The 7 Structures Distal to the Elbow That Are Critical to Successful Anterior Transposition of the Ulnar Nerve

Abstract

Background: Ulnar nerve transposition (UNT) surgery is performed for the treatment of cubital tunnel syndrome. Improperly performed UNT can create iatrogenic pain and neuropathy. The aim of this study is to identify anatomical structures distal to the medial epicondyle that should be recognized by all surgeons performing UNT to prevent postoperative neuropathy. Methods: Ten cadaveric specimens were dissected with attention to the ulnar nerve. Intramuscular UNT surgery was simulated in each. Distal to the medial epicondyle, any anatomical structure prohibiting transposition of the ulnar nerve to a straight-line course across the flexor-pronator mass was noted and its distance from the medial epicondyle was measured. Results: Seven structures were found distal to the medial epicondyle whose recognition is critical to ensuring a successful anterior transposition of the ulnar nerve: (1) Branches of the medial antebrachial cutaneous (MABC) nerve; (2) Osborne’s fascia; (3) branches from the ulnar nerve to the flexor carpi ulnaris (FCU); (4) crossing vascular branches from the ulnar artery to the FCU; (5) the distal medial intermuscular septum between the FCU and flexor digitorum superficialis (FDS); (6) the combined muscular origins of the flexor-pronator muscles; and (7) the investing fascia of the FDS. Measurements are given for each structure. Conclusions: Poor outcomes and unnecessary revision surgeries for cubital tunnel syndrome can be avoided with intraoperative attention to 7 structures distal to the medial epicondyle. Surgeons should expect to dissect up to 12 cm distal to the medial epicondyle to adequately address these and prevent kinking of the nerve in transposition.

Introduction

Debate continues regarding the comparative efficacy of various surgical techniques for treatment of cubital tunnel syndrome. Literature can be found to support both “”in situ” decompression and subcutaneous transposition as acceptable alternatives to the traditional submuscular transposition,^2,3,12 and transposition of the nerve is not necessary for effective treatment in every case. However, despite a variety of surgical options, ulnar nerve transposition (UNT) remains a commonly performed procedure for the treatment of ulnar neuropathy, particularly in the case of recurrent or subluxating cubital tunnel syndrome. In all cases, use of a technique that reliably produces clinical improvement is critical. Improperly performed UNT can cause significant patient morbidity with pain and neuropathy.⁴

In our experience, success of an UNT is not determined by properties of the tissue through which the nerve is transposed (eg, muscle versus subcutaneous), but rather by the ability to create a completely untethered path for the ulnar nerve, assuring no points of kinking or tension throughout its course.

While the medial intermuscular septum of the upper arm is routinely excised and known to be a “kink point” for the ulnar nerve in transposition,^16,18 comparatively little attention has been paid to the structures distal to the medial epicondyle that may result in kinking or tethering of the nerve. The aim of this study is to identify and describe the anatomic structures distal to the medial epicondyle that should be recognized and addressed by all surgeons performing UNT in order to prevent postoperative neuropathy and minimize the need for revision surgery.

Materials and Methods

Ten fresh-frozen cadaver specimens (5 left, 5 right) were dissected with attention to the ulnar nerve. Submuscular UNT was carried out according to the technique of the senior author (S.E.M.),^6,14,17 identifying any structures distal to the medial epicondyle that resulted in tethering of the nerve or caused an impediment to transposing the nerve in a straight line from its course above the medial epicondyle to below.

For purposes of dissection, a 25 cm curvilinear skin incision was made at the medial elbow centered between the medial epicondyle and the olecranon process. The ulnar nerve was identified proximally between the medial intermuscular septum and the triceps muscle. The medial intermuscular septum was dissected free and excised. The ulnar nerve was mobilized and followed distally toward the cubital tunnel. Osborne’s fascia was divided at the leading edge of the flexor carpi ulnaris (FCU). The superficial fascia overlying the FCU muscle was incised longitudinally over the expected distal course of the ulnar nerve.

Z-lengthening flaps were then designed on the surface of the flexor-pronator muscle fascia and elevated, exposing the underlying flexor-pronator muscle mass. These muscles were divided from their origin at the medial epicondyle and elevated and advanced as a muscle slide from proximal to distal an average of 3.2 cm to create an unobstructed “trough” in the line of planned UNT. Accompanying fascial septae between the median-innervated muscles of the flexor-pronator mass were also excised to clear a path for the nerve.

The distal intermuscular septum between the ulnar-innervated FCU muscle and the median-innervated flexor digitorum superficialis (FDS) muscle was dissected free from muscle attachments on both sides and its course and relationship to the ulnar nerve were followed.

With the course of the ulnar nerve within the FCU muscle exposed, tethering anatomical structures that overlay the distal ulnar nerve or impeded transposition were noted, and for each, the distance from the medial epicondyle to the point where the tethering structure crossed the ulnar nerve was noted. These structures were mobilized or excised as needed to provide a straight, lax, and untethered course for the ulnar nerve, which was verified by completing transposition of the ulnar nerve and observing its course.

Results

Seven structures (Table 1) were found distal to the medial epicondyle whose recognition is critical to ensuring a successful anterior transposition of the ulnar nerve.

Table 1.

Seven Structures Distal to the Medial Epicondyle That May Prevent a Straight-Line Anterior Transposition of the Ulnar Nerve.

Structure	Average distance measured distal from medial epicondyle
Medial antebrachial cutaneous nerve branches	2 cm, 3.7 cm, 7.7 cm
Osborne’s fascia	Spans from medial epicondyle to Olecranon process at entry point of ulnar nerve into flexor carpi ulnaris
Ulnar nerve motor branches to FCU muscle	2.4 cm, 3.6 cm, 4.5 cm, 6.3 cm
Crossing vascular branches from the ulnar artery	6 cm, 8.8 cm, 11.9 cm
Distal intermuscular septum between FCU and FDS	5 cm (pierced by ulnar nerve) 11.7 cm (total length of structure)
Flexor-pronator muscle mass origin	3.2 cm muscle slide required to clear path for nerve
Investing fascia of FDS muscle overlying ulnar nerve	Must be divided at most distal extent of dissection

Note. FCU = flexor carpi ulnaris; FDS = flexor digitorum superficialis.

Most superficially, branches of the medial antebrachial cutaneous nerve (MABC) were noted to cross the course of the ulnar nerve (Figure 1). The location of these branches were measured and then these cutaneous nerves were mobilized by longitudinal dissection from the subcutaneous tissues. Branching patterns varied between specimens, but between 1 and 3 branches were noted in each. On average, branches of the MABC crossed the course of the ulnar nerve at 2 cm, 3.7 cm, and 7.7 cm distal to the medial epicondyle.

Figure 1.

Branches of the medial antebrachial cutaneous nerve cross the path of the ulnar nerve in variable patterns distal to the medial epicondyle.

Note. These must be separated from subcutaneous tissue and mobilized to allow transposition of the ulnar nerve.

Between 1 and 4 motor branches were noted to exit the ulnar nerve within the proximal FCU muscle. On average, these were noted to arise at 2.4 cm, 3.6 cm, 4.5 cm, and 6.3 cm distal to the medial epicondyle. Beginning with the most distal of these branches, motor branches required neurolysis as a separate fascicular group from the main ulnar nerve for a distance of up to 6 cm to facilitate anterior transposition of the nerve.

Between 1 and 3 crossing vascular branches from the ulnar artery that pass superficial to the ulnar nerve and into the belly of the FCU muscle were noted to occur in all specimens (Figure 2). On average, these were located at 6 cm, 8.8 cm, and 11.9 cm. In all specimens, division of these vessels was required for unobstructed transposition of the ulnar nerve.

Figure 2.

Vascular branches from the ulnar artery are seen to cross the ulnar nerve and must be divided to allow for untethered transposition.

The septum that forms the common aponeurosis between the FCU and FDS muscles was measured from the medial epicondyle to its distal visible extent, which on average was 11.7 cm (Figures 3a and 3b). The relationship of the ulnar nerve to this septum is variable, but in all cases, the nerve was either deep or medial to this septum, representing a barrier to the superficial and lateral direction of mobilization needed for transposition of the nerve. Most frequently, the nerve was noted to lie medial to the septum proximally after penetrating Osborne’s fascia and entering the FCU muscle. Then, after giving off motor branches to the FCU muscle, the ulnar nerve pierced the septum at an average distance of 5 cm and transitioned to lie lateral to it, along the medial surface of the FDS muscle. This is the septum which we consider the “distal medial intermuscular septum” of the forearm that affects ulnar nerve kinking after transposition in an analogous manner to the proximal medial intermuscular septum (medial brachial intermuscular septum).

Figure 3.

(a) Proximal, left; distal, right. The proximal medial intermuscular septum of the brachium (proximal forceps) is seen to run in continuity with the distal medial intermuscular septum between the flexor carpi ulnaris (FCU) and flexor digitorum superficialis (FDS) muscles, separated only by the bony medial epicondyle. The relationship of the ulnar nerve to these 2 structures is analogous. (b) Proximal, left; distal, right. The proximal medial intermuscular septum of the brachium (proximal forceps) is seen to run in continuity with the distal medial intermuscular septum between the FCU and FDS muscles, separated only by the bony medial epicondyle. The relationship of the ulnar nerve to these 2 structures is analogous.

The combined muscular origins of the flexor-pronator muscles were noted to obstruct the linear course of the ulnar nerve in its transposed position (Figure 4a). To create a “straight trough” for the transposed nerve, a forearm muscle slide was performed, releasing the origins of the palmaris longus, FDS, flexor carpi radialis (FCR), and pronator teres from the medial epicondyle and dissecting them free from underlying attachments for an average of 3.2 cm distally (Figure 4b). The T-shaped fibrous septum separating the deep surface of palmaris longus muscle from the surrounding FDS and FCR muscles was also excised over this 3.2 cm distance.

Figure 4.

(a) The flexor-pronator muscle origin creates an impediment to a straight-line positioning of the ulnar nerve in transposition. (b) Muscle slide of the flexor-pronator origin allows the nerve to lie straight.

Finally, the investing fascia of the FDS muscle was noted to overlie the ulnar nerve superficially after the nerve pierced the FCU septum and traversed the medial surface of the FDS muscle belly. This fascia was apparent as a thin, transparent, continuous sheet overlying the ulnar nerve as it lies between the FCU and FDS muscles. Once all other tethering structures had been removed and the nerve placed into its transposed position, this fascia was noted to act as a hinge point, potentially creating kinking at the most distal point of mobilization of the ulnar nerve (Figure 5). Division of this fascia at the most distal extent of dissection was noted to relieve kinking.

Figure 5.

The thin, transparent investing fascia of the flexor digitorum superficialis muscle is seen to create a kinking force on the ulnar nerve (denoted at the level of the asterisk) if not divided with anterior mobilization of the nerve.

Note. Branches of the medial antebrachial cutaneous nerve also overlie the nerve in this photo.

Discussion

Ulnar nerve transposition is indicated for recurrent cubital tunnel syndrome or symptomatic subluxation of the ulnar nerve. Transposition surgery is conceptually designed to alleviate the compression and tension that lead to neural ischemia, demyelination, and axon loss in cubital tunnel syndrome. However, when performed without an intimate knowledge of the structures that may cause kinking of the nerve in its transposed position, either subcutaneous or submuscular transposition may sustain or worsen ulnar neuropathy, causing loss of function or pain.

An important concept for transposition is that normal structures, which are not sources of compression when the nerve lies in its native anatomic position, may become the source of compression or kinking once the nerve is transposed. This concept was advocated in regard to the proximal medial intermuscular septum of the upper arm by Sunderland,¹⁸ and to the arcade of Struthers by Kane and Spinner.^9,16 The same concept is equally applicable distal to the elbow although comparatively little attention in the literature has been paid to investigating its anatomic basis in this region.

In considering the structures identified distal to the elbow in this study, a comparison can be drawn between the “distal intermuscular septum” separating the FCU and FDS muscles, and the medial intermuscular septum of the brachium, separating the triceps and biceps/brachialis. With the elbow extended, this distal intermuscular septum structurally runs in line with the medial intermuscular septum of the upper arm, interrupted only by the bony prominence of the medial epicondyle (Figure 3). Developmentally, this septum is a continuous structure and can be observed to separate the triceps and FCU from more anterior muscles of the arm in a 16 mm embryo (Figure 6).¹¹ The ulnar nerve typically lies medial to the distal intermuscular septum until piercing it at an average of 5 cm distal to the medial epicondyle. Less often, the nerve sits posterior with respect to the septum. In all cases, the septum poses a barrier to the anterior and lateral movement of the nerve needed to accomplish transposition. If this septum is not adequately excised, it remains an unyielding fibrous band around which the transposed ulnar nerve must angulate sharply, creating an entrapment point (Figure 7).

Figure 6.

Embryologically, the triceps and flexor carpi ulnaris muscles are distinctly formed and separated from the more anterior muscles of the arm and forearm by an apparently continuous septum.

Source. Reproduced with permission from Lewis.¹¹

Figure 7.

Inadequate excision of either the proximal medial intermuscular septum in the upper arm or the distal medial intermuscular septum between the flexor carpi ulnaris and flexor digitorum superficialis muscles can create a sharp kink in the ulnar nerve once it is transposed anterior to the medial epicondyle.

Other studies have noted the potential for compression of the ulnar nerve at its intersection with this distal intermuscular septum, which is variably named.^1,8,15 We prefer the name “distal medial intermuscular septum” because it emphasizes the idea that surgically, treatment of this structure should be analogous to the well-known proximal medial intermuscular septum that lies proximal to the medial epicondyle. The anatomy in this area is frequently described as variable, and descriptions are complicated by the multiple adjacent muscle surfaces in this area. Some series, such as that of Amadio and Beckenbaugh, note compression of the ulnar nerve by fascial bands in a tunnel at the area of the distal intermuscular septum that were surgically released at a point approximately 5 cm distal to the medial epicondyle.¹ We similarly observed the ulnar nerve to pierce this septum at an average of 5 cm distal to the medial epicondyle. Articles by Green and Rayan as well as Mahan et al confirm anatomically that this distal intermuscular septum lies between the FCU and FDS muscles and is traversed by the ulnar nerve.^8,15 Green and Rayan additionally demonstrated that the ulnar nerve is under increased pressure in this area.⁸ Karatas et al and Gonzalez et al describe similar findings in anatomical dissection studies, with variable frequency.^7,10

The precise anatomical description and frequency of each variation are likely unimportant. The importance of the combined findings of these studies is the clinical recognition that a septal structure exists distal to the medial epicondyle that may compress the ulnar nerve and that affects nerve transposition in an analogous fashion to the proximal medial intermuscular septum of the upper arm. Revision UNT is a frequently presenting clinical case in our tertiary referral peripheral nerve surgical practice, and extensile exploration in these cases (beyond the zone of prior scar tissue) has illustrated to us the propensity for an iatrogenic compression point to occur here if this septum is not recognized.¹³ Thus, knowledge of the distal medial intermuscular septum is important in both primary transposition surgery to avoid the need for revision and doubly so in revision surgery to effectively treat potential causes of continued symptoms.

To fully mobilize the ulnar nerve into a linear course anterior to the medial epicondyle requires knowledge of structures other than the distal intermuscular septum. Crossing branches of the MABC are in a more superficial plane than the transposed ulnar nerve and do not kink the nerve. However, they must be recognized and safely mobilized to permit dissection of the nerve distal to the medial epicondyle. Unintentional division of these branches may lead to chronic postoperative pain at the surgical scar and lessen the result of surgery.⁵

Osborne’s fascia, spanning from the medial epicondyle to the olecranon process, must be divided to decompress the ulnar nerve and allow access for distal dissection of the nerve. During intramuscular dissection of the ulnar nerve within the FCU muscle, surgeons must recognize crossing vessels from the ulnar artery and motor nerve branches from the ulnar nerve to the FCU as tethering structures that must be either ligated or neurolysed, respectively, to allow unrestricted mobilization of the nerve into an ideal linear position. Crossing vessels may be present up to 12 cm distal to the medial epicondyle, requiring extensive dissection to allow for ligation prior to transposition. The flexor-pronator origin itself creates an additional barrier that is effectively dealt with by performing a muscle slide of 3 to 3.5 cm. Finally, the thin, transparent investing fascia of the FDS muscle must be recognized and released along the entire course of dissection of the ulnar nerve to prevent kinking of the nerve at the most distal extent of the field of dissection.

Conclusion

Seven anatomical structures have been identified that must be surgically addressed when dissecting distal to the medial epicondyle to perform UNT without risk of creating an iatrogenic injury or kink point. The distal intermuscular septum between the FCU and FDS muscles may be viewed as surgically analogous to the medial intermuscular septum of the upper arm and should be excised before anterior transposition of the ulnar nerve. Dissection of up to 12 cm distal to the medial epicondyle may be necessary to address other tethering structures, including crossing vascular branches and investing fascia of the FDS muscle.