Abstract
Background
Cementless acetabular cups rely on press-fit fixation for initial stability; an essential pre-requisite to implant longevity. Impaction is used to seat an oversized implant in a pre-prepared bone cavity, generating bone strain, and ‘grip’ on the implant. In certain cases (such as during revision) initial fixation is more difficult to obtain due to poorer bone quality. This increases the chance of loosening and instability. No current study evaluates how a surgeon's impaction technique (mallet mass, mallet velocity and number of strikes) may be used to maximise cup fixation and seating.
Questions/purposes
(1) How does impaction technique affect a) bone strain & fixation and b) seating in different density bones? (2) Can an impaction technique be recommended to minimize risk of implant loosening while ensuring seating of the acetabular cup?
Methods
A custom drop tower was used to simulate surgical strikes, seating acetabular cups into a synthetic bone model (Fig. 1). Strike velocity (representing surgeon strike level) and drop mass (representing mallet mass) were varied through representative low, medium and high levels. Polar gap between the implant and bone was measured using optical tracking markers. Strain gauges were used to measure acetabular rim strain. Following seating, cup pushout force was measured in a materials testing machine. Both measurements were used to quantify the level of fixation of the implant for two conditions: For the first, the cup was optimally seated (moving no more than 0.1mm on the previous strike, representing ideal conditions); For the second the cup was impacted 10 times (excessively impacted). Repeats (N = 5) were conducted in low and high density bone; a total of 180 tests.
Results
For ideally impacted cups, increasing mallet mass and velocity improved fixation and reduced polar gap. However a phenomenon of bone strain deterioration was identified if an excessive number of strikes were used to seat a cup, resulting in loss of implant fixation. This effect was most severe in low density bone (Fig. 2). For high strike velocity and mallet mass, each excessive strike halved the measured bone strain (78 ± 7 με/strike). This reduced fixation strength from 630 ± 65 N (optimally seated) to just 49 ± 6 N at 10 strikes (Fig. 3).
Discussion
These results identify a possible mechanism of loss of implant stability with excessive acetabular impaction. A high mallet mass with low strike velocity resulted in satisfactory fixation (442 ± 38 N) and polar gap (1 ± 0.1 mm) whilst minimizing the fixation deterioration due to excessive mallet strikes. Extreme caution must be exercised to avoid excessive impaction high velocity strikes in low density bone for any mallet mass.
Conclusion & Clinical relevance
As it may be difficult for a surgeon to accurately infer when an implant is optimally seated, this study informs surgeons of the effects of different impaction techniques, particularly in lower density bones.
For any figures or tables, please contact the authors directly.
