The composite extensor retinaculum cutaneous flap: an anatomical cadaveric study

Background : Complex digital extensor tendon injuries are difficult to manage when adhesion formation and stiffness prevail. Vascularised tissue to reconstruct the skin and extensor defect would be the ideal reconstruction in both the acute and delayed settings. This anatomical study evaluates vascular supply to a suitable composite flap comprising skin, subcutaneous tissue and extensor retinaculum. Methods : An anatomical study of 18 cadaveric upper limbs was conducted to investigate the technical feasibility of a composite flap prior to its clinical application. The anterior (n=9) or posterior (n=9) interosseous artery was exposed and selectively injected with a coloured dye. Specimens were then dissected to raise the proposed composite flap of extensor retinaculum and the overlying integument. Specimens were subsequently assessed by digital subtraction angiography to evaluate the corresponding microvascular supply to the composite flap. Results : The anterior and posterior interosseous arteries supplied the extensor retinaculum through a dense network of vessels with choke anastomoses. The skin overlying the extensor retinaculum was predictably supplied by either artery through the perforator vessels between the fourth and fifth extensor tendon compartments. Conclusion : A composite unit of skin and extensor retinaculum can be harvested on either the anterior or posterior interosseous arteries. It can be employed for simultaneous vascularised tendon and skin reconstruction.


Introduction
Complex extensor tendon injuries of the fingers can be associated with a slow and unpredictable recovery. 1,2 The extensor mechanism consists of a flat, delicate and complex tendon system. It has a broad gliding interface with its overlying subcutaneous areolar tissue and the underlying metacarpal or phalangeal periosteum. Interruption of this gliding plane results in reduced range of movement, joint contracture, pain and functional loss in the affected digit. [2][3][4] While reconstruction of individual anatomical components is feasible, re-establishing these areolar gliding planes for tendon excursion remains a surgical challenge. In acute injuries, either staged reconstruction or the use of a nonvascularised tendon graft can be considered. In delayed operations, extensor tenolysis may be used to resolve adhesions interrupting tendon excursion.
However, due to poor tendon vascularity, a scarred tissue bed or underlying fractures, adhesion formation often prevails. 2,3, 5 We postulate the use of a vascularised composite flap of extensor retinaculum and the overlying skin to reconstruct complex dorsal digital injuries. This composite tissue flap would provide vascularised tendon graft, the overlying skin and a potential intervening gliding plane, thus enhancing tendon excursion. The present study aims to demonstrate the microvascular supply to this composite flap in a cadaveric model.

Materials and methods
Twenty upper limbs, disarticulated at the midarm level, from 10 fresh frozen cadavers were obtained through the University of Tasmania's body bequest program. The study was approved by the Tasmanian human research ethics committee (H0015063). All specimens were free from scars, previous surgery or trauma of the forearm, wrist and hands. Two upper limbs from one fresh frozen cadaver were used in a pilot study to establish the study's protocol. Once established, the anatomical study comprising the remaining 18 upper limbs from the other nine cadavers was conducted.
A proximal volar forearm incision was made at the midline of the cubital fossa, and the brachial artery was identified first. The ulnar artery was then further exposed by reflecting the flexor digitorum superficialis. The common interosseous artery was traced from the ulnar artery at the level of the radial tuberosity superficial to the flexor digitorum profundus. The short common interosseous artery was then followed deeper to its two terminal arteries, the anterior interosseous artery (AIA) and the posterior interosseous artery (PIA), at the interosseous membrane. The AIA (n=9) or PIA (n=9) was selectively cannulated at its origin using an 18-gauge cannula.
The ulnar artery was ligated just proximal to Guyon's canal through a volar wrist stab incision.
The radial artery was ligated just proximal to the anatomical snuff box through a stab incision between the flexor carpi radialis and the abductor pollicis longus tendons. The ulnar and radial arteries were ligated at the wrist to prevent retrograde flow, thus minimising artefact and isolating perfusion to either the AIA or the PIA.
The specimens' vasculature was prepared as described by Ross and colleagues. 6 The specimens were injected serially with warmed normal saline (37 °C) and subsequently with coloured ink. Any vascular leaks were identified and sealed, with either silk sutures or monopolar diathermy.

Results
Of the nine fresh frozen cadavers, four were female and five were male. Their mean age at death was 80 ± 10 years (SD). None had documented peripheral vascular disease, however, seven had cardiovascular disease.
Both AIA and PIA contributed to a dense network of vessels to the extensor retinaculum (Figure 1).
Terminal branches of both vessels also penetrated the extensor retinaculum and provided cutaneous perforators.

Anterior interosseous artery
The One to two septocutaneous perforators were  found, supplying the extensor retinaculum and overlying skin ( Table 1). The mean diameter of these perforators was 0.4 ± 0.1 mm (SD; Table 2).
Additional perforators were also invariably present between the second and third and the third and fourth extensor tendon compartments.

Posterior interosseous artery
The

Discussion
The extensor mechanism of the digits is a broad, flat structure that is reliant on the low-friction gliding planes between the tendon itself and its adjacent areolar tissues.

Conclusions
This study has established the arterial supply to