Five-year Migration Pattern and Changes in Bone Mineral Density for the Tri-Lock and Summit Femoral Stems - Radiostereometric Analysis and Dual-energy X-ray Absorptiometry of 50 Patients.

BACKGROUND

According to the Danish Hip Arthroplasty Registry 201 approximately 10.400 primary hip joint prostheses in Denmark are implanted annually, and the incidence is relatively stable. The incidence of primary hip joint prosthesis in population was 180 per. 100,000 inhabitants in 2014, with a slight predominance of female patients (60%). The average age for women is 70 and for men 67 years. Idiopathic arthritis is diagnosed in 80% of all patients receiving a hip joint prosthesis [1,2]. For older patients, the risk of later revision of a hip joint prosthesis is small. The overall survival of the primary hip joint prostheses is for all diagnoses 82% after 19 years. The prosthesis survival is poorer for younger patients, and for women. In male patients younger than 50 years the 10-years survival of the prosthesis is 89 % and 82 % over 15 years. In women younger than 50 years the 10-years survival of the prosthesis is 87 % and 78 % over 15 years. [2]. As revision surgery is associated with higher complication risk for the patient and poorer outcome and implant survival it is necessary to examine possible methods that may increase long-term survival of the primary hip prosthesis or facilitate better outcomes after revision of hip joint prostheses for younger patients.

In younger patients the prosthesis often is inserted without the use of cement. When not using cement, it is crucial for the final result, that there is a direct bone ingrowth of the prosthesis. The chance of getting bone ingrowth depends firstly on a good immediate mechanical fixation at surgery and secondly on osteoconductive abilities of the prosthetic surface. From a theoretical point of view it is also important that elastic properties of the prosthesis differ as little as possible from the elastic properties of the bone, in order to prevent a subsequent loss of bone.

INVESTIGATED IMPLANTS

The Tri-Lock femoral implant from Depuy ® is in its basic design a well-known implant that has been used since 1981 with documented good long-term results. It is an implant with bone-preserving properties and proximal coating, providing a predominantly metaphyseal anchoring. The Summit femoral implant from Depuy ®, is newer to the marked, and has a longer diaphyseal support but is predominantly metaphyseally fixed. A proximal porous implant coating will increase the surface area in contact with the metaphyseale bone and increase the coefficient of friction compared to mid-coated implants [3,4,5]. Proximal coatings may on the positive side lessen periprosthetic bone stress shielding, however, on the negative side, proximal coatings may reduce primary prosthesis fixation and lead to fibrous membrane formation with consequently implant micromotions that will eventually causing aseptic loosening of the im-plant. Hypothetical extensively coated implants have a larger porous surface and thus provide better and faster primary fixation, but there are no publications of randomized prospective clinical studies to support this. There are different porous coatings on the two investigated femoral components. Summit is coated with Porocoat porous coating, whereas Tri-Lock in the latest version is coated with the new GRIPTION technique for an even larger surface for bone ingrowth (Gription) with a new ultra-porous surface marketed as an improved immediate im-plant fixation in the bone. The total surface of the metaphyseal implant part is increased by approximately 25% with the Gription coating technique. The Gription technique may provide consistent implant seating height and additional initial stability, and these qualities may in long term ensure an improvement on implant survival.