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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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.

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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.

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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

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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 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

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“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 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

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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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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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Special Topics

Unicompartmental Knee Replacement

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Knee Implant Wear

Polyethylene Wear and Osteolysis

Although there have been many advances in Total Knee Implants, they are still mechanical devices that can wear over time or if implanted improperly.

Modern have been cited to have a greater than 90% survivorship at 15 years (1) however, over the long term, challenges still remain.

The “weak link” so to speak in a Total Knee is typically the tibial insert (or tibial bearing). The insert is made of ultra-high-molecular-weight-polyethylene or UHMWPE. As your knee moves (to the tune of 1 million steps or more/year!) the surface of the insert will wear and create tiny particles of UHMWPE.

The wear rate in modern knees that are implanted well is very small. But if a knee is unbalanced and/or of poor design, accelerated wear can occur. This accelerated wear, can in turn, lead to failure of the insert (i.e. cracking) or a biologic response to the wear particles known as osteolysis. In the presence of the small particles (recognized as foreign bodies) the bone can “resorb” or waste away around the implants leading to loosening of the implants from the bone.

Knee implant design is a balance between providing freedom of movement and congruity. The less congruity between the tibial insert and femoral component, the more movement can occur (sounds good, but…). However, to distribute forces over a wide enough area to minimize wear (forces at the knee can reach up to 5 times body weight!), a close mating of the femoral component and tibial insert is desired. These competing requirements lead to a design challenge that is constantly being optimized as new implants are developed. Also recent treatment to implant materials can also help.

Surgical technique is also important. The better aligned the components are, the more resistant they will be to wear. Modern instrumentation has made knee replacement very reproducible. In addition, new computer assisted techniques may also be improving accuracy. On the other hand, new “less invasive” techniques offer reduced scarring and quicker recovery, but as you might infer, could make it more challenging to obtain a well aligned knee and hence could lead to long term problems if done incorrectly.

As younger and more active patients wish to maintain active lifestyles in the face of debilitating joint pain, the issue of implant wear will continue to be at the forefront and inspire continued development of implant design and surgical technique.

(1)”Long-term followup of nonmodular total knee replacements.” Keating EM, Meding JB, Faris PM, Ritter MA. Clin Orthop. 2002 Nov; (404):34-9.

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The Parts of a Total Knee Replacement

The knee joint involves three bones; the femur, the tibia and the patella. Total Knee Replacement, each one of these bones is resurfaced with a separate component. To “resurface” the bone the surgeon uses a saw guide and small power saw to trim off the outer worn surface of each bone in the exact shape of the inside of the corresponding implant.

The resected bone can be several millimeters thick (about 1/4-1/3 inch).

The separate components work together to form the complete prosthetic device. In some cases the surgeon may choose not to replace the patella and let your native patella mate with the other artificial components (this is quite common in Europe, less common in the US).

The Femoral Component

Typically this is a Cobalt-Chrome metal component. The highly polished outer surface serves as the end of the femur, the other side is either “cemented” or “press-fit” onto the prepared bone surface.

Most implants in the US today are cemented. A polymer based compound is mixed in the OR forming a putty which hardens. The surgeon puts the putty in between the implant and bone to secure it.

In the “press-fit” variety, a roughened surface on the inside of the component has a porous 3-D structure designed to promote bony ingrowth.

There are pros and cons to each type of fixation. As mentioned, the cemented version is more common, but both have well established clinical histories

The Tibial Component

The tibial component is usually a two-piece metal and plastic implant although there are all plastic versions available.

The metallic portion of the implant can be titanium or cobalt-chrome and it is fixed to the tibial bone in much the same way as the femoral implant.

The plastic component is known as a “tibial bearing” or “tibial spacer” or simply “tibial insert.” It is a plastic component made of a special grade polyethylene (UHMWPE). Manufacturers have begun to “treat” the plastic with gamma radiation to improve wear properties.

The Patellar Component

The patellar component is typically an all plastic component that is fixed to the cut surface of the underside of the patella
or knee cap.

The component is secured with bone cement and articulates or “joins up” with the femoral component, as it would with the normal knee.

Because the patella moves up and down across the surface of the femoral component, the relative alignment of the two components is critical. Patella problems are among the most common complications following total knee replacement, although the overall complication rate is small.

Primary Knee Replacement

Primary Knee Replacement is, as the term suggests, the name given to a family of knee implants designed to be used as a first knee replacement. This type of knee replacement allows the remaining soft-tissue to provide some of the constraint to the knee joint and is usually implanted in patients who have not had a previous Total Knee. The term constraint, does not imply limiting  movement, rather it simply allows some of the normal stabilizers in the knee (namely the collateral ligaments, and sometimes the PCL) to still perform their function.

Revision Knee Replacement

Revision Knee Replacement is designed to replace a worn-out and/or loosened Primary Knee Replacement. This type of knee replacement has more intrinsic constraint and can “play the role” of some of the natural stabilizers which may be absent or no longer functioning. In some cases, a revision knee implant may be used as a patient’s first knee implant if the disease is advanced enough.

PS Knee Implants

  • PS, Posteriorly Stabilized, or PCL Substituting Knee Implants are designed to be implanted with the Posterior Cruciate Ligament or PCL removed. In this type of knee replacement, a protrusion on the tibial insert is designed to mate with a specially shaped bar (cam) on the femoral component. This “cam and post” interaction substitutes for the normal function of the PCL by:
    • Limiting anterior or “frontward” movement of thefemur (thigh) relative to the tibia (shin).
    • Promoting posterior or backward movement of the femur relative to the tibia as you bend or flex your knee. This movement is necessary to allow for deep flexion of the knee.
  • Studies have shown that PS knees tend to be more predictable and provide for slightly higher degrees of flexion compared to CR knees (see CR section). This is because the function of the knee is dependent on a fixed metal and plastic mechanism rather than the native PCL whose function and balance can be variable in a diseased knee.
  • For some photos and description of a PS knee you can visit:
    http://www.stryker. com/jointreplacements/sites/scorpioknee/scorpiops.php

CR Knee Implants

  • CR or Cruciate Retaining Knee Implants are designed to be implanted with an intact Posterior Cruciate Ligament or PCL. Often this ligament is still functioning to some degree in patients with advanced osteoarthritis. In this type of knee replacement, the PCL is allowed to act as the primary stabilizer limiting anterior or “frontward” movement of the femur (thigh) relative to the tibia (shin). This is the primary function of the PCL in the normal knee. Another function of the PCL is to promote posterior or backward movement of the femur relative to the tibia as you bend or flex your knee. This movement is necessary to allow for deep flexion of the knee.
  • Studies have shown that CR knees tend to have less predictable and slightly lower degrees of flexion compared to PS knees (see PS section). This is because the function of the knee is dependent on the native PCL whose integrity and “health,” if you will, can be somewhat variable. That being said, there are studies that have shown that the performance of CR knees and PS knees are comparable. If you are so inclined, you can view abstracts of publications at:http://www.ncbi.nlm.nih.gov/entrez/query.fcgi
  • For some photos of CR knee implants you can visit:http://www.stryker.com/jointreplacements/sites/scorpioknee/scorpiocr.php

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