This simply means that only a part of the knee joint is replaced through a smaller incision than would normally be used for a total knee replacement. The knee joint is made up of 3 compartments, the patellofemoral and medial and lateral compartments between the femur and tibia (i.e. the long bones of the leg). Often only one of these compartments wears out, usually the medial one. If you have symptoms and X-ray findings suggestive of this then you may be suitable for this procedure.
The hip joint is often compared to a ball in socket articulation.
The ball is replacing your femoral head. It is perfectly round and can come in different sizes and made of two different materials:
Metal Heads
Ceramic Heads
The only difference between the two are that ceramic is smoother than metal therefore decreasing the wear on the plastic. However, ceramic is more fragile and can break, whether metal heads will not.
The socket is made of two components: the cup and the liner.
The cup is made of metal and sits into your acetabulum or pelvic bone. It is secured in place by the porous coating, which allows the bone to grow into, and screws to prevent it from moving initially.
The liner can be made of three materials and sits in the cup. It is the part onto which the ball will articulate with. Therefore it is important to use materials that are compatible between the head/ball and the liner so the new joint provides two critical desired outcomes:
Low Wear:
Your hip will last longer
High Stability:
Your hip will stay in place and provide you with
good range of motion
There are 3 main options of liners:
Plastic
Metal
Ceramic
But first, how does head/ball size matter?
The bigger the size, the greater the range of motion, possibly allowing a patient to return to more demanding activities.
Even though it might decrease the chance of dislocation (when the ball jumps out of the socket), it does not prevent it. Attention to follow your orthopedic surgeon activity precautions is extremely important.
More importantly, the size of the liner determines the size of the head. Each liner material offers some benefits but also its own limitations.
Plastic Liners (made of polyethylene or "poly liners" for short)
Benefits:
The new poly liners are now engineered to last longer by decreasing the amount of wear the ball generates over time. It should last longer with a ceramic head.
The science and adverse reactions from poly/plastic particles released in the body is well known.
A lot of options in sizes make it convenient for your surgeon to better match your anatomy for the best function.
Limitations:
Being a softer material than either the metal or the ceramic head, plastic will eventually wear out. It is important to maintain a certain thickness of plastic to prevent the ball to wear through it.
Remember the liner limits the head size. In this case, the larger the head:
The thinner the poly
The higher the wear (large heads increase wear on poly)
The sooner the head could wear through
But don’t be alarmed! It still offers outstanding range of motion and has been the standard of care of several decades.
A review of Medicare usage shows that:
Over 60% of hips uses a poly liner articulation (1).
And that large heads have contributed to reducing dislocation significantly in the last decade (2).
AAHKS Paper presentation: "The Comparative Effectiveness of Total Hip Arthroplasty Bearing Surfaces in the Medicare Population" Kevin Bozic, MD, MBA – Nov 6-8, 2009 – Dallas, TX
AAHKS Paper presentation: "Short-term Dislocation Risk Following Primary Hip Arthroplasty in the Medicare Population" Arthur Malkani, MD – Nov.6-8, 2009 – Dallas, TX
Metal liners – Metal on Metal Hips (metal head coupled with metal liner)
Benefits:
Metal liners are as hard as the metal head and can't wear through. Therefore a much larger head can be used to increase range of motion and reduce dislocation
Low wear: metal on metal articulation wear very little
Metal on Metal articulations have been used for over 40 years
Reversely to poly, the larger the head, the lower the wear in metal on metal couplings
Limitations:
The science of metal ions released in the body is not as well known
Woman of childbearing age are usually advised against it as well as individual with known metal sensitivity
Metal sensitivity or/and allergy reaction to metal particles can be common in as much as 4% of patients
A review of Medicare usage shows that:
Over 30% of hips uses a metal on metal articulation (1).
And that large heads have contributed to reducing dislocation significantly in the last decade (2).
AAHKS Paper presentation: "The Comparative Effectiveness of Total Hip Arthroplasty Bearing Surfaces in the Medicare Population" Kevin Bozic, MD, MBA – Nov 6-8, 2009 – Dallas, TX
AAHKS Paper presentation: "Short-term Dislocation Risk Following Primary Hip Arthroplasty in the Medicare Population" Arthur Malkani, MD – Nov.6-8, 2009 – Dallas, TX
Ceramic Liners – Ceramic on ceramic hips (ceramic head coupled with ceramic liner)
Benefits:
Ceramic liners probably offer the lowest wear
Being a hard material they will not likely wear through either
Limitations:
Ceramic is fragile and can break if too thin
Certain ceramic on ceramic have been reported to "squeak" loudly
New ceramics are becoming available in the US while they have been used in Europe successfully for much longer.
In Summary, the 3 main hips used in the US are:
Metal on Poly - Proven Materials:
Polyethylene has been available for over
25 years and provides a wide range of
intraoperative choices to optimize joint
stability while balancing wear performance
with function.
Ceramic on Poly - Advanced Technology:
BIOLOX delta ceramic femoral heads
on cross-linked polyethylene combine the
latest ceramic technology with the advanced
cross-linked polyethylene to optimize joint
stability and minimize wear.
Metal on Metal - Enhanced Function:
Metal-on-metal implants have been available
for over 40 years and combine enhanced joint
stability with excellent wear performance to
provide outstanding functional performance
Ceramic on Ceramic - Reduced Wear:
Ceramics
combine high material
hardness with smooth surface
finishes to minimize wear.
Current version of Ceramic on Ceramic’s brittle nature and squeaking problems have reduced its usage while waiting for FDA approval of new materials.
Dr. Altenburg takes time to match each patient with the articulation that will provide for the best outcome possible.
The new poly liners are now engineered to last longer by decreasing the amount of wear the ball generates over time. It should last longer with a ceramic head. 
