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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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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