Mobile Bearing Knee Replacement Offers no Benefits vs. Fixed Bearing

In a previous posting 1, the question was raised as to which is better, fixed or mobile bearing . In the end, the answer was not very clear. But in the view of one implant manufacture, the choice is simple: fixed bearing.

In a recent press release, a major U.S. Orthopedic implant company (Stryker Corporation) has decided not to offer their mobile bearing knee replacement in the U.S. market. Although the company has executed a clinical trial in an effort to gain FDA approval for the device, data gathered during the study has demonstrated no clinical advantages for the mobile bearing knee compared to a comparable fixed bearing design.

The company also cited higher costs and higher revision rates for mobile bearings as further reasons why the mobile bearing design will not be pursued in the U.S. market.

The original press release can be found here:
http://www.prnewswire.com/news-releases/us-ide-study-indicates-mobile-bearing-knees-offer-no-clinical-advantages-over-fixed-bearing-knees-86866577.html

More on Stryker can be found here:
http://www.stryker.com/en-us/products/Orthopaedics/KneeReplacement/index.htm

1 Is One Really Better? Fixed vs. Mobile Bearing Knee replacement

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Press-Fit Total Knee Replacement: Working to Prevent Loosening and Bone Loss

In , the worn ends of the bones that make up the joint are replaced with metal and plastic implants. These implants are secured to the bone either with “bone cement” which the surgeon applies to the implant and bony surfaces or they are simply “press-fit” into place. In the press-fit scenario, the surface of the implant which contacts the bone is typically coated with a porous structure. This structure is designed to mimic the porous or sponge-like structure of the prepared bone in order to promote bony in-growth into the implant coating.

In addition to utilizing a bone-like porous structure for press-fit implant coatings, it is desirable that the “flexibility” or “elasticity” of the implant material also approach that of bone. This is because bone, which is a dynamic living tissue, actually responds and adjusts to the stresses placed upon it. If an implant is too stiff relative to the bone, then over time, the bone will become less and less dense, literally disappearing under the implant due to what the body perceives as lack of use (the old “use it or loose it” adage). This is known as “stress-shielding.” The resulting bone loss may eventually result in loosening of the implant.

It is possible that such bone loss and resulting implant failure can be avoided by using materials and coatings which better approximate the properties of real bone. This is because as the implant material properties approach that of normal bone, loads are better transferred through the implant to the bone, thus allowing the bone to perceive more normal use and encourage the continued maintenance of bone under the implant.

In a recent article in the Journal of Bone and Joint Surgery (American) Minoda et. al1. looked at how bone in the tibia (shin bone) responded to a particular type of press-fit implant. The implant utilized a tantalum porous coating. The property of the bone that was measured to gage how it responded to the implant was bone mineral density (BMD). In the study, a group of implanted knees (28) receiving the press-fit implant was followed and compared over a period of two years, two a group of knees (28) with a cemented implant. BMD scans taken were taken at various intervals.

The researchers found that BMD decreased in the tibia in both groups. However, in the press-fit (porous tantalum) group, the decrease was only 6.7% +/- 22.9% vs. 36.8% +/- 24.2% in the cemented group. So it appears that this particular porous coated implant may have been able to allow more natural loading of the tibia, thereby reducing bone loss compared to the cemented implant. Only time will tell if these apparent benefits continue long-term.

References:
1. Yukihide M, Kobayashi A, Iwaki H, Ikebuchi M,I nori F, Takaoka K. Comparison of Bone Mineral Density Between Porous Tantalum and Cemented Tibial Total Knee Arthroplasty Components. Journal of Bone and Joint Surgery (American). 2010;92:700-706

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How Are the Ligaments Affected in Total Knee Replacement?

A common question regarding (also known as total knee arthroplasty) is what happens to the ligaments in the knee as a result of knee replacement?

First, it is helpful to understand the location and function of these important soft-tissue structures.  There are four major ligaments in the knee joint.  Two of these ligaments act as “side-to-side” stabilizers in the knee:  the medial collateral ligament (MCL) and the lateral collateral ligament (LCL).  These ligaments connect the femur (thigh-bone) to the tibia (shin-bone).  They are located on the inside (medial) and outside (lateral) parts of the knee.

The other two ligaments, the anterior cruciate ligament and the posterior cruciate ligament (ACL and PCL) limit “front-to-back” or anterior-to-posterior movement of the knee.  These ligaments are named based on their attachment site on the tibia and are found roughly in the middle of the joint.  The PCL runs from the back or posterior part of the tibia to the front of the femur.  The ACL runs in the opposite way.  These ligament cross, hence the name “cruciate”.

So what happens to these ligaments as a result of total knee replacement?   The collateral ligaments (ACL and MCL) are left intact as they are important for proper function and longevity of a total knee replacement since the function of these ligaments is, for the most part, not replaced by the prosthetic components.

The ACL is removed since the portions of the bone where it typically attaches are removed and replaced by the implant components.  Often this structure is severly compromised in patients requiring total knee replacement.

What is done with the PCL is not as straightforward.  Depending on the particular patient as well as surgeon preference, the PCL may either be retained or sacrificed. Implants specific to either scenario are available for the surgeon’s use.

For more on types of implants see our section on this topic.

So in summary, in a standard total knee replacement..

  • The collateral ligaments (LCL and MCL) are preserved
  • The ACL is removed
  • The PCL is either retained or sacrificed.

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Manipulation Following Total Knee Replacement Surgery

is generally very safe and effective, but it is a major operation.  For unknown reasons, scar tissue formation within the knee joint may be excessive in some patients (a condition known as athrofibrosis).  This can result in joint pain and stiffness and reduced range of motion (ROM).

This stiffness is sometimes addressed by a forcible, closed manipulation of the knee joint by an orthopedic surgeon.  In a recent study in the journal Orthopedics, the authors looked at the effectiveness of manipulation and sought to determine if any patient factors could be used to predict range of motion improvements following this procedure.

In this study the authors describe how manipulation following knee replacement is performed.  Patients are given either epidural or general anesthesia.  An assistant holds the heel while the surgeon gently extends or straightens the leg.

While straitening the leg, the surgeon applies pressure to the anterior (front) of the knee joint.  The knee is then flexed or bent with the assistant holding the thigh while the surgeon applies pressure with his chest on the anterior of the tibial (shin) area to, as they say “allow slow, steady stretching and tearing of the intra-articular scar tissue” (“intra-articular” means within the joint).

This process is repeated until full Range of motion is achieved or until the surgeon “no longer feels creptiation” (crepitation is the dry, crackling sound or sensation resulting from rubbing bones or irregular cartilage surfaces together as in arthritis).

Some interesting findings and observations from this study:

  • The surgeons looked at a consecutive series of 767 patients.  Forty-six required manipulation.  So this condition was observed in 6% of patients.
  • On average, patient’s ROM increased significantly after manipulation.  Patients were able to bend or flex their knee 22 degrees further, and extend or straighten their knee 4 degrees more.
  • The flexion achieved after manipulation was similar to that which the patient had before having knee replacement.  (Pre-operative ROM is often a predictor of post-op ROM).
  • Several patient factors were not associated with the effectiveness of manipulation.  These included patient sex, BMI and alignment of the knee joint before surgery (i.e. the amount of knock-kneed (valgus) or bow-legged (varus) deformity).
  • The authors used several different types of implants.  Implant design or type did not influence the effectiveness of manipulation.
  • Patients manipulated less that 8 weeks after surgery gained more flexion than those manipulated after 8 weeks following their knee replacement.
  • The patients were followed for 1 year, and those whom had full extension before manipulation wound up with more flexion at 1 year post-op.
  • To avoid the need for manipulation (and presumably the formation of excessive, debilitating scar tissue), the authors suggest aggressive pain management for the first few days after surgery followed by ambulation and some exercise.

This study can be found in the June 2009 issue of Orthopedics.

References
1.Cates HE, Schmidt, JM.  Closed Manipulation after Total Knee Arthroplasty: Outcome and Affecting Variables.  Orthopedics. 2009; 32:398.
2.Arthritis of the Knee Joint. Complications of Knee Replacement Surgery.  http://www.hipsandknees.com/knee/kneesurgerycomplications.htm#scar. 28-Feb-10

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Beer….Tastes Great, Less Filling and can Treat Osteoporosis?

A commonly occurring element, silicon, is present in relatively high quantities in beer. It is also a necessary ingredient for proper bone density. Researchers at the Department of Food Science & Technology at the University of California, Davis have proposed that drinking beer is an excellent source of dietary silicon. So next time your grab that cold one, think of it as a refreshing bone builder….

More from Science Daily

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Osteoporosis Cure on the Way?

Most osteoporosis treatments are aimed at slowing down bone loss, but a new investigational study in mice looked at a drug which could actually increase bone mass.

Serotonin is most often associated with the brain.  However, it is also synthesized in the gut.  Previous studies revealed that serotonin in the gut, worked to limit bone formation.  Researches theorized that reducing serotonin in the gut could increase the body’s ability to form bone thereby combating osteoporosis.  An existing compound, LP533401, had already been developed for another indication and was effective in reducing serotonin in the gut.  When the compound was tested in mice, it was found to increase bone formation.

More on this very exciting study can be found here.

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More “Joint-Soothing” Exercise

If your sore joints are getting you down and limiting your ability to exercise or even perform simple activities of daily living, then some “low-key” martial arts may be your answer.  A recently published study in Arthritis Care & Research found that Tai Chi was effective in reducing in a group of elderly participants.  Compared to a control group, the study group, who participated in one-hour classes two times a week for three months, reported a significant decrease in knee pain at the end of the study period.  So delay that knee replacement surgery and go Tai Chi!

Follow this link for more on this story…

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Wait…So Running Isn’t Bad for My Knees?

In another “counterintuitive conclusion” researchers from some recently published studies are not finding a clear correlation between running and bad knees.  In fact, runners may actually be at lower risk for developing compared to non-runners.  Go figure.  Then go running….

Read more About This Story Here

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Weight Gain After Knee Replacement?

A recent study out of the University at Delaware found that patients actually gained weight following knee replacement surgery. Conventional wisdom was that after suffering years of debilitating pain, formerly sedentary arthritis sufferers would resume activity and exercise following their knee replacement. However, researchers found that patients actually gained an average of 14 pounds after a two year follow up period…

Follow this link for more information on this topic

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Patellar Clunk Syndrome: Complication Following Knee Replacement Surgery

In general, (TKR) is a highly successful procedure. However, complications, although relatively rare, can sometimes occur. One such complication is known as .

To understand this condition, it helps to first understand a little bit about knee anatomy and function (Figure 1). During knee flexion and extension, the patella (knee-cap) rides within a groove in the femur (thigh bone).

Knee Anatomy

Figure 1: Basic Knee Anatomy

In knee replacement the worn ends of the bones are “resurfaced” with metal and plastic implants. The patellar groove is part of the metal femoral component that is implanted onto the prepared femur during the total knee replacement procedure. At the end of this groove there is a transition between the metallic implant and the native bony surface.

Following knee replacement, scar tissue can sometimes form at the top or “superior pole” of the patella. During particularly deep flexion, this scar tissue may move below the end of the groove in the femoral component and then “catch” on the end of the groove as the patella moves back with knee extension. It is this catching and then forceful release with extension that results in the “clunk” and pain characteristic of this condition.

This condition is more prevalent in a type of knee replacement known as Posterior Stabilized, or “PS” knee replacement, although it has also been reported in Cruciate Retaining (CR) knee designs as well. 1 In PS knee designs, where the posterior cruciate ligament is removed, the patellar groove tends to be shorter to avoid contact (in extension) between the end of the groove and a plastic post on the tibial component found in these types of designs. Consequently, the patella can come off the end of the groove during lesser degrees of flexion.

The publication cited at the end of this article provides more detailed information and a review of some of the literature on patellar clunk.

1 Niikura T, Tsumura N, Tsujimoto K, Yoshiya S, Jurosaka M, Shiba R. Patellar Clunk Syndrome After TKA with Cruciate Retaining Design: A Report of Two Cases. Orthopedics. 2008: 31:90

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