Case #06 Discussion:

Identification & Indications for Exam:

Patient is a 69 year old white female who is status post left total hip arthroplasty. The patient had a left total hip replacement due to progressive joint disease. The patient had follow-up X-rays on two months later to assess the status of the total hip replacement and to check for complications associated with the placement of artificial joints.

Background information on hip replacements: Types (Two main types):

I. Hemi-arthroplasties of the hip (Used for disease of the proximal femur in which the native acetabulum is intact):

A. Two types (Simple femoral prosthesis and bipolar femoral prosthesis):

1. A Simple femoral prosthesis consists of a head that articulates with the native acetabulum and a stem that is fitted into the medullary canal of the femur.

2. A Bipolar femoral prosthesis consists of a femoral component compromised of a stem and a head, and an acetabular component compromised of a metal socket with a polyethylene liner. The metal socket articulates with the patient's acetabulum, and the head of the femoral component articulates with the polyethylene liner. Due to this design most of the motion takes place between the head and liner thus preserving the native acetabular cartilage. Of note, a bipolar prosthesis may be converted to a THR with interchangeable components.

II. Total hip replacement. This type of artificial joint has both femoral and acetabular components that replace the native articular surfaces. The prosthesis has a metal acetabular component with a polyethylene liner that forms the articular surface.. The components can be fixed to the bone with or without cement. The cement used is PMMA (polymethylmethacrylate), and it is made radiopaque by the addition of barium sulfate.

Complications of joint replacements and their associated radiographic findings:

• Loosening of prosthetic components:

• Widening of the lucent zone at the cement-to bone or metal-to-bone interfaces greater than 2 mm.
• Migration of components from their original position.
• Cement fracture
• Periosteal reactive bone

• Osteolysis
• Dislocation: Trauma to limb can cause dislocation.
• Fracture: Fractures usually occur at a bone-metal interface that acts as a stress riser.
• Infection: Often requires arthrocentesis for diagnosis. If infection is still suspected and cultures from the arthrocentesis are negative, then an open synovial biopsy may be necessary. If the joint becomes infected, the prosthesis is usually removed.

The X-rays for this case demonstrate a normal total hip replacement with no radiographic evidence suggestive of complications.