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The pyramidalis muscle is introduced as a new small muscle free flap, with description of its anatomy, the surgical technique and the clinical results in five different cases in which this flap was used to treat small recalcitrant wounds in the foot/ankle region. The pyramidalis muscle can be an alternative option in selective cases to reduce donor site morbidity as compared with more traditional free flaps.
The pyramidalis muscle, although well known to the anatomists, does not show up in many clinical papers, probably due to its small size and unknown function. On the other hand, these two characteristics, and the fact that it can be harvested through a relatively inconspicuous Pfannenstiehl incision with negligible donor site morbidity make it an appealing donor muscle for microsurgical transfer.
We present five cases of free pyramidalis muscle transfer in the treatment of small chronic wounds on the foot or ankle where it proved valuable. Although the muscle seems quite variable, if necessary its presence can easily be demonstrated by noninvasive techniques.
1. Patients and methods
In the last three years, six free pyramidalis muscle transfers were performed in six different patients, all male ranging in age from 24 to 52 years. In all cases there was a need for a small muscle flap to fill a small chronic defect in the foot and ankle region, including two cases of chronic osteomyelitis of a calcaneal fracture, one midfoot chronic chemical burn ulcer, two cases of residual fistula of a distal tibial fracture and one exposed lateral malleolus. Previous surgery or trauma contraindicated the use of local flaps. In none of the cases was preoperative identification of the M. pyramidalis done as the surgery was planned as if a small medial slip of the M. rectus abdominis would be harvested. Only five transfers are considered here, as in the sixth patient the M. pyramidalis was covered with a free skin flap, which made the assessment of its eventual viability impossible. In a seventh case the plan to use the M. pyramidalis had to be abandoned due to the bilateral absence of the muscle. In this case a small medial piece of rectus abdominis muscle was harvested and used instead. In all but the first case the muscle was approached through a low Pfannenstiehl incision. Mean operative time was 4 h, dissection in itself taking about 1 h. All but one procedure went uneventfully with healing of the wounds without residual fistulas. In one case, however, venous thrombosis on the third postoperative day, probably due to deep venous thrombosis of the leg, resulted in total failure of the flap which was replaced some weeks later by a split M. Latissimus Dorsi free flap covered with a skin graft.
Though the M. pyramidalis and its variability have been well described, most contemporary anatomical textbooks hardly mention it. Most descriptions date back to the 19th or early 20th century, and although they describe its vascular supply through the deep inferior epigastric or spermatic vessels no detailed anatomical studies of its vascular pedicle exist. The motor nerve supply may occur in a variety of ways, being through one of the lowermost thoracic nerves (11th or 12th), the subcostal nerve, the first and/or second lumbar nerves, the iliohypogastric, the ilioinguinal, and/or genitofemoral nerves.
The Mm. pyramidalés are small triangular muscles on either side of the abdominal midline, lying in a space within the aponeurosis of the M. transversus abdominis, part of the anterior rectus sheath caudal to the arcuate line (Fig. 1) . The muscle originates with its base from the anterior surface of the superior ramus and symphysis of the pubic bone. The origin may extend inferiorly upon the ramus of the pubic bone for a distance of 0.5–2.0 cm where some of its tendinous fibres blend with those of the suspensory ligament of the penis. The course of the muscle is vertical, the medial margin lying close to the midline, and the muscle fibres converging proximally into a small tendon with its aponeurotic fibres of insertion interdigitating at the linea alba somewhere halfway the umbilicus and the pubis. This tendon can be duplicated as was seen in one of our cases; variations in muscle form and insertion seem to exist occasionally.
The M. pyramidalis is present on one or both sides of the body in 82.6% of cases, more frequently absent in the female than in the male. It tends to be more constant in native Africans (83.7%) and Asians (96.2%) than in Caucasians (79.3%).
The size of the muscle seems to vary between 20 and 138 mm in length; 91% are between 3.0 and 9.9 cm, 53% in between 5.0 and 7.9 cm. The average length is 6.82 cm, and the average width 1.98 cm. There is no correlation with general height or size of the abdomen.
The muscles can be best approached through a low Pfannenstiehl incision about 2–3 cm above the pubis through the pubic hair. After dissecting the anterior rectus fascia on both sides of the midline, the anterior rectus sheath should be incised longitudinally about 1 cm lateral of the midline to identify the underlying M. pyramidalis. If present the muscle is clearly separated from the M. rectus both posteriorly and laterally as a discrete muscle belly, the course of its fibres clearly different from those of the M. rectus. Different small vascular pedicles enter the muscle from its lateral side, perforating the underlying M. rectus abdominis. These branches eventually come out of one of the side branches of the proximal part of the inferior epigastric vessels (close to their origin from the iliac vessels) on the undersurface of the M. rectus abdominis. As such, the muscle can be harvested as is a DIEP (Deep Inferior Epigastric Perforator) flap; if necessary it may possibly be combined with a small DIEP skin paddle. The M. rectus abdominis fibres are split lengthwise over a small distance around the perforator, taking care to preserve its innervations and carefully ligating the different muscle branches until one reaches the undersurface of the M. rectus, the technique being the same as in the elevation of a DIEP flap. The length of the perforator in itself makes up most of the length of the pedicle which varied between 6 and 11 cm in our cases. Additional arterial length can be obtained by harvesting the inferior epigastric pedicle more distally (towards its entrance into the muscle) on the undersurface of the M. rectus abdominis and using it in a reverse way. Due to the existence of valves in the deep epigastric vein, the venous pedicle cannot be augmented in this way. In one of our cases, the venous outflow drained into a branch, which in itself entered the inguinal canal extended along the spermatic cord and drained into the femoral vein. The branch itself was of sufficient size to allow for a regular microanastomosis. In all cases, one perforator proved to be sufficient to vascularise the entire muscle belly.
In none of our cases could the nerve supplying the muscle be clearly identified.
After elevation of the muscle the anterior rectus sheet is closed primarily.
As to its function, judged by the relations of the muscle, it is generally stated that the pyramidalis muscle acts as a tensor of the linea alba, in tensing the linea alba it may assist in increasing the intraabdominal pressure as well as locally compressing the bladder.
Contrary to earlier beliefs, anthropological studies and comparative anatomy indicate this muscle to be a progressive structure becoming more firmly entrenched as a normal part of human anatomy, rather than a regressive vestige of a promammalian M. pyramidalis. This could have to do with the erect posture of man.
Its clinical use has been limited to occasional reports of a variation of the Marshall–Marchetti operation for urinary incontinence.
A 35-year-old male patient was the victim of a work accident three years previously in which he sustained a third degree chemical burn on the dorsum of his left foot due to sulphuric acid.
The wound was treated elsewhere, initially conservatively, later on by debridement and cover with a split thickness skin graft. As closure was not obtained, further attempts were made, first by application of a local skin rotation flap from the dorsum of the foot, then of a local fasciocutaneous transposition flap from the distal lateral leg and finally of a free lateral arm flap, all without success though it seems all flaps initially healed well.
At the time of referral, the patient presented with a small defect at the lateral dorsal side of the foot up to the underlying bone and surrounded by atrophic scar tissue (Fig. 2(A)) . The patient was a smoker and was suspected of automutilation. A preoperative angiography showed no apparent lesions of the distal leg vessels, a combined technetium bone and leukocyte scan showed no sign of underlying osteomyelitis.
At operation, the ulcer was debrided to healthy tissue (Fig. 2(B)) and a free pyramidalis muscle flap was harvested through a lower abdominal vertical midline incision (Fig. 2(C–E)), inset and the pedicle anastomosed end-to-side both onto the anterior tibial artery and vein (Fig. 2(F)). The flap was covered with a full sheet split thickness skin graft. Total intervention time was three and a half hours. The postoperative course was uneventful but due to the possibility of automutilation the patient was kept in a plaster cast for 14 days and only discharged on the 18th postoperative day. At four months follow-up there were no signs of recurrence or sinus formation (Fig. 2(G)).
3.2 Case 2
A 52-year-old male patient was seen two years after he sustained an open calcaneal fracture of the right foot. The fracture was treated by open osteosynthesis with plate and screws. In a second operation these were eventually removed. The patient developed osteomyelitis with a sinus, which was debrided and antibiotic treatment was started. An unsuccessful attempt was made to close the wound with a local extensor digitorum brevis muscle flap since when the patient had received intermittent antibiotics for sepsis.
At referral he presented with an unstable scar with a sinus at the lateral heel region of the right foot, underneath the lateral malleolus and parallel with the border of the foot (Fig. 3(A)) .
The wound was debrided to macroscopically vital bone (Fig. 3(B)), preoperative cultures were obtained and the empty space was filled with a free pyramidalis muscle flap (Fig. 3(C) and (D)) and covered with a split skin graft. The patient was kept on oral antibiotics for three months. The flap healed in well (Fig. 3(E)) and showed no signs of underlying residual infection at six months follow-up. Donor site scar was inconspicuous already at one month (Fig. 3(F)).
Multiple treatment modalities exist to cover small defects in the foot and ankle region, including pedicled muscle and fasciocutaneous flaps. However, sometimes previous surgery or the trauma itself preclude the use of local flaps. Sometimes local flaps tend to further injure an already compromised limb or are insufficient or inadequate to obliterate dead space with the required well vascularised tissue.
When using free flaps, we make a distinction between covering a defect and filling up dead space. In the second case there is a need to obliterate dead space with well vascularised tissue and a muscle flap still seems to be the most suitable. Even if a combination of both skin cover and fill is needed, a small piece of muscle will most often suffice. Myocutaneous flaps tend to be too bulky and the connection of muscle and overlying skin makes exact positioning of the muscle part into the dead space difficult.
Different options do exist in the choice of free muscle flaps. One option is to use only part of a larger muscle, e.g. Gracilis, disregarding functional anatomic subunits. Although the functional deficit in adduction power is small, patients may be dissatisfied with the donor scar at the medial thigh.
The other option is to carefully dissect part of the muscle and to leave as much as possible of the remaining muscle functionally intact as is the case with e.g. the split latissimus dorsi, the distal belly of the serratus Ant., or medial strip of the rectus Abdominis. Donor site morbidity in these functional dissections should be less than in taking the whole muscle but will still not be zero. Moreover, donor-site morbidity is not only related to the functional deficit but involves also size and location of the scar, frequency of seromas, secondary sensory loss and esthetic considerations.
As to the smaller free muscle flaps, options are more limited. The pectoralis minor free flap is mainly used in facial reanimation, its dissection is not always straightforward and one has to be very careful not to injure the nerve supply to the pectoralis major. Elevation of the extensor digitorum brevis
on the foot requires harvesting and ligating the anterior tibial artery both as a local or as a free flap, compromising eventual vascularisation of the foot. There is also a functional deficit in extension of the toes. Less well known small free muscle flaps include the anconeus,
The exact function of the M. pyramidalis is not known, the fact that it is bilaterally absent in some individuals without any known functional deficit suggests minimal functional donor-site morbidity as long as one takes care not to injure the underlying rectus abdominis. There is no need to harvest any fascia, which can be closed primarily. The donor scar is well hidden in the pubic area, there is no need to turn the patient and dissection is not difficult once one is comfortable dissecting out the perforators of a DIEP flap. The muscle belly can be used in combination with the skin island of the DIEP flap, with sufficient pedicle length in between them to allow for independent positioning of the muscle and skin, as opposed to other myocutaneous flaps.
Arguments against the pyramidalis are its small size, which makes it suitable only for small defects, its presumed variability and the absence of exact anatomical studies as to its vascularisation and nerve supply. However, preoperative evaluation of the muscle by ultrasonography can be performed, and even if the muscle is absent one can always revert to a medial piece of rectus abdominis, through the same incision. As to its vascularisation, our cases show that isolating the muscle on one of its pedicles is sufficient for its complete survival, the perforators eventually branch from the inferior epigastric pedicle, a well-known, expendable and reliable vascular axis of suitable size for microvascular anastomosis.