Abstract
INTRODUCTION:
In order to obtain a secure taper connection it is advised to clean and dry the metal cup before assembling a ceramic insert. A slight axial tap using a plastic impactor completes the insertion procedure.
There are a few reported cases that the taper connection failed intraoperatively although it was inserted and impacted as recommended. A conceivable reason seems to be a high amount of fluid in the gap between insert and cup (e.g. from rinsing process, blood) that prevent the insert from being securely fixed due to its incompressibility.
Methods:
Cups embedded in a cast resin have been used in an appropriate impaction test setup. Four different amounts of 1.75% polyvinyl pyrrolidone solution with comparable viscosity to that of blood were filled into the metal cups (figure 1). To obtain reference values, tests were made with dry metal cups (0%), too.
Three different in-vivo like test conditions were considered:
The fluid
- 1
cannot escape from the gap
- 2
can permeate through a low permeable screen cloth
- 3
can permeate through a high permeable screen cloth.
The screen cloth should represent different cancellous bone densities. Ten Ceramic inserts of each size (28 and 36 mm) made of pure alumina (BIOLOX® forte) were impacted axially into the cups resulting in a peak force of approximately 1200 N, measured by a load cell (see figure 2). Ensuring the exact level of fluid before impaction for conditions 2 and 3, two different hydrophobic screen clothes were fixed across the central hole of the cup. During impaction the fluid could permeate through the screen cloth. To assess the connection strength after impaction, push-out forces have been measured.
RESULTS
The values of condition 1 showed higher impaction forces at higher fill levels. In contrast, the values for condition 3 decrease at a higher filling level (100%, p = 0.03). The standard deviations of the impaction forces increased with increasing filling level. At 90% and 100% filling level the push-out forces were very low (see figure 3). Condition 1 even showed a push-out force of 0 N (filling level 100%). All values also showed very high standard deviations.
DISCUSSION
The results show that a low level of fluid only had a minor influence on impaction and push-out forces. They also show that impacting against a fluid filling (filling level 90–100%) does not lead to sufficient connection strength. Most likely, the fluid reduces the relative motion between insert and cup due to its incompressibility, leading to lower locking strength. The possibility of drain through the cup holes reduces the risk of insecure connection strength (see figure 3). The findings and the proposed mechanism suggest that the same risk exists for non-ceramic inserts.
In conclusion, it is necessary to clean and dry the cup intraoperatively before assembling the insert. Nevertheless, the paper does only reflect one aspect of the surgical procedure. For proper handling of the ceramic components, the IFU of the manufacturer have to be followed.
