JY LAZENNEC1, A RANGEL1, A BAUDOIN2, D MITTON2, W SKALLI2,Y CATONNE1
1. Service Chirurgie Orthopédie – Hôpital pitié Salpêtrière – Paris
Accurate evaluation of pelvis position in functional situations as standing, lying or sitting positions may help the surgeon for optimization of THP implants adjustment. The analysis of complications after total hip arthroplasty (THA), such as prosthesis dislocation , lower limb length discrepancy or rotational problems, is limited to standard frontal X-rays and classical Computed Tomography Scanner (CT-scan). In order to conclude, the surgeon needs to match the 2D data of the plane X-Rays and the transverse cuts of the CT scan in lying position. Large AP X-Rays of the full lower limbs Xrays are not currently available. In addition, plane X-Rays are only a projection on a plane and the valgus or varus angle of the knee may be artificially increased by the rotation of the lower limb. Moreover, a complete exploration induces significant irradiation on the patient. In addition, pelvic tilt and rotation are not considered and CT Scan in supine position does not replicate the functional configurations, especially in sitting position.
The EOS® system is based on gaseous detector technologies, discovered by Nobel Prize laureate G Charpak , and 3D stereo radiographic reconstruction techniques . The EOS™ low irradiation 2D-3D X-ray scanner is an innovative technology already used for global spinal evaluation; this system can also be used for lower limbs exploration.
EOS® is a digital X-Ray imaging device that allows the simultaneous acquisition of two orthogonal planar images in a vertical scanning mode, including the lower limbs. Imaging can be done in standing, sitting or squatting positions with a very low artifact level in case of metallic implants.
The native images obtained by the EOS® system are strictly comparable to plane X-Rays and the same measurements can be done. EOS provides low-dose high-definition digital radiographs (0.5 mm resolution, 30 000 gray levels, 200 μGy per shot).
The computerized treatment of the signal generates a 3D virtual reconstruction of the corresponding bony shapes. Transverse reconstructions, like the ones obtained with a CT scan, can be obtained as well but in standing ,sitting or squatting positions. Thanks to these 3D models, a quantification of morphological and positionnal parameters is avalaible.
In our experience, the EOS® imaging device has been used as a new diagnosis system for patients undergoing postoperative THA dysfunction when an anatomical problem was suspected. In some cases it has been used to diagnose combined spine-hip or hip-knee problems
Morphological parameters (pelvic incidence, femoral torsion and cumulative rotations of the lower limbs) can be obtained from the native images or from the 3D models and compared with the data issued from plane X Rays and CT Scans. Positionnal parameters (sacral slope, anterior pelvic plane, acetabular functional anteversion) can be calculated in the standing, sitting and squatting positions.
The radiation dose of an EOS® radiograph is about 10 times lower than that of a conventional radiograph and 800 to 1000 times less for 3D reconstruction from CT scan cuts. The mean error between the EOS® 3D models and the CT-scan was 3.5° to 5° according to the studied parameters. 21% of the patients presented with a significant pelvic axial rotation in standing position ; the evolution of this rotation was unpredictible in sitting or squatting positions. The anterior pelvic plane is classically used as a reference plane for THP navigation systems; it is supposed to be vertical in standing position. This orientation was not observed in 65% of the patients ; modifications of the anterior pelvic plane orientation were significant and unpredictible in sitting and squatting positions.
In some cases, the influence of hip-spine pathological relationships was highly significant to explain THA instability. The side effects of THA implantation on knee function could be demonstrated in some cases.
The biplanar X-Ray device with 3D reconstruction provides accurate 3D reconstructions which could be of major interest to evaluate the functionnal positions of the implants after THA and the influence of the 3D pelvis position and lower limb torsions. New questions are pointed out regarding the reference planes for THA evaluation. The use of EOS® for the global functionnal imaging of the lower limbs is very promising especially for the analysis of native knee and TKP dysfunctions.