Abstract
Puropose
Three-dimensional (3D) templating based on computed tomography (CT) in total hip arthroplasty improves the accuracy of implant size. However, even when using 3D-CT preoperative planning, getting the concordance rate between planned and actual sizes to reach 100% is not easy. To increase the concordance rate, it is important to analyze the causes of mismatch; however, no such studies have been reported. This study had the following two purposes: to clarify the concordance rate in implant size between 3D-CT preoperative planning and actual size; and to analyze risk factors for mismatch.
Materials and Methods
A single surgeon performed 149 THAs using Trident Cup and Centpillar Stem (Stryker) with CT-based navigation between September 2008 and August 2011. Minimal follow-up was 2 years. Patients with incomplete postoperative CT were excluded from this study. Based on these criteria, the study examined 124 hips in 111 patients (mean age, 60 years, mean BMI 23.2 kg/m2). The preoperative diagnosis was primary osteoarthritis in 8 hips, secondary osteoarthritis in 102 hips, osteonecrosis in 9 hips, rapidly destructive coxopathy in 4 hips and rheumatoid arthritis in 1 hip. We compared cup and stem sizes between preoperative planning and intraoperatively used components. Radiological evaluations were cortical index and canal flare index on preoperative X-rays. We evaluated preoperative planning and postoperative components for cup orientation, cup position, and stem alignment (anteversion, flexion and varus angle) on the CT-navigation system. Fixation of the stem was evaluated by X-ray radiography at 2 years postoperatively according to Engh's criteria. Statistical analysis was performed with the Mann-Whitney U test, and values of P<0.05 were considered statistically significant.
Results and Discussion
The concordance rate in cup size between preoperative planning and used implants was 94.4% (117/124 hips) (CS group). A one-size larger cup was used in 4 hips (CO group), and a one-size smaller cup was implanted in 3 hips (CU group). No significant difference was seen between the CS group and the CO or CU groups in change of cup orientation and cup position from planning (P>0.05) (Table 1). The concordance rate of stem size between preoperative planning and used stem was 85.5% (106/124 hips) (SS group). A one-size larger stem than the plan was used in two hips (SO group), and a one-size smaller stem than the plan was implanted in 16 hips (SU group). Significant differences were seen between the SU and SS groups in flexion angle, varus angle, and canal flare index (P<0.05, Table 2). Extension or varus of the stem, or an increase in canal flare index, were risk factors for the used stem size being smaller than planned. On the latest follow-up X-rays, all 124 hips showed bone ingrown stability of the implants.
Conclusion
The accuracy of implant size selection was 94.4% and 85.5% for the cup and stem, respectively. No factors associated with cup size mismatch were identified. Flexion angle, varus angle, and canal flare index were associated with stem size mismatch between preoperative planning and the actual used size.
