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* Joby John, Jacob
Varughese
*Amrita Institute of Medical Sciences, Kochi,
India.
Address for Correspondence
Joby John,
Amrita Institute of Medical Sciences, Kochi, India. |
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Abstract
Background:
Tricompartmental knee replacement has been shown to produce good
function and have a good survivorship. The limiting factor in
the function of knee replacements is polyethylene wear and
subsequent loosening at the bone prosthesis interface. Mobile
bearing prosthesis were developed to decouple the translational
and rotational motions, thus decreasing the wear. This would be
effective only if the bearings continue to remain mobile invivo.
Methods: 10 LCS knees implanted in the period beginning
2000 with at least 1 year follow-up were examined
fluoroscopically to determine if the rotating platform was
indeed rotating in vivo.
Results: All the knees examined were found to have
polyethylene bearings which were still mobile.
Conclusions: Rotating platforms in LCS knees continue to
rotate at early follow-up. Further studies have to be conducted
to examine whether these rotating platforms are still mobile at
further longer follow-up.
J.Orthopaedics 2006;3(2)e6
Introduction:
Replacement arthroplasty of the knee is a
successful procedure. The relief of pain and the restoration of
function can be dramatic, and the rate of survival of the
implants is long enough to satisfy most patients requiring knee
replacement1.The general consensus is that total condylar (tricompartmental)
knee arthroplasty substantially changes the kinematic profile of
the knee2. This may be due to several factors including the
differences between the geometry of the normal articular surface
of the knee and the replacement prosthesis, loss of the anterior
and/or posterior cruciate ligaments, and altered neuromuscular
patterns due to preexisting disease. All knee replacements are
subject to problems of polyethylene wear. Rotating platform
knees were designed and implanted from 1978 onwards to minimize
the wear and loosening problems of earlier designs3. Increasing
the surface congruity during major load bearing activities was
the mechanical concept for the New Jersey Low Contact Stress (LCS)
total knee system. Such an increase in congruity during peak
loading of walking and running allows reduction of contact
stresses within tolerable limits, while allowing unconstrained
mobility of the fixtured components to avoid loosening
stresses4.
Material and Methods :
 Between
2000 and 2002 10 LCS knee replacements, were performed by the
same surgeon (JV). These knees were fluoroscopically assessed
for mobility of the polyethylene bearing. The knee was placed in
leg capture to minimize errors secondary to rotations of the
tibia. The radio opaque markers present in the mobile bearing
were examined fluoroscopically to assess rotation.
The set up to assess mobility of the LCS knee bearing.
Results :
  All
knees examined were found to have mobile bearings from 0 degrees
to 70 degrees, that could be examined on the fluoroscope. The
fluoroscopic image showing radioopaque markers in line on the
left and out of line with further flexion on the left, as an
evidence for mobility of the rotating platform.
Discussion :
Rotating platform mobile knee prostheses
reduce contact stresses by decoupling sagittal plane motion from
rotation, thus reducing loosening stresses on the bone
prosthesis interface5.Volumetric wear is an important
consideration in the longevity of the prosthesis. The volume of
wear debris generated in fixed bearing prosthesis increases
significantly when subjected to internal- external rotations and
anterior– posterior displacements. Inspite of having two
surfaces where potential wear is possible rotating platforms
exhibited a mean wear of approximately one-third of the fixed
bearing prosthesis6. Clinical performace and survivorship of the
LCS rotating platform knees into the second decade is very good
4, 5. The biomechanical advantages of decoupling of motion in
the sagittal plane from rotations and the resultant decrease in
the volumetric wear can only be realized if the rotating
platform remains mobile. In this fluoroscopic analysis we have
analysed whether rotating platforms remain mobile in vivo.
Conclusion:
All the knees examined were found to be
mobile at a follow-up ranging from 12 to 30 months. These knees
would need to be examined with much longer follow-up to
establish if at some point in time these mobile platforms become
fixed.
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