Authors of section


Denis Marcellin-Little

Executive Editor

Amy Kapatkin

General Editor

Noel Moens

Open all credits

Lag screws

1. General considerations

The patella fracture is under tension and distracted apart by the pull of the quadriceps muscle group. Acute patella fractures can be reduced. Chronic patella fractures may not be reducible.
The fixation method must resist the large tension forces constantly acting on the patella.
Lag screws use is limited to large avulsed fragments.
Lag screws fixation of the patella

2. Preparation and approach

The patient is positioned in dorsal recumbency. The pelvic limb is extended.

Dog in dorsal recumbency
A sagittal or parasagittal skin incision is made over the patella.
Sagittal or parasagittal skin incision

A lateral parapatellar or medial parapatellar approach is performed along the patellar ligament and patella proximally to the quadriceps muscle.

An arthroscopic assisted minimally invasive approach can be used for some fractures.

Lateral parapatellar dissection

3. Reduction

The proximal fragment migrates proximally under the tension of the quadriceps muscles. It can be repositioned distally by extending the leg and by using pointed reduction forceps.

Reduction of patella fragments
Non-reconstructible avulsed fragments are removed using forceps.
Non-reconstructible fragments removal

The stifle is fully extended, and traction is applied to the proximal fragment to reduce it.

Stifle extension and reduction

The reconstructible fragments are anatomically reduced and stabilized using small pointed reduction forceps.

Note: Attention should be paid to preserve the patellar ligament fibers.
Reduction of the first two fragments

4. Fixation

The reduction forceps should be removed to allow distraction of the fracture site. This prevents the glide hole from being accidentally extended into the opposite fragment.

A glide hole is drilled medially or laterally in the smaller fragment, cranial to the articular cartilage. A power drill is used at low speed (less than 300 rpm), using irrigation to minimize heat generation.

The hole should be perpendicular to the fracture line, avoiding the articular surface.

The hole is drilled away from the other fracture lines
The patella is reduced again, and a thread hole is drilled through the far patellar fragment using the appropriate drill sleeve.
A thread hole is drilled along the remainder of the patella
A depth gauge is used to determine the screw length. The far cortex is tapped.
Depth gauge for screw length
A screw is placed and tightened, achieving interfragmentary compression.
Screw placement
The procedure is repeated on the opposite side of the patella. The second glide hole is drilled parallel and to the same depth as the first one. The thread hole preparation and screw insertion are identical to the first screw. 
In case of a transverse fracture, screw diameter should be approximately 25% of the width of the patella. For example, a 3.5 mm screw may be used in a German shepherd.
In small dogs and in cats, a K-wire and tension band procedure may be a more appropriate choice of fixation.
Lag screws

5. Closure

The stifle joint is sutured using monofilament absorbable suture material. The wound is closed in layers.
The stifle joint is sutured

6. Aftercare

General considerations

Limb disuse after the repair of a patellar fracture is unusual. Most dogs start weight bearing within 3 days of surgery.

The focus of physical rehabilitation is to:

  • Avoid loss of stifle joint motion, particularly loss of extension
  • Avoid excessive stress on the surgical repair during the healing period
  • Promote low-impact weight bearing pelvic limb exercise

The stifle joint is protected and partially immobilized if deemed necessary for approximately 14 days.

Cryotherapy and passive range of motion are indicated but not possible if immobilization is used.

Activity is limited to leash walks until bone healing is radiographically evident.

Phase 1: 1-3 days after surgery

The aim is to reduce the edema, inflammation and pain using cryotherapy anti-inflammatory and analgesic medications.

Gentle passive range of motion of the hip and stifle joint can be initiated to promote mobility and joint health.

Phase 2: 4-14 days after surgery

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

Anti-inflammatory and analgesic medications may still be needed.

If the dog is not starting to use the limb within a few days after surgery, re-evaluation is recommended.

Phase 3: 14 days-8 weeks after surgery

Rehabilitation therapy is continued.

10-14 days after surgery the sutures are removed.

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

Implant removal

If the implants interfere with soft tissues, they should be removed after the fracture is healed.