A Ferreira, Th Aslanian (Lyon)
For more than two decades, calcium phosphate ceramics, particularly hydroxyapatite, have been used in Europe to accelerate what is commonly referred to as the osteointegration of prosthetic components. Once implanted in the bone, these thin ceramic layers are transformed into a crystalline form (carbonated apatite), which facilitates the adsorption of proteins to the prosthesis and the rapid recruitment of osteogenic cells. At this point, if the primary stability of the implant is not compromised, these cells synthesize an osteoid matrix on the coating. Within a few weeks, mineralization of this matrix leads to the formation of bone lamellae and integration of the stem into the femoral diaphysis.
To support osseous fixation in the more proximal part of the implant, the placement of an under-layer between the prosthetic substrate and the hydroxyapatite coating makes it possible to obtain a surface landscape that will serve as an “anchoring volume” for newly formed bone.
In keeping with these objectives, we have defined a coating specification that is composed of a porous titanium layer of decreasing thickness, which is covered with a continuous layer of hydroxyapatite applied by vacuum plasma spraying. However different published studies have demonstrated that the metallurgical modifications generated by this type of deposit can sometimes be unfavorable to the fatigue resistance of the interfaces or the implant itself. It seemed absolutely necessary to quantify the mechanical effects of the various layers according to their projected characteristics, to confirm that these implants are not weaker.
This was an exploratory study with 150 consecutive primary intervention cases (mostly primary coxarthrosis) from June 2001 to March 2005. Radiological (cortical modifications, migration, edging and ossification) and clinical (objective and subjective) endpoints were evaluated.
With an average follow-up of 65 months (minimum 15, maximum 106), no revision was necessary and no acetabular migration was demonstrated from a clinical standpoint. No cortical bone remodeling was observed at the last radiological follow-up. The improvement of the average Merle d’Aubigné hip score at the last follow-up was very significant (from 9.5 to 17.8, p< 0.05).
The fixation of the acetabular component requires that the components be force fit (or press-fit), but imperfections in the preparation of the osseous bed and early loading of the implants can sometimes condemn it. The radiological parameters collected in our study demonstrated the stability of the bone-prosthesis interface. This series proves, in the medium-term, that a bilayer coating is clinically effective at providing reliable and reproducible cementless tertiary fixation that begins when hydroxyapatite is resorbed.