A noncemented femoral stem in total hip arthroplasty (THA) needs to achieve fixation immediately after implant to enable bone ingrowth. But proximal bone loss due to traumatic injury, disease, or previous surgery may prevent bone-implant integration and lead to implant failure.

Limited data exist on how much bone loss precludes noncemented arthroplasty or how well different stem designs function with varying degrees of bone loss. Benjamin C. Bengs, MD, assistant professor at the University of California, Los Angeles, received a 2010 Orthopaedic Research and Education Foundation (OREF) Career Development Grant in Total Joint and Trauma Surgery to study in vitro fixation of three stem designs using a biomechanical composite bone model.

The 1-year grant was one of six awarded in 2010, made possible by support from Zimmer, Inc. Each grant provided up to $50,000 to advance the training of orthopaedists who demonstrated a clinical or scientific interest in total joint surgery early in their careers.

Modeling stem stability
“Several processes can destroy the proximal femur—including infection, osteolysis, trauma, and previous surgery,” explained Dr. Bengs. “Understanding how much effective bone remains is essential in selecting a femoral stem that will result in sufficient fixation. With our research, we were hoping to guide clinicians to using the proper stem.”

Dr. Bengs and his colleagues used composite femurs—synthetic femurs simulating the biomechanical properties of human bone—to avoid the normal variability that would result with cadaveric specimens. They tested the following three common femoral stem implants, each based on a different method of promoting fixation within the femoral canal:

• Proximal, calcar-based fixation (near the top of the femoral shaft)
• Dual taper wedge fixation (metadiaphyseal)
• Fully porous-coated fixation

Each type of stem was implanted in six femurs for a total of 18 implantations. After implantation, the researchers tested fixation with a biomechanical simulator that applied a combined dynamic axial and torsional load. “We stressed the stems just like a human would do when walking,” noted Dr. Bengs. “We had sensors on the stems that could detect motion.”

For each stem design, the researchers conducted a load test on three implanted femurs to determine a baseline of stability. Then they made a series of transverse cuts in the femurs to simulate bone loss—up to six cuts on each specimen—and repeated the load tests, comparing failure thresholds. The team defined failure as either a fracture or motion at the point of contact between the bone and implant that exceeded 200 micrometers per cycle of load. “Motion is bad. Motion means that the stem is loosening. So we were looking at the conditions under which each of these stems starts to lose fixation,” explained Dr. Bengs.

The investigators altered the three remaining femoral specimens in each set to simulate the removal of a dynamic hip screw from previous surgery, a common bone loss scenario for THA. After implanting the femoral stems, they repeated the same biomechanical load tests for assessing fixation.

Translating data into stem selection
Dr. Bengs reported, “We observed that the dual tapered stem designed for metadiaphyseal fixation remained well-fixed even with severe bone loss. We were surprised because we had assumed that a fully porous-coated stem, the gold standard for revision surgery, would perform best.”

He cautioned, however, that the research only tested initial stem fixation and not the long-term stability of the implant. Although initial stem fixation is thought to be the essential factor for ingrowth and subsequent biologic stability, an in vitro study with composite bone could not address questions about implant-bone integration.

“Even though our study isn’t definitive, it at least suggests that we, as orthopaedic surgeons, should rethink the paradigm,” noted Dr. Bengs. “The biomechanical data we gathered should help surgeons who are performing revision THA surgeries to plan and select the best stem for a given environment.”

Supporting research
Dr. Bengs stated that he would not have been able to pursue this research project at all without the OREF Career Development Grant. “Funding is drying up to a large degree, and the competition for resources is very difficult. The logistics of setting up, funding, and sustaining a research program while maintaining a clinical career is an incredible challenge.”

He believes that OREF’s strength is derived from the input of the orthopaedic community itself. “The review process for grant applications enables orthopaedic surgeons to ensure that the organization’s funds are spent appropriately. Funding is awarded to worthy, applicable projects that help us with what we do every day.”

Jay D. Lenn is a contributing writer for OREF and can be reached at communications@oref.org