Knee Replacement Loosening and Pain

Total knee replacements have been a medical success story. About 85 to 90 percent of total knee replacements are successful up to ten years. A major reason for artificial knee failure is a loosening where the metal or cement meets the bone.

Loosening can occur because either the cement crumbles (like old mortar in a brick building) or the bone melts away (resorbs) from the cement. For the majority of failed knee implants, a physician can diagnose problems using conventional X-ray pictures.

Loosening occurs in about 1 percent of patients per year. After ten years, 25 percent of total knee replacements may appear loose in x-rays. About 10 percent will be painful and require revision surgery.

In the case of the Zimmer NexGen CR-Flex knee, one study has shown that the loose knee failure rate can be as high as 9% within the first two years. That same study suggests that the failure rate may be even higher. Another 36% of the study population showed signs of early loosening.

	The left picture shows a firmly seated knee implant. The implant is in direct contact with the bone. The X-ray below it shows direct contact of the white shadows of the knee implant with the gray shadow of the bone.
The right picture shows a loose implant. Both implant components have lost contact with the surrounding bone. The components are surrounded by loose connective tissue. The X-ray below shows the gray shadows of the bone from around the implant have been replaced by the dark shadows of the connective tissue. Also, both components of the artificial knee have changed position.

A loose knee implant:

• Causes pain and erosion of the bone.
• Can severely restrict daily activities.
• Can impose serious psychological burdens.

Once pain becomes unbearable, another operation may be required. A loose, painful artificial knee can usually—but not always—be replaced.

Mechanical Loosening

Because there are many possible causes, even when pain is serious many doctors wait until absolutely necessary to perform surgery again.

Mechanical loosening can occur in both cemented and uncemented replacement knees. In uncemented knees, movement between the surface of the implant and the leg bones can cut off blood supply to the ingrown bone tissue. As a result, only soft fibrous tissue may grow between the implant surface and the skeleton. This means the implant is not stable because only soft tissue binds it to the skeleton.

How Do I Know If My Replacement Knee Is Loose?

• Stiffness in the total knee.
• Increasing, lasting pain in the whole knee area.
• Difficulty putting weight on the knee.
• Diminished motion in the joint.
• Limping.

A physician will normally confirm a loose artificial knee with bone scans or X-rays. Bone scans use an injected radioactive substance to highlight likely areas of loosening on images.

X-ray pictures, on the other hand, identify a loose knee by showing one or more radiolucent lines around the contours of the artificial knee.

Sometimes, the knee replacement changes its position on successive X-ray pictures. This is another sign that the prosthesis has lost its fixation to the skeleton.

Source:
http://www.eorthopod.com/content/revision-arthroplasty-knee
http://www.oralchelation.com/calcium/DegenerativeKneeJoint/p12.htm
http://www.ehealthmd.com/library/totalkneereplacement/TKR_risks.html#loose
http://www.uihealthcare.com/topics/medicaldepartments/orthopaedics/
kneereplacement/index.html
http://www.lifebridgehealth.org/body_rubin.cfm?id=4095
http://www.totaljoints.info/LOOSE_TOTAL_KNEE.htm
http://www.totaljoints.info/IF_TKNEE_FAILS.htm
http://www.totaljoints.info/cemented_and_cementless_thr.htm#3