With the spreading use of total hip arthroplasty, the number of revision for aseptic loosening is growing year by year; unfortunately the clinical results of the revisions are definitely worse than the first implants [13].
These remarks led research to develop several systems of fixation, which could warrantee a longer survivorship of the implant, leaving a sufficient bone stock for revision. Particular interest was devoted to hydroxyapatite (HA), which could be fixed to the metal surfaces of the components using different techniques [14], specially plasma spray one.
HA coatings have been shown to induce strong union with bone and to promote early stable fixation of the implant in an animal study [15], in a human retrieval study [16] and in early-term clinical follow-up studies [17–19]. So, it was hypothesized that the use of HA coverings could enhance biologic fixation of the implants, improving thus the longevity after midterm follow-up.
Although good medium and long term results with HA coated femoral stems have been reported [20, 21], the use of HA coating on smooth hemispheric acetabular components does not seem as successful as in femoral ones [9, 10, 18, 20–24].
Some authors reported satisfactory short term results using HA coated smooth hemispheric implants, noticing a reduction of cup migration and of periacetabular radiolucent lines [25–27]. In a multicentric study, D'Antonio et al. reported that, at two years follow-up, in a cohort of 320 HA coated cups, only three patients showed a significant migration, but none required a revision [26]. However, these initial encouraging results were not confirmed in mid and long term follow-up: poor results have been reported with HA-coated smooth press-fit cups from different manufacturers, with a revision rate ranged from 20% to 30%, after 7 to ten years follow-up [9, 23, 24, 28–30]. Recently, Kim et al. reported poor results with the same cup of our study after midterm follow-up with a 13% of revision rate and 60.5% survival at 8 years with any revision as end points. In our study the rate of revision at an average follow-up of 10 years was 12%, but we noticed a higher rate of osteolysis, which interested both the cup and the proximal femur (respectively 89% of the cups and 70% of the stems). In literature the rate of osteolysis range from 28% to 66% [23, 24]. This date could be partially explained with our longer follow-up. The periacetabular radiolucent lines incidence is comparable to the one found in other studies on HA coated cups; even the location of the line is mostly in the Zone 3 [9, 23, 27].
Only in 4 implants we reported variation of the acetabular angle higher than 5°, compatible with implant loosening according to the limits founds in literature [7]. In these patients, the angle variation was associated by linear migration of 0,3 mm, 1,3 mm, 1,8 mm and 1,4 mm. Anyway, in none the radiographic pattern was related to a low clinical evaluation (HHS 100, 97, 97, 90). It has been observed that all these four patients were in origin affected by dysplasia.
The polyethylene wear was slightly higher in our study than the one found in literature for HA coated cup with the same follow-up [9, 31, 32].
There are several possible reasons for failure of the HA-coated smooth hemispheric acetabular cups used in literature [33, 34]. Manley et al. [23] evaluated 377 patients (428 hips) with a porous coated, press-fit acetabular cup, an HA-coated threaded screw-in cup, or one of two similar designs of HA-coated press-fit cups after an average of 7,9 years of follow-up. In this study, the probability of revision due to aseptic loosening was significantly greater for the HA-coated press fit cups, than for the HA-coated threaded cups or the porous-coated, press-fit cups (p < .001 for both comparisons). The HA-coated threaded cups and the porous coated press-fit cups continued to perform well more than 5 years after the operation.
The unsatisfactory results on the acetabular component suggest that in the specific biomechanical environment of the acetabulum, physical interlocking between the cup and the supporting bone beneath it may be a prerequisite for long-term stability; thus cup design is very critical for its performance [35, 36]. Therefore, despite the good short term results with HA-coated press-fit cups (2–3 years), fatigue failure between the metal surface and the HA coating, arising in response to prolonged distractional stress medially imposed by the patient's activity, was thought to be responsible for the separation of the socket from the bone in the case of press-fit cups in the long term [24, 37]. In other words, continued application of physiologic loads, especially tension and torsion, will cause motion and distraction between the acetabular components and the osseous structures beneath it, and progressive loosening at the interface and failure of fixation may occur. Initial stability dependent on a press fit and screws will necessarily fail [38]. The HA-coated threaded cups achieved sufficient bony and/or soft tissue interlock to resist the force load on the acetabular cup, whereas the HA-coated smooth hemispheric acetabular cups in many cases did not [9, 39, 40].
In HA-coated implants, one of the most important events occurring at the bone-implant interface is the resorption of the HA coating, also called "degradation or coating loss", sometimes with the presence of HA particles. Although it is essential for the establishment of bone-implant bonding, this has been one of the main concerns for the durability of the HA-coated implants.
Some studies have shown resorption of HA coatings up to 2 years after implantation [41–43] and a complete loss of a 60-mm-thick HA coating after 4 years [44].
Therefore, the long-term durability of the fixation enhanced by the HA coating is questionable [45, 46]. Direct contact of bone trabeculae with the surface of the implant after degradation of the HA coating is dependent on implant material, texture, and design. Application of an HA coating to an implant with a smooth surface increases the risk of delamination of the coating compared with its application to a porous surface [46, 47]. Resorption of the HA may cause micromotion with an increase in shear stresses, resulting in delamination of the HA, especially on the medial side of the cup.
An unacceptable accelerated polyethylene wear rate and high prevalence rate of pelvic osteolysis is described. Some authors suggested that HA particles could move and cause third-body abrasive wear, which subsequently could cause accelerated polyethylene wear and development of osteolysis [48, 49].
The use in our department of a protocol for the examination of the retrieved implant and the bone-implant interface, give us the possibility pointed out something about the mechanism of failure.
The SEM examination of the cups showed the complete disappearance of the coating, as observed in other studies [44], and the complete absence of bone ongrowth. No HA particles were found on polyethylene and the microanalysis of the waste on the liner pointed out not only Ca e P, but also other elements such as Ti, Al, C, O, which can be decay products also of the metallic alloys forming the metal back and the screws. Therefore, it is impossible to assert with certainty HA may cause increased polyethylene wear.
Many polyethylene debris were found in periacetabular tissue, using polarized light microscopy (Fig. 10).
Some authors believe that the incremented rate of osteolysis could be attributed to the fretting between the screws and the dome holes [50, 51]: we can't confirm this hypothesis, because no association between the use of screws and both the presence and the dimension of osteolysis were found (p <0,05). Manley himself had stated in his study [23] that the dome hole could not considered a way of passage of wear of polyethylene.
The most interesting aspect of our study is the discordance the clinical and X-Ray results.
In spite of the incidence of osteolysis, most patients are absolutely asymptomatic and satisfied with their life quality. These bone rarefaction areas do not weaken the mechanical stability, but being progressive [9], when the revision is performed, we may risk to face such poor bone-stock as to spoil the result of revision operation. Thus, revision rate is lower than other study, as it's very difficult to give such indication in asymptomatic patients.