Bone metastases in the femur - a CT-based finite element analysis

Objectives Mirels’ score is known for its low sensitivity in identifying impending fracture risk in patients with metastatic tumors in the femur. CT-based finite element analysis (CTFEA) is an emerging alternative as a patient specific quantitative decision support tool. Patient’s femurs are automatically segmented from the CT scan, material properties are assigned and a stance position load is applied according to patient’s weight. The CTFEA was validated in a retrospective study on the largest cohort of patients to date (50). Using a quantitative measure, strain fold ratio, it distinguishes between high and low risk of fracture.

Purpose: This multicenter study retrospectively identified 12 patients who presented with a pathologic fracture and had a CT scan of the femur within 6 months prior to the fracture. It is the largest such cohort to be analyzed using CTFEA to further substantiate the value of CTFEA.

Methods Twelve patients with metastatic tumors in their femurs and a CT scan within a period of 6 months prior to pathologic fracture were included: 6 males and 6 females. Metastatic disease diagnoses were: 4 multiple myeloma, 3 lung, 2 breast, 1 prostate, 1 TCC, 1 sarcoma. Pathologic fractures were located: 5 femoral head and neck, 5 mid and distal diaphysis, 2 subtrochanteric region. CTFEA was performed for each patient to determine the risk of fracture and the anticipated location of fracture. A strain fold ratio (SFR), defined as the ratio between the strain at the highest location (obtained in the vicinity of the tumor) and the typical strain in that area in a healthy femur, was computed for all patients. A SFR>1.48 defined a high risk of fracture based on published data. Mirels’ score was determined for all patients by experienced orthopaedic oncologists.

Results and Conclusion All patients who experienced a pathologic fracture had a SFR>1.48. Figure 1 presents the SFR vs Mirels score for 11 patients (one patient had Mirels score 12 and SFR 9.6 and was excluded). Most of the patients (9 out of 11) had

1.48 < SFR < 2.5, two additional patients had SFR 3.2 and 3.6. The location of the fracture was accurately determined by CTFEA in all cases.

Conclusions: CTFEA accurately predicts the risk of fracture in patients with metastatic tumors to their femurs. A quantitative measure of SFR>1.48 can be classified with much higher confidence, as a cut-off, for high risk of fracture suggesting prophylactic nailing.CTFEA not only predicts the risk of a pathological fracture, but in cases of multiple metastases in both femurs, it can also identify the ones at the highest risk of fracture and the location of the fracture.

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Publication History

Article published online:
15 October 2020