Prosthesis – Mobile Bearing

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What is a prosthesis?

The prosthesis is the replacement of a joint with an implant that reproduces its movement and that is as close as possible to its physiology. The treatment of osteoarthritis and of certain types of ankle deformities may require the use of this mobile bearing technique.

The online informations

By researching information on the Internet about implant prostheses, one can read many opinions that are contrary to this type of intervention. This school of thought is wrong and is rooted in time due to the old types of prostheses, used more than 15 years ago.

Mobile bearing nowadays

However, today, with particular reference to the mobile bearing procedure, prosthesis is a reliable and effective solution. This also thanks to a new, high-performance type of prosthesis: the mobile-bearing one.

Mobile bearing; three components

This implant consists of three basic components: a tibial component, a talar one and a mobile bearing polyethylene insert (hence the term “mobile”) called “meniscus”.

 

Hintegra® “mobile bearing”

Hintegra® “mobile bearing”

 

Hintegra® “mobile bearing” ankle prosthesis – talar component

Hintegra® “mobile bearing” ankle prosthesis – talar component

 

Hintegra® “mobile bearing” ankle prosthesis - tibial component

Hintegra® “mobile bearing” ankle prosthesis – tibial component

 

Hintegra® “mobile bearing” ankle prosthesis - insert

Hintegra® “mobile bearing” ankle prosthesis – insert

 

The first generation ankle prostheses resulted from a partial redesign of the geometry and tribology (i.e. the study of friction, lubrication and wear) of the old hip prosthesis and is a lot different to modern day ankle prosthesis procedures.

The main problem turned out to be the mobilization of the components in the medium term, together with an incomplete understanding of the anatomy of the ankle in the designs of the time.

A modern mobile bearing answer

The European research has found an answer to this problem in the late ’90s, leading to the design of the mobile-bearing procedure. To achieve this result, it was fundamental to work on the “constraint” and “congruency”, i.e. joint bond and congruity: two antithetical principles to balance.

Put simply, a prosthetic implant must find a balance between the need to replicate the anatomy of a physiological ankle (congruency) and the need to achieve stability through a constraint (constraint).

The European solution of prosthesis with a mobile bearing insert has reduced the bulkiness of previous designs, thus saving bone tissue and using a “press-fit” attachment to the skeleton i.e. without the need of cementing.

Last but not least, the result obtained thanks to this new innovative design means a longer lasting implant.

Three key features make a mobile-bearing prosthesis reliable and efficient

They are all present in the Hintegra® implant I use:

  1. It must comply with the anterior cortex of the tibia, through which is transmitted more than 30% of the load supported by the tibia itself. In practical terms, it means avoiding the anchoring of the tibial component with the use of tracks sliding from front to rear. The Hintegra® prosthesis, in fact, consists of a front shelf perpendicular to the articular surface itself, leaning on to the tibia without requiring any bone interruption in front.
  2. It must have a talus in the shape of a truncated cone and not a cylinder, considering that, in nature, the talus is a bone with a median diameter inferior to the lateral one. Recreating this feature allows to promote in the patient a physiological walk.
  3. It must restore proper alignment between the tibia and talus.

For these three reasons I use Hintegra® implants that allow me to operate on ankles affected by large deformity, without having to sacrifice the movement of my patients.

 

ateral X-ray - Hintegra®

ateral X-ray – Hintegra® “mobile bearing” ankle prosthesis

 

Antero-posterior X-ray - Hintegra®

Antero-posterior X-ray – Hintegra® “mobile bearing” ankle prosthesis

 

The movement is a very important factor because, if you compare them to patients with hip or knee osteoarthritis, those suffering from ankle osteoarthritis may be much younger and therefore have more mobility needs in their daily lives. And, as mentioned previously, these new-generation implants, reduce degenerative stress and stimuli on other joints nearby the ankle and allow you to preserve the patient’s mobility as opposed to what happens using arthrodesis (fusion).

Preparation of the talus and tibia for the insertion of the Hintegra®

Preparation of the talus and tibia for the insertion of the Hintegra® “mobile bearing” prosthesis

Positioning of the tibial and talar components

Positioning of the tibial and talar components

Inserting of the polyethylene insert (meniscus)

Inserting of the polyethylene insert (meniscus)

The only two factors that may prevent a patient from choosing the prosthesis, and then possibly make them go for an ankle arthrodesis, are:

Possible lack of “bone-stock” in the talus (can be verified with a CT scan), i.e. bone reserve on which the implant is placed; 

The presence of an ongoing septic process (infection).

Finally, because of the small case series compared to other joints (hip and knee), it is very important in the case of the ankle to highlight the importance of going to a specialized centre and relying on surgeons who are already at the end of their learning curve.

What is a learning curve in surgery?

It is the path that each surgeon must necessarily follow before reliably mastering a particular surgical procedure. In this specific case, we will be with a professional who has done his learning path only after having witnessed and executed at least 10 prosthetic implants; to maintain this level, they will then have to perform at least 7/8 of them a year.

I recently explored this theme in a study I presented at the American National Congress (AOFAS, Chicago 2014), at a seminar held online on Footinnovate (American portal of foot surgery) and at the Italian National Congress (SIOT, Rome 2014).