Does shape and size of the stems affect the stress-shielding around press-fit radial head arthroplasty?

Aims It has been hypothesized that proximal radial neck resorption (PRNR) following press-fit radial head arthroplasty (RHA) is due to stress- shielding. We compared two different press-fit stems by means of radiographs to investigate whether the sha pe and size of the stems are correlated with the degree of PRNR. Methods The radiographs of 52 RHAs were analyzed both at 14 days postoperatively and after two years. A cylindrical stem and a conical stem were implanted in 22 patients (group 1) and 30 patients (group 2), respectively. The PRNR was measured in the four quadrants of the radial neck and the degree of stem filling was calculated by analyzing the ratio between the prosthetic stem diameter (PSD) and the medullary canal diameter (MCD) at the proximal portion of the stem (level A), halfway along the stem length (level B), and distally at the stem tip (level C). Results Overall, 50 of the 52 patients displayed PRNR. The mean PRNR observed was 3.9 mm (0 to 7.4). The degree of endomedullary stem filling at levels A, B, and C was 96%, 90%, and 68% in group 1, and 96%, 72%, and 57%, in group 2, with differences being significant at levels B (p < 0.001) and C (p < 0.001). No significant correlations emerged between the severity of PRNR and the three stem/canal ratios either within each group or between the groups. Conclusion PRNR in press-fit RHA appears to be independent of the shape and size of the stems. Other causes besides stem design should be investigated to explain completely this phenomenon.