The present study found a high amount of osteolysis around the neck of the press-fit prosthesis used. In conclusion from the analysis of the imaging and clinical records, proximal osteolysis is common in the investigated implant and is not indicative of loosening within the investigated follow-up period. It does not correlate with patient demographics in the present cohort.
Pathologies of the radial head, like fractures or degeneration, represent clinically relevant entities. In the case of radial head fractures, which are common lesions accounting for up to 20% of all elbow injuries, anatomical reconstruction and repair of the commonly accompanying lesions should always be the primary goal if significant displacement is present [
24]. Only unreconstructable fractures should be replaced by a prosthetic implant, as the authors do not believe that the high energy injuries that often accompany traumatic joint instability can be treated with head resection alone. Several studies have shown the biomechanical benefits of replacing an absent radial head with an implant, both for valgus stability and for function of the interosseous membrane [
5,
6,
25]. However, as clinical studies found, prosthetic replacement of the radial head comes with complications, with loosening being the most common one together with overstuffing [
26]. The most commonly observed loosening is aseptic loosening [
9].
The study by Popovic et al. investigated in detail the radiological and functional outcome of cemented bipolar radial head prostheses [
20]. The authors included 51 patients that had received a radial head replacement for unreconstructable fractures. An intensive analysis was performed regarding signs of osteolysis and bone loss around the stem of the implant. The results showed that in 16 patients, accounting for 31%, bone loss around the neck of the prosthesis was found. Besides that, 53% of the patients showed peri-prosthetic lucencies, and in 10% progressive loosening was present. The authors concluded that the proximal bone loss might be caused by wear debris and a subsequent inflammatory response, or due to altered loading at the neck, as a result of the modified mechanical properties that arise with the implant. Aseptic loosening is most likely a product of mechanical failure of the implant, where the fixation of the implant is overcome by its load [
27]. But also, as shown for other joints, inflammatory factors may play a role as a response to particle wear [
27]. Loosening can also be as a result of infection, where inflammatory agents and bacteria lead to bone resorption and, consecutively, loss of fixation of an implant [
28]. However, the aforementioned patterns of loosening at the elbow usually happen along the shaft of the prosthesis. Osteolysis below the neck of a radial head implant has only been investigated in a handful of studies. Laumonerie et al. found osteolysis around the neck of the implant in 33% of patients after implantation of the MopYc prosthesis [
29]. The authors performed a meta-analysis, reporting on a total of 171 patients, and explained the high rate of proximal loosening with the auto-expanding mechanism of the press-fit implant, leading to stress shielding. Based on the authors’ findings, they also do not believe that the described bone loss is a sign of loosening. They believe that the observed osteolysis is occurring according to Wolff’s law (Julius Wolff 1836–1902), as a result of decreased mechanical loading in that area [
30]. As frequently observed with hip replacement, prosthetic implants reduce the mechanical stress on certain areas of bone as the load is transferred via the prosthesis [
31]. This phenomenon is known as “stress shielding”. Wolff’s law says that alteration of mechanical stress will lead to bone remodelling. In the case of reduced load, bone resorption takes place. This might be the case in the investigated press-fit implant. As it finds stable anchorage at the mid-section of the shaft component, the area under the neck sees a reduction in load, thus resulting in bone resorption. The data presented here can neither confirm nor exclude whether the proximal osteolysis leads to loosening of the implant. In total hip arthroplasties it could be shown that a high amount of stress shielding contributes to or even causes loosening [
30]. Although on the basis of the current data it is tempting to see no association between proximal osteolysis and loosening in general, due to only one case of loosening in the cohort, the limitations of the study do not allow a final assessment. On the one hand, with 30 patients, a limited cohort size is presented. Implantation numbers for radial head prostheses are small compared to other implants like in the knee or the hip. Therefore, it is always challenging to achieve large cohorts for elbow prosthetic implants. It would be of benefit to have a larger cohort to investigate in this regard. Multicentric studies would likely be beneficial to this end. Due to the number of cases, it was not possible to create subgroups according to accompanying lesions or pathologies, which might obscure the results of the present study. Another limitation is the lack of clinical outcome data, which would enable correlations to be drawn between the amount of resorption and functional scores. This would make it easier to judge the effect of that bone resorption on the patient’s well-being. Also, only one implant was investigated; therefore, it remains to be investigated whether other press-fit prostheses show similar areas of resorption.