Abstract
INTRODUCTION
Glenosphere disengagement can be a potential serious default in reverse shoulder arthroplasty [1]. To ensure a good clinical outcome, it is important for the surgeon to obtain an optimal assembly of the glenosphere - base plate system during surgery. However interpositioning of material particles (bone, soft tissue) between the contact surface of the glenosphere and the base plate and/or a misalignment of the glenosphere relative to the base plate can result in a suboptimal assembly of the glenosphere – base plate system [2]. This misalignment is typically caused by unwanted contact between the glenosphere and the scapula due to inadequate reaming. Both defects prevent the Morse taper from fully engaging, leading to a system configuration for which the assembly was not designed to be loaded in vivo. This study quantifies the influence these defects have on the relative movement between the glenosphere and metaglene.
MATERIALS AND METHODS
A biaxial test setup [Fig. 1] was developed to mechanically load the glenoidal assembly (base plate + glenosphere) of 5 Depuy® Delta Xtend 38 prostheses. The setup allows applying a cyclic loading pattern to the glenoidal component with a constant actuator load of 750 N. Each of the 5 samples was tested for 5000 cycles on 3 defects: an interpositioning of 150 µm thick (0.48 mm3) and two local underreaming defects, pushing one side of the glenosphere up 0.5 mm and 1 mm respectively, hence causing a misalignment. The relative movement was recorded using 4 Linear Variable Differential Transducers (LVDTs). The cycling frequency is 1 Hz.
RESULTS
A mean increase in relative movement of 26.84% (standard deviation: +- 18.2 %) and 38.04% (standard deviation +- 28.73%) was measured for respectively the 0.5 and 1 mm misalignment defect. The interpositioning of material with a thickness of 150 µm thick caused the relative movement between glenosphere and metaglene to increase by 38.5 % (standard deviation +-26.56 %). For each sample and each defect the changes in relative movement between an optimal assembly and the suboptimal assemblies were significant at the 1% level.
DISCUSSION AND CONCLUSIONS
Relative cyclic movement between two components is an important wear and fatigue parameter. An increase in this parameter might lead to increased wear and fatigue problems. The results show how interpositioning and misalignment defects are linked to an important increase in relative cyclic movement between the glenosphere and metaglene and thus underline the importance of avoiding both defects leading to a suboptimal assembly during surgery.
