Is Knee Replacement in Your Future?

Most knee replacements are performed for the treatment of where the smooth cartilage in the knee joint breaks down resulting in pain, stiffness and swelling. The x-ray below is of a knee where osteoarthritis is present.  Note the close proximity of the upper (femur) and lower (tibia) bones.  In this knee the cartilage, which is not visible on an x-ray has worn away.  If this had been a healthy knee, there would be a distinct gap between the two bones.

intro-re-write-21-jan-09_html_2834b341

Osteoarthritis is usually progressive.  It often begins mildly, but over time, wear in the cartilage starts to result in an uneven distribution of weight across the knee joint; often with more damage observed on one side versus the other.  This in turn causes even more force to pass through the affected side, resulting in even faster degeneration of the joint.

This localized damage is most commonly found on the inside or “medial” part of the knee (), but it can also occur on the “lateral” or outside part of the knee (lateral compartment).  When it occurs on the medial side, the patient may have a varus or “bow-legged” deformity.  When lateral osteoarthritis is present, a distinct “knock-kneed” or valgus deformity is observed.

These deformities are usually corrected at the time of .

It is interesting to consider what, if anything, may initiate this cycle of uneven wear, worsening load distribution, and further wear.  Is it possible that some folks may be predisposed to this condition due to their bony anatomy?  A presentation at this years AAOS meeting looked to answer this very question. 1 The researchers examined the anatomy in patients receiving knee replacements whose pre-operative varus and valgus deformities were significant.  In those patients, they found that key functional axes in the knee were not aligned normally.  Specifically, they found that the alignment of the “mechanical axis” of the lower limb to the in the knee was not normal. To understand these axes and their observed relationship, a review of basic knee anatomy is required.

In the normal lower limb, the mechanical axis is defined as an imaginary line originating at the center of the femoral head (the “ball” on the “ball-and-socket hip joint) passing through the center of the knee and ending at the center of the ankle.  When the hip, knee and ankle line up in this way, the lower limb and knee joint alignment are considered “normal”.  In a varus or valgus knee, the center of the knee is no longer on this line and is pushed “outward” in varus and “inward”  in valgus.

The epicondylar axis is an imaginary line connecting the femoral (thigh-bone) origins of the medial collateral and lateral collateral ligaments (MCL and LCL).  One can think of these ligaments as the “ropes” or “chains holding a swing.  In this analogy, the epicondylar axis is like the top bar of the swing.  The tibia or thigh bone is the swing which rotates about the bar (epicondylar axis) during knee flexion and extension.

is-knee-replacement-in-your-future-23-mar-09_html_m53f606bc

In their study, the authors observed that in patients with severe varus or valgus deformities, the epicondylar axis was “misaligned” relative to the mechanical axis in the same direction (albeit smaller in magnitude) as in the knee joint deformity.  For example, if a patient had severe osteoarthritis in the medial or inside part of the knee, a significant bow-legged or varus deformity was observed and the epicondylar axis was “tilted” in the same direction.  In other words, the inherent bony anatomy appeared to predispose certain patients to uneven loading across the knee joint (a “crooked swing”), which, in turn appears to have made them more susceptible to the resulting osteoarthritic deformity.  The results were not statistically significant, but a noticeable trend was observed.  So to answer the question “is knee replacement in your future?” one may not have to look much further than one’s own bony anatomy.

1. Beyers-Thering MT, Krackow KA, Mihalko WM. “Relationship of the Femoral Epicondylar Axis to the Mechanical Axis in Deformed Osteoarthritic Knees.” 2009 AAOS Annual Meeting, Poster No. P201

Posted by admin | Filed Under What's New? | 2 Comments 

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.

Posted by admin | Filed Under What's New? | 2 Comments 

Knee Replacement FAQ

Q. What, exactly is “replaced” in knee replacement?

A. The worn ends of the femur ( thigh bone), tibia (shin) and patella (knee cap) are removed with a saw and replaced with metal and plastic. More on Knee Replacement Surgery

Q. What, is a Knee Replacement Implant made of?

A. Implants are typically made of a combination of Cobalt Chrome and/or Titanium metals and UHMWPE (ultra- high-molecular-weight polyethylene) plastic. More on Knee Implants.

Q. Are there different kinds of implants?

A. Yes! There are many types of implants that fit into broad categories related to their interaction with the remaining natural soft tissue. For example a CR or Cruciate Retaining knee implant is designed to work with the PCL (posterior cruciate ligament) intact. Other types of implants are designed to work without it. In addition, there are many manufacturers of implants which create several brands of knees in each of the broadcategories. Surgeons have many, many choices. More on Types of Knee Implants. Links to Implant Manufacturers

Q. How long will my Knee Replacement last?

A. There are many studies on this topic and the actual time an implant will survive before needing revision will vary on a case by case basis. Several studies demonstrate greater than 90% survivorship at 15 years (1)

Q. How are the implants held in place?

A. implants are secured to the bone either with a special polymeric compound (“bone cement”) or are “press-fit” into place. Press fit implants have rough or porous under surfaces into which bone will grow. More on Knee Implants.

Q. What is a Uni (or Partial or Half-knee) Replacement?

A. A “Uni” or unicompartmental knee replacement replaces only the medial (inner) or lateral (outer) half of the knee joint (the tibio-femoral joint). The knee cap or patella is not replaced. More on Unicompartmental Knee Replacement

Q. What is MIS or Minimally Invasive Knee Replacement?

A. MIS or Minimally Invasive Knee Replacement is like standard knee replacement except that the incision is smaller, and care is taken not to resect the quadriceps muscle (also known as “quad sparing”). Also, the patella is not “everted” or “flipped” over This is all intended to result in less trauma and speed recovery. More on Minimally Invasive Knee Replacement.

Q. What about recovery, pain etc?

A. Every case is unique and this discussion is best left for a medical professional. You can read about the experience of other recipients at a very interesting discussion forum at Robin’s Total Knee Replacement.

References 1) Dixon MC, Brown RR, Parsch D, Scott RD. Modular fixed-bearing  total knee arthroplasty with retention of the posterior cruciate  ligament. A study of patients followed for a minimum of fifteen years. J Bone Joint Surg Am. 2005 Mar;87(3):598-603

Posted by admin | Filed Under Uncategorized | Leave a Comment 

Normal Knee Anatomy

  • The is the mating and movement of three bones; the femur or thigh bone, the tibia or shin bone and the patella or knee-cap. The end of the femur rides on the top surface of the tibia and the patella moves within a groove on the femur.
  • The bones are joined together by ligaments and tendons. These soft-tissue structures guide the movement of the bones and provide the stability needed for normal knee motion. Unlike the ball and socket hip joint, the bones are not interconnected; it is the soft-tissue around the bones which holds the joint in place. This is why one of the key factors in a successful total knee replacement is surgical management and later rehabilitation of the remaining soft-tissue (i.e. muscles, ligaments and tendons).

  • In a normal knee, the tendons attach the muscles to the bones and the ligaments attach the bones to one another. During the knee replacement operation, one or more of the ligaments is removed. Often the normal function of one or more of the ligaments is severely compromised due to the deterioration in the knee joint. One of the main jobs of the knee implants is to recreate the normal function of these removed ligaments.
Knee Joint Anatomy
  • Between the femur and tibia is a soft-tissue pad known as the meniscus. This pad acts as a “shock absorber” helping to evenly distribute the loads or forces between the bones of the knee. The meniscus also provides additional stability during motion.
  • The link below provides a more in-depth review of basic knee anatomy: http://www.kneepaininfo.com/kneeanatomy.html

Posted by admin | Filed Under Uncategorized | Leave a Comment