2.2. The Priciple of Primary Stability
2.2.1. Summary
Much more than simply primary "immobility" tested manually, the Primary Stability in my view is obtained if, at the end of the operation, the prosthesis is fully capable of fulfilling its mechanical function of support of the constraints exerted by the patient on the bone and the implant. ( Of course, for many other reasons, the patient can not be loaded immediately. )
To obtain the Primary Stability, the contact area between implant and cortical bone should be large enough that the bone is in the Field of Bone Vitality at all points of contact and nowhere overloaded, and in Prestressing so that micromovements can not appear.
If the implant would require a bone growth to be stable and be in charge, primary stability is absent. With the Primary Stability, bone growth is optional, but welcome.
2.2.2. The real primary stability
I think is the ability of the prosthesis to fullfill, immediately after the operation, the mechanical function of support from all the constraints exerted by the patient on the bone and the implant. This theory of immediate loading can obviously not be completed because the limitations due to hemostasis, the drains, the successive layers of sutures and the risk of skin infection delay the loading of a few days.
Much more than a simple primary âimmobilityâ, Primary Stability, in my opinion, is obtained so as of the first day:
a) The pressures applied to the bone by the prosthesis are for all the zones of the prosthesis, without exception, in the Field of Osseous Vitality
b) The bone is Prestressed on all the zones of contact
c) The micromovements between the implant and the bone are eliminated
d) By the preceding properties, bone regrowth is made optional, but welcome. If this implant required regrowth to be stable and be put in charge, primary stability would be non-existent, we could only speak about secondary stability and non Primary Stability.
It is obvious that surface microstructure is necessary for the medium-term Osseointegration and secondary consolidation of the implant.
In the case of custom prostheses made ââby computerized copy of the internal shape of the complex nature of the bone cavity, the Primary Stability can not be reached, because the Prestressing is absent and Micro-Movements are not eliminated.
2.2.3. The Partial Correction
The Primary Stability of the implant is obtained by a partial correction of the complex natural forms of the osseous bed to approach the more geometrical shapes of the implant.
Primary Stability by partial correction can be reached only for stems whose fixing is of conical or pyramidal nature, whether rectilinear or curved.
Partial correction is obtained by a working of the osseous cavity by rasping or boring without necessarily seeking the perfection of the form of prepared surface.
It is obvious that the osseous cavities, before preparation, comprise interior asperities and variations of form. It is not necessary to continue correction until the disappearance of hollow zones where the close contact will obviously not take place.
The conservation of space between the bone and the implant in the noncarrying and not prepared zones, like part of the former and posterior faces and allowing intramedullary blood circulation is an important advantage.
The interior surface of the osseous cavity must be prepared, by taking account of the concepts of Impaction Reserve and Prestressing.
It is essential that there is sufficient rectified surface so that the whole of the constraints exerted by the prosthesis on the bone are distributed on sufficient square millimeters so that the bone is in all contact points subjected to pressures lower than the threshold of resorption per excess of constraints and remains in the Field of Osseous Vitality.
An excessive correction of the osseous cavity could have like consequence a total weakening of the bone over the entire length of the stem, an useless loss of osseous capital and an useless reduction in intramedullary blood circulation and would lead to the installation of a stem of unnecessarily thick size causing an excessive rigidity of the prosthezed zone and a too abrupt fall of rigidification in the vicinity of the point of the stem.
With a too thick stem, the dynamic stress of the bone surrounding the prosthesis is insufficient so that the bone is reinforced, in the long run, around the prosthesis.
The key is to prepare a cavity which does not comprise any more localized asperities which would encroach in geometrical volume that the prosthesis will occupy and whose consequence would cause premature blocking, local osseous pains and risk of cracks at the time of the impaction of a stem of pyramidal form.
It is for these reasons that the aspect of the stems with Geometrical Anchoring, at first sight, is of a great simplicity.
2.2.4. Preparation for Osteointegration by the Primary Stability
To stimulate a reconstructive reaction of the bone to the immediate vicinity of the implant, it is essential that the bone is put in tension vis-a-vis the implant. This setting in tension can be provided by junction of conical type, obtained on the one hand by the geometrical form of the fixation zone of the implant itself, and on the other hand by the precise preparation of the osseous bed which will constitute the female conical part of the junction.
This geometry allowing the constitution of a conical junction is not sufficient. It is the impaction of the implant of the additional axial distance permitted by the Impaction Reserve and envisaged when designing the rasp, which brings the prestressing of the bone, therefore the prestressing of the assembly which is essential to the primary stability and the suppression of the micromouvements.
2.2.5. Primary Stability should not be confused with primary immobility
The primary immobility is only the result of a manual test of the Operator at the end of the implantation. In the best of the cases, this manual test is applied with a force not exceeding ten kilos to the implant tested in place. True Primary Stability must practically support the efforts and the weight of the patient and the dynamic constraints which he exerts on the implant, level of resistance which cannot be tested of a simple manual gesture.
Primary stability should either not be confused with the resistance of the stem to a manual traction of axial extraction. A stem fixed by junction of the conical type does not require this type of test, being in theory never subjected in vivo to tractive efforts, the prosthesis being always subjected to an axial compression.(except retentive prostheses)
Any conical junction is not very resistant to axial traction. The axial tensile strength is brought only by the friction between surfaces in presence in the conical junction, friction due to the microstructure of the implant and its incrustation on the surface of the bone whose hardness is less. This friction must be exerted on a sufficiently wide surface of contact.
A stem, whose rasp is not calculated to create a conical junction, is wedged more in the osseous bed because of the differences in form between the rasp and the stem and resists paradoxically better the manual test of traction, certain points of the stem much more âbeing blocked or being wedgedâ in the bone. It was the case of the ZweymĂŒller stems of first generation.
Secondary stability for cups with thin hull, will be obtained in a few weeks and a few months for the stems without cement, because the osseous cells could be fixed on the biocompatible surface of the implant without being periodically destroyed by micromovements or macromovements permitted by an insufficiency of Primary Stability.
I am persuaded that the excellent statistical results of the stems AlloClassic then SL Plus and the cups Bicon Plus are the result of their excellent Primary Stability for all the patients but especially for those whose bone growth is diminished or nonexistent.
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