Objective: The objective of the present study was to evaluate and compare the initial stability of a novel, custom-fit 3D-printed femoral stem to a commercially available stem with a successful long-term clinical history, under combined dynamic axial and torsional loading.
Methods:  A custom designed 3D-printed short femoral stem and a commercially available non-cemented flat-tapered wedge stem were used. The custom stem was designed and printed according to the dimensions of the femur, in order to achieve maximal cortical contact with the proximal femur, based on the 3D geometry of the composite bones (Sawbone Model number 3406-5), as it has a weaker synthetic cancellous structure, thought to be more representative of the poor quality bone. A combined dynamic axial and torsional load was applied cyclically in increasing increments at a rate of 1 Hz. Loading commenced at 200 N and 1 Nm of external rotation, increasing in amplitude with time until reaching a peak axial load of 2000 N and a peak torque of -5/+20Nm internal/external rotation. At each level of loading, 500 cycles were applied for a total of 5,500 cycles. Two microminiature DVRT transducers were placed in the proximal bone to measure proximal anteroposterior and mediolateral motion of the stem relative to the cortical shaft, and two were placed at the distal tip of the stem. 
RESULTS:  The peak per-cycle motion at the implant-bone interface was calculated at each of the four transducer locations, during the 200N compressive loading cycles. During these initial cycles under minimal loads, the micromotion of all specimens with custom stems remained below 3 microns, at all transducer locations. For the specimens with standard stems, the micromotion generally stayed below 5 microns, but distally, one specimen underwent approximately 20 microns of motion, and another, 11 microns. Despite these low values, the standard stems displayed statistically significantly greater motions at the proximal lateral location (p=0.004) and the distal anterior location (p=0.006). Similarly, the peak per-cycle motion at the implant-bone interface was also calculated for the final cycles of compressive loading at 2,000N. During these final cycles of high loads, the micromotion of all specimens with the 3D custom stems remained below 25 microns at all transducer locations. Conversely, for specimens implanted with the standard stems, the micromotion was much higher, remaining below 100 microns in most cases, but exceeding 100 at the distal lateral location in one specimen and the proximal lateral location in another specimen. Accordingly, the standard stems displayed statistically significantly greater motions at all transducer locations than the custom stems (p<0.04 in all cases).
Conclusion:  As expected, micromotion levels at every location increased at the end of the cyclic loading. Specifically, at all four transducer locations, with the custom stem, micromotion remained below 30 microns. In contrast, with the standard stems, micromotion increased to above 50 microns in all locations, and above 150 microns in one specimen. The cyclic motions measured for the specimens with the standard stems, were statistically higher in every location compared to the custom stem (p<0.04). Previous studies have established the threshold for implant stability to achieve successful bone ingrowth to range from 28-40 microns and for fibrous ingrowth, as high as 150 microns [1, 2]. In light of these thresholds, the custom stem in the present study met or exceeded these standards. On the other hand, the design of the standard stem used in this study has been well established in the literature, with some authors reporting success as high as 100% for aseptic loosening at 29 years [3]. 

SIGNIFICANCE/CLINICAL RELEVANCE: The findings from the present study are very promising with regard to the custom stem; however, more studies are needed to evaluate the potential and feasibility of this novel custom 3D-printed technology for total hip replacement. Clearly as every stem is intended to be unique for individual patients, a wide array of specifications that vary in proximal anatomy and bone quality will be necessary to evaluate.

1886 - Mechanical Testing of a Novel 3D-Printed Femoral Stem Under Dynamic Loading