Who Should Undergo Total Knee Replacement?

Total knee replacement surgery is the quickest and perhaps most effective solution to relieve pain in the damaged knee joint. However, it’s absolutely not the best solution for anyone who has a knee problem. It is strongly suggested that patients consider other treatments (anti-inflammatory medication, physical therapy as examples) as priority to a knee surgery. There are several reasons why you need a total knee replacement, however:

  • Severe knee pain or stiffness that limits your everyday activities, including walking, climbing stairs, and getting in and out of chairs.
  • Chronic knee inflammation and swelling that does not improve with rest or medications.
  • Knee deformity — a bowing in or out of your knee.
  • Failure to substantially improve with other treatments.
  • You are feeling mentally depressed or the pain heavily interferes with your social life.

Moreover, most patients who undergo total knee replacement are aged between 50 and 80. Young patients who suffer from severe arthritis problem should also take the surgery, but this is a relatively rare case. Whether it is necessary to perform a surgery depends largely upon the orthopedic surgeon’s evaluation, and discussions with family, friends and others who are closely related to your life.

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Cemented and cementless knee replacement

There are mainly two types of fixation to hold the prostheses in position: cemented fixation, in which

fast curing bone cement is used to hold the prostheses in place, and cementless fixation, which relies

on bone growing into the surface of the implant for fixation. Both of the two have their own advantages

and shortcomings.

Most knee replacement implants today are cemented, and normally they act very well. The stability of

cemented fixation relies on the connection between the prostheses and the cement, and the adhesion

between cement and bone is also very important.

Cementless implants have a surface topography that is conducive to attracting new bone growth. Most

are textured or coated so that the new bone actually grows into the surface of the implant. They may also

use screws or pegs to stabilize the implant until bone ingrowth occurs. Since they depend on new bone

growth for stability, cementless implants require a longer healing time than cemented replacements.

Both of the two type’s implants may suffer from loosening and wear. In daily movement, the implant is

under considerable amount of load, and the bone may not be able to support the implant, so there may

be relative motion between the implant and the bone, after a long time, the implant will become loose,

then the bone may be damaged and brings new pain. Although the surfaces of both the metal and plastic

part are already polished, there will still be significant friction to produce debris, and the debris will

damage the bone and leads to new disease.

Problem of TKA

The overall mortality rate with TKA is less than 1%, but this figure increases with age, male sex, and t

he number of preexisting medical conditions. Identification and optimization of such conditions prior

to surgery is important to reduce perioperative complications.

 

Complications of TKA include the following:

· Thromboembolism: Thromboembolism includes deep vein thrombosis (DVT) with subsequent

life-threatening pulmonary embolism (PE). Predisposing factors for increased risk of DVT include

age older than 40 years, female sex, obesity, varicose veins, smoking, past history of DVT, diabetes

mellitus, and coronary artery disease. Overall incidence of DVT following total knee replacement

without any prophylaxis has been reported at 40-88%. Most of these are calf thromboses. The risk

of fatal PE, however, is the important figure and varies from 0.1-1%.

 

· Infection: Prevention of infection in TKA begins in the preoperative examination to exclude

intercurrent infection. In the operating room, personnel should be kept to the smallest number,

and traffic in and out of the room should be kept to a minimum. Use of vertical laminar flow in

operating theaters, prophylactic antibiotics, ultraviolet light, body exhaust systems to prevent

bacterial shedding, and meticulous and expeditious surgery all help to reduce the occurrence of

infections to less than 1% of operations performed.

 

· Patellofemoral complications: Patellofemoral complications include patellofemoral instability

(see the image below), patellar fracture, patellar component failure, patellar clunk syndrome,

and extensor mechanism tendon rupture. All of these complications have been cited as the common

reasons for reoperation. These can be avoided by attention to detail, meticulous technique, and

avoidance of component malposition.

 

· Aseptic loosening: Loosening leads to the ultimate failure of the prosthesis and occurs in approximately

5-10% of patients at 10-15 years. It may be complicated by bone loss or osteolysis, which can lead

to catastrophic deterioration and make revision surgery difficult. The etiology of this problem is not

entirely understood but is related to polyethylene debris causing cellular alterations that result in bone

resorption. Once a component is loose, it becomes mechanically unstable with worsening osteolysis.

Treatment is revision with bone grafting.

Surgical Alternatives to Total Knee Replacement

There are surgical treatments other than a total knee replacement to alleviate pain and regain mobility in a

damaged knee. Torn cartilage and other matter can be removed from the knee during arthroscopic surgery.

Another procedure, an osteotomy, realigns the knee by cutting bone off either the femur or tibia.

 

A less common procedure, the unicompartmental knee arthroplasty, has shown promise in treating knee

joints with arthritic damage on only one side of the knee. For instance, if you had arthritis damage on the

left side of your knee, only the damaged portions on the left side would be removed, reshaped and replaced

by metal and polyethylene replicas. About 6 percent of patients with arthritic knees are candidates for

unidepartmental knee arthroplasty. If the untreated side eventually becomes arthritic, the patient can

still have a total knee replacement.

 

One relatively new technique that requires less tissue damage during the procedure is called a minimally

invasive knee replacement. Although the same type of implant is inserted, the surgeon works with a

smaller incision. There is less scarring with this method and less overall tissue damage, resulting in a

shorter hospital stay. The procedure is difficult to perform and requires different tools and instruments

to operate on the joint. Depending on the joint damage and other factors such as obesity, a minimally

invasive knee replacement may not be the ideal choice for some patients.

Knee replacement implant

Up to three bone surfaces may be replaced in a total knee replacement:

 

· The lower ends of the femur. The metal femoral component curves around the end of the femur (thighbone).

It is grooved so the kneecap can move up and down smoothly against the bone as the knee bends and straightens.

 

· The top surface of the tibia. The tibial component is typically a flat metal platform with a cushion of strong,

durable plastic, called polyethylene. Some designs do not have the metal portion and attach the polyethylene

directly to the bone. For additional stability, the metal portion of the component may have a stem that inserts

into the center of the tibia bone.

 

· The back surface of the patella. The patellar component is a dome-shaped piece of polyethylene that duplicates

the shape of the patella (kneecap).

 

Components are designed so that metal always adjoins with plastic, which provides smooth movement and results

in minimal wear. The material of the implant must be biocompatible,  in order to be staying in human body without

a rejection response. They must be able to duplicate the knee structures which they are designed to replace.

And they also need to retain their strength and shape for a long time. So for the metal parts of the implant, titanium

or cobalt-chromium based alloys are always chosen, and the plastic parts are normally made of ultra high molecular

weight polyethylene,because they have suitable material properties.

Why do we use vibration test

Vibration tests are normally used in modal analysis, a process in which we describe a structure in terms of its dynamic properties like natural frequency, damping and mode shapes. To introduce modal analysis clearly, let’s consider a simple example. Imagine there is a freely supported flat plate, and we apply a force that varies according to sinusoidal pattern. So the plate will start to vibrate, and we measure the response of the plate with an accelerometer attached to one corner of the plate.

Now if we keep the amplitude of the exciting force the same, but change its frequency, we would notice that the amplitude of the response will also change. This is actually reasonable: the response is amplified more when the excitation frequency is closer to the natural frequency of the structure, and the response will be the maximum when the excitation frequency is the same as the natural frequency.

 

To do some further analysis, I would like to introduce a function which represents the relationship between input and output signal: frequency response function or FRF. To get the FRF of a system, we measure both the applied force and the response of the structure at the same time. Then the measured data is transferred from the time domain to the frequency domain using Fourier Transform algorithm. If we calculate the FRF of the flat plate, we will notice that the peaks in the FRF are at the natural frequencies of the system.

 

So we can see using the vibration tests on one structure we can get the FRF, and then we can find the natural frequencies according to the peaks of FRF.

Then why do we need the natural frequencies? And what’s its relation with THA? First, the natural frequencies are related to the stiffness of the structure: the higher the stiffness is, the higher the natural frequencies are. Second, the stiffness of the structure is related to the fixation between the implant and the bone: the better the fixation is, the higher the stiffness is. So we can use the vibration tests to get the natural frequencies of the implant-bone structure, then we can use the value of the natural frequencies to evaluate the fixation of the implant during THA.

Source: Peter Avitabile, Experimental Modal Analysis, 2001

Knee implant, total knee arthroplasty and post-surgery vibration test

We have discussed the hip implant, THA and its related vibration assessing technique. So, now, what about the knee joint? Is it different from the hip? What is the difference? And what are the similarities that we can take advantage of in surgery of bad knee joints?

Principally, TKA, a surgical procedure to replace the weight-bearing surfaces of the knee joint with a prosthesis, is more or less the same as THA. Old people who suffer from diseases such as osteoarthritis or rheumatoid arthritis will come across the problem of pain in their knee joint, as well as a reduction of range of motion. In the USA, more than 230,000 TKA’s are performed every year. Of all these surgeries, approximately 5 to 10% of the prostheses need to be replaced within 10 to 15 years due to the worn-out connections between implant and bone. Either loosening or infection can cause bad connections.

Two photos are displayed to give a first impression of the hip implant and knee implant.

                      

There are several methods currently used in detect the worn-out of medical implants, including physical examination, radiography(most widely used), laboratory tests, and vibration techniques (latest). Physical examination is a relative subjective and inconclusive assessing method, and laboratory tests mainly focus on the diagnosis of infection in the bone.

Radiography is the most classic way of loosening diagnosis. It is used to evaluate prosthesis alignment, fixation,gross polyethylene wear and quality of periprosthetic bone. But it is subjective, inconclusive and less sensitive( e.g., a sensitivity of 83% has been reported for the tibial component and a specificity of 72%).

Vibration analysis has been used to determine bone mechanical properties, to monitor fracture healing, and to assess the stability of  dental implants. It is also used to assess the initial stability of the femoral THA component, intra-operatively. Two differences exist between a TKA and a THA. Firstly, knee joint is not accessible, so a small incision should be cut to use vibration test post-operatively. Secondly, in THA, vibration technique is used to  as a comparison between two succeeding insertion stages during surgery as a comparative method, while in TKA it is used in an absolute way.

References:

Baré J, MacDonald S, Bourne R. Preoperative evaluations in revision total knee arthroplasty. Clin Orthop Rel Res, Vol 446, p40-46, 2006.

Marx A, Saxler G, Landgraeber S, . Comparison of subtraction arthrography, radionuclide arthrography and conventional plain radiography to assess loosening of total knee arthroplasty. Biomedizinische Technik, Vol 50, No 5, p 143-147, 2005.

Pastrav L, Monitoring of the fixation of orthopaedic implants by vibration analysis, promoters G. Van der Perre, R. Van Audekercke, 28 October 2010.

Pastrav L, Jaecques S, Mulier M, Van der Perre G. Detection of the insertion end point of cementless hip prostheses using the comparison between successive frequency response functions. J Applied Biomat & Biomech, Vol 6, No 1, p 23-29, 2008.

http://bonesmart.org/joint-replacement-surgery/hip/