The flap, designed astride the intermuscular septum, is raised under tourniquet. Its breadth is limited to 6cm because of the need to close the secondary defect directly; its average length is 10 cm. As a first step, the line joining the lateral epicondyle and the deltoid insertion, representing the line of the intermuscular septum, is marked out on the skin.
Behind this line the flap contains no structures of note, and this makes it convenient to raise this seg-ment first, and establish the plane between the investing layer of fascia and triceps at the outset, dissecting forward until the lateral intermuscular septum is reached. The muscle fibres can then be separated from the septum over its full depth and over the length of the flap, exposing the vessels and the nerves in the septum.
The skin incision is extended proximally to just behind the posterior border of deltoid, and this allows the vessels and nerves to be dissected out proximally, separated from the radial nerve, and traced back into the spiral groove. In carry-ing out this dissection triceps and deltoid are separated, and any fibres of triceps attached to the septum which are obscuring the groove are divided.
With the vascular pedicle defined, the flap anterior to the septum can be freed from brachialis and brachioradialis, and its elevation completed. Throughout the dissection numerous small branches of the perfusing vessels supply-ing the surrounding muscles have to be divided.
When the transfer is as an osteofasciocutan-
eous flap, a strip of muscle is left attached to each side of the septum.
These strips, carried down to the bone over the length to be raised, provide protection for the vessels in the septum which are perfusing the two elements of the composite transfer, skin and bone. As a prelimin-ary to this part of the dissection the radial nerve should be retracted out of the way. A 1 cm broad, and up to 10 cm long, strip of the humeral shaft can be raised without compromising the strength of the bone. Clinical usage
The skin element is thin and the underlying layer of fat is generally thin, the combination making for a flexible flap. The scar which represents the sec-ondary defect, though in a site which may regularly be exposed, is not unduly obtrusive. The small-ness of the diameters of its perfusing vessels is its major drawback, the trend generally being towards flaps with larger perfusing vessel diameters.