2.1 General information
25 DDH patients treated in The Second Affiliated Hospital of Soochow University for DDH in the period of February 2017 to May 2018 were involved in this study. The cases (Crowe Ⅱ of 5 cases, Crowe Ⅲ of 14 cases and Crowe Ⅳ of 6) were randomly divided into the 3D printing guide plate group (12 cases) and the traditional total hip arthroplasty group (13 cases), all patients were treated with THA by the same operators. There were 21 males and 4 females, aged from 40 to 75 years with an average of 61.7 years. The post-operation follow-up was carried out from 12 to 18 months, 14.6 months averaged. Because of the need to compare the position of the acetabular cup with the contralateral acetabulum, patients with bilateral DDH or born structural abnormality of contralateral acetabulum were excluded.
2.2 3D printing
DICOM data obtained by CT scan of patients’ pelvises were loaded into Mimics15.0 software (Materialise, Leuven, Belgium) for 3D reconstruction (Fig. 1). With the reference to the contralateral acetabulum center, the real acetabulum at the affected side was located, after setting the position, set the angle according to the contralateral acetabular angle, the size of the acetabular cup would not break through the anterior and posterior walls of the affected real acetabulum. Then, a ring was made and it fell on the same level as the rim of the cup and 2mm larger than the diameter of the cup, this ring represents the position and size of the cup. Using the irregular bone surface of the fake acetabulum as the foundation bed, reverse-forming technique was used to make the matching surface with it, the base was extended and connected with the ring, shaping a guide plate. The base and the ring were detachable with a card slot, 3 K-wires fixing holes were reserved on the base (Fig. 1). The data was saved as STL format and loaded into the 3D printer, a guide plate and actual-sized pelvis model were generated, the printing material was polylactide.
2.3 Operation
At first, simulating operation was performed on the pelvis model (Fig. 2). Then an anterolateral incision of the hip was used to reveal the hip joints, the soft tissues surrounding the fake acetabulum were cleaned up and the bony structure was revealed, the base of the guide was completely matched with the fake acetabulum and fixed by 3 K-wires. The ring was installed into the card slots on the base, fixed with 3 K-wires, the ring pointed to the real acetabulum, then put the acetabular grinding drill into the ring, the grinding plane was parallel to the ring and at the center of the ring. As the grinding diameter was 2mm smaller than that of the ring, took off the guide, the metal cup was placed in the same direction as the ring (Fig. 3). Followed by the completion of acetabular processing, the femur side was treated routinely until the THA was completed (Fig. 4).
2.4 Clinical assessment
For each of those patients, operation time, intra-operative hemorrhage, post-operative drainage and Harris scores were evaluated, CT scanning of bilateral hip joints were used to evaluate the abduction and anteversion angle of the acetabulum, as well as the distance from rotation center to ischial tuberosity connection.
2.5 Statistics
SPSS 17.0 statistical software (SPSS Inc., USA) was applied. All measuring data were expressed as Mean ± Standard deviation. Comparison among groups shall be performed with independent samples t-test and intra-group comparison before and after the surgery with paired t test. As for measuring data, comparison among groups was performed with χ2 test, with the testing level α = 0.05.