Arthritis, Osteoarthritis and Osteoporosis….What’s the Difference anyway?

In this post we will provide some simple definitions for similar sounding terms that are related to the knee joint and overall bone/joint health.

Arthritis

The term simply means an inflammation (-itis) of a joint (arthro-). There are over 100 kinds of arthritis. A joint is an area in the body where two bones meet. In the knee, there are actually three bones which come together. There is the mating of the femur (thigh bone) and tibia (shin bone) forming the tibio-femoral joint and the mating of the femur with the patella (knee cap) forming the patella-femoral joint. Either one or both of these joints can be inflamed and, therefore, be arthritic.

Osteoarthritis

is a specific type of arthritis caused by the breakdown and possibly eventual loss of articular cartilage. Articular cartilage is a special kind of cartilage that covers the ends of the bones where they meet or articulate. In the knee, articular cartilage is found at the mating ends of the femur, tibia and patella. When this cartilage begins to wear down (which can be caused by a number of things such as age and obesity), pain, stiffness, and swelling (arthritis), can occur. This type of arthritis is also called degenerative arthritis, and as the name suggests, usually worsens over time. If it becomes severe enough, Total Knee Replacement may be indicated. For more information, see our section on Osteoarthritis.

Osteoporosis

Unlike arthritis and osteoarthritis, is not related to the joints or joint degradation. It is a change in the actual structure of bone itself. Our bone is a living, dynamic tissue that changes over time. It has a complex structure specifically designed to serve its weight bearing function. Within a long bone like the femur, for example, there are two types of bony structures. Cancellous or spongy bone is found in the interior near the ends of the bones. This type of bone is porous, hence the name. This porous interior is covered by a layer of dense bone known as cortical or compact bone. This bone also forms the hallow shaft of long bones. The picture below shows a cut away view of bone and it various structures.

illu_compact_spongy_bone

In osteoporosis, the major impact on bone is that the porosity of the spongy bone increases. That is, the pores or holes in the “sponge” get bigger. This leads to an overall weakening of the bone and can result in fractures. Common sites for such fractures are the vertebrae, hips and forearm. For more information on Osteoporosis, visit our dedicated section on that topic.

Hopefully these descriptions will clarify some confusion related to these similar terms and conditions.

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Is One Really Better? Fixed vs. Mobile Bearing Knee Replacement

In our recent post “What’s the ‘Best’ Total Knee Replacement?” it was suggested that more often than not, it “may simply be the one which your experienced surgeon has chosen to utilize”. This sentiment was echoed in a recent publication. 1

In the January 2007 Journal of Orthopedic Surgery and Research article, the authors reviewed two major types of total knee replacements: fixed bearing and mobile bearing designs. The theoretical advantages of the mobile bearing design are discussed and the review is broken down into two main sections, a “Biomechanical Review” and a “Clinical Review” where the results of the two types of designs are compared. (See our section on Mobile Bearing Knee Replacement Here, for a brief description of this type of implant).

In the “Biomechanical Review”, laboratory testing of relative wear rates of the plastic components in each type of design is discussed. Also, studies of knee motion comparing mobile and fixed bearings implanted in both cadaver experiments and in actual patients are summarized. Here, the mobile bearing design seems to have less wear and more natural motion.

In the “Clinical Review”, the results of some earlier clinical studies are examined. In these studies, increased wear rate and subsequent osteolysis (see our section on Implant Wear) in the mobile bearing design was observed. At first, this seems to contradict the previously discussed lab studies on wear (where the mobile bearing was superior). However, as the article goes on to explain the increased wear and osteolysis can be attributed to the presence of the additional wear surface on the underside of the mobile bearing as well as to size of the wear particles. Mobile bearings tend to produce smaller particles which actually result in a greater biologic response and osteolysis. Also, the possibility of tibial insert dislocation or “spinout” in mobile bearings is discussed and some observed cases of insert dislocation are shown.

So at this point in the article, the advantage of one type of design over the other is not very clear. The “Clinical Review” continues on with a review of long term clinical results. Unfortunately, that section concludes with the statement: “So far, the theoretical advantages for mobile bearing design to provide long-term durability have not been demonstrated by any outcome study.” Long term performance simply does not help in differentiating the two designs, with both performing similarly. At the end of the day, the study concludes, (as suggested in the previous post) “For the experienced surgeon, one familiar surgical protocol and instrumentation is suggested rather than implant design, either in fixed bearing or mobile bearing.”

1. Huang CH, Liau JJ, Cheng CK: Fixed or Mobile-bearing Total Knee Arthroplasty. J of Ortho Surg and Res 2007, 2:1

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What’s the “Best” Total Knee Replacement?

(TKR) is an operation where the worn ends of the bones that make up the knee joint are resurfaced with metal and plastic implants.  Surgeons have many options to choose from.  Not only do several manufactures make different brands of knee replacements, but within each brand are different types.

For example, in a “PCL sacrificing” knee replacement, the PCL (posterior cruciate ligament) in the knee is removed and its function is replaced by the special geometry of the implant.  There are also a group of implants known as “mobile bearing” or rotating platform” where one of the components (the tibial insert) actually moves or rotates relative to the metal tibial implant affixed to the tibia (shin bone).

Given the myriad of implants available, it is natural to ask, “Which total knee replacement is best?”  Unfortunately, the answer to that question is not straightforward.  While each manufacture will tout the benefits of their particular design, long-term follow-up of most contemporary implants yields similar results.  In reality, it is probably more important for perspective patients to seek out a good, experienced surgeon, rather than a particular product.

This is because factors such as proper alignment of the implant components and restoration of appropriate soft tissue tension (things which the surgeon directly controls) are more likely to influence the outcome of a total knee replacement than the particular brand of implant.

At the end of the day, perspective knee replacement recipients should seek out a doctor they are comfortable with and one that has a lot of experience with a particular implant system.  Experience is important because the technique of total knee replacement is rather involved.   The surgeon must utilize a series of complex instruments to perform the operation.  Often, these instruments are “implant specific” so surgeons will usually become loyal to a single brand allowing them to develop familiarity and expertise with a particular system.

So the answer to the question “what is best” may simply be the one which your experienced surgeon has chosen to utilize.

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New Total Knee Replacements Specially Designed for Female Anatomy

Recently, two major orthopaedic implant manufacturers, (Stryker Corp.  and Zimmer Holdings Inc.) announced the availability of implants specifically designed for women.  We all know that men and women are different, but how does this relate to knee implants?

The key difference is the relative size of the end of the femur or thigh bone.  For a given front-to-back or anterior-posterior (AP) width, the femurs of females tend to be narrower in the medial-lateral (ML) or side-to-side direction compared to males.  An implant which is proportioned based on male anatomy would, therefore, be too wide on a similarly sized female.  The result is that when a surgeon sizes the implant based on the AP width of the femur, the implant could “overhang” the narrower female bone and cause irritation of the surrounding soft-tissue.  Faced with this situation, orthopaedic surgeons are sometimes forced to “downsize” the femoral component during total knee replacement surgery.  In downsizing the femoral component, the doctor chooses a smaller implant, so that the implant does not overhang the sides of the bone.

There are, however, some potential disadvantages to downsizing depending upon the type of instrumentation system the surgeon is using during the knee replacement procedure.  If the surgeon is using an implant system where the femoral component is placed at a particular distance relative to the anterior surface of the femur (anterior referencing), excessive laxity in flexion can occur with downsizing.  Alternatively, if the doctor is using a system which references the posterior aspect of the femur, the resulting femoral preparation could “notch” or undercut the anterior aspect of the femur.  This could, in turn, lead to fracture of the femur.

Recognizing that neither of the above scenarios is desirable, the makers of implants specifically designed for females have sought to remedy this by making the implants more narrow in the ML direction.  On a male knee, the implant could “underhang” leaving some distal femoral bone exposed, but this is generally not a problem.

In the company announcements, both manufacturers noted that their new “Female” knee replacements are designed to address the more narrow female femur.  Zimmer also noted that differences extend beyond femur width to things such as the angle of the femur in relation to the tibia.  It will be interesting to see how these new implants fair clinically and whether the potential advantages are realized.

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