The sites that are most accessible for bone marrow aspiration in the dog are the proximal humerus (a), proximal femur (b) and the wing of the ilium (c), approached either from the dorsal crest or lateral face. The easiest sites from which to obtain bone marrow in the cat are the proximal femur and proximal humerus.

The most common needle types used for aspiration of bone marrow are the Illinois sternal-iliac and the Rosenthal needles (pictured). Needles are available in 16 and 18 gauges, 1 or 1-15/16 inch long. Aspiration-style needles have an inner stylet that is used to penetrate the cortex of the bone, then removed to aspirate marrow. Before introducing the needle into the bone, check to make sure the stylet totally occludes the lumen of the needle. If the lumen is not filled by the stylet, pieces of cortical bone may enter the lumen of the needle as it is driven into the bone. These pieces of cortical bone will plug the needle, preventing aspiration of marrow.

If you have relatively short fingers, the Rosenthal needle is held with the back of the needle pressed at the junction between fingers and palm. This gives you maximum driving force and stability. The stylet does not lock in place and must be held in the needle with digital pressure as the needle is advanced into the bone to prevent the lumen of the needle from becoming obstructed with cortical bone

For individuals with longer fingers, the needle can be held between the index and middle finger, and the thumb can be used to hold the stylet in place.

Bone marrow aspiration is performed in the dog with lidocaine local anesthesia and in cats, under short duration general anesthesia. The skin surface is clipped of hair and aseptically prepared as for surgery. Sterile gloves are usually worn. If local anesthesia is being used, the agent is placed in the skin, muscle and on the periostium of the bone.

A small skin incision is made with a scalpel blade. The bone marrow needle is used to bluntly cut through the muscle. Once the needle is in contact with the surface of the bone it is rotated into the bone with a clockwise/counter-clockwise motion. It takes a considerable amount of force to drive the needle through the cortex of the bone.

To obtain marrow from the lateral aspect of the wing of the ilium the animal is restrained in lateral recumbency.

Palpate the dorsal crest of the wing of the ilium and move down approximately 1 cm (* marks the site of bone penetration on photo below). The marrow cavity is shallow in this location (see below). The needle cannot be seated very deeply or it will pass through the marrow cavity and into or through the cortical bone on the opposite side of the wing. This technique is not recommended for use in small dogs (~25 lb or less) or cats due to the tendency to penetrate both cortices. Obese or very large dogs may have too much paralumbar fat &/or muscle for the needle to pass through before reaching the surface of the bone.

Bone marrow can be aspirated from the dorsal crest of the wing of the ilium. The animal is restrained in sternal recumbancy or standing. Hold the crest of the ilium between your fingers. The dorsal surface of the crest is rounded, and the needle tends to slide off the bone into soft tissues if you don't maintain control of it. The needle is directed parallel to the long axis of the wing of the ilium. The needle can be firmly seated in this location.

Bone marrow can be obtained from the proximal humerus of most animals, including those of small body size or obese condition. The bone marrow needle can be seated firmly in this location as the marrow cavity is thick. Run your finger down the spine of the scapula. The first prominance you feel is the acromion (a). The next prominance is the greater tubercle of the humerus (b). The needle is inserted at the distal end of this bony prominance (c). Maintain control of the needle as you are seating it into the bone. There is a tendency for the needle to slide down the surface of the bone instead of penetrating the cortex, causing damage to adjacent soft tissues.

The needle should be inserted at an angle ~450 from a line parallel to the long axis of the humerus. Needle insertion too proximal may result in entry into the scapulohumeral joint. Needle insertion at an angle perpendicular to the humerus may result in entry into the bicipital bursa that communicates with the scapulohumeral joint on the medial side of the limb.

To obtain bone marrow from the proximal humerus, the animal is restrained in lateral recumbency. The elbow (a) is rotated inward such that the shoulder joint is turned outward. This positions the greater tubercle (b) in a location that is easier for you to get the bone marrow needle seated in the bone.

The proximal femoral shaft usually contains red marrow and is relatively accessible for aspiration in the cat. The needle may not be of sufficient length to reach this site in large dogs. The needle is seated in the trochanteric fossa between the lesser and greater trochanters.

Hold the stifle in one hand with your thumb laying along the long axis of the femur and the thumb nail over the greater trochanter. Make a small skin incision with a scalpel blade and insert the needle under your thumb until the tip of the needle contacts the periostium of the intertrochanteric fossa. Keep the needle parallel to your thumb. If you do not remain parallel to your thumb (and hence the femur) the needle may exit the cranial or caudel cortex of the bone. Remember that the sciatic nerve is caudel to the femur and can be injured if the needle slips caudel to the femur.

A 12 or 20 ml syringe is used to aspirate bone marrow. It may be necessary to pull back on the syringe plunger to 10-15 ml to create enough negative pressure to break marrow particles loose from the endosteal surface. Aspirate only a small amount of marrow. Aspiration of large volumes results in dilution of the marrow sample with peripheral blood. When marrow just enters the barrel of the syringe (arrow), stop aspirating. Larger volumes of marrow can be drawn into an EDTA solution and the marrow particles (spicules) picked out with a needle or pipette.

A small drop of marrow is placed on each of several slides. Marrow clots very rapidly. It is important to make the smears immediately after obtaining the marrow sample.

If the marrow sample was heavily contaminated with peripheral blood during sampling it still may be possible to "salvage" such a sample. Smearing a drop of marrow that has heavy blood contamination results in a cytology sample of low cellularity with regard to marrow elements. The sample may be salvaged using the following technique.

Procurement of a core of marrow

A Jamshidi needle is used to obtain a core of marrow for histologic evaluation. The stylette (a) is locked into the hollow part (b) of the needle with a threaded cap, not shown in the photo. (c) is a thin wire used to dislodge the core of marrow from the needle.

A core of marrow can be obtained from the proximal humerus, proximal femur or dorsal crest of the wing of the ilium. The landmarks for starting the needle are the same as described for aspiration of marrow from the proximal humerus. Once the needle penetrates the cortex of the bone, the stylette is removed. The needle is advanced 1 to 2 cm. At that point, the entire needle is "stirred" in the marrow to break loose the core of marrow. Do this by rotating the blue cap of the needle in a circle which will also rotate the tip of the needle in a circle in the marrow, breaking loose a core in the lumen of the needle. The needle is removed from the marrow using a clockwise or counterclockwise (not back and forth) motion. Rotating the needle out in one direction allows a second opportunity to break loose the core. If the core is still attached and you withdraw the needle, rotaing back and forth, you may pull the needle off the core and leave the core in the marrow cavity.

The stylette is inserted into the tapered tip of the needle and the core is pushed out of the needle.

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