Variations of the flexor digitorum superficialis of the small finger

Variations of the flexor digitorum swperficialis of the small finger Normal functional variations of the flexor digitorum superficialis were clinically determined by use of the standard and modified superficialis flexion tests in 50 normal subjects. Forty hands of 20 cadavers were dissected to correlate the anatomic variations with the clinical findings. A flexor digitorum superficialis-independent pattern was found 58% of the time. A flexor digitorum superficialis-common pattern was found 21% of the time. A flexor digitorum superficialis-absent pattern was found 21% of the time. The right and left hands were asymmetric 26% of the time. All cadaver hands had a flexor digitorum superficialis tendon present in the palm and finger. The variability in flexor digitorum superficialis function may be explained by interconnections between the flexor digitorum superficialis of the small finger and either the flexor digitorum superficialis of the ring finger or the flexor digitorum profundus of the small finger. (J HAND SURC 1989;14A:262-7.)

Gregory J. Austin, MD, Bruce M. Leslie, MD, and Leonard K. Ruby, MD, Boston, Mass.

1 he normal functional variations of the flexor digitorum superficialis (FDS) of the small finger present a challenge to the physician faced with its malfunction or injury. Normal functional variations of the FDS can be detected using two tests of flexion, the standard and modified FDS tests. In a clinical survey, Baker et al. ,’ found that 33.8% of the population were unable to achieve a normal range of flexion at the proximal interphalangeal joint when the small finger FDS was tested in the standard fashion. When the ring and small fingers were submitted to the superficialis test together (the modified supeficialis test), there was still 15.7% of the population who exhibited deficiency of the FDS of the small finger. It is now generally appreciated that there are a number of formal variations in the FDS of the small finger, and that special care must be taken to make a correct diagnosis of the cause of From the Department of Orthopaedic Surgery, New England Medical Center, Boston, Mass.

Tufts University,

This paper was presented at the Seventeenth Annual Meeting of the American Association of Hand Surgery, 1987, San Juan, Puerto Rico. Received for publication 18, 1988.

May 4, 1988; accepted in revised form July

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Reprint requests: Bruce M. Leslie, MD, Department of Orthopaedic Surgery, Tufts University, 750 Washington St., Boston, MA 02111.

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any malfunction. It has been assumed that the varations of the FDS of the small finger are symmetric. This study was designed to test that assumption. Materials

and methods

The study had two parts. The first was a clinical survey of variations in FDS function. The second was an anatomical survey based on multiple dissections. In the clinical portion, 50 randomly selected subjects, with no previous history of hand injury were bilaterally tested (a total of 100 hands). Both the standard and the modified superficialis tests were done to classify function of the small finger FDS. The right and left hands were compared. Results were recorded as FDSindependent, FDS-common, or FDS-absent. In the standard FDS test, all fingers except for the small finger were held out in extension. The wrist was kept in full supination and neutral flexion. The subject was then asked to flex the small finger. If PIP flexion occurred with no distal interphalangeal (DIP) flexion, this was interpreted as showing independent FDS function and was recorded as FDS-independent (Fig. 1, A). If PIP flexion did not occur or if PIP flexion occurred only in conjunction with DIP flexion, this was interpreted as showing no independent FDS function (Fig. 1, B). The hands demonstrating no independent FDS function were subjected to the modified FDS test. In the modified FDS test, all fingers except for the ring and small fingers were held out in extension. The wrist was kept in full supination and neutral flexion. The

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Variations of FDS of small jinger

263

Fig. 1. A-B, The standard FDS test. A, Demonstrates independent FDS function in the small finger. Note PIP flexion without DIP flexion. B, Demonstrates the absence of both PIP and DIP flexion indicating no independent FDS function to the small finger.

subject was then asked to flex the small finger. If the small finger PIP flexion occurred with no DIP flexion, this was interpreted as showing a common FDS function and recorded as FDS-common (Fig. 2, A). If the small finger PIP flexion did not occur or if PIP flexion occurred only in conjunction with DIP flexion, this was believed to be evidence of no FDS function and was recorded as FDS-absent (Fig. 2, B). There was a subgroup with definite FDS function as evidenced by isolated PIP flexion, but the PIP was not able.to be fully flexed and had very little power. These were noted as being nonfunctional FDS to the small finger but were included in the FDS-independent group. In the anatomic portion of the study, 20 cadavers had both hands dissected for a total of 40 hands. The presence or absence of the FDS was recorded as was the symmetry between both the right and the left hands. In the forearm, the origin of the FDS tendons and any interconnections or variations in origin were recorded. Results Table I shows the results of the clinical survey. Fiftyeight hands demonstrated independent FDS function.

Table I. Clinical variations in FDS function the standard and modified FDS tests Functional pattern

FDS-independent FDS-common FDS-absent TOTAL

using

Hands (no.)

58* 21 21 100

*Includes 11 hands in the “nonlimctional” group exhibiting definite but very weak FDS function.

Twenty-one hands demonstrated FDS-common function. Twenty-one hands demonstrated an FDS-absent function. The presence or absence of symmetry in the clinical portion of the study is recorded in Table II. In nearly three fourths (74%) of the patients the right hand was the same as the left hand. Twenty-six percent of the time the clinical findings were asymmetric when comparing the right hand with the left hand (Fig. 3). Forearm dissection demonstrated that the muscle belly of the FDS to the small finger was generally contiguous with the muscle belly to the index finger. In most cases, the attachment was distal. In some cases,

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The Journal of HAND SIJRGERY

Fig. 2. A-B, The modified FDS test. A, Demonstrates full PIP flexion without DIP flexion. This

hand showed no independent FDS function with the standard FDS test but here shows full FDScommon function. B, Demonstrates FDS-absent function of the small finger. Note that the examiner’s hand can hyperextend the DIP joint of the ring finger indicating that the profundus is lax, while in the small finger the profundus is contracting. causing DIP flexion.

however, the muscular attachment was more proximal and occurred in the proximal third of the forearm. In 29 hands, the FDS tendon passed from the muscle belly through the fibro-osseous tunnel, with no additional attachments or interconnections demonstrated. In eight hands, the FDS tendon to the small finger had an additional muscle slip from the FDS of the ring finger. In three hands, there was a definite narrow, tendinous interconnection between the FDS tendon to the small finger and the underlying FDP to the small finger. Table III demonstrates the comparison of anatomic findings seen in both the right and left hands.

Discussion Our survey has shown that 26% of the population are asymmetric with respect to the pattern of FDS function found in the small finger. This is of particular importance in the emergency room setting. At first glance, this seems to call into question the usefulness

of using the uninjured hand as a “normal” comparison when an abnormality of the injured hand small finger FDS is suspected. On closer evaluation, the data show that comparison is accurate when the uninjured hand shows an FDS-independent pattern but less accurate when the uninjured hand shows an FDS-common or FDS-absent pattern. Table IV shows the probability of predicting the pattern of function of the FDS of the small finger in one hand after the pattern of function of the other hand is determined by testing with both the standard and the modified superficialis tests. In the traumatic setting, the uninjured hand can be tested and a functional pattern can be determined. If an FDS-independent pattern is found on the uninjured hand, there is a high probability (86%) that the opposite hand was FDS-independent before the injury. If an FDS-common or FDS-absent pattern is found in the uninjured hand, the predictability is less certain (57%).

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Variations of FDS of small jinger

Table II. Combination

of bilateral

265

FDS function

in patients

Symmetric I-I

c-c A-A Asymmetric I-A I-C C-A TOTAL

Total

Total %

37

74%

13

26%

25 6 6 4 4 J 50

I. FDS-Independent: C, FDS-common; A, FLXhbsent.

Fig. 3. Standard superficialis test demonstrates an example Table III. Comparison

of anatomic

findings

of asymmetry between the right and left hands. The right hand shows independent FDS function and the left shows no independent FDS function.

in

right and left hands

Independent FDS Tendon to small finger FDS Tendon with additional muscle slip from FDS ring finger FDS Tendon with additional tendinous slip to FDP small finger

Right

Lef

12

17

6

2

Table IV. Probability

of various FDS function patterns in the opposite hand Tested FDS function

2

1

There are several normal variations of the FDS function in the small finger. Most frequently an independent pattern is seen (58%). The remainder had either a common function with the ring finger (21%) or no discernible function (21%). An awareness of these frequent variations is essential to correctly interpret results of any hand examination. The close functional link between the ring and small finger FDS explains how an injury to either the ring or small finger can cause malfunction of the adjacent finger. The treating surgeon should consider both digits when-dealing with an injury to either finger. Unlike others who have suggested an absence of the FDS to the small finger, we were able to demonstrate an FDS tendon in 40 consecutive bilateral hand dissections.** 3 This compares favorably with results of Kaplan’s dissection of 41 cadavers.4 Others who use the FDS of the small finger as a free tendon graft relate that they have never encountered an absent tendon (H. K. Watson, personal communication 1987). The anatomic dissections suggest an explanation for the clinical finding of an FDS-independent, FDS-common, and FDS-absent small finger. The FDS-independent

Independent Common Absent

Independent

Common

Absent

86.2% 19.0% 19.0%

6.9% 57.1% 23.8%

6.9% 23.8% 57.1%

small finger was the most frequent. The tendon passed freely from the muscle belly through the fibro-osseous tunnel, with no attachment to the adjacent structures (Fig. 4, A). In 11 of 40 specimens the tendon either had a dual origin or had a concomitant slip to the underlying FDP to the small finger. When a dual origin was noted, the FDS tendon took origin from both the FDS to the small finger and the FDS to the ring finger (Fig. 4, B). The common origin explains the inability to independently flex the middle joint of the small finger without simultaneously flexing the middle joint of the ring finger. The common origin may also explain the shearing sensation that patients with FDS-common fingers have in the distal forearm when trying to unsuccessfully demonstrate FDS-independent function. In three specimens we demonstrated a tendinous slip from the FDS to the FDP of the small finger (Fig. 4, C). These slips were very definite and very narrow and prevented FDS function independent of the FDP function to the small finger. Similar slips have been reported between the FPL and the index FDP.5 It is difficult to correlate the clinical findings with the anatomic dissections unless the dissections are per-

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The Journal of HAND SURGERY

Austin, Leslie, and Ruby

Flexor

Digitorum

Fig. 4. A-C, The presumed correlation between the clinical and the anatomic findings. A, Demonstrates independent FDS function in the small finger. B, Demonstrates the additional muscle slip from the ring finger FDS to the small finger FDS tendon. The additional slip could explain why the small finger FDS would be nonfunctional according to the standard test but demonstrate common function with the modified FDS test. C, Demonstrates interconnection between the FDS to the small finger and the underlying FDP to the small finger. The additional tendinous interconnection could explain why small finger PIP flexion occurred only in conjunction with DIP flexion. The additional tendinous slip would make it very difficult to isolate FDS function to the small finger.

formed on the volunteers who demonstrated the clinical findings. Without accurate correlation we can only speculate as to the anatomic explanation of functional patterns.

On the basis of this study, it appears that if an FDSindependent pattern is present, there is a high probability that the opposite hand will also have an FDSindependent pattern. If, however, the hand demon-

Vol. 14A, No. 2, Part I March 1989

Variations of FDS of small jinger

strates either an FDS-common or FDS-absent pattern, there is an increased chance of asymmetry between the right and Ieft hands. An anatomical explanation is provided. REFERENCES 1. Baker II, Gaul JS, WilliamsV, Graves M. The little finger superficialis. Clinical investigation of its anatomic and functional shortcomings. J HANDSURG 1981;6:374-8.

3. Shrewsbury MM, Kuczynski K. Flexor digitorum super-

ficialis tendon in the fingers of the human hand. Hand 1974;6:121-33. 4. Kaplan EB. Muscle and tendinous variation of the flexor digitorum superficialis of the small finger of the hand. Bull Hosp Joint Dis 1969;30:59-66. 5. Linburg RM, Comstock Brian E. Anomalous tendon slips from the flexor pollicis longus to the flexor digitontm profundus. J HANDS~RG 1979;4:79-83.

2. Long C. Intrinsic-extrinsic muscle control of the fingers. J Bone Joint Surg 1968;50A:973-84.

Surgical anatomy of the medial antebrachial cutaneous nerve In standard anatomy textbooks the course and distribution of the medial antebrachii cutaneous nerve and its branches are glossed over in a vague fashion as if they are of little importance. There are, however, clinical circumstances in which a knowledge of the anatomy of this nerve is invaluable. Sacrifice of the posterior branch in a medial approach to the elbow or cubital tunnel surgery can lead to annoying numbness over the olecranon and a symptomatic neuroma. The medial antebrachial cutaneous nerve is also frequently used in nerve grafting, especially in brachial plexus reconstruction, in which it is beneficial to know the available length and size of donor nerve. Fifty fresh cadaveric arms were dissected to define the course, distribution, size, and branches of the nerve. (J HAND SURC 1989;14A:267-71.)

Victoria R. Masear, MD, Richard D. Meyer, MD, and David R. Pichora, MD, FRCSC,

Birmingham, Ala. and Kingston, Ont., Canada

The intricate

details of the medial ante-

(MACN) and its branches have not been carefully described. This study was undertaken to outline the course and distribution of the MACN and its major branches in an effort to correlate brachial

cutaneous

nerve

From the Division of Orthopaedic

Surgery, University of Alabama, Birmingham, Ala.; and the Department of Surgery, Queen’s University, Kingston, Ont., Canada.

Received for publication July 5, 1988.

March 30, 1988; accepted

in revised form

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Reprint requests: Victoria R. Masear, MD, Division of Orthopaedics, MEB 505, 1813 6th Ave. S., University of Alabama, Birmingham, AL 35233.

this anatomy with surgical approaches and use of the nerve as a donor in nerve grafting. Materials

and methods

Fifty limbs were explored from the axilla to the wrist, tracing out the MACN. The dissections were done on fresh cadaveric specimens at the University of Paris anatomy laboratory. Dissection was carried out under 3.5 times loupe magnification to visualize smaI1 communicating branches from the MACN to other nerves and to follow small cutaneous branches. Particular attention was directed to the origin of the MACN from the brachial plexus and to communications and branches in the arm. Both the anterior and posterior branches were followed as far distally as possible and their pathways in relation to other structures were identified. Specifically, the course of the posterior branch THEJOURNAL OF

HAND

SURGERY

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