Tag: zirconia

Alternative bearing surfaces- Ceramic on Ceramic (CoC) Hip Replacement

Major Points

Ceramic on ceramic articulation has 200 times less wear than metal on polyethylene coupling.
Improvements in taper technology, ceramic quality, quality control has lead to greater confidence among surgeons.
Ceramics are biologically inert
Two types of ceramics available are alumina and zirconia.
Alumina most commonly used.
Accepted only in the hip replacement.

Introduction

Total hip replacement is the treatment of choice in advanced hip arthritis. Ceramics was introduced in the 1970s as an option for THR. Boutin implanted the first ceramic prosthesis in 1970. But poor quality of taper design, surface finish and ceramic grade resulted in unsatisfactory outcome with early designs. The huge success of metal on polyethylene designs combined with poor outcome with early ceramic implants soon made them unpopular. Metal head on polyethylene cup was the first bearing surface combination to prove successful. But subsequently polyethylene wear leading to osteolysis and component loosening was identified as the major cause of failure on long term follow up. This lead to the search for alternate bearing surfaces and the redesigned and upgraded ceramics reentered as an alternative.

Advantages

Advantage of ceramics is their excellent biocompatibility, hardness, wear resistance and lubrication properties. The bulk material as well as particles of ceramic have excellent biocompatibility. Hardness of the ceramic gives it high abrasion and scratch resistance. Surface tension gives it excellent lubrication properties. Ceramic on poly shows lower wear rates than metal on poly and ceramic on ceramic show very low rates of wear. Lower wear rate and biocompatibility leads to lower wear rate which in turn lead to better long term survival of components.

Disadvantages

Disadvantages of ceramics are component fracture, squeaking and unreliable fixation of ceramic to bone. Component fracture reported with earlier designs were as high as 15%. But with the present generation of implants it has come down to 0.02%. As the ceramic cannot be reliably fixed to bone; ceramic liners are fixed to metal cups in the acetabular side and inserted on the metal stems on the femur side.

Another problem noted is chipping of the rim of acetabular liner during the insertion. This complication has come down with careful surgical technique. This is thought to be a design specific problem. It is also due to different mechanical properties of acetabular shell and liner. As ceramic is harder than metal, during insertion the cup may expand and this may lead to unsatisfactory locking of liner to the shell.

Another disadvantage of ceramic on ceramic coupling is squeaking, which is unique to hard on hard bearings. It is reported in 1-10% of cases. Squeaking is thought to be due to 1) mismatch between liner and head 2) insufficient lubrication 3) due to debris and 4) due to stripe wear. The causes and implications of squeaking is not yet determined. All patients should be counselled regarding squeaking prior to surgery.

Another disadvantage of ceramic is that due to manufacturing process constraints, only a one to two liner options are available for each size of head. These leads to reduced ability to customise to the patient on the table. According to many experts this is probably the single most important disadvantage of ceramic on ceramic.

Another important issue is revision. Revision of liner alone may not be possible as removal of liner may lead to damage to the locking mechanism. Hence the shell as well as liner should be exchanged. In case failed ceramic bearing surface, total synoviectomy should be done as ceramic wear particles are hard and may lead to accelerated wear of revised bearing couple if left behind.

Types of Ceramics

Two types of ceramics are available, alumina and zirconia. Alumina is the most popular. The latest generation of ceramics such as Biolox Delta have a mixture of alumina and zirconia. Alumina has several advantages. It is harder than metal; resulting in remarkably less wear and resistance to third body wear. The wear rate of alumina is 0.001mm/year which is significantly less than alumina on polyethylene (0.1mm/year) and metal on polyethylene (0.2mm/year). It has high density which allows very good surface finish. It is hydrophilic leading to better lubrication. Main defect of alumina is its brittleness or low ability to withstand deformation. Zirconia is tougher but it has low thermal stability and cannot be coupled with itself.

Recent Advances

The recent advances in ceramics are smaller grain size which is 3 times less than early designs, better design of Morse taper decreasing risk of fracture, Other modifications are clean room processing, hot isostatic pressing (HIP), laser marking instead of engraving earlier and improved sintering techniques. Another improvement is quality control by 100% proof test; which is a nondestructive overload testing of all ceramic components to identify defects.
Ceramic can be used as a ceramic head on ceramic acetabular cup or as ceramic head on polyethylene cup. At present one million total hip replacements are done worldwide, of these about 25% are ceramic on polyethylene and another 10% is ceramic on ceramic.

At present ceramics are used only in the hip because hip components have simple design and shape; there is minimal dimensional mismatch between the head and cup and there is no translational movement. Hip designs can withstand large loads as they have an axial symmetric structure of spheres and cones leading to very favourable contact conditions in spherical hinges and conical couplings. Kinematics of knee is translational and rotational, hence the design is complex. Hence ceramics is knee is still in the experimental stage.

Present recommendations in technique of ceramic arthroplasty

The modifications in technique include more horizontal (<45 degree) placement of cup to prevent edge loading, and the anteversion should be 20deg to compensate for lack of elevated posterior rim. Any anterior osteophytes present be removed to prevent impingement.

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Copyright @Dr Rajesh Purushothaman, Associate Professor, Government Medical College, Kozhikode, Kerala, India