Heterotopic ossifications after vertebroplasty using calcium phosphate in osteoporotic vertebral compression fractures: Report of 2 cases

1. Introduction 

Polymethylmethacrylate (PMMA) does not have osteoconductivity, and injected PMMA cement may interrupt normal spinal biomechanics [6]. Therefore, osteoconductive filler materials, such as calcium phosphate (CaP) and calcium sulfate, have been used in vertebroplasty and kyphoplasty [4]. Numerous studies have reported the advantages of osteoconductive calcium phosphate cement [2], [5]. However, CaP may have the potential for ectopic or heterotopic ossification [3], [7]. Injected CaP cement can induce unwanted heterotopic ossifications in the human body. Here, we report the case of 2 patients who developed an unwanted heterotopic ossification around a vertebral body after vertebroplasty using CaP.

2. Case 1 

A 69-year-old woman presented with severe upper back pain after slipping and falling. Her T score from bone densitometry was −2.5. Initial lumbar plain x-ray films showed an L2 compression fracture with an air cleft. She did not have a medical illness history and was not taking any medications (including steroids). We successfully performed percutaneous vertebroplasty with CaP cement (55% dicalcium phosphate dehydrate and 45% tricalcium phosphate, JectOS®, Kasios, France) through a unilateral posterolateral extrapedicular approach (posterolateral vertebral body access method) 8 days after the trauma. A total of 3.7 mL of CaP cement was injected unilaterally (Fig. 1A). Her back pain was significantly improved after the vertebroplasty, and she was discharged without any complications. After 3 months, she felt mild upper back pain. Both legs were neurologically intact. Lumbar x-ray films, which were taken 3 months after the vertebroplasty, showed that a heterotopic ossification had developed around the L2 vertebral body and that the injected CaP cement was partially absorbed. Also, the cemented L2 vertebral body had recollapsed (Fig. 1B). Her upper back pain improved after conservative managements including bed rest, analgesics, and brace wearing. Thirty-two months after the vertebroplasty, she complained of upper back pain again without specific trauma history. The follow-up plain films and CT showed a condensed and reduced heterotopic ossification mass with increased density of the L2 compressed vertebral body, and subsequent T12 compression fracture (Fig. 1C and D). We treated the subsequent T12 compression fracture conservatively. Recently, she has been receiving follow-up care at our hospital regularly and has had no recent problems.

View Large Image Download to PowerPoint

Fig. 1. Immediate postoperative lateral plain x-ray (A). Lumbar plain x-ray (B) taken at 3 months after the vertebroplasty showed a heterotopic ossification around the L2 vertebral body. Injected CaP cement had partially absorbed. Also, the cemented L2 vertebral body had progressively collapsed. Lumbar lateral x-ray (C) and CT (D) that were taken at 32 months after the vertebroplasty showed a condensed and reduced heterotopic ossification mass, and subsequent vertebral compression fractures occurred at the T12.

3. Case 2 

An 80-year-old man presented with severe upper back pain after slipping. Lumbar plain x-ray films showed an L1 compression fracture. His T score from bone densitometry was −3.5. He was taking medications for Parkinson's disease. We successfully performed percutaneous vertebroplasty with CaP cement (55% dicalcium phosphate dehydrate and 45% tricalcium phosphate, JectOS®) through a unilateral posterolateral extrapedicular approach 10 days after the trauma. A total of 3.3 mL of CaP cement was injected unilaterally (Fig. 2A). His back pain was significantly improved and he was discharged without any complications. After 26 months, he revisited our hospital for severe back pain after slipping. Both legs were neurologically intact. We took follow-up plain x-ray films, CT, and MR imaging for the evaluation of the back pain. Lumbar x-ray films showed that a recollapse of L2 vertebral body and a heterotopic ossification had developed. Also, the injected CaP had been reabsorbed and subsequent vertebral compression fractures had occurred at the L3 and L4 vertebrae (Fig. 2B). CT and MR imaging revealed bone fusion from T12 to L2 and acute vertebral compression fractures of L3 and L4 (Fig. 2C and D). We performed percutaneous kyphoplasty with PMMA cement at L2 and L3.

View Large Image Download to PowerPoint

Fig. 2. Immediate postoperative lateral plain x-ray (A). The lateral plain x-ray (B), CT (C), and MR imaging (D) were taken at 26 months after the vertebroplasty. The injected CaP was reabsorbed. Heterotopic ossification progressed and bone fusion developed from T12 to L2. Subsequent vertebral compression fractures occurred at the L3 and L4 vertebrae.

4. Discussion 

Several studies have reported the ectopic bone formation effect of CaP in animal models, and CaP may have osteoinductive properties [3], [7]. We suggest that injected CaP can induce ectopic or heterotopic ossification in humans because of the osteoconductive and osteoinductive properties of CaP. In our cases, injected CaP cement did not leak to adjacent soft tissue during vertebroplasty. Although the CaP did not leak in these cases, we think that injected CaP cement may leak out through a fractured site or needle puncture site after vertebroplasty and that the leakage of CaP cement after vertebroplasty may induce a heterotopic ossification. Another complication of vertebroplasty with CaP was the recollapse of the cemented vertebral body [1]. The stiffness of CaP cement was weaker than that of PMMA [1], [2], [5]. Therefore, the CaP cement may not provide enough initial stiffness in a compressed vertebral body. Consequently, the cemented vertebra may recollapse after vertebroplasty. In the first case, the final x-ray films showed a condensed and reduced heterotopic ossification mass and an increase in the density of the L2 compressed vertebral body (Fig. 1C and D). These findings suggested that heterotopic ossification may not progress continuously. In the second case, however, heterotopic ossification resulted in bony fusions (Fig. 2C and D). In both cases, subsequent vertebral compression fractures occurred after heterotopic ossification developed (Fig. 1, Fig. 2). We suggest that heterotopic ossification or bony fusion after heterotopic ossification may alter the normal biomechanics.

We think that the properties of CaP, such as osteoconductivity, were not always favorable. We suggest that heterotopic ossification may be complications of vertebroplasty with CaP. Therefore, we strongly recommended that the patients who undergo a vertebroplasty with CaP need strict observation, evaluation of serial follow up x-ray films, and long-term brace-wearing after vertebroplasty. Also, leakage of CaP cement into adjacent soft tissue should be prevented during vertebroplasty.