Fitmore® Cup

Firm seating and transmission of forces as in a natural joint

The press-fit system ensures high primary stability. This is a fundamental pre-requisite for secondary stability by means of osseointegration and ensures a long working life for the cup. The press-fit is due to the oversizing of the implant compared to the last reamer used. The cup is impacted that a close bone-implant contact is achieved in the equatorial region. With the press-fit, the transmission of the forces from the cup to the bone takes place on the peripheral equator, as in a natural hip joint.

Large contact surface, but no direct contact on the bottom of the acetabulum

The Fitmore cup is meant to be anchored more firmly in the acetabulum by an over-sizing of 1.5mm, and to come into more extensive and closer surface contact with the bone. The cup is chosen that its radius coincides with the last reaming pass. This gives rise to pre-tensioning in the equator, which holds the cup firmly in a bony seat over a large surface area.

In order to avoid a reduction of the pre-tension, the cup may not touch the bottom of the acetabulum. This requirement is met by the design of the Fitmore cup. The centre points of the curvature of the cup are eccentrically displaced outwards by 0.75mm, and the cup is slightly flattened in the polar region.

The slight narrowing of the cup on its equator due to the presence of the weld seam causes a lowering of the pressure on the rim of the cup imparted by the bone. The bone reaches slightly beyond the rim of the cup. This increases the retentive force exerted on the cup.

Depending on the type of Fitmore cup, anchorage may be achieved by stabilising fins or screw cones. This means that in special cases screws can be implanted in order to further decrease the risk of tilting or rotation.

Design Rationale 2 

Thanks to its press-fit effect, the Fitmore cup provides a guarantee of sufficient primary stability. For difficult cases, however, additional safety against tilting moments and rotation is desirable. The shell of the Fitmore cup is therefore equipped with additional elements for maximising mechanical stability.

Two mechanisms are available: a cup with sharp-edged stabilising fins and a cup with sharp-edged screw cones, through which insertion of cancellous screws is also possible.   
   
Either

Increased safety due to fin-like elements. The Fitmore cup is available in the version with self-tapping stabilising fins. These are welded into place on the titanium shell and consists of a pair of fins offset by 20° from one another, and pointing towards the pole of the shell. They are triangular in shape, and sharpened on their outer edges. The stabilising fins are made of Protasul™-100, since this high-quality alloy has a higher mechanical resistance than pure titanium. When the Fitmore cup is impacted into place, the stabilising fins penetrate into the acetabulum in an axial direction, and effectively prevent the implant from twisting or tilting over. The additional stabilising of the cup by the fins takes place in the equatorial region near the rim of the acetabulum, where the bone has its most favourable mechanical properties.  

Or

Increased safety with the screw cones and screw option.

As an alternative to the type of cup with stabilising fins, there is the Fitmore cup with screw cones in the cranial region. Depending on the size of the cup this version is fitted with one or two screw cones. The drilling axes run at an angle of 10° to one another. The outer edges on the screw cones are sharply cut and extend for a maximum of 0.9mm beyond the external radius of the shell. When the shell is impacted into place, the screw cones penetrate slightly into the cancellous bone of the acetabulum and have an immediate additional stabilising effect.

The system is absolutely stable without being screwed down. The authors only recommend the use of screws in exceptional cases in which the condition of the bone warrants additional stabilising. The insertion of countersunk cancellous bone screws, made of Protasul-100, achieves a closer bond between the cranial bony seat and the implant. This leads to consolidation in the cranial region. The conical drilling of the holes for the screws provides a margin of 15° for the direction of the screws. The screw axis can therefore be chosen freely within this range at the time of surgery on the basis of the individual anatomical conditions
 
Increased press-fit effect due to higher pre-tension

In order to optimise primary stability and achieve an even higher level of safety, it is possible to increase the press-fit effect. To do this, a cup with a larger radius than that created by the last reamer in the acetabulum has to be chosen. It is possible, for example, to implant a size 52 cup in an acetabulum prepared using the size 50 reamer.

Increasing the press-fit effect is specifically recommended by the authors, and has proved its worth in practice. In most cases the risk of a fracture of the acetabulum can be excluded. According to the authors’ recommendations, a press-fit using the same sizing for both the implant and the acetabulum is indicated only in extremely unfavourable conditions. For instance, with severely sclerotic, polyarthritic, rheumatic or cortisone-treated bone.