A 75-year-old female presented to the emergency department in September 2018 after a domestic trauma. She had the history of right hip severe coxarthrosis treated with cementless THA in 1998.
The patient underwent anterior-posterior (AP) radiographs of the pelvis and a cross-leg view of the hip. No signs of fracture or any sign of loosening around the acetabulum and the stem were evident Fig. 1.
CT scan of the pelvis with MAR protocol revealed a fracture of the posterior wall of the acetabulum apparently with an non-displaced acetabular cup Fig. 2.
CT parameters on the GE Optima 580 W (General Electric Healthcare, Chicago, IL, USA) were kept constant: Helical scan, 120 kv, 0.5 sec of rotation time, Pitch 1.375:1. Slice thickness was set to 1 mm and spacing at 0.8 mm.
Specific MAR algorithm, Smart Metal Artifact Reduction software (General Electric Healthcare, Chicago, IL, USA) was used to reduce noise and improve the image quality.
3D Modeling and Rapid Prototyping Process
3D digital model of the pelvis based on CT scan was performed using a medical image processing software (Mimics Innovation Suite 14.12, Materialise, Leuven, Belgium). Segmentation differentiate bone from prosthetic implants and the surrounding soft tissue.
The first phase is thresholding, which includes all voxels whose density is within a specified range of Hounsfield Unit (HU) values. We used a mask with a HU range from 130 to 1750 in order to exclude metallic and ceramic implants and include both cancellous and cortical bone.
The final segmentation, with the removal of soft tissues and artifacts, was manually performed using additional tools of the software (Fig. 3 a,b). Eventually, both femurs and metal implants were digitally removed from the corresponding pelvis and a 3D image of the isolated region of interest (ROI) was created.
A bone quality map of the acetabulum was obtain according to the overall bone thickness and cortical thickness of the different regions using a color gradient from red (inferior) to green (excellent; Fig. 3 c,d). Further analysis included measures of shape, area and spatial location of the fracture and the acetabular bone loss and the center of rotation, compared to the contralateral acetabulum ( Fig. 3 e,f).
At last a STL. (stereolithographic) file format was transmitted to a 3D printer (Form 3L, Formlabs) for the realization of a life-size model of the entire pelvis Fig. 4.
Classification, Planning and Surgical Procedure
The analysis of the 3D images and the 3D printed life size model showed additional information.
- On the 3D images the posterior wall fracture was re-classified as an incomplete posterior column and medial wall acetabular fracture.
- On the digital 3D model the fracture was classified as a “spontaneous fracture with less than 50% of bone stock loss” according to Della Valle and Paprosky [13, 14].
- Bone quality map showed poor bone quality of the posterior and medial wall, and global bone loss.
Preoperative templating was performed on the 3D printed model.
The treatment strategy was chosen according to the algorithm proposed by Simon et al. [14, 15, 16], which suggest the acetabular revision surgery bridging or distracting the fracture, without fracture fixation. The medial wall bone loss was filled with a patch of synthetic bone substitute (RegenOss, FinCeramica Faenza S.p.A.) and a cementless acetabular cup with three iliac flanges and a caudal hook, manufactured in trabecular titanium, was press fitted in the acetabulum (Delta Revision TT, Lima), augmented trough four additional iliac screws.
Outcome
Postoperative AP radiographs of the pelvis and the right hip, showed a well-positioned and fixed implant Fig. 5.
After 3 months from the surgery, the patient underwent to a CT scan of the pelvis, which showed the complete healing of the fracture, with callus formation, and the bone integration of the trabecular cup ( Fig. 5 b,c). The DICOM images were used to build a 3D digital model, which confirmed the CT scan findings ( Fig. 6 ).