Press-Fit Total Knee Replacement: Working to Prevent Loosening and Bone Loss
In 
total knee replacement, 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
Introductory Topics
Below are Introductory Topics for those who are interested in learning more about Total Knee Replacement:
Total Knee Replacement FAQ’s (Frequently Asked Questions)
The Diseased Knee: Osteoarthritis
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 
osteoarthritis compared to non-runners. Go figure. Then go running….
Read more About This Story Here
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
Knee Replacement Complications
Total Knee Replacement (also known as total knee arthroplasty) is generally a highly successful procedure with excellent long-term results. In a recent study, 97% of patients remained “revision-free” at 10 years 1. Despite these favorable outcomes, complications may still occur. Listed below are some of the more common complications of total knee replacement surgery.
Infection
Several steps are taken to prevent infection. For example, antibiotics are often given before, during and after knee replacement surgery. Still, a small percentage of knee replacements become infected. In a recent study of over 3000 replacements, 2.9% experienced “superficial” infections while 0.8% of infections occurred within the joint. An increased rate of infection was observed in patients who experienced complex surgery or who had certain characteristics such as obesity or poor pre-operative health. 2
Loosening
Knee replacement components are affixed to the bone in two ways. In “press-fit” implants, long term fixation relies on the growth of bone into what is typically a porous surface or coating on the underside of the implant. Alternatively, the implants may be “cemented” into place with a strong polymer (PMMA) which is mixed and applied at the time of surgery. Both of these fixation methods can fail over time for a variety of reasons causing the implant to become loose and requiring revision (see Implant Poly Wear)
DVT
Deep Vein Thrombosis (DVT) is a rare but potentially very serious complication. Blood clots (thrombus) can sometimes form in the “deep” veins near the surgical site. This can cause pain, redness and swelling. Sometimes, the clot can become dislodged and travel through the bloodstream and into the lungs where it can become trapped. This is called a pulmonary embolism and can be fatal. Obviously, many, many precautions are taken to prevent this serious complication.
Patellar Clunk Syndrome
This complication involves interaction of the scar tissue at the top of the patella with the femoral component. See our article on Patellar Clunk Syndrome for more information.
Joint stiffness
Obviously, the joint will be soar and relatively stiff immediately after surgery, however, limited flexion after the immediate post-operative period is undesirable. To read more on stiffness following knee replacement surgery, visit our article joint stiffness following knee replacement surgery.
Allergic Reaction
A small number of patients may experience an allergic reaction or hypersensitivity to the metallic elements in the implants. A more detailed explanation of this complication can be found our article on this topic.
1 Barrington JW, SahA, Malchau H, Burke DW. Contemporary cruciate-retaining total knee arthroplasty with a pegged tibial baseplate. Results at a minimum of ten years. JBJS (Am.). 2009;91:874-878.
2. Jämsen E, Varonen M, Huhtala H, Lehto MU, Lumio J, Konttinen YT, Moilanen T. Incidence of Prosthetic Joint Infections After Primary Knee Arthroplasty. J Arthroplasty.2008 Dec 3.
Patellar Clunk Syndrome: Complication Following Knee Replacement Surgery
In general, Total knee replacement (TKR) is a highly successful procedure. However, complications, although relatively rare, can sometimes occur. One such complication is known as 
Patellar Clunk Syndrome.
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).
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
Is Knee Replacement in Your Future?
Most knee replacements are performed for the treatment of osteoarthritis 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.
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 (
medial compartment), 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 
knee replacement surgery.
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 
epicondylar axis 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.
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
Knee Replacement Allergies – Nothing to Sneeze At?
Knee Replacement Allergies – Nothing to Sneeze At?
Each year, hundreds of thousands of knee replacements are performed. In knee replacement, the worn ends of the bones that make up the knee joint are replaced with metal and plastic implants. In most cases, results are excellent, however in rare cases, an immune response may arise and can become problematic. That is, the patient may suffer from an allergy or hypersensitivity to the metallic components of the implant. Symptoms may include warmth or swelling at the knee, a skin rash and even loosening of the implant.1
This phenomenon is so rare, that its very existence has been debated. But at this year’s annual meeting of the American Academy of Orthopaedic Surgeons, a paper was presented documenting what appear to be allergic reactions to knee replacements.2 Dr. Alvin Ong et. al. recorded 19 cases from 2005-2008 where total knee replacement failures were attributed to metal sensitivity. The authors ruled out all other possible causes of implant failure and then performed a special test, the lymphocyte transformation test (LTT) which looks at a patients sensitivity to certain metals. Seventeen of the nineteen patients with failed knee replacements were tested. Ten patients recorded high sensitivity while the remaining seven indicated mild sensitivity.
Following the sensitivity testing, 16 of the patients had a second procedure where the traditional metal 
knee replacement was removed and replaced with a non-metallic implant (the ceramic Oxinium implant from manufacturer Smith and Nephew). Following this knee replacement, the authors reported good results and elimination of the inflammation that was present with the traditional metal implant.
So although an allergic reaction to metal knee replacement implants may be rare, it is not unheard of. Unfortunately, diagnostic tests like LTT are relatively expensive and persons testing positive are not always symptomatic.1 In their article, Dr. Ong et. al. suggest that surgeons be mindful of the possibility of metal hypersensitivity especially when failures of implants are noted and cannot be attributed to other factors.
1 Rabin SI, Graf CN, Hopkinson, WJ, Hallab NJ “Immune Response to Implants.” emedicine.medscape.com. 08 Mar. 2009. <http://emedicine.medscape.com/article/1230696-overview>
2 Jafari SM, Della Valle CJ, Orozco F, Ong AC. “Metal Hypersensitivity Following Total Knee Arthroplasty: A Real Phenomenon?” 2009 AAOS Annual Meeting, Poster No. P134
“Total Knee Replacement: Get it While You Can”
Each year, the American Academy of Orthopaedic Surgeons (AAOS) holds its annual meeting. This year, two presentations highlighted a pending “perfect storm”: the demand for total knee replacement (total knee arthroplasty) will increase dramatically while the supply of Orthopaedic Surgeons trained to do them will decrease sharply.
In a paper entitled, “Joint Replacement Access in 2016: A Supply Side Crisis,” 1 Dr. Thomas K Fehring et. al. stated that “demand for arthroplasty is expected to double in 10 years.” By 2016, this translates into an annual demand of 1,046,000 knee replacement procedures. At the same time, the expected supply of Orthopaedic surgeons is expected to fall such that if they were to work at current rates, they will only be able to perform 287,759 knee replacements, leaving many “waiting in pain”.
In a related presentation, “National Projections of Younger Patient Demand for Primary and Revision Joint Replacement,” 2 Dr. Steven Kurtz, et. al. explained the “demand side” of this equation. Overall demand for knee replacement is growing, but not just because the population is aging (i.e. a greater number of people over 65), but also because the demand for knee replacement in folks under 65 is growing as well. That is, it will become increasingly more common for folks under 65 to be total knee replacement recipients.
Hip replacement was also highlighted in both studies; however the shortfall is not estimated to be as severe. Ultimately, both papers warn that something must be done to alleviate this pending crisis.
References:
1 Odum S.M., Iorio R., Fehring T.K. “Joint Replacement Access in 2016: A Supply Side Crisis” 2009 AAOS Annual Meeting”, Poster Presentation P043
2 Kurtz S., Lau E., Ong K., Kelly, M.P., Bozic K.J. “National Projections of Younger Patient Demand for Primary and Revision Joint Replacement”, 2009 AAOS Annual Meeting, Podium No. 183
What’s the “Best” Total Knee Replacement?
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 
knee replacement is best” may simply be the one which your experienced surgeon has chosen to utilize.
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Introductory Topics
Introduction - Home Normal Knee Anatomy The Diseased Knee: Osteoarthritis The Knee Replacement Procedure
Surgery Overview Knee Replacement Complications Minimally Invasive Knee Replacement Unicompartmental Knee Replacement Computer Assisted Knee Replacement Knee Replacement Implants
The Parts of a Total Knee Replacement Mobile Bearing Knee Replacement Knee Implant Wear Knee Implant Manufacturers Related Topics
Knee Arthroscopy Total Hip Replacement Osteoporosis -
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