Knee replacements in young adult patients

Introduction
Traditional wisdom dictates that knee replacements should be reserved for young patients. How young is young? Well, generally, knee replacements are considered appropriate for anyone above 65 years of age. The problem here is that with the wildly successful application of knee replacements in older patients, younger patients who have traditionally been managed with any number of biological measures like stem cell therapy, realignment procedures and ligament repairs can continue to be in pain albeit in a reduced fashion. It therefore becomes difficult to convince patients after a while that the best measure for them is the one that will allow them to have good high level activity but with significant pain. These sorts of consults are in my experience only useful in the initial 2 or 3 years of the onset of pain. After this the patient is typically disgruntled and moves on for a second opinion or seeks an opinion for a joint replacement - I have been on the receiving end of both sorts of patients and feel this is more a matter of education than competency. Should we therefore do knee replacements on such patients early? The answer is just not so simple......

Traditional approaches to young patients with arthritis in the knee
Young patients with arthritis in the knee generally fall along two categories those that have rheumatic conditions and those that have had trauma. There can be additional aggravating conditions like obesity but these don't usually cause arthritis. 

Rheumatic conditions like gout and rheumatoid arthritis typically affect patients at a younger age. This damage is more inflammatory and treatment before the age of forty is heavily weighted toward medical therapy. Accordingly the patient with gout is treated with medication to lower uric acid levels in the body and anti-inflammatory medication controls disease activity. The rheumatoid patient is treated with anti-rheumatic drugs. In general there is little place for surgical measures. Viscosupplementation is contraindicated in these patients.

In the patient who has had trauma, the idea is to restore the damaged anatomical structure to prevent  arthritis from happening. That is to say menisci (meniscus singular) are repaired, fractured are reduced and ligaments reconstructed. Stem cell therapy falls in this category as well. When arthritis does set in there is a possibility to perform various realignment procedures. Viscosupplementation may still buy a some time. By en large it becomes a waiting game as conservative measures fail give sufficient symptom relief and a knee replacement becomes imminent. It is apparent that most of these biological measures variably affect pain - realignments and viscosupplementation reduce pain by 30%. By contrast knee replacements can remove pain completely.

The ideal joint replacement for young patients
With this success in joint replacement surgery it can be difficult to deny a young patient joint replacement surgery. As a global community, joint replacement surgeons have had to seriously look at measures that may be advantages in implementing joint replacements in the younger patient.

Bone conservation
When one considers that the newer joint replacements are now lasting up to 15 years, there is a need to consider revision knee surgery. A person who has a knee replacement at the age of 50, would likely have a revision at 65 and then possibly another at 80. With each revision there is necessarily more and more bone loss. Thankfully, the real number of revisions is much less. Nevertheless, it would be advantageous to begin with a design that conserves the most bone. In that regard we embarked on a study looking at the bone removed in joint replacement surgery and discovered that the Stryker Scorpio NRG removes about 15% less bone then the standard total condylar knee designs in use by the majority of surgeons. By removing less bone therefore, the next operation should be more amenable to a more conservative approach.

Partial knee replacements
Related to this question of bone conservation is the use of partial knee replacements or unicompartmental knee replacements. My considered opinion is the design concepts around these joint replacements are flawed. The idea is such patients have arthritis in one compartment (usually the medial compartment). One should be able therefore to replace just one compartment. The problem is the femoral part of the medial compartment extends into the patello-femoral compartment. It therefore is very rarely purely a single compartment. Surgeons who advocate this technique therefore would advocate the use of such designs accepting that 'some amount of failure is to be expected'. In fact of all the knee replacement models available, these designs have been shown to have the highest failure rates. Some surgeons would advocate replacing two compartments of the three but this has always been an indication for total knee replacements. When these joints fail, it is not like you can put in a traditional normal knee replacement as one is forced to make an equivalent 15 mm tibial cut. Hence the revision option for such a failed knee is a much more expensive revision implant - which is a significant disadvantage to high tibial osteotomy realignment procedures where normal implants can be used to salvage the joint. Another odd issue with partial joint replacements is that unlike total joint replacements where the knee (which is usually bow-legged) is straightened out, in partial knee replacements the knee is actually kept bow-legged. Finally, the partial knee replacement is actually more costly than a partial knee replacement. All these issues have been seriously considered in our practice and we have come to the conclusion that a bone conserving total knee replacement is the better more cost-effective option for knee replacements in the young. This is supported by most joint replacement registries that are showing that such partial knee replacements have high revision rates.

Wear characteristics
Knee replacements tend to wear by fretting - the plastic is pounded until it cracks. Therefore there has been increasing interest in providing for articulating surfaces that are harder and less prone to wear. One approach is to use a ceramic surfaced implant which has excellent waer characteristics but they are nearly twice the price of a normal knee. The other option is the use of a harder plastic or highly-cross-linked polyethylene (eg. Scorpio NRG). The danger of course is for these surfaces to become so hard that they are brittle and more prone to cracking.

Related to this is the design of the knee. The more restrictive older total condylar designs do tend to have higher wear than the new rotating platform (eg. J&J RPF PFC) and single radius designs (eg. Scorpio NRG).

Range of motion
Younger patients tend to desire a higher range of motion to allow their more aggressive activities of daily living like squatting on the floor. Once again, the more restrictive older total condylar designs do tend to have lower range than the single radius designs (eg. Scorpio NRG). The rotating platform (eg. J&J RPF PFC) designs have not delivered on heir promise of better range of motion. One should be cautious about using high mobility knee designs in rheumatic patients in whom the added attendant ligamentous laxity can cause instability.

Anecdotally, it is apparent that younger patients can also find it difficult to have good range of motion in the immediate post-operative period because ironically they have better muscle function. Therefore they go into spasm easily and this sets up a vicious cycle of spasm, reduced range of motion more pain. Typically however range of motion returns with time and stretching and accommodation of the new implant.

   

 

Commonly Available Implants

 

Zimmer LPS Flex

DePuy PFC Sigma

Stryker Scorpio NRG

DePuy PFC Sigma RP-F

BBraun Allergy Solution knee

Uni-compartmental designs

(All makes, Non-robotic)

Remarks

Traditional total condylar design modified to increase range of motion

Traditional total condylar design

Single radius design employing highly cross-linked polyethylene

The dominant rotating platform design originally intended to give high range of motion but presently marketed as a ‘reduced wear’ design

A ceramic coated knee that gives excellent wear characteristics

Systems designed to replace single compartments (medial, lateral, patella-femoral)

  
 
 
 
 
 

Bone conservation

Moderate

Moderate

Good

Poor

Moderate

Very good

Wear characteristics

Poor

Poor

Good

Good

Moderate

Very poor (highest failure rate)

Range of motion

Average

Average

Good

Poor

Average

Average

 

Table. This is a personal experience of a number of designs vaunted for use in young patients with osteoarthrtitis in the knee. Most of these concepts are verifiable in the literature after careful analysis but necessarily represents our own analysis. Of note the cruciate ligament retaining designs have been omitted. These can provide good bone conservation but otherwise are similar to the cruciate sacrificing equivalent. They also have poor range of motion.