Lorenzo Spotorno*, Franco Astore*, Guido Grappiolo° *Istituto Clinico Humanitas, Rozzano (Mi) °Ospedale Santa Corona, Pietra Ligure (Sv)
The CLS has today become, in virtue of numerous polycentric experiences, above all in Europe, The Golden Standard of non-cemented hip prosthesis 3,4,8. Besides this, it is the prosthesis that has resisted the severe market selection the best. Therefore in reflection has satisfied the users.
In the history of prosthesis surgery, many prosthesis designs have been tested but only a few have resisted time and wear.
In particular, in terms of survival, like the Golden Standard, in cases of cemented prosthesis there are the Charnley and the Exeter and in non-cemented, the CLS. This achievement is based on both the numerous comparisons of scientific work carried out by more than one author, of which we are part and on the influential evidence found in “ The Swedish National Hip Arthroplasty Register”.
It is true to be said that in this register it is possible to see clearly the survival of CLS, although in different casuistics, and in vertex of all the prosthesis, comparable only to the cemented Golden Standard. It is possible to analyse and compare our survival curve at fifteen years (fig.1) with those of other authors who have presented casuistics on CLS 1,3,4,7,8. There are highlighted curves very similar to those of cemented prosthesis.
With this data, we can establish the projections that we have obtained from CLS are certified.
These results have been obtained by clinical observations and a lot of work in the research laboratory, as shown in the large number of implants carried out on average and long term (over 250,000). To confirm this success rate, many companies have made copies of this implant. They are , above all, the princes of the “press-fit” and “surface micro construction” that have world wide success like the standard. The first aim of a non-cemented prosthesis is to form an osteointegration (2). This, however, already becomes trapped to an average term due to the possibility of an onset of a biomechanical pathology of the periprosthetic bone. On the base of experience gained, in many cases, this was caused by a biohumoral pathology from debris damaging the bone. As far as the biomechanical aspect is concerned, we know that an organ maintains trophic if it is kept in working order. The job of the bone is to transmit its load, while the prosthesis is a load mediator that disturbs the physiology. The stability and the osteointegration must be obtained by a prosthetic model which causes the least amount of disturbance possible to the normal physiology of the bone. It is common notion that the distal anchorage as other prosthesis carries a higher invasion risk and favours the stress-shielding and atrophy of the bone. Since its birth, the CLS system has been thought to prevent the manifestation of these effects, which could be defined as iatrogenic. From the numerous clinical observations and experimental tests, the idea was born and went on to suggest that the concept of the proximal anchorage appears to guarantee a para physiological transmission of weight. As the years went by, this concept of proximal anchorage has always been shared, developed and was associated to the concept of functional reserve. To obtain these results the stem was made in titanium alloy, due to its elasticity and with a conical-trunk form without an under proportioned neck and tail. Fundamental is its wrinkled surface, micro constructed by the implant. From the birth of the first stem to this present day 1-4, the continuous work of clinical experience and research have lead to the introduction of three fundamental modifications :
The modification of the proximal portion of the stem in radiator form obtaining a higher rotary stability .
The modification of the rasps’ design, obtaining a compression of the spongy mechanical proximal saving its use, so to speak “intelligent”.
The introduction of the improved offset in light to anatomic observations of the statistic distribution of the femur morphology as suggested by Noble, we have individualized three types of families: Valgus with important trochanter and a high centre of rotation; Normotype; Varus with a relatively increased offset but with a lower centre of rotation compared to the gran trochanter. Having made stems with different offsets has definitely improved the reconstruction of biomechanics, which is not a simple way to the offset but is considered the interdependence of all parameters of which have centre of rotation, offset, dysmetria, cervic-diaphysial angle and antiversion. (Fig2).
At this moment in time we believe that the life expectancy of a non-cemented implant depends on three factors: osteointegration, maintenance of the biotrophics and control of usage. In particular the last, has a great possibility to influence the first two.
The osteointegration is the function of stability and the osteofilia of the surface, whereas its conservation through time is regulated by the laws of biomechanics. In our project the stability is obtained through press-fit. The concept can still be broken down in the “fit”, that is in fairness to the two osseo-prosthesis systems and “press” which represents the pre-loading kilogrammes which are gained during surgery of the implant .
To obtain stability through the press-fit, space must be left to reactivate the guest bone. Since 1985, we have already highlighted the fact that it is necessary to protect the valid mechanically spongy part, for its high intrinsic potential to adapt to its working load. Naturally, osteointegration favours the characteristics of the implants’ surface. Indeed it has been demonstrated that an implant made of smooth titanium is less osteofilico compared to a mesh of titanium to bone- in growth.
CLS with micro pores, due to the sanding of titanium, was presented in numerous histological studies with very good signs of bone neoapposition and the percentage of this in the case of explant reaches 1.80% (Fig3).
In light to this fact, it is not therefore necessary to use superficial coverings with substances that are more osteophilic and paradoxically with superior mechanical risk. Indeed, although in less frequent cases, there has been a detachment of a layer of plasma spray from the titanium in the prosthesis (Fig4). We believe that the surface wrinkling obtained by the sandblasting with a coned jet gives both the necessary and sufficient to guarantee osteointegration abolishing an eventual crisis of interface .
However, we also know that a stable and cyclical compression is a condition essential for osteointegration.
If we observe implant failures, we can see that the main problem still remains that of the cup8,9. If the concepts of stress-shielding are by now universally excepted regarding the stem not the same thing could be said about the socket, which we believe are both as fundamental in respect to the acetabular kinetics, radiographically expressed by persistence or reappearance of the warhead. It was indeed highlighted that throughout the passing phase the weight causes elastic deformation in the physiological hemi pelvis as described by the Muller Foundation. With the expansion cup, the elastic deformation of the pelvis has the same sign of on going as the physiological ones.
A cup with a rigid hemisphere in the opposite position determines its strength at a ischiadic and pubic level. Therefore apart from the values measured , the expansion cup behaves better to the physiological prosthesis of the acetabular than to the biological one. If we can theoretically define all of these observations, we can of course establish the three-dimensional stability of this cup is decidedly superior to any other, for example in Fig 5, it is highlighted how in respect to the acetabular physiology allows restoring of the voluminous deficit at the base of the socket.
These days we tend to emphasize the problem of debris, it is well-known that it comes from a friction torch of from the interface of polyethylene -metal back. The debris induces bone spotting in the direction from the proximal to the distal which then goes on to associate to the normal migration in the distal direction, distributing the weight load caused by the biomechanics throughout time. These two factors interact and produce a decline in bone quality, proximal-distal direction, which is well highlighted in the densitometric test (DEXA).
It is important to analyse the cup considering these facts, taking into account the changing of the friction torch between the head and the polyethylene both comparable to the other types of cup and concentrating , above all on the interface of polyethylene -metal back. With regards to this other than the laboratory studies carried out, we examined six, normally working cup explants, which were not damaged, with a life span between six months and nine years (Fig 6). It highlights the volumetric consumption that comes from the face of polyethadene in contact with the metal-back, recording on average, only 5.4% of the total volume.
In hindsight, this consumption depends almost entirely on a normal plastic deformation, which is produced during screwing in procedure, while the real and true debris produced by the interface polyethilene-metal-back is insignificant. To this cup many windows and risk of tiny breakings have attributed to this. The experimental findings show a good source of survival (Fig 1). We can also add that the introduction of Durasul in the interface head cup has ultimately minimize the problem, drastically reducing the total production of debris.
In conclusion to all that regards the breakings, we can affirm that out of about 150.000 implants the statistical survey was low8.
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Fig 1. Curve of survival from the stem and CLS cup.
Fig 2. An example of how to obtain an excellent reconstruction of the biomechanical physiology, thanks to the availability of CLS stems with three different cervic-difisari angles, which allow the morphotype physiology Valgus-neutral-varo.
Fig 3. Histological tests of transversal sections of the CLS stem, highlighted in all samples present a very good bone intergration.
Fig 4. In this test taken under the microscope, one side highlights the condition of integration of the plasma spray with the bone. On the other the catastrophic detachment of a thin layer of plasma spray from the metal of the cup, a condition which could be favoured by a modest deformation of the metal cup both during the implant and in passing time.
Fig 5. These images highlight how in respect to the acetabular physiology restoring the voluminous bone deficit at the base of the socket.
Fig 6. This table highlights the volumetric consumption that comes from the face of polyethylene in contact with the metal back cutting in a minimum percentage in respect to the total volume, which, above all, depends on the interface of the polyethylene head.