Research topic: Studying the role of implant design in achieving fixation for total hip arthroplasty patients who have sustained proximal bone loss
Research results: Pre-operative guidance for choosing the type of femoral stem that will result in the greatest fixation for patients with proximal bone loss
Patient care application of results: Patients receive implants that achieve sufficient fixation despite proximal bone loss
Simplified patient care application: Knowledge of which type of femoral stem will help THA patients avoid pain, fractures and further surgery
Stemming the Effects of Bone Loss
OREF-funded study may lead to more informed preoperative THA decision making
Jay D. Lenn
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.
There are limited data 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 the fixation of three stem designs using a biomechanical composite bone model.
The one-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 early in their careers who demonstrated a clinical or scientific interest in total joint surgery.
Modeling stem stability
“A number of processes can actually destroy the proximal femur—infection, osteolysis, trauma, previous surgery,” explained Dr. Bengs. “Understanding how much effective bone is left 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 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.”
First, the researchers conducted the load test on three implanted femurs to determine a baseline of stability for each stem design. Then they repeated the load tests after making a series of transverse cuts in the femurs to simulate bone loss—up to six cuts on each specimen, 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 becoming loose. 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. While 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. Dr. Bengs noted, “Even though our study isn’t definitive, it at least suggests we should rethink the paradigm. The biomechanical data we gathered should help surgeons who are doing revisions to plan and utilize the best stem for a given environment.”
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 the organization’s funds are spent appropriately. Funding is awarded to worthy, applicable projects that help us with what we do every day.”