The Study of Morphology of supinator muscle and the arcade of Frohse

Abstract               Background: Supinator muscle is important from surgical point of view. Sometimes, the superior border is thick and fibrous as described by Frohse. This is an anatomic variation and not so common. The Deep Branch of Radial Nerve (DBRN) enters deep to the superior border border of supinator and passes between the two heads of supinator. The DBRN nerve is called posterior interosseous nerve (PIN) at the lower border of supinator muscle. Radial Tunnel Syndrome and PIN Syndrome both have same medical and surgical treatment. Surgical exploration is by three method or approaches. For this few parameters, distance from radial head and lateral epicondyle of humerus to the entry and exit of DBRN respectively are of paramount importance. Material and Methods: 22 upper limbs were dissected to explore the radial tunnel and supinator muscle. Superior border, superficial and deep head of supinator were observed. The superior border of the supinator for the arcade of Frohse. The composition of the superficial and deep head was noted. The entry and exit of the DBRN was observed and was measured from the lateral epicondyle of humerus and radial head. These landmarks are important to explore the DBRN or PIN in surgical approach as a treatment for decompression. Lengthening of the supinator and splitting of arcade of Frohse is done to relieve the Radial Tunnel Syndrome or PIN Syndrome. Result:  Superficial head of supinator: 1 muscular, 13 fibrous and 6 musculofibrous, and 2 fibrousmuscular.  The arcade of Frohse was observed in only one cadaver out of 22. The breadth of the supinator muscle is 4.41 cm. The distance between the lateral epicondyle to the entry of the DBRN is 4.46 cm. The distance between lateral epicondyle to exit of DBRN is 7.72 cm. The distance between radial head and entry of DBRN is 2.98 cm. The distance between radial head and exit of DBRN is 6.5 cm. Conclusion: The arcade of Frohse is the variation at the superior border of the supinator. The superficial head of supinator is of different composition either muscular or fibrous or intermediate. The distance between lateral epicondyle of humerus, radial head and entry and exit of DBRN at superior border and at lower border of supinator respectively are important landmarks during surgical intervention as a treatment of Radial Tunnel Syndrome or PIN Syndrome.

Key Word: supinator muscle, arcade of Frohse.

INTRODUCTION

Supinator is inadequately described in the standard anatomical books. It is described as the two strata of fibers- superficial head which is fibrous or membranous and deep head only.¹ We found out that superficial head is wholly membranous or wholly muscular or intermediate in composition. We observed variation at the superior border of supinator, and composition of superficial head of supinator. The radial nerve gives three branches in the radial tunnel. These three branches are Deep Branch of Radial Nerve (DBRN) and sensory branch of radial nerve and branch to supinator. Only DBRN enters the superior border of supinator and passes in between two strata of muscle and emerges at the lower border of muscle the same nerve is called as posterior interosseous nerve (PIN). Few authorities do not used DBRN term, they use PIN. And thus PIN Syndrome described as the compression of posterior interosseous nerve at the arcade of Frohse. The common sites where the deep branch of radial nerve (DBRN) or posterior interosseous nerve is compressed in forearm are radial tunnel, arcade of Frohse and in between two heads of the supinator muscle. Radial tunnel (5cm) extends from the anterior edge of humeroradial joint and to proximal edge of supinator muscle. ² The arcade of Frohse is defined as the proximal edge of the supinator muscle which is fibrous in nature. This was first described by Frohse in 1908.³ Decompression of the DBRN or PIN (in PIN Syndrome) is done by two surgical approaches - anterior and posterior. Two bony landmarks lateral epicondyle and radial head are important during the surgical exploration and decompression procedure to locate and identify the entry and exit points DBRN at superior and inferior border of supinator.⁴

MATERIAL AND METHODS

We included 22 cadaveric upper limbs in our study. All the upper limbs were dissected keeping it   in the supine position. The midline incision was taken from 2 inches above the line joining the two epicondyle of humerus and 4 inches below it. Two horizontal incisions oneat upper end and other at lower end of midline incisions were carried out, skin was reflected to exposed the brachialis, brachioradialis, Extensor Carpi Radialis Brevis and Longus. We preferred to cut these three muscles transversely at the junction of their fleshy bellies and tendons. This leads to the radial tunnel where the radial nerve, its superficial and deep branch were observed. This also exposed the supinator muscle - upper border, superficial head and entry of DBRN and exit of posterior interosseous nerve. We measured the distance between lateral epicondyle of humerus to the two points - entry of DBRN at the upper border of the supinator muscle and the PIN (posterior interosseous nerve) at the lower border of the supinator. We have also taken the measurements from radial head to the same points mentioned above. We noted the composition of superficial head of supinator and the nature of the superior or upper border of the supinator muscle. We have categorized the superficial head into four types: muscular, fibrous, musculofibrous and fibromuscular. Arcade of Frohse was noted. The DBRN enters deep to the superior border supinator muscle and passes obliquely in between two heads. It comes out at the lower border and the nerve now called as PIN. The distance between entry and exit of the nerve is also the length of the nerve passing between the two heads of supinator. It also gives us the idea of the breadth of supinator.

RESULTS

Mean breadth of the supinator muscle is 4.41 cm with the standard deviation 0.87. Mean distance between the lateral epicondyle to the entry of the DBRN is 4.46 cm with standard deviation of 0.50. Mean distance between lateral epicondyle to exit of DBRN is 7.72 cm with standard deviation of 1.0. Mean distance between radial head and entry of DBRN is 2.98 cm with standard deviation 0.51. Mean distance between radial head and exit of DBRN is 6.5 cm with standard deviation 1.06. Superficial head of supinator: 1 muscular, 13 fibrous and 6 musculofibrous, and 2 fibrous. Arcade Frohse was observed in only one cadaver out of 22.

                        Figure 1                                                                Figure 2                                                                Figure 3

DISCUSSION

Surgical treatment of the radial tunnel syndrome is by the lengthening the supinator that reduces the the pressure inside the tunnel. It has 3 approaches. First, dorsal approach, arcade of Frohse and superficial portion of supinator is identified and divided, through the plane of separation between the brqchioradialis, ECRL/B.Second Transmuscular Brachioradialis-Splitting Approach. By splitting these muscle arcade of Frohse is divided. Third is anterior approach, when the compression is at two places, first in the radial tunnel and second at the aracade of Frohse. The radial nerve is exposed between brachialis and brachioradialis and then reaching to the point where radial nerve branch into superficial and PIN/DBRN. Arcade of Frohse and supinator is released.² The bony landmarks lateral epicondyle and radial head are very useful to reach the entry and exit of the DBRN / PIN at the superior and inferior border of supinator muscle. The presence of the thick, fibrous arcade of Frohse may compressed the posterior interosseous nerve or by making the space  narrow because of  oedema of the adjacent structures, neoplasm or inflammatory swelling which may compress against the unyielding tendinous arch.² Arcade of Frohse is explore and split to treat the paralysis of extensor muscles supplied by the PIN when there is no evidence of recovery either by clinical or electromyographic after  six weeks. Arcade of Frohse is an anatomical variant and present in 30 - 50 % of population. ⁵ In our study of 22 cadavers, only one arcade of Frohse was observed at the proximal border of the superficial head of supinator rest was muscular. We observed only one case out of 22 upper limbs. PIN is caused not only by arcade of Frohse but also by other factors: leash of Henry from radial artery, the edge of ECRB muscle.⁶. PIN is caused by repetitive movements as well as trauma. ⁷. Other causes of the PIN Syndrome are overuse, external compression, radial head fracture, soft tissue tumors.⁵ So it not only the arcade of Frohse which may be responsible of pathology but the fibrous superficial head of supinator to which PIN is fixed it  becomes elongated and compressed.⁸We have found that the  course of the PIN in between the two heads of supinator  is oblique. We are not sure about clinical implications of the oblique course the PIN.

CONCLUSION

Arcade of Frohse is the variant. The superficial head of supinator may be only muscular, fibrous only fibromuscular or musculofibrous. The distance between the radial head to the entry and exit of PIN/DBRN is 2.98 cm and 6.25 cm respectively. The distance between lateral epicondyle of humerus and entry and exit of DBRN/ PIN is 4.46 and 7.72 cm respectively. This distances are helpful in various surgical approaches to divide the arcade of Frohse or lengthening of supinator.

REFERENCES