THR: Cemented and uncemented

[Josephine]

Hips can be inserted with or without cement.

NB This article applies equally to knees though the surgeon would not be using the cement gun or a pressurizing tool.

Uncemented hips
These are fixed into the bone using very accurate reaming of the bones, a super accurate fit and a special coating on the prostheses into which the bone will grow. This is a material called hydroxyapatite which is a porous coating, sometimes made of very tiny beads formed in the manufacture of the implants. The intention is that new bone will grow into this coating thus making a strong bond between the implant and the bone. In order to facilitate this growth, it is necessary for the preparation of the bone be done with care and accuracy, ensuring a close, tight fit.

Reasons for use:
~ surgeon's choice
~ age: sometimes it is thought that using an uncemented hip in younger patients gives them a advantage in that there is some bone conservation
~ activity: it is thought that a patient more inclined to sporting activities is less likely to have problems with loosening with an uncemented hip
~ in cases where patients have an allergy to methylmethacrylate

Advantages:
~ no cement to fail
~ in theory a more secure fixation of the implants

Disadvantages:
~ some surgeons require the patient to be non-weight bearing for some weeks after surgery in order to avoid interruption of the bony ingrowth
~ bony ingrowth can be interrupted for a variety of reasons including:

~ movement of the implants before bony ingrowth is established
~ inaccurate fit between the implant and the bone
~ presence of debris, blood and fat which can obstruct the bony ingrowth. ​

~ if this happens, it is unlikely the ingrowth will subsequently re-establish itself


Cemented hips
An epoxy mix is used called methyl methacrylate. This starts life as a bag of powder and an ampoule of fluid which are mixed together at the table in a vacuum vented bowl or syringe.

The mix progresses from a thick runny cream, similar to double cream, to warm plasticine then to a semi-malleable rubbery consistency and then, of sudden, becomes as hard as marble. As the process evolves and the short molecular chains join into long chains, a great deal of heat is produced, a thermochemical process.

The entire curing process from adding the liquid to the powder to the final, marble like set, takes about 8-10 minutes depending upon the ambient temperature and humidity. The mix will set more quickly in a hot environment than in a cool one.

In the first period, the mix is too fluid to be handled.
The middle period is when the mix is inserted into the bone and the implant also inserted.
The last period, when the final stage of curing is happening, the implant and bone must be held still and firm until curing is complete.
The end result is very solid and excellently resistant to fracture or cracking.

However, the success of using cement is generally influences by three issues

If the surgeon moves the prosthesis or applies blows with a mallet with the intention of getting it to seat more, he is likely to cause it to 'bounce back' in the rubbery mix, thus creating a potential gap between the prosthesis and the cement.

The crucial part of this process is preparation of the bone and the pressurization of the cement into the bone. This is done by using special tools in the acetabulum and femoral shaft. In the knee, it is done simply by using the pressure of the surgeon's thumb! But the bone preparation must be equally carefully done.

After the bone is prepared for the implants, it is then power washed with a pulse lavage gun which, having forcibly sprayed the operation site with pulses of water, then removes by suction all blood, fat and debris from the wound and from the matrix of the bone. Where any of this material is in the matrix of the bone, the cement cannot properly embed itself.

For the acetabulum, a mushroom instrument like this is used for the pressurizing the cement

And a gun identical to that used for caulking in your bathroom is used for the femoral shaft.

To stop the cement being forced too far down the shaft and also to provide some back pressure to force the cement sideways, a small plug of plastic, steel or bone is placed in the shaft before the cement is inserted.

By this means, the cement is forced into the bony spicules of the honeycomb (cancellous) bone. This results in the fragile spicules of bone being encompassed and protected by the bone cement and the cement therefore being firmly anchored by those same spicules of bone.

If the bone is not protected by the cement in this fashion, the tiny spicules risk being snapped off during weight bearing, resulting in a loose implant.

Most loosening of cemented implants occurs at the cement/bone interface, rarely at the cement/implant interface.

Reasons for use:
~ surgeon's choice
~ the extra 'filler' removes the necessity for such exact reaming of the bone as is required for uncemented implants

Advantages:
~ as stated above
~ immediate weight bearing

Disadvantages:
~ if not applied carefully, cement/bone interface can break down
~ rarely, some people have an allergy to methyl methacrylate

 

[Josephine]

The bone is a hollow cylinder filled with fine threads and spicules of bone and marrow fat like this

When the surgeon prepares this for the implant, he uses special rasps to clear away most of this fragile stuff to make a bed for the stem. When he has reached the area where the bone isn't so fragile any more, he can insert the cement under pressure which forces it into the honeycomb of the bone like so .... and then forces the implant into the cement as well which further compresses the cement into the bone ....

This has a dual effect:
1. the cement encompasses and protects the bony spicules and prevents them from breaking off under the load when the patient bears weight on the implant (which, if it happens, occurs over extended periods of time)
and
2. anchors the cement firmly deep in the bone.
This way both the bone and the cement remain secure and can last years and years.

 

[Josephine]

My reply to a member who had loose knee replacement implant

He showed me that not only has the cement debonded from the side of the stem. He does not believe that any activity I have done is the cause. "It is just one of not expected failures".

My friend, let me expand on some things about what you've been told.

1. If cement is correctly applied it cannot 'debond'. Cement is an inert substance, not dynamic. It's like polyfilla. Once it's put where you want it, it stays there no matter what until/unless regular movement of the screw or hook causes it to work loose. If bone cement is not pressed into the properly prepared bone, then it will lie across the top of the delicate bony spicules which cannot tolerate loading like that. The result is that these spicules break up and die back, hence the creation of space between the bone and the cement.

I still have a cemented hip which was removed back in the days before we properly understood about the finite issues around cement security. The cement came out of the femur in one piece like a dagger sheath and the side that was against the bone is completely smooth. Cement is all but impossible to dig out of the bone if it's properly embedded.

2. whilst not specifically caused by your activity, the low standard of cement security made it almost inevitable that your activity would have contributed to the loosening if only making it show earlier than had you been an 80yr old popping down to the shops every other day. But you most certainly did not cause it to loosen.

3. "one of not expected failures" is a bit of an disingenuous statement. It is well reported in medical literature and teachings that loosenings of cemented implants will always occur in the cement/bone interface in the presence of poor cement security.