Simplified Countersinking
of an IM Pin
Submitted by Toby Eshelman, DVM | Veterinary Specialty Center
Radiograph of a dog with plate and rod fixation of a chronic left femur fracture
Images courtesy of Toby Eshelman, DVM | Veterinary Specialty Center
Countersinking of intramedullary pins is occasionally required in femoral, tibial, and humeral fractures to minimize morbidity due to joint interference or soft tissue impingement, most notably sciatic nerve irritation dorsal to the trochanteric notch. This procedure is traditionally done with a mallet and a pointed countersink, similar to a carpenter’s tool, which is placed against the end of the intramedullary pin. Difficulty visualizing the pin end in soft tissues and oblique or irregular (cut) ends of the intramedullary pin can make this a frustrating exercise. Small diameter pins may be inadvertently bent if struck slightly off-axis, and confirmation of satisfactory depth can be difficult, particularly in the trochanteric fossa of the femur.
An IMEX drill sleeve and Interface no-point pin of appropriate size can be used to simplify the process, at least for IM pins of the appropriate diameter:
| IM Pin Size | Drill Sleeve Size | Interface No-Point ESF Pin Size |
|---|---|---|
| 5/64" | 2.0 mm | 2.0 mm or 5/64" |
| 3/32" | 2.3 mm | 2.4/3.2 mm (3/32" | 1/8") |
| 1/8" | 3.1 mm | 3.2/4.0 mm (1/8" | 5/32") |
| 5/32" | 3.9 mm | 4.0/4.8 mm (5/32" | 3/16") |
STEP 1 | An IM pin is placed in the bone in routine fashion, and seated securely in the distal metaphyseal bone.
STEP 2 | The pin is cut at a convenient location about 1-2 cm from the entry point into the cortex. In the femur, this is usually just proximal to the level of the greater trochanter.
STEP 3 | The exposed pin length (call this length X) is measured with a K-wire or similar pin for probing the cortex.
STEP 4 | The IM pin is withdrawn a sufficient distance to permit removal of the excess length X and then driven as far back into the bone as achievable with a Jacobs chuck or similar instrument.
STEP 5 | The blunt end of an appropriate IMEX Interface no-point pin (see chart above) is passed through the matching drill sleeve until the positive-profile threads contact the sleeve. Duraface or other negative-threaded pins will not hit the sleeve and will not work as simply.
STEP 6 | The Interface no-point pin is cut 2-3mm beyond the end of the drill sleeve. This excess pin length will correspond to the desired depth of countersinking.
STEP 7 | The sleeve is placed over the end of the IM pin and pushed firmly until it contacts the cortex. The shortened Interface no-point pin is placed into the sleeve; the two pins are now end-to-end within the sleeve, preventing an off-axis strike and pin bending.
STEP 8 | The threaded end of the Interface® no-point pin is struck with a mallet until the threads contact the sleeve.
STEP 9 | The IM pin is now satisfactorily seated and countersunk.
STEP 10 | The Interface NP pin is discarded.
FIGURE 1 | Two-week recheck radiographs of a dog who underwent plate and rod fixation of a chronic (> three weeks) left femur fracture. Severe lameness and sciatic pain was noted at this recheck exam. Pin removal was not considered prudent due to slight IM pin bending noted and limited screw purchase in the proximal fragment (2/3 were monocortical). The IM pin was countersunk in the manner described.
FIGURE 2 | IM pin after countersink