Abstract
INTRODUCTION
Immediate post-operative stability of a cementless hip design is one of the key factors for osseointegration and therefore long-term success [1]. This study compared the initial stability of a novel, shortened, hip stem to a predicate standard tapered wedge stem design with good, long-term, clinical history. The novel stem is a shortened, flat tapered wedge stem design with a shape that was based on a bone morphology study of 556 CT scans to better fit a wide array of bone types [2].
METHODS
Test methods were based on a previous study [3]. Five stems of the standard tapered wedge design (Accolade, Stryker Orthopaedics, Mahwah, NJ) and the novel stem (Accolade II, Stryker Orthopaedics, Mahwah, NJ) were implanted into a homogenous set of 10 synthetic femora (Figure 1) utilizing large left fourth generation
composite femurs (Sawbones, Pacific Labs, Seattle, WA). The six degrees-of-freedom (6 DoF) motions of the implanted stems were recorded under short-cycle stair-climbing loads. Minimum head load was 0.15 kN and the maximum load varied between 3x Body Weights (BW) and 6 BW. Loading began with 100-cycles of “normal” 3 BW and was stepped up to 4 BW, 5 BW & 6 BW for 50-cycles each. Prior to each load increase, 50 cycles of 3 BW loading was applied. This strategy allowed a repeatable measure of cyclic stability after each higher load was applied.
The 6 DoF micromotion data, acquired during the repeated 3 BW loading segments, were reduced to four outcome measures: two stem migrations (retroversion and subsidence at minimum load) and two cyclic motions (cyclic retroversion and cyclic subsidence). Data were analyzed using repeated measures ANOVA with a single between-subjects factor (stem type) and repeated measures defined by load-step (3 BW, 4 BW, 5 BW 6 BW).
RESULTS
Both stems retroverted under increasing load (p = 0.0011, Fig 2). Retroversion of the novel stem was significantly smaller than that of the standard tapered wedge stem (p = 0.023). The rate of increase in retroversion with increasing load was significantly lower for the novel stem (p = 0.026). In addition, both stems subsided under increasing load (p = 0.0015, Fig 3). Subsidence of the novel stem was significantly smaller than that of the standard tapered wedge stem (p = 0.016). The rate of increase in subsidence with increasing load was significantly lower for the novel stem (p = 0.022). With regard to cyclic motions, both cyclic retroversion and cyclic subsidence were significantly lower for the novel stems (p = 0.0033 & p = 0.0098). In addition, the rate of increase in cyclic motion was significantly lower for the novel stems for both cyclic retroversion (p = 0.0021) and cyclic subsidence (p = 0.023).
DISCUSSION
In this study, the novel tapered wedge stem demonstrated an improved stability compared to the clinically successful predicate design. It appears that through optimization of the proximal geometry, a reduction in the length of the stem can be accomplished without jeopardizing initial stability.
