Long-Term Results of Cementless Hip ReplacementHip joint replacement has entered a new age: the completely cementless implant. Fifteen years ago, surgeons were combining the use of a cemented stem (down into the femur or thigh bone) and cementless cup. That combination has gradually been replaced with all cementless implants. In fact, up to 90 per cent of today's hip replacements are cementless.
In this report, surgeons from a large joint replacement center (Center for Joint Preservation and Replacement at The Rubin Institute for Advanced Orthopedics, Sinai Hospital in Baltimore, Maryland) review the long-term results of six different cementless hip joint implants.
The implants are compared based on their surfaces and coating, geometric designs, and technique of preparation. Other factors that affect implant fixation and stability such as the patient's bone quality, age, activity level, and any deformities present at the time of the replacement procedure.
In order to study and compare the various types of cementless implants available, six separate groups or classifications have been established. By name, these include: 1) single wedge, 2) double wedge, 3) tapered, 4) cylindrical, 5) modular, and 6) anatomic. The tapered implant has three separate types: round, cone, or rectangle (referring to the top of the stem that fits into the round ball that replaces the head of the femur).
Each of the unique design features of these six types has important characteristics and purposes. The surgeon chooses the implant design that is best for each patient on a case-by-case basis. The different shapes allow for different areas of bone-to-implant fixation needed for a stable unit.
The shape of the stem (whether tapered, round, curved, or straight) also influences bone-to-implant fixation by changing the contact points between these two surfaces. For example, tapered stems are wider at the top and narrower at the bottom. Bone fixation is greater at the top where there is more surface to latch onto.
Problems may develop in the future as newer, different designs are developed that don't necessarily fit into one of the current six implant types. The classification scheme may have to be changed over time as newer materials and designs become available.
For now, let's look at what the authors can tell us about the long-term (10 to 15 years or longer) results for these six groups. The overall goal of all implants (no matter what their design) is to make contact with the bone and stabilize the joint.
As it turns out, all six types have equally good rates of survival and success. The average patient will respond well to any of these implants. When there are deformities and/or problems with the bone, then the surgeon may need to select a specific implant based on its design.
For the general population, materials and fixation aren't as important as the geometric design. For example, type one (single wedge) is flat and thin with less surface space for contact with bone. Type three (tapered) can be fluted at the end with multiple slots and edges to make greater contact with bone. Type four (cylindrical) is a fully coated porous surface (bumpy, not smooth) that touches the bone along the entire length of the stem.
The outcomes for these implants also depends on the preparation required before inserting the stem down into the shaft of the femur. The femur has to be reamed out to make a channel for the stem to fit.
The surgeon must understand how the implant geometrical design and stem shape determine surgical preparation of the femur. Preserving as much bone as possible is a key feature of each procedure. For surgeons interested in reviewing the six types of stems, the authors provide an in-depth discussion and comparison of each one to the others.
The bottom-line is patients want a hip replacement that is sturdy, strong, and holds up for many years. Both surgeon and patient want to avoid implant loosening, sinking down into the bone, or breaking. Often, the first sign that something is wrong is thigh pain.
The authors conclude by saying that future studies will need to pay more attention to the bone type of patients when selecting the an implant that will give the best results. Long-term studies are needed to show outcomes based on bone quality (e.g., thick versus thin, osteoporotic versus normal).
Harpal S. Khanuja, MD, et al. Cementless Femoral Fixation in Total Hip Arthroplasty. In The Journal of Bone and Joint Surgery. March 2, 2011. Vol. 93. No. 5. Pp. 500-507.
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