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Cause of Rare Implant Fracture After Revision Surgery

Posted on: 02/16/2011
It doesn't happen very often but sometimes patients who have a total hip replacement end up having a second or revision surgery because of a failed implant. Even more rare is the occasional revision surgery that fails. In this series of six cases, the stem of the femoral implant broke in half. The implant fractures occurred early on (within the first six years). The surgeons in charge of the cases wanted to know why this happened in hopes of preventing such problems in the future.

Previous studies have shown that certain risk factors are linked with stem fracture. The most common of these factors are being overweight, high levels of physical activity, and malposition of the first implant. Implants have improved in design to help counter these problems but there are still a few now and then. And any number of failures (no matter how few) are unacceptable to the surgeon as well as to the patient.

One of the features of the new implant designs is a concept called modularity. These implants are somewhat adustable to offset leg length differences from bone loss. This feature allows for improved joint movement that mimics normal motion more closely than previous less adjustable implants.

The modular units (like a modular home) come with interchangeable parts that can be assembled and adjusted at the time of the surgery. A prosthetic femoral stem component is made up of a body, a neck, and a stem.

The height of the neck is adjustable (high or low) and the stem can be made longer or shorter according to what the patient needs.For patients with bone loss, the newer revision implants don't require cement to hold them in place. Instead, the surgeon can use hardware or fixation devices such as pins, wires, and screws to secure them.

The separate pieces of the modular implant are held together with a modular connection and that connector has a taper junction and an engaged-fit junction. Femoral stem fractures are a problem at these modular junctions. Implant manufacturers have worked with surgeons to find a design strong enough to withstand the forces at the intersection between two component parts.

At the same time, studies like this one attempt to identify all the risk factors for stem fractures. With this information, they may be able to incorporate features in the new designs that will prevent such problems. In order to accomplish this, the surgeons involved in this study took a very close look at fractured stem implants after they were removed.

First they made note of the location of the breaks. Then they used high-powered microscopes to examine the surface of the implants very carefully. They found that the fractures occurred most often just above the body-stem junction (hidden under the main body of the stem). They realized that a force strong enough to bend the stem at that point was contributing to the fractures.

Chemical analysis of the implants was also studied. Tests of "hardness" were conducted near the site of the fractures and compared with similar measurements far away from the fracture site. The strength of the implant material was not a cause of the fractures. Tensile strength was maintained at the fracture site and there was no sign of corrosion or softening of the metal.

It turns out that the biggest risk factor for stem fractures was the patient's body weight. All of the patients in the study who experienced failure of their revision implant were either overweight or obese. The second risk factor was poor bone structure that could not support the junction area of the stem. There was also visible evidence of wear and tear on the implant stem near the junction.

As a separate point of interest, analysis of the data showed that age was not a risk factor. Patients ranged in ages from 37 to 88. A recent history of trauma to account for the fractures was not present either. Many of the patients reported thigh pain as the first symptom of a problem. X-rays, of course, showed the fractures.

Given the results of this study, the authors make several suggestions for ways to prevent the problem in the future. They suggest surgeons who assemble the modular unit before putting it in the patient make sure the component parts fit perfectly. Where the body and stem fit together must be tight without even the smallest of motion between them. It may be necessary to fill in the crack at the junction. Other specific surgical techniques of interest to the surgeon are outlined.

Patients getting a total hip replacement and who suffer more than one implant failure don't have to fear losing hip motion and function. There are always options. New implant designs developed as a result of research results from studies like this one will help prevent future cases of implant failure.

Dror Lakstein, MD, et al. Fracture of Cementless Femoral Stems at the Mid-Stem Junction in Modular Revision Hip Arthroplasty Systems. In The Journal of Bone and Joint Surgery. January 5, 2011. Vol. 93(1):57-65.

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