Aneurysmal Bone Cyst Following Tibial Fracture: A Case Report

Abstract

A 14 year-old boy developed a tibial aneurysmal bone cyst (ABC) following a closed tibia fracture. The tumor formed in a site remote from the fracture and was not radiographically apparent until one year following the traumatic event. Most ABC's present due to fracture or pain, but this lesion was discovered during routine follow up films of the tibial shaft fracture. This case lends support to the debated theory that ABC's are reactive bone lesions and provides a rare radiographic glimpse at the lesion's early rate of development.

INTRODUCTION

The aneurysmal bone cyst (ABC) is a benign solitary osteolytic and expansile bone tumor. It is extremely rare, occurring in 1.5 per one million persons per year¹¹. It most commonly occurs in the metaphysis of long bones during the first two decades of life and usually presents due to pain, swelling, or fracture². The etiology of the ABC is unknown but numerous authors have proposed that it forms in response to vascular disruption in the bone due to a preexisting primary tumor or a traumatic insult^1,3,4,12,13. The early rate of growth of the ABC is also unknown. This case lends support to the theory that ABC's are reactive bone lesions and provides a rare glimpse at the early radiographic appearance of the lesion before a patient becomes symptomatic.

CASE REPORT

A 14 year-old healthy eighth grade boy suffered a closed left tibial shaft fracture at the junction of the middle and distal thirds in September of 2001 while playing football. There was no radiographic evidence of pathologic fracture. He was treated with closed reduction and long leg casting followed by functional bracing. The fracture healed uneventfully and by February of 2002, the fracture had healed radiographically (Figures 1a,b) and he had returned to all activities without any pain or limitations. During his routine final follow up visit in November of 2002, the fracture remained well healed, and he remained asymptomatic. However, the new x-rays revealed an eccentric osteolytic lesion in the proximal lateral tibial metaphysis (Figures 2a,b). The lesion is demonstrated well by CT scan (Figure 3). The patient was non-tender over this area.

Figures 1a and 1b.

AP and lateral x-rays of the tibia demonstrate a well healed distal tibial shaft fracture five months after injury. There is no sign of a lytic lesion in the proximal metaphysis.

1a.

1b.

Figures 2a and 2b.

AP and lateral x-rays one year after injury demonstrate an eccentric, geographic lytic lesion in the proximal lateral tibial metaphysis.

2a.

2b.

Figure 3.

CT scan of the lesion one year after injury.

After discussion with the family and patient, we performed an open biopsy. Frozen and permanent sections had all of the typical characteristics of an ABC. No evidence of a coexisting lesion was seen. Sections demonstrated multiloculated cyst-like walls without endothelial lining. Bands of cellular tissue separated fragments of bone and cartilage. This cellular tissue was composed of multinucleated giant cells and reactive fibroblasts showing moderate mitotic activity but no nuclear atypia. The lesion was thoroughly curettaged, soaked with ethanol, and then filled with a calcium sulfate paste. The patient recovered quickly and returned to all activities without restriction within six weeks of surgery.

DISCUSSION

Jaffe and Lichtenstein first described the ABC as a distinct pathologic lesion in 1942⁹. While initially thought the be an isolated primary tumor of bone, evidence mounted over the ensuing three decades that suggested many ABC's are reactive lesions, caused by the presence of a preexisting bone tumor. Biesecker et al. observed 32% of their cases associated with other benign lesions of bone¹. Mirra noted 40% of his cases with similar associations¹³. Others have reproduced these observations^3,4,5,12.

The pathogenesis of the ABC continues to be debated but many consider it to be a kind of arteriovenous malformation within the bone, likely caused by a previous or preexisting insult to the bone. This theory was supported by Biesecker et al.'s work in which they measured the intralesional pressure within three ABC's to be increased in the range of typically found in arteriovenous malformations¹. The lead author wrote:

Because ABC's are frequently accompanied by associated lesions and because these associated lesions are rarely accompanied by ABC's, it is probable that ABC's are secondary to the associated lesions of bone . . . Therefore, the inaugural event of the genesis of ABC's most likely is an antecedent, primary lesion of bone. The next step in the pathophysiologic development of an ABC is probably the production of an abnormal vascular component by the precursor lesion of bone . . . an arteriovenous fistula.

A few case reports in the last decade lend further support to Biesecker's theory. They have confirmed the presence of a posttraumatic ABC following an injury in which initial x-rays show no suggestion of a bone lesion. In one case, an ABC developed at the fracture site in the proximal tibia within six months of the injury¹⁵. In two cases, the lesions appeared on the proximal tibia following anterior cruciate ligament rupture and reconstruction^7,18. In another case, the frontal bone of a soccer player developed an ABC, presumably after repetitive trauma from heading the ball¹⁷. In all of these cases, the tumor appeared at the local site of injury, suggesting that the injury led to the formation of the lesion.

In a new twist, the current case documents the appearance of an ABC at a site remote from the injury in the tibia. Given the rare nature of ABC's and the temporal relationship, it seems probable that this lesion was secondary to recent fracture. Again, this case provides support for the notion that the tumor is a reactive lesion from an antecedent insult to the bone and its vascular supply.

This case also suggests a timeline for pathogenesis of the posttraumatic ABC. In this patient, the lesion developed sometime between six months and one year after the injury. In other case reports, the lesions were noted between three months and one year after injury. If the cells making up the lesion are indeed reparative tissues responding to the primary injury, it makes sense that the ABC would take several months to become mature and radiographically apparent.

While the circumstantial evidence above makes Biesecker's theory very compelling, it does not explain the etiology of the greater than 50% of ABC's with no apparent associated lesion. There have been a few case reports of a multiple ABC's within a single family suggesting a genetic cause or predisposition ^8,10. These cases may eventually prove to be the key to the puzzle. A genetic predisposition could explain why most of these apparent arteriovenous malformations of bone arise denovo, while other ABC's seem to require a major inciting insult to the bone. As with many tumors, modern genetics may eventually confirm or debunk our etiologic theories that have historically been based on strong reason but limited scientific evidence.