Intra-operatively, a standard dorsal longitudinal mid-line approach to the wrist was performed Listers tubercle was identified and a longitudinal incision was made 4 cm distally and 4 cm proximally to the radiocarpal joint over the 3rd/4th extensor compartment. She underwent a scapho-lunate ligament reconstruction using the tri-ligament tenodesis (3LT) technique which is a modification of the Brunelli procedure. A wrist arthroscopy confirmed the scapho-lunate ligament injury. The Kirk-Watson test was positive and radiographs of the wrist revealed no evidence of scapho-lunate advanced collapse (SLAC) wrist. This resulted in scapho-lunate diastasis and symptomatic instability. We present a case of a previously undescribed EDBM muscle function of pure finger abduction with no extension of the middle finger and a surgical technique of preserving its origin.Ī 25-year-old right hand dominant radiographer had a fall several years ago and sustained a scapho-lunate ligament injury which was not treated. Variations in intrinsic hand muscle anatomy can be encountered during common surgical approaches such as the dorsal approach to the carpus. There was no significant association found between EDBM and gender, ancestry or laterality. In a recent systematic review and meta-analysis, the EDBM muscle has an overall true cadaveric prevalence of 2.5% and a bilateral occurrence in 26.3%. When the EIP was absent, the EDBM was found to be present in 50% in cadaveric specimens which suggests that it compensates for EIP and supports the notion that EDBM is a finger extensor. It is innervated by the posterior interosseous nerve (PIN) and takes its blood supply from a posterior branch of the anterior interosseous artery. Type III: The EIP inserts onto the index finger but the EDBM inserts onto the middle (long) finger ± an accessory EIP to the middle finger. Type IIc: EIP inserts normally onto the index finger along with a thin EDBM tendon also inserted more ulnarly than the EIP tendon. Type IIb: The distal end of the EDBM muscle belly joins the EIP tendon. Type IIa: A vestigial EIP is confluent with an EDBM muscle belly and inserted onto the index finger. It has three subtypes which describe how they interact. Type II is where both EDBM and EIP insert onto the index finger. Type I: Absent EIP with EDBM attached to index finger dorsal aponeurosis. Ogura classified the EDBM according to its distal insertion and relationship with extensor indicis proprius (EIP) muscle. It can have up to four tendons with the most commonly occurring pattern being a single tendon to the index or middle finger. The EDBM typically originates from the dorsal wrist capsule, the dorsal distal radius, dorsal metacarpal surface or proximal radiocarpal ligament overlying the fourth extensor compartment. The name EDBM was ascribed to the muscle by Macalister in 1875 which has been widely used by authors in the literature. It was first described by Bernard Siegfried Albinus in 1758 who named it “Extensor brevis digiti indicis vel medii”. The extensor digitorum brevis manus (EDBM) muscle is a rare anatomical variant of the extensor compartment of the wrist and hand. We propose that the middle finger variant of the EDBM should be re-named the extensor digitorum brevis medius to reflect our findings. We present a case of a previously undescribed EDBM muscle function of pure finger abduction with no extension and a surgical technique of preserving its origin. Where possible these variant muscles should be carefully dissected off underlying structures, preserved and repaired at the conclusion of a procedure to ensure no perceived functional deficit to the patient. The EDBM is a rare anatomical variant of the extensor compartment of the wrist and may be encountered during surgical approaches. Electrical stimulation of the muscle belly demonstrated abduction of the middle finger. During a standard dorsal longitudinal mid-line approach to the carpus, an extensor digitorum brevis manus (EDBM) muscle was found taking its origin from the dorsal wrist capsule overlying the lunate with innervation from the posterior interosseous nerve (PIN). A 25-year-old female presented with a chronic scapho-lunate ligament injury with development of carpal instability requiring reconstruction.
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