Lag screw and K-wire

Fractures of the femoral neck are highly unstable because of:

• the long moment arm acting at the fracture plane (the entire femoral neck length), and
• the location of the fracture plane along the lines of maximum shear stress.

Compression of the fracture surface is needed to resist the high shear stress. Compression is achieved by using a lag screw, and stability is improved by adding an antirotational K-wire.

Although Salter-Harris type 1 fractures of the femoral head are generally treated with parallel K-wires only, physeal fractures in a more mature animal in which minimal growth potential remains can be stabilized using a lag screw and k-wires.

This procedure is performed with the patient in lateral recumbency, and through the craniolateral approach.

Although partially threaded screws or cannulated screws can be used, large cortical screws are often used in a lag fashion because of their large core diameter.

If a partially threaded screw is used, a screw with a large core diameter must be chosen. One must ensure that the threaded portion of the screw fully crosses the fracture plane to only engage the neck/head.

A K-wire is placed just proximal to the intended screw insertion site so it lays at the most proximal level of the fracture surface. Advancement of the wire stops when it reached the fracture surface.

The placement of the K-wire can be performed in a retrograde or normograde fashion.

Retrograde fashion ensures easier positioning of the pins within the femoral neck but requires a more aggressive exposure of the proximal femur to expose the fracture surface. An aiming device is often used to accurately place the implants in a normograde fashion.

The fracture is reduced and secured with pointed reduction forceps. The K-wire is driven into the femoral head without penetrating the articular surface.

The glide hole for the screw is drilled into the femoral metaphysis below the level of the K-wire.

The insertion point for the screw on the femoral shaft is located just distal and slightly cranial to the third trochanter. The implants are oriented to follow the angle of inclination and anteversion of the femoral neck to ensure that the implants remain within the narrow femoral neck.

The drilling of the glide hole can be performed in a retrograde or normograde fashion.

The hole is measured and tapped if necessary. A screw just slightly shorter than the depth of the hole must be chosen to allow compression of the fracture before the tip of the screw contacts the cortex of the femoral head.

Although a partially threaded or a cannulated screw can sometimes be used, large cortical screws are preferred because of their large core diameter.

If a partially threaded screw is used, a screw with a large core diameter must be chosen and one must ensure that the threaded portion of the screw fully crosses the fracture plane and only engages the neck/head to achieve compression.

Read more about lag screw fixation.

Pin is bent and cut short.

Postoperative orthogonal radiographs are taken to assess fixation.

A common mistake is to start the hole for the screw too high on the femur. This high location results in an inappropriate screw angle that cannot follow the axis of the femoral neck and results in penetration of the trochanteric fossa.

Activity restriction is indicated until radiographs indicate bone healing of the fracture.

Aim is to reduce the edema, inflammation and pain.

Integrative medical therapies, anti-inflammatory and analgesic medications.

Note: Nonsteroidal anti-inflammatory medications can be toxic in the cat and should only be used as labeled for the cat.

Aim is to resolve the hematoma, edema and control pain, and prevent muscle contracture.

Anti-inflammatory (see nonsteroidal warning) and analgesic medications may still be needed. Rehabilitation and integrative medical therapies can be used.

Special attention should be given to patients less than 1 year of age with a femoral fracture. Rehabilitation is strongly recommended to help prevent quadriceps muscle contracture.

If the cat is not starting to use the limb within few days after surgery, a careful evaluation is recommended.

10-14 days after surgery the sutures are removed.

Radiographic assessment is performed every 4-8 weeks until bone healing is confirmed.

Implants may cause discomfort of the adjacent soft tissue. If this occurs, implants can be removed after bone healing is observed. In case of infection, implants must be removed after healing.

If there is no implant failure or infection, there is no need for implant removal.