Knee arthroplasty

Design of a knee prosthesis: 

Munier Hossain, 2005

There are certain mechanical factors that need to be considered when designing a knee prosthesis: 

1. Degree of constraint
2. Surface contact area 

Degree of constraint: 

A normal knee joint has six degrees of freedom in three axes. When designing the artificial prosthesis, the dilemma is how much to replicate the normal freedom. There is a fine balance between freedom and instability. If the joint has large freedom, then , in other words, it is rather unstable and may result in undue stress to the local soft tissues. 

On the other hand, if the joint is too constrained, then it would act like a rigid structure in loading. This would mean no load transfer to the soft tissues and undue stress at the prosthesis-bone interface.   
From the picture, one can see that the knee can have Translational and Rotational movements in X,Y,Z axes- resulting in six degrees of freedom. If T and R is nil in either axis, joint acts as a rigid body in that mode of loading. This means no energy transfer to the soft tissues and load transmission is completely borne by the prosthesis-bone-cement interface. 
  
Metal and cement being stiff and non-viscoelastic, they do not absorb loads well, this can give rise to high local stress and eventual failure of bone-prosthesis interface. A way to prevent this is to have large surface area of contact to prevent localised heavy loading. That is why constrained prostheses are designed with large stems. 
  
The wear debris of HDP particle usually determines prosthesis life. So, it is important to know how this can be reduced. 
  
The volume of wear is proportional to the sliding distance.                    
  
V ∞ s   
So, wear volume can be reduced by keeping sliding distance to the minimum. 
However, HDP bearing life is more a reflection of depth of wear and not the volume of wear. Volume and depth of wear are related. 
  

Wear vol.= Contact area times Wear depth

So, for a given wear volume, wear depth can be reduced by increasing contact area. To reduce the depth of wear, the best option is to have a prosthesis with large contact area. 
  
But if we increase the sliding distance the wear volume would be increased, so the option is to increase the total surface contact area, but to keep the sliding distance to the minimum possible.

 So, prosthesis life depends on wear rate of HDP. 
     

Wear rate of HDP depends on sliding distance 
        

But failure depends NOT on wear volume, BUT

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