Abstract
Failed femoral total hip replacement components are frequently associated with bone loss. Bone loss is most often due to osteolysis or stress shielding. However, it can occur as the result of fracture and infection, as well. Most often, bone loss is proximal and medial, however, it can occur at any anatomic point along the distribution of the femoral stem. At the time of revision surgery, the goals are to create a construct that relieves pain, is stable, and preserves and enhances bone stock.
The internally compromised femur offers an inferior opportunity for bone/cement interlock. Thus, cemented revisions into a deficient cortical tube can be unreliable.
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A distally fixed cobalt chrome prosthesis with extensive porous coating has been advocated as a means of dealing with the structurally deficient femur. This can lead to further bone loss, secondary to stress shielding, and in dramatic instances where a large stem diameter is used, to complete dissolution of the proximal femur (
Figs. 1 and
2). Cortical onlay bone grafts preserve and enhance bone stock when used with a proximal load-bearing cementless prosthesis. Grafts used in this fashion typically unite, revascularize, remodel, and respond to physiologic stresses (
Figs. 3,
4, and
5). Onlay grafts used with distally fixed cobalt chrome stems also unite, but may then disappear because of stress shielding and resorption (
Figs. 6,
7, and
8).
A classification system must be used when presenting data on femoral revision surgery. We use the classification of Mallory, which is quite similar to the AAOS classification.
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The Type III femur with deficient cortices and deficient contents are those that require onlay grafting. The cortical onlay grafts are affixed to the femur with cerclage Luque wires. Morselized autograft/allograft slurry is placed at the interface in order to increase bony contact. The freeze-dried cortical strut is somewhat plastic and conforms to the surface of the host femur, as the wires are progressively tightened. It is necessary to create a small divot into the graft at the site of each wire. This prevents the wire from walking down the graft as the wires are tightened. We prefer wires to cables because they can progressively be tightened in place.
The use of cortical onlay grafts in hip revision is a technique of relatively recent vintage. Cortical plate allografts themselves have been investigated for as long as four decades by Kreutz et al to describe the features of freeze-dried allografts in an experimental setting.
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Grisham reported 85% healing in 80 patients with nonunion of forearm fractures treated with onlay cortical plate allografts. He noted revascularization and replacement of the host bone, using this technique.
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Much of the anticipated and subsequently realized success with this technique in revision hip surgery is based upon the information gained from experience with fractures in a canine model.
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The human grafts are allografts prepared from selected cadaver donors under sterile conditions. The bone is not subjected to treatment with any chemical agents and is freeze-dried using a slow freeze-drying cycle. This method of preparation allows for preservation of Bone Morphogenic Protein, as has been determined by implantation of the tested material into a semipermeable membrane capsule in rodent muscle.
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