Changes of the bone structure after cap resurfacing arthroplasty of the humeral head.

INTRODUCTION: Although humeral head resurfacing with a cap is relatively common in clinical practice, clinical studies about the changes of the bone are rare. The aim of this study was to analyse qualitative and quantitative changes of the bone after cup arthroplasty. Our hypothesis is that the implant leads to a new functional load with remarkable changes of the bone underneath the cap.

MATERIAL AND METHODS: Overall, 12 explanted caps with adherent bone tissues, retrieved at the revision surgery, were collected for histological examination. None of these implants were revised for loosening. Afterwards, there was a qualitative evaluation of the bone as well an assessment of established quantitative bone tissue parameters. Results were analyzed in correlation to the time to revision surgery, as well as to patient's age.

RESULTS: A significant decrease of the bone volume and trabecular architecture underneath the cap was already observed after a relatively short lifetime. The quantitatively measured bone loss does not depend on the patient's age, but correlates significantly with the lifetime of the implants. In contrast to these findings within the central areas, an increased bone remodeling with bone densifications and microcallus formations was noticed at the edge of the cap in most cases.

DISCUSSION: A significant reduction of the bone volume underneath the cap and remarkable changes of the trabecular architecture confirm our hypothesis. Even if these changes did not lead to a clinically relevant loosening in our series, they might influence revision surgeries. Thus, these findings might be of interest, especially when a non-stemmed reversed or anatomical revision arthroplasty with a central metaphyseal press-fit fixation is chosen.

LEVEL OF EVIDENCE: IV - retrospective or historical series.