Morphological Study of Anomalous Deep Extensors of the HandGaddam Vijaya Lakshmi1 Stelin Agnes Michael1
1Department of Anatomy, Pushpagiri Institute of Medical Sciences and Research Centre, Thiruvalla, Kerala, India
Address for correspondence Gaddam Vijaya Lakshmi, MD, Department of Anatomy, Pushpagiri Institute of Medical Sciences and Research Centre, Thiruvalla, 689101, Kerala, India (e-mail: [email protected]).
Background Anomalous muscles on the dorsum of the hand include the extensor medii proprius (EMP), extensor indicis et medii communis (EIMC), extensor digitorum brevis manus (EDBM), and anomalous extensor indicis proprius (aEIP). They are commonly seen during routine cadaveric dissections and hand surgeries. There are very few studies on them in the recent past. Hence, this study was conducted with the aim of studying their incidence and morphology.Materials and Methods This study was conducted at the Pushpagiri Institute of Medical Sciences and Research Centre, Thiruvalla, Kerala, over a period of 15 months, on 80 free upper limbs. Deep extensor muscles of the forearm were dissected. The frequency of the anomalous muscles was noted. Length and width of the muscles and tendons were measured using thread, ruler, and vernier calipers. The median and range were calculated. The musculotendinous (MT) junction of the EIP was observed. It was considered to be aEIP if its MT junction was beyond the distal edge of the extensor retinaculum in wrist flexion.Results EMP was seen in 8.75% of the limbs, with average muscle length and width of 4.53 ± 0.7 cm and 3.7 ± 0.8 mm and average tendon length and width of 9.7 ± 1.2 cm and 1.6 ± 0.8 mm, respectively. Frequency of EIMC was 3.75%, its average muscle length and width being 3.67 ± 1.14 cm and 2.3 ± 0.6 mm and its tendon length and width being 9.8 ± 0.61 cm and 2.0 ± 1.7 mm. Incidence of EDBM was 2.5%; its average muscle length and width being 4.5 ± 0.71 cm and 1.25 ± 0.3 mm and tendon length and width being 3.15 ± 0.91 cm and 3.5 ± 0.7 mm. Frequency of aEIP was observed to be 6.25%.Conclusion Anomalous extensors have a frequency of less than 10%. Tendons of the EMP and EIMC are very thin and asymptomatic. The EDBM belly is distal to the retinaculum and does not cause symptoms, unless hypertrophied. aEIP passes through the narrow compartment and is more likely to cause pain.
Several variations are observed in the muscles and tendons of the extensor compartment of the forearm. Anomalous muscles are often noticed during surgeries on the wrist and hand. The anomalous extensors of the hand reported in the literature include the extensor medii proprius (EMP),
extensor indicis et medii communis (EIMC), extensor digitorum brevis manus (EDBM), and anomalous extensor indicis proprius (aEIP).1
The EMP and EIMC take origin from the dorsal surface of the ulna, distal to the origin of the EIP. The tendons are thin and pass through the fourth compartment of the extensor retinaculum. Tendon of the EMP is inserted into the middle
Original Article
Published online: 2019-04-28
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finger whereas that of EIMC splits into two for insertion into the index and middle fingers. They are inserted into the extensor expansion, or into the tissues near the metacarpo-phalangeal joint.1,2
The EDBM takes origin from the dorsal radiocarpal liga- ment and capsule of the wrist joint. It is tendinous within the fourth compartment, the belly being distal to the retinaculum. It is inserted into the index or middle finger. It is classified into three types based on its association with the EIP.3
The EIP takes origin from the dorsal surface of the shaft of the ulna distal to the extensor pollicis longus. The tendon pass-es through the fourth compartment, and joins the tendon of the extensor digitorum to the index finger. When the muscle belly passes through the retinaculum, and emerges at its distal border in fully flexed wrist, then it is considered to be aEIP.1
The importance of these muscles lies in that they pose a diagnostic challenge to the clinician, as they can resemble ganglion or other soft tissue conditions. Knowledge of these anomalous muscles is required in assessing patients with dorsal wrist pain. Rarely, they can cause compression of the posterior interosseous nerve (PIN).4
These anomalous muscles have to be kept in mind in surgical approach to the dorsum of the hand. They can be utilized for tendon graft and transfer without compromising the functions of the hand.5 Moreover, there are no compre- hensive studies in the recent past on these anomalous extensors. Hence, this study was undertaken to have better understanding on their incidence and morphology.
Materials and MethodsThe study was conducted in the Department of Anatomy, Pushpagiri Institute of Medical Sciences and Research Centre, Thiruvalla, Kerala, over a period of 15 months from July 2017 to September 2018, following ethical clearance from the institution. Dissection of the posterior compartment of the forearm and hand following the Cunningham’s Manual was performed on 80 free upper limbs, used for routine undergraduate teaching purpose. Upper limbs with deformities were excluded from the study.
Anomalous muscles like the EMP, EIMC, and EDBM, if present, were identified and photographed. Their attachments, the compartment through which they pass through, and their nerve supply were noted. The length of the muscle bellies and tendons were measured using a thread and a ruler, and their widths were measured using vernier calipers. All measurements were tabulated in centimeters. Median and range of the above parameters were calculated.
The musculotendinous (MT) junction of the EIP with reference to the extensor retinaculum in both neutral and flexed positions of the wrist was looked into. When the wrist was flat on the table, that is, neither flexed nor extended, it was considered neutral position in this study. An aEIP was defined to be present when the junction was noted beyond the distal edge of the retinaculum, in com-pletely flexed position of the wrist. Incidence of the above anomaly was noted down.
ResultsThe EMP was seen in 6 left and 1 right upper limbs (8.75%). In all the cases, it took origin from the ulna, distal to the attachment of the EIP; passed through the fourth compartment as a single tendon and received a branch from the PIN. In four of them, the tendon got inserted into the deeper tissues of the metacarpophalangeal joint of the middle finger (►Fig. 1). But in two specimens, the tendon split into two after leaving the compartment, and both of them were inserted into the extensor expansion of the middle finger (►Fig. 2). In all cases, the insertion was on the ulnar side of the extensor digitorum communis (EDC) tendon to the middle finger. The length and width of the muscle bellies and tendons are given in ►Table 1.
The EIMC was noticed in 2 right and 1 left upper limbs (3.75%). In all specimens, the muscle took origin from the dorsal surface of the ulna, distal to the attachment of the EIP. In two specimens, the tendons were single in the fourth compartment. Distally, they split into two tendons each, the lateral tendon joined the tendon of the EIP, while the medial one was flat and inserted into the deeper tissues at the head of the third metacarpal (►Fig. 3). In the third specimen, there were two tendons in the compartment. The lateral tendon was very thin and inserted into the index finger, while the medial one bifurcated into two, both of which passed into the middle finger for insertion into the extensor expansion. A thin muscular slip was seen passing from this anomalous muscle to the tendon of the EIP (►Fig. 4). All of them received innervations from the PIN. The length and width of the muscle bellies and tendons are given in ►Table 2.
The EDBM was seen in 1 right and 1 left upper limbs (2.5%). In the left-sided specimen, the muscle had origin
Fig. 1 Extensor medii proprius (EMP) inserting into the middle finger.
Fig. 2 Split tendon of the extensor medii proprius (EMP).
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from the distal carpal bones as a short tendon. The muscle belly began at the distal edge of the retinaculum, and converged to a tendon for insertion into the middle finger on the ulnar side of the EDC tendon (►Fig. 5). But in the other specimen on the right side, the origin was aponeurotic from the base of the third metacarpal. Muscle was inserted through a short tendon on the radial side of the EDC tendon to the middle finger (►Fig. 6). Both of them were supplied by branches from the PIN. The tendon of origin on the left side and the tendon of insertion on the right side were not measured as they were too short. The length and width of the muscle bellies and tendons are given in ►Table 3.
In neutral wrist position, the MT junction of the EIP was observed to be within the compartment in 58.5% and distal to the retinaculum in 3.75% of the limbs studied (►Table 4). When the wrist was flexed by the observer, the MT junction was noticed to move distally, and was observed within the compartment in 91.25% (►Fig. 7) and distal to it in 6.25% (►Fig. 8) of the limbs. Thus, the crite-rion for defining an aEIP was fulfilled in 6.25% (five upper limbs). In all cases, the muscle attachments were same as reported in the literature for the normal EIP, and they received nerve supply from the PIN.
Other variations of EIP like absence (1.25%), double tendons (2.5%), and tendon bifurcation distal to the retinaculum
Table 1 Morphometry of the extensor medii proprius
S. no. Side R/L Muscle bellylength (cm)
Muscle bellywidth (cm)
Tendonlength (cm)
Tendonwidth (cm)
1 L 4.5 0.4 10.5 0.1
2 L 4.8 0.4 9.0 0.2
3 L 4.6 0.4 10.5 0.1
4 L 3.0 0.2 8.5 0.1
5 L 4.8 0.4 8.3 0.1
6 L 5.0 0.4 8.0 up to bifurcation3.6 each up to insertion
0.3 up to bifurcation0.15 cm each up to insertion
7 R 5.0 0.4 8.0 up to bifurcation1.5 each up to insertion
Distal to the retinaculum 1 (2.5) 2 (5) 3 (3.75) 2 (5) 3 (7.5) 5 (6.25)
Abbreviation: EIP, extensor indicis proprius.
(3.75%) were noted. Since, these variations were different from the criterion for defining aEIP, they are not described further in this study.
Discussion
EMP and EIMCThe incidence of the EMP has been reported differently in the range of 0.8 to 10.3%.1 Moreover, there are very few studies in the recent past to compare the incidence of these anomalies. The frequency observed in this study was 8.75%, and closer to the observations of Wood6 (8%), Straus7 (10.4%), and von Schroeder and Botte8 (10.3%). Cauldwell et al9 reported very low frequency (0.8%), whereas Wagenseil10 reported a higher frequency of 13%. In 2011, Dass et al11 observed an incidence of 5%.
Very few studies are available on the length and width of the muscle bellies and tendons. The average muscle length and width of EMP in the present study was 4.5 cm and 3.7 mm, respectively, and similar to the values reported by Li and Ren12 (4.9 cm and 4.1 mm, respectively). The mean tendon length obtained in our study was 9.7 cm which was
slightly higher than that observed by Tiwari et al.13 in their case report (8.4 cm).
The EIMC was seen in 3.75% of the specimens, and was well within the frequency range of 2 to 6.5% given in the literature.1 Similar frequencies were reported by von Schroeder and Botte8 (3.4%) and Yalçin et al14 (4.8%). The incidence observed by us is slightly higher than that of Cauldwell et al9 (1.14%). Dass et al11 in 2011 did not observe any case of EIMC in their study on 100 upper limbs.
The mean muscle length and width of EIMC in the present study were 3.7 cm and 2.3 mm, respectively. The muscle bellies were shorter and thicker than those observed by Yalçin et al.14 The corresponding values obtained by them are 4.5 cm and 0.8 mm.
The muscle and tendons of both the EMP and EIMC lie deep to the EIP and EDC, and usually go unnoticed during
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routine dissection.15 Clinically, they are asymptomatic due to thin tendons. Their importance lies in the fact that they can be utilized for tendon graft or tendon transfer.16,17
The development of the anomalous muscles in the limbs is attributed to the aberrant splitting of the myo-genic mass that has migrated from the myotome, with contributory influences from the limb mesenchyme.5 Anomalies of the deep extensors are more common than the superficial group, which can be explained on the basis of developmental patterns and phylogenetic relationships.7 The forearm extensors develop from a muscle mass which splits into a radial part, a superficial part, and a deep part. The radial part forms the brachioradialis and extensor carpi radialis longus and brevis. The superficial part gives rise to the extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris. The deep part gives rise to the deep muscles and has been subject to numerous evolutionary changes, conferring instability, and maximum number of variations.6,7,9,18
Phylogenetic analysis shows that the EMP, EIMC, and EIP share a common origin. The EMP and EIMC are well developed in old world monkeys, whereas the EIP was observed only in apes and humans.1 Lower primates had a separate tendon for each digit in the deep muscle group.2 As evolution progressed to humans, a significant enhancement of the deep muscles supplying the thumb and the index fingers was noticed, with simultaneous reduction in the muscles supplying the other digits.9 It may be noted that the EIP enables independent extension of the index finger that is essential for the pinch grip, unique to the human hand. These factors suggest that the two anomalous muscles are embryological remnants of the deep group rather than variations of the EIP.1,9
EDBMThe EDBM is relatively rare, with frequencies ranging from 1 to 9%.1 It was seen in 2.5% in the present study. The frequencies observed by different authors include 1.1% (Gama19), 1.14% (Cauldwell et al9), 2% (Stith and Browne20), 2.3% (Rodríguez-Niedenführ et al21), 4.2% (Ranade et al4), 3% (Dass et al11), and 2.6% (Vinaychandra et al3). The muscle was described under the term musculus extensor brevis digiti indicis vel medii till the mid-nineteenth century. Later, the term extensor digitorum brevis manus was coined by Macalister22 in 1866.
In the present study, the muscle belly had an average length and width of 4.5 cm and 1.3 mm, respectively, and was comparable to that obtained by Mao and Li23 and Stith and Browne.20 The corresponding values obtained by them were 3.95 cm and 1.05 mm and 5 cm and 1.2 mm, respectively.
The tendons of origin observed by Stith and Browne20 were short in both the specimens observed by them, whereas we observed very short tendon in one specimen, and 2.5 cm long in the other one. The tendons of insertion observed by them had a mean length of 3.1 cm and mean width of 2.5 mm, whereas we observed too short a tendon in one specimen and 3.8 cm long and 3 mm wide tendon in the other.
The EDBM usually takes origin from the dorsal radiocar-pal ligament and capsule of the wrist joint, though origin from distal row of carpal bones, bases of metacarpals, and the distal end of radius have also been observed.1,6,9,19,20,22,24,25 Literature review reveals bilateral occur-rence in 61% of cases,20 with no difference in laterality and sex preponderance.15,19
The EDBM is classified based on the presence or absence of EIP in its association and its mode of insertion.15 Accordingly, the EDBM is classified into:
• Type I–EIP absent and EDBM is inserted into the dorsal digital expansion of the index finger.
• Type II–EIP present and both the EIP and EDBM are inserted into the index finger.
• Type III–EIP is present and inserted into the index finger and the EDBM into the middle finger.
Type II is further subdivided into:
• Type IIa (EIP rudimentary, inserts into EDBM). • Type IIb (distal ends of tendons of EIP and EDBM unite and
get inserted into the index finger). • Type IIc (EDBM tendon thin and joins the tendon of the
EIP).
In the present study, the EDBM in both cases were associated with normal EIP, indicating type III variant. Though the above classification bears no functional significance,1 it may be noted that in the absence of EIP, the EDBM may be the sole independent extensor of the index finger.4
The tendon of origin of the EDBM passes through the fourth compartment, the muscle belly being distal to the retinaculum.26 It is often misdiagnosed as a ganglion, tendon sheath cyst, tenosynovitis, or other soft tissue conditions, resulting in unnecessary surgery.21,23,27 It presents as a soft swelling that moves longitudinally with extension of the digit. It becomes more prominent, often painful, when the finger is actively extended against resistance.25,28 Transillumi-nation is negative29 and direct electromyogram can be used to diagnose the muscle precisely along with the subtypes.15,28
Hypertrophied EDBM is believed to cause pain in the dominant hand of the manual laborers due to repeated constriction of the hypertrophied muscle against the distal edge of the retinaculum and the resultant synovitis.15,19,23,27 Pain can be reproduced by asking the patient to push against the table with wrist in extension.15 Surgical excision is required in severe cases. Ross and Troy noted that surgical excision was required in 75% of the cases.25 Surgical excision should be considered with care, since EDBM can be used in tendon transfer.30
The EDBM muscle is an atavistic muscle, representing the remnant of extensor brevis manus of the amphibians. Subsequent evolution noticed proximal migration of the muscle into the antebrachium followed by disappearance in humans.1,7,18, Some consider this muscle to represent delamination of the extensor muscles of the forearm,24 while others consider it as a variant of the EIP.15
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Anomalous EIPThere are very few studies on the aEIP, most of them being case reports of anatomical descriptions and clinical implications. aEIP was noted in 6.25% in the present study in contrast to 3.75% observed in 1943 by Cauldwell et al.9
The MT junction of the EIP with reference to the retinaculum in both neutral position and flexed position of the wrists was studied and is represented in ►Table 4. The MT junction was observed to move distally on flexion at the wrist. In the present study, the MT junction in flexed wrist position, was found to be within the compartment in 91.25% and beyond the retinaculum in 6.25%; in contrast to 75 and 3.75%, respectively, observed by Cauldwell et al.9
The long tendons to the index finger are generally loose within the fourth compartment. But when aEIP is present, on flexion, the compartment gets tightly packed as the muscle fibers move into it. Repeated constrictions of these fibers result in tenosynovitis and dorsal wrist pain, described as the EIP syndrome.28,31,32 Moreover, this muscle moves in the direction opposite to the other tendons as in pointing with the index finger. This also predisposes the development of tenosynovitis.1 aEIP has also been implicated in subluxation of the EDC tendons to the ring and little fingers.33
aEIP can be clinically diagnosed by EIP test. A positive test is indicated by pain on extending the index finger against resistance, with the wrist in flexion.34 Decompression with partial excision of the muscle is considered when conservative management fails to produce relief from symptoms.1 Partial excision is preferred over complete excision in view of two factors: (1) the EIP is frequently used for tendon transfer and (2) to avoid risk of losing independent extension of the index finger.16 Moreover, the EIP is reported to be the best choice for surgical reconstruction of the abductor pollicis longus and opponensplasty.17
Clinical ImplicationsAwareness about these anomalous muscles and their clinical presentation is necessary in making accurate diagnosis in cases with swelling and pain on the dorsum of the hand.23 Knowledge of these anomalous muscles and their tendons is essential while performing surgeries on the hand and also while contemplating tendon transfer and tendon repair surgeries in patients with rheumatoid arthritis.35
Though clinical examination techniques reveal anomalous muscles like aEIP and EDBM, diagnosis is often confusing especially in cases with associated ganglion.1 Ultrasound sonography is often helpful in preoperative diagnosis.16 Electromyogram can be used for confirmation in cases with clinically positive tests.15,19 Magnetic resonance imaging technique remains the ultimate authority in the diagnosis in most of the cases.36
Limitation of the StudyA bigger sample size would have been better to comment on the incidence. Moreover, since the study was conducted
Fig. 7 Normal extensor indicis proprius (EIP); black arrow–musculotendinous (MT) junction; dotted line–proximal edge of the retinaculum.
Fig. 8 Anomalous extensor indicis proprius (EIP); black arrow–musculotendinous (MT) junction; dotted line–distal edge of the retinaculum.
in free upper limbs, neither the sex preponderance nor the laterality could be commented upon.
ConclusionThe EMP, EIMC, EDBM, and aEIP occur with an incidence of 8.75, 3.75, 2.5, and 6.25%, respectively. The EMP and EIMC muscle bellies are small and their tendons are long and thin. They usually do not cause pain or compression symptoms, but can be harvested for tendon graft or transfer. The EDBM muscle belly lies distal to the retinaculum, with little chances of producing pain, unless it is hypertrophied. The common anomalous muscle implicated in dorsal wrist pain is aEIP, by virtue of bulky muscle passing through the fourth compartment. Knowledge of these anomalies helps to arrive at accurate diagnosis in approach to patient with dorsal wrist pain, as well as planning for tendon transfer surgeries.
Conflict of InterestNone.
FundingNone.
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