Modular parts are interchangeable pieces that go together to make up one structure. Both the femoral and the acetabular components of a total hip arthroplasty may be modular.
The original total hip components were monoblock. Each component was one single piece and a surgeon had relatively few choices. The addition of modularity gives the surgeon more options and allows him to best match the components to the patient’s individual anatomy. A surgeon may use modular components to help him solve specific problems such as leg length discrepancy or an unusual angle (version) of the femur.
Modular components can be used to provide a better fit and to create greater stability in the hip replacement. The option of modularity gives the surgeon greater flexibility. He can fine tune the arthroplasty or replacement to meet the specific needs of the patient. Some of the features that can be addressed by modularity are:
Size of the femoral head (affects both the head and the acetabular liner)
Length of the femoral neck
Offset of the femoral neck
Width of the proximal or metaphyseal part of the femoral component
Fit of the femoral stem
Length of the femoral stem
Shape of the femoral stem—straight or bowed
Shape of the liner of the socket—elevated rim or lateralized
Angle or the version of the femoral neck
Most acetabular components have two modular parts, the outer metal shell and the inner polyethylene liner.
Femoral components also have two parts, the femoral head and the stem of the component. Some femoral components, which are mostly used for revision cases, have three modular parts.
In these components, the middle section below the head, called the body or metaphysis, is separate from the femoral stem. The middle component comes in several shapes and sizes that can be used to make up for bone loss or bone height in a revision case.
Three piece modular femoral components can be a handy tool for a surgeon in a situation where bone loss makes it hard to fit normal components. The weakest points of these components are the junctions between the modular parts.
Modularity means that most setups for a routine total hip replacement may have over one thousand possible combinations of head, neck, acetabular, and femoral stem sizes. The surgeon can make the best possible choices for fit and fixation of all components.