A trephine is a surgical instrument with a cylindrical blade

Blade

A blade is that portion of a tool, weapon, or machine with a cutting edge and/or a pointed tip that is designed to cut and/or puncture, stab, slash, chop, slice, thrust, or scrape animate or inanimate surfaces or materials...

. It can be of one of several dimensions and designs depending on what it is going to be used for. They may be specially designed for obtaining a cylindrically shaped core of bone

Bone

Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

 that can be used for tests and bone studies, cutting holes in bones

Rochester bone biopsy trephine

The Rochester bone biopsy trephine is a surgical instrument that enables a 7.5 mm percutaneous bone biopsy of the iliac crest, or hip, in a human patient. Samples are used for bone histology and histomorphometry.Stephen F...

 (i.e. the skull

Human skull

The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...

) or for cutting out a round piece of the cornea

Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 for eye surgery

Eye surgery

Eye surgery, also known as orogolomistician surgery or ocular surgery, is surgery performed on the eye or its adnexa, typically by an ophthalmologist.-Preparation and precautions:...

.

A cylindrically shaped core of bone (or bone biopsy

Biopsy

A biopsy is a medical test involving sampling of cells or tissues for examination. It is the medical removal of tissue from a living subject to determine the presence or extent of a disease. The tissue is generally examined under a microscope by a pathologist, and can also be analyzed chemically...

) obtained with a bone marrow trephine is usually examined in the histopathology

Histopathology

Histopathology refers to the microscopic examination of tissue in order to study the manifestations of disease...

 department of a hospital under a microscope

Microscope

A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

. It shows the pattern and cellularity of the bone marrow

Bone marrow

Bone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells. On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg , bone marrow accounts for approximately 2.6 kg...

as it lay in the bone and is a useful diagnostic tool in certain circumstances such as bone marrow cancer and leukemaeia!