13 Mar 25-27.2. 2021 | EKB Endocongress | ARTIQO Short Shaft Symposium
25.–27.2. 2021 | EKB Endokongress | ARTIQO Short Stem Symposium
The short shaft as a new standard
The A2® short stem prosthesis not only scores very well in the German Endoprosthesis Register, but also has a well thought-out range of products for a broad spectrum of applications. At the ARTIQO symposium “Short stems – the new standard – cementless to cemented” at the Endoprosthetics Congress Berlin on 26 February 2021, the experts chaired by Dr Frank Horst, Sendenhorst, discussed whether the short stem has what it takes to become the standard stem. For the A2® stem, the answer seems to be clear.
Short stems are becoming increasingly popular in hip arthroplasty. According to the German Endoprosthesis Register, more than 10% of the stems are now of this type. In his lecture, Mario Frank, member of the management and developer of the ARTIQO company, reported that conceptual advantages are attributed above all to the calcium-guided short stems in comparison to conventional stems (M. Frank / Development of a modern short stem system). In addition to the good usability for minimally invasive approaches, there are advantages in the reconstruction of the femoral offset and femoral torsion. In addition, these can be adapted to the individual CCD angle and ensure a more physiological application of force.
The anchorage of the calcar-guided short stems is ideally a longest possible support on the calcar, a lateral support on the femoral neck and a lateral-cortical support of the prosthesis stem in the lower third. In addition, there is proximal conical clamping and cortical support in the axial view. Compared to distally anchoring stems, calcaneally guided short stems have a more favourable biomechanical pressure distribution, Frank explained.
A range of sockets for (almost) all cases
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This means that the lime-guided short shaft has the potential to become the standard shaft. With a view to a wide range of applications, however, this cannot be addressed with just one shank shape, Frank emphasised. For the development of the A2® stem, the ARTIQO team therefore analysed proximal femora and measured a variability of the calcar radius from 34mm to 134mm for the typical user spectrum of 120 degrees to 135 degrees CCD angle. “Especially the calcar radius is enormously important as a guiding structure for the positioning of the short stems and the bony support,” Frank emphasised.
The conclusion of the ARTIQO development team was to not only limit the range variability to the taper angle and offer two anatomical neck angles with an angle difference of 6 degrees, but also to include the calcar radius. The “two body” design concept of the A2® stem therefore has stem type B for more varicose and normal femora and stem type G for more valgus femora. Finally, the support of the prosthesis tip has also been optimised for the two stem types.
Another development goal was optimised size growth. The growth factors were adapted in such a way that the cortical femoral neck ring on the resection surface can be retained as far as possible. As a result, the A2® stem system has 19 finely tuned stem sizes, subdivided into type B and G, which can be comprehensibly distinguished by a colour code.
The first cemented short socket
In the five years since its market launch, the A2® stem has been implanted approximately 14,000 times in Germany. The new cemented short stem version, which has been available since January 2021, rounds off the system. Gerade für schwierige Femurmorphologien mit weitem Markkanal und dünner Kortikalis (Dorr Klassifikation Typ C) könne das neueste A2® Familienmitglied eine gute Lösung sein, konstatierte Frank.
Prof. Dr. Johanneszeichen, Minden, also referred in his lecture (Prof. Dr. med. J.zeichen / Why a cemented short stem?). Complications such as greater trochanter avulsion or intraoperative fractures, which still occur with cementless stem insertion and often in elderly patients. Limited bone quality and pronounced osteoporosis are still considered indications for a cemented implant. The cemented A2® short stem was developed in order to be able to provide these patients with good anatomical care in the future. Sign explained that medial femoral neck fractures in combination with osteoporosis and coxarthrosis in combination with pronounced osteoporosis could now be treated with an anatomical short shaft.
Not all short shafts are the same
The crucial question for the short stem is: how does it hold up with the proximal anchoring and the primary stability? The two concepts are supposedly in conflict with the short shaft, explained Priv-Doz. dr Stefan Budde, Hanover, in his lecture (PD Dr. med. St. Budde / Data on the short stem – where is the A2® stem? FEM analysis, EPRD data, RSA study). However, short stems are not just short stems, and above all, not only the length is decisive. There are shortened straight shafts that behave like straight shafts (distal force transmission) versus “real” short shafts that also make a biomechanical difference.
And this is how the A2® stem makes a biomechanical difference. Using a finite element analysis (FEM), Budde showed that there is a very slight reduction in density of the bone surrounding the prosthesis and that there is no distal load transfer. In addition, the support reaction on the lateral cortical ring protects against valgus sintering. With the FEM, the implant component is mathematically divided into the smallest sub-areas (“finite elements”) (wikipedia.org /wiki/discretization) so that their physical response to forces and loads can be calculated. The physical behavior of the entire implant component results from the loads and reactions at the interface between the smallest elements. “The special coating design of the A2® socket is the result of a simulation in which various coating options were calculated. The coating that prevailed was the most beneficial in biomechanical FEM,” explained Budde. When asked if he thought FEM would make sense for all new implants to be introduced, Budde replied: “Yes, that’s recommended, even if it’s complex and requires expert knowledge. It offers itself pre-market without patient involvement. In this way, design changes could be tested in advance.”
A2® shaft shows very good primary stability
The second aspect of primary stability can be examined with radio stereometry analysis (RSA). With this method, the postoperative migration of stems in the bone can be checked over time. According to Budde, there are essentially two migration schemes. “One type of migration initially moves after implantation and then stops. This benign migration type grows firmly and promises a long service life. The other type moves strongly at first and then steadily moves on – an alarm sign of impending loosening at an early stage.”
The RSA offers a high level of information about the migration and long-term behavior of implants, even in small patient groups and short follow-ups of two years. Budde emphasized that experts demanded that RSA studies should be mandatory for the launch of new implants. In the method, the smallest marker beads made of biologically non-toxic and chemically inert tantalum (Ø 1mm) are inserted into the bone surrounding the prosthesis. Immediately after the implantation and at defined postoperative times, two simultaneous X-rays are taken. The patient lies on a calibration box. With special evaluation software, the movement of the implant can be calculated with an accuracy of 0.06mm. The RSA for the A2® socket is still in progress. However, the preliminary results showed very good stabilization of the implant and very favorable migration behavior, explained Budde.
A2® socket is the leader in the endoprosthesis register
Apart from science, what does the reality of care look like? Budde recommended reading the EPRD annual report 2020 (Very good EPRD results for fittings with the A2® short stem). Overall, the register indicates a significantly lower probability of failure of short sockets compared to standard sockets in patients under 70 years of age. The A2® stem shows the lowest probability of failure of all cementless stems examined after one, two and three years. “With the results, you have to come up with good arguments if you don’t want to implant the A2 stem,” concluded Budde.
A2® socket as standard in clinical use
dr Frank Horst, Sendenhorst is already convinced of the potential of the short stock as a standard stock. In his lecture (Dr. med. F. Horst / The short stem as a standard stem Clinical experiences / Data and range of applications), the co-developer of the A2® stem system shared his extensive Short shaft experience of the past 15 years. The A2® stem is now also being used in Sendhorst for varying osteotomies, for people over 80, as post-traumatic care and as a replacement implant for aseptic short stem loosening. Of the 14,000 A2® sockets in total, 3,813 were implanted in Sendenhorst.
According to Horst, the preoperative planning determines whether the A2® socket type B or G is to be used. “You have to pay attention to whether there is a rotation of the femur on the X-ray. It may be necessary to plan for the opposite side.” The prosthesis is planned along Adam’s arch. “You have to be aware that you can only plan two-dimensionally, while intraoperatively you have a four-dimensional situation that includes the tissue consistency,” emphasized Horst.
Accordingly, the size of the implants can essentially only be determined intraoperatively. “We strive to insert the smallest possible cup – since 80% of the loosening takes place at the cup – and the largest possible shaft,” Horst explains his approach. To protect the calcar, the rasp is inserted along Adam’s arch. It is advisable to start with 1G after placing the awl in order to be able to orient yourself as closely as possible to Adam’s arch. An essential point is the combined anteversion of the shaft anteversion and the anteversion of the cup, which is approx. 35-40 degrees: “It is therefore recommended to carry out the short shaft preparation before the cup implantation”, advised Horst.
Very good results confirm the procedure
With this approach, Horst and his team achieve very satisfactory results, certified by the EPRD. With regard to the number of changes to be expected (82.18) with 2,673 registered first implantations, the St. Josef-Stift Sendenhorst performs significantly better than the average. In the EPRD Clinic Evaluation 2020-2, the hospital has a switching probability of 1.1 after two years for operations between October 2017 and March 2020 for elective total hip replacement with a cementless shaft, while in other hospitals there is a switching probability of 3.1 after two years.
Horst reports that an observational study on the A2® socket is currently underway. A total of 172 patients in 6 centers are planned for the clinical and radiological follow-up after 3 and 12 months, 3, 5 and 10 years. The short-term results for 159 patients at 3 months and 114 patients at 1 year each showed a 100% stem survival rate. Both the Harris Hip Score (HHS) and the Hip Osteoarthritis Outcome Score (HOOS-PS) improved significantly after three months and again after one year (HHS at 96.7 and HOOS-PS at 95.8 at 12 months ).
