand and Microsurgery and University of Louisville, School of Medicine, Louisville, KY. Received for publication May 21, 2003; accepted in revised form January 8, 2004. No benefits in any form have been received or will be received by a commercial party related directly or indirectly to the subject of this article. Reprint requests: Amit Gupta, MD, FRCS, Christine M. Kleinert Institute for Hand and Microsurgery, 225 Abraham Flexner Way, Ste 850, Louisville, KY 40202. Copyright © 2004 by the American Society for Surgery of the Hand 0363-5023/04/29A03-0026$30.00/0 doi:10.1016/j.jhsa.2004.01.006
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The complex anatomic variations of the ulnar nerve and the hypothenar muscles in the hand have been described in the literature.1– 6 The purposes of this study are to (1) show the variations in the hypothenar muscles and the arborization patterns and course of the ulnar nerve and (2) to investigate the relationship between the hypothenar muscles and the ulnar nerve. This knowledge can assist the surgeon in the diagnosis and treatment of conditions affecting the ulnar aspect of the hand such as tumors, trauma, nerve compression, abductor digiti quinti opposition trans-
Murata, Tamai, and Gupta / Anatomy of Hypothenar Muscles and Ulnar Nerve
Table 1. Number of Origins and Insertions ADM Number of origins Number of hands Number of insertions Number of hands
FDM
1 2 3 0 1 6 28 1 8 24 1 2 3 0 1 4 31 — 8 27*
ODM 2 1 2 3 — 35 2 1 2 — 35 —
3 — 3 —
*Patterns of fusion in FDM insertions and number of hands: fused with ADM, 22; fused with ODM & ADM, 2; independent, 3; absent, 8.
fer, and palmar fasciectomy for Dupuytren’s contracture.
Materials and Methods We dissected 35 hands (15 right, 20 left) from 20 embalmed cadavers. The age of the deceased subjects ranged from 56 to 99 years (average, 76 years). Dissection was performed at the ulnar side of the hand under loupe magnification (3.5⫻). After the skin of the distal forearm and palm was removed a radial-based flap (a roof of the Guyon’s canal) containing subcutaneous tissue and the palmaris brevis (PB) muscle was elevated to expose the hypothenar muscles and the ulnar neurovascular bundle. For each hand we recorded the number of hypothenar muscles and their sites of origin and insertion, the existence of aberrant muscles, the structural patterns of the hiatus for the deep branch of the ulnar nerve, and the branching patterns of the superficial and deep branch of the ulnar nerve. Based on these findings we investigated the anatomic relationship between the hypothenar muscles and the ulnar nerve.
Results The abductor digiti minimi (ADM) muscle originated from the pisiform, the tendon of the flexor carpi ulnaris, and the pisohamate ligament. In 31 hands this muscle had 2 slips of insertion. One slip inserted into the ulnar side of the proximal phalanx base of the small finger and the other slip inserted into the extensor apparatus of the small finger. In 4 hands the muscle had 1 slip of insertion into the ulnar side of the proximal phalanx base of the small finger. In 28 hands the ADM had 2 bellies. These 2 bellies were identified as the palmar and dorsal portions, as Landsmeer reported.7 The ADM had 1 belly in 6 hands and 3 bellies in 1 hand (Table 1). The flexor digiti minimi brevis (FDM) muscle was absent in 8 hands. In the remaining 27 hands the FDM originated from the hook of the hamate, the adjacent ulnar portion of the flexor retinaculum,
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and/or the radial portion of the pisiform. The FDM had 1 belly in 24 hands and 2 bellies in 3 hands. The FDM was fused distally with the ADM in 22 hands. When the FDM fused with the ADM it had common insertions with the ADM. In 2 hands the FDM was fused with the opponens digiti minimi muscle (ODM) at the place of origin and fused distally with the ADM. In 3 hands the FDM existed independently. All of the independent FDM muscles inserted onto the volar aspect of the head of the fifth metacarpal (Table 1). Additionally, in some cadavers the FDM had a different number of bellies in the right hand than in the left hand of the same body. The ODM had 2 layers of origin in all hands. The superficial layer originated from the distal part of the hook of the hamate and inserted into the distal ulnar side of the fifth metacarpal shaft. The deep layer originated from the part of the ulnar flexor compartment wall that is adjacent to the hook of the hamate and it inserted into the proximal ulnar side of the small finger metacarpal shaft (Table 1). When we dissected the muscle fibers distally we observed that the ODM had 2 layers. The width of the deep layer’s insertion to the fifth metacarpal varied greatly (Fig. 1). In 9 hands the deep layer was so thin that it looked like membrane. In even the thinnest deep layer, however, we found muscle fibers on the metacarpohamate ligament macroscopically. The deep branch of the ulnar nerve passed between the superficial and the deep layers of the ODM in all hands. We found 4 variations of an aberrant muscle in 9 hands (Table 2). In variation 1 (5 hands) the aberrant muscle originated from the flexor retinaculum (Fig. 2). In variation 2 (2 hands) the muscle originated from the antebrachial fascia (Fig. 3). These 7 muscles should be classified as the accessory abductor digiti minimi muscle because they were fused with the ADM distally. They lie volar to the ulnar neurovascular bundle in Guyon’s canal. In variation 3 (1 hand) the aberrant muscle originated from the pisiform and inserted into the flexor retinaculum. This muscle passed between the ulnar nerve and artery, lying over the ulnar nerve (Fig. 4). In variation 4 (1 hand) the aberrant muscle originated from the pisiform and inserted onto the volar aspect of the head of small finger metacarpal dorsally to the ADM (Fig. 5). We classified the structural patterns of the hiatus into 3 types (Table 3). In type 1 (25 hands) the hiatus consisted of the intermuscular space between the ADM and the FDM (Fig. 6). In this type the fascial continuity between the ADM and the FDM formed the arched structure (fascial arch) across the hiatus
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Figure 1. Two layers of the ODM.
and the deep branch of the ulnar nerve passed under this fascial arch. In type 2 (2 hands) the hiatus consisted of the fascial arch between the 2 origins of the FDM, the pisiform and the hook of the hamate (Fig. 7). In this type the deep branch of the ulnar nerve passed under this fascial arch. In type 3 (8 hands) the hiatus consisted of the intermuscular space between the ADM and the ODM (Fig. 8). In these 8 hands the FDM was absent. In 3 of the hands classified as type 3 the fascia of the superficial layer of the ODM continued to the fascia of the ADM and this fascial continuity formed the fascial arch. These hands had very thin deep layers and the ADM and the superficial layer of the ODM existed closely. In the remaining 5 hands that were type 3 we did not find the fascial arch. We also classified the arborization patterns of the ulnar nerve into 5 types according to their morpho-
Table 2. Aberrant Muscle Variations Variation
No. of Hands
1
5
2
2
3 4
1 1
Origin Flexor retinaculum Antebrachial fascia Pisiform Pisiform
Insertion Fused with ADM Fused with ADM Flexor retinaculum Head of 5th metacarpal
Figure 2. Aberrant muscle (AM): variation 1. The muscle originates from the flexor retinaculum and is fused with the abductor digiti minimi distally. It lies volar to the ulnar neurovascular bundle.
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logic characteristics (Figure 9). In type 1 (27 hands) the ulnar nerve bifurcated into a deep branch and a superficial trunk just distal to the distal edge of the pisiform (Fig. 10). The superficial trunk bifurcated distally into 2 sensory branches: the fourth common digital nerve and the ulnar proper digital nerve of the small finger. In 1 hand with a type 1 pattern we found a communicating branch between the 2 sensory branches to the fingers. In type 2 (3 hands) the ulnar nerve trifurcated into the ring finger common digital nerve, the ulnar proper digital nerve of the small finger, and a deep branch just distal to the distal edge of the pisiform (Fig. 11). In type 3 (3 hands) the ulnar nerve bifurcated into radial and ulnar trunks just distal to the distal edge of the pisiform (Fig. 12). The radial trunk continued to the ring finger common digital nerve and the ulnar trunk bifurcated distally into the ulnar proper digital nerve of the small finger and a deep branch. In one type 3 hand we found a communicating branch between the 2 sensory branches to the fingers. In this hand the ulnar sensory branch pierced the ADM. In type 4 (1 hand) an additional superficial branch originated from the main trunk of the ulnar nerve in Guyon’s canal and
Figure 3. AM: variation 2. The muscle originates from the antebrachial fascia and is fused with the abductor digiti minimi. It lies volar to the ulnar neurovascular bundle.
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Figure 4. AM: variation 3. The muscle originates from the pisiform and inserts into the flexor retinaculum. It passes between the ulnar nerve and artery. a, artery; n, nerve.
communicated with the ulnar proper digital nerve of small finger distally (Fig. 13). The main trunk trifurcated into the ring finger common digital nerve, the ulnar proper digital nerve of small finger, and a deep branch just distal to the distal edge of the pisiform. In this hand the additional branch passed through the muscle belly of the ADM. We also found a communicating branch between the 2 sensory branches to the fingers in this hand. In type 5 (1 hand) the dorsal sensory branch of the ulnar nerve connected with the ulnar proper digital nerve of the small finger (Fig. 14). The main trunk trifurcated into the ring finger common digital nerve, the ulnar proper digital nerve of the small finger, and a deep branch just distal to the distal edge of the pisiform. In this hand a communicating branch between the dorsal sensory branch of the ulnar nerve and the ulnar proper digital nerve of the small finger passed through the muscle belly of the ADM. We also identified 4 branching patterns of the motor branch to the ADM (Figure 15). In type 1 (16 hands) a motor branch to the ADM originated from the deep branch distal to the hiatus. In type 2 (1 hand) a main motor branch to the ADM originated from the deep branch proximal to the hiatus in the Guyon’s
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Figure 5. AM: variation 4. The muscle originates from the pisiform and inserts onto the volar aspect of the fifth metacarpal head.
canal and an additional motor branch to the ADM originated from the deep branch distal to the hiatus. In type 3 (8 hands) the ulnar nerve trifurcated into a superficial trunk, a deep branch, and a motor branch to the ADM in Guyon’s canal. In type 4 (10 hands) a main motor branch to the ADM originated from the
Table 3. Structural Patterns of the Hiatus Type
No. of Hands (%)
1
25 (71%)
2
2 (6%)
3
8 (23%)
Pattern Fascial arch between ADM and FDM Deep branch of ulnar n. passes under arch Fascial arch between 2 origins of FDM Deep branch of ulnar n. passes under arch Intermuscular space between ADM & ODM Arch: 3 hands-arch between ADM and ODM 5 hands-no arch
Figure 6. Pattern of the hiatus (PH): type 1. The hiatus consists of the intermuscular space between the ADM and the FDM. The fascial continuity between these 2 muscles forms the fascial arch and the deep branch passes under this arch.
main trunk of the ulnar nerve proximal to the hiatus in Guyon’s canal. In 4 hands classified as type 4 an additional motor branch to the ADM originated from the deep branch distal to the hiatus (type 4a). The deep branch of the ulnar nerve supplied 1 branch to the hypothenar muscles in 8 hands, 2 branches in 17 hands, 3 branches in 9 hands, and 4 branches in 1 hand. Many muscles received crossinnervation from multiple motor branches and many motor branches innervated multiple muscles. We found no correlation between the number of the hypothenar muscles and the number of hypothenar nerve branches. A superficial communication branch between the median and ulnar nerves was found in 22 hands. We did not dissect the innervation of the interosseous and adductor pollicis muscles.
Discussion Some articles8,9 and anatomy textbooks7,10 –13 have described the origins or insertions of the hypothenar muscles in different ways. In such situations counting the number of each hypothenar muscle is diffi-
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cult. The distinction between the ADM and the FDM is confusing because these muscles are fused frequently to each other. To prevent this confusion we defined the FDM as the muscle that exists next to the ADM and originates separately from the hook of the hamate regardless of whether it has an additional origin and/or fuses to the ADM or the ODM. Based on this definition of the FDM we counted the number of hands in which this muscle was present. It is well known that the FDM is absent in some hands. Blair et al2 reported that the FDM was present in 13 of the 21 specimens they examined. In our study the FDM was present in 27 of the 35 hands we dissected. Classic anatomy textbooks10 –13 state that the ADM has 2 slips of insertion: into the ulnar aspect of base of the proximal phalanx and into the dorsal apparatus. Recently Kanaya et al14 reported that the insertion of ADM has 2 variations. According to their study the ADM had 2 slips of insertion, as classic textbooks have mentioned, in only 58 of 70 hands. In the remaining 12 hands the ADM had only 1 slip of insertion into the ulnar aspect of the base of the proximal phalanx. Our results agree with Kanaya Figure 8. PH: type 3. The hiatus consists of the intermuscular space between the ADM and the ODM. The fascial arch is absent in this specimen.
Figure 7. PH: type 2. The hiatus consists of the fascial arch between 2 origins of the FDM. The deep branch of the ulnar nerve passes under this arch.
et al: in our study the ADM had 2 slips of insertion in 31 of 35 hands and it had only 1 slip of insertion in 4 hands. The morphology of the ODM is described in different ways in the literature. Some textbooks7,11–13 mention that this muscle has 2 origins: from the hook of the hamate and from its distal continuity with the ulnar flexor compartment wall. The deep branch of the ulnar nerve passes through these 2 layers. Blair,2 however, described the ODM as having 2 layers in 18 of 21 hands and only 1 layer in the remaining 3 hands. To clarify this point we dissected the muscle fibers distally from the ulnar edge of the origin of superficial layer to the metacarpal shaft and measured the width of the insertion of the proximal head of the ODM. In 9 hands the deep layer was so thin that it had no insertion into the ulnar aspect of the fifthmetacarpal shaft and it inserted into the metacarpohamate ligament. In even the thinnest deep layer, however, we found muscle fibers on the metacarpohamate ligament macroscopically. Consequently the ODM of all hands in our study had 2 layers. This finding is consistent with the textbooks.7,11–13
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Figure 9. Arborization patterns of the ulnar nerve.
Many reports3,12,15–25 describe the presence of an aberrant muscle in the hypothenar region. Dodds et al3 found a 22% incidence of aberrant muscles in a series of 58 specimens. In our study 9 out of 35 hands had aberrant muscles. Of these we discovered a muscle that had not been described previously: one that originated from the pisiform and inserted onto the head of fifth metacarpal dorsal to the ADM. We also found an aberrant muscle that passed between the ulnar nerve and artery lying over the ulnar nerve
Figure 10. Arborization pattern of the ulnar nerve (APUN): type 1. The ulnar nerve bifurcates into a deep branch and a superficial trunk, which bifurcates distally into 2 sensory branches. sm, small.
characteristically. Others have reported this variation.18,24 Fahrer and Milroy18 performed a dissection study using 400 hands and found the incidence of this variation to be 1.5%.Clinically, the existence of an aberrant muscle— especially variations 1, 2, and 3 of our study—may cause symptoms of compression neuropathy of the ulnar nerve as previously reported.15–18,20 –22,24 Some studies have shown that the presence of the fibrous arch over the deep branch of the ulnar nerve at the hiatus predisposes individuals to compression of the motor branch of the ulnar nerve when Guyon’s
Figure 11. APUN: type 2. The ulnar nerve trifurcates into 2 sensory branches and a deep branch.
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canal is subjected to increased pressure.26 –29 The presence of this fascial arch, however, is not constant. Dellon et al29 reported that only 40% of the hands in their study of 50 specimens contained the fascial arch at the site of hiatus. Ko¨ nig et al4 reported that a fascial arch was present in only 61% of the hands in their 23 specimens. Although our study agreed that the presence of the fascial arch was not constant our incidence differed. We found that the fascial arch existed in this region in 30 of the 35 hands in our study. In all 5 of the hands in our study that had no fascial arch at the site of hiatus the FDM was absent. After the deep branch of the ulnar nerve of these hands passed through Guyon’s canal it went over the pisohamate ligament without any restricting fibrous tissue coverage for some distance and then entered the space between the 2 layers of the ODM. When the fascial arch is not present there is less chance of entrapment of the deep branch of the ulnar nerve at this site. Without knowledge of such a variation surgeons could be confused about the location of the deep branch and misdiagnose the location of nerve compression. Figure 13. APUN: type 4. The main trunk trifurcates. An additional branch originates from the main trunk of the ulnar nerve and communicates with the ulnar proper digital nerve of the small finger distally. A communicating branch between the sensory branches is present.
Figure 12. APUN: type 3. The ulnar nerve bifurcates proximally into the radial and ulnar trunks. The deep branch of the ulnar nerve arises from the ulnar trunk.
Classically, the course of the ulnar nerve in the hand has been described as follows. The ulnar nerve divides into superficial sensory and deep branches proximal to the hiatus. The superficial sensory branch runs distally over the hypothenar muscles and divides into the fourth common digital nerve and the ulnar proper digital nerve of the small finger. The deep branch passes through the hiatus and then curves deeply and radially around the hook of the hamate. Recently, however, several articles1,4 – 6 have discussed various arborization patterns of the ulnar nerve in the hand. Most of them have classified the divisions of the ulnar nerve into 3 types: bifurcated, trifurcated, and anomalous. They have reported that the bifurcated type is the most common, occurring with an incidence of 66% to 81% (Bonnel and Vila,1 76%; Lindsey and Watumull,5 81%; Gotani et al,6 66%). The incidence of the trifurcated type has been reported as 13% to 22% (Bonnel and Vila,1 22%; Ko¨ nig et al,4 13%; Lindsey and Watumull,5 19%; Gotani et al,6 18%). In our study the incidence of the
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Figure 14. APUN: type 5. The main trunk trifurcates. The dorsal sensory branch of the ulnar nerve connects with the ulnar proper digital nerve of the small finger.
bifurcated and trifurcated types was 86% (30/35) and 14% (5/35), respectively. To avoid injury to the nerve branches when dissection is performed at the ulnar side of the hand and wrist these anatomic variations should be considered. When a communicating branch is present between the 2 sensory branches to the fingers nerve injury to 1 of the sensory branches can cause sensory distur-
bance on both the ring and the small fingers. When an abductor digiti quinti opposition transfer is being performed and the ulnar sensory branch pierces the ADM the sensory branch should be dissected from the muscle belly of the ADM before the muscle is transferred. Additionally, if a connection between the dorsal sensory branch and the ulnar proper digital nerve of the small finger is present (type 5) injury or compression of the dorsal branch of the ulnar nerve may cause sensory deficit on the ulnar side of the small finger. Knowledge of this variation would help surgeons diagnose and understand a confusing symptom that cannot be explained by the standard anatomic concept of the ulnar nerve. The literature that mentions anatomic variations of the motor nerve branch to the hypothenar muscles is scarce. Blair et al2 and Lindsey and Watumull5 have reported the number of the motor branches to the hypothenar muscles. In types 1, 2, and 4a of our study the motor branch to the ADM originated from the deep branch distal to the fascial arch of the hiatus. In these cases if the abductor digiti quinti opposition transfer is performed the fascial arch should be divided and the motor branch to the ADM should be dissected distally so that the motor branch is not impaired during the transfer of the muscle. Additionally, in types 2, 3, and 4 of our study the function of the ADM would be preserved even if a compressive lesion were present in the pisohamate tunnel. The incidence of superficial communication between the median and ulnar nerves varies throughout the literature. Bonnel and Vila1 mentioned that this connection was constant in 50 cadaveric hands and Meals and Shaner30 reported that this connection was found in 80% of their 50 cadaveric specimens. In our study, however, 63% of the 35 hands had this connection. To determine an accurate incidence a study
Figure 15. Branching patterns of motor branch to ADM.
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must be performed with a larger number of specimens than the ones reported previously. The detailed anatomy in our study can assist the surgeon in the diagnosis and treatment of conditions affecting the ulnar aspect of the hand, including hypothenar trauma, compression of Guyon’s canal, and malignant and benign tumors in the hypothenar area. This knowledge also can assist the surgeon who is performing an abductor digiti quinti opposition transfer or a palmar fasciectomy for Dupuytren’s contracture. We would like to acknowledge the University of Louisville Body Bequeathal Program (Anatomical Science/Neurobiology) and Dr Robert Acland, Director, Fresh Tissue Dissection Laboratory.
References 1. Bonnel F, Vila RM. Anatomical study of the ulnar nerve in the hand. J Hand Surg 1985;10B:165–168. 2. Blair WF, Percival KJ, Morecraft R. Distribution pattern of the deep branch of the ulnar nerve in the hypothenar eminence. Clin Orthop 1988;229:294 –301. 3. Dodds GA 3rd, Hale D, Jackson WT. Incidence of anatomic variants in Guyon’s canal. J Hand Surg 1990;15A:352–355. 4. Ko¨ nig PS, Hage JJ, Bloem JJ, Prose LP. Variations of the ulnar nerve and ulnar artery in Guyon’s canal: a cadaveric study. J Hand Surg 1994;19A:617– 622. 5. Lindsey JT, Watumull D. Anatomic study of the ulnar nerve and related vascular anatomy at Guyon’s canal: a practical classification system. J Hand Surg 1996;21A:626 – 633. 6. Gotani H, Yamano Y, Gilbert A. Anatomy of the ulnar nerve in the hand: study of anomalous sensory branches. J Jpn Soc Surg Hand 2001;18:670 – 674. 7. Landsmeer JMF. Atlas of anatomy of the hand. Edinburgh: Churchill Livingstone, 1976:43– 48, 264 –277. 8. Denman EE. The anatomy of the space of Guyon. Hand 1978;10:69 –76. 9. Gross MS, Gelberman RH. The anatomy of the distal ulnar tunnel. Clin Orthop 1985;196:238 –247. 10. Deaver JB. Surgical anatomy: a treatise on human anatomy in its application to the practice of medicine and surgery. Vol.1. Philadelphia: Blakiston’s Son & Co, 1901:152–180. 11. Spinner M. Kaplan’s functional and surgical anatomy of the hand. 3rd ed. Philadelphia: Lippincott, 1984:107–109, 206 – 235. 12. Tountas CP, Bergman RA. Anatomic variations of the upper extremity. New York: Churchill Livingstone, 1993:170 – 175.
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13. Williams PL, ed. Gray’s anatomy. 38th ed. New York: Churchill Livingstone, 1995:858 – 862. 14. Kanaya K, Wada T, Isogai S, Murakami G, Ishii S. Variation in insertion of the abductor digiti minimi: an anatomic study. J Hand Surg 2002;27A:325–328. 15. Thomas CG, Hill C. Clinical manifestations of an accessory palmaris muscle. J Bone Joint Surg 1958;40A:929 –930. 16. Jeffery AK. Compression of the deep palmar branch of the ulnar nerve by an anomalous muscle. Case report and review. J Bone Joint Surg 1971;53B:718 –723. 17. Turner MS, Caird DM. Anomalous muscles and ulnar nerve compression at the wrist. Hand 1977;9:140 –142. 18. Fahrer M, Milroy PJ. Ulnar compression neuropathy due to an anomalous abductor digiti minimi— clinical and anatomic study. J Hand Surg 1981;6:266 –268. 19. Martin AH. An accessory hypothenar muscle. Plast Reconstr Surg 1981;67(2):219 –220. 20. Tonkin MA, Lister GD. The palmaris brevis profundus. An anomalous muscle associated with ulnar nerve compression at the wrist. J Hand Surg 1985;10A:862– 864. 21. James MR, Rowley DI, Norris SH. Ulnar nerve compression by an accessory abductor digiti minimi muscle presenting following injury. Injury 1987;18(1):66 – 67. 22. Luethke R, Dellon AL. Accessory abductor digiti minimi muscle originating proximal to the wrist causing symptomatic ulnar nerve compression. Ann Plast Surg 1992;28(3): 307–308. 23. Sanˇ udo JR, Mirapeix RM, Ferreira B. A rare anomaly of abductor digiti minimi. J Anat 1993;182:439 – 442. 24. Netscher D, Cohen V. Ulnar nerve compression at the wrist secondary to anomalous muscles: a patient with a variant of abductor digiti minimi. Ann Plast Surg 1997;39:647– 651. 25. Curry B, Kuz J. A new variation of abductor digiti minimi accessorius. J Hand Surg 2000;25A:585–587. 26. Hayes JR, Mulholland RC, O’Connor BT. Compression of the deep palmar branch of the ulnar nerve. Case report and anatomical study. J Bone Joint Surg 1969;51B:469 – 472. 27. Uriburu IJ, Morchio FJ, Marin JC. Compression syndrome of the deep motor branch of the ulnar nerve (piso-hamate hiatus syndrome). J Bone Joint Surg 1976;58A:145–147. 28. McFarlane RM, Mayer JR, Hugill JV. Further observations on the anatomy of the ulnar nerve at the wrist. Hand 1976; 8:115–117. 29. Dellon AL, Mackinnon SE. Anatomic investigations of nerves at the wrist: II. Incidence of fibrous arch overlying motor branch of ulnar nerve. Ann Plast Surg 1988;21(1):36 – 37. 30. Meals RA, Shaner M. Variations in digital sensory patterns: a study of the ulnar nerve-median nerve palmar communicating branch. J Hand Surg 1983;8:411– 414.
Purpose: To show the variations in the hypothenar muscles and the arborization patterns of the ulnar nerve and to investigate the relationship between the hypothenar muscles and the ulnar nerve. Methods: We performed an anatomic study of 35 hands from embalmed cadavers. After dissecting the ulnar side of the hand we recorded the number of hypothenar muscles and their variations, the site of the hiatus for the deep branch of the ulnar nerve, and the branching patterns of the ulnar nerve in each hand. We then investigated the relationship between the variations in the hypothenar muscles and the arborization patterns of the ulnar nerve. Results: The abductor digiti minimi (ADM) had 1 belly in 6 hands, 2 bellies in 28 hands, and 3 bellies in 1 hand. The flexor digiti minimi brevis was absent in 8 hands, had 1 belly in 24 hands, and had 2 bellies in 3 hands. The opponens digiti minimi had 2 layers of origin and the deep branch of the ulnar nerve passed between these 2 layers in all hands. We classified the structural patterns of the hiatus into 3 types, the arborization patterns of the ulnar nerve into 5 types, and the branching patterns of the motor branch to the ADM into 4 types according to their morphologic characteristics. We found variations of the arborization pattern in which the deep branch originated from the ulnar trunk of the ulnar nerve distal to bifurcation in 3 hands, a communicating branch was present between the 2 sensory branches to the fingers in 3 hands, and the ulnar sensory branch pierced the ADM in 2 hands. Conclusions: We confirmed that the anatomic relationship between the ulnar nerve and the hypothenar muscle is complex and that the formation of the hiatus varies. This knowledge can assist the surgeon in the diagnosis and treatment of conditions associated with the ulnar aspect of the hand. (J Hand Surg 2004;29A:500 –509. Copyright © 2004 by the American Society for Surgery of the Hand.) Key words: Aberrant muscle, Guyon’s canal, hypothenar muscle, ulnar nerve.
From the Christine M. Kleinert Institute for Hand and Microsurgery and University of Louisville, School of Medicine, Louisville, KY. Received for publication May 21, 2003; accepted in revised form January 8, 2004. No benefits in any form have been received or will be received by a commercial party related directly or indirectly to the subject of this article. Reprint requests: Amit Gupta, MD, FRCS, Christine M. Kleinert Institute for Hand and Microsurgery, 225 Abraham Flexner Way, Ste 850, Louisville, KY 40202. Copyright © 2004 by the American Society for Surgery of the Hand 0363-5023/04/29A03-0026$30.00/0 doi:10.1016/j.jhsa.2004.01.006
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The complex anatomic variations of the ulnar nerve and the hypothenar muscles in the hand have been described in the literature.1– 6 The purposes of this study are to (1) show the variations in the hypothenar muscles and the arborization patterns and course of the ulnar nerve and (2) to investigate the relationship between the hypothenar muscles and the ulnar nerve. This knowledge can assist the surgeon in the diagnosis and treatment of conditions affecting the ulnar aspect of the hand such as tumors, trauma, nerve compression, abductor digiti quinti opposition trans-
Murata, Tamai, and Gupta / Anatomy of Hypothenar Muscles and Ulnar Nerve
Table 1. Number of Origins and Insertions ADM Number of origins Number of hands Number of insertions Number of hands
FDM
1 2 3 0 1 6 28 1 8 24 1 2 3 0 1 4 31 — 8 27*
ODM 2 1 2 3 — 35 2 1 2 — 35 —
3 — 3 —
*Patterns of fusion in FDM insertions and number of hands: fused with ADM, 22; fused with ODM & ADM, 2; independent, 3; absent, 8.
fer, and palmar fasciectomy for Dupuytren’s contracture.
Materials and Methods We dissected 35 hands (15 right, 20 left) from 20 embalmed cadavers. The age of the deceased subjects ranged from 56 to 99 years (average, 76 years). Dissection was performed at the ulnar side of the hand under loupe magnification (3.5⫻). After the skin of the distal forearm and palm was removed a radial-based flap (a roof of the Guyon’s canal) containing subcutaneous tissue and the palmaris brevis (PB) muscle was elevated to expose the hypothenar muscles and the ulnar neurovascular bundle. For each hand we recorded the number of hypothenar muscles and their sites of origin and insertion, the existence of aberrant muscles, the structural patterns of the hiatus for the deep branch of the ulnar nerve, and the branching patterns of the superficial and deep branch of the ulnar nerve. Based on these findings we investigated the anatomic relationship between the hypothenar muscles and the ulnar nerve.
Results The abductor digiti minimi (ADM) muscle originated from the pisiform, the tendon of the flexor carpi ulnaris, and the pisohamate ligament. In 31 hands this muscle had 2 slips of insertion. One slip inserted into the ulnar side of the proximal phalanx base of the small finger and the other slip inserted into the extensor apparatus of the small finger. In 4 hands the muscle had 1 slip of insertion into the ulnar side of the proximal phalanx base of the small finger. In 28 hands the ADM had 2 bellies. These 2 bellies were identified as the palmar and dorsal portions, as Landsmeer reported.7 The ADM had 1 belly in 6 hands and 3 bellies in 1 hand (Table 1). The flexor digiti minimi brevis (FDM) muscle was absent in 8 hands. In the remaining 27 hands the FDM originated from the hook of the hamate, the adjacent ulnar portion of the flexor retinaculum,
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and/or the radial portion of the pisiform. The FDM had 1 belly in 24 hands and 2 bellies in 3 hands. The FDM was fused distally with the ADM in 22 hands. When the FDM fused with the ADM it had common insertions with the ADM. In 2 hands the FDM was fused with the opponens digiti minimi muscle (ODM) at the place of origin and fused distally with the ADM. In 3 hands the FDM existed independently. All of the independent FDM muscles inserted onto the volar aspect of the head of the fifth metacarpal (Table 1). Additionally, in some cadavers the FDM had a different number of bellies in the right hand than in the left hand of the same body. The ODM had 2 layers of origin in all hands. The superficial layer originated from the distal part of the hook of the hamate and inserted into the distal ulnar side of the fifth metacarpal shaft. The deep layer originated from the part of the ulnar flexor compartment wall that is adjacent to the hook of the hamate and it inserted into the proximal ulnar side of the small finger metacarpal shaft (Table 1). When we dissected the muscle fibers distally we observed that the ODM had 2 layers. The width of the deep layer’s insertion to the fifth metacarpal varied greatly (Fig. 1). In 9 hands the deep layer was so thin that it looked like membrane. In even the thinnest deep layer, however, we found muscle fibers on the metacarpohamate ligament macroscopically. The deep branch of the ulnar nerve passed between the superficial and the deep layers of the ODM in all hands. We found 4 variations of an aberrant muscle in 9 hands (Table 2). In variation 1 (5 hands) the aberrant muscle originated from the flexor retinaculum (Fig. 2). In variation 2 (2 hands) the muscle originated from the antebrachial fascia (Fig. 3). These 7 muscles should be classified as the accessory abductor digiti minimi muscle because they were fused with the ADM distally. They lie volar to the ulnar neurovascular bundle in Guyon’s canal. In variation 3 (1 hand) the aberrant muscle originated from the pisiform and inserted into the flexor retinaculum. This muscle passed between the ulnar nerve and artery, lying over the ulnar nerve (Fig. 4). In variation 4 (1 hand) the aberrant muscle originated from the pisiform and inserted onto the volar aspect of the head of small finger metacarpal dorsally to the ADM (Fig. 5). We classified the structural patterns of the hiatus into 3 types (Table 3). In type 1 (25 hands) the hiatus consisted of the intermuscular space between the ADM and the FDM (Fig. 6). In this type the fascial continuity between the ADM and the FDM formed the arched structure (fascial arch) across the hiatus
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Figure 1. Two layers of the ODM.
and the deep branch of the ulnar nerve passed under this fascial arch. In type 2 (2 hands) the hiatus consisted of the fascial arch between the 2 origins of the FDM, the pisiform and the hook of the hamate (Fig. 7). In this type the deep branch of the ulnar nerve passed under this fascial arch. In type 3 (8 hands) the hiatus consisted of the intermuscular space between the ADM and the ODM (Fig. 8). In these 8 hands the FDM was absent. In 3 of the hands classified as type 3 the fascia of the superficial layer of the ODM continued to the fascia of the ADM and this fascial continuity formed the fascial arch. These hands had very thin deep layers and the ADM and the superficial layer of the ODM existed closely. In the remaining 5 hands that were type 3 we did not find the fascial arch. We also classified the arborization patterns of the ulnar nerve into 5 types according to their morpho-
Table 2. Aberrant Muscle Variations Variation
No. of Hands
1
5
2
2
3 4
1 1
Origin Flexor retinaculum Antebrachial fascia Pisiform Pisiform
Insertion Fused with ADM Fused with ADM Flexor retinaculum Head of 5th metacarpal
Figure 2. Aberrant muscle (AM): variation 1. The muscle originates from the flexor retinaculum and is fused with the abductor digiti minimi distally. It lies volar to the ulnar neurovascular bundle.
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logic characteristics (Figure 9). In type 1 (27 hands) the ulnar nerve bifurcated into a deep branch and a superficial trunk just distal to the distal edge of the pisiform (Fig. 10). The superficial trunk bifurcated distally into 2 sensory branches: the fourth common digital nerve and the ulnar proper digital nerve of the small finger. In 1 hand with a type 1 pattern we found a communicating branch between the 2 sensory branches to the fingers. In type 2 (3 hands) the ulnar nerve trifurcated into the ring finger common digital nerve, the ulnar proper digital nerve of the small finger, and a deep branch just distal to the distal edge of the pisiform (Fig. 11). In type 3 (3 hands) the ulnar nerve bifurcated into radial and ulnar trunks just distal to the distal edge of the pisiform (Fig. 12). The radial trunk continued to the ring finger common digital nerve and the ulnar trunk bifurcated distally into the ulnar proper digital nerve of the small finger and a deep branch. In one type 3 hand we found a communicating branch between the 2 sensory branches to the fingers. In this hand the ulnar sensory branch pierced the ADM. In type 4 (1 hand) an additional superficial branch originated from the main trunk of the ulnar nerve in Guyon’s canal and
Figure 3. AM: variation 2. The muscle originates from the antebrachial fascia and is fused with the abductor digiti minimi. It lies volar to the ulnar neurovascular bundle.
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Figure 4. AM: variation 3. The muscle originates from the pisiform and inserts into the flexor retinaculum. It passes between the ulnar nerve and artery. a, artery; n, nerve.
communicated with the ulnar proper digital nerve of small finger distally (Fig. 13). The main trunk trifurcated into the ring finger common digital nerve, the ulnar proper digital nerve of small finger, and a deep branch just distal to the distal edge of the pisiform. In this hand the additional branch passed through the muscle belly of the ADM. We also found a communicating branch between the 2 sensory branches to the fingers in this hand. In type 5 (1 hand) the dorsal sensory branch of the ulnar nerve connected with the ulnar proper digital nerve of the small finger (Fig. 14). The main trunk trifurcated into the ring finger common digital nerve, the ulnar proper digital nerve of the small finger, and a deep branch just distal to the distal edge of the pisiform. In this hand a communicating branch between the dorsal sensory branch of the ulnar nerve and the ulnar proper digital nerve of the small finger passed through the muscle belly of the ADM. We also identified 4 branching patterns of the motor branch to the ADM (Figure 15). In type 1 (16 hands) a motor branch to the ADM originated from the deep branch distal to the hiatus. In type 2 (1 hand) a main motor branch to the ADM originated from the deep branch proximal to the hiatus in the Guyon’s
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Figure 5. AM: variation 4. The muscle originates from the pisiform and inserts onto the volar aspect of the fifth metacarpal head.
canal and an additional motor branch to the ADM originated from the deep branch distal to the hiatus. In type 3 (8 hands) the ulnar nerve trifurcated into a superficial trunk, a deep branch, and a motor branch to the ADM in Guyon’s canal. In type 4 (10 hands) a main motor branch to the ADM originated from the
Table 3. Structural Patterns of the Hiatus Type
No. of Hands (%)
1
25 (71%)
2
2 (6%)
3
8 (23%)
Pattern Fascial arch between ADM and FDM Deep branch of ulnar n. passes under arch Fascial arch between 2 origins of FDM Deep branch of ulnar n. passes under arch Intermuscular space between ADM & ODM Arch: 3 hands-arch between ADM and ODM 5 hands-no arch
Figure 6. Pattern of the hiatus (PH): type 1. The hiatus consists of the intermuscular space between the ADM and the FDM. The fascial continuity between these 2 muscles forms the fascial arch and the deep branch passes under this arch.
main trunk of the ulnar nerve proximal to the hiatus in Guyon’s canal. In 4 hands classified as type 4 an additional motor branch to the ADM originated from the deep branch distal to the hiatus (type 4a). The deep branch of the ulnar nerve supplied 1 branch to the hypothenar muscles in 8 hands, 2 branches in 17 hands, 3 branches in 9 hands, and 4 branches in 1 hand. Many muscles received crossinnervation from multiple motor branches and many motor branches innervated multiple muscles. We found no correlation between the number of the hypothenar muscles and the number of hypothenar nerve branches. A superficial communication branch between the median and ulnar nerves was found in 22 hands. We did not dissect the innervation of the interosseous and adductor pollicis muscles.
Discussion Some articles8,9 and anatomy textbooks7,10 –13 have described the origins or insertions of the hypothenar muscles in different ways. In such situations counting the number of each hypothenar muscle is diffi-
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cult. The distinction between the ADM and the FDM is confusing because these muscles are fused frequently to each other. To prevent this confusion we defined the FDM as the muscle that exists next to the ADM and originates separately from the hook of the hamate regardless of whether it has an additional origin and/or fuses to the ADM or the ODM. Based on this definition of the FDM we counted the number of hands in which this muscle was present. It is well known that the FDM is absent in some hands. Blair et al2 reported that the FDM was present in 13 of the 21 specimens they examined. In our study the FDM was present in 27 of the 35 hands we dissected. Classic anatomy textbooks10 –13 state that the ADM has 2 slips of insertion: into the ulnar aspect of base of the proximal phalanx and into the dorsal apparatus. Recently Kanaya et al14 reported that the insertion of ADM has 2 variations. According to their study the ADM had 2 slips of insertion, as classic textbooks have mentioned, in only 58 of 70 hands. In the remaining 12 hands the ADM had only 1 slip of insertion into the ulnar aspect of the base of the proximal phalanx. Our results agree with Kanaya Figure 8. PH: type 3. The hiatus consists of the intermuscular space between the ADM and the ODM. The fascial arch is absent in this specimen.
Figure 7. PH: type 2. The hiatus consists of the fascial arch between 2 origins of the FDM. The deep branch of the ulnar nerve passes under this arch.
et al: in our study the ADM had 2 slips of insertion in 31 of 35 hands and it had only 1 slip of insertion in 4 hands. The morphology of the ODM is described in different ways in the literature. Some textbooks7,11–13 mention that this muscle has 2 origins: from the hook of the hamate and from its distal continuity with the ulnar flexor compartment wall. The deep branch of the ulnar nerve passes through these 2 layers. Blair,2 however, described the ODM as having 2 layers in 18 of 21 hands and only 1 layer in the remaining 3 hands. To clarify this point we dissected the muscle fibers distally from the ulnar edge of the origin of superficial layer to the metacarpal shaft and measured the width of the insertion of the proximal head of the ODM. In 9 hands the deep layer was so thin that it had no insertion into the ulnar aspect of the fifthmetacarpal shaft and it inserted into the metacarpohamate ligament. In even the thinnest deep layer, however, we found muscle fibers on the metacarpohamate ligament macroscopically. Consequently the ODM of all hands in our study had 2 layers. This finding is consistent with the textbooks.7,11–13
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Figure 9. Arborization patterns of the ulnar nerve.
Many reports3,12,15–25 describe the presence of an aberrant muscle in the hypothenar region. Dodds et al3 found a 22% incidence of aberrant muscles in a series of 58 specimens. In our study 9 out of 35 hands had aberrant muscles. Of these we discovered a muscle that had not been described previously: one that originated from the pisiform and inserted onto the head of fifth metacarpal dorsal to the ADM. We also found an aberrant muscle that passed between the ulnar nerve and artery lying over the ulnar nerve
Figure 10. Arborization pattern of the ulnar nerve (APUN): type 1. The ulnar nerve bifurcates into a deep branch and a superficial trunk, which bifurcates distally into 2 sensory branches. sm, small.
characteristically. Others have reported this variation.18,24 Fahrer and Milroy18 performed a dissection study using 400 hands and found the incidence of this variation to be 1.5%.Clinically, the existence of an aberrant muscle— especially variations 1, 2, and 3 of our study—may cause symptoms of compression neuropathy of the ulnar nerve as previously reported.15–18,20 –22,24 Some studies have shown that the presence of the fibrous arch over the deep branch of the ulnar nerve at the hiatus predisposes individuals to compression of the motor branch of the ulnar nerve when Guyon’s
Figure 11. APUN: type 2. The ulnar nerve trifurcates into 2 sensory branches and a deep branch.
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canal is subjected to increased pressure.26 –29 The presence of this fascial arch, however, is not constant. Dellon et al29 reported that only 40% of the hands in their study of 50 specimens contained the fascial arch at the site of hiatus. Ko¨ nig et al4 reported that a fascial arch was present in only 61% of the hands in their 23 specimens. Although our study agreed that the presence of the fascial arch was not constant our incidence differed. We found that the fascial arch existed in this region in 30 of the 35 hands in our study. In all 5 of the hands in our study that had no fascial arch at the site of hiatus the FDM was absent. After the deep branch of the ulnar nerve of these hands passed through Guyon’s canal it went over the pisohamate ligament without any restricting fibrous tissue coverage for some distance and then entered the space between the 2 layers of the ODM. When the fascial arch is not present there is less chance of entrapment of the deep branch of the ulnar nerve at this site. Without knowledge of such a variation surgeons could be confused about the location of the deep branch and misdiagnose the location of nerve compression. Figure 13. APUN: type 4. The main trunk trifurcates. An additional branch originates from the main trunk of the ulnar nerve and communicates with the ulnar proper digital nerve of the small finger distally. A communicating branch between the sensory branches is present.
Figure 12. APUN: type 3. The ulnar nerve bifurcates proximally into the radial and ulnar trunks. The deep branch of the ulnar nerve arises from the ulnar trunk.
Classically, the course of the ulnar nerve in the hand has been described as follows. The ulnar nerve divides into superficial sensory and deep branches proximal to the hiatus. The superficial sensory branch runs distally over the hypothenar muscles and divides into the fourth common digital nerve and the ulnar proper digital nerve of the small finger. The deep branch passes through the hiatus and then curves deeply and radially around the hook of the hamate. Recently, however, several articles1,4 – 6 have discussed various arborization patterns of the ulnar nerve in the hand. Most of them have classified the divisions of the ulnar nerve into 3 types: bifurcated, trifurcated, and anomalous. They have reported that the bifurcated type is the most common, occurring with an incidence of 66% to 81% (Bonnel and Vila,1 76%; Lindsey and Watumull,5 81%; Gotani et al,6 66%). The incidence of the trifurcated type has been reported as 13% to 22% (Bonnel and Vila,1 22%; Ko¨ nig et al,4 13%; Lindsey and Watumull,5 19%; Gotani et al,6 18%). In our study the incidence of the
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Figure 14. APUN: type 5. The main trunk trifurcates. The dorsal sensory branch of the ulnar nerve connects with the ulnar proper digital nerve of the small finger.
bifurcated and trifurcated types was 86% (30/35) and 14% (5/35), respectively. To avoid injury to the nerve branches when dissection is performed at the ulnar side of the hand and wrist these anatomic variations should be considered. When a communicating branch is present between the 2 sensory branches to the fingers nerve injury to 1 of the sensory branches can cause sensory distur-
bance on both the ring and the small fingers. When an abductor digiti quinti opposition transfer is being performed and the ulnar sensory branch pierces the ADM the sensory branch should be dissected from the muscle belly of the ADM before the muscle is transferred. Additionally, if a connection between the dorsal sensory branch and the ulnar proper digital nerve of the small finger is present (type 5) injury or compression of the dorsal branch of the ulnar nerve may cause sensory deficit on the ulnar side of the small finger. Knowledge of this variation would help surgeons diagnose and understand a confusing symptom that cannot be explained by the standard anatomic concept of the ulnar nerve. The literature that mentions anatomic variations of the motor nerve branch to the hypothenar muscles is scarce. Blair et al2 and Lindsey and Watumull5 have reported the number of the motor branches to the hypothenar muscles. In types 1, 2, and 4a of our study the motor branch to the ADM originated from the deep branch distal to the fascial arch of the hiatus. In these cases if the abductor digiti quinti opposition transfer is performed the fascial arch should be divided and the motor branch to the ADM should be dissected distally so that the motor branch is not impaired during the transfer of the muscle. Additionally, in types 2, 3, and 4 of our study the function of the ADM would be preserved even if a compressive lesion were present in the pisohamate tunnel. The incidence of superficial communication between the median and ulnar nerves varies throughout the literature. Bonnel and Vila1 mentioned that this connection was constant in 50 cadaveric hands and Meals and Shaner30 reported that this connection was found in 80% of their 50 cadaveric specimens. In our study, however, 63% of the 35 hands had this connection. To determine an accurate incidence a study
Figure 15. Branching patterns of motor branch to ADM.
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must be performed with a larger number of specimens than the ones reported previously. The detailed anatomy in our study can assist the surgeon in the diagnosis and treatment of conditions affecting the ulnar aspect of the hand, including hypothenar trauma, compression of Guyon’s canal, and malignant and benign tumors in the hypothenar area. This knowledge also can assist the surgeon who is performing an abductor digiti quinti opposition transfer or a palmar fasciectomy for Dupuytren’s contracture. We would like to acknowledge the University of Louisville Body Bequeathal Program (Anatomical Science/Neurobiology) and Dr Robert Acland, Director, Fresh Tissue Dissection Laboratory.
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