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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Introductory Topics

Below are Introductory Topics for those who are interested in learning more about Total Knee Replacement:

Introduction

Total Knee Replacement FAQ’s (Frequently Asked Questions)

Normal Knee Anatomy

The Diseased Knee: Osteoarthritis

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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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Knee Replacement “Clicking”

Following , patients sometimes report hearing clicks or a clicking sound during certain activities or at a certain point or points during their gait cycle.  In most cases, this sound is believed to be benign and is not associated with pain or other adverse consequences.

So what causes this clicking?

To answer this question, it is helpful to understand a little bit about the nature of total knee replacement.  In knee replacement, the ends of the bones that make up the knee joint are “resurfaced” with metal and plastic components.  The surgeon uses specialized instrumentation and surgical technique to properly align the implants to the bones and the bones to one another.

This alignment is not straightforward because in the replaced knee joint, as in the normal knee, the bones are not “mechanically interconnected”; rather the joint is constrained by the remaining soft tissue (ligaments, muscles/tendons) and the conformity of the implant components.  During the knee replacement operation, the surgeon works to optimize range of motion and joint stability through proper alignment and sizing of the implant.

The resulting tension in the replaced joint can, therefore, vary slightly from patient to patient and can be different within the range of motion of a single patient.  That is, some patients may have “tighter” knees or “looser” knees or a single knee can be “tight” in flexion, but “loose” in extension and vice versa, but in general, some degree of laxity is desirable to allow for adequate motion.

Because there is almost always some laxity in a replaced knee, clicking can sometimes be heard as a result of contact between the metal and plastic components during activity.  Often it is heard during a transition from low or non-weight bearing to weight bearing.  For example, a patient may hear the click while walking as the leg comes out of swing phase and makes contact at heal strike.  For the most part, the clicking is usually not associated with any adverse conditions.

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