Value of imaging study assessment of transarticular invasion of primary bone tumors abutting the sacroiliac joint
Among imaging techniques (X-ray, CT, MRI and radionuclide scans) for the assessment of bone tumors, MRI has unique advantages with respect to showing the range of intramedullary and surrounding soft tissue invasion of the bone tumor, invasion of the adjacent joints and bone metastasis. Abnormal signal changes on MRI can not only clearly reveal the location, size and involved range of bone tumors abutting the sacroiliac joint but also clearly show that a tumor destroying the ipsilateral cortical bone invades the contralateral bone of the joint across different structures of the sacroiliac joint. According to the literature, the sensitivity and specificity of MRI are 100% and 92%[2, 4], respectively, if a strict definition of transarticular invasion of the bone tumor abutting the sacroiliac joint is used. In this study, 33 of 128 patients with primary tumors and tumor-like lesions abutting the sacroiliac joint presented with transarticular invasion in MRI studies, were all confirmed by surgical findings and pathological examination. These results indicate that MRI can accurately indicate transarticular invasion of a bone tumor abutting the sacroiliac joint with very high sensitivity.
Comparison of transarticular invasion incidence in different locations
The transarticular incidence of primary bone tumors was higher in the ilium than in the sacrum (29.9% vs. 17.1%), but this difference was not statistically significant (P > 0.05). In Group 1, significantly more patients with iliac lesions had transarticular invasion than those with sacral lesions. This group may be primarily responsible for the difference in the transarticular invasion incidences of iliac and sacral tumors. In Groups 2 and 3, the transarticular invasion incidences of iliac and sacral lesions did not differ significantly (P > 0.05). This study demonstrated that transarticular invasion of iliac and sacral tumors is not associated with the anatomical structure, i.e., the thinner iliac cartilage compared with the sacral cartilage, but is mainly associated with the degree of malignancy.
Comparison of transarticular invasion incidence by different pathological type and grade
In this study, the incidence of transarticular invasion was highest in Group 1 (conventional osteosarcoma and Ewing’s sarcoma). The incidences of conventional osteosarcoma and Ewing’s sarcoma were similar. The transarticular invasion incidence was lower in well-differentiated central chondrosarcoma. This finding is consistent with the biological characteristics of tumor malignancy and invasion. In this study, 27 (29.0%) of 93 patients with malignant bone tumors appeared with transarticular invasion. This incidence is similar to the value of 29.4% (15/51) reported in the literature[2, 4]. The transarticular invasion incidence reached 51.7% (15/29) in patients with osteosarcoma, also similar to the incidence of 53.8% (7/13) reported in the literature[4]. However, the transarticular invasion incidence in patients with chondrosarcoma or Ewing’s sarcoma obviously differed from that in the literature. Ozaki et al. reported in two studies[2, 4] that the transarticular invasion incidence of chondrosarcoma was 47.1% (8/17) and 46.7% (7/15), whereas the transarticular invasion incidence of Ewing’s sarcoma was 4.3% (1/23) and 8.7% (2/23). In this study, the transarticular invasion incidence of chondrosarcoma and Ewing’s sarcoma was 11.4% (4/35) and 43.8% (7/16), respectively. These incidences are obviously different from those in the literature. Other studies related to the transarticular incidence of Ewing’s sarcoma are case reports or had small sample sizes[5–7]. These discrepancies may be attributable to differences in inclusion criteria. First, Ozaki’s study only included patients with transarticular invasion of the primary iliac tumor, whereas the present study included patients with transarticular invasion of primary iliac and sacral tumors. Second, the study by Ozaki, et al [2] did not include patients with tumor margins 2 cm away from the joint surface, but these patients were included in their subsequent studies[4]. Next, patients with high-grade chondrosarcoma accounted for 80% of all patients with chondrosarcoma in the studies[2, 4] by Ozaki et al, whereas patients with low-grade chondrosarcoma were predominant in the current study. Hence, the degree of malignancy of chondrosarcoma is mainly attributable to the incidence of transarticular invasion.
The transarticular incidence of chordoma has not been reported. In this study, the transarticular incidence of chordoma was low, only 7.7% (1/13). This finding may be associated with the low-grade malignant nature of chordoma, which is less invasive. In addition, chordoma is usually located in the midline of the lower sacrum [8], distant from the sacroiliac joint.
Few cases of transarticular invasion of intermediate bone tumors across the sacroiliac joint have been reported in the literature[3]. The transarticular invasion incidence has not been discussed. Of 35 patients with intermediate bone tumors abutting the sacroiliac joint in this study, only 6 (17.1%) presented with transarticular invasion, including 4 (20%) patients with giant cell tumor of bone, 1 (16.7%) patient with chondroblastoma and 1 (11.1%) patient with aneurysmal bone cyst. These results indicate that intermediate bone tumors are somewhat aggressive but the transarticular invasion incidence across the sacroiliac joint was significantly lower than that of highly malignant bone tumor.
Comparison of transarticular invasion incidence between different invasion types of primary bone tumors abutting the sacroiliac joint
Of 33 patients with tumor invasion across the sacroiliac joint, although a few patients showed involvement of two or more invasion types, but the statistical results after patients were sub-grouped showed differences in incidence among the 3 types of transarticular invasion of primary bone tumors, i.e., invasion across the ligamentous portion, the articular cartilage and the periarticular tissue. Invasion across the ligamentous portion was most common, followed by across the articular cartilage and, finally, across the periarticular tissue. In addition, the incidences of the 3 types of invasion were not associated with benignity and malignancy or the malignant degree of a tumor.
Transarticular invasion of primary bone tumors across articular cartilage is usually complicated by invasion across the ligamentous portion. Isolated invasion across the articular cartilage is very rare. These findings are consistent with those from the previous literature[2, 4], indicating that the ligamentous portion is vulnerable to invasion of a bone tumor near the sacral iliac joint and that cartilage may prevent tumor invasion to some extent. The mechanism of tumor invasion prevention by cartilage may be related to the following factors. ① There are no blood vessels in the articular cartilage. Thus, a direct anatomical basis and blood supply for tumor invasion and tumor cell growth are lacking [8]. ② Cartilage cells produce a substance to inhibit tumor angiogenesis[9] and collagenase activity[10–12]. In addition, the isolated invasion of primary bone tumors across the periarticular tissue is rare. Of 13 patients with chordoma, only 1 patient presented with transarticular invasion across the periarticular tissue. A possible explanation is that chordoma is usually located in the midline of the lower sacrum and distant from the sacroiliac joint. The probability of invading the sacroiliac joint is relatively low for small-sized chordoma. Moreover, chordoma is a low-malignant and less-invasive tumor. It has a weak ability to invade into the ligamentous portion and the articular cartilage but may invade across the sacroiliac joint via the periarticular tissue.