Abstract
Introduction
Proper femoral stem and acetabular implant orientation is critical to the initial and long-term success of THA. Post-operative determination of cup and stem anteversion is important in cases of hip instability and planning isolated component revisions. At ISTA 2010 Dubai, we introduced a novel, simple stem modification that can be added to any stem design to help assess stem, and possibly cup anteversion with plain post-operative radiographs throughout the lifespan of the implant. [Figure 1] As the stem is rotated, the visible hole pattern changes. [Figure 2] This study was performed to further validate the accuracy and potential usefulness of this design.
Methods
We prospectively reviewed 100 consecutive THA cases using the stem reference hole modification on rectangular tapered Zweymuller-type stems implanted from September 2010 to May 2012. Post-operative hip/femur CT scans were obtained to determine the true cup and stem orientation to validate and quanitify the precision of the reference holes. Intra-operative estimates of stem anteversion and combined anteversion (Ranawat Sign) were recorded. Post-operative radiograph measurement of stem anteversion (AP hip x-ray with leg in neutral rotation) was obtained and compared to the CT scan measurement referencing stem rotation relative to the knee epicondylar axis. [Figure 3] In addition, we compared the modified reference hole anteversion assessment to a control group of original unmodified stems assessed using the same methods.
Results
All 100 patients had post-operative CT scans and ‘neutral’ rotation AP hip radiographs. The modified reference hole design was accurate to within 4.1 degrees compared to CT measurements. Estimates of stem anteversion in the control group (original Alloclassic or SL-Plus stems) was accurate to only 19.6 degrees with wide variablity as expected. The difference was statistically significant. Residual hip flexion contracture (2 patients) made the reference holes undetectable on radiographs. Morbid obesity did not decrease accuracy but required x-ray beam intensity modification. There was no statistical difference between standing and supine x-ray ‘neutral’ rotation radiograph measurements. The Ranawat combined stem and cup anteversion value could not predict cup anteversion reliably when subtracting the stem rotation. Two patients sustained post-operative THA dislocations that required closed reduction (occuring 2 months and 15 months after index THA).
Conclusion
We conclude that hip stems with this pattern of modified anteversion reference holes provides an accurate and reliable method of determining stem component orientation post-operatively by using only simple plain radiographs. Initial finite element analysis of the modified stem hole pattern predicted that the fatigue strength was actually higher than that of the original unmodified implants indicating it is safe in the square taper design. The clinical usefulness became apparent when two hips in this series suffered dislocations. Review of the ‘neutral’ rotation xrays indicated the stem was placed in the ‘safe zone’ from 15–25 degrees and the hips would not likely need stem implant revision. We will continue to test this technology and improve the measuring techniques to accurately predict implant position post-operatively.
