July 19, 2020
osteoid osteoma cover

What are Osteoid Osteomas?

The osteoid of bone is a matrix of Type I collagen that is embedded in a glycosaminoglycan gel that contains proteins that bind strongly to calcium (1). The osteoid is made by the osteoblast cells (1). Bone is made up of the combination of osteoid, osteoblasts, osteoclasts, and mineral salts (1).
An osteoid osteoma consists of an area of abnormal osteoid surrounded by dense sclerotic bone tissue (2). Rarely are multiple nidus found within one sclerotic margin (5). These are benign tumors that are typically less than 2cm in size (3). They affect men more than women and typically occur in a patient’s 20s (3).
Osteoid osteomas are most commonly found in the metaphyseal and diaphyseal regions of the tibia and femur (2). However, they can also be found in the spine, hands, pelvis, sacrum, and feet (2). Intra-articular osteoid osteomas differ from intra-cortical osteoid osteomas and are recognized as separate entities (11). They are a benign tumor and do not metastasize or have the capacity to become malignant (5). They are typically categorized based on their location within bone (11). This includes either the cortical portion, subperiosteal, or medullary portion (11).

Introduction

The most common presentation of an osteoid osteoma is a dull and local pain that increases at night (4). The area of abnormal osteoid, known as the nidus, is innervated by sensory nerve fibers that produce prostaglandins (2). Levels of prostaglandins as high as 100 to 1000 times normal have been documented within the nidus of an osteoid osteoma (7). In a survey of patients with hip osteoid osteomas, they found that 90% of the affected population had pain at nighttime and 88% felt that the pain was relieved with NSAIDs (2).
Physical exam findings can sometimes reveal a knee effusion due to the osteoid osteoma’s proximity to the joint (5). There can also be local bony tenderness and soft tissue swelling (5). Physicians should also evaluate for limb-length discrepancy, which could be the result of a femoral osteoid osteoma (5).
osteoid osteoma tibia shaft xray radiograph

Image 1. Osteoid osteoma of the tibia as seen on xray (courtesy of sciencedirect)

osteoid osteoma femoral neck CT

Image 2. Osteoid osteoma of proximal femur on CT (courtesy of radiopaedia.org)

Imaging

First line imaging for osteoid osteomas is a plain radiograph (5). The nidus is seen as an oval radiolucency and is surrounded by bony sclerosis (5). When evaluating, second line imaging CT scans have been found to be more accurate in diagnosing these tumors than MRIs (5). In a study comparing the two modalities, CT scans identified the tumor in 67% of the cases and MRI only identified them in 3% of the cases (6). However, due to the small size of the osteoid osteomas, 1mm slices on a CT would be required and this can lead to large radiation doses for the patient (10). The small size of the tumors is also the reason why an MRI may miss the nidus of the tumor (10). Frequently an MRI will only identify an area of non-specific edema within bone, which can be misinterpreted as a stress fracture (10).
A bone scan will also show an area up increased radioisotope uptake, which represents the nidus of the tumor (10).
osteoid osteoma of femur MRI

Image 3. Osteoid osteoma as seen on MRI (courtesy of researchgate)

Treatment

First line treatment for osteoid osteomas is typically non-operative (2). In 1992, a study published in the Journal of Bone and Joint Surgery looked at the medical management of osteoid osteomas with salicylates and NSAIDs. What they found was that the group of patients treated medically had symptom relief after on average 30-40 months of medication (7). However, medical therapy can sometimes fail and in those patients, three alternative options exist. These include open surgical excision, CT-guided percutaneous excision, or CT-guided radiofrequency ablation (5).
Previously, the gold standard was surgical excision if a patient failed medical management (5). There is also a risk of developing arthritis or growth disturbance in tumors that are not resected (9). Despite the small size of the tumor, they can be difficult to resect. This can lead to incomplete removal or in some cases, too much bone can be removed, increasing the risk of developing a post-operative fracture (9).
In order to avoid an open procedure, radiologists began to perform CT-guided percutaneous excision (9). These procedures are generally well tolerated and have few complications (9). However, due to mis-reads of the CT scan, the procedure can sometimes leave the nidus (9).
Currently, the gold standard for symptomatic osteoid osteomas is thermal radiofrequency ablation under CT guidance (3). The procedure is performed under a general anesthetic (10). Under CT guidance, a drill bit is inserted into the cortex of the bone (10). Once within the nidus, a radiofrequency probe is inserted into the nidus and heated to 90 degrees Celsius for 4 minutes (10). Research finds that the procedure is typically successful in 90% of the cases (3). In a study done by Rosenthal in the Journal of Bone & Joint Surgery, there was no difference in the rate of recurrence when comparing open excision to radiofrequency ablation (9).

Conclusion

Osteoid osteomas are one of the most common benign bone tumors found. They have a characteristic presentation of nocturnal pain relieved with NSAIDs. Diagnosis can sometimes be difficult due to their location or small size on radiographs. In the future, treatment options will continue to shift towards minimally invasive options, such as arthroscopic excision.

REFERENCES

1) Lowe, James, and Peter Anderson. “Musculoskeletal System.” Stevens & Lowe’s Human Histology, Fifth, Elsevier, 2020, pp. 222–45.

2) May, Collin J., et al. “Osteoid Osteoma About the Hip in Children and Adolescents.” The Journal of Bone and Joint Surgery. American Volume, vol. 101, no. 6, Mar. 2019, pp. 486–93. PubMed, doi:10.2106/JBJS.18.00888.

3) Masciocchi, Carlo, et al. “Radiofrequency Ablation versus Magnetic Resonance Guided Focused Ultrasound Surgery for Minimally Invasive Treatment of Osteoid Osteoma: A Propensity Score Matching Study.” European Radiology, vol. 26, no. 8, Aug. 2016, pp. 2472–81. PubMed, doi:10.1007/s00330-015-4111-7.
4) Ahlfeld, S. K., et al. “Osteoid Osteoma of the Femoral Neck in the Young Athlete.” The American Journal of Sports Medicine, vol. 18, no. 3, June 1990, pp. 271–76. PubMed, doi:10.1177/036354659001800309.
5) Atesok, Kivanc I., et al. “Osteoid Osteoma and Osteoblastoma.” The Journal of the American Academy of Orthopaedic Surgeons, vol. 19, no. 11, Nov. 2011, pp. 678–89. PubMed, doi:10.5435/00124635-201111000-00004.
6) Hosalkar, Harish S., et al. “The Diagnostic Accuracy of MRI versus CT Imaging for Osteoid Osteoma in Children.” Clinical Orthopaedics and Related Research, no. 433, Apr. 2005, pp. 171–77. PubMed, doi:10.1097/01.blo.0000151426.55933.be.
7) Kneisl, J. S., and M. A. Simon. “Medical Management Compared with Operative Treatment for Osteoid-Osteoma.” The Journal of Bone and Joint Surgery. American Volume, vol. 74, no. 2, Feb. 1992, pp. 179–85.
8) Frassica, F. J., et al. “Clinicopathologic Features and Treatment of Osteoid Osteoma and Osteoblastoma in Children and Adolescents.” The Orthopedic Clinics of North America, vol. 27, no. 3, July 1996, pp. 559–74.
9) Rosenthal, D. I., et al. “Percutaneous Radiofrequency Coagulation of Osteoid Osteoma Compared with Operative Treatment.” The Journal of Bone and Joint Surgery. American Volume, vol. 80, no. 6, June 1998, pp. 815–21. PubMed, doi:10.2106/00004623-199806000-00005.
10) Rankine, James J. “The Diagnosis and Percutaneous Treatment of Osteoid Osteomas.” Current Orthopaedics, vol. 21, no. 6, Dec. 2007, pp. 464–70.
11) Chai, Jee Won, et al. “Radiologic Diagnosis of Osteoid Osteoma: From Simple to Challenging Findings.” Radiographics: A Review Publication of the Radiological Society of North America, Inc, vol. 30, no. 3, May 2010, pp. 737–49. PubMed, doi:10.1148/rg.303095120.