Artificial hip joint

Introduction to failures in THR

Entire hip replacing ( THR ) or entire hip arthroplasty ( THA ) is one of the major solution for the patients enduring from assorted bone diseases like degenerative arthritis, arthritic arthritis, osteonecrosis, inborn dysplasia etc. These diseases may ensue in complete or partial malfunctioning of the hip articulation and this may even happen due to trauma. Even though THA is a alleviation for such patients, it has many restrictions. These are chiefly caused by the disruption of the joint after the replacing or due to the erosion of the prosthetic cotyloid cups. The factors on which the efficiency of arthroplasty relies on are the stuffs chosen to construct cotyloid cup and femoral caput, the radius of femoral caput and cotyloid cup, the lubrication efficiency of the stuffs, thickness of cup wall, maximal burden applied etc. After the replacing surgery the patients are followed for many old ages in order to analyze about the prosthetic device and the follow up may go on to, state 10 to 20 old ages or until the individual dice. This is really much required in instance of doing any betterments in the bing method of replacing or to make alteration surgeries.

Literature reappraisal on the affect of femoral caput size in entire hip articulation replacings

The major cause of the failure in THA is due to the changing femoral caput diameters used in the prosthetic device. THA has been rapid since 1940 ‘s but the success rate was really less at that times. The innovators in this field are Charnley and McKee who brought first-class developments through their plants. Charnley introduced low clash arthroplasty ( LFA ) . There existed broad differences in the sentiments about utilizing the different diameters of femoral caput. Apparently the largely used 1s are 22mm, 24mm, 28mm and 32mm. Early surveies about hip prosthetic device reveal that the femoral caput size used in THA were relatively really smaller to the current 1s. So it makes really clear that the failures occurred during the early age of hip prosthetic device were due to the little sized femoral caputs.

The prosthetic implants are observed and studied through figure of clinical surveies or in other instances by utilizing simulators. The hip joint simulators are widely used presents in order to do a elaborate survey about the prosthetic constituents before engrafting them in the patients. These devices can mensurate the wear rate happening in implants and it is given in millimeters per twelvemonth. 1 million rhythms is approximately tantamount to one twelvemonth of clinical usage.

Charnley used polytetrafluroethylene ( PTFE ) for cotyloid cups during his early phases of work. He used femoral caputs of changing diameters runing from 22mm to 41.5mm. However, the success rate was really low in many joint replacings. He shortly realised that the femoral caput size plays a critical function in the efficiency of the prosthetic device and after his farther surveies on the same he concluded that upper limit wear life is obtained by doing the femoral caput diameter half of the cotyloid cup diameter. Then he started utilizing 22mm diameter femoral caputs which could harvest high rate of success after the replacings. The wear rate of 22mm femoral caput was merely 0.07mm/year but the larger femoral caputs produced a wear rate of 0.2mm/year and supra.

The elaborate survey on femoral caput size affects made it clear that the larger femoral caputs can cut down the rate of disruption of the prosthetic constituents, addition scope of gesture when compared to smaller sizes and give more stableness to the articulations. But the use of larger caputs were non outstanding because they produce more additive and volumetric wear particularly in cotyloid cups due to greater frictional torsion which eventually leads to the relaxation of cotyloid cup. Component relaxation occurs after the erosion of prosthetic constituents which may take to osteolysis induced by the worn out polythene atoms. So until the recent yesteryear, smaller femoral caput sizes ( 20mm to 26mm ) were more conspicuously used.

Further more surveies on the failure of THR revealed the fact that non merely the femoral caput size but besides the stuffs used in fabricating prosthetic constituents contribute to the operation of the implants. In the yesteryear, metal on metal hip prosthetic device was good known. During that clip the femoral caput diameters widely used were 16mm, 22.225mm, 28mm and 36mm. But the wear rate was improbably more so. This was reduced by simply increasing the diameter of the femoral caput. Later research workers used extremist high molecular weight polythene ( UHMWPE ) for cotyloid cup. The UHMWPE/steel hip prosthetic device was heartily appreciated in the field of joint replacing because of its high success rate. But it had few failures due to the UHMWPE wear caused by the raggedness of femoral caput surface. Lancaster demonstrated that skiding distance between the cup and the femoral caput is a chief factor in bring forthing UHMWPE wear. The lone solution for this was to cut down the femoral caput size which can widen the life of prosthetic constituents. Likewise, there existed many sentiments in taking an appropriate diameter for femoral caput.

Assorted clinical studies have shown that there was a close relation between the addition in femoral caput size and addition in wear rates. Harmonizing to those studies, wear rate is straight relative to the femoral caput size where as disruption rate is reciprocally relative to the caput size. A finite component survey has showed that the capacity of the prosthetic implants to defy the disruption additions by 3.6 % for every 1mm addition in the femoral caput diameter. The hazard for disruption is more when the difference between the inner and outer cup diameter is more. All these factors wholly made the research workers hard to convey out a better solution for the job.

Hip prosthetic device in the yesteryear used smaller femoral caputs and line drives. The line drives helped in cut downing the wear to a little extent but found to hold figure of restrictions. Nevertheless, elevated cross linked polythene line drives used along with larger femoral caputs reduced the wear rates relatively. But one status prevails that the line drives should be of minimal thickness or else the full prosthetic device will fall in instead than have oning. The similar instances used 32mm and 28mm femoral caputs with elevated cross linked polyethylene line drive for the cup which dramatically reduced the wear rates by 85 % and 90 % severally and besides reduced the hazard of disruption in THA. This shows that usage of smaller femoral caputs along with cross linked PE line drives can cut down the wear rate every bit good as the hazard of disruption at the same clip. The farther research wholly eliminated the use of line drives because they were more hazardous to be made and installed, and besides due to the failure rate.

When the femoral caput articulates with the cotyloid cup, at the point of using burden, a contact force per unit area is applied. The contact force per unit area should be minimised in order to obtain better life efficiency for the prosthetic device. The contact force per unit area can be reduced by increasing the diameter of femoral caput. A instance survey was undergone for parametric optimization of stuffs used for cotyloid cup. In this a 28mm femoral caput was used. The bed thickness of the cotyloid cup was half of the radius of the caput which is 7mm. Besides that, a unvarying metal backup of 3mm was given to the cup to defy more force per unit area from the caput. Comparing to the contact force per unit area values obtained for several other diameters of caput, the prosthetic device given supra has a relatively low contact force per unit area which is given by an equation ( with no regard to Elastic modulus and Poisson ‘s ratio of stuffs ) ,

P= 12W/5 & A ; pi ; R?

Where, P is the contact force per unit area at a contact half angle of 60 & A ; deg ; , W is the fixed burden ( 2500N ) and R is the femoral caput radius.

Another instance survey was taken into consider, in which 385 hips were followed for about 10 old ages after the joint replacing. 22mm and 32mm femoral caputs were used in this. As antecedently said additive wear and volumetric wear occur during class of clip after the arrested development of prosthetic device. Linear wear was more outstanding in the 22mm prosthetic device whereas volumetric wear was more in 32mm prosthetic device.

The old surveies showed that the lesser the femoral caput radius is, the lesser the wear rate is. But since the opportunity of disruption is more in smaller caputs, usage of larger caputs became more fruitful. However, the wear rate of 22mm caput was 0.1mm/year ( with scope of 0.07 to 0.21mm ) and that of 32mm was 0.2mm/year harmonizing to the earlier surveies. Taking both the appendages, it was found that the 28mm femoral caput significantly has lesser wear rate and hazard of disruption. So 28mm femoral caput was chosen as the mean and better one by Livermore and Morrey ( 1990 ) . Even Andrew and others ( 1998 ) suggested that the 28mm femoral caput had better consequences than the so existed 1s since the polythene wear rate was merely 0.11 to 0.17mm/year.

The present surveies are now chiefly focussed on larger femoral caputs with really immune stuff combination of femoral caput and cotyloid cup. Very recent survey upon this conducted by Tarasevicius and spouses demonstrated that the cumulative alteration rate is 2.8 times more in 32mm than those with 22mm. Besides, the additive wear and volumetric wear rate in 32mm caput is 3 times more than the other. This study was given after the follow up conducted in patients for 9 to 21 old ages. After all, it is sawbones ‘s pick to choose the appropriate femoral caput diameter after sing the patient ‘s anatomy and the chemical science of the disease effects.

Harmonizing to a really recent research, 21047 primary THAs were undertaken from 1969 to 1999 ( 30 old ages ) , in which, 8691 used 22mm caput, 8797 used 28mm caput and 3559 used 32mm caput. The chief complications in these were due to disruption of prosthetic device which was caused by the smaller femoral caput size. The patients with a disruption ne’er go for a reoperation since many who had undergone alteration surgery after the disruption had perennial disruptions. This once more shows the prevalence of larger femoral caputs.


22, 26, 28, and 32 millimeters are the most normally used diameters for the femoral caput. Harmonizing to Charnley ( 1979 ) , the 22mm caput produced lesser abrasion but more disruption. Subsequently 26-28 millimeter was considered to be the optimum caput diameter because it represents the best via media between abrasion and disruption. But the recent hip prosthetic device chiefly introduces a 32mm caput in order to avoid the hazard of disruption but at the same clip this showed a disagreement due to an increased wear rate of polythene. Finally it is proved that taking the intermediate value for femoral caput diameter is more applicable.

Optimum radius of the femoral caput

After making the literature reappraisal, I found that the optimal femoral radius is 14mm ( diameter is 28 millimeter ) . This produces lower wear rate, when compared to the 32mm diameter caput and hazard of disruption is low when compared to the still smaller 1s ( 22mm and 26mm ) . The cotyloid wall thickness can besides be taken in needed value whereas in instance of larger caputs, the cup thickness should be much more decreased. There is adequate scope of gesture and stableness to articulations. The additive wear and volumetric wear is relatively really low. There is adequate lubrication infinite in between the cup and the caput.


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