These flaps are perfused by branches of the circumflex scapular artery and its associ-ated veins. The artery is formed by division of its parent vessel, the subscapular, itself a branch of the axillary artery.
The subscapular artery passes down on the posterior wall of the axilla for approximately 4 cm, where it divides into the thoracodorsal artery which continues down to reach the latissimus dorsi muscle, and the circumflex scapular artery, the latter the larger branch of the two. Almost immediately the artery passes into the triangular intermuscular space.
This space is bounded above by subscapularis and teres minor, below by teres major, laterally by the long head of triceps, and the artery, while curving round the lateral border of the scapula inside the space, gives off musculoskeletal branches. Emerging from between teres major and minor into the subcutaneous tissues overlying the scapula, it divides into a horizontal branch which runs towards the midline approximately 2 cm below the spine of the scapula, and a des-cending branch which runs obliquely down-wards, parallel to the lateral border of the scapula.
These terminal cutaneous branches have each been used as the basis of a fasciocutaneous free flap, one the horizontal flap, the other the parascapular flap. The main musculoskeletal branch of the circumflex scapular artery is given off in the trian-gular space, and passes deep to infraspinatus into the infraspinous fossa.
Before doing so, it gives off a branch which runs down parallel to the lateral border of the scapula as far as its inferior angle, where it anastomoses with the deep branch of the transverse cervical artery. In its course it gives off a series of small branches which provide a periosteal supply to the scapula along its lateral border, through its muscle attachments.
These provide the perfusion source which allows a strip of the lateral border of the scapula to be trans-ferred, usually as a vascularised composite along with one of the fasciocutaneous flaps. The horizontal flap is designed approximately midway between the spine of the scapula and its inferior angle; the parascapular flap runs parallel to the lateral border of the scapula, both as ellipses.
The two flaps share a common centre, the site where the cutaneous branch of the parent artery emerges from the intermuscular space, between the teres muscles. Identification of the space provides the point from which the central line of the ellipse is drawn on the skin, parallel to the spine of the scapula in the case of the horizon-tal flap, parallel to its lateral border in the case of the parascapular flap.
The width of each flap is limited in practice by the need to be able to close the secondary defect directly, 24cm x 12cm being considered an absolute maximum.
Raising the flap
Both flaps are raised from their distant ends towards the point of entry of the vessels, the plane of elevation baring the underlying muscles to ensure that the flaps contain their axial ves-sels. As the vascular pedicle is approached the vessels become visible on the deep surface of the flap. It is a short pedicle, but it can be lengthened if the vessels are traced back to their subscapular origin in the axilla.
Extension of the dissection in this way increases the length of the pedicle and the calibre of the vessels, both making for ease in transfer as a free flap. The degree of technical dif-ficulty involved in the dissection is largely dependent on the amount of fat which sur-rounds the vessels, and in the obese patient this can be considerable.
When the transfer is as a fasciocutaneous flap, the musculoskeletal branches of the circumflex scapular vessels are divided to allow the vessels to be mobilised, but when the transfer includes bone these vessels have to be carefully pre-served. The segment of bone transferred, approximately 1.5 cm in width, is cut from the lateral border of the scapula from just below the origin of the long head of triceps, a length of up to 14cm.
This part of the scapula is perfused through its muscle attachments, and these must be preserved intact when the bone is being cut.
The skin which both flaps transfer has the thick-ness and lack of flexibility typical of dorsal skin generally, and this may be a factor of importance when the method is being considered for use. The thickness of the subcutaneous fat is variable but can be substantial.
The shortness of the pedi-cle and the small calibre of the vessels involved, unless extended to the origin of the circumflex scapular vessels, make for a degree of technical difficulty in the transfer, and the extension adds considerably to the difficulty of dissection. A fur-ther adverse factor is that both flaps involve turning the patient, unless the surgeon is willing to carry out the entire procedure, creation of the defect and elevation of the flap, in the lateral position, a position which for most surgeons lacks the virtue of familiarity.
In its osteofasciocu-taneous version a serious question mark concer-ning its use relates to the considerable disruption of the musculature of the shoulder girdle which it leaves in the scapular area. The potential it cre-ates for frozen shoulder must be a major deter-rent to its use, particularly in the older patient.