Harpal S. Khanuja, MD, et al.
In The Journal of Bone and Joint Surgery. March 2, 2011. Vol. 93. No. 5. Pp. 500-507.
In this report, surgeons reviewed 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.
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 stabilise 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 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).