Patient-specific instrument for acetabular cup orientation: accuracy analysis in a pre-clinical study.
Takehito Hananouchi, MD, PhD1, Elke Giets, Eng, PhD2, Johan Ex2, Hendrik Delport, MD, PhD3
1. Medical Engineering Laboratory, Department of Mechanical Engineering, Faculty of Engineering, Osaka Sangyo University, Daito, Japan.
2. Orthopedics Department, Materialise N.V., Leuven, Belgium.
3 Faculty of Engineering Science, Biomechanics Section, KU Leuven, Leuven, Belgium.
Abstract: Back ground Optimal alignment of the acetabular cup component is crucial for good outcome of total hip arthroplasty1. Increased accuracy of implant positioning may improve clinical outcome by decreasing wear and reducing dislocation rate. To achieve this, patient specific instrumentation (PSI) is developed. Methods A patient-specific guide (fig.1) manufactured by 3D printing was designed to aid in positioning of the cup component with a pre-operatively defined anteversion and inclination angle. The guide fits perfectly on the acetabular rim. An alignment K-wire in a pre-operatively planned orientation is used as a visual reference during cup implantation2. Accuracy of the device was tested on 6 cadaveric specimens. During the experiment, cadavers were positioned in lateral decubitus and stabilized to allow for a THA procedure using a posterolateral approach. A normal-sized incision was made and soft tissue was removed from the acetabular region as in the conventional surgical procedure for THA. The PSI was subsequently fitted onto the acetabular rim and secured into its unique position due to its patient specific design. Then, using a power drill, the metallic pin was placed in a drill hole of PSI. Wounds were closed and cadavers were taken for postoperative CT imaging. Postoperative CT image data of each acetabulum with the placed pin were transferred to Mimics and the 3D model was registered to the pre-operative one. The anteversion and inclination of the placed pin was calculated and compared to the pre-operatively planned orientation. The absolute difference in degrees was evaluated. A secondary test was carried out to assess the error during impaction while observing the alignment K-wire as a visual reference. In a laboratory setting, an error during impaction with a visual reference of the K-wire was measured. Results Deviation from planning showed to be on average 1.04° for anteversion and 2.19° for inclination. By visually aligning the impactor with this accurately placed alignment K-wire, the surgeon may achieve cup placement as pre-operatively planned. The effect of the visual alignment itself was also evaluated in a separate test-rig showing minimal deviations in the same range. The alignment validation test resulted in an average deviation of 1.2° for inclination and 1.4° for anteversion between the metallic alignment K-wire used as visual reference and the metallic K-wire impacted by the test subjects. The inter-user variability was 0.9° and 0.8° for anteversion and inclination respectively. The intra-user variability was 1.6° and 1.0° for anteversion and inclination respectively. Tests per test subject were conducted in a consecutive manner. Discussion and conclusion We investigated the accuracy of the two main factors affecting accuracy in the cup insertion with PSI, i.e. accuracies of the errors of bony fitting and cup impaction. Since the accuracy of the two major contributing factors (i.e. fitting on bony surfaces and cup impaction) to the overall accuracy of PSI for cup insertion with linear visual reference of a metallic K-wire was within the acceptable range of 2 to 3 degrees, we state that the PSI we have designed assists to achieve the preoperatively planned orientation of the cup and as such leads to the reduction of outliers in cup orientation. As such, the acetabular cup orientation guide can transfer the pre-operative plan to the operating room. References 1. Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am., 1978 Mar, 60(2):217-20. 2. Hananouchi T, Saito M, Koyama T, Sugano N, Yoshikawa H. Tailor-made surgical guide reduces incidence of outliers of cup placement. Clin Orthop Relat Res (2010) 468:1088-1095.