Constrained cup implantation

It is important to achieve appropriate fracture reduction to restore length, alignment, and rotation and fixation of the femur around the implant. 

Revision of the femoral component can require polyethylene exchange or revision of the bearing surfaces to enhance hip stability and improve longevity of the prosthesis.

The goal is to achieve:

• Secure seating and engagement of the new polyethylene bearing into the retained acetabular cup
• Secure engagement of new femoral head on previous femoral prosthesis

The fracture fixation should be addressed first to reconstruct a stable femur.

Revision of the femoral head and polyethylene is then performed to reconstruct the hip joint.

The surgeon should use the surgical approach that is the most familiar to him/her for any total hip arthroplasty.

Extensile approach is necessary to access the fracture site for exposure, reduction, and placement of fixation devices, such as cerclage wires, cables, strut grafts, or plates for fracture fixation.

The surgeon should assess the position and the fixation stability of the acetabular cup.

The accepted "safe zone" is:

• cup inclination 40° to 55° (a)
• cup anteversion 20° to 40° (b)

A constrained system limits range of motion and risks implant impingement. Use trial components to confirm the most appropriate position of the cup and the new liner.

If the femoral stem is not revised, it may be necessary to exchange the head, in order to couple to the acetabular liner, and to reduce the risk of dislocation after surgery.

The hip is dislocated.

Bone tamp, and mallet are used to disengage the femoral head from the Morse taper of the femoral prosthesis.

Following removal of the head, the surgeon can place the acetabular retractors to retract the femur with the femoral prosthesis and to retract the surrounding soft tissues.

Once exposed, debride the scars and the soft tissues covering the polyethylene/acetabular component interface.

There are two commonly used techniques for liner removal:

• Use of an osteotome at the liner-cup junction to lever and to remove the liner

• Use a threaded extraction tool or cancellous screw to push out the liner

The constrained liner is selected when there is irreparable, and/or incompetent abductor mechanism. Moreover, the constrained liner should be considered if the patient has high risks for hip dislocation.

The new polyethylene liner must have the correct locking mechanism and geometry for the acetabular cup.

The new liner is inserted into the cup using implant system specific instruments.

If there is no appropriate liner or the locking mechanism is disrupted, a new constrained liner can be cemented into the old acetabular cup.

Trial components can be used to determine the most appropriate size for retained acetabular cup.

The backside of the new liner should be scored to allow for proper surface texture for cement fixation.

Cement mixing is done following standard protocol for routine hip and knee arthroplasty.

Cement is applied to both the liner and the cup.

The liner is inserted into the cup and held in place until the cement hardens.

Following completion of the liner exchange, the surgeon should do a trial reduction.

With the hip reduced, confirm the range of motion and hip stability. Adjust the neck and head if necessary.
Once satisfactory, attach the definitive femoral head to the stem, and reduce the hip. Confirm complete reduction, stability, and range of motion again.

If the femoral stem is not being revised, it is critical to identify the manufacturer to ensure use matching femoral head taper. A failure to do so will lead to inability to complete the procedure.