What is the optimal shape and size of a cortical window that least affects bone strength? A biomechanical study.

Abstract

Background: Biopsy and curettage are frequently used in orthopaedic oncology surgeries. However, these procedures reduce bone strength and result in pathological fractures. Therefore, meticulous planning of the size and shape of the cortical window is paramount to preserve bone strength. However, few studies have examined the shape and size of the windows created for biopsy and curettage procedures. The purpose of this study was to evaluate the effect of cortical window shape (rectangular versus elliptical) on bone strength and to evaluate the effect of ellipses with different dimensions on bone strength.

Hypothesis: The hypothesis of this study was that an elliptical cortical window would have a lesser impact on bone strength than a rectangular window, particularly in cases where the length of the ellipse increases while its width decreases.

Materials and methods: Sixty-four synthetic femur models were divided into four groups (n = 8) for compression and torsional tests. Four equal-area cortical windows were created. G1 (rectangular), G2 (elliptical, 1/1.5 ratio), G3 (elliptical, 1/3 ratio), and G4 (elliptical, 1/6 ratio). Compression tests (10 mm/min) were used to assess the maximum load, stiffness, yield load, and fracture energy. Torsion tests (50º/min) were used to measure the maximum torque, stiffness, and work done. Biomechanical performance was compared using load-displacement and torque-angular displacement data.

Results: All bone models were fractured along the cortical window. G3 (elliptical, 511.09 ± 55.07 N) had higher maximum load than G1 (rectangular, 389.18 ± 88.46 N, p = 0.003). The elliptical groups (G2:71.21 N/mm, G3:71.04 N/mm, G4:84.10 N/mm) showed greater compression stiffness than G1 (52.60 N/mm, p ≤ 0.05). G3 had higher yield load (458.72 ± 43.42 N) and work done (2.6 J) than G1 (352.43 ± 91.24 N, 1.78 J, p ≤ 0.05). No significant difference in torsion test results was observed between G3 and G1 (p > 0.05). G2 exhibited lower maximum load, yield load, and work than G3 and G4 (p ≤ 0.003). G4 exhibited higher torque (17.08 Nm) than G2 (11.73 Nm, p = 0.02) and G3 (12.62 Nm, p = 0.018). The torsional stiffness was similar across the elliptical groups.

Discussion: This biomechanical study demonstrated that elliptical cortical windows, especially those with higher length-to-width ratios, outperform rectangular windows in terms of strength under compression and torsional loads. These findings suggest that elliptical cortical windows may provide biomechanical advantages in terms of preserving bone strength and reducing fracture risk. However, as these results are based on an experimental model, further in vivo studies are needed to confirm their clinical applicability.

Level of evidence: V; Comparative laboratory study.