CASE REPORTS: The Split Flexor Carpi Ulnaris as a Local Muscle Flap

Abstract

The flexor carpi ulnaris is a useful local muscle flap in the forearm and elbow. It is, however, an important palmar flexor and ulnar deviator of the wrist, and functional loss may arise from the use of this muscle in its entirety. The flexor carpi ulnaris is made up of two distinct neuromuscular compartments. This arrangement allows for splitting of the muscle and the potential use of the larger ulnar compartment as a local muscle flap while maintaining the humeral compartment as an ulnar deviator and palmar flexor of the wrist. We report two cases illustrating the clinical use of the split flexor carpi ulnaris as a local muscle flap.

The flexor carpi ulnaris has been used as a local muscle rotational flap to cover the elbow, arteriovenous shunts, and vascular prosthetic grafts and to treat infected nonunion of the proximal ulna.^6,8,9 However, the flexor carpiulnaris is the dominant palmar flexor and ulnar deviator of the wrist, and several authors have expressed concerns about functional loss arising from the use of this muscle in its entirety.^1,9

A detailed anatomic study of the flexor carpi ulnaris has established the presence of two distinct neuromuscular compartments.² This arrangement allows for splitting the muscle and the potential use of the larger ulnar compartment as a local muscle flap, while maintaining the humeral compartment as an ulnar deviator and palmar flexor of the wrist.

We report using the split flexor carpi ulnaris as a local muscle flap.

Case Report

Patient 1

A 54-year-old, right-hand-dominant, male welder sustained a closed comminuted fracture of his left olecranon when he fell from a height of 3 m onto his left elbow. He gave a history of noninsulin-dependent diabetes mellitus. We performed open reduction and internal fixation with a 3.5-mm locked compression plate (Synthes Inc, West Chester, PA). One month after surgery, a superficial wound infection developed, which was treated with oral antibiotics. The fracture united after an additional 2 months.

One year after surgery, the patient presented with increasing pain and swelling in his left elbow. He was advised to have the implants removed. At surgery, we observed infected granulation tissue over the plate. The screws were loose and pus was extruding from the screw holes. We removed the implants, and the bone was thoroughly débrided. Although there was no overlying soft tissue defect, a local muscle flap, consisting of the split flexor carpi ulnaris, was fashioned to facilitate healing by enhancing vascularity and obliterating dead space.

We extended the old incision to the pisiform and identified the distal tendon of the flexor carpi ulnaris. The larger ulnar compartment, which extends to the distal aspect of the forearm, was separated from the ulna by sharp dissection. We then split the flexor carpi ulnaris tendon along the bicompartmental plane, and turned the muscle belly on itself to cover the elbow. The vascular pedicle at the junction of the proximal third and the distal part of the muscle was preserved. The muscle was denervated by sharp transection of the nerve branch to the ulnar compartment. It reached as far as 3 cm proximal to the olecranon. The humeral compartment was preserved in its anatomic location (Fig 1). The skin was closed primarily over the muscle flap.

Fig 1A: B. (A) The flexor carpi ulnaris has been split along the bicompartmental plane. (B) The muscle belly of the ulnar compartment has been turned on itself to cover the olecranon.

Postoperatively, the patient reported resolution of pain and the wound healed without complication. On review 7 months after the procedure, the grip strength in his left hand was 235 N, as compared with 294 N on the contralateral side. The strength of ulnar deviation at the left wrist was 58 N, as compared with 71 N on the contralateral side. He had full range of motion (ROM) of the left wrist in flexion, extension, radial deviation, and ulnar deviation.

An electromyogram showed normal activity in the remaining compartment of the flexor carpi ulnaris (Fig 2A).

Fig 2A: B. (A) The electromyogram of Patient 1 shows normal voluntary compound muscle action potentials in the humeral compartment, and no substantial voluntary muscle action potentials in the ulnar compartment because of its denervation at surgery. (B) The electromyogram of Patient 2 is similar to that of Patient 1.

Patient 2

A 23-year-old, right-hand-dominant, male technician sustained an open comminuted fracture of his right olecranon in a motor vehicle accident. It initially was treated with débridement and external fixation. One week after the initial surgery, the external fixator was removed, and we performed open reduction and internal fixation with a 3.5mm limited contact-dynamic compression plate (Synthes Inc). Three weeks later, the patient presented with a dehisced wound and visible plate. At surgery, the wound first was débrided. A local muscle flap, consisting of the split flexor carpi ulnaris, was then fashioned to cover the soft tissue defect overlying the exposed plate. Skin coverage was achieved with a transposition flap instead of a skin graft in this case, as a more durable alternative was preferred given the location of the wound over the posterior aspect of the elbow, which is a weightbearing area of the upper limb.

Postoperatively, the wound healed without complication and the fracture united. At review 16 months after the flap coverage, the grip strength in his right hand was 333 N, as compared with 338 N on the contralateral side. The strength of ulnar deviation at the right wrist was 85 N, which was identical to that on the contralateral side. He had full ROM of the right wrist in flexion, extension, radial deviation, and ulnar deviation. An electromyogram showed normal activity in the remaining compartment of the flexor carpi ulnaris (Fig 2B).

DISCUSSION

The flexor carpi ulnaris has two clearly defined neuromus-cular compartments. There is a larger ulnar compartment originating from the ulnar shaft with a mean physiologic cross-sectional area of 1.71 cm² and a smaller humeral compartment originating from the medial epicondyle with a mean physiologic cross-sectional area of 1.65 cm².² The nerve branches to the muscle arise from the ulnar nerve either as a single branch which subsequently splits into two, or as two separate branches. These branches penetrate the muscle belly at its proximal quarter, and run parallel to each other on either side of the central tendon to the musculotendinous junction.^2,10 The blood supply occurs via a dominant pedicle from the posterior ulnar recurrent artery, which enters the muscle at its proximal third and splits into two branches. These branches run distally toward the musculotendinous junction and supply each compartment separately.⁷ A previous anatomic study suggests adequate perfusion of each compartment from its corresponding branch even after splitting the muscle from distal to proximal.²

These features enable the muscle to be split along the line of the central tendon into two separate compartments, each with its own vascular and nerve supply.⁵ In this way, one compartment may be used for resurfacing, while the other can be maintained as a palmar flexor and ulnar deviator of the wrist. (Fig 3) Thus, at least part of its function is preserved. The ulnar compartment is chosen for flap coverage as it is broader and extends farther distally than the smaller humeral compartment.

Fig 3: The anatomic basis of the split flexor carpi ulnaris flap is shown.

We found splitting the flexor carpi ulnaris easily achieved surgically. The larger ulnar compartment can be mobilized to cover the olecranon, although it also can be expected to address defects in the antecubital fossa. The preserved humeral compartment continues to function as a palmar flexor and ulnar deviator, as shown by our patients' positive electromyograms.

The use of the split flexor carpi ulnaris as a local muscle flap is one of several clinical applications envisioned for the muscle in a previous anatomic study.² The other potential applications include split muscle tendon transfers in the ipsilateral limb³ and free vascularized split muscle transfers to the contralateral limb⁴ in circumstances where there are deficiently functioning muscle units at those sites.

The flexor carpi ulnaris is a useful local muscle flap in the forearm and elbow. Splitting it provides good coverage from the ulnar compartment and preserves wrist flexion and ulnar deviation from the intact humeral compartment.

Acknowledgments

We thank Associate Professor Einar Wilder-Smith and Dr. Chan Yee Chuen for performing the electromyograms for our patients, and Linda Lim for assisting in preparation of the manuscript. We also thank Dr. Sandeep Sebastin for preparing the line drawing of the neuromuscular compartments of the flexor carpi ulnaris.