The patient's medical history was significant for an abdominal operation two years prior for a sigmoid diverticular bleed. At that time, he had a midline abdominal incision, bowel resection, end colostomy, and a rectal pouch. In the ensuing two years, he found that his colostomy was malpositioned, located on a skin crease, and, therefore, he had difficulty with stoma hygiene. Under our care, the patient was taken operating room for an exploratory laparotomy, tumor resection, and stoma revision.

Upon abdominal exploration, lysis of adhesions resulted in several serosal tears that were repaired primarily. One large tear required a partial small bowel resection and primary hand-sewn anastomosis. The tumor was encountered adherent to the aorta and carefully resected. The end colostomy was resited, and careful exploration of the abdominal cavity ensured no missed enterotomies. The fascia was closed with two types of absorbable suture, and the skin was closed with staples.

On postoperative day five, he was noted to have leakage of serosanguinous fluid from the midline wound. Some staples were removed from the skin, and the wound was packed. In the interim, the colostomy had regained function. On postoperative day six, succus was found to be draining from the inferior portion of the wound, and the patient was taken back to the operating room for a re-exploration with a pre-operative diagnosis of anastomotic leak or failure of enterotomy repair.

Upon re-exploration, the remaining staples were removed. The inferior portion of the midline wound was opened first, as it was draining succus. We quickly identified, immediately under the fascia, an enterotomy that was draining succus. This area was oversewn in two layers. The remainder of the wound was opened, and a thorough exploration of the abdominal cavity did not reveal any other evidence of leakage or perforation. All previous anastomosis and enterotomy repairs were intact. During that search, a hard 3mm × 1cm irregularly shaped lesion with a sharp point was encountered along the inferior portion of the midline wound directly adjacent to the noted enterotomy (Figure 1(a)). The remainder of the midline wound was explored, and a similarly hard, larger 2.3cm × 7mm lesion was found at the superior portion of the wound (Figure 1(b)). Both specimens had the consistency of bone or plastic. They were both removed and submitted to pathology. The wound was closed with a biosynthetic patch underlay. Subsequently, the patient recovered and regained enteral autonomy.

Reports of heterotopic ossification specifically within abdominal wall can be found in the literature dating back to the 1940s [18] with a surge of reports in the 1970s [11–13, 15, 19–21]. Since that time, there have been subsequent reports with different inciting mechanisms and complications. Development of heterotopic ossification within the abdominal wall following penetrating abdominal trauma makes up the majority of reports, but development after blunt abdominal trauma has also been noted [10]. Heterotopic ossification has been described in conjunction with burns as well [14, 22, 23]. Various uncommon locations for heterotopic, such as the hand, head, kidney, and even the popliteal fossa have been reported [8, 9, 16, 24–26] in addition to the more commonly reported cases following orthopedic procedures such as total hip arthroplasties and open repair of acetabular fractures [2]. Penetrating trauma to the abdomen has been reported to cause ossification within the mesentery [27].

Focusing solely on abdominal heterotopic ossification, one of the earliest case series conducted in 1975 looked at heterotopic ossification in 23 abdominal incisions. They found a male preponderance (79% versus 21%) with an average age of 55. They noted the possibility of reabsorption and disappearance in some patients but urged removal if symptomatic and cautioned against mistaking it for a malignant lesion [13]. Male preponderance is further supported from subsequent case series in the gynecologic literature [7] along with a radiologic review where ten of the eleven cases were male [5]. A study based upon radiographic review showed a median age of 40 years-old (range 20–76 years) with a scar size of 2.2cm (range 0–4.9cm) and found the time from traumatic event to development of radiologic findings may be as early as eleven days (mean 6.8 months) [5]. The most recent series of 3 cases included patients with ages between 51 and 74 years old and found development of ossification between 2 and 4 months following the traumatic event [28]. All were treated with excision with the one recurrence receiving adjuvant radiotherapy of the excision site. Although the majority of these cases and series include adults, the pediatric population is not immune, as there are reports of heterotopic ossification in children as young as two years of age [29, 30].

There are two proposed mechanisms for the development of heterotopic ossification in abdominal wounds. The first is that heterotopic ossification develops from liberated bone fragments from the periosteum or perichondrium of either the xiphoid process or symphysis pubis that are deposited within and along an incision. This theory is supported by the fact that all reported cases have been associated with midline laparotomy wounds and none have been reported within transverse incisions [28]. The second theory is founded in the belief that the heterotopic ossification develops from immature pluripotent mesenchymal cells that are triggered by the inciting trauma to differentiate into either osteoblasts or chondroblasts [31]. A similar proposal has been made in the orthopedic literature. Kaplan et al. have suggested that four factors are necessary for the development of heterotopic ossification [6]:

Signals for osteoblast and chondroblast differentiation from mesenchymal cells may include bone morphogenetic protein-2 (BMP-2) as heterotopic ossification has been seen in patients receiving recombinant human bone morphogenetic protein-2 (rhBMP-2) for orthopedic procedures [32]. Although trauma is believed to be an underlying inciting factor, there may also be a genetic predisposition to the development of heterotopic ossification, as the presence of such ossification in a midline abdominal wound may indicate the possibility of similar deposits elsewhere in the body that have not previously been operated upon [31].

Regardless of the underlying etiology, management of a mass believed to be heterotopic ossification must take several factors into account. First, one must differentiate between benign heterotopic ossification and the possibility of a malignancy [13]. If the first operation was performed for cancer, one must exclude the possibility of scar recurrence following surgery for abdominal malignancy [33]. Excision or resection of the mass is usually indicated with symptoms (discomfort or pain) or to rule out malignancy. Following resection, the use of etidronate disodium may be useful in the prophylactic setting to help prevent recurrence [34], while radiotherapy may be used if and when those recurrences present themselves [28].