1 Introduction

The first dorsal interosseous (FDI) muscle is one of the four dorsal interossei of the hand, whose main function is abducting digits from the middle finger.¹ The FDI muscle is also responsible for stabilizing the index finger during pinch grip and thus, its functional loss can present with ulnar deviation of the index finger due to unopposed rotary forces from the ulnar collateral ligament and the palmar interossei.¹ The dorsal interossei also act as flexors of the metacarpophalangeal joint (MCPJ) and extensors of the interphalangeal joints (IPJ) due to having two distal insertions – a bony one into the proximal phalanx base and a soft tissue insertion into the extensor aponeurosis.²

Pathology of the FDI muscle is commonly attributed to trauma and ulnar nerve palsy which may be due to injury, compressive masses, or central neurological causes.³ Other causes of functional loss include inflammatory joint disease and iatrogenic injuries.⁴ Surgical reconstruction is typically indicated if there exists functional limitation despite a trial of conservative treatment.

Since Bunnell introduced the extensor indicis proprius (EIP) as a tendon graft in 1942,⁵ we describe an innovative technique to reanimate the ruptured FDI muscle, using a free graft of palmaris longus (PL) tendon.

2 Surgical technique

Through a dorsoradial incision over proximal phalanx extending proximally to the MCPJ, the radial sagittal band is incised and elevated to expose the FDI muscle insertion. 0.5cm stab incisions are made at the proximal wrist crease overlying the distal end of the PL tendon, and in the mid forearm along its proximal end. The tendon is cut proximally and retrieved through the distal incision. The harvested free PL tendon is stitched from the point of its rupture to the extensor expansion with figure of 8 stitches using Prolene 5-0 (Fig. 1).

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Fig. 1 Palmaris Longus tendon graft stitched to the end of the ruptured first dorsal interosseous muscle and the right index finger metacarpophalangeal joint extensor expansion.

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A 56-year-old right hand dominant male sustained a traumatic injury of his right FDI muscle during a fencing competition. His right index finger was hyperextended due to the impact from a clashing opponent. On examination, there was scissoring of his right index finger dorsal to his middle finger, limited flexion with finger pointing and tensing of his FDI muscle during attempted fine pinch. The presence of a PL tendon was confirmed preoperatively.

Magnetic resonance imaging revealed injuries of the right index finger MCPJ ulnar and radial collateral ligaments (both proper and accessory components respectively), volar plate and dorsal capsular ligaments. Chronic inflammatory changes, such as dorsal capsular ligament fraying, diffuse thinning of the second MCPJ articular cartilage and osseous irregularity and subchondral bone cyst of the radial proximal phalanx, were noted (Figs. 2 and 3). Initially, the patient opted for conservative treatment involving heat therapy, strengthening exercises, activity modification and rest. However, the deformities did not improve and necessitated surgery.

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Fig. 2 Diffuse thinning of the second metacarpophalangeal joint (MCPJ) articular cartilage and osseous irregularity and subchondral bone cyst of the index finger proximal phalanx.

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Fig. 3 Dorsal capsular ligament fraying of the second MCPJ.

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He underwent exploration of the right index finger one year post-injury under general anaesthesia and arm tourniquet. Intraoperative findings were consistent with his pre-operative imaging but also showed rupture of the FDI tendon from its muscle belly (Fig. 4). Scarring of MCPJ extensors and collaterals was also noted. Branches of the radial and ulnar nerves were not encountered.

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Fig. 4 Intraoperative finding of ruptured first dorsal interosseous muscle represented by white arrows and extensor tendon represented by white dashed line.

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The FDI muscle was reconstructed using a free 5-cm PL graft according to the steps described above (Fig. 1), with the index finger MCPJ positioned in slight abduction and flexion. Antibiotic prophylaxis was not required. Passive range of motion was 70° in the MCPJ, 100° in the proximal IPJ and 60° in the distal IPJ. The tension was deemed satisfactory as scissoring was corrected at rest.

1 week post-operatively, passive range of motion exercises were initiated with a focus on index finger flexion and abduction, as tolerated by pain. No specific rehabilitation protocol was used as our institution did not have a standard protocol for tendon transfer and this technique is the first of its kind. 1 month postoperatively, a splint for separating the index and middle fingers was customized to be worn at night. During the patient's most recent follow-up eight months after surgery, there was good FDI muscle bulk (Fig. 5). Index finger abduction and full active extension at the MCPJ and IPJ could easily be achieved without pain (Fig. 6). Active flexion was satisfactory as well, measuring 100° in the MCPJ, 90° in the proximal IPJ and 55° in the distal IPJ. Scissoring between his right index and middle fingers was not obvious during this range of motion. The wounds over his index finger MCPJ and PL donor site have healed without complications. Key and tip pinch grip strengths in the affected hand were 4.5kg on a hand dynamometer (Baseline, White Plains, NY, USA). No functional limitation was reported post-reconstruction and the patient has resumed fencing.

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Fig. 5 Post-operative examination of patient's right hand shows good first dorsal interosseous muscle bulk 8 months after reconstruction.

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Fig. 6 Patient's right hand in abduction after reconstruction of ruptured first dorsal interosseous muscle.

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3 Discussion

Due to the complexity of its anatomy, the FDI muscle is responsible for performing more than one function and therefore, acts as a balancing vector force to dynamic movements of the hand.⁶ In the release of an intrinsic plus hand, failure to identify all insertions might lead to incomplete correction of deformity.⁷ Whereas if surgical reconstruction is indicated, its success is dependent on the length, strength, excursion and moment of the selected donor tendon or muscle.⁸

An early reconstructive technique was described by Bunnell in 1942 and uses the EIP as a tendon graft.⁵ Typically, the graft is passed subcutaneously and sutured palmar to the axis of rotation of the MCPJ. However, the resultant vector force is ulnar to the index finger MCPJ (instead of radial) and confers poor strength and moment.⁹ Some authors have modified this technique by using thumb extensors as a pulley,¹⁰ while others have introduced new tendon transfer techniques using the flexor digitorum superficialis (FDS) IV,¹¹ extensor pollicis brevis (EPB)¹² and abductor pollicis longus (APL), also known as the Neviaser technique.¹³ However, in cases where the length of the tendon transferred is inadequate, a PL autograft is usually used.¹⁴ The PL graft acts more as a tenodesis than active tendon transfer for index finger MCPJ abduction and thus, limitations in restoring strength have been reported.⁹ To address this, Hirayama et al. described a tendon transfer method using only the PL with more favourable outcome.¹⁵

Our reported case is the first to use PL as a standalone, free tendon graft in FDI reconstruction and shows favourable outcomes. We demonstrated that a free PL tendon graft can adequately restore pinch strength and provide stability during index finger abduction without complications. This could be because a free tendon graft can be placed in such a way to create a rectilinear force that produces satisfactory moment. Additionally, the PL tendon is reliable for providing adequate length and thus, an optimal excursion can be achieved with this technique. The main advantage of using the PL as a donor tendon is that other functions of the hand are not necessarily sacrificed. Its condition is also independent of the condition of the muscle that it serves to reanimate and therefore, could be suitable for patients who have an underlying chronic pathology. A limitation of this paper is the lack of preoperative clinical photograph to compare the FDI muscle bulk before and after reconstruction.

In this case report, we have shown that a free PL tendon graft can restore FDI muscle function one year after injury, despite chronic inflammatory changes and scarring. In the future, longer follow-up duration and more widespread adoption of this technique for patients with functional loss of the FDI muscle and failure to conservative treatment can further substantiate our findings. Recruitment of patients who present for surgery later could also prove its benefit in restoring function beyond one year after injury.

Financial support and sponsorship

The authors have no relevant financial or non-financial interests to disclose.

All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.

The authors have no financial or proprietary interests in any material discussed in this article.

Authors contribution

1st author: protocol/project development, data collection or management, data analysis, literature search, manuscript writing.

2nd author: conceptualization, manuscript editing, supervision.

Informed consent

The patient has signed an informed consent regarding publishing his data and photographs.

Institutional ethical clearance

Our institution does not require ethical approval for reporting individual cases or case series.