If the image quality is good enough then thresholding, smoothing, and inverted island removal may produce good enough results. See detailed description here: 2D/3D fill tool
If image quality is very low and/or bone density is low then you may need to use Grow from seeds effect (growing bone from seed regions), as described here: Bone segmentation to create 3D-printable STL
Exporting to 3D-printable STL file and many other tips are described in segmentation tutorials.